Modified CI and Modulation Order Replacement for Enhancing OFDM-IM Performance

Recently, a lower order modulation aided bit error rate reduction scheme was proposed for orthogonal frequency division multiplexing with index modulation (OFDM-IM). Here, the key idea was to replace the conventional modulation order in OFDM-IM with a lower order modulation and its rotated versions. In this paper, we incorporate on the aforementioned system, a novel coordinate interleaving (CI) scheme, where instead of the conventional application of CI to the data symbols alone, it is applied to the entire sub-block containing data symbols and zero-padded sub-carriers. Also, we replace the conventional modulation order in OFDM-IM with a higher order signal constellation. This leads to an increase in the number of symbol selecting bits and a subsequent decrease in the number of index combinations required for mapping. Next, the resulting excess inactive sub-carriers are used to carry more copies of the same data symbols in order to provide diversity. Finally, this modulation order replacement scheme is extended to the in-phase quadrature variant of OFDM-IM. Numerical results indicate that up to 7-dB transmit power gains over OFDM-IM and also over the existing lower order constellation aided scheme can be accrued.