  Ambient backscatter communications have been identified for ultra-low energy
wireless communications. Indeed, a tag can send a message to a reader without
emitting any wave and without battery, simply by backscattering the waves
generated by a source. In the simplest implementation of such a system, the tag
sends a binary message by oscillating between two states and the reader detects
the bits by comparing the two distinct received powers. In this paper, for the
first time, we propose to study an ambient backscatter communication system, in
the presence of a diffusing surface, a simple flat panel that diffuses in all
directions. We establish the analytical closed form expression of the power
contrast in the presence of the surface. We show that the diffusing surface
improves the power contrast. Moreover our approach allows us to express the
contrast to noise ratio, and therefore to establish the BER performance.
Furthermore, we derive the optimum source transmit power for a given target
power contrast. This makes it possible to quantify the amount of energy that
can be saved at the source side, thanks to the diffusing surface.
