rom the region of delay spread. As the blank spaces cannot be left in the signal without a guard interval between successive OFDM symbols, intersymbol interference from the (i - 1)th symbol gives a distortion to the ith symbol. Therefore a guard interval (no signal transmission) with length Ag > Tmax should be applied, where Tmax is the maximum time delay due to multipath fading. This completely eliminates any intersymbol interfer- ence that may arise, but results in a sudden change of waveform containing higher spectral components, thereby leading to intersubcarrier interference. However, to counter intersubcarrier interference, cyclic prefixing is performed where the original signal is extended in the guard time (by approximately 1.25 times the original signal). Here, the end part of the symbol is copied and joined to the start of the signal until the signal length becomes 1.25 times the original length. The symbol source becomes continuous and it would appear that the symbol period is becoming longer. Given the channel frequency selectivity, the channel time selectivity and the symbol transmission rate, the transmission performance becomes more sensitive to the time selectivity as the number of subcarriers increases because the wider symbol duration is less robust to random FM noise, whereas it becomes poor as the number of subcarriers decreases because the wider power spectrum of each subcarrier is less robust to the frequency selectivity. On the other hand, the transmission performance becomes poor as the length of the guard interval increases because the signal transmission in the guard interval introduces a power loss, whereas it becomes more sensitive to the frequency selectivity as the length of the guard interval decreases because the shorter guard interval is less robust to the delay spread. Weighting/ 0010110 Channel 'H' 0010110 Coding, Modulation De-mapping Weighting/mapping De-Modulation Decoding N TXers M RXers Figure 5.3 MIMO system 5.5.2 MIMO Systems The MIMO systems need to form an integral part