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Multipath comparison of IEEE802.11g High Rate Proposals

This study compares the performance of CCK-OFDM and PBCC-22 in multipath conditions. Results show PBCC-22's superiority over CCK-OFDM.

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Multipath comparison of IEEE802.11g High Rate Proposals

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  1. Multipath comparison ofIEEE802.11g High Rate Proposals Sean Coffey, Anuj Batra, Srikanth Gummadi, Chris Heegard, Matthew Shoemake Texas Instruments 141 Stony Circle, Suite 130 Santa Rosa California 95401 (707) 521-3060,coffey@ti.com Coffey et al, Texas Instruments

  2. Contents • CCK-OFDM is not 802.11a • Receiver structures • Multipath performance comparisons • Conclusions Coffey et al, Texas Instruments

  3. What is wrong with CCK-OFDM that is right with pure 11a OFDM? Coffey et al, Texas Instruments

  4. Overhead and Data Payloads PBCC 11a CCK-OFDM No acks, 500 byte packets Coffey et al, Texas Instruments

  5. Relative throughputs Coffey et al, Texas Instruments

  6. The CCK-OFDM Dilemma Any receiver requires overhead for channel estimation, tracking, etc: • 802.11a is “pay as you go”-ultra-short preamble, 16 musecs • 802.11b and PBCC-22 are “pay all up front”-11b “short” preamble, 96 musecs • CCK-OFDM is “pay up front and again as you go”-11b “short” preamble, plus (non-standard) OFDM preamble, 110 musecs • “Double the pain” Coffey et al, Texas Instruments

  7. Packet size & system performance Compare performances – results are critically dependent on packet size • Short packets (e.g., MPEG-4 packets of 188 bytes) strongly favor “pay-as-you-go” approach  802.11a/Hiperlan 2 • Long packets increasingly favor “pay-all-up-front” approach”  PBCC-22 aimed at this application • Short or long, it won’t work well if it’s CCK-OFDM Coffey et al, Texas Instruments

  8. PBCC-22 features • Excellent performance in full range of multipath conditions - much better than CCK-OFDM at comparable rates • This is the central technical point in dispute • Range advantage of PBCC 22 Mbps over CCK-OFDM 24 Mbps in multipath conditions is 30-40%. • These claims documented later; standard IEEE models were used. Coffey et al, Texas Instruments

  9. Multipath comparison of the proposals First, for each proposal, assume same ground rules: • floating point implementation • full channel knowledge • standard IEEE multipath model • off-the-shelf algorithms • assume each uses receiver structure presented by proposers Coffey et al, Texas Instruments

  10. PBCC-22 Receiver: • treat multipath and code as forming a composite state machine, or “super code” • decode the “super trellis” using any standard reduced state algorithm Simulation results here assume whitened matched filter plus M-algorithm; standard material, very well understood: • whitened matched filter - Forney, 1972. • M-algorithm - Anderson, 1969. Coffey et al, Texas Instruments

  11. M-algorithm decoder background: • M-algorithm operates like regular trellis decoder, but retains only best “M” paths at each depth • No restrictions on choice of M • straightforward way of trading performance versus complexity • natural receiver upgrade path • Main results use M = 64 • we also present M = 8, M = 16, M = 32, M = 128. Coffey et al, Texas Instruments

  12. M-algorithm decoder: • Assume the “state” consists of input data bits at last 8 time units • Compare last 4 time units to represent pure code state • Choice of 8 is arbitrary, other values possible • Assume each “state” remembers the full impact of the past on the future • Curve shown in Doc. 01/140 assumes instead that multipath is regenerated from last 8 time unit inputs Coffey et al, Texas Instruments

  13. Baseline comparisons, IEEE multipath model, 100 ns From Doc. 00/392r1 5dB Ideal channel knowledge, floating point implementations Coffey et al, Texas Instruments

  14. Baseline comparisons, 100 ns, contd. From Doc. 00/392r1 Coffey et al, Texas Instruments

  15. Implications of 5 dB advantage: 5 dB translates to a factor of 3.1 • For similar throughput and range, PBCC requires 3 times less received power than CCK-OFDM 24 Mbps – translates to greater battery life • For similar throughput and received power, PBCC has 40% more range than CCK-OFDM 24 Mbps • Assuming the “power of 3.3” model for path loss – this is the standard model used in 802.15.2 (Doc. 802.15/138r0) Coffey et al, Texas Instruments

  16. 100ns:40% PBCC range advantage PBCC 22 Mbps CCK-OFDM 24 Mbps Double the coverage Coffey et al, Texas Instruments

  17. Relative throughputs: Coffey et al, Texas Instruments

  18. Actual PBCC receiver algorithms, 100 ns 2.9dB Ideal channel knowledge, floating point implementation for CCK-OFDM Coffey et al, Texas Instruments

  19. Baseline comparisons, contd: 250 ns 4.5dB Ideal channel knowledge, floating point implementations Coffey et al, Texas Instruments

  20. Baseline comparisons, 250 ns, contd. Coffey et al, Texas Instruments

  21. Conclusions • PBCC-22 has a natural superiority over CCK-OFDM in multipath. • Established IEEE multipath model used. • CCK-OFDM tries to juggle two incompatible things – makes an underperforming system out of a merger of two otherwise good components • A better way of doing things – PBCC-22 + .11a! Coffey et al, Texas Instruments

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