A simple technique to improve performance of RAKE receivers in CDMA PCS indoor systems

In channels where the delay spread is smaller than the chip interval, spread spectrum signals do not give rise to path diversity, thus a classical maximal ratio combiner (MRC)-RAKE receiver sets to unity the number of branches. In this situation, MRC-RAKE receivers with resolution reduction (RR) of the diversity branches may be used to significantly improve performance in the forward link of DS-CDMA systems (Yang 1995). As a result of the use of RR techniques, the noise components at the finger outputs are correlated. It has been shown that the MRC is suboptimal in the presence of correlated noise components. In this paper, a minimum mean squared error combiner (MMSEC) RAKE receiver with RR is proposed. The performance of the RAKE receiver with two branches is studied by means of theoretical analysis and computer simulations. To allow a simple practical implementation, a suboptimal structure of the MMSEC is also proposed. Frame error rate (FER) is obtained by using a simulation system based on the forward link of the digital cellular standard IS-95 in a typical indoor radio channel. Our results show that this new receiver scheme can achieve a gain of 3.6 dB over a conventional MRC-RAKE receiver at an FER of 0.01, with small increase in complexity.