onversion and filtering S(N-1)/2-2 S(N-1)/2-1 Modulated S(N-1)/2 (m+ 1)T, Input bits symbols Xm 'm (N-1 Transmitter One useful OFDM symbol Frequency domain Time domain Discard Serial to Frequency Parallel to CP removal parallel domain serial Modulated symbols converter equalization conversion Discard Receiver Figure 3.11 OFDM signal generation and reception process. The mapping of the modulated data symbol into multiple subcarriers also allows an increase in the symbol duration. The symbol duration obtained through an OFDM scheme is much larger than that of a single-carrier modulation technique with a similar transmission band- width. In general, when the channel delay spread exceeds the guard time, the energy con- tained 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, that is, 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; therefore, this will introduce the inter-subcarrier orthogonality loss. This can affect the performance in fast fading conditions. Hence, the symbol duration should be kept smaller than the minimum channel coherence time. Since the channel 320 Chapter 3 coherence time is inversely proportional to the maximum Doppler spread, the symbol dura- tion Ts must, in general, be chosen such that << / VRMS. The large number of OFDM subcarriers makes the bandwidth of the individual subcarriers small relative to the overall signal bandwidth. With an adequate number of subcarriers, the inter-carrier spacing is much smaller than the channel coherence bandwidth. Since the chan- nel coherence bandwidth is inversely proportional to the channel delay spread TRMS, the