A Molecular Code Index Modulation Scheme for Molecular Communications

This paper proposes a molecular code index modulation (MCIM) scheme to achieve reliable transmission for molecular communication systems. The MCIM system, using molecules of two distinct types, encodes information on the indices of the activated spreading code and the activated molecular type, where its receiver samples the concentration difference between the molecules of two types to generate a real-valued signal. Two low-complexity detectors dispensing with the channel state information are proposed based on the correlation detection principles. Furthermore, considering that the orthogonality between spreading codes may be disrupted by the severe inter-symbol interference (ISI), an enhanced stepwise detector is developed to correct the error estimation of the activated code index caused by ISI. The bit error rate (BER) upper bound is derived to analyze the MCIM system performance. Numerical simulations demonstrate that the proposed MCIM scheme achieves superior BER performance compared with the existing modulation schemes employing two molecular types, such as molecular shift keying and molecular type permutation shift keying, while maintaining low computational complexity.