Improving Wireless Link Throughput via Interleaved FEC Ling-Jyh Chen, Tony Sun, M. Y. Sanadidi, Mario Gerla Computer Science Department, University of California at Los Angeles

Background Dealing with packet losses: End-to-end Link Layer Level Improving Wireless Link Throughput via Interleaved FEC Background Dealing with packet losses: End-to-end ARQ: Retransmission in TCP FEC: Robust Audio Tool (RAT) [14] Partial Correctness: Interleaving as in Video transfer protocols [RFC 2354] Link Layer Level ARQ: as in HDLC, or Bluetooth link layer protocol FEC: as in 802.11a and Bluetooth RAT: http://www-mice.cs.ucl.ac.uk/multimedia/software/rat/ June 2004 ISCC 2004

Background Problems: ARQ does not perform well for high error rates Improving Wireless Link Throughput via Interleaved FEC Background Problems: ARQ does not perform well for high error rates FEC works if errors are not bursty Interleaving introduces latency Our Proposal: I-FEC combines FEC and Interleaving at Bluetooth link layer for high-rate bursty errors June 2004 ISCC 2004

Background In Bluetooth DM mode, FEC uses a (15, 10) Hamming code Improving Wireless Link Throughput via Interleaved FEC Background In Bluetooth DM mode, FEC uses a (15, 10) Hamming code Each block of 10 information bits is encoded into a 15 bit codeword Capable of correcting single bit error in each block Mode FEC Packet Symmetric Throughput (kbps) Asymmetric Throughput (kbps) Size (bytes) Length (slots) DM1 Yes 17 1 108.8 DM3 121 3 258.1 387.2 54.4 DM5 227 5 286.7 477.8 36.3 DH1 No 27 172.8 DH3 183 390.4 585.6 86.4 DH5 339 433.9 723.2 57.6 DM: has FEC coding; DH: has no FEC coding Time slots accommodate one packet, different packet lengths… feedback (occupy one time slot) at end of the number of slots == and of a packet Symmetric means data in two directions June 2004 ISCC 2004

Gilbert-Elliott Burst Error Model Improving Wireless Link Throughput via Interleaved FEC Gilbert-Elliott Burst Error Model June 2004 ISCC 2004

Improving Wireless Link Throughput via Interleaved FEC Burst Error Length At high Pbb, burst length are too long; do not expect FEC to work June 2004 ISCC 2004

Packet Error Rate after FEC Improving Wireless Link Throughput via Interleaved FEC Packet Error Rate after FEC Pgb = 0.0005 Packet Size FEC Level Pbb = 0.1 Pbb = 0.3 Pbb = 0.5 Pbb = 0.7 Pbb = 0.9 100 bytes (15,10) 3.25 % 9.42 % 15.44 % 21.47 % 28.61 % (7,4) 3.39 % 10.24 % 16.92 % 23.45 % 30.18 % None 32.92 % 32.93 % 32.94 % 32.98 % 33.23 % 200 bytes 7.40 % 20.48 % 31.84 % 42.00 % 51.16 % 6.76 % 19.39 % 30.79 % 41.28 % 50.91 % 55.09 % 55.10 % 55.11 % 55.13 % 55.29 % 500 bytes 16.86 % 42.59 % 60.57 % 73.33 % 82.68 % 16.09 % 41.90 % 60.51 % 73.76 % 83.15 % 86.49 % 86.50 % 86.55 % Simulation results; Monte Carlo, multiple runs, average the results obtained When Pbb is high, FEC does not help much! June 2004 ISCC 2004

Proposed Approach: I-FEC Improving Wireless Link Throughput via Interleaved FEC Proposed Approach: I-FEC Inherits both the robustness to random errors from FEC and the survivability to burst errors from Interleaving Simulated I-FEC in Bluetooth, and to Bluetooth DH (no FEC coding) and DM (FEC coding) modes June 2004 ISCC 2004

Proposed Approach: I-FEC Improving Wireless Link Throughput via Interleaved FEC Proposed Approach: I-FEC Bluetooth DM mode (FECed already!) (b) I-FEC Interleaving by transposition The FEC overhead is the same as DM mode The latency caused by interleaving is negligible, since I-FEC interleaves data in bit level within one link layer packet, instead of packet level cross different packets June 2004 ISCC 2004

Improving Wireless Link Throughput via Interleaved FEC Evaluation Comparison of I-FEC, DH, and DM mode packets using 5-timeslot Bluetooth packets. PER with different Pgb and Pbb (using Monte Carlo Simulation) TCP Throughput with different Pgb and Pbb (using NS2 simulator) Simulation Topology: (a) one hop (b) two hops I-FEC is implemented here on each link in (b) TCP is end-to-end June 2004 ISCC 2004

Packet Error Rates (1 hop) Improving Wireless Link Throughput via Interleaved FEC Packet Error Rates (1 hop) Pgb = 0.0005 Pbb = 0.2 June 2004 ISCC 2004

TCP Performance Improving Wireless Link Throughput via Interleaved FEC Pgb not Pab Pbb = 0.2 Pbb = 0.2 June 2004 ISCC 2004

TCP Performance Improving Wireless Link Throughput via Interleaved FEC FEC overhead will cause its performance to be worse than no FEC after some Pbb, that is after some average burst length is exceeded; in that range application throughput will drop sue to useless use of FEC redundancy bits Pgb = 0.0003 Pgb = 0.0003 June 2004 ISCC 2004

Improving Wireless Link Throughput via Interleaved FEC Conclusion I-FEC, a hybrid approach incorporating the robustness of FEC coding to random errors and the survivability of interleaving to burst errors I-FEC is simple and applicable to other link technologies, as long as FEC is already there June 2004 ISCC 2004

T h a n k s Improving Wireless Link Throughput via Interleaved FEC June 2004 ISCC 2004