Index Modulation for Backscatter Communication Using Multiple Tags

Backscatter communication (BC) is a promising method of providing low-power interconnection for green or Internet of Things applications. To combat the strong interference from the direct link, it often uses a small constellation size for modulation, leading to a low data rate. In this article, backscatter index modulation (BIM) is proposed to increase the data rate of BC by employing multiple single-antenna tags and activating one tag a time to carry extra data bits for information delivery. Both coherent detection with full channel knowledge and noncoherent detection with partial channel knowledge are studied. In each case, ambient BC with either Gaussian distributed or constant modulus source signals, as well as dedicated BC with known source signals, are considered. The optimal detectors are derived using the maximum-likelihood principle, and their symbol error rate (SER) performances are analyzed in closed-form expressions. Numerical results are presented to examine the effects of different system parameters on the SER. In particular, BIM performs well in coherent detection with full channel knowledge but could quickly reach error floors in noncoherent detection with partial channel knowledge. Also, the number of tags is more important than the number of samples in some cases. The channel asymmetry in the tag-to-reader links can also play an important role in the SER performance.