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MAC FEC Performance

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MAC FEC Performance

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  1. MAC FEC Performance Sean Coffey, Ph.D., Chris Heegard, Ph.D. Texas Instruments 141 Stony Circle, Suite 130 Santa Rosa, CA 95401 coffey@ti.com Sean Coffey, Texas Instruments

  2. Simulation assumptions • Service field problem fixed • 1452-byte packets (7 RS codewords) • Simulations with and without interleaver • Interleaver simulated is full (depth-7) byte interleaver Sean Coffey, Texas Instruments

  3. Key • All plots use Es/No • Green: standard mode • Blue: FEC, no interleaving (current draft) • Red: FEC, byte interleaving to maximum depth Sean Coffey, Texas Instruments

  4. Low rate modes, AWGN Sean Coffey, Texas Instruments

  5. Low rate modes, heavy multipath Sean Coffey, Texas Instruments

  6. High rate modes, AWGN Sean Coffey, Texas Instruments

  7. High rate modes, heavy multipath Sean Coffey, Texas Instruments

  8. Notes • FEC modes have lower rate • Note that use of Es/No implies no rate compensation • Difference in Es/No translates into range difference • E.g., 1 dB translates into 7% range difference using power law model with exponent 3.3 • Graphs plotted to different scales Sean Coffey, Texas Instruments

  9. FEC performance overview • FEC trades 10% rate reduction for robustness improvement • Robustness improvement roughly 2–3 dB as rule of thumb • Maximum depth byte interleaving has approx 1 dB advantage over no interleaving • FEC option is aimed at low PER • Increasing advantage is seen in AWGN as PER decreases • Parallel curves in heavy multipath Sean Coffey, Texas Instruments

  10. Curve steepness in MP vs. AWGN • Note that IEEE 100 ns delay spread “channel” is actually a set of channels • Simulation assumes new channel for each packet • PER significantly affected (dominated?) by probability of choosing bad channel • Not necessarily valid for fixed channel with multipath • Is there an agreed model? Sean Coffey, Texas Instruments

  11. Performance vs. standard modes • Most natural comparison is FEC+54 (= 50.5 Mbps) vs. standard 48 • Clear advantage to FEC+54 in AWGN, even at PER = 0.01: higher range and rate • In (channel-averaged) heavy multipath, FEC performs in line with rate, but no better Sean Coffey, Texas Instruments

  12. Performance, contd. • FEC+24 (= 22 Mbps) requires Es/No midway between 24 and 12 • FEC adds new rates, e.g., FEC+48 (=43 Mbps) fills gap in standard rates • has performance matching standard 36 Mbps in multipath Sean Coffey, Texas Instruments

  13. Es/No vs. Eb/No • Can compare FEC+(standard mode) to (standard mode) in Eb/No • Rate compensation subtracts 10 log10 (224/208) = 0.32 dB from Es/No difference • Using this metric, FEC is always better • However, this comparison is not really appropriate as signalling interval cannot be changed • Eb/No comparisons typically flatter the lower rate system, e.g., 48 Mbps is much “better” than 54 Mbps in existing 11a standard Sean Coffey, Texas Instruments

  14. Interleaving • Byte interleaving seeks to break up “bursts” of errors in decoded inner code stream • Note that the relevant bursts are in decoded information bits, not coded bits: bursts may be very short • Distribution of bursts changes with SNR • Shorter bursts with larger SNR • Distribution depends on code, puncturing, and modulation scheme Sean Coffey, Texas Instruments

  15. Interleaving, contd. • Results shown are for full-depth interleaving • Byte interleaving gains some performance • Adds delay: amount depends on implementation • No results presented for depth-2 byte interleaving • Probably a better choice than full-depth interleaving Sean Coffey, Texas Instruments

  16. Pro: Interleaving adds some performance In some cases, further increases slope, advantage increases at lower PERs Con: Adds considerable delay, depending on interleaver chosen Restricts broad applicability of option Interleaving pros & cons Sean Coffey, Texas Instruments

  17. Other possible changes • Possible performance gain from altering code parameters • E.g., (255, 235) RS code has approximately same rate and corrects 10 errors Sean Coffey, Texas Instruments

  18. Conclusions • Solid gains achievable using MAC FEC • Other design decisions are possible, may provide limited further performance gains Sean Coffey, Texas Instruments