A new construction of signature waveforms for synchronous CDMA systems

We consider synchronous code-division multiple access (CDMA) systems over an additive white Gaussian noise (AWGN) channel, where all users are divided into groups of small size. The signature waveforms for users in each group are constructed from the same signature sequence but with different chip waveforms. To minimize the multiple access interference (MAI) at the output of the correlators, Welch-bound-equality (WBE) sequences and chip waveforms having optimal correlation property are employed. The main idea behind the proposed construction is to suppress the inter-group interference from users in different groups as much as possible (even to remove it completely) at the expense of introducing the intra-group interference among the users in the same group. The intra-group interference, however, can be easily handled by a low-complexity, optimal (or suboptimal) multiuser detector(s) if the group size is kept small enough. As special cases, the proposed constructions correspond to the optimal design of the signature waveforms and the conventional system that uses a single chip waveform, respectively. Thus the proposed construction offers a flexibility to trade performance for complexity. In particular, it is demonstrated that, while the conventional system's error performance is very sensitive to even a small amount of overload, the proposed system with two users per group can have up to 100% overload with an excellent error performance.