Qualcomm hsdpa

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Qualcomm hsdpa

  1. 1. HSDPA Protocols and Physical Layer UMTS University WCDMA (UMTS) HSDPA: Protocols and Physical Layer Student Guide Book 1 80-W0331-1 Rev B
  2. 2. Export of this technology may be controlled by the United States Government. Diversion contrary to U.S. law prohibited. QUALCOMM is a registered trademark and registered service mark of QUALCOMM Incorporated. gpsOne, QCTest, repeaterOne, and Retriever are trademarks of QUALCOMM Incorporated. cdma2000® is a registered certification mark of the Telecommunications Industry Association. Used under license. All other trademarks and registered trademarks are the property of their respective owners. Material Use Restrictions These written materials are to be used only in conjunction with the associated instructor-led class. They are not intended to be used solely as reference material. No part of these written materials may be used or reproduced in any manner whatsoever without the written permission of QUALCOMM Incorporated. Copyright © 2005 QUALCOMM Incorporated. All rights reserved. QUALCOMM Incorporated 5775 Morehouse Drive San Diego, CA 92121 U.S.A.
  3. 3. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 UMTS.HELP UMTS University HSDPA Protocols and Physical Layer UMTS.HELP@QUALCOMM.COM Email hotline resource to assist our UMTS customers worldwide. Experienced UMTS engineers in our Engineering Services Group will answer your technical questions on topics including: –Industry Standards –Network Planning –Infrastructure Design –Network Optimization –Voice Quality –Test Engineering –System Design –Training © 2005 QUALCOMM Incorporated iii
  4. 4. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 This page left blank intentionally. © 2005 QUALCOMM Incorporated iv
  5. 5. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 Table of Contents Section 1: Introduction .......................................................................... 1-1 HSDPA Protocols and Physical Layer – Course Overview..................... 1-2 Section Learning Objectives .................................................................... 1-3 High Speed Downlink Packet Access (HSDPA) ..................................... 1-4 WCDMA Evolution................................................................................. 1-5 UMTS Standards...................................................................................... 1-6 Release 99 Packet Data ............................................................................ 1-7 Release 99 Downlink Limitations............................................................ 1-8 High Speed Downlink Packet Access (HSDPA) ..................................... 1-9 HSDPA Basic Concepts......................................................................... 1-10 Comparison Summary ........................................................................... 1-11 Introduction – What We Learned .......................................................... 1-12 Section 2: HSDPA Concepts ................................................................. 2-1 Section Learning Objectives .................................................................... 2-2 References .......................................................................................... 2-3 UMTS Network Architecture .................................................................. 2-4 UMTS Network Architecture with HSDPA ............................................ 2-5 UMTS Protocol Stack .............................................................................. 2-6 HSDPA Protocol Stack ............................................................................ 2-7 Release 99 Channels ................................................................................ 2-8 HSDPA Channels..................................................................................... 2-9 HSDPA Channel Timing ....................................................................... 2-11 HS-DPCCH ........................................................................................ 2-12 HS-SCCH ........................................................................................ 2-13 HS-PDSCH ........................................................................................ 2-14 Data Rate Quiz 1.................................................................................... 2-15 Data Rate Quiz 1 – Answer ................................................................... 2-16 Theoretical HSDPA Maximum Data Rate............................................. 2-17 Multi-code Transmission ....................................................................... 2-18 Consecutive Assignments ...................................................................... 2-19 Hybrid Automatic Repeat Request (HARQ) ......................................... 2-20 Lower Coding Gain................................................................................ 2-21 16-QAM ........................................................................................ 2-23 Theoretical HSDPA Maximum Data Rate............................................. 2-24 Data Rate Quiz 2.................................................................................... 2-25 Answer ....................................................................................... 2-26 More Data Rate Factors ......................................................................... 2-27 Inter-TTI Interval ................................................................................... 2-28 Retransmissions ..................................................................................... 2-29 ACK/NAK Repetitions .......................................................................... 2-30 Node B Considerations .......................................................................... 2-31 OVSF Allocation ................................................................................... 2-32 © 2005 QUALCOMM Incorporated v
  6. 6. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 Node B Transmit Power Allocation....................................................... 2-33 CQI Report Processing .......................................................................... 2-34 Node B Scheduler .................................................................................. 2-35 HSDPA Cell Re-pointing Procedure ..................................................... 2-36 Review Quiz ........................................................................................ 2-37 Answers ..................................................................................... 2-38 HSDPA Concepts – What We Learned ................................................. 2-41 Section 3: Physical Layer Channels ..................................................... 3-1 Section Learning Objectives .................................................................... 3-2 References .......................................................................................... 3-3 HSDPA Physical Layer Model Downlink...................................................................................... 3-4 Uplink .......................................................................................... 3-5 Physical Layer Frame Timing.................................................................. 3-6 Downlink HS-PDSCH ............................................................................. 3-7 HS-DSCH Channel Coding and Physical Channel Mapping .................. 3-8 HS-DSCH Channel Coding ..................................................................... 3-9 Physical Layer HARQ Functionality......................................... 3-12 HARQ Combining Schemes ...................................................... 3-13 Segmentation and Interleaving .................................................. 3-14 16-QAM Constellation Rearrangement ..................................... 3-15 HS-DSCH Physical Channel Mapping .................................................. 3-16 Downlink HS-SCCH.............................................................................. 3-17 HS-SCCH Channel Coding.................................................................... 3-18 HS-PDSCH and HS-SCCH Spreading and Modulation........................ 3-19 HS-PDSCH and HS-SCCH Timing....................................................... 3-20 Uplink HS-DPCCH ............................................................................... 3-21 HS-DPCCH Channel Coding................................................................. 3-22 HS-DPCCH Spreading and Modulation ................................................ 3-23 HS-DPCCH Timing ............................................................................... 3-24 HARQ Transmission Quiz..................................................................... 3-25 HARQ Transmission Quiz – Answers................................................... 3-26 Review Quiz ........................................................................................ 3-27 Answers...................................................................................... 3-29 Physical Layer Channels – What We Learned ...................................... 3-31 Section 4: UE Physical Layer Processing............................................. 4-1 Section Learning Objectives .................................................................... 4-2 UE Physical Channel Processing ............................................................. 4-3 UE HS-SCCH Monitoring ....................................................................... 4-4 UE HS-DSCH Decoding.......................................................................... 4-6 HARQ Processing.................................................................................... 4-7 UL Feedback Signaling............................................................................ 4-8 Channel Quality Indicator (CQI) Measurement ...................................... 4-9 CQI Reporting ........................................................................................ 4-10 © 2005 QUALCOMM Incorporated vi
  7. 7. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 CQI Mapping Table ............................................................................... 4-11 UE Categories ........................................................................................ 4-12 DL Transmit Diversity........................................................................... 4-13 Review Quiz ........................................................................................ 4-14 Answers...................................................................................... 4-16 UE Physical Layer Processing – What We Learned.............................. 4-18 Section 5: Layer 2 Protocols.................................................................. 5-1 Section Learning Objectives .................................................................... 5-2 References .......................................................................................... 5-3 HSDPA Protocol Stack ............................................................................ 5-4 UTRAN MAC Architecture..................................................................... 5-5 UTRAN MAC-hs Architecture................................................................ 5-6 UE MAC-hs Architecture ........................................................................ 5-7 Data Flow Example.................................................................................. 5-8 RNC MAC-d PDU to Node B Priority Queue............................. 5-9 Node B MAC-hs PDU Assembly .............................................. 5-10 Node B HARQ Process ............................................................. 5-11 UE HARQ Process..................................................................... 5-12 UE Re-ordering Queue .............................................................. 5-13 UE MAC-hs PDU Disassembly................................................. 5-14 MAC-hs Quiz ........................................................................................ 5-15 Answer ....................................................................................... 5-16 MAC Multiplexing................................................................................. 5-17 HARQ Protocol...................................................................................... 5-18 UE HARQ Process Flowchart ............................................................... 5-19 HARQ Protocol Signaling on HS-SCCH .............................................. 5-20 HARQ Protocol Errors .......................................................................... 5-21 HARQ Protocol Error Impact ................................................................ 5-23 Re-ordering Protocol.............................................................................. 5-24 MAC-hs Header ......................................................................... 5-25 In-sequence Delivery of MAC-hs PDUs ................................... 5-26 Re-ordering Protocol Quiz..................................................................... 5-27 Answer ....................................................................................... 5-28 Re-ordering Protocol – Transmit Window ............................................ 5-29 Flushing the Re-ordering Queue Window Method ........................................................................ 5-30 Timer Method ............................................................................ 5-32 Summary of Re-ordering Queue Flushing Methods.............................. 5-33 RLC Considerations............................................................................... 5-34 HSDPA Cell Re-pointing Procedure ......................................... 5-35 Sequence Number Space and Data Rates .................................. 5-36 Review Quiz ........................................................................................ 5-37 Answers ..................................................................................... 5-38 Layer 2 Protocols – What We Learned.................................................. 5-40 © 2005 QUALCOMM Incorporated vii
  8. 8. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 Section 6: Layer 3 Protocols and Procedures...................................... 6-1 Section Learning Objectives .................................................................... 6-2 References .......................................................................................... 6-3 HSDPA RRC Functions........................................................................... 6-4 Mobile Originated PS Data Call Setup .................................................... 6-5 Establish HSDPA Operation from Cell_DCH......................................... 6-6 UE Measurements Change of Best Cell (Event 1d) ................................................... 6-7 Measurement Control Message.................................................... 6-8 Event 1d Parameters .................................................................... 6-9 HSDPA Configuration Messages .................................................................................... 6-10 Radio Bearer Reconfiguration Message .................................... 6-11 Layer 1 Information Elements ................................................... 6-12 Layer 2 Information Elements ................................................... 6-13 UE State Transitions .............................................................................. 6-14 Stopping HS-DSCH in Cell_DCH............................................. 6-15 Cell_DCH to Cell_FACH .......................................................... 6-16 Cell_FACH to Cell_DCH with HS-DSCH................................ 6-17 HSDPA Cell Re-pointing Procedure Overview ................................................................................... 6-18 Messages and Information Elements ......................................... 6-19 Synchronized vs. Unsynchronized............................................. 6-20 HSDPA Cell Repointing Procedure Synchronized Inter-Node B ....................................................... 6-21 Unsynchronized Inter-Node B ................................................... 6-22 Problem Areas in Release 5 Signaling................................................... 6-23 Review Quiz ........................................................................................ 6-24 Answers...................................................................................... 6-26 Layer 3 Protocols and Procedures – What We Learned ........................ 6-28 Section 7: Summary............................................................................... 7-1 High Speed Downlink Packet Access (HSDPA) ..................................... 7-2 HSDPA Protocol Stack ............................................................................ 7-3 Theoretical HSDPA Maximum Data Rate............................................... 7-4 HS-DSCH Channel Coding and Physical Channel Mapping .................. 7-5 UE MAC-hs Architecture ........................................................................ 7-6 Re-ordering Protocol – In-sequence Delivery of MAC-hs PDUs ........... 7-7 HSDPA Cell Re-pointing Procedure – Overview ................................... 7-8 © 2005 QUALCOMM Incorporated viii
  9. 9. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 Acronyms and Abbreviations 16-QAM 3GPP ACK ACLR AICH AM AMC AMR ARQ AS AuC BCCH BCH BLER BTFD CC CCCH CLTD CM CPICH CQI CS CTCH DCCH DCH DPCCH DPDCH DTCH DTX EDGE EUL EVM FACH FEC GGSN GMM GMSC GPRS GSM GTF H-RNTI HARQ HLR 16-Quadrature Amplitude Modulation 3rd Generation Partnership Project ACKnowledge Adjacent Channel Leakage Ratio Acquisition Indicator Channel Acknowledged Mode Adaptive Modulation and Coding Adaptive Multi-Rate Automatic Repeat Request Access Stratum Authentication Center Broadcast Control Channel Broadcast Channel Block Error Rate Blind Transport Format Detection Call Control Common Control Channel Closed Loop Transmit Diversity Connection Management Common Pilot Channel Channel Quality Indicator Circuit Switched Common Traffic Channel Dedicated Control Channel Dedicated Channel Dedicated Physical Control Channel Dedicated Physical Data Channel Dedicated Traffic Channel Discontinuous Transmit Enhanced Data rates for GSM Evolution Enhanced Uplink Error Vector Magnitude Forward Access Channel Forward Error Correction GPRS Gateway Support Node GPRS Mobility Management Gateway Mobile Switching Center General Packet Radio Service Global System for Mobiles Generalized Transport Format High Speed Radio Network Temporary Identity Hybrid Automatic Repeat Request Home Location Register © 2005 QUALCOMM Incorporated ix
  10. 10. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 HS-DPCCH HS-DSCH HS-PDSCH HS-SCCH HSDPA IR ISDN kbps L1 MAC MAPL Mbps Mcps MM ms MSC NACK NAK NAS NDI OVSF PA PAR PCCH PCCPCH PCH PDU PICH PRACH PS PSTN QID QoS QPSK RAB RACH RLC RM RNC ROT RRC RSCP RV SAW SCCPCH SCH High Speed Dedicated Physical Control Channel High Speed Downlink Shared Channel High Speed Physical Downlink Shared Channel High Speed Shared Control Channel High Speed Downlink Packet Access Incremental Redundancy Integrated Services Digital Network Kilobits Per Second Physical Layer Medium Access Control Maximum Allowable Path Loss Megabits Per Second Megachips per second Mobility Management Millisecond Mobile Switching Center Negative Acknowledgment Negative Acknowledgment Non-Access Stratum New Data Indicator Orthogonal Variable Spreading Factor Power Amplifier Peak-to-Average-Power Ratio Paging Control Channel Primary Common Control Physical Channel Paging Channel Protocol Data Unit Paging Indicator Channel Physical Random Access Channel Packet Switched Public Switched Telephone Network Queue Identifier Quality Of Service Quadrature Phase Shift Keying Radio Access Bearer Random Access Channel Radio Link Control Reed-Muller coding Radio Network Controller Rise Over Thermal Radio Resource Control Received Signal Code Power Redundancy Version Stop And Wait Secondary Common Control Physical Channel Synchronization Channel © 2005 QUALCOMM Incorporated x
  11. 11. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 SF SGSN SID SIR SM SMS SPER SRB SS STTD TFCI TFRC TFRI TM TPC TSN TTI UE UM UMTS USIM UTRAN VF VLR WCDMA Spreading Factor Serving GPRS Support Node Size Index Identifier Signal-to-Interference Ratio Session Management Short Message Service Sub-Packet Error Rate Signal Radio Bearer Supplementary Services Space Time Transmit Diversity Transport Format Combination Indicator Transport Format Resource Combination Transport Format Resource Indicator Transparent Mode Transmit Power Control Transmission Sequence Number Transmission Time Interval User Equipment Unacknowledged Mode Universal Mobile Telecommunications Systems Universal Subscriber Identity Module Universal Terrestrial Radio Access Network Version Flag Visitor Location Register Wideband Code Division Multiple Access © 2005 QUALCOMM Incorporated xi
  12. 12. WCDMA (UMTS) HSDPA: Protocols and Physical Layer 80-W0331-1 Rev B Table of Contents – Book 1 This page left blank intentionally. © 2005 QUALCOMM Incorporated xii
  13. 13. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction UMTS University Section 1: Introduction 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 1-1 SECTION 1 Introduction Notes © 2005 QUALCOMM Incorporated 1-1
  14. 14. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction HSDPA Protocols and Physical Layer – Course Overview UMTS University 1. 2. 3. 4. 5. 6. 7. 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 1-2 Introduction HSDPA Concepts Physical Layer Channels UE Physical Layer Processing Layer 2 Protocols Layer 3 Protocols and Procedures Summary Notes © 2005 QUALCOMM Incorporated 1-2
  15. 15. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction Section Learning Objectives UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 1-3 Review WCDMA and HSDPA evolution and standards. Review Release 99 packet data service methods. Define HSDPA data transfer model. Describe how HSDPA addresses the Release 99 limitations for packet data service. Notes © 2005 QUALCOMM Incorporated 1-3
  16. 16. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction High Speed Downlink Packet Access (HSDPA) UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 1-4 What are the drivers and motivations for migrating to HSDPA? • Data Rate – Demand for high data rate multimedia services – Demand for higher peak data rates • Throughput – Cost per megabyte • Capacity – Improved Link Adaptation dependent on Radio Conditions Data Services and High Speed Downlink Packet Access (HSDPA) Data Services are expected to grow significantly within the next few years. Current 2.5G and 3G operators are already reporting that a significant proportion of usage is now due to data, implying an increasing demand for high-data-rate, content-rich multimedia services. Although current Release 99 WCDMA systems offer a maximum practical data rate of 384 kbps, the 3rd Generation Partnership Project (3GPP) have included in Release 5 of the specifications a new high-speed, low-delay feature referred to as High Speed Downlink Packet Access (HSDPA). HSDPA provides significant enhancements to the Downlink compared to WCDMA Release 99 in terms of peak data rate, cell throughput, and round trip delay. This is achieved through the implementation of a fast channel control and allocation mechanism that employs such features as Adaptive Modulation and Coding and fast Hybrid Automatic Repeat Request (HARQ). Shorter Physical Layer frames are also employed. © 2005 QUALCOMM Incorporated 1-4
  17. 17. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction WCDMA Evolution UMTS University GSM GPRS EDGE WCDMA Release 99 HSDPA Downlink Peak Data Rate (Typical Deployment) 9.6 kbps 40kbps 120 kbps 384 kbps 10.0 Mbps 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 1-5 Downlink Peak Data Rate (Theoretical Maximum) 9.6 kbps 171 kbps 473 kbps 2.0 Mbps 14.4 Mbps WCDMA Evolution WCDMA evolved from GSM/GPRS, inheriting much of the upper layer functionality directly from those systems. The first commercial deployments of WCDMA are based on a version of the standards called Release 99. Enhanced Data rates for GSM Evolution (EDGE) is another system in the GSM/GPRS family that some operators have deployed as an intermediate step before deploying WCDMA. HSDPA was introduced in WCDMA Release 5 to offer higher speed Downlink data services. Release 6 introduces the Enhanced Uplink (EUL) that will provide faster data services for the Uplink. © 2005 QUALCOMM Incorporated 1-5
  18. 18. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction 80-W0331-1 Rev B UMTS Standards HSDPA Protocols and Physical Layer UMTS University Topic Specifications Series Number RF Performance 25.1xx Physical Layer 25.2xx Layer 2 and Layer 3 25.3xx UTRAN Section 1-6 Specifications are segmented by layers and are available on the ftp site: ftp://ftp.3gpp.org/Specs/ See Rel-5 folder for HSDPA versions. 25.4xx NAS Layer (CC, SS, SMS, MM) 22.xxx, 23.xxx, 24.xxx Packet Switched Data Service 22.060, 23.060 Circuit Switched Data Service 23.910 Voice Service 26.xxx USIM 31.xxx UE Conformance 34.xxx For a complete list of Release 99 specifications, see 21.101. For a list of acronyms, see 21.905. UMTS Standards The 3rd Generation Partnership Project (3GPP) is responsible for writing and maintaining the UMTS specifications. Revisions of the specifications are published every three months, as contributing members of 3GPP suggest enhancements and corrections. When a set of features is deemed completed, the current revision is designated as the next release and the specifications are frozen. Corrections are allowed after the release is frozen, but no enhancements may be added. HSDPA features are described in Release 5. © 2005 QUALCOMM Incorporated 1-6
  19. 19. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction Release 99 Packet Data UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 1-7 How is Packet Data handled in Release 99 (FDD)? • DCH (Dedicated Channel) – Spreading codes assigned per user – Closed loop power control – Macro diversity (soft handover) • FACH (Common Channel) – Common spreading code – User ID detected by MAC layer – No closed loop power control – No soft handover Release 99 Packet Data There are three different techniques defined in the Release 99 specification to enable Downlink packet data. Most commonly, data transmission is supported using either the Dedicated Channel (DCH) or the Forward Access Channel (FACH). The DCH is the primary means of supporting packet data services. Each user is assigned a unique Orthogonal Variable Spreading Factor (OVSF) code dependent on the required data rate. Fast closed loop Power Control is employed to ensure that a target Signal to Interference Ratio (SIR) is maintained in order to control the block error rate (BLER). Macro Diversity is supported using soft handover. Data transfer can also be supported on the FACH. This common channel employs a fixed OVSF code. As it needs to be received by all UEs, higher data rates are generally not supported. Macro Diversity is also not supported and the channel operates with a fixed (or slow changing) power allocation. Each data block contains a unique UE identifier that allows a given UE to keep its own data and discard that belonging to other UEs. © 2005 QUALCOMM Incorporated 1-7
  20. 20. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction Release 99 Downlink Limitations 80-W0331-1 Rev B HSDPA Protocols and Physical Layer UMTS University Section 1-8 Dedicated Channel Features (DCH) • • • • Maximum implemented Downlink of 384 kbps OVSF Code limitation for high speed users Rate switching response to bursty throughput is slow Outer loop power control responds slowly to channel conditions Common Channel Features (FACH) • • • Good for bursty data applications Only low data rates supported Fixed transmit power Release 99 Downlink Limitations Although WCDMA Release 99 standard allows for maximum data rates of up to 2.0 Mbps, it has only been widely implemented with a maximum data rate of 384 kbps. This data rate is achieved by allocating a dedicated channel to each user. The use of dedicated resources can be a limitation, especially for data applications with bursty characteristics. Each dedicated channel uses an OVSF code. Shorter codes are used for higher data rates and longer codes for lower data rates. When an OVSF of a particular length is used, all longer OVSF codes derived from that code become unavailable. This limits the number of simultaneous high speed data users in a given cell. The Release 99 standards provide support for a Secondary Scrambling Code, which eases this limitation, but it has not been widely implemented in commercial systems and will likely be removed from future versions of the specification. The data rate of a dedicated channel can be adjusted to accommodate varying requirements of a data service application, but the procedure for doing so is slow and thus inefficient. Capacity is controlled both by the maximum amount of PA power that is available and by the power requirement of each data service. In dedicated mode, fast power control is used so that a target Eb/No is achieved on the Downlink. However, the required Eb/No setpoint changes at a much slower rate. This can result in wasted resources whereby a better than required Eb/No is achieved for the required BLER. © 2005 QUALCOMM Incorporated 1-8
  21. 21. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction High Speed Downlink Packet Access (HSDPA) 80-W0331-1 Rev B HSDPA Protocols and Physical Layer UMTS University Section 1-9 • Set of high speed channels • Channels are shared by multiple users • Each user may be assigned all or part of the total bandwidth every 2 ms. High Speed Downlink Packet Access (HSDPA) In HSDPA, the Node B allocates a set of high speed channels. These channels are assigned to a user using a fast scheduling algorithm that allocates the channels every 2 ms. All or part of the channels may be assigned to a given user during any 2 ms period. The rapid scheduling of HSDPA is well-suited to the bursty nature of packet data. During periods of high activity, a given user may get a larger percentage of the channel bandwidth, while it gets little or no bandwidth during periods of low activity. © 2005 QUALCOMM Incorporated 1-9
  22. 22. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction HSDPA Basic Concepts UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 1-10 How will HSDPA address the limitations of Release 99? • • • Adaptive modulation and coding – Fast feedback of channel condition – QPSK and 16-QAM – Coding from R=1/3 to R=1 Multi-Code operation – Multiple codes allocated per user – Fixed spreading factor Node B scheduling – Physical Layer HARQ HSDPA Basic Concepts In HSDPA a common channel with fixed power is employed for data transfer. Users are separated in both the time and code domains. A fixed spreading factor is employed but multicode operation is possible for increased data rates. Adaptive Modulation and Coding (AMC) replaces the role of power control so that the modulation and coding rate are changed depending on the channel condition. This is accomplished by locating the scheduling algorithm for channel allocation at the Node B instead of the RNC in Release 99. © 2005 QUALCOMM Incorporated 1-10
  23. 23. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction 80-W0331-1 Rev B Comparison Summary UMTS University Mode Channel Type Power Control Soft Handover Suitability for Bursty Data Data Rate HSDPA Protocols and Physical Layer Section 1-11 HSDPA DCH FACH Dedicated Common Common Closed Inner Loop Fixed Power at 1500 Hz - Slow None with link Outer Loop adaptation Supported Not Supported Not Supported Poor Good Good Medium Low High Comparison Summary DCH and FACH are the two Release 99 channels typically used for packet switched data in practice. The advantages and disadvantages of each approach are apparent. Whereas DCH is suited for medium high data rates (with a maximum rate of 384 kbps), rate switching is slow, making it unsuitable and inefficient for bursty data such as a Web browsing application. By contrast, FACH provides good support for bursty data but is a common channel without power control or other mechanism to account for channel conditions. This makes it unsuitable for higher data rates. Switching from DCH to FACH is slow and inefficient, due in part to the typical timer values used to detect inactivity (may be as much as 5 seconds). HSDPA is suitable to high date rates for a bursty application, though we will see that the absence of soft handover makes it more suitable for stationary or low-mobility users than for highly mobile users. HSDPA typically operates at a fixed power, but feedback from the UE can instruct the Node B to use lower power when the UE is in good channel conditions. Link adaptation is used to adjust data rate, coding, and modulation to quickly respond to changing channel conditions. © 2005 QUALCOMM Incorporated 1-11
  24. 24. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 1: Introduction Introduction – What We Learned UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 1-12 WCDMA and HSDPA evolution and standards. Release 99 data service review. HSDPA data transfer model. HSDPA solutions to Release 99 limitations. Notes © 2005 QUALCOMM Incorporated 1-12
  25. 25. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts UMTS University Section 2: HSDPA Concepts 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-1 SECTION 2 HSDPA Concepts Notes © 2005 QUALCOMM Incorporated 2-1
  26. 26. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Section Learning Objectives UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-2 Review UMTS network architecture and protocols. Define HSDPA protocol stack. Review UMTS Release 99 channels. Define HSDPA channels. Illustrate theoretical maximum HSDPA data rate. Show how theoretical data rate is reduced to a practical data rate in a real world scenario. Describe Node B enhancements to support HSDPA. Notes © 2005 QUALCOMM Incorporated 2-2
  27. 27. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B References UMTS University HSDPA Protocols and Physical Layer Section 2-3 3GPP Release 5 Specification References 25.308 HSDPA overall description stage 2 25.858 HSDPA physical layer aspects Notes © 2005 QUALCOMM Incorporated 2-3
  28. 28. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts UMTS Network Architecture UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-4 UMTS Network Architecture A UMTS system consists of three major subsystems: • User Equipment (UE) – May be a mobile, a fixed station, a data terminal, etc. Includes a Universal Subscriber Identity Module (USIM), which contains all of a user’s subscription information. • Universal Terrestrial Radio Access Network (UTRAN) – Includes all of the radio equipment necessary for accessing the network. Consists of Node Bs that provide radio links to the UEs, and Radio Network Controllers (RNC) that control the radio resources and interface to the Core Networks. • Core Network – Includes all of the switching and routing capability for connecting to either the PSTN (circuit-switched calls) or to a Packet Data Network (packet-switched calls), for mobility and subscriber location management, and for authentication services. © 2005 QUALCOMM Incorporated 2-4
  29. 29. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B UMTS Network Architecture with HSDPA HSDPA Protocols and Physical Layer Section 2-5 UMTS University Node B Node B USIM RNC Iups SGSN Node B Node B Mobile Equipment User Equipment Iub Hardware and Software Changes Software Changes Node B Node B Node B Node B Internet GMSC PSTN/ ISDN HLR/ AuC Iur Uu GGSN Iub RNC Iucs MSC/ VLR Core Network UTRAN UMTS Network Architecture with HSDPA Adding HSDPA to an existing UMTS network requires no new network entities, but hardware and/or software changes may be required for each entity. The changes are concentrated in the UE, Node B, and RNC. Interface changes are concentrated on the Uu interface between UE and Node B and on the Iub interface between Node B and RNC. • UE and Node B – Require hardware and software changes to support the new channels and functionality of HSDPA. • RNC – Requires software changes to support the new signaling messages used to configure and manage HSDPA channels. • Uu Interface – Requires new signaling messages exchanged over existing signaling channels and new transport and physical channels to support high-speed operation. • Iub Interface – Requires a new frame protocol for sending high-speed user data from the RNC to the Node B. No functional changes to the Iups are required, although there may be bandwidth issues to support higher data rates to multiple users. © 2005 QUALCOMM Incorporated 2-5
  30. 30. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B HSDPA Protocols and Physical Layer UMTS Protocol Stack Section 2-6 UMTS University Circuit Switched Connection Management (CM) NonAccess Stratum Packet Switched Session Management (SM) Short Message Services (SMS) Call Control Supplementary Short Message Services (SS) Services (SMS) (CC) Mobility Management (MM) GPRS Mobility Management (GMM) Radio Resources Control (RRC) Access Stratum Radio Link Control (RLC) Layer 2 Medium Access Control (MAC) Physical Layer (L1) UMTS Protocol Stack The UMTS signaling protocol stack is divided into Access Stratum (AS) and Non-Access Stratum (NAS). The Non-Access Stratum architecture evolved from the GSM/GPRS upper layers and is divided into Circuit Switched (CS) and Packet Switched (PS) protocols. The Access Stratum consists of three layers: • Layer 3 – The Radio Resource Control (RRC) layer handles establishment, release, and configuration of radio resources. • Layer 2 – Consists of two sublayers. The Radio Link Control (RLC) sublayer provides segmentation, re-assembly, duplicate detection, and other traditional Layer 2 functions. The Medium Access Control (MAC) sublayer multiplexes data and signaling onto the appropriate channels and controls access to the Physical Layer. • Layer 1 – The Physical Layer transfers data over the radio link. © 2005 QUALCOMM Incorporated 2-6
  31. 31. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts HSDPA Protocol Stack UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-7 HSDPA Protocol Stack In a Release 99 PS network, the NAS layer protocols are terminated at the SGSN. The RRC, RLC, and MAC protocols are terminated at the RNC. The Physical Layer protocol is terminated at the Node B. The Release 5 specifications define a new sublayer of MAC called MAC-hs, which implements the MAC protocols and procedures for HSDPA. This sublayer operates at the Node B and the UE. The location of MAC-hs in Node B has an important implication for HSDPA operation. In Release 99, a UE may be in soft handover with multiple Node Bs. Transport channel frames are constructed by the MAC sublayer in the RNC and sent over the Iub interface to each Node B with which the UE is in soft handover. The UE receives identical Transport channel frames from each Node B. HSDPA requires fast scheduling of the shared channels, and allocates the channels in 2 ms intervals called subframes. To meet this requirement, the Transport channel frames are constructed by the MAC-hs sublayer operating in the Node B. By design, the HSDPA channels cannot operate in soft handover because the MAC-hs sublayer of each Node B operates independently. © 2005 QUALCOMM Incorporated 2-7
  32. 32. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Release 99 Channels UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-8 Release 99 Channels This diagram shows possible mappings of logical, transport, and physical channels in the control and user planes for UMTS Release 99. Some channels exist only in Physical Layer (CPICH, SCH, DPCCH, AICH, PICH). These channels carry no upper layer signaling or user data. Transport channels carry the following types of information: Broadcast Control Channel (BCH) – Broadcast information that defines overall system configuration. Paging Channel (PCH) – Paging notification messages. A Paging Indicator Channel (PICH) is associated with a PCH to allow a UE to quickly determine whether it needs to read the PCH during its assigned paging occasion. Forward Access Channel (FACH) – Common Downlink signaling messages. Also carries dedicated Downlink signaling and user information to a UE operating in Cell_FACH state. An Acquisition Indicator Channel (AICH) is associated with a FACH channel. Random Access Channel (RACH) – Common Uplink signaling messages. Also carries dedicated Uplink signaling and user information to a UE operating in Cell_FACH state. Dedicated Channel (DCH) – Dedicated signaling and user information for a UE operating in the Cell_DCH state. DCH is mapped to a Dedicated Physical Data Channel (DPDCH). An associated Dedicated Physical Control Channel (DPCCH) carries Physical Layer control information, such as power control commands. © 2005 QUALCOMM Incorporated 2-8
  33. 33. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts HSDPA Channels 80-W0331-1 Rev B HSDPA Protocols and Physical Layer UMTS University Section 2-9 New HSDPA Channels • High Speed Downlink Shared Channel (HS-DSCH) – Downlink Transport Channel • High Speed Shared Control Channel (HS-SCCH) – Downlink Control Channel • High Speed Physical Downlink Shared Channel (HS-PDSCH) – Downlink Physical Channel • High Speed Dedicated Physical Control Channel (HS-DPCCH) – Uplink Control Channel HSDPA Channels HSDPA introduces three new Downlink channels and one new Uplink channel: • High Speed Downlink Shared Channel (HS-DSCH) – A Downlink transport channel shared by several UEs. The HS-DSCH is associated with one or several Shared Control Channels (HS-SCCH). It operates on a 2 ms Transmission Time Interval (TTI). High Speed Shared Control Channel (HS-SCCH) – A Downlink physical channel used to carry Downlink control information related to HS-DSCH transmission. The UE monitors this channel continuously to determine when to read its data from the HS-DSCH, and the modulation scheme used on the assigned physical channel. • High Speed Physical Downlink Shared Channel (HS-PDSCH) – A Downlink physical channel shared by several UEs. It supports Quadrature Phase Shift Keying (QPSK) and 16-Quadrature Amplitude Modulation (16-QAM) and multi-code transmission. It is allocated to a user at 2 ms intervals. • High Speed Dedicated Physical Control Channel (HS-DPCCH) – An Uplink physical channel that carries feedback from the UE to assist the Node B’s scheduling algorithm. The feedback includes a Channel Quality Indicator (CQI) and a positive or negative acknowledgement (ACK/NAK) of a previous HS-DSCH transmission. © 2005 QUALCOMM Incorporated 2-9
  34. 34. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts HSDPA Channels (continued) UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-10 HSDPA Channels (continued) Only dedicated logical user data channels may be mapped to HS-DSCH. When DTCH is mapped to HS-DSCH, only Unacknowledged Mode (UM) and Acknowledged Mode (AM) channels may be used. A UE operating in HSDPA mode also has at least one Release 99 dedicated channel (DCH/DPDCH) allocated, to ensure that RRC and NAS signaling can always be sent, even if the UE is not able to receive the high speed channels. The HS-DPCCH is a Physical Layer control channel. It carries no upper layer information, and therefore has no logical or transport channel mapping. © 2005 QUALCOMM Incorporated 2-10
  35. 35. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts HSDPA Channel Timing UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-11 HSPDA Channel Timing HSDPD channel timing is based on a time interval of 2 ms, or 3 slots. This slide illustrates a single HSDPA channel assignment. Consecutive assignments to a single UE allow the theoretical maximum HSDPA data rate to be achieved. 1. The UE measures the Downlink channel quality and sends a CQI report on the HSDPCCH. An ACK or NAK from a previously received block may also be included in this transmission. 2. If the Node B decides to send data to the UE, it will send information on the HS-SCCH to assign the physical channel and give the UE information about how the data was encoded. The earliest that this assignment can be made is in the subframe following the end of CQI report. 3. During the next 2 ms HS-DSCH transmission time, one or more HS-PDSCHs carry the UE’s data. The HS-SCCH transmission overlaps the HS-PDSCH transmission. 4. After the UE decodes the data, it sends an ACK or NAK on the HS-DPCCH. The UE must send the ACK or NAK 5 ms after the end of the HS-DSCH transmission. If the UE sends a NAK, the Node B may send the data again during a later time slot, or may choose not to retransmit the data. A CQI report may also be included in this transmission. © 2005 QUALCOMM Incorporated 2-11
  36. 36. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B HS-DPCCH HSDPA Protocols and Physical Layer UMTS University Section 2-12 High Speed Dedicated Physical Control Channel (HS-PDCCH) • • 1st slot carries ACK or NAK for received HS-DSCH blocks 2nd and 3rd slots carry Channel Quality Indicator (CQI) – UE measures Downlink CPICH channel quality – CQI indicates the highest data rate for error rate < 10% – Frequency of CQI reports configured by UTRAN • • DTX during ACK/NAK and CQI slots if nothing to send Uses Spreading Factor = 256 HS-DPCCH Whenever the UE is operating in HSDPA mode, it uses the HS-DPCCH to give feedback to the serving Node B. This feedback consist of two parts: ACK/NAK – The UE sends a positive or negative acknowledgement for each HS-DSCH assignment. UTRAN may configure the UE to repeat the ACK/NAK, up to a maximum of 4 transmissions. The first ACK/NAK for a given HS-DSCH assignment is sent 5 ms (7.5 slots) after the end of the HS-DSCH transmission. Channel Quality Indicator (CQI) – The UE measures the channel quality of the Downlink CPICH and computes a CQI value. The value is an index into a table, and corresponds to the maximum data rate that the UE can decode with an error rate of less than 10%, assuming the channel conditions don’t change. UTRAN may configure the UE to repeat the CQI, up to a maximum of 4 transmissions. UTRAN may also configure the periodicity of CQI reporting, ranging from 2 ms to 160 ms. © 2005 QUALCOMM Incorporated 2-12
  37. 37. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B HS-SCCH UMTS University HSDPA Protocols and Physical Layer Section 2-13 High Speed Shared Control Channel (HS-SCCH) • 1st part carries modulation information • • • • – OVSF code assignment – Modulation scheme 2nd part carries transport block size, Hybrid ARQ parameters UE Identity encoded over each part – UE decodes each part independently UE assigned up to 4 HS-SCCHs to monitor Uses Spreading Factor = 128 HS-SCCH Whenever the UE is operating in HSDPA mode, it continuously monitors up to four HS-SCCHs. Each HS-SCCH transmission carries scheduling information about the next HS-DSCH assignment and the Physical Layer parameters of the associated HS-PDSCH. OVSF Code Assignment – The HS-SCCH indicates which of the OVSF codes allocated to the HS-PDSCHs will be used. HS-PDSCH uses multi-code transmission, which means that multiple OVSF codes may be assigned to one UE at the same time Modulation Scheme – HS-PDSCH uses either QPSK or 16-QAM modulation. This can change from one assignment to the next, and HS-SCCH indicates which method will be used. Transport Block Size – The HS-SCCH indicates how much data will be sent during the next assignment Hybrid ARQ (HARQ) Parameters – The HARQ protocol supports retransmissions and incremental redundancy. These parameters allow the UE to differentiate new transmissions from retransmissions. UE Identity – Multiple UEs may be monitoring the same set of HS-SCCHs. Each UE has an assigned identity called the H-RNTI. The first part of the HS-SCCH is scrambled using the H-RNTI so that an UE can determine whether the corresponding HS-DSCH assignment carries its data or data belonging to another UE. The second part contains additional information to allow the UE to decode the block, and a CRC masked with the H-RNTI. © 2005 QUALCOMM Incorporated 2-13
  38. 38. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts HS-PDSCH UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-14 High Speed Physical Downlink Shared Channel (HS-PDSCH) • Carries UE data • Up to 15 HS-PDSCH may be assigned simultaneously • – UE capability indicates maximum number of codes it supports Uses Spreading Factor = 16 HS-PDSCH When the UE decodes the HS-SCCH and determines that there is an HS-DSCH assignment in the next TTI, it decodes the assigned HS-PDSCHs. Each HS-PDSCH uses an OVSF of length 16. If multiple HS-PDSCHs are assigned simultaneously to one UE, they must use consecutive OVSF codes. The HS-SCCH indicates the first OVSF code and the number of codes for each assignment. A UE is a member of one of 12 categories, as a function of its hardware capabilities. Each category represents different values of the following parameters: Number of simultaneous HS-PDSCH codes (5, 10, or 15) Maximum transport block size Inter-TTI interval – minimum time between consecutive assignments. Incremental redundancy buffer size – used to soft-combine symbols from retransmissions. © 2005 QUALCOMM Incorporated 2-14
  39. 39. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B Data Rate Quiz 1 UMTS University HSDPA Protocols and Physical Layer Section 2-15 Question: Assuming a transport block size of 320 bits, what HSDPA data rate can be achieved by a single UE using the channel allocation timing shown above? Data Rate Quiz 1 Determine the HSDPA data rate achieved by a single UE, assuming the following parameters: 1. Each HSDPA assignment is for a single HS-PDSCH (no multi-code). 2. Each HS-PDSCH block carries 320 bits. 3. Each transport block is successfully decoded after the first transmission (the UE always sends an ACK and the Node B never retransmits any block). 4. The Node B schedules an assignment as early as possible following the ACK transmission, as shown in the above timing diagram. © 2005 QUALCOMM Incorporated 2-15
  40. 40. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B Data Rate Quiz 1 – Answer UMTS University HSDPA Protocols and Physical Layer Section 2-16 Answer: 320 bits are transmitted every 10 ms, so the maximum data rate is 32 kbps. Data Rate Quiz 1 – Answer Using the allocation scheme and block size given in this example, the UE achieves only 32 kbps! Obviously, this is not the whole story of HSDPA. © 2005 QUALCOMM Incorporated 2-16
  41. 41. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B Theoretical HSDPA Maximum Data Rate HSDPA Protocols and Physical Layer UMTS University Section 2-17 Theoretical Maximum is 14.4 Mbps! How do we get from 32 kbps to 14.4 Mbps? • Multi-code transmission • Consecutive assignments using multiple Hybrid • • Automatic Repeat Request (HARQ) processes Lower coding gain 16-QAM Theoretical HSDPA Maximum Data Rate The theoretical maximum data rate is 14.4 Mbps. The following techniques are used to achieve this data rate: Multi-code transmission – Up to 15 HS-PDSCH channels may be assigned to a single UE during one 2 ms TTI. Consecutive assignments – The HARQ procedure allows the Node B to send back-toback assignments at 2 ms intervals. Lower Coding Gain – The block size of 320 bits was chosen assuming a turbo code rate of 1/3. Higher data rates can be achieved by puncturing more bits for a higher effective code rate (and thus lower coding gain). 16-QAM – This modulation scheme increases the data rate over QPSK by a factor of 2. © 2005 QUALCOMM Incorporated 2-17
  42. 42. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Multi-code Transmission UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-18 Data Rate with 15-code Multi-code 32 kbps X 15 = 480 kbps Multi-code Transmission HSDPA allows up to 15-code multi-code. Each HS-PDSCH uses an OVSF of length 16. The Node B signals the number of codes to the UE in the HS-SCCH. The number of codes supported by the UE is one factor in determining the UE’s HSDPA category. The allowed choices are 5, 10, or 15 codes. For a UE capable of the maximum number of codes, the data rate in the above example is 15 times greater than the single code assignment, or 480 kbps. © 2005 QUALCOMM Incorporated 2-18
  43. 43. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Consecutive Assignments UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-19 Data Rate with Consecutive Assignments 480 kbps X 5 = 2.4 Mbps Consecutive Assignments HSDPA allows the channels to be assigned in consecutive TTIs to the same UE. In the UE, up to six simultaneous HARQ processes operate in parallel to decode consecutive assignments. Each HARQ process is responsible for decoding one assignment, and transmitting the associated ACK or NAK 5 ms after the end of that assignment. The UE can achieve a maximum data rate that is five times greater than in the previous example, or 2.4 Mbps, if all of the following conditions are met: The UE supports 15-code multi-code. The Node B assigns all 15 OVSF codes every TTI. Every data block is correctly decoded (the UE always sends an ACK). © 2005 QUALCOMM Incorporated 2-19
  44. 44. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Hybrid Automatic Repeat Request (HARQ) 80-W0331-1 Rev B HSDPA Protocols and Physical Layer UMTS University Section 2-20 Hybrid Automatic Repeat Request (HARQ) • Each HSDPA assignment is handled by a HARQ process – HARQ Processes run in Node B and UE – Up to 8 HARQ processes per UE – Number configured by Node B when HSDPA operations begin • The UE HARQ process is responsible for: – Attempting to decode the data – Deciding whether to send ACK or NAK – Soft-combining of retransmitted data • The Node B HARQ process is responsible for: – Selecting the correct bits to send according to the selected retransmission scheme and UE capability Hybrid Automatic Repeat Request (HARQ) To support consecutive assignments, HSDPA defines a Hybrid Automatic Repeat Request (HARQ) protocol. This protocol is implemented in both the Node B and the UE, and consists of procedures implemented in both the MAC-hs sublayer and the Physical Layer. When the Node B assigns an HSDPA subframe to a UE, it also assigns a HARQ process to handle the data transfer. The UE HARQ process is responsible for Decoding the initial transmission Sending an ACK or NAK Soft-combining retransmissions of the data packet until it is successfully decoded or until Node B aborts the packet. Up to 8 HARQ processes may run simultaneously. At least 6 simultaneous processes are required to sustain consecutive HSDPA assignments. Depending on its implementation, the Node B scheduler algorithm may require more than 6 HARQ processes to sustain consecutive assignments. When HSDPA operations begin, the RNC configures the number of HARQ processes in a signaling message to the UE. The maximum number of HARQ processes that a UE supports is a function of its HSDPA category. The minimum number of HARQ processes supported by any UE is 2, which corresponds to a UE that uses an inter-TTI interval of 3. © 2005 QUALCOMM Incorporated 2-20
  45. 45. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Lower Coding Gain 80-W0331-1 Rev B HSDPA Protocols and Physical Layer UMTS University Section 2-21 R=1/3 Turbo Coding and QPSK Modulation Lower Coding Gain All examples so far have assumed a turbo code rate of 1/3 and QPSK modulation. If we assume a single HS-PDSCH and a transport block containing 320 data bits, rate 1/3 turbo coding produces 960 symbols. QPSK modulation maps two symbols onto one modulation symbol, which then gets spread by the OVSF of length 16. This results in 7680 chips sent every 2 ms, corresponding to the fundamental WCDMA chip rate of 3.84 Mcps. If the transport block is not exactly 320 data bits, the rate matching step adjusts the number of symbols after turbo coding to produce 960 symbols. If multiple HS-PDSCHs are used, the rate matching step produces an integer multiple of 960 symbols, and blocks of 960 symbols are mapped to each HS-PDSCH. © 2005 QUALCOMM Incorporated 2-21
  46. 46. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Lower Coding Gain (continued) UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-22 Data Rate with Rate 1 Turbo Coding and QPSK Modulation 2.4 Mbps X 3 = 7.2 Mbps Lower Coding Gain (continued) HSDPA allows the initial transmission of a data block to contain no parity bits, only systematic bits. Systematic bits are the original data bits that are input into the turbo encoder. Sending only systematic bits produces an effective code rate of 1, resulting in a data rate 3 times the previous example, or 7.2 Mbps. The HARQ procedure provides a mechanism for sending the parity bits in a later assignment if the UE is not able to decode the block using only systematic bits; however this will reduce the UE’s overall throughput. © 2005 QUALCOMM Incorporated 2-22
  47. 47. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B 16-QAM UMTS University HSDPA Protocols and Physical Layer Section 2-23 Data Rate with 16-QAM 7.2 Mbps X 2 = 14.4 Mbps 16-QAM HSDPA supports higher order modulation on HS-PDSCH. Where QPSK maps 2 bits onto one of 4 modulation symbols, 16-QAM maps 4 bits onto one of 16 modulation symbols. The resulting data rate is two times that of QPSK, or 14.4 Mbps. 16-QAM is more sensitive to both channel conditions and interference than QPSK, and therefore is only useful in very good channel conditions (e.g., close to the cell site, low speed). Support for 16-QAM is one factor in determining the UE’s HSDPA category. © 2005 QUALCOMM Incorporated 2-23
  48. 48. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Theoretical HSDPA Maximum Data Rate 80-W0331-1 Rev B HSDPA Protocols and Physical Layer UMTS University Section 2-24 Review: How do we get to 14.4 Mbps? • Multi-code transmission – Node B must allocate all 15 OVSF codes of length 16 to one UE • Consecutive assignments – Node B must allocate all time slots to one UE – UE must decode all transmissions correctly on the first transmission • Lower Coding Gain – Effective code rate = 1 – Requires very good channel conditions to decode • 16-QAM – Requires very good channel conditions Theoretical HSDPA Maximum Data Rate The following assumptions are needed to achieve the theoretical maximum data rate of 14.4 Mbps: Multi-code transmission – All 15 HS-PDSCH channels must be assigned to a single UE during one 2 ms TTI. This uses up a significant portion of the OVSF tree, leaving very few codes for non-HSDPA users and overhead channels. Consecutive assignments – The Node B must send back-to-back assignments to a single UE, and the UE must be able to correctly decode every block without requiring retransmission. Lower Coding Gain – Using an effective code rate of 1 increases the data rate, but the channel conditions must be very good for the UE to correctly decode every data block on the first transmission. 16-QAM – This modulation scheme works well only in very good channel conditions. In a practical scenario, the practical maximum data rate will be considerably less than 14.4 Mbps, due to less than ideal channel conditions, the need for retransmission, and the need to share the channel with other HSDPA users and Release 99 users. Other factors that reduce the practical maximum data rate will be discussed in subsequent slides. © 2005 QUALCOMM Incorporated 2-24
  49. 49. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Data Rate Quiz 2 UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-25 Calculate the data rate for one UE assuming: • 5 OVSF codes • Consecutive assignments • QPSK modulation • Turbo Code R = 1/3 • Retransmission statistics: – 75% of blocks decoded on first transmission – 25% of blocks decoded on second transmission Data Rate Quiz 2 Calculate the data rate achieved by one UE using the assumptions given above. © 2005 QUALCOMM Incorporated 2-25
  50. 50. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Data Rate Quiz 2 – Answer UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-26 Start from 14.4 Mbps and decrease • 5 code Multi-code – 14.4 Mbps / 3 = 4.8 Mbps • QPSK modulation – 4.8 Mbps / 2 = 2.4 Mbps • Turbo Code R = 1/3 – 2.4 Mbps / 3 = 800 kbps • Retransmission statistics – 800 kbps * 0.8 = 640 kbps Data Rate Quiz 2 – Answer Starting with the 14.4 Mbps figure, decrease the data rate for each of the stated assumptions: 5-code multi-code – Using 5 codes instead of 15 reduces the data rate by a factor of 3, to 4.8 Mbps. QPSK Modulation – Using QPSK instead of 16-QAM reduces the data rate by a factor of 2, to 2.4 Mbps. Turbo Code R = 1/3 – Using R = 1/3 decreases the data rate by a factor of 3, to 800 kbps. Retransmission Statistics – If every 4th block needs to be retransmitted, the data rate is reduced by 20%, to 640 kbps. © 2005 QUALCOMM Incorporated 2-26
  51. 51. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts More Data Rate Factors UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-27 More Factors that Affect Data Rate • Inter-TTI Interval • Retransmissions • ACK/NAK Repetition More Data Rate Factors Other factors that influence the maximum data rate are: Inter-TTI Interval – The interval between consecutive assignments is called the interTTI interval. If the UE supports an inter-TTI interval of 1, then it is capable of receiving a new HSDPA assignment every 2 ms. Allowed values of the inter-TTI interval are 1, 2, and 3. Retransmissions – If the UE NAKs a transmission, the Node B may retransmit that data in a subsequent assignment. The retransmission may consist of identical symbols that were sent previously, or may be a different redundancy version of the turbo coded output symbols. ACK/NAK Repetition – The Node B may configure the UE to send the ACK/NAK transmission up to four times. © 2005 QUALCOMM Incorporated 2-27
  52. 52. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Inter-TTI Interval UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-28 Inter-TTI Interval = 2 Inter-TTI Interval One parameter of the UE’s HSDPA capability is its inter-TTI interval. This parameter determines the interval between consecutive assignments that the UE is capable of decoding. Allowed values are 1, 2, and 3. The diagram above illustrates inter-TTI interval equal to 2. This reduces by half the maximum data rate achieved by the UE, all other parameters being equal. The UE’s signals its HSDPA capability to the Node B before beginning HSDPA operation, to allow the Node B to correctly schedule HSDPA assignments. © 2005 QUALCOMM Incorporated 2-28
  53. 53. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Retransmissions UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-29 Retransmissions If the UE is unable to decode an HSDPA data block, it sends a NAK 5 ms after the end of the received block. The Node B may choose to retransmit the data as early as the next HS-SCCH assignment following the NAK. The earliest a retransmitted block may be sent is 10 ms after the beginning subframe boundary of the previous transmission. The retransmitted block may be identical to the previous transmission, or it may be a different redundancy version. This means that a different combination of systematic and parity bits are sent. In either case, the UE retains the symbols from the first transmission and uses either Chase combining or incremental redundancy to increase the probability that the data will be decoded correctly on the 2nd attempt. Retransmissions decrease the data rate, as the retransmitted data occupies an interval that would otherwise be used to transmit new data. © 2005 QUALCOMM Incorporated 2-29
  54. 54. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts ACK/NAK Repetitions UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-30 ACK/NAK Repetitions The Node B may configure the UE to transmit the ACK/NAK up to four times, to increase the reliability of decoding the ACK/NAK. Using an ACK/NAK repetition greater than one has the same effect on data rate as the UE’s inter-TTI interval. In the example shown above, the Node B cannot transmit a data block to the same UE in subframe 2, because the ACK/NAK slot for that subframe is occupied by the repetition of the ACK/NAK corresponding to subframe 1. It may, of course, send data to a different UE in subframe 2. The Node B signals the ACK/NAK repetition rate to the UE before the UE begins HSDPA operation. © 2005 QUALCOMM Incorporated 2-30
  55. 55. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Node B Considerations UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-31 Node B Considerations • OVSF Code Allocation • Power Allocation • CQI Report Processing • Scheduler • HSDPA Cell Re-pointing Procedure • Compressed Mode Node B Considerations Most of the changes to support HSDPA on network side are implemented in the Node B. Things to consider are: OVSF Code Allocation – HSDPA uses OVSF codes of length 16. The number of HS-PDSCH codes allocated affects the number of other users that can be supported for Release 99 operations (including voice). Power Allocation – HSDPA channels may be allocated all the remaining transmit power on a 2 ms basis. CQI Report Processing – Node B uses the CQI reports from the UE to determine when to schedule the HSDPA channels and what data rate to use. Scheduler – The scheduler in the Node B must allocate the channels as a function of the number of HSDPA users in the cell, the channel conditions reported by each user, and available transmit power. HSDPA Cell Re-pointing Procedure – HSDPA channels do not operate in soft handover, but there is a mechanism to re-point the serving Node B to support the UE’s mobility. Compressed Mode – The Node B should not schedule an HSDPA assignment during a UE’s compressed mode gaps. © 2005 QUALCOMM Incorporated 2-31
  56. 56. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts OVSF Allocation UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-32 OVSF Allocation Each HS-PDSCH uses an OVSF of length 16, which blocks all codes above and below it in the OVSF code tree. Each HS-SCCH uses an OVSF of length 128. The illustration above shows a possible OVSF allocation if 15 HS-PDSCH codes are used and only 1 HS-SCCH. If only one HS-SCCH is used, then only one UE can operate in HSDPA during each 2 ms TTI. The overhead channels CPICH, PICH, AICH, and PCCPCH require codes of length 256. SCCPCH spreading factor is configurable, but SF = 128 is typical. Each HSDPA user requires a DPCH in addition to its high speed channel. The spreading factor of this channel is configurable. If voice users are supported in the same cell, they typically use codes with SF = 128. The conclusion to be drawn is that using 15 HS-PDSCH codes is not practical unless the cell is dedicated to HSDPA users. © 2005 QUALCOMM Incorporated 2-32
  57. 57. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Node B Transmit Power Allocation UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-33 Node B Transmit Power Allocation The Node B transmit power allocation algorithm is not specified by the standard, but two possible schemes are likely: Static – A fixed amount of power is allocated to the HS-PDSCHs and HS-SCCHs. Remaining power is distributed among common channels and power controlled dedicated channels. The overall transmit power fluctuates as a function of the power controlled channels. Dynamic – HS-PDSCH and HS-SCCH power is allocated dynamically as a function of the remaining available power, which fluctuates due to the power controlled dedicated channels. The overall transmit power of the cell remains constant. The above diagram does not consider the Node B’s power margin, whereby the Node B’s power fluctuates. The Node B power doesn’t really remain constant, due to the peak-to-average ratio of transmit power. © 2005 QUALCOMM Incorporated 2-33
  58. 58. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts CQI Report Processing UMTS University • • • 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-34 UE measures CPICH strength – Measurement reference period is 3 slots, ending 1 slot before CQI is sent UE reports index into CQI Table – Highest data rate for which UE can guarantee error rate < 10% Node B may filter CQI reports – Varying CQI means UE is in a fast changing environment – Steady CQI means UE is in a stable environment CQI Report Processing The Node B may use the UE’s CQI reports in its scheduling algorithm. The details of this is implementation dependent. When the UE is required to perform CQI reporting, the measurement reference period consists of 3 slots ending 1 slot before the CQI is sent. The value reported is an index into a table, where each row of the table maps to a combination of: Transport block size Number of HS-PDSCH codes Modulation Scheme (QPSK or 16-QAM) Reference power adjustment The CQI reported corresponds to the highest data rate that the UE can decode with an error rate less than 10%, assuming the channel conditions and transmit power stay at the same level as in the reference period. The reference power adjustment maps to a negative value when the channel conditions are so good that the UE can decode the highest data rate at a lower power level than is currently being used. © 2005 QUALCOMM Incorporated 2-34
  59. 59. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Node B Scheduler UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-35 Node B Scheduler The Node B scheduler is responsible for deciding how to allocate the available HSDPA channels and transmit power among users. The standard puts no requirements on this algorithm, leaving it entirely implementation dependent. Some possible schemes: Round Robin – Each user is allocated the channel in a fixed rotation. The scheme could be simple, or modified to account for CQI and/or user priorities. Proportional Fair – Each user sees a throughput proportional to the peak rate that its link can sustain. CQI Based – Channel is allocated to the user in the best radio condition. This scheme provides the highest cell throughput, though at the cost of not serving users in located in areas of poor coverage. Scheduling algorithms for systems such as HSDPA are the subject of much research and analysis in the wireless industry. © 2005 QUALCOMM Incorporated 2-35
  60. 60. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts HSDPA Cell Re-pointing Procedure UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-36 HSDPA Cell Re-pointing Procedure HSDPA channels do not operate in soft handover. For a given UE, the Node B from which it receives the HSDPA channels is called the Serving Node B. The UE may be in soft handover on the associated DPCH. If the radio conditions change such that there is a better cell on another Node B for HSDPA operations, the HSDPA Cell Re-pointing Procedure is performed. This procedure occurs independently from the Active Set update procedure. © 2005 QUALCOMM Incorporated 2-36
  61. 61. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Review Quiz 80-W0331-1 Rev B HSDPA Protocols and Physical Layer UMTS University Section 2-37 1. List the HSDPA channels and describe the purpose of each. 2. What is the fundamental time unit of HSDPA channels? 3. List five assumptions necessary to achieve the theoretical maximum data rate of 14.4 Mbps. 4. What effect does an inter-TTI interval of 3 have on the data rate? 5. What effect does an ACK/NAK repetition factor of 4 have on data rate? 6. What is the minimum interval between first transmission and retransmission of the data block? 7. List three parameters that may be used by the Node B scheduler to allocate the channel. 8. How many HS-SCCHs are needed if HSDPA channels are allocated to three users simultaneously? Notes © 2005 QUALCOMM Incorporated 2-37
  62. 62. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Review Quiz – Answers UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-38 1. List the HSDPA channels and describe the purpose of each. • • • • HS-DPCCH Uplink channel carries ACK/NAK and CQI report HS-SCCH Downlink channel assigns HSDPA channel to a user HS-DSCH Downlink transport channel that carries HSDPA data HS-PDSCH Downlink physical channel that carries HSDPA data 2. What is the fundamental time unit of HSDPA channels? • 2 ms 3. List five assumptions necessary to achieve the theoretical maximum data rate of 14.4 Mbps. • • • • • 15 OVSF codes allocated Consecutive assignment and inter-TTI interval = 1 Turbo code effective rate = 1 16-QAM All data decoded correctly on first transmission Notes © 2005 QUALCOMM Incorporated 2-38
  63. 63. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B Review Quiz – Answers UMTS University HSDPA Protocols and Physical Layer Section 2-39 4. What effect does an inter-TTI interval of 3 have on the data rate? • Reduces by a factor of 3 5. What effect does an ACK/NAK repetition factor of 4 have on data rate? • Reduces by a factor of 4 6. What is the minimum interval between first transmission and retransmission of the a data block? • 10 ms Notes © 2005 QUALCOMM Incorporated 2-39
  64. 64. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts Review Quiz – Answers UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-40 7. List three parameters that may be used by the Node B scheduler to allocate the channel. • • • CQI Report Code Tree Utilization User priority 8. How many HS-SCCHs are needed if HSDPA channels are allocated to three users simultaneously? • 3 Notes © 2005 QUALCOMM Incorporated 2-40
  65. 65. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts HSDPA Concepts – What We Learned UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 2-41 UMTS network architecture and protocols. HSDPA protocol stack. UMTS Release 99 channels. HSDPA channels. Theoretical maximum HSDPA data rate. Practical maximum HSDPA data rate. Node B enhancements to support HSDPA. Notes © 2005 QUALCOMM Incorporated 2-41
  66. 66. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 2: HSDPA Concepts 80-W0331-1 Rev B Comments/Notes © 2005 QUALCOMM Incorporated 2-42
  67. 67. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels UMTS University Section 3: Physical Layer Channels 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 3-1 SECTION 3 Physical Layer Channels Notes © 2005 QUALCOMM Incorporated 3-1
  68. 68. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels Section Learning Objectives UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 3-2 Identify HSDPA Physical Layer channels. Describe channel coding for individual channels. Understand the timing relation of HSDPA channels. Notes © 2005 QUALCOMM Incorporated 3-2
  69. 69. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels References UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 3-3 3GPP Release 5 Specification References 25.211 25.212 25.213 25.214 25.306 25.308 25.321 25.331 25.858 Physical channels and mapping of transport channels onto physical channels (FDD) Multiplexing and channel coding (FDD) Spreading and modulation (FDD) Physical layer procedures (FDD) UE Radio Access Capabilities HSDPA overall description stage 2 Medium Access Control (MAC) protocol specification Radio Resource Control (RRC) protocol specification HSDPA physical layer aspects Notes © 2005 QUALCOMM Incorporated 3-3
  70. 70. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels 80-W0331-1 Rev B UMTS University Node B PHY HS-DSCH HS-DSCH coding and modulation HS-PDSCH Section 3-4 UE PHY HS-DSCH … HSDPA Protocols and Physical Layer HSDPA Physical Layer Model – Downlink HS-DSCH decoding and demodulation HS-DSCH associated control signaling HS-PDSCH HS-SCCH … HS-PDSCH HS-DSCH associated control information HS-PDSCH HS-SCCH HS-PDSCH – carries actual information payload from HS-DSCH HS-SCCH – carries Physical Layer control information including HARQ parameters, channelization codes, and UE ID HSDPA Physical Layer Model – Downlink In 3GPP Release 5, two new Downlink physical channels have been introduced to enable HSDPA. In addition, the existing R99 channels are also required for HSDPA operation. HS-PDSCH – Transmitted by Node B to send HS-DSCH data to UEs in the HSDPA serving cell. Unlike a dedicated channel, this shared channel is assigned to a user for a 2 ms period and may be assigned to another user in the next 2 ms period. This fast user switching rate is well suited for the bursty packet data and helps increase the capacity of a cell. There can be multiple (up to 15) HS-PDSCHs in a serving cell, which enables use of both time division and code division multiple access methods. HS-PDSCH carries user data and has a transport channel HSDSCH mapped on it. HS-SCCH – Transmitted by Node B to signal control information to the users in the HSDPA serving cell. This channel is shared by multiple users and the control information sent on it is masked with a UE ID. The mask allows a UE to identify if there is HS-DSCH data for it in the upcoming HS-PDSCH subframe and the control information tells how to decode that data. The control information is transmitted in two parts: Part 1 consists of HS-PDSCH channelization codes and modulation scheme Part 2 consists of HARQ parameters and HS-DSCH transport block size HS-SCCH does not have a transport channel mapped on it. © 2005 QUALCOMM Incorporated 3-4
  71. 71. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels 80-W0331-1 Rev B UMTS University Node B PHY Section 3-5 UE PHY DCH DCH DCH decoding and demodulation DCH coding and modulation HARQ and channel quality feedback information UL DPCH HSDPA Protocols and Physical Layer HSDPA Physical Layer Model – Uplink HS-DPCCH HARQ and channel quality feedback signaling UL DPCH HS-DPCCH HS-DPCCH – carries feedback signaling consisting of HARQ acknowledgement and channel quality indicator (CQI) HSDPA Physical Layer Model – Uplink In 3GPP Release 5, there is one new Uplink physical channel. The existing R99 channels are required for the HSDPA operation. HS-DPCCH – Transmitted by the UE to signal feedback information to Node B. The feedback information consists of: acknowledgement of data received by the UE on HS-PDSCH Downlink channel quality indicator (CQI) Node B uses this feedback information to send retransmissions and to schedule HS-PDSCH transmissions to UEs. HS-DPCCH doesn’t carry any higher layer control or traffic and doesn’t have a transport channel mapped on it. © 2005 QUALCOMM Incorporated 3-5
  72. 72. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels UMTS University Physical Layer Frame Timing 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 3-6 Frame Timing • 10 ms radio frame, 15 slots • 2 ms HSDPA subframe, 3 slots – 1 HS-DSCH Transmission Time Interval (TTI) Slot Timing • 2560 chips per slot, 0.67 ms – 7680 chips per HSDPA subframe Symbol Timing • QPSK: 2 bits per symbol • 16-QAM: 4 bits per symbol • OVSF spreads symbol to chips Physical Layer Frame Timing A basic WCDMA radio frame is 10 ms long and has 15 slots. HSDPA introduces the notion of subframes within a WCDMA radio frame. An HSDPA subframe is 2 ms (3 slots) long and all the HS-channels use this subframe timing. The subframe allows fast user switching where the shared channel can potentially be assigned to a different user every subframe. As the HSDPA subframe is only 2 ms long, it alleviates the need for power control. HS-DSCH has a fixed TTI of 2 ms. Each HS-DSCH transport block is mapped to an HS-PDSCH subframe. HS-SCCH and HSDPCCH also use the 2 ms subframe to transmit control and feedback respectively. Each HSDPA subframe has 3 slots and each slot is comprised of symbols. The number of symbols in a slot depends on the spreading factor used for that channel. HS-PDSCH, HS-SCCH, and HS-DPCCH use SF 16, 128, and 256 respectively, giving number of symbols per slot as 160 (HS-PDSCH), 20 (HS-SCCH), and 10 (HS-DPCCH). A symbol is made up of 1 or more bits and each bit is spread using SF to an equivalent number of chips. A QPSK symbol consists of two consecutive bits, one bit each mapped onto the I and Q branch. A 16-QAM symbol, on the other hand, has four consecutive bits with two bits on each branch. © 2005 QUALCOMM Incorporated 3-6
  73. 73. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels UMTS University Downlink HS-PDSCH 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 3-7 High Speed Physical Downlink Shared Channel (HS-PDSCH) • • • • Fixed spreading factor SF 16 with 2 slot formats Up to 15 HS-PDSCHs under a cell May use QPSK or 16-QAM modulation scheme Node B responsible for transmitting HS-PDSCH at reasonable power DL HS-PDSCH – High Speed Physical Downlink Shared Channel An HS-PDSCH channel carries the actual user payload to the UE. One HS-PDSCH subframe contains one TTI (2 ms) of HS-DSCH transport channel payload. There is no transport channel multiplexing in HSDPA so the information contained in HS-PDSCH subframe is from a single HS-DSCH transport channel. An HS-DSCH serving cell can have as many as 15 channelization codes assigned to HS-PDCH. The HS-PDSCH channels are shared among different users by using time division, code division, or a combination of the two multiple access methods. The number of HS-PDSCHs that can be simultaneously decoded by a UE depends on the HS-DSCH UE Category. The HS-PDSCH power control depends on the Node B’s implementation and is not specified by standards. The UE may assume that the power is kept constant during the corresponding HS-PDSCH subframe. If multiple HS-PDSCHs are allocated to one UE, all the HS-PDSCHs intended for that UE shall be transmitted with equal power. The phase reference used for demodulating HS-PDSCH is the same as for the associated DL DPCH. By default, P-CPICH is used as the phase reference. UE is informed through higher layer signaling if S-CPICH or dedicated Pilot is to be used as the phase reference. © 2005 QUALCOMM Incorporated 3-7
  74. 74. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels UMTS University Release 99 Channel 80-W0331-1 Rev B HS-DSCH Channel Coding and Physical Channel Mapping HSDPA Protocols and Physical Layer Section 3-8 Release 5 HS-DSCH New Physical Layer blocks in Release 5: • Bit Scrambling • Physical Layer HARQ functionality • HS-DSCH Interleaving • Constellation re-arrangement for 16-QAM HS-DSCH Channel Coding and Physical Channel Mapping Channel coding is done to add robustness to the information bits. Usually, channel coding is performed by adding redundant bits determined by an FEC coding scheme such as CRC, convolutional coding, turbo coding, etc. The HS-DSCH channel coding involves a number of other functions performed by the Node B’s Physical Layer. The main reason for this additional processing is the dynamic size of the transport block transmitted in an HS-DSCH TTI. Other reasons include large HS-DSCH payload size and the possible use of 16-QAM modulation for HS-PDSCH. Comparing the coding chain for the Release 99 channel with the Release 5 HS-DSCH channel, some blocks have been removed and some new blocks have been added. HS-DSCH coding chain does not require: 1. Concatenation, because there is always only one transport block per HS-DSCH TTI. The transport block size, however, varies from 137 bits to 27952 bits. In case of retransmission, the transport block size remains the same as of the original transmission. 2. First DTX insertion, because HS-DSCH doesn’t support fixed position transport channel and thus Blind Transport Format Detection (BTFD). 3. Second DTX insertion, because there is just one transport channel mapped on to HS-PDSCH. 4. Radio frame segmentation, because HS-DSCH has a fixed TTI of 2 ms, which is equal to the HS-PDSCH subframe duration. 5. Transport channel multiplexing, because there is just one transport channel mapped on to HS-PDSCH. © 2005 QUALCOMM Incorporated 3-8
  75. 75. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels UMTS University HS-DSCH Channel Coding 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 3-9 MAC delivers 1 HS-DSCH Transport Block per TTI to Physical Layer CRC Attachment • 24-bit CRC is added per Transport Block Bit Scrambling • Facilitates uniform distribution of 16-QAM symbols at receiver Code Block Segmentation • • FEC encoder has a fixed maximum code block size of 5114 bits If bit scrambled data is more than 5114 bits, need to segment into equal code blocks HS-DSCH Channel Coding Node B’s MAC-hs delivers the HS-DSCH transport channel data to the Physical Layer in Node B. The Physical Layer then performs a number of functions on the HS-DSCH TTI data before the data is finally mapped to one or more HS-PDSCH physical channels. CRC Attachment – A fixed 24-bit Cyclic Redundancy Check (CRC) is attached to HS-DSCH TTI data. There is only one transport block per HS-DSCH TTI. Bit Scrambling – Done to avoid non-uniform symbol distribution over 16-QAM constellation at the receiver. A uniform symbol distribution helps the UE efficiently decode the received HS-DSCH bits. Typically, the received symbols are uniformly distributed over the entire constellation. However, certain degenerate HS-DSCH bit sequences (e.g., the all-ones or all-zeroes sequences) could violate this condition, leading to an asymmetric HS-DSCH bit distribution (over {0,1}) and hence a non-uniform 16-QAM symbol distribution at the receiver input. This is true regardless of the use of turbo-encoding on the HS-DSCH, due to the possibility of transmitting turbo-codewords comprised predominantly of systematic bits. The estimated performance loss due to the non-uniform distribution in such very unlikely cases is between 1.0-1.5 dB. Code Block Segmentation – It is done if the number of bits output from the bit scrambler is more than the maximum input code block size of the FEC encoder. The maximum encoder code block size in case of HS-DSCH is 5114 bits. If segmentation is performed, all the resulting segments are of equal size and may require adding some filler bits to the beginning of 1st code block. The filler bits are all 0s and are transmitted along with data. © 2005 QUALCOMM Incorporated 3-9
  76. 76. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels 80-W0331-1 Rev B HS-DSCH Channel Coding (continued) UMTS University HSDPA Protocols and Physical Layer Section 3-10 FEC Coding • • • • Rate 1/3 Turbo Coder used for Forward Error Correction (FEC) Tail bits are padded to perform trellis termination Effective code rate changes after HARQ Code blocks from the same transport block are concatenated after encoding. Code Blk Turbo Coder (r = 1/3) Parity 1 bits Parity 2 bits Systematic bits Serial concatenation of encoded blocks HARQ HS-DSCH Channel Coding (continued) FEC Coding – Rate 1/3 turbo coder is used for encoding HS-DSCH bits. FEC coding is done on one or more code blocks, where code blocks are formed by segmenting bit scrambled HS-DSCH data (if more than 5114 bits). The minimum HS-DSCH transport channel data rate is 68.5 kbps, which makes turbo coding a better choice than convolutional coding. The turbo coder uses two parallel concatenated convolutional coders, each with constraint length K = 4. The output from turbo coder consists of Systematic bits (original input data bits) and Parity bits. For each input bit, there is 1 Systematic bit and 2 Parity bits. Twelve tail bits are added per block after encoding for the trellis termination. The encoded blocks, when more than one, are serially concatenated and fed to the HARQ block. The code rate after turbo encoding is 1/3 but the effective coder rate after HARQ rate matching may be different. An effective code rate of close to 1 is required to achieve peak throughput of 14.4 kbps. © 2005 QUALCOMM Incorporated 3-10
  77. 77. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels UMTS University 80-W0331-1 Rev B HS-DSCH Channel Coding (continued) HSDPA Protocols and Physical Layer Section 3-11 Hybrid ARQ (HARQ) • • • • Combines ARQ with adaptive coding (FEC) Transmitter sends new set of parity bits if the previous transmission failed (NAK’d) Receiver buffers the failed decodes for soft combining with future retransmissions Soft combining is done before each FEC decoding attempt Serial concatenation of encoded blocks HARQ To Physical Channel Segmentation HS-DSCH Channel Coding (continued) Hybrid ARQ (HARQ) – HARQ is a technique combining FEC and ARQ methods that save information from previous failed decode attempts to be used in the future decoding. There are two different HARQ schemes, Chase and IR, depending on which bits are chosen to be sent over the air to UE. The redundancy version (RV) parameters, r and s, indicate to the UE the HARQ scheme used for the current transmission. Both HARQ combining schemes soft combine bits from the previous failed decodes with the currently received retransmission. Soft combining helps minimize the number of retransmissions. For a retransmission, HARQ uses the same transport block size and consequently the same number of HS-DSCH bits that were used in the initial transmission. However, it may use a different modulation scheme, channelization code set (including the size of the channelization code set), or transmission power. Thus, HARQ implicitly provides a link adaptation technique called Adaptive Modulation and Coding (AMC). The number of physical channel bits available for a retransmission may differ from that of the previous transmission. © 2005 QUALCOMM Incorporated 3-11
  78. 78. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels UMTS University 80-W0331-1 Rev B HS-DSCH Channel Coding – Physical Layer HARQ Functionality HSDPA Protocols and Physical Layer Section 3-12 Physical Layer HARQ consists of two rate matching stages and a virtual buffer • 1st stage: matches number of input bits to the virtual IR buffer size – IR buffer size is determined by UE’s soft memory capability – Puncturing is done if inputs bits exceed the virtual IR buffer size • 2nd stage: matches numbers of bits to the number of HS-PDSCH bits in the given TTI – Redundancy Version (RV) parameters control the output from 2nd stage – Repetition or puncturing is done to perform 2nd stage rate matching RV Parameters HS-DSCH Channel Coding – Physical Layer HARQ Functionality The Physical Layer HARQ functionality in Node B matches the number of bits at the output of the channel coder to the total number of HS-PDSCH physical channel bits in a subframe. It consists of two rate matching stages and a virtual IR buffer. 1st rate matching stage matches the number of input bits to the virtual Incremental Redundancy (IR) buffer size, information about which is provided by the higher layers. The systematic bits remain untouched in this stage but some parity bits (P1 and P2) may be punctured if the total number of input bits (including S1, P1, P2) is more than the virtual IR buffer size. 2nd rate matching stage matches the number of bits after the 1st rate matching stage to the number of available HS-PDSCH bits. Given a fixed number of input and output bits for the 2nd rate matching stage, the exact set of output bits depends on the RV parameters s and r. Either puncturing or repetition is performed to accomplish the 2nd stage rate matching. © 2005 QUALCOMM Incorporated 3-12
  79. 79. WCDMA (UMTS) HSDPA: Protocols and Physical Layer Section 3: Physical Layer Channels HS-DSCH Channel Coding – HARQ Combining Schemes UMTS University 80-W0331-1 Rev B HSDPA Protocols and Physical Layer Section 3-13 First Transmission • Always self-decodable, RV parameters s = 1 Chase Combining • Each retransmission is self decodable, RV parameter s = 1 – Systematic bits are prioritized • Same coded data packet may be sent in each retransmission • Retransmission with a different r value implies different set of punctured bits Receiver attempts to decode by soft combining multiple copies – Using the same RV parameter r in each retransmission • Incremental Redundancy (IR) • Retransmissions are not self decodable, RV parameter s = 0 – Parity bits are prioritized • • Redundant information is incrementally transmitted if initial decoding fails Each retransmission provides additional redundant bits to the receiver – RV parameter r is different for different set of redundancy bits • Receiver attempts to decode based on accumulated bits HS-DSCH Channel Coding – HARQ Combining Schemes HARQ combining refers to the combining of the HS-DSCH soft bits in the receiver (UE). If an HS-DSCH subframe transmission is not correctly decoded (CRC failure) by the UE’s Physical Layer the soft bits from this failed decode are buffered in the IR buffer to be combined with the soft bits from the future retransmissions. This type of combining changes the effective received code rate with each retransmission and helps in minimizing the number of retransmissions. There are different types of HARQ combining schemes: Chase combining (also called HARQ Type III) requires each retransmission to be selfdecodable. The transmitter may retransmit the same coded data packet in which case the decoder at the receiver combines multiple copies of the same transmitted packet weighted by the received SNR. Time diversity gain is thus obtained. Using a different redundancy version parameter r, a different set of puncture bits can be used in each retransmission. Incremental Redundancy (IR) (also called H-ARQ Type II) is another implementation of the HARQ technique where retransmissions are not self decodable, i.e., they may have a very low proportion (or none) of the systematic bits. Additional redundant information, prioritizing the parity bits, is incrementally transmitted if the decoding fails on the prior attempt. Retransmitted subframes are soft combined with the buffered soft bits to achieve additional coding gain, which helps the UE to successfully decode the subframe. The Node B’s proprietary algorithm in MAC-hs determines which HARQ scheme to use to transmit an HS-DSCH subframe. The RV parameter signaled to the UE indicates the HARQ scheme used, allowing the UE to use the same scheme for HARQ combining. © 2005 QUALCOMM Incorporated 3-13

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