Transmitter-based multiuser interference rejection for the down-link of a wireless CDMA system in a multipath environment

In wireless code division multiple access (CDMA) communications systems, th ere has been recent interest in processing the transmitted down-link signal in order to shift signal processing to the transmitter where power and com putational resources are plentiful, thus simplifying receiver operation and reducing the power it requires. Multiuser interference (MUI) and multipath effects observed by the receiver are anticipated and suppressed at the tra nsmitter; channel equalization and multiuser detection are therefore not re quired. This paper introduces two methods that are able to combat both degr adations, yet allow the receiver to remain as simple as a single user recei ver for a perfect channel. For mild multipath channels, the performance of the algorithms is excellent, within a few decibels of the single user ideal channel case, at the cost of additional computation at the base station at which complete knowledge of the channels and the receiver codewords is req uired. One method, the decorrelating prefilter, is most flexible and applic able to existing systems yet less powerful than other recently published me thods. The second, the jointly optimized sequences algorithm, has performan ce on average superior to published methods. In addition to theoretical ana lysis and simulation of the algorithms' potential, these algorithms have al so been implemented and tested on a software radio testbed and experimental data is shown. The jointly optimized sequences performed particularly well even in severe multipath and multiuser interference environments.