Enhanced antenna selection techniques for energy-efficient code index modulation aided spatial modulated wireless communication systems

This study proposes an integrated multiple-input multiple-output (MIMO) transceiver framework, termed CIM-HQAM-SM, which combines code index modulation (CIM) and spatial modulation (SM) with energy-efficient hexagonal quadrature amplitude modulation (HQAM). In the proposed bit mapping, the information bits jointly select (i) the active transmit-antenna index, (ii) the Walsh–Hadamard spreading-code indices for the in-phase and quadrature branches, and (iii) an HQAM symbol. Hence, the payload is conveyed through the constellation symbol as well as through antenna and code indices. For the considered Rayleigh-fading scenarios and matched spectral-efficiency settings, the proposed framework offers BER improvements over conventional SM and quadrature SM (QSM), while achieving uncoded BER performance comparable to CIM-SM with improved transmission-side energy efficiency due to the lower average energy of HQAM. Euclidean distance-based antenna selection (EDAS) and capacity-optimized antenna selection (COAS) are further incorporated to enhance reliability and capacity. The performance is evaluated under both perfect and imperfect channel state information (CSI), and the throughput, computational complexity, and transmission-side energy-saving characteristics are analyzed.