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8 - Spread spectrum

Published online by Cambridge University Press:  05 June 2012

Tri T. Ha
Tri T. Ha
Affiliation:
Naval Postgraduate School, Monterey, California
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Summary

Introduction

A spread spectrum modulated signal has a bandwidth much larger than the bandwidth of its narrowband counterpart for transmitting the message signal. For example, a spread spectrum PSK signal employed in the forward link of the 2G cellular standard IS-95 carrying a coded message signal of symbol rate 19.2 ksps has a bandwidth of 1.25 MHz. The narrowband PSK signal would only require a bandwidth of 19.53 kHz. This is an increase of bandwidth by a factor of 64. What is the reason behind the use of spread spectrum modulation? Historically, spread spectrum modulation originated from military applications. The purpose is to protect the received signal's integrity by reducing the effectiveness of a jamming signal. In order to jam a spread spectrum signal, the jamming signal must distribute its fixed transmitted power over a larger spread bandwidth. This would lower the magnitude of its power spectral density and correspondingly its jamming power in the smaller message bandwidth. There are basically two types of spread spectrum modulated signals: direct sequence (DS) and frequency hop (FH).

Direct sequence modulation

In direct sequence modulation the data 0s and 1s of bit rate Rb = 1/Tb are used to modulate a periodic pseudo-noise (PN) sequence of N chip 0s and 1s. The chip rate is Rc = 1/Tc = NRb, and the sequence period is the same as the bit time.

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Theory and Design of Digital Communication Systems , pp. 432 - 472
Publisher: Cambridge University Press
Print publication year: 2010

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References

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  • Spread spectrum
  • Tri T. Ha, Naval Postgraduate School, Monterey, California
  • Book: Theory and Design of Digital Communication Systems
  • Online publication: 05 June 2012
  • Chapter DOI: https://doi.org/10.1017/CBO9780511778681.009
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  • Spread spectrum
  • Tri T. Ha, Naval Postgraduate School, Monterey, California
  • Book: Theory and Design of Digital Communication Systems
  • Online publication: 05 June 2012
  • Chapter DOI: https://doi.org/10.1017/CBO9780511778681.009
Available formats
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Save book to Google Drive

To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.

  • Spread spectrum
  • Tri T. Ha, Naval Postgraduate School, Monterey, California
  • Book: Theory and Design of Digital Communication Systems
  • Online publication: 05 June 2012
  • Chapter DOI: https://doi.org/10.1017/CBO9780511778681.009
Available formats
×