Preamble detection using the Wireless Testbench

Dear all,

I am trying to use the preamble detection function in Matlab Wireless Testbench, with 2 USRPs N310. In the transmitter side, I generate a baseband signal, wchich I transmit using the transmit function:

transmit(bbtx,signal,"continuous");

stopTransmission(bbtx);

In the receiver side I use the preamble detection function:

recordLen = milliseconds(1000)

timeout = milliseconds(1000)

[data, timestamp, ~, status] = capture(pd,recordLen, timeout);

At the moment, the two USRPs are connected with cable. Unfortunatelly, only once I managed to get the result given in teh attachment, and most of teh time, I do not detect packets. Could you help me understand the problem?

[Jieming Xu]

Hi Giti,

I noticed that you are you baseband transceiver to transmit signals. If possible, please check the transmitter is working properly. You can use internal loopback or transmit and receive by the same devices to check the transmitter works properly.

As you said, the two USRP devices are wired by cables. Please make sure an attenuator is placed between the transmitter and receiver to avoid it damage the RF receiver.

Best regards,

Jieming Xu

Answer 1

[Jieming Xu]

Hi Giti,

In preamble detector, if you don't receive anything, you can check the thresholding value and correlator output by using: pd.plotThreshold. From the result, you can check and tune the threshold if it is too high or too low.

From you attachment, I saw you are using the plot threshold function. From the picture you provide, I can see very clear peaks for detection, which mean the transmission and receive of the two N310 devices you are using at that time works corretly. The picture shows me the threshold you set is low, which causes many false alarm detection point where there are no peaks.

If it still doesn't work, please tell me which threshold method (fixed, adaptive) you are using. Also with a plotThreshold picture when you cannot make a detection.

[Giti Dimrov]

Dear Jieming,

Thank you very much for your answer. I connected the devices with each other using a cable and 30 dB attenuator. However, today I noticed something weird. Now when I try to rerun the examples provided by matlab for the preamble detector, I can not detect anymore any packet. It always says: Detection failed.

I tried to reinstall matlab, set up the USRP again. Could you please help he understand the problem? I even tried to use different antenna: e.g., RF1: RX2 etc.

If it would be possible, can we have a zoom call?

Best Regards,

Giti

[Giti Dimrov]

Also, I now tried to run the loopback example provided under Wireless Testbench site, and the result is attached. But whenever I try to run other examples, I can not detect any packet

[Giti Dimrov]

Dear Jieming,

I tried today to set the transmit/receive gain to high values such as 65 dB, and finally I am getting the following results from the Matlab examples (please see atached):

https://www.mathworks.com/help/wireless-testbench/gs/loopback-transmit-and-capture.html

https://www.mathworks.com/help/wireless-testbench/gs/triggered-capture-using-preamble-detection.html

From the results I see that the signal power is very low.

[Neil MacEwen]

Hi Giti,

Looking at your spectrum plot, you are correct, the power is very low. We reproduced the same example on our N310 which has antennas connected and see much higher powers at much lower gains. You may want to investigate the hardware you are using (loopback cable + attenuator).

If you have more detailed questions please contact us via our support pages.

Thanks,

Neil

[Neil MacEwen]

This was resolved with the connection of antennas on transmit and receive ports for the loopback case.