8. Thus, the integration example, nothing forbids the implementation of an interval will not necessarily correspond to an integer OFDM modulator as a set of parallel modulators as number of periods of complex exponentials of that path illustrated in Figure 4.4-3. Also nothing prevents the use as the modulation symbols ak may differ between con- of a larger IFFT size, e.g. a size-2048 IFFT size, even in secutive symbol intervals. As a consequence, in case of case of a smaller number of OFDM subcarriers. a time-dispersive channel there will not only be inter- Similar to OFDM modulation, efficient FFT symbol interference within a subcarrier but also processing can be used for OFDM demodulation, interference between subcarriers. replacing the bank of No parallel demodulators of Another way to explain the interference between Figure 4.4-5 with sampling with some sampling rate fs = subcarriers in case of a time-dispersive channel is to have 1/Ts, followed by a size-N DFT/FFT, as illustrated in in mind that time dispersion on the radio channel is Figure 4.4-7. equivalent to a frequency-selective channel frequency response. As clarified in Section 4.4.2, orthogonality between OFDM subcarriers is not simply due to fre- 4.4.4 Cyclic-prefix insertion quency-domain separation but due to the specific frequency-domain structure of each subcarrier. Even if As described in Section 4.4.2, an uncorrupted OFDM the frequency-domain channel is constant over a band- signal can be demodulated without any interference width corresponding to the main lobe of an OFDM between subcarriers. One way to understand this subcarrier and only the subcarrier side lobes are subcarrier orthogonality is to recognize that a modulated corrupted due to the radio-channel frequency selectivity, subcarrier xk(t) in (4.4.1) consists of an integer number the orthogonality between subcarriers will be lost with of periods of complex exponentials during the inter-subcarrier interference as a consequence. Due to demodulator integration interval Tu = 1/Af. the rel