Detected an invalid packet at item

[Hamed Al-Zubi]

Hello, 

I am utilizing two USRPs, specifically the X300 models, along with GNURadio for the purpose of transmitting and receiving OFDM signals. I have implemented the OFDM flowgraphs available on Github for this purpose. The transmission and reception setup involves LOS (Line-of-Sight) configuration, where the transmit and receive antennas are positioned with a separation distance of 2 meters. To minimize the impact of multipath interference, the antennas are surrounded by absorption boards. Although the transmission and reception process itself is functioning without issues, I have encountered a problem when displaying the channel state information. In particular, there are instances where invalid packets are detected, as evidenced by the following occurrences.

packet_headerparser_b :info: Detected an invalid packet at item 167472

header_payload_demux :info: Parser returned #f

I have searched the possible causes of the error I'm encountering, and there are a few factors that have been suggested:

1- Interference--> I used different frequencies and ensured they are not in use.

2- Multipath --> absorption boards were used to minimize the impact of multipath.

3- Some people suggested to change   the FFT length and CRC length as a potential solution, however, modifying these papermeters did not resolve the issue. 

When I used a channel model instead of USRPs, the error did not occur. 
I am still uncertain about the exact cause of the error I am experiencing.

Thanks, 

HZ