Index Modulated Spectral Efficient Frequency Division Multiplexing Assisted by Lower Order Modulations

Spectral efficient frequency division multiplexing with index modulation (SEFDM-IM) transmits information through predefined subcarriers’ activation patterns along with M-ary constellation symbols. It employs non-orthogonal sub-carriers to enhance the spectral efficiency (SE) of classical orthogonal frequency division multiplexing with index modulation (OFDM-IM). SEFDM-IM improves the bit-error rate (BER) when compared to classical SEFDM at the same spectral efficiency (SE). The inactive subcarriers of SEFDM-IM aid in reducing inter-carrier interference; however, higher-order modulations to improve SE can erode this achievement and contribute to BER degradation. This paper suggests lower-order modulation-assisted SEFDM-IM (LSEFDM-IM) to achieve high SE by increasing index bits and sending fewer bits via the constellation symbols. For this purpose, certain index combinations of conventional SEFDM-IM are reused with the help of distinguishable lower-order constellations. LSEFDM-IM outperforms conventional SEFDM-IM and traditional lower-order modulation-aided OFDM-IM in terms of inter-symbol interference, BER, and SE.