Adaptive spatial modulation with optimized switching thresholds for MIMO systems

Spatial modulation (SM) is a simple and spectral efficient modulation technique that has received much interest recently. By introducing adaptive modulation in the SM-MIMO system, an adaptive spatial modulation (ASM) scheme is presented, and the corresponding performance is investigated in Rayleigh fading channel. To simplify the calculation of average bit error rate (BER), a simple 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 may match the simulation well in the SNR range of interest. With the above results, the closed-form average spectrum efficiency (SE) and BER are obtained, respectively. Besides, the optimized switching thresholds for maximizing SE subject to an average BER constraint are also derived by means of the Karush Kuhn Tucker conditions, and the resultant SE performance can be improved greatly. Simulation results show that the derived average SE and BER are in agreement with the simulations. The presented ASM scheme with the optimized thresholds outperforms the existing scheme with fixed threshold, and has higher SE while maintaining the target BER.