Secure multiple-mode OFDM with index modulation

Multiple-mode orthogonal frequency division multiplexing with index modulation (MM-OFDM-IM) is a promising IM technique for OFDM systems to improve the spectral efficiency. In this paper, a secure index and data symbol modulation (DSM) scheme based on MM-OFDM-IM systems is proposed by employing the notion of the channel reciprocity assuming time division duplexing mode. The channel state information of the legitimate link is adopted as a secret key for protecting data transmission. We investigate randomized mapping rules for both IM and DSM, based on which even though an eavesdropper has estimated the permutations of modes and their carrying symbols, he cannot decode the message correctly. Particularly, the randomized mapping rules for IM are designed by exploiting the redundancy of mode permutations. Both numerical simulations and theoretical analysis for achievable secure rate and bit error rate are processed, whose results demonstrate that the proposed scheme can enhance the physical-layer security of the considered communication system.