Hi Sarah - A few things to note on using the default GR OFDM using real
SDR devices that could be relevant here:
*  tx data amplification: This needs to be such that the data heading to
   UHD doesn't saturate on conversion. You can visually see this if you
   look at the raw Rx signal ... it will look mostly like OFDM in the
   center but then have significant side bands. With too little Tx data
   amplification, the signal will look very OFDM, but it might end up
   with the average signal peaks being not far enough above the noise
   floor. The Tx data amp value depends on which constellation you're
   using for the payload. An easy way to see all of this is to use a
   graphical slider to set this value & then watch the raw Rx signal.
* rx data amplification: You generally want this "high", but you can
  play with this value to see what works best. This value generally
  isn't as critical as the Tx data amp value.
* Since OFDM sends data in packets, you really care about the Packet
  Error Rate (PER). Lose 1 packet, and you've lost a bunch of bits. The
  OFDM Rx uses the tag "packet_num" to show successful packet receipt,
  and increments this value by 1 for each packet detected but invalid
  (whether in the meta-data header or in the payload). So you should be
  able to create a PER block that just watches for skips in the Rx
  "packet_num" tag.
* The meta-data you mention is the header, which is indeed inserted into
  the OFDM signal. It is removed on Rx, so it should not be the cause of
  your issues.
* It sounds almost as if the OFDM Rx is losing sync after 20% of the
  data is received, for some reason. Usually Rx sync is lost because the
  USRP's Tx gain isn't high enough, and so the Rx signal's "SNR" isn't
  high enough to always meet the sync's criteria. Maybe try playing with
  the USRP Tx gain & see if that helps.
I';m sure there are other things relevant here, but these are the ones
that come to mind. Hope this helps! - MLD
On Tue, Mar 6, 2018, at 3:53 PM, Sarah Tran via USRP-users wrote:
> I am trying to use the ofdm_tx.grc and ofdm_rx.grc to transmit data
> from one N210 controlled by a host to another N210 controlled by a
> separate host. Eventually I want to build an 8x8 MIMO system using
> OFDMA. I want to verify that the correct data is being received so I
> sent all of the decoded payload data into a file sink. The file I sent
> has 10e6 random samples from 0-255 being stored into bytes. I received
> at least 5x that amount to ensure that I got all the data. I imported
> the received data and the known sent data into Matlab and performed
> the cross-correlation. The figure looks kind of okay as it showed
> multiple peaks (see figure below). However when I went to actually
> calculate the BER, I never got more than 20% of the correct bits. It
> was behaving strangely as 20% of the bits would be correct in a row,
> and then after that it would just be completely wrong. I know the
> ofdm_tx.grc graphs inserts metadata into the signal, and my theory is
> that the meta data is also being decoded and it won’t match exactly
> what was sent in the file because of the extra information. My
> understanding might be completely off, so if anyone can clarify it
> would be greatly appreciated! I just want to be able to have a way
> that calculates the BER between what is transmitted and what is
> received. Thank you for your time!!
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