Zynq SDR Receiver Block outputs data regardless of hardware state/cannot detect 802.11 beacons.

I'm trying to detect the 802.11 beacon from my iphone 'hotspot' (2.4GHz, channel 1) using the ' Targeting the HDL Optimized 802.11 WLAN Beacon Frame Receiver Using Analog Devices AD9361/AD9364' example design.

I've used HDL Workflow Advisor to load the image onto the FPGA fabric without any problems but when I run the retargeted simulation, without error, there are no beacons to be found, on any channel.

To trouble shoot this I've tried using a 'to workspace' block to write the output of the SDR Receiver block to the workspace and it does show activity but it continues to do so even if I ground the antenna connector.

I now don't know how to continue troubleshooting since it seems the signal passing into the system is not coming from my antenna. Has anyone got any ideas for a sensible next step?

Answer 1

[Neil MacEwen]

Hi Harry,

This example will only receive 802.11b standard waveforms. I believe many routers do not transmit that beacon by default, you may have to edit the router setup to transmit this beacon. For this reason we are planning to remove this example in a future release.

This example is also designed to always receive data. Whether the data samples coming from the board to the host contain valid data or not is encoded into the sample stream. If you look in the subsystem "zynqRadioQPSKRxFPGAAD9361AD9364SL/QPSK Receiver/HDLRx/FPGA to Host" you will see that the valid signal is packed into one of the data samples. The host will receive a constant stream of data, but that doesn't necessarily mean that the data is good. Rest assured that the data path from your antenna, through the algorithm and back to MATLAB/Simulink should be good!

Cheers,

Neil

[Hewy.j]

Thanks Neil!

That all makes sense - I'll see if I can find something that broadcasts 802.11b and give that a try.

I accept what you've said about the data being continuously output, good or bad but I am still misunderstanding something... using the same SDR Receiver in:

>>zynqRadioQPSKRxAD9361AD9364SL

Or indeed the model you mentioned above:

>>zynqRadioQPSKRxFPGAAD9361AD9364SL

...while running a Transmit-Repeat in MATLAB, displays the indexed Hello World whether the antenna loopback cable is connected or not. Am I being daft? Does the Transmit-Repeat not tell the hardware to transmit out into the real world? My signal seems to be bypassing the over-the-air component of things.

I'll add the MATLAB code below. But thanks Neil, I appreciate the help.

Regards, Harry.

 load('zynqRadioQPSKTransmitData.mat');

 tx = sdrtx('ADI RF SOM', ...
              'BasebandSampleRate',      600.76e3, ... 
              'CenterFrequency',         2.4e9, ...
              'ChannelMapping',          1, ...
              'ShowAdvancedProperties',  true, ...
              'BypassUserLogic',         true);

 transmitRepeat(tx, zynqRadioQPSKTransmitData);

[Neil MacEwen]

Nope, transmitRepeat will indeed send the signal out the antenna. What you're seeing is just leakage between the transmit and receive paths on the hardware, it's enough to receive the data. In a real system, transmitting and receiving on the same frequency on the same hardware at the same time wouldn't be recommended, but it is useful for loopback tests like we are doing here.

If you wanted to test it out, you could try reducing the transmit power and seeing where it fails. It may be that you see it fail at a higher power when you don't have the cable connected. It also may be that the leakage is still enough.

Cheers,

Neil

[Hewy.j]

Success!

I'm detecting beacon packets from a router! Maybe just stating in the example that it only works with 802.11b would be better than deleting the whole thing?

I should have realised about the leakage, it seems obvious now you say it.

Thank you Neil, I appreciate all your help!

Regards, Harry.