  This paper focuses on the non-coherent detection in ambient backscatter
communication, which is highly appealing for systems where the trade-off
between signaling overhead and the actual data transmission is very critical.
Modeling the time-selective fading channel as a first-order autoregressive (AR)
process, we propose a new receiver architecture based on the direct averaging
of the received signal samples for detection, which departs significantly from
the energy averaging-based receivers considered in the literature. For the
proposed setup, we characterize the exact asymptotic bit error rate (BER) for
both single-antenna (SA) and multi-antenna (MA) receivers, and demonstrate the
robustness of the new architecture to timing errors. Our results demonstrate
that while the direct-link (DL) interference from the ambient power source
leads to a BER floor in the SA receiver, the MA receiver can remove this
interference by estimating the angle of arrival (AoA) of the DL. The analysis
further quantifies the effect of improved angular resolution on the BER as a
function of the number of receive antennas. A key intermediate result of our
analysis is the derivation of a new concentration result for a general sum
sequence that is central to the derivation of the conditional distributions of
the received signal.
