Complex Hadamard Transform-Based Quaternary Quasi-Orthogonal Sequences and Allocation for DS-CDMA Systems

The capacity of downlink direct-sequence code-division multiple access (DS-CDMA) system is often limited by the available number of Walsh-Hadamard (WH) codes. Using quasi-orthogonal sequences (QOSs) is a practical solution to enhance the capacity. In this paper, a new construction for a set of quaternary quasi-orthogonal sequences (QQOSs) is proposed based on a complex Hadamard transform (CHT) matrix. This approach is simple and easy to generate QQOSs with any length in {2^m, m=1, 2,...,}. Meanwhile, allocation schemes of proposed QQOSs are investigated for downlink DS-CDMA systems over frequency selective fading channel. Simulation shows that different sequence allocation schemes result in different bit error rate performance