Hello: I have a question regarding filtfilt function in Matlab. Due to forward and backward filtering, filtfilt function will have zero-phase response at any frequency. If I apply this filter to a time domain OFDM sequence, does that mean I can set the FFT start window the same as the transmitted IFFT starting point? In my test, I have an OFDM sequence of 5 symbols each with 80 samples at 20 MHz clock. Only the third symbol has valid data, all others set to zero. After passing though a 5th order Butterworth filter implemented using filtfilt function, the data are spread over previous symbols and following symbols. But for filtfilt, there should be no phase distortion. What the data outside of 3th symbol represent? Should I call them intersymbol interference? Does the third symbol itself suffer from intercarrier interference as well? I have another question about phase delay and group delay. We know linear phase FIR filter has a constant group delay. If the phase response is (angle(w)=a*w). The phase delay and group delay both equals a. There is no time delay distortion or envelop delay distortion. If (angle(w)=a*w+c). The group delay is still a, and it is a linear phase filter. But the phase delay (-angle(w)/w) is not a constant among frequencies. Does this filter has phase distortions? Thanks very much X.Y

# Using Filtfilt function for OFDM signals

Started by ●June 19, 2008