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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 Rev A 30-November-2001 1 On the Performance of UMTS Packet Data Services Wolfgang Granzow Ericsson Research Corporate Unit Ericsson Eurolab Deutschland, Nürnberg ([email protected]) Rev A 30-November-2001 2 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 Rev A 30-November-2001 3 UMTS network architecture MSC GSN RNC Node B Mobile Services Switching Center GPRS Support Node MSC/GSN Radio Network controller Base Node RNC RNC Radio network System (RNS) Node B Node B Node B Node B Node B Node B Node B Rev A 30-November-2001 4 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 Rev A 30-November-2001 5 Radio interface architecture (simplified) CTRL USER USER RRC CTRL RRC Signaling Radio Bearers Radio Bearers RLC PDCP PDCP RLC RLC RLC Logical Channels Control channels Traffic channels MAC MAC Transport Channels Common Dedicated Shared PHY PHY Physical Channels UE Rev A 30-November-2001 UTRAN 6 UMTS Protocol Architecture (user plane) UTRAN Packet switched Core Network IP server UE Application Applic. GGSN TCP Radio Access Bearers IP IP TCP IP IP SGSN RNC Radio Bearers PDCP RLC Logical channels MAC Transport channels PHY PDCP Iu UP Iu UP RLC GTP-U GTP-U MAC UDP UDP Node B PHY FP FP PHY AAL2/ ATM AAL2/ ATM IP IP AAL5/ ATM AAL5/ ATM GTP-U GTP-U GPRS IP backbone UDP/ TCP IP routing IP IP IP UDP/ TCP IP Physical channels Uu Rev A Iub Gn Iu 30-November-2001 7 Gn/Gp Gi Data flow for packet data (UE side) 40 bytes typ. 512 bytes (MSS = 1460 bytes) TCP/IP header TCP/IP 2 or 3 bytes L2 PDCP PDCP header PDCP header PDCP PDU L2 MAC … typ. 40 bytes 2–4 bytes 0 or 3 bytes MAC header … RLC header … MAC SDU L1 CRC 30-November-2001 8 RLC header MAC header … Transport block (MAC PDU) Rev A PDCP PDU RLC SDU RLC SDU L2 RLC … TCP/IP header payload (application data) … MAC SDU Transport block (MAC PDU) 2 bytes CRC Transmission Format UTRA FDD 1 radio frame (10 ms), 15*2560 chips (3.84 Mcps) Slot 1 Slot 2 Slot i Uplink frequency Microcell layer 5 MHz 5 MHz 5 MHz Duplex distance, e.g. 190 MHz • bit level QPSK (downlink) or dual-channel BPSK (uplink) • modulation rates 15 ... 960 Ksps for spreading factors 256 ... 4 Rev A time Downlink Macrocell layers 5 MHz Slot 15 30-November-2001 9 Principal Mobile Station Transceiver Structure Physical Layer Processing Encoding Interleaving Rate matching Multiplexing Spreading & Baseband Modulation Power control commands Higer layers Code generator Decoding Deterleaving Demux Transport channels Rev A D/A Up conversion & Power Amplification Freq. Synthes. Time Sync. Despreading & Baseband Demodulation A/D Physical channels 30-November-2001 Power setting 10 Duplexer Down conversion Coding, Interleaving, Rate Matching, Multiplexing TFI1 TFI2 TFCI TFCI Coding .. . TFIN TrCh1 CRC attachment TrCh2 .. . TrChN Coding Inter-frame interleaving CRC attachment Coding .. . .. . Inter-frame interleaving CRC attachment Coding .. . Inter-frame interleaving TF: Transport Format TrCh: Transport Channel TFCI: Transport Format Combination Indicator Rev A 30-November-2001 11 Rate Matching (repetition and puncturing) Multiplexing Intra-frame interleaving Principle of spectral spreading Spreading code Tchip Tsymbol Data Baseband modulation Pulse shaping Sample-and -hold Frequency Frequency Rev A 30-November-2001 12 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 Rev A 30-November-2001 13 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, SpaceTime Transmit Diversity (STTD) 10 0 target BLERreq = 10 % 10-1 10 BLER Example: performance of a 144 kbps DCH in vehicular environment (120 km/h) with turbo coding 10 -2 -3 -4 10 -5 10 0 0.5 1 1.5 2 2.5 3 (Eb/N0)req Rev A 30-November-2001 14 3.5 4 4.5 Eb/N0 5 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 data arrival in tx buffer inititial setup delay time Variable-rate tx on the radio link, slope: Rlink average user throughput = „total amount of served data“ „time to deliver data“ (average packet bit rate for one user) average link throughput Rlinktrp = Rlink / Ntransm = Rlink (1 – BLER) number of transmissions/block: Ntransm = 1/(1 – BLER) Rev A 30-November-2001 15 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 Rev A 30-November-2001 16 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) Rev A 30-November-2001 17 Dedicated Channel (FDD downlink) 10 ms 1-rate 1/2-rate fixed spreading factor 0-rate Variable rate R=1 R=0 R = 1/2 R=1 : user data (Dedicated Physical Data Channel, DPDCH) : physical control info (Dedicated Physical Control Channel, DPCCH) (Pilot+TPC+TFCI) DL DCH Features: Rev A • • • Fast closed-loop power control Macro diversitity potential blocking due to insufficient spreading codes 30-November-2001 18 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 : DPDCH (Data) : DPCCH (Pilot+TFCI+FBI+TPC), fixed spreading factor 256 UL DCH Features: Rev A • • Fast closed-loop power control Macro diversitity 30-November-2001 19 DCH characteristics • Lowest delay among all transport channels • Large overhead in Eb/N0 at low data duty cycle due to physical control : 12 11 10 [dB] overhead DPCCH DPCCH overhead (dB) Example: UL RDPCCH = 15 kbps, independent of RDPDCH (w = 1) ( Eb / N 0 ) total RDPCCH 1 w ( Eb / N 0 ) DPDCH Rs data ( E / N 0 ) DPCCH w s ( E s / N 0 ) DPDCH Rev A 9 8 7 6 5 4 3 2 1 0 0 20 40 60 80 100 120 mean modulation rate 140 160 mean modulator data rate [kbits] 30-November-2001 20 180 200 Impact of overhead on capacity 400 Example: K max 350 M G F ( E s / N 0 ) req α w R DPCCH / R data 300 no overhead capacity, Kmax Capacity, K (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 max 250 200 min overhead 150 100 full overhead 50 0 -2 10 Rev A 30-November-2001 21 -1 10 activity activityfactor, factor, 0 10 Random Access Channel (FDD uplink) Message 38400 chips (10 ms) or 20 ms Preamble 4096 chips “Access slot” 5120 chips DPDCH DPCCH Physical Random Access Channel (PRACH) -Uplink - Timing offset Acquisition Indicator Channel (AICH) - Downlink Acquisition Indicator (AI) 4096 chips Rev A 30-November-2001 22 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 Rev A 30-November-2001 23 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): 6 S=G S = G eG/16 (K = 16 signatures), no interference 5 Throughput S 4 simulation for 10 ms messages, SF = 128, K = 16 3 2 no load control S = G eG (K = 1) 1 0 0 5 10 15 20 25 30 35 40 Offered Traffic G max normalized throughput S = 3.3 corresponds to 2475 messages/s (80 user data bits/message, 198 kbps) Rev A 30-November-2001 24 45 50 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 S-CCPCH physical control Rev A data of other users data of desired users TTI 2 TTI 1 TTI 1 30-November-2001 25 TTI 3 Downlink Shared Channel (DSCH) • • Format indicator (TFCI) on associated DPCH includes assignment of PDSCH spreading code Jointly power controlled with the associated DCH 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 UL-DPCH Rev A 30-November-2001 26 DPCCHs 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 SF = 2 SF = 1 Rev A 30-November-2001 27 OVSF codes allocated to PDSCHs (example) UE modes and RRC States („activity levels“) UTRAN Connected Mode – – 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 URA_PCH CELL_PCH CELL_DCH Release RRC Connection CELL_FACH Establish RRC Release RRC Connection Connection Idle Mode Rev A 30-November-2001 28 – – – – – UE listens to PCH in DRX mode location known on cell level UE continuously monitors FACH on downlink RACH (and/or CPCH) can be used anytime location known on cell level Establish RRC Connection – – UE not registered to the network Cell broadcast info can be received UTRAN Service example: e.g. email download, WAP browsing Cell_FACH: UE S-CCPCH data of other users data of desired users TTI 2 TTI 1 TTI 1 PRACH DPDCH DPCCH TTI 3 physical control DPDCH DPCCH AICH TCP acks RLC acks RRC measurement reports Rev A 30-November-2001 29 4.5 - 6.5 kbps average load UTRAN Service example: ftp or email download, Web browsing Cell_DCH: UE DL-DPCH TTI 2 TTI 1 UL-DPCH DPCCH Rev A DPCCH DPCCH DPDCH DPCCH DPCCH 30-November-2001 30 DPCCH DPCCH DPCCH DPCCH DPDCH DPCCH UTRAN Service example: ftp or email upload Cell_DCH: UE DL-DPCH UL-DPCH DPDCH DPCCH Rev A DPDCH DPCCH DPDCH DPDCH DPDCH DPDCH DPCCH DPCCH DPCCH DPCCH 30-November-2001 31 DPCCH DPCCH DPDCH DPCCH DPDCH DPCCH UTRAN Service example: e.g. file download, Web browsing Cell_DCH: UE Data for the considered user Data for other users PDSCH 2 Data for the considered user 10 ms PDSCH 1 DL-DPCH 10 ms DPCCHs UL-DPCH Rev A 30-November-2001 32 DPCCHs 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 Rev A 30-November-2001 33 Up-switching (Cell_FACH Cell_DCH) time switching decision (RRC/SRNC) Power PDSCH assigned to the given user DPCCH downlink PDSCH and DPCH DPCH switching command SCCPCH measurement report PRACH ramping confirm uplink DPCH Cell_FACH Rev A transport and DPCH synchr. radio interface delay delay 30-November-2001 34 Cell_DCH (DCH + DSCH) Down-switching (Cell_DCH Cell_FACH Cell_PCH) PDSCH assigned to the given user Power downlink PDSCH and DPCH time switching decision (RRC/SRNC) DPCCH switching command Paging indicator On PICH RLC ack SCCPCH confirm PRACH measurement report ramping RLC ack uplink DPCH Cell_DCH (DCH + DSCH) Rev A inactivity interval transport and radio interface delay 30-November-2001 35 Cell_FACH Cell_PCH Throughput illustration for Web page download 600 completed generated arrived at RLC delivered Downlink bits [kbit] Downlink Traffic Volume [kbits] 500 400 50 kBytes data packet 300 200 slope: Rlinktrp = Rlink (1-BLER) Rlinkmax = 64 kbps 100 0 Rev A 1 2 3 30-November-2001 4 36 6 5 Time [s] 7 8 9 10 System throughput vs. user throughput 50 10% best packets median 10% worst packets Example configuration: 64 kbps DCH, 30 codes available per cell with SF= 32 (kbps/cell) ratethroughput bit user packet average (kbps/cell) 45 20 30 35 40 50 35 30 25 20 30 35 20 45 15 20 10 0 50 30 users per cell 35 45 5 Note: 45 50 0 100 200 300 400 500 system throughput (kbps/cell) 600 700 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. Rev A 30-November-2001 37 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) Rev A 30-November-2001 38 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 Rev A 30-November-2001 39 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 dedicated channels dedicated channels Rev A 30-November-2001 40 HS-DSCH physical layer processing chain Adaptation Algorithm Ch. Code #1 Mapping on Code-Tree Turbo Encoding CRC Puncturing and Repetition gain Modulation Interleaving Ch. Code #N Coding Rate: 1/3 - 1 Rev A scrambling QPSK 16QAM (64QAM opt.) 30-November-2001 41 Example: 12 out of 16 codes with SF=16 other channels Adaptive Modulation and Coding Modulation and Coding Schemes (Example) C/I 64QAM, 64QAM, 16QAM, 16QAM, QPSK, QPSK, C/I time Rev A 1 30-November-2001 0.1 42 0.01 FER R=0.75 (12.96 Mbps) R=0.50 (8.64 Mbps) R=0.63 (7.20 Mbps) R=0.38 (4.32 Mbps) R=0.50 (2.88 Mbps) R=0.25 (1.44 Mbps) for 12 codes (of 16) 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 Rev A 30-November-2001 43 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 Rev A 30-November-2001 44 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) Rev A 30-November-2001 45