Large-System Comparison of Space-Time Transmit Diversity and Orthogonal Transmit Diversity for Downlink W-CDMA

In this paper, we continue our study of the performance of W-CDMA transmit diversity (TD) that we started in B. Mouhouche et al. (2005). We compare the performance of two open-loop TD techniques for W-CDMA: The space time transmit diversity (STTD) and the orthogonal transmit diversity (OTD). We use the output SENR provided by a RAKE receiver when combined with each of these diversity techniques. It is impossible to obtain useful conclusions about the performance of the two techniques because the SENR depend in a complex way on many parameters like the spreading codes, the channel, etc. In order to obtain positive results, we follow the classical approach used for the first time in D.N.C. Tse and S. Hanly (1999) and assume that the spreading codes are random following a certain distribution. In this case, the SINRs can be interpreted as random variables. We consider the limit of the these SENRs when the spreading factor N and the number of users K tend to oo with fixed ratio. Under these conditions, the output SENRs can be shown to converge to deterministic limits independent of the "random" spreading codes. We interpret the output SENRs and discuss the different parameters influencing the performance of the two techniques