20-03-2012, 02:10 PM
Spread Spectrum Modulation
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Introduction
Spread spectrum communication systems are widely used today in a variety of applications for different purposes such as access of same radio spectrum by multiple users (multiple access), anti-jamming capability (so that signal transmission can not be interrupted or blocked by spurious transmission from enemy), interference rejection, secure communications, multi-path protection, etc. However, irrespective of the application, all spread spectrum communication systems satisfy the following criteria-
(i)As the name suggests, bandwidth of the transmitted signal is much greater than that of the message that modulates a carrier.
(ii)The transmission bandwidth is determined by a factor independent of the message bandwidth.
The power spectral density of the modulated signal is very low and usually comparable to background noise and interference at the receiver.
Advantages of Spread Spectrum (SS) Techniques
Reduced interference: In SS systems, interference from undesired sources is considerably reduced due to the processing gain of the system.
b) Low susceptibility to multi-path fading: Because of its inherent frequency diversity
properties, a spread spectrum system offers resistance to degradation in signal quality due to multi-path fading. This is particularly beneficial for designing mobile communication systems.
c) Co-existence of multiple systems: With proper design of pseudo-random sequences, multiple spread spectrum systems can co-exist.
d) Immunity to jamming: An important feature of spread spectrum is its ability to withstand strong interference, sometimes generated by an enemy to block the communication link. This is one reason for extensive use of the concepts of spectrum spreading in military communications.
Types of SS
Based on the kind of spreading modulation, spread spectrum systems are broadly classified as-
(i)
Direct sequence spread spectrum (DS-SS) systems
(ii)
Frequency hopping spread spectrum (FH-SS) systems
(iii)
Time hopping spread spectrum (TH-SS) systems.
(iv)
Hybrid systems
Direct Sequence (DS) Spread Spectrum System (DSSS)
The simplified scheme shown in Fig. 7.38.1 is of this type. The information signal in DSSS transmission is spread at baseband and then the spread signal is modulated by a carrier in a second stage. Following this approach, the process of modulation is separate from the spreading operation. An important feature of DSSS system is its ability to operate in presence of strong co-channel interference. A popular definition of the processing gain (PG) of a DSSS system is the ratio of the signal bandwidth to the message bandwidth.