Both macrodiversity and microdiversity can effectively overcome the harmful effect of fading. Much of previous work focused on their benefits to the reverse link in CDMA systems. However, their effects on the forward link are less well understood. In this paper, we analyze the CDMA forward-link capacity with macrodiversity and microdiversity. It is shown that macrodiversity causes forward-link capacity loss since the extra forward-link channels supported by the involved base stations enhance not only the received signal power, but also the total interference. Unfortunately the latter gains more whatever power allocation scheme is adopted. Based on the analysis of the cause of capacity loss, we further present a new transmission scheme, in which some joint control among the involved base stations is made to assure that the signals arrived at the desired mobile in phase and simultaneously. The simulation results show that in the new transmission scheme much higher capacity can be achieved with macrodiversity and the capacity increases rapidly with the number of involved base stations. A comparison of the forward-link capacity with microdiversity and macrodiversity indicates that both types of diversity can bring benefits to the forward-link capacity. However, with macrodiversity higher capacity can be obtained at the cost of complexity.

Conventional multiuser interference canceling is always done in the uplink. In this paper, a novel multiuser interference canceling scheme is introduced. It can be used in the downlink of the TDD-CDMA systems. The multiuser interference canceling is done through the joint optimization of both the spreading and despreading waveform for different users, instead of the despreading code only. The idea is to take the advantages of exploitation, rather than mitigation, of the multipath propagation effects. In TDD-CDMA systems, the channel parameter of the downlink can be estimated from the uplink. We can calculate the spreading and despreading code by using the instantaneous channel parameters, so they can accommodate to the characteristic of the multipath channel. The maximum receive signal to interference plus noise ration (SINR) criterion is used to optimize the spreading and despreading code. An iteration algorithm is given to create the optimal spreading and despreading code. Numerical results show that our scheme can greatly decrease the multiple access interference (MAI), hence greatly increase the SINR, compared with the conventional spreading code such as Gold code. Numerical results of the proposed scheme assuming different number of active users, Doppler frequency and coefficient estimation error are also given.

A novel multipath diversity scheme used in TDD-CDMA systems is presented. A FIR filter is added in the transmitter, while a RAKE combiner is used in the receiver. The optimum FIR filter problem may be viewed as an eigenvalue problem. The multiple antennas system is also analyzed. Results show that this new scheme can greatly improve the output Signal to Noise Ratio (SNR) compared with conventional RAKE receiver or Pre-RAKE diversity system.