Xandem Technology

Synergistic Device-Free Sensing

The name "Xandem" comes from the word "tandem," which means to harness individuals into a more powerful framework. Xandem technology surrounds areas with small, inexpensive, and low-power wireless transceivers. Each of the devices communicates with the others, forming a dense web of radio waves that blanket the area. When people enter the network area, the mass of their bodies disturb the radio field and Xandem's algorithms can both detect and locate the disturbance. We call it "Synergistic Sensing" because it's not the individual transceivers doing the sensing, the network itself is the sensor working in tandem.

Xandem's technology has the very powerful ability of "seeing" through opaque obstructions like walls. Radio waves are a form of electromagnetic energy that can penetrate solid materials, which is why a cell phone can successfully make a call from inside a building. Since Xandem's sensors rely on radio frequency measurements, the sensing is successful even when the device is placed around the external perimeter of an enclosed area.

Competitive Technologies

A variety of other sensor technologies exist which are capable of detecting and locating people and objects that do not carry a device. Below is a summary of these technologies.

Passive infrared (PIR) motion sensors
Passive infrared (PIR) sensors are the most commonly used motion sensors available today. They work by detecting changes in thermal energy within the view of the sensor. A human body is a different temperature than a typical background environment, and when a person moves, the change in the thermal scene is detected by the sensor.

PIR sensors suffer from false alarms and detection vulnerabilities. Since they work off thermal energy, abrupt changes in temperature cause the sensor to trip. For example HVAC vents and sunlight coming in through a window would cause a PIR sensor to trigger.

PIR sensors are also extremely easy to defeat. Glass, foam, and other insulating materials can allow an intruder to easily walk by undetected. This technique was also demonstrated by the television Discovery Channel show "MythBusters" in an episode entitled "Crimes and Mythdemeanors."

In addition to the false alarm issues and vulnerabilities, PIR sensors are too coarse to use for tracking location. They are simply yes/no detection mechanisms, and can not quantify the amount of movement or its location.

Cameras
Cameras excel in applications where a high level of visual detail is required, but they are weak for motion detection and tracking applications. Even a small fly landing on a camera lens can cause a false alarm, and changes in lighting (sunlight, headlights) are problematic. Cameras cannot see through opaque obstructions and even a small amount of blockage can hinder a camera's ability to sense.

Using cameras to track the location of people is problematic in many situations. They must be carefully installed at a very particular location above an area of interest, and require complex video analytic software to track people. Tracking an entire floor plan requires stitching of video data, which is very expensive and inflexible. Blockages, obstructions, moved furniture, and changes in lighting can render a camera-based tracking system totally useless. Cameras can also be invasive of privacy in many situations.

Thermal and infrared cameras suffer from the same challenges of regular cameras, except for the fact that visible lighting of the scene is not necessary.

Dual-tech (hybrid) motion sensors
Dual-tech (hybrid) sensors combine multiple forms of motion detectors to try to reduce the probability of a false alarms. The most typical configuration is a microwave and PIR sensor both housed in the same unit. For an alarm to occur, both sensors must detect motion. If an HVAC vent turns on, for example, the microwave will not detect motion. If motion happens on the other side of the wall, the PIR sensor will not activate, thus no false alarm. Some forms of motion are still problematic, however, like balloons. When a heating vent turns on, balloons move, and the combination of microwave reflection and heat causes a false alarm.

By attempting to mitigate false alarms in dual-tech sensors, vulnerabilities are looked over. A dual-tech sensor is only as strong as its weakest link. For example, if one blocks the sensor with a piece of foam or glass, the PIR sensor will remain untriggered. Even though the microwave sensor is triggered, the system as a whole does not react. Thus, each time a new sensor is combined in a hybrid configuration, the detection capabilities are weakened.

Radar
Radar is a technology that, like Xandem, can be used to see through walls and obstructions to locate people. Radar works by sending pulses of radio energy into an area, then recording the time it takes for the radio wave to reflect off an object.

Since radio waves travel at the speed of light, this requires radar devices to use very fast and power-hungry computing hardware. This computational complexity and speed translates to expensive equipment, making it impractical for many applications.

Ranges for through-wall radar devices are typically very low. Some of the best through-wall radar products available today have a range of less than 10 meters.

A New Wireless Sensing Network Paradigm

When computer networks were developed decades ago, organizational rules and protocols were designed for wired connections. Networks were organized into 5 (sometimes 7) "layers" of functionality. They are:

Application: This is the stuff the end-user sees. For example, when you open up your email program, you're using the application layer.
Transport: This layer provides a mechanism for transferring data to/from end-users.
Network: This is where packets of information are routed through many devices.
Link: This is the lowest level of "1s" and "0s."
Physical: This is the part where electromagnetic signals travel through space or material from transmitter to receiver.

In general, a layer does not "know" anything about the internal workings of another layer. The application layer, for example, does not know how information is actually sent, it simply knows that it can communicate with others by passing data to the transport layer. It's kind of like sending a letter to someone. You don't actually know how the letter gets there, you just know that if you put an address on it and place it in your mailbox, it gets there. You don't need to worry how the post office actually routes it.

Under this paradigm, it's easy to see how the application layer would have no business getting involved in the physical layer. That's like a mail sender telling the postal service which truck to use and what kind of gas to put in it. In wireless networks, however, strict layering breaks down. Since nodes are mobile and transmit through space instead of a wire, layers must pass information to each other in a "cross-layer" architecture.

While wireless networks and cross-layer architecture present many challenges, Xandem views the wireless channel as a fertile ground for innovation. Specifically, there are many opportunities that arise when the application and physical layer are connected. By measuring how strong the physical layer signals are, for example, technologies like radio tomographic imaging and device-free localization are possible.