Message Protection by Spread Spectrum Modulation in a Packet Voice Radio Link

In spread spectrum communication systems the bandwidth of the transmitted signal is far in excess of the information bandwidth itself. The spectrum spreading is controlled by a pseudonoise (PN) code. Knowledge of this code allows authorized receivers to process the arriving signal with a significant gain in signal-to-noise ratio by correlating it with a locally generated reference waveform. The inherent interference suppression capability has been the primary motivation for the development of spread spectrum techniques /1/. The two most common forms of spread spectrum modulation are direct-sequence (DS) and frequency-hopping (FH), the first of which is considered in this paper. In DS systems the carrier is phase-modulated by a PN code with a code rate (chip rate) much higher than the data rate . The term ‘chip’ is used to distinguish between code and data stream. Although the codes most frequently used are not secure in a cryptographic sense, protection against unauthorized message access is associated with the low power spectral density of the wideband DS signals. This attribute applies even more to systems employing non-repeating spreading codes.