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Outline. UMTS architecture and main features (FDD mode) Discussion of packet performance issues Present concepts for support of packet-switched services UMTS evolution Conclusions. On the Performance of UMTS Packet Data Services. Wolfgang Granzow Ericsson Research Corporate Unit

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  1. Outline • UMTS architecture and main features (FDD mode) • Discussion of packet performance issues • Present concepts for support of packet-switched services • UMTS evolution • Conclusions

  2. On the Performance of UMTS Packet Data Services Wolfgang Granzow Ericsson Research Corporate Unit Ericsson Eurolab Deutschland, Nürnberg (wolfgang.granzow@eed.ericsson.se)

  3. UMTS – enabler for the mobile Internet • Mobile Internet: the unification of Cellular Networks and the Internet, enabling use of Internet services when mobile • Examples of packet data services • Conventional internet services • Web-browsing, electronic mail, file transfer, video/audio streaming, e-commerce, ... • Combination with location information and mobility • Location based services, navigation

  4. MSC Mobile Services Switching Center GSN GPRS Support Node RNC RNC MSC/GSN RNC Radio Network controller Node B Base Node Radio network System (RNS) Node B Node B Node B Node B Node B Node B UMTS network architecture Node B

  5. UMTS – Main Features • New radio access technology using new spectrum • spectrum allocation around 2 GHz • two radio transmission modes • Frequency Division Duplex (FDD): 2  60 MHz • Time Division Duplex (TDD): 15 + 20 MHz • Wideband Code Division Multiple Access (WCDMA) • Chip rate 3.84 Mcps  Channel bandwidth 4.4 – 5 MHz • Built on GSM Core Network technology • Support of user data rates 0 – 2 Mbps • Multi-call, multimedia capability

  6. Radio interface architecture (simplified) CTRL UE USER USER UTRAN CTRL RRC RRC Signaling Radio Bearers Radio Bearers PDCP PDCP RLC RLC RLC RLC Logical Channels Control channels MAC MAC Traffic channels Transport Channels Common Dedicated PHY PHY Shared Physical Channels

  7. UMTS Protocol Architecture (user plane)

  8. Data flow for packet data (UE side)

  9. time frequency 1 radio frame (10 ms), 15*2560 chips (3.84 Mcps) Uplink Downlink Macrocell layers Microcell layer Duplex distance, e.g. 190 MHz 5 MHz 5 MHz 5 MHz 5 MHz Slot i Slot 1 Slot 2 Slot 15 Transmission Format UTRA FDD • bit level QPSK (downlink) or dual-channel BPSK (uplink) • modulation rates 15 ... 960 Ksps for spreading factors 256 ... 4

  10. Higer layers Physical Layer Processing Power setting Encoding Interleaving Rate matching Multiplexing Spreading & Baseband Modulation Up conversion & Power Amplification D/A Freq. Synthes. Code generator Power control commands Time Sync. Duplexer Decoding Deterleaving Demux Despreading & Baseband Demodulation Down conversion A/D Transport channels Physical channels Principal Mobile Station Transceiver Structure

  11. TFI1 TFCI Coding TFI2 CRC attachment CRC attachment Coding Coding Inter-frame interleaving Inter-frame interleaving TFCI . . . TFIN TrCh1 Multi- plexing Rate Matching (repetition and puncturing) TrCh2 Intra-frame interleaving . . . . . . . . . . . . TrChN CRC attachment Coding Inter-frame interleaving Coding, Interleaving, Rate Matching, Multiplexing TF: Transport Format TrCh: Transport Channel TFCI: Transport Format Combination Indicator

  12. Principle of spectral spreading Spreading code T T chip symbol Baseband Pulse Sample-and modulation shaping - hold Data Frequency Frequency

  13. General design objectives for packet services • High spectral efficiency, i.e. low Eb/N0 for desired error rate (low overhead, efficient radio link adaptation, diversity, ...) • Low delay (interaction between TCP and RLC) • High throughput (system and users) • Simplicity and effectiveness of radio resource management (including QoS management) • Efficient usage of channelization codes on the downlink • Efficient usage of BTS transmitter power • Efficient usage of hardware resources (especially in the Node B) • Low terminal power consumption

  14. 0 10 -1 10 -2 10 BLER -3 10 -4 10 -5 10 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 (Eb/N0)req Eb/N0 Performance measures • Link performance (BER/BLER vs. Eb/N0) • Advanced coding (turbo) • High degree of diversity (multipath, Rx antenna) • Optional enhancements: interference cancellation, adaptive antennas, Space-Time Transmit Diversity (STTD) target BLERreq = 10 % Example: performance of a 144 kbps DCH in vehicular environment (120 km/h) with turbo coding

  15. „total amount of served data“ average user throughput = „time to deliver data“ Packet data throughput definitions slope: average throughput wrt. single packet („packet bit rate“) data volume (e.g. # bytes) slope: average link throughput Rlinktrp e.g. retransmissions time inititial setup delay data arrival in tx buffer Variable-rate tx on the radio link, slope: Rlink (average packet bit rate for one user) average link throughput Rlinktrp =Rlink / Ntransm = Rlink (1 – BLER) number of transmissions/block: Ntransm = 1/(1 – BLER)

  16. Performance measures • System throughput (capacity): • Average throughput of all users in the system • Maximum system throughput is reached when the packet delay can grow unbounded • Capacity definitions: • average number of users, or • system throughput when user quality drops to an unacceptable level („outage“) • Outage can be defined in terms of a delay and/or throughput threshold that should be met with a certain probability • Capacity defined as system throughput is less sensitive to the traffic load generated per user

  17. UTRA transport channels categories • Common channels • Multiplexed users (user ID in the MAC header) • Forward Access Channel (FACH) • Random Access Channel (RACH) • Common Packet Channel (CPCH) • Dedicated channels (DCH) • Assigned to a single user (identified by the spreading code) • Shared channels • „Sharing“ of code resource by several users by fast re-assignment scheduling • Downlink Shared Channel (DSCH)

  18. 10 ms 1-rate 1/2-rate 0-rate Variable rate R = 1 R = 0 R = 1/2 R = 1 : physical control info (Dedicated Physical Control Channel, DPCCH) (Pilot+TPC+TFCI) : user data (Dedicated Physical Data Channel, DPDCH) Dedicated Channel (FDD downlink) fixed spreading factor DL DCH Features: • Fast closed-loop power control • Macro diversitity • potential blocking due to insufficient spreading codes

  19. : DPCCH (Pilot+TFCI+FBI+TPC), fixed spreading factor 256 : DPDCH (Data) Dedicated Channel (FDD uplink) 10 ms 1-rate 1/2-rate variable spreading factor on DPDCH 1/4-rate 0-rate Variable rate R = 1 R = 1/2 R = 0 R = 0 R = 1/2 UL DCH Features: • Fast closed-loop power control • Macro diversitity

  20. DCH characteristics • Lowest delay among all transport channels • Large overhead in Eb/N0 at low data duty cycle due to physical control : Example: UL RDPCCH= 15 kbps, independent of RDPDCH (w = 1) DPCCH overhead [dB] mean modulator data rate [kbits]

  21. Impact of overhead on capacity Example: (Es/N0)req = 3 dB M = 0.6 (load margin) F = 1.5 (ratio of inter-cell to intra-cell interference) RDPCCH = 15 kbps Rdata = RDPDCH(UL) =120 kbps

  22. Message 38400 chips (10 ms) or 20 ms Preamble 4096 chips Physical Random Access Channel (PRACH) -Uplink - “Access slot” 5120 chips DPDCH DPCCH Timing offset Acquisition Indicator Channel (AICH) - Downlink - Acquisition Indicator (AI) 4096 chips Random Access Channel (FDD uplink)

  23. RACH characteristics • Slotted-ALOHA type of contention-based channel • No power control during message transmission • reasonable initial message power derived via preamble ramping • Access delay due to ramping 5 – 50 ms (mean  15 ms) • Increased delay in case of message collisions

  24. RACH throughput performance • Example (derived with a specific choice of parameter settings, throughput S and offered load G normalized to 1 message per access slot): max normalized throughput S = 3.3 corresponds to 2475 messages/s (80 user data bits/message, 198 kbps)

  25. S-CCPCH data of other users data of desired users TTI 3 TTI 2 physical control TTI 1 TTI 1 Forward Access Channel (FDD downlink) • Several FACHs with different transport format can be multiplexed on the physical layer • Mapped to Secondary Common Control Physical Channel (S-CCPCH) • No fast power control, no macro diversity (transmitted at broadcast power level, i.e. on average rather high energy per data bit Eb spent) • Scheduling delay

  26. 10 ms Data for the considered user Data for other users PDSCH 2 Data for the considered user PDSCH 1 DL-DPCH 10 ms DPCCHs DPCCHs UL-DPCH Downlink Shared Channel (DSCH) • Format indicator (TFCI) on associated DPCH includes assignment of PDSCH spreading code • Jointly power controlled with the associated DCH

  27. DSCH characteristics • No macrodiversity • mostly suitable in the inner cell area • then approximately same spectral efficiency as a DCH with the same rate • Avoids capacity limitation due to non-availability of codes • Scheduling delay • Aimed to be compensated by higher link data rate SF = 256 SF = 128 SF = 64 SF = 32 SF = 16 SF = 8 SF = 4 OVSF codes allocated to PDSCHs (example) SF = 2 SF = 1

  28. UE modes and RRC States(„activity levels“) • UE listens to PCH in DRX mode • location known on cell level • UE listens to PCH in DRX mode • location known on URA level • DPCH allocated • location known on cell level Dedicated (DCH) or Shared (DSCH) Transport Ch. can be used • UE continuously monitors FACH on downlink • RACH (and/or CPCH) can be used anytime • location known on cell level • UE not registered to the network • Cell broadcast info can be received

  29. Cell_FACH: Service example: e.g. email download, WAP browsing S-CCPCH data of other users data of desired users TTI 3 TTI 2 physical control PRACH TTI 1 TTI 1 DPDCH DPDCH DPCCH DPCCH AICH TCP acks RLC acks RRC measurement reports 4.5 - 6.5 kbps average load

  30. Cell_DCH: Service example: ftp or email download, Web browsing DL-DPCH TTI 2 TTI 1 UL-DPCH DPDCH DPDCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH

  31. Cell_DCH: Service example: ftp or email upload DL-DPCH UL-DPCH DPDCH DPDCH DPDCH DPDCH DPDCH DPDCH DPDCH DPDCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH DPCCH

  32. Cell_DCH: Service example: e.g. file download, Web browsing Data for the considered user Data for other users PDSCH 2 Data for the considered user PDSCH 1 10 ms DL-DPCH 10 ms DPCCHs DPCCHs UL-DPCH

  33. Channel Switching • Dynamic switching between common and dedicated channels, i.e. common channel RRC states (Cell_FACH and Cell_PCH) and dedicated channel RRC state (Cell_DCH) • provides radio link adaptation to different levels of data transmission activity, in order to achieve at low transmission activity following objectives: • efficient utilisation of BTS TRX hardware resources dedicated to each cell • high utilisation of the downlink channelization codes available in a cell • low terminal power consumption • reasonable high spectral efficiency and reasonable delay compared to dedicated channel transmissions • channel type switching is a special type of intra-cell handover

  34. Up-switching (Cell_FACH  Cell_DCH)

  35. Down-switching (Cell_DCH  Cell_FACH  Cell_PCH)

  36. Throughput illustration for Web page download 50 kBytes data packet Downlink Traffic Volume [kbits] slope: Rlinktrp = Rlink (1-BLER) Rlinkmax = 64 kbps

  37. System throughput vs. user throughput Example configuration: 64 kbps DCH, 30 codes available per cell with SF= 32 Note: This example shall just illustrate the principal characteristics of throughput performance which were obtained for some specific assumptions not discussed here; absolute capacity figures depend strongly on the choice of simulation parameters and assumptions. packet bit rate (kbps/cell) users per cell

  38. Summary on packet data performance • Results from system-level simulations: • Data on dedicated channels • throughput very much dependent on configuration and traffic characteristics, • can be very low if only a few channels for very high rate are configured due to code limitation effects • Data on common channels • inefficient for large amount of data due to lack of tight power control (especially FACH) • Data on shared channels • can reach approx. same system throughput as a configuration with low-rate dedicated channels, at much higher peak data rate per user • scheduling policy affects performance („fairness“ reduces system throughput)

  39. UMTS Evolution (Release 4 and 5) • Main future new features (affecting packet services): • All-IP transport in the Radio Access and Core Networks • Enhancements of services and service management • High-speed Downlink Packet Access (HSDPA) • Introduces additional downlink channels: • High-Speed Downlink Shared Channel (HS-DSCH) • Shared Control Channels for HS-DSCH

  40. dedicated channels dedicated channels HS-DSCH shared control channels HS-DSCH characteristics • Provision of 8 –10 Mbps peak user data rate by • Fast selection of modulation and coding scheme depending on channel conditions (no fast power control) • Short transmission time interval (2 ms) • Fast hybrid ARQ (incremental redundancy and/or Chase combining) • Fast scheduling • Fast cell selection/handover

  41. Ch. Code #1 HS-DSCH physical layer processing chain Adaptation Algorithm Mapping on Code-Tree Turbo Encoding Puncturing and Repetition Interleaving Modulation gain scrambling CRC other channels Ch. Code #N Example: 12 out of 16 codes with SF=16 QPSK 16QAM (64QAM opt.) Coding Rate: 1/3 - 1

  42. C/I C/I 1 0.01 0.1 FER time Adaptive Modulation and Coding Modulation and Coding Schemes (Example) 64QAM, R=0.75 (12.96 Mbps) 64QAM, R=0.50 (8.64 Mbps) 16QAM, R=0.63 (7.20 Mbps) 16QAM, R=0.38 (4.32 Mbps) QPSK, R=0.50 (2.88 Mbps) QPSK, R=0.25 (1.44 Mbps) for 12 codes (of 16)

  43. Scheduling Strategies Transmission time interval 3 slots (2 ms) Example: Max C/I scheduling C/I time C/I served mobile time C/I time

  44. Conclusions • UMTS provides the presently most advanced cellular radio access technology • Mature technology, already proven to work • Prepared for future evolution • Fine-tuning of parameters in order to optimize end-to-end user and overall system performance still remains a challenging task

  45. Conclusions (cont.) • But ... the market success will primarily not depend on technology • Marketing strategy of network operators and service providers • Charging policy and tariffing • Availabilty of handsets in large volumes at low price at UMTS introduction • Early provision of useful and inventive new services • Simplicity to apply the offered services • Readiness of the subscribers to get used to new services • Social factors: openess to modern technology (provision of high level of security to resolve all doubts on potential hazards, EMC, confidentiallity)

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