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Re: [Discuss-gnuradio] larger cyclic prefix in OFDM example degrades per

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] larger cyclic prefix in OFDM example degrades performance
Date: Wed, 7 Nov 2012 11:09:43 -0500

On Tue, Nov 6, 2012 at 9:20 PM, David Andersson <address@hidden> wrote:
> I've been working with the OFDM examples that come with GNURADIO
> (benchmark_tx.py and benchmark_rx.py).  I've noticed that my bit error rate
> (BER) increases as the length of my cyclic prefix (CP) increases.  This is
> opposite of what theory predicts, since larger CP decreases inter-symbol
> interference.
> I googled the issue and found this paper document the same issue (page 31):
> http://upcommons.upc.edu/pfc/bitstream/2099.1/12261/1/Master_Thesis_Marcos_Majo.pdf
> The paper explains this by saying longer CP adds additional overhead by
> making the frames longer, thus leading to worse performance.  I'm having a
> hard time understanding how this explains a performance increase.  I see
> little change in system performance between CP=1 and 128 when monitoring
> system resources, and the number of packets received and number of correct
> packets doesn't change much from run to run.  Just the number of errors in
> packets that do fail.
> I played around with a lot of different input parameters (FFT size, tx/rx
> gain, occupied tones, bandwidth, etc) but the issue persists.  There's also
> no direct correlation between these parameters and BER, and all these
> parameters can increase the complexity of the system.
> I'm using a USRP2 with a xcvr2450 daughterboard and the latest version of
> GNURADIO as of august (I think 3.6?) on ubuntu.
> Has anyone else had this issue?  Any solutions or insight into the source of
> the problem?
> Regards,
> Dave.

Hi Dave,

What you'll want to do is take a look at the frequency, phase, and
timing estimates of the receiver over the length of a packet. These
values are estimated once at the beginning of the packet with the
preamble symbols and then held through the rest of the symbols in a
packet. As you make the packet longer, you're more likely to rotate in
phase of slip in time due to small inaccuracies in the estimates.


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