Spread Spectrum Techniques

Spread spectrum signals have the distinguishing characteristic that the bandwidth used to transmit a message is much greater than the message bandwidth. This band spread is achieved by using a spreading code or pseudo-noise (PN) sequence that is independent of the message and is known to the receiver. The receiver uses a synchronized replica of the PN sequence to despread the received signal allowing recovery of the message. The chapter begins with an introduction to direct sequence (DS) and frequency hop (FH) spread spectrum. PN sequences are fundamental to all spread spectrum systems and are treated in detail. A variety of sequences are considered including m-sequences, Gold sequences, Kasami sequences, Barker sequences, Walsh-Hadamard sequences, variable length orthogonal codes, and complementary code keying. The remainder of the chapter concentrates on DS spread spectrum. The power spectral density of DS spread spectrum signals is considered. Afterwards, the bit error rate performance of DS spread spectrum signals is considered in the presence of tone interference. Afterwards, the performance of point-to-point DS spread spectrum on frequency-selective fading channels is discussed and it is shown how a RAKE receiver can be used to gain multipath diversity. The chapter concludes with a discussion of CDMA multiuser detection techniques, including optimum CDMA multiuser detection, decorrelation detection and minimum mean square error detection.