Albert Kai-Sun Wong

Received Signal Strength-Based Wireless Localization via Semidefinite Programming: Noncooperative and Cooperative Schemes

The received signal strength (RSS)-based approach to wireless localization offers the advantage of low cost and easy implementability. To circumvent the nonconvexity of the conventional maximum likelihood (ML) estimator, in this paper, we propose convex estimators specifically for the RSS-based localization problems. Both noncooperative and cooperative schemes are considered. We start with the noncooperative RSS-based localization problem and derive a nonconvex estimator that approximates the ML estimator but has no logarithm in the residual. Next, we apply the semidefinite relaxation technique to the derived nonconvex estimator and develop a convex estimator. To further improve the estimation performance, we append the ML estimator to the convex estimator with the result by the convex estimator as the initial point. We then extend these techniques to the cooperative localization problem. The corresponding Cramer-Rao lower bounds (CRLB) are derived as performance benchmarks. Our proposed convex estimators comply well with the RSS measurement model, and simulation results clearly demonstrate their superior performance for RSS-based wireless localization.

Scalable architectures for integrated traffic shaping and link scheduling in high-speed ATM switches

Emerging broad-band switches must accommodate the diverse traffic parameters and quality-of-service requirements of voice, data, and video applications. End-to-end performance guarantees depend on connections complying with traffic contracts as their cells travel through the network. This paper presents a leaky-bucket shaper architecture that scales to a large number of connections with diverse burstiness and bandwidth parameters. In contrast to existing designs, the proposed architecture arbitrates fairly between connections with conforming cells by carefully integrating leaky-bucket traffic shaping with rate-based scheduling algorithms. Through a careful combination of per-connection queueing and approximate sorting, the shaper performs a small, bounded number of operations in response to each arrival and departure, independent of the number of connections and cells. When the shaper must handle a wide range of rate parameters, a hierarchical arbitration scheme can reduce the implementation overheads and further limit interference between competing connections. Through simulation experiments, we demonstrate that the architecture limits cell-shaping delay and traffic distortions, even in periods of heavy congestion. The efficient combination of traffic shaping and link scheduling results in an effective architecture for managing buffer and bandwidth resources in large, high-speed ATM switches.

Indoor Location Estimation with Reduced Calibration Exploiting Unlabeled Data via Hybrid Generative/Discriminative Learning

For indoor location estimation based on wireless local area networks fingerprinting, how to reduce the offline calibration effort while maintaining high location estimation accuracy is of major concern. In this paper, a hybrid generative/discriminative semi-supervised learning algorithm is proposed that utilizes a large number of unlabeled samples to supplement a small number of labeled samples. This hybrid method allows us to combine the modeling power and flexibility of generative models with the superior performance of discriminative approaches. Other related issues, such as learning efficiency enhancement and distribution estimation smoothing, are also discussed. Extensive experimental results show that our proposed method can effectively reduce the calibration effort and exhibit superior performance in terms of localization accuracy and robustness.

A Novel Threshold-Based Coherent TOA Estimation for IR-UWB Systems

Precise time-of-arrival (TOA) estimation is challenging in dense multipath environments because the leading-edge path of a received signal may not be the strongest path. In this paper, we propose a threshold-based coherent TOA estimation scheme for impulse radio-based ultrawideband (UWB) systems. The scheme uses a dedicated ranging sequence with a good cross-correlation property and a search-back process that considers the count of threshold crossings to find the leading edge of the received signal. We derive an approximate closed-form expression for the optimal threshold that restrains the mean absolute error. We show by simulations that high-precision short-distance ranging is achieved without introducing too much latency.

An AGPS-based elderly tracking system

The design of an experimental AGPS-based (Assisted Global Positioning System) elderly tracking system is described. The system includes: a wearable AGPS terminal with HSPA two-way communication capability and designed for 10 days of continuous battery operation, a GPS assistance data server with reference GPS stations, location database and server, application server, and web server and client. Assistance data is retrieved by the wearable AGPS terminal using the SUPL protocol (Secured User Plane Location). This paper describes the design of each component based on key considerations such as accuracy, availability, battery life-time, and user behavior.

Adaptive Mobility Mapping for People Tracking Using Unlabelled Wi-Fi Shotgun Reads

Mobility tracking plays an important role in identifying human activities and providing location-based services (LBSs). Up to now, with the help of special infrastructures or hardware devices, many existing tracking systems involve some cumbersome work of fingerprint or landmark calibration. In this letter, we present an adaptive mobility map construction scheme for large-scale Wi-Fi mobility tracking in indoor areas which does not require any off-line fingerprinting effort or deployment of landmarks. Our scheme works by collecting a large data-set of Wi-Fi received signal strength using a large number of cell phones carried by users during their normal daily activities.

A scalable architecture for fair leaky-bucket shaping

This paper presents a shaper architecture that scales to a large number of connections with diverse burstiness and bandwidth parameters. The architecture arbitrates fairly between connections with conforming cells by carefully integrating leaky-bucket traffic shaping with rate-based scheduling algorithms. Through a careful combination of per-connection queueing and approximate sorting, the shaper performs a small, bounded number of operations in response to each arrival and departure, independent of the number of connections and cells. To handle a wider range of rate parameters, a hierarchical arbitration scheme can reduce the implementation overheads and the interference between competing connections. Simulation experiments demonstrate that the architecture limits shaping delay and traffic distortions, even under heavy congestion.

GPS Localization Accuracy Improvement by Fusing Terrestrial TOA Measurements

This paper explores the use of terrestrial time of arrival (TOA) measurements to improve the initial Global Positioning System (GPS) location fix accuracy. First, we present a geometric approach when a GPS location fix and one TOA measurement are available. Then, a more general hybrid GPS/TOA method via the Weighted Least Square Estimator (WLSE) is proposed. To simplify the calculation, a closed-form solution based on the two-step Least Square approach is also designed. The Cramer-Rao Lower Bound (CRLB) is derived as a performance benchmark. Simulation results exhibit excellent performance of the proposed methods which attain the CRLB in different scenarios. The proposed methods work even if only one TOA measurement (in addition to a GPS location fix) is available and the corresponding accuracy improvement (compared with the initial GPS location fix) can be as much as 30%.

Routing and time-slot assignment in optical TDM networks

Optical time-division multiplexing (O-TDM) networks can provide a finer bandwidth granularity than wavelength-division multiplexing networks, and will play an important role in future all-optical networks. Since optical buffers are expensive, a small buffer size will be the characteristic of O-TDM systems. This paper analyzes the problem of routing and time-slot assignment in O-TDM networks. The results lead to the proposal of a Dijkstra-like shortest-path routing scheme that intends to maximize the performance of an optical network with a small number of optical buffers. Performance evaluation of the proposed scheme is also presented.

Mobility tracking using GPS, Wi-Fi and Cell ID

Capability to track the mobility pattern of children and elders can enable many useful applications. Assisted GPS (AGPS) and Wi-Fi can be used to precisely calculate the location of a person, but AGPS consumes a lot of power and Wi-Fi coverage is often spotty. On the other hand, Cell ID offers ubiquitous coverage in most inhabited environments, low power consumption, but has the disadvantage of lower accuracy. This paper introduces an energy-efficient cellular and AGPS/Wi-Fi tracking system based on the construction of a mobility map from a persons historical location data. A location map consists of a set of crucial locations (CL), personal common locations (PCL), and routes. CL is a point that terminates several routes and PCL is a place where the person spends a lot of time in. The tracking process is divided into four parts: data collection, offline data processing, online tracking, and path reconstruction. With the use of the mobility map, AGPS fixes can be significantly reduced and a more accurate recognition of the persons location can be provided during the tracking phase.