Index and Constellation Order Lowering for OFDM With Index Modulation

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a promising variant of OFDM where indices of symbol carrying sub-carriers also convey information bits besides the complex symbols themselves. Recently, a lower order modulation variant has been proposed for OFDM-IM, where the baseband constellation is lowered with appropriate rotations and index reuse, so as to lower the number of sub-blocks whose pairwise error probability is inversely related to the signal to noise ratio (SNR). In this work, we demonstrate that the number of symbol carrying sub-carriers and hence the number of used indices can also be lowered along with modulation order lowering. This reduces the aforementioned number of worst case sub-block pairs even further. The resulting unused sub-carriers can now be used to enhance the spectral efficiency as well. Numerical results reveal SNR gains up to 5dB for uncoded systems and 2dB for coded systems.