Hi Michael, Thank you for your quick response! I wanted the payload to be QPSK modulated. I put a QT Range blocks in for the gain settings on the ofdm_tx and ofdm_rx flowgraphs. I still can’t seem to get any better than 20% BER though, but I think the advice about doing a PER calculator is pointing me in the right direction as a lost packet would definitely screw up the BER. So thank you very much for the advice!!
I did have another question though regarding the metadata that still has me confused. If the header data is taken out of the payload, why does it still show up when I put a time sink after the payload gettings decoded (after the constellation decoder/stream CR32 blocks)? It plots the decoded bits as well as the packet num, carrier_offset, etc. Did I put my time sink in the wrong place? Thank you! Sarah From: Michael Dickens [mailto:michael.dick...@ettus.com] Sent: Tuesday, March 6, 2018 5:24 PM To: Sarah Tran <st...@oceanit.com>; email@example.com Subject: Re: [USRP-users] OFDM TX/RX BER Calculator 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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