1
Chapter one
1. 1 Introduction
1.1.1 over view
In modern digital communication and storage systems design, information theory is
becoming increasingly important .In recent years; there has been an increasing demand for
efficient and reliable digital data transmission. This demand has been accelerated by the
emergence of large scale, high speed data networks for the exchange and processing of digital
information in the military, governmental and private spheres. A major concern is control of
errors, so that reliable reproduction of data can be obtained. Recent developments have
contributed toward achieving the reliability required by today’s high speed digital systems and
the use of coding for error control has become an integral part in the design of modern
communication systems and digital computers.
When we are watching TV, enjoying music on CD, talking with friend by cellular phone
etc., we encounter problem due to the impact of the noisy environment. A CD player includes
semiconductor memories, optical media, laser device, disk files, and so on. Each of them is subject
to various types of noise disturbance may come from switching impulse noise, thermal noise, dust
on disk surface, or even lightening. Error control coding (also referred to as channel coding) is
used to detect, and often correct, symbols which are received in error on digital communications
channels.
A typical digital communications system is shown below in Figure 1.1. The source
information is usually composed of binary or decimal digits or alphabetic information in either a
continuous waveform or a sequence of discrete symbols. The encoder transforms these messages
into a sequence of binary digits (bits) to the channel. The sequence enters the channel and is
perturbed by noise. The output enters the decoder, which makes a decision concerning which
message was sent and delivers this message to the sink.
