# LimeSDR-USB OFDM TX/RX noise

**URL:** <https://discourse.myriadrf.org/t/limesdr-usb-ofdm-tx-rx-noise/7526>\
**Category:** LimeSDR\
**Created:** [15 February 2022 19:12 UTC](https://discourse.myriadrf.org/t/limesdr-usb-ofdm-tx-rx-noise/7526 "2022-02-15T19:12:02Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![r0ymanesco](https://avatars.discourse-cdn.com/v4/letter/r/9d8465/32.png) [@r0ymanesco](https://discourse.myriadrf.org/u/r0ymanesco)\
**Post date:** [15 February 2022 19:12 UTC](https://discourse.myriadrf.org/t/limesdr-usb-ofdm-tx-rx-noise/7526/1 "2022-02-15T19:12:02Z")

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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.

 ![Screenshot_2022-02-15_19-06-56](https://canada1.discourse-cdn.com/flex034/uploads/myriadrf/original/2X/e/eba2dea592c97a09b3d7a1f0531e8c6d761e9064.png)  
 ![Screenshot_2022-02-15_19-08-36](https://canada1.discourse-cdn.com/flex034/uploads/myriadrf/original/2X/8/85263ccf93c1a1e84c67fc92a4c9227f7d635c55.png)  
 ![Screenshot_2022-02-15_19-08-51](https://canada1.discourse-cdn.com/flex034/uploads/myriadrf/original/2X/a/a4ffe0897ed9d8d96a208cd16770bfcad01c9209.png)

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!