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Packet service in UMTS: delay-throughput performance of the downlink shared channel

Packet service in UMTS: delay-throughput performance of the downlink shared channel. Flaminio Borgonovo, Antonio Capone, Matteo Cesana, Luigi Fratta. 1. Introduction. In the last ten years: IP applications has pushed the data traffic to grow quickly

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Packet service in UMTS: delay-throughput performance of the downlink shared channel

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  1. Packet service in UMTS: delay-throughput performance of the downlink shared channel Flaminio Borgonovo, Antonio Capone, Matteo Cesana, Luigi Fratta

  2. 1. Introduction • In the last ten years: • IP applications has pushed the data traffic to grow quickly • 2G cellular systems have heavily changed the way in which users access the network. • The challenge of third generation mobile communication systems is to provide access for a wide range of multimediaapplications and services.

  3. 1. Introduction • UMTS: 3G mobile communication system developed by ETSI, extend the present GSM service to include multimedia. • UMTS provides great flexibility and a variety of different physical and logical channel types. • Several user rates and protections are possible by choosing suitable parameters. • More implementation complexity.

  4. 2. UTRA basics (1/3) • Two access scheme for the radio interface: • W-CDMA scheme • 60MHz for downlink and 60MHz for uplink • 3,84 Mchips/s, 5MHz for each channel, QPSK modulation • TD-CDMA scheme • 35MHz for downlink and uplink • Physical channels are defined by the associated spreading and scrambling codes. • Spreading sequence : input dataXORspreading code XOR scrambling code • Spreading code of CDMA: channelization code • Scrambling code of CDMA: PN code

  5. 2. UTRA basics (2/3) • Transport channels: • Dedicated channels (DCH) • Devoted to the connection between a single mobile station and the UTRA Network. • Mapped into two physical channel :DPDCH, DPCCH • Common transport channel • Broadcast channel (BCH), paging channel (PCH) • Random access channel (RACH), forward access channel (FACH): control information or packet • Common packet channel (CPCH), downlink shared channel (DSCH) : packet only

  6. 2. UTRA basics (3/3) • Power control • DCH: TPC (Transmit power control) symbols in each slot carry a command for increasing or decreasing • DSCH: computed on the basis of the power of the DCH

  7. 3. Downlink packet data services (1/3) • Three channels for downlink direction • DCH: • assigned to single users through set-up and tear down procedures, subject toclosed loop power controlandservice such as voice. • DSCH • No set-up, tear down procedure • Doesn’t carry power control signaling, but must have an associated active DCH • FACH • Shared by many users to transmit short bursts of data • No DCH must be activated

  8. 3. Downlink packet data services (2/3) • For real-time circuit traffic—DCH • Well known results show that CDMA with closed-loop power control is very effective.[14] • Efficiency can be further enhanced by using powerful FEC codes. [15] • For packet service—DSCH • Due to the burstiness, the number of interferingchannels, its power level, errors can be more efficiently obviated by ARQ than FEC[16][17]

  9. 3. Downlink packet data services (3/3) • it is interesting to investigate whether UMTS achieves the highest data throughput with circuit or packet switching technology • For circuit switching, the additional of any further channel beyond the capacity cannot be accepted • since the BER will increase • For packet switching, occasional increases in BER over its target value can be to tolerated • Because of the use of ARQ techniques • This paper provide a quantitative evaluation of the performance of the different alternatives.

  10. 4. Simulator description (1/4) • Propagation model • The received power Pr is given by • The path loss L is expressed as • Each cell is assigned a signal tree of orthogonal variable spreading factors, so that channels in the same cell are always orthogonal

  11. 4. Simulator description (2/4) • Traffic model • The performance of the UMTS downlink heavily depends on the input traffic characteristics. • Users become active according to a Poisson point process of intensity λ

  12. 4. Simulator description (3/4) • Receiver model • Carrier to interference ratio: • Block error rate: • Assumes an ideal ARQ procedure • When system operates far from capacity: • Increase power on the same channel to maintain SIR • Retransmitting the packets. • maximum interference tolerable is attained with a channel traffic G less than 1, and a further increase in retransmission would causes a strong decrease in throughput

  13. 4. Simulator description (4/4) • Power control model • Closed loop power control mechanism • Inner loop: controls the transmitted power to maintain the SIR at target value • Outer loop: controls the SIR to provide a target BLER →provide different qualities to different services. • Each channel cannot exceed a transmitted power of 30 dBm, whereas the overall power transmitted by a BS is limited to 43 dBm. • SIR • Proportionally reduced

  14. 5. Simulation results (1/4) • Effect of codes-1 • Channel codes : Convolutional codes • The encoded bits depend not only on the current k input data bits but also on past input bits • Coding rate: • Decoding strategy for convolutional codes: based on Viterbi algorithm

  15. Light codes Heavier code 5. Simulation results (1/4) • Effect of codes-1

  16. 5. Simulation results (1/4) • Effect of codes-2

  17. Wrongly designed: Require very low interference G: 0.955~0.97 Reach 1, but throughput is limited 5. Simulation results (1/4) • Effect of codes-3

  18. 5. Simulation results (2/4) • Effect of user traffic on downlink shared channel • If the amount of information is lower than the space available, the efficiency is reduced due to the unused space. • Consider sources that generate an average number of packets Np in the range from 1 to 25…

  19. 5. Simulation results (2/4) • Effect of user traffic on downlink shared channel

  20. 5. Simulation results (2/4) • Effect of user traffic on downlink shared channel

  21. 5. Simulation results (3/4) • Effect of control channels • Too many users would reduce the system throughput. • Limit the number of DCHs. (User arrived system is queued and wait for DCH availability) • Power control commands indicate changes in the transmitted power level • Both the power PDCH and PDSCH must change in the same way. • The SIR achieved after despreading on the two channels are related as:

  22. 5. Simulation results (3/4) • Effect of control channels • Interference generated by the related DSCH: (loss-of-orthogonality factor = 0.4 [20]) • Assume

  23. 5. Simulation results (3/4)

  24. 5. Simulation results (3/4) • Effect of control channels • Trade off: interference and multiplexing effect • SF, coding rate, =>maximum user number minimum delay

  25. 5. Simulation results (4/4) • Effect of power control • The closed-loop power control: introduced to increase the system capacity • The burstiness of data transmission may jeopardize the gain achieved. • DCH introduce additional interference • Thus DSCH is compared with FACH.

  26. 5. Simulation results (4/4) • Effect of power control

  27. 6. Conclusions (1/2) • Present some preliminary results obtained by simulation on the delay-throughput curves • Focus on the system parameters and channel configurations • Closed-loop power control mechanism • SIR increases as the speed of the physical channel increases • It has been verified the mechanism is very efficient even with low interference protection • If multiple physical channels is allowed • Intra-cell interference is improved by the closed-loop power control mechanism

  28. 6. Conclusions (2/3) • When the system operates with the highest bearable interference • A backoff mechanism is crucial to let the system operate close to capacity. • The use of DCH for power control with DSCH may lead to instability if the number of DCH is not limited. • If low speed users are served • A great reduction of capacity may be observed because of the minimum unit that a single user may use • The reduced frame filling degree does not reduce the interference, but yielding a net decrease in throughput

  29. 6. Conclusions (3/3) • In spite of the several limitations of packet switching, due to its intrinsic flexibility, better adapts to interference limited systems than circuit switching.

  30. Reference [14]. Erlang Capacity of a power controlled CDMA system (1993) [15]. Throughput analysis for Code Division Multiple Access of the Spread Spectrum Channel (1984) [16]. Channels with block interference [17]. Retransmissions versus FEC plus interleaving for real-time applications: a comparison between CDMA and MCD-TDMA cellular systems (1999) [20].3rd Generation Partnership Project, RF system scenarios, 3G TR 25.942, Dic. 1999

  31. Normalized energy per information:

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