Binary signature sets for increased user capacity on the downlink of CDMA systems

We compare the maximum achievable common signal to interference and noise ratio (SINR) target level K_max in the downlink of various code division multiple access (CDMA) systems with binary sequence sets. We derive an upper bound to K_max for binary systems with symbol-aligned users, and we compare this upper bound to the value of K_max for quasi-orthogonal sequences (QOS), Grassmannian signatures and binary Pados-Karystinos signatures. Secondly, we turn our attention to systems where the (binary) signatures of the users are displaced with respect to each other. In this context, we show that the value of K_max of improved random O(rthogonal)CDMA/OCDMA (O/O) can further be increased as compared to the original improved. O/O system (with square-root cosine rolloff pulses) by an appropriate choice of the chip pulse. In addition to this, improved O/O systems with fixed signature set can achieve values of K _max that are as high as those in the improved random O/O system, while not having the drawback of a spread on K_max. Applying a steepest descent search to such a fixed improved O/O system, can result in a system with a binary signature set that has even higher values for K_max well beyond the upper bound on K_max for systems with aligned chip pulses