Generalization of Index-Modulation: Breaking the Conventional Limits on Spectral and Energy Efficiencies

In the context of orthogonal frequency-division multiplexing with index modulation (OFDM-IM), this paper first presents an improved index modulation (IIM) technique to address the well-known low spectral efficiency problem of IM techniques in general and OFDM-IM in particular. OFDM-IIM is realized by deriving the minimum required number of subcarriers per group, and the minimum number of active subcarriers in each group, of the conventional OFDM-IM technique. It is proven that OFDM-IIM can deliver higher spectral and energy efficiencies than classical OFDM and OFDM-IM. The gains come at the expense of increased detection complexity. Therefore, a proposal of OFDM with generalized IM (OFDM-GIM) technique is presented, where a variable number of subcarriers per group are allowed to be activated. Two variants of OFDM-GIM are designed based on the activation pattern of the subcarriers: the first achieves superior spectral efficiency with reduced detection complexity as compared to OFDM-IIM, and the second is tailored for low data rate communications with higher energy efficiency requirements. Besides optimal maximum likelihood detection, a less-complex sub-optimal detector is proposed. A closed-form expression for the bit error rate (BER) of OFDM-IIM and the two variants of OFDM-GIM is derived, considering transmissions over Nakagami fading channels and multiple-antenna reception. Finally, OFDM-IIM and OFDM-GIM are compared with state-of-the-art techniques to validate their superiority in terms of BER, achievable rate, and energy efficiency.