e received signal as a function of delay, A(v) identifies expected power of the received signal as a function of frequency. Similarly, the spaced-frequency correlation function I(() and the spaced-time correlation function P(t) are dual functions. It implies that, as (f) represents channel correlation in frequency, P(t) corresponds to the channel correlation function in time in a similar manner. 9.3 SC-FDMA AND OFDMA PRINCIPLES The Orthogonal Frequency Division Multiplexing (OFDM) is a form of multi-carrier modulation technique that was first introduced more than four decades ago. In a conventional serial data trans- mission system, the information bearing symbols are transmitted sequentially, with the frequency spectrum of each symbol occupying the entire available bandwidth. Figure 9-4 illustrates an unfiltered Quadrature Amplitude Modulation (QAM) signal spectrum. It is in the form of sin(f with zero crossing points at integer multiples of 1/T where Tu is the QAM symbol period. The concept of OFDM is to transmit the data bits in parallel QAM modulated sub-carriers using frequency division multiplexing. The carrier spacing is carefully selected SO that each sub-carrier is located on all the other sub-carriers' zero crossing points in the frequency domain. Although there are spectral overlaps among sub-carriers, they do not interfere with each other if they are sampled at the sub-carrier 9.3 SC-FDMA and OFDMA principles Fourier Transform -3-2-1 TUTUTU A Rectangular Pulse An Unfiltered QAM Signal Spectrum OFDM Signal Spectrum FIGURE 9-4 An illustration of OFDM Concept frequencies. In other words, they maintain spectral orthogonality. As shown in Figure 9-4, the OFDM signal in the frequency domain is generated through aggregation of NFFT parallel QAM-modulated sub-carriers, where adjacent sub-carriers are separated by sub-carrier spacing 1/T. An example OFDM signal spectrum as seen on a vector signal analyzer is shown in Figure 9-5. Since an OFDM signal consists of many parallel QAM sub-carriers, the mathematical 