TABLE OF CONTENTS In Section 4.5, we saw the BER performance of DPSK-based OFDM systems in Rayleigh fading channels. Here, we will discuss the robustness of an OFDM system against frequency selective fading.
Assume a time-invariant impulse response of a channel that has a support shorter than the guard interval (in other words, the largest time delay is smaller than the length of the guard interval). In this case, the impulse response is written as
| (4.43) |  |
Substituting (4.1), (4.7) and (4.43) into (4.8), we have
| (4.44) |  |
| (4.45) |  |
Equation (4.44) clearly shows that if the guard interval absorbs all the time delays of the channel, no ISI and no intersubcarrier interference due to multipath propagation occurs. It is also interesting to compare (4.44) with (3.4).
