Recently the area of wireless sensor networks (WSNs) has gained a lot of mo- mentum and has become increasingly attractive for many emerging applications due to its low cost, small size, light weight, and potential to be deeply embedded into the environment for a variety of applications. In many of these applications, localization has been an active area of research due to the fact that without the knowledge of sensor location, data passed by sensors will be meaningless. Here, determining the physical location of subject nodes will provide additional information in order to quantify the measured data. One of the important tasks for real time localization in WSNs is the precision and accuracy of range mea- surement. Hence, for efficient localization, it is imperative to understand the performance limits of ranging in realistic environments.
This chapter reports on round-trip time-of-flight (RT-ToF) and received signal strength (RSS) for point to point range estimation using 2.4GHz IEEE 802.15.4 compliant transceivers. Firstly, the performance limits for RT-ToF and RSS based range measurements are compared with the fundamental Cram´er-Rao Lower Bound (CRLB). Secondly the range where the error for RSS ranging is expected
to be greater than the error for ToF ranging is considered. We term this the ‘cross-over’ range (CR) of RSS and ToF ranging, where ToF ranging becomes
more accurate than the RSS ranging. Thirdly, using a site survey application, a series of experiments has been conducted in different environments to make it pos- sible to determine which parameters of the system lead to improved performance and successful ranging polls. Performance results and channel parameters have been obtained in outdoor and indoor for the line-of-sight (LOS) and non-line-of- sight (NLOS) environments. Both indoor and outdoor experimental results and analysis are presented. Based on the experimental results for outdoor and indoor unobstructed (LOS) environment over short range, it is demonstrated that RSS is a good candidate for range estimation at ranges less than 7m. Uncertainty in RSS based range estimation increases with distance and beyond 7m severely limits RSS performance. Further analysis over long range (i.e. up-to 100m) demonstrate that ToF is a good candidate for range estimation at greater than 7m.
3.2
Introduction
Localization is mainly categorized into range-free and range-based localization schemes, that differ in what kind of geometric information they use to estimate locations. The former is based on the radio connectivity information, where each node estimates the location based on the information received from the neighbour nodes. The accuracy of range free localization depends on assumption that nodes in a dense network with radio connectivity are typically in close proximity. How- ever, node density negatively effects traffic overhead [10]. The latter approach is based on using angle estimates [14], or range measurements, which can be derived from measuring point-to-point propagation time [16, 17] or using RSS [14]. In range-based localization, the process to determine the physical location of sen- sor nodes consists of two main phases. During the first phase of localization, a sensor node performs range estimation to a set of anchor nodes, whose positions are known. Sensor nodes are typically equipped with extra hardware capable of estimating distance or angle [14,88]. The second phase uses the range estimation
as an input to determine the position estimation through different localization strategies such as trilateration, multi-lateration and triangulation [55].
For the first phase of localization, literature has focused on two classes of ranging techniques: RF based ranging and acoustic ranging [89, 90]. The RF method is more cost effective than acoustic signal based ranging schemes, as it does not require ultrasonic transducers [90]. The basic RF-based techniques available for the first phase of localization are angle-of-arrival (AoA) [14], time- difference-of-arrival (TDoA) [91], time-of-flight (ToF) [90,92] , and received signal strength (RSS) [12, 93].
The RSS is a standard parameter available on most wireless devices [94]. It is a popular method of ranging because it does not require additional hardware which makes it cheap as compared to other methods. Due to the complex behaviour of RF signal propagation (reflection, diffraction, refraction and scattering) and different application environments, there are several propagation models devel- oped to predict signal decay with distance. It is a challenge to set up a model to predict RSS appropriate to the environment in which the system will be used. The RSS measurement model and principle of operation is discussed in section 3.6.
In ToF ranging, measurements based on propagation time are used to esti- mate the distance between neighbouring devices. ToF is classified as either one way propagation time or two-way propagation time measurement based on the number of packet transmission for range estimation. In one way, the node A transmits the time-stamped signal at t1 and is received at node B at t2, the dis-
tance between the nodes is given by the equation d = c × ToF2 − ToF1. As
compared to one-way ToF, where two highly synchronized clocks are needed, in two-way ToF the same clock is used to calculate the round-trip time [16, 17]. Consequently synchronization between different clocks is not necessary. The ToF principle of operation is discussed in section 3.5.1. In the context of WSNs, range based techniques are more suitable due to the requirement of high accuracy and simple measurement hardware. The two most widely used and accepted tech- niques in wireless networks are ToF and RSS [90]. These two techniques have shown great potential for numerous emerging WSN applications, so are the focus of this chapter.
This chapter is organized as follows. Section 3.2 followed by the sources of ranging errors in section 3.3. Section 3.4.2 describes the experimental infras- tructure and test beds used for point-to-point ranging. In section 3.5 and 3.6 respectively, the principle of operation for ToF and RSS is discussed. Section 3.8.1 explains the cross-over range whereas results and analysis from site survey are presented in section 3.7. Finally, experimental results and conclusions are provided in section 3.8 and 3.9 respectively.