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OFDM Question

Started by Bob the Builder February 4, 2006
I was wondering about demodulation. We woudl need phase coherence of both
the main carrier and the sub-carriers. I assume the main carrier can be
locked onto with a PLL but what about the sub-carriers? Do they use a pilot
sequency of some sort when multiply up to get the other frequencies?

Thanks


Tom


Bob the Builder said the following on 04/02/2006 04:35:
> I was wondering about demodulation. We woudl need phase coherence of both > the main carrier and the sub-carriers. I assume the main carrier can be > locked onto with a PLL but what about the sub-carriers? Do they use a pilot > sequency of some sort when multiply up to get the other frequencies? >
In OFDM, once you've mixed down by the "main" carrier to baseband, the sub-carriers are demodulated in parallel using an FFT. This of course relies on the fact that you are able to exactly identify the main carrier frequency - any error and the whole thing screws up. With a multi-carrier scheme, it's not as simple as locking onto the main carrier with a PLL, there is no identifiable "main carrier" as there is in a single-carrier scheme. Typically, some sub-carriers have to be dedicated as pilot tones (i.e. no user data is transmitted), and the PLL can lock onto those. Alternatively, training sequences with particular properties are used that allow the frequency error to be estimated and corrected. There are ways of doing it non-data-aided, i.e. not using pilot tones. These schemes typically use other forms of redundancy inherent in most OFDM implementations - virtual carriers, or the correlation of the CP. See for instance "ML Estimation of time and frequency offset in OFDM systems", J.J. van de Beek, et al, or "A high-efficiency carrier estimator for OFDM communications", H. Liu, et al. -- Oli
"Oli Filth" <catch@olifilth.co.uk> wrote in message
news:Pg2Ff.1035$Z7.998@newsfe3-win.ntli.net...
> Bob the Builder said the following on 04/02/2006 04:35: > > I was wondering about demodulation. We woudl need phase coherence of
both
> > the main carrier and the sub-carriers. I assume the main carrier can be > > locked onto with a PLL but what about the sub-carriers? Do they use a
pilot
> > sequency of some sort when multiply up to get the other frequencies? > > > > In OFDM, once you've mixed down by the "main" carrier to baseband, the > sub-carriers are demodulated in parallel using an FFT. > > This of course relies on the fact that you are able to exactly identify > the main carrier frequency - any error and the whole thing screws up. > > With a multi-carrier scheme, it's not as simple as locking onto the main > carrier with a PLL, there is no identifiable "main carrier" as there is > in a single-carrier scheme. Typically, some sub-carriers have to be > dedicated as pilot tones (i.e. no user data is transmitted), and the PLL > can lock onto those. Alternatively, training sequences with particular > properties are used that allow the frequency error to be estimated and > corrected. > > There are ways of doing it non-data-aided, i.e. not using pilot tones. > These schemes typically use other forms of redundancy inherent in most > OFDM implementations - virtual carriers, or the correlation of the CP. > See for instance "ML Estimation of time and frequency offset in OFDM > systems", J.J. van de Beek, et al, or "A high-efficiency carrier > estimator for OFDM communications", H. Liu, et al. > > > > -- > Oli
Thanks - so the main carrier is found with a PLL?
"Oli Filth" <catch@olifilth.co.uk> wrote in message
news:Pg2Ff.1035$Z7.998@newsfe3-win.ntli.net...
> Bob the Builder said the following on 04/02/2006 04:35: > > I was wondering about demodulation. We woudl need phase coherence of
both
> > the main carrier and the sub-carriers. I assume the main carrier can be > > locked onto with a PLL but what about the sub-carriers? Do they use a
pilot
> > sequency of some sort when multiply up to get the other frequencies? > > > > In OFDM, once you've mixed down by the "main" carrier to baseband, the > sub-carriers are demodulated in parallel using an FFT. > > This of course relies on the fact that you are able to exactly identify > the main carrier frequency - any error and the whole thing screws up. > > With a multi-carrier scheme, it's not as simple as locking onto the main > carrier with a PLL, there is no identifiable "main carrier" as there is > in a single-carrier scheme. Typically, some sub-carriers have to be > dedicated as pilot tones (i.e. no user data is transmitted), and the PLL > can lock onto those. Alternatively, training sequences with particular > properties are used that allow the frequency error to be estimated and > corrected. > > There are ways of doing it non-data-aided, i.e. not using pilot tones. > These schemes typically use other forms of redundancy inherent in most > OFDM implementations - virtual carriers, or the correlation of the CP. > See for instance "ML Estimation of time and frequency offset in OFDM > systems", J.J. van de Beek, et al, or "A high-efficiency carrier > estimator for OFDM communications", H. Liu, et al. > > > > -- > Oli
I will re-phrase that -I meant the modulation carrier of transmission - does this get demodulated with a PLL?
What flavour of OFDM are you talking about?  Is this a general
question?  Are you considering what would be done for a specific
standard?  Depending on the flavour, the technique can be different.