WO2012175352A1

WO2012175352A1 - Localization system

Info

Publication number: WO2012175352A1
Application number: PCT/EP2012/060907
Authority: WO
Inventors: Josua Petrus VAN VELZEN; Jan Willem Houwer DE GEUS
Original assignee: Salland Electronics Holding B.V.
Priority date: 2011-06-22
Filing date: 2012-06-08
Publication date: 2012-12-27

Abstract

A localization system (100) for locating a receiver (130), the system comprising a first beacon (110) configured for wirelessly transmitting a first localization message and a second beacon (120) configured for wirelessly transmitting a second localization message, the receiver being configured for wireless reception, the localization system comprises a synchronization system (115, 125) for synchronizing the transmitting of the first localization message and the transmitting of the second localization message so that the first and second localization message at least partially overlap causing a collision between the first and second localization message, the receiver being configured for wirelessly receiving at most one of the first localization message and the second localization message.

Description

LOCALIZATION SYSTEM

FIELD OF THE INVENTION The invention relates to a localization system for locating a receiver, the system comprising a first beacon configured for wirelessly transmitting a first localization message and a second beacon configured for wirelessly transmitting a second localization message, the receiver being configured for wireless reception. The invention also relates to a beacon, a receiver, a method for locating a receiver and a corresponding computer program product.

BACKGROUND OF THE INVENTION

A variety of methods have been employed for electronic localization of objects. For outdoor localization, the GPS system is popular. However, for indoor localization this method is less suitable. In fact, indoors GPS may not be useable at all.

Localization systems suitable for indoor localization of objects are often based on Radio Frequency signals (RF signals). The object to be localized is provided with a receiver for receiving wireless signals transmitted from a plurality of beacons that are deployed in various fixed locations. RF based localization is typically based on the detected strengths of received wireless signals transmitted from the plurality of beacons. A measure of the received signal strength of an RF signal is referred to as a Received Signal Strength Indication (RSSI).

The signal strength of an RF signal decreases as the distance between transmitter (i.e. the beacon) and the receiver increases. One way to use the RSSI values is to interpret them directly as a measure for distance and employ triangulation. From the RSSI values for signals obtained from some of the beacons, preferably at least three, an estimate of the location can be obtained.

In a more advanced system, a training phase may be used. In the training phase the signal strength of beacons is recorded at various training locations around the area in which localization is desired. During a subsequent localization phase, the receiver measures the signal strength of the received signals and compares the measured values with values obtained during the training phase. From the measurements recorded during the training phase there will be one set of measurements that best matches the measurements currently obtained in the localization phase. That set of measurements was recorded at one of the training locations. That one training location is an indication of the current position of the receiver.

There are various problems associated with using a RSSI for localization. First, the relationship between the signal strength of an RF signal at a receiver and the distance of the receiver to the transmitter of that signal is not linear. Furthermore, signal strength is also influenced by other factors. Those factors include, among others, the presence of conductive objects near the antenna of the transmitter or the receiver, and of obstacles that may or may not be in the path between the transmitter and receiver.

Furthermore, the received signal strength is influenced by changes in the environment. Environmental factors, such as people moving around, changed pieces of furniture and even whether a door is open or closed will change the RSSI. As a result the RSSI will be different and the receiver will arrive at an estimate of the distance to the beacon having a large error. Incorporating a training phase does not help against dynamic changes in the RSSI caused by current changes in the environment since the environment may be different during the training phase than during the localization phase. Various solutions to address these problems have been suggested.

US 2010/0134356 A1 discloses an indoor localization system including a radio badge and a plurality of beacons. The beacons are deployed in an indoor space. Each of the beacons periodically transmits a localization signal comprising an ID of the

transmitting beacon. Collisions among localization signals transmitted by the beacons are prevented by configuring the beacons to transmit the localization signals asynchronously. The timing at which each of the beacons transmits the localization signal thereof is adjusted according to a comparison with timings at which neighboring beacons transmit localization signals, such that collisions among the localization signals transmitted by the beacons are prevented. This system thus reduces interference caused by self-collisions, i.e., collisions between two beacons of the same system.

WO 2010/022797 and US 2010/0303129 disclose two other indoor localization systems. Both use a plurality of beacons and measurement of RSSI to localize a mobile receiver. To reduce interference caused by other devices transmitting on the same frequency as the localization system, these localization systems offer the possibility to switch to another frequency. At the other frequency, hopefully, the interference is less severe, and the correspondence between RSSI and distance to a beacon is improved.

These developments further complicate localization using RSSI. Moreover, even using these countermeasures the localization nevertheless remains sensitive to

disturbances of the RSSI.

SUMMARY OF THE INVENTION

An improved localization system for locating a receiver comprises a first beacon configured for wirelessly transmitting a first localization message and a second beacon configured for wirelessly transmitting a second localization message. The receiver is configured for wireless reception. The localization system comprises a synchronization system for synchronizing the transmitting of the first localization message and the transmitting of the second localization message so that the first and second localization message at least partially overlap causing a collision between the first and second localization message. The receiver is configured for wirelessly receiving at most one of the first localization message and the second localization message. By transmitting the first and second localization messages at the same time the system intentionally causes a collision between these two signals. The collision causes interference. As a result of the collision the receiver will not be able to correctly receive both the first localization message and the second localization message. A wireless signal attenuates as it moves away from the sender. In a region close enough to the first beacon and far away enough from the second beacon, reception of the first localization message will be much stronger than reception of the second localization message. In that region, the first localization message drowns out the second localization message and the receiver can correctly receive the first localization message.

Similarly, in a region close enough to the second beacon and far away enough from the first beacon, reception of the second localization message will be much stronger than reception of the first localization message. In that region, the second localization message drowns out the first localization message and the receiver can correctly receive the second localization message.

An advantage of the localization system according to the invention is that it is less sensitive to changes in signal strength due to a changing environment. Since the first and second localization messages are sent very close together in time, the

environment will be the same for both. However, for an RSSI based system there may be large differences in environment between messages, and especially so between training phase and localization phase.

Locations are separated by blind spots that function as buffer zones between locations. The localization system according to the invention is more robust against false positives as it guarantees equal circumstances for all beacon messages. An advantage of the localization system according to the invention is that it can correctly localize moving receivers. Other localization systems that rely on multiple messages can produce incorrect results for moving receivers, especially if the receiver is able to move from one location to another, while the messages are being received. Because the localization system according to the invention uses only one received message, it will produce correct localization results even when the receiver is moving.

An advantage of the localization system according to the invention is that it requires less energy than conventional systems. For example, an RSSI based system requires multiple messages and advanced computation. Since RSSI based systems are more sensitive to interference they requires multiple messages to reduce errors in the localization. Multiple messages require the system to be running in a powered mode for a longer time. With a receiver according to the invention the location can be determined using a single received message. After a message has been received the receiver may enter a lower-power stand-by mode for some amount of time. The time spent in stand-by may be determined by a desired location update interval of tracking the receiver. The time spent in stand-by mode by the receiver may be increased if a battery power level is below a certain power limit, e.g., a predetermined power limit.

Furthermore, an RSSI based system requires advanced computation since they determine location based on triangulation or comparing messages with trained data. This requires the receiver to perform mathematical calculations. Long calculations use more power.

Since the localization system according to the invention can be made to work with the receiver powered for less time and requiring less computation this will result in significant reduction in energy consumption.

Reduction in energy consumption is advantageous since a mobile receiver is often battery powered.

Increased energy efficiency leads to longer lifetimes, i.e. it is not required to change the battery as often. The system is also less complicated. Battery-less receivers are more likely to harvest sufficient energy for operation.

An advantage of the localization system is that it does not need electronics to measure reception strength such as would be required for an RSSI based system.

An advantage of the localization system is that it may be applied both indoor and outdoor. The localization system may be applied in agriculture. Using this system one can track the whereabouts of animals and farming robots may rely on this system for navigation, both indoors and outdoors.

The receiver needs only little power for at least two reasons. First, the receiver needs only to be on for the time needed to receive one message instead of many. The location can be instantly derived from a single message, thus greatly reducing the number of messages per second needed to detect the location. Second, the location may directly be obtained from the message. No triangulation is required and no lookup database is used, this saves a significant amount of energy.

The system is very simple which results in a significant cost reduction. A simple microcontroller with little RAM and FLASH is sufficient.

These advantages allow for very small and portable receiver units, e.g., key fob size.

The walls, floors and ceilings help to weaken signals from other rooms. The system uses these barriers to aid localization. Thus instead of complicating the system and requiring a training phase, the natural reduction in signal strength due to the environment improves localization in the invention.

In regions where both localization messages are of comparable strength the interference will be so severe that it impedes the correct reception of either signal. A person skilled in the art may verify with direct experimentation with a given receiver, in which regions he will be able to receive the first localization message and in which regions he will be able to receive the second localization message. The receiver can correctly receive at most one of the localization messages that were transmitted simultaneously. If the receiver receives the first localization message, one can deduce that the receiver is located in a region close to the first beacon. If the receiver receives the second localization message, one can deduce that the receiver is located in a region close to the second beacon. If no localization message is received other conclusions may be drawn, although less firm. Not receiving a localization message may indicate that the receiver is located in a 'blind zone' in between two beacons. It may also indicate that the receiver is outside of reception altogether, or that the system is malfunctioning. By placing the first and second beacons at places where localization of a receiver is desired, an area can be portioned into regions in which the receiver will be localized as soon as the receiver moves into the region.

The localization system is able to localize a receiver as being present in one of multiple regions. It is not necessary for the first and second localization message to overlap with perfect precision. It is sufficient that they at least partially overlap, as long as the overlap causes such interference for the receiver to receive only the strongest of the two localization messages.

The localization system for locating a receiver may comprise multiple beacons, each beacon of the multiple beacons configured for wirelessly transmitting a respective localization message, the receiver being configured for wireless reception, the localization system comprises a synchronization system for synchronizing the transmitting of respective localization messages so that the respective localization messages at least partially overlap, the receiver being configured for wirelessly receiving at most one of the respective localization messages. The number of multiple beacons, may be two as described above, but may also be more than 2 , e.g. , more than 3, more than 4, 8 or more, 16 or more, 10 or more, 50 or more, etc. Multiple receivers may be used at the same time in the system.

Preferably, the beacon transmits omnidirectional signals. In an embodiment, the localization messages contain a checksum in order to detect errors in the transmission of the localization message. The checksum depends on the content of the localization message in which it is comprised. The receiver may comprise a checksum checker for verifying that the checksum corresponds to the content of the received localization message. A localization message for which the checksum cannot be verified is discarded. A checksum can be a cyclic redundancy check (CRC), a hash, a digital signature etc. A localization message may contain a variable number, such as a nonce (number used once), a random number, a time stamp, etc. A variable number increases the effectiveness of the checksum.

A message with a checksum that does not correspond to the content of the message is not correctly received and ignored by the system.

In an embodiment, the first and second beacons are configured for sending RF signals. The first and second localization messages are RF signals and the receiver is configured for receiving an RF signal. The interference between the first and second localization message is caused by overlap in time and frequency of the two messages. In an embodiment, the first beacon is placed in a first room and the second beacon is placed in a second room different from the first room. In a further embodiment, the power level of the first and second beacon is proportional to the room in which they are placed.

In an embodiment, the receiver is an electronic mobile receiver. A mobile receiver can move from one region to another. The moved receiver can be detected by the localization system. For example, in a first region the receiver receives the first localization messages, as the receiver moves from the first region to the second region, the receiver will stop receiving the first localization message and start receiving the second localization message. By detecting this change, the movement of the receiver is detected. The localization system may also be used for normally stationary objects, for example, as part of an alarm system wherein the localization system detects unauthorized movement of an object provided with a receiver.

In an embodiment, the receiver is configured for correct reception of one signal of the first and second localization message provided that reception of that one signal is sufficiently stronger than a reception of the other signal of the first and second localization message.

The first and second beacon may transmit the first and second localization messages at first and second power levels respectively. The first and second power level may be different. This is convenient, for example, if the beacons are placed in different rooms of different sizes. The power levels are chosen such that in the immediate vicinity of the first beacon reception of the second localization message is drowned out, i.e. suppressed, by the first localization message; in the immediate vicinity of the second beacon reception of the first localization message is drowned out, i.e. suppressed, by the second localization message. This guarantees that in at least one region only the first localization message can be received, while in another region only the second localization message can be received. Since the wireless signals attenuate with distance, this can always be achieved by placing the first and second beacon far enough apart or by lowering their transmitting power levels sufficiently. For example, in an embodiment, the first and second beacons are placed at a distance from each other and transmit at an equal power level. The localization may be one at various places in the system. For example, in an embodiment, the localization system comprises a localization module for localizing the receiver. The localization module may be comprised in the receiver. For example, the receiver may comprise a device, such as a display, to inform a user of the receiver of his current location. The localization module may also be comprised in a server. For example, the receiver may transmit the localization messages it receives to a server. The server and/or the localization module may also be comprised in a beacon. The receiver leverages the so-called capture effect, which is an inherent feature for most radio transceivers. The capture effect is that only the stronger of two signals at, or near, the same frequency will be demodulated with complete suppression of the weaker signal at the receiver.

There are several ways to configure a receiver such that it correctly receives the stronger of two colliding messages. For example, the messages may start with a stronger message detection phase. During the stronger message detection phase, the receiver determines which one of the messages is the stronger and commits to receiving that message. If that stronger message is sufficiently stronger it will be correctly received. Using the capture effect in this way requires accurate time synchronization to within stronger message detection phase margins. If the overlap is not exact enough the receiver will not receive any of the signals (this does not apply to MiM receivers see below). The stronger message detection phase may contain any one of a starting period having a raised noise level, a preamble, and a network address.

An alternative way to configure a receiver such that it correctly receives the stronger of two colliding messages is to configure the receiver with so-called message-in- message (MiM) capabilities. Such a receiver will terminate reception of a first message as soon as a subsequent stronger message arrives. If the receiver is configured with Message-in-Message reception, then time synchronization could be relaxed a bit. Message-in-Message reception also decreases the size of the blind spot, because MiM requires less difference in signal strength between two messages. In an embodiment, the first localization message comprises a first identifier and the second localization message comprises a second identifier. The first identifier is different from the second identifier. The first and second identifiers may be associated to the first and second beacons respectively, to allow easy identification of the nearest beacon. By receiving a localization message, the first or second identifier comprised in the stronger signal is obtained.

It is also possible to obscure the relation between identifier and beacon, e.g., by randomizing the identifiers so that the correlation between identifier and beacon is obscured. In this way, only the randomizing party, typically a server, can use the localization system.

The location identifier may comprise geographical coordinates, e.g., geographical coordinates of the beacon transmitting the location identifier. In an embodiment the location identifier was complemented with location information formatted to be compatible with GPS information, such that a mobile device could use GPS outdoors and the localization system indoors. Even though the system is a localization system it may be used as a positioning system if coarse positioning is all that is required. The localization module is configured for establishing that the receiver is located in the vicinity of the first beacon in case of the receiver receiving the first identifier and for establishing that the receiver is located in the vicinity of the second beacon in case of the receiver receiving the second identifier. In an embodiment, the first localization message comprises a first header and a first body and the second localization message comprises a second header and a second body. The first identifier is contained in the first body and the second identifier is contained in the second body. The synchronization system is configured for synchronizing the transmitting of the first localization message and the transmitting of the second localization message so that first header and the second header at least partially overlap. The headers function as stronger message detection phases.

The receiver is configured such that reception of a weaker signal is canceled in favor of a stronger signal. By synchronizing such that the collision already occurs during the header the collision occurs before either body has started transmitting. This means that the receiver need not be configured to anticipate a stronger signal during reception of the body part. This allows a simpler, i.e., cheaper receiver to be used. In an embodiment, the header comprises, e.g. starts with, a preamble comprising a bit sequence used to detect 0 and 1 levels in the receiver. For example, the preamble may comprise a predetermined number of 0 ,1 transitions. In an embodiment, the preamble is one byte long and is either 01010101 or 10101010. In an embodiment, the synchronization system is configured for synchronizing the transmitting of the first localization message and the transmitting of the second localization message so that their respective preambles at least partially overlap.

A header may also comprise a network address. Before transmitting data the beacon may enter a settling mode, i.e. a starting period having a raised noise level, in which weaker signals are already drowned out. We will consider such settling time as part of the header. The network address may be ignored by a receiver for localization messages. The network address may be a special broadcast address. The network address may be omitted. The network address may be an address of the receiver.

The network address may be the address of the entire network. For example, the Nordic transceiver is configured in this way. This implies the address is the same for all messages from all devices in the network. In effect, the network address can thus function as an extension of the preamble. By choosing a long network address that differs from the preamble, any incorrectly received bit causes the receiver to stop receiving the weaker message and start receiving the stronger message. Synchronization may be established in a variety of ways. In an embodiment, the synchronization system comprises a first synchronization module comprised in the first beacon and a second synchronization module comprised in the second beacon.

For example, the first and second synchronization module may each comprise a clock. The first and second synchronization module may be configured to transmit the respective localization messages at predetermined moments in time. The clocks are synchronized with any suitable synchronization mechanism, for example, using an online synchronization protocol, or offline synchronization. For example, a server or one of the beacons may transmit a synchronization message to another one of the beacons; the synchronization message comprising a timestamp, the receiving beacon synchronizing its clock with the timestamp.

In an embodiment, the first beacon is configured to transmit a first localization message after each elapse of a predetermined amount of time, each first localization message comprising a first identifier, the second beacon is configured to transmit a second localization message after each elapse of the predetermined amount of time, each second localization message comprising a second identifier. A next one of the first localization messages overlaps with a next one of the second localization messages.

The first and second beacons may be configured to repeatedly transmit a first and second localization message respectively. The first and second beacon may be configured to transmit a localization message at regular intervals.

If the beacons comprise a clock, then the clock may be configured for sending at a particular predetermined moments so as to cause overlap.

In an embodiment, the localization system comprises a third beacon configured for wirelessly transmitting a third localization message, the localization system being configured such that the third localization message does not interfere with the first and second localization messages.

One potential drawback of the localization system according to the invention is the existence of blind zones. Between two simultaneously transmitting beacons there are some regions in which neither beacon can be received. The size of a blind spot depends on the ability of the receiver to discern signals with different strength. A radio which is able to distinguish radio signals with less difference in strength has a reduced size of the blind spots.

If a receiver stays in a blind zone, it cannot be localized. This disadvantage can be somewhat mitigated by monitoring and logging the location of the receiver. As soon a receiver moves into a blind zone, its location can be estimated, in a first approximation, by its last known location. This may be sufficient, since receivers are not likely to remain in a blind zone indefinitely. A more refined solution comprises assigning the beacons to two or more groups. The system is configured such that localization messages from one group do not interfere with those of another group. The beacons can now be placed such that blind zones of one group are covered by beacons of the other group. A receiver has an increased chance of correct reception of a beacon of at least one group.

The localization system may be configured so that the third localization message does not interfere with the first and second localization messages, by transmitting the third localization message on a different frequency and/or in a non-overlapping time-slot. In a preferred solution the third localization message uses the same frequency as the first and second localization message but a non-overlapping time-slot; this reduces system complexity. Also the third localization message may be an RF signal.

In an embodiment, the synchronization system is configured for desynchronizing the transmitting of the third localization message with the transmitting of the first and second localization message so that the third localization message does not overlap with the first and second localization messages.

In an embodiment, the first localization message comprises a first identifier and the third localization message comprises a third identifier which is equal to the first identifier.

One application of the localization system is to use multiple beacons and to place a beacon of the multiple beacons in each one of multiple rooms of a building, e.g., in each room of a building in which localization is desired. All beacons could transmit a localization message simultaneously. The walls would significantly reduce the occurrence of blind zones. Blind zones could still result, but those would likely be in the vicinity of walls.

To cover a larger room however, it may be desired to use two beacons, i.e., to place two beacons in the same room. For example, this may occur if one only wants to use low-power beacons. However, if those two beacons are of the same group, there may be blind spots in the middle of a room, which is undesirable. By placing two beacons of a different group, i.e., that do not interfere with each other, this is avoided. The two beacons in the same room could transmit the same identifier, i.e., an identifier associated with the room, so that localization would still correctly place the receiver in the room.

In one embodiment of such a system, the first localization message comprises a first identifier, the second localization message comprises a second identifier, third localization message comprises a third identifier, the first and third identifier are equal, the first and second identifier are different. The localization module is configured for establishing that the receiver is located in the vicinity of the first beacon and third beacon in case of the receiver receiving the first identifier and for establishing that the receiver is located in the vicinity of the second beacon in case of the receiver receiving the second identifier.

In an embodiment, the localization system comprises a data transfer system for transporting data from the receiver to a server, the receiver being configured to transfer a received one of the first or second identifier to the server with the data transfer system. More in particular, in an embodiment the localization system comprises a data transfer system for wirelessly transporting data from the receiver to a server, the receiver being configured to transfer a received one of the first or second identifier to the server with the data transfer system, wherein the data transfer system is configured to transmit wireless using the same frequency as the first and second beacon.

The ability of transferring data from the receiver to a server, allows tracking of the receiver. The receiver can be attached to any object or person for which localization is desired. The data transfer can be wireless, e.g., using TDMA. The TDMA allows part of the bandwidth to be used for data transfer, in which collision is avoided, and part of the bandwidth for localization in which collision is actively sought and exploited.

An aspect of the invention concerns a first beacon for use in a localization system for locating a receiver, the first beacon being configured for wirelessly transmitting a first localization message, the first beacon comprises a synchronization module for synchronizing the transmitting of the first localization message with the transmitting of the second localization message by a second beacon so that the first and second localization message at least partially overlap.

An aspect of the invention concerns a receiver for use in a localization system for locating the receiver, the receiver being configured for wirelessly receiving at most one of a first localization message and a second localization message, the first and second localization message at least partially overlapping.

In an embodiment the receiver is configured to transfer a received one of the first or second identifier to a server using a data transfer system.

An aspect of the invention concerns a method for locating a receiver configured for wireless reception, the method comprising wirelessly transmitting a first localization message by a first beacon, wirelessly transmitting a second localization message by a second beacon, synchronizing the transmitting of the first localization message and the transmitting of the second localization message so that the first and second localization message at least partially overlap.

In an embodiment the method comprises wirelessly receiving at most one of the first localization message and the second localization message by the receiver, and correctly receiving a stronger signal of the first and second localization message provided a reception of the stronger signal is sufficiently stronger than a reception of the other weaker signal of the first and second localization message. The invention has been successfully implemented on a 2.4Ghz radio based sensor network but may work with any technology wherein signal strength reduces exponentially with the distance to the signal source. Such technology includes radio, acoustic and light based systems. A beacon and/or receiver according to the invention may be an electronic device. The receiver may be a mobile electronic device, such as a mobile phone, or laptop. The beacon may be a transceiver, e.g. a wireless sensor, WIFI router etc.

A method according to the invention may be implemented on a computer as a computer implemented method, or in dedicated hardware, or in a combination of both. Executable code for a method according to the invention may be stored on a computer program product. Examples of computer program products include memory devices, optical storage devices, integrated circuits, servers, online software, etc. In a preferred embodiment, the computer program comprises computer program code means adapted to perform all the steps of a method according to the invention when the computer program is run on a computer. Preferably, the computer program is embodied on a computer readable medium. An aspect of the invention concerns one or more processor readable storage devices having processor readable code non-transiently embodied thereon for programming one or more processors to perform a method according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail by way of example and with reference to the accompanying drawings, wherein:

Figure 1 is a block diagram schematically illustrating an embodiment of the localization system according to the invention,

Figure 2 is a schematic diagram illustrating simultaneously transmitting beacons, Figure 3 is a schematic top view of two rooms,

Figure 4 is a diagram illustrating a TDMA schedule,

Figure 5a is a top view of a floor having six rooms.

Figure 5b is a top view of a hall way,

Figure 5c is a top view of a large meeting room,

Figure 6 is a top view of two rooms, a hallway and a meeting room,

Figure 7 is a flow chart illustrating a method according to the invention,

Figure 8 is a diagram illustrating partially overlapping localization messages.

Throughout the Figures, similar or corresponding features are indicated by same reference numerals.

List of Reference Numerals: 100 a localization system for locating a receiver

1 10 a first beacon

1 15 a first synchronization module

120 a second beacon

125 a second synchronization module

130 a receiver

135 a localization module

140 a third beacon

145 a third synchronization module

170 a server

210 a range of beacon 1 10

220 a range of beacon 120

232, 234 a border indicating a blind zone

400 a TDMA schedule

410 a data transfer period

420 a first localization period

422 a second localization period

700 a flow chart

710 synchronizing a first beacon and a second beacon

720 wirelessly transmitting a first localization message by a first beacon,

730 wirelessly transmitting a second localization message by a second beacon,

740 enabling a receiver for reception of a stronger signal of the first and second localization message,

745 determining if a localization message has been received,

750 locating the receiver

755 signaling a problem

812, 814 stronger message detection phase

842, 844 data

DETAILED EMBODIMENTS

While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail one or more specific embodiments, with the understanding that the present disclosure is to be considered as exemplary of the principles of the invention and not intended to limit the invention to the specific embodiments shown and described.

Figure 1 illustrates a localization system 100 for locating a receiver 130. The system comprises multiple beacons. Three beacons are shown: a first beacon 1 10 comprising a first synchronization module 1 15, a second beacon 120 comprising a second synchronization module 125 and a third beacon 140 comprising a third

synchronization module. The system further comprises a receiver 130.

Receiver 130 is currently in range of beacons 1 10, 120 and 140.

The number of beacons and receivers shown in Figure 1 is only exemplary. The invention can be practiced with larger numbers of beacons or receivers.

The beacons of localization system 100, including beacons 1 10, 120 and 140 have been configured for wireless transmission of localization messages. In this

embodiment, the localization messages are transmitted as RF signals at a radio frequency of 2.4 GHz. This allows the system to be integrated in wireless radio networks, such as wireless sensor networks (WSN's), without adding a second additional hardware. Other radio frequencies are possible.

The beacons have been configured to repeat sending their localization message according to a fixed TDMA schedule, this will be further detailed below. The beacons of localization system 100 have been assigned to two groups. Two beacons of different groups have been configured such that their localization messages do not interfere with each other. In Figure 1 , beacon 1 10 and 120 belong to the same group. Beacon 140 belongs to a second group. A group may contain multiple beacons, e.g., more than 2. Multiple groups may be used, e.g., more than 2, say 3. A group could also contain only a single beacon, in which case its messages are desynchronized with the beacons in other groups.

At least one group comprises two or more beacons. If only one group with only one beacon were used, it would still be possible to detect whether or not the receiver is in range of the beacon or not. The beacons from the same group have been synchronized such that they transmit their localization messages so that they at least partially overlap. Receiver 130, regardless of its location can receive at most one of the messages sent by the multiple beacons in a group. For example, receiver 130 can receive at most one of the messages sent by beacons 1 10 and 120.

In this embodiment, the beacons from the same group are configured to transmit their localization messages at the same time and at the same frequency. However, due to limitations in synchronization precision, the localization messages may only partially overlap. In this embodiment all overlapping messages overlap in their header.

If the receiver receives a localization message, then it knows its location with near certainty. As all beacons in a single group are transmitting simultaneously the conditions under which the beacons were transmitted were identical for all beacons. All beacons suffer from the same environmental interference (Wi-Fi etc.). This implies that the system cannot return false positives (incorrect locations) due to environmental interference. It is not immune to the interference, but if the interference is too strong this will result in no message being received and therefore no information on the location will be available. This results in the system not having to filter for this interference and therefore the system requires only one message each time to detect the location.

In a single group scenario, the energy required to locate a receiver each second is the energy consumed by a single transmission in each beacon and listening to just one message in each receiver and processing the information. This information may also contain, in addition to a location identifier, a synchronization timestamp and directions for transmitting localization data to the network. In an embodiment this required less than 2 milliseconds, mainly due to receiver startup-time (1 .5ms). The remainder of the period the system is in an idle state, using almost no energy. This implies that, if the period would be, for example, one second and the system would be compared to a technology that requires the receiver to actively listen continuously, that this system is a factor 500 more energy efficient. This does not apply to passively listening devices such as RFID tags. In the invention, all that is required in a receiver is enough memory, say RAM, to receive one beacon message, a timer and almost zero processing capacity as the location can be directly extracted from the received beacon. In other systems filtering, triangulation and lookup of trained data require both intensive processing and more memory. This requires more complex and more expensive processors and more energy to power the processor and the memory. This in turn requires bigger, less portable devices as they need to contain larger or more batteries. Therefore all objects and persons of which the location needs to be detected require these bigger more expensive receivers and in most applications a great many receivers are used.

Synchronization

One way to achieve synchronization is to have a clock in each one of the

synchronization modules. The beacons may then be configured, e.g. programmed, to transmit at fixed predetermined moments in time, i.e., according to a predetermined schedule.

Another way to achieve synchronization is to have all beacons connected to a synchronization master, say using a wired network. The synchronization master sends the correct time to the beacons. Alternatively, the synchronization master may broadcast a transmit command to all beacons in a group in response to which each beacon in that group transmits a localization message.

Another way to achieve synchronization is to wirelessly distribute synchronization messages. This may use a TDMA schedule having a data phase in which the beacon can send and/or transmit data messages which do not interfere with each other. Each synchronization module comprises a clock and a memory storing a quality number. During the data phase each beacon wirelessly transmits a synchronization message containing a time stamp indicating the moment of sending according to its clock and copy of its quality number. If a beacon receives a synchronization message with a higher quality number than its own quality number, then it will use the time stamp in the received message to set its clock, the quality number is set to the received quality number minus 1. Preferably, the quality number is also lowered if an expected synchronization message is not received. One central node is selected, the so-called synchronization master that has a fixed quality value, higher than the quality values of the other beacons. The clock used in the synchronization module needs only to be accurate for several synchronization intervals as it is constantly adjusted. This implies a far simpler and cheaper clock than the one that would be required to keep beacons synchronized without communication.

In figure 1 , first beacon 1 10 has been selected as synchronization master. The synchronization master is started with a higher quality number than the other beacons. The synchronization master can never receive a synchronization message from a beacon with a higher quality number and will not set its clock. As synchronization messages are distributed in the network all beacons become synchronized, even though they are not necessarily in direct range of the synchronization master.

For example, beacon 140 may receive a synchronization message of beacon 1 10, and beacon 120 may receive a synchronization message of beacon 140.

Synchronization message can typically reach further than localization messages since they are not interfered with.

However, any data transfer protocol that embodies a sufficient level of synchronization will do. Examples include: the MyriaNed and other wireless network protocols. Other methods are also possible such as wires with synchronization pulses.

The receiver units do not need to use the synchronization method used to synchronize the beacons with each other.

It is not necessary for the receivers to synchronize. However, having the receivers synchronized has the advantage that the receiver can predict when the next localization message will be sent. The time between two localization messages the receiver may be in a stand-by mode, i.e. sleep mode, in which it does not listen for messages and consumes less power.

Synchronizing a receiver may be done with using the above mentioned

synchronization methods. The receivers may use a timestamp from a beacon message too. As all beacons transmit at the same time, synchronization for a receiver is very simple. It just needs to wait a fixed interval before starting to listen again. Note that in an embodiment all information required by the receiver is transmitted as part of the beacon messages, this includes the moment that the receiver may transmit the detected location to the server in a tracking application, but is not limited to this. The beacon messages may even include data such as date and time.

Localization

Localization system 100 comprises a localization module 135. Localization module 135 may be comprised in receiver 130. Optionally, localization system 100 may comprise a server 170 in which localization module 135 may be comprised.

Localization module 135 may be comprised in a beacon as well.

In this embodiment, the localization messages comprise an identifier which carries information about the location and/or identity of the beacon. If receiver 130 is able to receive a localization message, the identifier contained in it can be obtained. The identifier may uniquely identify a beacon within its group. In that case localization module 135 can deduce that receiver 130 is closest to that particular beacon than to any other beacon in that group. Similarly, if receiver 130 next receives a localization message from a beacon in the second group, localization module 135 can deduce that receiver 130 is closer to that beacon than to any other beacon in the second group. Using multiple groups helps reducing the number of blind spots.

A receiver typically passes through a blind zone to get from one region in which the receiver may be localized to a next region. One possible implementation uses the following algorithm:

If a beacon is received, return location.

If no beacon has been received for less than T seconds and the network is in range (e.g. data phase messages are received) return the last known location.

If no beacon has been received for more than T seconds and the network is in range, return a signal indicating unknown location.

If no beacon has been received and no network data messages were detected, return a signal indicating out of network range. The value of T depends on the application of the localization system. For example, 5 minutes may be appropriate for tracking persons, but different values may be used for tracking objects or animals.

Message Reception

Figure 2 helps explaining the principle of colliding messages. Figure 2 shows two beacons: beacon 1 10 and beacon 120. Receiver 130 is configured to receive localization messages. A range of beacon 1 10 is indicated with line 210. A range of beacon 120 is indicated with line 220. The range is defined as the area in which a message of the beacon could be received in the absence of interference of another beacon, in this case the other beacon. Note that the ranges are not perfectly circular, since the range is influenced by the terrain, e.g., by walls etc. In this case the locations may well be separated by a wall with a door in the middle. The two beacons shown are in the same group and have synchronized the sending of localization messages.

The two beacons need not transmit at the same power level, but neither one completely overpowers the other. That is, in the immediate vicinity of beacon 120, the reception of beacon 120 is stronger than that of first beacon 1 10 so that in at least one region the second localization message will be received. In the immediate vicinity of beacon 1 10, the reception of beacon 1 10 is stronger than that of first beacon 120 so that in at least one region the second localization message will be received. In this case, beacons 1 10 and 120 lie in each other range. In figure 1 , beacon 140 is in range of beacon 1 10.

In the region marked I in figure 2, reception of beacon 1 10 is so much stronger than that of beacon 120, that even though the two beacons are in range in that region only the message of first beacon 1 10 will be correctly received.

The region marked III, which is between the dotted lines 232 and 234, is a blind zone. Although both beacons are in range, each one of the beacons blocks reception of the other beacon In the region marked II only beacon 1 10 is in range, so messages of the beacon will be correctly received.

In the region marked V only beacon 120 is in range, so messages of the beacon will be correctly received.

If receiver 130 is moved into regions IV and V, it can deduce that it is near beacon 120 since it can only receive localization messages of beacon 120. If receiver 130 is moved into region III, it cannot make a straightforward deduction since receiver 130 cannot receive messages from beacon 1 10 nor beacon 120. Once receiver 130 moves into region I or II, it can deduce that it is near beacon 1 10 since it can only receive localization messages of beacon 1 10.

The presence of a blind zone can be mitigated by using multiple groups.

Figure 3 shows a simplified reception figure. It shows only the localization ranges as influenced by the walls. In figure 3, each one of the beacons 1 10 and 120 have been placed in different rooms. The wall reduces the power of a signal as it passes from one room to another. Although there is a blind zone in which no localization is possible, it is small and only close to the wall.

TDMA

It has been found advantageous to use Time Division Multiple Access (TDMA) to build a beacon network. TDMA is a channel access method for shared medium networks. It allows several beacons to share the same frequency channel by dividing the signal into different time slots. The beacons transmit localization messages at the same time, but transmit data messages in succession, one after the other, each using its own time slot. This allows multiple beacons to share the same transmission medium (e.g. radio frequency channel) while using only a part of the channel capacity.

The beacons are preferably connected in a network which allows the beacons to receive data from a central server and transmit data to a central server. The

connection between a beacon and the server need not be direct, but may go via one or more other beacons, referred to as 'hopping'. The latter is especially advantageous in wireless networks. Although TDMA is an effective low cost solution to create a data transfer system for transporting data from the beacons to a server, there are other options. For example, the data transfer system may comprise a wired network between the beacons and the server. For example, the network may be running Ethernet. Using the data transfer system for transporting data from the beacons to a server, a data transfer system for transporting data from the receiver to a server may be created. The receiver may wirelessly forward data, such as a received localization message or the identifier contained therein, to a beacon, which may then forward it to the server. The server may be one of the beacons.

Figure 4 shows a TDMA schedule 400 that may be used with the system shown in Figure 1 . The horizontal axis is a time axis. The time axis runs from 0 to 1 and illustrates one TDMA cycle. This TDMA cycle is repeated, possibly indefinitely. The TDMA cycle preferably lasts 1 second; the cycle may be faster, say 30 ms or slower, say 2 seconds.

A slower cycle conserves more energy at the beacon. The shown TDMA cycle uses two localization phases and only one data phase, to reduce the number of messages for processing at the receiver. The receiver may save energy by not listening to all beacon phases. For example, a receiver that is energy constrained may only listen to some proportion of receivable localization messages, say one of every four localization phases. TDMA schedule 400 shows a data phase 410 and two localizations phases 420 and 422. There may be more data phases, and more or less localizations phases. TDMA schedule 400 shows time slots for three beacons 1 10, 120 and 140, although more beacons may be present. The TDMA schedule runs from the points marked 0 to the point marked 1. The TDMA schedule is repeated for multiple cycles. One cycle may last e.g. 1 second. The beacons are synchronized such that the start of a cycle is synchronized across all beacons in all groups. However, beacons transmit at different times in the schedule depending on their group.

A data phase, such as data phase 410, is a time period during which, each one of the beacons 140, 1 10 and 120 is assigned a timeslot in which only that one beacon may send data. Sending data during the data phase has the advantage of no interference from other beacons. During a localization phase, such as localization phases 420 and 422 each group is assigned a timeslot in which only that group transmits a localization message. Figure 4 shows two groups: a first group comprising beacons 1 10 and 120, and a second group comprising beacon 140.

Figure 4 shows that during localization phases 420 and 422, the beacons of the first group transmit simultaneously. Correct reception of a localization message is only possible if one message is sufficiently stronger than the other. Beacon 140 transmits a localization message during a different timeslot. TDMA schedule 400 may be modified to omit beacon 140 and its corresponding timeslots.

Since beacons 1 10 and 120 transmit a localization message at the same time, the reception of a localization message contains information on the location of the receiver. The receiver must be located in a position in which reception of the received beacon is stronger. If beacons 1 10 and 120 are placed in different rooms, the reception of a localization message is a strong indication that the receiver is also in that room. Reception in a particular room of a beacon which is not in that room is significantly attenuated compared to a beacon which is in the room. This allows correct reception of a beacon in the same room as the receiver.

If beacon 140 is the only beacon in its group, its localization messages will not be interfered with. Accordingly, the localization messages of that beacon are likely to be received whenever a receiver is in range. Having such a beacon confers several advantages. First of all, the messages of beacon 140 may incorporate a time stamp, which allows the localization messages of beacon 140 to be used as synchronization messages. Second, since a localization message of beacon 140 is likely to be received, it gives the information that the receiver is in range of the system. If no other localization message is received, it can be deduced that the receiver is in range of the system but currently in a blind zone.

The slots for data transfer may have fixed positions in time in the schedule but a data transfer slot may be dynamically assigned to a beacon or receiver. In an embodiment, the interval between beacon transmissions may be incremented, e.g., in factors of two, to save energy. To save even more energy the localization system may be disabled, e.g. at night or during weekends, by disabling the beacons.

The TDMA schedule shown is only an example. Completely different schedules work just as well. Preferably, the receiver knows when to listen for beacons though, so that it does not need to be permanently powered.

In an embodiment a localization message contains a cycle time length. The cycle time length indicating that after each elapse of the cycle time length, a localization message will be transmitted by the beacons in that group. The receiver may comprise stand-by means for switching to a stand-by mode while no localization messages are expected.

Figures 5a, 5b, 5c and 6 show several different ways of arranging beacons in a building. All figures show one floor of a building. The localization system can also be used in three dimensions and/or across multiple floors. The beacons are labeled, A, B or C. Two beacons labeled with the same letter are in the same group and transmit at the same time. Figure 5a, shows a top view of a floor of a building with multiple rooms. For this particular floor, 6 rooms are shown. In each room a beacon is placed. This system can localize in which room a receiver is placed. The walls cause sufficient attenuation of other beacons that in most places exactly one localization message is correctly received.

Figure 5b shows a hallway. Covering the hallway with a single beacon is less advantageous, since the beacon would have to be quite strong to cover the entire hallway. If only beacons A were used (omitting beacons B), there would be blind zones in the hall way. By using two groups, blind zones are avoided.

For the identifiers that are transmitted by the beacons various choices are possible. For example, all beacons in a localization system may have a unique identifier assigned to it. Applying this principle to Figure 5b has the advantage of highest possible resolution. At some places, approximately indicated by the dashed lines, two identifiers would be received (recall that groups A and B transmit in different time slots and do not interfere).

To reduce administration overhead, one could also assign a different identifier to beacons in different rooms. Beacons in the same room would be assigned the same identifier. Applying this principle to Figure 5b has the advantage of having an immediate correspondence between received identifier and a room. In this case, all shown beacons would use the same identifier. Figure 5c, shows a possible arrangement of beacons in three groups, suitable for localization in large room such as a co-working space, auditorium, meeting room etc. The beacons are placed on the vertices of triangles in a regular triangular tiling, such that each triangle of the tiling has one beacon of each group on a vertex. This arrangement of beacons results in a honeycomb pattern of localization regions.

Experimentation showed that, when the power of all beacon signals is tuned to the same power level, a blind zone is typically between one quarter and at most half of the range diameter of a beacon. From this it can be deduced that a honeycomb pattern of three beacon groups will not leave blind spots when used in a large area.

In an embodiment, each localization message contains a list of multiple location identifiers. Each one of the multiple location identifiers corresponds to a region. The list includes the identifier of the region that the beacon is in, in a distinguished position, say as the first in the list. The list also includes the identifier of regions that are neighboring regions of the region that the beacon is in. The receiver uses the following algorithm to determine the location: If only one beacon message is received, the identifier in the distinguished position is the location; If multiple beacon messages are received, the identifier that occurs in all messages is returned; If no beacon occurs in all messages, or if multiple beacons occur in all messages, then no location is detected.

Figure 6 shows a floor with two smaller rooms, a hallway and a large meeting room. Beacons are used in three groups. A different identifier is used for each room (four in total). The identifier sent by each beacon is indicated in the figure, as a number running from 1 to 4. The room with beacon A1 is smaller than the room with beacon A2. To compensate for the difference in room size, A2 transmits at a higher power level than A1 .

Transmitting at a power level proportional to the size, e.g. floor area size, of a room has the advantage that reception in a room is good in a large part of the room, while the size of blind zones is decreased.

Another method to compensate for size is by placing the beacon in a larger room, say room A2, near the outside wall and placing the beacon in, the smaller room, say room A1 , in the center of the room. As signal strength attenuates with distance this has a similar effect

Figure 8 is a diagram illustrating partially overlapping localization messages or, in other words, a collision. Figure 8 shows a first beacon, indicated by A and a second beacon, indicated by B, which have been arranged by a synchronization system so that localization messages sent by beacon A, such as the first localization message, at least partially overlap localization messages sent by beacon B, such as the second localization message. In figure 8 time is progressing from left to right. Transmission of the message of beacon B is started during transmission of the message of beacon A, causing a collision of the messages during transmission. In figure 8, reference numbers ending in a '2' refer to beacon A whereas reference numbers ending in a '4' refer to beacon B. Note that beacon A may be beacon 1 10 and beacon B may be beacon 120 to which reference was made earlier. Beacon A and B are in the same group.

The figure shows two times lines, one for beacon A and one for beacon B. The timelines show transmitting of a localization message. Beacon B sends its message slightly later than beacon A. This may be by design, but may also arise due to limitations in the synchronization mechanism.

In periods 842 and 844 data is transmitted, preferably comprising a checksum.

Although the checksum is preferred, the system may be used without one. The data contains a location identifier. Before the data parts 842 and 844 a stronger message detection phase 812 and 814 is transmitted. A stronger message detection phase may comprise several parts. For example at the start, before a beacon transmits information the transmitter may already become active. During this period no data is transmitted. This period may last several tens of microseconds. For the Nordic transceiver this period occurs at the end of a so-called TX settling period.

Next, a so-called preamble may be transmitted as part of periods 812 and 814. The preamble comprises a bit sequence used to detect 0 and 1 levels in the receiver. Finally, also during periods 812 and 814a network address is transmitted.

For the localization system, any one of the preamble, network address, starting period etc are optional; As long as stronger message reception of two colliding messages is possible for the receiver at least in some region.

The periods 812, and 814 respectively are referred to as the headers of the two messages. Note that the headers, as defined here, include only adjacent fields that can be used as part of a stronger message detection phase. All other fields are considered to be part of the data.

A receiver who is receiving the message of beacon A, will switch to receiving a stronger message if that stronger message commences during the header of beacon A. In other words, if the start of stronger message detection phase 814 of the second message occurs before the end of the stronger message detection phase 812 of the first message, the receiver will switch to the stronger message. If the first message by beacon A is stronger, it will continue receiving with the first message. If a chosen receiver has a final switching time, then the final switching time occurs before start of the data, i.e., body. If no switch occurs, e.g., because start of period 814 occurs after the end of period 812 then the receiver may not start to receive message B, even if beacon B is stronger. However, the interference may well cause the reception of the first message to be incorrect, i.e., as indicated by the checksum. In that case no correct, i.e., valid message is received. A receiver can also be configured to always switch to the stronger message even if the strong message starts during a data period of the weaker message. This reduces accuracy demands on the synchronization. This would be the case in a message-in message-radio: this type of radio should be more sensitive and therefore the blind spots should be smaller.

A beacon may be a transceiver. A suitable transceiver is for example a 2.4GHz transceiver, such as the nRF24L01 (+) made by Nordic Semiconductor. There are many other types of suitable transceivers.

Figure 7 is a flow chart illustrating a method according to the invention. The flow charts shows 5 steps. Step 710 comprises synchronizing the transmitting of the first localization message and the transmitting of the second localization message so that the first and second localization message at least partially overlap. For example, Step 710 could comprise synchronize a clock of a first and second beacon. Step 720 comprises wirelessly transmitting a first localization message by a first beacon. Step 730 comprises wirelessly transmitting a second localization message by a second beacon. Step 740 comprises enabling a receiver for reception of a stronger signal of the first and second localization message. In Step 745 it is determined if a localization message has been received during step 740. In case a localization message has been received, the receiver configured for wireless reception is localized in step 750.

It is possible that nothing is received during step 740. To detect this, the receiver must be synchronized so it can detect the absence of a message. In case no localization message has been received, this is signaled in step 755. In step 755 it may be determined that the received is currently located either in a blind zone, is outside of range, or there is a malfunction and message reception is not possible for other reasons.

Preferably, step 740 is executed for a limited time period. Steps 720 and 730 are executed, at least partially, in parallel. Also Receiving 740 will happen largely in parallel to steps 720 and 730. This has been indicated with a dotted line from 710 to 740. Synchronization 710 is required before beacon transmission, but only one synchronization action may be required for several beacon transmissions. For example, synchronization is done once every 8 seconds and the beacons are sent once every second. Therefore step 710 is not required for every localization action.

Many different ways of executing the method are possible, as will be apparent to a person skilled in the art. For example, the order of the steps can be varied or some steps may be executed in parallel. Moreover, in between steps other method steps may be inserted. The inserted steps may represent refinements of the method such as described herein, or may be unrelated to the method. Moreover, a given step may not have finished completely before a next step is started. A result of the localization may be processed further. For example, the location of the receiver may be displayed, possibly overlaid on a map. The location of the receiver may be logged, e.g., in a database.

The receiver may be tracked by repeatedly localizing it and by communicating the detected location or received identifier to a server or other device. For communicating to the server, the receiver could utilize the data phase. However, if the receiver updates its location frequently, the system needs a data phase capable of transporting the data of all receivers in the network. A method according to the invention may be executed using software, which comprises instructions for causing a processor system to perform method 700.

Software may only include those steps taken by a particular sub-entity of the system, such as the beacon or the receiver. The software may be stored in a suitable storage medium, such as a hard disk, a floppy, a memory etc. The software may be sent as a signal along a wire, or wireless, or using a data network, e.g., the Internet. The software may be made available for download and/or for remote usage on a server.

It will be appreciated that the invention also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source code, object code, a code intermediate source and object code such as partially compiled form, or in any other form suitable for use in the implementation of the method according to the invention. It will also be appreciated that such a program may have many different architectural designs. For example, a program code implementing the functionality of the method or system according to the invention may be subdivided into one or more subroutines. Many different ways to distribute the functionality among these subroutines will be apparent to the skilled person. The subroutines may be stored together in one executable file to form a self-contained program. Such an executable file may comprise computer executable instructions, for example, processor instructions and/or interpreter instructions (e.g. Java interpreter instructions). Alternatively, one or more or all of the subroutines may be stored in at least one external library file and linked with a main program either statically or dynamically, e.g. at run-time. The main program contains at least one call to at least one of the subroutines. Also, the subroutines may comprise function calls to each other. An embodiment relating to a computer program product comprises computer executable instructions corresponding to each of the processing steps of at least one of the methods set forth. These instructions may be subdivided into subroutines and/or be stored in one or more files that may be linked statically or dynamically. Another embodiment relating to a computer program product comprises computer executable instructions corresponding to each of the means of at least one of the systems and/or products set forth. These instructions may be subdivided into subroutines and/or be stored in one or more files that may be linked statically or dynamically.

The carrier of a computer program may be any entity or device capable of carrying the program. For example, the carrier may include a storage medium, such as a ROM, for example a CD ROM or a semiconductor ROM, or a magnetic recording medium, for example a floppy disc or hard disk. Furthermore, the carrier may be a transmissible carrier such as an electrical or optical signal, which may be conveyed via electrical or optical cable or by radio or other means. When the program is embodied in such a signal, the carrier may be constituted by such cable or other device or means.

Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing, or for use in the performance of, the relevant method. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1 . A localization system (100) for locating a receiver (130), the system

comprising a first beacon (1 10) configured for wirelessly transmitting a first localization message and a second beacon (120) configured for wirelessly transmitting a second localization message, the receiver being configured for wireless reception,

characterized in that

the localization system comprises a synchronization system (1 15, 125) for synchronizing the transmitting of the first localization message and the transmitting of the second localization message so that the first and second localization message at least partially overlap causing a collision between the first and second localization message, the receiver being configured for wirelessly receiving at most one of the first localization message and the second localization message.

2. A localization system as in Claim 1 , wherein the receiver is configured for correct reception of a stronger signal of the first and second localization message provided a reception of the stronger signal is sufficiently stronger than a reception of the weaker signal of the first and second localization message.

3. A localization system as in any one of the preceding claims, wherein the first localization message comprises a first identifier and the second localization message comprises a second identifier, the first identifier being different from the second identifier, the localization system comprising a localization module for localizing the receiver, the localization module being configured for establishing that the receiver is located in the vicinity of the first beacon in case of the receiver receiving the first identifier and for establishing that the receiver is located in the vicinity of the second beacon in case of the receiver receiving the second identifier.

A localization system as in Claim 3, wherein the first localization message comprises a first header and a first body and the second localization message comprises a second header and a second body, the first identifier being contained in the first body and the second identifier being contained in the second body, the synchronization system being configured for synchronizing the transmitting of the first localization message and the transmitting of the second localization message so that first header and the second header at least partially overlap.

A localization system as in any one of the preceding claims, wherein the synchronization system comprises a first synchronization module comprised the first beacon and a second synchronization module comprised in the second beacon.

6. A localization system as in any one of the preceding claims, wherein the first beacon is configured to transmit a first localization message after each elapse of a predetermined amount of time, each first localization message comprising a first identifier,

the second beacon is configured to transmit a second localization message after each elapse of the predetermined amount of time, each second localization message comprising a second identifier.

7. A localization system as in any one of the preceding claims comprising a third beacon configured for wirelessly transmitting a third localization message, the localization system being configured such that the third localization message does not interfere with the first and second localization messages.

8. A localization system as in Claim 7, wherein the synchronization system is configured for desynchronizing the transmitting of the third localization message with the transmitting of the first and second localization message so that the third localization message does not overlap with the first and second localization messages.

9. A localization system as in Claim 7 or 8, wherein the first localization message comprises a first identifier and the third localization message comprises a third identifier which is equal to the first identifier.

10. A localization system as in any one of the preceding claims, comprising a data transfer system for wirelessly transporting data from the receiver to a server, the receiver being configured to transfer a received one of the first or second identifier to the server with the data transfer system, wherein the data transfer system is configured to transmit wireless using the same frequency as the first and second beacon.

1 1 . A localization system as in any one of the preceding claims, comprising a number of additional beacons, the first, second and additional beacons being configured for wirelessly transmitting a respective localization message, the synchronization system being configured for synchronizing the transmitting of the respective localization messages so that the respective localization messages at least partially overlap, the receiver being configured for wirelessly receiving at most one of the respective localization messages.

12. A first beacon for use in a localization system for locating a receiver, the first beacon being configured for wirelessly transmitting a first localization message,

characterized in that

the first beacon comprises a synchronization module for synchronizing the transmitting of the first localization message with the transmitting of the second localization message by a second beacon so that the first and second localization message at least partially overlap.

13. A receiver for use in a localization system for locating the receiver, the receiver being configured for wirelessly receiving at most one of a first localization message and a second localization message, the first and second localization message at least partially overlapping. A receiver as in Claim 13 configured to transfer a received one of the first or second identifier to a server with a data transfer system.

A method for locating a receiver configured for wireless reception, the method comprising

wirelessly transmitting a first localization message by a first beacon, wirelessly transmitting a second localization message by a second beacon, and

synchronizing the transmitting of the first localization message and the transmitting of the second localization message so that the first and second localization message at least partially overlap.

A method for locating a receiver as in Claim 15, comprising

wirelessly receiving at most one of the first localization message and the second localization message by the receiver, and

correctly receiving a stronger signal of the first and second localization message provided a reception of the stronger signal is sufficiently stronger than a reception of the other weaker signal of the first and second localization message.

A computer program comprising computer program code means adapted to perform all the steps of claim 15 or 16 when the computer program is run on a computer.

A computer program as claimed in claim 17 embodied on a computer readable medium.