1. Sean Coffey, Ph.D., Chris Heegard, Ph.D.
Month 1998
doc.: IEEE /xxx
March 2002
MAC FEC Performance
Sean Coffey, Ph.D., Chris Heegard, Ph.D.
Texas Instruments
141 Stony Circle, Suite 130
Santa Rosa, CA 95401
Sean Coffey, Texas Instruments

2. Simulation assumptions
March 2002
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
March 2002
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. March 2002
Low rate modes, AWGN
Sean Coffey, Texas Instruments

5. Low rate modes, heavy multipath
March 2002
Low rate modes, heavy multipath
Sean Coffey, Texas Instruments

6. March 2002
High rate modes, AWGN
Sean Coffey, Texas Instruments

7. High rate modes, heavy multipath
March 2002
High rate modes, heavy multipath
Sean Coffey, Texas Instruments

8. Notes FEC modes have lower rate
March 2002
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
March 2002
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
March 2002
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
March 2002
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. March 2002
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. March 2002
Es/No vs. Eb/No
Can compare FEC+(standard mode) to (standard mode) in Eb/No
Rate compensation subtracts 10 log10 (224/208) = 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. March 2002
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
March 2002
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. Interleaving pros & cons
March 2002
Interleaving pros & cons
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
Sean Coffey, Texas Instruments

17. Other possible changes
March 2002
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
March 2002
Conclusions
Solid gains achievable using MAC FEC
Other design decisions are possible, may provide limited further performance gains
Sean Coffey, Texas Instruments