Re: OFDM results

GNU Radio
1

Hi Andreas,

where is the inaccuracy in sampling frequency exactly happening?
In USRP?

Regards,
Hoo Chang Shin.

----- Original Message ----
From: Andreas Müller removed_email_address@domain.invalid
To: removed_email_address@domain.invalid
Cc: removed_email_address@domain.invalid
Sent: Wednesday, June 4, 2008 12:48:00 PM
Subject: Re: [Discuss-gnuradio] OFDM results.

Hello Per

What is the cause of the problems. Clock jitter ?, non-linearities ?

No idea what could be the problem in your case, but maybe try also
plotting the phase of all symbols in one OFDM symbol vs the offset from
the central carrier.

I’m currently working on DAB and I found some problems by doing this,
e.g. in [1] there is a frequency dependent phase offset due to an
inaccurate the sampling frequency.

Regards, Andreas

[1] http://people.ee.ethz.ch/~andrmuel/tmp/phase-vs-subcarrier.png

2

where is the inaccuracy in sampling frequency exactly happening?
In USRP?

Yes - the sampling frequency is inaccurate, because the crystal on the
USRP is not oscillating at exactly 64 MHz.

FYI, the samples in the plot are actually not from an USRP (I didn’t
record them myself) - the inaccuracy is about 74 PPM. With samples from
my USRP I found that the sampling frequency is off by about 14 PPM, and
in some samples that Jens E. recorded with his USRP I found an
inaccuracy of about 9 PPM.

Depending on the number of subcarriers and constellation density, 9 PPM
can be enough to make the constellation a little more cloudy, if the
sample rate is not corrected.

Regards, Andreas