Positioning in Wireless Communications Systems

About the Authors

Stephan Sand (MSc EE 2001, Dipl-Ing 2002, Dr ETH Zurich 2010) studied electrical engineering with focus on communications technology, digital signal processing, and wireless communications at the University of Ulm, Germany (1996–2002), the University of Massachusetts Dartmouth, MA, USA (1999–2001), and the Swiss Federal Institute of Technology (ETH) Zurich, Switzerland (2005–2009). In 2002, he joined the Institute of Communications and Navigation, German Aerospace Center (DLR), Oberpfaffenhofen, Germany. Currently, he is managing and working on cooperative positioning and swarm navigation research projects at DLR. He was visiting researcher at NTT DoCoMo R&D Yokosuka, Japan in 2004 and at the Swiss Federal Institute of Technology (ETH) Zurich, Switzerland in 2007 working in the area of wireless communications. Stephan was actively involved in several research projects on mobile radio funded by the European Commission (4MORE, NEWCOM, COST289, PLUTO) and by international industry cooperation. In the GJU/GSA project GREAT and the EU FP7-ICT collaborative project WHERE, he lead the work on hybrid location determination. He was the coordinator of the recent EU FP7 project GRAMMAR on Galileo mass-market receivers. Currently, he leads the work on cooperative swarm navigation in the Valles Marineris Explorer Project. Stephan is a founding member of the International Conference on Localization and GNSS (ICL-GNSS) steering committee and was program committee co-chair of ICL-GNSS 2011 and general chair of ICL-GNSS 2012. His research interests and activities include wireless communications, multi-sensor navigation, cooperative positioning, and swarm navigation. Stephan has authored and coauthored more than 90 technical and scientific publications in conferences and journals, and obtained several patents on his inventions.

Armin Dammann (Dipl-Ing 1997, Dr-Ing 2005) studied electrical engineering with the main topic information- and microwave-technology at the University of Ulm, Germany from 1991–1997. In 1997, Armin joined the Institute of Communications and Navigation of the German Aerospace Center (DLR). Since 2005 he has been head of the Mobile Radio Transmission Research Group. His research interests and activities include navigation signal design for the European satellite navigation system Galileo, PHY/MAC layer design for terrestrial communications systems based on OFDM, antenna diversity techniques for wireless communications/broadcast systems and synchronization/positioning in wireless communications. Armin has authored and co-authored more than 120 technical and scientific publications in conference proceedings and journals in the fields of wireless communications and (cooperative) positioning. In these areas, he additionally holds several international patents. He has coorganized and cochaired the MC-SS workshop series on multi-carrier systems and solutions. Armin has been active in several EU research projects, for example, MCP, 4MORE, WINNER, NEWCOM, PLUTO, GREAT, GRAMMAR, and WHERE/WHERE2. For the latter, he has also been involved in management and coordination.

Christian Mensing (BSc 2002, Dipl-Ing 2004, MSc 2005 and Dr-Ing 2013) studied electrical engineering and information technology focusing on signal processing and high frequency technology at the Munich University of Technology (TUM), Germany. He received the BSc, Dipl-Ing, MSc, and Dr-Ing degree from TUM in 2002, 2004, 2005, and 2013, respectively. In 2005, he joined the Institute of Communications and Navigation of the German Aerospace Center (DLR) as a research engineer. His main interests included location estimation strategies for cellular networks and satellite based navigation systems, and detection techniques for communications. He has authored and co-authored more than 40 publications in conference proceedings and journals, and holds several patents. Christian has been involved in various European research projects on positioning and communications, for example, GREAT, WINNER, GRAMMAR, and WHERE. Since 2011, he has been working as development engineer at Rohde & Schwarz, Germany.

Preface

Since the advent of smartphones and tablet computers, such as Apple's iPhone and iPad or Google's Android devices, location based services have been widely used. Currently, the Global Positioning System (GPS) receivers in smartphones primarily provide the location information for these services. Usually, GPS only works well if the smartphone has an unobstructed clear sky view. Besides GPS, smartphones support many additional communications systems such as GSM, UMTS, LTE, WiFi, Bluetooth, and NFC. These communications technologies complement GPS for location based services, especially in urban and indoor environments. Hence, companies like Apple or Google already exploit the identity of WiFi hotspots and cellular base stations for a fast, sometimes crude, first position fix. Besides that, regulatory bodies such as US Federal Communications Commission require operators of cellular communications networks to guarantee, for emergency calls, a service quality of the measured location (FCC 1999). Hence, there exists a strong market and regulation driven development of positioning with current and future wireless communications technologies.

Personally, we started working on localization as early as 1997 with some early signal design studies on Galileo at the German Aerospace Center (DLR). Then, positioning with wireless communications, in particular with cellular communications technologies complementing GPS and the future Galileo system, became our focus from 2005. As our backgrounds are in information and communication theory as well as signal processing, we were not familiar with the specific challenges and requirements of positioning and corresponding signal processing. For example, communications engineers often model the wireless channel by a tapped delay line starting with delay zero. However, they do not take into account the delay of the first arriving path. This path is proportional to the distance between transmitter and receiver. Thus, it is essential to determine the location of a mobile terminal. Hence, this book reflects our learning process on positioning with wireless communications. It also shows our work experience through several projects on positioning.

The content of this book is organized into nine chapters. Through Chapters 1 and 2, the reader will quickly get acquainted with the topic of positioning with wireless communications. Chapter 1 introduces past, current, and future satellite and ground based radio positioning systems as well as critical environments for satellite based positioning systems. Next, Chapter 2 discusses the fundamental positioning principles. These principles are the basis for positioning in today's wireless radio systems.

Then, Chapters 3, 4, and 5 will enable the reader familiar with communication technology or signal processing to achieve a deep technical understanding of the basic positioning technology. Chapter 3 formulates the parameter estimation problem for obtaining position dependent measurements from wireless communications systems. In Chapter 4, positioning algorithms use these measurements to estimate a mobile terminal's location assuming the mobile terminal does not move during the positioning process. Chapter 5 extends the previously static positioning process to dynamic position tracking of moving mobile terminals.

More advanced topics on positioning with wireless communications are addressed in Chapters 6–9. First, Chapter 6 discusses in detail the scenarios and environments in which positioning with satellite and wireless communications systems takes place. It also presents corresponding propagation models for the radio signals and movement models for the mobile user. Second, Chapter 7 presents advanced positioning algorithms such as hybrid data fusion of satellite navigation and positioning with wireless communications, cooperative positioning among mobile terminals, and multipath and non-line-of-sight mitigation concepts. Subsequently, Chapter 8 surveys positioning with various wireless communications systems that are currently widely deployed and in use, or will be in the near future. The book concludes with an introduction to applications of positioning with wireless communications in Chapter 9.

Acknowledgements

The authors would like to thank the many direct and indirect contributors to this book. Many thanks go to Helena Leppäkoski from Tampere University of Technology for allowing us to reproduce her work on WLANs from the Galileo Ready Advanced Mass Market Receiver (GRAMMAR) project. Many thanks also go to Jimmy J. Nielsen from Aalborg University for permitting us to replicate his work on location-aided relay selection and location assisted handover prediction from the Wireless Hybrid Enhanced Mobile Radio Estimators (WHERE) project. Further, we would like to express our sincere thanks to Loïc Brunel, Nicolas Gresset, and Mélanie Plainchault from Mitsubishi Electric R&D Centre Europe, who granted us their permission to reproduce their work on location based inter-cell interference coordination from the WHERE project.

Many thanks to our colleagues from the Mobile Radio Transmission Group, the Department of Communications Systems, and the Institute of Communications and Navigation of DLR for helpful technical discussions. In particular, we thank Simon Plass for helping us on the application of position information for cellular diversity and Wei Wang on positioning with triangulation. Further, we would like to thank the members of the GREAT (Galileo REceiver for mAss markeT), GRAMMAR, WHERE, and WHERE2 project teams, whose work we have cited in this book.

Finally, many thanks to the Wiley team who made this book possible.

List of Abbreviations

Chapter 1

Introduction

The determination of position is an art that has fascinated scientists for centuries. First positioning methods were probably developed several millennia ago when people realized the necessity of knowing their position for systematic travel. Orientation at natural landmarks such as mountains, rivers, or coastlines are straightforward methods for that purpose. Early man made landmarks were trails and ways that were often built for trading, for example, the famous Silk Road, which has its origins around 500 B.C., and connected Europe and Eastern Asia. Other man made landmarks are lighthouses. They provide orientation in monotonous environments even at night, for example, for ships relatively close to the coastline. On the high seas, however, landmarks are missing. Keeping track of a journey by measuring direction and velocity, called the dead reckoning method, was the straightforward approach used by early ocean navigators. Celestial navigation is another method that utilizes well-known objects as position references. Measuring the angle of the pole star above the horizon directly provides the latitude. The major problem for a long time has been the determination of the longitude directly related to the exact measurement of time due to the Earth's rotation. As the Earth rotates around c01-math-0001 each day, a deviation of 4 s in time keeping results in a position error of c01-math-0002 that is 1 nautical mile or 1.852 km at the equator. At that time, the longitude problem was so severe that several prizes were offered for the development of more precise longitude determination methods. In 1714 the British government rewarded £10 000 for a method capable of determining the longitude within a range of 60 nm (nautical miles), £15 000 for a deviation of 40 nm and £20 000 for 30 nm during a six week journey to the West Indies. Famous scientists like Isaac Newton and Edmond Halley proposed and promoted the use of astronomic methods, that is, predictable astronomic occurrences, for time determination. The ‘lunar distance’ relative to a fixed star or the ecliptic of Jupiter's moons are such ideas. The invention of chronometers with sufficient accuracy solved the problem and made astronomical methods needless. In 1761 John Harrison's H.4 marine chronometer, constructed in 1759, showed a time deviation of 5 s during a five-week journey to Jamaica. All methods that at least partially rely on visual observations require clear sight. This limits the usability of these methods to certain times of a day or to good weather conditions. The discovery of radio waves in the late nineteenth century opened the door for the field of radio navigation. Radio beacons take the role of man made landmarks. Radio frequency bands provide a propagation range exceeding that of visible light. Dependent on the frequency band, radio waves are able to travel through clouds or fog, or even propagate as ground waves over a long distance. This solved the range problem even for ground based radio navigation systems. Nowadays, satellite navigation systems provide global coverage with accuracy in the range of meters. Some of the positioning principles, however, remain the same as for traditional landmark or celestial navigation. In particular these are angular methods, where the angle of arrival of radio waves are determined. Today, radio navigation is mainly based on radio propagation time measurements, by which the knowledge of propagation speed (speed of light) provides distance measures related to the radio beacons.

The civil availability of accurate satellite navigation together with chip-sets and navigation receivers for consumer applications have formed the basis of a rapidly growing navigation market in recent years. Indicated by this market growth, the availability of position information will play an increasingly important role in current and future mobile information systems. Information about the position of a user or a mobile terminal (MT) can be exploited in a multiplitude of ways. Navigation services for both the consumer and professional market are probably the most well-known applications for positioning systems. Such services can be classified into the following categories:

Positioning: Determining solely the location of a person or object.

Tracking: Monitoring the movement of a person or object.

Navigation: Routing and guidance from an origin to a destination.

These categories are listed regarding increasing usage of auxiliary information and mutual dependency. As an example, tracking requires position determination of a target but usually also incorporates the movement history and a movement prediction of that target in order to achieve a more accurate estimation of the target's trajectory than independent sequential position measurements would. Maps, for instance, provide additional information about environment, especially the traffic infrastructure. This enables route planning, which together with accurate localization and tracking, is the the core of navigation applications. Mobile communications devices are equipped more and more with positioning capabilities that make information about mobiles' positions ubiquitous. The integration of positioning and communications in one device leads to an increasing number of location based applications and services. Service providers and end users are not the only ones who can benefit from added value of positioning information. Even network operators can take advantage from the knowledge of mobile devices. Spectrum is an extremely valuable resource and its availability is essential for wireless communications. Information about the position of the mobile communication devices allow an efficient usage of communication resources through the optimization of resource management, handover, or routing procedures.

Algorithms for communications systems, which for instance take into account the position of MTs in order to optimize the assignment of radio resources or location and context aware services, are typical examples that show the value of accurate positioning in different layers of a communications system. A simple example, shown in Figure 1.1 points out the added value of position information. An MT moves through an environment covered by a macro-cell with base station c01-math-0003 and a pico or femto hotspot cell c01-math-0004 . The hotspot cell, which could be a WLAN, provides a much higher data rate c01-math-0005 than the much bigger macrocell does ( c01-math-0006 ). At position A of the movement trajectory, the MT becomes aware of the hotspot and starts a handover procedure. This procedure takes some time, during which the MT has moved into the hotspot cell until position B but without getting the higher data rate c01-math-0007 . This data rate can be exploited from position B until position C, where the MT leaves the hotspot coverage area. Due to the small size of the hotspot, this probably happens too rapidly so there is not enough time for a seamless handover back to the macrocell. This handover is completed at position D. The waste of throughput due to handover latency depends on the size of the hotspot cell in relation to the speed of the mobile terminal. Information about the position of both the mobile device and the hotspot area enables us to predict entry into and exit of the hotspot cell and allows us increase the throughput.

Figure 1.1 Example: Handover procedure

Although not primarily designed for positioning, terrestrial communications systems can be used to obtain the position of a mobile terminal in a radio access network (RAN). Compared to positioning systems, the main part of transmitted signals are unknown at the receiver. This signal part is information to be transferred from the transmitter to the receiver. Nevertheless, today's wireless communications systems specify well-known signal components, called pilots, which are used in a receiver for synchronization and channel estimation purposes. Similar to positioning systems, these signal components can be used for propagation timing measurements and, therefore, positioning. As already mentioned, such systems are designed for communication. Here, the requirements for timing accuracy, in particular synchronization, are usually much weaker than for positioning.

This book focuses on the utilization of terrestrial wireless communications systems for positioning. Before we discuss this as the main topic in the next chapters, we introduce already existing radio positioning systems and environments that are critical for positioning systems to work in.

1.1 Ground Based Positioning Systems

Prior to satellite based positioning systems, radio signals transmitted from terrestrial stations were used for positioning purposes. One challenge for terrestrial positioning systems is to achieve a sufficient coverage and accuracy under certain constraints. Building up dense networks of terrestrial radio beacons either becomes expensive or even impossible, such as on the high seas. For maritime positioning in particular, it is obvious that transmitters for radio positioning have to cover relatively wide areas. Signals radiated in the long wave radio band are well suited to covering large areas. Signals mainly propagate as ground waves, that is, the electromagnetic waves follow the Earth's surface. This allows us to measure their traveling distances, the important figure for ranging, by their traveling times, basically the measurable signal value. This is in contrast to the short wave band, where signals propagate around the globe by sky waves. Sky waves are reflected multiple times between the ionosphere and the Earth's surface, depending on ionospheric properties. However, these reflections are less predictable and make reliable ranging impossible. Therefore, terrestrial wide area radio positioning systems operate at the long wave band ( c01-math-0008 ).

1.1.1 DECCA

The DECCA was developed by the British company Decca and deployed during World War II, mainly in the North Sea for maritime navigation in coastal regions. The Allied Forces needed a system for accurate landing operations. It was first used for the landing operation in Normandy in June 1944. On the day prior to D-Day, the first DECCA stations were switched on. Civilian use post World War II has been for fishing vessels or aviation. The system was shut down in Spring 2000.

A DECCA positioning system consists of a number of stations that are organized into so-called chains. A chain consists of a master station and usually three slave stations, which have been termed ‘Red’, ‘Green’, and ‘Purple’. Geometrically the slave stations are located at the vertices of an equilateral triangle. The master station position is the center of that triangle. The master-slave distance, that is, the baseline length, was about 60–−20 nm. Each station transmits a continuous wave signal. A receiver compares the phase difference of the master and a slave signal. Hyperbolas with foci at the master-slave (respectively) positions describe locations of equal signal phase differences. Three hyperbola patterns, associated with the three master-slave pairs ‘Red’, ‘Green’, and ‘Purple’ were drawn on nautical charts. Intersections of the hyperbolas resulting from phase difference measurements provided the position estimate.

It is not desirable for the four stations (one master and three slaves) to transmit their continuous waves using the same frequency. The signals would not have been separable at the receiver in that case. Thus, for simple signal separation, the stations used different frequencies. In order to provide simple phase relations for waveforms the frequencies of the stations in a chain had to be chosen properly. For that,