Optimum Combining in Digital Mobile Radio with Cochannel Interference

Abstract-This paper studies optimum signal combining for space diver-sity reception in cellular mobile radio systems. With optimum combining, the signals received by the antennas are weighted and combined to maxi-mize the output signal-to-interference-plus-noise ratio. Thus, with cochan-ne1 interference, space diversity is used not only to combat Rayleigh fading of the desired signal (as with maximal ratio combining) but also to reduce the power of interfering signals at the receiver. We use analytical and computer simulation techniques to determine the performance of optimum combining when the received desired and interfering signals are subject to Rayleigh fading. Results show that optimum combining is significantly better than maximal ratio combining even when the number of interferers is greater than the number of antennas. Results for typical.cellular mobile radio systems show that optimum combining increases the output signal-to-interference ratio at the receiver by several decibels. Thus, systems can require fewer base slation antennas and/or achieve increased channel capacity through greater frequency reuse. We also describe techniques for implementing optimum combining with least mean square (LMS) adaptive arrays. I