Performance of a practical RAKE receiver for W-CDMA downlink in the presence of interfering user

The RAKE receiver is the most common receiver employed in wideband CDMA (W-CDMA) systems. Traditionally, multiple access interference (MAI) in spread spectrum systems is modelled as white Gaussian noise. This presumption is valid when the number of users is large and interference signals have large spreading factors. But the W-CDMA air interface for 3rd generation mobile communication systems supports low spreading factors, high and variable data rates, which make MAI non-Gaussian. We analyze the bit error rate (BER) performance of a downlink receiver in the presence of an interfering user when the Gaussian assumption does not hold any more. The receiver is a maximum ratio combining RAKE receiver. First we study the performance of the ideal RAKE receiver, which has exact estimates of delays, amplitudes and phases of the multipath components. Then we consider the effect of MAI when non-data aided (NDA) and pilot aided decision directed (PADD) algorithms are used for delay and channel coefficients estimation, respectively. It is shown that imperfect power control can significantly affect the BER performance, while practical single-user channel estimation algorithms, such as the NDA-PADD algorithm, behave well also in the presence of non-Gaussian MAI.