A DESIGN PRINCIPLE FOR COLORED-NOISE-TOLERANT OPTIMUM DESPREADING-CODE SEQUENCES FOR SPREAD-SPECTRUM SYSTEMS

To improve the demodulated signal-to-noise ratio, SNR, for colored noi se environments, Lye present a new direct-sequence spread-spectrum rec eiver system, whose construction is based on the concept of Shaped hi- sequence Demodulation (SMD). This receiver has the function for shapin g the local despreading-code waveform, This method can modify the freq uency transfer function from a received input to the damp-integrated o utput according to the pou er spectrum of colored noise added in the t ransmission process. SMD performs the combined function of a whitening filter and a matched filter, which can be used to implement an optimu m receiver. For the case when the additive colored-noise power spectru m is known and the transmission channel is non-band-limited, a design theory is derived that provides the maximum SNR by choosing the best d espreading-code sequence corresponding to a given signature spreading- code sequence. The noise power component produced in the receiver damp -integrated-output is anayzed by introducing the auto-correlation matr ix of the additive noise. The SNR performance of systems, one using no n-optimized codes and the other using optimized codes. is examined and compared for various noise models. It is verified by analysis and com puter simulation that, compared to a conventional system using non-opt imized codes, remarkable SNR improvements can be achieved due to the w hitening effect acquired without producing inter-symbol interference. in contrast, ifa transversal whitening filter is front-ended, it produ ces inter-frame interference, degrading the SNR performance. The band- limiting effect of the transmission channel is also analyzed. and we c onfirmed that the codes optimized for the non-band-limited channel can be applied to the band-limited channel with little degradation of SNR . SMD is inherently tolerant of fast-changing noise such as fading: du e to its frame-by-frame operation. Considering this function as a gene ral demodulation scheme, it may be called ''Local Code Filtering.''