Rake receiver performance for CDMA downlink

This paper presents a closed -form expression of the average SIR of the optimum CDMA downlink Rake receiver [1] for identically independently distributed (iid) Raleigh fading and a tight bound for the non-iid case. It shows that the average SIR decreases as the number of multipaths. L. increases and achieves its minininin when the channel multipath intensity profile (VIP) is constant. This paper also proves, both in theory and with simulations that the BER increases as, L increases, and that it is a convex function of the multipath channel gains and achieves its maximum when the channel MIP is constant. fit addition, this paper derives a decision threshold for MRC, which results in a non-optimum Rake, to determine if a multipath is to be combined without loss of SIR. In summary, this paper proves, against conventional wisdom, that a Rake receiver for the CDMA downlink does not achieve the Lth order diversity gain. In fact. its performance varies inversely to an Lth order diversity system, such as CDMA uplink.