Preview and Introduction - Digital Communication: Principles and System Modelling - page 11

transmit logic 0 and S 1 (t) waveform is transmitted to transmit logic 1. This technique
of modulation can also be called as binary modulation.
Communication channel is the physical medium that is used to communicate
the digital modulated/un-modulated signal from transmitter to the receiver end.
In wireless communication, free space is considered as the channel. On the other
hand, telephone channels employ a variety of physical media, including wired
transmission line, optical fiber cables and microwave. The characteristic of the chan-
nel is randomly corrupting the transmitted signal in a variety of possible manner,
such as additive thermal noise generated by electronic deices, human made noise
like automobile ignition noise, atmospheric noise like lightning discharges due to
thunderstorm.
The digital demodulator processes the corrupted digital waveform and gener-
ates output bit stream. The bit stream is passed then through the channel decoder
to reconstruct correct information after removing the redundant bits from the
knowledge of encoding scheme used in the channel encoder end.
To get natural (analog) signal at the output, the last and final stage employed
is source decoder. Like channel decoding, source decoder also decodes the input
signal according to the source encoding scheme applied at the transmitter end.
The schemes may be anything out of PCM, Delta Modulation, Adaptive Delta
Modulation (ADM), sigma delta modulation (SDM). The performance of the chan-
nel decoder-source decoder pair depends upon the statistical prediction of the
amount of error introduced per unit time, i.e., bit error rate. The parameter bit error
rate (BER) is highly dependent upon the waveform used at the time of source encod-
ing, the modulation scheme applied, etc. When the two extreme end signals (input
analog signal at the transmitter and recovered analog signal at the receiver) are com-
pared, the difference is the measure of the total distortion introduced by the digital
communication system.
1.6 MATLAB Programs
1.6.1 Time and Frequency Domain Representation of Signals
% In CD: ch1_1.m
% Time & frequency domain representation of signals
% Output: Fig. 1.3
% Programed by Apurba Das (Aug,'09)
clc;
clear all;
close all;
t = 0:.001:5;
T = 2;
f = 1/T;