> I just read the Pactor 3 specification. I am not sure that it is OFDM. It is > multi-tone but the spacing of the tones seems wider than OFDM requires.
The spacing is derived from the "integration time" only, while the baudrate is the sum of "integration time" plus "guard interval". (integretion time = FFT length for one symbol) You will always see the spacing is higher than 1/T if there is a guard interval involved. This makes it pretty difficult to determine if a signal is FDM or OFDM, since you can turn any FDM signal into an OFDM signal by defining an appropriate guard interval. Example: a FDM system with 20ms symbol time and 75 hz spacing is exactly the same as an OFDM system with a "guard interval" of 6.6ms and an "integration time" of 13.3 ms. total symbol time is 6.6 + 13.3 = 20ms, spacing = 1000/13.33 = 75 hz. OFDM and FDM is the same signal ...
