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Re: [Discuss-gnuradio] Invalid packets in ofdm_rx example
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From: |
Michael Dickens |
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Subject: |
Re: [Discuss-gnuradio] Invalid packets in ofdm_rx example |
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Date: |
Thu, 05 Jul 2018 11:06:18 -0400 |
The "Parser returned #f" means that the OFDM header didn't pass CRC, which generally means that the "SNR" isn't adequate, but it could mean other things too. There are numerous possible issues that you will encounter when using GR's OFDM.
* Start with the -default- OFDM settings (e.g., FFT of 128, C/P of 32) and see if those work.
* Add a "gain" block immediately after the Tx OFDM block, with settings from (say) -10 dB to +10 dB; play around & see what happens on Rx. You'll want the Tx OFDM block abs() output to average around sqrt(1/2) or so, but less than 1.0; peaks will typically be over 1.0. Get the gain from above such that this is the case & you're one step closer. This value decreases the chances of saturating the USRP on Tx.
* After you've gotten a stable system working (gain is the key when using default OFDM parameters), start playing with other OFDM parameters: constellation, FFT length, and so forth --> BUT: one at a time.
* If you find one that doesn't work, reset its values and go to another one. If some parameter continues to be stubborn, post back here with your progress, tests & actual GRC script asking for help again.
Hope this is useful! - MLD
On Thu, Jul 5, 2018, at 4:57 AM, Usama Saeed wrote:
Hello, this is my first time posting a question:
I tried using the ofdm_tx and ofdm_rx examples with two USRP B200s, but even though I could see the ofdm spectrum on the receiver end, the time graph at the end of the demodulation was blank (straight line at 0) and I received the error
"gr::log :INFO: header_payload_demux0 - Parser returned #f
gr::log :INFO: packet_headerparser_b0 - Detected an invalid packet at item 6720"
The associated screenshots are posted on this page:
Thank you for any assistance.
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