N. Kuroyanagi et al., A DESIGN PRINCIPLE FOR COLORED-NOISE-TOLERANT OPTIMUM DESPREADING-CODE SEQUENCES FOR SPREAD-SPECTRUM SYSTEMS, IEICE transactions on communications, E79B(10), 1996, pp. 1558-1569
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.''