A novel CDMA multi-user detection scheme, orthogonal decision-feedback detector (ODFD), is proposed for a synchronous CDMA system in this paper. It is robust for its near-far resistance and high multi-user detection efficiency with a performance similar to that of decorrelating decision-feedback detector (DDFD) but with a reduced complexity. The ODFD scheme employs a match-filter bank that matches a set of ortho-normal sequences. The ortho-normal sequences are generated by the Gram-Schmidt orthogonalisation procedure based on the spreading codes. The ODFD algorithm involves only with the ortho-normal coefficient-matrix which requires no frequent recalculations even when system parameters change. Successive decision-feedback detection is carried out immediately at the output of the ODFD match-filter bank without matrix inversion operations, resulting in a much simplified structure.

A new scheme to study the performance of a DS/CDMA indoor wireless system, the correlation statistics distribution convolution(CSDC)modeling, is introduced in this paper. With the aid of the CSDC modeling, the bit error rate versus number of simultaneous interfering transmitters can be directly evaluated, considering the effects of Rayleigh fading, power control, multipath and co-channel interference. The performance of two CDMA receiver structures, conventional correlator and RAKE receiver, is compared. It is shown that the RAKE receiver is effective in improving the system performance under indoor multipath fading. However, its effectiveness under transmitter power control is sensitive to the severity of multipath interference in the indoor channel. When the multipath fading is severe, a tight power control over the main paths may not be able to improve the performance of the RAKE receiver.

Either GMW sequence or m-sequence possesses a 2-valued auto-correlation function which helps to improve the performance of a RAKE receiver. However, their cross-correlation functions are less well controlled. Before they can be applied to a CDMA system, it is necessary to construct their sub-families (taking advantage of their large family size) which offer satisfactory cross-correlation functions. This paper studies several algorithms for constructing those quasi-optimum sub-families in terms of minimized bit error rate under co-channel interference. The study shows that the performance of resultant sub-families is sensitive to sub-family sizes and algorithms. A new criterion based on combined (even and odd) maximum cross-correlation for code selection is introduced, and highest-peak-deleting and most-peak-deleting algorithms are suggested for constructing quasi-optimum sub-families of GMW and m-sequences.