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Bug report: 802.11ac with LDPC

Question asked by hmc on Feb 21, 2018
Latest reply on Apr 4, 2018 by jonathan.falco

I have a N5182B signal generator with a WiFi licence, allowing me to generate WiFi packets using N7617B Signal Studio for Wireless LAN (latest version 3.4.0.0).

 

I believe there is a bug affecting many configurations of 802.11ac with LDPC encoding. More specifically, it appears that under certain configurations, an insufficient number of OFDM symbols is generated (1 fewer than is suggested by the 802.11ac standard, according to my interpretation). I have attached a WLAN configuration file which I believe reproduces the issue (Note: changing Data Length from 64 to 65 or 66 seems to produce valid packets).

 

My other main point of reference is Matlab's WLAN toolbox. Out of the box, their implementation is incorrect, but after a series of communications with Mathworks, three bugs have been fixed and I now agree with their interpretation of the 802.11ac standard (i.e. for the attached configuration, their transmissions would contain one extra OFDM symbol).

 

I know the procedure for determining the number of OFDM symbols in LDPC packets is slightly convoluted (and in 802.11ac there is an extra flag in the VHT-SIG whose role I may have misinterpreted). Therefore, I would greatly appreciate any assistance in establishing what the correct behaviour should be.

 

Many thanks in advance.

 

Harry

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