On sequence sharing for multi-code DS/CDMA systems

This paper focuses on sequence sharing, a technique of sequence reuse for multi-code DS/SSMA systems. Each user in the system owns a subset of orthogonal sequences, whereby the different subsets of sequences are not disjoint-a given sequence may be shared by more than one user. Thus, the system needs fewer orthogonal sequences, compared to a system which does not employ sequence sharing. Since, in our system, sequence collision is an inevitable problem, we investigate branch-, user- and system-collision-free probabilities for each sequence assignment. In order to achieve a collision-free design at the system level, a low-complexity protocol for coordinating users´ start times is put forward. With this protocol, codes (sequences) and times-of-arrival are used in combination to distinguish the signals of different users. Numerical results and comparisons are provided to illustrate the performance of the scheme, and our analytical results are supported by simulations