Constrained adaptive algorithms for code acquisition and interference cancellation in asynchronous DS-CDMA systems

A technique for the estimation of desired-user bits in asynchronous DS-CDMA systems is developed. It does not assume an explicit bit synchronization between the receiver and the user. A bit value is estimated based on the observation window with size 2N-1, where N is the length of bit period. The window shifts for N in order to estimate a new bit. A constrained adaptive constant modulus algorithm (CMA) is used to estimate bits. The constraint limits the maximum elevation a weighting vector that multiplies the observations can have with respect to the hyperplane defined by shifted copies of the desired-user spreading sequence. The constraint is parameterized so the monitoring is not needed. The constrained CMA is compared with the unconstrained CMA. It is shown that the former one can lock to the desired user when the latter one locks to the wrong user. Moreover, the constrained CMA can estimate the desired-user signal delay, which can be used to switch to the synchronous DS-CDMA mode.