802.11n Link in Simulink -- interference case scenario
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First of all I am using Matlab 2019a with the latest update. I reused the Simulink example that's provided by Matlab (wlan/HTLinkModelWLANExample). However, I modified it to simulate packet error rate (PER) in case of very narrow band (VNB) GMSK-based interference (~100Hz) instead of AWGN noise (SNR is actually set to 50 dB). Based on my thorough practical measurements in the anechoic chamber, I was expecting similar results in Simulink, but I got different ones as well as different/unexplained behavior.
The two main issues in Simulink I have trouble with are:
1. Given the same SIR, the PER performance in case the VNB interference coincides with a pilot sub-carrier is better than that when it coincides with a data sub-carrier. This is contradicting with my measurements which clearly indicate the opposite.
2. Given the same SIR, some data sub-carriers (e.g. 12 -- 15) are clearly more resilient to the VNB interference compared to other data sub-carriers (e.g. 16 -- 19). This is again contradicting with my measurements which indicate that the PER performance is more or less the same regardless of which data sub-carrier is interfered with.
I would appreciate any help/suggestion/clarification. Please let me know if more clarification about the simulation/measurements is needed.
p.s. I attached the modified Simulink file.
SIR = total WiFi-signal-power to VNB-interference-power ratio
Thanks in advance.
Shashwat Bajpai on 19 Nov 2019
I believe performing smoothing on the channel estimate will lead to performance closer to your expectation. In the simulation the channel estimate does not perform any smoothing (averaging over subcarriers). Without smoothing, when a subcarrier is affected by narrowband interference the channel estimate assumes this interference is caused by the channel and treats it as a complex gain for the subcarrier, rather than an offset (which is is). When equalizing the data subcarrier, this incorrect estimate is used to scale and rotate the subcarrier. This in turn causes the decoding to fail. The reason some subcarriers are more susceptible than others is due to the properties of the noise after demodulation. Depending on the subcarrier index the phase of the equalized interference may change per symbol leading to a messy constellation, while on some there is no phase change per symbol and the constellation has a constant offset. The symbol demapper seems to deal with these cases differently, leading to different error rates.
Smoothing the channel estimate will minimize the incorrect gain and rotation applied to the subcarrier during equalization with the narrow band interference as the interference is averaged out. In WLAN Toolbox you can enable smoothing by passing the smoothing span as the third argument to the estimator: wlanHTLTFChannelEstimate(demodHTLTF,cfgHT,span);
Hope this helps!