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

[Morgan Redfield]

On Tue, Jun 7, 2011 at 6:03 PM, Tom Rondeau <address@hidden> wrote:
> On Tue, Jun 7, 2011 at 5:31 PM, Morgan Redfield <address@hidden> wrote:
>>
>> On Tue, Jun 7, 2011 at 1:50 PM, Tom Rondeau <address@hidden>
>> wrote:
>> > On Tue, Jun 7, 2011 at 3:24 PM, Morgan Redfield <address@hidden>
>> > wrote:
>> >>
>> >> 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
>> >
>> > You can use the averaging in the uhd_fft plot to help smooth out the
>> > signal
>> > to see if it's centered. The OFDM transmitter we have notches out the
>> > center
>> > two subcarriers, so you will hopefully be able to see a small gap in the
>> > middle of the signal.
>> > You might have the transmitter power set too high. OFDM really needs to
>> > operate in the linear range of the transmitter, so keeping the power
>> > down
>> > helps. I thought of this when you said "messy," which could be
>> > influenced by
>> > this factor.
>> > In general, though, no, 3 kHz offset should not be a problem for this.
>> > Tom
>> >
>> Hi Tom,
>>
>> The transmitter power was one of my problems. I had it set to the max.
>> I've adjusted it to be about a quarter of the range, and I'm looking
>> at the FFT of the OFDM signal. I'm seeing a spike that's significantly
>> higher than the rest of the carriers right at my center frequency. It
>> looks like there may be a small gap in it, but I'm not sure if that's
>> what I'm supposed to be getting. I'm attaching a screenshot of the FFT
>> so you can see what I mean.
>>
>> Thanks for your help,
>> Morgan
>
> I'm not entirely sure what I'm seeing in that picture. It looks like we're
> seeing 200 kHz of bandwidth in that scope view. What bandwidth are you
> transmitting at? That should give some indication of what we should be
> seeing.
> If it's working correctly, you should see a flat-top signal out of the noise
> floor that has a very sharp rolloff. You should be able to see the notch
> around DC, as well, but you could see it distorted by the DC offset, which I
> think is part of what I'm seeing in that picture.
> Tom
>

I'm currently transmitting with a sample rate of 1MHz. I was looking
at only 200kHz to try and get a good view of the center frequency.

I'm including a couple of screenshots of the FFT with 1MHz and 2MHz
bandwidth. The signal doesn't seem to look like what you've described.
Could I have broken something when I converted it to use UHD?

The code that I'm using to transmit is here:
https://github.com/mogar/uhd_ofdm/blob/master/benchmark_ofdm_tx.py
I invoked it with: python benchmark_ofdm_tx.py -f 650M -v --rate=1M

Thanks,
Morgan

ofdm_fft_1M_bandwidth.tiff
Description: TIFF image

ofdm_fft_2M_bw.tiff
Description: TIFF image