High-Speed Downlink Packet Access (HSDPA)

Transcription

1 High-Speed Downlink Packet Access (HSDPA) HSDPA Background & Basics Principles: Adaptive Modulation, Coding, HARQ Channels/ UTRAN Architecture Principles: Fast scheduling, Mobility Performance Results

2 Motivation (as of 2000) Voice, low speed packet data GSM/GPRS No Multimedia, Limited QOS Medium rate Packet data UMTS Rel. 99 Theoretical 2 Mbps but ~384 kbps subjected to practical constraints As the UMTS networks are rolled out, the demand for high bandwidth services is expected to grow rapidly. By 2010, 66% of the revenues will come from data services (source: UMTS forum). Release 99/4 systems alone will not be capable to meet these demands. (Realistic outdoor data rates will be limited to 384kbps). A more spectral efficient way of using DL resources is required. Competition with CDMA x EV-DO/DV 2

3 HSDPA Background Initial goals Establish a more spectral efficient way of using DL resources providing data rates beyond 2 Mbit/s, (up to a maximum theoretical limit of 14.4 Mbps) Optimize interactive & background packet data traffic, support streaming service Design for low mobility environment, but not restricted Techniques compatible with advanced multi-antenna and receivers Standardization started in June 2000 Broad forum of companies Major feature of Release 5 Enhancements in R7 HSPA+ Advanced transmission to increase data throughput Signaling enhancements to save resources 3

4 HSDPA Basics Evolution from R99/ R4 5MHz BW Same spreading by OVSF and scrambling codes Turbo coding New concepts in R5 Adaptive modulation (QPSK vs. 16QAM), coding and multicodes (fixed SF = 16) Fast scheduling in NodeB (TTI = 2ms) Hybrid ARQ Enhancements in R7 HSPA+ Signaling enhancements 64QAM MIMO techniques, increase of the bandwidth 4

5 Higher Order Modulation Standard modulation scheme in UMTS networks QPSK 2 bit per symbol With HSDPA, modulation can be switched between two schemes QPSK 2 bit per symbol 16-QAM 4 bit per symbol Low bitrate robust High bitrate Sensitive to disturbances 5

6 Key Principles Adaptive Modulation and Coding (Mother Turbo code rate = 1/3) For wireless data, link adaptation through Rate Control is more effective then Power Control. Users in favorable channel conditions (based on Channel Quality indication) are assigned higher code rates and higher order modulation (16QAM). This means higher data rates = Reduced latency But what about when channel is changing at high rate; Can AMC guarantee reliability? 6

7 Hybrid ARQ No. In fast fading conditions, AMC alone is not enough. H-ARQ automatically adapts to instantaneous channel conditions by: fast retransmissions at physical layer adding redundancy only when needed The retransmitted packets are combined with original packet to improve the decoding probability. Simple form of Hybrid ARQ shows significant gains over link adaptation alone. Different schemes can be used for retransmission of original data packet. Chase combining Incremental Redundancy 7

8 Fast Scheduling Fading is good in multiuser environment!! Channels are uncorrelated Multi-user diversity Assign the resources to the best user(s) in time to maximise throughput Gains increase with number of users Max C/I Proportional fair Round Robin With HSDPA Scheduling function is moved from RNC to Node-B. 10 8

9 HS-DSCH Principle I Channelization codes at a fixed spreading factor of SF = 16 Up to 15 codes in parallel SF=2 SF=4 SF=8 SF=16 C 16,15 Physical channels (codes) to which HS-DSCH is mapped C 16,0 CPICH, etc. OVSF channelization code tree allocated by CRNC HSDPA codes autonomously managed by NodeB MAC-hs scheduler Example: 12 consecutive codes reserved for HS-DSCH, starting at C 16,4 Additionally, HS-SCCH codes with SF = 128 (number equal to simult. UE) 9

10 HS-DSCH Principle II Resource sharing in code as well as time domain: Multi-code transmission, UE is assigned to multiple codes in the same TTI Multiple UEs may be assigned channelization codes in the same TTI Code Time (per TTI) Data to UE #1 Data to UE #2 Data to UE #3 not used Example: 5 codes are reserved for HSDPA, 1 or 2 UEs are active within one TTI 10

11 UMTS Channels with HSDPA Cell 1 = Serving HS-DSCH cell UE Cell 2 Rel-5 HS-DSCH DL PS service (Rel-6: DL DCCH) R99 DCH (in SHO) UL/DL signalling (DCCH) UL PS service UL/DL CS voice/ data 11

12 HSDPA Channels HS-PDSCH Carries the data traffic Fixed SF = 16; up to 15 parallel channels QPSK: 480 kbps/code, 16QAM: 960 kbps/code HS-SCCH Signals the configuration to be used next: HS-PDSCH codes, modulation format, TB information Fixed SF = 128 Sent two slots (~1.3msec) in advance of HS-PDSCH HS-DPCCH Feedbacks ACK/NACK and channel quality information (CQI) Fixed SF = 256, code multiplexed to UL DPCCH Feedback sent ~5msec after received data 12

13 Timing Relations (DL) T slot (2560 chips) Downlink DPCH 3 T slot (2 msec) HS-SCCH ch. code & mod TB size & HARQ Info HS-DSCH TTI = 3 T slot (2 msec) HS-PDSCH DATA HS-DSCH-control = 2 T slot NodeB Tx view Fixed time offset between the HS-SCCH information and the start of the corresponding HS-DSCH TTI: HS-DSCH-control (2 T slot = 1.33msec) HS-DSCH and associated DL DPCH not time-aligned 13

14 Timing Relations (UL) T slot (0.67 ms) Uplink DPCCH 3 T slot (2ms) HS-PDSCH DATA UEP = 7.5 T slot (5ms) chips HS-DPCCH CQI A/N CQI A/N CQI A/N CQI A/N m 256 chips UE Rx view Alignment to m 256 to preserve orthogonality to UL DPCCH HS-PDSCH and associated UL DPCH not time-aligned (but quasi synch ) 14

15 HSDPA Architecture Evolution from R99/R4 SRNC RRC PDCP HSDPA functionality is intended for transport of dedicated logical channels Logical Channels DCCH DTCH RLC BCCH Takes into account the impact on R.99 networks MAC-d DCH HSDPA in R5 Additions in RRC to handle HSDPA RLC nearly unchanged (UM & AM) Modified MAC-d with link to MAC-hs entity CRNC NodeB w/o MAC-c/sh Transport Channels MAC-hs HS-DSCH Upper phy MAC-c/sh DSCH FACH MAC-b BCH New MAC-hs entity located in the Node B 15

16 MAC-hs in NodeB MAC-d flows MAC-hs UE #1 UE #2 Priority Queue UE #N Priority Queue distribution Priority Queue Priority Queue Scheduling MAC Control MAC-hs Functions Priority handling Flow Control To RNC To UE Scheduling Select which user/queue to transmit Assign TFRC & Tx power HARQ handling Service measurements e.g. HSDPA provided bitrate HS-DSCH TFRC: Transport Format and Resource Combination 16

17 MAC-hs in UE To MAC-d MAC-hs Disassembly Disassembly MAC Control MAC-hs Functions HARQ handling ACK/ NACK generation Reordering Reordering Reordering buffer handling Re-ordering queue distribution HARQ Associated to priority queues Flow control per reordering buffer HS-DSCH Memory can be shared with AM RLC Associated Downlink Signalling HS-SCCH Associated Uplink Signalling HS-DPCCH Disassembly unit 17

18 Data Flow through Layer 2 Higher Layer PDU Higher Layer PDU Higher Layer Reassembly RLC SDU RLC SDU Segmentation & Concatenation RLC header RLC header L2 RLC (non-transparent) MAC-d header MAC-d SDU MAC-d header MAC-d SDU L2 MAC-d MAC-d PDU MAC-d PDU (non-transparent) MAC-hs header MAC-hs SDU MAC-hs SDU L2 MAC-hs (non-transparent) Transport Block (MAC-hs PDU) CRC L1 18

19 Hybrid Automatic Repeat Request HARQ is a stop-and-wait ARQ Up to 8 HARQ processes per UE Retransmissions are done at MAC-hs layer, i.e. in the Node B Triggered by NACKs sent on the HS-DPCCH The mother code is a R = 1/3 Turbo code Code rate adaptation done via rate matching, i.e. by puncturing and repeating bits of the encoded data Two types of retransmission Incremental Redundancy Additional parity bits are sent when decoding errors occured Gain due to reducing the code rate Chase Combining The same bits are retransmitted when decoding errors occured Gain due to maximum ratio combining HSDPA uses a mixture of both types 19

20 HARQ Processes RTT HARQ Data HS-PDSCH ACK/NACK HS-DPCCH HARQ is a simple stop-and-wait ARQ Example RTT min = 5 TTI Synchronous retransmissions (MAC-hs decides on transmission) UE support up to 8 HARQ processes (configured by NodeB) Min. number: to support continuous reception Max. number: limit of HARQ soft buffer Number of HARQ processes configured specifically for each UE category 20

21 HSDPA UE Categories The specification allows some freedom to the UE vendors 12 different UE categories for HSDPA with different capabilities (Rel.5) The UE capabilities differ in Max. transport block size (data rate) Max. number of codes per HS-DSCH Modulation alphabet (QPSK only) Inter TTI distance (no decoding of HS-DSCH in each TTI) Soft buffer size The MAC-hs scheduler needs to take these restrictions into account 21

22 HSDPA UE Physical Layer Capabilities HS-DSCH Category Maximum number of HS-DSCH multi-codes Minimum inter- TTI interval Maximum MAC-hs TB size Total number of soft channel bits Theoretical maximum data rate (Mbit/s) Category Category Category Category Category Category Category Category Category Category Category 11* Category 12* Note: UEs of Categories 11 and 12 support QPSK only cf. TS

23 Channel Quality Information (CQI) Signalled to the Node B in UL each 2ms on HS-DPCCH Integer number from 0 to 30 corresponds to a Transport Format Resource Combination (TFRC) given by Modulation Number of channelisation codes Transport block size For the given conditions the BLER for this TFRC shall not exceed 10% Mapping defined in TS for each UE category 23

24 CQI Mapping Table CQI value Transport Block Size Number of HS-PDSCH Modulation Reference power adjustment 0 N/A Out of range QPSK QPSK QPSK 0 NIR XRV Tables specified in TS For each UE category Condition: BLER 10% Example for UE category QPSK QAM QAM QAM QAM QAM QAM QAM QAM QAM 0 24

25 HSDPA Fast Scheduling 3G (Rel.99) with dedicated channels Note: No fast channel quality feedback 3G with high speed feedback/scheduling on shared channels k 64k 64k CQI CQI CQI C/I C/I C/I 25

26 Scheduler Inputs QoS & Subscriber Profile QoS: guar. bitrate, max. delay GoS: gold/ silver/ bronce History How long had the user been waiting? Traffic Model Morning Afternoon Evening Off peak Feedback from UL (CQI, ACK/NACK) Scheduler UE capability Buffer Status Radio resources Power, OVSF codes Scheduled Users & Packet Formation Strategy 26

27 Packet Formation Strategy Scheduler Outputs Selected User Adaptive Transport Block size Adaptive Coding or redundancy Adaptive Modulation (QPSK, 16 QAM) # of OVSF codes So that QoS/GoS constraints are satisfied and Network throughput is maximized, while Subject to constraints (standards restrictions and service requirements) Maintain fairness across UEs and traffic streams 27

28 Classical Scheduling Disciplines HSDPA scheduler runs every TTI (2 msec) Round Robin: allocate the users consecutively Advantage: - Offers fair time allocation - One of the simplest solutions Disadvantage: - Low cell and user throughput Best Effort scheduler: prefer the users with good channel conditions Advantage: - Highest system throughput and easy to implement Disadvantage: - Starvation to users with low C/I Proportional Fairness: equalise the channel rate / throughput ratio Advantage: - Higher throughput than Round Robin Disadvantage: - Does not use QoS information 28

29 Comparison of Schedulers user perceived throughput aggregated cell throughput 100% 2500 Round Robin Percentage of users receiving throughput 80% 60% 40% 20% Proportional Fair QoS aw are average throughput [kbps] % average throughput [kbps] 0 Round Robin Proportional Fair QoS aw are Simple Round Robin doesn t care about actual channel rate Proportional Fair offers higher cell throughput QoS aware algorithm offers significantly higher user perceived throughput than PF with similar cell throughput 29

30 Mobility Procedures I HS-DSCH for a given UE belongs to only one of the radio links assigned to the UE (serving HS-DSCH cell) The UE uses soft handover for the uplink, the downlink DCCH and any simultaneous CS voice or data Using existing triggers and procedures for the active set update (events 1A, 1B, 1C) Hard handover for the HS-DSCH, i.e. Change of Serving HS-DSCH Cell within active set Using RRC procedures, which are triggered by event 1D 30

31 Mobility Procedures II CRNC CRNC Source HS- DSCH Node B Target HS- DSCH Node B MAC-hs MAC-hs NodeB NodeB NodeB NodeB s t Serving HS-DSCH radio link Serving HS-DSCH radio link Inter-Node B serving HS-DSCH cell change Note: MAC-hs needs to be transferred to new NodeB! 31

32 HS-DSCH Serving Cell Change Measurement quantity CPICH 1 Hysteresis CPICH 2 CPICH3 Time to trigger Reporting event 1D Time Event 1D: change of best cell within the active set Hysteresis and time to trigger to avoid ping-pong (HS-DSCH: 1 2 db, 0.5 sec) 32

33 Handover Procedure UE Target HS-DSCH cell Source HS-DSCH cell SRNC = DRNC RL Reconfiguration Prepare RL Reconfiguration Ready ALCAP Iub HS-DSCH Data Transport Bearer Setup Serving HS-DSCH cell change decision i.e. event 1D If new NodeB RL Reconfiguration Prepare Radio Bearer Reconfiguration RL Reconfiguration Commit RL Reconfiguration Ready RL Reconfiguration Commit Synchronous Reconfiguration with Tactivation Radio Bearer Reconfiguration Complete Reset MAChs entity DATA ALCAP Iub HS-DSCH Data Transport Bearer Release Example: HS-DSCH hard handover (synchronized serving cell change) 33

34 HSDPA Managed Resources a) OVSF Code Tree Border adjusted by CRNC SF=2 SF=4 SF=8 SF=16 C 16,15 Codes reserved for HS-PDSCH/ HS-SCCH b) Transmit Power Border adjusted by CRNC Codes available for DCH/ common channels C 16,0 Tx power available for HS-PDSCH/ HS-SCCH Tx power available for DCH/ common channels Note: CRNC assigns resources to Node B on a cell basis 34

35 Cell and User Throughput vs. Load Throughput [kbit/sec] Mean User Throughput Aggregated Cell Throughput Load Impact Number of Users/ Cell 36 cells network UMTS composite channel model FTP traffic model (2 Mbyte download, 30 sec thinking time) The user throughput is decreased when increasing load due to the reduced service time The cell throughput increases with the load because overall more bytes are transferred in the same time 35

36 HSDPA Performance per Category Mean User Throughput Peak User Throughput Aggregated Cell Throughput Cat 6 - Cat 8 Comparison 36 cells network UMTS composite channel model FTP traffic model (2 Mbyte download, 30 sec thinking time) throughput (kbps) Higher category offers higher max. throughput limit Cat.6: 3.6 MBit/sec Cat.8: 7.2 MBit/sec Cat 6/ 10 users Cat 8/ 10 users Cat 6/ 20 users Cat 8/ 20 users Max. user perceived performance increased at low loading Cell performance slightly better 36

37 Coverage Prediction with HSDPA Example Scenario 15 users/cell Pedestrian A channel model Plot generated with field prediction tool HSDPA Throughput depends on location 37

38 HSDPA Summary New downlink transmission concept Optimised for interactive & background, support of streaming Design for indoor & urban environment Improved PHY approach New DL transport channel: HS-DSCH Additional signalling channels to support fast adaptation Advanced architecture MAC-hs entity located in NodeB Radio Resource Control procedures similar to DCH HSDPA Resource Management Cell resources managed by Controlling-RNC Re-use of principles for DCH control (handover, state transition) Significant improved performance 38

39 HSDPA References Papers: Arnab Das et al: Evolution of UMTS Toward High-Speed Downlink Packet Access, Bell Labs Technical Journal, vol. 7, no. 3, pp , June 2003 A. Toskala et al: High-speed Downlink Packet Access, Chapter 12 in Holma/ Toskala: WCDMA for UMTS, Wiley 2010 T. Kolding et al: High Speed Downlink Packet Access: WCDMA Evolution, IEEE Veh. Techn. Society News, pp. 4 10, February 2003 H. Holma/ A. Toskala (Ed.): HSDPA/ HSUPA for UMTS, Wiley 2006 Standards TS 25.xxx series: RAN Aspects TR HSDPA PHY Aspects TR HSDPA: UTRAN Overall Description (Stage 2) TR Iub/Iur protocol aspects 39

40 Abbreviations ACK (positive) Acknowledgement IE Information Element ALCAP AM AMC CAC CDMA CQI DBC DCH DPCCH FDD FEC FIFO GoS HARQ H-RNTI HSDPA HS-DPCCH HS-DSCH HS-PDSCH HS-SCCH Access Link Control Application Protocol Acknowledged (RLC) Mode Adaptive Modulation & Coding Call Admission Control Code Division Multiple Access Channel Quality Information Dynamic Bearer Control Dedicated Channel Dedicated Physical Control Channel Frequency Division Duplex Forward Error Correction First In First Out Grade of Service Hybrid Automatic Repeat Request HSDPA Radio Network Temporary Identifier High Speed Downlink Packet Access High Speed Dedicated Physical Control Channel High Speed Downlink Shared Channel High Speed Physical Downlink Shared Channel High Speed Signaling Control Channel MAC-d MAC-hs Mux NACK NBAP OVSF PDU PHY QoS QPSK RB RL RLC RRC RRM SDU SF TB TFRC TFRI TTI UM 16QAM dedicated Medium Access Control high-speed Medium Access Control Multiplexing Negative Acknowledgement NodeB Application Part Orthogonal Variable SF (code) Protocol Data Unit Physical Layer Quality of Service Quadrature Phase Shift Keying Radio Bearer Radio Link Radio Link Control Radio Resource Control Radio Resource Management Service Data Unit Spreading Factor Transport Block Transport Format & Resource Combination TFRC Indicator Transmission Time Interval Unacknowledged (RLC) Mode 16 (state) Quadrature Amplitude Modulation 40