ABSTRACT
Spectral leakage, that is, leakage of spectral energy into undesired parts of the spectrum, can significantly lower spectral efficiency in practical systems. Figure 7.1 illustrates the importance of spectral leakage in OFDMA systems. Like all communication systems, also OFDMA-based systems have to maintain spectral compatibility with systems occupying neighboring bands-both in downlink (DL) and in uplink (UL) directions (cf. Figure 7.1a). Spectral energy outside the band covered by the subcarriers is eliminated by continuous-time postfiltering, which constructs the analog transmit signal by suppressing the mirror images of the digital-to-analog converter’s output. A primary design goal in practical systems is to keep the requirements for the continuous-time postfilter fairly relaxed for the sake of hardware complexity and power consumption. Therefore, often up to a fifth of the available subcarriers are nulled to form guard bands with reduced spectral energy close to the two band edges.
