A Survey of Indoor Positioning Systems for Wireless Personal Networks

A Survey of Indoor Positioning Systems for Wireless Personal Networks
IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
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A Survey of Indoor Positioning Systems for
Wireless Personal Networks
Yanying Gu, Anthony Lo, Senior Member, IEEE, and Ignas Niemegeers
Abstract—Recently, indoor positioning systems (IPSs) have
been designed to provide location information of persons and
devices. The position information enables location-based protocols for user applications. Personal networks (PNs) are designed to meet the users’ needs and interconnect users’ devices
equipped with different communications technologies in various
places to form one network. Location-aware services need to
be developed in PNs to offer flexible and adaptive personal
services and improve the quality of lives. This paper gives a
comprehensive survey of numerous IPSs, which include both
commercial products and research-oriented solutions. Evaluation
criteria are proposed for assessing these systems, namely security
and privacy, cost, performance, robustness, complexity, user preferences, commercial availability, and limitations. We compare the
existing IPSs and outline the trade-offs among these systems from
the viewpoint of a user in a PN.
Index Terms—Personal Networks, Indoor Positioning Systems,
Location Techniques.
I. I NTRODUCTION
CCURATE, reliable and real-time indoor positioning and
position-based protocols and services are required in the
future generation of communications networks [1], [2], [3].
A positioning system enables a mobile device to determine
its position, and makes the position of the device available
for position-based services such as navigating, tracking or
monitoring, etc. Location information of devices or users could
significantly improve the performance of wireless network for
network planning [4], network adaptation [5], load balancing
[6], etc. Some position-based indoor tracking systems have
been used in hospitals, where expensive equipment needs to
be tracked to avoid being stolen, and the patients can get
guidance to efficiently use the limited medical resources inside
complex environments of the hospitals. Indoor navigation
systems are also needed in a large public area to provide
position indications for the users. For example, tourists need
indoor navigation services in some large museums to see the
artifacts in different places in sequence. In addition, position information brings benefits to self-organization and selfformation of ad hoc networks in the future communications
systems.
The needs of users are highly addressed by the rapid
development of integrated networks and services in personal
networks (PNs) [7]. Much more attention has been paid to
context-aware intelligent services for personal use, which
A
Manuscript received 14 May 2007; revised 1 February 2008.
Yanying Gu, Anthony Lo and Ignas Niemegeers are with the Faculty
of Electrical Computer Engineering, Mathematics and Computer Science,
Delft University of Technology, The Netherlands e-mail: {Y.Gu,A.C.C.Lo,
I.G.M.M.Niemegeers}@tudelft.nl.
Digital Object Identifier 10.1109/SURV.2009.090103.
make the persons’ behaviors more convenient and simple.
Position information in indoor environments is of course an
essential part of the contexts. The uncertainty in dynamic and
changing indoor environments is reduced by the availability of
position information. And valuable position-based applications
and services for users in PNs are enabled by location context
offered by IPSs in various places such as homes, offices, sports
centers, etc.
Global positioning system (GPS) [8] is the most widely
used satellite-based positioning system, which offers maximum coverage. GPS capability can be added to various
devices by adding GPS cards and accessories in these devices,
which enable location-based services, such as navigation,
tourism, etc. However, GPS can not be deployed for indoor
use, because line-of-sight transmission between receivers and
satellites is not possible in an indoor environment. Comparing
with outdoor, indoor environments are more complex. There
are various obstacles, for example, walls, equipment, human
beings, influencing the propagation of electromagnetic waves,
which lead to multi-path effects. Some interference and noise
sources from other wired and wireless networks degrade the
accuracy of positioning. The building geometry, the mobility
of people and the atmospheric conditions result in multi-path
and environmental effects [9]. Considering these issues, IPSs
for indoor applications raise new challenges for the future
communications systems.
Some articles [1], [10], [11] have given an overview of
various available technology options for the design of an IPS
such as infrared (IR), ultrasound, radio-frequency identification (RFID), wireless local area network (WLAN), Bluetooth,
sensor networks, ultra-wideband (UWB), magnetic signals,
vision analysis and audible sound. Based on these fundamental
technologies, numerous IPSs have been developed by different
companies, research centers and universities. Each system
takes advantage of a particular positioning technology or
combining some of these technologies, which also inherits the
limitations of these technologies. The designers make tradeoff between the overall performance and the complexity of the
IPSs. In this paper, we systematically introduce and explain
various commercially available and research-oriented IPSs. We
also discuss the advantages and disadvantages of these IPSs
and compare them in terms of the services design for the users
in PNs.
The remainder of this paper is organized as follows. An
overview of indoor positioning systems is presented in Section II. In Section III, we describe 17 existing IPSs and classify
them into 6 categories according to their main medium used
to sense location. The advantages and disadvantages of each
c 2009 IEEE
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Fig. 1.
IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
Personal Network
of the IPSs are also included. Section IV evaluates each of
the IPSs from the viewpoint of PNs. Finally, Section V summarizes our work and presents recommendations for future
work.
II. A N OVERVIEW OF I NDOOR P OSITIONING S YSTEM FOR
P ERSONAL N ETWORKS
In this section we describe and explain IPSs and personal
networks. We address why PNs need position information.
The existing IPSs are classified. Various evaluation criteria
are proposed to compare them for the services demanded by
the users in PNs.
A. What is a Personal Network?
To meet the demands of users, personal networks (PNs)
[7], interconnect various users’ personal devices at different
places such as home, office, vehicle, etc., into one single
network, which is transparent to the users, as shown in
Figure 1. Through PNs, users can have global access to
public and personal services in different types of networks
with their personal devices. Personal devices may be equipped
with different cellular and wireless networking technologies
including wireless personal area network (WPAN), WLAN
and the third-generation (3G) cellular networks. PNs connect
personal devices with different networking technologies and
form dynamic, private and secure networks. Thus PNs with
the user-centric perspectives can facilitate personal ubiquitous
communications anywhere and at anytime.
The success of PNs is highly dependent on the optimal
organization of the personal devices to achieve efficient communication over various types of communications networks.
Using different networking technologies, personal devices in
each place form a personal area network (PAN), a vehicle
area network, a home area network, a company area network,
etc. Personal devices in the same or different places should
cooperate with each other to form one single network for
the user. Thus interconnecting numerous types of networks
enables personal devices in these networks to communicate
with each other and offer flexible personal services.
B. Why does a PN require an Indoor Positioning System?
In order to meet the user’s needs and offer adaptive and
convenient personal services, the location information of the
persons and their devices at different places such as home,
office, etc., can be provided by the IPSs to any applications
in PNs. Although the GPS system can provide location information for users in outdoor environments, GPS can not
give accurate positioning estimations for indoor use. Thus,
IPS is required to support location-based services when the
PN is located in indoor area. The need of IPS in PN is
further illustrated by two typical scenarios, namely fitness
center and conference. These scenarios are selected from a
set of scenarios envisioned by the Information Societies and
Technologies (IST) MAGNET Beyond project [15].
1) Fitness Center Scenario: Many people are keen to keep
a healthy lifestyle by working out in a fitness center. A person
named John is in his early 20’s and would like to exercise for
the purpose of losing weight. He has thus been a member
of the local fitness center for the past two years. Today
is one of his two weekly exercising days. As John enters
the fitness room, his personal mobile device estimates his
location that he is in the fitness center, which enables locationbased services to provide the information of all networkenabled fitness equipment and displays this to John. Before
commencing his fitness program, he needs to be weighed
in order to track physical changes (i.e., weight loss) over
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GU et al.: A SURVEY OF INDOOR POSITIONING SYSTEMS FOR WIRELESS PERSONAL NETWORKS
time. He steps onto a scale capable of communicating with
his mobile device. Based on the location context that he is
on the scale, the personal service can communicate with the
scale to get the measured weight and save it in his personal
database for further comparison and exercising guidance. John
can use various equipment in the fitness center. When he uses
any equipment, John.s location information is required by the
personal service to offer an adaptive personal training guide.
For example, John steps on the threadmill, then the personal
service automatically detects the position of John and monitors
his heart rate during running. If his heart rate is too fast, which
means John can not afford the running speed, the service will
inform the threadmill to decrease the speed.
2) Conference Scenario: The conference is a typical scenario, which needs location information to offer flexible services for the users. For example, a journalist named Lily is
visiting a technical conference. As she enters the event area,
the local services can get her location information. Based on
the location context information that she is in the service
range of the local services, the services communicate with
the devices carried by Lily and offer service-related information. Lily’s devices receive the service-related information
so that she can use various types of services provided by
the conference. Using IPSs, an indoor navigation service can
be provided to Lily to find the right presentation room in a
conference hall. Her devices can be monitored and tracked by
the positioning system to avoid them being stolen by other
people. Furthermore, the temporary wireless network is selforganized based on the location information of the devices of
the user to offer convenient and secure personal services. For
example, Lily has the right of using the printing services in
the conference, if her PN is formed and connected with the
conference network so that a document in her laptop can be
printed by a printer provided by the conference organizers.
Through the use cases, the location context awareness
should be implemented in PN services, which offers comfort
and efficiency to the end-user. However, IPSs enable locationbased services and applications in PNs, which also raise
significant security and privacy risks [16]. For example, in
the fitness center scenario, when the user is out of the fitness
center, he or she does not want the services to track him or
her any more. So the location-aware services are required to
ensure users’ privacy.
C. What is an Indoor Positioning System?
An indoor positioning system (IPS) considers only indoor
environments such as inside a building. The location of users
or their devices in PNs can be determined by an IPS by
measuring the location of their mobile devices in an indoor
environment. Dempsey [13] defines an IPS as a system that
continuously and in real-time can determine the position of
something or someone in a physical space such as in a
hospital, a gymnasium, a school, etc. [1]. From this definition,
an IPS should work all the time unless the user turns off
the system, offer updated position information of the target,
estimate positions within a maximum time delay, and cover
the expected area the users require to use an IPS.
An IPS can provide different kinds of location information
for location-based applications required by the users. The ab-
Fig. 2.
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Location-aware Computing System Architecture
solute location information is provided by some IPSs. Before
the position can be estimated, the map of the locating area
such as an office, a floor, a building, etc., should be available
and saved in the IPS. With respect to the map, the absolute
position of a target can be measured and displayed. Usually,
the absolute position information with respect to the map
of a coverage area is offered by indoor positioning tracking
systems and indoor navigation systems, because tracking and
guiding services need the exact positions of the targets. The
relative position information is another kind of outputs offered
by the IPSs, which measure the motion of different parts of
a target. For example, an IPS which tracks whether the door
of a car is closed or not, needs to give the relative position
information of the tracked point on the door with respect to
the body of the car. The third kind of position information
is proximity location information, which specifies the place
where a target is. Sometimes, IPSs do not need to provide
absolute or relative position information. The position monitoring and tracking systems in hospitals are such examples.
The IPS should provide the room where a patient is. Thus
location-based applications in hospital can monitor whether
the patient enters a correct room for diagnoses or operations.
The success of IPSs is starting to enable the locationaware computing systems in indoor situations. The system
architecture of the location-aware computing systems [17]
is illustrated in the Figure 2, which includes 3 layers, the
location sensing systems, the software location abstractions
and the location-based applications. At the location sensing
systems layer, different location sensing technologies are
used to perform measurements of the location of the users
and their devices. The software location abstractions layer
converts the data reported from the location sensing systems
layer into a required presentation of the locations [18]. An
example of the software location abstractions layer is the Java
location application programming interface (API) [19]. The
Java location API can produce the location information of
targets in a standard format and provide access to a database of
landmarks. Thus the developers can use this Java location API
to develop location-based applications for resource limited
devices. Moreover, the location-based applications, such as
navigation and geographical advertising [20], are implemented
at the highest layer, which use the location context information
measured and calculated by the lower layers.
D. Location Technologies, Location Techniques and Location
Algorithms
As the need of IPS is to enable location-awareness in
computing systems, a number of wireless technologies have
been developed for indoor location sensing. These technolo-
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Fig. 3.
IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
Triangulation Positioning Techniques
(a) The Experiment Environment
gies include IR, ultra-sound, RFID, WLAN, Bluetooth, UWB,
magnetic technology, etc. Each technology has unique advantages in performing location sensing for indoor use. For
example, an IPS using WLAN technology does not need new
infrastructures, because it can reuse the devices equipped with
WLAN technology, which are widely deployed. At the same
time, they have some limitations because of their properties.
Various IPSs using one or combining two of these wireless
technologies will be presented in detail in the following
sections. And the influence of the wireless technologies on
the performance of these IPSs will be discussed.
Equipped with one or several location technologies, IPSs
use location techniques to locate objects and offer absolute,
relative and proximity location information. There are four
techniques for indoor position estimations: triangulation, fingerprinting, proximity and vision analysis [10], [50]. Triangulation, fingerprinting and vision analysis positioning techniques can provide absolute, relative and proximity position
information. The proximity positioning technique can only
offer proximity position information. In the design of IPSs,
some IPSs use one positioning technique; others combine
some of these positioning techniques to compensate for the
limitations of single positioning technique.
Based on the geometric properties of triangles, three methods can be used to calculate the position, namely received signal strength (RSS), angle of arrival (AOA) and time of arrival
(TOA) [1]. The basic principle of triangulation method for a
2-D position measurement is demonstrated in Figure 3. If the
geographical coordinates (xi , yi ) of three reference elements
A, B, C are known, the absolute position E1 can be calculated
by using either the length [10] or the directions [10] of R1 , R2
and R3 . Based on the information of the coverage area of an
IPS, absolute, relative and proximity position information can
be provided by the IPS using the triangulation method. Each
triangulation method has advantages and limitations. TOA is
the most accurate technique, which can filter out multi-path
effects in the indoor situations. However, it is complex to
implement [10]. RSS and TOA need to know the position of
at least three reference elements, such as A, B, C in Figure 3,
to estimate the position of an object. AOA only requires two
position measuring elements to perform location estimation.
However, when the target object to be located is far away, the
AOA method may contain some errors, which will result in
lower accuracy [21].
Fingerprinting positioning technique is proposed to improve
the accuracy of indoor position measurements by using pre-
(b) Signal Strength for AP1
Fig. 4.
Fingerprinting Positioning Techniques [23]
measured location related data. Fingerprinting includes two
phases: offline training phase and online position determination phase [50]. In the offline phase, useful location related
data with respect to different places in the position estimation
area is measured and collected for the position estimation.
During the online position determination phase, the location
related data of a target object is measured and compared with
the pre-measured data collected in the offline phase to get a
similar case in the database to make the location estimations.
For example, in an IPS [23], WLAN technology is used in the
position estimation. In Figure 4 (a), three access points (APs)
are fixed in the different places in an area of 25 m x 25 m. In
the offline phase, a laptop equipped with a WLAN card was
moved to various sample points to measure the strength of the
signals received from different APs, as shown in Figure 4 (a).
These pre-measured signal strength values are used to make
the fingerprinting maps of the area with respect to different
APs. Figure 4 (b) shows the received signal strength from
AP1 with respect to various sample points in the IPS working
area. In the online position determination phase, based on
the fingerprinting maps of the area, the IPS [23] uses the knearest-neighbours location algorithm [22] to locate the target
node.
The proximity location sensing technique examines the
location of a target object with respect to a known position
or an area. The proximity location technique needs to fix a
number of detectors at the known positions. When a tracked
target is detected by a detector, the position of the target is
considered to be in the proximity area marked by the detector.
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GU et al.: A SURVEY OF INDOOR POSITIONING SYSTEMS FOR WIRELESS PERSONAL NETWORKS
Fig. 5.
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Proximity Positioning Technique
As shown in the Figure 5, E2 and E3 are the tracked targets.
A proximity area of the detector D is specified and shown
by the dotted square in the Figure 5. E2 and E3 are located
by monitoring whether they are in the proximity area or not.
Thus the target E2 is in the area of D and E3 is not. Thus the
proximity location sensing technique can not give absolute or
relative position estimations as with the other three positioning
techniques. The proximity location information provided is
useful for various location-based services and applications. For
example, a sensing area of a location measuring element is a
room. Thus proximity sensing can accurately specify whether
a tracked target is in the room or not.
The vision analysis estimates a location from the image
received by one or multiple points [10] as shown in Figure 6.
Vision positioning [84]-[87] brings the comfort and efficiency
to the users, since no extra tracked devices are needed to
be carried by the tracked persons. Usually, one or multiple
cameras are fixed in the tracking area of an IPS to cover
the whole place and take real-time images. From the images,
the tracked targets are identified. The observed images of the
targets are looked up in the pre-measured database to make the
position estimations. In addition, vision positioning technique
can provide useful location context for services based on
the captured images. For example, in Figure 6, the vision
positioning technique can observe that the girl is sitting on
her sofa and using her laptop.
The location algorithms are specifically designed to specify
how to calculate the position of a target object. For example,
in the triangulation technique, when the distance between a
target object and each reference point is obtained, the location
algorithm calculates the location of the object. Researchers
have developed various location algorithms to improve the
accuracy of location calculation. The accuracy of the location
information depends on whether the location data, such as
the distance between a target object and each reference point,
contains errors or not. If the initial location data includes a
mix of correct and erroneous data, a priori knowledge of an
IPSs behaviors and the properties of the coverage area are
needed to improve the accuracy.
E. How to Classify Indoor Positioing System?
The IPSs can be categorized according to different criteria.
One way to classify them is based on whether an IPS uses an
existing wireless network infrastructure to measure the position of an object. The IPSs can be grouped as network-based
approach and non-network-based approach. The networkbased approach takes advantage of the existing network
infrastructure, where no additional hardware infrastructure
is needed. For cost reasons, the network-based approach
Fig. 6.
An Example of Image used in Vision Positioning Technique
is preferred. However the non-network-based approach uses
dedicated infrastructure for positioning and has the freedom
of physical specifications by the designers, which may offer
higher accuracy.
Another way of classifying IPSs is on the system architecture. There are three kinds: self-positioning architecture, infrastructure positioning architecture and self-oriented
infrastructure-assisted architecture. Self-positioning calculates
the positions by the targets themselves and takes advantage of
the infrastructures of positioning systems, which provide high
security and privacy. The infrastructure positioning estimates
the positions of the targets using the infrastructures, which
can automatically track the position of devices if they are
in the coverage positioning area. In the self-oriented and
infrastructure-assisted architecture, a tracked target sends a
request to the positioning system to start the position measurements, and then gets its location information from the system.
The key point of the third architecture is that unless the device
allows a positioning system to track it, no positioning activities
for the device can be carried out.
In this article, we classify the IPSs based on the main
medium used to determine location, which include six categories: IR signals, ultrasound waves, radio frequency, electromagnetic waves, vision-based analysis and audible sound.
In Section III, we will explain and compare the advantages
and disadvantages of these media used in indoor positioning.
And numerous IPSs in each category will be introduced.
F. What are the Criteria of Evaluating Indoor Positioning
Systems for PNs?
To evaluate the IPSs for PNs, various important system performance and deployment criteria are proposed and described
in this section. These criteria are proposed fully focusing on
user preference and experience, and are used to evaluate if
IPSs can meet the need of users in PNs.
1) Security and Privacy: Security and privacy [24]-[28]
are important issues for IPSs in PNs, because PNs focus on
the needs of users. private and social activities, who want to
have full control of the usability of their personal location
information and history. The user cares if someone tracks
him/her and gets his/her history of all past activities. Controlling access to the location information [29] and distribution
of the information [11] can improve the privacy in IPSs. The
enhancements of security and privacy could be carried out
from the software side and system architecture side [23]. For
example, self-localized position system architecture [11] can
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IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
ensure the privacy by performing location estimations in the
target device. Unless the target device gives its location information to an entity, no one can access the information. Thus
IPSs with self-localized location computation architecture can
offer a high degree of security and privacy for the users in
PNs.
2) Cost: The cost of an IPS contains several parts: the cost
of the infrastructure components, the cost of a positioning
device for each user and the cost of system installation and
maintenance. Some positioning systems, such as GPS, have
a large infrastructure to support the location measurement,
which is expensive and complex. Some IPSs reuse the existing infrastructures such as WLAN, are more cost-effective,
because there is no extra cost incurred by the infrastructure
of IPSs. The cost of every positioning device at the user’s
side contains the device and maintenance cost, which are
important for an individual person. Often, the device cost is
specified when a person buys the device and starts to use
the service of an IPS. But the users do not consider much
about the device maintenance cost such as the battery cost
and life time. For example, a device with longer battery life
time needs less frequency of changing the batteries and lower
maintenance cost. Positioning devices with self-positioning
calculation ability are preferred to offer privacy of the end
users, which raise the price of the devices and decrease the
battery life time duration, because the devices are responsible
for more complex positioning calculations. In addition, the
cost of a positioning system installation and maintenance
needs to be addressed for the long-term use of the system.
Some IPSs include extra infrastructure to be installed such
as sensor-based positioning systems, which need complex and
expensive installations of fixing various sensors in different
places in indoor situations. And some IPSs need professional
engineers to support the operation of the IPS, which brings
the cost of system maintenance higher.
The cost of the system can be addressed in different ways.
Time and space costs are also factors indicating the efforts
for the operation of an IPS. The time cost involves the time
requirements of system installation and the time length of the
positioning system in case of the system fails to work because
of some serious faults. Space cost contains requirements of
the size and the place, where the installed infrastructure
components and user devices occupy. A large positioning
device is not convenient for a user to carry it in his/her daily
lives.
3) Performance: The accuracy and precision are two main
performance parameters to evaluate an IPS, where the accuracy means the average error distance, and the precision
is defined as the success probability of position estimations
with respect to predefined accuracy. Moreover, the delay of
an IPS is another performance aspect, which contains the
delay of measuring, calculating positions of estimated target
and forwarding position information to the requesting parts.
There are two reasons: one is that the tracked target may
move quickly; another one is that indoor environments are
also dynamically changing. Scalability, defined as the number
of objects that an IPSs can locate with a certain amount
of infrastructure devices and within a given time period, is
another issue of the performance evaluations for IPSs. A
stable IPS, which can simultaneously locate a large number of
objects, is preferred. For example, the orientation calculation
of an object is required in a motion tracking application, which
needs at least three, non-collinear, located targets mounted
on the object to perform orientation calculation. Thus the
deployed IPS needs to simultaneously locate, at least, three
targets and offers higher scalability for the location sensing
and location-based applications.
The performance of an IPS should be evaluated in order
to examine whether it meets the requirements of the locationbased services and applications in PNs or not. For example, an
application for PNs in the home only needs room level accuracy. So the positioning system should offer the information,
in which room person A is. Some indoor spaces cover only
one floor, but others may have multiple floors. A positioning
system offering 2-D position estimations can not meet the
requirements of giving specifications of which floor the target
is. Depending on the needs of users, 3-D positioning systems
are preferred in some cases.
Usually, there is a trade-off between the price and the
performance of an IPS. A system has higher performance
also has higher cost. For example, the accuracy of an IR
positioning system can be improved by adding filters to reduce
the influence from florescent light and sunlight. However, the
price of the whole system is increased because of these extra
filters [11].
4) Robustness and Fault Tolerance: A robust IPS should
be able to keep on operation even in some serious cases
such as some devices in the system are malfunctioned, or a
mobile device runs out of battery energy. For example, the IR
positioning technique needs line-of-sight signal transmission
between the emitters and the tags. In the Active Badge system
[11], a user wears an active badge. If the badge is covered by
his/her thick clothes, it can not get location information from
the system, since the line-of-sight communications are not
possible between the active badge and the emitters. Thus for
these serious situations and faults in the system, the positioning system should still offer positioning services. In a sensorbased positioning system, if some sensors in a public area
are stolen, the positioning system should still provide position
information, which may have a lower accuracy, because the
number of sensors in the measurement is reduced. In addition,
the services and applications design for PNs needs to consider
the situation that the location information of users and devices
are not available, and the location related components do not
stop functioning and can work in another way to support the
demands of the users.
5) Complexity: An aspect of the complexity of IPSs is
about the human intervention/efforts during the deployment
and maintenance of the IPS. For IPS deployment, a rapid
set-up of a system is required with a low number of fixed
infrastructure components and easily used software platform
for the users. Another issue during the IPS deployment is
to enable optimum performance, such as accuracy, in each
part of the entire deployment space [30]. For example, the
WLAN-based IPSs reuse the existing access points (APs) of
WLAN as reference locations and positioning measuring units,
which do not need much infrastructure installation. Proper
signal coverage should be offered by an IPS to cover all the
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desirable area, so that the performance can be ensured over
all the coverage area [30].
Another aspect of the complexity indicates the required
computing time of the device carried by the user to determine
his/her position. Because of the limited CPU processing and
battery power of the mobile devices, an IPS uses positioning
methodology with lower calculation complexity is desired.
Using positioning system in a large space requires the
system to be scalable with the increasing size of working
area. For example, a positioning system deployed in a large
building having many floors should not contain a large number
of infrastructure devices and need time-consuming installation
and maintenance. If the number of tracked components is
large, an IPS should have the ability of offering precise
location measurements for these elements at the same time.
In addition, the sensing rate of positioning systems should
be enough for the fast moving devices in complex indoor
environments.
6) User Preference: Since personal networks are defined
and developed for the needs of users, the IPSs should consider
the users’ requirements of the tracked devices, infrastructures
and software. For the comfort of the users, the devices should
be wireless, small, light weight, lower power consumption and
computational powerful to offer rapid, accurate and real-time
positioning services. For example, in some IPSs, tags are taken
by persons to track their positions. Thus these tags should be
easily wearable and fulfil the requirements described above.
In addition, the infrastructure components and software used
by people should be easily learnt and user interface friendly.
7) Commercial Availability: Among the existing IPSs,
some are commercially available, and others are researchoriented, which are not available in the market. For the
commercially available products, we can buy their devices
and deploy the positioning systems. The designers consider
and address multiple aspects of an IPS to make it popular
in the market. But most of the companies keep the working principles of their commercial IPSs as secrets due to
the competition among companies. For the research-oriented
positioning systems, we can know their design details clearly,
which is valuable for the future improvement of IPSs.
8) Limitations: Although the proposed IPSs have achieved
various valuable improvements, they still have some limitations due to the positioning technology and other issues
in the systems. One of the fundamental limitations is the
medium used in position sensing. For example, using WLAN
technology in positioning systems leads to great interest in
the design of IPSs, because the system can reuse the existing
infrastructure of WLAN and reduce the cost of positioning
services. However, the radio frequency based positioning has
multi-path and reflection effects resulting in a relatively higher
error range. Another kind of limitations is the scope provided
by the positioning systems. For example, some positioning
systems only cover a short range. For large areas, these
systems are not scalable. Another example is that some positioning systems are designed for a small number of persons
or devices simultaneously using the positioning estimation
services, which can not afford a large number of targets. In
addition, some potential limitations should be considered in
the evaluation of the positioning systems. For instance, using
19
a positioning system may influence the performance of other
wireless communications systems.
III. I NDOOR P OSITIONING S YSTEMS FOR P ERSONAL
N ETWORKS
Many positioning systems have been developed over the
years for indoor location estimations. We introduce a variety
of IPSs in this section. The location technology and technique
used in each IPS are addressed to give a scientific overview of
the system. Since the evaluation of these IPSs is focusing on
the need of users in PNs, these IPSs are explained according
to the criteria and requirements as specified in the subsection II-F. Thus we can know the advantages and limitations
of these IPSs from the view of users in PNs.
A. Infrared (IR) Positioning Systems
Infrared (IR) positioning systems [30]-[41] are the most
common positioning systems, because IR technology is available on board of various wired and wireless devices, such
as TV, printer, mobile phones, PDAs, etc. An IR-based positioning system, which offers absolute position estimations,
needs line-of-sight communication between transmitters and
receivers without interference from strong light sources [30].
Thus the coverage range per infrastructure device is limited
within a room. In this section, we describe some IR-based
IPSs.
Active Badge: The Active Badge system [32]-[35] is one
of the first indoor badge positioning systems designed at
AT&T Cambridge in 1990s, which covers the area inside
a building and provides symbolic location information of
each active badge such as the room where the active badge
is. The Active Badge system uses diffuse IR technology to
realize location sensing [32]. By estimating the location of
the active badges taken along with the persons, the Active
Badge system can locate persons in its coverage area. An
active badge transmits a globally unique IR signal every 15
seconds [32]. In each located place such as a room, one
or more sensors are fixed and detect the IR signal sent by
an active badge. The position of an active badge can be
specified by the information from these sensors, which are
connected with wires and forwards the location information
of the tracked active badges to a central server. Based on
the location information, some location-aware applications can
be designed. For example, a location tracking application for
helping a telephone receptionist has been proposed in [32].
Using the measured location of the employees in the building,
the application displays a table onto a PC, which contains the
names of these employees, their location (room numbers) and
the nearest telephone. Thus, using the location information
from the application, the user, a telephone receptionist, can
forward the phone call to the expected employee.
Although the price of active badges and networked sensors
are cheap, the cables connecting sensors raise the cost of the
Active Badge system. The active badges taken by persons to
locate themselves are light weight and have acceptable size. If
the transmission frequency of an active badge is about every
15 seconds, the battery life time is half to one year, which is
convenient for the users.
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IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
Fig. 8.
Fig. 7.
Firefly Motion Tracking System Architecture [37]
The Active Badge system is an old project, which has
been closed down. There is no commercial product of the
system available. The system provides room level accuracy.
However, the coverage range of IR signal is about several
meters, positioning in a large room such as lecture room, needs
more than one networked sensor to cover the whole area. An
IR signal is influenced by fluorescent light and sunlight.
Firefly: Firefly designed by Cybernet System Corporation
is an IR-based motion tracking system [36], [37]. The Firefly
system uses IR tracking technology to offer high accuracy.
The Firefly system animates complex motion of an object
by locating the small tags emitting IR light and mounted on
the object. The 3-D position information, which is generated
by the system, can be used as an input to track the motion
of moving objects. The position information can be used
by virtual reality applications, such as computer games and
computer animation. Since the Firefly system is a commercial
product: its location techniques and algorithms are proprietary
and have not been published, and so can not be described in
this article.
As shown in Figure 7, the Firefly system contains a tag controller, tags and one camera array. The tag controller is carried
by the tracked person, and it is a small, light weight (about
425 g) and battery-powered. Tags are IR emitters, which are
supported by tag controller and mounted on different tracked
parts of the person. Three cameras installed on a 1 m bar
as a camera array receive the IR signals sent by tags fixed
on different parts of the person.s body, and estimate the 3D position of them. In addition, tags are much smaller than
coins.
The Firefly system can offer a high level accuracy of about
3.0 mm. The position tracking is carried out in a high-speed
and real-time way with measurement delay of 3 ms and
sampling rate of 30 scans per second, if 30 tags are tracked.
The system is easy to install and maintain. The cost of a Firefly
system with a camera array, one tag controller and 32 tags is
$27500.
Although the tag controller and the tags are small and
portable, they are not comfortable to be worn in our daily
lives, because they are connected using cables. The system
can only operate correctly in a normal lighting environment.
In addition, the coverage area is limited to within 7 m and the
OPTOTRAK PROseries System
field of view is 40◦ × 40◦ . Thus the system is not suitable for
the implementation in a large public area such as a shopping
mall.
OPTOTRAK PROseries: OPTOTRAK PROseries [38] system is one of the IPSs designed by Northern Digital Inc. for
congested shops and workspaces. The OPTOTRAK system
uses a system of three cameras as a linear array to track 3D positions of numerous markers on an object. As shown in
Figure 8, the optical tracker includes 3 cameras can cover a
volume of 20 m3 , and a maximum distance between tracked
targets and the tracker is about 6.0 m. The system is a type of
active system, where markers mounted on different parts of a
tracked object emits IR light that is detected by the camera
to estimate the location of them. The triangulation technique
is used in the positioning process to calculate the positions of
IR light emitters in the space.
OPTOTRAK system takes advantage of dynamic referencing, which is used to automatically compensate the movement,
to measure relative motion. There is an example of dynamic
referencing in Figure 8, where three emitters A, B and C
mounted on the surface of a car form a dynamic reference
with static relative positions, and the tracked emitter E fixed
on the car door can be measured with relative position changes
with respect to the formed dynamic reference. Thus even if
the car is slightly moving, opening the door of the car can be
accurately measured.
The system can offer a high accuracy of 0.1 mm to 0.5 mm
with 95% success probability [39]. The IR emitters used in the
system are small and light-weight with a diameter of 16 mm
and the weight is 6 g. The OPTOTRAK system still covers
a limited area (20 m3 ). A disadvantage of OPTOTAK system
is the line-of-sight requirement between the objects and the
tracking system. By using a large number of IR markers, this
problem can be partly solved.
Infrared Indoor Scour Local Positioning System
(IRIS LPS): Infrared Indoor Scour Local Positioning
System (IRIS LPS) [40] is an optical IR local positioning
system. Cheap stationary mounted stereo-cameras receive
IR signals from a tag carried by a target object to measure
the angle of arrival and calculate the location of the tag by
triangulation technique. The IRIS LPS was tested in a lecture
hall covering an area of 15 m × 9 m. Two cameras with IR
filter and 120◦ wide angle lenses are mounted on a rail with
a distance of 20 cm, which are fixed on the wall at a height
of 3 m. And the cameras are connected to a computer, which
extracts and processes the data to estimate the position of
an object. In this situation, the system can offer accuracy
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of about 16 cm covering 100 m2 , which is larger than the
covered areas of Firefly and Optotrak.
The system is cheap and easy to install and maintain.
The system can support multiple tags being tracked at the
same time. Although the IRIS LPS can cover a larger area
than Firefly and Optotrak, it results in less accurate position
estimations, which shows a trade-off between accuracy and
coverage area. This IPS can locate an object in a still mode
with acceptable accuracy. For the moving object, the system
needs to be improved to offer real-time motion tracking.
Summary of IR-based Positioning Systems: Using IR signal
in the measurement of locations, the systems described in this
section have some common advantages. The IR-based systems
perform positioning estimations in a very accurate way. IR
emitters are small, light-weight and easy to be carried by a
person. The system architecture is simple, which does not need
time-consuming installation and maintenance.
However, there are still some disadvantages with these
indoor IR positioning systems. There are little considerations
of security and privacy issues in the design of IR-based
positioning systems. IR signals have some limitations for
sensing location, for example, interference from florescent
light and sunlight [31]. This problem can be solved by using
optical and electronic filters to reject the disturbance from the
light sources [41], and implementing noise cancelling signal
processing algorithm at the receivers [31], which raise the
cost of the positioning system. Another disadvantage is the
expensive system hardware requirements. Although the IR
emitters are cheap, the whole system using camera array and
connected via wires is expensive comparing to the coverage
area. There should be a transmitter or receiver in every
measured place such as a room equipped with at least one IR
device to locate whether the target persons or devices are in
the room or not. These transmitters or receivers fixed in each
place are connected using special wire. In addition, when an
IR device taken by a person is covered by his/her clothes,
the system fails to work since the IR wave can not penetrate
opaque materials.
B. Ultra-sound Positioning Systems
Using ultrasound signal [42]-[48] is another way of position
measurement. Ultrasound signals are used by bats to navigate
in the night, which inspire people to design a similar navigating system in the last hundreds of years. In this section, several
ultrasound positioning systems are introduced and their design
principles and aims are addressed.
Active Bat: Active Bat positioning system [42] designed by
researchers at AT&T Cambridge provides 3-D position and
orientation information for the tracked tags. The Active Bat
System uses ultrasonic technology and triangulation location
technique to measure the location of a tag carried by a person.
A tag periodically broadcasts a short pulse of ultrasound.
The short pulse of ultrasound is received by a matrix of
ceiling mounted receivers at known positions as shown in
Figure 9. The distances between the tag and the receivers
can be measured by the ultrasonic waves. TOA. Since all
receivers are mounted on the ceiling, the tags are below the
receiver matrix. The distance between a tag and three receivers
Fig. 9.
21
Active Bat System
is needed to calculate the 3-D position of the tag based on the
principles of multilateration [43].
Tags are small and convenient tracked devices carried by
persons with a volume of 7.5 cm × 3.5 cm × 1.5 cm. In the
testing of the Active Bat system, the active tag is powered by
a single 3.6 V Lithium Thionyl Chloride cell with a life time
of around 15 months. So the users do not need to frequently
change the batteries. 720 receivers are fixed on the ceiling to
cover a 1000 m2 floor, where 75 tags can be tracked with an
accuracy of about 3 cm for 95% of the measurements. Each
central controller can locate 3 tags at the same time with
50 times per second. In the maintenance phase, the battery
voltage of each tag is monitored by the central controller,
which is wired to all the receivers, to know the condition of
the battery. Thus the battery voltage change does not influence
the accuracy of position estimations.
However, the performance of this technology is influenced
by the reflection and obstacles between tags and receivers,
which degrades the system accuracy. From the view of a user,
deploying a large number of sensors on the ceiling in each
room is a time-consuming task, which degrades the scalability
of this system. The receivers also need to be accurately placed,
which results in complex and costly installation.
Cricket: Cricket system [44], [45] is a location system with
the aim of offering user privacy, efficient performance and
low cost. The cricket system uses TOA measuring method
and triangulation location technique to locate a target. The
cricket system includes ultrasound emitters as infrastructure
attached on the walls or ceilings at known positions, and a
receiver mounted on each object to be located. This approach
provides privacy for the user by performing all the position
triangulation calculation locally in the located object. Thus
the located object owns its location information and can
decide how and where to publish its location information. The
emitters also transmit RF messages for synchronization of the
TOA measurement and forwarding their location information
in a decentralized fashion. Thus when there are not enough
emitters for the triangulation location calculation, the receiver
can use the semantic string forwarded by the radio link to get
proximity location information.
The Cricket system addresses the issues of fault tolerance by
using RF signals as a second method of proximity positioning
in the case of not enough emitters available. Unlike the Active
Bat system using a grid of receivers, the Cricket system uses
less number of emitters fixed on the ceiling, because the target
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IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
Fig. 10.
Radio Frequency and Ultrasound Signal Comparison inside a
building from a top view [46]
object receives and processes the ultrasound signals to locate
itself. Thus the system is scalable for large area deployment
inside a building. And the object receiver is cheap, about
$10. So the cost of the whole system is low. Moreover, the
Cricket system can provide a position estimation accuracy
of 10 cm and an orientation accuracy of 3◦ . However, the
located receivers in the system perform location estimations
and receive both ultrasound and RF signal at the same time.
Thus a receiver in the cricket system consumes more power,
and its power supply needs to be designed in an efficient
way to bring convenience to the users in stead of frequently
changing batteries in the receiver.
Sonitor: The Sonitor ultrasound IPS [46] is an indoor tracking and positioning solution provided by Sonitor Technologies
Inc. The Sonitor system can locate and track people and
devices in real-time and offer proximity location information
with room level accuracy. The ultrasound signals are suitable
for room level location tracking. Comparing with the coverage
range of a radio frequency signal as shown in Figure 10, the
ultrasound signal can give a simple and accurate solution for
room level positioning, because the ultrasound signals can
not penetrate through the walls. Unlike Active Badge, the
ultrasound technology does not need line-of-sight transmission
between tracked targets and detectors as IR technology in the
system. Thus the Sonitor system enables hidden targets to be
tracked. For example, equipment in a drawer can be tracked
by the Sonitor system, which is sometimes not possible by an
IR positioning system.
In the Sonitor ultrasound IPS, tags attached to people or
equipment are tracked by wireless detectors fixed in various
rooms or places in an open public indoor area. A tracked
tag transmits ultrasound signals with unique identification of
each person or device. The transmitted ultrasound signals are
received by a detector in the same room. The detector forwards
this information through the existing wired or wireless LAN
to a central positioning calculation and management element,
which stores the tag’s location and associated time. In addition,
a Sonitor patented digital signal processing algorithm [46] is
designed to protect the ultrasound signals from interference
and help the detectors receive these signals successfully and
correctly.
An energy-efficient method is proposed by the Sonitor
ultrasound IPS, where the tags are activated by inside motion
sensors, and transmit ultrasound signals in the case the tracked
targets change locations. A sleeping mode is proposed by
the designers to save power for the tags. Thus battery life
time is extended, which can last up to 5 years with 600,000
transmissions. The size of each tag is 57.7 mm × 32.9 mm
× 19.5 mm and the weight is 28 g, which is convenient for
the users to carry. However, the Sonitor system can not give
absolute position of a target. And the system needs numerous
detectors fixed in each place of the tracking coverage area.
Summary of Ultrasound-based Positioning Systems: Ultrasound positioning systems give a kind of inexpensive positioning solutions. Usually the ultrasound signals used to locate
objects need to be combined with RF signals, which perform
synchronization and coordination in the system. These ultrasound positioning systems increase the system coverage area.
However, ultrasound-based positioning systems have lower
measurement accuracy (several centimeters) than IR-based
systems (several millimeters). These ultrasound positioning
systems suffer from reflected ultrasound signals and other
noise sources such as jangling metal objects, crisp packets,
etc.
C. Radio Frequency (RF) Positioning Systems
Radio frequency (RF) technologies [49], [50] are used
in IPSs, which provide some advantages as follows. Radio
waves can travel through walls and human bodies easier,
thus the positioning system has a larger coverage area and
needs less hardware comparing to other systems. RF-based
positioning systems can reuse the existing RF technology systems such as APs in WLAN. Triangulation and fingerprinting
techniques are widely used in RF-based positioning systems.
For complicated indoor environments, location fingerprinting
is an effective position estimation method, which uses location
related characteristics such as RSS and location information of
the transmitters to calculate the location of a user or a device.
1) Radio Frequency Identification (RFID): The radio frequency identification (RFID) is a means of storing and
retrieving data through electromagnetic transmission to an
RF compatible integrated circuit [51]. The RFID positioning
systems are commonly used in complex indoor environments
such as office, hospital, etc. RFID as a wireless technology
enables flexible and cheap identification of individual person
or device [52]. RFID technology can replace the identification
technique such as the barcodes, and be used to design various
products and services [53]. There are two kinds of RFID
technologies, passive RFID and active RFID [51]-[53]. With
passive RFID, a tracked tag is a receiver. Thus the tags with
passive RFID are small and inexpensive. But the coverage
range of tags is short. Active RFID tags are transceivers, which
actively transmit their identification and other information.
Thus the cost of tags is higher. On the other hand, the coverage
area of active tags is larger. In this section, positioning systems
[54]-[56] based on active RFID technology is explained in
detail.
WhereNet: WhereNet positioning system [54], [55] is offered by Zebra Technology Company to provide various
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Fig. 11.
WhereNet’s Real Time Locating System
equipment to support indoor and outdoor real-time positioning.
RFID technology is employed in WhereNet IPS to identify
various located units, called tags, which can be mounted
on the target located objects, such as a device or a person.
WhereNet IPS uses sophisticated differential time of arrival
(DTOA) algorithm [55] to calculate the locations of these tags.
WhereNet IPS produces absolute location information of tags,
which can be used by a number of location-based applications.
For example, the Visibility Server Software, a location-based
application, provides the visibility of the tracked tags and
efficiently uses the location information from the WhereNet
IPS.
WhereNet’s Real Time Locating System (RTLS) [54], [55]
consists of the following parts: tags, location antennas, location processors, servers, and Where Ports, which are shown
in Figure 11. Tags are attached to their objects such as
persons, devices, etc., so that it is possible to track location.
In Figure 11, long range spread spectrum radio beacons are
sent by tags with a unique identification number with respect
to each tag to identify and locate them. Location antennas
mounted on the ceiling at fixed positions receive signals from
tags and forward the data to a location processor. The location
processor uses the information from the location antennas
to perform location calculation and can simultaneously track
many tags. A location processor can connect with up to
8 location antennas via coaxial cable. Location processors
transmit the calculated location information of tags to the
server, where the location information can be saved and used
by location-based services such as real-time tracking services.
Where Ports fixed in different locations send low frequency
electromagnetic signals to the tags to indicate the required
behaviors of the tags based on the users’ applications. For
example, a Where Port offers specifications of the transmission
frequency of the tags based on the location and the needs of
the location-based services.
WhereNet tag III, which is a kind of tags [55] used in
WhereNet IPS, is a small and convenient device for users. It
has the size of 6.6 cm × 4.4 cm × 2.1 cm and the weight of
53 g. The tags are powered by batteries, which can last up to 7
years depending on the transmission rate of the tags. Based on
the decision of the Where Ports, the transmission frequency of
the tags is varying from every 5 seconds to one hour. However,
the WhereNet offers an error range around 2 m to 3 m,
23
which is not very accurate in indoor situations. The system
is complex with numerous infrastructure components fixed in
different locations. Thus the installation of these devices is
time consuming.
Summary of RFID Positioning Systems: The RFID technology is not only for the indoor positioning applications, but also
provides many potential services for the demands of users. The
advantage of an RFID positioning system is light and small
tags that can be taken by people to be tracked. The RFID
system can uniquely identify equipment and persons tracked
in the system. However, the proximity and absolute positioning
techniques need numerous infrastructure components installed
and maintained in the working area of an RFID positioning
system.
2) WLAN: WLAN technology is very popular and has been
implemented in public areas such as hospitals, train stations,
universities, etc. WLAN-based positioning systems reuse the
existing WLAN infrastructures in indoor environments, which
lower the cost of indoor positioning. The accuracy of location
estimations based on the signal strength of WLAN signals is
affected by various elements in indoor environments such as
movement and orientation of human body, the overlapping
of APs, the nearby tracked mobile devices, walls, doors,
etc. The influence of these sources and their impacts have
been discussed and analyzed in the literature [57]-[61]. In
this section, some WLAN-based IPSs are introduced and
discussed.
RADAR: RADAR [57] positioning system was proposed
by a Microsoft research group as an indoor position tracking
system, which uses the existing WLAN technology. RADAR
system employs signal strength and signal-to-noise ratio with
the triangulation location technique. The multiple nearest
neighbors in signal space (NNSS) location algorithm was
proposed, which needs a location searching space constructed
by a radio propagation model. The RADAR system can
provide 2-D absolute position information and thereby enable
location-based applications for users.
In the experiments of the RADAR system, 3 PCs are used
as APs and one laptop is tracked as the target object. The
system was tested on a floor inside a building, which is a
typical indoor environment. The three APs measure the signal
strength of the RF signals from the target. These measurements
are used to calculate a 2-D position of the object. The system
achieves an accuracy of about 4 m with about 50% probability.
The major advantages of RADAR system are that the
existing indoor WLAN infrastructures are reused and it requires few base stations to perform location sensing. Thus
the RADAR system is easy to be set up. However, the
limitation is that the located object needs to be equipped with
WLAN technology, which is difficult for some lightweight
and energy-limited devices. There is also no consideration
of privacy issues in the design of RADAR system, where a
person using a device with WLAN interface may be tracked,
even he/she does not want any one know his/her location. In
addition, the RADAR system suffers from the limitations of
RSS positioning methodology [50].
Ekahau: The Ekahau positioning system [58] uses the
existing indoor WLAN infrastructures to continually monitor the motion of WiFi devices and tags. The triangulation
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Fig. 12.
IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
System Architecture of Ekahau Positioning System
positioning techinique is used for locating any WiFi enabled
device in Ekahau positioning system. The received signal
strength indication (RSSI) values of the transmitted RF signals
recorded at different APs are used to determine the targets.
locations. The Ekahau positioning system offers 2-D location
information, which can be used by location aware services and
applications.
This solution is inexpensive and flexible to perform indoor
positioning by tracking the tags with respect to reference
devices, which are standard APs. The Ekahau system consists
of three parts: site survey, WiFi location tags and positioning
engines as shown in Figure 12. Site survey is a software tool,
which provides site calibration before the real-time position
estimations, and demonstrates the network coverage area,
signal strength, SNR, data rate and overlapping of the WLAN
network in users. social and professional places. The mapping
of the network environment is quick with about 1,111 m2
per hour. Another part is WiFi location tag, which can be
attached to any tracked object to enable real-time positioning.
The tags transmit RF signals. APs measure signal strength
of the received RF signals. The measured data is forwarded
via WLAN to the third part, which is a positioning engine, a
software tool, offering real-time positioning to any device such
as laptop, PDA, etc., using WLAN technology. Combining
signal strength and site calibration done by site survey, the
positioning engine calculates and displays the locations of
WiFi location tags mounted on devices on the map of the
local place.
The accuracy of the positioning system can achieve 1 m, if
there are three or more overlapping APs that can be used to
locate objects. The engine can simultaneously track thousands
of devices. The Ekahau system achieves low cost by sharing
the existing WLAN APs. The tags tracked are comfortable
for the users to take them, with a size of 45 mm × 55 mm
× 19 mm and weighing 48 g. The battery life time can last
up to 5 years with low battery warning alerts to avoid the
performance degradation because of the low power level of the
battery. When the tags move, they start to work and be tracked,
which offer an energy efficient solution and less influence to
other WLAN-based communication.
COMPAS: The COMPASS system [61] takes advantages of
WLAN infrastructures and digital compasses to provide low
cost and relative high accurate positioning services to locate
a user carrying a WLAN-enabled device. Position estimations
are based on the signal strength measured by different APs.
The COMPASS system uses fingerprinting location technique
and a probabilistic positioning algorithm to determine the
location of a user [61]. A major contribution of the COMPASS
system is that the user’s orientation is considered in the
location sensing process. A user’s orientation is measured by a
digital compass to reduce the human body blocking influence
to the positioning process. A digital compass is a low cost and
low power consumption component with small size, because
digital compass is integrated into a chip.
For the tracking of a mobile user, the orientation impact
is highly addressed and analyzed in detail by the designers
of both RADAR and COMPASS system. As human body
contain more than 50% water, which absorbs the 2.4 GHz
radio signal, the clocking effect of human body influences the
measurement accuracy. COMPASS system aims at solving this
problem by increasing the number of signal strength samples
in each position with different orientations.
The test experiments were taken in an area of 312 m2 on
a floor inside a building. In this situation, the COMPASS
system achieves an accuracy of about 1.65 m. But the RADAR
system only has an error distance of 2.26 m in the same case.
However, the COMPASS system only considers tracking a
single user. Locating multiple users at the same time has not
been discussed. Thus the scalability of the COMPASS system
is low to provide location sensing for multiple targets.
Summary of WLAN-based Positioning Systems: IPSs have
the goal of increasing the location estimation performance,
and at the same time reducing the cost of system. WLANbased indoor positioning [57]-[61] is an example of low cost
positioning technology, which uses the existing infrastructures
in indoor environments. WLAN technology is widely used
and integrated in various wireless devices such as PDAs,
laptops, mobile phones, etc. Thus the WLAN-based positioning systems can also reuse these wireless devices as tracked
targets to locate persons. However, because of complex indoor
environments consisting of various influenced sources [57][61], the performance of the positioning systems are not very
accurate with an accuracy of several meters. And using the
stored information and fingerprinting technique in the location
estimations is complex and costly if the number of users of
the positioning system is increasing significantly.
3) Bluetooth: Bluetooth, the IEEE 802.15.1 standard, is a
specification for WPAN. Bluetooth enables a range of 100 m
(Bluetooth 2.0 Standards) communication to replace the IR
ports mounted on mobile devices. Piconets are formed under
Bluetooth specifications by using a master/slave based MAC
protocol. Bluetooth technology has been implanted in various
types of devices such as mobile phone, laptop, desktop, PDA,
etc. In addition, Bluetooth chipsets are low cost, which results
in low price tracked tags used in the positioning systems.
In Bluetooth-based positioning systems [62]- [73], various
Bluetooth clusters are formed as infrastructures for positioning. The position of a Bluetooth mobile device is located by
the effort of other mobile terminals in the same cluster. In this
section, a Bluetooth-based IPS is introduced.
Topaz: The Topaz location system [74] uses Bluetooth
technology to locate tags in indoor environments. By using
Bluetooth technology, Topaz can only provide 2-D location
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GU et al.: A SURVEY OF INDOOR POSITIONING SYSTEMS FOR WIRELESS PERSONAL NETWORKS
Fig. 14.
Fig. 13.
25
System overview of evaluation of OPT on a floor
The System Architecture of Topaz IPS
information with an error range of around 2 m, which is
not sufficient to provide room level accuracy in a multiobstacle indoor environment. Thus the Topaz system combines
the Bluetooth-based positioning with IR-based positioning
technique, where IR location techonology is suitable for this
aim. IR can not penetrate the walls of the rooms, which
offer perfect room level accuracy. The Topaz location system
consists of software and hardware parts for local positioning
of Bluetooth tags or any device equipped with Bluetooth
technology.
Figure 13 shows the system components and the architecture
of Topaz indoor location system. In the system, tags are
located by numerous Bluetooth and IR enabled APs fixed in
different places. Typically, 32 APs are associated with one
Bluetooth server, which is responsible for performing Bluetooth functionalities such as managing APs. The Bluetooth
servers receive the measured signal strength and forward the
raw data to the Location server. The location server calculates
the location of the tags. Bluetooth servers, location servers
and location clients are connected with LAN.
Combining Bluetooth technology and IR technology, the
target device can be located in the correct room. Tens of
objects can be tracked at the same time. However, the tags
using batteries need to be charged once per week, which is
a short period compared with tags used in other positioning
systems. The delay of calculating the position of a tag is quite
long, around 10 s to 30 s.
Summary of Bluetooth-based Positioing System: Using
Bluetooth technology in location sensing can reuse the devices
already equipped with Bluetooth technology. Since Bluetooth
is a low-cost and low-power technology, it is an efficient way
to design IPSs using Bluetooth. However, a disadvantage of
Bluetooth-based positioning system is that the syatem can
only provide accuracy from 2 m to 3 m with the delay of
about 20 s. The Bluetooth positioning systems suffer from the
drawbacks of RF positioning technique in the complex and
changing indoor situations [62].
4) Sensor Networks: Sensors are devices exposed to a
physical or environmental condition including sound, pressure,
temperature, light, etc., and generate proportional outputs.
Sensors are typically divided into two kinds: active sensors and
passive sensors. Active sensors can interact with the environment such as radars. Passive sensors only receive information
from the outside world. The sensor-based positioning systems
consist of a large number of sensors fixed in predefined
locations [75]. From the measurements taken by these sensors,
a person or device can be located. Positioning methods using
sensor networks were discussed in [76]. This section will
introduce a sensor-based IPS.
Online Person Tracking (OPT) System: Online Person
Tracking (OPT) system [77] is designed to provide location
information to the context-aware applications in PNs. OPT
is a low-cost positioning system with a number of sensors
deployed at fixed positions in indoor environments. OPT uses
cheap and small sensors called T-mote [78]. The sensors are
employed and mounted at fixed positions in the OPT system.
These sensors take advantages of RSSI to measure the distance
between a transmitter sensor and a receiver sensor. Based on
these determined distances, triangulation location technique
with a weighted minimum mean square error (W-MMSE)
location algorithm [77] is used.
A prototype was designed and implemented to test the
performance of real-time person tracking in OPT system. The
testing experiments were taken on a floor inside a building,
which is shown in Figure 14. The triangulation method is
used in the OPT system based on the RSSI value measured
by 3 or more sensors. A target node carries a T-mote to
be tracked by OPT system. Various T-motes receive the
signal from the target node and measure the RSSI values of
these signals. Then T-motes forward these data to a locationbased application, which is software implemented in a laptop,
through the wireless sensor network. The location application
calculates the position of the target node, and displays its
position on the floor map through GUI.
OPT system gives a low cost location sensing solution that
reuses the sensors deployed at fixed positions in the indoor
environments. However, the accuracy of the system varies
from 1.5 m to 3.8 m, which is not very accurate to offer room
level location information. And installing sensors in fixed
locations and maintaining numerous sensors in OPT system
is complex.
Summary of Sensor-based Positioning Systems: Sensorbased positioning [75]-[77] provides a cost effective and
convenient way of locating persons and devices due to the
decreasing of the price and the size of sensors. At the same
time, cheap and small sensors have limited processing capability and battery power comparing to other wireless devices
such as mobile phone, PDA, etc. Thus using sensor technology
in indoor positioning has some drawbacks: less accuracy, the
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IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
Fig. 15. Hardware of the Ubisense System in a Academic Research Package:
five tags (left) and four sensors (right)
limited battery power in the case of real-time tracking, lower
computational ability, etc., which needs further improved to
offer precise and flexible indoor positioning services.
5) UWB: The RF positioning systems suffer from the
multi-path distortion of radio signals reflected by walls in
indoor environments. The ultra-wideband (UWB) [79] pulses
having a short duration (less than 1 ns) make it possible
to filter the reflected signals from the original signal, which
offer higher accuracy. Using UWB technology in positioning
systems has been a popular way of improving the positioning
accuracy [80]. In this section, a UWB-based positioning
system is introduced.
Ubisense: The Ubisense Company, which is funded by
engineers from AT&T Cambridge, provides a new real-time
positioning system based on UWB technology [81]. The triangulation locating technique, which takes advantages of both
the time difference of arrival (TDOA) and AOA techniques,
is employed in the system to provide flexible capability of location sensing. Since Ubisense can measure signal angles and
difference in arrival times, and complex indoor environments
including walls and doors do not significantly influence the
performance [81], the accuracy offered by Ubisense is about
tens of centimeters.
The Ubisense system consists of three parts: the sensors, the
tracked tags and the Ubisense software platform as shown in
Figure 15. The active tags transmit UWB pulses. The sensors,
which are fixed in the known locations, receive the UWB
signals from the tracked tags. Then the location data of the tags
is forwarded from these sensors via existing Ethernet to the
Ubisense software platform, which analyses and displays the
location of the tags. The software platform includes two parts:
Location Engine and Location Platform. Location Engine is
a run-time component and enables sensors and tags to be
set up. Location Platform gathers location data from for
location-aware applications. For example, one location-aware
application is real-time monitoring and displaying the location
the target tags according to the imported coverage area model.
The visualization of the location of tags is provided, and
absolute, relative and proximity location information can be
abstracted and offered to various location-aware applications.
Comparing with the other RF-based positioning systems, the
Ubisense system results in a higher accuracy of about 15 cm in
3-D. The time delay of the position estimations is short and the
sensing rate can be up to 20 times per second. The Ubisense
sensors are organized into cells. In each cell, there are at least
four sensors, which cover an area of up to 400 m2 . Thus the
Fig. 16.
Components of MotionStar Wireless System: transmitter and
controller (left), base station (right up), mounted sensors and RF transmitters
(right down) [83]
coverage range per infrastructure element is large. The system
is scalable with respect to a large position monitoring area. The
tracked tags are wireless, easily wearable, and light weight
(45 g) and a long battery life time of about 1 year. However,
the price of this high performance positioning system is also
high. An active research package as shown in Figure 15 costs
about $16,875.
Summary of UWB-based Positioing Systems: UWB technology offers various advantages over other positioning technologies used in the IPSs: no line-of-sight requirement, no multipath distortion, less interference, high penetration ability, etc.
Thus using UWB technology provides a higher accuracy.
Furthermore, the UWB sensors are cheap, which make the
positioning system a cost-effective solution. In addition, the
large coverage range of each sensor results in that the UWBbased positioning system is scalable.
D. Magnetic Positioning System
Using magnetic signals is an old and classic way of position measuring and tracking [82]. The magnetic positioning
systems offer high accuracy and do not suffer from the line-ofsight problems, where the positions are measured in the case
of an obstacle between the transmitters and receivers. This
section introduces a magnetic positioning system in detail.
MotionStar Wireless: MotionStar Wireless [83] is a motion
tracking system that uses pulsed DC magnetic fields to simultaneously locate sensors within 3 m coverage area. MotionStar
Wireless is an improved version of the original wired motion
tracking system named MotionStar designed by Ascension
Technology Corporation. By modifying the design of the old
version, MotionStar, there is no wire between a tracked person
and a base station tracking the person’s motion. MotionStar
wireless system provides precise body motion tracking by
measuring numerous sensors mounted on the different parts of
a person. Thus the position information of sensors determined
by the MotionStar Wireless system can be used by various
applications, such as Animation, Biomechanics, virtual reality,
etc.
The MotionStar wireless system tracks multiple targets (up
to 120 sensors) at the same time and in real-time. The systems
consist of a transmitter and controller, a base station, mounted
sensors and RF transmitters, as illustrated in Figure 16. The
transmitter and controller part sends magnetic pulses to the
body mounted sensors. Then each sensor mounted on a particular body part receives magnetic pulses from the transmitter
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GU et al.: A SURVEY OF INDOOR POSITIONING SYSTEMS FOR WIRELESS PERSONAL NETWORKS
and controller. The sensors are connected through wires to the
RF transmitter, which is carried by the tracked person. The
RF transmitter can connect up to 20 sensors and transmits
the measured data to the base station. Finally, the base station
calculates the position and orientation of sensors and transfers
the measured data to the user’s computer through RS232
or Ethernet interface. These estimated data can be used for
animation and tracking applications.
The error range of the static position estimating is about
1 cm. The update rate of the position measurements is up
to 120 measurements per second. The sensors carried by a
tracked person are small (2.54 cm × 2.54 cm × 2.03 cm) and
light weight (21 g), which are highly portable and comfortable
wearing devices. These mounted sensors are connected via
wires with the back pack RF transmitter, which has the size
of 17.5 cm × 14 cm × 4.54 cm and the weight of 0.99 kg.
However, the disadvantage of the Motion Star system is that
the magnetic trackers are quite expensive. The battery life time
for continuous motion tracking is around 1 hour or 2 hours,
which is a short period for daily position estimations. And
the performance of the Motion Star system is influenced by
the presence of metal elements in the positioning estimating
area. In addition, the coverage range of each transmitter is
limited within 3 m, which is not scalable for large indoor
public applications and services.
Summary of Magnetic Positioning Systems: The magnetic
sensors are small in size, robust and cheap, which bring benefits for positioning estimations in indoor environments. The
magnetic-based positioning systems can offer higher accuracy
and afford multi-position tracking at the same time. However,
the limited coverage range is a drawback for the performance
of the magnetic IPSs. Thus increasing the coverage range of
each magnetic transceiver or using various magnetic infrastructures to cover enough area for indoor use needs further
study, design and development.
E. Vision-based Positioning System
Vision-based positioning is a way of tracking the locations
and identifying persons or devices in a complex indoor environment [84]-[86]. The vision-based positioning does not need
the tracked person carrying or wearing any device. And vision
can easily provide some location-based information such as
person A is drinking wine and sitting on his/her sofa. In
this section, an example of vision-based positioning system is
explained, and the pros and cons of vision-based positioning
are discussed.
Easy Living: Microsoft research group designed the Easy
Living positioning system based on vision-based location
techniques [87]. Vision-based location techniques can capture
the motion of the targets with data from a single perspective or
multiple perspectives. Easy Living systems use the multipleperspective vision-based location technique with two cameras
covering the whole measuring area. The location estimation in
Easy Living system combines color and depth from the two
cameras to provide position sensing and target identification
services.
The components of the Easy Living system are demonstrated in Figure 17. In the evaluation of the Easy Living
Fig. 17.
27
Easy Living System Components [87]
system, two stereo cameras are mounted on the ceiling of
a room. Thus every part of the room is covered by at least
one camera. Two real-time 3-D cameras are responsible for
covering the measured area and providing updated visions,
which are raw data to be used in the position estimations. To
reduce the influence of changes in the background, depth and
color pixels are used in the modeling of the background. Then
PCs running the stereo module receive the images taken by the
cameras and process these raw data. To identify each tracked
person, Easy Living system defines a ”person creation zone”,
which is normally near the entrance of the room. Thus, when
a person enters the room, in this ”person creation zone”, the
stereo module creates the vision instance of the person. Then
the stereo model tracks the motion of the person and keeps
the location history of the person. Using the saved location
information of the person, the Easy Living system can correct
some mistaken location estimations.
Although Easy Living system is very convenient for the
users, there are still some disadvantages of the system. The
Easy Living system needs substantial processing power to
process the images taken by the stereo cameras, because image
processing is complex. And the system’s accuracy can not
be guaranteed due to the interference of dynamic changing
environment to the vision data.
Summary of Vision-based Positioning Systems: In visionbased positioning systems, a low price camera can cover
a large area. The users do not need to carry any location
device, and can be located by the vision-based IPS. However,
these systems still have some drawbacks. Firstly, the privacy
of people is not provided by the vision-based positioning.
Secondly, the system is not reliable in a dynamic changing
environment. Since the position estimations are based on the
saved vision information in a database, which needs to be
updated due to the changing in the environment such as
changing the place of your desk in your office. The visionbased positioning is influenced by many interference sources
such as weather, light, etc. For example, the turning on and off
a light in a room reduces the accuracy of tracking a person.s
motion. In addition, tracking multiple persons moving round at
the same is still a challenge for the vision-based positioning,
which needs higher computational ability of the positioning
system.
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IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
penetration ability, so the scope of an infrastructure component
is within a single room. Transmitting audible sound is a kind
of noise to indoor environments, where people would not like
to hear audible sound made by the positioning services.
IV. C OMPARISONS
Fig. 18.
System Architecture of Beep [90]
F. Audible Sound Positioning System
Audible sound is a possible technology for indoor positioning [88]. Nearly every mobile device has the ability of emitting
audible sound such as mobile phone, PDA, etc. The audible
sound-based positioning system can reuse these devices owned
by the users for indoor positioning. Wearable tracked tags are
no longer needed resulting in a low-cost system.
Beep: A 3-D IPS named Beep [89], [90] was designed as
a cheap positioning solution using audible sound technology.
Triangulation location technique is used in Beep with a standard 3-D multilateration algorithm based on TOA measured
by the sensors in Beep system. The 3-D location information
determined by Beep can be used by various practical applications as proposed by Beep in the situations, such as office
building, shopping center, etc.
Figure 18 shows the architecture of the Beep positioning
system. A roaming device is used as a tracked target in the
system to send audible sound. Various acoustic sensors (Si )
are pre-installed at fixed positions in the measuring area and
connected to the central server through wireless connection
(Ri ). These sensors receive the audible sound transmitted from
the tracked device and forward these data to the central server
through WLAN. TOA technology and triangulation method
are used to estimate the position of the device. Finally, the
roaming device can get its position information from the
central server via WLAN. The testing experiments were taken
in a 20 m × 9 m room. The positioning system can achieve
up to an accuracy of 0.4 m with 90% of all cases. In addition,
the effect of sound noise and obstacles reduce the positioning
accuracy by 6-10%.
One of the benefits brought by the Beep system is that the
privacy of the users is considered by avoiding them being
tracked automatically. The users can stop their devices from
sending audible sound; if they do not want the system knows
their location.
Summary of Audible Sound Positioning Systems: Audible
sound is an available service in various mobile devices used
in our daily lives. Thus the users can use their personal devices
in an audible sound positioning system to get their positions.
Because of properties of audible sound, using it for indoor
positioning has some limitations. The audible sound can be
interfered by the sound noises in the dynamic changing and
public indoor situations. Audible sound does not have high
In this section, the existing IPSs described in Section III are
evaluated from the viewpoint of user needs in PNs. IPSs are
compared with respect to various aspects as introduced in the
section II-F, including security and privacy, cost, performance,
robustness, complexity, user preference, availability, and limitations. These criteria are proposed based on the user need,
preference and convenience. The evaluation and comparison
results are shown in Table I and Table II. For each design
requirement of PN, these IPSs are compared. Thus, in the
future IPS design for PNs, we can use the comparisons in
these tables to easily find the perfect location methods and
system design issues with respect to each evaluation criteria
to enable location-aware intelligence in PNs. For example, for
the ”Fitness Center Scenario” described in subsection II-B, an
IPS can be chosen based on the eight evaluation criteria. From
Table I and Table II, we can obtain the best IPS that fulfils
the requirement of the IPS for ”Fitness Center Scenario”.
V. C ONCLUSION
In the next generation communications networks, the
telecommunications applications require various types of context information of the environments, persons and devices to
offer flexible and adaptive services in PNs. Location context
is a kind of context information, which enables locationaware intelligence to improve the quality of lives. A personal
network focuses on the demands of a user to integrate all
his/her personal devices at various places in different types of
networks into one single network, which provides a private
and user-centric solution. The IPSs produce absolute, relative
and proximity location information for the users and their
devices in indoor environments. Based on the measured location information, tracking, navigation, monitoring and other
location-aware services can be designed for the users in PNs.
In this article, we describe the concept of IPSs and introduce
the types of location data offered by IPSs. Typical scenarios
and use cases are explained to show the requirements of
location data in PNs. Then we classified 17 existing IPSs into
6 categories based on the main medium used to sense location.
We explained the system architecture and working principles,
and discussed the advantages and disadvantages of each IPS.
From this survey, we can see that each medium used in position estimations has its limitations. None of the technologies
can satisfy the system requirements of performance and cost.
Instead of using a single medium to estimate the locations
of the targets, combining some positioning technologies can
improve the quality of positioning services [91]. For example,
the SVG system [92] combines the advantages of WLAN and
UWB based positioning technologies, where WLAN technology can provide positioning services covering large area and
UWB can give highly accurate position estimated in some
small required areas.
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29
TABLE I
S UMMARY AND C OMPARISON OF IPS S IN S ECURITY AND P RIVACY, C OST, P ERFORMANCE AND ROBUSTNESS
System Name
Active Badge [32]-[35]
Security
and
Privacy
No
Cost
Performance
Robustness
Resonable price with cheap
tags and sensors
A tag controller and 32 tags
cost $27,500
Room level accuracy
Line of sight requirements and influence
from light source
Influence from light source
Firefly [36], [37]
No
OPTOTRAK [38]
No
Expensive
IRIS LPS [40]
No
Less than Firefly and
OPTOTRAK
Active Bat [42]
No
Expensive
Cricket [44], [45]
Yes
Cheap
Sonitor [46]
WhereNet [54], [55]
No
No
Inexpensive
Not Cheap
RADAR [57]
No
Ekahau [58]
No
Research-oriented solution,
no products
Inexpensive
COMPASS [61]
No
Inexpensive
Topaz [74]
No
High
Error range is about 2 m to 3 m,
room level accuracy
OPT [77]
No
Cheap
Ubisense [81]
No
MotionStar [83]
No
An active research package
containing 5 tags and
4 sensors cost $18,354
Expensive magnetic
trackers
Easy Living [87]
No
Inexpensive stereo cameras
are used
Beep [89], [90]
Yes
Inexpensive sensors
Error range is about 1.5 m to
3.8 m
The accuracy is about 15 cm with
with short location estimation
delay
The accuracy is about 1 cm; both
the position and orientation are
estimated
The system accuracy cannot be
guaranteed due to various
interference sources
The accuracy is up to 0.4 cm with
with 90% cases
Error range below 3.0 mm;
high positioning frequency;
short delay
An accuracy of 0.1 mm to
0.5 mm with 95%
Error range is about 16 cm;
IRIS LPS is accurate than Firefly
and OPTOTRAK
The accuracy is about 3.0 cm with
95% probability
An accuracy of 10 cm and an
orientation accuracy of 3◦
Room level accuracy
Error range of 2 m to 3 m;
position estimation frequency is
every 5 seconds to 1 hour
The accuracy is about 4 m with
50% probability
The accuracy is up to 1 m, and the
system can simultaneously track
thousands of devices
The accuracy is about 1.65 m
Since people have certain habits in their daily lives and
follow routines in their living and working environments, the
position of users can be predicted based on the previous
position information. Position prediction is described in [93],
which supports efficient and cognitive applications for the
users in PNs. Furthermore, the position prediction can be used
together with other types of context information such as user
preference, time, weather, etc., to consider multiple issues of
a user and the situation in which the user is. Another future
research issue is to consider the influence of mobility in IPSs,
which includes the mobility of the located object and the
mobility of people and equipment in indoor situations. The
design of an IPS should ensure optimum performance for a
moving target in a varying indoor situation. Thus the position
measurements, position prediction and other related context
will be used together to enhance the context-aware intelligence
in PNs.
Taking location sensing technology and system design,
some on-going research projects are trying to improve the
performance of IPSs. A project named ”Development of
Line of sight requirement
IRIS LPS can only locate a static object
with acceptable accuracy. For moving
objects, the system need to be improved
Influenced by reflection from and
obstacles between a tag and a receiver
Good
Hidden targets can be tracked
Instead of using RFID technology in
positioning magnetic signals are used to
give the location zone of a tracked target
As the accuracy is low, the position
measurement are not reliable
Only if there are enough APs
(more than 3), the system can locate
a target with an accuracy of up to 1 m
The system considers the human body
blocking effect and use digital compasses
to improve the performance
Using Bluetooth and IR technologies
at the same time to achieve higher
robustness
The system needs at least three sensor
measurements to locate target
Good
Influenced by metal elements
The system is not reliable in a dyanmic
changing environment
Influenced by sound sources in the same
place
Location Centric Networks” [94] is undertaken by Mitsubishi
Electric Research Laboratories (MERL) to estimate the location of transceivers in an UWB impulse radio network.
Currently, the location system proposed by this project can
determine the location of an object with an accuracy of 15
centimetres and cover large area. Another on-going research
project is ”Indoor position location technology project” [95]
taken by the Commonwealth Scientific and Industrial Research
Organization (CSIRO) in Australia. The aim of the project is to
improve the accuracy of location estimation, at the same time,
reduce the cost of indoor tracking systems. The third project
is a ”Real-Time Location System (RTLS) Healthcare”, which
is undertaken by AeroScout Company [96], an industry leader
of WiFi-based active RFID solutions. The project focuses on
patients care in digital hospitals to propose advanced locating
technologies and methods.
From this survey, readers can have a comprehensive understanding of the existing IPSs, especially the 17 IPSs
surveyed and discussed in this paper. Eight criteria, which
include security and privacy, cost, performance, robustness,
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IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 11, NO. 1, FIRST QUARTER 2009
TABLE II
S UMMARY AND C OMPARISON OF IPS S IN C OMPLEXITY, U SER P REFERENCE , AVAILABILITY AND L IMITATIONS
System Name
Active Badge [32]-[35]
Complexity
Low
User Preference
The active badges are lightweight and have
acceptable size; the battery life is about
half to one year
Tag controller and tags are not comfortable
to be worn, because they are wired
No battery; the emitters are small and
wired connected to the power supply
No battery; emitters are small
Availability
Not Available
Limitations
Absolute location
information is not available
Firefly [36], [37]
Low
Commercially
available
Commercially
available
Not available
The scope of this system is
limited within 7 m
Limited coverage area
OPTOTRAK [38]
Low
IRIS LPS [40]
Low
Active Bat [42]
Lots of receivers need to be
mounted on the ceiling
The battery life is about 15 months
Not available
Cricket [44], [45]
Low
Not available
Sonitor [46]
Numerous detectors
fixed in each place of
tracking area
Complex
The object receiver consumes more power,
because the position calculation is done
by itself
Tags are small and lightweight; the
battery life can last up to 5 years
WhereNet [54], [55]
RADAR [57]
Ekahau [58]
COMPASS [61]
Topaz [74]
OPT [77]
Ubisense [81]
MotionStar [83]
The system uses PC as APs,
and does not address the
issues with installation
and maintenance
The system reuses the WLAN
infrastructures; the system
needs several hours of site
The system reuses the WLAN
infrastructures
There are many IR APs and
servers need to be
installed
Low
Low, only sensors are used
as infrastructures and four
sensors can cover an area
400 m2
Small coverage area with
3 m in length; it is not
scalable
Easy Living [87]
Low, for example, two
stereo cameras can cover a
a single room
Beep [89], [90]
Not complex
Commercially
available
Tags are small, lightweight and long
battery life of up to 7 years
The system uses laptops as tracked targets,
and does not consider the user preference
Commercially
available
Not available
Any WiFi device can be used as a tracked
target; the tags are small, lightweight;
the battery life is up to 5 years
A device located at the end-user should
contain a digital compass
The tracked tags need to be recharged
every week
Commercially
available
Not Available
Commercially
available
A trade-off between
accuracy and coverage
Deploying large
numbers of sensors on the
ceiling for each room is a
time-consuming task
Mobile device’s power
consumption
The system cannot give
absolute position
measurements
The accuracy of the system
is not good enough
The system does not take
advantages of the existing
WLAN infrastructure in
indoor environments
The system needs site
calibration time in the
installation phase
The system does not give
real-time tracking services
The delay of calculating the
position of a tag is long
(around 10 s - 30 s)
The location measurement
is not reliable
The tracked T-motes are small and
lightweight, but its battery life is not
long
The tracked tags are small and lightweight
with battery life around 1 year
Not available
Commercially
available
The UWB technology is
new and the price of the
system is high
The tracked sensors are connected via wire
to RF transmitters; RF transmitters are
heavy (about 1 kg) to wear; the battery
life is around 1-2 hours
The users do not need to carry any
positioning device
Commercially
available
The system is designed for
short range mobility
tracking
Commercially
available
The users can use their own devices such
as PDA, mobile phone, etc., as positioning
devices
Not available
The image processing is
complex and needs
substantial processing
power
The audible sound
technology is influenced
by sound noise in indoor
environments
complexity, user preference, availability and limitations, have
been proposed to evaluate and compare these IPSs from the
view of users in PNs. Each IPS, which uses a certain type of
technology or a combination of two or more technologies, has
its design purpose, and works well under certain situations.
It is desirable that the location estimation service can work
for different indoor environments and provide scalable positioning services. Thus, in future research, a combination of
different existing communications technologies and location
information from different sources should be considered to
increase the scalability and availability of location estimation
services [97]. In the conclusion part, the current location
sensing technology developing interests are described, and
some on-going location system projects are introduced. We
hope that researchers can use the information in this survey
paper to propose more accurate and flexible positioning and
position-based services for the users in the future PNs.
ACKNOWLEDGMENT
This work was partially funded by the EU IST MAGNET
Beyond Project and the Dutch Freeband PNP2008 Project.
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Yanying Gu received her BSc degree in the Department of Electronic
Engineering of Dalian University of Technology, Dalian, China in 2004.
She received her MSc degree in Telecommunications Engineering from Delft
University of Technology, Delft, The Netherlands in 2006. She is currently a
Ph.D student at Delft University of Technology, The Netherlands. Her research
interests include clustering and routing protocols in wireless ad hoc networks,
context-aware intelligence, mobility modelling and indoor position sensing
techniques.
Anthony Lo received his combined BSc/BE degree with first class Honours in
Computer Science and Electronics Engineering in 1992 and his Ph.D degree
in Protocol and Network Engineering in 1996, all from La Trobe University,
Australia. He is currently an assistant professor at Delft University of Tecnology in the Netherlands. Prior to that, he was a Wireless Internet Researcher at
Ericsson EuroLab, where he worked on research and development of UMTS
and beyond 3G systems.
Ignas Niemegeers is currently Chair Professor of Wireless and Mobile
Communications at Delft University of Technology in The Netherlands, where
he is heading the Centre for Wireless and Personal Communications (CWPC)
and the Department of Telecommunications. He is an active member of the
Wireless World Research Forum (WWRF) and IFIP TC-6 Working Group on
Personal Wireless Communications. He has been involved in many European
research projects, in particular, ACTS TOBASCO, ACTS PRISMA, ACTS
HARMONICS, RACE MONET, RACE INSIGNIA and RACE MAGIC.
Presently, he participates in the IST projects MAGNET, on personal networks
and EUROPCOM on emergency ad hoc networks.
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