Re: [Discuss-gnuradio] OFDM receiver on USRP2

Matt,

There was a frequency offset of ~30 KHz at the Rx w.r.t Tx so I compensated for it and it worked!.

The settings I am using is as follows:

./benchmark_ofdm_tx.py -f 2.45G --tx-amplitude 0.9 -M 8 -s 200 -m bpsk --fft-length=512 --occupied-tones=80 -i 64 --tx-gain=10 --cp-length=128
./benchmark_ofdm_rx.py -f 2.45G -m bpsk --fft-length=512 --occupied-tones=80 -d 64 --rx-gain=20 --cp-length=128

I calculate the data-rate for OFDM as follows Data rate R = (ADC sampling rate x Occupied Tones) / (Nfft x Decimation)
For the above setting it is 244 KHz.

Am I right with the data rate calculation ?

Thanks very much for your time,

On Thu, Feb 11, 2010 at 10:26 PM, Matt Ettus <matt@ettus.com> wrote:

On 02/11/2010 04:45 PM, Srinivas wrote:

Hi All,

I have 2 pairs of USRP2s with GNURadio-3.2 installed on their hosts. On
one pair I am able to successfully run OFDM (benchmark_ofdm_tx & rx)
with almost 95+% packet success rate. However on the other pair I am not
receiving even 1 packet!

I am using the same host machines and scripts. I also tried swapping the
daughtercards (XCVR2450) and the firmwares with the working pair, but
the problem remains.

Does any one have a clue of where the problem might be ?

PS: The received signal spectrum (usrp2_fft.py) on one of the
non-working USRP2s is attached herewith. Besides this I plotted the
spectrum of the received data from usrp2_rx_cfile.py at the receiver
using MATLAB. The spectrum is of the same shape and strength as
usrp2_fft.py displays.

Srinivas,

It looks like you are using a very narrow signal.  The frequency offset of the USRP2s giving you trouble may be enough that you are outside of the search range of the OFDM receiver (which is a percentage of the bandwidth of the signal).

You could try any or all of the following:

- increasing the data rate by a factor of 2 or 4
- modifying the OFDM code to widen the search range
- locking the usrps to a common reference
- measure the frequency offset of the transmitter, and run the receiver with the actual frequency.  For example, if the receiver sees the signal 30 kHz high using usrp2_fft.py, call the ofdm receiver with

   -f 2.450030G
on the command line


Matt

--
Srinivas
WINLAB, Rutgers University
New Jersey