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Drop HARQ feedback for aggressive HARQ transmission IEEE 802.16 Presentation Submission Template (Rev. 9) Document Number: IEEE S802.16m-09/0048 Date Submitted:

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Presentation on theme: "Drop HARQ feedback for aggressive HARQ transmission IEEE 802.16 Presentation Submission Template (Rev. 9) Document Number: IEEE S802.16m-09/0048 Date Submitted:"— Presentation transcript:

1 Drop HARQ feedback for aggressive HARQ transmission IEEE 802.16 Presentation Submission Template (Rev. 9) Document Number: IEEE S802.16m-09/0048 Date Submitted: 2009-1-5 Source: Zheng Yan-Xiu, Yu-Chuan Fang, Chang-Lan Tsai, Chung-Lien Ho, Hsi-Min Hsiao, Ren-Jr Chen, Richard Li, E-mail: zhengyanxiu@itri.org.tw.zhengyanxiu@itri.org.tw ITRI Venue: IEEE Session #59, San Diego. Base Contribution: N/A Re: 802.16m-08/052, Call for Comments on 802.16m SDD (802.16m-08/003r6), Section 11.13.2.6 HARQ Feedback Purpose: To be discussed and approval by IEEE 802.16m TG Notice: This document does not represent the agreed views of the IEEE 802.16 Working Group or any of its subgroups. It represents only the views of the participants listed in the “Source(s)” field above. It is offered as a basis for discussion. It is not binding on the contributor(s), who reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor grants a free, irrevocable license to the IEEE to incorporate material contained in this contribution, and any modifications thereof, in the creation of an IEEE Standards publication; to copyright in the IEEE’s name any IEEE Standards publication even though it may include portions of this contribution; and at the IEEE’s sole discretion to permit others to reproduce in whole or in part the resulting IEEE Standards publication. The contributor also acknowledges and accepts that this contribution may be made public by IEEE 802.16. Patent Policy: The contributor is familiar with the IEEE-SA Patent Policy and Procedures: and.http://standards.ieee.org/guides/bylaws/sect6-7.html#6http://standards.ieee.org/guides/opman/sect6.html#6.3 Further information is located at and.http://standards.ieee.org/board/pat/pat-material.htmlhttp://standards.ieee.org/board/pat

2 Aggressive HARQ Transmission Aggressive HARQ transmission removes HARQ soft buffer constraint to achieve higher MS transmission rate –Less HARQ soft buffer implies lower MS complexity –Higher transmission rate improves user experience However, aggressive HARQ transmission introduces buffer overflow –Received samples are stored in the HARQ buffer if they are not decodable –Buffer overflow occurs when MS reception status becomes bad temporarily due to channel variation and erroneous RSSI report, e.g. deep fade, shadowing, channel estimation error, etc And then, packets drop –Packet dropping results in unreliable HARQ transmission

3 Packet Dropping Rate Analysis 4 HARQ buffers are assumed 4 ~ 24 HARQ processes are analyzed –Multi-carrier may support up to 32 HARQ processes Packet error rate=0.01, 0.1, 0.2, 0.3, 0.5 As packet error rate is high, the packet dropping becomes significant The error rate also increases with number of HARQ processes Error rate= 0.01 Error rate= 0.1 Error rate= 0.2 Error rate= 0.3 Error rate= 0.5 HARQ Processes =4 00000 HARQ Processes =8 6.9e-105.7e-050.00150.00880.0684 HARQ Processes =12 6.3e-094.1e-040.00800.03700.1746 HARQ Processes =16 2.5e-080.00130.01990.07340.2508 HARQ Processes =20 7.0e-080.00290.03490.10760.3001 HARQ Processes =24 1.6e-070.00510.05070.13570.3333

4 The Bit Error Rate Performance of Lost of Packet in the First Transmission Observation: disaster occurs if the first transmission is lost –Turbo decoder can not correctly decode packet if complete systematic part is not received –If HARQ-IR is used and the first packet is dropped, the next redundancy version (RV1) can not carry complete systematic part and the redundancy version is not decodable

5 An Example for Aggressive HARQ Transmission 4 HARQ soft buffers A/X denotes ACK with X buffer used Red N/X denotes NACK with X buffer used Blue N/X denotes NACK with drop and X buffer used 12 HARQ processes 17 Packets pass –At least 5 more packets compared to conventional HARQ scheduling 9 Packets are dropped –Dropped packets may deteriorate reception performance and cause large latency –It may further worsen HARQ-IR performance

6 Solution: HARQ Buffer Management for Aggressive HARQ Transmission MS-assisted HARQ flow control –Avoid HARQ failure due to buffer overflow HARQ buffer synchronization –Acknowledge BS MS buffer state by an extra HARQ feedback –Clean up the obsolete HARQ soft buffer while necessary (C802.16m-08/1328)

7 MS-assisted HARQ Flow Control Concept: MS feedbacks current buffer status to BS to facilitate HARQ scheduling –Drop feedback (DROP): inform BS the overflow of MS HARQ soft buffer –DROP is only applied for aggressive HARQ transmission Normal HARQ transmission only applies 1-bit ACK/NACK feedback BS schedules aggressive HARQ transmission based on feedback –Retransmit if NACK is received –Reinitiate a dedicated HARQ process if DROP is received MS sends DROP feedback to notify BS of overflow dropping

8 Example Revisited with DROP Feedback 4 HARQ soft buffers 12 HARQ processes A/X denotes ACK with X buffer used Red N/X denotes NACK with X buffer used Blue D/X denotes DROP with X buffer used DROP signal can reinitiate HARQ transmission to avoid transmission failure due to the 2nd transmission 22 Packets pass –At least 10 more packets transmitted compared to conventional HARQ scheduling –5 packets transmitted compared to aggressive HARQ transmission 4 Packets are dropped –Less packets are dropped –It maintains HARQ-IR performance –HARQ reliability can be maintained for aggressive HARQ transmission

9 Performance Evaluation for HARQ-CC Throughput and Block Error Rate Round Trip Delay=10 ms MCS selection is based on predetermined SNR No feedback error When 10K soft bits are used in soft buffer, –drop feedback does not throughput gain –drop feedback maintains HARQ fail rate at 1%-0.1% instead of more than 0.1 HARQ fail rate Conventional scheduling can only provide maximum throughput 500Kbps with 10K soft bits We push throughput to 2X~3X If MCS selection can be better, more throughput and less HARQ fail rate are achievable

10 Performance Evaluation for HARQ-IR Throughput and Block Error Rate Round Trip Delay=10 ms MCS selection is based on predetermined SNR No feedback error When 10K soft bits are used in soft buffer, –drop feedback drives extra 33% throughput gain from retransmission of the first packet –drop feedback maintains HARQ fail rate at 1% instead of more than 0.1 HARQ fail rate Conventional scheduling can only provide maximum throughput 500Kbps with 10K soft bits We push throughput to 2X~3X If MCS selection can be better, more throughput and less HARQ fail rate are achievable

11 Example of Feedback Channel Design Conventional bi-state feedback: 12 orthogonal sequences for 6 for ACK and 6 for NACK Tri-state HARQ feedback: –Method: 12 orthogonal sequences for 4 ACK and 4 NACK and 4 DROP –Pros: Compatible with conventional design –Cons: Increased overhead Nice-State two channel HARQ feedback: –Method: two feedback channel joint coding Choose four sequence from 12 orthogonal sequences Construct nine sequences to represent two feedback channels –Pros: Compatible with conventional design Similar error rate performance –Cons: Even feedback channels

12 Error Rate Performance for Tri-State Feedback The proposed design provides similar performance comparing to 1-bit ACK/NACK Channel Bandwidth10MHz Over-sampling Factor28/25 FFT Size1024 Cyclic prefix (CP) ratio1/8 Channel conditionPB3, VA120, VA350 The number of antennasTx:1, Rx:2 ModulationBPSK FMT size6x2 Block size6x6 ReceiverHARQCH: non-coherent detection, MLD

13 Overhead issue Tri-state feedback channel is only applied for aggressive HARQ transmission mode, e.g. long burst service –Less resource shared by other users –More concurrent HARQ feedback channels Bi-state feedback channel is applied for normal HARQ transmission mode, e.g. VoIP Configuration example: 4 LRUs is configured for UL feedback channels –Tri-State HARQ feedback : 24 tri-state HARQ feedbacks (2 LRU)+36 1-bit HARQ feedbacks (2LRU) Transmitting large packet ( ≧ 50Kbits) with limited overhead is reasonable –Nice-State two channel HARQ feedbacks: 36 tri-state HARQ feedbacks (2LRU)+36 1-bit HARQ feedbacks (2LRU) Transmitting more HARQ processes to increase throughput is reasonable

14 An Example of Overhead Calculation 96 LRU per subframe @ 20MHz 100Mbps @ Downlink 500Kbps per 5ms frame –16 concurrent HARQ processes per 5ms 31250 bps per HARQ process 16 HARQ feedbacks per 5ms –16 Tri-State feedback occupies 4/3 LRU => 2LRU = 2.08% per UL subframe –8 Nice-State feedback occupies 8/9 LRU => 1LRU = 1.04% per UL subframe –Transmission (bits)/HARQ overhead (LRU) > 250K bits/LRU (high efficiency) –8 concurrent HARQ processes per 5ms 62500 bps per HARQ process 8 HARQ feedbacks per 5ms –8 Tri-State feedback occupies 2/3 LRU => 1LRU = 1.04% per UL subframe –4 Nice-State feedback occupies 4/9 LRU => 1LRU = 1.04% per UL subframe –Transmission (bits)/HARQ overhead (LRU) > 500K bits/ LRU (high efficiency) VoIP: –26Kbps per user and 18 concurrent users –520bits per 20ms frame –18 concurrent users share one LRU for HARQ feedback (Intel, Samsung, LGe,…etc) –Transmission (bits)/HARQ overhead (LRU)= 9.36K bits/LRU (Low efficiency) Feedback overhead is very minor for high throughput transmission

15 Conclusions Drop HARQ feedback alleviates buffer overflow issue due to lack of soft buffer –Drop feedback is introduced for HARQ flow control in case of buffer overflow –BS can reinitiate HARQ process to ensure MS receiving maximum 4 transmissions if MS discards HARQ packets –Drop HARQ feedback maintains link reliability for aggressive HARQ transmission –Drop HARQ feedback further drives extra throughput gain for HARQ-IR Drop HARQ feedback only introduces less HARQ feedback overhead for high throughput scenario Two exemplary 1-bit feedback compatible HARQ feedback designs are introduced –Two channel joint coding further provide similar error rate performance to 1- bit feedback design

16 Proposed text 11.13.2.7 Aggressive HARQ Transmission 16m BS can transmit coded bits more 16m MS soft buffer capability. DROP feedback is used to notify BS the MS buffer overflow when aggressive HARQ transmission is used. BS can restart the HARQ process to maintain identical link error rate performance to the normal HARQ transmission.

17 Backup Slides

18 Example of HARQ Flow Chart When most HARQ processes are incorrect received concurrently, buffer outage still occurs. When the case occurs, dropping HARQ feedback indicates the process is dropped to assist BS to reschedule Extra Advantages: –The successful transmission would not be dropped –The failed transmission could be safely reinitiated –ARQ-introduced latency is further reduced –Dropping signaling can further reinitiate HARQ process if DL allocation signal might be missed by MS, or MS ACK/NACK signal is missed

19 HARQ Feedback Errors and Response ACK => DROP : BS/MS restarts the process in the next transmission (no significant influence) ACK => NACK : send ACK at the next transmission (same as conventional HARQ feedback) NACK => DROP : BS/MS restarts the process in the next transmission (no significant influence) NACK => ACK : apply ARQ mechanism to restore the lost packet (same as conventional HARQ feedback) DROP => ACK : apply ARQ mechanism to restore the lost packet (same as conventional HARQ feedback) DROP => NACK : Send DROP in the next transmission to reinitiate the process (no significant influence)

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