0195136047.pdf

710 COMMUNICATION SYSTEMS

AMPS, D-AMPS, or PCS areas) has been developed. TheGSM(Global System for Messaging

communications) is the most widely accepted transmission method for PCS phones.

The latest wireless communications technology is thepersonal satellite phone.The coverage

is planetary and one can reach anywhere on earth. Examples include the Iridium satellite handset

developed by Motorola and others from the Teledesic constellation. Special requirements, such as

sending and receiving data or faxes, can be handled by the new handheldcomputer-and-mobile-

phone hybridssuch as Nokia’s 9000i and the Ericsson DI27. Mobile phones with a Web browser

capability are also available. We have yet to see the more exciting new developments in the

telecommunications industry with computers, networking, and wireless technology.

15.3 Digital Communication Systems

A digital signal can be defined as having any one of a finite number of discrete amplitudes at any

given time. The signal could be a voltage or current, or just a number such as 0 or 1. A signal for

which only two amplitudes are possible is known as abinary digital signal, the type of which is

commonly used in computers and most digital communication systems. A communication system

that is designed to process only digital signals (or messages) to convey information is said to be

digital. The recent trend is to make as much of the system digital as possible, because:

• Discrete data are efficiently processed.


• Analog messages can also be converted to digital form.


• Digital systems interface well with computers.


• Digital systems offer great reliability and yield high performance at low cost.


• Being flexible, digital systems can accommodate a variety of messages with ease.


• Security techniques are available to offer message privacy to users.


• Advanced signal-processing techniques can be added on.
  However, the most serious disadvantages are the increased complexity needed for system
  synchronization and the need for larger bandwidths than in an equivalent analog system. A
  digital system can directly interface with a source having only discrete messages, because of
  the inherent characteristic of the digital system. With suitable conversion methods, however,
  systems currently exist that can simultaneously transmit audio, television, and digital data over
  the same channel. Figure 14.1.2 illustrates the basic elements of a digital communication system,
  which was introduced in Section 14.1.
  Before we begin discussing digital systems, it is helpful to talk about the methods by which
  analog messages are converted into digital form.Sampling, quantization,andcodingare the three
  operations needed for the transmission of an analog signal over a digital system.

Sampling

This method was introduced in Section 14.2. Sampling of an analog signal makes it discrete in

time. A bandlimited signal can be recovered exactly from its samples, taken periodically in time

at a rate at least equal to twice the signal’s bandwidth. If a messagef(t) has a spectral extent of

Wfrad/s, the sampling ratefs(samples per second) must satisfy

fs≥

Wf

π

(15.3.1)

from the sampling theorem. The minimum rateWf/π(samples per second) is known as the

Nyquist rate. If the exact message samples could be transmitted through the digital system, the