Design of spreading codes for quasi-synchronous CDMA with intercell interference

The recently proposed loosely synchronized (LS) spreading code can in principle realize an intracell-interference-free quasi-synchronous code-division multiple-access (QS-CDMA) system by creating a wide enough interference-free window (IFW). However, the problem of minimizing intercell interference (ICI) in a cellular QS-CDMA system remains an open issue. Addressing the problem from a sequence design viewpoint, the key challenge is how to generalize the known construction of a single LS code to the design of many families of generalized LS (GLS) codes so that a desirable code family can be selected for the realization of a low-ICI cellular QS-CDMA system. Our main contribution is a systematic construction of new families of GLS codes with favorable intercode cross-correlation properties within a certain window, while maintaining the desirable IFW property. Many such code families can be obtained from our construction by choosing different Hadamard matrices and different uncorrelated complementary pairs. Their effectiveness with respect to some meaningful evaluation criteria are compared. In particular, by a simplified system bit-error rate analysis, it is demonstrated that a new GLS code family significantly outperforms the conventional scrambled LS codes.

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