  Multiple-antenna backscatter is emerging as a promising approach to offer
high communication performance for the data-intensive applications of ambient
backscatter communications (AmBC). Although much has been understood about
multiple-antenna backscatter in conventional backscatter communications (CoBC),
existing analytical models cannot be directly applied to AmBC due to the
structural differences in RF source and tag circuit designs. This paper takes
the first step to fill the gap, by exploring the use of spatial modulation (SM)
in AmBC whenever tags are equipped with multiple antennas. Specifically, we
present a practical multiple-antenna backscatter design for AmBC that exempts
tags from the inter-antenna synchronization and mutual coupling problems while
ensuring high spectral efficiency and ultra-low power consumption. We obtain an
optimal detector for the joint detection of both backscatter signal and source
signal based on the maximum likelihood principle. We also design a two-step
algorithm to derive bounds on the bit error rate (BER) of both signals.
Simulation results validate the analysis and show that the proposed scheme can
significantly improve the throughput compared with traditional systems.
