X(N-1)/2-2 X(N-1)/2-1 Input Symbols (N-1)/2 (m+1)Tu sm(0), sm(1), sm(2), (N-1) One Useful OFDM Symbol Frequency Domain Time Domain FIGURE 9-6 OFDM signal generation the multipath delay, the ISI can be effectively eliminated. The ratio of the guard interval to useful OFDM symbol duration depends on the deployment scenario and the frequency band. Since the insertion of guard intervals will reduce data throughput, Tg is usually less than Tu/4. The cyclic prefix should absorb most of the signal energy dispersed by the multipath channel. The entire ISI energy is contained within the cyclic prefix if its length is greater than that of the channel RMS delay spread, i.e., Tg > TRMS. In general, it is sufficient to have most of the delay spread energy absorbed by the guard interval, considering the inherent robustness of large OFDM symbols to time dispersion. The mapping of the modulated data symbol into multiple sub-carriers also allows an increase in the symbol duration. Since the throughput on each sub-carrier is greatly reduced, the symbol duration obtained through an OFDM scheme is much larger than that of a single carrier modulation technique with a similar overall transmission bandwidth. In general, when the channel delay spread exceeds the guard time, the energy contained in the ISI will be much smaller with respect to the useful OFDM symbol energy, as long as the symbol duration is much larger than the channel delay spread, i.e., Ts > > TRMS. Although large OFDM symbol duration is desirable to mitigate the ISI effects caused by time dispersion, large OFDM symbol duration can further reduce the ability to alleviate the effects of fast fading, particularly if the symbol period is large compared to the channel coherence time; then the channel can no longer be considered as time-invariant over the OFDM symbol duration and this will introduce inter-sub-carrier orthogonality loss. This can affect the performance in fast fading conditions. Hence, the symbol duration should be kept smaller than the minimum channel cohere