Hi all, I've been developing an OFDM receiver for some time. I wrote demodulation routines in the complex baseband, where I can access a digital baseband signal resulting from a general purpose RF digitizer or a vector analyser. As the bandwidth of my digitizer is much larger than my signal bandwidth and there is no IF bandpass filter I decided to implement an FIR lowpass filter in baseband. But if the FIR length exceeds the guard interval inter-symbol-interference will be introduced. Does it mean it's better to insert an analog bandpass in the IF instead of an FIR lowpass in the baseband? LB.
Lowpass filtering for OFDM introduces interference?
Started by ●April 18, 2006
Reply by ●April 18, 20062006-04-18
lanbaba wrote:> Hi all, > > I've been developing an OFDM receiver for some time. I wrote demodulation > routines in the complex baseband, where I can access a digital baseband > signal resulting from a general purpose RF digitizer or a vector analyser. > As the bandwidth of my digitizer is much larger than my signal bandwidth > and there is no IF bandpass filter I decided to implement an FIR lowpass > filter in baseband. But if the FIR length exceeds the guard interval > inter-symbol-interference will be introduced. Does it mean it's better to > insert an analog bandpass in the IF instead of an FIR lowpass in the > baseband? > > LB. >Why do you need the filter? From the little bit that I've paid attention to OFDM receivers (DRM radio receivers, specifically) the demodulation process ends up being a gazzilion correlation detectors, which automatically filter just the right amount. Any additional filtering would then only make sense if it were done before a time-varying or nonlinear block that would allow out of band noise to get into your signal -- if you're not resampling then once you've sampled your data it's too late -- extra filtering only degrades your signal. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/
Reply by ●April 18, 20062006-04-18
Tim Wescott wrote:> lanbaba wrote: > > Hi all, > > > > I've been developing an OFDM receiver for some time. I wrote demodulation > > routines in the complex baseband, where I can access a digital baseband > > signal resulting from a general purpose RF digitizer or a vector analyser. > > As the bandwidth of my digitizer is much larger than my signal bandwidth > > and there is no IF bandpass filter I decided to implement an FIR lowpass > > filter in baseband. But if the FIR length exceeds the guard interval > > inter-symbol-interference will be introduced. Does it mean it's better to > > insert an analog bandpass in the IF instead of an FIR lowpass in the > > baseband? > > > Why do you need the filter? > > From the little bit that I've paid attention to OFDM receivers (DRM > radio receivers, specifically) the demodulation process ends up being a > gazzilion correlation detectors, which automatically filter just the > right amount.Indeed. These correlation filters are almost always implemented as an FFT. Just extend your FFT size to include the empty bandwidth. Obviously, this will create extra frequency-domain components, but you may just discard them. These are commonly known as virtual sub-carriers. -- Oli
Reply by ●April 18, 20062006-04-18
>Why do you need the filter? > > From the little bit that I've paid attention to OFDM receivers (DRM >radio receivers, specifically) the demodulation process ends up being a >gazzilion correlation detectors, which automatically filter just the >right amount. Any additional filtering would then only make sense if it>were done before a time-varying or nonlinear block that would allow out >of band noise to get into your signal -- if you're not resampling then >once you've sampled your data it's too late -- extra filtering only >degrades your signal. > >-- > >Tim Wescott >Wescott Design Services >http://www.wescottdesign.com > >Posting from Google? See http://cfaj.freeshell.org/google/ >Why I need the filter. Well, I am playing with the sampling rate a bit now. I oversample the bandpass signal and want to denoise the signal before downsampling in baseband. Averaging two noise currupted samples, being the simplest LP filter, should give me 3 dB gian. Knock me if I'm wrong. LBB
Reply by ●April 18, 20062006-04-18
lanbaba wrote:>>Why do you need the filter? >> >>From the little bit that I've paid attention to OFDM receivers (DRM >>radio receivers, specifically) the demodulation process ends up being a >>gazzilion correlation detectors, which automatically filter just the >>right amount. Any additional filtering would then only make sense if it > > >>were done before a time-varying or nonlinear block that would allow out >>of band noise to get into your signal -- if you're not resampling then >>once you've sampled your data it's too late -- extra filtering only >>degrades your signal. >> >>-- >> >>Tim Wescott >>Wescott Design Services >>http://www.wescottdesign.com >> >>Posting from Google? See http://cfaj.freeshell.org/google/ >> > > > Why I need the filter. Well, I am playing with the sampling rate a bit > now. I oversample the bandpass signal and want to denoise the signal > before downsampling in baseband. Averaging two noise currupted samples, > being the simplest LP filter, should give me 3 dB gian. Knock me if I'm > wrong.Averaging two samples gives you an extra bit of significance. If you discard that bit, you haven't gained. Even if you save it, averaging reduces noise only if the noise in the two samples is substantially uncorrelated. After a bandpass filter that removes out-of-band noise, only the low-frequency components of in-band noise will be uncorrelated. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 18, 20062006-04-18
>Averaging two samples gives you an extra bit of significance. If you >discard that bit, you haven't gained. Even if you save it, averaging >reduces noise only if the noise in the two samples is substantially >uncorrelated. After a bandpass filter that removes out-of-band noise, >only the low-frequency components of in-band noise will be uncorrelated. > >Jerry >-- >Engineering is the art of making what you want from things you can get. >����������������������������������������������������������������������� >Correct. I don't have a bandpass filter to remove the out-of-band noise. All I want to do is to reduce the out-of-band noise by a lowpass filter in the baseband.
Reply by ●April 18, 20062006-04-18
lanbaba wrote:>>Why do you need the filter? >> >>From the little bit that I've paid attention to OFDM receivers (DRM >>radio receivers, specifically) the demodulation process ends up being a >>gazzilion correlation detectors, which automatically filter just the >>right amount. Any additional filtering would then only make sense if it > > >>were done before a time-varying or nonlinear block that would allow out >>of band noise to get into your signal -- if you're not resampling then >>once you've sampled your data it's too late -- extra filtering only >>degrades your signal. >> >>-- >> >>Tim Wescott >>Wescott Design Services >>http://www.wescottdesign.com >> >>Posting from Google? See http://cfaj.freeshell.org/google/ >> > > > Why I need the filter. Well, I am playing with the sampling rate a bit > now. I oversample the bandpass signal and want to denoise the signal > before downsampling in baseband. Averaging two noise currupted samples, > being the simplest LP filter, should give me 3 dB gian. Knock me if I'm > wrong. > > LBB >As Jerry pointed out you'll only get that gain if the noise is white -- if the noise is concentrated in the filter passband it won't do you any good at all. Of course if the noise is concentrated _outside_ of the filter passband it'll get you huge gains. But again, the OFDM demodulation process is already a matched filter, so you shouldn't intrude on its passband with your filtering, and you shouldn't downsample in a way that aliases into your OFDM demodulation's passband, either. The only reason to filter/downsample is to save yourself computational resources, or to make the FFT embedded in the OFDM demodulation line up better with the demodulation. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/
Reply by ●April 18, 20062006-04-18
lanbaba said the following on 18/04/2006 17:05:>> Why do you need the filter? >> >> From the little bit that I've paid attention to OFDM receivers (DRM >> radio receivers, specifically) the demodulation process ends up being a >> gazzilion correlation detectors, which automatically filter just the >> right amount. Any additional filtering would then only make sense if it >> were done before a time-varying or nonlinear block that would allow out >> of band noise to get into your signal -- if you're not resampling then >> once you've sampled your data it's too late -- extra filtering only >> degrades your signal. >> >> > > Why I need the filter. Well, I am playing with the sampling rate a bit > now. I oversample the bandpass signal and want to denoise the signal > before downsampling in baseband. Averaging two noise currupted samples, > being the simplest LP filter, should give me 3 dB gian. Knock me if I'm > wrong. >In an OFDM system, the baseband DFT is a matched filter - i.e. it already optimally removes noise. There is no need to perform low-pass pre-filtering on your oversampled baseband signal - it will not improve SNR. To show this, say your received freq-domain values are: Y[k] = DFT{y[n]} = DFT{x[n] + v[n]} = X[k] + V[k] [Where v[n] is the time-domain noise sequence, and V[k] is the freq-domain noise sequence]. Evaluating this at some arbitrary k = k_0, this relationship is clearly not affected by the size of the DFT, nor by the amount of noise at any other k. Therefore, there is no performance advantage in low-pass filtering and downsampling at baseband. -- Oli
Reply by ●April 18, 20062006-04-18
Jerry Avins said the following on 18/04/2006 17:15:> lanbaba wrote: >>> Why do you need the filter? >> >> Why I need the filter. Well, I am playing with the sampling rate a bit >> now. I oversample the bandpass signal and want to denoise the signal >> before downsampling in baseband. Averaging two noise currupted samples, >> being the simplest LP filter, should give me 3 dB gian. Knock me if I'm >> wrong. > > Averaging two samples gives you an extra bit of significance. If you > discard that bit, you haven't gained. Even if you save it, averaging > reduces noise only if the noise in the two samples is substantially > uncorrelated.However, the receiver DFT already does this for you, in an optimal way. An extra step of pre-DFT averaging won't offer any benefit, even in the case of completely uncorrelated noise. -- Oli
Reply by ●April 18, 20062006-04-18
You will get some level of ISI regardless of whether you use an analog or digital filter. Assuming that there is not too much out-of-band (OOB) energy aliasing into your bandwidth of interest at the digitizer and that you do not have dynamic range issues due to this OOB energy I would go with the FIR. If you use it as part of a decimation process, you can even reduce the size of your FFT in the process. Also, if the OOB energy does not cause problems for you, skipping the filters and using a large FFT is also possible if your detection mechanism lies behind the FFT and not before it. If your detection mechanism precedes the FFT, it is desireable to limit the noise bandwidth in the received signal to get better performance. Phil
