  Ambient backscatter communication (AmBC) enables radio-frequency (RF) powered
backscatter devices (BDs) (e.g., sensors, tags) to modulate their information
bits over ambient RF carriers in an over-the-air manner. This technology also
called "modulation in the air", thus has emerged as a promising solution to
achieve green communications for future Internet-of-Things. This paper studies
an AmBC system by leveraging the ambient orthogonal frequency division
multiplexing (OFDM) modulated signals in the air. We first model such AmBC
system from a spread-spectrum communication perspective, upon which a novel
joint design for BD waveform and receiver detector is proposed. The BD symbol
period is designed to be in general an integer multiplication of the OFDM
symbol period, and the waveform for BD bit `0' maintains the same state within
a BD symbol period, while the waveform for BD bit `1' has a state transition in
the middle of each OFDM symbol period within a BD symbol period. In the
receiver detector design, we construct the test statistic that cancels out the
direct-link interference by exploiting the repeating structure of the ambient
OFDM signals due to the use of cyclic prefix. For the system with a
single-antenna receiver, the maximum-likelihood detector is proposed to recover
the BD bits, for which the optimal threshold is obtained in closed-form
expression. For the system with a multi-antenna receiver, we propose a new test
statistic, and derive the optimal detector. Moreover, practical timing
synchronization algorithms are proposed, and we also analyze the effect of
various system parameters on the system performance. Finally, extensive
numerical results are provided to verify that the proposed transceiver design
can improve the system bit-error-rate (BER) performance and the operating range
significantly, and achieve much higher data rate, as compared to the
conventional design.
