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ABSTRACT
Wi-Fi signals are typically information carriers between a transmitter
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 demonstrate
the validity of our design by building it into USRP software
radios and testing it in office buildings.
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 personnel
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 detection,
privacy-enhanced monitoring of children and elderly, or personal
security when stepping into dark alleys and unknown places.
The concept underlying seeing through opaque obstacles is similar
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 variations
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 community
has been investigating these issues, and has recently introduced
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 reflections
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 technology
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.
So, how does Wi-Vi eliminate the flash effect without using GHz
of bandwidth? We observe that we can adapt recent advances in
MIMO communications to through-wall imaging. In MIMO, multiple
antenna systems can encode their transmissions so that the signal
is nulled (i.e., sums up to zero) at a particular receive antenna.
MIMO systems use this capability to eliminate interference to unwanted
receivers. In contrast, we use nulling to eliminate reflections
from static objects, including the wall. Specifically, a Wi-Vi device
has two transmit antennas and a single receive antenna. Wi-Vi operates
in two stages. In the first stage, it measures the channels from
each of its two transmit antennas to its receive antenna. In stage 2,
the two transmit antennas use the channel measurements from stage
1 to null the signal at the receive antenna. Since wireless signals (including
reflections) combine linearly over the medium, only reflections off objects that move between the two stages are captured in
stage 2. Reflections off static objects, including the wall, are nulled
in this stage. In §4, we refine this basic idea by introducing iterative
nulling, which allows us to eliminate residual flash and the weaker
reflections from static objects behind the wall.
Second, how does Wi-Vi track moving objects without an antenna
array? To address this challenge, we borrow a technique
called inverse synthetic aperture radar (ISAR), which has been used
for mapping the surfaces of the Earth and other planets. ISAR uses
the movement of the target to emulate an antenna array. As shown in
Fig. 1, a device using an antenna array would capture a target from
spatially spaced antennas and process this information to identify
the direction of the target with respect to the array (i.e., θ). In contrast,
in ISAR, there is only one receive antenna; hence, at any point
in time, we capture a single measurement. Nevertheless, since the
target is moving, consecutive measurements in time emulate an inverse
antenna array – i.e., it is as if the moving human is imaging
the Wi-Vi device. By processing such consecutive measurements
using standard antenna array beam steering, Wi-Vi can identify the
spatial direction of the human. In §5.2, we extend this method to
multiple moving targets.
Additionally, Wi-Vi leverages its ability to track motion to enable
a through-wall gesture-based communication channel. Specifically,
a human can communicate messages to a Wi-Vi receiver via
gestures without carrying any wireless device. We have picked two
simple body gestures to refer to “0” and “1” bits. A human behind
a wall may use a short sequence of these gestures to send a message
to Wi-Vi. After applying a matched filter, the message signal
looks similar to standard BPSK encoding (a positive signal for a
“1” bit, and a negative signal for a “0” bit) and can be decoded by
considering the sign of the signal. The system enables law enforcement
personnel to communicate with their team across a wall, even
if their communication devices are confiscated.
We built a prototype of Wi-Vi using USRP N210 radios and evaluated
it in two office buildings. Our results are as follows:
• Wi-Vi can detect objects and humans moving behind opaque
structural obstructions. This applies to 8!! concrete walls, 6!! hollow
walls, and 1.75!! solid wooden doors.
• A Wi-Vi device pointed at a closed room with 6!! hollow walls
supported by steel frames can distinguish between 0, 1, 2, and 3
moving humans in the room. Computed over 80 trials with 8 human
subjects, Wi-Vi achieves an accuracy of 100%, 100%, 85%,
and 90% respectively in each of these cases.
• In the same room, and given a single person sending gesturebased
messages, Wi-Vi correctly decodes all messages performed
at distances equal to or smaller than 5 meters. The decoding
accuracy decreases to 75% at distances of 8 meters, and
the device stops detecting gestures beyond 9 meters. For 8 volunteers
who participated in the experiment, on average, it took a
person 8.8 seconds to send a message of 4 gestures.
• In comparison to the state-of-the-art ultra-wideband see-throughwall
radar [27], Wi-Vi is limited in two ways. First, replacing the
antenna array by ISAR means that the angular resolution in WiVi
depends on the amount of movement. To achieve a narrow
beam the human needs to move by about 4 wavelengths (i.e.,
about 50 cm). Second, in contrast to [27], we cannot detect humans
behind concrete walls thicker than 8!!. This is due to both
the much lower transmit power from our USRPs and the residual
flash power from imperfect nulling. On the other hand, nulling
the flash removes the need for GHz bandwidth. It also removes
clutter from all static reflectors, rather than just one wall. This includes
other walls in the environments as well as furniture inside
and outside the imaged room. To reduce clutter, the empirical results
in past work are typically collected using a person-height standing wall, positioned either outdoors or in large empty indoor
spaces [27, 41]. In contrast, our experiments are in standard
office buildings with the imaged humans inside closed fullyfurnished
rooms.
Contributions: In contrast to past work which targets the military,
Wi-Vi introduces novel solutions to the see-through-wall problem
that enable non-military entities to use this technology. Specifically,
Wi-Vi is the first to introduce interference nulling as a mechanism
for eliminating the flash effect without requiring wideband spectrum.
It is also the first to replace the antenna array at the receiver
with an emulated array based on human motion. The combination
of those techniques enables small cheap devices that operate in the
ISM band, and can be made accessible to the general public. Further,
Wi-Vi is the first to demonstrate a gesture-based communication
channel that operates through walls and does not require the
human to carry any wireless device.