Dual-Mode Index Modulation Aided OFDM With Constellation Power Allocation and Low-Complexity Detector Design

Abstract

Dual-mode index modulation aided orthogonal frequency division multiplexing (DM-OFDM) is recently proposed, which modulates all subcarriers to eliminate the limits of spectrum efficiency (SE) in OFDM with index modulation (OFDM-IM). In DM-OFDM, the subcarriers within each subblock are divided into two groups, which are modulated by two distinguishable constellation alphabets drawn from the inner and the outer constellation points of a given M-ary QAM constellation. In this paper, a new DM-OFDM scheme, called DM-OFDM with constellation power allocation (DM-OFDM-CPA) is proposed, where the two groups of subcarriers within each subblock are set at different power levels and modulated by different M-ary PSK symbols, leading to the improvement of the system performance at low-order modulation. At the receiver, a low-complexity maximum likelihood (LC-ML) detector and two reduced search complexity detectors, based on energy detection and log-likelihood ratio (LLR) criterion, respectively, are employed for demodulation. Then, the bit error rate (BER) analyses based on pairwise error probability are provided for the proposed DM-OFDM-CPA, and the power ratio between the two groups is optimized to maximize achievable BER performance for a given signal to noise ratio. The simulation results confirm that at a given SE, the proposed DM-OFDM-CPA achieves a significantly better BER performance than the existing OFDM-IM. When the modulation order is lower than 16, it outperforms the classic DM-OFDM. The results also show that two reduced search complexity detectors, especially the LLR detector, provide similar performance to the LC-ML detector in the DM-OFDM-CPA.