1. An evaluation of 802.11 error-correcting codes
March 2018
doc.: IEEE /0303r5
November 2018
An evaluation of error-correcting codes
Date: 11/12/2018
Authors:
Sigurd Schelstraete, Quantenna
Dorothy Stanley (HP Enterprise)

2. Introduction: 802.11 FEC generations
November 2018
Introduction: FEC generations
802.11a defines BCC codes with coding rates 1/2, 2/3 and ¾
BCC is the only FEC scheme
802.11n added BCC with rate 5/6 and introduced LPDC codes as an optional FEC scheme
Block sizes 648, 1296 and 1944 bits
802.11ac maintained the n FEC schemes
802.11ax makes LPDC mandatory for a majority of cases
Sigurd Schelstraete, Quantenna

3. How well does the current FEC perform?
November 2018
How well does the current FEC perform?
Typically, coding performance is measured against the Shannon limit
Theoretical SNR limit at which error-free transmission can be achieved
Any gap between the performance of the code at a given BER (e.g. 10-6) and the Shannon limit can be seen as a measure of quality for the code
Some caveats:
Comparing error-free performance vs. performance at chosen BER is not unambiguous
The Shannon limit does not assume any specific modulation
Sigurd Schelstraete, Quantenna

4. November 2018
The Shannon limit
The Shannon limit defines the capacity of the channel at a given SNR as:
𝐶=𝑙𝑜𝑔 1+ 𝑆 𝑁
There is also a discrete-constellation version of the Shannon limit that applies when the modulation is fixed (e.g. QAM, PAM, …)
Needs to be derived numerically
For K equiprobable signal points:
𝐶=𝑙𝑜𝑔 𝐾 − 1 𝐾 𝑎 𝑘 𝑛 𝑙𝑜𝑔 𝑎 𝑘 ′ 𝑒𝑥𝑝 − 𝑛− 𝑎 𝑘 ′ + 𝑎 𝑘 2 − 𝑛 2 2 𝜎 2
Sigurd Schelstraete, Quantenna

5. November 2018
The Shannon limit(s)
Uncoded performance at 10-6 is about 8-10 dB away for the Shannon limit
Sigurd Schelstraete, Quantenna

6. Performance of current coding schemes
November 2018
Performance of current coding schemes
LDPC improves about 2 dB over BCC
About 1.5 to 2 dB gap with the “discrete constellation” limit
About 3.5 dB gap to “unconstrained” Shannon limit
Sigurd Schelstraete, Quantenna

7. November 2018
Conclusions
Current FEC compare relatively well to Shannon limit
LDPC has narrowed the gap
Some gap to full capacity remains
“Anecdotical evidence” that further coding gain is possible
The complexity needed to narrow this gap is unknown
Possible considerations:
Typically, longer LDPC codes improve performance
Note that 802.3ca has already adopted a 18K LDPC block code [1]
3GPP has defined LDPC with block size 8448 [2]
Would TCM in combination with LDPC result in further improvement?
Sigurd Schelstraete, Quantenna

8. November 2018
Recommendation
Incremental improvement in coding gain may be possible over the currently defined codes for
Given that:
Current FEC schemes haven’t been updated in 10 years
Other projects have adopted newer codes
Further improvements appear possible
We recommend that EHT considers review of FEC within scope of the project
Sigurd Schelstraete, Quantenna

9. November 2018
References
[1] FEC Proposal for NGEPON, laubach_3ca_1a_1117, [2] 3GPP TS , NR; Multiplexing and channel coding
Sigurd Schelstraete, Quantenna