Achieving Transmit Diversity in OFDM-IM by Utilizing Multiple Signal Constellations

Orthogonal frequency division multiplexing (OFDM) index modulation (IM) is a novel multicarrier modulation, which employs both the indices of the active subcarriers and conventional constellation symbols to convey information. Although OFDM-IM shows advantages in many aspects, it cannot provide transmit diversity (TD). To address this problem partly, in this paper, we propose a modified scheme, termed as OFDM-IM with TD (OFDM-IM-TD), through utilizing silent subcarriers and multiple signal constellations (SCs). Specifically, in OFDM-IM-TD, besides the active subcarriers, the same information is transmitted over the silent subcarriers, while using a different SC. The design of the SC for the silent subcarriers is further illustrated. Next, the union bound of the average bit error probability is derived, based on which the transmit diversity gain of the proposed scheme over OFDM-IM is theoretically verified. Moreover, a low-complexity log-likelihood ratio-based detector is developed. Finally, simulation results are given to demonstrate the performance of the proposed scheme.