OFDM with Generalized Cascade Index Modulation for Secure Communications

Cascade index modulation (CIM) is a recently proposed improvement of orthogonal frequency division multiplexing with index modulation (OFDM-IM) and achieves better error performance. In CIM, at least two different IM operations construct a super IM operation or achieve new functionality. First, we propose a OFDM with generalized CIM (OFDM-GCIM) scheme to achieve a joint IM of subcarrier selection and multiple-mode (MM) permutations by using a multilevel digital algorithm. Then, two schemes, called double CIM (D-CIM) and multiple-layer CIM (M-CIM), are proposed for secure communication, which combine new IM operation for disrupting the original order of bits and symbols with conventional OFDM-IM, to protect the legitimate users from eavesdropping in the wireless communications. A subcarrier-wise maximum likelihood (ML) detector and a low complexity log-likelihood ratio (LLR) detector are proposed for the legitimate users. A tight upper bound on the bit error rate (BER) of the proposed OFDM-GCIM, D-CIM and MCIM at the legitimate users are derived in closed form by employing the ML criteria detection. Computer simulations and numerical results show that the proposed OFDM-GCIM achieves superior error performance than OFDM-IM, and the error performance at the eavesdroppers demonstrates the security of D-CIM and M-CIM.