Description
PHICH
Sunday, February 26, 2017 4:45 PM
I explained the gains of optimizing the PDCCH (control part) of the LTE subframe in my previous article. Let’s have a look at the data part (PDSCH) and find out various ways to improve it’s efficiency. The spectral efficiency is simply the number of bits transmitted over a frequency bandwidth in a specific time and is measured in bits/s/Hz. It is proportional to throughput as the throughput is also bits per time transmitted in a certain bandwidth. From LTE’s perspective, if the number of bits transmitted in a subframe (time) over a specific number of Resource Blocks (frequency bandwidth) is high, then it will correspond to higher throughput and higher spectral efficiency. Let’s understand the various factors impacting the spectral efficiency and ways to perform LTE throughput optimization.
– Signal To Noise & Interference Ratio: The most basic and common factor that controls the spectral efficiency and throughput is the SINR (Signal to Noise and Interference Ratio). If the SINR of a network is bad, then that puts a
limit on the throughput gain that it can achieve. So, the first thing to verify is the average SINR of the network. Let’s check some of the factors that impact SINR Inter-site distance This one is a basic thing. If sites are too close to each other, they will have a higher tendency to interfere with each other and will require aggressive down-tilts to limit overshooting. The distance is something that is usually fixed as LTE sites mostly use the previously deployed network. So, there is not much to do at this level other than downtilts to improve SINR and reduce overshooting.
Electrical Tilt Ports: It is better to use antennas which assign a different RET port (electrical tilt) to LTE. That provides flexibility for optimization. If the network uses the same RET port for LTE and other RATs (3G or 2G) then any change on LTE tilt will impact the other RAT and it takes away the flexibility. So, it is a good idea to keep this in mind in the design or expansion phase.
Pa & Pb: Another thing that can be done in case of smaller inter-site distance is to use a more balanced RS power (Reference Signal). There are two parameters in LTE Pa and Pb which define the power of the Reference Signals against the other symbols e.g. PDSCH Symbols. I will just explain with an example. If the Pa is -3 and Pb is 1, then that means that the Reference Signals will be having 3 dB higher power than the PDSCH symbols. When the intersite distance is low, then high reference signal power can result in higher interference. If the inter-site distance is large then this configuration can be helpful as a 3dB Reference Signal boost will improve coverage as LTE coverage is controlled with RSRP and RSRP is the direct outcome of RS power. However, in case of small inter-site distance, Pb and Pa values of 0 might provide a more optimized solution as in this case, the RS power will not be boosted compared to the PDSCH symbols. Moreover, the PDCCH/PDSCH symbols in which Reference Signals are present will have a slightly higher power for 2 and 4 antenna port systems. This happens because previously with 0,-3 configuration, the RS were taking the extra available power but now with the 0,0 configuration, the extra power is used by the other channels instead of RS. So, that improves the credibility of PDSCH and can result in better throughput results. This is a big topic so I am just touching it here and will cover this in more details in the future articles. For Example Live values of Pa & Pb
Cell Name
RS Power (dBm)
Pb
Pa
G_15_2-0
21.2
1
-3
G_15_2-1
21.2
1
-3
G_15_2-2
18.2
0
0
Islamabad_Homes-0
21.2
1
-3
Islamabad_Homes-1
18.2
0
0
Islamabad_Homes-2
21.2
1
-3
Motorway_Chowk-0
18.2
0
0
Motorway_Chowk-1
18.2
0
0
Motorway_Chowk-2
18.2
0
0
Load & Utilization Second factor is the load in the area or cluster. Higher the load, higher the interference to the neighbouring cells. As the load increases, the power per Resource Element increases which will result in higher aggregate power in the area increasing the RSSI. For neighbouring cell, such a power is considered interference. So, if the load increases above a threshold, it is better to add another carrier or if another carrier already exists, then it will be better to offload the congested carrier and shift the load to the uncongested carrier. This can be done using Load Balancing features or by tuning the cell reselection or mobility parameters. Sometimes, the actual traffic volume is not that high but the utilization of the cell is still very high. This is usually caused due to low signal quality as the users with bad SINR will take a lot RBs at a lower modulation. In case, the traffic is not high but utilization is still high, it is a good idea to see the TA and CQI for the cell. If TA is pretty high and CQI is below 8 (value depends on the frequency layer) then it might be a better idea to physically optimize the area or cell. Introducing PDCCH optimization also helps in such cases as it can add to the PDSCH capacity relieving congestion to an extent. PCI Planning As described in my PCI planning article, if the adjacent cells with overlapping coverages have same PCI modulo 3, then there is a probability of RS interference between them. Such an interference will reduce the overall RS SINR and demodulation capability resulting in throughput degradation. So, it should be tried to avoid PCI modulo3 conflicts wherever
possible. In FDD networks, it is better to ensure that time synchronization is not enabled as that adds a randomness to the system and PCI mod3 impact is reduced significantly. – CQI & MCS Mapping: The next step is the CQI (Channel Quality Indicator). Once the UE measures it’s SINR, it will convert it to a CQI value so it can report to the eNB. The eNB will take this CQI and map it to a MCS (Modulation & Coding Scheme) value. A higher SINR will result in a higher CQI value and consequently, a higher MCS index. As MCS increases, the throughput usually increases so we need to ensure that we have the most optimum CQI and MCS indexes for each SINR value. In LTE, there are 16 CQI indexes and 32 MCS indexes. Usually, the CQI value of below 7 is considered bad and CQI value of around 10 is considered fair. CQI Adjustment Algorithms The eNB adjusts the raw CQI value shared by the UE to find an optimum CQI and this provides a higher spectral efficiency. There are basically two scenarios where this comes into play Consider a UE-1 that measures its SINR value to be around 10 dB and based on that it calculates a CQI of 9 and sends it to the eNB. Another UE, let’s call it UE-2, measures its SINR value to be around 8 dB but based on that it sends CQI of 9 as the UEs have different chipsets from different vendors and can have a different CQI value for same SINR indexes. The eNB will have two UEs with same CQI value and if the eNB provides both of them with the same MCS (for example MCS20) then it is possible that the UE-1 might be able to work with MCS20 but the UE-2 will not be able to decode MCS20 properly at 8 dB SINR. So, to address this issue, the eNB maintains another index which is like the outer loop of BLER (Block Error Rate). Most of the vendors maintain a BLER target of 10%. Now consider the same scenario, both UEs get MCS20 and UE-1 works with a BLER value of 10% but the UE-2 had lower SINR so it will have a relatively higher BLER. Let’s say, the eNB calculates the BLER to be around 13% so the eNB will lower the MCS for the UE-2 and make it 19. If the BLER still remains above 10%, the eNB will reduce it further to ensure that the BLER target is maintained. Similarly, if the UE sends a CQI value of 8 and eNB initiates downlink data with a MCS of 16 and it finds out that the BLER value is below 10%, it will increase MCS to 17 or 18 until the BLER target is achieved. This scenario will increase the spectral efficiency and the throughput. So, we need to ensure that CQI adjustment or dynamic CQI assignment algorithms or outer loop control based on BLER is activated to achieve maximum gains from the channel.
CQI Convergence Another important thing is that some vendors use low CQI values initially. For example, if the UE has just accessed the cell and it shares a CQI value of 9, the eNB will treat it as a CQI of 7 and a corresponding MCS will be allocated to it. Then after subsequent transmissions, the eNB will keep monitoring BLER and once the credibility of the UE’s CQI is ensured, the eNB will converge to the effective CQI. Some vendors keep this as a hard-coded algorithm while others provide parameters to tune this and then these parameters can be tuned to limit this behaviour resulting in faster convergence and higher throughputs especially for small packet data transfers. For instance, a UE which has a small amount of data accesses the cell and gets its data within two to three TTIs (subframes), then the eNB will not have enough CQI samples to converge quickly. The same UE will try again next time and the eNB will keep using a conservative CQI and MCS for such a UE. So, if the delta for initial CQI value is reduced, such UEs will get a less conservative CQI and MCS resulting in better data rates. CQI Periodicity Another thing that helps is the CQI periodicity or the frequency of CQI reporting from the UE. If the UE reports CQI after a large interval, then the eNB might not have the most accurate CQI to begin with and it will take longer time to converge to the optimum MCS. Usually CQI reports are shared every 40 or 80 ms but if the UE is moving or if the channel is fluctuating then 40 or 80 ms can be considered a large interval. If we shift the CQI period to a smaller value like 20ms or 10ms, then the CQI will be more accurate and that should improve the spectral efficiency. However, the lower the interval, higher the number of CQI reports and higher the PUCCH utilization. Periodic CQI reports are sent over PUCCH in uplink so if we reduce the CQI reporting interval, that will increase the load on PUCCH. This can lead to
interference on PUCCH and it can also result in RRC rejections due to PUCCH congestion. eNB needs PUCCH for CQI, HARQ & SRIs so if the PUCCH is congested, then it will have to reject new incoming access requests. This can be solved by using the following two approaches
Adaptive or Dynamic PUCCH : This is introduced by vendors to resolve the RRC Rejections due to PUCCH overload. This allows the PUCCH to expand and it can consume more Resource Blocks if required. The down side is that the PUCCH takes the Resource Blocks from the PUSCH which can then limit the uplink throughput. However, usually the networks require higher downlink capacity so uplink can be compromised to an extent. Adaptive CQI Period : This is another enhancement that some vendors have. This makes the CQI reporting interval dynamic and the eNB can adjust it based on the user’s characteristics. This way, if the eNB finds a UE that has no channel fluctuation (mostly stationary), it can use longer CQI reporting interval like 80ms and eNB can reduce the interval to 10ms for a UE that has high fluctuation. This provides an optimum performance gain in CQI accuracy without impacting the PUCCH load to that extent.
There is another type of CQI reports known as Aperiodic CQIs but we will discuss that in the next episode of the throughput optimization. Adaptive BLER Targets Firstly, lets understand the concept of BLER. It can be divided into two categories:
Initial BLER: When the eNB sends data to the UE and UE is unable to decode it, then it will send a HARQ NACK to the eNB. A NACK means that the eNB will have to retransmit the data and this NACK is considered IBLER or Initial Block Error. Residual BLER: If the UE is unable to decode the data even after retransmission, the UE will send another NACK and the eNB will have to retransmit again. However, there is a limit to these retransmissions and usually they are configurable. Commonly, these retransmissions are set to 4 and after 4 retransmissions, the eNB will not retransmit at HARQ level and consider this as a Residual Block Error. The BLER target is maintained by the IBLER so this means that the eNB tries to maintain an IBLER of 10% for each UE. RBLER is usually very low and it is supposed to be less than 0.5%. The question may arise that why don’t we reduce the IBLER further and make it low as that should reduce retransmissions. The problem here is that lowering IBLER means that we need to lower the MCS. Even a very low MCS will not ensure a linear decrease in IBLER but it will degrade throughput excessively. So, various simulations and field trials were done to come up with an optimum target of 10% for IBLER which is followed by most of the vendors. However, recently it has been found that BLER target of 10% works fine in fair conditions but when the radio conditions are bad or good, other BLER targets provide higher gains. For instance, if the radio conditions are bad, a BLER target of 10% keeps the MCS very conservative and increasing the BELR target, increases the MCS and it provides higher throughput gains. So, such parameters can be tuned if available to get better results. – Mobility Strategy: One thing that can really help in increasing the throughput is the optimum mobility strategy. Transition to Higher CQI layer Consider two LTE layers, for instance L800 and L1800 with same bandwidth. In this case, L800 will have a higher coverage as it is a lower frequency. So, the user count on L800 will be higher compared to L1800. However, the lower frequency layer also has higher interference since it has a bigger coverage radius. So, that will result in a lower CQI and a bad throughput. L1800 throughput will usually be better even with same bandwidth because it will have better CQI. So, the most important thing is to ensure that the layer with the better CQI gets most of the traffic. This can be done in many ways and I have jotted down a few of those.
The easiest way is to give a higher priority to L1800 and that will shift most of the UEs in L1800 coverage away from L800. This will ensure better CQI for users and thus a better throughput. Another way would be to keep them on same priority and provide a frequency offset to move the users to L1800. This is more reasonable if L1800 is also getting overloaded then the amount of load to be shifted can be tuned by varying the offsets. I prefer load shifting by cell reselection instead of handovers. If the handover thresholds are changed or frequency priority based handovers are used, then it initiates gap periods. For UE, to move from one frequency to another frequency in connected mode, it needs to measure the target frequency. In order to measure the target frequency, the UE goes into a gap mode of 6 ms. This gap mode repeats itself after every 40 or 80ms. So, if it repeats every 40ms then that means that the UE cannot be scheduled for 6ms in every 40ms. Moreover, when the UE gets data, it needs to send a HARQ ACK/NACK after 4ms. So, it means that since the eNB knows that the UE will be in gap mode so the eNB will not schedule any data for the UE 4ms before the gap mode. That makes it 10ms in each 40ms that the UE cannot be scheduled which is around 25% of the time. So, inter-frequency handovers should be minimized as it can
cause a 25% degradation in throughput. Cell reselection works in idle mode so it is a much better way to move users between the layers. Load Balancing Another way is to enable load balancing between the layers and ensuring that the higher CQI layer gets more load. Load balancing usually also comes in two modes
Connected Mode: In this case, the eNB calculates the PRBs or user count and tries to maintain target load values by performing load based handovers between the layers. Idle Mode: In this case, the eNB sends the frequency in the RRC Release command to the UE. eNB increases the priority of the target frequency for that UE temporarily and the UE tries to reselect to that frequency in idle mode. Once again, I prefer idle mode based load balancing as it does not introduce the interfrequency handovers and also gets the work done. But idle mode based load balancing will not have significant impact in case the layers have different priorities since one layer already has higher priority and idle mode based load balancing also moves users by increasing the priority. So, if the UEs are not moving to higher priority layer than that means that the layer has coverage constraints and then the idle mode based load balancing will also be unable to shift the load. Vertical Beam-Width Another important factor is that many times, the low band like L800 has a bigger vertical beamwidth than the corresponding higher band. This effectively means that at the same tilt value, the L800 will have a much bigger coverage foot print than the L1800. So, before making any mobility strategy, it is important to verify the antenna patterns especially the vertical beam-width for all the layers. If the beam-width of one layer is significantly wider than the other, then ensure to put a tilt offset between the two to keep an optimum and balanced coverage. – Scheduler Fairness: Another important factor is the scheduler type. A scheduler can work in multiple modes Round Robin: In this mode, the scheduler provides equal resources to all users. This is not an optimum algorithm as different users have different data requirements. Max C/I: This mode provides significantly higher resources to users in good coverage conditions. This mode can starve the cell edge users and they will not get enough data resulting in degradation in user experience. Proportion Fair: This scheme maintains a fairness between all users maintaining a healthy resource sharing between all user types. The basic concept of this mode is to strike a balance between users and it does that by prioritizing based on CQI and data rates. So, if the CQI is high, it will give resource to that user first but since it needs to maintain a fair data rate for all users, the cell edge users will also be scheduled. This scheme is essentially a combination of both round robin and Max C/I as it provides more resources to users with higher CQI as
compared to round robin but it also provides more resources to cell edge users when compared to Max C/I. Hence, it gets the name Proportional fair. The user throughput KPI improves with Max C/I scheduler as it provides more resources to good users resulting in higher user throughput but the cell throughput is improved with Proportional Fair algorithm as it strikes a balance between all users. So, if the user throughput KPI is to be improved then the scheduler can be tilted towards Max C/I while Proportional Fair can be used if cell throughput gain is required. The optimization at this level really needs deep understanding of the scheduler’s algorithm and it also depends if the specific vendor provides the options to play with the scheduling weights. These are the basics to improve the spectral efficiency for a network. In the next part, I will explain the features that can be used to improve throughput along with the scenarios where they will be applicable From
1. Throughput Optimization Signal To Noise & Interference Ratio - (Physical Redesign for getting best SINR values) Electrical Tilt Ports - (Different technologies have different E-tilt antenna ports) Pa & Pb value - (for city/dense area use Pa Pb 0,0 (same power of PDSCH and RS 18.2 dBm power) for rural use Pa Pb -3,1 (increased RS power 21.2 dBm than PDSCH Load & Utilization - (If utilization is high PCI Planning - (if MOD 3 overlap then RS interference will increase and resultingly decrease in RS SINR and demodulation capability. CQI Adjustment Algorithms - (UE measure RS SINR and send CQI value according to that MCS will be decided it is vendor specific ) CQI Periodicity - (Use dynamic CQI for varying and static conditions (For static CQI update with delay for moving CQI update ) S1 interface fault (Zero Traffic after occurring this
From
MIB(Master Information Block)
MIB is special signal that carries the following information. As you see, you can get the System Bandwidth and SFN by decoding MIB.
i) DL Bandwidth, Number of Transmit Antenna ii) System Frame Number (SFN) iii) PHICH Configuration iv) Transmit every 40 ms , repeat every 10 ms MasterInformationBlock ::= SEQUENCE { dl-Bandwidth ENUMERATED { n6, n15, n25, n50, n75, n100}, systemFrameNumber BIT STRING (SIZE (8)), phich-Config PHICH-Config, spare BIT STRING (SIZE (10)) }
Following is the cycles in which MIB and SIB is transmitted.
From
MIB
Carries physical layer information of LTE cell which in turn help receive further SIs, i.e. system bandwidth
SIB1
Contains information regarding whether or not UE is allowed to access the LTE cell. It also defines the scheduling of the other SIBs. carries cell ID, MCC, MNC, TAC, SIB mapping.
SIB2
Carries common channel as well as shared channel information. It also carries RRC, uplink power control , preamble power ramping, uplink Cyclic Prefix Length, sub-frame hopping, uplink EARFCN
SIB3
carries cell re-selection information as well as Intra frequency cell re-selection information RESELECTION
SIB4
carries Intra Frequency Neighbors(on same frequency); carries serving cell and neighbor cell frequencies required for cell reselection as well handover between same RAT base stations(GSM BTS1 to GSM BTS2) and different RAT base stations(GSM to WCDMA or GSM to LTE or between WCDMA to LTE etc.) . Covers E-UTRA and other RATs as mentioned INTRA FREQ
SIB5
Carries Inter Frequency Neighbors(on different frequency); carries E-UTRA LTE frequencies, other neighbor cell frequencies from other RATs. The purpose is cell reselection and handover. 4G HO
SIB6
carries WCDMA neighbors information i.e. carries serving UTRA and neighbor cell frequencies useful for cell re-selection . 3G HO
SIB7
carries GSM neighbours information i.e. Carries GERAN frequencies as well as GERAN neighbor cell frequencies. It is used for cell re-selection as well as handover purpose. 2G HO
SIB8
carries CDMA-2000 EVDO frequencies, CDMA-2000 neighbor cell frequencies.
SIB9
carries HNBID (Home eNodeB Identifier)
SIB10 carries ETWS prim. notification SIB11 carries ETWS sec. notification
From
System information block 2 (SIB2) in LTE
After initial cell synchronization process is completed, UE will read master information block which contains important information regarding downlink cell bandwidth, PHICH configuration and System frame number. Then UE can read System information block 1 and System information block 2 to obtain useful information related to cell access, SIB scheduling and radio resource configuration System information block 2 carries radio resource configuration information which is common for all UEs. SIB2 information can be divided in following sub categories Random access channel (RACH) related parameters Idle mode paging configurations Uplink physical control channel (PUCCH) and shared channel (PUSCH) configurations Uplink power control and Sounding reference signal configurations Uplink carrier frequency / Bandwidth Cell barring information
SIB2 Example Example SIB2 info is shown below (Taken from UE logs). This SIB2 does not represent any real network value BCCH-DL-SCH-Message ::= message c1 : systemInformation : criticalExtensions systemInformation-r8 : sib-TypeAndInfo sib2 : radioResourceConfigCommon rach-ConfigCommon preambleInfo numberOfRA-Preambles n40, preamblesGroupAConfig sizeOfRA-PreamblesGroupA n32, messageSizeGroupA b144, messagePowerOffsetGroupB dB10 , powerRampingParameters powerRampingStep dB2, preambleInitialReceivedTargetPower dBm-104 , ra-SupervisionInfo preambleTransMax n10, ra-ResponseWindowSize sf5, mac-ContentionResolutionTimer sf32 , maxHARQ-Msg3Tx 3 , bcch-Config modificationPeriodCoeff n8 , pcch-Config defaultPagingCycle rf64, nB oneT , prach-Config rootSequenceIndex 30,
prach-ConfigInfo prach-ConfigIndex 4, highSpeedFlag FALSE, zeroCorrelationZoneConfig 8, prach-FreqOffset 3 , pdsch-ConfigCommon referenceSignalPower 11, p-b 1 , pusch-ConfigCommon pusch-ConfigBasic n-SB 1, hoppingMode interSubFrame, pusch-HoppingOffset 6, enable64QAM FALSE , ul-ReferenceSignalsPUSCH groupHoppingEnabled FALSE, groupAssignmentPUSCH 0, sequenceHoppingEnabled FALSE, cyclicShift 0 , pucch-ConfigCommon deltaPUCCH-Shift ds2, nRB-CQI 1, nCS-AN 0, n1PUCCH-AN 36 , soundingRS-UL-ConfigCommon release : NULL, uplinkPowerControlCommon p0-NominalPUSCH -100, alpha al1, p0-NominalPUCCH -100, deltaFList-PUCCH deltaF-PUCCH-Format1 deltaF0, deltaF-PUCCH-Format1b deltaF1, deltaF-PUCCH-Format2 deltaF0, deltaF-PUCCH-Format2a deltaF0, deltaF-PUCCH-Format2b deltaF0 , deltaPreambleMsg3 1 , ul-CyclicPrefixLength len1 , ue-TimersAndConstants t300 ms200, t301 ms200, t310 ms500, n310 n10, t311 ms3000, n311 n1 , freqInfo
ul-CarrierFreq 20600, ul-Bandwidth n50, additionalSpectrumEmission 12 , timeAlignmentTimerCommon sf10240
Definition of important Parameters Rach Configurations numberOfRA-Preambles: Total number of random access preambles available for contention based random access. Since there are maximum 64 preambles sequences available, others could be reserved by eNB for Non-Contention based random access. Range of this parameter is 4 to 64 sizeOfRA-PreamblesGroupA: Total number of random access preambles sequences available within Group A. Preambles are divided into Group A and Group B. Group A preambles are intended for sending small packets and Group B preambles are intended for sending large packets. Range of this parameter is 4 to 60 messageSizeGroupA: Message size threshold for selecting preamble Group A in term of bits (56, 144, 208 or 256 bits) messagePowerOffsetGroupB: Power offset for selecting preamble Group B (0, 5, 8, 10, 12, 15 or 18 dB) powerRampingStep: power ramping step size with possible values of 0, 2, 4 or 6 dB preambleInitialReceivedTargetPower: Preamble initial received target power with values from -120 dBm to -90 dBm with step size of 2 dBm preambleTransMax: Maximum number of preambles transmissions. Possible values are 3, 4, 5, 6, 7, 8, 10 ,20, 50, 100, 200. ra-ResponseWindowSize: Duration of RA response window. RA response window size is in unit of subframes (2, 3, 4, 5, 6, 7, 8, or 10 subframes) mac-ContentionResolutionTimer: Mac contention resolution timer in unit of subframes (8, 16, 24, 32, 40, 58, 56 or 64 subframes) maxHARQ-Msg3Tx: Maximum number of HARQ retransmissions for message 3 of RACH process (contention-based Random access) with possible values from 1 to 8 in step of 1
BCCH Configurations modificationPeriodCoeff: The value (2,4,6,8) of this parameter is multiplied with default DRX cycle (e.g. 320ms, 640ms) to generate the BCCH modification period. It is the period in which the change in SI is repeated to UEs so that the change in SI is acquired by UE. BCCH modification period = modificationPeriodCoeff x idle mode DRX cycle
PCCH Configurations defaultPagingCycle: The default DRX cycle in idle mode in unit of radio frames (rf64 means 640ms ) nB: This parameter value is used in finding the actual paging frames and paging occasions in RRC idle mode with the following formula SFN modT = (T/N) x (UE_ID mod N) Where T = Drx cycle N = Min (T, nB) (nB is broadcasted in SIB2) UE_ID = IMSI mod 1024
PRACH Configurations
rootSequenceIndex: RA preambles are generated from Zadoff Chu sequence which consists of series of root sequences. Each root sequence can be cyclic shifted to obtain preamble sequence. Range of rootSequenceIndex is 0 to 837. prach-ConfigIndex: This parameter defines exactly when UE should send RACH in frequency/time grids (Details TS36.211 Table 5.7.1-2) highSpeedFlag: For high speed UEs , as this can impact the correlation between cycles zeroCorrelationZoneConfig: The zero correlation zone is used to guarantee orthogonality of generated sequences. The value depends on particular condition in the cell prach-FreqOffset: With this information cell informs UE and other neighbor cells know about which PRB is available for RACH access
PDSCH Configurations referenceSignalPower: This defines the energy per resource element for the reference signal using a range from -60 to 50 dBm. p-b: It is used to calculate the power difference between PDSCH and Reference Signal. Value is from 0 to 3
PUSCH Configurations n-SB: Number of subbands (range 1 to 4) hoppingMode: Hopping mode can be inter-subframe, intra or inter-subframe pusch-HoppingOffset: Offset values range from 1 to 98 enable64QAM: if 64QAM capable UE should use it (True or False) groupHoppingEnabled: True or False groupAssignmentPUSCH: Gives sequence shift pattern for group hopping (0 to 29) sequenceHoppingEnabled: True or False cyclicShift: Frequency shift for demodulation (0 to 7)
PUCCH Config deltaPUCCH-Shift: 1,2 or 3 nRB-CQI: Number of PRBs per slot for PUCCH2 (0 to 98) nCS-AN: Cyclic shift used for PUCCH1 (0 to 7) n1PUCCH-AN: PUCCH to be used for HARQ (0 TO 2047)
Sounding Reference Signaling Configurations: The uplink Sounding Reference Signal (SRS) is configured in terms of bandwidth and subframes
Uplink Power Control p0-NominalPUSCH: It impacts the calculation of PUSCH transmit power and applicable to non-persistent scheduling only (-126 to 24 dBm) alpha: It also impacts the calculation of PUSCH transmit power and also scales the contribution of path loss. Possible values are 0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1 p0-NominalPUCCH: This parameter impacts the calculation of PUCCH transmit power (-127 to -96 dBm) deltaFList-PUCCH: These parameters impacts the calculation of PUCCH transmit power deltaPreambleMsg3: It impacts the transmit power of PUSCH when responding to random access response grant (-1 to 6dB) ul-CyclicPrefixLength: To differentitate between normal (len1) OR extended (len2) cyclic prefix for uplink transmission
UE Timers and Constants T300: Time during which UE waits for RRC connection request message response (100, 200, 300, 400, 600, 1000, 1500, 2000 ms) T301: Started after RRC Connection Reestablishment request message. On expiration UE will go to RRC idle (100, 200, 300, 400, 600, 1000, 1500, 2000 ms) T310: Started after receiving N310 out of sync indications (0, 50, 100, 200, 500, 1000, 2000 ms) T311: Started after initiating connection re-establishment procedure. On expiration UE goes to RRC idle mode if it is unable to locate suitable cell (1, 3, 5, 10, 15, 20, 30 seconds) N310: Consecutive out of sync indications (1, 2, 3, 4, 6, 8, 10, 20) N311: Consecutive in-sync indications (1, 2, 3, 4, 6, 8, 10, 20)
Frequency Information ul-CarrierFreq: Defined in terms of EARFCN ul-Bandwidth: Defined in terms of resource blocks additionalSpectrumEmission: This allows spectrum emission limits to be configured according to local requirements (1 to 32) timeAlignmentTimerCommon: it tells UE how long it should consider itself to be time aligned in uplink in unit of subframes. (500, 750, 1280, 1920, 2560, 5120, 10240 or infinity subframes)
1. 2. 3. 1. 2.
Resource Allocation :- In control information or more specifically inside DCI , eNodeB has sent the details regarding resources on which UE can decode its downlink data or UE can send their uplink data. As control information data is made up of CCE's , similarly actual data information is composed of RB's. We have already discussed about RB ( Resource Block ) in basic LTE section. In 20 Mhz we have 100 RB's . Suppose eNodeB wants to allocate 20 RB's to a UE . To send this information inside DCI such that UE can figure out which 20 RB's out of 100 are allocated to that UE , eNodeB needs to send 100 bits each bit representing 1 RB. But this procedure will increase too much data on control information . To reduce the size of control information , LTE introduces different resource allocation strategies:Resource Allocation Type 0 Resource Allocation Type 1 Resource Allocation Type 2 Localized Distributed According to spec 36213 section 7.1.6 , There is a fixed mapping exist between DCI format used in downlink and the RAT policy used for allocation in downlink. How UE comes to know about the RAT policy used by eNodeB : In PDCCH (or In DCI ) There is a field present called Resource allocation field which includes two parts
1. Resource Allocation Header Field :- It is 1 bit information in which value 0 is indicated for RAT0 and RAT1 is indicated otherwise. This field is not present for RAT2 2. Actual resource block assignment :- It contains the actual resource information. For Example:- The example clearly illustrates the presence of Header field and the actual allocation in blue color for DCI format 1. How the actual resource block assignment is decoded will be explained below in each resource allocation policy. From
Device ON Power ON Synch PSS, SSS BCH MIB (DL RB, PHICH, SFN, HARQ) SIB The first thing UE needs to do is to search for Primary Synchronization Signal (PSS). The location of PSS is in the 1st and 6th subframe of LTE and within the subframe it exists on the last symbol of the slot. So, once the UE has decoded the PSS, it gets the following information.
PSS gives the slot boundary timing independant of the CP length, so syncs at slot level. It provides the center frequency as it is around the DC carrier. After this, the UE starts looking for the Secondary Synchronization Signal (SSS) which is just one symbol before the PSS. The PSS is on the 7th symbol of the 1st slot of 1st & 6th subframe while SSS lies on the 6th symbol. After decoding SSS, the UE gets the following information.
UE gets to know the CP length as it has the duration of two consecutive symbols (SSS and PSS) so it can derive whether the eNB is using normal CP or extended CP. Since the location of SSS and PSS differs in TDD and FDD so the UE can also find out the frame type. Next thing to find out is the start of the frame. As SSS & PSS exist in both Subframe 1 and 6, so the UE needs to know which of the frame is the 1st subframe. That is why the SSS is different in both subframes and therefore, after decoding the SSS, the UE can understand which is the 1st subframe. So, this ensures that the UE is synchronized at both frame and symbol level. The PCI (Physical Cell Identity) is made up of a combination of PSS & SSS with the following equation
PCI = 3(SSS) + PSS
Once, the UE has decoded both the PSS and SSS, it can derive the PCI which tells the location of the RS and the PCFICH. This lets the UE get the RSRP and verify that the cell is above the cell selection threshold. Then it goes for PBCH which is after the PSS of the first subframe. PBCH tells about the system BW, System frame number, PHICH config and number of Tx. Now that the UE knows PHICH, PCFICH and RS location, all the other REs belong to PDCCH. The UE looks for the DCI for SIB-1 by decoding the DCI masked with CRC of SI-RNTI. The SIB-1 is sent after every 20 ms but the TTI is 80 ms (like PBCH – comes every 10 ms but the TTI is 40 ms). The copies of SIB-1 after 20 ms are different redundacny versions of the same SIB-1. The SIB-1 tells about the other SIBs (SI periodicity and SI Window length), including SIB-2 which tells about the RACH information required for uplink synchronization. The location of RACH is determined by the following parameters in SIB-2 PRACH CONFIGURATION INDEX ==> Tells the SFN (even/odd) and subframe number – thus the location in time domain PRACH FREQUENCY OFFSET ==> Tells the PRB offset and thus the location in frequency domain
NCS VALUE ==> Tells the NCS value and the number of root sequences per cell needed to generate 64 preambles ROOT SEQUENCE INDEX ==> Tells the starting root sequence index for the cell Based on these values, the UE generates a random preamble and sends a RACH request. After the RACH request, the UE needs to start reading PDCCH for its RA-RNTI after 3 subframes (3 SF after the prach preamble transmission is finished). The RA RSP WINDOW SIZE tells the maximum number of subframes within which the eNB needs to send the RAR. Usually, it is set to 10 SF and therefore the eNB needs to respond to a PRACH request within 12 SF. The RAR contains RA-RNTI or temporary C-RNTI and RAPID (which contains the preamble ID that UE sent). Once RAR is received, the UE sends msg3 which is RRC Conn Req message that contains UE ID (TMSI or random value). eNB responds with a MCE Contention resolution message before RRC Conn Setup and that contention resolution message contains the same UE ID that is sent by UE in RRC Conn Req message. So, if there are two UEs using the same preamble, then at this step the contention will be resolved. As the UE with the same ID will send the HARQ ACK to Contention resolution message but the other UE will consider RACH failure and re-initiate RACH. In response to RRC Connection Request, eNB sends a RRC Connection Setup which carries SRB1 (Signalling Radio Bearer) addition parameters. Before this, the UE uses SRB0 to send the RRC message. Once the UE gets RRC Connection Setup message, the UE responds with the RRC Setup Complete message. It is this message that carries NAS messages. At this moment, the RRC setup is completed and SRB1 is also setup. Based on this, the eNB initiates S1 Initial UE message to the MME and MME can respond to this message in different ways but the most common response is S1 Initial Context Setup Request. This message is considered as the ERAB Setup Request and it usually contains the ERAB-ID and QCI that has to be setup for the UE along with MBR configuration of the bearer.Consequently, the eNB reconfigures UE using RRC Connection Reconfiguration message which contains the addition for SRB2 and DRB (data radio bearer) based on the QCI requirement. eNB also sends Security Mode Command to UE to configure the security context at this stage. Once this is done, the eNB responds to MME with S1 Initial Setup Response and at this point the ERAB Setup is considered successful.
Primary Synchronization Signal (P-SS) Sequences • Three PSS sequences are used in LTE, corresponding to the three physical layer identities within each group of cells.
• The PSS is constructed from a frequency-domain ZC sequence of length 63. • Transmitted on 6th symbol of slot 0 and slot10 of each radio frame on 72 subcarriers centered around DC.
Secondary Synchronization Signal (S-SS) Sequences
• SSC1 and SSC2 are two codes are two different cyclic shifts of a single length-31 M sequence. • Each SSS sequence is constructed by interleaving, in the frequencydomain, two length-31 BPSK-modulated secondary synchronization codes • Two codes are alternated between the first and second SSS transmissions in each radio frame • This enables the UE to determine the 10 ms radio frame timing from a single observation of a SSS • Transmitted on 5th symbol of slot 0 and slot10 of each radio frame on 72 subcarriers centered around DC. PSS carries physical layer identity (NID)(2) SSS carries physical layer cell identity group (NID)(1) Cell identity is computed using (NID)cell = 3*NID(1) + NID (2) ,
Where (NID)(1) = 0,1,.....,167 and (NID)(2) = 0,1,2
Signal Full Form
Direction Position
Modulation/ Coding scheme
P-SS
Primary Synchronization Signal
Downlink 6th symbol of Zadoff Chu slot 0 and sequence of slot 10(time length 63 exis) mapped on 72 subcarriers centered around DC(frequency axis)
S-SS
Secondary Downlink 5th symbol of Synchroslot 0 and nization slot 10(time Signal exis) mapped on 72 subcarriers centered around DC(frequency axis)
BPSK modulated length-31 M sequence
Function
UE first finds the primary synchronization signal (PSS) which is located in the last OFDM symbol of first time slot of the first and 5th sub-frames This enables UE to be synchronized on sub-frame level Primary Synchronization Signal helps for Slot Timing Detection and Physical Layer ID (0,1,2) detection From SSS, UE is able to obtain physical layer cell identity group number (0 to 167) It helps for Radio Frame Timing detection,
find Physical Layer Cell ID, cyclic prefix length detection, FDD or TDD detection
From
Trial 3 # Background Application Scenarios: Receiving diversity is a technique to monitor multiple frequencies from the same signal source or multiple radios and antennas monitoring the same frequency, in order to combat signal fading and interference. Receive diversity is a way to enhance the reception of uplink channels, including the PUSCH, physical uplink control channel(PUCCH), physical random access channel (PRACH) , and sounding reference signal(SRS). Benefit This feature can improve the uplink coverage and throughput. Dependency RX diversity requires the eNodeB to provide enough RF channels and demodulation resources that can match the number of diversity antennas. This feature has no special requirements on UEs. This feature is only applicable to Macro eNodeB.
Summary
Significant improvement in Uplink User and Cell Throughput Significant increase in LTE Volume (MB) Increase in DL Cell throughput is observed Significant improvement in UL HARQ & UL RLC retransmissions Improvement in PDCCH CCE usage rate Increase in PDCCH aggregation level 1 and decrease in level 8 Significant increase in UL 16QAM share and decrease in UL QPSK share Significant increase in PDSCH and PUSCH MCS
Automatic Neighbor Relation (ANR) in LTE Manually adding neighbor cells in network is indeed a very hectic process and prone to errors as well. While networks are becoming more and more complex, it is required to find an automatic and a more optimized way of adding neighbor cells.
ANR comes under the umbrella of Self Organizing Networks ( SON) features. ANR relies on UE to detect unknown cells and report them to eNB. There are two major types: i) UE based ANR ii) ANR with OAM Support
UE based ANR
No OAM support is required. UE detects PCI of unknown cell when it needs to do measurement (as configured by network) In case of inter-frequency or inter-RAT measurements, eNB needs to configure measurement gaps/or DRX so UE can detect PCI to different frequencies as well. UE reports the unknown PCI to eNB via RRC-Reconfiguration message. eNB request UE to report Eutran Cell Global ID (ECGI). UE reports ECGI by reading BCCH channel. eNB retrieves the IP address from MME to further setup the x2 interface.
ANR with OAM Support
OAM support is required Every new eNB registers to OAM and download the table with information of PCI/ECGI/IP related to neighbors Neighbors also update their own table with new eNB information Now like "UE based ANR", UE will detect unknown PCI and report it to the eNB
eNB doesn't request for ECGI and does not need support from MME eNB setups x2 interface with the help of mapping table created in second step above
From
All About Automatic Neighbor Relation (ANR)
Automatic Neighbor Relation (ANR): - Automatic Neighbor Relation (ANR) function is to relieve the operator from the burden of manually managing Neighbor Relations (NRs). - The ANR function resides in the eNB and manages the conceptual Neighbor Relation Table (NRT). - The Neighbor Detection Function finds new neighbors and adds them to the NRT. - ANR also contains the Neighbor Removal Function which removes outdated NRs. - An existing Neighbour Relation from a source cell to a target cell means that eNB controlling the source cell: Knows the ECGI/CGI and PCI of the target cell. Has an entry in the Neighbour Relation Table for the source cell identifying the target cell. Has the attributes in this Neighbour Relation Table entry defined, either by O&M or set to default values. - NRT contains : Target Cell Identifier (TCI) Identifies the target cell. For E-UTRAN, the TCI corresponds to the E-UTAN Cell Global Identifier (ECGI) and Physical Cell Identifier (PCI) of the target cell. Each NR has three attributes: NoRemove : eNB shall not remove the Neighbor cell Relation from the NRT. NoHO : Neighbor cell Relation shall not be used by the eNB for handover reasons. NoX2 attribute : Neighbor Relation shall not use an X2 interface in order to initiate procedures towards the eNB parenting the target cell.
NR Table
- Part of Self-Configuring Networks in SON. - Manual Addition of cells in the neighbor list is a tough task. It is becomes very difficult when there are exiting networks like 2G,3G. - For LTE there can be Intra LTE, Inter LTE and Inter RAT neighbors. - There can be: Intra-Freq Automatic Neighbor Relations. Inter-Freq/Inter System Automatic Neighbor Relations. Intra-Freq Automatic Neighbor Relations: - Intra-Frequency neighbors can be added automatically as part of Intra-Freq handover procedure. - As a process for Intra Frequency HO procedure the serving eNodeB instructs each UE to perform Intra-Frequency Measurement on neighboring cell by sending RRC Connection Reconfiguration message with measurement control information.
Automatic Neighbour Relation Function
- Consider as in the above picture, Serving Cell is CellA(UE is in RRC Connected State) and Cell B is the Target Cell: 1. The UE sends a measurement report regarding cell B. This report contains Cell B’s PCI, but not its ECGI. eNodeB checks if the reported PCI is already included in the Neighbor Database, then the HO proceeds in the normal way. If reported PCI is not included in the Neighbor Database then eNB proceeds to add the PCI to its NRT. 2. Once eNB receives a UE measurement report containing the PCI, the eNB instructs the UE with another RRC Connection Reconfiguration Message, using the newly discovered PCI as parameter. Instruct UE to read the ECGI, the TAC and all available PLMN ID(s) of the related neighbor cell.
To do so, the eNB may need to schedule appropriate idle periods to allow the UE to read the ECGI from the SIB1 of the detected neighbor cell. The UE reads the requested information from SIB1 on PDSCH. UE needs to read MIB on PBCH, then DCI with SI-RNTI on PDCCH to read SIB1 on PDSCH. 3. When the UE has found out the new cell’s ECGI, the UE reports the detected ECGI to the serving cell eNB. In addition the UE reports the tracking area code and all PLMN IDs that have been detected. If the detected cell is a CSG or hybrid cell, the UE also reports the CSG ID to the serving cell eNB.
4. The eNB decides to add this neighbor relation, and can use PCI and ECGI to: Look up a transport layer address to the new eNB. Update the Neighbor Relation List. If needed, setup a new X2 interface towards this eNB. Inter-Freq/System Automatic Neighbor Relations: - Can be done as a part of the normal inter-frequency and inter-system handover procedure. - Inter-frequency and Inter-RAT measurement requires compressed mode to be configured. - The eNodeB instructs UE to start inter-frequency and inter-RAT measurement using RRC Connection Reconfiguration message. - The UE searches for the neighbor cells, identifies and reports them to eNodeB. - The format of PCI depends upon the RAT of the Cell being measured.
Measurement Info Reported By UE
- Neighbor cell addition procedure:
Automatic Neighbour Relation Function in case of Inter Frequency/System Neighbor
- Once UE receives the Measurement Report for the Inter Frequency/RAT cell, eNodeB checks if the reported PCI is already included in the Neighbor Database, then the HO proceeds in the normal way. If reported PCI is not included in the Neighbor Database then eNB proceeds to add the PCI to its NRT
- eNodeB instruct UE using another RRC Connection Reconfiguration message to decode the Global Cell Identity (CGI) from the system information. - The UE may be required to report additional information depending on the system being measured. - The eNB updates its inter-RAT/inter-frequency Neighbour Relation Table. - In the inter-frequency case and if needed, the eNB can use the PCI and ECGI for a new X2 interface setup towards the new eNB.
From
LTE CARRIER AGGREGATION BANDWIDTH CLASSES
CARRIER AGGREGATION BANDWIDTH CLASS
AGGREGATED TRANSMISSION BW CONFIGURATION ATBC
NUMBER OF COMPONENT CARRIERS CC
A
1
B
≤100
C
100 - 200
2
≤100
2
From
https://www.electronics-notes.com/articles/connectivity/4g-lte-long-term-evolution/4g-lte-carrieraggregation-ca.php
View more...
Comments
