Enhanced Coordinate Interleaved OFDM With Index Modulation

Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has attracted much attention in recent years. In OFDM-IM, the information bits of a block are generally divided into several subblocks, each of which is further split into two parts, i.e., the index bits and the symbol bits. By performing a coordinate interleaving operation over each two constellation symbols, the bit-error-rate (BER) performance of the symbol bits is significantly improved in an existing OFDM-IM scheme, which is called coordinate-interleaved OFDM-IM (CI-OFDM-IM). In this paper, we propose a novel OFDM-IM scheme termed enhanced CI-OFDM-IM (ECI-OFDM-IM). In ECI-OFDM-IM, to improve the BER performance of the index bits, we adopt a special subcarrier-activation pattern (SAP) lookup table in which the Hamming distance of any pair of SAPs is at least four. To improve the BER performance of the symbol bits conveyed by the active subcarriers, the operations of rotating and coordinate interleaving are conducted twice over each four constellation symbols. Based on the maximum likelihood (ML) detection, a theoretical upper bound on the average bit-error probability of ECI-OFDM-IM is derived in closed form. By using the log-likelihood ratio method for SAP detection, a semi-ML detection scheme is proposed. Both the theoretical and simulation results show that ECI-OFDM-IM outperforms CI-OFDM-IM with additional diversity gains over frequency-selective Rayleigh fading channels.