Hi Everyone,

I’m new to the forum and the LimeSDR device itself and I have a few questions about using the LimeSDR-USB device for TX/RX OFDM symbols.

I am using GNURadio to write a OFDM Tx/Rx pipeline, where the source is a set of custom symbols that I modulate onto OFDM subcarriers. I have tried the pipeline by looping the Tx time domain signal directly to the Rx input and everything works as expected. I even tried adding a simulated channel and it worked as well. However, when I connect the Tx time domain signal to the LimeSDR sink block and try to receive using the corresponding source block, I find that the receiver PSD has very low power even though the antennas are only 5cm apart from each other (see attached PSD plot). I am using one LimeSDR-USB as the Tx and another as Rx. I also noticed that the Rx PSD is fluctuating a lot, sometimes dropping very low, perhaps indicating it is not able to receive the data quickly enough. Because of this my received data is estimating very low SNR values (<0dB). I am using 2.5M sampling rate and 440MHz RF frequency. Tx gain=50dB, Rx gain=30dB.

As a sanity check, I used the SDRs to transmit and receive music using FM and that worked well so I think I am configuring the source/sink blocks correctly. Can anyone please advise how I should go about debugging this issue? Thanks in advance!