  Ambient backscatter communication (AmBC) leverages the existing ambient radio
frequency (RF) environment to implement communication with battery-free
devices. One critical challenge of AmBC systems is signal recovery because the
transmitted information bits are embedded in the ambient RF signals and these
are unknown and uncontrollable. To address this problem, most existing
approaches use averaging-based energy detectors and consequently the data rate
is low and there is an error floor. Here we propose a new detection strategy
based on the ratio between signals received from a multiple-antenna Reader. The
advantage of using the ratio is that ambient RF signals are removed directly
from the embedded signals without averaging and hence it can increase data
rates and avoid the error floor. Different from original ratio detectors that
use the magnitude ratio of the signals between two Reader antennas, in our
proposed approach, we utilize the complex ratio so that phase information is
preserved and propose an accurate linear channel model approximation. This
allows the application of existing linear detection techniques from which we
can obtain a minimum distance detector and closed-form expressions for bit
error rate (BER). In addition, averaging, coding and interleaving can also be
included to further enhance the BER. The results are also general, allowing any
number of Reader antennas to be utilized in the approach. Numerical results
demonstrate that the proposed approach performs better than approaches based on
energy detection and original ratio detectors.
