Two-Level Index Modulation for Noncoherent Reception

Index modulation (IM) is a simple technique for improving the spectral efficiency of digital communications. IM can be combined with various types of digital modulation, both linear and nonlinear. However, in the classical version of IM the maximum available spectral efficiency cannot be achieved due to the presence of inactive elements in the IM pattern. In this paper, the use of two-level index modulation, where all elements are active and carry additional data, is proposed. Such two-level IM patterns are combined with codebooks for noncoherent reception obtained using a neural network. For all combinations of IM and codebook parameters, the optimal ratio between two levels of the IM pattern is determined. Moreover, the addition of differential phase shift keying between high- and low-level portions of the IM pattern further improves the spectral efficiency (SE). The relationship between spectral and power efficiency for the case of an energy-based receiver is obtained. The results show that SE can be doubled without degradation of power efficiency. A comparison of the computational complexity of two reception methods, optimal and simplified energy-based, is also made.