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ABSTRACT
Wi-Fi signals are typically information carriers between a trans-
mitter and a receiver. In this paper, we show that Wi-Fi can also
extend our senses, enabling us to see moving objects through walls
and behind closed doors. In particular, we can use such signals to
identify the number of people in a closed room and their relative
locations. We can also identify simple gestures made behind a wall,
and combine a sequence of gestures to communicate messages to
a wireless receiver without carrying any transmitting device. The
paper introduces two main innovations. First, it shows how one can
use MIMO interference nulling to eliminate reflections off static
objects and focus the receiver on a moving target. Second, it shows
how one can track a human by treating the motion of a human body
as an antenna array and tracking the resulting RF beam. We demon-
strate the validity of our design by building it into USRP software
radios and testing it in office buildings.
Categories and Subject Descriptors C.2.2 [Computer
Systems Organization]: Computer-Communications Networks.
H.5.2 [Information Interfaces and Presentation]: User Inter-
faces - Input devices and strategies.
Keywords Seeing Through Walls, Wireless, MIMO, Gesture-
Based User Interface
1. INTRODUCTION
Can Wi-Fi signals enable us to see through walls? For many
years humans have fantasized about X-ray vision and played with
the concept in comic books and sci-fi movies. This paper explores
the potential of using Wi-Fi signals and recent advances in MIMO
communications to build a device that can capture the motion of
humans behind a wall and in closed rooms. Law enforcement per-
sonnel can use the device to avoid walking into an ambush, and
minimize casualties in standoffs and hostage situations. Emergency
responders can use it to see through rubble and collapsed structures.
Ordinary users can leverage the device for gaming, intrusion detec-
tion, privacy-enhanced monitoring of children and elderly, or per-
sonal security when stepping into dark alleys and unknown places.
The concept underlying seeing through opaque obstacles is sim-
ilar to radar and sonar imaging. Specifically, when faced with a
non-metallic wall, a fraction of the RF signal would traverse the
wall, reflect off objects and humans, and come back imprinted with
a signature of what is inside a closed room. By capturing these re-
flections, we can image objects behind a wall. Building a device
that can capture such reflections, however, is difficult because the signal power after traversing the wall twice (in and out of the room)
is reduced by three to five orders of magnitude [11]. Even more
challenging are the reflections from the wall itself, which are much
stronger than the reflections from objects inside the room [11, 27].
Reflections off the wall overwhelm the receiver’s analog to digital
converter (ADC), preventing it from registering the minute varia-
tions due to reflections from objects behind the wall. This behavior
is called the “Flash Effect" since it is analogous to how a mirror in
front of a camera reflects the camera’s flash and prevents it from
capturing objects in the scene.
So how can one overcome these difficulties? The radar com-
munity has been investigating these issues, and has recently in-
troduced a few ultra-wideband systems that can detect humans
moving behind a wall, and show them as blobs moving in a dim
background [27, 41] (see the video at [6] for a reference). Today’s
state-of-the-art system requires 2 GHz of bandwidth, a large power
source, and an 8-foot long antenna array (2.4 meters) [12, 27].
Apart from the bulkiness of the device, blasting power in such a
wide spectrum is infeasible for entities other than the military. The
requirement for multi-GHz transmission is at the heart of how these
systems work: they separate reflections off the wall from reflec-
tions from the objects behind the wall based on their arrival time,
and hence need to identify sub-nanosecond delays (i.e., multi-GHz
bandwidth) to filter the flash effect.1 To address these limitations,
an initial attempt was made in 2012 to use Wi-Fi to see through a
wall [13]. However, to mitigate the flash effect, this past proposal
needs to install an additional receiver behind the wall, and connect
the receivers behind and in front of the wall to a joint clock via
wires [13].
The objective of this paper is to enable a see-through-wall tech-
nology that is low-bandwidth, low-power, compact, and accessible
to non-military entities. To this end, the paper introduces Wi-Vi,2 a
see-through-wall device that employs Wi-Fi signals in the 2.4 GHz
ISM band. Wi-Vi limits itself to a 20 MHz-wide Wi-Fi channel,
and avoids ultra-wideband solutions used today to address the flash
effect. It also disposes of the large antenna array, typical in past
systems, and uses instead a smaller 3-antenna MIMO radio.