Adaptive spatial modulation and thresholds optimization for MIMO systems in correlated Rayleigh channels

Spatial modulation (SM) is a simple and spectral efficient modulation technique that has received much interest recently. In this paper, an adaptive SM (ASM) scheme is presented by combining adaptive modulation with conventional SM. The performance of ASM is analyzed in spatially correlated Rayleigh fading channels. In order to reduce the computational complexity of average bit error rate (BER), an approximate expression of error probability of the antenna index estimation, which contributes one of two components of the average BER, is derived by using the fitting method and matches well with simulation results in the SNR range of interest. With the above results, the closed-form spectrum efficiency (SE) and overall average BER are obtained, respectively. Besides, the optimized switching thresholds for maximizing SE under an average BER constraint are achieved by means of the Karush Kuhn Tucker conditions, and the resultant SE performance can be improved greatly when compared to the ASM system with fixed thresholds. Simulations indicate that the theoretical SE and BER are effective and agree well with the corresponding simulations. Moreover, the SE and BER performance of ASM under spatially correlated channel are poorer than those under spatially independent channel because of the influence of spatial correlation.