 structure of the OFDM subcarriers, e.g. due to equal to 2m for some integer m, the OFDM modulation a frequency-selective radio channel, may lead to a loss of can be implemented by means of implementation- inter-subcarrier orthogonality and thus to interference efficient radix-2 Inverse Fast Fourier Transform (IFFT) between subcarriers. To handle this and to make an processing. It should be noted that the ratio N/Nc, which OFDM signal truly robust to radio-channel frequency could be seen as the over-sampling of the time-discrete selectivity, cyclic-prefix insertion is typically used, as will OFDM signal, could very well be, and typically is, a non- be further discussed in Section 4.4.4. integer number. As an example and as already men- tioned, for 3GPP LTE the number of subcarriers No is 4.4.3 OFDM implementation using approximately 600 in case of a 10 MHz spectrum allo- cation. The IFFT size can then, for example, be selec- IFFT/FFT processing ted as N = 1024. This corresponds to a sampling rate fs = N.Af = 15.36 MHz, where Af = 15 kHz is the LTE Although a bank of modulators/correlators according to subcarrier spacing. Figures 4.4-3 and 4.4-5 can be used to illustrate the basic It is important to understand that IDFT/IFFT-based principles of OFDM modulation and demodulation, implementation of an OFDM modulator, and even more respectively, these are not the most appropriate modu- SO the exact IDFT/IFFT size, are just transmitter- lator/demodulator structures for actual implementation. implementation choices and not something that would An OFDM signal defined according to (4.4.1) in theory has an infinite bandwidth and thus the sampling theorem can never be fulfilled completely. From now on the index m on the modulation symbols, indicating the OFDM-symbol number, will be ignored unless especially needed. CHAPTER 4.4 OFDM transmission Size-N D/A conversion (IFFT) aNc- 1 Figure 4.4-6 OFDM modulation by means of IFFT processing. be mandated by any radio-access specification. As an path, as illustrated in Figure 4.4-