GB2510373A

GB2510373A - A sensing device in a wireless local area network for determining the location and identity of other wireless devices in communication with an access point

Info

Publication number: GB2510373A
Application number: GB1301757.9A
Authority: GB
Inventors: Kate Wilkins
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2013-01-31
Filing date: 2013-01-31
Publication date: 2014-08-06
Also published as: WO2014119142A1; GB201301757D0

Abstract

A method performed by a sensing device 4 in a wireless local area network, WLAN, 10 the method comprising determining the position of another wireless device 1, 2, 3 relative to the sensing device 4, and determining the identity of the other wireless device using data within a packet transmitted by the other wireless device to an access point 5 of the wireless local area network. Means for determining a bearing of the other wireless device comprises a plurality of antennas and a Doppler direction finding circuit. Means for determining a distance of the other wireless device may comprise means for processing received signal strength indicator, RSSI, data and obtaining information relating to the transmission power level of the other wireless device. Also disclosed are location determining means for determining a location of the wireless sensing device 4 (a mobile communications device) relative to the wireless access point 5 in dependence upon: (i) information indicating a transmission power of the wireless access point; (ii) an indication of the strength of a wireless signal received from the access point, and (iii) a direction from which the wireless signal transmitted by the access point is received by the sensing device.

Description

COMMUNICATION SYSTEM

The present invention relates to communications devices operating in a wireless local area network (WLAN). The invention is particularly applicable, but by no means limited, to devices operating in accordance with the Institute of Electrical and Electronics Engineers' (IEEE) 802.11 family of standards, such technology being commonly referred to as "WiFi". However, the term "WiFi" used herein should be interpreted broadly to encompass any WLAN, and not just those configured in accordance with the 802.11 standards.

In order to provide a more engaging user experience or enhanced functionality, there is a desire to be able to determine the physical position (i.e. spatial location) of a device within a WLAN network, relative to other devices within the network.

This has potential applications in many areas, such as games, education, security, advertising, and mapping. For example, one possible application would be to provide exhibit information in a museum, such that, when a user with a suitably equipped mobile device comes close to an exhibit, they are offered data on the exhibit via their device. A second example would be a game involving a plurality of devices in a WLAN network, whereby each device (or the game played thereon) reacts or responds in some way to the motion and position of the other device(s).

According to a first aspect of the present invention there is provided a sensing device for use in a wireless local area network, the sensing device comprising: means for sending data packets to, and receiving data packets from, an access point of said wireless local area network; means for detecting the timing of the start of a data packet transmitted by another wireless device to the access point; means for determining the bearing and distance of the other wireless device relative to the sensing device; and means for determining the identity of the other wireless device using data within the transmitted packet.

By detecting the timing of the start of a data packet transmitted by the other wireless device to the access point, determining the identity of the other wireless device using data within the transmitted packet, and determining the position of the other wireless device relative to the sensing device, this enables the identity of

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the other wireless device to be associated with the determined positional information. This in turn enables greater functionality to be achieved than would be the case from finding just the relative position of the devices. A further benefit of associating the transmitted packet with the positional information is that information coded within the packet, such as the transmitter power level, can be used in determining the position of the device.

Preferably the means for determining the bearing of the other wireless device comprises a plurality of antennas, for example arranged to enable Doppler direction finding.

Preferably the means for determining the distance of the other wireless device from the sensing device comprises means for determining the strength of the wireless signal received from the other wireless device, for example by processing received signal strength indicator (RSSI) data.

Alternatively, or additionally, the means for determining the distance of the other wireless device from the sensing device may comprise means for obtaining power level information based on data within a received data packet. Thus, a transmitted packet may be used to obtain both the identity of the transmitter and information indicative of its distance from the sensing device.

The method may further comprise means to decode the transmitted packet to the media access control (MAC) layer, in order to extract transmitter identification and/or power level data from the packet.

The sensing device is preferably a mobile communications device. For example, it may be a mobile telephone, smartphone, user equipment, personal digital assistant, laptop computer, portable web browser, tablet device, or such like.

However, the sensing device may alternatively be a "non-mobile" device, such as a desktop computer, a dedicated sensing "tag", or some other piece of hardware in communication with the wireless local area network.

According to a second aspect of the invention there is provided a method performed by a sensing device in a wireless local area network, the method comprising: detecting the timing of the start of a data packet transmitted by another wireless device to an access point of said wireless local area network; determining the bearing and distance of the other wireless device relative to the sensing device; and determining the identity of the other wireless device using data within the transmitted packet.

According to a further aspect of the invention there is provided a sensing device comprising: receiving means for receiving a wireless data packet transmitted by another wireless device to a wireless access point; direction determining means for determining a direction from which the wireless signal is received by the wireless sensing device; decoding means for decoding the wireless data packet transmitted by the other wireless device to the access point, to determine an identity of the other wireless device; location determining means for determining the location of the other wireless device relative to the sensing device; and memory means for storing the determined location of the other wireless device in association with the determined identity of the other wireless device.

According to a further aspect of the invention there is provided a sensing device comprising: receiving means for receiving wireless signals carrying wireless data packets transmitted by a wireless access point; signal strength determining means for determining an indication of a strength of a wireless signal received by the receiving means; direction determining means for determining a direction from which a wireless signal is received by the wireless sensing device; and location determining means for determining a location of the wireless sensing device relative to the wireless access point; wherein the receiving means is further arranged to receive a wireless signal carrying a wireless data packet transmitted from another wireless device to the wireless access point; wherein the sensing device is arranged to process the wireless data packet transmitted by the other wireless device to the access point, to determine an identity of the other wireless device; wherein the direction determining means is arranged to determine a direction from which the wireless signal transmitted by the other wireless device to the access point is received by the sensing device; and wherein the location determining means is arranged to determine the location of the other wireless device relative to the sensing device.

According to a still further aspect of the invention there is provided a sensing device comprising: receiving means for receiving wireless signals carrying wireless data packets transmitted by a wireless access point; signal strength determining means for determining an indication of a strength of a wireless signal received by the receiving means; direction determining means for determining a direction from which a wireless signal is received by the wireless sensing device; and location determining means for determining a location of the wireless sensing device relative to the wireless access point; wherein the receiving means is further arranged to receive wireless signals carrying wireless data packets transmitted from one or more other wireless devices to the wireless access point; wherein the sensing device is arranged to process wireless data packets transmitted by the one or more other wireless devices to the access point, to determine an identity of the other wireless device that transmitted the wireless data packet; wherein the direction determining means is arranged to determine a direction from which each wireless signal transmitted by the one or more other wireless devices is received by the sensing device; and wherein the sensing device is arranged to update the determined locations of the one or more other wireless devices relative to the sensing device using subsequently received wireless signals, in order to track the locations of the one or more other wireless devices relative to the sensing device.

The invention also provides, for all methods disclosed, corresponding computer programs or computer program products for execution on corresponding user equipment or network communications devices.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 illustrates schematically a wireless local area network (WLAN) including a base station and a plurality of mobile devices; Figure 2 illustrates an arrangement of antennas within a mobile device, for determining the bearing of the device relative to a transmitter in a WLAN using Doppler direction finding; Figures 3a and 3b illustrate principles of determining distance information in respect of mobile devices in a WLAN; Figure 4 is a data flow diagram in respect of a mobile device within a WLAN, enabling bearing, distance and identification information to be determined; and Figure 5 is a schematic block diagram of a mobile device.

Oven/jew Figure 1 illustrates an example of a wireless local area network (WLAN) 10 including a WiFi base station or access point 5, and mobile devices 1-4. The mobile devices 1-4 are all equipped with WiFi transmitters and receivers, enabling them to be in data communication with the network (e.g. to send and receive data packets via the base station 5).

Of the mobile devices 1-4, device 4 will subsequently be referred as a sensing device, as it is configured to sense its own position relative to the access point 5 and to sense the position of the other devices 1-3 relative to the sensing device 4.

The present method provides a way for the sensing device 4 to uniquely identify the ID and relative position of the other WiFi transmitters (i.e. access point 5 and/or mobile devices 1-3) within the WLAN 10, based on determining the direction, distance and unique ID of the other devices using a combination of technologies, namely: * WiFi -to provide transmitter ID information; * direction finding -to determine bearing, for example using Doppler direction finding (DDF) or other solid state methods, or other methods such as movable directional antenna etc; * distance determination -for example using received signal strength indicator (RSSI) information; and * timing -to manage measurement amongst transmitters More particularly, the present method uses positional data (from suitable direction and bearing finding methods) together with decoded WiFi packet data (e.g. MAC layer data) to uniquely identity devices on the WLAN network 10 along with their bearing and distance trom the sensing device 4. This combination of various sources of information, along with timing information, enables multiple devices on a WiFi network to be individually identified and located. This is illustrated in the enlarged portion of Figure 1 which depicts the screen 6 of the sensing device 4, on which the relative positions and distances of the other mobile devices 1, 2, 3 and the base station 5 are determined and shown in real-time.

Use of direction tinding and RSSI distance can be used with triangulation.

However the present method does not rely on triangulation or a constant transmission. Instead, the positional data is combined with timing information to correlate a transmitter with its MAC address, to identify multiple devices on the WLAN 10. This allows identitication of all transmitting devices 1-5 attached to the network 10, rather than just the base station 5.

Detailed embodiment In more detail, a presently-preferred embodiment combines position determination techniques with the detection of the timing of the start of a transmitted packet, enabling distance and direction measurement for the duration of that packet. The packet is then decoded to media access control (MAC) level to ascertain the transmitter's ID and power level, and this is then associated with the transmitter's direction and distance to identify the relative position of the transmitting device relative to the detecting device.

The system provides for relative location between devices and, as such, the detecting system can be implemented in a sensing device 4 that is either fixed in position or that is free to move within the WLAN 10.

Method for determining bearing In the presently-preferred embodiment, bearing data is only captured when the sensing device 4 has been determined that a packet is actually being transmitted by a sender, and only during the duration of that transmitted packet. The bearing data is then associated with a sender ID as determined by decoding the received packet to identify the MAC address of the sender.

With reference to Figure 2, in the presently-preferred embodiment the bearing of the mobile device 4 relative to a transmitter (e.g. base station 5) is determined using Doppler Direction Finding (DDF). For this purpose, the sensing device 4 has, in this embodiment, a plurality of (in this case four) antennas A0-A3 tuned to the WiFi (2.4 GHz) frequency band. The sensing device 4 has antenna switching and Doppler shift decode circuitry, as well as conventional WiFi communication capabilities. The Doppler shift decode circuitry uses knowledge of the WiFi modulation scheme to adjust the way that it determines Doppler shifts in the signals received by the different antennas, from which the bearing information is determined. The antennas A0-A3 are arranged so that the bearing determination is performed in a plane that is parallel with the flat plane of the display of the sensing device 4 and determines the angle a between antenna A0 and the transmitting device 5. More details of the way in which the sensing device 4 can determine the bearing information using such DDF techniques can be found in the paper by Mike Kossor entitled "A Doppler Radio-Direction Finder" published in the QST magazine, May 1999, pages 35 to 40, the content of which is incorporated herein by reference.

Method for determining distance A) Distance between sensing device and Access Point In the presently-preferred embodiment, the distance of the sensing device 4 from the access point 5 is determined using calibrated received signal strength indicator (RSSI) level data, whereby the sensing device 4 converts received signal level to distance data based on knowledge of the transmission power of the access point 5. In practice, the accuracy of the distance information may be limited due to the way 802.11 networks are controlled depending on their version. For example: * 802.lla -No power control -54 mpbs, 5 GHz band.

* 802.11 b -No power control -11 mpbs, 2.4 GHz.

* 802.llg -Contains power control in the physical layer -54 mbps, 2.4 GHz.

* 802.11 n -Contains power control in the physical layer -600 mbps, 2.4 GHz & 5 GHz.

Where power control is not present then the distance of the sensing device 4 from the access point 5 can only be crudely estimated from the RSSI data -by assuming that the access point 5 transmits at a known power level. Where power control is used, the sensing device 4 will know (from system information broadcast by the access point 5) the current transmit power of the access point 5 and so can determine its distance from the access point 5 using the RSSI data it calculates for the signal received from the access point 5.

B) Distance between sensing device and other mobile device If the sensing device 4 knows or if it can determine information relating to the transmit power of another mobile device (assumed to be mobile device 1 in the following discussion), then the sensing device 4 can determine the distance between itself and that other mobile device 1 in a similar way as it determines the distance between itself and the access point 5. The sensing device 4 can obtain this transmit power information by eavesdropping on the wireless signals transmitted between the other mobile device 1 and the access point 5. For example, the sensing device 4 may be able to eavesdrop a wireless data packet sent from the access point 5 to the other mobile device 1 telling it to transmit at a certain power level (using the defined TXPWRLVL data parameter). The sensing device 4 can then assume that the next transmission made by the other mobile device 1 to the access point 5 will be at the commanded power level. Therefore, when the sensing device 4 subsequently detects a transmission made by the other mobile device 1 to the access point 5, the sensing device 4 can determine the distance between itself and the other mobile device 2 using the commanded power level and the received power level (RSSI) at the sensing device 4 of the wireless signal transmitted by the other mobile device 1.

Alternatively, if the sensing device 4 eavesdrops a transmission sent from the other mobile device 1 to the access point 5 that contains the RSSI or the RCPI (Received Channel Power Indicator) determined by the other mobile device 1 for a wireless signal the other mobile device 1 received from the access point 5, the sensing device 4 can use that information to work out the distance between the other mobile device 1 and the access point 5 (as it knows the transmit power of the access point 5). The sensing device 4 can then determine the distance between itself and the other mobile device 1 using the bearing information for the signals transmitted by the access point 5 and the other mobile device 1 and a simple trigonometric calculation. An example of this principle is illustrated in Figure 3a. In this example, the angle B corresponds to the difference in the bearing determined for the signals coming from the access point 5 and those coming from the other mobile device 1. The distance x between the sensing device 4 and the other mobile device 1 is calculated based on the distances between the access point 5 and each of the sensing device 4 and the other mobile device 1, and the angle e.

This simple calculation works if the sensing device 4 is closer to the access point 5 than the other mobile device 1 is to the access point 5. However, if the sensing device 4 is further away from the access point 5 than the other mobile device 1, then the bearing information will not give an unambiguous position. The reason for this is illustrated in Figure 3b. As can be seen from Figure 3b, the distance between the other mobile device 1 and the access point 5 (represented in Figure 3b as the black circle) defines a circle 8 of possible locations for the other mobile device 1 and when the sensing device 4 is located further away from the access point 5 than the other mobile device 1, it is located outside this circle 8 and so the bearing information determined (represented by the line 10 in Figure 3b) by the sensing device 4 for the wireless signal transmitted by the other mobile device 1 identifies two possible locations, the actual location and a false location marked with an "X" in Figure 3b. (When the sensing device 4 is closer to the access point than the other mobile device 1, the sensing device will be located within the circle 8 and so the bearing information will uniquely identify the location of the other mobile device 1). The sensing device 4 can differentiate between the actual location and the false location using the RSSI level at the sensing device 4 of the wireless signal transmitted by the other mobile device 1.

Method for capturing device ID In the presently-preferred embodiment, distance data is only determined when the sensing device 4 determines that a packet is actually being transmitted by a transmitting device, and only during the duration of that transmitted packet. The determined distance data is then associated with the corresponding device, as determined from the analysis of the received packet.

All devices transmitting on an 802.11 WiFi network transmit a media access control (MAC) address (SRC:DST). As MAC level protocol information can be encrypted, the present system may be limited to WiFi networks to which the sensing device 4 is, or has been, attached (i.e. the password/encryption information is known to the sensing device 4).

In the presently-preferred embodiment, the sensing device 4 decodes the transmitted signal to determine the MAC address of the transmitting device (e.g. base station 5) and/or of the destination device (for example if the packet is transmitted from the access point to a mobile device to set its transmit power level). This ID information, combined with bearing and distance information, enables the device to be uniquely identified.

Combining the concepts As shown in Figure 4, to participate in the present method the sensing device 4 first scans the WiFi environment and selects a particular service set identification (SSID) and attaches to that network.

WiFi is a CSMA-CD packet based system (and therefore only one transmitter is allowed to transmit on the channel at any one time), and the presently-preferred embodiment combines timing information with received packets, RSSI and Doppler data to determine bearing/distance information and ID.

With reference to Figure 4, the sequence of events is as follows (the steps corresponding to the numbers in the circles): 1. The sensing device 4 attaches to the WiFi network 10.

Known parameters include encryption keys, channel, spreading sequence, etc. 2. When a synchronisation detector 44 detects the start of a transmitted packet (by detecting the WiFi sync sequence on the selected channel), the synchronisation detector 44 triggers the Doppler Direction Finder circuit 45 (which uses antenna switcher 46 to switch between antennas A0-A3) and the WiFi demodulation circuit 42 to operate and collect data.

3. Doppler data can either be decoded on the CPU 47 or within the Doppler Direction Finder circuit 45, to determine the bearing information for the wireless packet currently being received.

4. As normally there is only one WiFi transmitter transmitting at any one time on the given channel, the CPU 47 (having WiFi data decoder 48) can combine the RSSI data for the received wireless signal carrying the data packet and the sender MAC address from the decoded WiFi MAC layer packet, with the bearing data obtained for the transmitted packet, to determine bearing, distance and ID (in the manner discussed above). If the received data packet includes a commanded transmit power for another wireless device, the transmit power may be stored in association with the MAC address for the other wireless device so that it can be used to calculate the distance to the other wireless device the next time a wireless signal transmitted by that other wireless device is detected by the sensing device 4.

5. The CPU 47 keeps a table of data of identified transmitters (access points and other fixed or mobile devices) that is constantly updated as new signals are detected by the sensing device 4. In this way, the location of the sensing device relative to the access point and the locations of the other wireless devices relative to the sensing device can be tracked as one or more of the devices move relative to the other devices. If required, a mapping can be maintained between the device MAC addresses and either a device name or a user name depending on the situation. If geographical coordinates are known for the access point, then the sensing device 4 may determine absolute location information for itself and for the other wireless devices.

Mobile device architecture Figure 5 is a block diagram illustrating the main components of the sensing device 4 shown in Figures 1-4. As shown, the sensing device 4 has a transceiver circuit that is operable to transmit signals to and receive signals from the access point via one or more antenna 43. The transceiver is also able to receive other wireless signals transmitted by other wireless devices -even when not destined for the sensing device 4. The sensing device 4 also has the antenna array and DDF circuit 45, 46 for use in determining the bearing of a wireless signal received by the array. The sensing device 4 has a controller 54 (functionally equivalent to CPU 47 in Figure 4) to control the operation of the sensing device 4. The controller 54 is associated with a memory 56 and is coupled to the transceiver circuit 50 and to the antenna array 45. Although not necessarily shown in Figure 5, the sensing device 4 will of course have all the usual functionality of a conventional mobile device (such as a user interface 52 (e.g. display and/or keyboard)) and this may be provided by any one or any combination of hardware, software and firmware, as appropriate. Software may be pre-installed in the memory 56 and/or may be downloaded via the telecommunications network or from a removable data storage device (RMD), for example.

The controller 54 is configured to control overall operation of the sensing device 4 by, in this example, program instructions or software instructions stored within memory 56. As shown, these software instructions include, among other things, an operating system 58, a communications control module 60, a WLAN module 62, a sync detector/timing control module 66, a bearing determination module 68, a distance determination module 70 and an identified transmitter table 72.

The communications control module 60 is operable to control the communication between the sensing device 4 and the base station of a telephone network (not shown) -to allow the sensing device to operate as a mobile telephone device.

The WLAN module 62 comprises a WLAN client 64 and is operable to control communication between the sensing device 4 and the access point 5 and to eavesdrop on the communications between the access point 5 and other wireless devices in the WLAN 10. The functionality of the WLAN module 62 includes the functionality of the WiFi decoder shown in Figure 4. The WiFi demodulator 42 shown in Figure 4 forms part of the transceiver circuit 50 shown in Figure 5.

The sync detector/timing control module 66 is configured to operate the synchronisation detector 44 (which forms part of the transceiver circuit 50), to detect a WiFi synchronisation sequence on a selected channel, and to trigger the antenna array and the DDF circuit 45, 46 and the WiFi demodulation circuit 42.

The functionality of module 66 includes detecting the timing of the start of a transmitted packet sent from a transmitting device.

The bearing determination module 68 is configured to operate the Doppler Direction Finder circuit 45. The functionality of module 68 includes determining the bearing of the sensing device 4 relative to the transmitter during the duration of the transmitted packet, for example using the principles described above with reference to Figure 2.

The distance determination module 70 is operable to determine the distance of the sensing device 4 from the transmitter in the manner discussed above.

The identified transmitter table 72 contains the details (bearing, distance and ID) of identified transmitters, the identification information being derived from the decoded WiFi MAC layer packets as described above. This table is updated as new wireless signals are received and positions change. Historical locations that are determined may be kept or discarded as desired.

Example applications

Museum I exhibition I retail location In a first example, the sensing device is located at an exhibit in a museum or exhibition, or in a retail location such as a shopping centre. (Thus, in this example, the sensing device is effectively fixed or "non-mobile" during use.) In this example scenario, the user has a mobile phone having a suitably programmed software application. When the sensing device at the exhibit determines that the user is in front of it (or within a predefined range of the sensing device), they are offered information on the exhibit. This could be displayed to the user on a display next to the exhibit or even on the display of the user's mobile phone. The information that is displayed can also be used for advertising/offers in shops, and in indoor locations where GPS is not suitable.

Location Aware Games A second example provides an onscreen card game, where users can pass representations of cards to each other by using the device screen to "flick" a card in the direction of another device. Through the present method, each user's device is able to know which device is in the direction of (or closest to the direction of) the "flick" and should be the given the card. This concept may readily be extended to other applications in which information is passed/flicked in the direction of a receiving device -for example enabling the distribution of electronic business cards in a conference or business networking event.

Other potential applications include security (e.g. determining who is connected to a WLAN and where they are located), and the use of multiple devices to measure or map a room or building.

Modifications and Alternatives Detailed embodiments have been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. By way of illustration only some of these alternatives and modifications will now be described.

In the above embodiments, the sensing device 4 and the other wireless devices 1- 3 are cellular telephones. It will be appreciated that the above embodiments could be implemented using devices other than mobile telephones such as, for example, personal digital assistants, laptop computers, web browsers, tablet devices, etc. In the above embodiments, the access point 5 is assumed to be fixed in location.

However, in other embodiments the access point 5 may be move within the WLAN.

In the above description, sensing device 4 is described for ease of understanding as having a number of discrete functional components or modules. Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the invention, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities.

In the above embodiments, one sensing device was provided. In other embodiments, more than one sensing device may be provided.

In the above embodiments, the term access point has been used for illustrative purposes only and in no way shall be considered limiting the invention to any particular standard. Embodiments of the invention are applicable to systems using any type of node for accessing a wireless local area network irrespective of the access technology used thereon. The above embodiments are applicable to non-mobile or generally stationary user equipment as well.

In the above embodiments, a number of software modules were described. As those skilled in the art will appreciate, the software modules may be provided in compiled or un-compiled form and may be supplied to the base station or to the mobile device as a signal over a computer network, or on a recording medium.

Further, the functionality performed by part or all of this software may be performed using one or more dedicated hardware circuits. However, the use of software modules is preferred as it facilitates the updating of the access point 5 and of the sensing device 4 in order to update their functionalities.

Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

Claims

CLAIMS1. A sensing device for use in a wireless local area network, the sensing device comprising: means for sending data packets to, and receiving data packets from, an access point of the wireless local area network; means for receiving a wireless signal carrying a data packet transmitted by another wireless device to the access point; means for detecting the timing of the start of the data packet transmitted by the other wireless device to the access point; means, responsive to the detected start of the transmitted data packet, for determining a bearing of the other wireless device relative to the sensing device; means, responsive to the detected start of the transmitted data packet, for determining the distance of the other wireless device from the sensing device; means for determining the identity of the other wireless device using data within the transmitted packet; and means for storing the determined distance and the bearing in association with the determined identity of the other wireless device.
2. A sensing device as claimed in claim 1, wherein the means for determining the bearing of the other wireless device relative to the sensing device comprises a plurality of antennas.
3. A sensing device as claimed in claim 2, wherein the means for determining the bearing of the other wireless device relative to the sensing device comprises a Doppler direction finding circuit.
4. A sensing device as claimed in any of claims 1, 2 or 3, wherein the means for determining the distance of the other wireless device from the sensing device comprises means for determining the strength of the wireless signal received from the other wireless device.
5. A sensing device as claimed in claim 4, wherein the means for determining the strength of the signal comprises means for processing received signal strength indicator data.
6. A sensing device as claimed in claim 4 or claim 5, wherein the means for determining the distance of the other wireless device from the sensing device comprises means for obtaining information relating to the power level of the other wireless device based on data within a received data packet.
7. A sensing device as claimed in any preceding claim, further comprising means to decode the transmitted packet to the media access control (MAC) layer.
8. A sensing device as claimed in any preceding claim, being a mobile communications device.
9. A method performed by a sensing device in a wireless local area network, the method comprising: receiving a wireless signal carrying a data packet transmitted by another wireless device to an access point of the wireless local area network; detecting the timing of the start of the data packet transmitted by the other wireless device to the access point; in response to detecting the start of the transmitted data packet, determining a bearing of the other wireless device relative to the sensing device; in response to detecting the start of the transmitted data packet, determining the distance of the other wireless device from the sensing device; determining the identity of the other wireless device using data within the transmitted packet; and storing the determined distance and the bearing in association with the determined identity of the other wireless device.
10. A method as claimed in claim 9, wherein determining the bearing of the communications device comprises using a plurality of antennas coupled to a direction finding circuit.
11. A method as claimed in claim 9 or claim 10, wherein determining the distance of the sensing device from the other wireless device comprises determining the strength of the signal received from the other wireless device.
12. A method as claimed in claim 11, wherein determining the strength of the signal comprises processing received signal strength indicator data.
13. A method as claimed in claim 11 or claim 12, wherein determining the distance of the other wireless device from the sensing device comprises obtaining information relating to the power level of the other wireless device based on data within a received data packet.
14. A method as claimed in any of claims 9to 13, further comprising decoding the transmitted packet to the media access control (MAC) layer.
15. A method as claimed in any of claims 9 to 14, wherein the sensing device is a mobile communications device.
16. A computer program product comprising computer implementable instructions for causing a programmable computer device to perform all the method steps of any of claims 9 to 15.
17. A sensing device comprising: receiving means for receiving a wireless signal carrying a wireless data packet transmitted by another wireless device to a wireless access point; direction determining means for determining a direction from which the wireless signal is received by the wireless sensing device; decoding means for decoding the wireless data packet transmitted by the other wireless device to the access point, to determine an identity of the other wireless device; location determining means for determining the location of the other wireless device relative to the sensing device using: (i) information relating to the transmission power of the wireless signal transmitted by the other wireless device to the access point; and (ii) the direction from which the wireless signal transmitted by the other wireless device to the access point is received by the sensing device, as determined by the direction determining means; and memory means for storing the determined location of the other wireless device in association with the determined identity of the other wireless device.
18. A sensing device according to claim 17, configured to determine the information relating to the transmission power of the other wireless device by reading a commanded transmission power level sent from the access point to the other wireless device.
19. A sensing device according to claim 17 or 18, further comprising signal strength determining means for determining an indication of a strength of the wireless signal received by the receiving means; and wherein the location determining means is arranged to determine the location of the other wireless device relative to the sensing device using an indication of the strength of the wireless signal received from the other wireless device, as determined by said signal strength determining means.
20. A sensing device according to claim 17, wherein the information relating to the transmission power of the other wireless device comprises RSSI/RCPI data included in the data packet transmitted by the other wireless device to the access point and the transmission power of the access point.
21. A sensing device according to claim 20, wherein the location determining means is arranged to determine the distance between the other wireless device and the access point and the distance between the sensing device and the access point, and is arranged to determine the location of the other wireless device relative to the sensing device using the determined distances and the relative bearings of the wireless signals received from the other wireless device and from the access point and a trigonometric calculation.
22. A sensing device according to claim 21, configured to use an indication of the signal strength of the wireless signal transmitted by the other wireless device to the access point, at the sensing device to resolve any ambiguity in the determined location of the other wireless device.
23. A sensing device according to any of claims 17 to 22, wherein the sensing device is arranged to update calculation of the location of the other wireless device relative to the sensing device using subsequently received wireless signals, in order to track the location of the other wireless device relative to the sensing device.
24. A sensing device according to any of claims 17 to 23, wherein the receiving means is arranged to receive wireless signals carrying wireless data packets transmitted from a plurality of other wireless devices to the wireless access point and wherein the sensing device is configured to determine location information for each of the other wireless devices.
25. A sensing device comprising: receiving means for receiving wireless signals carrying wireless data packets transmitted by a wireless access point; signal strength determining means for determining an indication of a strength of a wireless signal received by the receiving means; direction determining means for determining a direction from which a wireless signal is received by the wireless sensing device; and location determining means for determining a location of the wireless sensing device relative to the wireless access point in dependence upon: (i) information indicating a transmission power of the wireless access point; (ii) an indication of the strength of a wireless signal received from the access point, as determined by said signal strength determining means; and (Di) a direction from which the wireless signal transmitted by the access point is received by the sensing device, as determined by the direction determining means; wherein the receiving means is further arranged to receive a wireless signal carrying a wireless data packet transmitted from another wireless device to the wireless access point; wherein the sensing device is arranged to process the wireless data packet transmitted by the other wireless device to the access point, to determine an identity of the other wireless device; wherein the direction determining means is arranged to determine a direction from which the wireless signal transmitted by the other wireless device to the access point is received by the sensing device; and wherein the location determining means is arranged to determine the location of the other wireless device relative to the sensing device using: (i) information relating to the transmission power of the wireless signal transmitted by the other wireless device to the access point; and (ii) the direction from which the wireless signal transmitted by the other wireless device to the access point is received by the sensing device, as determined by the direction determining means.
26. A sensing device comprising: receiving means for receiving wireless signals carrying wireless data packets transmitted by a wireless access point; signal strength determining means for determining an indication of a strength of a wireless signal received by the receiving means; direction determining means for determining a direction from which a wireless signal is received by the wireless sensing device; and location determining means for determining a location of the wireless sensing device relative to the wireless access point in dependence upon: (i) information indicating a transmission power of the wireless access point; (ii) an indication of the strength of a wireless signal received from the access point, as determined by said signal strength determining means; and (Di) a direction from which the wireless signal transmitted by the access point is received by the sensing device, as determined by the direction determining means; wherein the receiving means is further arranged to receive wireless signals carrying wireless data packets transmitted from one or more other wireless devices to the wireless access point; wherein the sensing device is arranged to process wireless data packets transmitted by the one or more other wireless devices to the access point, to determine an identity of the other wireless device that transmitted the wireless data packet; wherein the direction determining means is arranged to determine a direction from which each wireless signal transmitted by the one or more other wireless devices is received by the sensing device; wherein the location determining means is arranged to determine the location of the one or more other wireless devices relative to the sensing device using: (i) information relating to the transmission power of the wireless signal transmitted by the other wireless device; and (ii) the direction from which the wireless signal transmitted by the other wireless device is received by the sensing device, as determined by the direction determining means; and wherein the sensing device is arranged to update the determined locations of the one or more other wireless devices relative to the sensing device using subsequently received wireless signals, in order to track the locations of the one or more other wireless devices relative to the sensing device.