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Re: [Discuss-gnuradio] OFDM Implementation
From: |
sumitstop |
Subject: |
Re: [Discuss-gnuradio] OFDM Implementation |
Date: |
Mon, 26 Sep 2011 06:10:21 -0700 (PDT) |
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
>
>
>
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>
>
-----
Sumit Kr.
Research Assistant
Communication Research center
IIIT Hyderabad
India
--
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- Re: [Discuss-gnuradio] OFDM Implementation, (continued)
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/05
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/07
- Re: [Discuss-gnuradio] OFDM Implementation, sumitstop, 2011/09/07
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/08
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/08
- Re: [Discuss-gnuradio] OFDM Implementation, sumitstop, 2011/09/09
- Re: [Discuss-gnuradio] OFDM Implementation, Tuan (Johnny) Ta, 2011/09/09
- Re: [Discuss-gnuradio] OFDM Implementation, Marcus D. Leech, 2011/09/09
- Re: [Discuss-gnuradio] OFDM Implementation, Tuan (Johnny) Ta, 2011/09/09
- Re: [Discuss-gnuradio] OFDM Implementation, Colby Boyer, 2011/09/09
- Re: [Discuss-gnuradio] OFDM Implementation,
sumitstop <=
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/11
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/11
- Re: [Discuss-gnuradio] OFDM Implementation, sumitstop, 2011/09/11
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/11
- Re: [Discuss-gnuradio] OFDM Implementation, Tuan (Johnny) Ta, 2011/09/11
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/11
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/11
- Re: [Discuss-gnuradio] OFDM Implementation, Tuan (Johnny) Ta, 2011/09/11
- Re: [Discuss-gnuradio] OFDM Implementation, Tuan (Johnny) Ta, 2011/09/11
- Re: [Discuss-gnuradio] OFDM Implementation, waqasme, 2011/09/11