BER performance analysis of the discrete time and the continuous time Rake receivers for the UMTS downlink

The paper gives the bit error rate (BER) expressions of the continuous time and discrete time Rake receivers (CTR and DTR) for the UMTS downlink. The former receiver is the most used and consists in combining the output of correlators locked on the most significant paths. Since the conventional synchronization module of the CTR is hot able to distinguish close paths, a discrete time implementation of the Rake receiver which circumvents the problem of path delay estimation was proposed in previous papers. The DTR consists in a matched filter to the discrete time equivalent model of the global channel resulting from the convolution of the impulse response of the transmitter shaping filter and the multipath channel. Using the assumption of Gaussian noise on the receiver output, we derive the BER expressions of both the CTR and DTR. The obtained expressions are valid for any multipath channel and take simultaneously into account path gain estimation noise, the intersymbol interference and the multiuser interference which is a realistic scenario for the UMTS downlink. Based on this theoretical study, a power control policy is also formulated.