B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

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SPREAD SPECTRUM
COMMUNICATIONS
In traditional digital communication systems, the design of baseband pulse-shaping and
modulation techniques aims to minimize the amount of bandwidth consumed by the modulated
signal during transmission. This principal objective is clearly motivated by the desire
to achieve good spectral efficiency and thus to conserve bandwidth resource. Nevertheless,
a narrowband digital communication system exhibits two major weaknesses. First, its concentrated
spectrum makes it an easy target for detection and interception by unintended users
(e.g., battlefield enemies and unauthorized eavesdroppers). Second, its narrow band, having
very little redundancy, is more susceptible to jamming, since even a partial band jamming can
ruin the signal reception.
Spread spectrum technologies were initially developed for the military and intelligence
communities to overcome the two aforementioned shortcomings against interception and jamming.
The basic idea was to expand each user signal to occupy a much broader spectrum than
necessary. For fixed transmission power, a broader spectrum means both lower signal power
level and higher spectral redundancy. The low signal power level makes the communication
signals difficult to detect and intercept, whereas high spectral redundancy makes the signals
more resistant to partial band jamming, whether intentional or unintentional.
There are two dominant spread spectrum technologies: frequency hopping spread spectrum
(FHSS) and direct sequence spread spectrum (DSSS). In this chapter, we provide detailed
descriptions on both systems.
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. 1 FREQUENCY HOPPING SPREAD SPECTRUM
{FHSS) SYSTEMS
The concept of frequency hopping spread spectrum (FHSS) is in fact quite simple and easy
to understand. Each user can still use its conventional modulation. The only difference is that
now the carrier frequency can vary over regular intervals. When each user can vary its carrier
frequency according to a predetermined, pseudorandom pattern, its evasive signal effectively
occupies a broader spectrum and becomes harder to intercept and jam.
The implementation of an FHSS system is shown in Fig. 11.1. If we first ignore the two
frequency converters, this system is no different from a simple digital communication system
with an FSK modulator and a demodulator. The only difference in this FHSS system lies in
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