Threshold-based substream selection combining for adaptive spatial multiplexing

We propose a novel closed-loop spatial multiplexing method with limited feedback and low bit error rates over MIMO fading channels. The transmitter adaptation is designed to satisfy spectral efficiency and average transmit power constraints. The transmit adaptation is performed simply by selecting the number of sub-streams based on a given threshold with a fixed nonadaptive constellation and transmit power per substream. The proposed system chooses the substream selection threshold as a function of the slowly changing average SNR. We analyze the performance of the proposed system by deriving closed-form expressions for spectral efficiency, average transmit power, and BER. Under spectral efficiency and average transmit power constraints, we show that the average BER decreases exponentially in terms of average SNR over fading channels. We present numerical and simulation results that validate our analysis.