Novel orthogonal codes for DS-CDMA with improved crosscorrelation characteristics in multipath fading channels

In today´s DS-CDMA systems, Hadamard-Walsh (HW) codes are employed as spreading codes in the downlink to maintain the orthogonality between users. However, after transmission through multipath fading channels, interference arises between different paths, leading to MPI (multi-path interference) in the DS-CDMA RAKE receiver. This, in turn, degrades the DS-CDMA system´s probability of error performance. We propose a set of novel complex spreading codes called carrier interferometry (CI) codes, and we demonstrate how these novel orthogonal spreading codes achieve crosscorrelations independent of the phase offsets between different paths (after transmission over a multipath fading channel). This improved crosscorrelation property (relative to HW codes) leads to higher SIR in the DS-CDMA RAKE receiver, and, as a direct result, better performance in terms of probability of error.