Localization and Communication for UWB-based Wireless Sensor Networks

Abstract

The great demand for location-aware wireless sensor networks (WSNs) motivates the research in this thesis. The unique characteristics of WSNs impose numerous challenges on localization and communication. In this thesis, we handle some key challenges and provide affordable solutions. Impulse radio ultra wideband (IR-UWB) is employed as the fundamental technology for both localization and communication due to its distinctive advantages in accurate ranging and reliable communication. The following aspects are treated in this thesis. Transmitted-reference (TR) UWB communication systems: IR-UWB processing in the digital domain usually asks for very high sampling rates. The TR-UWB scheme allows for sub-Nyquist rate sampling by correlating the received pulse sequence with its delayed version in the analog domain. Thus, it avoids the daunting Nyquist sampling rate, relaxes the stringent synchronization requirements, and only asks for aggregate channel coefficients. A data model including various kinds of interferences is employed, and then a complete receiver is proposed including signal detection, channel estimation, synchronization and equalization. Theoretical ranging bounds and practical ranging methods based on IR-UWB: We investigate the theoretical ranging accuracy of a novel method, which exploits the range information in both the amplitude and the time delay of the received signal. The investigations are conducted not only for an additive white Gaussian noise (AWGN) channel with attenuation, but also for an AWGN channel with both attenuation and shadowing. Furthermore, a practical ranging method based on time-of-arrival (TOA) estimation using UWB IRs is developed. Stroboscopic sampling is employed to sacrifice transmission efficiency for a lower sampling rate. Moreover, it can maintain the same ranging resolution as Nyquist sampling can achieve. Due to the long preamble required by stroboscopic sampling, the clock drift, which is an accumulative effect over time caused by the relative clock skew between different clocks, is one of the main error sources in TOA estimation. Therefore, TOA estimation methods with clock drift calibration are explored to dramatically mitigate the influence of the drift. Various localization and tracking methods: Extended multi-dimensional scaling (MDS): Since the classical MDS cannot be applied to general networks with missing links, we extend the classical MDS algorithm to deal with a special kind of network with specific missing links. Our goal is to jointly estimate the positions of all the nodes given partial pairwise distance measurements up to a translation, rotation, and reflection. Reference-free time-based localization: Low-complexity least-squares (LS) estimators based on time-of-arrival (TOA) or time-difference-of-arrival (TDOA) measurements have been developed in literature to locate a target node with the help of anchors (nodes with known positions). They require to select a reference anchor in order to cancel nuisance parameters or relax stringent synchronization requirements, and suffer from a poor reference selection. We propose reference-free localization estimators based on TOA measurements to decouple the reference dependency. Furthermore, we generalize existing reference-based closed-form localization estimators using TOA or TDOA measurements, and shed new light on their relations to clarify some confusions that still persist in recent literature. Robust time-based localization: Time-based localization approaches attract a lot of interest due to their high accuracy and potentially low cost for WSNs. However, time-based localization is tightly coupled with clock synchronization. Thus, the reliability of timestamps in time-based localization becomes an important yet challenging task to deal with. Regardless of the reliability of the timestamps from the target node, we propose a novel ranging protocol, namely asymmetric trip ranging (ATR), which leads to localization methods that are naturally immune to internal attacks mounted by a compromised target node. Robust localization strategies using the ATR protocol based on TOA measurements are proposed to localize a target node with the help of anchors for asynchronous networks. Kalman tracking: Due to the nonlinearity of the localization problem, a Kalman filter (KF) is usually replaced by an extended KF (EKF) for tracking a mobile target. However, the modeling errors inherently contained in the EKF degrade the tracking performance. Therefore, we make use of the ATR protocol again, carry out exact linearizations, and achieve a KF based on a linear measurement model to track a mobile target with the aid of fixed anchors.