RAN
HSDPA Parameter Description
Issue
02
Date
2009-06-30
Copyright © Huawei Technologies Co., Ltd. 2009. All rights reserved. No part of this document may be reproduced or transmitted in any form or by any means without prior written consent of Huawei Technologies Co., Ltd.
Trademarks and Permissions and other Huawei trademarks are trademarks of Huawei Technologies Co., Ltd. All other trademarks and trade names mentioned in this document are the property of their respective holders.
Notice The purchased products, services and features are stipulated by the contract made between Huawei and the customer. All or part of the products, services and features described in this document may not be within the purchase scope or the usage scope. Unless otherwise specified in the contract, all statements, information, and recommendations in this document are provided “AS IS” without warranties, guarantees or representations of any kind, either express or implied. The information in this document is subject to change without notice. Every effort has been made in the preparation of this document to ensure accuracy of the contents, but all statements, information, and recommendations in this document do not constitute the warranty of any kind, express or implied.
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RAN HSDPA
Contents
Contents 1 Introduction to This Document...............................................................................................1-1 1.1 Scope.............................................................................................................................................................1-1 1.2 Intended Audience.........................................................................................................................................1-1 1.3 Change History..............................................................................................................................................1-1
2 Overview of HSDPA .................................................................................................................2-1 2.1 General Principles of HSDPA .......................................................................................................................2-1 2.2 HSDPA Channels ..........................................................................................................................................2-2 2.2.1 HS-DSCH and HS-PDSCH .................................................................................................................2-3 2.2.2 HS-SCCH.............................................................................................................................................2-3 2.2.3 HS-DPCCH..........................................................................................................................................2-3 2.2.4 DPCCH and DPCH/F-DPCH...............................................................................................................2-4 2.3 Impact of HSDPA on NEs .............................................................................................................................2-4 2.4 HSDPA Functions .........................................................................................................................................2-4 2.4.1 HSDPA Control Plane Functions .........................................................................................................2-4 2.4.2 HSDPA User Plane Functions ..............................................................................................................2-6
3 Control Plane ..............................................................................................................................3-1 3.1 Bearer Mapping.............................................................................................................................................3-1 3.2 Access Control ..............................................................................................................................................3-2 3.3 Mobility Management ...................................................................................................................................3-2 3.4 Channel Switching ........................................................................................................................................3-3 3.5 Load Control .................................................................................................................................................3-5 3.6 Power Resource Management .......................................................................................................................3-5 3.7 Code Resource Management.........................................................................................................................3-6 3.7.1 HS-SCCH Code Resource Management..............................................................................................3-6 3.7.2 HS-PDSCH Code Resource Management ...........................................................................................3-7 3.7.3 RNC-Controlled Static Code Allocation ..............................................................................................3-7 3.7.4 RNC-Controlled Dynamic Code Allocation ........................................................................................3-7 3.7.5 NodeB-Controlled Dynamic Code Allocation .....................................................................................3-9 3.7.6 Dynamic Code Tree Reshuffling........................................................................................................3-10
4 User Plane ....................................................................................................................................4-1 4.1 Flow Control and Congestion Control ..........................................................................................................4-1
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Contents
4.1.1 Flow Control ........................................................................................................................................4-2 4.1.2 Congestion Control ..............................................................................................................................4-3 4.2 RLC and MAC-d...........................................................................................................................................4-3 4.2.1 RLC......................................................................................................................................................4-3 4.2.2 MAC-d .................................................................................................................................................4-4 4.3 MAC-hs Scheduling......................................................................................................................................4-4 4.3.1 Determining the Candidate Set ............................................................................................................4-4 4.3.2 Calculating Priorities ...........................................................................................................................4-5 4.3.3 Comparison of Four Algorithms ..........................................................................................................4-8 4.4 HARQ ...........................................................................................................................................................4-9 4.4.1 HARQ Retransmission Principles........................................................................................................4-9 4.4.2 Soft Combining During HARQ .........................................................................................................4-10 4.4.3 Preamble and Postamble ....................................................................................................................4-10 4.5 TFRC Selection ...........................................................................................................................................4-11 4.5.1 Basic Procedure of TFRC Selection...................................................................................................4-11 4.5.2 Determining the TBSmax .....................................................................................................................4-11 4.5.3 Determining the TBSused, Modulation Scheme, Power, and Codes....................................................4-13 4.5.4 Determining the Number of MAC-d PDUs .......................................................................................4-14
5 QoS and Diff-Serv Management ............................................................................................5-1 5.1 QoS Management..........................................................................................................................................5-1 5.2 Diff-Serv Management..................................................................................................................................5-3 5.2.1 SPI Weight Description........................................................................................................................5-3 5.2.2 Differentiated Services Based on Service Types..................................................................................5-4 5.2.3 Differentiated Services Based on User Priorities .................................................................................5-4 5.3 QoS Parameter Mapping and Configuration .................................................................................................5-5
6 Parameters ...................................................................................................................................6-1 7 Counters .......................................................................................................................................7-1 8 Glossary .......................................................................................................................................8-1 9 Reference Documents ...............................................................................................................9-1
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1 Introduction to This Document
1
Introduction to This Document
1.1 Scope This document describes the HSDPA functional area. It provides an overview of the main functions and goes into details regarding HSDPA control and user plane functions.
1.2 Intended Audience It is assumed that users of this document are familiar with WCDMA basics and have a working knowledge of 3G telecommunication. This document is intended for: System operators who need a general understanding of HSDPA Personnel working on Huawei products or systems
1.3 Change History This section provides information on the changes in different document versions. There are two types of changes, which are defined as follows: Feature change: refers to the change in the HSDPA feature. Editorial change: refers to the change in wording or the addition of the information that was not described in the earlier version.
Document Issues The document issues are as follows: 02 (2009-06-30) 01 (2009-03-30) Draft (2009-03-10) Draft (2009-01-15)
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02 (2009-06-30) This is the document for the second commercial release of RAN11.0. Compared with 01 (2009-03-30) of RAN11.0, this issue incorporates the changes described in the following table. Change Type
Change Description
Parameter Change
Feature change
None.
None.
Editorial change
The description of MAC-hs Scheduling is optimized. For details, see section 4.3 MAC-hs Scheduling.
The deleted parameters are as follows: MaxDchVoipHarqRt MaxDchAmrHarqRt The added parameters are as follows: 8KRSCLMT 16KRSCLMT 32KRSCLMT 64KRSCLMT 128KRSCLMT 256KRSCLMT 384KRSCLMT
The description of QoS and Diff-Serv Management is optimized. For details, see section 5.2 Diff-Serv Management and 5.3 QoS Parameter Mapping and Configuration.
The added parameters are as follows: SingalUlMBR SingalDlMBR StreamUlMBR StreamDlMBR ConverUlMBR ConverDlMBR ARP1Priority ARP2Priority ARP3Priority ARP4Priority ARP5Priority ARP6Priority ARP7Priority ARP8Priority ARP9Priority ARP10Priority ARP11Priority ARP12Priority ARP13Priority
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Change Type
Change Description
Parameter Change ARP14Priority TrafficClass THP USERPRIORITY UlGBR DlGBR SPI FACTOR HappyBR THPClass
The structure of the document is adjusted.
None.
01 (2009-03-30) This is the document for the first commercial release of RAN11.0. Compared with draft (2009-03-10), this issue incorporates the following changes: Change Type
Change Description
Parameter Change
Feature change
None
None
Editorial change
The structure of the docuement is adjusted.
None
Draft (2009-03-10) This is the second draft of the document for RAN11.0. Compared with draft (2009-01-15), draft (2009-03-10) optimizes the description.
Draft (2009-01-15) This is the initial draft of the document for RAN11.0. Compared with issue 03 (2008-11-30) of RAN10.0, draft (2009-01-15) incorporates the following changes: Change Type
Change Description
Parameter Change
Feature change
The description of dynamic code tree reshuffling is added in section 3.7.6 "Dynamic Code Tree Reshuffling."
The added parameters are as follows:
The description of setting the maximum number of retransmissions
The added parameters are as follows:
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CodeAdjForHsdpaUserNumThd CodeAdjForHsdpaSwitch
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Change Type
Change Description
Parameter Change
on a service basis is added to section 4.3.1 "Determining the Candidate Set."
Editorial change
1-4
MaxDchVoipHarqRt MaxDchAmrHarqRt MaxNonConverHarqRt
The description of HBR-based resource allocation is added to section 4.3.2 "Calculating Priorities."
The added parameter is HappyBR.
The description of a new resource allocation method is added to section 4.5.3 "Determining the TBSused, Modulation Scheme, Power, and Codes."
The parameter RscAllocM is added with a new value PowerCode_Bal.
The description of HSDPA is rewritten for readability.
None
All the parameter names are replaced with the corresponding parameter IDs.
None
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2
Overview of HSDPA
2.1 General Principles of HSDPA To meet the rapidly growing demands for data services on the mobile network, 3GPP Release 5 introduced HSDPA in 2005. HSDPA improves the downlink capacity, increases the user data rate greatly, and reduces the transmission delay on the WCDMA network. The characteristics of HSDPA are as follows:
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Fast scheduling
Fast scheduling introduced into the NodeB determines the UEs for data transmission in each TTI (2 ms) and dynamically allocates resources to these UEs. It improves the usage of system resources and increases the system capacity. For details about how Huawei RAN implements fast scheduling, see section 4.3 "MAC-hs Scheduling."
Fast HARQ
Fast hybrid automatic repeat request (HARQ) is used to rapidly request the retransmission of erroneously received data. Specifically, when the UE detects an erroneous data transmission, it saves the received data and requests the NodeB to retransmit the original data at the physical layer. Before decoding, the UE performs soft combining of the saved data and the retransmitted data. The combining makes full use of the data transmitted each time and thus increases the decoding success rate. In addition, the retransmission delay at the physical layer is reduced greatly, compared with that at the RLC layer. For details about how Huawei RAN implements fast HARQ, see section 4.4 "HARQ."
Fast AMC
To compensate for channel variations, the DCH performs power control. To achieve this goal, HSDPA also performs fast adaptive modulation and coding (AMC), that is, adjusts the modulation scheme and coding rate in each TTI. AMC is based on the channel quality indicator (CQI) reported by the UE, and its purpose is to select an appropriate transmission rate so as to meet channel conditions. When the channel conditions are good, 16QAM can be used to provide higher transmission rates. When the channel conditions are poor, QPSK can be used to ensure the transmission quality. For details about how Huawei RAN implements fast AMC, see section 4.5 "TFRC Selection."
The MAC-hs, a new MAC sublayer, is introduced into the UE and NodeB to support HSDPA.
2.2 HSDPA Channels To support the HSDPA technologies, 3GPP defines one transport channel (HS-DSCH) and three physical channels (HS-PDSCH, HS-SCCH, and HS-DPCCH). Figure 2-1 shows the physical channels of HSDPA in the shaded area.
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Figure 2-1 Physical channels of HSDPA
2.2.1 HS-DSCH and HS-PDSCH HS-DSCH is a high speed downlink shared channel. Its TTI is fixed to 2 ms. It may be mapped onto one or more HS-PDSCHs. HS-PDSCH is a high speed physical downlink shared channel. Its spreading factor is fixed to 16. According to 3GPP TS 25.433, a maximum of 15 HS-PDSCHs can be used for transmission at the same time. The number of HS-PDSCHs per cell is configurable. Generally, the NodeB can use the HS-PDSCH codes only allocated by the RNC. The NodeB-controlled dynamic code allocation, however, allows the NodeB to temporarily allocate idle codes to the HS-PDSCH. "Dynamic Code Allocation Based on NodeB" is an optional feature. The use of 2 ms TTI reduces the round trip time (RTT) on the Uu interface and, together with AMC, improves the tracking of channel variations. In addition, the use of 2 ms TTI enables fast scheduling and resource allocation and thus improves the usage of transmission resources. In each TTI, HSDPA assigns the HS-PDSCHs onto which the HS-DSCH maps. More HS-PDSCHs can provide higher transmission rates. Unlike the DCH, the HS-DSCH cannot support soft handover. The reason is that this type of handover requires different cells to use the same radio resource for sending the same data to the UE, but the scheduling function can be performed only within the cell.
2.2.2 HS-SCCH HS-SCCH is a high speed shared control channel. It carries the control information related to the HS-DSCH. The control information includes the UE identity, HARQ-related information, and information about transport format and resource combination (TFRC). For each transmission of the HS-DSCH, one HS-SCCH is required to carry the related control information. One cell can be configured with a maximum of four HS-SCCHs. The number of HS-SCCHs determines the maximum number of UEs that can be scheduled simultaneously in each TTI.
2.2.3 HS-DPCCH HS-DPCCH is a high speed dedicated physical control channel. In the uplink, each HSDPA UE must be configured with an HS-DPCCH. This channel is mainly used by the UE to report Issue 02 (2009-06-30)
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the CQI and whether a transport block is correctly received. The information about the transport block is used for fast retransmission at the physical layer. The CQI is used for AMC and scheduling to allocate Uu resources.
2.2.4 DPCCH and DPCH/F-DPCH DPCCH is a dedicated physical control channel in the uplink. DPCH is a dedicated physical channel in the downlink. F-DPCH is a fractional dedicated physical channel in the downlink. The HSDPA UE must be configured with dedicated physical control channels in both the uplink and the downlink. The uplink DPCCH is used for providing reference information about the transmit power of HSDPA channels. In addition, it is used for closed-loop power control by working with the DPCH or F-DPCH. In SRB over HSDPA mode, the downlink channel can be established on the F-DPCH without the dedicated assisted DPCH. In this case, a maximum of 10 UEs use an SF256 to transmit the TPC, thus saving a large amount of downlink codes.
2.3 Impact of HSDPA on NEs HSDPA has an impact on the RNC, NodeB, and UE. On the control plane of the network side, the RNC processes the signaling about HSDPA cell configuration, HS-DSCH related channel configuration, and mobility management. On the user plane of the network side, the RLC layer and MAC-d of the RNC are unchanged. At the NodeB, the MAC-hs is added to implement HSDPA scheduling, Uu resource allocation, AMC, and Iub flow control. The MAC-hs implements these management functions in a short time. Thus, it reduces both unnecessary delays and processing complexity caused by Iub message exchange. On the UE side, the MAC-hs is added between the MAC-d and the physical layer for data reception. To support HSDPA, 3GPP defines 12 UE categories. These UEs support different peak rates at the physical layer, ranging from 912 kbit/s to 14 Mbit/s. The UE of category 10 supports the highest rate. The UE of category 11 or 12 supports only the QPSK mode. For details, see 3GPP TS 25.306. Huawei RAN supports all the UE categories.
2.4 HSDPA Functions HSDPA functions are implemented on the HSDPA control plane and user plane.
2.4.1 HSDPA Control Plane Functions The control plane is responsible for setting up and maintaining HS-DSCH connections and managing cell resources. Figure 2-2 shows the HSDPA control plane functions based on the service connection setup and maintenance procedure.
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Figure 2-2 HSDPA control plane functions
The HSDPA control plane functions are described as follows: Bearer mapping The bearer mapping is used by the network side to configure the RAB during the setup of a service connection in the cell. The network side then configures bearer channels for the UE based on the requested service type, service rate, UE capability, and cell capability. For details, see section 3.1 "Bearer Mapping." Access control Access control, a sub-function of load control, checks whether the current resources of the cell are sufficient for the service connection setup. If the resources are insufficient, intelligent access control is triggered. If the resources are sufficient, the service connection can be set up. For details, see section 3.2 "Access Control." Mobility management For the established HS-DSCH connection, mobility management decides whether to switch it to another cell for providing better services, based on the channel quality of the UE. For details, see section 3.3 "Mobility Management." Channel switching Channel switching is responsible for switching the transport channel among the HS-DSCH, DCH, and FACH based on the requirements of mobility management or load control. For details, see section 3.4 "Channel Switching." Load control When the cell load increases, the load control function adjusts the resources configured for the established radio connections to avoid cell overload. For details, see section 3.5 "Load Control." Resource management
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Resource management coordinates the power resource between the HS-DSCH and the DCH and the code resource between the HS-SCCH and the HS-PDSCH. The downlink power and codes are the bottleneck resources of the cell. Resource management can increase the HSDPA capacity. Power resource management reserves power for channels of different types and allocates power for them. For details, see section 3.6 "Power Resource Management." Code resource management allocates and reserves code resources for channels of different types. In addition, it collects and reshuffles idle code resources. For details, see section 3.7 "Code Resource Management."
2.4.2 HSDPA User Plane Functions After the service is set up, the user plane is responsible for implementing data transmission. Figure 2-3 shows the HSDPA user plane functions based on the data processing procedure. Figure 2-3 HSDPA user plane functions
The service data carried on the HS-DSCH is passed to the RLC layer and MAC-d of the RNC for processing and encapsulation. Then, the MAC-d PDU is formed and passed through the Iub/Iur interface to the NodeB/RNC. To avoid congestion, the flow control and congestion control functions control the traffic on the Iub/Iur interface through the HS-DSCH frame protocol (3GPP TS 25.435). After the MAC-d PDU is received by the NodeB, it is passed through the MAC-hs to the physical layer and then sent out through the Uu interface. The MAC-hs provides MAC-hs scheduling, TFRC selection, and HARQ. MAC-hs scheduling determines the HSDPA users in the cell for data transmission. TFRC selection determines the transmission rates and Uu resources to be allocated to the HSDPA UEs. HARQ is used to implement the hybrid automatic repeat request function.
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Control Plane
This chapter consists of the following sections: Bearer Mapping Access Control Mobility Management Channel Switching Load Control Power Resource Management Code Resource Management
3.1 Bearer Mapping The HS-DSCH can carry services of multiple types and service combinations, as listed in Table 3-1. Table 3-1 Bearer mapping CN Domain
Service Type
Can Be Carried on HS-DSCH?
Optional Feature?
-
Signaling (SRB)
Yes
Yes Feature name: SRB over HSDPA
CS
Voice
Yes
Yes Feature name: CS Voice over HSPA/HSPA+
PS
Videophone
No
No
Streaming
No
No
Conversational
Yes
Yes Feature name: VoIP over HSPA/HSPA+
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CN Domain
Service Type
Can Be Carried on HS-DSCH?
Optional Feature?
Streaming
Yes
Yes Feature name: Streaming Traffic Class on HSDPA
Interactive
Yes
No
Background
Yes
No
IMS signaling
Yes
Yes Feature name: IMS Signaling over HSPA
MBMS PTP
Yes
Yes Feature name: MBMS P2P over HSDPA
During the service setup, the RNC selects appropriate channels based on the UE capability, cell capability, and service parameters to optimize the use of cell resources and ensure the QoS. Huawei RAN supports the setting of the types of RABs carried on the HS-DSCH according to service requirements. For details, see the Radio Bearers Parameter Description. Huawei supports bearer management of HSDPA over Iur. "HSDPA over Iur" is an optional feature.
3.2 Access Control Access control determines whether an HS-DSCH connection can be set up under the precondition that the QoS is ensured. The determination is based on the status of cell resources and the situation of Iub/Iur congestion. When the resources are insufficient, the HS-DSCH is switched to the DCH and only the DCH connection is set up. When the resources are sufficient, the DCH is switched to the HS-DSCH. The implementation of this function requires the support of channel switching. Access control allows the HSDPA UE to access an inter-frequency neighboring cell that has the same coverage area as the source cell. The purpose is to achieve load balance between the cells and improve HSDPA user experience. This is HSDPA directed retry decision (DRD), an optional feature. For details, see the Load Control Parameter Description.
3.3 Mobility Management The DCH supports soft handover, and therefore downlink data can be concurrently sent out from all the cells in the active set in DCH transmission. In comparison, the HS-DSCH does not support soft handover, and therefore downlink data can be sent out only from the HS-DSCH serving cell and inter-cell handover has to be performed through the change of the serving cell. Thus, HSDPA mobility management focuses on the change of the HS-DSCH serving cell.
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For the UE with the HS-DSCH service, the best cell in the active set acts as the HS-DSCH serving cell. When the best cell changes, the UE disconnects the HS-DSCH from the source cell and attempts to set up a new HS-DSCH connection with the new best cell. For details, see the Handover Parameter Description. By changing the HS-DSCH switching threshold, you can modify the conditions for triggering the change of the best cell. Lowering this threshold can increase both the handover frequency and the sensitivity of HS-DSCH switching to signal variations in the serving cell. Raising this threshold can reduce the handover frequency but may increase the probability of the HS-DSCH service being discontinuous or even dropping on the cell edge. For the HS-DSCH service, Huawei supports inter-cell intra-frequency handover, inter-cell inter-frequency handover, and inter-RAT handover. Mobility management may trigger the switching from the HS-DSCH to the DCH. If the UE with the HS-DSCH service cannot set up the HS-DSCH connection with the target cell, the channel switching function, together with mobility management, switches the HS-DSCH to the DCH. When the HS-DSCH connection is available, the channel switching function switches the DCH back to the HS-DSCH. When the HSDPA user returns from the DCH cell to the HSDPA cell, the DCH is set up to ensure successful handover. A certain period later after the handover, the channel switching function switches the DCH to the HS-DSCH. For details, see section 3.4 "Channel Switching." "HSDPA over Iur" is an optional feature.
3.4 Channel Switching After the HS-DSCH is introduced, the UE can stay in a new state, CELL_DCH (with HS-DSCH). Thus, there are additional transitions between CELL_DCH (with HS-DSCH) and CELL_FACH and transitions between CELL_DCH (with HS-DSCH) and CELL_DCH even when both the cell and the UE support the HS-DSCH, as shown in Figure 3-1. Figure 3-1 UE state transition
Table 3-2 lists new state transition and new channel switching. Table 3-2 New state transition and new channel switching New State Transition
New Channel Switching
CELL_DCH (with HS-DSCH) CELL_FACH
HS-DSCH FACH
CELL_DCH (with HS-DSCH) CELL_DCH
HS-DSCH DCH
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Here, the switching between HS-DSCH and FACH can be triggered by traffic volume, which is similar to the switching between DCH and FACH. When the cell load is too high, load control may also trigger the switching from the HS-DSCH to the FACH to relieve congestion. For details, see the Load Control Parameter Description. When the cell load becomes low, channel switching aids load control in attempting to switch the transport channel back to the HS-DSCH. For details, see the Rate Control Parameter Description. As the HS-DSCH is introduced later, it is inevitable that some cells support the HS-DSCH but others do not. This is also the case with UEs. When a service is set up, the channel switching function selects an appropriate bearer channel based on the cell capability and UE capability to ensure the QoS while efficiently using the cell resources. When the user is moving, the channel switching function adjusts the channel type based on the UE capability to ensure service continuity while improving user experience. Figure 3-2 Relations between channel switching and other functions
Triggers for switching from the HS-DSCH to the DCH are as follows: The HS-DSCH is selected during the service setup but neither the resources of the serving cell nor the resources of the inter-frequency same-coverage neighboring cell are sufficient. In such a case, the HS-DSCH is switched to the DCH. The HS-DSCH serving cell changes. The UE attempts to set up a new HS-DSCH connection with the new best cell. In such a case, the possible scenarios are as follows: −
If the new best cell does not support the HS-DSCH, the UE cannot set up the HS-DSCH connection. In this case, the HS-DSCH is switched to the DCH.
−
If the new best cell supports the HS-DSCH but a new HS-DSCH connection cannot be set up because the resources are insufficient, the DCH connection is set up and the HS-DSCH is switched to this DCH.
The user moves from a cell supporting the DCH but not supporting the HS-DSCH to a cell supporting the HS-DSCH. In this case, the DCH connection is also set up because the DCH supports soft handover, which can increase the inter-cell handover success rate. In one of the cases described previously, the DCH connection is set up in a cell supporting the HS-DSCH or in an inter-frequency same-coverage neighboring cell supporting the HS-DSCH. Then, the DCH is switched to the HS-DSCH by either of the following mechanisms:
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Channel switching based on timer After the DCH connection is set up, this mechanism periodically attempts to switch the DCH to the HS-DSCH. Channel switching based on traffic volume When the traffic volume of the UE increases and the RNC receives an event 4A report, this mechanism attempts to switch the DCH to the HS-DSCH. For details on the event 4A report, see the Rate Control Parameter Description.
3.5 Load Control When the cell is congested, load control selects some users (including HSDPA users) for congestion relief. The selection is based on the integrated priority, which considers the allocation retention priority (ARP), traffic class (TC), traffic handling priority (THP), and bearer type. When the cell load is high, the basic congestion control selects some HSDPA users for handover to an inter-frequency same-coverage neighboring cell or an inter-RAT neighboring cell with lower load. When the cell load is too high, the overload congestion control selects some HSDPA BE services for the switching to a common channel or releases some HSDPA services. For details, see the Load Control Parameter Description.
3.6 Power Resource Management Power resource management determines the transmit power of the HS-PDSCH, HS-SCCH, and HS-DPCCH. Generally, an HSDPA cell has the same coverage as the corresponding R99 cell. To improve the resource usage in this case, the downlink power resources of HSDPA can be dynamically allocated as follows: 1.
The downlink power resources are first reserved for common physical channels and allocated to the DPCH. The remaining power resources are available for HSPA, including HSUPA and HSDPA.
2.
The HSPA power resources are first allocated to the HSUPA downlink control channels, including the E-AGCH, E-RGCH, and E-HICH. The remaining power resources are available for HSDPA.
3.
The HSDPA power resources are first allocated to the downlink control channel HS-SCCH. For details, see the Power Control Parameter Description. The remaining power resources are allocated to the traffic channel HS-PDSCH. For details on power resource allocation, see section 4.5 "TFRC Selection."
Figure 3-3 shows the dynamic HSDPA power resource allocation.
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Figure 3-3 Dynamic HSDPA power resource allocation
Every TTI, the NodeB detects the power usage of R99 channels to determine the power available for HSPA. To reserve the power for R99 power control itself, the power margin PwrMgn needs to be set on the NodeB side. In addition, the power allocated to HSPA must not exceed the maximum permissible power HspaPower, which can be set on the RNC side. For details on uplink HS-DPCCH power control, see the Power Control Parameter Description. "HSDPA over Iur" is an optional feature.
3.7 Code Resource Management Code resource management allocates code resources to the HS-SCCH and HS-PDSCH. The NodeB supports HS-DSCH transmissions to multiple users in parallel in a TTI. If more than one HS-PDSCH code can be allocated by the NodeB, then code multiplexing can be used to allocate the codes to multiple users so as to improve resource usage and system throughput. "Time and HS-PDSCH Code Multiplexing" is an optional feature.
3.7.1 HS-SCCH Code Resource Management Each HS-SCCH uses an SF128 code. The number of HS-SCCHs determines the maximum number of HSDPA users that can be scheduled simultaneously in a TTI. Generally, the number of HS-SCCHs depends on the traffic characteristics of the cell. The default number is 4, which is specified by the parameter HsScchCodeNum on the RNC side. If the default setting is used, the HS-PDSCH can use only 14 SF16 codes. To enable the HS-PDSCH to use 15 SF16 codes, you are advised to configure 2 HS-SCCHs.
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3.7.2 HS-PDSCH Code Resource Management The DPCH and the HS-PDSCH coexist in a cell. Therefore, sharing the cell code resources between them to improve the resource usage is of critical importance in HSDPA code resource management. Huawei supports both RNC-level and NodeB-level code resource management. RNC-controlled static or dynamic code allocation is enabled through the parameter AllocCodeMode. NodeB-controlled dynamic code allocation is enabled through the parameter DynCodeSw. For details, see the following sections. The dynamic code allocation controlled by the NodeB is more flexible than that controlled by the RNC. It shortens the response time and saves the Iub signaling used for code reallocation. Huawei recommends the following code allocation modes, where the first mode is preferred: Configure the RNC to use static code allocation and the NodeB to use dynamic code allocation. If the NodeB does not support dynamic code allocation, configure the RNC to use dynamic code allocation. If not all the NodeBs controlled by an RNC support dynamic code allocation, the RNC-controlled dynamic code allocation is recommended. In this case, the NodeB-controlled dynamic code allocation can also be enabled for those supporting NodeBs.
3.7.3 RNC-Controlled Static Code Allocation If the RNC-controlled static code allocation is used, the number of reserved HS-PDSCH codes is specified by the parameter HsPdschCodeNum on the RNC side. Based on the number, the RNC reserves codes for the HS-PDSCH. The DPCH, HS-SCCH, and common channels use the other codes. The parameter HsPdschCodeNum can be set on the basis of the traffic characteristics of the cell. If there are more HSDPA users and the traffic is high, the parameter value can be increased. If there are more DCH users and the HSDPA traffic is low, the parameter value can be decreased. A maximum of 15 codes can be allocated to the HS-PDSCH. Figure 3-4 shows the RNC-controlled static code allocation. Figure 3-4 RNC-controlled static code allocation
3.7.4 RNC-Controlled Dynamic Code Allocation If the RNC-controlled dynamic code allocation is used, the minimum number of available HS-PDSCH codes is specified by the parameter HsPdschMinCodeNum on the RNC side. The purpose of this setting is to prevent too many DCH users from being admitted and to ensure the basic data transmission of the HS-PDSCH. In addition, the maximum number of available HS-PDSCH codes is specified by the parameter HsPdschMaxCodeNum. The
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purpose of this setting is to prevent too many codes from being allocated for the HS-PDSCH and to prevent DCH users from preempting codes during admission. The number of codes that can be shared between HS-PDSCH and DPCH is equal to the value of HsPdschMaxCodeNum minus the value of HsPdschMinCodeNum, as shown in Figure 3-5. When a code that can be shared is idle, it can be allocated to the HS-PDSCH if the idle code is adjacent to the allocated HS-PDSCH codes. Figure 3-5 RNC-controlled dynamic code allocation
Adding an HS-PDSCH Code Figure 3-6 shows how to add an HS-PDSCH code. The solid dots represent the allocated codes, and the circles represent the idle codes. Figure 3-6 Adding an HS-PDSCH code
After a DCH RL is released or reconfigured (for example, because the spreading factor becomes larger), the RNC adds an HS-PDSCH code if the following conditions are met: The code adjacent to the allocated HS-PDSCH codes is idle. After the code is added, the minimum spreading factor of the remaining codes is smaller than or equal to the value of CellLdrSfResThd. The parameter CellLdrSfResThd set on the RNC side is used to reserve codes for new users, to avoid congestion due to code insufficiency, and to avoid unnecessary reshuffling of the code tree.
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Releasing an HS-PDSCH Codes Figure 3-7 shows how to release an HS-PDSCH code. The solid dots represent the allocated codes, and the circles represent the idle codes. Figure 3-7 Releasing an HS-PDSCH code
If idle DPCH codes are insufficient when a DCH RL is set up, added, or reconfigured (for example, because the spreading factor becomes smaller), the RNC preempts HS-PDSCH codes in the shared codes for the DPCH. In addition, if the minimum spreading factor of idle DPCH codes is greater than the value of CellLdrSfResThd, the RNC can also reallocate some HS-PDSCH codes to the DPCH. The reallocated code number must be the smallest one of the available shared codes.
3.7.5 NodeB-Controlled Dynamic Code Allocation Generally, the NodeB can use the HS-PDSCH codes only allocated by the RNC. The NodeB-controlled dynamic code allocation, however, allows the NodeB to temporarily allocate idle codes to the HS-PDSCH. Figure 3-8 NodeB-controlled dynamic code allocation
Every TTI, the NodeB detects the SF16 codes that are not allocated to the HS-PDSCH. If such an SF16 code or any of its subcodes is allocated by the RNC to the DCH or a common channel, this SF16 code is regarded as occupied. Otherwise, it is regarded as unoccupied. Therefore, the available HS-PDSCH codes include the codes reserved by the RNC and the idle codes adjacent to the allocated HS-PDSCH codes. Every time the RNC allocates or release HS-PDSCH codes, it notifies the NodeB through Iub signaling and the NodeB performs the corresponding processes. For example, the RNC reserves the SF16 codes numbered 11 to 15 for the HS-PDSCH and those numbered 0 to 5 for the DCH and common channels in a TTI. Thus, the HS-PDSCH can use the codes numbered 6 to 15 in this TTI.
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If the setup of an RL requires a DPCH code that is already allocated by the NodeB to the HS-PDSCH, the NodeB releases this code and sends an NBAP message to the RNC, indicating that the RL is set up successfully. Then, the DCH uses this code. After the DCH releases it, the HS-PDSCH can use this code again. "Dynamic Code Allocation Based on NodeB" is an optional feature.
3.7.6 Dynamic Code Tree Reshuffling Regardless of whether dynamic code allocation is controlled by the RNC or the NodeB, the number of continuous codes available for the HS-PDSCH shall be maximized. The dynamic code tree reshuffling function can achieve this goal by reallocating DPCH codes. When the minimum spreading factor of the remaining idle codes in a cell is greater than the value of CellLdrSfResThd, the RNC reshuffles the codes used by the DPCH to provide more continuous SF16 codes for HSDPA. This function can be enabled or disabled by the parameter CodeAdjForHsdpaSwitch on the RNC side. In addition, the threshold number of users that can be reshuffled needs to be specified by the parameter CodeAdjForHsdpaUserNumThd. If the number of users on a subtree is smaller than or equal to this parameter value, this subtree can be reshuffled. Otherwise, it cannot be reshuffled. This parameter limits the number of users that can be reshuffled each time, to prevent too many users from being reshuffled in a short time and thus to avoid affecting user experience. Figure 3-9 Dynamic code tree reshuffling
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User Plane
This chapter consists of the following sections: Flow Control and Congestion Control RLC and MAC-d MAC-hs Scheduling HARQ TFRC Selection
4.1 Flow Control and Congestion Control HSDPA flow control and congestion control are used to control the HSDPA data flow on the Iub and Iur interfaces. HSDPA data packets are sent through the Iub interface to the NodeB and then through the Uu interface to the UE. Thus, congestion may occur on the Uu, Iub, or Iur interface. Flow control is used to relieve Uu congestion, and congestion control is used to relieve Iub/Iur congestion. The two types of control are implemented by the NodeB. HSDPA flow control and congestion control are part of the HSDPA Iub frame protocol (3GPP TS 25.435). They are implemented for each MAC-hs queue through the Capacity Request message sent by the RNC and the Capacity Allocation message sent by the NodeB. Figure 4-1 shows the basic principles of flow control and congestion control.
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Figure 4-1 Basic principles of Iub flow control and congestion control
4.1.1 Flow Control For each MAC-hs queue, flow control calculates the pre-allocated Iub bandwidth based on the Uu transmission rate and the amount of data buffered in the NodeB. The Uu transmission rate of the MAC-hs queue is determined by the scheduling algorithm. For each MAC-hs queue, if the Iub transmission rate is higher than the Uu transmission rate, the data packets are buffered. Too much data buffered in the NodeB leads to transmission delay and even packet loss. Therefore, each MAC-hs queue should not have too much data buffered in the NodeB. On the other hand, it should keep a certain amount of data to avoid wasting the Uu resources due to no data to transmit. The flow control procedure is as follows: 1.
The NodeB measures the buffered data amount of each MAC-hs queue and the average Uu transmission rate.
2.
The NodeB estimates the buffering time based on the measurements.
3.
The NodeB adjusts the Iub bandwidth pre-allocated to the MAC-hs queue.
The pre-allocated Iub bandwidth is adjusted as follows: If the buffering time is too short, you can infer that the RNC slows down the data transmission, that is, the Iub transmission rate is lower than the Uu transmission rate. In such a case, the pre-allocated Iub bandwidth is adjusted to a value greater than the average Uu transmission rate. If the buffering time is appropriate, the pre-allocated Iub bandwidth is adjusted to the average Uu transmission rate. If the buffering time is too long, the pre-allocated Iub bandwidth is adjusted to a value smaller than the average Uu transmission rate.
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4.1.2 Congestion Control The Iub bandwidth may be lower than the Uu bandwidth. If the RNC uses the Iub bandwidth pre-allocated to each MAC-hs queue, the Iub bandwidth for HSDPA is insufficient. This may lead to congestion and even packet loss. The amount of data to be transmitted is sent by the RNC to each MAC-hs queue through the Capacity Request message. Based on this amount and the total Iub bandwidth available for HSDPA, the congestion control function adjusts the bandwidth pre-allocated to each MAC-hs queue. Thus, congestion control ensures that the total bandwidth actually allocated to all the MAC-hs queues is not higher than the total available Iub bandwidth. The total Iub bandwidth available for HSDPA depends on the variations in HSDPA packet delay and the situation of packet loss. HSDPA shares the bandwidth with the DCH and control signaling, and the DCH and control signaling has higher priorities than HSDPA. Thus, when the HSDPA packet delay or packet loss increases, you can infer that the number of DCHs or the amount of control signaling increases. In such a case, the bandwidth available for HSDPA decreases and the bandwidth actually allocated for HSDPA decreases. For details on congestion control, see the Transmission Resource Management Parameter Description. For the Iur interface, flow control and congestion control are also applied. The control principles and processing procedures are the same as those for the Iub interface.
4.2 RLC and MAC-d 4.2.1 RLC One of the main purposes of HSDPA is to reduce latency by handling retransmissions at NodeB level. Retransmissions, however, may still be triggered at the RLC layer of the RNC under the following circumstances: The NodeB misinterprets an NACK sent by the UE. The number of HARQ retransmissions exceeds the maximum permissible number. The data buffered in the NodeB is lost when the HS-DSCH serving cell changes. Therefore, HARQ retransmission cannot totally replace RLC retransmission, which is described in 3GPP TS 25.322. For services with high requirements for data transmission reliability, Huawei recommends that the RLC acknowledged mode (AM) also be used to ensure correct transmission on the Uu interface even when the services such as the BE service are carried on HSDPA channels. Before the introduction of HSDPA, the size of an RLC PDU is usually 336 bits, where 320 bits are for the payload and 16 bits for the RLC header. Without additional overhead, the MAC PDU is of the same size as the RLC PDU. According to the 3GPP specifications, a maximum of 2,047 RLC PDUs can be transmitted within an RLC window, and the RTT at the RLC layer is about 100 ms (50 TTIs). In this condition, the maximum peak rate can only be 336 bits x (2047/50)/2 ms = 6.88 Mbit/s. To reach higher rates, an RLC PDU of 656 bits is introduced, where 640 bits are for the payload and 16 bits for the RLC header. The RLC PDU size can be set for each typical service. For high-speed services, the size is set to 656 bits by default.
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4.2.2 MAC-d The MAC-d functionality is unchanged after the introduction of HSDPA. The HS-DSCH bearers are mapped onto MAC-d flows on the Iub/Iur interface. Each MAC-d flow has its own priority queue. The theoretical peak rate of HSDPA on the Uu interface is 14.4 Mbit/s. It is calculated on the assumption that the chip rate of WCDMA is 3.84 Mcps, the spreading factor for HSDPA is SF16, the maximum number of available codes is 15, and the gain of 16QAM is 4. Thus, the rate is 3.84 Mcps/16 x 15 x 4 = 14.4 Mbit/s. Limited by many factors, the theoretical peak rate of 14.4 Mbit/s is unreachable in actual situations. The UE capability is one factor. For example, 3GPP specifies that the UE of category 10 can use a maximum of 15 codes and receive a transport block with a maximum of 27,952 bits. For details, see 3GPP TS 25.306. Thus, the theoretical peak rate is 27952 bits/2 ms = 13.976 Mbit/s. In addition, the RLC PDU size is fixed to 656 bits, and a transport block of 27,952 bits can contain a maximum of 42 PDUs. Thus, the maximum RLC payload rate is (656 bits – 16 bits) x 42/2 ms = 13.44 Mbit/s. In practice, the radio channel quality, retransmission probability, and available power also need to be considered. Therefore, the UE of category 10 cannot reach 13.44 Mbit/s at the RLC layer in most tests.
4.3 MAC-hs Scheduling With the limited Uu resources for HSDPA in a cell, the user expects to maximize the service rate while the telecom operator expects to maximize the system capacity. MAC-hs scheduling is used to coordinate the Uu resources, user experience, and system capacity. It is implemented at the NodeB MAC-hs. The scheduling algorithm consists of two steps. At first, the algorithm determines which initial transmission queues or retransmission processes can be put into the candidate set for scheduling. Then, the algorithm calculates their priorities based on factors such as the CQI, user fairness, and differentiated services. If the algorithm is weighted more towards the channel quality of the UE, the HSDPA cell can have a higher capacity but user fairness and differentiated services may be affected. If the algorithm is weighted more towards user fairness and differentiated services, the system capacity may be affected. Huawei provides four scheduling algorithms: maximum C/I (MAXCI), round-robin (RR), proportional fair (PF), and Enhanced Proportional Fair (EPF). The EPF algorithm is optional.
4.3.1 Determining the Candidate Set The candidate for scheduling contains new data packets (hereinafter referred to as initial transmission queues) or data packets to be retransmitted (hereinafter referred to as retransmission processes), with the following exceptions: If the UE starts the compressed mode, its data cannot be put into the candidate set during the GAP. If the UE category requires the UE to wait for several TTIs before it can be scheduled again, its data cannot be put into the candidate set in this period. The UE of category 1 or 2 needs to wait for 3 TTIs, and the UE of category 3, 4, and 11 must wait for 2 TTIs.
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If the number of retransmissions of a data packet reaches or exceeds the maximum number, the data of this UE cannot be put into the candidate set. The data should be discarded. Huawei supports that the maximum number of retransmissions is set on a service basis: −
MaxNonConverHarqRt: the maximum number of non-conversational service retransmissions in the CELL_DCH state
Other user data can be put into the candidate set.
4.3.2 Calculating Priorities Four algorithms are available for calculating the priorities of data packets in the candidate set. The scheduling policies vary according to the algorithms for calculating the priorities of data packets. The algorithm to be used is specified by the parameter SM on the NodeB LMT.
MAXCI Algorithm The retransmission processes unconditionally have higher priorities than the initial transmission queues. The retransmission processes are sorted in first-in first-out (FIFO) mode. The initial transmission queues are sorted in the CQI order. A higher CQI means a higher data priority. The MAXCI algorithm aims to maximize the system capacity but cannot ensure user fairness and differentiated services.
RR Algorithm The retransmission processes unconditionally have higher priorities than the initial transmission queues. The retransmission processes are sorted in FIFO mode. The initial transmission queues are sorted in the order of the waiting time in the MAC-hs queue. A longer waiting time means a higher data priority. The RR algorithm aims to ensure user fairness but cannot provide differentiated services. Not considering the CQI reported by the UE leads to lower system capacity.
PF Algorithm The retransmission processes unconditionally have higher priorities than the initial transmission queues. The retransmission processes are sorted in FIFO mode. The initial transmission queues are sorted in the order of R/r. Here, R represents the throughput corresponding to the CQI reported by the UE, and r represents the throughput achieved by the UE. A greater R/r value means a higher data priority. The PF algorithm aims to make a tradeoff between system capacity and user fairness. It provides the user with an average throughput that is proportional to the actual channel quality. The system capacity provided by PF is between the system capacity provided by RR and that provided by MAXCI.
EPF Algorithm The EPF algorithm can meet the requirements of telecom operators related to user fairness and differentiated services and also provide a high system capacity.
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Firstly, priorities are determined on the basis of service types. The EPF algorithm distinguishes between delay-sensitive data and throughput-sensitive data based on the QoS requirements. The amount of delay-sensitive data is generally small. The transmission delay of delay-sensitive data should be as short as possible. When the transmission delay reaches a specified threshold, data packets are discarded. The delay-sensitive data includes the following data: SRB signaling VoIP and AMR service data whose waiting time approaches the value of the discard timer The amount of a throughput-sensitive data is generally small. A higher transmission rate brings greater user satisfaction. The throughput-sensitive data includes the following data: BE service data Streaming service data IMS data VoIP and AMR service data whose waiting time is far from the value of the discard timer The EPF algorithm meets the basic QoS requirements of users. For delay-sensitive data, the transmission delay must not exceed the maximum permissible delay. For throughput-sensitive data, the transmission rate must not be lower than the GBR. Users require higher QoS for delay-sensitive data. Therefore, the delay-sensitive data has a higher priority than the throughput-sensitive data. Secondly, for delay-sensitive data or throughput-sensitive data, the EPF algorithm distinguishes between retransmission processes and initial transmission queues. The retransmission processes unconditionally have higher priorities than the initial transmission queues. Thirdly, the priorities of the initial transmission queues are calculated for delay-sensitive data or throughput-sensitive data. The following factors are considered: the waiting time, CQI reported by the UE, throughput achieved by the UE, guaranteed bit rate (GBR), scheduling priority indicator (SPI) weight, happy bit rate (HBR), and power consumed in the queue for a certain period. The impacts of these factors on the priority calculation are as follows: For the delay-sensitive data, a longer waiting time means a higher data priority. For the throughput-sensitive data, a greater R/r value means a higher data priority. Here, R represents the throughput corresponding to the CQI reported by the UE, and r represents the throughput achieved by the UE. The UEs with the rates lower than the GBR have higher priorities than those with the rates already reaching the GBR. A higher SPI weight means a higher data priority. A larger difference between the actual rate and the HBR means a higher data priority. When the resource limitation switch (RscLmSw) is on, the algorithm allocates the lowest priority to a queue whose power consumption exceeds the threshold. RscLmSw is used to prevent the users in areas with poor coverage from consuming too many cell resources so that there is no decrease in system capacity. The ratio of the maximum available power of a queue to the total power of the cell depends on the GBR, as listed in Table 4-1. By calculating the priority of each queue, the scheduling algorithm achieves the following:
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When the system resources are sufficient to meet the basic QoS requirements of all users, the transmission delay of delay-sensitive data is within the permissible range and the transmission rate of throughput-sensitive data is not lower than the GBR. High-priority users can obtain more resources for higher QoS. When the system resources are insufficient to meet the basic QoS requirements of all users, delay-sensitive data has higher priorities than throughput-sensitive data. High-priority users can obtain more resources to ensure the basic QoS. Fourthly, special processing is performed. Differentiated services based on SPI weights are provided. Different services have different service types, and different users have different user priorities. Therefore, the scheduling function needs to consider these two factors to provide differentiated services. SPI is a parameter specified on the basis of service types and users priorities. The parameter SPIweight can be specified according to the SPI to provide differentiated services. This parameter is specified on the RNC, and its value ranges from 0% to 100%. The SPI weight affects the calculation of queue priorities. It is used to quantify the differentiated services. If all the rates of throughput-sensitive services with different SPI weights exceed or none of the rates exceeds their GBRs, the proportion of SPI weights determines the proportion of rates among users. For example, for three throughput-sensitive service users with the same channel quality, if their GBRs are not configured and the proportion of SPI weights is 100:50:30, the proportion of actual rates is close to 100:50:30. Differentiated services based on SPI weights are optional. Users with poor channel quality are prevented from consuming too many radio resources. If a user in a poor-coverage area, for example, at the edge of a cell, has a high priority, too many radio resources may be consumed to meet the QoS requirement. In this case, the QoS of other users may be affected. To solve this problem, resource restriction parameters such as 8KRSCLMT, 16KRSCLMT, 32KRSCLMT, 64KRSCLMT, 128KRSCLMT, 256KRSCLMT, and 384KRSCLMT are defined to restrict the maximum power consumption of each user. They are configured on the NodeB according to the GBRs. Table 4-1 Default maximum ratios based on the GBR GBR (kbit/s)
Maximum Ratio
8
10%
16
10%
32
15%
64
15%
128
20%
256
25%
384
30%
The HBR is configured. The HBR determines the throughput expected by the user based on a study on user experience. When the rate for a user reaches the HBR, the scheduling probability for the user is decreased. Therefore, the scheduling probability of the users with rates lower than the HBR is increased. In this way, more users can obtain satisfying Issue 02 (2009-06-30)
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services. The HBR is specified by the parameter HappyBR on the RNC side. The setting can be based on user levels, including gold, silver, and copper. For details on the parameters related to QoS management, such as the GBR, SPI, SPI weight, and HBR, see section 5.3 "QoS Parameter Mapping and Configuration." The EPF algorithm is optional.
4.3.3 Comparison of Four Algorithms Table 4-2 lists the factors considered in the four scheduling algorithms. Table 4-2 Factors considered in the four scheduling algorithms Factor
MAXCI
RR
PF
EPF
Service type
No
No
No
Yes
Initial transmission or retransmission
Yes
Yes
Yes
Yes
Maximum power
No
No
No
Yes
Waiting time
No
Yes
No
Yes
CQI
Yes
No
Yes
Yes
Actual throughput
No
No
Yes
Yes
SPI
No
No
No
Yes
GPR
No
No
No
Yes
HBR
No
No
No
Yes
Table 4-3 lists the effects of the four scheduling algorithms. Table 4-3 Effects of the four scheduling algorithms
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Item
MAXCI
RR
PF
EPF
System capacity
Highest
High
Higher
Higher
User fairness
Not guaranteed
Best
Guaranteed
Guaranteed
Differentiated services
Not guaranteed
Not guaranteed
Not guaranteed
Guaranteed
Real-time services
Not guaranteed
Not guaranteed
Not guaranteed
Guaranteed
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4.4 HARQ The main purpose of introducing HARQ is to reduce the retransmission delay and improve the retransmission efficiency. HARQ enables fast retransmission at the physical layer. Before decoding, the UE combines the retransmitted data and the previously received data, thus making full use of the data transmitted each time. In addition, HARQ can fine-tune the effective rate to compensate for the errors made by TFRC section.
4.4.1 HARQ Retransmission Principles The HARQ process of HSDPA involves only the NodeB and the UE, without involving the RNC. After receiving a MAC-hs PDU sent by the NodeB, the UE performs a CRC check and reports an ACK or NACK on the HS-DPCCH to the NodeB: If the UE reports an ACK, the NodeB transmits the next new data. If the UE reports an NACK, the NodeB retransmits the original data. After receiving the data, the UE performs soft combining of this data and the data received before, decodes the combined data, and then reports an ACK or NACK to the NodeB. RLC retransmission on the DCH involves the RNC, and therefore the RTT is relatively long. In comparison, HARQ involves only the physical layer and MAC-hs of the NodeB and those of the UE, and therefore the RTT is reduced to only 6 TTIs. After a transmission, the HARQ process must wait at least 10 ms before it can transmit the next new data or retransmit the original data. Therefore, to improve transmission efficiency, other HARQ processes can transmit data during the waiting time. A maximum of six HARQ processes can be configured in each of the NodeB HARQ entity and the UE HARQ entity. Note that not all UE categories support six HARQ processes. For example, the UEs of some categories can receive data every one or two TTIs. Thus, only two or three HARQ processes can be configured. The RAN can automatically choose the most appropriate configuration based on UE capability. Figure 4-2 HARQ retransmission principle
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4.4.2 Soft Combining During HARQ Before decoding a MAC-hs PDU, the UE performs soft combining of all the data received before to improve the utilization of Uu resources and thus increase the cell capacity. The size of the UE buffer determines the number of coded bits or the size of transport blocks. For HARQ retransmission between the NodeB and the UE, two combining strategies are available. They are Chase Combining (CC) and Incremental Redundancy (IR). In the case of CC, all retransmitted data is the same as previously transmitted data. In the case of IR, the retransmitted data may be different from the previously transmitted data. In comparison, IR has a higher gain than CC but requires more buffer space. CC can be regarded as a special case of IR. The IR strategy is hard-coded in Huawei RAN.
4.4.3 Preamble and Postamble If the HS-SCCH is received, the UE checks whether the HS-PDSCH is also correctly received and then reports an ACK or NACK in the first slot of the HS-DPCCH subframe. If the HS-SCCH is erroneously received, the UE does not report any information in the first slot of the HS-DPCCH subframe. This type of transmission is called DTX. In the case of high interference, the NodeB may demodulate DTX as ACK by mistake when demodulating the HS-DPCCH. Thus, the lost data blocks cannot be retransmitted through HARQ retransmission, and the reception can be ensured only through RLC retransmission. To meet the requirement of the 3GPP specifications for a low DTX misjudgment probability, more power has to be allocated for HS-DPCCH ACK/NACK. To solve this problem, 3GPP TS 25.214 introduces preamble and postamble. When the NodeB demodulates an HS-DPCCH ACK/NACK, it considers the subframe prior to and the subframe next to the HS-DPCCH subframe in addition to the HS-DPCCH subframe itself. Thus, for a certain DTX misjudgment probability, the introduction of preamble and postamble reduces the power required by ACK/NACK, lower the downlink load level, and increase the uplink capacity. "HS-DPCCH Preamble Support" is an optional feature. Figure 4-3 HS-DPCCH preamble and postamble
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4 User Plane
4.5 TFRC Selection The TFRC selection algorithm handles the MAC-hs queues in descending order of their priorities determined by the scheduler. The main tasks of the algorithm for each queue in each TTI are as follows: Determining the amount of data that can be transmitted by the queue Determining the modulation scheme of the queue Allocating appropriate power and channelization codes to the queue During the handling, the TFRC selection algorithm considers the following factors: Channel conditions of the UE Available resources Amount of data buffered in the MAC-hs queue Based on these factors, the algorithm allocates appropriate resources and selects appropriate transport block sizes to ensure the transmission quality and avoid wasting the resources. When the channel conditions are bad, the algorithm selects small transport block sizes to ensure that the data is received correctly and transmitted continuously. When the channel conditions are good, the algorithm selects large transport block sizes for higher transmission rates and QoS.
4.5.1 Basic Procedure of TFRC Selection The basic procedure of the TFRC selection algorithm is as follows: Step 1 Based on the CQI reported by the UE, available power, and available channelization codes, the algorithm searches a CQI mapping table for the TBSmax, that is, the maximum MAC-hs transport block size (TBS). For details, see section 4.5.2 "Determining the TBSmax." Step 2 Based on the TBSmax and the amount of data buffered in the queue, the algorithm determines the most appropriate MAC-hs TBS (TBSused). Here, TBSused
