02-BSS Function description
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Feature Description M900/M1800 Base Station Subsystem
Table of Contents
Table of Contents Chapter 2 BSS Functions ............................................................................................................. 2-1 2.1 Basic Functions.................................................................................................................. 2-1 2.1.1 Overview ................................................................................................................. 2-1 2.1.2 Channel ................................................................................................................... 2-2 2.1.3 System Information ................................................................................................. 2-9 2.1.4 Idle Mode Behavior ............................................................................................... 2-15 2.1.5 PLMN Selection..................................................................................................... 2-18 2.1.6 Cell Selection and Reselection ............................................................................. 2-19 2.1.7 Location updating .................................................................................................. 2-24 2.1.8 Access................................................................................................................... 2-32 2.1.9 Paging ................................................................................................................... 2-33 2.1.10 Immediate assignment ........................................................................................ 2-35 2.1.11 Assignment.......................................................................................................... 2-44 2.1.12 Authentication...................................................................................................... 2-45 2.1.13 Ciphering ............................................................................................................. 2-48 2.1.14 DTX ..................................................................................................................... 2-52 2.1.15 Frequency hopping ............................................................................................. 2-55 2.2 Extended Functions ......................................................................................................... 2-60 2.2.1 Handover............................................................................................................... 2-60 2.2.2 Power Control........................................................................................................ 2-74 2.2.3 Extended Cell ........................................................................................................ 2-86 2.2.4 IUO ........................................................................................................................ 2-89 2.2.5 "HW-IUO Property"Satellite Transfer .................................................................... 2-95 2.2.6 Diversity Receiving................................................................................................ 2-97 2.2.7 Aggressive Frequency Reuse Pattern .................................................................. 2-99 2.2.8 Multiband Network .............................................................................................. 2-104 2.2.9 Carrier Mutual-assistance ................................................................................... 2-116 2.2.10 Cell Broadcast ................................................................................................... 2-119 2.2.11 Radio Channel Allocation.................................................................................. 2-121 2.2.12 Half Rate ........................................................................................................... 2-125 2.2.13 E1 Ring Topology.............................................................................................. 2-127 2.2.14 GSM900/GSM1800 Co-cell............................................................................... 2-129 2.2.15 Multi-MNC ......................................................................................................... 2-131 2.2.16 E-GSM/R-GSM.................................................................................................. 2-135 2.3 GPRS Function .............................................................................................................. 2-137 2.3.1 Supported Packet System Information ............................................................... 2-137 2.3.2 Supported GPRS MS Modes .............................................................................. 2-141
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Feature Description M900/M1800 Base Station Subsystem
Table of Contents
2.3.3 Supported RLC Modes........................................................................................ 2-143 2.3.4 Supported Channel Coding Scheme .................................................................. 2-144 2.3.5 Supported Network Control Modes ..................................................................... 2-148 2.3.6 Supported Network Operation Mode .................................................................. 2-148 2.3.7 Supported QoS.................................................................................................... 2-150 2.3.8 Supported Assignment........................................................................................ 2-150 2.3.9 Supported Paging ............................................................................................... 2-151 2.3.10 Timing Advance ................................................................................................ 2-152 2.3.11 Measurement Report ........................................................................................ 2-153 2.3.12 Supported Flow Control .................................................................................... 2-153 2.3.13 Supported Dynamic Handover between TCH and PDCH ................................ 2-155 2.3.14 Supported Packet Access Function .................................................................. 2-155
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Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Chapter 2 BSS Functions BSS is a bridge between MS and NSS, which performs mainly the management of radio links and conversion of radio links and wire links. It is responsible for the communication of MS. BSS system functions can be divided into basic functions, extended functions and GPRS functions.
2.1 Basic Functions 2.1.1 Overview Figure 2-1 illustrates the GSM Protocol. MS
L3 L2 L1
MSC
BSC
BTS
CM
CM
MM
MM
RR
RR RR
LAPDm Sign. Layer1
BTSM
BTSM
LAPDm LAPD Sign. Sign. Layer1 Layer1
LAPD Sign. Layer1
SCCP
SCCP
MTP
MTP
Abis
Um
BSSMAP
B BSSMAP
A
MS: Mobile Station BTS: Base Transceiver Station BSC: Base Station Controller RR: Radio Resource Management MSC: Mobile services Switching Centre, Mobile Switching Centre MTP: Message Transfer Part (MTP) SCCP: Signaling Connection Control Part LAPD: Link Access Procedure on the D channel MM: Mobility Management LAPDm: Link Access Procedure on the Dm channel CM: Connection Management BSSMAP: Base Station Subsystem Management Application Part BTSM: Base Transceiver Station Site Management
Figure 2-1 GSM protocol stack According to GSM 04.07, the functions of BSS on layer 3 and related sub-layers on the radio interface (Um) are classified into: 1)
RR: Radio Resource Management
2)
MM: Mobility Management
3)
CM: Communication Management
Where the functions on the MM and CM sub-layers are supported by the DTAP between A- and Um interfaces. The functions of RR sub-layer that include the Huawei Technologies Proprietary 2-1
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
maintenance and release of radio resources are mainly carried out by BSS. There are corresponding communication management protocol for A interface and Abis interface to realize the air interface between GSM network and MS. The other functions of BSS are also essential for establishing communication between the GSM network and MS. The functions (RR) that BSS involves are mainly as follows: z
Radio channel management
z
Channel coding/decoding
z
Transcoding & Rate Adaptation
z
Full-rate & half-rate coding of speech and enhanced full-rate coding
z
Encryption/Decryption
z
Frequency hopping
z
Antenna Diversity
z
RF Power control and handover management
2.1.2 Channel I. Types of Radio Channels According to GSM/GPRS specifications, the radio channels fall into two major categories, which are Traffic Channel and Control Channel. A traffic channel s further divided into Speech Traffic Channel, Circuit Data Traffic Channel and Packet Data Traffic Channel, while the Control Channel is subdivided into Broadcast Channel, Common Control Channel and Dedicated Control Channel. Logical channel
CCH
TCH
DCCH BCCH
CCCH
SDCCH Downlink
SCH FCCH BCCH (BCCH1) (BCCH2) (BCCH3)
PCH
ACCH
Uplink
AGCH
RACH
SACCH
FACCH
Downlink Downlink/Uplink
Figure 2-2 GSM/GPRS channel classification Huawei Technologies Proprietary 2-2
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Figure 2-2 illustrates the logical channels. Below is the introduction.
II. Traffic Channel 1)
Speech traffic channels
In the latest GSM 05.02, the speech traffic channels are divided into: z
TCH/FS: full rate traffic channel for speech.
z
TCH/HS: half rate traffic channel for speech.
z
TCH/EFS: enhanced full rate traffic channel for speech.
z
TCH/AFS: adaptive full rate traffic channel for speech.
z
TCH/AHS: adaptive half rate traffic channel for speech.
Huawei BSS currently supports three types of traffic channels for speech: TCH/FS, TCH/HS and TCH/EFS. 2)
Circuit data traffic channel
In the most updated GSM 05. 02, the circuit data traffic channels are divided into: z
TCH/F9.6: full rate traffic channel for 9.6 kbit/s user data.
z
TCH/F4.8: full rate traffic channel for 4.8 kbit/s user data.
z
TCH/H4.8: half rate traffic channel for 4.8 kbit/s user data.
z
TCH/H2.4: half rate traffic channel for 2.4 kbit/s user data.
z
TCH/F2.4: full rate traffic channel for 2.4 kbit/s user data.
z
TCH/F14.4: full rate traffic channel for 14. 4 kbit/s user data.
z
E-TCH/F28.8: enhanced circuit switched full rate traffic channel for 28.8 kbit/s user data.
z
E-TCH/F32.0: enhanced circuit switched full rate traffic channel for 32.0 kbit/s user data.
z
E-TCH/F43.2: enhanced circuit switched full rate traffic channel for 43.2 kbit/s user data.
Huawei BSS currently supportsTCH/F9.6, TCH/F4.8 and TCH/F2.4. 3)
Packet Data Traffic Channel
There are two rates for the PDTCH: PDTCH: full-rate PDTCH. With GMSK modulation it can carry packet data whose momentary rates are 0~22.8 kbit/s, while PDTCH with an 8PSK modulation system can carry packet data whose momentary rates are 0~69.6 kbit/s. PDTCH is a one-way channel and categorized by the direction as: z
PDTCH/D: downlink PDTCH, for MS terminated packet transmission.
z
PDTCH/U: uplink PDTCH, for MS originated packet transmission.
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Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
III. Broadcast Channel (BCH) BCH is used to transmit broadcast messages to the MS in down link direction. It includes the following logical channels: 1)
FCCH (Frequency Correction Channel): This channel is responsible for transferring the frequency correction signals to the MS so that the MS can be adjusted to the corresponding frequency.
2)
SCH (Synchronization Channel): This channel is responsible for transmission of the frame synchronization number (TDMA frame number) and the Base Station Identity Code (BSIC) to the MS.
3)
BCCH (Broadcast Control Channel): This channel transmits the information common to all cells, such as Location Area Identity (LAI), cell maximum allowable output power, BCCH carrier frequency of the adjacent cells, and packet service system parameters.
4)
PBCCH (Packet Broadcast Control Channel): This channel transfers the messages related to packet services.
5)
Cell Broadcast Channel (CBCH): This channel is used for the cell broadcast short message services. It uses the same physical channels as SDCCH.
The channels introduced above are downlink channels.
IV. Common Control Channel (CCCH) CCCH are classified into the following four channels: 1)
Paging Channel (PCH): Downlink channel. MS tunes to and receives the information from this channel to check for any call from MSC at regular intervals.
2)
Random Access Channel (RACH): Uplink channel, through which an MS accesses the network and requests for allocating SDCCH.
3)
Access Grant Channel (AGCH): Through which the network notifies the MS about the allocation of the dedicated channel.
4)
NCH (Notification Channel): Downlink channel used for Voice Group Call Service (VGCS) and Voice Broadcast Service (VBS).
V. Packet Common Control Channel (PCCCH) PCCH includes the following four channels: 1)
PPCH (Packet Paging Channel): Downlink packet paging channel. MS tunes to the PPCH channel at a regular interval to check if there is any call from SGSN.
2)
PRACH (Packet Random Access Channel): Uplink packet random access channel. MS requests to access the network via the PRACH channel.
3)
PAGCH (Packet Access Grant Channel): Downlink channel. The network notifies the MS of the allocation of the packet data traffic channels via the PAGCH channel.
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Feature Description M900/M1800 Base Station Subsystem
4)
Chapter 2 BSS Functions
PNCH (Packet Notification Channel): Downlink channel, designed for point-to-multipoint multicast call.
Huawei BSS supports PPCH, PRACH and PAGCH.
VI. Dedicated Control Channel (DCCH) DCCH consists of the following channels: 1)
SACCH (Slow Associated Control Channel): Associated with the SDCCH or TCH. This channel is designed for MS to send received signal quality and signal intensity of adjacent BTSs to the network, and meanwhile receives the system information including transmission power, power adjustment and timing advance.
SACCH can be further divided into: z
SACCH/TF: SACCH associated with TCH/F.
z
SACCH/TH: SACCH associated with TCH/H.
z
SACCH/C8: SACCH associated with SDCCH/8.
z
SACCH/C4: SACCH associated with SDCCH/4.
z
SACCH/M: SACCH associated with TCH/F for multi-TS configuration.
2)
FACCH (Fast Associated Control Channel): FACCH implements transmission by occupying a part on TCH, mostly for transmitting handover command.
FACCH can be further divided into: z
FACCH/F: FACCH associated with TCH/F;
z
FACCH/H: FACCH associated with TCH/H.
3)
SDCCH (Standalone Dedicated Control Channel): it serves to transmit the signaling such as short message information, location updating information, etc. between the MS and the network, prior to the call setup.
z
SDCCH/8SDCCH/8
z
SDCCH/4SDCCH/4
VII. Packet Dedicated Control Channel 1)
PACCH (Packet Associated Control Channel): Downlink channel serving to transmit the signaling, including response messages and power control messages, to the MS. PACCH can also transmit the resources allocation and re-allocation messages. PACCH shares the resource with the PDTCH currently allocated to MS. When MS is in transmission mode, SGSN can page the MS via PACCH to initiate CS service.
2)
PTCCH/U (Packet Timing Advance Control Channel Uplink): PTCCH/U sends the timing advance by way of random access burst when the MS operates in a transmission mode.
3)
PTCCH/D (Packet Timing Advance Control Channel Downlink): PTCCH/D is designed to send transmission timing advance to several MSs. One PTCCH/D corresponds to several PTCCH/Us.
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Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
VIII. Radio channel management Radio channel management involves the management of diverse radio channels in the GSM/GPRS. This process occurs in the phase of connection setup, maintenance, modification and release.
IX. Radio channel combination As per the logical channel types as listed above, a user can configure the following channel combinations in the M900/M1800 BSS. z
TCH/F+FACCH/F+SACCH/TF
z
SDCCH/8+SACCH/C8
z
FCCH+SCCH+BCCH+CCCH
z
FCCH+SCCH+BCCH+CCCH+SDCCH/4+SACCH/C4
z
BCCH+CCCH
z
BCCH+CBCH
z
SDCCH+CBCH
z
PBCCH+PCCCH+PDTCH+PACCH+PTCCH
z
PCCCH+PDTCH+PACCH+PTCCH
z
PDTCH+PACCH+PTCCH
X. Traffic channel management BSS is in charge of all the configured traffic channels. When a call is established, MSC sends the channel type, channel code and other parameters regarding the call to BSS, which chooses a traffic channel based on the messages. BSS also assumes the task for the measurement and release of these traffic channels.
XI. Dedicated control channel management BSS manages all the available dedicated control channels. After MS has sends a random access request via RACH or PRACH, BSS will allocate a DCCH for the MS. Besides, BSS is also responsible for monitoring and releasing the link of DCCH.
XII. Broadcast channel and common control channel management The management of the available broadcast channels and common control channels by the BSS involves DRX management, paging message dispatching, AGCH and PAGCH control, RACH and PRACH control, and BCCH message broadcast.
XIII. Terrestrial channel management The management of terrestrial channels between BSS and MSC is to keep the terrestrial circuit states at BSS and MSC consistent so that an idle circuit can be
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Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
available when MSC makes a call (“assign circuit”) and when MS performs handover (“assign terrestrial circuit”). This is to ensure the success for the call and the handover. Procedures included in the A-interface circuit resource management are Circuit Block/Unblock, Circuit Group Block/Unblock, Unequipped Circuit, and Reset Circuit. General principles of the circuit control includes: z
Circuit management message is normally initiated by BSC. While resetting circuit can be initiated either by MSC or BSC,
z
MSC can only block or unblock its circuits without affecting the circuits at the BSS side.
z
The BSS can not change the circuit state that has been changed at the local end of the MSC. For circuits blocked on the maintenance console at MSC side, the BSS has no authority to unblock or reset the circuit.
XIV. Channel Coding & Decoding The messages are encoded/decoded before being transmitted on the radio channel to avoid radio channel interference. There are various coding and interleaving methods for different logical channels (speech, data and signaling). For a detailed description of the coding methods for various channels, please refer to the specifications GSM 05. 03.
XV. Transcoding & Rate Adaptation Transcoding (TC) and Rate Adaptation provides an interface between the standard 64 kbit/s transmission at NSS side and the lower rate transmission at BSS side. The conventional voice-coding mode is PCM with a rate of 64 kbit/s. It is widely applied to PSTN. Pulse Code Modulation (PCM) is used for normal speech in PLMN, at a rate of 64 kbit/s whereas in GSM, RPE-LTP or CELP coding with much lower rate (16 kbit/s) is used due to the limitation of radio channel resources. To further improve the voice quality, EFR (Enhanced Full Rate) is introduced. To implement EFR, newly designed algorithms are used but it does not affect the coding rate on the Um interface. When adopting EFR, the compression algorithm for the MS and Transcoder & Rate Adaptor Unit (TRAU) must be modified. Generally, 3.6 kbit/s and 6 kbit/s data rates on the Um interface are arranged for the 8 kbit/s or 16 kbit/s channel (for transmission either on the full-rate channel or the half-rate channel), while the 12 kbit/s rate is for the 16 kbit/s channel. If a PSTN subscriber wants to call an MS, rate adaptation must be performed for the voice. The TRAU is introduced to complete this function. When the BTS and the TRAU are physically detached, these conversions will be especially important. A detailed description of the conversions on the interfaces is given in the related GSM specifications. Huawei Technologies Proprietary 2-7
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Since the rate of each channel of existing terrestrial lines is 64 kbit/s, it is a waste if one channel is used to carry one 16 kbit/s GSM channel. To save terrestrial line resources, sub-multiplexer (SMUX) is used between MSC and BSC to multiplex 4 % 16 kbit/s channels to transmit four speech channels over one terrestrial channel. In general, TRAU and SMUX are integrated in one unit called TCSM, i. e., it handles both rate conversion and multiplexing. Table 2-1 introduces the full-rate coding/decoding process and enhanced full-rate coding/decoding process. Table 2-1 Voice coding comparison FR (Full Rate)
EFR (Enhanced Full Rate)
Algorithm
RPE-LTP algorithm (regular impulse excitation-long term prediction)
ACELP algorithm (arithmetic code book excitation linear prediction)
Coding Process
TRAU converts the voice signal received from MSC into frames in the format of 20 ms/fr. A frame of voice data contains 160 PCM sampling points, making up 1280 bit. The output parameters after encoding are 260 bit, making up the 320 bit TRAU frame together with the synchronous header and control parameter.
TRAU converts the voice signal received from MSC into frames in the format of 20 ms/fr. A frame of voice data contains 160 PCM sampling points, making up 1280 bit. The output parameters after encoding are 244 bit, making up the 320 bit TRAU frame together with the synchronous header and control parameter.
Decoding is a reverse process of coding. After TRAU receives the TRAU frames sent Decoding from the BSC, it restores them into speech Process data by applying decoding algorithm before sending them to MSC.
Decoding is a reverse process of coding. After TRAU receives the TRAU frames sent from the BSC, it restores them into speech data by applying decoding algorithm before sending them to MSC.
In the occasion of MS-MS session, the TRAU coding / encoding can be omitted. As the coding / encoding process will degrade the voice quality, it is possible to improve the voice quality by removing TRAU coding/decoding with Tandem Free Operation (TFO). TFO is implemented by FTC via in-band signaling to reduce the primary coding/decoding during MS-MS session and improve the voice quality. To set up TFO status, the following should be realized: Both parties of the session should subscribe to the same service (i.e. both to FR or EFR service). The FTCs seized by the two MSs should support TFO function. There should be no other equipment that is capable of changing the PCM signal on the PCM link between the FTCs of the MSs, i.e., it should be a direct link, because TFO message and frame are transmitted with the low bit of the PCM sampling value. If these conditions are not satisfied, FTC will perform the normal coding/decoding. TFO features: Realized in the occasion of MS-MS session; Huawei Technologies Proprietary 2-8
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
TFO can improve the voice quality of both FR and EFR, especially the former with the MOS can be improved by 0.5 points (totally 5 points).
2.1.3 System Information I. Overview System information contains the major wireless network parameter on the air interface, including network identifier parameter, cell selection parameter, system control parameter and network function parameter. By receiving system information, MS can be properly accessed and perform network selection so that it can make full use of the services and cooperate with network. There are two modes for the transmission of system information: broadcast message and channel associated message. In idle mode, MS communicates with the network via the broadcasting of system information. The network sends system information to MS so that MS knows its current position and the service type available. Some parameters can also control the cell reselection of MS. When MS is establishing calls, the communication between network equipment is realized with the channel associated system information. Network equipment sends some contents in the channel-associated message to MS so as to control the behaviors such as transmission, power control and handover of MS. The broadcast system information is closely related to the channel-associated message. The content in the broadcast system information can overlap with that in the channel associated message. While the content in the channel associated message can be inconsistent with that in the broadcast system information, because the channel associated message has the effect on only one MS, while the broadcast system information affects all MSs in idle mode.
II. Types and content of system information There are totally 13 types: 1, 2, 2bis, 2ter, 3, 4, 5, 5bis, 5ter, 6, 7, 8 and 9. Among them, 1, 2, 2bis, 2ter, 3, 4, 7, 8 and 9 are broadcast information transmitted via BCCH under idle mode; 5, 5bis, 5ter and 6 are channel associated information transmitted via SACCH in active mode. Type 1: Cell channel description + RACH control information (optional) Cell channel description: all frequencies used by this cell, including BCCH frequencies and FH frequency to provide the frequency reference for MS Frequency Hopping (FH).
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Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
RACH control information: parameters such as maximum times of retransmission (MAX RETRANS), number of transmission timeslots (TX Integer), Cell Bar Access, bit allowed for call reestablishment (RE), bit allowed for emergency call (EC) and access restricted user level (AC). These parameters are used to control the behavior of MS in the initial access. Type 2: Adjacent cell BCCH frequency description + Network color code allowed + RACH control information (mandatory) Adjacent cell BCCH frequency description: the BCCH frequency used by the adjacent cell. Network color code allowed: NCC allowed for the MS test on the BCCH carrier in the cell. Type 2bis: Adjacent extended cell BCCH frequencies description + RACH control information (optional) Extended adjacent cell BCCH frequency description: the number of frequencies described in the frequency allocation table in system information type 2 is limited, therefore system information type 2bis contains the information of other frequencies in BA1 which are in the same frequency segment as system information type 2. RACH control information: contains the maximum times of parameter retransmission (MAX RETRANS), number of retransmission timeslot (TX Integer), Cell Bar Access, bit allowed for call reestablishment (RE), Restricted user level, bit allowed for emergency call (EC) to control the MS behavior during initial access. Type 2ter: Attached multi-frequency information + extended cell BCCH frequency description 2 (optional) Attached multi-frequency information: Number of the multi-frame measurement needed. Extended adjacent cell BCCH frequency description 2: describes the extended frequency allocation table of the adjacent cell (part of BA1 table). The frequency contained in this information is located at the different frequency segment as the current cell. Therefore, only the multiband MS can read this information. The single-band GSM 900 of GSM 1800 MS will skip this information. Type 3:Cell ID + LAI + control channel description + cell option + cell selection parameter + RACH control information (mandatory) Cell ID: identifier of the current cell. LAI: location area identifier of the current cell. Control channel description: contains the MS attach/detach allowed indication (ATT, Attach-Detach Allowed), number of blocks reserved for AGCH (BS AG BLKS RES),
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Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
common control channel configuration (CCCH CONF), number of 51 TDMA multi-frames reserved for the same paging group in the paging information (BA PA MFRMS) and the interval of periodic location update. Cell option: includes the power control indication (PWRC), discontinuous transmission (DTX) and radio link timeout value (Radio Link Timeout). Cell selection parameter: includes the cell reselection hysteresis value, maximum Tx power level allowed for MS access to the cell (MS TXPWR MAX CCH) and minimum access level allowed for MS to access system (RXLEV Access MIN). RACH control information: contains the maximum times of parameter retransmission (MAX RETRANS), number of retransmission timeslot (TX Integer), Cell Bar Access, bit allowed for call reestablishment (RE), Restricted user level, bit allowed for emergency call (EC) to control the MS behavior during initial access. System information type 3 rest bytes: cell reselection parameter information and type 3 MS control information. Type 4: LAI + cell selection parameter + RACH control information + CBCH description + CBCH dynamic allocation information (mandatory) LAI: the location area identifier of the current cell. Cell selection parameter: includes the cell reselection hysteresis value, maximum Tx power level allowed for MS access to the cell (MS TXPWR MAX CCH) and minimum access level allowed for MS to access system (RxLEV Access MIN). RACH control information: contains the maximum times of parameter retransmission (MAX RETRANS), number of retransmission timeslot (TX Integer), Cell Bar Access, bit allowed for call reestablishment (RE), Restricted user level, bit allowed for emergency call (EC) to control the MS behavior during initial access. CBCH description: includes the channel type and TDMA offset (which type of dedicated channel combination), timeslot No. (TN), training sequence code (TSC), FH channel indication (H), mobile allocation index offset (MAIO), FH serial No. (HSN) and absolute RF channel No. (ARFCN). CBCH mobile allocation information: the relation between the sequence of frequencies used for FH and cell channel description. System information types 4 rest bytes: cell reselection parameter. Type 5: Adjacent cell BCCH frequency description (mandatory) Adjacent cell BCCH frequency description: the BCCH frequency used by the adjacent cell. Comparing with system information type 2, the difference is that MS can get the frequencies described in system information type 5 in active mode, and report the related information of the adjacent cell in the measurement report as the reference of Huawei Technologies Proprietary 2-11
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
handover. Similarly, the GSM900 MS in Phase 1 recognizes only the adjacent cell frequencies described in system information type 5 and ignore those contained in 5bis and 5ter. Type 5bis: Extended adjacent cell BCCH frequency description (optional) Extended adjacent cell BCCH frequency description: the number of frequencies described in the frequency allocation table in system information type 5 is limited, therefore system information 5bis contains the information of other frequencies in BA2 which are in the same frequency segment as system information 5. Type 5ter: Attached multi-frequency information + extended cell BCCH frequency description 2 (optional) Attached multi-frequency information: Number of the multi-frame measurement needed. Extended adjacent cell BCCH frequency description 2: describes the extended frequency allocation table of the adjacent cell (part of BA2 table). The frequency contained in this information is located at the different frequency segment as the current cell. Therefore, only the multiband MS can read this information. The single-band GSM 900 of GSM 1800 MS will skip this information. Type 6: Cell ID + LAI + cell option (mandatory) Cell ID: identifier of the current cell. LAI: the location area identifier of the current cell. Cell option: includes the power control indication (PWRC), discontinuous transmission (DTX) and radio link timeout value (Radio Link Timeout). Type 7: Cell reselection parameter Cell reselection parameter: includes cell reselection indication (PI), Cell Bar Qualify (CBQ), Cell Reselect Offset (CRO), Temporary Offset (TO) and Penalty Time (PT). Type 8: Cell reselection parameter Cell reselection parameter: includes cell reselection indication (PI), Cell Bar Qualify (CBQ), Cell Reselect Offset (CRO), Temporary Offset (TO) and Penalty Time (PT). Type 9: RACH control information + broadcast channel parameter RACH control information: contains the maximum times of parameter retransmission (MAX RETRANS), number of retransmission timeslot (Tx Integer), Cell Bar Access, bit allowed for call reestablishment (RE), Restricted user level, bit allowed for emergency call (EC) to control the MS behavior during initial access. Broadcast channel parameter Huawei Technologies Proprietary 2-12
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
III. Meaning and function of wireless network parameter 1)
Network identification parameters
Network identification parameters include CGI and BSIC. CGI consists of LAI and CI. LAI is composed of MCC, MNC and LAC. System information type 3,4 and 6 include all or part of CGI information. MS decodes the system information to get the CGI. MS decides whether to connect to the network in this cell according to the MCC and MNC indicated by CGI. It is also used to check whether the current location area has changed so as to initialize the location updating process. MCC, consisting of three decimal digits, is allocated worldwide in unified way. MNC, consisting of two decimal digits, is allocated by the country in unified way. LAC and CI, both consisting of 2 bytes, are arranged by GSM carrier in unified way. Note that the value range of CI is 0X0001~0XFFFE, while 0X0000 and 0XFFFF are reserved. BSIC identifies the local color code of each BTS in the GSM system. In GSM system, frequencies are multiplexed to different extents according to the different requirements in network plan. MS differentiates two cells' same frequency with their BSICs. Therefore, it is necessary to guarantee the uniqueness of BSICs of the cells using the same BCCH carrier frequency. BSIC is transmitted on the SCH of each cell. It consists of NCC (3 bits) and BCC (3 bits). Note that the TSC described in system information type 4 is the BCC of the current cell. 2)
System control parameter
System control parameter is transmitted to MS with system information via air interface by BTS. It serves to keep contact between MS and BTS. Besides, these parameters have the direct effect on the service bearing and signaling flow of various part of system. Therefore, reasonable setting of these parameters is important in maintaining of the normal operation of GSM system. IMSI attach and detach allowed (ATT) is used to notify MS whether the local cell allows IMSI attach/detach process. It is transmitted in control channel description in the system information type 3. ATT has 1 bit. "0" stands for IMSI attach/detach process not allowed, and "1" stands for the process allowed. CCCH CONF decides the integration mode of the CCCH in the cell. It is transmitted in the control channel description in the system information type 3. CCCH CONF is a 3 bit code. For details, see Table 2-2.
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Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Table 2-2 CCCH code meaning CCCH CONF
Meaning
Number of CCCH information blocks in BCCH multiframe
000
CCCH uses a basic physical channel which is not shared with SDCCH.
9
001
CCCH uses a basic physical channel which is shared with SDCCH.
3
010
CCCH uses two basic physical channels which are not shared with SDCCH.
18
100
CCCH uses three basic physical channels which are not shared with SDCCH.
27
110
CCCH uses four basic physical channels which are not shared with SDCCH.
36
Others
Reserved
Note: The CCCH CONF setting of a cell should be in line with the actual setting of the cell's CCCH. It is decided by the traffic module of the cell.
BS AG BLKS RES is transmitted in the control channel of system information type 3. It is used together with CCCH CONF to decide the number of information blocks in each BCCH of the current cell. After setting CCCH CONF, BS AG BLKS RES will be used to arrange the occupancy ratio between AGCH and PCH on CCCH. It is possible to adjust this parameter to achieve the bearing balance between AGCH and PCH. BS PA MFRAMS is transmitted in the control channel description in system information type 3. It decides how many multiframes making up a cycle of a page sub-channel. This parameter actually decides how many sub-channels the PCH of a cell will be deviled into. BS PA MFRAMS is a 3 bit code. The value range is 0~7, respectively meaning that the number of multi-frame of a paging group cycled on the PCH is 2~9. Periodic location updating timer (T3212) decides the frequency of periodic location updating. It is transmitted in the control channel description in system information type 3. It is an 8-bit code. The value range is 0~255, each unit of which is the duration of six minutes, and 0 means no location updating. Cell Channel Description, transmitted in system information type 1, describes the RF channel No. of the local cell. It is used in frequency hopping. Note that the maximum number of channels configured in cell channel description is 64.
Huawei Technologies Proprietary 2-14
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Neighbor Cells Discretion, transmitted in system information type 2, 2bis, 2ter, 5, 5bis and 5ter, describes the absolute channel No. of the BCCH TRX of the cell adjacent to the current cell. Huawei BSS supports at most 32 adjacent cells. Extension Indication, transmitted in system information type 2 and 5, indicates whether there are still extended adjacent cells to be transmitted in system information type 2bis and 5bis. It is a 1-bit code. "0" means that system information type 2 and 5 contains the complete BA table, and "1" means that type 2 and 5 contains part of BA table. BA Indication transmitted in system information type 2 and 5. It is a 1-bit code, used for MS to select the data in BA 2 before or after modification. In another word, if the adjacent cell relation of the current cell and the BA2 table is changed during a session, the BA Indication in system information type 5 will be 1 instead of stead of 0. This indicate that MS perform decoding in the adjacent cell indicated in the system information type 5 again. Multiband Reporting (MBR), transmitted in system information type 2ter and 5ter. It is a 2-bit code, indicating MS to report adjacent cell information on multiple frequency bands. It is applicable to multiband MS only.
2.1.4 Idle Mode Behavior I. Overview A powered on mobile station (MS) that does not have a dedicated channel allocated is defined as being in idle mode. The purpose of the tasks performed in the idle mode is to be able to access the system and be reached by the system from any location in the network. When a mobile is powered on, it immediately attempts to make contact with a GSM Public Land Mobile Network (PLMN). The particular PLMN contacted may be selected either automatically or manually. The MS will look for and select a suitable cell of the chosen PLMN. It will then tune to the control channel of the cell to receive information about the available services provided by the PLMN. This selecting is known as “camping” on a cell. When an MS is in idle mode it will always try to camp on the best cell according to a signal level based criterion. The idle mode behavior is managed by the MS. It can be controlled by parameters which the MS receives from the base station on the Broadcast Control Channel (BCCH). All the main controlling parameters for idle mode behavior are transmitted on the BCCH carrier in each cell. When the MS is powered on but neither making nor receiving any calls (idle mode) there has to be a mechanism that always selects the best cell on which to camp. Moreover, to be able to access the system from anywhere in the network, regardless of where the MS was powered off, it has to be able to select a Huawei Technologies Proprietary 2-15
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
specific GSM base station, tune to its frequency and listen to the system message informations transmitted in that cell. It must also be able to register its current location to the network so that the network knows where to route incoming calls. The PLMN selection mechanism, the cell selection and reselection algorithms in addition to the location updating procedure are the core of the idle mode behavior. The purpose is to always ensure that the mobile is camped on the cell where it has the highest probability of successful communication.
II. Usage 1)
High signal level when accessing the system
The MS will at all times try to obtain the highest possible signal level when accessing the system. This is achieved by means of the idle mode cell selection and reselection algorithms. These algorithms will enable the MS to choose the most suitable cell to camp on, based on signal level. A cell is suitable if certain criteria are satisfied. Camping on the most suitable cell provides the MS with a high probability of good communication with the system. The cell selection and reselection algorithms are governed by parameter settings. Using these parameters an operator can, on a per cell basis, make a specific cell more or less attractive to camp on for the MS. This makes it possible for the operator to achieve similar behavior for MSs in idle mode as in active mode. Well-designed parameter settings for cell selection and reselection in idle mode, will make the MS to camp on the cell that would have been chosen if the MS had been in active mode. 2)
Control of the paging load
In idle mode the MS will notify the network whenever it changes location area by the location updating procedure. Thus, the network will be kept updated concerning which location area the MS is presently in. When the system receives an incoming call it knows in which location area it should page the MS, and does not need to page it throughout the whole MSC service area. This reduces the load on the system. If the MS does not respond to the first paging information, then the network can send a second paging information. The MS can also, periodically and when powered on or off, notify the network of its present status by the location updating procedure. This prevents the network from doing unnecessary paging of MSs that have been powered off or left the coverage area. This would otherwise cause unnecessary load on the system. 3)
Low idle mode power consumption
In idle mode, the MS only occasionally monitors the system information being transmitted in the current cell or does measurements on neighboring cells to see if a cell change should be initiated.
Huawei Technologies Proprietary 2-16
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
However, most of the time it will be in “sleep mode”. Hence, the power consumption during idle mode will be low. This is also referred to as discontinuous reception (DRX).
III. Technical description While the MS is in idle mode it will continuously make measurements on the BCCH-carriers of serving and neighboring cells to decide on which cell to camp on. It will also, if necessary, register its presence in the location area of the chosen cell by performing a location updating. The purpose of camping on a cell is threefold: 1)
It enables the MS to receive system information from the PLMN
2)
The MS can initiate a call by accessing the network on the Random Access Channel (RACH) of the cell on which it is camped,
3)
The PLMN will know the location area of the cell in which the MS is camped (unless the MS has entered a limited service state) and can therefore page the MS when an incoming call is received.
The idle mode task can be subdivided into four processes: z
PLMN selection
z
Cell selection
z
Cell reselection
z
Location updating.
The relationship between these processes is illustrated in Figure 2-3.
Huawei Technologies Proprietary 2-17
Feature Description M900/M1800 Base Station Subsystem
Service indication to User
Chapter 2 BSS Functions
Automatic/Manual Mode Selection User Selection of PLMN Indication to User
PLMN Selection PLMN Selection
PLMN Available Cell Selection
New Location Area
Initial Cell Selected Cell Reselectin Location Updating Responses
Cell & Location Area Changes Periodic Registration
Location Updating
Figure 2-3 Overall idle mode processes
2.1.5 PLMN Selection I. Overview The MS will select a PLMN when it is powered on or upon recovery from a lack of coverage. It will first try to select and register on the registered PLMN if one exists. If registration on a PLMN is successful, the MS indicates this PLMN (the “registered PLMN”) and is capable of making and receiving calls on it. If there is no registered PLMN, or if the registered PLMN is unavailable, the MS will try to select another PLMN either automatically or manually depending on its operating mode, The MS normally operates on its home PLMN. However, another PLMN may be selected if, for example, the MS loses coverage. The MS will register on a PLMN if the MS finds a suitable cell to camp on and if a location-updating request is accepted. Registration has to be successful in order for the MS to be able to access that network. However, it does not need to perform location updating if it is in the same location area belonging to the same PLMN as it was before it entered the inactive state. The MS can select and register on another PLMN of its home country than its home PLMN if national roaming or international roaming is permitted. However, the MS will then do periodical attempts to return to its home PLMN. This is controlled by a timer. Huawei Technologies Proprietary 2-18
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
The interval between attempts is stored in the Subscriber Identity Module (SIM). Only the service provider is able to set the timer value for return to home PLMN. There are two modes for PLMN selection; automatic and manual. The automatic mode utilizes a list of PLMNs in an order of priority whereas the manual mode leaves the decision to the user and only indicates which PLMNs that are available.
II. Automatic mode In automatic mode, the MS will select PLMN if available and allowable, in the following order if no registered PLMN exists or is available: Home PLMN 1)
Each PLMN that has been stored in the Subscriber Identity
Module (SIM) in priority order 2)
Other PLMNs with received signal level above -85 dBm in random order
3)
All other PLMNs in order of decreasing signal level.
III. Manual mode In manual mode, the MS will first try to select the registered PLMN or home PLMN (if no registered PLMN exist). If this registration fails or if the user has initiated a PLMN reselection the MS will indicate to the user all available PLMNs. The user can then select a desired PLMN which causes the MS to initiate a registration on this PLMN. If the selected PLMN is not allowable, an indication to the user to select another PLMN will be made. The user can at any time request the MS to initiate reselection and registration onto an alternative available PLMN. This is done either using automatic or manual mode, depending on the mode selected by the user.
2.1.6 Cell Selection and Reselection I. Overview The purpose of cell selection and reselection is to enable MS to find a most suitable cell on which MS can reliably decipher the downlink data and maintain a high communication rate on uplink (so as to realize various telecom services). Once MS has selected a cell as its serving cell, its communication with the network becomes possible on this cell. MS will tune to the BCCH to receive the paging message and the system information broadcast on BCCH and use the RACH to send access request after it has selected this cell.
Huawei Technologies Proprietary 2-19
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
MS implements cell reselection according to the message in BA table in the system broadcast information from the serving cell. There are two BA tables in GSM network. One is transmitted in the system information via BCCH. It includes the BCCH carrier used in a certain physical area for the MS in idle mode to implement cell selection and reselection. The other one is transmitted in the system information via SACCH. It is used to indicate the MS in active mode about the BCCH carrier for handover monitoring. In active mode, MS obtains the information of adjacent cell BCCH frequency through BA (BCCH). The process will not stop until MS receives the first BA (SACCH) information.
II. Cell selection procedure When MS is powered on and move from blind spot of coverage to the serving area, it will search for all available frequencies in the PLMN and select the suitable cell to camp on. This is the procedure of "cell selection". Cell selection procedure in the case of no BCCH information in MS MS first searches the 124 RF channels of GSM 900(if the MS is a multiband one, MS searches 374 GSM 1800 RF channels), and compares the signal level on the channels to calculate the average level. The entire measurement procedure lasts 3~5 s, during which, at least five sampling points will be extracted from different RF channels. After MS has tuned to the maximum carriers of the receiving level, it will first judge which one is the BCCH carrier (by searching FCCH burst). If so, MS will attempt to decode SCH to obtain the BCCH system broadcast information synchronous with this carrier. If the MS can properly decode BCCH data, and make sure that this cell belongs to the selected PLMN, parameter C1>0, and this cell has no access barring, MS can camp on this cell. Otherwise, MS will keep tuning to the next highest carriers until it reaches the available cell. If no suitable cells are found after searching 30 RF channels with the highest level, MS will monitor the level of all channels and search for the BCCH of C>0 and no access barring. After finding this carrier, MS will camp on this cell without considering its PLMN ID. In this occasion, only emergency call can be implemented. Case 1: If the access level of the MS is barred at the cell, the cell selection algorithm will not be affected, i. e., when the cell satisfies the criterion, MS will still camp on it. Case 2: If the cell selected by MS belongs to PLMN, but access is barred (parameter CBA is set as "bar") or algorithm C1 Lev_Thr and RxQual > Qual_Thr, the interference handover is triggered. Interference handover is illustrated in Figure 2-16. Receiving quality (dtqu)
Qual_Thr
0
Receiving level
Lev_Thr
Figure 2-16 Interference handover zone The shadowed part in the figure stands for zones within which interference handover occurs. e) Edge handover This is a level-based handover and is rescue handover. If edge handover is to be triggered, the level of the destination cell is required to be higher than that of serving cell for at least one hysteresis value (inter-cell handover hysteresis).
Huawei Technologies Proprietary 2-64
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
The criterion for triggering edge handover: When the receiving level of the serving cell is lower than the edge handover threshold, and fulfilling the P/N criterion within a certain measurement period, the edge handover will be triggered to ensure the communication quality. Edge handover is illustrated in Figure 2-17. Cell1
Cell2 -97dBm
-85dBm
Figure 2-17 Edge handover f) PBGT handover PBGT also belongs to better cell handover, a handover based on path fading. PBGT handover algorithm searches for the cell with lower path loss and satisfying the system requirement on real-time basis so as to judge whether handover is needed. Difference from other handover algorithms: the trigger condition is path loss and receiving power. Triggering condition of PBGT handover: The path loss of the adjacent cell is smaller than the threshold of the serving cell and the P/N criterion is satisfied within a period of measurement time. P/N criterion is that there are P satisfying the criterion during N measurements. PBGT(n) > PGBT_HO_Margin (n) In the inequality above, P, N and PBGT_HO_Margin (n) are configured at data configuration console. PBGT (n) calculates according to the control parameter and the information reported by BTS. The method of calculating PBGT (n): PBGT (n ) = (Min(MS _ TxPWR _ MAX , P ) − RxLEV _ DL − PWR _ C _ D ) − (Min(MS _ TxPWR _ MAX (n ), P ) − RxLEV _ NCELL(n ))
Meanings of the parameters: z
MS TxPWR MAX: maximum transmitting power allowed in the serving cell
z
MS TxPWR MAX (n): maximum transmitting power allowed in the adjacent cell n
z
RxLEV_DL: downlink receiving power of the serving cell
z
RxLEV_NCELL (n): downlink receiving power of the adjacent cell n
z
PWR_C_D: difference between the maximum downlink transmitting power caused by power control and the actual downlink transmitting power of the serving cell.
Huawei Technologies Proprietary 2-65
Feature Description M900/M1800 Base Station Subsystem z
Chapter 2 BSS Functions
P: maximum transmitting capability of MS
PBGT handover occurs only between cells of the same layer and same level. g) Load handover There may be cells with heavy load while their upper layer cell and the adjacent cell bears less load. To achieve load balance between cells by sharing the load with upper layer and adjacent cell, the traffic load handover is applied. Its aim is to hand over part of the traffic in the heavily loaded cell to less loaded cells, and preventing the traffic of the adjacent cells being handed over to this cell. Load handover can be implemented between cells within the same BSC. Load handover is illustrated in Figure 2-18.
High traffic cell Low traffic cell
Low traffic cell Heavy traffic cell
High traffic cell
Low traffic cell Low traffic cell
Figure 2-18 Load handover The method of realizing load share: by heightening the edge handover threshold towards that of the serving cell, the traffic at the cell edge will be handed over those with less traffic. The basis for judging the traffic of a cell is the cell flow (i. e. TCH occupation rate) and the preset threshold. If the cell flow of a cell is higher than the heavy traffic load threshold (Load HO Start Threshold ), this cell is consider to have a heavy traffic load, and the load handover algorithm needs to be activated. If the cell flow of a cell is lower than the low traffic threshold (Load HO Rx Threshold), it is consider having a low traffic load, and is allowed to accept the traffic handed over from other heavy traffic load cells. Since the load handover mechanism is likely to trigger a good number of handovers, the situation of system CPU load should be taken into consideration before triggering handover, i. e. system flow level. In addition, to avoid too many handovers happening simultaneously, the load handover is implemented step by step, i. e. edge handover threshold will increase by certain step length (CLS_Ramp) and period (CLS_Period).
Huawei Technologies Proprietary 2-66
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
The increase ends when the threshold reaches the load handover bandwidth (CLS_Offset). Load handover is illustrated in Figure 2-19. Load HO zone
Normal HO border
Cell A
Cell B
CONF_HO_RXLEV
CONF_HO_RXLEV+CLS_Offset
CONF_HO_RXLEV+CLS_Ramp
Figure 2-19 Load handover h) Hierarchical handover The GSM network is classified into layers, so as to flexibly direct its traffic and fulfill the needs of different network structure. If a cell has a high priority and its signal level is higher than a threshold (Inter-layer HO Threshold) and satisfy the P/N criterion, the traffic will be handed over to this cell even if the serving cell can still provide normal services. The purpose of hierarchical handover is to direct the traffic to the cell with higher priority so that the traffic can be distributed more reasonably. i) Fast moving handover This kind of handover is carried out for fast moving MS to reduce the number of handover and hence reduced call drop rate. If MS is moving quickly with micro cell as the reference, it will be handed over to the macro cell. If the fast moving MS registered in the macro cell, time penalty will be implemented to the micro cell so that the MS will stay in the macro cell. Fast moving handover is illustrated in Figure 2-20.
Huawei Technologies Proprietary 2-67
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Umbrella Cell Micro Cell
Figure 2-20 Fast moving handover There are two principles for fast moving handover: z
If the MS is moving fast with the micro cell as the reference, it will be handed over to the macro cell.
z
To avoid the fast moving MS registered in macro cell enter the micro cell, time penalty will be implemented to micro cell.
If the duration of MS camping in a cell is lower than a certain threshold (Fast Moving Time Threshold), this MS is considering to be moving fast with this cell as the reference. To avoid miscarriage of justice, P/N measurement will be implemented to several cells. If the criterion of fast moving is satisfied, this MS will be handed over the macro cells. For MS registered in macro cell, the method of "timer + penalty" is applied. Before the speed sensitive timer of a certain micro cell times out, this receiving level of this micro cell will be punished, so that the position of this micro cell in the cell sequencing will be lowered. Fast moving handover algorithm can only perform accumulation judgement to the MS within the same BM and same BSC. When MS moves to another BM, it is necessary to re-judge. j) Other handovers Other handovers include IUO handover, directed retry, forced handover, and extended cell handover. 3)
Handover procedure
Handover decision algorithm enables the preprocessing of the input MR and decides whether handover should be done and which type of handover it should be (intra-cell handover, inter-cell handover in the same BSC, outgoing BSC handover, etc.) according to the various conditions. Handover decision algorithm sends the message of decided handover result to call process module, which will complete handover-signaling process together with BTS, MSC and MS. If a handover fails for a Huawei Technologies Proprietary 2-68
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
certain reason, call process module will notify the handover result to the algorithm, which will decide how to further process this handover. Handover process is as shown in Figure 2-21.
MR preprocessing Handover decision algorithm starting decision MR averaging procesing Penalty processing of cell measurement value
Basic cell sorting Adjustment according to network features Handover decision Sending handover commands to the call handling module Call control Processing of handover results
Figure 2-21 Handover decision process flow chart Each phase of the process is described as follows. a) MR preprocessing. MR provides basic parameters needed in handover decision. MS measures the receiving quality (RxQual) and receiving level (RxLev) of the downlink of the serving cell as well as the downlink RxLev of the BCCH carrier frequency of adjacent cells (best six adjacent cells average). Then MS sends these measurement results to BTS through SACCH once every 480ms. If SACCH is used for the transmission of other signals, MS sends the measurement results once every 960ms. BTS measures the RxQual and RxLev of the corresponding uplink. BTS combines the uplink measurement value and the downlink measurement value to form a MR message.
Huawei Technologies Proprietary 2-69
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Note: If messages transmitted on the uplink SACCH do not include the MR (transmitted by MS), the uplink measurement result will indicate that the MR transmitted by MS is lost.
MR should be preprocessed so that it can have a better reflection of radio links. MR preprocessing process can be realized in both BTS and BSC and controlled by OMC. The preprocessing of the MR includes the following three procedures: MR interpolation processing: When discontinuous MRs are received by BTS or BSC, lost MRs should be interpolated so as to guarantee the continuity of the whole MR processing process. This procedure is called MR interpolation. If the number of lost MRs exceeds a limit, previously received MRs will be regarded as invalid ones and re-collection is needed. To eliminate the uncertainty in handover decision, it is necessary to perform smooth processing over the MRs, or filtering. A simple and practical algorithm is weighted filtering. Different filter lengths can be respectively defined for different types of measurement values like the receiving level, receiving quality and TA, or different channel types like signaling channels, speech and data channels. The receiving level (RxLev) and receiving quality (RxQual) use corresponding assumed value for calculations, as shown in Table 2-7, and Table 2-8. Table 2-7 Receiving level calculation assumed value RxLev number
Implication
Assumed value
0
< -110dBm
-110dBm
1
-110dBm~109dBm
-109dBm
2
-109dBm~-108dBm
-108dBm
…
…
…
62
-49dBm ~ -48dBm
-48dBm
63
> -48dBm
-47dBm
Table 2-8 Receiving quality calculation assumed value RxQual number
BER range
Assumed value
Calculated value
0
< 0. 2%
0. 14%
0
1
0. 2% ~ 0. 4%
0. 28%
10
2
0. 4% ~ 0. 8%
0. 57%
20
Huawei Technologies Proprietary 2-70
Feature Description M900/M1800 Base Station Subsystem
RxQual number
Chapter 2 BSS Functions
BER range
Assumed value
Calculated value
3
0. 8% ~ 1. 6%
1. 13%
30
4
1. 6% ~ 3. 2%
2. 26%
40
5
3. 2% ~ 6. 4%
4. 53%
50
6
6. 4% ~ 12. 8%
9. 05%
60
7
> 12. 8%
18. 10%
70
The MR represents the condition of radio channels in the previous measurement cycle, so it is of hysteresis to some extent. The prediction algorithm is mainly responsible for MR values for the next cycle(s) based on the radio environment changes prediction. MR prediction is a process that can be selected by the operator. When the multiplexing on the Abis interface is 15:1, every 4 signaling links multiplex a 64kbit/s timeslot statistically. MR is transmitted through RSL. In order to minimize signaling transmission error bit, when Abis interface multiplexing mode is 15:1, MR processing mode requires that MR should be a preprocessed one instead of the original one. Moreover, MR reporting frequency can adopt interval reporting. It can be realized with data configuration: in [Cell/Modify Cell's Handover parameter/Modify Handover Parameter/HO Control Data], modify [BTS Measurement Report Preprocessing], [Transfer Original Measurement Report] and [Report Freq. of Preprocessed Measurement Report]. Accordingly, the emergency handover due to fast level dropping is decided by BTS. And the BSC will forward the decision. For other handovers completed within BSC, handover decisions and processing are still carried out in the BSC. b) Handover decision algorithm starting decision Judge whether basic conditions for handover are satisfied, such as whether there are enough MRs. If conditions are satisfied, handover decision algorithm is started. c) MR averaging processing Filter MRs according to a certain algorithm, cancel their noise, and smooth MRs, thus to prevent incorrect handovers due to individual interference. d) Penalty processing of cell measurement value Practically there is a possibility that a handover can not be successful. In case the handover to the selected target cell fails, the MS will stick to the original serving cell. After the cycle of a handover decision is finished, the system might try to hand over the MS to the above-mentioned target cell again, which might cause invalid handover attempt or handover failure, or even interruption. Therefore, the target cell shall be
Huawei Technologies Proprietary 2-71
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
punished, which is to reduce the receiving power of the corresponding cell by a set penalty value for a period (called the penalty time). Penalty types include the penalty of the forsaken cell due to TA value, penalty of the forsaken cell due to bad quality (BQ), penalty to the failed cell due to ordinary handover failure, and fast-moving penalty (this is a penalty imposed on the microcell in the candidate queue in order to prevent frequent handover when the fast moving MS accesses a cell of small coverage). e) Basic cell sorting. Adjacent cells that have been imposed penalty and the serving cell are sorted through a certain algorithm, thus the position of each cell is located. This is to get ready for final handover. f) Adjustment according to network features To adjust candidate queue through a certain algorithm according to hierarchical network, cell priority, speed sensibility, and the specific network environment. g) Handover decision Handover decision algorithm is used to decide the time to start handover and the target cell to be handed over. Confirm the candidate cell queue list, adjust cells adjusted in last procedure and finalize a uniform clear list of cells that are ready to be handed over. h) Sending handover commands to the call handling module After making the handover decision with the algorithm and deciding to execute the handover, BSC sends handover message containing the type of incoming handover to the call-handling module, then the latter starts the signaling procedures for this handover. i) Processing of handover results After the call handling module has processed handover signaling, it returns the result to the handover decision module. If the handover fails, the handover decision module will start penalty to the cell responsible for the failure. If the handover is successful, the module will set a new handover interval timer to avoid frequent handovers.
III. Parameter 1)
TA Handover
“TA Thrsh ” in [Handover\Emergency Handover Table] “Filter Length for TA ” in [Handover\Filter Table] “Penalty Time after TA HO ” in [Handover\Penalty Table] “Penalty Level after TA HO ” in [Handover\ Penalty Table] Huawei Technologies Proprietary 2-72
Feature Description M900/M1800 Base Station Subsystem
2)
Chapter 2 BSS Functions
BQ Handover
“UL Qual. Thrsh ” in [Handover\Emergency Handover Table] “DL Qual. Thrsh ” in [Handover\ Emergency Handover Table] “Filter Length for TCH Level ” in [Handover\Filter Table] “Filter Length for SD Qual ” in [Handover\Filter Table] “Penalty Level after BQ HO ” in [Handover\Penalty Table] “Penalty Time after BQ HO ” in [Handover\Penalty Table] 3)
Level rapid dropping handover
“Rx_Level_Drop HO Allowed ” in [Handover\Handover Control Table] “Filter parameter A1~A8 ” in [Handover\Emergency Handover Table] “Filter parameter B ” in [Handover\Emergency Handover Table] 4)
Interference handover
“UL Qual. Thrsh. for Interf. HO ” in [Handover\Emergency Handover Table] “DL Qual. Thrsh. for Interf. HO ” in [Handover\Emergency Handover Table] “UL RX_LEV Thrsh. for Interf. HO ” in [Handover\Emergency Handover Table] “DL RX_LEV Thrsh. for Interf. HO ” in [Handover\Emergency Handover Table] 5)
Edge handover
“Edge HO UL RX_LEV Thrsh ” in [Handover\Normal Handover Table] “Edge HO DL RX_LEV Thrsh ” in [Handover\Normal Handover Table] “Edge HO watch time” in [Handover\Normal Handover Table] “Edge HO valid time” in [Handover\Normal Handover Table] “Inter-cell HO Hysteresis” in [Handover\Adjacent Cell Relation Table] 6)
PBGT handover
“PBGT HO Allowed” in [Handover\Handover Control Table] “PBGT HO Thrsh” in [Handover\Adjacent Cell Relation Table] “PBGT Watch Time” in [Handover\Normal Handover Table] “PBGT Valid Time” in [Handover\Normal Handover Table] 7)
Load handover
“load HO Allowed ” in [Handover\Handover Control Table] “System Flux Thrsh. for Load HO” in [Handover\Load Handover Table]
Huawei Technologies Proprietary 2-73
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
“Load HO Thrsh” in [Handover\Load Handover Table] “Load Req. on Candiate Cell” in [Handover\Load Handover Table] “Load HO Bandwidth” in [Handover\Load Handover Table] “Load HO Step Period” in [Handover\Load Handover Table] “Load HO Step Level” in [Handover\Load Handover Table] 8)
Layered and hierarchical handover.
“Layer of The Cell” in [Handover\Cell Description Table] “Cell Priority” in [Handover\Cell Description Table] “Inter-layer HO Thrsh” in [Handover\Cell Description Table] “Inter-layer HO hysteresis” in [Handover\Cell Description Table] “Layer HO watch time” in [Handover\Normal Handover Table] “Layer HO valid time” in [Handover\Normal Handover Table] 9)
Fast Moving handover
“MS Fast Moving HO Allowed” in [Handover\ Handover Control Table] “MS Fast-moving Watch cells” in [Handover\Fast-moving handover Table] “MS Fast-moving Valid cells” in [Handover\Fast-moving handover Table] “MS Fast-moving time Thrsh” in [Handover\Fast-moving handover Table] “Penalty on MS Fast Moving HO” in [Handover\Cell Description Table] “Penalty Time on MS Fast moving HO” in [Handover\Cell Description Table]
2.2.2 Power Control I. Overview As an important method to control radio link, power control adjusts the transmit power of MS and BTS according to the expected value configured in OMC data management system, the receiving level (including uplink and downlink) from BTS and the MR of receiving quality. Basic rules for power control are: 1)
When the level or signal quality is higher than the expected value, the power should be decreased accordingly.
2)
When the level or signal quality is lower than the expected value, the power should be increased accordingly.
3)
The level and signal quality should be both considered so as to improve the accuracy and effectiveness. Huawei Technologies Proprietary 2-74
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
The nature of a cellular system requires that the output power of the BSC and MS should be set as low as possible. With the limited resource of the RF spectrum cellular systems depend upon the reuse of the RF channels. The reuse distance between these channels mainly upon the subscriber density in a particular area, the greater the density the shorter the reuse distance. By keeping the MS and BSC at the minimum acceptable power output it reduces the chances of interference, particularly co-channel. Another benefit of effective power control is that the battery of MS is extended, thus maximizing available talking time. Huawei BSS offers three different algorithms for the implementation of power control, which is GSM 0508 power control algorithm, and Huawei I (HW_I) and Huawei II (HW_II) algorithms. Any algorithm can be selected among these three algorithms. HW_I and HW_II algorithms are recommended due to their flexible configurations, effectiveness, easy operations and easy command. These Huawei-developed algorithms are compatible nicely with the GSM900 and GSM1800 systems.
II. Technical description 1)
Power control classification
Power control comprises uplink and downlink power controls, which are executed separately. The uplink power control is for MS while the downlink power control is for BTS. a) MS power control The purpose of MS power control is to adjust the MS output power in order to achieve the stable receiving signal so as to reduce the interference from subscribers of adjacent channels, decrease the saturation degree of BTS multicoupler and reduce MS power consumption. The MS power control is divided into two adjusting stages, i.e., the stable adjusting stage and the initial adjusting stage. The stable adjusting is the normal method for performing the power control algorithm, while the initial adjusting is used in the time when the call connection is initially started. When a connection is performed, MS is output as the nominal power of the cell where it is located (the nominal power indicates that the MS transmitting power is the MS maximum transmitting power MS_TXPWR_MAX_CCH in the broadcast system messages on the BCCH channel of the cell where it is located. If MS does not support this power class, the supported power class that is nearest to it will be utilized, such as the maximum output power class supported by the reported MS Classmark in the establishment indication message). However, since BTS may simultaneously support multiple calls, the receiving signal intensity should be reduced in a new connection as quick as possible, otherwise, the quality of other call supported by this BTS may be deteriorated due to
Huawei Technologies Proprietary 2-75
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
the saturation of the BTS multi-coupler, and the call quality of other cells may be affected due to the high interference. Therefore, the purpose of the initial stage power control adjusting is to reduce the MS transmitting power as quick as possible until the stable measurement report is obtained, so that the MS can be adjusted according to the stable power control algorithm. The parameters that must be selected in the uplink power control, such as the expected desirable uplink receiving level, desirable uplink receiving quality, etc. , are all set by the O&M data management console, the data configuration can be dynamically carried out according to the actual situations of the cell. After a given number of the uplink measurement reports is received, by the processing methods such as interpolation and filtering, the actual uplink receiving level and the receiving quality are obtained, then they are compared with the desirable uplink receiving level and the receiving quality, with the power control algorithm, the power class to which the MS should be adjusted is calculated; if it is different from the current MS output power class and meets a given application restricted conditions (such as the power adjusting step length restriction, MS output power range restriction), the power adjusting command is sent. The essence of the uplink power control adjusting is to enable the actual uplink receiving level and receiving quality obtained from interpolation and filtering to progressively approach the desirable uplink receiving level and receiving quality set by O&M. The purpose for the interpolation and filtering of the measurement reports is to process the lost measurement report, clear the temporary nature (spiffiness), so as to ensure the stability of the power control algorithm. The difference between power controls in initial phase and stable phase is that their expected uplink receiving levels and receiving qualities, filter lengths are different, and the former one only adjusts downwards. b) BTS power control The BTS power control is an optional function. The base station power control is basically identical to the MS power control, except that the base power control utilizes only the stable power control algorithm. The parameters that must be selected in the power control include the receiving level threshold (lower limitation) to be performed the power control and the receivable maximum sending level threshold (upper limitation). The receiving level RXLEV is divided into 64 classes, with numbers from 0 to 63, class 0 of the receiving level is the lowest, while the class 63 of the receiving level is the highest. The base station power control is divided into the static power control and the dynamic power control, the later is the fine adjusting based on the former. The GSM 05.05 protocol specification specifies that the base station static power class is divided into 6 (2dB/per class), when the maximum power output by the base station is 46dBm (40W), the class 6 is 34 dBm. The static power level is defined in the cell attribute table of the data management console, i. e., the maximum output power value Pn of the current dynamic power control is specified. As the dynamic power control classes are
Huawei Technologies Proprietary 2-76
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
set to 15, the range of the dynamic power control is Pn-Pn-30dB. If the requirements cannot be satisfied when the dynamic power control reaches its maximum value, the static power control classes should be adjusted to increase the maximum output power value Pn of the dynamic power control. enable the actual uplink receiving level and receiving quality obtained from interpolation and filtering to progressively approach the desirable uplink receiving level and receiving quality set by O&M. The purpose for the interpolation and filtering of the measurement reports is to process the lost measurement report, clear the temporary nature (spilliness), so as to ensure the stability of the power control algorithm. 2)
Execution process of power control
There are 3 MR cycles from sending command to execution, as shown in Figure 2-22. In the 26 multiframe the 12th frame is for Report period of SACCH: sending SACCH 26× 4＝104 frame (480ms) SA0
SA1 SA2
BTS transmifs the command of adjust power and TA at SACCH header
SA3
SA0
SA1
SA2 SA3
SA1 SA2
SA3
MS starts to send the messurement report of the previous multi-frame
MS adopts new powerand TA
MS obtains SACCH block
SA0
MS Generates new SACCH BTS receives the header to report new TA and measurement report power control message
Figure 2-22 Power control execution process a) In the first MR cycle, MS receives the power regulation message carried by SACCH header on dedicated channel and the first layer header carried by a downlink SACCH message block. MS will execute the power control command in next cycle instead of upon the receipt of these headers in first cycle. b) In the second MR cycle, power control is executed. The maximum rate of change of MS power is 2dB/13 frame (60ms). If the regulation step length is 8, i. e. 8×2=16dB. It needs 104 frames (i. e. 480ms, one MR cycle) to complete power regulation. If the regulation step length is 16, i. e. 16×2=32dB. It takes 2 MR cycleS to complete power regulation. c) In the third cycle, the current transmit power (refers to the power level used by the last burst pulse of SACCH MS cycle) is stored, which will be reported to BTS in next SACCH uplink MR.
Huawei Technologies Proprietary 2-77
Feature Description M900/M1800 Base Station Subsystem
3)
Chapter 2 BSS Functions
Power Control Algorithm
BSC can dynamically implement power control on each MS and BTS Three algorithms can be adopted as power control algorithm: GSM 0508 algorithm, HW_I algorithm and HW_II algorithm. Algorithm process is as shown in Figure 2-23 MR Preprocessing
Power Control Algorithm selection
GSM0508 power control algorithm
HW_I Power control algorithm
HW_II Power control algorithm
Figure 2-23 Power control algorithm selection Power control algorithm is specified in the 0508 protocols (for further details refer to the related specifications). For uplink, upper limit and lower limit thresholds are set for the receiving signal level and the receiving signal quality. Counters P' and N' are used to count the MR and the values of these counters can be set through OMC. When N' MRs in the consecutively received P' MRs exceeds the above threshold, power regulation will be executed. Usually the steps for power control are: z
MR preprocessing
z
Power calculation
z
Power control decision
z
Adjustment by sending power control commands
4)
Huawei HW_I algorithm
Huawei HW_I has following features: z
Compared with protocol algorithm, the initial state regulation is added.
z
Data configuration is rather complicated. The power control adjustment involves many parameters and complicated calculation.
z
Power control decision is the sum of the level and quality, and the expected value is just a specified value instead of a range. Once the adjustment results of the receiving level and receiving quality are contrary, the power control will never stop and the level fluctuates with the expected value.
Power control decision process HW_I algorithm power control decision process is as shown in Figure 2-24.
Huawei Technologies Proprietary 2-78
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
MR pre-processing
Y
Satisfying power control target N Power control calculation and regulation cinitial state and stable state
Figure 2-24 HW_I algorithm power control decision process b) Measurement Report In order to implement power control decision, various kinds of information about the current communications status from MS and BTS should be collected, including receiving signal level, and communication quality etc. Network side on SACCH will receive MRs from MS and BTS every 480 ms, in which various kinds of information needed for power control decision are contained. The process of MS reporting is as shown in Figure 2-25.
MR
MR
MR
MR Uplink measurement
Downlink measurement
Figure 2-25 Reporting MR An example of BSS MR is shown in Figure 2-26. Huawei Technologies Proprietary 2-79
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Figure 2-26 MR example c) MR preprocessing. Interpolation: each MR has a serial number. If the serial numbers of received MRs are found not continuous, this means that there are some MRs gets lost. In this case, network will add all lost MRs according to interpolation algorithm. Filtering: Several continuous MRs results will be used to reflect the state of MS in a period of time thus to avoid the one-sidedness caused by judging the state of MS according to only one MR result. d) Power control decision Number of transmit power to be adjusted (Expected stable signal level - current receiving signaling level) × uplink (downlink) compensating factor + (current actual receiving uplink (downlink) quality – expected uplink (downlink) quality) × 10 × uplink (downlink) quality compensating factor
Caution: The last regulated power level cannot exceed the maximum power control step length.
Huawei Technologies Proprietary 2-80
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Actual stable level equals to the sum of current actual level and transmit power to be regulated During the process of judging power control level to be adjusted, it needs to search tolerance table according to the level of current transmit power. If the final power regulation level is with the tolerance range, the regulation is unnecessary. GSM1800 tolerance table is shown in Table 2-9. Table 2-9 GSM1800 tolerance table Level
0
1
2
3
4
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
Tolerance
2
2
2
2
2
2
2
2
2
3
3
3
3
3
4
4
4
2
2
2
GSM900 tolerance table is shown in Table 2-10. Table 2-10 GSM900 tolerance table Level
0
1
2
3
4
5
6
7
8
9
1 0
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
Tolerance
2
2
2
4
4
4
4
4
4
4
4
4
4
4
4
4
6
6
6
6
The similarities and difference of HW_I algorithm uplink power control and downlink power control is as follows: Similarities: z
In order to avoid the fluctuation caused by power controls, the interval between two continuous controls is specified for both uplink and downlink.
z
In order to not being affected by unexpected factors, all MRs should be filtered.
z
Both uplink and downlink power controls have power control on level and quality respectively.
z
Both have maximum power control step length and compensating factor.
Difference: z
MS has power control not only for stable state but also for initial connecting phase before a call is connected. The purpose is to lower MS transmit power as soon as possible.
z
Uplink has measures to improve transmit power in the case of MS handover failure.
z
Downlink has the restriction for both maximum and minimum MS transmit power.
5)
Huawei HW_II algorithm
Compared with HW_I, HW_II has following advantages: z
MR compensation, which makes the power control decision more accurate. Huawei Technologies Proprietary 2-81
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
z
MR prediction, which reduces power control delay.
z
Adaptive power control, which sufficiently guarantees the algorithm stability and high efficiency.
z
Keep power control target within the range between upper limit and lower limit so as to avoid power control fluctuation.
z
Easy and flexible data configuration, which guarantees effective regulation of network optimized parameters.
z
Configure the upward and the downward power control step respectively.
a) Power control decision process HW_II power control decision process is as shown in Figure 2-27.
MR pre-processing
Power control requested by receiving level
Power control requested by receicing quality
Conprehensive decision of power control
Figure 2-27 HW_II power control decision process b) Request power control according to level z
After the preprocessing of MR, power control module compares the current receiving level with expected value.
z
Then the transmit level step length is calculated. The regulation is to make the receiving level closer to the expected value.
z
When receiving level regulates transmit power, variable step length can be adopted so that the quick power control can be obtained.
c) Request power control according to receiving quality After the preprocessing of MR, power control module compares the evaluation value of the current receiving quality with expected value. z
Calculate the transmit step length to be regulated
z
Improving transmitting power for low receiving quality. Huawei Technologies Proprietary 2-82
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
z
Decreasing transmitting power for high receiving quality.
z
Do not adjust the transmitting power when the receiving quality falls between the upper/lower thresholds.
d) Comprehensive decision of power control Comprehensive decision of power control is shown in Table 2-11. Table 2-11 Table of comprehensive decision of power control Receiving level power control regulation
Receiving quality power control regulation
Comprehensive decision of power control
οAdjStep_Lev
οAdjStep_Qul
οMAX(AdjStep_Lev, AdjStep_Qul)
οAdjStep_Lev
μAdjStep_Qul
No action
οAdjStep_Lev
No action
οAdjStep_Lev
μAdjStep_Lev
οAdjStep_Qul
μAdjStep_Lev
μAdjStep_Lev
μAdjStep_Qul
μMAX(AdjStep_Lev, AdjStep_Qul)
μAdjStep_Lev
No action
μAdjStep_Lev
No action
οAdjStep_Qul
οAdjStep_Qul
No action
μAdjStep_Qul
μAdjStep_Qul
No action
No action
No action
e) MR compensation Power control module will extract the receiving level and receiving quality of some history MRs when it implements power control decision. These MRs might be obtained in different transmit powers. In order to guarantee the accuracy of receiving level to be used, if the transmit powers in these MRs are different, the receiving level value of history MRs should be compensated. The interpolated and compensated MRs are filtered so as to make control power decision more effective. f) Predict filtering The power control is a process of transmitting power control based upon the current received level and the receiving quality. The sending and transmission of power control command and power adjustment will take certain period of time, so there will exist certain hysteresis between the receiving change and corresponding transmitting power adjustment. Filtering prediction enables MR on which power control decision is based to get closer to the state of power regulation so as to erase delay effectively.
Huawei Technologies Proprietary 2-83
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
MR filtering prediction is implemented in a very short time and changes of receiving level and quality are likely to be continuous. N MRs before the current time are used for weighted filtering, then 0~3 MRs of after the current time are predicted. Generally, there are 3 MRs between power control decision and power regulation, which is about 1.5 second. As a consequence, the accuracy of prediction is guaranteed. Power control decision is made after the filtering of predicted MRs, interpolated MRs, and compensated history MRs. g) Dual threshold power control algorithm Dual threshold power control algorithm adopts the following three strategies: Adjust power control step length according to receiving level: The final purpose of power control is to obtain the best communication quality at the lowest level. However, due to the instability of radio link and the external interference, transmit power cannot be lowered greatly. Therefore, HW_II adopts the power control strategy of dual threshold so as to try to keep receiving within two thresholds. Adjust power step length according receiving quality: Generally, the change of receiving quality is associated with interference. The main interference of GSM comes to same frequency interference generated from frequency multiplexing. This interference is interactive. One call increases its power means that it exerts a stronger interference on the other call. Therefore, the power regulation caused by the change of receiving quality should avoid the group effect of increasing transmit power due to bad quality. Receiving quality threshold is also set with dual thresholds. Receiving quality with the range between two thresholds needs not to adjust transmit power. While receiving quality beyond the range should be adjusted. For the power regulation caused by quality factor should use fixed step length to avoid. Considering both power control strategies of receiving level and receiving quality regulation. Considering the requirements of both level and quality. On one hand, both requirements should be satisfied as much as possible; on the other hand, in the case that the requirements are not consistent or completely contrary, the stability should be fully considered to prohibit the unstable regulation process. Therefore, the effect on power control caused by level and quality should be both considered. h) Variable step length power control When variable step length regulation is adopted, if that the level or quality is greatly different from its expected value, use the larger step length to quickly adjust power; in the case that the level or quality is slightly different from its expected value, use the smaller step length to adjust power. Thus, quick and accurate power regulation is achieved. i) Adaptive power control
Huawei Technologies Proprietary 2-84
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Adaptive power control is to change power control strategy according different communication environments. This fact leads to a more effective and more stable power control. This is reflected in following two aspects: z
Power control adjustable maximum step length can be adjusted automatically according to the different communication environments.
z
The different power control strategies are adopted for different communication environments.
j) Adjustment of upper threshold of signal strength Double-threshold power control algorithm is adopted for power control. For level, there are upper threshold of uplink (downlink) signal strength and lower threshold of uplink (downlink) signal strength. When the receiving quality is rather poor, the value of upper threshold can be increased furthermore. When the receiving quality is good, the lower value of upper threshold can be adopted so as to reduce the transmit power of mobile phone or base station. When the receiving quality is rather poor, the higher value of upper threshold can be adopted so as to improve the communication quality. k) Configure the upward and the downward power control step respectively System configure the upward and the downward power control step respectively, this enable the system can control the power rapidly and flexibly according to the actual network. When the up/down link signal quality or receiving quality become worse suddenly, system increase the power rapidly to avoid call drop.
III. Parameter 1)
HW_I algorithm parameters
“Initial RX_LEV Expected ” in [Power\MS Power Control Table] “Stable RX_LEV Expected ” in [Power\MS Power Control Table] “Uplink RX_LEV Compensation ” in [Power\MS Power Control Table] “UL Qual. Expected ” in [Power\MS Power Control Table] “UL Qual. Compensation ” in [Power\MS Power Control Table] “Max PC Step ” in [Power\MS Power Control Table] 2)
HW_II algorithm parameters
“Filter Length for UL RX_LEV ” in [Power\HWII Power Control Table] “Filter Length for DL RX_LEV ” in [Power\HWII Power Control Table] “Filter Length for UL Qual ” in [Power\HWII Power Control Table] “Filter Length for DL Qual ” in [Power\HWII Power Control Table] “MR Compensation Allowed ” in [Power\HWII Power Control Table] Huawei Technologies Proprietary 2-85
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
“UL M.R. Number Predicted ” in [Power\HWII Power Control Table] “DL M.R. Number Predicted ” in [Power\HWII Power Control Table] “PC Interval ” in [Power\HWII Power Control Table] “UL RX_LEV Upper Thrsh ” in [Power\HWII Power Control Table] “UL RX_LEV Lower Thrsh ” in [Power\HWII Power Control Table] “UL Qual.Upper Thrsh ” in [Power\HWII Power Control Table] “UL Qual. Lower Thrsh ” in [Power\HWII Power Control Table] “DL RX_LEV Upper Thrsh ” in [Power\HWII Power Control Table] “DL RX_LEV Lower Thrsh ” in [Power\HWII Power Control Table] “DL Qual. Upper Thrsh ” in [Power\HWII Power Control Table] “DL Qual. Lower Thrsh ” in [Power\HWII Power Control Table] “MAX Down Adj. value Qual. zone 0” in [Power\HWII Power Control Table] “MAX Down Adj. value Qual. zone 1” in [Power\HWII Power Control Table] “MAX Down Adj. value Qual. zone 2” in [Power\HWII Power Control Table] “MAX Down Adj. PC Value by Qual.” in [Power\HWII Power Control Table] "MAX Up Adj. PC Value by RX_LEV" in [Power\HWII Power Control Table] "MAX Up Adj. PC Value by Qual." in [Power\HWII Power Control Table] “UL Qual. Bad TrigThrsh ” in [Power\HWII Power Control Table] “UL Qual. Bad UpLEVDiff ” in [Power\HWII Power Control Table] “DL Qual. Bad TrigThrsh ” in [Power\HWII Power Control Table] “DL Qual. Bad UpLEVDiff ” in [Power\HWII Power Control Table] BTS PC class
2.2.3 Extended Cell I. Overview In GSM specifications, the TA of cell has a restriction of 63 bit at the radio interface, which results that the cell coverage radius should be within 35km. In regions such as vast land, with scattered subscribers, with low traffic, and the infrastructure facilities such as transmission and power supply are hard to be constructed or unavailable, the cell with radius over 35km should be provided. The extended cell breaks the restriction Huawei Technologies Proprietary 2-86
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
of radius within 35km. Supported by BTS hardware, it can cover a range with radius of 120km under its ideal state. Carriers can use this technology to fast construct their GSM networks with fewer stations and at lower cost, and to attract the mobile subscribers in special regions so as to improve profit.
II. Technical description When the cell coverage radius exceeds 35km, signal delay will exceed the duration corresponding with the maximum value 63 bit specified in GSM specifications. If an MS reaches the ordinary coverage verge, it will transmit at the maximum TA allowed by system; if the MS continues to move outside of cell range, the system is no longer able to implement adaptive regulation on TA value due to the TA has reached its maximum, and part of signaling transmitted by MS will reach BTS receiver at next time slot. It is this principle that extended cell uses to realize the cell extension, i. e. two continuous time slots in BTS are specified for each MS call, and the receiving window of BTS receiver is also extended to a width of two time slots thus the cell coverage radius is extended to over 35km. In order to enable MSs in extended range to initiate call at any time, two time slots should be always distributed to BCCH, CCCH and SDCCH. The frame TDMA of GSM radio interface is composed of 8 time slots. Each time slot is a channel. Normally, the system uses TA to make the uplink signals of MSs with different distances reach within the corresponding local time slot. TA supports a maximum of 63 bit. In order to support the extended MS signals over 63 bit, dual time slot solution binds odd and even time slots and regards each TDMA frame as only with four channels: 0/1, 2/3, 4/5, 6/7. . For MS, only channel 0, 2, 4, and 6 are distributed. The MS in the range 0~35km, its TA value changes within the range 0~63. The TA value of MS with radius over 35km is always maintained as 63. While BTS demodulates uplink data in two continuous time slots. TA value of TA in MS has a maximum of 63+156. 25 = 219. 25 bit. The principle of extended cell delay regulation is as shown in Figure 2-28.
Huawei Technologies Proprietary 2-87
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
unlink data
DELAY63
After TA adjustment
TS0
TS2
TS1 Dual times lot extendend cell
Figure 2-28 Principle of extended cell delay regulation If all carrier frequencies in a cell are set as ordinary ones, this is called cell level dual time slot solution. If part of carrier frequencies in a cell are set as ordinary ones and other carrier frequencies are configured as dual time slot ones, and BCCH is located in dual time slot carrier, then this is called carrier level dual time slot solution. When carrier level dual time slot extended cell is adopted, there are ordinary carrier and dual time slot ones. BCCH in dual time slot guarantees the random access of any areas. The calls within TA value accessed randomly being within 35km radius are distributed to ordinary carrier; while the calls within 34~120km radius and the incoming handovers are distributed to dual time slot carrier. For the incoming handovers to be found as 0~35km ones, the system can handover them again to ordinary carrier. When the calling MS crosses 35 km line, this will lead to an intra-cell handover, which is from the dual time slot frequency to the ordinary one or from the ordinary to dual time slot frequency. The conversion of carrier frequencies between ordinary one and dual time slot one can be set through BSC data configuration
III. Parameter “Cell Extension Type ” in [Cell\Cell Attribute Table] “CH Type ” in [Local Office\Radio Channel Configuration Table] “TA Thrsh ” in [Handover\Emergency Handover Table] “TA Thrsh ” in [Handover\Concentric cell Handover Table] Huawei Technologies Proprietary 2-88
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
“TA Hysteresis ” in [Handover\ Concentric cell Handover Table]
2.2.4 IUO I. Overview With the development of GSM network, the number of subscribers increases gradually, so the contradict between short frequency resource and great demand is particularly obvious. In order to increase capacity, the technology of aggressive frequency reuse should be used to improve the frequency utilization. However, the aggressive frequency reuse increases the radio interference greatly and even to affect the communication quality seriously. Under the circumstance of aggressive frequency reuse, the IUO technology can be used to avoid or decrease radio interference so as to guarantee communication quality. The IUO technology divides an ordinary cell into two service layers: OverLaid subcell and UnderLaid subcell. For the MS in the UnderLaid subcell, try to distribute the less reuse frequency, such as BCCH frequency; for the MS in the OverLaid subcell, try to distribute the more reuse frequency, such as frequency except BCCH. The frequency inside the OverLaid subcell adopts aggressive frequency reuse mode, which can improve system capacity effectively.
II. Technical description IUO refers to the different carrier circle cells formed by different carrier frequencies in a cell with difference on coverage. Logically, OverLaid subcell and UnderLaid subcell can be regarded as two cells because their coverage areas are different, The OverLaid subcell is the main traffic carrier layer because it has many channels. Its function is to absorb the most subscribers within the cell coverage area. UnderLaid subcell solve the problem of coverage and provide service for the areas not covered by overlaid cell. e The technical description of IUO is as shown in Figure 2-29. UnderLaid subcell Cell A
Cell B
OverLaid subcell
Signal
Interference
Figure 2-29 Aggressive Frequency Reuse of IUO cell
Huawei Technologies Proprietary 2-89
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
As shown above, the IUO divides the cell coverage into OverLaid subcell and UnderLaid subcell. The carrier frequencies of OverLaid subcell and UnderLaid subcell can adopt different multiplexing modes. For the OverLaid subcell cell, it adopts more reuse frequency mode such as 1x3 due to its small coverage. For the UnderLaid subcell cell, it adopts less reuse frequency mode such as 4x3. After the IUO technology is employed, compared with Multiple Reuse Pattern (MRP), it can greatly increase the network capacity and guarantee the network quality because the OverLaid subcell employs of aggressive frequency reuse mode. In some special cases, the UnderLaid subcell is configured with only one carrier BCCH with the multiplexing mode of 4x3 being adopted and the rest TCH carrier frequencies are configured in OverLaid subcell with the multiplexing mode of 1x3 being adopted, then the IUO cell is completely the same as the cell with the multiplexing mode of 1x3 adopted and the average frequency multiplexing ratio is the same as that of 1x3 multiplexing. Therefore, in this case, the IUO can effectively reduce the interference for the whole network and obtain the better network quality than 1x3 multiplexing without the decrease of network capacity. The wider coverage can be realized through having the carrier in which BCCH is used large power amplifier. The power that provided by BCCH carrier is greater than other carriers, so the coverage distance of different carrier is different. While the cell coverage area depends on the carrier of smaller coverage, so the coverage area is greatly restricted. When the IUO technology is employed, the carrier with wide coverage can be used to serve as UnderLaid subcell to realize the far end coverage of site; while the carrier with small coverage can be used to serve as OverLaid subcell to increase the near end capacity of site. In this way, the cell coverage area can be increased.
Underlaid Overlaid
Figure 2-30 IUO coverage After the employment of the IUO cell, the cell coverage area can be greatly increased. The theoretically added coverage of various typical stations with different combining modes is shown in Table 2-12. Huawei Technologies Proprietary 2-90
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Table 2-12 Coverage changes for typical sites after the employment of IUO cell Number of cell carrier frequencies
Combining mode
Loss of low loss carrier
Loss of high loss carrier
Added coverage area after the employment of IUO
3
CDU+CDU
1. 0dB
4. 5dB
27%
4,5
CDU+CDU+SCU
1. 0dB
8. 0dB
60%
4,5
CDU+CDU+CD U
1. 0dB
4. 5dB
27%
5,6
CDU+CDU+SCU
4. 5dB
8. 0dB
27%
The division of OverLaid subcell and UnderLaid subcell is based on the MS downlink receiving level, downlink receiving quality and TA. The division of OverLaid subcell and UnderLaid subcell of common IUO is based on the "RX-LEV Thrsh." and "RX-LEV Hysteresis". The division of OverLaid subcell and UnderLaid subcell of enhanced IUO is based on the "U to O HO received level Thrsh." and "O to U HO received level Thrsh.". as shown in Figure 2-31.
Figure 2-31 Division of OverLaid subcell and UnderLaid subcell in a common IUO cell
Huawei Technologies Proprietary 2-91
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Note: The division foundation of OverLaid subcell and UnderLaid subcell is as follows: OverLaid subcell: Receiving Level>= RX_LEV Thrsh. + RX_LEV Hysteresis and TA=TA Thrsh + TA Hysteresis or Receiving Quality >= Receiving Quality Thrsh.
RX_LEV Thrsh., Receiving Quality Thrsh. and TA Thrsh. can be adjusted through data configuration. Therefore, under the precondition of without affecting the network performance indexes, the boarders of UnderLaid subcell and OverLaid subcell can be adjusted flexibly to let OverLaid subcell and UnderLaid subcell rationally share the traffic.
Underlaid subcell U to O HO received level Thrsh. O to U HO received level Thrsh. Receiving Quality Threshold TA Threshold
Overlaid subcell
TA Hysteresis
Figure 2-32 Division of OverLaid subcell and UnderLaid subcell in a enhanced IUO cell
Huawei Technologies Proprietary 2-92
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Note: The division foundation of OverLaid subcell and UnderLaid subcell is as follows: OverLaid subcell: Receiving Level>= U to O HO received level Thrsh. and TA=TA Thrsh + TA Hysteresis or Receiving Quality >= Receiving Quality Thrsh.
1)
Channel assignment technology of IUO cell
This technology can adopt different assignment strategies in various channel assignment cases in fully consideration of features of IUO. The following are the main cases: a) Immediate assignment System assigns channel through access_delay in Channel Request message. System assigns the channel in overlaid subcell for the MS in the overlaid subcell; system assigns the channel in underlaid subcell for the MS in the underlaid subcell. System always selects the appropriate service layer for MS. There is no reference receiving level, receiving quality and TA for immediate assignment. In order to guarantee the service quality, the SDCCH of UnderLaid subcell is assigned preferentially. Only when there is no signaling channel available in the UnderLaid subcell, will the signaling channel in the OverLaid subcell be assigned. b) Assignment The channel assignment strategy of IUO is used to assign channels. The OverLaid subcell channel will be assigned as far as possible when the subscriber is in the OverLaid subcell coverage. The UnderLaid subcell channel will be assigned when no OverLaid subcell channel is available. Similarly, the UnderLaid subcell channel will be assigned as far as possible when the subscriber is in the UnderLaid subcell coverage. The OverLaid subcell channel will be assigned when no UnderLaid subcell channel is available. Select the suitable service layer to serve the subscriber. c) Intra-BSC handover Intra-BSC handover is applicable to the non-IUO handover and the handover from the OverLaid subcell directly to an adjacent cell. Use the IUO channel assignment strategy to assign channels and select the suitable service layer to serve the MS. d) Inter-BSC handover
Huawei Technologies Proprietary 2-93
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Being unable to get the receiving level, receiving quality and TA of adjacent cells, the system selects the preferential UnderLaid subcell, or preferential OverLaid subcell, or non-strategy mode through switch. 2)
IUO cell handover technology
Huawei handover algorithm has the IUO handover judgement function to realize the IUO technology. When the MS crosses the boundary between OverLaid subcell and UnderLaid subcell, the IUO handover can be initiated to enable the MS to setup a call at a suitable service layer. If the object handover layer is congested, the handover will not be initiated. With the IUO cell handover technology, BSC can intelligently direct the traffic so as to utilize the frequency resource effectively.
III. Parameter Parameters in [Handover/ Concentric Cell Handover Table]: "Direction for IUO HO – UL to OL HO Allowed" "Direction for IUO HO – OL to UL HO Allowed" "Criterion for IUO HO – Rx_Lev for UO HO Allowed" "Criterion for IUO HO – Rx_Qual for UO HO Allowed" "Criterion for IUO HO – TA UO HO Allowed" "UO signal intensity difference " "RX_LEV Thrsh." "RX_LEV Hysteresis" "Receiving Quality Thrsh." "TA Thrsh." "TA Hysteresis" "IUO HO Watch Time" "IUO HO Valid Time" "Assign optimum layer" "Assign-optimum-level thrsh." "Assign-Optimum-TA Thrsh" "TA pref. Of Imme-Assign Allowed" "TA Thrsh. Of Imme- Assign pref." "Incoming-to-BSC HO optimum layer" Huawei Technologies Proprietary 2-94
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
"Pref. subcell in HO of intra-BSC " "Enhanced IUO allowed" "O to U HO received level Thrsh." "U to O HO received level Thrsh." "U to O Traffic HO Allowed" "Traffic Thrsh. of underlay" "Underlay HO Step period" "Underlay HO Step level" "Penalty Time of U to O HO (S)" Parameters in [Handover/Penalty Data Table]: "Penalty time after IUO HO Fail." Parameters in [Handover/Cell Description Data]: "Cell Type" Parameters in [Site/Carrier Configuration Table]:
2.2.5 "HW-IUO Property"Satellite Transfer I. Technical description Satellite communication is the development and the special form of microwave communication, the supplement and backup to conventional communication means. Satellite communication features wide coverage, little effected by landform, fine mobility, and flexible link calling. Meanwhile, it has the problems such as delay, jitter, and bit error, which leads to the Abis interface of ordinary GSM equipment not supporting satellite transfer. Huawei BSS adopts dedicated satellite transfer equipment to realize the satellite transfer of Abis interface according to the features of satellite transfer. The solution principle is described as follows: 1)
LAPD protocol processing
During the LAPD protocol process, the timer duration is prolonged and the value of slide window is increased to resist delay. 2)
TRAU frame algorithm
The adjustment algorithm of the TRAU frame is modified from fixed cycle adjustment to self-adaptive adjustment. Huawei Technologies Proprietary 2-95
Feature Description M900/M1800 Base Station Subsystem
3)
Chapter 2 BSS Functions
BTS clock work mode
The transmission between BSC and BTS can only occupy 19 time slots of DDN circuit (TS1~18, TS31) and the time slot 0 of DDN circuit is used for the synchronization of DDN instead of transmitting service. Therefore, BTS can only use the clock of DDN. However, the accuracy of DDN clock is only 10E-7, which cannot satisfy the requirement of GSM protocol. BTS adopts internal clock, which accuracy meets the requirement of GSM protocol. 4)
Voice quality
When the transmission bit error is less than 10E-6, the Voice quality is not affected. Usually, the transmission bit error of satellite circuit is less than 10E-8. As the link lease is very expensive and the quality is particularly sensitive to environments, the solution of Abis interface transmission by using satellite transfer should be positioned for the special areas where the ordinary transmission means is dissatisfactory and for the emergency communication. When the satellite transfer is used for networking, the star networking mode is usually adopted. The typical satellite transfer networking diagram is shown in Figure 2-33.
Satelite
Earth Station
MSC
Earth Receiving E1 Station BTS BSC BTS SDH/PDH /HDSL/Microware /E1 BTS
Earth Receiving E1 BTS Station
Figure 2-33 Typical satellite transfer networking diagram Satellite communication is composed of satellite and ground station. Generally, the satellite communication adopts synchronous satellite, i.e. the satellite orbit plane is on the equator plane, the satellite is 35786. 6km from the earth surface, the flying direction is the same as the earth rotation, and the duration of satellite rotation
Huawei Technologies Proprietary 2-96
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
cycle is the same as that of the earth. The satellite consists of control system, communication system (antenna and trunk equipment), telemetry system, power supply system and temperature control system. The ground station consists of antenna system, transmitter, receiver, channel terminal equipment (modem), communication control system and power supply system. The ground station of ordinary satellite communication is a kind of large-sized international or European standard communication station. It has such features as high transmission rate, antenna of large caliber, and expensive cost of equipment. The subscriber data are connected to the ground station through the ground communication network to complete communication. The subscribers in VSAT system form a dedicated network to communicate through satellite respectively. This mode is featured by its low cost of equipment, antenna of small caliber, and flexible application.
II. Parameter 1)
“Transfer Mode ” in [Site\Site Description Table]
2)
“Immediate Assignment opt ” in [Cell\Cell Call Control Table]
3)
“MS MAX retrans ” in [Cell\System Information Table]
4)
“Tx-integer ” in [Cell\System Information Table]
5)
“CCCH_CONF ” in [Cell\System Information Table]
2.2.6 Diversity Receiving I. Technical description In radio waves propagation, fading (including slow fading and fast fading) may impact on the communication quality and may even interrupt the communication. In this technique, the system receives two or more input signals, which carry identical information but irrelevant random fading features. To minimize these impacts and enhance the transmission quality, diversity technique is used. It is an effective approach to overcome fading, encompasses frequency diversity, time diversity, polarization diversity and space diversity. 1)
Space diversity
Space diversity is implemented by providing two sets of stand-alone receiving equipment concurrently, including antenna, tower amplifier (optional), feeder, DMUX and receiver. The receiver is made up of two completely independent paths. The input signals of the two channels come from the master and diversity antennas. The two signals of space diversity receiving have different propagation environments and different kinds of fading so they have the feature of coherence or little coherence. It lowers the impact of propagation factor to adopt diversity combining technology and Huawei Technologies Proprietary 2-97
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
make it output powerful useful signals. In the mobile communication, the wider spacing interval, the more different multipath propagation, and the less relativity. The interval between antennas can be either vertical or horizontal. The vertical interval has a poor performance of diversity, so it is rarely used. In the same BTS or cell, if two sets of antennas with an interval of dozens of wavelength are used to receive the same signal, the most powerful signals or combined signals with minimum fading can be selected through diversity combining technology. The diversity gain can be used to indicate the improvement of space diversity, which value is related with adopted combing technology. However, the improvement depends on the ratio between the effective height of diversity antenna (he) and level interval (d), and the incoming wave angle α. When the frontal signal (i. e. α=0o) is received, the signal coherence coefficient on two sets of antennas is the smallest one and the gain is the greatest one; when the lateral signal (α=90o) is received, the coherence coefficient is the greatest one and the gain is the smallest one. Space diversity is the most effective and most common mode in the mobile communication. 2)
Time diversity
Time diversity can be used to send the same message through a certain delay, or send a part of message at different times within the allowed range of delay. Interleaving technology is used to realize time diversity. 3)
Frequency diversity
Frequency diversity is realized through frequency hopping. 4)
Polarization diversity
It can get a better diversity gain to set two sets of antenna to form a certain angle. Moreover, the two sets of antenna can be integrated as one set of antenna. Therefore, for a sector, only one set of Tx antenna and one set of Rx antenna are needed. If the duplexer is used, only one set of antenna integrated by Tx and Rx antennas is needed. Huawei BTS uses dual polarization antenna to realize polarization diversity. This can realize the combination of antenna, tower top amplifier (optional), feeder, and divider. When the complicated radio transmission conditions result in deterioration in a path of the received signals, another path of received signals may vary in signal quality as they are from an irrelevant transmission path. The BTS receives two paths of signals: main and diversity signals, demodulates and combines them. This gives 3~5dB diversity gain. It has been proven that for the space diversity, a better diversity can be achieved when the distance between 2 sets of antenna is greater than 10 wavelengths. For the polarization diversity, it has the advantage of convenient antenna extension and saving hoist space and is increasedly used.
Huawei Technologies Proprietary 2-98
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
II. Parameter The system needs no extra data configuration to realize the diversity receiving.
2.2.7 Aggressive Frequency Reuse Pattern I. Aggressive Frequency Reuse With the development of network, the subscribers increase gradually, the contradict between short frequency resource and great demand is particularly obvious. In order to increase capacity, the technology of aggressive frequency reuse should be used to improve the frequency utilization. According to the actual network circumstance and requirements, the system can adopt hierarchical aggressive frequency reuse and 1x3 multiplexing technology. The comparison of adopting different aggressive frequency reuse is as shown in Table 2-13. Table 2-13 The maximum configuration under different bandwidths Frequency band
4x3 multiplexing
Hierarchical multiplexing
1x3 multiplexing
6MHz
S(2/2/2)
S(3/3/3)
S(4/4/4)
10MHz
S(4/4/4)
S(6/6/6)
S(8/8/8)
Note: z z
S(4/4/4) indicates three synchronous cells with each carrier number being 4. In 4x3 multiplexing, 4 indicate four sites, 3 indicates three cells, and totally there are twelve cells as frequency cluster. Different cells in the same cluster have different frequencies; while cells of other clusters reuse one certain group of frequency in these twelve frequency clusters.
II. Advanced aggressive frequency reuse technology 1)
Hierarchical aggressive frequency reuse
Hierarchical aggressive frequency reuse supports that there can be several different frequencies multiplexing modes working simultaneously in the same GSM network. For example BCCH adopts 4x3 multiplexing mode and TCH adopts 3x3 and 2x3 modes. The nature of hierarchical aggressive frequency reuse is a method of frequency planning. It has no special requirements of software and hardware for equipment. Hierarchical aggressive frequency reuse divides all available frequencies into several groups and each group serves as a carrier layer. The principle of hierarchical aggressive frequency reuse is as shown in Figure 2-34.
Huawei Technologies Proprietary 2-99
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
(1,2,3,4,...36,37) 1 2 3 4 5 6 7 8 9 10 11 12 BCCH BCCH
TCH1 TCH1
TCH2 TCH2
TCH3
MICRO
Figure 2-34 Principle of hierarchical aggressive frequency reuse After the hierarchical aggressive frequency reuse is used, frequency hopping, DTX and dynamic power control should be started to improve C/I thus to satisfy the requirement of C/I>12dB. The frequency hopping can get the frequency diversity gain and interference diversity gain. For example: the maximum configuration S (4/4/4) packet mode can be divided into: BCCH, TCH1, TCH2 and TCH3. There are two modes of carrier packet: z
Continuous packet: The ARFCNs of frequencies assigned in the same layer are continuous.
z
Interval packet: The ARFCNs of frequencies assigned in the same layer have intervals.
The following examples illustrate these two packets. Provided that frequency range is 512~561, totally 50 frequencies. 12 frequencies are assigned for BCCH, 38 for TCH. a) Continuous packet mode BCCH (12): 512~523; TCH (38): 524~561. b) Interval packet mode BCCH (12): 512, 514, 516, 518, 520, 522, 524, 526, 528, 530, 532, 534; TCH: 513, 515, 517, 519, 521, 523, 525, 527, 529, 531, 533, 535~561. Both these two packet modes have their advantages and disadvantage. The comparison is made as follows. In the case of continuous packet, the interference between BCCH carrier layer and TCH carrier layer is little. However, both same frequency and adjacent frequency interference should be considered as a restriction for the planning of BCCH layer. Meanwhile, BCCH layer and TCH layer are quite independent and there is only one
Huawei Technologies Proprietary 2-100
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
frequency between BCCH and TCH layers, therefore, BCCH layer can be easily modified without interference to TCH layer. The employment of interval packet mode can guarantees that there is no adjacent frequency interference between BCCHs. Moreover, the planning of BCCH carrier layer is relatively easy since the same frequency interference is considered as a main restriction. However, the interference between BCCH and TCH layers is strong. Therefore, the planning of TCH layer after the planning of BCCH layer becomes difficult. Under the condition of the same number of frequencies, the continuous packet mode of BCCH carrier layer is more difficult than the interval packet mode, for more consideration should be given to the restriction of adjacent frequency interference. (In the system with frequency hopping adopted, the less consideration can be given to the restriction of adjacent frequency interference. The principle for different carrier layer multiplexing ratios: Assign frequency layer by layer, try to apply different multiplexing ratios for different layers, and realize aggressive frequency reuse layer by layer. General principle: BCCH>TCH1>TCH2>TCH3 When multiple frequency multiplexing is adopted, C/I value will be decreased due to the aggressive frequency reuse being adopted for each TCH layer. Then the requirement that the same frequency interference C/I is greater or equal to 12dB in GSM system is not guaranteed. Moreover, the different frequencies have different interference situations. The less frequencies in the layer, the more serious interference. If frequency hopping and other measures are not adopted, the above-mentioned interference between frequencies will take place thus the communication quality is not guaranteed. Therefore, the system must adopt measures such as frequency hopping, discontinuous transmission, and dynamic power control to minimize these kinds of interference. Frequency hopping can get the frequency diversity gain and interference diversity gain so as to avoid Rayleigh fading and same frequency interference. It should be noted that the purpose for different carrier layers using different multiplexing ratios is to avoid interference at most. This is shown in the flowing aspects. Under the circumstance of non-uniform network sites, it is not the case for every cell to use the TRX of last layer or the most last layers. So the TRX of last layer or the most last layers can realize a higher aggressive frequency reuse ratio (even without the employment of frequency hopping). Since the system tries to s are tried to use different multiplexing modes for each carrier layer, frequencies of any two cells in network are not completely the same, i. e. there is no the real same frequency cell. After multiple frequency multiplexing is realized, though interference is increased, the TRX also increased. This makes more frequencies to participate in frequency hopping and the gain is increased. If the frequency with weak interference and frequency with strong interference coexist in the Huawei Technologies Proprietary 2-101
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
same cell, they will be mixed after frequency hopping is adopted. The system can still use the interfered frequencies according to the feature of decoder. So for each burst, C/I is changeable. But for a specified connection, its quality depends on C/I equalizing value and the equalizing value is not fluctuated.
III. 1x3 frequency multiplexing technology 1x3 frequency multiplexing technology is a kind of aggressive frequency reuse. The following is a simple example to illustrate the principle of 1x3 frequency multiplexing. Provided that the maximum configuration site is S (8/8/8) in a certain area, the available frequency band is 14. 4MHz, 9 frequencies are reserved for micro-cellar, 12. 6MHz is left, and there are totally 63 frequencies. Among these 63 frequencies, 15 are assigned for BCCH carrier (the assignment on the frequency is continuous), and 48 TCH frequencies are left. Then frequencies are divided into 3 groups (combiner hopping mode is adopted): Group 1: 29, 32, 35, 38, 41, 44, 47, 50, 53, 56, 59, 62, 65, 68, 71, 74; Group 2: 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, 63, 66, 69, 72, 75; Group 3: 31, 34, 37, 40, 43, 46, 49, 52, 55, 58, 61, 64, 67, 70, 73, 76. 1x3 frequency multiplexing has the advantage of high frequency efficiency, easy planning method, and easy assignment of frequency. Meanwhile, HSN and MAIO should be carefully planned and the BTS should support radio frequency hopping. In large cities, there are many BTSs and the site is complicated. The employment of 1x3 frequency planning method can greatly reduce workload and good performance can be achieved in the case of small multiplexing ratio. 1x3 multiplexing uses the principle that the number of FH frequencies is greater than the number of carrier frequencies in the cell to avoid interference and to reduce same frequency collision probability. For a specified connection, its quality depends on C/I equalizing value. It has been proven that whether the C/I is good or not depends on same frequency collision probability after radio frequency hopping. And the collision probability is only related with the frequency utilization. 1x3 frequency multiplexing mode is as shown in Figure 2-35.
Huawei Technologies Proprietary 2-102
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
Figure 2-35 1x3 frequency multiplexing mode The frequency planning of 1x3 frequency multiplexing is easy and practical, as well as some disadvantages. For example: when sites are distributed irregularly and the landforms are greatly different, the collision probability will be greatly increase Moreover, in the network in which 1x3 planning is implemented, there is also requirement for network load. When TCH multiplexing ratio is over 40% and the load is over 80%, the network quality will be decreased rapidly. If the TCH multiplexing ratio is higher, for example, over 50% and the load is over 60% or 70%, the network quality will also be decreased rapidly.
IV. Applied conditions for aggressive frequency reuse To adopt aggressive frequency reuse to improve the frequency utilization and the network capacity, a series of anti-interference measures should be taken to reduce the same frequency and adjacent frequency interference caused by aggressive frequency reuse. According to the specifications, carrier interference ratio index (engineering value) is: Same frequency carrier interference ratio: C/I is greater than or equal to 12dB; Adjacent frequency carrier interference ratio: C/I is greater than or equal to-6dB; Carrier interference ratio when carrier has an offset of 400 kHz: C/I is greater than or equal to -38dB. Currently, the following measures are taken to improve the network anti-interference capability so as to satisfy the carrier interference ratio index: Frequency hopping, DTX and power control. The following introduces the effect on improvement of network same frequency C/I and adjacent frequency C/I by frequency hopping. Frequency hopping has two functions: frequency diversity and interference diversity. The frequency diversity gain of frequency hopping depends on propagation environment, MS speed, frequency number of frequency hopping sequence number,
Huawei Technologies Proprietary 2-103
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
and the inter-frequency relativity. It is no greater than 6dB. When MS has a high, frequency hopping has no function of frequency diversity. Generally, the electromagnetic wave of mobile communication consists of direct wave component and scattered wave component. When direct wave is in a dominant position, the frequency diversity of frequency hopping is not obvious. Its gain is about 0~3dB. On the contrary, when scattered wave in a dominant position, the gain is obvious, which is about 3~6dB. For a typical environment in which propagation environment, MS speed, and interval between frequencies are satisfied to achieve the maximum FP frequency gain, the maximum gain for three-frequency hopping reaches 3. 3dB, 6dB for four-frequency hopping, no greater than 5.5dB for 9-frequency hopping. The maximum gain of frequency diversity is no greater than 6dB. The interference diversity capability of frequency hopping is related to interference distribution, frequency number of frequency hopping sequence number, and the inter-frequency relativity. Generally, for the narrow band interference, interference diversity functions apparently; for the broadband interference, it does not function apparently. It has been proven that when interference is distributed as narrow band and the number of FH frequencies is 3, 5, 7, the interference diversity gain for interfered frequency is 3.2 dB, 4.6 dB , 5.5 dB respectively. The function of interference diversity is shown on the equalization of interference. Therefore, the interference diversity gain for a single frequency is 0 by default and is sent in the system information
2.2.8 Multiband Network I. Overview The multiband network is a network combined GSM900 and GSM1800 In the multiband network, GSM multiband MS can communicate in either GSM900 frequency band or GSM1800 frequency band. Each cell in a multiband network has frequencies from only one frequency band. The multiband network allows cell reselection, distribution and handover between GSM900 cell and GSM1800 cell. The application of multiband network is as shown in Figure 2-36.
Huawei Technologies Proprietary 2-104
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
GSM900
GSM1800
MSC BSC
BSC
MSC BSC
BSC
GSM900 Cell GSM1800 Cell Figure 2-36 GSM900/GSM1800 multiband network The multiband network can be used to utilize the abundant frequency resources in GSM1800 frequency band, to absorb network traffic, and to satisfy the increasing demand of network capacity.
II. Technical description To guarantee the stable operation of multiband network, it is of utmost importance to correctly configure the parameters related with the multiband network operation at the stage of network commissioning. Given below is a description of the technical principles governing the multiband network. 1)
MS Classmark
In the GSM system, MS Classmark represents the MS services, supported bands, power, and encryption capability. The Classmark of the mobile station falls into three categories: Classmark1, Classmark2 and Classmark3. The network can interrogate the Classmark of MS and realize its capabilities. In addition, the network can request the mobile station to report its Classmark3 immediately after creating a link by setting the parameter “Early Classmark Sending Control”. Since the important messages in Classmark3 are created specially for multiband applications, it is required that in the multiband network the equipment should support the processing of MS Classmark. Huawei BSS supports ECSC, processing of MS Classmark3, etc. 2)
BA list
In the GSM system, the BA (BCCH Allocation) list is a set of all the carrier channel numbers of adjacent cells of each cell. The network carries out compatibility handling for various types of MSs through system information control. It also guides the MSs to access and handover correctly so that good services of the radio network can be guaranteed. Huawei Technologies Proprietary 2-105
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
BA defines the absolute channel numbers used by carrier of all adjacent frequency cell BCCHs, which is used for cell selection and handover. It is the system that informs MS the BA list through system information. There are two types of BA list: z
The BA1 mainly contains the list of adjacent cells searched by the MSs in idle mode. It is transmitted periodically in the system information type 2, 2bis or 2ter, and used for cell re-selection in the idle mode.
z
The BA2 mainly contains the list of adjacent cells searched by the MSs in active mode. It is transmitted in the system information 5, 5bis or 5ter, and used for handover in active mode.
When an MS is in active mode, It extracts the parameters of adjacent cells from the associated channel system information type 5, 5bis or 5ter on the SACCH, instead of from the system information type 2, 2bis and 2ter. In accordance with the actual network status, the BA list in the system information type 5, 5bis and 5ter can be either identical to or varied with that in the system information type 2, 2bis and 2ter. The BA list shall be set in accordance with the network design requirements and the actual status of adjacent cells. Otherwise, there might be inappropriateness in handover or cell re-selection, or even handover failure. In this case, it may impact the services provided by the network. The number of adjacent cells on each BA list shall not exceed 32. 3)
Support of system Information for multiband network
The network carries out compatibility handling of MSs of various classes through system information (type 2 / 2bis / 2ter and 5 / 5bis / 5ter). The radio network controls the MSs to access and handover correctly and guarantees good services. Huawei GSM system carries out thorough compatibility processing of Phase 1 and Phase 2 900 MSs, Phase 1 and Phase 2 1800 MSs and multiband MSs, and supports system information type 2 / 2bis / 2ter and 5 / 5bis / 5ter. The BA1 list is sent in system information type 2 for re-selection. The BA2 list is sent in the system information type 5 for handover. In GSM900 system, the frequency channels are numbered from 1 to 124. Coding can be done on one list without 2bis / 2ter / 5bis / 5ter when the bitmap format is used. However, this should be adjusted after the multiband system is employed. For the GSM900 cells, the GSM1800 frequency channels on its adjacent cell list are for multiband MSs. They are transmitted via the system information type 2ter / 5ter. Only a multiband MS supports the system information type 2ter / 5ter. Whereas the frequency channels of its GSM900 adjacent cells are placed in the system information type 2 / 5 and can be coded in the bitmap format. The Phase 1 MS recognizes the bitmap format only. This ensures compatibility with Phase 1 GSM900 MSs.
Huawei Technologies Proprietary 2-106
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
For the GSM1800 cells, they are handled in a similar way. The 900M frequency channels on the list of its adjacent cells are for multiband MSs, transmitted through the system information type 2ter / 5ter. Whereas the frequency channels of its GSM1800 adjacent cells are placed in the system information type 2/5. As they cannot be coded on one list, the BA list needs to be split into two parts, transmitted respectively in the system information type 2 (or 5) and 2bis (or 5bis). The system information type 2bis (5bis) is for single-band M1800 MSs and multiband MSs only. For the multiband network, it is required that the equipment should support the system information type 2ter/5ter. 4)
ECSC (Early Classmark Sending Control)
ECSC indicates whether MS is required to report the MS Classmark3 voluntarily and early. For further details refer to the protocol 0408. On receipt of the Classmark change message, MS will send the additional Classmark message to the network as early as possible. Classmark3 information includes the power messages of various frequency bands of multi-frequency MS. In the handover between different frequency bands, power level should be correctly described. It is essential to know the Classmark3 message when making a paging call or sending the BA2 list in different bands. The sampling range of the ECSC is as follows: ECSC=1, transmission required. ECSC=0, transmission not required. This function comes into beings for the multiband-networking situation. And the information in Classmark3 is for multiband application. In case of single-band networking, it is advisable to set this parameter to 0. In case of multiband networking, the recommended value of ECSC is 1 so that signaling flow can be reduced. The parameter ECSC is transmitted in system information type 3. 5)
MBR (Multi-Band Report)
MBR serves to help the network to notify the MS that the 6 adjacent cells reported must cover multiple bands. In the single-band GSM system, when the MS reports the adjacent cell measurement results to the network, it need only report the 6 adjacent cells with strongest signals in a band. When there is a multi-band network, the operator will usually expect the MS to have the priority to enter a band in time of handover depending on the actual status of the network. Therefore, the MS is expected to report the measurement results based not only on the level of the signals but also on the band of the signals. The system parameter “Multi-band Report”, therefore, serves to notify the mobile station to report the multi-band adjacent messages.
Huawei Technologies Proprietary 2-107
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
In the multiband network, the following situation often occurs because the propagation loss in the 1800 MHz band is larger than that of the 900MHz band: among the 6 adjacent cells with strongest signals as reported by MSs, none of them is a GSM 1800 cell. This will affect the absorption of traffic by the GSM1800 network. In this case, the network can request the multiband MSs to send the MR about the adjacent 1800MHz cells by setting the MBR value. By setting different values for MBR, the MSs can report the messages of the adjacent cells of different bands as required when submitting the MRs of 6 best adjacent cells. MBR is represented in decimal digits, with the ranges from 0 to 3. Its implication is shown in Table 2-14. Table 2-14 MBR implication MBR
Implication
0
MS shall report the measurement results of 6 adjacent cells with strongest signals known and allowed by NCC depending on the signal level of the cells, regardless of which band the cells are in.
1
MS shall report the measurement results of an adjacent cell in each band with strongest signals, which are known and allowed by NCC on the adjacent cell list. Then it shall report the adjacent cells in the band used by the current service area in the remaining space of the report. If there is still space left, it shall report the status of the other adjacent cells, regardless of which band they are in.
2
The MS shall report the measurement results of two adjacent cells in each band with strongest signals known and allowed by NCC on the adjacent cell list. Then it shall report the adjacent cells in the band used by the current service area in the remaining space of the report. If there is still space left, it shall report the status of the other adjacent cells, regardless of which band they are in.
3
The mobile station shall report the measurement results of three adjacent cells in each band with strongest signals known and allowed by NCC on the adjacent cell list. Then it shall report the adjacent cells in the band used by the current service area in the remaining space of the report. If there is still space left, it shall report the status of the other adjacent cells, regardless of which band they are in.
6)
PI (Cell Reselection Parameter Index)
PI is used to notify MS whether to adopt C2 as cell reselection parameter and to calculate whether the parameter of C2 exist. Value range of PI: Y or N. Y indicates that MS should extract parameters from broadcasting of system information in cell to work out C2 value and use the value to serve as the standard of cell reselection; N indicates that MS should use C1 to serve as cell reselection standard (i. e. C2 = C1). Generally, PI is set as Y in multiband network.
Huawei Technologies Proprietary 2-108
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
III. Traffic guide strategy in multiband network In the multiband networking, one of the most important purposes is to try to let GSM1800 network absorb or share traffic so as to satisfy the increasing requirement of network capacity and quality. The following principles should be followed. z
In the early stage of multiband network construction, try to let GSM1800 cells absorb multiband subscribers.
z
Realize the continuous coverage of GSM1800 network in hot spot areas.
z
When the number of multiband subscribers reaches a certain level, use different bands to share traffic thus to reduce handover and provide better service.
The carrier can realize different traffic control strategy through real-time adjustment of related parameters. Different traffic control methods are used for different MS states. GSM1800 cell can have higher priority or better adjacent cell measurement comparison value through the configuration of system parameters. So when the subscriber turns on the mobile to select cell in idle mode or reselects cell in standby state, GSM1800 cell can be more likely to be the serving cell for multiband subscribers. In this way, the subscriber is more likely to wait at GSM1800 before a call connection; during the connection of MS call, the traffic distribution can be adjusted by directed retry. In connected state, try to connect as much as possible traffic to high level GSM1800 cells in lower layers through cell hierarchy and specifying different hierarchical cell structures (HCS); the multiband traffic handover can be used to make traffic load more rational. The following describes in detail the cell selection, cell reselection, directed retry, cell hierarchy and specifying HCS, and multiband handover. 1)
Cell Selection and Cell Reselection
In idle mode, the system guides the traffic absorption by controlling the process of MS cell selection and cell reselection. When MS turns on, it first needs to select cell so that to confirm its serving cell. Principle of cell selection: cells allowing to be accessed and cells with high priority are first selected; for the cells with the same priorities, the cell with maximum C1 value is first selected. The C1 value of selected cell should be greater than zero. C1 value is calculated as follows: C1 = RxLEV − RxLEV _ Access _ MIN − MAX ((MS _ TxPWR _ MAX _ CCH − P ),0 )
RxLEV Access MIN range: 0~63, 0 is corresponding with -110dBm, 63 is corresponding with -47dBm. MS TxPWR MAX CCH value range: z
GSM900: 0~19 available, 0 is corresponding to 43dBm, 1 is corresponding to 41dBm. The value of higher level is 2dB greater than that of lower level.
Huawei Technologies Proprietary 2-109
Feature Description M900/M1800 Base Station Subsystem
Chapter 2 BSS Functions
GSM1800: 0~15 available, 0 is corresponding to 30dBm, 1 is corresponding to
z
28dBm. Step is 2dB. In the multiband network, owing to the strong fading of signals in GSM1800 frequency band, signals in GSM900 frequency band is stronger. In order to enable MS can be accessed to GSM1800 system, the cell selection priority can be controlled by setting value of cell bar qualify (CBQ) and cell bar access (CBA). The signals in GSM1800 cell are generally weaker than that in GSM900 cell. To enable the multiband MS to select GSM1800 cell preferentially, GSM1800 cell can be set as Normal and GSM900 cell as Low. Table 2-15 Cell selection/reselection hierarchy Case
CBQ
CBA
Cell selection
Cell reselection
1
0
0
Normal
Normal
2
0
1
Barred
Barred
3
1
0
Low
Normal
4
1
1
Low
Normal
When selecting cell, GSM900 cell is set as CBQ=1, CBA=0 and GSM1800 is set as CBQ=0, CBA=0. This enables GSM1800 cell to have a higher priority. After MS completes cell selection, it should reselect cell in standby state in order to select a better serving cell. The parameter that decides cell reselection is C2. MS reselection principle is to select the cell with maximum C2 value as the serving cell. C2 depends on the following factors: z
C2=C1+CRO-TO×H(PT-T) (PT
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