Local Positioning for Wireless Sensor Networks F. Ellinger1, R. Eickhoff1, R. Gierlich2, J. Hüttner2, A. Ziroff2, S. Wehrli3, T. Ußmüller5, J. Carls1, V. Subramanian4, M. Krcmar4, R. Mosshammer5, S. Spiegel6, D. Doumenis7, A. Kounoudes7, K. Kurek8, Y. Yashchyshyn8, C. B. Papadias9, P. Tragas, A. Kalis9, E. Avatagelou10 1
Dresden University of Technology, 01062 Dresden, Germany,
[email protected] 2 Siemens, CT PS7, 81730 Munich, Germany 3 Swiss Federal Institute of Technology (ETH) Zürich, Electronics Laboratory, Zürich, Switzerland 4 Microwave Engineering Group, Technical University of Berlin, Germany 5 Friedrich Alexander University of Erlangen-Nuremburg, Institute of Electronics Engineering, Erlangen, Germany 6 RIO Systems, Tel Aviv, Israel 7 SignalGeneriX Ltd, Limassol, Cyprus 8 Warsaw University of Technology, Poland 9 Athens Information Technology, Athens, Greece 10 Exodus, Athens, Greece
ABSTRACT — This workshop paper gives an overview of local positioning and tracking principles for wireless sensor networks including recent results of the European project RESOLUTION (Reconfigurable Systems for Mobile Local Communication and Positioning). Measurements of a first demonstrator applying a frequency modulated continuous wave (FMCW) radar principle are presented. The unlicensed ISM band around 5.8 GHz, 150 MHz bandwidth and less than 25mW effective isotropic radiated transmit power are used. Excellent 3-D positioning accuracies in the order of 4 cm in an anechoic chamber and 18 cm in a conference hall with strong multipath and area of 800 m2 are measured. Furthermore, the results of optimized radio frequency integrated circuits and a suitable compact flash card are outlined. Index Terms — Radar, FMCW, radio frequencies, local positioning, tracking, integrated circuits, BiCMOS.
I. INTRODUCTION Positioning based on radar waves has been extensively used since Word War II [1-2]. Today, increasing efforts are made to apply radar for local positioning in industrial and commercial wireless sensor networks [3-14]. Corresponding applications are outlined in Section II. RT1
RT2
RT3
MTi
RT4
RTn
Fig. 1. Illustration of wireless sensor network, RTi: reference terminals with known position, MTi: mobile terminals with position to be determined.
As depicted in Fig. 1, the unknown position of mobile terminals can be determined by means of reference terminals with known position. For 3-D positioning, at least 3 reference terminals are required. Further reference terminals improve the positioning accuracy and the coverage range. In Section III and Section IV, the most common positioning approaches, and the general functionality of positioning radar are discussed, respectively. Then, the recent insights and results gained in the framework of the ongoing EU project RESOLUTION [15-20] are presented focusing on two specific FMCW radar positioning systems. a. Transceivers with full processing/communication functionality applying synchronized frequency ramps are treated in Section V. First demonstration trials conducted in an anechoic chamber, and in a conference hall with strong multipath propagation, are presented. b. Simple, pulsed and modulated active reflector circuits with very low power consumption, low costs and circuit size are outlined in Section VI. The power consumption, form factors and costs of positioning systems can be decreased by using silicon integrated circuits [21]. In the framework of RESOLUTION, radio frequency integrated circuits (RFICs) optimized for positioning sensors are designed in 0.18 μm BiCMOS technology [22-25]. Corresponding results of first prototypes are included in Sections V and VI. In Section V, the results of an application specific compact flash card are included. II. APPLICATIONS Target applications within RESOLUTION are smart factories and interactive and cultural guiding. A smart factory is based on knowing the position of every workpiece, fabrication tool, transport machinery or maintenance worker at any time and everywhere. Position tracking managed by a global host enables efficient use of
fabrication tools and machines, optimization of material flow, reduced fabrication time and costs by high level of automation, collision avoidance and increased security. Fig. 2 outlines a typical application scenario within a fabrication hall. DUT
Excellent positioning accuracies in the order of 15 cm have been achieved on the time of arrival (TOA) approach based on UWB pulses [26]. However, due to the very low possible transmit power of UWB systems, the coverage ranges are typically limited to distances of around 10 m. TABLE I COMPARISON OF POSITIONING SYSTEMS
Transponder Term
Fig. 2. Smart factory application.
A second promising application is illustrated in Fig. 3. Realtime based active mapping, e.g. for advanced sightseeing, in museums, shopping malls and amusement parks are useful for guiding of pedestrians. The positioning is marked in a PDA based map and can be transferred to a server. Location-aware services can be applied to increase the amusement and information quality. The services can be combined with location-aware billing, which decreases personnel costs and improves the billing system.
Laser
System/ standard Individual
Ultrasonic
Individual
Satellite
(D)GPS Gallileo GSM UMTS
Receive strength Trilateralisation d of assigned basestations
Receive Strength
WLAN Bluetooth
Power ~1/d x≥2
UWB
WLAN UWB Aviation
Narrow pulses FMCW
Cell-ID
FM Radar
Principle
Accuracy@ max distance d 0.1-1cm@10m
Roundtrip time of flight
x,
5cm@5m 1m@100m >1m@unlimited Depending on basetation d 50m – 1km 5-100m
15cm@10m 30cm@1km
Comments Object must be located prior to measurement, dirt problems Strong air losses Dependent on service provider Dependent on service provider Low coverage range, d and environment have strong impact Low coverage range since low TX power Excellent 3D accuracy at relatively high d
GPS: Global Positioning System, Cell-ID: Cell-identification, UWB: Ultra-Wideband, FM: Frequency Modulation, GSM: Global System for Mobile communication, d: distance between device under test and transmitter Tracking Routing Guiding
Automation Enhanced guiding Augmented reality
Fig. 3. Interactive guiding application.
III. OVERVIEW OF POSITIONING APPROACHES In Table I, major positioning principles are outlined. Further information can be found in [1-2, 20]. The methods are mainly based on radio frequencies (RF), ultrasonic waves or light. Due to the widespread use of RF devices such as mobile phones or WLAN, RF positioning approaches can benefit by corresponding synergies.
Outdoor global Outdoor local
1k 100 10
GPS Galileo DGPS GSM/3G
Cell ID + GSM/3G
Pulse+ Field-strength + UWB WLAN, Bluetooth
Indoor
Fig. 2
RESOLUTION FMCW + WLAN
1 0.1
Coverage range [m]
10k
Positioning + mobile communication
0.1
0.3
1
10
30
100
1k
3k
Positioning accuracy [m]
Fig. 4. Positioning accuracy and coverage range of systems merging mobile communication and positioning.
Expanded coverage ranges at higher spectral efficiencies are feasible by means FMCW approaches, which are well known in the area of avionics. Novel positioning systems based on FMCW radar are developed in the framework of RESOLUTION. The design goals are as follows: 3-D positioning accuracy in the
centimeter range, operation in indoor environments with area of up to 1000 m2, real-time ability and self-sustaining operation independent on any external operator. There is the trend to combine radio-location and mobile communications yielding enhanced and novel services. Examples of such systems are: GPS and GSM/3G, Cell-ID and GSM/3G, field strength and WLAN, TOA and UWB. In Fig. 4, the positioning accuracies and coverage ranges of these merged systems are illustrated and compared with the objectives of the RESOLUTION approach.
components have a higher IF frequency than the target object. Third, we have to consider reflections at undesired objects not hitting the target object. Smart algorithms have to be applied to avoid corresponding measurement errors. Target
Undesired mutipath
IV. GENERAL FUNCTIONALITY OF FMCW RADAR
~ Ramp generator
TX mode d
LO Circulato
RX mode fRF
fLO
Several FMCW radar approaches are investigated in RESOLUTION. Among the most promising with respect to accuracy is the one based on synchronized frequency ramps. This approach employs equal hardware for both the transmitter and receiver unit. In Fig. 7, the corresponding generic architecture of a system unit is shown. The unit is capable to work both as transmitter and active reflector recovering the received signal. synchronisation f
fIF
T
4. STA1 mixes synchronized ramps for distance measurement
B=140MHz
fmax
fmin
measurement
1. STA1 transmits frequency ramp
fIF
BW
TX/RX
V. TRANSCEIVERS USING SYNCHRONIZED FREQUENCY RAMPS
Mixer
LP Filter
LO
Undesired target
Fig. 6. Types of reflections.
In Fig. 5, the basic approach of a FMCW positioning radar is illustrated. An oscillator is modulated by a ramp generator yielding a reference signal, which is transmitted and reflected back. The transmitted and reflected signals are denoted by LO and RF, respectively. VCO
REF2
Desired
REF1
T=0.5ms-1ms
Ts=0.5ms-1ms
Δt RF
t
2. STA2 mixes received frequency ramp with internal ramp
f
3. STA2 transmits synchronized ramp
Received signal 1
Fig. 5. Functional principle of FMCW radar for positioning, TX: transmitter, RX: receiver.
Due to the time delay Δt, the two signals have a frequency offset fIF, which can be extracted by mixing. Suppose that the mixer acts as frequency subtractor yielding f IF =f LO -f RF .
t
τ
T
ττ
Transmitted signal Internal signal
(1)
Attributed to the linear dependence between d, Δt and fIF, the distance can be determined by d ∼ f IF .
STA2 internal ramp for synchronisation
(2)
According to Fig. 6 we can identify three types of reflections. First, the desired one carrying the distance information. Second, unwanted multipath reflections. In the FMCW based approach, a major part of the mutipath components can be suppressed by lowpass filters, since the delayed multipath
Fig. 7. Functionality of synchronized frequency ramp approach, STA: station.
A. Measurements The synchronization and distance measurement approach employing these units is explained in Fig. 8. In addition to the positioning functionality, the system includes also WLAN functionality. A prototype test system has been measured in two environments: 1. Anechoic chamber, which can be seen as a best-case scenario revealing the lower limits of the approach.
2. Large conference hall with area of 800m2 and strong multipath components. This hall serves as a kind of worst-case scenario. By means of an automated measurement vehicle, thousands of measurements have been performed throughout the full area of the rooms.
A bandwidth of 150 MHz and less than 25 mW of effective isotropic radiated power (EIRP) has been applied in the unlicensed ISM band around 5.8 GHz. At a probability of occurrences of 1 σ, positioning accuracies of 4 cm and 18 cm have been measured in the anechoic chamber and the conference hall, respectively.
RF Frontend
Fig. 8. Architecture of system unit.
TABLE II MEASURED RESULTS RESOLUTION COMPARED TO UWB RESOLUTION Verified 800m2 Coverage Accuracy 4-18cm @1σ Req.uired
