We have seen in Chapter 6 that delay spread causes intersymbol interference (ISI), which can cause an irreducible error floor when the modulation symbol time is on the same order as the channel delay spread. Signal processing provides a powerful mechanism to counteract ISI. In a broad sense, equalization defines any signal processing technique used at the receiver to alleviate the ISI problem caused by delay spread. Signal processing can also be used at the transmitter to make the signal less susceptible to delay spread: spread-spectrum and multicarrier modulation fall in this category of transmitter signal processing techniques. In this chapter we focus on equalization; multicarrier modulation and spread spectrum are the topics of Chapters 12 and 13, respectively.

Mitigation of ISI is required when the modulation symbol time Ts is on the order of the channel's rms delay spread σTm. For example, cordless phones typically operate indoors, where the delay spread is small. Since voice is also a relatively low–data-rate application, equalization is generally not needed in cordless phones. However, the IS-136 digital cellular standard is designed for outdoor use, where σTm ≈ Ts, so equalization is part of this standard. Higher–data-rate applications are more sensitive to delay spread and generally require high-performance equalizers or other ISI mitigation techniques. In fact, mitigating the impact of delay spread is one of the most challenging hurdles for high-speed wireless data systems.