Orthogonal Spreading Code for Quasi-synchronous CDMA Based on Scrambled Walsh Sequence

摘要

Orthogonal spreading code based on scrambling on Walsh sequence is proposed for quasi-synchronous (QS) CDMA. We derive scramble patterns which achieve zero cross correlation within lower and upper half rows of Walsh-Hadamard sequence matrix for single chip timing offset. By assigning the proposed codes to the users in the QS CDMA, multi-access interference free communication is possible with the number of simultaneous users less than one half of the code length, which is the nominal uplink simultaneous user density in frequency reuse-efficient cellular systems. Even with more than one chip timing offset, the proposed scheme achieves still lower BER compared to the conventional spreading codes. Furthermore, there exist numerous multiple scramble patterns satisfying zero periodic cross correlation among user spreading codes. This allows us to utilize the intrinsic function of long code such as inter-cell interference randomization in multi-cell environment.