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Cancellation of aggregate Multicast feedback – measurement results

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1 Cancellation of aggregate Multicast feedback – measurement results
July 2010 doc.: IEEE /0768r0 July 2010 Cancellation of aggregate Multicast feedback – measurement results Date: Authors: Jochen Miroll Jochen Miroll, Saarland University

2 July 2010 doc.: IEEE /0768r0 July 2010 Abstract This presentation is an update on the Leader-based aggregate feedback Protocol (LBP) proposal previously made to TGaa by the authors and provides measurement results obtained on a consumer hardware test bed The feedback cancellation probability in the worst case of LBP is measured and compared to previous theoretical / simulation results These results have also been published and presented at the IEEE ISCE 2010 conference in June 2010 Jochen Miroll Jochen Miroll, Saarland University

3 Motivation 11aa is standardizing Multicast ARQ: MRG
July 2010 Motivation 11aa is standardizing Multicast ARQ: MRG Gathering per-receiver feedback, the overhead due to the positive ACKs grows linearly with the number n of receivers How does 11aa MRG compensate for this increased overhead? Aggregation of multiple frames: single-TID, uncompressed Block-ACK (802.11n) for MRG Per-frame ACK becomes multi-frame Block-ACK bitmap for the last k frames Still: overhead increases linearly with receivers n How to get rid of the dependency on n? We have previously proposed a leader-based Multicast retransmission scheme to 11aa Jochen Miroll

4 Feedback aggregation in the same time slot
July 2010 Feedback aggregation in the same time slot All receivers provide feedback, but it is aggregated in the same time slot - then (n = number of receivers) overhead(n) = overhead(1) Idea: Introduce NACK Transmit a data frame Then, ask for ACK/NACK If STA i has received the frame: it responds with an ACK If STA j did not: it responds with NACK at the same time ? AP1 ACK STA 1 NACK STA 2 STA 3 NACK STA 4 Jochen Miroll

5 Feedback cancellation premise
July 2010 Feedback cancellation premise If ACK and NACK are approx. equally strong Is it possible to cancel an ACK by a simultaneous NACK and thus enforce a retransmission? The „capture effect“: Describes the phenomenon that a frame (e.g. ACK) may be received correctly in the presence of another, similarly strong (e.g. NACK) Main reasons for „imperfect collision“ Locking the preamble and then Viterbi decoding the locked-onto frame is a very robust mechanism. E.g.: ACK is BPSK, rate ½ and only 14 Bytes in length. It is the most robust frame (OFDM: few dB difference between ACK and NACK may suffice to „capture“) Jochen Miroll

6 Earlier comments from TGaa (resolved)
July 2010 Earlier comments from TGaa (resolved) Will feedback cancellation actually work? answer: Yes, collisions are happening all of the time answer: No, due to the capture effect We have consequently provided Matlab and ns-2 results for feedback cancellation to Tgaa cf. doc.: IEEE /1150r2 Provided in this document: measurement results using real and cheap hardware Jochen Miroll

7 Leader-based feedback cancellation
July 2010 Leader-based feedback cancellation Idea: Multicast is essentially handled as a unicast connection to a „leader receiver“ „Non-leaders“ transmit a NACK if a frame is lost Target: Larger Multicast groups (large n) If ACK survives the somewhat weaker NACK, does it survive many? Does it survive many equally strong, many somewhat stronger? Intuitive leader selection: choose the „weakest“ receiver (as seen by the AP, no power control, just due to path loss) If no loss: Leader’s ACKs can be received (ACKs are most robust) Else: Expect a good chance that whenever several somewhat stronger NACKs are transmitted at the same time, the Leader’s ACK will be cancelled Jochen Miroll

8 July 2010 doc.: IEEE /0768r0 July 2010 Aggregation through Leader-Based feedback cancellation Protocol (LBP) cf. doc.: IEEE /0290r1 optional SEQ# indicator and NAV updater to synchronize aggregate feedback Jochen Miroll Jochen Miroll, Saarland University

9 Feedback cancellation constraint
July 2010 Feedback cancellation constraint Failure of feedback cancellation results in uncorrectable packet loss at non-leaders (i.e. capture of ACK happens, no collision) Question that arises: What is the error floor in the worst case? What is the worst case for the leader-based feedback cancellation approach? Intuitively: the „weakest“ receiver can not be distinguished All receivers on average experience the same SNR We assume that all are sending approx. equally strong feedback Jochen Miroll

10 Feedback cancellation measurements
July 2010 Feedback cancellation measurements Examine two different cases of how feedback aggregation may be implemented In the WLAN card‘s real-time OS In the WLAN card‘s host OS (e.g. Linux) Implications Cards allow for strict timing constraints (similar to ACK, ±900ns), so we can examine short feedback Host OS is less accurate in timing, thus we examine feedback cancellation with frames of several tens of Bytes Jochen Miroll

11 Feedback cancellation test setup (1)
July 2010 Feedback cancellation test setup (1) We have used real consumer hardware Limited freedom in implementing MAC algorithms But: We can fix some parameters in cancellation experiments Here: Non-leaders transmit different frames Examine different frame sizes and timings with what is possible… …out of the box: Let positive feedback be a 6 Mbps ACK and the negative feedback be a 12 Mbps ACK …own implementation: Driver level ACK/NACK implementation Jochen Miroll

12 Feedback cancellation test setup (2)
July 2010 Feedback cancellation test setup (2) SEQ frame triggers feedback, assume this is the question „did you get the data frame“ Jochen Miroll

13 Feedback cancellation test setup (3)
July 2010 Feedback cancellation test setup (3) To obtain independence from the (fading) environment: Move receivers slowly around the AP, changing their positions in the environment Periodically change the roles (leader, non-leader) of the receivers (always have exactly 1 leader) Non-leader 1 Non-leader n-1 Leader AP Jochen Miroll

14 Validation of test setup
July 2010 Validation of test setup CDF of SNR at receivers is very steep ~identical channel conditions for all receivers on average Error free reception rates of different frames at the end of measurement run yield valid results SEQ (trigger) loss? loss rate < 0.1% Jochen Miroll

15 Test results (representative example)
July 2010 Test results (representative example) Parameter Value wireless channel 40 (5.2 GHz) AP transmit power 17 dBm Station transmit power 8 dBm Number of stations/non- leaders 4/3 SEQ frequency 10 Hz SEQs transmitted ~26000 Role switching interval 100 s 1 leader, 3 non-leaders Why? An example, assume But: Assume large n Virtually no SEQ loss ~89% feedback cancellation success probability Result seems independent of frame length and timing Worst case results (where leader-selection would not work) Measurement parameter Rate LBP ACK loss avg. LBP NACK loss avg. Hardware ACK-6 loss avg. Hardware ACK-12 loss avg. LBP SEQ loss at station 1 LBP SEQ loss at station 2 LBP SEQ loss at station 3 LBP SEQ loss at station 4 Jochen Miroll

16 Theoretical / Simulation results
July 2010 Theoretical / Simulation results Compare with ns-2 results Scenario: Rayleigh fading channel, equal AP-STAs distance feedback cancellation rate is about 76% for 2, more than 90% for more than 2, and already 99% for 5 receivers Again: worst case where leader selection fails Jochen Miroll

17 July 2010 Conclusion Scalable Multicast error correction can be achieved by aggregation through cancellation Real test bed results are backed up by simulations Channel will not be arbitrarily reliable but limited by an error floor Combined MAC-layer and “Application Layer” error correction feasible Assume overlay packet erasure FEC Audio/Visual streams typically can tolerate errors Residual error requirement can be dealt with on layers above MAC Jochen Miroll

18 July 2010 Questions? (a further presentation will propose how this scheme should be incorporated into 11a) Jochen Miroll

19 Recap: Hybrid LBP (HLBP)* cf. doc.: IEEE 802.11-09/0290r1
July 2010 Recap: Hybrid LBP (HLBP)* cf. doc.: IEEE /0290r1 Phase I Transmit a block of frames, as in MRG BA. Here: systematic FEC part Phase II Parity phase. Instead of BAR/BA, do AggregateAckRequest/AggregateAck * Assume e.g. DVB-IPDC or Raptor code on upper layer, MAC somehow knows which packets are systematic (DATA) or parity Jochen Miroll

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