nce time. Since the channel coherence time is inversely proportional to the maximum Doppler spread, the symbol duration Ts must, in general, be chosen such that Ts VRMS The large number of OFDM sub-carriers makes the bandwidth of the individual sub-carriers small relative to the overall signal bandwidth. With an adequate number of sub-carriers, the CHAPTER 9 The IEEE 802. 16m Physical Layer (Part I) inter-carrier spacing is much smaller than the channel coherence bandwidth. Since the channel coherence bandwidth is inversely proportional to the channel delay spread TRMS, the sub-carrier separation is generally designed such that 1/TRMS. In this case, the fading on each sub- carrier is flat and can be modeled as a complex-valued constant channel gain. The individual reception of the QAM symbols transmitted on each sub-carrier is therefore simplified to the case of a flat-fading channel. This enables a straightforward introduction of advanced MIMO schemes. Furthermore, in order to mitigate Doppler spread effects, the inter-carrier spacing should be much larger than the RMS Doppler spread Afc 1/VRMS. Since the OFDM sampling frequency is typically larger than the actual signal bandwidth, only a subset of sub-carriers are used to carry QAM symbols. The remaining sub-carriers are left inactive prior to the IFFT and are referred to as guard sub-carriers. The split between the active and the inactive sub-carriers is determined based on the spectral constraints, such as the bandwidth allocation and the spectral mask. An OFDM transmitter diagram is shown in Figure 9-6. The incoming bit stream is first QAM modulated to form the complex-valued QAM symbols. The QAM symbols are converted from serial to parallel with N < NFFT complex-valued numbers per block, where NFFT is the number of FFT/IFFT points. Each block is processed by an IFFT and the output of the IFFT forms an OFDM symbol, which is converted back to serial data for transmission. A guard interval is inserted between symbols to eliminate ISI effects caused by multipat