Group Transmission in Downlink of Overloaded CDMA Systems

This paper proposes two different transmit precoding schemes for downlink of overloaded CDMA systems i.e. CDMA systems in which the number of users is more than the processing gain of the system. These schemes are based on user grouping technique. In this technique, K users in the system are partitioned into G groups and a unique spreading sequence is assigned to each group. Using multiple antenna in the base station transmitter, we can separate users in each group by their different spatial characteristics. This separation is performed by some linear spatial pre-processing techniques in the transmitter. In addition, to suppress the multiple access interference between users in different groups, we use temporal linear transmit precoding method. Using precoding method in the base station´s transmitter makes the mobile receivers very simple and low cost. We present the bit error rate (BER) performance of the proposed schemes in a flat fading channel by computer simulations. It is demonstrated that a considerable improvement in the performance and increasing in the system capacity will be obtained by using the proposed group transmission schemes