Unable to decode OFDM based wifi packets

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rohan
Posts: 1
Joined: Mon Aug 31, 2015 3:05 am

Unable to decode OFDM based wifi packets

Post by rohan »

The project I am currently working on requires precise estimation of time of arrival of wifi packets at multiple bladeRFs. Following this project by Bastian, I decided to give it a try with my bladeRF x40 instead of USRP board to receive wifi packets suggested by Bastian.

The problem is, I am receiving very random constellation instead of the constellation actually used by the wifi USB sticks, when I run the wifi_rx.grc(provided in the examples).

This is my first hand with bladeRF and am not sure if I am following the correct steps. The steps I follow are as such:
1. Replace the uhd Source block in the wifi_rx.grc with osmocom source block. The only modification I do is the sample rate, center frequency and bandwidth to match with the replaced uhd block.
2. Listen to the wifi traffic and then create a adhoc network using FritziWlan USB sticks on channels that are not being used by the wifi infrastructure at my place(e.g 2.422GHz). I could see the beacons generated by these sticks using my laptop's wifi adaptor(monitor mode).
3. I run the flowgraph at the adhoc network frequency to see a very random constellation :(
I see the bit rate of beacon signals as 12 Mbps, which I believe is provided through a QPSK constellation using OFDM modulation. To ensure that my host machine receives the samples fast enough, I use a USB 3.0 port.

Please find attached the results I see, i.e
the constellation plot
Image
and FFT
Image

Hope to hear some suggestions from the bladerf community!

P.S I am working on Ubuntu machine(14.04 LTS) with GRC version 3.7.8
jynik
Posts: 455
Joined: Thu Jun 06, 2013 8:15 pm

Re: Unable to decode OFDM based wifi packets

Post by jynik »

Could you share what the expected output should look like?

With a cursory glance at the GRC file, I see there's some offset tuning parameters being used with the UHD source -- perhaps that's something to ensure you're matching.

Just a thought - why not take an intermediate step? If it's working with the UHD and a USRP, try getting it working with gr-osmosdr and a USRP. From there, then get it working with the gr-osmosdr and the bladeRF. Hopefully that might expose some variable or setting that was easy to overlook.
rup2014
Posts: 1
Joined: Tue Aug 01, 2017 8:15 am

Re: Unable to decode OFDM based wifi packets

Post by rup2014 »

Rohan did you end up getting this to work? I am currently trying to get the same flowgraph working for myself. Any help would be appreciated.
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