5
Equalization
As discussed in Chapter 2, from the theoretical point of view the transmission of a digital signal requires infinite bandwidth. However, in real life the bandwidth is a shared and very expensive resource. So, in real life we deal only with bandlimited systems. The result of this is that each received pulse is smeared by adjacent pulses. This phenomenon called intersymbol interference (ISI) was described in Chapter 2. The ISI is one of the major obstacles to reliable data transmission over bandlimited channels. One of the ways to eliminate or reduce the ISI is to use for transmission the waveforms obeying Nyquist criterion, which was described in Section 2.1.3. Unfortunately, the use of pulse-shaping allows the ISI to be eliminated only in the case of the ideal channel, that is, 
is the channel frequency function and W is the channel bandwidth. In this chapter we consider the methods of reducing the ISI in case of nonideal channel. The general name for these methods is channel equalization or simply equalization. These methods can be roughly divided into two broad categories: methods based on sequence estimation, and methods based on equalization with filters. For the first group of equalization methods it is assumed that the data stream at the receiver is detected as a sequence rather than each symbol being detected separately. ...
