Complexity reduction in subspace-based blind channel identification for DS/CDMA systems

Direct-sequence code-division multiple access is emerging as a potential mu ltiple-access communication scheme for future digital wireless communicatio ns systems, Such wide-band systems usually operate in a frequency-selective fading channel that introduces intersymbol interference and thus potential performance degradation, Previously proposed subspace-based blind channel identification algorithms, which provide estimates of channel parameters fo r effective equalization, suffer from high numerical complexity for systems with large spreading gains. In this paper, it is shown that, through the u se of matched filter outputs, reduction in numerical complexity can be obta ined. The complexity reduction is considerable when the channel length is s mall and the system is moderately loaded. The results show that the neu alg orithm suffers a slight performance loss. Although the employed matched fil ter outputs do not form a set of sufficient statistics for the unknown chan nels, the difference between the matched filter outputs and the sufficient statistics becomes negligible for large observation lengths and the asympto tic normalized Fisher information does not change. Performance is evaluated through simulations, the derivation of a tight approximation of the mean-s quared channel estimation error, and through comparisons to the Cramer-Rao bound for the estimation error variance. It is shown that the approximation of the mean-squared error can be obtained in terms of the correlation of t he spreading codes and the channels. This representation of the error suppl ies a tool for investigating the relationship between performance and sprea ding sequence correlations.