A simple and new optimum combining scheme for CDMA down link signals

This paper presents a new optimum combining scheme for a CDMA wireless mobile receiver. The new combining scheme, compared to conventional maximal ratio combining (MRC), not only combats multi-path fading, but also mitigates interference by further scaling the finger output inversely proportional to its total average interference power. This interference power is found to be mutually uncorrelated among fingers and its average power is proportional to its instantaneous gain. The reason being that the interference power varies at the chip rate (MHz) while channel gain varies at the fading rate (hundreds of Hz). It is also found that the interference caused by non-orthogonality due to multi-path delays is same for all the fingers. This makes the optimum weights depend only on the channel impulse response and renders an easy way to obtain the optimum weights without actually measuring or estimating the interference power. In this paper, we show how the proportional scaling finger outputs can be performed based on a finger´s own instantaneous channel gain. Therefore the new combining RAKE receiver is simply an MRC RAKE receiver with each finger´s weight further scaled proportionally to the instantaneous channel gain. Compared with other optimum combining schemes, such as DMI or LMS the new scheme has much lower cost, does not have sample support problems and converges to MRC when there is no interference or the multi-paths have equal average power. Simulation results show that the proposed optimum combining RAKE receiver has a 0.3 dB to 2.5 dB gain over the MRC RAKE receiver.