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Abstract: Can WiFi signals be used for sensing purpose? The growing PHY layer capabilities of WiFi has made
it possible to reuse WiFi signals for both communication and sensing. Sensing via WiFi would enable remote
sensing without wearable sensors, simultaneous perception and data transmission without extra communication
infrastructure, and contactless sensing in privacy-preserving mode. Due to the popularity of WiFi devices and the
ubiquitous deployment of WiFi networks, WiFi-based sensing networks, if fully connected, would potentially rank as
one of the world’s largest wireless sensor networks. Yet the concept of wireless and sensorless sensing is not the
simple combination of WiFi and radar. It seeks breakthroughs from dedicated radar systems, and aims to balance
between low cost and high accuracy, to meet the rising demand for pervasive environment perception in everyday
life. Despite increasing research interest, wireless sensing is still in its infancy. Through introductions on basic
principles and working prototypes, we review the feasibilities and limitations of wireless, sensorless, and contactless
sensing via WiFi. We envision this article as a brief primer on wireless sensing for interested readers to explore this
open and largely unexplored field and create next-generation wireless and mobile computing applications.
Introduction
Technological advances have extended the role of
wireless signals from a sole communication medium to
a contactless sensing platform, especially indoors. In
indoor environments, wireless signals often propagate
via both the direct path and multiple reflection
and scattering paths, resulting in multiple aliased
signals superposing at the receiver. As the physical
space constrains the propagation of wireless signals,
the wireless signals in turn convey information that
characterizes the environment they pass through. Herein
the environment refers to the physical space where wireless signals propagate, which includes both
ambient objects (e.g., walls and furniture) and humans
(e.g., their locations and postures). As shown in Fig. 1,
sensorless sensing with WiFi infers the surrounding
environments by analyzing received WiFi signals, with
increasing levels of sensing contexts.
It is not a brand-new concept to exploit wireless
signals for contactless environment sensing. Aircraft
radar systems, as a representative, detect the presence
of outdoor aircrafts and determine their range, type,
and other information by analyzing either the wireless
signals emitted by the aircrafts themselves or those
broadcast by the radar transmitters and reflected by the
aircrafts afterwards. Recent research has also explored
Ultra-Wide Band (UWB) signals for indoor radar
systems[1]. Primarily designed for military context,
however, these techniques either rely on dedicated
hardware or extremely wide bandwidth to obtain high
time resolution and accurate range measurements,
impeding their pervasive deployment in daily life.
On the other hand, contactless sensing technology is
of rising demand in our everyday world. For instance,
passive human detection has raised extensive research
interest in the past decade[2–5]. By Passive (also termed
as device-free or non-invasive), it refers to detecting
users via wireless signals, while the users carry no
radio-enabled devices[2]. Such contactless and privacypreserving
mode can stimulate various applications
including security surveillance, intrusion detection,
elderly monitoring, remote health-care, and innovative
human-computer interaction.
One solution to passive human detection is to deploy
extra sensors like UWB radar systems. Yet a more
attractive alternative is to reuse the ubiquitous WiFi
infrastructure for pervasive, cost-effective, and easy-touse
passive human sensing. Such WiFi-based sensing
is challenging in two aspects: Standard WiFi signals
have limited bandwidth and insufficient time resolution
compared with dedicated radar signals; commercial
WiFi hardware often fails to support sophisticated radar
signal processing. It is thus urgent to break away
from traditional radar systems and develop theory and
technology for high-resolution wireless sensing with
off-the-shelf WiFi infrastructure.
Although neither WiFi nor radar alone yields
new concepts, their combination sparks interesting
innovations in mobile computing. Pioneer researchers
have termed this largely unexplored field as Wireless
Sensing, Sensorless Sensing or Radio Tomography
Imaging[3], and we will use wireless sensing and
sensorless sensing throughout this paper. In this paper,
we reviewed the emergence of wireless, sensorless, and
contactless sensing via WiFi. We focus on the principles
and the infrastructure advances that enable wireless and
sensorless sensing on commodity devices. Over the past
five years researchers have developed a series of WiFibased
contactless sensing prototypes with increasing
functionalities[6–10] and we expect wireless, sensorless,
and contactless sensing to leap towards industrial products in the coming few years.
2 From Received Signal Strength (RSS) to
Channel State Information (CSI)
How can we infer environment information from
wireless signals? As a toy example, weak WiFi
signal strength may indicate long distance from the
access point. Though intuitive, RSS is widely used
to infer environment information such as propagation
distances. The past two decades have witnessed
various sensing applications using RSS, with RSSbased
localization as the most representative.
2.1 Received signal strength
RSS acts as a common proxy for channel quality and
is accessible in numerous wireless communication
technologies including RFID, GSM, WiFi, and
Bluetooth. Researchers also utilize RSS for sensing,
such as indoor localization and passive human
detection. In theory, it is feasible to substitute RSS
into propagation models to estimate propagation
distance, or take a set of RSS from multiple access
points as fingerprints for each location, or infer human
motions from RSS fluctuations. However, in typical
indoor environments, wireless signals often propagate
via multiple paths, a phenomenon called multipath
propagation. In presence of multipath propagation,
RSS may no longer decrease monotonically
with propagation distance, thus limiting ranging
accuracy. Multipath propagation can also lead to
unpredictable RSS fluctuations. Studies showed that
RSS can fluctuate up to 5 dB in one minute even
for a static link[11]. Such multipath-induced RSS
fluctuation may cause false match in fingerprint-based
localization. Since RSS is single-valued, it fails to
depict multipath propagation, making it less robust and
reliable. Hence RSS-based sensing applications often
resort to dense deployed wireless links to avoid the
impact of multipath via redundancy