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
How to decide FFT window size in OFDM system receiver
Started by ●May 27, 2014
Reply by ●May 27, 20142014-05-27
There is no upper limit to the FFT size. It's an implementation tradeoff. _____________________________ Posted through www.DSPRelated.com
Reply by ●May 27, 20142014-05-27
On Tue, 27 May 2014 05:54:45 -0500, "runinrainy" <100271@dsprelated> 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?The FFT does not need to include the cyclic prefix and you'll make life a lot harder on yourself if you do. The FFT in the receiver can be the same size as the FFT in the transmitter. Since the cyclic prefix is a portion of the FFT output repeated in the output sequence, the receiving FFT can be the same size and operate on the same portion of the signal. That means discarding the cyclic prefix after the FFT is synchronized to the correct portion of the received signal. Synchronizing the FFT to the correct chunk of signal in the receiver is the hard part. ;) Eric Jacobsen Anchor Hill Communications http://www.anchorhill.com
Reply by ●June 23, 20142014-06-23
On Tuesday, May 27, 2014 9:25:28 PM UTC+5:30, Eric Jacobsen wrote:> On Tue, 27 May 2014 05:54:45 -0500, "runinrainy" <100271@dsprelated> > > 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? > > > > The FFT does not need to include the cyclic prefix and you'll make > > life a lot harder on yourself if you do. > > > > The FFT in the receiver can be the same size as the FFT in the > > transmitter. Since the cyclic prefix is a portion of the FFT output > > repeated in the output sequence, the receiving FFT can be the same > > size and operate on the same portion of the signal. That means > > discarding the cyclic prefix after the FFT is synchronized to the > > correct portion of the received signal. > > > > Synchronizing the FFT to the correct chunk of signal in the receiver > > is the hard part. ;) > > > > > > Eric Jacobsen > > Anchor Hill Communications > > http://www.anchorhill.comFFT size used in the Receiver has to be the same as used in Transmitter. -Advantage/s of Increased FFT size(in the communication scheme) : Provides longer delay profile support. Same fraction of CP can provide immunity against longer delay profiles as compared to choosing smaller FFT sizes. -Disadvantage/s of Increased FFT size(in the communication scheme) : Rx System will be more prone to Sampling frequency offsets since the carriers will be more closely sampled in frequency domain. Also, there may be higher memory requirements in the Demodulator implementation.
Reply by ●June 23, 20142014-06-23
>> FFT size used in the Receiver has to be the same as used inTransmitter. nope. _____________________________ Posted through www.DSPRelated.com
Reply by ●June 23, 20142014-06-23
Reply by ●June 23, 20142014-06-23
Alright, unless you don't upsample the time domain signal, the receiver should use the same fft size. But one can't surely apply a lesser size fft wrt tx.
Reply by ●June 24, 20142014-06-24
On Tuesday, June 24, 2014 7:47:22 AM UTC+5:30, Rakesh Joshi wrote:> Alright, unless you don't upsample the time domain signal, the receiver should use the same fft size. But one can't surely apply a lesser size fft wrt tx.One advantage I see with using a smaller fft size in the transmitter is to provide better doppler support. This is due to the quick Channel frequency updates that one can get using a smaller FFT size.
Reply by ●June 24, 20142014-06-24
sorry, but again, "nope". Let's say I receive a 300 subcarrier signal with a 512-bin FFT. Assume the signal is filtered, containing no energy outside the subcarrier range. There are 512-300 = 212 unused subcarriers / FFT bins, 106 on either side, that don't correlate with the signal (because we assume it's filtered) and give zero. In reality we put them there because using an oversized FFT is easier than implementing a steeper filter. Now I change the FFT size to 513, 1024 or 987654321 bins. It doesn't matter - all I do is add subcarrier bins that are uncorrelated with the signal and give a zero result. The receiver always picks the subcarriers where it expects the signal and doesn't care about the unused ones. What is maybe not obvious is that changing the FFT size scales the sampling rate, if the "numerology" of the signal (from the radio standard), i.e. symbol length, number of SCs etc remains unchanged. For example, in LTE, 5 MHz mode with a 512 bin FFT, the correct sampling rate is 7.68 MSPS. If I chose - for whatever reasons - to use a 2048 bin FFT, the correct sampling rate would be 30.72 MSPS. Looking the other way, it means that an FFT can resample a cyclic (that's the catch!) sampled signal to any integer number of samples. For example, I take a 3-minute audio file, fft(), insert one sample, ifft(). The result is exactly one sample longer. Try it, it really works. _____________________________ Posted through www.DSPRelated.com
Reply by ●June 25, 20142014-06-25
@runinrainy : i confused your question regarding choosing fft size at the rx of ofdm systems. My point about dis/adv of choosing a smaller/bigger fft windows were related to the no. of carriers used to form an ofdm symbol at the transmitter. My apologies for the same. As far as the question of choosing an fft window at the rx for a given mode of symbol size(@ baseband rate) is concerned one can use the fft window size used in tx or an integral multiple of that size should provide you the required data carriers with respective up sampling in the freq domain. ANY FFT SIZE ISNT OK! Else one will potentially end up losing the transmitted carriers in the fractional frequency space. Also, as Eric rightly points out, cyclic prefix is supposed to be used to align the fft window such that there isn't any effect due to ISI. If one takes the CP also while doing fft, isi gets into the signal.
