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Optimizing the ARQ Performance in Downlink Packet Data Systems With Scheduling

Optimizing the ARQ Performance in Downlink Packet Data Systems With Scheduling. Haitao Zheng, Member, IEEE Harish Viswanathan, Senior Member, IEEE IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS VOL. 4, NO. 2, MARCH 2005 Presented by Li-Yi Lin. Outline . Introduction

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Optimizing the ARQ Performance in Downlink Packet Data Systems With Scheduling

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  1. Optimizing the ARQ Performance in Downlink Packet Data Systems With Scheduling Haitao Zheng, Member, IEEE Harish Viswanathan, Senior Member, IEEE IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS VOL. 4, NO. 2, MARCH 2005 Presented by Li-Yi Lin OPLAB, Dept. of IM, NTU

  2. Outline • Introduction • Optimizing the mapping between SINR and MCS • Optimizing the scheduler ranking for HARQ • Simulation and discussion OPLAB, Dept. of IM, NTU

  3. Introduction • Two main considerations of designing systems which provide high-speed packet data service on the downlink - the selection of MCS based on the channel quality of the link - the selection of the user to whom a particular slot is assigned • Adaptive techniques - Dynamic Link Adaptation of Adaptive Modulation and coding - Automatic Repeat reQuest (ARQ) or Hybrid ARQ (HARQ) - Scheduler OPLAB, Dept. of IM, NTU

  4. Dynamic Link Adaptation of Adaptive Modulation and coding • Link adaptation continuously adjusts the modulation and coding scheme (MCS) • The transmitter selects an appropriate MCS, based on the user’s channel quality feedback • The performance of link adaptation largely depends on the accuracy of channel quality measurement. OPLAB, Dept. of IM, NTU

  5. Automatic Repeat reQuest (ARQ) or Hybrid ARQ • Packet data is delay-tolerant - feasible to use retransmission schemes to recover erroneous packets. • HARQ can compensate for link adaptation errors and provide a finer granularity of coding rate • HARQ - simple ARQ - chase combining - incremental redundancy OPLAB, Dept. of IM, NTU

  6. Simple ARQ - simple ARQ based transmitter retransmits the same packet and repeats the procedure until the packet is received successfully. • Chase combining - the base station repeatedly sends the same packet and the receiver aggregates the energy from the (re)transmissions to improve signal to noise ratio (SNR) • Incremental redundancy (IR) - transmits additional redundant information in each retransmission and gradually refines coding rate and SNR till a successful decoding is achieved. OPLAB, Dept. of IM, NTU

  7. Scheduler • Scheduler can take advantage of channel variations by giving certain priority to the users with transitorily better channel conditions. • The choice of the scheduling algorithm critically impacts the system performance. OPLAB, Dept. of IM, NTU

  8. Introduction (cont’) • Aggressive MCS • The conventional mapping design fails to take into account the performance improvement by HARQ. • The instantaneous rate does not fully represent the true data throughput. OPLAB, Dept. of IM, NTU

  9. Outline • Introduction • Optimizing the mapping between SINR and MCS • Optimizing the scheduler ranking for HARQ • Simulation and discussion OPLAB, Dept. of IM, NTU

  10. Optimizing the mapping between SINR and MCS • Assume that the channel quality feedback carries SINR. • The mapping is between the SINR and the MCS. • For both Chase combining and simple ARQ, the MCS used in retransmissions is the same as that in the original transmission. • Assume the channel condition stays constant during retransmissions as that of the initial transmission. OPLAB, Dept. of IM, NTU

  11. Notation OPLAB, Dept. of IM, NTU

  12. The traditional mapping selection criteria OPLAB, Dept. of IM, NTU

  13. A single unified mapping criterion • - The average throughput of an AWGN channel • using MCS i ：the average number of successfully received information bits ：the average time taken by the packet OPLAB, Dept. of IM, NTU

  14. TRPT-Chase： OPLAB, Dept. of IM, NTU

  15. TRPT-SARQ： (6) OPLAB, Dept. of IM, NTU

  16. OPLAB, Dept. of IM, NTU

  17. Outline • Introduction • Optimizing the mapping between SINR and MCS • Optimizing the scheduler ranking for HARQ • Simulation and discussion OPLAB, Dept. of IM, NTU

  18. Optimizing the scheduler ranking for HARQ • Two important characteristics - Frame Error Rate Information - Retransmission Information • The scheduler design and the mapping selection can be conducted jointly to optimize the system performance. • But for simplicity, we assume the scheduler design and the mapping selection are performed independently. • Replacing the instantaneous rate with effective rate. OPLAB, Dept. of IM, NTU

  19. Ranking A: Instantaneous Rate • Ranking B:ARQ Success Probability Weighted Instantaneous rate , OPLAB, Dept. of IM, NTU

  20. Ranking C: Average Packet Throughput Based Effective Throughput OPLAB, Dept. of IM, NTU

  21. Ranking C: OPLAB, Dept. of IM, NTU

  22. Ranking C: OPLAB, Dept. of IM, NTU

  23. Ranking D: Success Probability Weighted Instantaneous Rate • Ranking E: Approximated Average Packet Throughput Based Effective Throughput OPLAB, Dept. of IM, NTU

  24. Outline • Introduction • Optimizing the mapping between SINR and MCS • Optimizing the scheduler ranking for HARQ • Simulation and discussion OPLAB, Dept. of IM, NTU

  25. Simulation and discussion • The simulated radio network - Opnet network simulation tool - a radio network controller (RNC) - a base station (Node B) - mobile terminals - total networking delay = 50 ms - time multiplexing - each scheduling interval of frame lasts 2 ms - apply built-in module to IP, TCP, UDP, HTTP, FTP OPLAB, Dept. of IM, NTU

  26. - Radio Link Protocol (RLP) * performs data block segmentation and reassembly * The RLP PDU size is chosen to be 40 bytes - * performs scheduling, MCS selection and HARQ functionality * HARQ operates in terms of three Stop And Wait process * the Maximum number of retransmissions is 4 * MAC scheduler makes scheduling decision about 1 ms prior to the actual transmission - PHY * assume the uplink channel operates at 64 kbps and 0% FER * For the downlink channel, the frame error is generated by relating the SINR at each mobile terminal to a link level performance curve * Physical mobility of the user has not been considered * 5-MHz spectrum and 2-ms frame OPLAB, Dept. of IM, NTU

  27. OPLAB, Dept. of IM, NTU

  28. 4MCS set • - 640kbps、1.28Mbps、1.92Mbps、2.56Mbps • 6MCS set • - 320kbps、480kbps、640kbps、1.28Mbps、1.92Mbps、2.56Mbps OPLAB, Dept. of IM, NTU

  29. OPLAB, Dept. of IM, NTU

  30. AGG criterion：3 dB more aggressive than TRPT-chase OPLAB, Dept. of IM, NTU

  31. OPLAB, Dept. of IM, NTU

  32. Three scenarios • Scenario A: Single User With TCP • Scenario B: Multiple Users With TCP and Modified Scheduler Rankings • Scenario C: Multiple Users With TCP Using Ranking C for Various Mapping Criterion OPLAB, Dept. of IM, NTU

  33. Scenario A: Single User With TCP • Performance measurement： OPLAB, Dept. of IM, NTU

  34. Scenario A: Single User With TCP OPLAB, Dept. of IM, NTU

  35. Scenario B: Multiple Users With TCP and Modified Scheduler Rankings • Fairness measurement OPLAB, Dept. of IM, NTU

  36. Scenario B: Multiple Users With TCP and Modified Scheduler Rankings Fig. 7. Multiple-user HTTP performance of various scheduler rankings using Chase combining and TRPT-chase mapping criterion, 6MCS set. (a) 3 km/h and (b) 30 km/h. OPLAB, Dept. of IM, NTU

  37. Scenario B: Multiple Users With TCP and Modified Scheduler Rankings Fig. 8. Multiple-user HTTP performance of various scheduler rankings using Chase combining and AGG mapping criterion, 6MCS set. (a) 3 km/h and (b) 30 km/h. OPLAB, Dept. of IM, NTU

  38. Scenario B: Multiple Users With TCP and Modified Scheduler Rankings OPLAB, Dept. of IM, NTU

  39. Scenario C: Multiple Users With TCP Using Ranking C for Various Mapping Criterion Fig. 10. User HTTP performance of various mapping criterion using Chase combining, proportional fair scheduler with ranking C, 4MCS set, 3 km/h. (a) System metric and (b) user throughput. OPLAB, Dept. of IM, NTU

  40. Summary & future work • The proposed mapping design achieves 5%-50% throughput improvement. • The modified proportional fair scheduler achieves 10%-30% performance improvement. • The sensitivity of the performance to the mapping depends on the granularity of the MCS set with decreasing sensitivity for larger MCS sets. • The channel estimation and prediction error can also be included in the mapping design by modifying the frame error rate accordingly OPLAB, Dept. of IM, NTU

  41. Thank You! OPLAB, Dept. of IM, NTU

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