Re: [Discuss-gnuradio] OFDM Implementation

[sumitstop]

Hey Marcus this was very very informative.This offset was really killing me
every time when I was running the OFDM  example.I think I need to order some
GPSDO :) 


Marcus D. Leech wrote:
> 
> On 09/09/2011 07:26 PM, Tuan (Johnny) Ta wrote:
>> As far as I know there's no open source code for an OFDM transceiver 
>> available. I was trying to build one half a year back but wasn't 
>> successful before I had to move on to something else. The 
>> benchmark_ofdm code will give you a simplex OFDM system. Ie you can 
>> run the transmitter on 1 USRP and receiver on another.
>>
>> Ie. run this on 1 USRP
>> ./benchmark_ofdm_tx.py -f 2.412G
>>
>> And this on the other
>> ./benchmark_ofdm_rx.py -f 2.412G
>>
>> The value of the frequency depends on the daughterboards you're using. 
>> If you're using USRP1 make sure the decimation rate is 1/2 of the 
>> interpolation rate as the ADC is 2 times faster than the DAC on the 
>> USRP1 (or the other way around, you should chek that).
> The DAC on the USRP1 runs at twice the rate of the ADC.
> 
>>
>> Watch out for the frequency offset, it killed the system for me. If 
>> the above doesn't work, run the transmitter on 1 USRP and usrp_fft.py 
>> on the other. Check the center frequency of the FFT plot and manually 
>> adjust the receiver center frequency. I used the RFX2400 boards and 
>> the offset for me was ~ 40kHz.
>>
>>
> Frequency offset comes up a lot on this list.  It's usually in the 
> context of someone who has up to this point in their DSP/SDR "career"
>    only been dealing with baseband signals inside a simulation 
> environment--and environment that doesn't always adequately reflect
>    what you'll experience in real-world systems, and real-world channels.
> 
> RF synthesizers are only as good as their reference clock.  The 
> reference clocks on most garden variety RF platforms are usually of
>    good-to-excellent quality.  But they may still have residual errors 
> of a few 10s of PPM.   So that means for every MHz of frequency,
>    the absolute, actual frequency could be "off" by a few 10s of Hz.  
> Multiply that up to typical channel frequencies for many experiments
>    in the modern communications domain of 1 to 3GHz or even higher, and 
> you can easily end up with 10s of Khz of absolute frequency offset,
>    and this applies to both the transmitter and receiver.
> 
> In typical cellular phone systems like LTE, and GSM and the like, the 
> base-station transmitters typically have really good reference clocks--
>    good to a few PPB--a local rubidium clock, or a GPSDO.  The the 
> hand-helds typically have cheap local reference clocks, in order to meet
>    the grueling BOM cost requirements of typical consumer electronics.
> 
> What that means is that the demodulation chain needs some mechanism to 
> deal with frequency offset, and provide feedback to "center"
>    the baseband signal--either by tweaking the RX hardware, or shifting 
> the baseband signal in software.  But the example code that's floating
>    around is typically *not* a *complete* system in this regard.  In 
> some sense, much of it was designed to work in the "fantasy" land of
>    the simulation environment, and may not work that well in the real 
> world.  In some OFDM systems, for example, I understand that there
>    is often a "pilot" carrier against which one can correlate some kind 
> of sequence, and once you've found the most-strongly-correlated
>    "bin" in the OFDM "comb", you can use that to estimate the frequency 
> offset relative to the transmitter.  Examples and simulations may or
>    may not have that covered.  College-level programs in DSP and SDR may 
> or may not discuss that important "real world" detail.
> 
> Physics, it turns out, is a harsh mistress...
> 
> 
> 
> 
> -- 
> Marcus Leech
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> http://www.sbrac.org
> 
> 
> 
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> 
> 


-----
Sumit Kr.
Research Assistant
Communication Research center
IIIT Hyderabad
India
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