  Ambient backscatter communication (AmBC) is becoming increasingly popular for
enabling green communication amidst the continual development of the
Internet-of-things paradigm. Efforts have been put into backscatter signal
detection as the detection performance is limited by the low
signal-to-interference-plus-noise ratio (SINR) of the signal at the receiver.
The low SINR can be improved by adopting a multi-antenna receiver. In this
paper, the optimum multi-antenna receiver that does not impose any constraints
on the types of binary modulation performed by the backscatter device and the
waveform used by the ambient source system is studied. The proposed receiver
owns a simple structure formed by two beamformers. Bit error rate (BER)
performances of the optimum receiver are derived under constant-amplitude
ambient signal and Gaussian-distributed ambient signal. Moreover, to facilitate
the implementation of the optimum receiver, a simplified receiver is proposed
and practical approximations to required beamformers are provided. The derived
optimum receiver avoids the complex direct path interference cancellation and
coherent reception, but exploits the fact that backscatter signal changes the
composite channel impinging at the receiver and the directivity of receiver
antenna array. Comparative simulation results show that the performance of the
optimum receiver achieves the same performance as the coherent receiver even
though it realizes non-coherent reception. The studied receivers provide high
flexibility for implementing simple and low-cost receivers in different AmBC
systems.
