mohatta2015.pdf

2 INTRODUCTION
Table 1.1
Possible messages
Index Representation Message
Si S2
1 +1—1 "Do you like classical music?"
2 -1 -1 "Do you like soccer?"
3 +1+1 "Do you like me?"
Figure 1.1
Dispersive scenario.
Which message was sent? How would you figure it out? Would it help if symbol So
were known or thought to be +1? Think about different approaches for determining
the transmitted symbols. Try them out. Do they give the same answer? Do they
give valid answers (the sequence si = — 1 S2 = +1 is not in the table)?
1.1 THE IDEA
Channel equalization is about solving the problem of intersymbol interference (ISI).
What is ISI? First, information can be represented as digital symbols. Letters
and words on computers are represented using the symbols 0 and 1. Speech and
music are represented using integers by sampling the signal, as shown in Fig. 1.2.
These numbers can be converted into base 2. Thus, the number 6 becomes 110
(0x1 + 1x2+1x4). There are different ways of mapping the symbols 0 and 1
into values for transmission. One mapping is to represent 0 with +1 and 1 with
— 1. Thus, 110 is transmitted as using the series —1 —1 +1. The symbols 0 and 1
are often referred to as Boolean values. The transmitted values are called modem
symbols or simply symbols.
ISI is the interference between symbols that can occur at the receiver. In the
Alice and Bob example, we saw that one symbol was interfered by a previous symbol
due to a second signal path. This is a problem in cell phone communications, and
we will refer to it as the dispersive channel scenario. A cell tower transmitter sends