Backscatter Communications Over Ambient
OFDM Signals: Transceiver Design and
Performance Analysis
Gang Yang, Member, IEEE, and Ying-Chang Liang, Fellow, IEEE
University of Electronic Science and Technology of China (UESTC), Chengdu, P. R. China
Email: yanggang@uestc.edu.cn, liangyc@ieee.org
Abstract—Ambient backscatter communications (AmBC)
enables radio-frequency (RF) powered devices (e.g., tags,
sensors) to modulate their information bits over ambient
RF carriers in an over-the-air manner. This system, called
“modulation in the air”, thus has emerged as a promising
technology for green communications and future Internet-of-
Things. This paper studies the AmBC system over ambient
orthogonal frequency division multiplexing (OFDM) carriers
in the air. We first establish the system model for such
AmBC system from spread-spectrum perspective, from which
a novel joint design for tag waveform and reader detector is
proposed. 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.
The maximum-likelihood detector is proposed to recover the
tag bits, for which the optimal threshold is obtained with
closed-form expression. Also, we analyze the effect of various
system parameters on the transmission rate and detection
performance. Finally, extensive numerical results show that
the proposed transceiver design outperforms the conventional
design.
I. INTRODUCTION
Ambient backscatter communications (AmBC) enables
radio-frequency (RF) powered devices to harvest power
from ambient RF signals (e.g., TV signal and WiFi signal),
and to transmit information to nearby readers over the am-
bient RF carriers [1]. Different from traditional backscatter
communications such as radio-frequency-identification (R-
FID) systems [2], AmBC can exempt the reader from gen-
erating RF sinusoidal carriers, thus enables low-cost and
energy-efficient ubiquitous communications. Thus, AmBC
is a promising technology for green communications with
great potential for applications in next-generation Internet
of Things (IoT) [3].
There are some related literature on receiver design
for AmBC [1], [4]–[6]. In [1], an averaging detector is
proposed to decode the tag bits by treating the strong
direct-link interference as noise, which results in very low
decoding signal-to-noise-ratio (SNR) and thus low data
rate. A maximum-likelihood (ML) detector is proposed
in [4] for an AmBC system in which the tag adopts
differential modulation. However, the proposed detection
scheme suffers from severe performance degradation when
the relative difference of the backscatter channel and the
direct-link channel is small. An ambient WiFi backscatter
system is proposed in [5], in which a WiFi helper (e.g.,
smart phone) decodes the tag bits by detecting the changes
in received signal strength indication (RSSI). However, this
system has very low data rate and very limited commu-
nication range, since the detection of the RSSI changes
suffers from the strong direct-link interference from the
WiFi access point (AP). Recently, a new WiFi backscatter
system is proposed in [6], in which the WiFi AP decodes
the received backscattered signal while simultaneously
transmitting WiFi packages to a standard WiFi client. This
design relies on the self-interference-cancellation for full-
duplex radios, resulting in high complexity and cost.
In this paper, we are interested in the system modelling,
transceiver design and performance analysis for AmBC
systems over ambient orthogonal frequency division mul-
tiplexing (OFDM) carriers in the air. OFDM is a widely
used modulation scheme in current wireless systems such
as WiFi and DVB [7]. The main contributions of this paper
are summarized as follows:
• We first establish the system model for AmBC from
spread-spectrum perspective. We view the backscatter
operation at the tag as a modulation technique, called
“modulation in the air”, i.e., the backscattered signal
is viewed as the multiplication of a low-rate tag
data signal and a high-rate spreading signal (i.e., the
received ambient signal at the tag from the RF source)
in an over-the-air manner.
• Then, based on the established system model, we
propose a novel joint design for tag waveform and
reader detector, which cancels out the direct-link
interference. We construct the test statistic for tag
signal detection that cancels out the direct-link in-
terference by exploiting the repeating structure of
the ambient OFDM signals due to the use of cyclic
prefix (CP). The ML detector is further proposed,
for which the optimal detection threshold is obtained
in closed-form. Our joint transceiver design cancels
out direct-link interference without increasing the
hardware complexity. To the best of our knowledge,