OFDM packets

GNU Radio
1

Hi,
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
receiver.
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.

2

On Thu, Feb 28, 2008 at 12:51:18PM +0100, Crespi F. wrote:

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.

Hi Crespi,

You have neglected to tell us virtually anything about your problem.
Could you please tell us:

which version of GNU Radio? trunk, tarball (which one), etc?
what operating system, distribution and architecture?
the command line you issued
any modifications you may have made to distributed code?
anything else that might be relevant

Thanks,
Eric

3

Eric B. wrote:

I use revision 6845 and RFX 2400.

anything else that might be relevant

it seems to me like you’re using antennas, you can try coax to isolate
the problem… maybe you’re getting interference at 2.4GHz. But answer
Eric’s questions too.

  • George
4

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George N. wrote:

it seems to me like you’re using antennas, you can try coax to isolate
the problem… maybe you’re getting interference at 2.4GHz. But answer
Eric’s questions too.

With suitable attenuation of course – I think Matt says at least 30 dB.

  • -Dan
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5

Crespi F. wrote:

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.

Crespi,

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.

Thanks,
Tom