Re: [Discuss-gnuradio] Multiple-Transmitter OFDM

[Michael Dickens]

Hi Ramazan - The "#f" generally indicates that you the Rx SNR isn't high enough ... nearly, but not quite. The sync symbols are being detected, but the header itself can't be decoded cleanly (CRC fails).

In your "test" script, you don't have to worry so much about gains. When using actual over-the-air USRPs, gains become quite important! I'd advise you to add in 2 gains to the Tx, and 1 to the Rx:

Tx: change the "Multiply Const" to a gain slider, and then add a Qt time sink to see the actual Tx signal amplitude. You want the values going into the USRP to be in [-1, +1], backed off just a touch is probably wise. Anything above |1| will result in clipping on the USRP, which will cause strange spectrum glitches & if too much clipping will lead to bad decoding.

Tx: add a slider for the USRP's Tx gain setting. 30 is probably good, but it really depends on the actual USRP in use. There will be a range that works well.

Rx: add a slider for the USRP's Tx gain setting. 0 is probably not enough for actual Rx, but it really depends on the actual USRP in use. There will be a range that works well.

WIth those sliders in place, play with them until the signal levels look good: on Tx you want the signal hitting the USRP to be near but within [-1, +1], and on Rx you need "enough" SNR to do decoding .. there are websites and papers that list the lower end SNR for normal OFDM ... add a few (3) dB to those values for the default GR OFDM for various reasons.

Hope this is useful! - MLD

On Mon, Jun 24, 2019, at 8:01 AM, Ramazan Çetin wrote:

Hi Michael,

Firstly, thank you for your suggestion. I have tried your solution and i have transmitted two signals using PFB synthesizer. At the receiver, i have used a PFB channelizer. In simulation, this system works perfectly. I could receive two different files at the receiver from two different transmitters.

But when i tried this system on E310s, it fails. At the transmitter, E310 gives tGtGtGtGtGtGtGtG errors (I know this is related with length tags but i have already provided tag to USRP). At the receiver, it cannot receive signals. It gives this error:

gr::log :INFO: header_payload_demux1 - Parser returned #f

gr::log :INFO: packet_headerparser_b1 - Detected an invalid packet at item 38

gr::log :INFO: header_payload_demux1 - Parser returned #f

gr::log :INFO: packet_headerparser_b1 - Detected an invalid packet at item 76

gr::log :INFO: header_payload_demux1 - Parser returned #f

gr::log :INFO: packet_headerparser_b1 - Detected an invalid packet at item 114

gr::log :INFO: header_payload_demux1 - Parser returned #f

gr::log :INFO: packet_headerparser_b1 - Detected an invalid packet at item 152

gr::log :INFO: header_payload_demux1 - Parser returned #f

gr::log :INFO: packet_headerparser_b1 - Detected an invalid packet at item 190

gr::log :INFO: header_payload_demux1 - Parser returned #f

gr::log :INFO: packet_headerparser_b1 - Detected an invalid packet at item 228

This is obviously related with HPD. But, it does not give this error continuously while signal is transmitting. Normally, if header cannot be decoded, this error should be seen continuously. Instead, it just gives at the beginning of signal.

I have attached three GRC FGs. pfb_test is simulation environment that works perfectly. pfb_transmitter and pfb_receiver are USRP FGs.

Do you have any idea about this situation?

Best regards.

Ramazan