Re: [Discuss-gnuradio] OFDM receiver on USRP2

[Guanbo]

Thanks Tom. I tried to see how much the carrier frequency offset would be. 
As you suggested above, I output the coarse freq offset bins, which was
similar as Srinivas's result. It is constant "-2". 

Does it means the carrier offset can be computed as: ADC rate /( Decim_rate
* FFT_len) * 2  ?
If so, in my case, it is around 4KHz. From my knowledge, it seems too good
to be true. 

>From your reference, I put on the channel filter code:
--------------------------------------------------------------
        bw = (float(occupied_tones) / float(fft_length)) / 2.0
        tb = bw*0.08
        chan_coeffs = gr.firdes.low_pass (1.0,                     # gain
                                          1.0,                     #
sampling rate
                                          bw+tb,                   #
midpoint of trans. band
                                          tb,                      # width
of trans. band
                                          gr.firdes.WIN_HAMMING)   # filter
type
        self.chan_filt = gr.fft_filter_ccc(1, chan_coeffs)
        
        win = [1 for i in range(fft_length)]

--------------------------------------------------------------
Also, my hardware is USRP2+ XCVR2450.
$ sudo python benchmark_ofdm_tx_new.py --mac-addr=xxxx  -f 2.462G -m qpsk -i
100 --tx-gain=30 -M 8 -s 1000
FFT_len = 512 (default)
occupie_tones=200






Tom Rondeau wrote:
> 
> On Wed, Jan 12, 2011 at 7:36 PM, Guanbo <address@hidden> wrote:
>>
>> Hi, Tom
>>
>> I am not quite understand the ofdm_sync_xx.cc in
>> python/gnuradio/blks2impl/.
>>
>> What is the difference between frequency synchronization and carrier
>> frequency offset.
>> Why we have to do the timing and frequency synchronization before it?
>>
>> Thanks,
>> Guanbo
> 
> There is the difference between the fine frequency tuning (centering
> the signal inside of a subcarrier) and coarse frequency tunning
> (making sure that subcarrier 0 is at the correct location). The
> ofdm_sync_xx blocks perform the timing and fine frequency calculations
> (the self.nco in ofdm_receiver.py is generated from the fine frequency
> offset from the sync block). This is required before being able to
> perform the FFT demo, or else you'll get inter-carrier interference.
> 
> We can then go into the frequency domain (after the FFT) and work on
> the subcarrier bins to handle the coarse frequency offset, since it is
> easier to do in this domain.
> 
> Tom
> 
> 
> 
>> Tom Rondeau wrote:
>>>
>>> On Tue, Feb 16, 2010 at 5:45 PM, Srinivas <address@hidden> wrote:
>>>> Matt,
>>>>
>>>> Thanks for verifying the data rate calculation!
>>>>
>>>> I tried the other solutions that you suggested, namely,
>>>>
>>>> - increasing the data rate by a factor of 2 or 4
>>>> It works.
>>>>
>>>> - modifying the OFDM code to widen the search range - How do I widen
>>>> the
>>>> search range ?
>>>> Should I be looking in the "ofdm_sync_" blocks in "blks2impl" folder ?
>>>> If
>>>> yes, which synchronizer is currently used with ofdm_examples ?
>>>
>>> You need to add an argument to gr.ofdm_frame_acquisition in
>>> ofdm_receiver.py (in python/gnuradio/blks2impl).
>>>
>>> In the current Git master, this is located on line 109 of
>>> ofdm_receiver.py. After the "ks[0]" argument, you can put in an
>>> integer. This is the maximum number of bins the receiver will search
>>> over for correlation. It defaults to 10.
>>>
>>>
>>>> - locking the usrps to a common reference
>>>> My usrp2s are located wide apart so I guess this solution is not
>>>> practical.
>>>>
>>>> Besides, this confirms that the problem is somewhere in the USRP2
>>>> board,
>>>> right ? (as I tried swapping the daughter cards & firmware with the
>>>> working
>>>> pair)
>>>>
>>>> Thanks,
>>>> Srinivas
>>>
>>> Nope, this is typical of radio hardware. They are always off
>>> frequency. If two oscillators are off frequency and then multiplied up
>>> to another frequency, the difference will also be magnified. So a 2.4
>>> GHz board will have a larger frequency offset than if you ran it just
>>> through the BasicTx/Rx boards (even though the ratios should be the
>>> same).
>>>
>>> Tom
>>>
>>>
>>> _______________________________________________
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>>>
>>>
>>
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