 sub- carrier separation is generally designed such that 1/TRMS- In this case, the fading on each subcarrier is flat and can be modeled as a complex-valued constant channel gain. The individual reception of the modulated symbols transmitted on each subcarrier 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 VRMS. Since the OFDM sampling frequency is typically larger than the actual signal bandwidth, only a subset of subcarriers is used to carry modulated symbols. The remaining subcarriers are left inactive prior to the IFFT and are referred to as guard subcarriers. The split between the active and the inactive subcarriers is determined based on the spectral sharing and regula- tory constraints, such as the bandwidth allocation and the spectral mask. An OFDM trans- mitter diagram is shown in Fig. 3.11. The incoming bit stream is channel coded and modulated to form the complex-valued modulated symbols. The modulated symbols are converted from serial to parallel with N <NFFT complex-valued numbers per block, where NFFT is the size of FFT/IFFT operation. 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 transmis- sion. A guard interval or CP is inserted between symbols to eliminate ISI effects caused by multipath distortion. The discrete symbols are windowed/filtered and converted to an analog signal for RF upconversion. The reverse process is performed at the receiver. A one-tap equalizer is usually used for each subcarrier to correct channel distortion. The tap coeffi- cients are calculated based on channel information. When there is multipath distortion, a conventional single-carrier wideband transmission sys- tem suffers from frequency-selective fading. A complex adaptive equalizer must be used to equalize the in-band fading