10. Multi-Carrier Techniques

This chapter begins with a discussion of OFDM on frequency-selective channels, and describes how a cyclic guard interval can be used to completely remove any intersymbol interference (ISI) in a very efficient fashion provided that the length of the cyclic guard interval is at least as long as the length of the overall discrete-time channel impulse response. The performance of OFDM on static ISI channels and fading ISI channels is then considered, in cases where the length of the guard interval is less than the length of the overall discrete-time channel impulse response. In this case, residual ISI is present which is shown to be devastating to the performance of OFDM. An effective technique is then presented to mitigate residual ISI, called residual ISI cancellation (RISIC) that uses a combination of tail cancellation and cyclic reconstruction. The chapter then considers the combination of single-carrier modulation with frequency domain equalization (FDE), a technique known as single-carrier frequency domain equalization (SC-FDE). FDE is especially attractive on channels having long impulse responses where the complexity of time-domain equalizers can become prohibitive. Afterwards, a variety of topics related to OFDMA are considered. The use of OFDMA on both the forward and reverse link is covered, and issues such as sub-carrier allocation and time-domain windowing are considered. The chapter concludes with a discussion of single-carrier frequency division multiple access (SC-FDMA), including multiplexing methods and analysis of peak-to-average power ratio.