WIRELESS NETWORK

Technical field

Examples of the present disclosure relate to methods and apparatus for performing timing measurement in a wireless network, and particularly to methods and apparatus for performing fine timing measurement (FTM) in a wireless local area network.

Background

It is frequently desirable to determine the location of a wireless communication device in a network. In some situations, the location may be determined accurately (i.e., in three dimensions) by assessing signals transmitted from or received at multiple, geographically dispersed sources and triangulating the location accordingly. In other situations, it may be sufficient to determine only the range of a wireless communication device from a particular source of wireless signals.

Figure 1 is a schematic diagram of the architecture for determining the position of a mobile station in a network according to IEEE 802.1 1x communication protocols (where x can be different suffixes such as a, b, g, ac, etc). Such communication protocols are commonly collectively referred to as "Wi-Fi".

The architecture comprises a mobile station 12a, whose position is to be determined, three access points (APs) 14, 16, 18, an access controller 20, and a positioning server 22. Also shown are two further mobile stations 12b, 12c.

The mobile station 12a may be any Wi-Fi-enabled device, such as a smartphone or other mobile computing device (e.g. tablet, laptop, etc) or indeed a stationary device such as a desktop computer, domestic appliance, etc. Mobile stations may also be termed non-AP STAs. The APs 14, 16, 18 exchange dedicated frames and/or beacons with the mobile station 12a. The frames/beacons contain positioning-related information, for example, time stamps, path loss information, etc, based on which the requesting party can perform the necessary measurements to determine the location of the mobile station 12a. Note that the mobile station 12a itself may wish to determine its location, in which case the mobile station 12a performs the necessary measurements based on the positioning-related information to determine its location. In other examples, however, the network may wish to determine the location of the mobile station 12a (and potentially also the locations of the other mobile stations 12b, 12c). In this case, the access controller 20 delivers configuration information to the APs 14, 16, 18 to control their behaviour for positioning, i.e. instructing them to transmit appropriate frames or beacons to the mobile stations in question. Measurement data is collected at the APs 14, 16, 18 and reported to the access controller 20, which processes the results and reports the processed data to the positioning server 22. The positioning server 22 then calculates the location of the mobile station 12a based on the reported data, and potentially other information that is available to it. In alternative configurations, the APs 14, 16, 18 may report the collected measurement data directly to the positioning server 22.

Several techniques have been used for positioning in Wi-Fi networks. Those techniques exploit different signal features and thus, may require different measurements and apply corresponding algorithms.

In the early version of the IEEE 802.1 1 standard, the measurement of signal strength, defined as received signal strength indicator (RSSI), was used to locate mobile stations. The signal strength of a signal transmitted between a mobile station and an AP is dependent on the distance between those two devices. In principle, this distance could be calculated based on the RSSI and a certain attenuation model. Thus each of the APs 14, 16, 18 transmits a reference signal at a known signal strength, and the mobile station 12a measures the received signal strength to determine its distance from each of the APs. However, RSSI is sensitive to the radio environment and frequently the actual behaviour of RSSI is a poor match to the model.

An alternative technique, known as "fingerprinting", utilizes the RSSI (and other measurements) in a different way. During an initial calibration phase, a map is constructed comprising a plurality of different measured parameters (such as, RSSI, angle of arrival, time of arrival, etc) at different geographical locations and for all APs within range of the mobile station at those locations. For example, a test mobile station may be utilized to collect reference measurements at different geographical locations (with the reference measurements corresponding to different "fingerprints" of the radio environment at each location). When later determining the location of a mobile station, the mobile station measures the same parameters and compares the values of those parameters to the map to find a matching fingerprint and therefore a matching location. Further alternative techniques measure the travel time of signals transmitted between the mobile station and the AP (or multiple APs) and translate that travel time into the distance between the two devices based on the approximation that the speed of light is a constant regardless of any changes in the medium between the two devices.

One such technique, known as the time of arrival (TOA) method, has been standardized in the IEEE 802.1 1x protocols and requires the inclusion of time-stamps in transmitted signals to determine the amount of time that the signal is in flight. A capable mobile station (such as the mobile station 12a) may transmit timing measurement frames addressed to a peer mobile station (such as the mobile stations 12b, 12c) or an AP (such as any of the APs 14, 16, 18). A higher-layer protocol has been established in order to synchronize the local times between mobile stations and APs.

An alternative, but similar, technique known as the round trip time (RTT) method measures the time spent by a specific frame in travelling from a transmitting node (e.g. the mobile station 12a) to a receiving node (e.g. an AP), and then back to the transmitting node. This method does not require synchronization between the two nodes. However, both this method and the TOA method can be inaccurate when the transmitting and receiving nodes do not have a direct line of sight, such that signals transmitted between the two suffer from reflections and refractions in flight.

Note that these timing methods can be used to determine the distance between two devices. If more accurate positioning is required for a particular mobile station, the process may be repeated with other devices (e.g. other APs or mobile stations), and the measurements combined using triangulation or trilateration to determine the position of the mobile station more accurately. For example, the measurements from multiple APs (e.g. up to three) may correspond to respective spheres centred on each of the APs, with the mobile station located at a location where those spheres intersect. Note that there may be more than one point of intersection (and thus more than one possible location), even if measurements are taken from three APs.

In future amendments to the IEEE 802.1 1x specifications, the feature of fine timing measurement (FTM) will be added. FTM is an agreed process characterized by a three-stage procedure comprising an initial negotiation, subsequent FTM implementation and final reporting of the time-stamps of the previous FTM exchange. The time-stamp resolution is expected to improve to the order of 100ps from that of 10ns in existing TOA and RTT methods. This will substantially decrease the theoretical limitation on the positioning accuracy.

The conventional FTM signalling procedure is illustrated in Figure 2 between two devices labelled STA1 and STA2. STA1 is the requesting device (i.e. the device that wishes to know the distance between it and STA2). STA1 and STA2 may each be mobile stations or APs, and thus the communications may take place between a requesting mobile station and a responding AP, a requesting AP and a responding mobile station, a requesting mobile station and a responding mobile station, or a requesting AP and a responding AP.

First, a handshake between the two devices is executed. Thus, in step 50, a request message is transmitted from STA1 to STA 2, requesting the initiation of a FTM procedure. STA2 acknowledges the request by transmitting an ACK message to STA1.

In a second FTM implementation stage, a first FTM message (FTM1) is sent by STA2 and received by STA1 (step 54). The time of departure (TOD) of this packet, t1 , is recorded by STA2; and the time of arrival (TOA) of this packet, t2, is recorded by STA1. In step 56, STA1 acknowledges the FTM message by transmitting an ACK message to STA2. Again, the TOD (t3) and the TOA (t4) are recorded by STA1 and STA2, respectively.

In a third, reporting stage, the values of t1 and t4 are reported by STA2 to STA1. Thus, in step 58, a further FTM message (FTM2) is transmitted by STA2 to STA1. The FTM2 message comprises the values of t1 and t4, i.e. the time of departure of the FTM1 message transmitted in step 54, and the time of arrival of the ACK message transmitted in step 56. In an optional step 60, this message is acknowledged by the transmission of an ACK message from STA1 to STA2.

Thus STA1 has knowledge of the values of t1 , t2, t3 and t4, and can calculate the average time of flight between STA1 and STA2 according to the equation:

((t4 - t3) + (t2 - t1))/2, and a corresponding distance using the speed of light. Note that this equation may not rely on synchronization between the clocks of STA1 and STA2 as any clock offset is automatically compensated by subtraction. In any case, a value for the offset can be calculated by the equation:

((t4 - t3) - (t2 - t1))/2.

Note that the FTM process can be repeated based on the TOD and TOA values (tV, t2', t3' and t4') associated with the FTM2 message transmitted in step 58 and its corresponding ACK message transmitted in step 60 (and may be repeated again as needed in a similar manner). The measurements for each round of FTM signalling may be combined, for example, by determining an average value of the round-trip time. However, in other examples the FTM process is carried out only once, with a single measurement of the round trip time.

Summary

The Fine Timing Measurement (FTM) protocol is a powerful protocol that is able to achieve very high positioning accuracy in many typical environments. However, in dense deployments where large numbers of user devices congregate in the same location (e.g., metro stations or internet of things (loT) environments), the resulting signalling overhead at the access points becomes expensive in terms of the resources required. For example, as suggested to be standardized, FTM requires a minimum of six physical layer convergence protocol (PLCP) Data Units (PPDUs) per mobile station per AP per positioning request. Assuming at least 384 με per FTM procedure (the minimum transmission time over the air, typically, for the FTM Request/ACK is 124με, the initial FTM/ACK is 132 με and the second FTM/ACK is 128 μβ) and that there are 1000 mobile stations in range of an AP with a 0.5 Hz refreshing rate, this will consume at least 19% of the medium time on that AP's channel. The overhead gets N-fold worse if there are N co-channel APs. Therefore, enhancements to the FTM protocol are needed which enable improved scalability in dense environments.

In one aspect of the present disclosure, there is provided a method in a first node of a wireless local area network, comprising: broadcasting a fine timing measurement, FTM, message forming part of a plurality of respective FTM processes with a plurality of second nodes of the wireless local area network; and receiving respective response messages from one or more second nodes of the plurality of second nodes during the respective FTM processes.

In another aspect of the disclosure, there is provided a method in a second node of a wireless local area network, the method comprising: receiving a broadcast fine timing measurement, FTM, message from a first node of the wireless local area network, the broadcast FTM message forming part of a plurality of respective FTM processes with a plurality of second nodes of the wireless local area network; determining that the broadcast FTM message is part of an FTM process between the first node and the second node; and transmitting a response message to the first node during the respective FTM process.

In a further aspect, there is provided a first node for a wireless local area network, comprising: processing circuitry; and a computer-readable medium coupled to the processor circuitry and storing code which, when executed by the processor circuitry, causes the first node to: broadcast a fine timing measurement, FTM, message forming part of a plurality of respective FTM processes with a plurality of second nodes of the wireless local area network; and receive respective response messages from one or more second nodes of the plurality of second nodes during the respective FTM processes.

In another aspect, there is provided a second node for a wireless local area network, the second node comprising: processing circuitry; and a computer-readable medium coupled to the processor circuitry and storing code which, when executed by the processor circuitry, causes the second node to: receive a broadcast fine timing measurement, FTM, message from a first node of the wireless local area network, the broadcast FTM message forming part of a plurality of respective FTM processes with a plurality of second nodes of the wireless local area network; determine that the broadcast FTM message is part of an FTM process between the first node and the second node; and transmit a response message to the first node during the respective FTM process.

Brief description of the drawings

For a better understanding of examples of the present disclosure, and to show more clearly how the examples may be carried into effect, reference will now be made, by way of example only, to the following drawings in which: Figure 1 is a schematic drawing of the positioning architecture for a wireless local area network; Figure 2 is a signalling diagram showing the signalling between two devices according to the conventional FTM procedure;

Figure 3 is a signalling diagram showing the signalling for an FTM procedure according to embodiments of the disclosure;

Figure 4 is a signalling diagram showing the signalling for an FTM procedure according to further embodiments of the disclosure;

Figure 5 is a flowchart of a method in a first node according to embodiments of the disclosure;

Figure 6 is a flowchart of a method in a second node according to embodiments of the disclosure; Figure 7 is a schematic diagram of a first node according to embodiments of the disclosure;

Figure 8 is a schematic diagram of a first node according to further embodiments of the disclosure;

Figure 9 is a schematic diagram of a second node according to embodiments of the disclosure; and

Figure 10 is a schematic diagram of a second node according to further embodiments of the disclosure.

Detailed description

The following sets forth specific details, such as particular embodiments or examples for purposes of explanation and not limitation. It will be appreciated by one skilled in the art that other examples may be employed apart from these specific details. In some instances, detailed descriptions of well-known methods, nodes, interfaces, circuits, and devices are omitted so as not obscure the description with unnecessary detail. Those skilled in the art will appreciate that the functions described may be implemented in one or more nodes using hardware circuitry (e.g., analog and/or discrete logic gates interconnected to perform a specialized function, ASICs, PLAs, etc.) and/or using software programs and data in conjunction with one or more digital microprocessors or general purpose computers. Nodes that communicate using the air interface also have suitable radio communications circuitry. Moreover, where appropriate the technology can additionally be considered to be embodied entirely within any form of computer- readable memory, such as solid-state memory, magnetic disk, or optical disk containing an appropriate set of computer instructions that would cause a processor to carry out the techniques described herein.

Hardware implementation may include or encompass, without limitation, digital signal processor (DSP) hardware, a reduced instruction set processor, hardware (e.g., digital or analogue) circuitry including but not limited to application specific integrated circuit(s) (ASIC) and/or field programmable gate array(s) (FPGA(s)), and (where appropriate) state machines capable of performing such functions.

According to embodiments of the disclosure, a plurality of respective fine timing measurement (FTM) processes are carried out between a first node and a plurality of respective second nodes in a wireless local area network. The first node may be the "requesting node", i.e. the node that wishes to determine the distances between it and other nodes; alternatively, the second nodes may be the "requesting nodes". According to embodiments of the disclosure, the first node is operative to broadcast an FTM message to the plurality of second nodes as part of those multiple respective FTM processes. Thus for a particular step of the FTM process, the first node transmits a single message that is utilized by multiple second nodes in their respective FTM processes. The FTM message that is broadcast may be different in different embodiments, depending, for example, on whether the first node is the requesting node or the second nodes are the requesting nodes. In one embodiment, particularly where the second nodes are the requesting nodes, the broadcast FTM message can be the first message in the second stage of the FTM process (i.e. equivalent to the FTM 1 message shown in step 54 of Figure 2), or the first message in the third stage of the FTM process (i.e. equivalent to the FTM2 message shown in step 58 of Figure 2). In further embodiments, both messages may be separately broadcast. In another embodiment, particularly where the first node is the requesting node, the broadcast FTM message can be the initial FTM request message (i.e. equivalent to the request message shown in step 50 of Figure 2) or the ACK message in the second stage of the FTM process (i.e. equivalent to the ACK message shown in step 56 of Figure 2). In still further embodiments, both messages can be separately broadcast.

Thus by using broadcast messages for one or more of the messages defined in the FTM procedure, and using each broadcast message for FTM processes that are ongoing with a plurality of second nodes, the resources necessary to service those FTM processes in the first node can be reduced.

Figure 3 is a signalling diagram showing the signalling for an FTM procedure according to embodiments of the disclosure. The signalling shown in Figure 3 applies particularly where a plurality of second nodes request initiation of an FTM process with a first node.

The signalling shown in Figure 3 illustrates signals being transmitted between a single access point (AP) and a plurality of mobile stations (STA1 , STA2 and STA3). The AP may be considered equivalent to any one of the APs 14, 16, 18 shown in Figure 1. The mobile stations may be considered equivalent to the mobile stations 12a, 12b, 12c shown in Figure 1. The mobile stations STA1 , STA2 and STA3 may form part of a basic service set with the AP.

In this example, the mobile stations are equivalent to the second nodes, i.e. the nodes that receive the broadcast message(s), whereas the AP is equivalent to the first node that transmits the broadcast message(s). However, it will be understood by those skilled in the art that, while this arrangement may be typical, alternative arrangements falling within the scope of the invention described herein may be based on mesh wireless local area networks that have no access point (in which case a mobile station transmits broadcast messages to other mobile stations), or a mobile station transmitting broadcast messages to multiple other nodes (which may include one or more APs and one or more mobile stations). Thus, while Figure 3 may illustrate the broadcast of one or more FTM messages by an AP, it will be understood that the broadcast of one or more FTM messages by a mobile station is also contemplated. The signalling begins in step 100, in which STA1 transmits to the AP a request for initiation of an FTM process between STA1 and the AP. The request may comprise an FTM request frame. The request message may request immediate initiation of such an FTM process (or initiation of an FTM process as soon as practicable or within a specific time frame of receipt of the request message), or may request initiation of an FTM process at a specific later time. In the latter case, the request message may contain an indication of the preferred time slot for initiation of the FTM process.

In step 102, the AP transmits an acknowledgement (ACK) message to STA1 , acknowledging the request message transmitted and received in step 100. The ACK message may contain an acknowledgement that the FTM process with STA1 has been initiated, and may begin within a specific time frame, or within a scheduled time slot at a later time. In steps 104 and 106, this process is repeated with a second mobile station STA2. Thus in step 104, STA2 transmits to the AP a request for initiation of an FTM process between STA2 and the AP. The request message may request immediate initiation of such an FTM process (or initiation of an FTM process as soon as practicable or within a specific time frame of receipt of the request message), or may request initiation of an FTM process at a specific later time. In the latter case, the request message may contain an indication of the preferred time slot for initiation of the FTM process.

In step 106, the AP transmits an acknowledgement (ACK) message to STA2, acknowledging the request message transmitted and received in step 104. The ACK message may contain an acknowledgement that the FTM process with STA2 has been initiated, and may begin within a specific time frame, or within a scheduled time slot at a later time.

Thus the AP has received two requests for initiation of an FTM process, from mobile stations STA1 and STA2. The third mobile station STA3 has not requested initiation of an FTM process. The AP may determine to transmit a single broadcast message forming part of both FTM processes. Such a determination may be based on, for example, the FTM request messages in steps 100 and 104 being received within a threshold time of each other. That is, a threshold time may be set in the AP (for example through signalling from an internet service provider, or hard-coding within the AP during initial configuration). Upon receipt of a first FTM request message, a timer may be activated in the AP and all further FTM request messages received while the timer is active (i.e. before it reaches the threshold time value) grouped together such that the AP transmits a single broadcast message in respect of those multiple grouped FTM processes. In alternative embodiments, the time resources of the AP may be divided into time slots, with all FTM request messages received within a particular time slot grouped together for the purpose of transmitting a broadcast message. The length of the time slot may be configured dynamically or hard-coded, as with the threshold time described above. Further, the mobile stations STA1 and STA2 may initiate corresponding timers, for example, upon receipt of the ACK messages in steps 102 and 106 respectively. The values of those timers may be set to a value that is greater than the threshold timer running in the AP. If a corresponding FTM1 message (see step 110 below) is not received before the timer runs out in STA1 or STA2, the affected mobile station may abort the current FTM process and transmit a further request message at a later time in order to initiate another FTM process.

Having decided to group the FTM processes requested by mobile stations STA1 and STA2, in an optional step 108 the AP transmits a broadcast announcement message. A broadcast announcement message (or packet) may be used to specify particular devices that should listen for a subsequent broadcast FTM message, and may thus comprise an indication of those devices. In the illustrated embodiment, therefore, the broadcast announcement message may comprise an indication of the mobile stations STA1 and STA2 (which have both requested initiation of FTM processes). For example, the indication may comprise identifiers for the mobile stations in question, such as association identifiers (AIDs). The broadcast announcement message may further instruct all devices (e.g. all devices in the basic service set) not to transmit messages in the following timeslot, or in a later timeslot that is specified, as the timeslot is to be used for FTM broadcasting. The broadcast announcement message may therefore be broadcast to all devices in the basic service set. Thus, following transmission of the broadcast announcement message, the AP has control of the channel in a particular timeslot (either the immediately following timeslot or a specified timeslot) and can avoid collisions with other devices in the basic service set. The broadcasting announcement packet can be a variant of the null-data packet (NDP) announcement packet used for beamforming. For example, the message may comprise a duration field, indicating the time period in which broadcast of an FTM packet is intended. Mobile stations that receive this message may set their network allocation vector (NAV) timers according to the values specified in the duration field such that they do not contend with the AP for use of the channel during the specified period. The broadcast announcement packet may further comprise "STA info" fields carrying information for each targeted mobile station (i.e. STA1 and STA2).

Alternatively, the broadcast announcement message can be carried as part of the beacon frame regularly transmitted by APs according to the IEEE 802.11x specifications.

In step 1 10, in the timeslot specified by the broadcast announcement message (if one is transmitted), the AP broadcasts an FTM message to all devices in the basic service set and records the time (based on a clock that is local to the AP) at which the FTM1 message is broadcast (e.g. equivalent to t1 in Figure 2). For example, the broadcast FTM message can be the first message in the second stage of the FTM process (i.e. equivalent to the FTM1 message shown in step 54 of Figure 2). In some embodiments, apart from being broadcast to the entire basic service set, the FTM message is substantially identical to the conventional FTM 1 message shown in step 54 of Figure 2. Thus the FTM message may comprise an FTM field (which is configurable to contain time-stamp information), and control signalling (which may specify the time length of the FTM message, for example).

In further embodiments, however, the FTM message may be adapted from the conventional FTM1 message shown in step 54 of Figure 2. For example, the FTM message may comprise mobile station information identifying the (multiple) mobile stations that are to use the FTM message in their respective FTM processes, i.e. mobile stations STA1 and STA2. This configuration is particularly useful in embodiments where the broadcast announcement message is not transmitted in step 108.

In some embodiments, the control signalling may also be adapted from the conventional FTM1 message to specify one or more mechanisms by which each mobile station should respond to the message. For example, different mechanisms may be specified for each mobile station, or a particular mechanism may be specified for multiple, or all mobile stations. The broadcast FTM message may comprise a time value within which the mobile stations should respond. Further detail regarding this embodiment will be described below.

In step 1 12, the mobile stations identified in the broadcast announcement message, or the broadcast FTM1 message (or both), and which successfully received the broadcast FTM1 message, record the time at which the FTM1 message was received (based on respective clocks that are local to the mobile stations; equivalent to t2 in Figure 2), transmit respective ACK messages to the AP to acknowledge safe receipt of the FTM 1 message broadcast in step 110, and record the respective times at which the ACK message is transmitted (equivalent to t3 in Figure 2). The AP further records the time(s) at which the ACK messages are received (equivalent to t4 in Figure 2). Thus, as mobile stations STA1 and STA2 were specified, and we assume that the broadcast FTM1 message was safely received by both stations, both mobile stations STA1 and STA2 transmit respective ACK messages to the AP in step 112. Note that mobile station STA3 does not transmit an ACK message (even though it may have received the broadcast FTM1 message), as it was not specified in the broadcast announcement message or the broadcast FTM1 message. The broadcast FTM1 message is therefore ignored by mobile station STA3. Note also that in general the broadcast FTM1 message may not be successfully received by all devices that it is intended for. Therefore, even though the broadcast FTM1 message is intended for use by multiple mobile stations in their respective FTM processes, not all those mobile stations may respond with an ACK message in step 1 12. The FTM processes for stations that do not respond with an ACK message may be cancelled, or restarted with transmission of a further FTM1 message at a later time. If no ACK messages are received (for example within a time window following broadcast of the FTM1 message), the AP may re-transmit the broadcast FTM1 message. The ACK messages transmitted in step 1 12 may be transmitted according to the mechanism or mechanisms specified in the control signalling of the broadcast FTM1 message. For example, mobile stations may be instructed to transmit their ACK messages in a contention-based way, i.e. sensing whether the transmission medium is active or idle, and transmitting the ACK message responsive to a determination that the medium is idle. Thus other devices transmitting on the medium, whether transmitting their own ACK messages or some other data message, may take priority over the transmission of an ACK message by a particular mobile station. As noted above, in some embodiments the broadcast FTM message may comprise a time value within which mobile stations should respond (i.e. by transmission of an ACK message). The time value may be an absolute time value, or a relative time value (i.e. relative to a time at which the broadcast FTM message is received). If a mobile station is unable to respond within the time value specified, the mobile station may abort the current FTM process and, if desired, request initiation of another FTM process in due course.

In other embodiments, each mobile station may be scheduled, in the control signalling of the FTM1 message, a respective time slot in which to transmit its respective ACK message to the AP. In these embodiments, the duration field in the broadcast announcement message, or the time length in the FTM1 message, may include, in addition to the time window in which the FTM 1 message is broadcast, the time window in which ACK messages are expected to be received from the mobile stations.

In other embodiments, multiple or all mobile stations may be instructed to transmit their respective ACK messages substantially simultaneously, either immediately upon receipt of the FTM1 message in step 1 10, or within a timeslot specified in the control signalling of the FTM1 message. Such simultaneous transmission may be facilitated by use of orthogonal frequency division multiple access (OFDMA) or multi-user multiple-input-multiple-output (MU-MIMO) techniques, in which case the particular multi-user mode to be utilized may be specified in the control signalling of the FTM1 message. In further embodiments, a mix of these mechanisms may be specified. For example, those mobile stations that are relatively insensitive to delay (e.g. because they are stationary or have low mobility) may be configured to transmit their respective ACK messages on a contention basis, i.e. as and when the medium is idle; whereas those mobile stations that are relatively sensitive to delay (e.g. because they have high mobility or need to know their position urgently) may be scheduled to transmit their respective ACK messages immediately or within a short time window of receiving the broadcast FTM1 message.

In still further embodiments, for example if no mechanisms are specified in the control signalling, one or more of the response mechanisms described above may be utilized as a default response mechanism. Once the ACK messages have been received by the AP, the final stage of the FTM process takes place, in which the AP transmits the FTM2 message comprising the time values (i.e. time stamps) at which the AP broadcasted the FTM1 message in step 110, and received the respective ACK messages in step 112.

The signalling diagram in Figure 3 contains two separate embodiments for this aspect of the process. In a first embodiment, shown in step 1 14, the AP broadcasts a single FTM2 message in respect of each of the ACK messages received in step 112. The AP may initiate broadcast of the FTM2 message responsive to a determination that all ACK messages have been received (i.e., a respective ACK message has been received for each of the mobile stations specified in the FTM1 message or in the broadcast announcement message). However, as noted above, it is possible that not all of the targeted mobile stations will successfully receive the broadcast FTM1 message. Thus, in some embodiments, the AP may initiate broadcast of the FTM2 message responsive to a determination that the timeslot(s) scheduled for transmission of the ACK messages have passed. Alternatively, if no timeslots are reserved for transmission of ACK messages (e.g. the mobile stations are configured to transmit ACK messages on a contention basis), the AP may initiate broadcast of the FTM2 message responsive to a determination that a time window since transmission of the FTM1 message has passed.

The FTM2 message that is broadcast in step 1 14 may take substantially the same form as the FTM1 message broadcast in step 1 10. However, the FTM2 message additionally comprises the time value at which the FTM1 message was broadcast in step 110 (i.e. t1) and respective time values at which each of the ACK messages was received by the AP in step 112 (i.e. t4). Thus, the broadcast FTM2 message comprises respective t4 values associated with each of the mobile stations for which ACK messages were received. For example, the t4 value may be contained within a respective field for each of the mobile stations, identified by the mobile station identity.

In some embodiments, the FTM2 message may be preceded by a further broadcast announcement message, substantially as described above with respect to step 108. In an alternative embodiment to that described above with respect to step 114, the AP may transmit separate, respective FTM2 messages to each of the mobile stations. In this embodiment, the FTM2 message can be transmitted at any time after the respective ACK message for the mobile station has been received in step 1 12. Thus in step 1 16, the AP transmits an FTM2 message to mobile station STA1 ; and in step 118, the AP transmits an FTM2 message to mobile station STA2. Each FTM2 message comprises the time data (i.e. t1 and t4) that is relevant to the recipient mobile station, i.e. the time t1 at which the FTM1 message was broadcast by the AP, and the time t4 at which the respective ACK message for that mobile station was received by the AP.

Thus at the conclusion of step 114, or steps 116 and 118, each mobile station STA1 and STA2 has the following information:

• The time t1 at which the FTM 1 message was broadcast in step 1 10;

• The time t2 at which the FTM1 message was received by the respective mobile station;

· The time t3 at which the respective ACK message was transmitted by the mobile station in step 112; and

• The time t4 at which the respective ACK message was received by the AP.

Based on this information, the mobile stations are able to determine the clock offset between their respective local clocks and the clock that is local to the AP, and also the time of flight for messages transmitted to and received from the AP. Based on the latter parameter, the mobile stations are able to determine their respective distances from the AP. The signalling shown in Figure 3 may continue in a number of ways. For example, as shown above with respect to Figure 2, the FTM process can continue for multiple rounds of transmission of an FTM1 , FTM2, FTM3 message, etc and corresponding ACK messages. The time data for each round of messages may be combined to form an average time (and a corresponding average distance from the AP). Further, in order to determine their position more accurately, the mobile stations may undertake corresponding FTM processes with other nodes (whether APs or peer mobile stations).

However, the signalling shown in Figure 3 has the advantage of combining the signals for multiple FTM processes into a single, broadcast message. This reduces the resources required of the "responding node" (i.e. the AP in the illustrated embodiment) in handling FTM processes for multiple requesting nodes (i.e. the mobile stations in the illustrated embodiment).

As discussed above, it is possible for the first node (that is, the node that broadcasts one or more messages as part of the FTM process) to respond to requests for initiation of respective FTM processes with one or more second nodes, or to request initiation of respective FTM processes with multiple second nodes itself. Figure 4 is a signalling diagram according to the latter case, showing the signalling for an FTM procedure according to further embodiments of the disclosure.

The same example is used as Figure 3, of an AP that broadcasts one or more messages to multiple mobile stations. Again, however, it should be noted that the broadcasting node may be an AP or a mobile station, and the other nodes may be APs or mobile stations (or both).

The signalling shown in Figure 4 begins with the AP determining that it needs to determine the distance between it and a plurality of mobile stations in its basic service set, and thus needs to initiate respective FTM processes with those devices. In the illustrated embodiment, FTM processes are required with mobile stations STA1 and STA2, but not mobile station STA3.

In an optional step 200, the AP transmits a broadcast announcement message. The broadcast announcement message may be substantially similar to that described above with respect to step 108. Thus, the broadcast announcement message may be used to specify particular devices that should listen for a subsequent broadcast FTM message, and may thus comprise an indication of those devices. In the illustrated embodiment, therefore, the broadcast announcement message may comprise an indication of the mobile stations STA1 and STA2 (with which FTM processes are to be initiated). For example, the indication may comprise identifiers for the mobile stations in question, such as association identifiers (AIDs). The broadcast announcement message may further instruct all devices (e.g. all devices in the basic service set) not to transmit messages in the following timeslot, or in a later timeslot that is specified, as the timeslot is to be used for FTM broadcasting. The broadcast announcement message may therefore be broadcast to all devices in the basic service set. Thus, following transmission of the broadcast announcement message, the AP has control of the channel in a particular timeslot (either the immediately following timeslot or a specified timeslot) and can avoid collisions with other devices in the basic service set.

As described above, the broadcasting announcement packet can be a variant of the null-data packet (NDP) announcement packet used for beamforming, or carried as part of the beacon frame regularly transmitted by APs according to the IEEE 802.11x specifications, for example.

In step 202, in the timeslot specified by the broadcast announcement message (if one is transmitted), the AP broadcasts an FTM message to all devices in the basic service set. For example, the broadcast FTM message can be the FTM request message (i.e. equivalent to the FTM request message shown in step 50 of Figure 2). In some embodiments, particularly if the broadcast announcement message comprises an indication of the target devices that should listen for the broadcast FTM request message, the FTM request message can be substantially similar to the conventional FTM request message shown in step 50 of Figure 2.

In further embodiments, however, the FTM request message may be adapted to include mobile station information identifying the (multiple) mobile stations with which FTM processes are requested to be initiated, i.e. mobile stations STA1 and STA2. This configuration is particularly useful in embodiments where the broadcast announcement message is not transmitted in step 200.

In some embodiments, the FTM request message may be further adapted to specify one or more mechanisms by which each mobile station should respond to the message, i.e. the mechanisms by which each mobile station should transmit an ACK message acknowledging receipt of the broadcast FTM request message, the mechanisms by which each mobile station should transmit respective FTM1 messages, or both. For example, different mechanisms may be specified for each mobile station, or a particular mechanism may be specified for multiple, or all mobile stations. Again, as above, the broadcast FTM request message may comprise a timer value (whether absolute or relative) within which mobile stations should respond to the FTM request message (or else abort the process). The FTM request message is broadcast to all devices within the basic service set, and is thus received by mobile stations STA1 , STA2 and STA3. Mobile station STA3, however, is not indicated in either the broadcast announcement message or the broadcast FTM request message, and can therefore ignore the request for initiation of an FTM process. In step 204, mobile stations STA1 and STA2 transmit respective ACK messages to the AP, acknowledging receipt of the broadcast FTM request message. These ACK messages can be substantially similar to those shown in step 52 of Figure 2. The ACK messages may be transmitted according to the mechanisms specified in the FTM request message, or according to a default mechanism if no mechanism is specified. The range of mechanisms is substantially the same as that described above with respect to step 112, e.g. contention-based, scheduled, multi-user, etc.

In steps 206 and 208, mobile stations STA1 and STA2 further transmit respective FTM1 messages to the AP, and record the times at which those messages were transmitted (according to their local clocks). Thus in step 206, mobile station STA1 transmits an FTM 1 message to the AP; and in step 208, mobile station STA2 transmits a further FTM1 message to the AP. The AP records the times at which those messages were received. The FTM 1 messages may be substantially conventional, i.e. as described above with respect to step 54 of Figure 2. The FTM 1 messages may be transmitted according to the mechanisms specified in the FTM request message, or according to a default mechanism if no mechanism is specified. The range of mechanisms is substantially the same as that described above with respect to step 1 12, e.g. contention-based, scheduled, multi-user, etc. In some embodiments, the ACK messages transmitted in step 204 may be omitted, and the FTM1 messages themselves used as an acknowledgement of the broadcast FTM request message transmitted in step 202.

Upon receipt of the respective FTM1 messages, according to the established FTM process the AP responds with an ACK message. Figure 4 illustrates two embodiments for this aspect. In a first embodiment, the AP broadcasts a single ACK message that is utilized by all mobile stations with which FTM processes are ongoing. In a second embodiment, the AP transmits respective individual ACK messages for each of the FTM1 messages received in steps 206 and 208. In the first embodiment, in step 210, the AP may initiate broadcast of the ACK message responsive to a determination that all FTM1 messages have been received (i.e., a respective FTM1 message has been received for each of the mobile stations for which ACK messages were received in step 204). However, it is possible that not all of the targeted mobile stations will successfully transmit a FTM1 message. For example, the mobile stations may fall out of coverage, or be powered down. Thus, in some embodiments, the AP may initiate broadcast of the ACK message responsive to a determination that the timeslot(s) scheduled for transmission of the FTM1 messages have passed. Alternatively, if no timeslots are reserved for transmission of FTM1 messages (e.g. the mobile stations are configured to transmit FTM1 messages on a contention basis), the AP may initiate broadcast of the ACK message responsive to a determination that a time window has passed since receipt of the first FTM 1 message.

The broadcast ACK message may comprise an indication of the mobile stations for which respective FTM1 messages were received in steps 206 and 208 (i.e. a list of the identities of the mobile stations in question). Note that the mobile stations listed in the broadcast ACK message may not include all of the mobile stations which were listed in the original broadcast announcement in step 200, or the broadcast FTM request message in step 202, for example in the event that a mobile station fails to transmit an ACK message in step 204 or an FTM1 message in steps 206 or 208.

The broadcast ACK message may be preceded by a corresponding broadcast announcement message transmitted by the AP, as described above with respect to step 200.

In the second embodiment, in step 212, the AP transmits individual ACK messages to each of the mobile stations STA1 and STA2 for which FTM1 messages were received in steps 206 and 208, i.e. without broadcasting messages. These messages may nonetheless be transmitted simultaneously in some embodiments, e.g. if using downlink multi-user transmission.

In both embodiments, the AP records the time (or times) at which the ACK message(s) were transmitted, and the mobile stations STA1 and STA2 record the times at which the ACK message(s) were received. In steps 214 and 216, the final stage of the FTM process is carried out, in which the mobile stations STA1 and STA2 report the time values at which the FTM1 messages were transmitted (i.e. t1) and at which the ACK message(s) were received (i.e. t4). Thus in step 214, mobile station STA1 transmits a respective FTM2 message to the AP; and in step 216, mobile station STA2 transmits a respective FTM2 message to the AP. Each FTM2 message comprises an indication of t1 and t4 for the mobile station in question.

Thus at the conclusion of step 216, the AP has the following information for each mobile station:

• The times t1 at which the respective FTM1 messages were transmitted in steps

206 and 208;

• The times t2 at which the respective FTM1 messages were received by the AP; · The time t3 at which the ACK message was broadcast by the AP in step 210, or the respective times t3 at which the respective ACK messages were transmitted by the AP in step 212; and

• The times t4 at which the ACK message or messages were received by the respective mobile stations.

Based on this information, the AP is able to determine the clock offset between its local clock and the respective local clocks that are local to the mobile stations, and also the times of flight for messages transmitted to and received from the mobile stations. Based on the latter parameters, the AP is able to determine the respective distances between it and the mobile stations.

Again, the signalling shown in Figure 4 may continue for multiple rounds of transmission of FTM1 , FTM2, FTM3 messages, etc and corresponding ACK messages. The time data for each round of messages may be combined to form an average time (and a corresponding average distance from the AP). Further, in order to determine their position more accurately, the process may be repeated with other requesting devices. The information from multiple devices (e.g. multiple APs) can then be combined to determine the position of the mobile stations, e.g. as described above with respect to Figure 1. Figure 5 is a flowchart of a method in a first node of a wireless local area network according to embodiments of the disclosure. The first node may, for example, correspond to the role of the AP as described above in Figures 3 and 4. However, it will be apparent from the comments above that the first node may also be a mobile station.

The method begins in an optional step 300, in which the first node broadcasts a broadcast announcement message. For example, the broadcast announcement message may be broadcast to a particular basic service set. The broadcast announcement message may comprise an indication of a plurality of second nodes of the wireless local area network which are to listen for a subsequent broadcast message forming part of a plurality of respective FTM processes with those second nodes. For example, the indication may comprise identifiers for the second nodes in question, such as association identifiers (AIDs). The broadcast announcement message may further instruct all devices (e.g. all devices in the basic service set) not to transmit messages in the following timeslot, or in a later timeslot that is specified, as the timeslot is to be used for FTM broadcasting. The broadcast announcement message may therefore be broadcast to all devices in the basic service set. Thus, following transmission of the broadcast announcement message, the first node has control of the channel in a particular timeslot (either the immediately following timeslot or a specified timeslot) and can avoid collisions with other devices in the basic service set.

The broadcasting announcement packet can be a variant of the null-data packet (NDP) announcement packet used for beamforming. For example, the message may comprise a duration field, indicating the time period in which broadcast of an FTM packet is intended. Second nodes that receive this message may set their network allocation vector (NAV) timers according to the values specified in the duration field such that they do not contend with the first node for use of the channel during the specified period. The broadcast announcement packet may further comprise second- node information fields carrying information for each targeted second node.

Alternatively, the broadcast announcement message can be carried as part of the beacon frame regularly transmitted by APs according to the IEEE 802.11x specifications. In step 302, the first node broadcasts (i.e. transmits to all devices in the basic service set) an FTM message forming part of a plurality of respective FTM processes with the plurality of second nodes. The FTM message may comprise an indication of the plurality of second nodes (e.g. a list of identities of the second nodes), particularly in embodiments where no broadcast announcement message is transmitted in step 300.

The FTM message that is broadcast may be different in different embodiments, depending, for example, on whether the first node is the requesting node (i.e. the node that wishes to determine its distance from a plurality of second nodes) or the second nodes are the requesting nodes (i.e. the plurality of second nodes wish to determine their respective distances from the first node). In one embodiment, particularly where the second nodes are the requesting nodes (i.e. respective FTM request messages have been received from the second nodes for initiation of FTM processes), the broadcast FTM message can be the first message in the second stage of the FTM process (i.e. the FTM1 message), or the first message in the third stage of the FTM process (i.e. the FTM2 message). In further embodiments, both messages may be separately broadcast.

In another embodiment, particularly where the first node is the requesting node, the broadcast FTM message can be the initial FTM request message or the ACK message in the second stage of the FTM process (i.e. transmitted in response to receipt of multiple FTM1 messages from respective second nodes). In still further embodiments, both messages can be separately broadcast. In step 304, the first nodes receives respective response messages from one or more of the plurality of second nodes. Although a plurality of FTM processes are initiated with the plurality of second nodes, it will be appreciated that not all of the plurality of second nodes may be able to transmit response messages. For example, a second node may fail to receive the broadcast FTM message, drop out of coverage (such that transmitted messages are not received by the first node), or be powered down.

The response messages that are transmitted by the second nodes and received by the first node may differ in different embodiments, depending on the nature of the FTM message broadcast in step 302. For example, if an FTM request message is broadcast in step 302, the response messages may comprise one or more of: ACK messages from those second nodes that received the FTM request message; and FTM1 messages from those second nodes. If an FTM1 message is broadcast in step 302, the response messages may comprise ACK messages for that FTM1 message. If an ACK message is transmitted in step 302, the response messages may comprise FTM2 messages, containing respective time information for times of transmission and receipt of the preceding messages in the FTM processes. Further detail regarding these embodiments can be found in the discussion of Figures 3 and 4 above.

In step 306, the FTM processes are completed. Depending on the nature of the FTM message broadcast in step 302, and the response messages received in step 304, step 306 may comprise one or more of: transmitting or broadcasting ACK message(s) for FTM1 messages received in step 304; receiving respective FTM2 messages from each of the second nodes and determining the distances to each of the second nodes based on the time information contained within those FTM2 messages; and transmitting or broadcasting an FTM2 message(s) to the second nodes.

Figure 6 is a flowchart of a method in a second node of a wireless local area network according to embodiments of the disclosure. The first node may, for example, correspond to the role of either of the mobile stations STA1 and STA2 as described above in Figures 3 and 4. However, it will be apparent from the comments above that the first node may also be an access point.

The method begins in an optional step 400, in which the second node receives a broadcast announcement message from a first node of the wireless local area network. For example, the broadcast announcement message may be broadcast to a particular basic service set. The broadcast announcement message may comprise an indication of a plurality of second nodes of the wireless local area network which are to listen for a subsequent broadcast message forming part of a plurality of respective FTM processes with those second nodes. For example, the indication may comprise identifiers for the second nodes in question, such as association identifiers (AIDs). The broadcast announcement message may further instruct all devices (e.g. all devices in the basic service set) not to transmit messages in the following timeslot, or in a later timeslot that is specified, as the timeslot is to be used for FTM broadcasting. The broadcast announcement message may therefore be broadcast to all devices in the basic service set. Thus, following transmission of the broadcast announcement message, the first node has control of the channel in a particular timeslot (either the immediately following timeslot or a specified timeslot) and can avoid collisions with other devices in the basic service set.

The broadcasting announcement packet can be a variant of the null-data packet (NDP) announcement packet used for beamforming. For example, the message may comprise a duration field, indicating the time period in which broadcast of an FTM packet is intended. Second nodes that receive this message may set their network allocation vector (NAV) timers according to the values specified in the duration field such that they do not contend with the first node for use of the channel during the specified period. The broadcast announcement packet may further comprise second- node information fields carrying information for each targeted second node.

Alternatively, the broadcast announcement message can be received as part of the beacon frame regularly transmitted by APs according to the IEEE 802.11x specifications.

In step 402, the second node receives a broadcasts FTM message forming part of a plurality of respective FTM processes with a plurality of second nodes (to which the second node belongs). The broadcast FTM message may comprise an indication of the plurality of second nodes (e.g. a list of identities of the second nodes), particularly in embodiments where no broadcast announcement message is transmitted in step 300.

The broadcast FTM message that is received may be different in different embodiments, depending, for example, on whether the first node is the requesting node (i.e. the node that wishes to determine its distance from a plurality of second nodes) or the second nodes are the requesting nodes (i.e. the plurality of second nodes wish to determine their respective distances from the first node). In one embodiment, particularly where the second nodes are the requesting nodes (i.e. respective FTM request messages have been received from the second nodes for initiation of FTM processes), the broadcast FTM message can be the first message in the second stage of the FTM process (i.e. the FTM1 message), or the first message in the third stage of the FTM process (i.e. the FTM2 message). In further embodiments, both messages may be separately broadcast. In another embodiment, particularly where the first node is the requesting node, the broadcast FTM message can be the initial FTM request message or the ACK message in the second stage of the FTM process (i.e. transmitted in response to receipt of multiple FTM1 messages from respective second nodes). In still further embodiments, both messages can be separately broadcast.

In step 404, the second node determines whether its identity is listed in either the broadcast announcement message optionally received in step 400, or the broadcast FTM message received in step 402. If the identity of the second node is not listed in either of those messages, the method proceeds to step 406 in which the broadcast FTM message received in step 402 is ignored.

If the identity of the second node is listed in either of those messages, the FTM message received in step 402 forms part of an FTM process between the first node and the second node, and the method proceeds to step 408 in which the second node transmits a response message to the first node.

The response message that is transmitted by the second node may differ in different embodiments, depending on the nature of the broadcast FTM message received in step 402. For example, if an FTM request message is received in step 402, the response message may comprise one or more of: an ACK message; and an FTM1 message. If an FTM1 message is received in step 402, the response message may comprise an ACK message for that FTM1 message. If an ACK message is received in step 402, the response message may comprise an FTM2 message, containing time information for the times of transmission and receipt of the preceding messages in the FTM process. Further detail regarding these embodiments can be found in the discussion of Figures 3 and 4 above.

In step 410, the FTM process between the second node and the first node is completed. Depending on the nature of the broadcast FTM message received in step 402, and the response message transmitted in step 408, step 410 may comprise one or more of: transmitting an FTM1 message; receiving an FTM2 message and determining the distance to the first node based on the time information contained within that FTM2 message; and transmitting an FTM2 message to the first node. Figure 7 is a schematic diagram of a first node 500 according to embodiments of the disclosure. For example, the first node 500 may be suitable for carrying out the method described above with respect to Figure 5, for example, or the signalling of the AP shown in Figures 3 and 4. The first node 500 may be a mobile station or an access point.

The first node 500 comprises processing circuitry 502 and a computer-readable medium 504 coupled to the processor circuitry 502. The computer-readable medium 504 stores code which, when executed by the processor circuitry 502, causes the first node 500 to: broadcast a fine timing measurement, FTM, message forming part of a plurality of respective FTM processes with a plurality of second nodes of the wireless local area network; and receive respective response messages from one or more second nodes of the plurality of second nodes during the respective FTM processes. In some embodiments, the first node 500 may also comprise one or more interfaces (not illustrated) over which packets can be transmitted. For example, such interfaces may comprise wired transceiver circuitry, or wireless transceiver circuitry and one or more antennas. Figure 8 is a schematic diagram of a first node 600 according to further embodiments of the disclosure. For example, the first node 600 may be suitable for carrying out the method described above with respect to Figure 5, for example, or the signalling of the AP shown in Figures 3 and 4. The first node 600 may be a mobile station or an access point.

The first node 600 comprises a first module 602 configured to broadcast a fine timing measurement, FTM, message forming part of a plurality of respective FTM processes with a plurality of second nodes of the wireless local area network. The first node 600 further comprises a second module 604 that is configured to receive respective response messages from one or more second nodes of the plurality of second nodes during the respective FTM processes.

In some embodiments, the first node 600 may also comprise one or more interface modules (not illustrated) over which packets can be transmitted. For example, such interface modules may comprise wired transceiver circuitry, or wireless transceiver circuitry and one or more antennas. Figure 9 is a schematic diagram of a second node 700 for a wireless local area network according to embodiments of the disclosure. For example, the second node 700 may be suitable for carrying out the method described above with respect to Figure 6, for example, or the signalling of the mobile stations STA1 or STA2 shown in Figures 3 and 4. The second node 700 may be a mobile station or an access point.

The second node 700 comprises processing circuitry 702 and a computer-readable medium 704 coupled to the processor circuitry 702. The computer-readable medium 704 stores code which, when executed by the processor circuitry 702, causes the second node 700 to: receive a broadcast fine timing measurement, FTM, message from a first node of the wireless local area network, the broadcast FTM message forming part of a plurality of respective FTM processes with a plurality of second nodes of the wireless local area network; determine that the broadcast FTM message is part of an FTM process between the first node and the second node; and transmit a response message to the first node during the respective FTM process.

In some embodiments, the second node 700 may also comprise one or more interfaces (not illustrated) over which packets can be transmitted. For example, such interfaces may comprise wired transceiver circuitry, or wireless transceiver circuitry and one or more antennas.

Figure 10 is a schematic diagram of a second node 800 for a wireless local area network according to further embodiments of the disclosure. For example, the second node 800 may be suitable for carrying out the method described above with respect to Figure 6, for example, or the signalling of the mobile stations STA1 or STA2 shown in Figures 3 and 4. The second node 800 may be a mobile station or an access point.

The second node 800 comprises a first module 802 configured to receive a broadcast fine timing measurement, FTM, message from a first node of the wireless local area network, the broadcast FTM message forming part of a plurality of respective FTM processes with a plurality of second nodes of the wireless local area network. The second node 800 further comprises a second module 804 configured to determine that the broadcast FTM message is part of an FTM process between the first node and the second node. The second node 800 further comprises a third module 806 configured to transmit a response message to the first node during the respective FTM process. In some embodiments, the second node 800 may also comprise one or more interface modules (not illustrated) over which packets can be transmitted. For example, such interface modules may comprise wired transceiver circuitry, or wireless transceiver circuitry and one or more antennas.

The disclosure thus provides methods and apparatus for performing timing measurement in a wireless local area network. According to embodiments of the disclosure one or more messages, forming part of a plurality of respective fine timing measurement processes with a plurality of second nodes of the network, are broadcast by a first node of the network, e.g. to all devices in the basic service set. Thus resources at the first node are considerably reduced when handling multiple concurrent FTM processes. It should be noted that the above-mentioned examples illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative examples without departing from the scope of the appended statements. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the statements below. Where the terms, "first", "second" etc are used they are to be understood merely as labels for the convenient identification of a particular feature. In particular, they are not to be interpreted as describing the first or the second feature of a plurality of such features (i.e. the first or second of such features to occur in time or space) unless explicitly stated otherwise. Steps in the methods disclosed herein may be carried out in any order unless expressly otherwise stated. Any reference signs in the statements shall not be construed so as to limit their scope.