Quasi-Synchronous CDMA Using Properly Scrambled Walsh Codes as User-Spreading Sequences

Abstract

An orthogonal spreading code family based on scrambled Walsh-Hadamard (WH) sequences is proposed for quasi-synchronous (QS) code-division multiple-access (CDMA) systems. With 1 chip offset, either the top or the bottom half rows of the WH matrix remain orthogonal if they are scrambled using the proposed scrambling patterns. With quadriphase scrambling applied on a quarter row of the WH matrix, the zero-correlation zone (ZCZ) is extended to 3 chips. The proposed spreading codes satisfy the theoretical limit of code size for a given code length and ZCZ. Compared with existing spreading codes for QS-CDMA, the proposed spreading codes have several advantages: 1) A special procedure for code family construction is not needed; it only requires a simple modification to the scrambling sequence of the conventional CDMA, which employs WH sequences as user signatures. 2) Flexible code family size; the size can easily be extended for multicell applications without having to consider existing codes used. Moreover, if synchronization is guaranteed to be within 1 chip, the code size can be doubled by allocating another quarter member of the WH sequence set. 3) The benefits of long code scrambling from the conventional cellular CDMA systems are inherited, such as robustness against interception and multiaccess interference randomization.