An Efficient Index Mapping Algorithm for OFDM-Index Modulation

An index mapping algorithm that efficiently associates the subcarrier activation pattern (SAP) or constellation mode pattern (CMP) sequences with the index selecting bit (ISB) sequences is proposed to improve the error performances of two OFDM-`index modulation' (IM) variants, OFDM-`in-phase/quadrature' (IQ)-IM and `coordinate interleaving' (CI)-`multiple-mode' (MM)-OFDM-IM. If the Hamming distances between every combinational pair of the Gray-coded ISB sequences are written in a 2-dimensional Hamming distance table, the minimum Hamming distances (i.e., 1's) are located in an X-shaped cross. In the proposed index mapping algorithm, the selection and ordering of the SAP or CMP sequences is determined by a mapping strategy of locating the minimum Hamming distances (i.e., 2's) of the SAP or CMP sequences in the same X-shaped cross as that of the minimum Hamming distances (i.e., 1's) of the Gray-coded ISB sequences. A pseudo-code is provided to reify the proposed index mapping algorithm. We show by numerical simulation that the proposed index mapping algorithm can provide substantial performance improvement for various IM schemes such as OFDM-IQ-IM, CI-MM-OFDM-IM, and OFDM-IM, especially when an error correction code is applied simultaneously.