On the performance of space time transmit diversity in the downlink of W-CDMA with and without equalization [3G communication applications]

In this paper, we discuss the performance of space time transmit diversity (STTD) in the downlink of DS-CDMA over frequency-selective fading channels. We consider two kinds of receiver: the RAKE receiver and the chip-level MMSE equalizer-based receiver. These two receivers comparison turns out to be a very difficult task because their output signal to interference plus noise ratios (SINRs) depend in a complex way on the spreading and scrambling codes. To obtain tractable expressions, we study the SINRs in the asymptotic regime, i.e. we suppose that the spreading factor and the number of users both tend to infinity while their ratio remains constant. We further suppose that the code matrix is a random matrix obtained by multiplying a random scrambling code by a Walsh-Hadamard matrix. Under these conditions, the SINRs of the two receivers tend to deterministic values. We compare the asymptotic SINRs and draw some conclusions about the effect of the channel transfer function on the performance. Simulation results show that the asymptotic results allow us to predict the performance of real life systems like the UMTS-FDD.