Signals and Information Group
K. J. Ray Liu

Recent Research Areas

Achieving (The World's First) Centimeter Accuracy for NLOS Indoor Locationing

Indoor positioning systems (IPSs) are becoming more and more popular which spawn numerous location-based services. The imperative demands on the high localization accuracy give rise to an extensive development of IPSs leveraging a variety of radio-frequency (RF) techniques. These approaches can be further classified into (i) ranging (ii) fingerprinting. Most of the techniques are only effective in the line-of-sight (LOS) scenarios with strong direct paths, and degrade significantly when multiple non-line-of-sight (NLOS) multipath components exist, which is in general the case for indoor spaces. In general, 1m is the cursed limit of the current accuracy of IPSs, no matter how complicated the schemes are, after more than two decades of intensive research.

In an indoor environment, there commonly exists a large amount of multipaths due to the rich scatterers. These multipaths make the indoor positioning problem very challenging. The main reason is that most of the transmitted signals are significantly distorted by the multipaths before arriving at the receiver, which causes the inaccuracy in the estimation of the positioning features such as time-of-arrival and angle-of-arrival.

On the other hand, the multipath effect can be very constructive as employed in the time-reversal (TR) radio transmission. By utilizing the uniqueness of the multipath profile at each location, TR can create a resonating effect of focusing the energy of the transmitted signal only onto the intended location, which is known as the spatial-temporal resonating effect. By utilizing such a unique location-specific CIR, the proposed time-reversal indoor position system (TRIPS) is able to position the target by matching the CIR with the geographical location. Since the spatial-temporal resonance is a half-wavelength focus spot, the proposed system can achieve a 1~2cm-level localization accuracy even with a single AP working in the NLOS condition.

To evaluate the performance of our proposed system, we build a prototype and the video below is the demo for the proposed TRIPS to perform the NLOS indoor locationing. Is it possible to achieve the centimeter-level accuracy on a WiFi platform by leveraging the TR technique? The answer is affirmative! We can utilize the antenna and/or frequency diversity of WiFi systems to achieve a large effective bandwidth to achieve the 1-2cm accuracy of indoor locationing.

Speaker: Dr. Quoc Lai, a member of SIG.

Wireless Event Detection

The past few decades have witnessed the increase in the demand of surveillance systems which aims to capture and to identify unauthorized individuals and events. With the development of technologies, traditional outdoor surveillance systems become more compact and of low cost. In order to guarantee the security in offices and residences, indoor monitoring systems are now ubiquitous and their demand is rising both in quality and quantity.

Currently, most device-free systems have limitations in that they either require multiple antennas and dedicated sensors or require LOS transmission environment and ultra-wideband to capture features that can guarantee the accuracy of detection. In contrast, we propose a time-reversal (TR) based indoor monitoring system, the proposed system, capable of accurate through-the-wall indoor monitoring with only one pair of single-antenna devices. TR technique treats each path of the multipath channel in a rich scattering environment as a widely distributed virtual antenna and provides a high-resolution spatial-temporal resonance. When the propagation environment changes, the involved multipath signal varies correspondingly and consequently the spatial-temporal resonance also changes.

To illustrate the capability, we conduct a real-time NLOS intruder monitoring experiment as shown in the video. The system provides a two-threshold alarm mechanism that will report for the existence of human movement and the state changes of doors, respectively.

Speaker: Dr. Quoc Lai, a member of SIG.

(The World's First) Realtime Centimeter-Accuracy NLOS Indoor Tracking

The indoor tracking problem is a derivation of indoor positioning problem in that it requires to a sequence of locations to form a trajectory for the target from the first to the most actual location. However, the indoor tracking is more difficult than the indoor positioning, owing to its extremely high sensitivity in computational delay.

In seeking of new indoor tracking technologies, our group has discovered in our preliminary work that the time reversal (TR) technique can be an ideal solution to the real-time indoor tracking problem with high accuracy and low computational complexity, because of its inherent spatial-temporal resonanting effect as a response to the surrounding environment.

Our proposed TR based indoor tracking system consists of two phases:

  • offline mapping phase where the multipath profile of each location on the trajectory path is stored in the database, and
  • online tracking phase where the current location is determined by matching the multipath profile with the database collected during offline phase.

By mapping each physical location into the space of the TR logical locations through the TR spatial-temporal resonances, a centimeter-accurate positioning and tracking can be achieved even in a NLOD transmission scenario.

In the indoor train tracking demo, the origin (receiver) is placed in the room at the southwest corner of the floorplan, whereas the bot (transmitter) is attached on the train to be tracked in the room on the northeast corner of the floorplan. Apparently, there is no LOS path between the origin and the bot, and they are far away from each other. The video below demonstrates how the TR based indoor tracking system works in the train tracking experiment.


The distance between the origin and the bot is about 20 meters.

Speaker: Dr. Quoc Lai, a member of SIG.