Wi-Vi : See Through Walls with Wi-Fi

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 Wi-Vi : See Through Walls with Wi-Fi

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 thesis 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 penetrate the wall, reflect off objects and humans, and come back imprinted with a signature of what is inside a closed room. By capturing these reflections, 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. Even more challenging are the reflections from the wall itself, which are much stronger than the reflections from objects inside the room [13, 28]. 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 [28, 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, 28]. 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.To address these limitations, an initial attempt was made in 2012 to use Wi-Fi to see through a wall. 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.

Seeing Through Walls with Wi-Fi   

The objective of this thesis 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 thesis introduces Wi-Vi, 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. 1-1, 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.

Evaluation of Wi-Vi   

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. The video available in [9] shows a demo of how Wi-Vi tracks a moving human from behind a wall.

• 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. The precisions with which Wi-Vi identifies each case, computed over 80 trials with 8 human subjects, are 100%, 100%, 85% and 90% respectively.

• In the same room, and given a single person sending gesture-based 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-through-wall radar [28], Wi-Vi is limited in two ways. First, replacing the antenna array by ISAR means that the angular resolution in Wi-Vi 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  [28], 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 typ-
ically collected using a person-height standing wall, positioned either outdoors or in large empty indoor spaces  [28,  41]. In contrast, our experiments are in standard office buildings with the imaged humans inside closed fully-furnished 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. 

Related Work

3.2   Device Operation

Wi-Vi can be used in one of two modes, depending on the user’s choice. In mode 1,
it can be used to image moving objects behind a wall and track them. In mode 2, on

Through-wall Radar

Interest in through-wall imaging has been surging for about a decade [5]. Earlier work in this domain focused on simulations [39, 29] and modeling [33, 34]. Recently, there have been some implementations tested with moving humans [28, 41, 14]. These past systems eliminate the flash effect by isolating the signal reflected off the wall from signals reflected off objects behind the wall. This isolation can be achieved in the time domain, by using very short pulses (less than 1ns) [42, 5] whereby the pulse reflected off the wall arrives earlier in time than that reflected off moving objects behind it. Alternatively, it may be achieved in the frequency domain by using a linear frequency chirp  [13,  28]. In this case, reflections off objects at different distances arrive with different tones. By analog filtering the tone that corresponds to the wall, one may remove the flash effect. These techniques require ultra-wide bandwidths (UWB) of the order of 2 GHz [13, 42]. Similarly, through-wall imaging products developed by the industry  [5,  7] hinge on the same radar principles, requiring multiple GHz of bandwidth and hence are targeted solely at the military.

As a through-wall imaging technology, Wi-Vi differs from all the above systems in that it requires only few MHz of bandwidth and operates in the same range as Wi-Fi. It overcomes the need for UWB by leveraging MIMO nulling to remove the flash effect. 

Researchers have recognized the limitations of UWB systems and explored the potential of using narrowband radars for through-wall technologies [30, 31]. These systems ignore the flash effect and try to operate in presence of high interference caused by reflections off the wall. They typically rely on detecting the Doppler shift caused by moving objects behind the wall. However, the flash effect limits their detection capabilities. Hence, most of these systems are demonstrated either in simulation [29], or in free space with no obstruction [22, 24]. The ones demonstrated with an obstruction use a low-attenuation standing wall, and do not work across higher attenuation materials such as solid wood or concrete [30, 31]. Wi-Vi shares the objectives of these devices; however, it introduces a new approach for eliminating the flash effect without wideband transmission. This enables it to work with concrete walls and solid wood doors, as well as fully closed rooms. 

The only attempt which we are aware of that uses Wi-Fi signals in order to see through walls was made in 2012 [14]. This system required both the transmitter and a reference receiver to be inside the imaged room. Furthermore, the reference receiver in the room has to be connected to the same clock as the receiver outside the room. In contrast, Wi-Vi can perform through-wall imaging without access to any device on the other side of the wall.

        Gesture-based Interfaces

Today, commercial gesture-recognition systems - such as the Xbox Kinect [10], Nin- tendo Wii [4], etc. - can identify a wide variety of gestures. The academic community has also developed systems capable of identifying human gestures either by employing cameras [25] or by placing sensors on the human body [16, 21]. Recent work has also leveraged narrowband signals in the 2.4 GHz range to identify human activities within line-of-sight using micro-Doppler signatures [22]. Wi-Vi, however, presents the first gesture-based interface that works in non-line-of-sight scenarios, and even through a wall, yet does not require the human to carry a wireless device or wear a set of sensors.

        Infrared and Thermal Imaging

Similar to Wi-Vi, infrared and thermal imaging technologies extend human vision beyond the visible electromagnetic range, allowing us to detect objects in the dark or in smoke. They operate by capturing infrared or thermal energy reflected off the first obstacle in line-of-sight of their sensors. However, cameras based on these technologies cannot see through walls because they have very short wavelengths (few µm to submm) [38], unlike Wi-Vi which employs signals whose wavelengths are 12.5 cm.

 

Wi-Vi Overview    

Wi-Vi is a wireless device that captures moving objects behind a wall. It leverages the ubiquity of Wi-Fi chipsets to make through-wall imaging relatively low-power, low-cost, low-bandwidth, and accessible to average users. To this end, Wi-Vi uses Wi-Fi OFDM signals in the ISM band (at 2.4 GHz) and typical Wi-Fi hardware. 
         

Device Description 

Wi-Vi is essentially a  3-antenna MIMO device: two of the antennas are used for transmitting and one is used for receiving. It also employs directional antennas to focus the energy toward the wall or room of interest.Its design incorporates two main components: 1) the first component eliminates the flash reflected off the wall by performing MIMO nulling; 2) the second component tracks a moving object by treating the object itself as an antenna array using a technique called inverse SAR. 

        Device Operation

Wi-Vi can be used in one of two modes, depending on the user’s choice. In mode 1, it can be used to image moving objects behind a wall and track them. In mode 2, on the other hand, Wi-Vi functions as a gesture-based interface from behind a wall that enables humans to compose messages and send them to the Wi-Vi receiver. 

Eliminating the Flash

In any through-wall imaging system, the signal reflected off the wall, i.e., the flash, is much stronger than any signal reflected from objects behind the wall. This is due to the significant attenuation which electromagnetic signals suffer when penetrating dense obstacles. Table 4.1 shows a few examples of the one-way attenuation experienced by Wi-Fi signals in common construction materials (based on [1]). For example, a one-way traversal of a standard hollow wall or a concrete wall can reduce Wi-Fi signal power by 9 dB and 18 dB respectively. Since through-wall systems require traversing the obstacle twice, the one-way attenuation doubles, leading to an 18-36 dB flash effect in typical indoor scenarios.

  

This problem is exacerbated by two other parameters: First, the actual reflected signal is significantly weaker since it depends both on the reflection coefficient as well as the cross-section of the object. The wall is typically much larger than the objects of interest, and has a higher reflection coefficient [13]. Second, in addition to the direct flash caused by reflections off the wall, through-wall systems have to eliminate the direct signal from the transmit to the receive antenna, which is significantly larger than the reflections of interest. Wi-Vi uses interference nulling to cancel both the wall reflections as well as the direct signal from the transmit to the receive antenna, hence increasing its sensitivity to the reflections of interest. 

Nulling to Remove the Flash

Recent advances show that MIMO systems can pre-code their transmissions such that
the signal received at a particular antenna is cancelled [37, 18]. Past work on MIMO
has used this property to enable concurrent transmissions and null interference [27,
23]. We observe that the same technique can be tailored to eliminate the flash effect
as well as the direct signal from the transmit to the receive antenna, thereby enabling
Wi-Vi to capture the reflections from objects of interest with minimal interference.

At a high level, Wi-Vi’s nulling procedure can be divided into three phases: initial nulling, power boosting, and iterative nulling, as shown in Alg. 1.

          Initial Nulling

In this phase, Wi-Vi performs standard MIMO nulling. Recall that Wi-Vi has two transmit  antennas and  one  receive  antenna.  First,  the  device  transmits  a  known preamble x only on its first transmit antenna. This preamble is received at the receive antenna as y = h₁x , where h₁  is the channel between the first transmit antenna and the receive antenna. The receiver uses this signal in order to compute an estimate of the channel hˆ₁. Second, the device transmits the same preamble x , this time only on its second antenna, and uses the received signal to estimate channel h₂  between thesecond transmit antenna and the receive antenna. Third, Wi-Vi uses these channel estimates to compute the ratio p = −hˆ₁/hˆ₂. Finally, the two transmit antennas transmit concurrently, where the first antenna transmits x and the second transmits px . Therefore, the perceived channel at the receiver.

Power Boosting

Simply nulling static reflections, however, is not enough because the signals due to
moving objects behind the wall are too weak. Say, for example, the flash effect was 30
to 40 dB above the power of reflections off moving objects.