WO2014135213A1

WO2014135213A1 - Positioning and tracking of nodes in a network

Info

Publication number: WO2014135213A1
Application number: PCT/EP2013/054647
Authority: WO
Inventors: Emanuele FRANCIONI; Sergey Igorevich VLADIMIROV; Fulvio VENTURELLI; Charlotte ALDERWISH
Original assignee: Hokuto Bv
Priority date: 2013-03-07
Filing date: 2013-03-07
Publication date: 2014-09-12

Abstract

An antenna-array system is proposed for determining a current location of a node. Two or more antenna elements are configured to receive an input signal from the node. The antenna elements are positioned in an antenna-array such that the input signal arrives at two neighboring antenna elements at different times. The time difference is measured and may be used to calculate an angle of arrival of the signal.

Description

Positioning and tracking of nodes in a network FIELD OF THE INVENTION

The present invention relates to positioning and/or tracking of nodes in a network. More specifically the invention relates to positioning and/or tracking of RFID nodes.

BACKGROUND

Systems for positioning and tracking of nodes in a network are known. Solutions based on Near Field ID Cards and barcodes e.g. can provide snapshots of discrete moments, but fail in providing a complete, up to date and accurate knowledge of the whereabouts of nodes. Systems based on e.g. WIFI,

Bluetooth and Zigbee provide positioning possibilities, but behave poorly in the neighborhood of walls, people and water sources. This is due to the fact that these technologies operate in a bandwidth that has been conceived for data transmission purposes and they are improperly employed in poorly designed short range tracking solution. Other known GPS or GPRS/3G solutions are expensive and provide best localization outdoors only .

An RFID tag reading system and method for position determination and tracking of passive RFID tags is disclosed in US 2005/0110641. The system uses phase-shifting technology and beam steering to detect the location of a tag. Its operating range is limited and the system is not suitable for

communication purposes.

There is a need for an improved communication system that allows positioning and/or tracking of nodes in a network.

SUMMARY OF THE INVENTION

According to an aspect of the invention an antenna- array system is proposed for determining a current location of a node. The antenna-array system can comprise two or more antenna elements configured to receive an input signal from the node. The antenna elements can be positioned in an antenna-array such that the input signal arrives at two neighboring antenna elements at different times resulting in a first intermediate signal from a first antenna element and a second intermediate signal from a second antenna element neighboring the first antenna element. The antenna-array system can further comprise comparator electronics configured to compare the first

intermediate signal and the second intermediate signal to obtain an output signal indicative of a time interval between reception of the input signal at the first antenna element and reception of the input signal at the second antenna element.

According to an aspect of the invention a method in an antenna-array system is proposed for determining a current location of a node. The method can comprise receiving in two or more antenna elements of an input signal from the node. The antenna elements can be positioned in an antenna-array such that the input signal arrives at two neighboring antenna elements at different times resulting in a first intermediate signal from a first antenna element and a second intermediate signal from a second antenna element neighboring the first antenna element. The method can further comprise comparing in comparator

electronics the first intermediate signal and the second

intermediate signal to obtain an output signal indicative of a time interval between reception of the input signal at the first antenna element and reception of the input signal at the second antenna element.

Thus, the signal transmitted by the node is received at the antenna elements in the antenna array. Variations in the timing of the reception of the signal at the different antenna elements are used to measure the direction from which the signal is received. Thus, the angle of the signal can be determined, which can be used for positioning and tracking of the node.

The embodiments of claims 2 and 17 advantageously enable the angle of the input signal to position the node.

The embodiments of claims 3 and 18 advantageously enable calibration of the analog parts of the antenna elements, resulting in more accurate timing measurements.

The embodiments of claims 4 and 19 advantageously enable an additional measurement of the angle of the signal, making the measurements more precise. The embodiment of claim 5 advantageously provides for an antenna-array configuration that reduces background

interference .

The embodiments of claims 6 and 7 advantageously enable more precise measurement of the input signal.

The embodiments of claims 8, 9, 20 and 21

advantageously enable the distance to the node to be determined. In combination with the angle of the signal this determines the position of the node.

According to an aspect of the invention a communication network is proposed comprising one or more antenna-array systems as described above.

The embodiment of claim 11 advantageously enables a server to determine the position of the node.

The embodiment of claim 12 advantageously enables tracking of nodes by their identifier.

The embodiment of claim 13 advantageously enables positioning and tracking of persons or pets.

The embodiments of claims 14 and 15 advantageously enable communication between nodes on top of tracking and/or tracing of the nodes.

Hereinafter, embodiments of the invention will be described in further detail. It should be appreciated, however, that these embodiments may not be construed as limiting the scope of protection for the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the invention will be explained in greater detail by reference to exemplary embodiments shown in the

drawings, in which:

Fig.l shows a network of an exemplary embodiment of the invention;

Fig.2a shows a front view of an antenna-array of an exemplary embodiment of the invention;

Fig.2b shows a side view of an antenna-array of an exemplary embodiment of the invention;

Fig.3, Fig.4 and Fig.6 show parts of a printed board circuit of an exemplary embodiment of the invention; Fig.5 shows signals before and after processing by parts of a printed board circuit of an exemplary embodiment of the invention;

Fig.7 shows an effect of a time difference of arrival of a signal and how this can be used to calculate an angle of arrival; and

Fig.8 shows a graph of energy levels at antenna elements . DETAILED DESCRIPTION

The invention enables positioning and tracking of nodes by using an antenna-array system as described in more detail below. Moreover, the nodes and antenna-array system can form a communication network wherein the nodes communicate with the antenna-array system or with other nodes directly or via the antenna-array system. The antenna-array system is preferably configured such that long-range positioning and tracking and communication are possible.

Fig.l shows an exemplary embodiment of a network with three antenna-array systems la-lc and six nodes 2a-2f that can be positioned and tracked by the antenna-array systems la-lc. Each of the nodes 2a-2f may be, depending on its location, capable of wirelessly communicating with one or more of the antenna-array systems la-lc. In Fig.l this is depicted by the arrows that, as an example, show a wireless communication

between node 2d and antenna-array systems la, lb and lc,

respectively, and a wireless communication between node 2e and antenna-array systems lb and lc, respectively. For positioning and tracking, a signal from a node 2a-2f may be received by at least one antenna-array system la-lc. Optionally two-way

communication is possible between a node 2a-2f and an antenna- array system la-lc. In case of two way communication the

antenna-array system la-lc may relay messages between nodes.

Preferably the nodes 2a-2f are small, i.e. sized such that they may be worn or carried by humans or pets. For health- safety reasons and increased battery life the nodes preferably have low gain requirements. The nodes may e.g. be active RFID tags embedded on flat badges or small garments like wristbands. The nodes may be used both indoor and outdoor and are preferably adaptable to noisy environments.

The communication between the antenna-array system la^¬ ic and a node 2a-2f may use one of the known RFID frequency bands as defined in the international standard ISO/IEC 18000 or any other suitable RFID frequency band. Preferably the free sub- Ghz ISM band standard is used to avoiding interference problems with other systems. In contrast to the sub-Ghz ISM band, use of RFID bands is becoming more and more problematic due to over^¬ crowding the frequency bands by systems including Wireless LAN systems, 2-way radios, Modems, Passive systems and Zigbee.

Another advantage of the sub-Ghz ISM band is its capability of achieving a long range of options with very limited power consumption .

An antenna-array 1 of an antenna-array system of an exemplary embodiment of the invention is shown in Figs.2a and 2b, wherein Fig.2a depicts a front view of the antenna-array 1 and Fig.2b depicts a side view of the antenna-array 1. The antenna-array 1 may be used with the sub-Ghz ISM RFID frequency band and may be a part of the antenna-array systems la-lc of Fig.1.

The antenna-array 1 may consist of at least two antenna elements 12 positioned on an antenna base 11. The antenna element 12 is preferably a quarter wave monopole antenna, also known as Marconi antenna, but may be any other suitable

omnidirectional antenna, such as e.g. a half-wave antenna or a co-linear antenna, with an omnidirectional radiation pattern in the azimuth plane, i.e. radiating equally well in all horizontal directions .

In case of a quarter wave monopole antenna element 12, the antenna element 12 is formed by a conductor λ/4 in length (λ being the wave length) , fed in the lower end, which is near the base 11 that works as a reflector. The current in the reflected image has substantially the same direction and phase as the current in the real antenna. The quarter-wave conductor and its image together form a half-wave dipole that radiates in the upper half of space. In this upper side of space the emitted field has substantially the same amplitude of the field radiated by a half-wave dipole fed with the same current. Therefore, the total emitted power is approximately one-half the emitted power of a half-wave dipole fed with the same current. As the current is the same, the radiation resistance (real part of series impedance) will be one-half of the series impedance of a half- wave dipole. The fields above ground are substantially the same as for the dipole. Because only half the power is applied the gain is twice (3dB over) that for a half-wave dipole (λ/2).

The quarter-wave antenna element 12 uses a ground plane to work properly. The ground plane typically extends a minimum of one wavelength in each direction from the base of the antenna element 12. The ground plane is a substantially flat conducting surface that serves as part of the antenna by reflecting radio waves from other antenna elements, and is formed by the base 11. Hereto the base 11 is preferably built on a plastic base 11a with a reflector layer lib of e.g. copper and a protective layer 11c, as shown in Fig.2b.

In the exemplary embodiment of Fig.2a there are

nineteen antenna elements 12 placed on the base 11 for sending and receiving signals to and from RFID nodes. It is to be understood that any other number of antenna elements 12 may be used instead.

A larger number of antenna elements 12 may be advantageous for aligning the analog signals that arrive at the antenna elements 12 before digitizing. Due to possible

imperfections in the production process there may be technical variations in the analog lines from the different antenna

element 12. As a result signals may travel at different speeds through the analog lines. Since time differences between signals are measured, variations in timing of the signal travelling from the antenna element to the digital processing part due to the imperfections are to be compensated for. This compensation may be achieved by using multiple measurements from multiple pairs of antenna elements 12 and averaging the signal delays from the different pairs of antenna elements.

When using multiple antenna element the distance between the antenna elements may be small, e.g. 10 cm. With a radio wave travelling at a high speed, the equipment has to be precise enough to distinguish times of 0.1 / 3*10⁸ = 3.3*10^~10. The best possible clock (oscillator) currently on the open market may be used as it oscillates with frequency of 3 Ghz, thus providing the required resolution of 3.3*10^~10 s.

A smaller number of antenna elements 12 may be used if the timing variation can be compensated otherwise, e.g. by using Time-to-Digital converters (TDC) . A TDC can precisely measure a time between two energy peaks and are known to have a lOps (i.e. 10^_11s) resolution, i.e. TDCs can measure a smallest time

difference of lOps. TDCs are currently being developed that have an even higher resolution of 3.6ps. A TDC may be thus used to measure a time difference very precisely. The TDC may be used as a calibration device by installing the TDC at the end of analog part, just before the digital processing part. The TDC is then used to calibrate the analog parts of the antenna elements 12. During the calibration phase the antenna element 12 is replaced by a signal generator to simulate analog signals being received at the antenna element. The TDC measures the time differences in the signals received from the generator for different antenna elements. Thus, for the different analog parts in between the antenna elements 12 and the digital circuitry boards the delays are measured. By comparing the measurement results the timing variations are calculated. With the timing variations known, a correctional value may be defined and applied to the timing of the signals at the different antenna elements 12 when the system is in operation.

The antenna elements 12 are typically placed such that each antenna element 12 is equally far away from another one at a distance equal to 1/2 wave length, which is a safe distance to prevent negative radio effects of one emitter antenna element 12 onto another.

Each antenna element 12 is connected to a printed board circuit which performs computational and controlling tasks of the antenna-array 1. The antenna array 1 and the printed board circuit together form the antenna-array system la-lc. The main parts of the printed board circuit and its computational and controlling tasks will be explained in Figs.3-6.

Fig.3 shows a signal 101 originating from one of the nodes 2a-2f. The signal 101 arrives at all antenna elements 12 of the antenna-array 1. In Fig.3 three of the antenna elements 12 are shown. Each antenna element 12 may pass the signal 101 to a high frequency amplifier HFA. Next the signal may be processed by a high frequency demodulator DM where the signal frequency is lowered and demodulated. The resulting signals 103-105 envelope the originally received signal 101, as depicted by the signal 102.

The signal 101 transmitted from the node 2a-2f is received by the different antenna elements 12 in the antenna array 1 at (slightly) different times due to the variation in distance between the antenna elements 12 and the node 2a-2f. This time difference is measurable.

Fig.4 shows an example of a comparator part of the printed circuit board that may be used to measure the time difference. Alternatively any other electronic circuit capable of measuring the time difference in a similar manner may be used. Each of the signals 103-105 from the demodulators DM may be passed to an operational amplifier OA and a trigger scheme. The outputs 106 and 107 of the comparator part may contain a signal that uniquely represents the time difference between arrival of original signal 101 onto the antenna elements 12. Outputs 106 and 107 are typically generated for each pair of neighboring antenna elements 12 in the antenna-array 1. For simplicity reasons Fig. only shows the comparator part for two neighboring antenna elements 12.

The trigger scheme in the example of Fig.4 has two D- type flip-flops, also known as data or delay flip-flops. The D- type flip-flops operate as follows. If input D equals a logical "0" and the input C has a rising edge (i.e. changing from "0" to "1"), then the output Q becomes "0". If input D equals a logical "1" and the input C has a rising edge, then the output Q becomes "1". If input C does not have a rising edge then the output Q keeps its value unchanged. Together with the output Q an

inverted Q value is output. The D-type flip flops can be forced to a set or reset state, wherein the D and C inputs are ignored. If input S equals a logical "0" and input R equals a logical "1", then output Q becomes "0". If input S equals a logical "1" and input R equals a logical "0", then output Q becomes "1". If input S equals a logical "1" and input R equals a logical "1", then output Q becomes "1". If inputs S and R both equal a logical "0" then the inputs D and C are evaluated as described above .

The wiring scheme as shown in Fig.4 results in the D- type flip-flops outputting signals 106 and 107 representing the time difference between the signals 103 and 104. Fig.5 shows an example of two input signals 103 and 104 to the comparator part of Fig.4 and the resulting outputs 106 and 107. Depending on which of the signals 103 and 104 comes first in time, one of the output signals 106 and 107 represents the time difference by having a value of "1" in the time interval representing the time difference. The other of the output signals 106 and 107 will have a value of "0".

The outputs 106 and 107 may be aligned and given a predefined output level by using a schematic as by example shown in Fig.6. In Fig.6 the relevant one of the outputs 106 and 107 from the comparator part is selected and converted to an output logic 108 of e.g. 0-3V. The resulting digital signal 108

representing the time difference is the output of the antenna- array system la-lc and is suitable for further digital

processing.

The time difference or delay directly depends on the angle at which the radio wave 101 arrived at the antenna

elements 12. With the layout of the antenna elements 12 in the antenna-array 1 known, the digital signal 108 may be used to calculate the angle from the receiving antenna-array system la- lc to the transmitting node 2a-2f, which essentially gives the direction in which RFID node is located.

Fig.7 illustrates the effect of a time difference of arrival of a signal at antenna elements and how this may be used to calculate the angle of arrival. A wave front 3 travelling in the direction indicated by the arrows hits an antenna element 12 at some moment in time. In Fig.7 two antenna elements 12 are shown as "i" and "j". The distance between the antenna elements i and j is predefined and is depicted as di_j . Radio waves C₀ travel at a speed of C_o=300000000 m/s . The radio wave 3 arrives at antenna element j first and moments later the same wave 3 arrives at antenna element i. The time difference Tj__j between these events is measured. To find the direction from which the wave was received the angle Φ is calculated, e.g. by using the formula: =arcsin (C₀Ti_j/di_j ) .

Instead of or in addition to measuring the signal delays from different pairs of antenna elements or using the TDC method described above, the signal strength at each antenna element may be used to determine the direction from which the wave was received. In Fig.8 the signal strength of three antenna elements are shown, which are physically aligned as described in Fig.2a. The three peaks are related to the three antenna element. The y-axis of the graph represents the energy level of the received signal (RSSI) at the specific antenna element, as measured at the antenna element. The middle antenna in this example receives the most energy. Taking into account that all antennas are capable of receiving substantially the same amount of energy from the signal wave, it may be concluded that the source of the signal is placed in front of the middle antenna element, which gives a direction from which the wave was

received. The more antenna elements are used, the more accurate the determination of the direction will be. The node that transmits the signal and thus radiates the energy will be in the direction where the measured energy level is at its maximum. An estimation of the distance to the source of the signal can be derived from the energy value (RSSI).

The angle alone may not provide enough data for determining the position of the node. The distance from the antenna-array system to the node may be needed as well. By using the angle information from at least two antenna-array systems the distance can be calculated using geometry mathematics. In such calculation the location of the antenna-array systems is supposed to be known. Because two antenna-array systems may be located such that they have substantially a same angle to a node, as is the case with node 2e and antenna-array systems la and lc in Fig.l, preferable at least three antenna-array systems la-lc are installed at the monitored area.

Alternatively just one antenna-array system la-lc may be used to perform a location search. In this case the RSSI (received signal strength index) may be used to calculate the distance between the RFID node and an antenna-array system.

Together with the determined direction (angle) the approximate position of the node may thus be determined. The accuracy of the position depends on the value of RSSI in the used environment and will often be less than when using multiple antenna-array systems .

The calculation of the distance to the node and the position of the node may be performed by one or more of the antenna-array systems la-lc. Alternatively the output signals 108 may be transmitted to a server where the calculations are made .

Typically all antenna elements 12 in the antenna-array

1 are active for input or output at the same time. It is

possible however to turn on sequential scanning of surface observing some limited angle at the same time - this may e.g. be used in case the environment is noisy, there are too many devices that are sending data or the signal strength from individual device is too weak (for example due to the

construction of that device) . In this case the antenna-array system la-lc may be configured to sequentially switch on and off one or more antenna elements 12 in the antenna array 1 such to emulate a rotation of the radiation lobe.

The following algorithm may be used in order to control transmissions of the nodes 2a-3f and prevent collisions of their signals :

1) The antenna-array system la-lc transmits a request to a node 2a-2f having a specific identifier X asking to

transmit a beacon;

2) The antenna-array system la-lc switches into

listening mode;

3) The node having identifier X sends a beacon containing the identifier;

4) The antenna-array system la-lc detects the location of the node and switches into transmitting node to start the whole procedure from step 1 for the next node.

When using this algorithm, the antenna-array system la- lc typically keeps identifiers of all known nodes 2a-2f in a memory. In order to simplify collection of identifiers in the antenna-array system la-lc it is possible to use reserved

periods of time for the antenna-array system la-lc to stay passively in receiving mode and wait for new nodes to send their identifier telling the system that they would like to be requested to send a beacon as the rest of the nodes joined in the network.

A decentralized wireless communication network comprising the antenna-array system la-lc and the nodes 2a-2f may be a long-range location aware network that may be used for e.g. people or pet seek and rescue. People or pets wearing a RFID node may be located and tracked, wherein the identifier stored in the node and communicated to the antenna-array system identifies the person or pet. The nodes and antenna-array systems may be configured to allow communication between nodes and between a node and an antenna-array system other than exchanging the identifier. One or more nodes may be configured to receive information about the whereabouts of another node. Information may be exchanged that is presented visually, by sound and/or by vibrations on the node. In the sub-Ghz band the DASH7 protocol may be used as a communication protocol.

Other example of use cases are: attendance evaluation and behavioral analysis at universities and campuses; child safety, customer behavior, event statistics and decision support in events, parks and fairs; and warehouse management, personal safety, asset tracking and sensor networks in logistics.

The range of the system, among other factors, typically depends on the possible output power of the nodes and the sensitivity of the antenna elements. With the current EU

limitations for RFID radio power output the controlled area range can be up to a few kilometers.

One embodiment of the invention may be implemented as a program product for use with a computer system. The program(s) of the program product define functions of the embodiments

(including the methods described herein) and can be contained on a variety of computer-readable storage media. Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, ROM chips or any type of solid-state non-volatile

semiconductor memory) on which information is permanently

stored; and (ii) writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive or any type of solid- state random-access semiconductor memory or flash memory) on which alterable information is stored. Moreover, the invention is not limited to the embodiments described above, which may be varied within the scope of the accompanying claims .

Claims

1. An antenna-array system (la-lc) for determining a current location of a node (2a-2f ) , comprising:

two or more antenna elements (12) configured to receive an input signal (101) from the node (2a-2f ) , wherein the antenna elements (12) are positioned in an antenna-array (1) such that the input signal (101) arrives at two neighboring antenna elements (12) at different times resulting in a first

intermediate signal (103) from a first antenna element (12) and a second intermediate signal (104) from a second antenna element (12) neighboring the first antenna element (12);

comparator electronics configured to compare the first intermediate signal (103) and the second intermediate signal (104) to obtain an output signal (108) indicative of a time interval between reception of the input signal (101) at the first antenna element (12) and reception of the input signal (101) at the second antenna element (12) .

2. The antenna-array system (la-lc) according to claim 1, further configured to calculate an angle from the antenna-array system (la-lc) to the node (2a-2f) based on the indication of the time interval in the output signal (108) and the locations of the first antenna element (12) and the second antenna element (12) in the antenna-array (1).

3. The antenna-array system (la-lc) according to claim 1 or claim 2, further comprising a time-to-digital

converter at each antenna element to determine a time difference in receiving the input signal at each antenna element and use the measured time difference to compensate for timing variations between the antenna elements.

4. The antenna-array system (la-lc) according to any one of the preceding claims, further configured to measure an energy level of the input signal at each antenna element and to determine the antenna element with the highest energy level.

5. The antenna-array system (la-lc) according to any one of the preceding claims, wherein the antenna-array (1) comprises an antenna base (11) configured as a ground plane to the two or more antenna elements (12).

6. The antenna-array system (la-lc) according to claim 5, wherein the antenna base (11) comprises a plastic base (11a), a reflector layer (lib) and a protective layer (11c) and wherein the two or more antenna elements (12) are quarter wave monopole antenna elements.

7. The antenna-array system (la-lc) according to claim 6, wherein the two or more antenna elements (12) are quarter wave monopole antenna elements that are positioned on the antenna base (11) such that the distance between two neighboring antenna elements is substantially equal to 1/2 of the wave length of the input signal (101) .

8. The antenna-array system (la-lc) according to any one of the claims 1-7, configured to calculate a distance from the antenna-array system (la-lc) to the node (2a-2f) based on the output signal (108) and a further output signal from another antenna-array system (la-lc) and the location of the antenna- array systems (la-lc) and the other antenna-array systems (la- lc) , and configured to calculate the position of the node (2a- 2f) based on the output signal (108), the calculated distance and the location of the antenna-array system (la-lc) .

9. The antenna-array system (la-lc) according to any one of the claims 1-7, configured to calculate a distance from the antenna-array system (la-lc) to the node (2a-2f) based on the output signal (108) and a receive signal strength index RSSI, and configured to calculate the position of the node (2a- 2f) based on the output signal (108), the calculated distance and the location of the antenna-array system (la-lc).

10. A communication network comprising one or more antenna-array systems (la-lc) according to any one of the preceding claims and one or more nodes (2a-2f ) .

11. The communication network according to claim 10, further comprising a server configured to calculate the distance from the antenna-array system (la-lc) to the node (2a-2f) based on the output signal (108) and the further output signal from the other antenna-array system (la-lc) and the location of the at least two antenna-array systems (la-lc), and configured to calculate the position of the node (2a-2f) based on the output signal (108), the calculated distance and the location of the antenna-array system (la-lc).

12. The communication network according to claim 10 or claim 11, wherein the antenna-array system (la-lc) is configured to:

transmit a request to the node (2a-2f ) , the request comprising an identifier;

switches into listening mode after transmitting the request; and

receiving from the node (2a-2f) that matched the identifier a beacon containing the identifier.

13. Use of the communication network according to any one of the claims 10-12 for seeking a person or a pet wearing or carrying the node (2a-2f ) .

14. Use of the communication network according to claim 13, further comprising sending information from one node to another node, from a node to an antenna-array system and/or from an antenna-array system to a node.

15. Use of the communication network according to claim 14, wherein the DASH7 protocol is used for sending the information .

16. A method in an antenna-array system (la-lc) for determining a current location of a node (2a-2f) , comprising:

receiving in two or more antenna elements (12) of an input signal (101) from the node (2a-2f ) , wherein the antenna elements (12) are positioned in an antenna-array (1) such that the input signal (101) arrives at two neighboring antenna elements (12) at different times resulting in a first intermediate signal (103) from a first antenna element (12) and a second intermediate signal (104) from a second antenna element

(12) neighboring the first antenna element (12);

comparing in comparator electronics the first intermediate signal (103) and the second intermediate signal

(104) to obtain an output signal (108) indicative of a time interval between reception of the input signal (101) at the first antenna element (12) and reception of the input signal (101) at the second antenna element (12) .

17. The method according to claim 16, further

comprising calculating an angle from the antenna-array system (la-lc) to the node (2a-2f) based on the indication of the time interval in the output signal (108) and the locations of the first antenna element (12) and the second antenna element (12) in the antenna-array (1) .

18. The method according to claim 16 or claim 17, further comprising determining a time difference in receiving the input signal using time-to-digital converter at each antenna element and using the measured time difference for compensating for timing variations between the antenna elements.

19. The method according to any one of the claims 16-

18, further comprising measuring an energy level of the input signal at each antenna element and determining the antenna element with the highest energy level.

20. The method according to any one of the claims 15-

19, further comprising calculating a distance from the antenna- array system (la-lc) to the node (2a-2f) based on the output signal (108) and a further output signal from another antenna- array system (la-lc) and the location of the antenna-array systems (la-lc) and the other antenna-array systems (la-lc), and calculating the position of the node (2a-2f) based on the output signal (108), the calculated distance and the location of the antenna-array system (la-lc).

21. The method according to any one of the claim 15- 19, further comprising calculating a distance from the antenna- array system (la-lc) to the node (2a-2f) based on the output signal (108) and a receive signal strength index RSSI, and calculating the position of the node (2a-2f) based on the output signal (108), the calculated distance and the location of the antenna-array system (la-lc).