>> Else one will potentially end up losing the transmitted carriers in thefractional frequency space I'm sorry, but it seems I failed to make my point: The length of the OFDM symbol in seconds is the same, regardless of the FFT size I use for reception (how should it know). The subcarrier spacing is the inverse of the symbol length and does not change with FFT size. There is no "fractional frequency spacing" problem or the like. _____________________________ Posted through www.DSPRelated.com
How to decide FFT window size in OFDM system receiver
Started by ●May 27, 2014
Reply by ●June 25, 20142014-06-25
Reply by ●June 25, 20142014-06-25
Lets say ifft size used @ the tx is 512.now this creates 512 orthogonal frequencies within the channel bw. Data is modulated using sine waves of these 512 frequencies. At the rx, i need to precisely generate these sine waves to correlate with the time domain signal. Can i generate these 512 unique frequency sine waves using 513,123456789 pt fft? Whereas with 512,1024,1536,2048 etc pt dft, one is sure to generate the required sine waves precisely for correlation.
Reply by ●June 25, 20142014-06-25
On Wed, 25 Jun 2014 10:50:14 -0700 (PDT), Rakesh Joshi <rakesh@saankhyalabs.com> wrote:>Lets say ifft size used @ the tx is 512.now this creates 512 orthogonal fre= >quencies within the channel bw. Data is modulated using sine waves of these= > 512 frequencies. At the rx, i need to precisely generate these sine waves = >to correlate with the time domain signal. Can i generate these 512 unique f= >requency sine waves using 513,123456789 pt fft? Whereas with 512,1024,1536,= >2048 etc pt dft, one is sure to generate the required sine waves precisely = >for correlation.As was previously mentioned, as long as the FFT window is exactly the time length of the FFT portion of the OFDM symbol (i.e., the symbol period less the CP), it doesn't matter how many points are in the FFT. Remember what the indices in the FFT/DFT mean, numbering the bins from -N/2, 0, N/2-1, bin 1 contains the energy for signals with 1 cycle/N samples, bin 2 contains the energy for signals with 2 cycles/N samples, etc., etc. Bins -1 and -2 have the energy for 1 and 2 cycles/N, respectively, with the opposite rotation. That is always true, regardless of N. So if you have subcarriers -52 to +52, they will occupy bins -52 to +52 regardless of N, but you'll have to adjust the sample rate if N changes, keeping the time length of the N samples constant. So using the minimum N minimizes computation complexity in a number of ways, by keeping the FFT size minimized and keeping the sample rate minimized, which affects complexity elsewhere. But it is not strictly necessary to use the minimum FFT size in either the modulator or demodulator, and they don't even have to match, it just makes things a bit harder in general. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●June 27, 20142014-06-27
On Tuesday, May 27, 2014 4:24:45 PM UTC+5:30, runinrainy wrote:> Could someone please explain me in details how we select the FFT window > > size at the receiver of the OFDM system? > > > > Does it include both the size of N parallel sub carriers and the Cyclic > > Prefix length? > > > > Thanks! > > > > > > > > _____________________________ > > Posted through www.DSPRelated.comHello Eric/mnentweg, Thanks for correcting the '512 unique freq sine waves are generated' statement of mine made earlier. That was a grave mistake on my part. Infact that helped me figure out one reason why no more than N/2 unique frequencies are not possible. Eg: If IFFT generates a sine wave of (N/2 + x) repitions per N samples, that would violate Nyquist Theorem! However, I still cant comprehend completely how a say, 513 pt dft would yield me carriers at exactly the frequencies generated by a 512 pt IDFT, even after considering the same time window and upsampling the signal in the time window by (513/512). Will work/contemplate on it more and see where it takes me. Thanks a lot for the insightful feedback.
Reply by ●April 30, 20152015-04-30
>On Tuesday, May 27, 2014 4:24:45 PM UTC+5:30, runinrainy wrote: >> Could someone please explain me in details how we select the FFT >window >> >> size at the receiver of the OFDM system? >> >> >> >> Does it include both the size of N parallel sub carriers and the >Cyclic >> >> Prefix length? >> >> >> >> Thanks! >> >> >> >> >> >> >> >> _____________________________ >> >> Posted through www.DSPRelated.com > >Hello Eric/mnentweg, >Thanks for correcting the '512 unique freq sine waves are generated' >statement of mine made earlier. That was a grave mistake on my part.Infact>that helped me figure out one reason why no more than N/2 uniquefrequencies>are not possible. Eg: If IFFT generates a sine wave of (N/2 + x)repitions>per N samples, that would violate Nyquist Theorem! >However, I still cant comprehend completely how a say, 513 pt dft would >yield me carriers at exactly the frequencies generated by a 512 pt IDFT, >even after considering the same time window and upsampling the signal inthe>time window by (513/512). Will work/contemplate on it more and see whereit>takes me. Thanks a lot for the insightful feedback.===================== Thanks all for your comments on this topic! Could you also please explain me the synchronization of the OFDM symbol? I am also trying to understand how to decide the OFDM symbol start/end time at the receiver just before going into FFT process. So far I understand that there two methods for time synchronization which are data-aided and non-data aided methods, and in the data-aided methods the training subcarriers are included in OFDM symbol and they are correlated with the known training subcarriers at the receiver and the time of the peak of the auto-correlation is the time delay (tao)of the channel and this means the OFDM symbol reaches the receiver after this tao delay. My questions here are; 1-For time synchronization, if the data-aided method is used, should the one complete OFDM symbol contain all training subcarriers and then all are correlated with the known trainings one by one at the receiver? 2-Or the OFDM symbol contains a few training subcarriers and when they are received by the receiver each of the trainings is correlated by the known pilots? Thanks! --------------------------------------- Posted through http://www.DSPRelated.com
Reply by ●April 30, 20152015-04-30
On Thu, 30 Apr 2015 04:15:39 -0500, "runinrainy" <100271@DSPRelated> wrote:>>On Tuesday, May 27, 2014 4:24:45 PM UTC+5:30, runinrainy wrote: >>> Could someone please explain me in details how we select the FFT >>window >>> >>> size at the receiver of the OFDM system? >>> >>> >>> >>> Does it include both the size of N parallel sub carriers and the >>Cyclic >>> >>> Prefix length? >>> >>> >>> >>> Thanks! >>> >>> >>> >>> >>> >>> >>> >>> _____________________________ >>> >>> Posted through www.DSPRelated.com >> >>Hello Eric/mnentweg, >>Thanks for correcting the '512 unique freq sine waves are generated' >>statement of mine made earlier. That was a grave mistake on my part. >Infact >>that helped me figure out one reason why no more than N/2 unique >frequencies >>are not possible. Eg: If IFFT generates a sine wave of (N/2 + x) >repitions >>per N samples, that would violate Nyquist Theorem! >>However, I still cant comprehend completely how a say, 513 pt dft would >>yield me carriers at exactly the frequencies generated by a 512 pt IDFT, >>even after considering the same time window and upsampling the signal in >the >>time window by (513/512). Will work/contemplate on it more and see where >it >>takes me. Thanks a lot for the insightful feedback. > > >===================== > >Thanks all for your comments on this topic! > >Could you also please explain me the synchronization of the OFDM symbol? I >am also trying to understand how to decide the OFDM symbol start/end time >at the receiver just before going into FFT process. > >So far I understand that there two methods for time synchronization which >are data-aided and non-data aided methods, and in the data-aided methods >the training subcarriers are included in OFDM symbol and they are >correlated with the known training subcarriers at the receiver and the >time of the peak of the auto-correlation is the time delay (tao)of the >channel and this means the OFDM symbol reaches the receiver after this tao >delay. > >My questions here are; > >1-For time synchronization, if the data-aided method is used, should the >one complete OFDM symbol contain all training subcarriers and then all are >correlated with the known trainings one by one at the receiver? > >2-Or the OFDM symbol contains a few training subcarriers and when they are >received by the receiver each of the trainings is correlated by the known >pilots?You can use whatever pilots are there, and that will depend on the system you're working on. You can also correlate the CP with the end of the symbol (from which it was copied) and get time synchronization that way. This is very commonly done. Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
