Abstract

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).

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