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Communications Letters
1
Is Backscatter Link Stronger than Direct Link in
Reconfigurable Intelligent Surface-Assisted System?
Wenjing Zhao, Gongpu Wang, Saman Atapattu, Senior Member, IEEE,
Theodoros A. Tsiftsis, Senior Member, IEEE, and Chintha Tellambura, Fellow, IEEE,
Abstract—This letter considers integrating a backscatter link
with a reconfigurable intelligent surface to enhance backscatter
communication while assisting the direct communication. We
derive the probability that the backscatter channel dominates
in the composite channel. This probability is a useful perfor-
mance measure to determine the number of reflectors. Since the
exact probability lacks a closed-form solution, we develop two
approximations by modeling the gain of the backscatter link
with a Gaussian or Gamma distribution. We found that these
approximations match well with the exact value. Importantly,
with a well-designed number of reflectors, the channel gain of
the backscatter link may be always stronger than that of the
direct one.
Index Terms—Backscatter communication, reconfigurable in-
telligent surface (RIS), parameter design.
I. INTRODUCTION
Reconfigurable intelligent surface (RIS) technology [1] en-
ables wireless charging, remote sensing and unprecedented
data transmission in line with the vision of future wire-
less communications that are low-cost, energy-efficient and
connectivity-ubiquitous. RIS consists of many nearly passive
reflectors, each of which can sensibly interact with the im-
pinging signal in a software-controlled way. Specifically, each
reflector can vary the phase of the incident signal with the
perceived channel state information (CSI) without requiring
special energy source, in order to realize a high-performance
communication link. Moreover, RIS can be easily embedded
into objects such as clothing, furniture, walls, and building
facades.
This study is supported in part by Key Laboratory of Universal Wireless
Communications (BUPT), Ministry of Education, P.R.China under Grant
KFKT-2018104, in part by the Natural Science Foundation of China (NSFC)
under Grants 61571037, 61871026 and U1834210, in part by NFSC Outstand-
ing Youth under Grant 61725101, and in part by the Australian Research
Council (ARC) through the Discovery Early Career Researcher (DECRA)
Award under Grant DE160100020. (Corresponding author: Gongpu Wang)
W. Zhao and G. Wang are with Beijing Key Lab of Transportation
Data Analysis and Mining, School of Computer and Information Tech-
nology, Beijing Jiaotong University, China (e-mail: {wenjingzhao, gp-
wang}@bjtu.edu.cn).
S. Atapattu is with the Department of Electrical and Electronic Engineer-
ing, The University of Melbourne, Parkville, VIC 3010, Australia (e-mail:
saman.atapattu@unimelb.edu.au).
T. A. Tsiftsis is with the School of Intelligent Systems Sci-
ence and Engineering, Jinan University, Zhuhai 519070, China (email:
theo tsiftsis@jnu.edu.cn).
C. Tellambura is with the Department of Electrical and Computer Engi-
neering, University of Alberta, Edmonton, AB T6G 1H9, Canada (e-mail:
chintha@ece.ualberta.ca).
State-of-the-art research on RIS includes data transmission
[1]–[3], energy efficiency of multi-user communications [4],
beamforming design [5]–[7], user assignment in distributed
RIS systems [8], secure [9] and ultra-reliable communication
[10]. Existing RIS techniques can provide extra paths to aid
transmission from the transmitter to the receiver. However, it
is often assumed that the RIS device itself cannot transmit
extra information to the receiver [4]–[7], [9], [11]. In con-
trast, references [2], [3] show that a RIS device can realize
information transfer by adopting spatial index modulation. By
virtue of the information transmission feature of the RIS, we
integrate backscatter technology with RIS. We thus introduce
a new paradigm that can not only enable RIS to aid an ongoing
data transmission, but also transmit its own binary signals or
sensor signals to the receiver by performing binary modulation
through RIS, embedding sensors into RIS, or connecting RIS
to surrounding sensors through wires [10], [12]. Specifically,
RIS synchronizes with the transmitter and conveys data {0,
1} to the receiver. In this way, loading information ‘1’ or ‘0’
means that RIS is in a reflective or absorptive state. The key
performance measure that helps of the design of this overall
system is the probability that the gain of backscatter link is
stronger than that of direct one. We derive this measure in
both exact and approximate forms and provide numerical and
simulations results. By using this measure, one can choose
the appropriate number of reflectors to ensure high-quality
communications even under extremely poor direct channel
conditions.
Notation: For random variable (RV) X, f
X
(·) and F
X
(·)
denote the probability density function (PDF) and cumulative
distribution function, respectively. A complex or real Gaussian
RV with mean µ and variance σ
2
is denoted by CN(µ, σ
2
)
or N(µ, σ
2
). The Exp(λ) density is f
X
(x) =
1
λ
e
−
x
λ
, x ≥ 0.
The Ral(1) density is f
X
(x) = 2xe
−x
2
, x ≥ 0. We write
X ∼ Gamma(α, β) if f
X
(x) =
β
α
Γ(α)
x
α−1
e
−βx
, x ≥ 0.
II. SYSTEM MODEL
Fig. 1 shows the proposed system consists of a backscatter
link, direct link, one RF source, one receiver and one RIS. The
RIS unit comprises of K passive reflectors {1, ··· , K}, and
also includes a controller for CSI acquisition and information
transmission [13]. When the RF source transmits its data
signal s(n), whose power is σ
2
s
, to the receiver, the RIS
will also receive the same signal and then adjust the phase
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