Unable to decode OFDM based wifi packets
Posted: Thu Sep 03, 2015 6:46 am
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
and FFT
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
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
and FFT
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