  Existing studies about ambient backscatter communication mostly assume
flat-fading channels. However, frequency-selective channels widely exist in
many practical scenarios. Therefore, this paper investigates ambient
backscatter communication systems over frequency-selective channels. In
particular, we propose an interference-free transceiver design to facilitate
signal detection at the reader. Our design utilizes the cyclic prefix (CP) of
orthogonal frequency-division multiplexing (OFDM) source symbols, which can
cancel the signal interference and thus enhance the detection accuracy at the
reader. Meanwhile, our design leads to no interference on the existing OFDM
communication systems. Next we suggest a chi-square based detector for the
reader and derive the optimal detection threshold. Simulations are then
provided to corroborate our proposed studies.
