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Re: [Discuss-gnuradio] OFDM packets

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] OFDM packets
Date: Fri, 29 Feb 2008 13:34:52 +0000
User-agent: Thunderbird (Windows/20071031)

Crespi Floriana wrote:
I have a problem with OFDM system.
I transmit OFDM signal at 2.4 GHz, but I receive half of the total packets.
Moreover, only half of the packets received are correct.
CRC controller is perfect but i want a ber estimate, so I change the
payload (binary file) and I  receive check bit by bit.
The result of my alteration test is similar than CRC test.
I have a lot of frequency offset, therefore I change manually the frequency
I use revision 6845 and RFX 2400.
Example with crc:
The final packet I receive:
ok: False      pktno 4602   n_rcvd 1300       n_right 83

Why can't i get all the packets?
Could you help me??

I'm testing OFDM system without USRP and none of the packets are lost.

Thanks a lot.


I'm not sure what state the OFDM code was in at that revision. That is likely a result of the timing issue I discussed in a previous email to the list.

The latest revision of the trunk has a much better implementation that works well over the air and perfectly in the loopback simulation. There is 1 minor problem left that I hope to get to over the weekend for the over the air operation. Remember, though, that we don't do any channel coding, so any wrong bit will cause a packet error. If you run the code currently over the air, BPSK is almost perfect while QPSK and QAMs will miss many packets due to only a few bit errors.

In general, the OFDM code is development work so it's likely to be buggy. We are working on it, but I can only spend a bit of my time with it currently. I would be very happy to see other people getting into the code and figuring out ways to make it better.


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