Index modulation multiple access in multi-user MIMO systems

A comprehensive performance evaluation of index modulation multiple access (IMMA) is presented for a multi-user (MU) multiple-input multiple-output (MIMO) communication system, in which each user is equipped with multiple transmit antennas and conveys information by activating a subset of antennas per symbol interval. Unlike conventional orthogonal multiple access (OMA) schemes such as time division multiple access (TDMA), the IMMA approach employs a non-orthogonal strategy that enables multiple users to share the same time-frequency resources by utilizing the principles of index modulation (IM). At the base station, a joint maximum likelihood (ML) detector is employed to identify the active antenna indices of all users simultaneously. The IMMA framework is analyzed in terms of spectral efficiency and average bit error rate (ABER), and its performance is benchmarked against a TDMA system in which users adopt the same IM scheme but transmit in orthogonal time slots. Several IM variants are considered, including space shift keying (SSK), generalized SSK (GSSK), and Steiner-SSK (S-SSK). Simulation results demonstrate that TDMA–SSK achieves better performance in scenarios with low spectral efficiency (i.e., $\eta \le 4$ bits/s/Hz/user) due to reduced interference. However, for higher spectral efficiency regimes ($\eta \ge 6$ bits/s/Hz/user), the IMMA–SSK configuration provides superior performance by leveraging the spatial domain for enhanced data multiplexing. These results validate the potential of IMMA as a scalable multiple access strategy for future high-throughput wireless systems.