MMSEC-RAKE receivers with resolution reduction of the diversity branches: analysis, simulation and applications

The DS-CDMA IS-95 standard with 1.23 MHz bandwidth was originally designed for an outdoor cellular system where the delay spread is usually in the range of 10 μs. The delay spread for an indoor environment is typically around 100 ns, which cannot be resolved by a CDMA receiver with an 813 ns chip interval. In this situation, the conventional maximal ratio combiner (MRC) RAKE receiver sets to unity the number of branches and diversity gain is not available. Resolution reduction (RR) of the diversity receiver branches is a technique that improves the performance of CDMA systems in channels where the delay spread is smaller than the chip duration. A new resolution reduction technique based on the use of a minimum mean square error diversity combiner (MMSEC) is proposed in this work. A detailed study of the performance of a dual-branch MMSEC-RAKE receiver with RR in a typical indoor office environment is presented. We show that, under very general assumptions, this new method of RR is optimal and provides a 1.2 dB improvement over the previously RR proposed technique based on MRC, and 3.6 dB improvement over conventional MRC-RAKE receiver without RR, at a frame error rate (FER) of 0.01 for the IS-95 system downlink