atively large side lobes of each OFDM subcarrier, However, in case of a time-dispersive channel the already a relatively limited amount of time dispersion or, orthogonality between the subcarriers will, at least partly, equivalently, a relatively modest radio-channel frequency be lost. The reason for this loss of subcarrier orthogo- selectivity may cause non-negligible interference nality in case of a time-dispersive channel is that, in this between subcarriers. case, the demodulator correlation interval for one path To deal with this problem and to make an OFDM will overlap with the symbol boundary of a different signal truly insensitive to time dispersion on the radio channel, so-called cyclic-prefix insertion is typically used in case of OFDM transmission. As illustrated in Figure 4.4-9, cyclic-prefix insertion implies that the last part of the OFDM symbol is copied and inserted at the beginning of the OFDM symbol. Cyclic-prefix insertion P-> S ao , NC-1 Size-N thus increases the length of the OFDM symbol from to Tu + TCB where TCP is the length of the cyclic prefix, (FFT) with a corresponding reduction in the OFDM symbol rate as a consequence. As illustrated in the lower part of Unused Figure 4.4-9, if the correlation at the receiver side is still only carried out over a time interval Tu = 1/Af, Figure 4.4-7 OFDM demodulation by means of FFT processing. subcarrier orthogonality will then be preserved in case of OFDM transmission CHAPTER 4.4 (m-1) (m+1) Direct path Reflected path Integration interval for demodulation of direct path Figure 4.4-8 Time dispersion and corresponding received-signal timing. a time-dispersive channel, as long as the span of the time of the OFDM transmission to fast channel variations, that dispersion is shorter than the cyclic-prefix length. is high Doppler spread, as well as different types of fre- In practice, cyclic-prefix insertion is carried out on the quency errors, see further Section 4.4.8. time-discrete output of the transmitter IFFT. Cyclic- It is also important to understa