Re: [Discuss-gnuradio] problems with benchmark_ofdm and N210

[Morgan Redfield]

On Tue, Jun 7, 2011 at 7:38 AM, Tom Rondeau <address@hidden> wrote:
> On Mon, Jun 6, 2011 at 8:33 PM, Morgan Redfield <address@hidden> wrote:
>>
>> Hi Everyone,
>>
>> I've been playing around with GNURadio and a couple of USRPs lately,
>> but I've run into some problems. I'm using a modified version of the
>> benchmark_ofdm_tx.py and benchmark_ofdm_rx.py scripts. I updated them
>> to use uhd, and I'm using them with two N210s. Each N210 has a WBX
>> daughterboard, and they're placed about a meter apart right now.
>>
>> I'm attempting to send data from one USRP to the other, but using
>> tunnel.py or the benchmark_ofdm files doesn't seem to work. I never
>> receive any packets correctly.
>>
>> With the benchmark_ofdm files, if I start receiving before I start
>> transmitting then I just get TIMEOUTs. If I start transmitting before
>> I start receiving, I get the following:
>>
>> $ python benchmark_ofdm_rx.py -f 650M -v --rate=1M
>> Mac OS; GNU C++ version 4.0.1 (Apple Inc. build 5490); Boost_104601;
>> UHD_003.001.000-4eb4025
>>
>> -- Opening a USRP2/N-Series device...
>> -- Current recv frame size: 1472 bytes
>> -- Current send frame size: 1472 bytes
>> -- mboard0 is MIMO master
>> >>> gr_fir_ccf: using SSE
>> >>> gr_fir_fff: using SSE
>>
>> OFDM Demodulator:
>> Modulation Type: bpsk
>> FFT length:      512
>> Occupied Tones:  200
>> CP length:       128
>> ok: False        pktno: 21611    n_rcvd: 1       n_right: 0
>> ok: False        pktno: 43626    n_rcvd: 2       n_right: 0
>> ok: False        pktno: 21611    n_rcvd: 3       n_right: 0
>> ok: False        pktno: 37548    n_rcvd: 4       n_right: 0
>> ok: False        pktno: 21909    n_rcvd: 5       n_right: 0
>> ok: False        pktno: 4473     n_rcvd: 6       n_right: 0
>> ok: False        pktno: 27253    n_rcvd: 7       n_right: 0
>> ok: False        pktno: 38378    n_rcvd: 8       n_right: 0
>> ok: False        pktno: 21909    n_rcvd: 9       n_right: 0
>> ok: False        pktno: 38486    n_rcvd: 10      n_right: 0
>> ok: False        pktno: 54634    n_rcvd: 11      n_right: 0
>> ok: False        pktno: 21909    n_rcvd: 12      n_right: 0
>> ok: False        pktno: 39158    n_rcvd: 13      n_right: 0
>> ok: False        pktno: 27237    n_rcvd: 14      n_right: 0
>> ok: False        pktno: 42410    n_rcvd: 15      n_right: 0
>> ok: False        pktno: 21909    n_rcvd: 16      n_right: 0
>> ok: False        pktno: 21994    n_rcvd: 17      n_right: 0
>> ok: False        pktno: 21652    n_rcvd: 18      n_right: 0
>> ok: False        pktno: 21097    n_rcvd: 19      n_right: 0
>> ok: False        pktno: 43626    n_rcvd: 20      n_right: 0
>> ok: False        pktno: 21909    n_rcvd: 21      n_right: 0
>> TIMEOUT
>> TIMEOUT
>>
>> I think those timeouts at the end there are from when the transmitter
>> stopped transmitting data. It looks like I'm receiving a few packets
>> (far fewer than I should), and all the packets I do receive are not
>> correct.
>>
>> Does anyone have any idea what's causing this?
>>
>> Thanks
>> Morgan
>
> TIMEOUTs occur when a preamble has been detected, but then the signal is
> lost.
> My thinking is that you are too far off frequency and so the received signal
> is outside the bandwidth of the receiver. Look at the signal in a FFT plot
> and see if you can adjust the transmitter's frequency to close the gap.
> Tom
>

I tried measuring the frequency offset of the USRPs by generating a
100KHz sine wave and mixing it up to my rf frequency (650MHz). When I
used uhd_fft.py to look at the signal at the receiving N210, I see the
peak pretty close to where it should be at 650.1MHz. I doubt the
signal is off by more than 3KHz. Could such a small frequency offset
really be causing me so many problems?

I also tried looking at my OFDM signal in uhd_fft.py, but it was
pretty messy and bounced around a lot as packets were transmitted. I'm
not sure how I would go about adjusting the transmitter's frequency
from just looking at that. Could you please give me a few more
details?

Morgan