多天线标签环境反向散射通信：信号检测与天线选择策略

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"这篇研究论文探讨了多天线标签在环境反向散射通信中的信号检测和最佳天线选择问题。环境反向散射通信是一种利用现有射频（RF）信号进行数据传输的技术，大多数先前的工作集中在单天线标签上，而本论文则深入研究了多天线标签的情况，这些标签能同时实现能量收集和数据传输。然而，由于多天线通道的复杂性以及来自不可控RF源（如位置和发射功率）导致的RF信号的不可预测性，信号检测变得极具挑战性。因此，检测过程被构架为一个具有未知参数的假设检验问题。论文提出了一种基于广义似然比测试（GLRT）的盲检测器设计，无需使用信道状态信息（CSI）。" 在这篇论文中，作者陈晨、王公甫（会员，IEEE）、帕纳吉奥蒂斯·迪米特拉科斯（资深会员，IEEE）、何瑞思（资深会员，IEEE）、乔治·卡里亚尼尼斯（会士，IEEE）和钦塔·泰拉姆波拉（会士，IEEE）首先阐述了多天线标签在环境反向散射通信中的优势与挑战。多天线配置可以提高通信系统的性能，通过空间分集和多径传播增加信号强度和可靠性，同时也能提供空间复用能力，增加数据传输速率。然而，这种系统面临的主要困难在于如何有效地检测和利用这些不可预测的RF信号。为了解决这一问题，论文提出了一个基于GLRT的盲检测算法。GLRT是一种统计测试方法，用于在不知道参数确切值的情况下对假设进行测试。在环境反向散射通信中，由于RF信号源的不可控性和多天线通道的动态性，GLRT成为了理想的工具。通过这种方法，可以构建一个检测器，它能够在不需精确的信道信息情况下，检测到标签发送的数据，并且适应环境变化。此外，论文还讨论了最佳天线选择策略，这是提高系统性能的关键。在多天线系统中，选择最佳天线可以显著提升信号质量，降低错误率。作者可能提出了基于某种准则（如最大信号强度、最小信噪比等）的天线选择算法，以优化通信性能。这篇论文为环境反向散射通信领域提供了一个重要的贡献，即在缺乏信道状态信息的情况下，通过设计高效的盲检测器和最佳天线选择策略，提高了多天线标签的通信效率和可靠性。这将有助于未来无线通信系统的设计，尤其是在能量受限和环境条件苛刻的应用场景下。

468 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 68, NO. 1, JANUARY 2020

optimal backscatter antenna selection scheme, that maximizes

the detection probability. Finally, simulation results are pro-

vided in Section V and conclusions are drawn in Section VI.

D. Notations

Scalars, vectors, and matrices are denoted by lowercase,

boldface lowercase, and boldface uppercase letters, respec-

tively. The Euclidean norm, statistical expectation, transpose,

and the Hermitian transpose are represented by ·

, E(·),

(·)

,and(·)

, respectively. The matrix I

represents the

N × N identity matrix. We denote the ith element of the

vector y as [y]

and the (i, j)th element of the matrix Y as

[Y]

i,j

. The set of real-valued M × N matrices is denoted

by R

M×N

and R

M×1

= R

. The car dinality of the set

A is |A| and the ith element of the set is A(i).Weuse

N(μ, σ

) or CN (μ, σ

), χ

and χ



(τ) to indicate the real or

complex Gaussian distribution with mean μ and variance σ

the central chi-squared distribution with degrees of freed om d,

and the non-central chi-squared distribution with the degrees

of freedom d and the noncen trality par a meter τ, respectively.

II. S

YSTEM MODEL

We consider an ambient system composed of an RF source,

a single-antenna read er and a multi-antenna tag (Fig. 1). We

assume that the tag is equipped with K (K ≥ 2) antennas.

These K antennas are divided into two sets: one set with M

(1 <M≤ K) antennas is for backscattering, and the other

set with K − M antennas is for wireless energy harvesting.

Thus, these two tasks occur simultaneously and independently.

In other words, the RF signals received by energy-harvesting

antennas are reserved purely for powering the circuit, while

the RF signals received by the active backscatter antennas are

entirely devoted to backscattering. We denote the whole can-

didate antenna set as A = {1, 2,...,K} and the backscatter

antenna set as S = { κ

,κ

,...,κ

},whereS⊆A.

Let h denote the channel between the RF source and the

reader. Let the channels from the antenna κ

∈Sof the

tag to the RF source and the reader be f

and g

,where

1 ≤ m ≤ M . Let the RF source transmit complex Gaussian

random signal x(n) with tran smitted power P

. The signal

received at the antenna κ

∈Sis given by [10]

(n)=f

x(n). (1)

This antenna then backscatters the signal r

(n) and transmits

its own binary signal B. Speciﬁcally, B =0indicates that

the tag changes its impedance so that solely a relatively very

small amount of the RF signal is reﬂected, and B =1implies

that the tag switches the impedance so as to backscatter some

of the RF signal. Consequently, the backscattered signal by

the antenna κ

∈Sis

(n)=αBr

(n), (2)

where α ∈ [0, 1] represents the attenuation inside the tag,

which is achieved by adjusting the load impedance at the port

of the antenna, and also depends on the structure mode of the

antenna [36], [37]. It determines the amount of the incident RF

Fig. 2. GBS of the tag in one time slot: (a) N RF signals received by the

tag, (b) backscattered signals by M backscatter antennas.

signals to be backscattered [38]. Finally, the received signal at

the reader can be expressed as

y(n)=hx(n)+g

(n)+ω(n)



hx(n)+ω(n),B=0,

(h + αf

)x(n)+ω(n),B=1,

(3)

where ω(n) ∼CN(0,σ

) is additive white Gaussian

noise (AWGN) at the r eader.

Because the data r ate of the tag is m uch lower than that of

the RF source, i.e., the tag signal B remains unchanged during

N consecutive x(n) in one time slot, we divide the N RF

signals in one time slot into M sets, where the mth set contains

an arbitrary number of signals N

such that



m=1

= N .

As Fig. 2 shows, the mth set is given by

=[x

(1),x

(2),...,x

)]

, (4)

where 1 ≤ m ≤ M .

When the tag receives the mth RF signal set, the mth

backscatter antenna κ

is active and reﬂects these RF signals;

in other words, only one antenna is selected at one time for

backscattering due to limited power. While the other K − 1

antennas are all connected to the energy harvester, and collect

energy from both the RF signals and the backscattered signals

of the antenna κ

∈S.

According the references [40], [41], the energy harvesting

efﬁciency depends on the value of load impedance in the

matching network, which is connected to the multiple antennas

to transfer maximum power from the antennas to the rectiﬁer.

As well, the rectiﬁer is equipped with multiple diodes, which

leads to nonlinear RF-to-DC conversion. Clearly, the energy

harvester is then a nonlinear device.

The energy captured

by both the K − M antennas dedicated to wireless energy

harvesting and the M −1 silent backscatter antennas provides

the p ower required for the backscatter communication w ith the

antenna κ

∈S.

The amount of harvested energy is typically on the order of micro-watts

when the battery-free tags locate close to the RF source, such as TV tower

with transmit power 10

W [42]. Thus, the harvested energy is sufﬁcient for

powering the analog components of the ambient backscatter devices, which

consume 0.79 μWto2 mW [2], [38].

Since the energy harvester and the transmitter operate independently and

simultaneously, the energy harvesting model has no effect on the backscattered

signal (2) by the backscatter antennas, and the received signal at the reader (3).

We only focus on the detector design in our work.

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