Design of Space-Frequency Index Modulation Waveforms for MIMO–OFDM Heterogeneous Networks

In this paper, a novel framework to efficiently design the waveforms toward heterogeneous networks (HetNets) is proposed. The proposed scheme combines the index modulation (IM) concept with multiple-input multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) techniques and jointly considers the resources on both spatial and frequency domains to optimize the transmission signals. Furthermore, a generalized union bound expression is derived first to disclose the potential diversity gain for the spatial-frequency index modulation (SFIM) system and based on the developed bound different impacts of activating spatial and frequency domains resources for transmission are analyzed. Following the theoretical insights, two novel transmit schemes, termed as enhanced SFIM with non-overlap (ESFIM-NO) and enhanced SFIM with overlap (ESFIM-O), toward higher diversity gains are proposed. We show that the proposed methods can significantly improve the diversity of both index and amplitude phase modulation (APM) domains for achieving higher reliability for the entire HetNets. Finally, the simulation results are given to verify the theoretical analysis and system error performance.