GB2588966A

GB2588966A - Time difference of arrival calculation

Info

Publication number: GB2588966A
Application number: GB1916722.0A
Authority: GB
Inventors: Topi Juhani Mattila Petri
Original assignee: Unipart Group Ltd
Current assignee: Unipart Group Ltd
Priority date: 2019-11-16
Filing date: 2019-11-16
Publication date: 2021-05-19
Also published as: GB2588988A; GB201916722D0; GB202007337D0

Abstract

A method, device, and computer program for calculating a time difference of arrival, TDOA, of a wireless signal are provided. A first wireless signal 412 is transmitted by a tag device 410 and received by a first anchor device 420a at a first time and by a second anchor device 420b at a second time. The second anchor device transmits a second wireless signal 414 at a third time, which is received by the first anchor device at a fourth time. A first time difference is the fourth time minus the first time. A second time difference is the third time minus the second time. A TDOA is calculated between the receipt of the first wireless signal at the first anchor device at the first time and at the second anchor device at the second time based on the first time difference the second time difference and a known time of flight between the first anchor and the second anchor. Preferably the tag device transmits a third signal 416 which is received by the anchors at a fifth and sixth time. Third and fourth time differences are determined as the fifth time minus the fourth time and the sixth time minus the fifth time. The TDOA being calculated based at least partly on the first and second time differences, the third and fourth time differences and the known time of flight between the anchors.

Description

TIME DIFFERENCE OF ARRIVAL CALCULATION TECHNICAL FIELD

The embodiments herein pertain to wireless communication systems adapted to perform time difference of arrival (TDOA) calculations.

BACKGROUND

Wireless technologies have enabled the distance between radio frequency (RF) devices to be determined based on the time of flight (TOF) of RF signals transmitted therebetween. Techniques that determine the distance between RF devices in this manner are commonly referred to as RF ranging.

RF ranging techniques measure the RF signal propagation time between RF devices and multiply this time by the speed at which the RF signal propagates, i.e., the speed of light, to yield a distance.

The RF signal propagation time is measured by clocks of the RF devices involved in transmitting and/or receiving the RF signals. Given the extremely high speed of light, however, any error in these measured times due to non-ideal clock behaviours can result in an unacceptable error in the calculated distance. Furthermore, the precision of geographic location positioning using RF ranging improves when timing errors are reduced.

The various methods of RF ranging include one-way ranging (OWR) and two-way ranging (TWR). In OWR, the RF device to be located transmits but does not receive RF signals. Conversely, in TWR, the RF device to be located may both transmit and receive RF signals as part of the signal exchange protocol for a TDOA calculation.

TWR involves measuring the time taken for an RF signal to travel from a tag device (the RF device to be located) to an anchor device (a fixed RF device used to locate the tag) and back to the tag, known as the round trip time (RTT). This involves the tag device both transmitting and receiving RF signals. Based on the measured RTT, the distance between the tag device and the anchor device can be determined. The tag device and the anchor device are electronic devices with wireless communication capabilities..

The latest TWR methods involve the transmission and reception of further RF signals that reduce errors in the calculated distance arising from the non-ideal behaviours of measuring clocks (sometimes denoted "clock skew" or "clock drift"). However, these methods fundamentally rely on both the transmission and the reception of RF signals by the tag device, and thus cannot be used for OWR localisation.

In contrast with TWR, OWR localisation techniques require only a tag that transmits RF signals; it need not receive them. The tag may have a transmitter but no receiver; alternatively, it may have a transmitter and a receiver that can be configured to be powered down or powered off for certain time periods To perform a single RF transmission, a tag only needs to be awake for the duration of the transmission, which is typically in the order of a few hundred microseconds. In contrast, to perform a single RF reception, a tag usually needs to be awake for a much longer time, typically in the order of a number of milliseconds. Thus, the power consumption of a tag involved in TWR localisation, which must be capable of both transmitting and receiving RF signals, may be significantly higher than that of a tag involved in TDOA localisation, which is only required to transmit RF signals.

However, OWR is potentially more susceptible to error in the calculated position due to certain non-ideal behaviours of the clocks used to measure the timings of the corresponding signals.

Thus, it may be desirable to modify existing OWR methods to reduce the likelihood of error in calculating the TDOA and in calculating geographical location using the TDOA where such error is caused by the non-ideal behaviours of the clocks of wireless communication devices.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments will now be described by way of example only, with reference to the accompanying figures, in which: FIG. 1 schematically illustrates an example system comprising a tag device, two anchor devices and a location server, which can be used to calculate a TDOA via a communication protocol; FIG. 2 schematically illustrates a communication protocol for calculating a TDOA in a system with a tag device, a first (receiver) anchor device, and a second (reference) anchor device, wherein the communication protocol comprises a two-transmission exchange with the transmission of a first wireless signal by the tag device and the transmission of a second wireless signal by the reference anchor device; FIG. 3 schematically illustrates a variant of the communication protocol shown in FIG. 2 comprising two signal transmissions, wherein the order of the first wireless signal and the second wireless signal differs relative to the FIG. 2 example; FIG. 4 schematically illustrates a communication protocol different from that shown in FIG. 2 and FIG. 3, comprising three signal transmissions instead of two, the transmissions comprising two tag device transmissions separated by one anchor device transmission; FIG. 5 schematically illustrates a variant of the three signal communication protocol shown in FIG. 4, wherein the transmission order of the first wireless signal, the second wireless signal, and the third wireless signal differs relative to FIG. 4 in that the first two signals are transmitted by the tag device, whereas the third signal is transmitted by the anchor device; FIG. 6 schematically illustrates another variant of the three transmission communication protocol shown in FIG. 4, in which the reference anchor performs a first transmission and the tag device performs the two subsequent transmissions; FIG. 7 schematically illustrates a three transmission communication protocol, different from those shown in FIG. 4 to FIG. 6, comprising two tag device transmissions and one reference anchor transmission, wherein a first transmission is performed by a tag device, followed by the transmission of two wireless signals by a reference anchor device; FIG. 8 schematically illustrates a variant of the three transmission communication protocol shown in FIG. 7, but the signal order involves a reference anchor transmission followed by a tag device transmission and then a further reference anchor transmission; FIG. 9 schematically illustrates a further variation of the three transmission communication protocol shown in FIG. 7, this example comprising two anchor device transmissions followed by a tag device transmission; and FIG. 10 schematically illustrates a three transmission communication protocol for calculating a TDOA with the same order as the protocol illustrated in FIG. 4, but with three rather than two anchor devices.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an example system 100 in accordance with various example embodiments. In some of the embodiments, the system comprises a tag device 110; a first anchor device, anchor device 120a; and a second anchor device, anchor device 120b. In some embodiments, anchor device 120b may be selected as a reference anchor and anchor device 120a as a receiver anchor. The selection of a reference anchor is arbitrary; in other embodiments, anchor device 120a may be selected as the receiver anchor and anchor device 120b as the reference anchor. The tag device 110,the anchor device 120a and anchor device 120b may be any electronic devices capable of wireless communication. In some of the embodiments, the tag device 110 is a battery-operated mobile device whose geographic location is to be tracked using the TDOA measurement(s). A one-dimensional location may be obtained using two anchor devices, whereas a three-dimensional (3D) location may be obtained using four anchor devices with one of the two signal transmission or three signal transmission TDOA calculation protocols described below. The anchor device 120a and the anchor device 120b may be mobile devices or devices with known fixed locations. If the 3D locations of the anchor device 120a and the anchor device 120b are known in advance because their locations are fixed, this may facilitate the determination of the geographical location of the tag device 110 using RF ranging, if it is mobile.

In some of the embodiments, the system 100 may comprise more than the two anchor devices 120a and 120b. In some embodiments, any one of the constituent anchor devices 120 in the system 100 may be selected as the reference anchor, and some or all of the remaining anchor devices 120 may be selected as receiver anchors. A reference anchor is an anchor device that performs one or more of the transmissions of the signal protocol for the TDOA calculation, whereas a receiver anchor receives signals from the tag device 110 and at least one other anchor device 120 as part of the signal protocol. A system may comprise more than one reference anchor and/or more than one receiver anchor.

As shown in FIG. 1, the tag device 110 comprises an antenna 112 communicatively coupled to a transmission entity 114, which comprises transmission circuitry, a clock 116, a set of processing circuitry 119, and a battery 118. The processing circuitry may be general purpose or special purpose processing circuitry operable to at least control the transmission of one or more RF signals by the tag device 110. The tag device 110 may lack a reception entity; alternatively, the tag device 110 may contain a reception entity that comprises reception circuitry and is thus capable of reception but is configured to not receive wireless signals for the purpose of the TDOA calculation according to the present technique. Thus, the tag device 110 may be configured to transmit but not receive messages.

As also shown in FIG. 1, the first anchor device 120a and the second anchor device120b each comprises an antenna ( antenna 122a and antenna 122b, respectively) communicatively coupled to a transmission and reception entity (reception entity 124a and reception entity 124b, respectively), a clock (clock 126a and clock 126b, respectively) and processing circuitry (processing circuitry 129a and processing circuitry129b, respectively).

The processing circuitry may be special purpose processing circuitry or general purpose processing circuitry operable to implement the TDOA calculation according to the present technique. The transmission and reception entity 124a comprises transmission circuitry and reception circuitry that may be composed of distinct entities or integrated as a single unit. The clock 116 of the tag device 110 and the clocks of the two anchor devices (clock 126a and clock 126b) may run faster or slower than an ideal clock (i.e., may suffer from so-called "clock drift"). For example, the frequency of the clock 126a of the first anchor device 120a may be higher or lower than the ideal frequency, of an ideal clock. This can be modelled as: [Eq. la] -11' where /i" is the frequency of the anchor clock 126a, iced is the frequency of an ideal clock, and k models the deviation from F",,", For example, the clock drift of anchor device 120a can be modelled as: [Eq. lb] F =k" F where F, is a frequency of the anchor device 120a, and models the deviation of Fc, from the ideal frequency Jcile", . Similarly, the clock drift of the anchor device 120b may be modelled as: [Eq. lc] Ph = The positions of the anchor device 120a and the anchor device 120b relative to each other may be fixed. Conversely, the position of the tag device 110 relative to any anchor device 120 may be variable due to the movement of the tag device 110.

The system 100 may optionally include a location server 130 or another data processing device with processing circuitry 139 that is capable of implementing at least part of a TDOA calculation according to the present technique. In some embodiments, the anchor devices 120 may be networked with each other and/or with the location server 130.

FIG. 2 illustrates a communication protocol that can be used to determine a TDOA between the reception of a first wireless signal 212 transmitted by a tag device 210 to a first anchor device, anchor device 220a, and the reception of the first wireless signal 212 transmitted by the tag device 210 to a second anchor device, anchor device 220b. The tag device 210 in FIG. 2 may correspond to the tag device 110 in FIG. 1. Similarly, the anchor device 220a and the anchor device 220b shown in FIG. 2 may correspond respectively to the anchor device 120a and the anchor device 120b in FIG. 1. The FIG. 2 protocol is a two-transmission protocol for deriving two time differences on respective local clocks, from which a TDOA can be determined for a wireless signal transmitted by the tag device 210 for arrival at the first anchor device 220a and the second anchor device 220b. The magnitude of the TDOA may vary depending on the current position of the tag device 210 relative to the anchor device 220a and the anchor device 220b.

In the example shown in FIG. 2, the anchor device 220b is a reference anchor (subsequently referred to as a reference anchor device 220b), whereas the anchor device 220a is a receiver anchor (subsequently referred to as a receiver anchor device 220a). The selection of reference anchor is arbitrary; in other embodiments, any other anchor device 120 may be selected as a reference anchor. The reference anchor device 220b performs both transmission and reception, whereas the receiver anchor device 220a is arranged only to receive, not to transmit, wireless signals during the example TDOA calculation protocol.

As shown in FIG. 2, the tag device 210 may transmit a first wireless signal 212 that may be received by anchor device 220a at a time T1 and received by the reference anchor device 220b at a time T2. The TDOA between the reception of the first wireless signal 212 at the time T1 by anchor device 220a and the reception of the first wireless signal 212 atthe time T2 by the reference anchor device 220b corresponds to a time difference T" )A, as illustrated in FIG. 2. The time T1 is measured by a local clock (not shown) of the receiver anchor device 220a, and time T2 is measured by a local clock (not shown) of the reference anchor device 220b.

The time difference Tm,a1 may vary depending on the position of the tag device 210 in relation to the receiver anchor device 220a and the reference anchor device 220b.

Before or after the transmission of the first wireless signal 212 by the tag device 210, the reference anchor device 220b may transmit a second wireless signal 214 at a time T3. In the example shown in FIG. 2, the reference anchor device 220b transmits the second wireless signal 214 after the transmission of the first wireless signal 212 by the tag device 210. However, in other embodiments, the reference anchor device 220b may transmit the second wireless signal 214 before the transmission of the first wireless signal 212 by the tag device 210. The second wireless signal 214 may be received by the receiver anchor device 220a at a time T4, as measured by a local clock (not shown) of the receiver anchor device 220a.

In the example shown in FIG. 2, the time difference between the reception of the first wireless signal 212 at the time T2 by the reference anchor device 220b and the transmission of the second wireless signal 214 at the time T3 by the reference anchor device 220b corresponds to a time difference D2. The time difference D, may be fixed or variable. D2 may be defined as: [Eq. 2a] D2 = 17-T2 ' The time difference D, may be measured by the clock of anchor device 220b. Alternatively, two timestamps of the clock of the anchor device 220b can be used to determine the time difference via subtraction or otherwise. This applies to any time difference described herein.

The measured time difference D2 may differ from the true time difference D2 if the clock of the reference anchor device 220b runs faster or slower than an ideal clock (i.e., due to clock drift or clock skew).

The time difference between the transmission of the second wireless signal 214 from the reference anchor device 220b and the reception of the second wireless signal 214 by the receiver anchor device 220a (i.e., the TOF from the reference anchor device 220b to the receiver anchor device 220a) corresponds to T,, which is a known constant. For example, T" may be known based on the fixed position of the reference anchor device 220b relative to the receiver anchor device 220a.

The time difference between the reception of the first wireless signal 212 at the time T1 by the receiver anchor device 220a and the reception of the second wireless signal 214 at the time T4 by the receiver anchor device 220a corresponds to time difference D, . The time difference Di may be defined as: [Eq. 2b] D D, may be measured by the clock of the receiver anchor device 220a either as a relative time or using two timestamps. Note also that Di is a vector quantity in the sense that it has a positive value when the time T4 is later than the time Ti; however, in examples such as FIG. 8 and FIG. 9 below, where the signal ordering is changed relative to FIG. 2 and FIG. 4, then the time difference Di as defined in Eq. 2b may have a negative sign because the time Ti is later than the time 14. Returning to the FIG. 2 embodiment, the measured time difference DI may differ from the true time difference D, if the clock of the receiver anchor device 220a runs faster or slower than an ideal clock (i.e., clock drift or clock skew).

In the example shown in FIG. 2, the time difference DI is a function of Ti."," , T2, and D2, wherein: [Eq. 3a] D1 = rTa21 Ti2±D2, which can be rearranged to give: [Eq. 3b] TTDOA =D -D -T2.

Thus 2 12 ' Thus the TDOA may be calculated based on the known TOF (11,) and by measuring two time differences via two respective local clocks. The time differences are associated with two signal transmissions in this example. In some examples, the calculation of the TDOA between the reception of the first wireless signal 212 at the time T1 by the receiver anchor device 220a and the reception of the first wireless signal 212 by the reference anchor device 220b is based, at least in part, on the measured time differences /A and D? and the known time difference 112. For example, by replacing the true values of D, and D2 in Eq. 3b with the corresponding measured values Di and nz, an estimate I IDOA of T",", can be calculated via [Eq. 4] t TDOA -D1-D2 -The calculation of the TDOA between the reception of the first wireless signal 212 by the anchor device 220a and the reception of the first wireless signal 212 by the anchor device 220b, based at least in part on the measured time differences Di and D, and the known TOF may be performed by the receiver anchor device 220b; alternatively, it may be performed on a location server networked to the receiver anchor device 220a and/or anchor device 220b.

If, for example, the TDOA is calculated by the receiver anchor device 220a, the reference anchor device 220b may provide the receiver anchor device 220a with the measured time difference D2 via a communication network to which both anchors are connected.

Alternatively, the measured time difference D2 may be embedded in the second wireless signal 214 transmitted from the reference anchor device 220b and received by the receiver anchor device 220a. As another alternative, the measured time difference a may be transmitted from the reference anchor device 220b to the receiver anchor device 220a in additional wireless signals or follow-up messages.

If the calculation of the TDOA is performed by the anchor device 220b, the receiver anchor device 220a may provide the measured time difference DI to the anchor device 220ab via a communication network to which both anchors are connected. Alternatively, the measured time difference Di may be transmitted from the receiver anchor device 220a to the reference anchor device 220b in additional wireless signals or follow-up messages.

The measured time differences Di and D2 are each determined on a single local clock, rather than being absolute time values. Thus the above embodiment may enable the calculation of the TDOA without requiring the clock of the tag device 210 to be precisely synchronised with the clock of the anchor device 220a orthe clock of the anchor device 220b, or requiring the clock of the anchor device 220a to be precisely synchronised with the clock of the anchor device220b.

In some embodiments, more than two anchor devices 220 may be used and the TDOA of the reception of the first wireless signal 212 may be calculated between each receiver anchor device 220 and the reference anchor device 220.

In some embodiments, the location of the tag device 210 in relation to the anchor devices 220 may be determined based on the calculated TDOA of the first wireless signal 212 between each receiver anchor device 220a and the reference anchor device 220b. (n+1) anchor devices can be used to obtain the n-dimensional location of the tag device 210.

In accordance with the communication protocol illustrated in FIG. 2, various embodiments may enable the discussed TDOA(s) to be calculated without requiring the precise synchronisation of the clock of the tag device 210 with the clock of the anchor device 220a or the clock of the anchor device 220b, or the clock of any anchor device 220 with that of any other anchor device 220. However, the accuracy of the calculated TDOA(s) may be degraded by clock drift (or clock skew). For example, the difference between the measured time difference D (as measured by the clock of the receiver anchor device 220a, which includes clock drift as specified by Eq. 1b) and the true time difference Di is given by: [Eq. 5a] D, = Similarly, the relationship between the measured time difference D2 (as measured by the clock of reference anchor device 220b, which includes clock drift as modelled by Eq. 1c) and the true time difference D2 is given by: [Eq. 5b] D2 = kvD2.

Hence, the calculated estimate of in Eq. 4 relates to the true time differences Di and D2, as follows: [Eq. 6] "'mai = -15 TZ=kDi-i Thus, the error in the calculated estimate of T7DU1 in Eq. 4 due to clock drift is given by: [Eq.7] "12»0.4 -12,,", = D,(lc -1)-D,(ki, -1).

The magnitude of this error determines the accuracy with which geographic location determination can be performed using the calculated TDOA. As the clock drift error in the TDOA calculation diminishes, location accuracy increases. Whilst the above equations used to calculate the TDOA, namely Eq. 3a, Eq. 3b, and Eq. 4, are derived in view of the example wireless signal order shown in FIG. 2, these expressions are also valid for any transmission order of the first wireless signal 212 and the second wireless signal 214 in relation to each other provided that the definitions of times T1 to T4 and the definitions of the time differences D, and D2 in terms of times T1-T4 (as defined in Eq. 2a and Eq. 2b) remain consistent.

For example, FIG. 3 schematically illustrates an alternative communication protocol in accordance with various embodiments of the present disclosure. The example shown in FIG. 3 differs from the example shown in FIG. 2 only in that the second wireless signal 214 is transmitted by the reference anchor device 220b before the transmission of the first wireless signal 212 by the tag device 210. Thus, the above description of the example shown in FIG. 2, excluding the statement that the first signal 212 is transmitted before the second signal 214, is also applicable to the example shown in FIG. 3. For brevity, therefore, this disclosure does not provide a full description of the example shown in FIG. 3. The same reference numerals are used for corresponding features of FIG. 2 and FIG. 3.

As shown in FIG. 3, the tag device 210 may transmit a first wireless signal 212 that may be received by the receiver anchor device 220a at time T1 and by the reference anchor device 220b at time T2. The TDOA between the reception of the first wireless signal 212 at time T1 by anchor device 220a and the reception of the first wireless signal 212 at time T2 by the reference anchor device 220b corresponds to the time difference T",, . As shown in FIG. 3, a second wireless signal 214 may be transmitted by the reference anchor device 220b before the transmission of a first wireless signal 212. As shown in FIG. 3, and consistent with the example shown in FIG. 2, the second wireless signal 214 is transmitted at time T3 by the reference anchor device 22Db and received by the receiver anchor device 220a at the time T4. The first wireless signal 212 is received by anchor device 220a at time T1 and by the reference anchor device 22Db at time T2.

The time difference between the transmission of the second wireless signal 214 from the reference anchor device 22Db and the reception of the second wireless signal 214 by the receiver anchor device 220a (i.e., the TOF from the reference anchor device 220b to anchor device 220a) corresponds to 7j,, which is a known constant. For example, 11,2 may be known based on the fixed position of the reference anchor device 22Db relative to the receiver anchor device 220a.

In the example shown in FIG. 3, the time difference D, may be defined in terms of the time T2 and the time T3 using Eq. 2a.L)2may be a signed quantity, and in the example shown in FIG. 3, it may be negative. The time difference D, may be measured by the clock of anchor device 22Db. Accordingly, the corresponding measured time difference a may differ from the true time difference 1), if the clock of the reference anchor device 22Db runs faster or slower than an ideal clock (i.e., clock drift).

In the example shown in FIG. 3, R may be defined in terms of the time T1 and the time T4 by Eq. 2b. D may be a signed quantity, and in the example shown in FIG. 3, it may be negative. The time difference R may be measured by the clock of the receiver anchor device 220a. Accordingly, the corresponding measured time difference Di may differ from the true time difference R if the clock of the receiver anchor device 220a runs faster or slower than an ideal clock (i.e., clock drift).

In the example shown in FIG. 3, the TDOA between the reception of the first wireless signal 212 at time T1 by the receiver anchor device 220a and the reception of the first wireless signal 212 by the reference anchor device 22Db may be calculated based, at least in part, on the measured time differences ni and D2 and the known time difference T,. For example, an estimate of the TDOA may be calculated according to Eq. 4. This TDOA estimate would differ from the true TDOA value by an amount described by Eq. 7.

FIG. 4 illustrates a different communication protocol that can be used to determine the TDOA between the reception of a first wireless signal 412 transmitted by a tag device 410 to a first anchor device, anchor device 420a and the reception of the first wireless signal 412 transmitted by the tag device 410 to a second anchor device, anchor device 420b, wherein the error due to clock drift may be reduced, in accordance with various embodiments of the present disclosure. The tag device 410 in FIG. 4 may correspond to the tag device 110 in FIG. 1. Similarly, the anchor device 420a and the anchor device 420b in FIG. 4 may correspond to the anchor device 120a and the anchor device120b in FIG. 1, respectively.

In the example shown in FIG. 4, the anchor device 420b is a reference anchor device (subsequently referred to as a reference anchor device 420b), whereas the anchor device 420a is a receiver anchor device (subsequently referred to as a receiver anchor device 420a).

The selection of reference anchor is arbitrary. In other embodiments, any anchor device that is not a reference anchor device may act as a receiver anchor device.

As shown in FIG. 4, the tag device 410 may transmit a first wireless signal 412 that may be received by the receiver anchor device 420a at a time T1 and received by the reference anchor device 420b at a time T2. The TDOA between the reception of the first wireless signal 412 at the time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at the time T2 by the reference anchor device 420b corresponds to the time difference "I,' no,.

The time difference Tmai may vary depending on the position of the tag device 410 in relation to the receiver anchor device 420a and the reference anchor device 420b.

Before or after the transmission of the first wireless signal 412 by the tag device 410, the reference anchor device 420b may transmit a second wireless signal 414 at a time T3. In the example shown in FIG. 4, the reference anchor device 420b transmits the second wireless signal 414 after the transmission of the first wireless signal 412 by the tag device 410. In other embodiments, however, the reference anchor device 420b may transmit the second wireless signal 414 before the transmission of the first wireless signal 412 by the tag device 410. The second wireless signal 414 may be received by the receiver anchor device 420a at a time T4.

In the example shown in FIG. 4, the time between the reception of the first wireless signal 414 at the time T2 by the reference anchor device 420b and the transmission of the second wireless signal 414 at the time T3 by the reference anchor device 420b corresponds to the time difference D,. D, may be fixed or variable. It can be defined as: [Eq. 8a] The time difference D2 may be measured by the clock of the reference anchor device 420b.

Accordingly, the corresponding measured time difference D2 may differ from the true time difference D, if the clock of the reference anchor device 420b runs faster or slower than an ideal clock (i.e., clock drift).

The time difference between the transmission of the second wireless signal 414 from the reference anchor device 420b and the reception of the second wireless signal 414 by the receiver anchor device 420a (i.e., the TOF from the reference anchor device 420b to the receiver anchor device 420a) corresponds to T,, which is a known constant. For example, T" may be known based on the fixed position of the reference anchor device 420b relative to the receiver anchor device 420a.

The time difference between the reception of the first wireless signal 412 at the time T1 by the receiver anchor device 420a and the reception of the second wireless signal 414 at the time T4 by the receiver anchor device 420a corresponds to the time difference Dl. Dl may be defined as: [Eq. 8b] Dl =T -24 The time difference Di may be measured by the clock of the receiver anchor device 420a.

Accordingly, the corresponding measured time difference DI may differ from the true time difference DI if the clock of the receiver anchor device 420a runs faster or slower than an ideal clock (i.e., clock drift).

Before or after the transmission of the second wireless signal 414, the tag device 410 may transmit a third wireless signal 416. In the example shown in FIG. 4, the tag device 410 transmits the third wireless signal 416 after the transmission of the second wireless signal 414, which itself is transmitted after the transmission of the first wireless signal 412. In other embodiments, however, the first wireless signal 412, the second wireless signal 414, and the third wireless signal 416 may be transmitted in any order.

Proceeding with the example shown in FIG. 4, the third wireless signal 416 may be received by the receiver anchor device 420a at a time difference Di after the receiver anchor device 420a receives the second wireless signal 414 at a time T4. Time differenceR may depend on the time between the transmission of the first wireless signal 412 by the tag device 410 and the transmission of the third wireless signal 416 by the tag device 410. The time between the transmission of the wireless signals may correspond to the tag signal period Ping, as shown in FIG. 4.

The time difference D, may be defined as: [Eq. 8c] D; =7; -T, . D; may be measured by the clock of the receiver anchor device 420a. Accordingly, the corresponding measured time difference, D3, may differ from the true time difference 1), if the clock of the receiver anchor device 420a runs faster or slower than an ideal clock (i.e., clock drift).

The TDOA between the reception of the third wireless signal 416 at a time T5 by the receiver anchor device 420a and the reception of the third wireless signal 416 at a time T6 by the reference anchor device 420b corresponds to time difference T'n"" . 1""")3 may vary depending on the position of the tag device 410 in relation to the receiver anchor device 420a and the reference anchor device 420b. In some embodiments, Ph,", may be sufficiently short that any movement of the tag device 410 in relation to the anchor devices 420 within a tag signal period may be negligible, and thus T,D,, and P TD"4 may be substantially similar. In some embodiments, it may even be assumed that T"," is equal to 7-"01.

The time between the transmission of the second wireless signal 414 at the time T3 by the reference anchor device 420b and the reception of the third wireless signal 416 at the time T6 by the reference anchor device 420b corresponds to the time difference D, . D, can be defined as: [Eq.8d] L3/4 = 1a -7; . The time difference D4 may be measured by the clock of the receiver anchor device 420a.

Accordingly, the corresponding measured time difference 1)4 may differ from the true time difference D4 if the clock of reference anchor device 420b runs faster or slower than an ideal clock (i.e., clock drift).

The difference between the measured time differences D1 and D3, as measured by the clock of the receiver anchor device 420a (which includes clock drift as modelled by Eq.

lb), and the true time differences Di and is given by: [Eq. 9a] 1), =k,/), [Eq. 9b] D, =1cD, , where k models the deviation of the frequency of the clock of the receiver anchor device 420a from the frequency of an ideal clock.

Similarly, the difference between the measured time differences D2 and D4, as measured by the clock of the reference anchor device 420b (which includes clock drift as modelled by Eq. 1c), and the true time differences D2 and D, is given by: [Eq. 9c] 1)2 =ki)2 [Eq. 9d] Da = kb') 4, where k models the deviation of the frequency of the clock of the reference anchor device 420b from the frequency of an ideal clock.

In various embodiments, the TDOA between the reception of the first wireless signal 412 at time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at time T2 by the reference anchor device 420b may be calculated based (at least in part) on the measured time differences D1, D2, D2, and D4 and the known time Tu. Doing so may reduce the error in the calculated TDOA arising from clock drift in the clock of the receiver anchor device 420a and/or the clock of the reference anchor device 420b.

In the example shown in FIG. 4, time difference D1 is a function of Tma, , T12, and D, , and time difference D4 is a function of T-"21,r12, and.D wherein: [Eq. 10a] Di =T".. + + D, [Eq. 10b] D4 = m+Ti, + If it is assumed that T =TTDOA Eq. 10a and Eq. 10b can be rearranged to give: [Eq. 11a] D,-D2-T2 [Eq. 11b] Tmai =D, -D-T2.

Eqs 10a and 10b can be readily understood by looking at the signal magnitudes in FIG. 4. These equations are fundamental to the estimation of the TDOA according to the present technique. These two equations hold true not only for the FIG. 4 signal ordering, but also for the signal orderings shown in FIG. 8 and FIG. 9. Note that in FIG. 8 the time differences D1, D2, D3, and D4 are negative in sign. For example, D1 = T4-T1, and by definition T4 is an earlier time than T4 in FIG. 8 and thus takes a negative value. However, the equation -1D31=-1D4I+Ti2+TATooA is rendered equivalent to Eq. 10b by swapping the sides of the negative time differences in the equation, and clearly holds true for FIG. 8, because the magnitude of D4 is greater than the magnitude of D3.

In some embodiments, an estimate of the TDOA between the reception of the first wireless signal 412 at the time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at the time T2 by the reference anchor device 420b can be calculated using Eq. 12a, in which the true values of DI and D2 in Eq. 11a have been replaced by the corresponding measured values DI and D2 [Eq. 12a] 7%01_1 = -1)2-Tu.

In other embodiments, an estimate of the TDOA between the reception of the first wireless signal 412 at the time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at the time T2 by the reference anchor device 420b may be calculated via Eq. 12b, wherein the true values of D4 and D" in Eq. 11b have been replaced by the corresponding measured values D4 and L)3: [Eq. 12b] typa4_2 = D4-D3-I,.

Eq. 12a and Eq. 12b relate to the true values of time differences D4, D2, D, , and D4 as follows: [Eq. 13a] T TD0.4 _1 = D1-D2 = T12 [Eq. 13b] 11TD124_2 = h4 -D3 TI1 Thus, the error due to clock drift in the estimated values of T7001_1 and T71)(11_2 in Eq. 12a and Eq. 12b in relation to the true TDOA, TTD04, is given by: [Eq. 14a] TDOA _1 -;T) 0 = Di (k, -1)-4(k, -1) [Eq. 14b] -1_2 -Tmai= D4 (kb -1) (k, -1).

Note that Eq. 14a and Eq. 14 specify errors in clock drift in which the time differences Di to D4 seem to be inextricably linked with the two clock skew errors ka and kb.

In embodiments in which the order of the first wireless signal 412, the second wireless signal 414, and the third wireless signal 416 is as shown in FIG. 4, the TDOA between the reception of the first wireless signal 412 at time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at time T2 by the reference anchor device 420b may be calculated based, at least in part, on the measured time differences Di, D3, D3, and Di and the known time T2, where the tag signal period Jco, and time difference D, are set to reduce the error in the calculated TDOA arising from clock drift as follows.

It is assumed that 7"""), =))W; then adding Eq. 11a and Eq. 11b and rearranging yields the following: [Eq. 15] TT 1 = -2 (DI-D2+124-D3)-Ti2 Thus, by replacing the true time differences /2, D D3, and L3/4 in Eq. 15 with the corresponding measured time differences Di, D2, D3, and Di, an estimate T TD(24_3 of T"ari can be calculated, in accordance with one embodiment of the present disclosure, via: [Eq. 16] tIT)04 = -12(151 4)2.

Eq. 16 relates to the true values of time differences Di, D, " and L3/4 as follows: [Eq. 17] TIDOA _ 3 = (ka 1 (I) -D3)-kb (T3/4-D))-T2 12 Thus, the error arising from clock drift in the estimated value of THX)14; in Eq. 16 in relation to the true TDOA, TIDOA 3 is given by: [Eq. 18] / 'DCA 1_3 1;o = 2-((k" -1)(11-D3) + (k -1)(D4 -D)) The parameters Ica and kb may be expressed as lc =(1+e,) and kb =(1+e,) respectively, where e" and eh are typically expressed in parts per million (PPM). Substituting these expressions for k a and kh, into Eq. 18, the expression for El into Eq. 10a, and the expression fork', into Eq. 10b, and assuming that that T 1, we have: [Eq. 19] T TWA _3 -T71)(24 =2 (ea + eh)(TT/DA + Ti2) (e" -eh)(D2 -D").

The magnitude of T TDO I I -T7,01 in Eq. 19, and thus the magnitude of the error in the calculated TDOA of Eq. 16, will be minimal or negligible when the time differences D, and D, are equal.

In the example shown in FIG. 4, 1)Z may be variable, and D, is a function of D2, 112 and the tag signal period Nag. Hence, through the judicious choice of time difference D2 in relation to P",,, , D2 and D, may be set as equal or substantially similar. In practice, it may be difficult to make time differences D2 and D; truly equal; however, if they are substantially similar, the error due to clock drift in the calculated TDOA may be significantly reduced using Eq. 16 in comparison with Eq. 4, Eq. 12a, or Eq. 12b.

In various other embodiments, the TDOA between the reception of the first wireless signal 412 at time T1 by the receiver anchor device 420a and the reception of the wireless signal 412 at time T2 by the reference anchor device 420b may be calculated based, at least in part, on the measured time differences D, D2, D3, and D4 and the known time Ti2, wherein at least one of the measured time differences D2 and D4 are scaled based on a determined ratio of the clock drift of the clock of the receiver anchor device 420a to the clock drift of the clock of the reference anchor device 420b (i.e., ), and/or at least one of the measured time differences D1 and is scaled based on the inverse of the above determined ratio (i.e., ). ka

From Eq. 14a, it can be seen that the TDOA estimate calculated using Eq. 13a differs from the true value of the TDOA by an amount dependent on k (the clock drift of the clock of receiver anchor device 420a) multiplied by time difference p in addition to an amount dependent on k, (the clock drift error of the clock of reference anchor device 420b) multiplied by time difference R. If the values of kb and kb could be determined, the measured values of D1 and D2 could be multiplied by the inverse of k" and k, respectively before calculating the TDOA according to Eq. 13a. Doing so would remove the error from the calculated estimate of TDOA using Eq. 13a. However, there is no convenient way to determine the values of k and k.

Whilst it may not be convenient to determine the values of ka and kb, the ratio of ka to k can be determined based on the measured time differences D, 2, 15, , and D4. For example, in various embodiments, the ratio of koto kb may be determined based on the ratio of /), + D2 to 02+ , which gives the following expression: [Eq. 20] _ kaD,+ kin; _ k"(D, +D3) D2 +D4 kbD2+k,,D4 ki,(D2+D4) Substituting the expressions for P and D4 from Eq. 10a and Eq. 10b into Eq. 20, and assuming that r",01 = T"ai, we have: [Eq. 21] i51+753 k (D +M) k (T +T +D +D) k 152 +154 kb(D2+ D4) kh(D, +I:112+ T,+ D3) kb Thus, by taking the ratio of the measured values of DI +D3 to D2 +D4, the ratio of ka to kb can be determined. The fact that this ratio can be measured via the signal protocols described herein, comprising a minimum of three signal transmissions, means that the TDOA can be calculated such that the clock drift error in the calculation is negligible, thus offering sufficient accuracy for location determination. In some embodiments, Eq. 21 is used to reduce the error in an estimate of TTDOA by measuring the time difference ratio and using the measured value to eliminate the dependence of TTDOA on one or the other of the anchor clock skew parameters ka and kb.

In some embodiments, the TDOA between the reception of the first wireless signal 412 at time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at time T2 by the reference anchor device 420b may be calculated based, at least in part, on the measured time differences n1, D2, D3, and D4 and the known time Tv, where the measured time difference D2 is scaled based on the ratio of the clock drift of the clock of the receiver anchor device 420a to the clock drift of the clock of the reference anchor device 420b.

According to the present technique, having recognised that D1+D3 k [Eq. 21'] kb and given that the quantity on the left-hand side of Eq. 21' is empirically measurable, a mathematical "trick" can be used to reduce the error in the estimated value of TDOA. Note that in the absence of clock skew in the two anchor clocks, the above ratio is equal to one; therefore, we can legitimately multiply just one of the terms in Eq. 21a below by this ratio.

-2 Di ± D3 Thus, the term D2 in Eq. 12a may be replaced by the scaled term D2( ) to give the D2 +Da following expression for calculating an estimate of Tma, : [Eq. 22] "M1 O = h2( ± 51) T2.

Dz +D4 Eq. 22 relates to the true values of the time differences as follows: [Eq. 23] TTDOA 4 = kJ), - 7;2 which can be simplified to give: [Eq. 24] TTne.i 4 = ka(1),-1)2)-Ti, . This simplification allows for a reduction in error in the TWA estimate because it eliminates the dependence of the estimate on kb, retaining a dependence on a single clock skew error, ka, rather than two different clock skew errors. Thus, the error arising from clock drift in the estimated value of T TD0_4_4 in Eq. 22 in relation to the true TDOA, , is given by: [Eq. 25] tip° 4 -rin",=(ka-1)(Di-D").

The parameter k" may be expressed as ka=(1+e"), where ea is typically expressed in PPM.

Substituting this expression fork into Eq. 25 gives: [Eq. 26] TD0.4_4 -77m0A = ea(T),-D").

Hence, by scaling the measured value D2 by the determined ratio of the clock drift of the clock of the receiver anchor device 420a to the clock drift of the reference anchor device 420b, an estimate of the TDOA can be calculated wherein the error in the estimate is dependent only on the clock drift of the clock of anchor 420a, not on the clock drift of the clock of anchor 420b. A practical application of this mathematical "trick" not only forms the empirical timing measurements of the left-hand side of Eq. 21', but also allows for a more accurate estimate of TWA than offered by previously known OWR techniques. Eq. 22 was formed using Eq. 12a, and by multiplying the term D2 by the ratio ka/kb (which approximates unity) to eliminate the clock skew error from one of the two anchor clocks. The same "trick" can be repeated in Eq. 12a by instead multiplying the term DI by the ratio kb/ka (i.e., the inverse ratio) to eliminate the clock skew error of the other of the two anchor clocks (giving Eq. 27). This gives a TDOA estimate with less error than a TDOA calculated using Eq. 12a directly. The same "trick" can be used by either multiplying 1)3 by the ratio ka/kb or multiplying 1)4 by the inverse ratio kb/ka to derive TDOA estimates with less error than those calculated directly from Eq. 12b.

Closer inspection of Eq. 26 reveals that the error in the TDOA that was previously dependent via Eq. 14a on the time differences Di and D2, due to the inability to disentangle the dependencies on ka and kb, has effectively been made independent of Di and D2, because, as shown in Eq. 3a, the quantity D, -D2 in Eq. 25 that specifies the error in the estimated TDOA is equal to T,-",+T,", and thus the error in the estimated TDOA calculated via Eq. 22 is proportional only to the values of Tmo,, and ru; it is not dependent on, for example, the absolute value of Di, D", D3, or D4. e" is typically less than 100 PPM, and thus, the error in Eq. 22, which is equal to e"(17,""+T"), is negligible.

In other embodiments, the TDOA between the reception of the first wireless signal 412 at time Ti by the receiver anchor device 420a and the reception of the first wireless signal 412 at time T2 by the reference anchor device 420b may be calculated based, at least in part, on the measured time differences D1, U2, 1) 3, and D4 and the known time 7i2, where the measured time difference L is scaled based on the inverse of the ratio of the clock drift of the clock of the receiver anchor device 420a to the clock drift of the clock of the reference anchor device 420b.

For example, the term D in Eq. 12a may be replaced by the scaled term a( D2 +D4) Di +D3 to give the following expression for calculating an estimate of 1,',", - +D4 [Eq. 27] 1' 11)0,4 _5 = Di( ) 2 = . + D3 Eq. 27 relates to the true values of the time differences as follows: [Eq. 28] 1; TD0.4 5 = ka 1),(-; -which can be simplified to give: [Eq. 29] TH)(1.4_5 = kb (1) D TI2 Similar to Eq. 24, this simplification allows for a reduction in error in the TTD0A estimate because it eliminates the dependence of the estimate on ka, retaining a dependence on a single clock skew error kb rather than two different clock skew errors. Thus, the error arising from clock drift in the estimated value of T TLX),1_5 in Eq. 27 in relation to the true TDOA, Trini, is given by: [Eq. 30] i'mo I s -Tffia, =(kb.

The parameter k may be expressed as k, =(1 +eb), where eb is typically expressed in PPM. Substituting this expression for k into Eq. 30 gives: [Eq. 31] 1; MO -TTD0A = eb (D1- . Hence, by scaling the measured value D1 by the inverse of the determined ratio of the clock drift of the clock of the receiver anchor device 420a to the clock drift of the reference anchor device 420b, an estimate of the TDOA can be calculated whose error is dependent only on the clock drift of the clock of anchor 420b, not on the clock drift of the clock of anchor S 420a.

Moreover, as can be seen from Eq. 3a, LI -D, is equal to Tmal +7;2 and thus the error in the estimated TDOA calculated via Eq. 27 is proportional only to the values of T2,-21 and 7;2; it is not dependent on, for example, the absolute values of 17, D, , D3, or D4. It is noted that eb is typically less than 100 PPM, and thus, the error in Eq. 22, which is equal to e b(Tmo.4+172), is negligible Eq. 22 may be rearranged to give: A D4 -2 D3 [Eq. 32] T 2D0 _4 = 712 1) 2 D 4 which may alternatively be derived by replacing the D4 term in Eq. 12b with the scaled term +173 D4( -), equivalent to multiplying D4 by one due to the equality of the error free D2 + D4 quantities D1+D3=D2+11, as is clear from the signal diagram in FIG. 4.

Eq. 27 may be rearranged to give: [Eq. 33] 2D0.4 _5 = T1)1 D4 -D2 D3 Tie + which may alternatively be derived by replacing the /33 term in Eq. 12b with the scaled term D2+1)4) . As before, the scaled term is equivalent to multiplying by one. DI + D3 In some embodiments, the TDOA between the reception of the first wireless signal 412 at time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at time T2 by the reference anchor device 420b may be calculated based, at least in part, on the measured time differences D1, D7, D3, and D4 and the known time 1;2, wherein Eq.

22, or equivalently Eq. 32, is used to calculate the TDOA. The resulting error in the calculated TDOA will thus be proportional only to the drift of the clock of anchor 420a, not to the drift of the clock of anchor 420b. Hence, if the clock of receiver anchor device 420a is known or suspected to be more accurate than the clock of reference anchor device 420b, the estimated TDOA 1',"), calculated via Eq. 22 or Eq. 32 may be closer to the true value of the TDOA 1;,"), than the estimated TDOA Twai calculated via Eq. 27 or Eq. 33.

In other embodiments, the TDOA between the reception of the first wireless signal 412 at time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at time T2 by the reference anchor device 420b may be calculated based (at least in part) on the measured time differences Di, D2, D3, and D4 and the known time 7i,, wherein Eq.

27, or equivalently Eq. 33, is used to calculate the TDOA. The resulting error in the calculated TDOA will thus be proportional only to the drift of the clock of anchor 420b, not to the drift of the clock of anchor 420a. Hence, if the clock of reference anchor device 420b is known or suspected to be more accurate than the clock of the receiver anchor device 420a, the estimated TDOA TH", calculated via Eq. 27 or Eq. 33 may be closer to the true value of the TDOA T, than the estimated TDOA T"", calculated via Eq. 22 or Eq. 32.

It can be seen from the error equations Eq. 14a and Eq. 14b that the error in the corresponding TDOA equations, Eq. 12a and Eq. 12b, is due to the clock drift of both the clock of the receiver anchor device 420a (ka) and the clock drift of the reference anchor device 420b (kb). An error reduction is achieved using the present technique because the equations for determining the TDOA, including Eq. 22, Eq. 27, Eq. 32, and Eq. 33, only include error due to clock drift in either the clock of the receiver anchor device 420a orthe clock of the reference anchor device 420b, not both. This is achieved by determining the ratio of the clock drift of the receiver anchor device 420a to the clock drift of the reference anchor device 420b based on Eq. 21 and Eq. 21', and using this empirically determined ratio (or the inverse of the ratio) to cancel the error due to the clock drift (kb) of the clock of the reference anchor 420b from Eq.

22 and Eq. 32, as shown in Eq. 23, and to cancel the error due to the clock drift (ka) of the clock the receiver anchor 420a from Eq. 27 and Eq. 33, as shown in Eq. 28.

If this cancellation is not performed, the clock drift error of the clock of the receiver anchor device 420a and the clock drift error of the clock of the reference anchor device 420b may compound each other such that the total error of the TDOA is dependent on the absolute time durations of Di, D,, D, , and R, as shown in Eq. 14a and Eq. 14b. Cancelling the clock drift error as described above removes this dependency from Eq. 22, Eq. 27, Eq. 32, and Eq. 33.

In other embodiments, the TDOA between the reception of the first wireless signal 412 at time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at time T2 by the reference anchor device 420b may be calculated based, at least in part, on the measured time differences Di, D2, D3, and 1)4 and the known time Tu. The first estimate of a TDOA is calculated based on one of Eq. 32 (or equivalently Eq. 22) and Eq. 33 (or equivalently Eq. 27). The second estimate of a TDOA is calculated based on the other of Eq. 32 (or equivalently Eq. 22) and Eq. 33 (or equivalently Eq. 27). Finally, the TDOA is calculated by taking an average of the first TDOA estimate and the second TDOA estimate. This may yield a calculated TDOA that is at least as accurate as the least accurate of the first and second estimates. For example, doing so would yield an error in the calculated TDOA that is equal to the average of the errors in the first and second estimates: ctb (D D)-e"±eb (TT"7.4+ TO, [Eq. 34] TTTX).4 vg = , 2 2 e 2 where: T7720.1_4 T77)(M5 [Eq. 35] 7 TTD(44sIvg -In the above example, the arithmetic mean is treated as the average of the first and second TDOA estimates. In other embodiments, the average may be any other type of mean value of the first and second TDOA estimates, such as the geometric mean or the harmonic mean.

In some embodiments, the TDOA between the reception of the first wireless signal 412 at the time T1 by the receiver anchor device 420a and the reception of the first wireless signal 412 at the time T2 by the reference anchor device 420b may be calculated based on an analytical expression derived by taking the average of Eq. 32 (or equivalently Eq. 22) and Eq. 33 (or equivalently Eq. 27).

In another example, an estimate of the TDOA may be calculated based on the analytical expression below, which is derived by taking the inverse average, or harmonic mean, of Eq. 32 (or equivalently Eq. 22) and Eq. 33 (or equivalently Eq. 27): [Eq. 36] i'mo.4 Di D2+ + 4 In various embodiments, the calculation of the TDOA between the reception of the first wireless signal 412 by the receiver anchor device 420a and the reception of the first wireless -1)", signal 412 by the reference anchor device 420b, which is based (at least in part) on the measured time differences DI, D2, D3, and D4 and the known time 72, may be performed by the processing circuitry of the receiver anchor device 420a, the processing circuitry of the reference anchor device 420b, or the processing circuitry of a location server communicatively coupled to both the receiver anchor device 420a and the reference anchor device 420b.

If, for example, the calculation of the TDOA is performed by the receiver anchor device 420a, the reference anchor device 420b may provide receiver anchor device 420a with the measured time differences D2 and D4 via a communication network to which both anchors are connected. Alternatively, the measured time differences D) and D4 may be transmitted from the reference anchor device 420b to the receiver anchor device 420a in additional wireless signals or follow-up messages.

If, for example, the calculation of the TDOA is performed by the reference anchor device 420b, the receiver anchor device 420a may provide reference anchor device 420b with the measured time differences Di and D3 via a communication network to which both anchors are connected. Alternatively, the measured time differences L) and 1i3 may be transmitted from the receiver anchor device 420a to the reference anchor device 420b in additional wireless signals or follow-up messages.

Whilst the equations shown in Eq. 12a, Eq. 12b, Eq. 16, Eq. 22, Eq. 27, Eq. 32, Eq. 33, and Eq. 36 for calculating the above-described TDOA are derived in view of the example wireless signal order shown in FIG. 4, these expressions, unless otherwise stated, are also valid for any order of the first wireless signal 412, second wireless signal 414, and third wireless signal 416 provided that the definitions of the times T1 to T4 and the definitions of the time differences Di 1)3 and D, in terms of times T1-T4 (as defined in Eq. 8a-d) remain consistent.

For example, FIG. 5 illustrates an example embodiment wherein the third wireless signal 416 is transmitted by the tag device 410 after the transmission of the first wireless 412 by tag device 420b but before the transmission of the second wireless signal 414. In the example shown in FIG. 5, the time differences and 1), may be negative.

As another example, FIG. 6 illustrates an embodiment wherein the second wireless signal 414 is transmitted before the transmission of the first wireless signal 412 by the tag device 410 and wherein the first wireless signal412 is transmitted before the third wireless signal 416. In the example shown in FIG. 6, the time differences R and R, may be negative.

The examples shown in FIG. 5 and FIG. 6 differ from the example shown in FIG. 4 only in terms of the order of the first wireless signal 412, second wireless signal 414, and third wireless signal 416. Thus, the above description in relation to the example shown in FIG. 4, except where otherwise stated, is also applicable to the examples shown in FIG. 5 and FIG. 6. Thus, for brevity, this disclosure does not provide a full description of the examples shown in FIG. 5 and FIG. 6. The same reference numerals are used for corresponding features of FIG. 4, FIG. 5, and FIG. 6.

In various embodiments of the present disclosure, the TDOA between the reception of a wireless signal transmitted by a tag device to a first anchor device and the reception of the wireless signal transmitted by the tag device by a second anchor device may be calculated based on a communication protocol comprising one wireless signal transmitted by a tag device and two wireless signals transmitted by anchor devices.

For example, FIG. 7 illustrates a variant of the example communication protocol shown in FIG. 4, wherein a third wireless signal 716 is transmitted by a second anchor device, anchor device 720b.The communication protocol shown in FIG. 7 may be used to determine the TDOA between the reception of a first wireless signal 712 (transmitted by a tag device 710) by a first anchor device, anchor device 720a, and the reception of the first wireless signal 712 (transmitted bythe tag device 710) by a second anchor device, anchor device 720b, wherein the error due to clock drift may be reduced, in accordance with various embodiments of the present disclosure. The tag device 710 in FIG. 7 may correspond to the tag device 110 in FIG. 1. Similarly, the anchor device 720a and the anchor device 720b in FIG. 7 may correspond to the anchor device 120a and the anchor device 120b shown in FIG. 1, respectively.

In the example shown in FIG. 7, the anchor device 720b is a reference anchor (subsequently referred to as a reference anchor device 720b), whereas the anchor device 720a is a receiver anchor (subsequently referred to as a receiver anchor device 720a). The selection of reference anchor is arbitrary; in other embodiments, any other anchor device 720 may be selected as a reference anchor. In other embodiments, any anchor device 720 that is not a reference anchor may act as a receiver anchor.

As shown in FIG. 7, the tag device 710 may transmit a first wireless signal 712 that may be received by the receiver anchor device 720a at a time T1 and received by the reference anchor device 720b at a time T2. The TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at the time T2 by the reference anchor device 720b corresponds to the time difference Ta4.

The time difference T",04 may vary depending on the position of tag device 710 in relation to the receiver anchor device 720a and the reference anchor device 720b.

At a time T3, before or after the transmission of the first wireless signal 712 by the tag device 710, the reference anchor device 720b may transmit a second wireless signal 714. In the example shown in FIG. 7, the anchor device 720b transmits the second wireless signal 714 after the transmission of the first wireless signal 712 by the tag device 710. In other embodiments, however, the reference anchor device 720b may transmit the second wireless signal 714 before the transmission of the first wireless signal 712 by the tag device 710. The second wireless signal 714 may be received by the receiver anchor device 720a at a time T4.

In the example shown in FIG. 7, the time between the reception of the first wireless signal 712 at time T2 by the reference anchor device 720b and the transmission of the second wireless signal 714 at the time T3 by the reference anchor device 720b corresponds to the time difference 02. 0, may be fixed or variable, and can be defined according to Eq. 8a.

The time difference D, may be measured by the clock of reference anchor device 720b. Accordingly, the corresponding measured time difference 02 may differ from the true time difference D if the clock of reference anchor device 720b runs faster or slower than an ideal clock (i.e., clock drift).

The time difference between the transmission of the second wireless signal 714 from reference anchor device 720b and the reception of the second wireless signal 714 by the receiver anchor device 720a (i.e., the TOF from the reference anchor device 720b to the receiver anchor device 720a) corresponds to T,, which is a known constant. For example, T" may be known based on the fixed position of the reference anchor device 720b relative to the receiver anchor device 720a.

The time difference between the reception of the first wireless signal 712 at time T1 by the receiver anchor device 720a and the reception of the second wireless signal 714 at the time T4 by the receiver anchor device 720a corresponds to the time difference /), . D, may be defined according to Eq. 8b, as Di =T4-T1.

The time difference D, may be measured by the clock of the receiver anchor device 720a. Accordingly, the corresponding measured time difference, Di, may differ from the true time difference, 12, if the clock of the receiver anchor device 720a runs faster or slower than an ideal clock (i.e., clock drift).

Before or after the transmission of the second wireless signal 714 by the reference anchor device 720b, the reference anchor device 720b may transmit a third wireless signal 716. In the example shown in FIG. 7, the reference anchor device 720b transmits the third wireless signal 716 after the transmission of the second wireless signal 714, which itself is transmitted after the transmission of the first wireless signal 712. In other embodiments, however, the first wireless signal 712, the second wireless signal 714, and the third wireless signal 716 may be transmitted in any order.

Proceeding with the example shown in FIG. 7, a time difference D may be defined as: [Eq. 37] D3 =1; -T.

Note that as illustrated in FIG. 7, the timestamp Ti is at an earlier time than the timestamp Te.

Thus the value of D3 in Eq. 37 may be a signed number with a magnitude and negative sign. A similar logic applies to D4 in the FIG. 7 embodiment, because 12 is now an earlier time than Ts. The negative signs of D3 and at in the FIG. 7 embodiment mean that Eq. 10b still holds true even though Ds is greater in magnitude than at in the FIG. 7 embodiment but as is greater in magnitude than D3 in the FIG. 4 embodiment. Therefore, one way of accommodating the alternative signal orderings of FIG. 7 relative to FIG. 4 is to treat the distances D3 and D4 as signed numbers in the equations. Alternatively, only the magnitudes of time differences may be taken into account, yielding a negative TDOA estimate for some embodiments. In this case, the magnitude of the calculated TDOA value may give the TDOA estimate. A similar logic applies to the time differences D1, D2, D3, and D4 schematically illustrated in the FIG. 8 and FIG. 9 embodiments, which each involve subtracting an earlier time from a later time, giving rise to negative time differences. The time difference D3 may depend on the time between the transmission of the first wireless signal 712 by the tag device 710 and the transmission of the third wireless signal 716 by the reference anchor device 720b, which in turn may depend on the time difference D,, the time between the transmission of the second wireless signal 714 by the reference anchor device 720b, and the time between the transmission of the third wireless signal 716 by the reference anchor device 720b. The time between the transmission of the second wireless signal and the third wireless signal by the reference anchor device 720b may correspond to an anchor signal period, Pato, , as shown in FIG. 7.

The time difference D3 may be measured by the clock of the receiver anchor device 720a. Accordingly, the corresponding measured time difference D3 may differ from the true time difference D, if the clock of the receiver anchor device 720a runs faster or slower than an ideal clock (i.e., clock drift).

The time difference between the transmission of the third wireless signal 716 from the reference anchor device 720b and the reception of the third wireless signal 716 by the receiver anchor device 720a (i.e., the TOF from reference anchor device 720b to receiver anchor device 720a) corresponds to 72, which is a known constant. 7; 2 may be known based on the fixed position of the reference anchor device 720b relative to the receiver anchor device 720a, for example.

A time difference D4 may be defined as: [Eq. 38] =T5 D4 may be measured by the clock of the receiver anchor device 720a. Accordingly, the corresponding measured time difference D4 may differ from the true time difference D4 if the clock of the receiver anchor device 720a runs faster or slower than an ideal clock (i.e., clock drift). Note that whilst the definitions of D1 and D2 in terms of timestamps are consistent for each of the embodiments in FIG. 2 to FIG. 9, the definitions of D3 and D4 differ in the FIG. 7, 8 and 9 embodiments relative to the FIG. 4, 5 and 6 embodiments. The difference is due to the fact that the FIGS 4, 5, and 6 embodiments involve two tag blinks, whereas the FIGS 7, 8, and 9 embodiments each involve two reference anchor blinks and a single tag blink.

The difference between the measured time differences DI and D3 (as measured by the clock of the reference anchor device 720a, which includes clock drift as modelled by Eq.

lb) and the true time differences DI and Di, is given by: [Eq. 39a] Di = k Di [Eq. 39b] D3 = kuDz, where k" models the deviation of the frequency of the clock of the receiver anchor device 720a from the frequency of an ideal clock.

Similarly, the difference between the measured time differences D2 and D4 (as measured by the clock of the reference anchor device 720b, which includes clock drift as modelled by Eq. 1c) and the true time differences D2 and D4 is given by: [Eq. 40a] h2 [Eq. 40b] D4 =Ici,D, where k models the deviation of the frequency of the clock of the reference anchor device 720b from the frequency of an ideal clock.

In various embodiments, the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at the time T2 by the reference anchor device 720b may be calculated based, at least in part, on the measured time differences D1, D2, D3, and D4 and the known time T", Doing so may reduce the error in the calculated TDOA arising from clock drift in the clock of the receiver anchor device 720a and/or the clock of the reference anchor device 720b.

In the example shown in FIG. 7, the time difference D, is a function of TTD4, T,, and, and the time difference D4 is a function of 7:"."24,712, and D, wherein: [Eq. 41a] Dl = Tm,0_,1+ /in +D2 [Eq. 41b] D4= TTD04 Ty, ± 1), , which can be rearranged to give the two expressions for Tm"4 shown in Eq. 11a and Eq. 11b.

In some embodiments, an estimate of the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at the time T2 by the reference anchor device 720b may be calculated via Eq. 12a, wherein the true values of DI and D2 in Eq. 11a have been replaced by the corresponding measured values a and D2.

In other embodiments, an estimate of the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at the time T2 by the reference anchor device 720b may be calculated via Eq. 12b, wherein the true values of D, and ZJ in Eq. 11b have been replaced by the corresponding measured values D4 and, provided that the definition of D3 from Eq. 37 and the definition of D4 from Eq. 38 are used.

For the example shown in FIG. 7, Eq. 12a and Eq. 12b relate to the true values of the time differences D4, D2, D", and D4, as follows: [Eq. 42a] Imacs = n1-1)2 -1;, = khDl- [Eq. 42b] 7%01_9 = 154 -1;,= kaD4-k,D,-1;, . Thus, the error arising from clock drift in the estimated values of TTDOA_8 and T TWA _9 in Eq. 42a and Eq. 42b in relation to the true TDOA, Triym, is given by: [Eq. 43a] i; TDOA _8 - = Di(k, -1)-D,(k a -1) [Eq. 43b] i'22)0.4_9 -1 Do.4 = " 4(k -1)-"3(kb -1) In various other embodiments, the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at the time T2 by the reference anchor device 720b may be calculated based, at least in part, on the measured time differences Di, D, , and D4 and the known time zip, wherein at least one of the measured time differences TA and al is scaled based on a determined ratio of the clock drift of the clock of the receiver anchor device 720a k to the clock drift of the clock of the reference anchor device 720b (i.e., = ), and/or at least one of the measured time differences D2 and D4 is scaled based on the inverse of the above k determined ratio (i.e., ).

For example, in some embodiments, the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at time T2 by the reference anchor device 720b may be calculated based, at least in part on the measured time differences DI, D2, D3, and D4 and the known time 1;, , wherein Eq. 22, or equivalently Eq. 32, is used to calculate the TDOA. The resulting error in the calculated TDOA is thus proportional only to the drift of the clock of the receiver anchor device 720a, not to the drift of the clock of the reference anchor device 720b. Hence, if the clock of the receiver anchor device 720a is known or suspected to be more accurate than the clock of the reference anchor device 720b, the estimated TDOA Tm", calculated via Eq. 22 or Eq. 32 may be closer to the true value of the TDOA T"", than the estimated TDOA 1 calculated via Eq. 27 or Eq. 33.

In other embodiments, the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at the time T2 by the reference anchor device 720b may be calculated based, at least in part, on the measured time differences a, n2, D 3, and D4 and the known time Tr, , wherein Eq. 27, or equivalently Eq. 33, is used to calculate the TDOA, provided that the definition of 0, from Eq. 37 and the definition of 14)4 from Eq. 38 are used. The resulting error in the calculated TDOA is thus proportional only to the drift of the clock of the reference anchor device 720b, not to the drift of the clock of the receiver anchor device 720a. Hence, if the clock of the reference anchor device 720b is known or suspected to be more accurate than the clock of receiver anchor device 720a, the estimated TDOA TDOA calculated via Eq. 27 or Eq. 33 may be closer to the true value of the TDOA Two, than the estimated TDOA TT x7a calculated via Eq. 22 or Eq. 32.

In other embodiments, the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at the time T2 by the reference anchor device 720b may be calculated based, at least in part, on the measured time differences D, /J2, D3, and D4 and the known time T", wherein: a first estimate of a TDOA is calculated based on one of Eq. 32 (or equivalently Eq. 22) and Eq. 33 (or equivalently Eq. 27); a second estimate of a TDOA is calculated based on the other of Eq. 32 (or equivalently Eq. 22) and Eq. 33 (or equivalently Eq. 27), provided that the definition of Di from Eq. 37 and the definition of D4 from Eq. 38 are used; and the TDOA is calculated by taking an average of the first TDOA estimate and the second TDOA estimate.

This may yield a calculated TDOA that is at least as accurate as the least accurate of the first and second estimates. For example, this method yields an error in the calculated TDOA that is equal to the average of the errors in the first and second estimates, which is given by the expression shown in Eq. 34. In some examples, taking the average of the first and second TDOA estimates may involve taking the arithmetic mean, the geometric mean, or the inverse mean of the first and second TDOA estimates. In some examples, an analytical expression may be derived based on taking an average of the first and second TDOA estimates, and this analytical expression may be used to calculate the TDOA. For example, the TDOA may be calculated via Eq. 36, provided that the definition of D, from Eq. 37 and the definition of /-24 from Eq. 38 are used.

In various embodiments, the calculation of the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at the time T2 by the reference anchor device 720b, based, at least in part, on the measured time differences Di, 1)2, 1)3, and a and the known time Ti2 may be performed by the receiver anchor device 720a, by the reference anchor device 720b, or on a location server communicatively coupled to both the receiver anchor device 720a and the reference anchor device 720b.

If, for example, the calculation of the TDOA is performed by the receiver anchor device 720a, the reference anchor device 720b may provide the receiver anchor device 720a with the measured time differences L) and D4 via a communication network to which both anchors are connected. Alternatively, the measured time differences a and D4 may be transmitted from the reference anchor device 720b to the receiver anchor device 720a in additional wireless signals or follow-up messages.

If, for example, the calculation of the TDOA is performed by the reference anchor device 720b, the receiver anchor device 720a may provide the reference anchor device 720b with the measured time differences DI and D3 via a communication network to which both anchors are connected. Alternatively, the measured time differences DI and D3 may be transmitted from the receiver anchor device 720a to the reference anchor device 720b in additional wireless signals or follow-up messages.

Whilst the equations shown or discussed in relation to calculating the above-described TDOA are derived in view of the example signal order shown in FIG. 7, these expressions, unless otherwise stated, are also valid for any order of the first wireless signal 712, second wireless signal 714, and third wireless signal 716, provided that the definitions of times T1 to T4 and the definitions of the time differences R and D, , as in Eq. 8a-b, and D3 and R, as in Eq. 37 and Eq. 38, remain consistent.

For example, FIG. 8 illustrates an example embodiment wherein the first wireless signal 712 is transmitted by the tag device 710 after the transmission of the second wireless signal 714 by the reference anchor device 720b and the third wireless signal 716 is transmitted after the first wireless signal 712.

In embodiments in which the order of the first wireless signal 712, the second wireless signal 714, and the third wireless signal 716 is as shown in FIG. 8, the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at time T2 by the reference anchor device 720b can be calculated based, at least in part, on the measured time differences DI, D2, Dl, and A and the known time 1;7, wherein the anchor signal period Yid, and time difference D2 are set to reduce the error in the calculated TDOA due to clock drift as follows.

For example, in some embodiments, the TDOA between the reception of the first wireless signal 712 at the time T1 by the receiver anchor device 720a and the reception of the first wireless signal 712 at time T2 by the reference anchor device 720b may be calculated based, at least in part on the measured time differences Di, D2, D, and D4 and the known time Tu, where Eq. 16 is used to calculate the TDOA, provided that the definition of 1), from Eq. 37 and the definition of D4 from Eq. 38 are used, and that 1',,d, and time difference /J2 are set such that D2 and IA may be equal or substantially similar. This may reduce the error in the calculated TDOA due to clock drift for receiver anchor device 720a and reference anchor device 720b in accordance with the corresponding error equation shown in Eq. 19.

FIG. 9 illustrates another example embodiment wherein the first wireless signal 712 is transmitted after the transmission of the third wireless signal 716, which itself is transmitted after the transmission of the first wireless signal 712.

The examples shown in FIG. 8 and FIG. 9 differ from the example shown in FIG. 7 only in the order of the first wireless signal 712, second wireless signal 714, and third wireless signal 716. Thus, the above description of the example shown in FIG. 7, except where stated otherwise, is also applicable to the examples shown in FIG. 8 and FIG. 9. Thus, for brevity, this disclosure does not provide a full description of the examples shown in FIG. 8 and FIG. 9.

The same reference numerals are used for corresponding features of FIG. 7, FIG. 8, and FIG. 9.

In some embodiments, more than two anchors may be used and the TDOA of the reception of a wireless signal transmitted by a tag device may be calculated between all pairs of receiver and reference anchors. In such embodiments, any of the previously described methods of calculating a TDOA may be used to calculate any one of the TDOAs between any constituent pair of receiver anchor device and reference anchor device.

FIG. 10 illustrates an example embodiment wherein, in addition to a first anchor device, anchor device 1020a, and a second anchor device, anchor device 1020b, a third anchor device, anchor device 1020c, may be used. A tag device 1010 in FIG. 10 may correspond to the tag device 110 shown in FIG. 1. Similarly, the anchor device 1020a, the anchor device 1020b, and the anchor device 1020c in FIG. 10 may correspond to the anchor devices 120 in FIG. 1. In the example shown in FIG. 10, the anchor device 1020c is a reference anchor (subsequently referred to as a reference anchor device 1020c), whereas the anchor device 1020a and the anchor device 1020b are receiver anchors (subsequently referred to as a receiver anchor device 1020a and a receiver anchor device 1020b, respectively). The selection of reference anchor is arbitrary; in other embodiments, any other anchor may be selected as a reference anchor. In other embodiments, any anchor that is not a reference anchor may act as a receiver anchor.

The example shown in FIG. 10 illustrates an embodiment comprising three anchor devices and a communication protocol comprising two wireless signals transmitted by the tag device 1010 and one wireless signal transmitted by the reference anchor device 1020c. In other embodiments comprising two or more anchors, a communication protocol may comprise two wireless signals transmitted by a reference anchor device and one wireless signal transmitted by a tag device.

As shown in FIG. 10, the tag device 1010 may transmit a first wireless signal 1012. The TDOA between the reception of the first wireless signal 1012 by the receiver anchor device 1020a at a time Tta and the reception of the first wireless signal 1012 by the reference anchor device 1020c at a time T2 corresponds to a first time difference The TDOA between the reception of the first wireless signal 1012 by the receiver anchor device 1020b at a time Tl_b and the reception of the first wireless signal 1012 by the reference anchor device 1020c at the time T2 corresponds to a second time difference 7:"",4," . The first and second time differences /;,,,u, and 1;7"), " may vary depending on the position of the tag device 1010 in relation to the receiver anchor device 1020a, the receiver anchor device 1020b, and the reference anchor device 1020c.

Before or after the transmission of the first wireless signal 1012 by the tag device 1010, the reference anchor device 1020c may transmit a second wireless signal 1014 at time T3. In the example shown in FIG. 10, the reference anchor device 1020c transmits the second wireless signal 1014 after the transmission of the first wireless signal 1012 by the tag device 1010. However, in other embodiments, the reference anchor device 1020c may transmit the second wireless signal 1014 before the transmission of the first wireless signal 1012 by the tag device 1010. The second wireless signal 1014 may be received by the receiver anchor device 1020a at a time T4_a and by the receiver anchor device 1020b and a time T4_b.

In the example shown in FIG. 10, the time between the reception of the first wireless signal 1014 at time T2 by the reference anchor device 1020c and the transmission of the second wireless signal 1014 at the time T3 by the reference anchor device 1020c corresponds to a time difference R. The time difference 4 may be fixed or variable. 4 may be defined according to Eq. 8a.

The time difference D may be measured by the clock of the reference anchor device 1020c. Accordingly, the corresponding measured time difference a may differ from the true time difference /1 if the clock of the reference anchor device 1020c runs faster or slower than an ideal clock (i.e., clock drift).

The time difference between the transmission of the second wireless signal 1014 from the reference anchor device 1020c at the time T3 and the reception of the second wireless signal 1014 by the first receiver anchor device 1020a at time T4_a (i.e., the TOF from the reference anchor device 1020c to the receiver anchor device 1020a) corresponds to 7;3, which is a known constant. For example, 71, may be known based on the fixed position of the reference anchor device 1020c relative to the receiver anchor device 1020a.

The time difference between the transmission of the second wireless signal 1014 from the reference anchor device 1020c at the time T3 and the reception of the second wireless signal 1014 by the receiver anchor device 1020b at the time T4_b (i.e., the TOF from the reference anchor device 1020c to the receiver anchor device 1020b) corresponds to T23, which is a known constant. For example, T" may be known based on the fixed position of the reference anchor device 1020c relative to the receiver anchor device 1020b.

The time difference between the reception of the first wireless signal 1012 at time T1_a by the receiver anchor device 1020a and the reception of the second wireless signal 1014 at the time T4_a by the receiver anchor device 1020a corresponds to a time difference D1 a: The time difference D may be defined as: [Eq. 40a] DI a = 1 -7), . Similarly, the time difference D1 b may be defined as: [Eq. 40b] "11, h 71 b The time differences D and D can be measured by the clock of the first receiver anchor device 1020a and the clock of the second receiver anchor device 1020b, respectively.

Accordingly, the corresponding measured time differences Di a and Di b may differ from the true time differences Di and D1 h if the clocks of anchor devices 1020a and 1020b run faster or slower than an ideal clock (i.e., clock drift).

Before or after the transmission of the second wireless signal 1014, the tag device 1010 may transmit a third wireless signal 1016. In the example shown in FIG. 10, the tag device 1010 transmits the third wireless signal 1014 after the transmission of the second wireless signal 1014, which itself is transmitted after the transmission of the first wireless signal 1012. In other embodiments, however, the first wireless signal 1012, the second wireless signal 1014, and the third wireless signal 1016 may be transmitted in any order.

Proceeding with the example shown in FIG. 10, the third wireless signal 1016 may be received by the receiver anchor device 1020a at a time difference D, after the receiver

G

anchor device 1020a receives the second wireless signal 1014 at the time T4_a.

The third wireless signal 1016 may be received by the receiver anchor device 1020b at a time difference D3 b after the receiver anchor device 1020b receives the second wireless signal 1014 at the time T4_b.

The time differences D3 a and D3 a may depend on the time between the transmission of the first wireless signal 1012 by tag device 1010 and the transmission of the third wireless signal 1016 by the tag device 1010. The time between the transmission of the wireless signals may correspond to the tag signal period pra as shown in FIG. 10.

The time differences D3 and D3 may be defined as:b [Eq. 41a] Di a = 7 -7 4 a [Eq. 41b] b = T5 b -T4 b The time differences D., a and a, may be measured by the clock of anchor device 1020a and the clock of anchor device 1020b, respectively.

The TDOA between the reception of the third wireless signal 1016 at a time T5_a by the receiver anchor device 1020a and the reception of third wireless signal 1016 at atime T6 by the reference anchor device 1020c corresponds to the time difference 1' ',Doc; . The TDOA between the reception of the third wireless signal 1016 at a time T5_b by the receiver anchor device 1020b and the reception of third wireless signal 1016 at the time T6 by the reference anchor device 1020c corresponds to the time difference 7 TDOA)3 * 7117-DOA _13 and 7"rvua23 may vary depending on the position of the tag device 1010 in relation to the receiver anchor device 1020a, the receiver anchor device 1020b, and the reference anchor device 1020c. In some embodiments, Piag may be sufficiently short that any movement of the tag device 1010 in relation to the anchor devices 1020 within a tag signal period may be negligible; thus Tiabi 13 and T '7D0_4 13 may be substantially similar and TID0,4 23 and T marl)3 may be substantially similar. In some embodiments, it may even be assumed that 7DC24 13 is equal to T '71,"A 13 and 7711)(21 23 is equal to T 'TD"A 23.

The time between the transmission of the second wireless signal 1016 at the time T3 by the reference anchor device 1020c and the reception of the third wireless signal 1016 at time T6 by the reference anchor device 1020c corresponds to the time difference R. R may be defined according to Eq. 8d and may be measured by the clock of the receiver anchor device 1020a.

In various embodiments, the TDOA between the reception of the first wireless signal 1012 by anchor device 1020a and the reception of the first wireless signal 1012 by the reference anchor device 1020c may be calculated based at least in part on the measured time differences Di,, D2, D3, and D 4 and the known time difference 7;3. For example, the TDOA may be calculated via an equivalent analytical expression to Eq. 16, wherein /agand time difference D, are set such thatD, and D, may be set to be equal or substantially similar.

For example, the following equation may be used to calculate TAB: [Eq. 42] TIDO-1_1'1 = (751_0 -52 + -h3_,/ )-71,3 Doing so may reduce the error in the calculated TDOA due to the clock drift of the clock of the receiver anchor device 1020a and the reference anchor device 1020c in accordance with the corresponding error equation shown in Eq. 19.

In other embodiments, the TDOA between the reception of the first wireless signal 1012 by the receiver anchor device 1020a and the reception of the first wireless signal 1012 by the reference anchor device 1020c may be calculated via an equivalent analytical expression to any one of Eq. 22, Eq. 27, Eq. 32, or Eq. 33 by replacing the D, and terms thereof with the corresponding terms Di_" and DLU.

In other embodiments, a first estimate of a TDOA may be calculated based on one of an equivalent analytical expression to Eq. 32 and Eq. 33, a second estimate of a TDOA may be calculated based on the other of the equivalent analytical expression to Eq. 32 and Eq. 33, and the TDOA is calculated based on taking an average of the first TDOA estimate and the second TDOA estimate.

Similarly, in various embodiments, the TDOA between the reception of the first wireless signal 1012 by the receiver anchor device 1020b and the reception of the first wireless signal 1012 by the reference anchor device 1020c may be calculated based at least in part on the measured time differences /kb, D2, /12,_b, and D4 and known time difference 723 in accordance with any of the above examples discussed in relation to the calculation of the TDOA.

In various embodiments, the calculation of the TDOA between each pair of receiver and reference anchors may be performed by the receiver anchor device 1020a, the receiver anchor device 1020b, or the reference anchor device 1020c; alternatively, it may be performed on a location server communicatively coupled to the receiver anchor device 1020a, the receiver anchor device 1020b, and the reference anchor device 1020c. One anchor may provide any other with the various measured time differences via a communication network to which all anchors are connected. Alternatively, the various measured time differences may be conveyed in additional wireless signals or follow-up messages.

In other embodiments, more than three anchors may be used, and the TDOA between the reception of the first wireless signal 1012 by each receiver anchor and the reception of the first wireless signal 1012 by the reference anchor may be calculated. The location of the tag device may be determined based on each of the calculated TDOAs. Irrespective of the number of anchors involved, only three wireless signals -1012 (first), 1014 (second), and 1016 -are used to determine the TDOA between the reception of the first wireless signal 1012 by each receiver anchor and the reception of the first wireless signal 1012 by the reference anchor.

In some embodiments, a communication protocol may be used where the transmission of a wireless signal by the tag device is prior to any transmission by a reference anchor device.

As it is not generally known when the tag will transmit the wireless signal, the protocol is easier to implement according to this ordering.

Example embodiments enable the accurate measurement of the TDOA between the time of reception of a wireless signal by a first anchor device (as measured by a clock of the first anchor device) and the time of reception of the same wireless signal by a second anchor device (as measured by a clock of the second anchor device).

Error can arise in determining the TDOA when the clocks of the two anchor devices run at slightly different frequencies due to non-ideal clock behaviour. The OWR TDOA calculation process based on a given signal protocol reduces error in the calculated TDOA in such a case.

This is achieved by transmitting and receiving specific signals between the two anchor devices and measuring time durations related to these specific signals by the clocks of both anchor devices. The specific signals are such that if the clocks of both devices were running at exactly the same frequency, the values measured by the clocks of the two anchor devices in relation to these signals in the signal protocol would be the same.

If, as is likely, the values measured by the clocks of both anchor devices in relation to these signals differ, the amount by which they differ corresponds to how much faster the clock of one anchor device is running than the clock of the other anchor device. Knowing this value allows the difference in frequency between each anchor device clock to be accounted for when calculating the above TDOA for a OWR signal exchange.

According to the example embodiments, error reduction techniques previously only exploited in relation to TWR are leveraged for OWR by modifying existing OWR signal protocols for TDOA calculation to utilise the ratio of clock errors of the reference anchor to those of the receiver anchor to compensate for clock drift when calculating the TDOA. This improves the accuracy of OWR. The OWR technique facilitates the use of battery-powered devices because the tag device need not be configured to receive to participate in the TDOA calculation according to the present technique. The transmit-only capabilities associated with the OWR TDOA calculation offer power savings relative to the TWR technique, extending battery life and making the use of battery-powered devices in the ranging calculation more viable. The improved accuracy afforded by the cancellation of clock drift using the protocol in the embodiments makes the OWR eminently suitable for 3D positioning with mobile tags, allowing tag location to be dynamically tracked in an energy efficient manner.

An output of the OWR algorithm according to the present technique is the TDOA of a tag blink between a reference anchor and a receiver anchor. To calculate the 3D location of a tag, TDOA measurements from six to eight anchors may be used. According to the present technique, a 3D tag position may be calculated using only three tag blinks, regardless of how many anchor devices are participating in the TDOA calculation. This can reduce RF utilisation relative to alternative TDOA calculation techniques. One skilled in the art may use one of many known methods, such as trilateration based positioning, to convert the TDOA measurement into a set of 3D coordinates for the tag device.

From a security point of view, the tag described in the present disclosure does not provide any information that is used as part of the TDOA calculations. Thus, the tag is not able to artificially influence or "fake" its determined 3D position.

In this specification, the phrase "at least one of A or B" and the phrase "at least one of A and B" should be interpreted to mean any one or more of the plurality of listed items A, B, etc., taken jointly and severally in any and all permutations.

Where functional units are described as circuitry, the circuitry may be general purpose processor circuitry configured by program code to perform specified processing functions. The circuitry may also be configured by modification to the processing hardware. The configuration of the circuitry to perform a specified function may be limited exclusively to hardware, limited exclusively to software, or a combination of hardware modification and software execution.

Program instructions may be used to configure the logic gates of general purpose or special purpose processor circuitry to perform a processing function.

Circuitry may be implemented, for example, as a hardware circuit comprising processors, microprocessors, circuits, circuit elements (e.g., transistors, resistors, capacitors, inductors, and so forth), integrated circuits, application specific integrated circuits, programmable logic devices, digital signal processors, field programmable gate arrays, logic gates, registers, semiconductor devices, chips, microchips, chip sets, and the like.

The processors may comprise general purpose processors, network processors that process data communicated over a computer network, or other types of processor, including reduced instruction set computers or complex instruction set computers. Each processor may have a single or a multiple core design. Multiple core processors may integrate different processor core types on the same integrated circuit die.

The TDOA calculations described herein may be implemented in whole or in part by machine-readable program instructions. Machine-readable program instructions may be provided on a transitory medium, such as a transmission medium, or on a non-transitory medium, such as a storage medium. These machine-readable instructions (computer program code) may be implemented in a high level procedural or object oriented programming language. However, the program(s) may be implemented in assembly or machine language, if desired. In any case, the language may be a compiled or interpreted language, and combined with hardware implementations.

Embodiments of the present invention are applicable for use with all types of semiconductor integrated circuit (IC) chips. Examples of these IC chips include but are not limited to processors, controllers, chipset components, programmable logic arrays, memory chips, and network chips. One or more of the components described herein may be embodied as a System On Chip (SOC) device. A SOC may include, for example, one or more Central Processing Unit cores, one or more Graphics Processing Unit cores, an Input/Output interface, and a memory controller. In some embodiments, a SOC and its components may be provided on one or more integrated circuit die; for example, they may be packaged into a single semiconductor device.

The following numbered examples relate to embodiments of the present technique: Example 1. A method of calculating a time difference of arrival, TDOA, of a wireless signal the method comprising: transmitting, by a tag device, a first wireless signal; receiving the first wireless signal at a first anchor device at a first time T1 and at a second anchor device at a second time T2; transmitting, by the second anchor device, before or after the transmission of the first wireless signal, a second wireless signal at a third time T3; receiving, by the first anchor device, the second wireless signal at a fourth time T4; determining a first time difference, D1, corresponding to the fourth time T4 minus the first time T1; determining a second time difference, D2, corresponding to the third time T3 minus the second time T2; and calculating a TDOA between the receipt of the first wireless signal at the first anchor device at the first time T1 and receipt of the first wireless signal at the second anchor device at the second time T2 based, at least in part, on the first time difference D1, the second time difference D2, and a known time of flight, TOF, between the first anchor device and the second anchor device.

Example I may be implemented as the embodiment of FIG.2.

Note that in any of the examples described herein, the first time difference may have a negative sign if the signal ordering is such that the fourth time is earlier than the third time and the first time difference may have a negative sign if the signal ordering is such that the third time is earlier than the second time. Similar logic applies to the third time difference and the fourth time difference for certain relative signal orderings. Alternatively, the signs may be disregarded and a negative TDOA may be obtained, and the magnitude of this value may be used as the estimated TDOA.

2. The method of example 1, or any other example described herein, further comprising: transmitting, by the tag device, before or after the transmission of the second wireless signal, a third wireless signal; receiving, by the first anchor device, the third wireless signal at a fifth time T5; receiving, by the second anchor device, the third wireless signal at a sixth time T6; determining a third time difference, D3, corresponding to the fifth time, T5, minus the fourth time, T4; determining a fourth time difference, D4, corresponding to the sixth time, T6, minus the fifth time T5; and calculating a TDOA between receipt of the first wireless signal at the first anchor device at the first time T1 and receipt of the first wireless signal at the second anchor device at the second time T2 based, at least in part, on the first time difference D1, the second time difference D2, the known TOF between the first anchor device and the second anchor device, the third time difference D3, and the fourth time difference D4.

This second example may correspond to the FIG. 4 embodiment 3. The method of example 2, or any other example described herein, wherein a time period between transmission of the first wireless signal and transmission of the third wireless signal by the tag device is sufficiently short such that any motion of the tag device within time period is negligible relative to a desired accuracy of the calculated tag location.

The third example applies to embodiments in which two tag blinks are used as part of the signalling protocol.

4. The method of example 3, or any other example described herein, further comprising: transmitting, by the second anchor device, before or after the transmission of the second wireless signal, a third wireless signal at a fifth time T5; receiving, by the first anchor device, the third wireless signal at a sixth time T6; determining a third time difference D3 corresponding to the first time T1 minus the sixth time T6; determining a fourth time difference, D4, corresponding to the second time, T2, minus the fifth time, T5; calculating a TDOA between receipt of the first wireless signal at the first anchor device at time T1 and receipt of the first wireless signal at the second anchor device at time T2 based, at least in part, on the first time difference D1, the second time difference D2, the known TOF between the first anchor device and the second anchor device, the third time difference D3, and the fourth time difference D4.

This fourth example may correspond to the FIG. 8 embodiment in which there are two reference anchor blinks and one intervening tag blink.

5. The method of example 2, or any other example described herein, wherein a time period between transmission of the second wireless signal and transmission of the third wireless signal by the second anchor device is sufficiently short such that any motion of the tag device within time period is negligible relative to a desired accuracy of the calculated tag location.

6. The method of any one of examples 2 to 5, or any other example described herein, wherein calculating the TDOA comprises: calculating a first TDOA estimate based on the first time difference D1 and the second time difference D2; calculating a second TDOA estimate based on the third time difference D3 and the fourth time difference D4, calculating the TDOA based on taking an average of the first TDOA estimate and the second TDOA estimate.

Example 6 may be implemented by Eqn. 15 and Eqn.16 where an average of the TDOA estimates specified by Eq.11a and Eq.11b have been taken.

7. The method of any one of examples 2 to 6, or any other example described herein, wherein calculating the TDOA comprises: determining a ratio of a first clock skew error, ka, of the clock of the first anchor device to a second clock skew error, kb, of the clock of the second anchor device by calculating a ratio of the first time difference D1 plus the third time difference D3 to the second time difference D2 plus the fourth time difference D4; and calculating a first TDOA estimate based on the determined ratio and one of: the first time difference D1 and the second time difference D2; or the third time difference D3 and the fourth time difference D4.

Example 7 and/or Example 8 may be implemented as described in relation to Eqn. 22 and Eqn. 27, where the ratio ka/kb has been used to scale the term D2 in Eq.12a or the inverse ratio kb/ka has been used to scale the term D1 in Eq. 12a. Alternatively, the ratio the ratio ka/kb may been used to scale the term D3 in Eq.12b or the inverse ratio kb/ka may be used to scale the term D4 in Eq. 12b.

8. The method of example 7, or any other example described herein, wherein the calculation of the TDOA comprises: scaling either the second or the third time difference by the determined ratio to cancel the second clock skew error kb; (as illustrated by Eq. 22 for the second time difference and using Eq. 12a rather than the alternative, Eq.12b) or scaling the first or the fourth time difference by the inverse of the determined ratio to cancel the first clock skew error, ka (as illustrated by Eq.27 for the first time difference and using Eq. 12a rather than the alternative, Eq. 12b).

9. The method of example 8, or any other example described herein, further comprising: calculating a second TDOA estimate based on the other of the first time difference D1 and the second time difference D2; or the third time difference D3 and the fourth time difference D4.

10. The method of example 9, or any other example described herein, wherein the second time difference or the third time difference is scaled based on the determined ratio to cancel clock skew errors of the clock of the second anchor device; or the first time difference or the fourth time difference is scaled based on an inverse of the determined ratio to cancel clock skew errors of the clock of the first device; and wherein the TDOA is calculated based on the average of the first TDOA estimate and the second TDOA estimate.

11. The method of any one of examples 1 to 10, or any other example described herein, wherein the tag device transmits wireless signals to determine the TDOA but is configured to not receive wireless signals as part of the TDOA determination.

12. The method of any one of examples 1 to 11, or any other example described herein, comprising using the calculated TDOA to calculate a geographical location of the tag device.

13. The method of example 12, or any other example described herein, wherein an n-dimensional geographical location of the tag device is calculated using signal exchanges between at least (n+1) anchor devices and the tag device, where n is a non-zero integer.

14. Machine readable instructions provided on a transitory or non-transitory machine-readable medium, the machine-readable instructions upon execution by processing circuitry to perform the method of any one of examples 1 to 13.

Example 15. Processing circuitry to calculate a time difference of arrival of a wireless signal between receipt of a wireless signal at a reference anchor device and receipt of the same wireless signal at a receiver anchor device, the processing circuitry being arranged to: obtain a first time difference, D1, based on a measurement of a first clock local of a first anchor device, the first time difference corresponding given by a time of receipt, T4, of a second wireless signal originating from a second anchor device at the first anchor device minus a time of receipt, T1, of a first wireless signal transmitted by a tag device at the first anchor device; obtain a second time difference, D2, based on a measurement of a second clock local of the second anchor device, the second time difference D2 given by a time of transmission, T3, of the second wireless signal by the second anchor device minus a time of receipt, T2, of the first wireless signal at the second anchor device; calculate a time difference of arrival, TDOA, between receipt of the first wireless signal at the first wireless device and receipt of the first wireless signal at the second wireless device based at least in part on the first time difference, the second time difference and a known time of flight of a wireless signal between the first anchor device and the second anchor device.

Example 16. A wireless communication device comprising the processing circuitry of example 15.

Example 17. A wireless communication system comprising: a tag device to transmit a first wireless signal; a first anchor device to receive the first wireless signal at a first time, T1, and to receive a second wireless signal at a fourth time, T4; a second anchor device to transmit, before or after the transmission of the first wireless signal, a second wireless signal at a third time, T3, and to receive the first wireless signal at a second time, T2; and processing circuitry to: determine a first time difference, D1, corresponding to the fourth time T4 minus the first time Ti; determine a second time difference, D2, corresponding to the third time T3 minus the second time T2; and calculate a TDOA between the receipt of the first wireless signal at the first anchor device at the first time,T1, and receipt of the first wireless signal at the second anchor device at the second time, T2, based, at least in part, on the first time difference D1, the second time difference D2, and a known time of flight, TOF, between the first anchor device and the second anchor device.

18. The wireless communication system of example 17, or any other example described herein, wherein the processing circuitry is located in at least one of the first anchor device and the second anchor device.

19. The wireless communication system of example 15, or any other example described herein, comprising a location server and wherein the processing circuitry is provided in at least one of the first anchor device, the second anchor device and the location server.

Note that in some examples, time differences may be absolute, such as timestamps whereas in other examples, time differences may be relative. The TDOA calculation may be performed on any processing device(s) using the specified time differences.

Claims

CLAIMS1. A method of calculating a time difference of arrival, TDOA, of a wireless signal the method comprising: transmitting, by a tag device, a first wireless signal; receiving the first wireless signal at a first anchor device at a first time T1 and at a second anchor device at a second time T2; transmitting, by the second anchor device, before or after the transmission of the first wireless signal, a second wireless signal at a third time T3; receiving, by the first anchor device, the second wireless signal at a fourth time T4; determining a first time difference, D1, corresponding to the fourth time T4 minus the first time T1; determining a second time difference, D2, corresponding to the third time T3 minus the second time T2; and calculating a TDOA between the receipt of the first wireless signal at the first anchor device at the first time T1 and receipt of the first wireless signal at the second anchor device at the second time T2 based, at least in part, on the first time difference D1, the second time difference D2, and a known time of flight, TOF, between the first anchor device and the second anchor device.
2. The method of claim 1 further comprising: transmitting, by the tag device, before or after the transmission of the second wireless signal, a third wireless signal; receiving, by the first anchor device, the third wireless signal at a fifth time T5; receiving, by the second anchor device, the third wireless signal at a sixth time T6; determining a third time difference, D3, corresponding to the fifth time, T5, minus the fourth time, T4; determining a fourth time difference, D4, corresponding to the sixth time, T6, minus the fifth time T5; and calculating a TDOA between receipt of the first wireless signal at the first anchor device at the first time T1 and receipt of the first wireless signal at the second anchor device at the second time T2 based, at least in part, on the first time difference D1, the second time difference D2, the known TOF between the first anchor device and the second anchor device, the third time difference D3, and the fourth time difference D4.
3. The method of claim 2, wherein a time period between transmission of the first wireless signal and transmission of the third wireless signal by the tag device is sufficiently short such that any motion of the tag device within the time period is negligible relative to a desired accuracy of the calculated tag location.
4. The method of claim 1 further comprising: transmitting, by the second anchor device, before or after the transmission of the second wireless signal, a third wireless signal at a fifth time T5; receiving, by the first anchor device, the third wireless signal at a sixth time T6; determining a third time difference D3 corresponding to the first time T1 minus the sixth time T6; determining a fourth time difference, D4, corresponding to the second time, T2, minus the fifth time, T5; calculating a TDOA between receipt of the first wireless signal at the first anchor device at time T1 and receipt of the first wireless signal at the second anchor device at time T2 based, at least in part, on the first time difference D1, the second time difference D2, the known TOF between the first anchor device and the second anchor device, the third time difference D3, and the fourth time difference D4.
5. The method of claim 2, wherein a time period between transmission of the second wireless signal and transmission of the third wireless signal by the second anchor device is sufficiently short such that any motion of the tag device within time period is negligible relative to a desired accuracy of the calculated tag location.
6. The method of any one of claims 2 to 5 wherein calculating the TDOA comprises: calculating a first TDOA estimate based on the first time difference D1 and the second time difference D2; calculating a second TDOA estimate based on the third time difference D3 and the fourth time difference D4; calculating the TDOA based on taking an average of the first TDOA estimate and the 30 second TDOA estimate.
7. The method of any one of claims 2 to 6 wherein calculating the TDOA comprises: determining a ratio of a first clock skew error, ka, of the clock of the first anchor device to a second clock skew error, kb, of the clock of the second anchor device by calculating a ratio of the first time difference D1 plus the third time difference D3 to the second time difference D2 plus the fourth time difference D4; and calculating a first TDOA estimate based on the determined ratio and one of: the first time difference D1 and the second time difference D2; or the third time difference D3 and the fourth time difference D4.
8. The method of claim 7, wherein the calculation of the TDOA comprises: scaling either the second or the third time difference by the determined ratio to cancel the second clock skew error kb; or scaling the first or the fourth time difference by the inverse of the determined ratio to cancel the first clock skew error, ka.
9. The method of claim 8, further comprising: calculating a second TDOA estimate based on the other of: the first time difference D1 and the second time difference D2; or the third time difference D3 and the fourth time difference D4.
10. The method of claim 9, wherein the second time difference or the third time difference is scaled based on the determined ratio to cancel clock skew errors of the clock of the second anchor device; or the first time difference or the fourth time difference is scaled based on an inverse of the determined ratio to cancel clock skew errors of the clock of the first device; and wherein the TDOA is calculated based on the average of the first TDOA estimate and the second TDOA estimate.
11. The method of any one of claims 1 to 10, wherein the tag device transmits wireless signals to determine the TDOA but is configured to not receive wireless signals as part of the TDOA determination.
12. The method of any one of claims 1 to 11, comprising using the calculated TDOA to calculate a geographical location of the tag device.
13. The method of claim 12, wherein a n dimensional geographical location of the tag device is calculated using signal exchanges between at least (n+1) anchor devices and the tag device, where n is a non-zero integer.
14. Machine readable instructions provided on a transitory or non-transitory machine-readable medium, the machine-readable instructions upon execution by processing circuitry to perform the method of any one of claims 1 to 13.
15. Processing circuitry to calculate a time difference of arrival of a wireless signal between receipt of a wireless signal at a reference anchor device and receipt of the same wireless signal at a receiver anchor device, the processing circuitry being arranged to: obtain a first time difference, D1, based on a measurement of a first clock local of a first anchor device, the first time difference corresponding given by a time of receipt, T4, of a second wireless signal originating from a second anchor device at the first anchor device minus a time of receipt, T1, of a first wireless signal transmitted by a tag device at the first anchor device; obtain a second time difference, D2, based on a measurement of a second clock local of the second anchor device, the second time difference D2 given by a time of transmission, T3, of the second wireless signal by the second anchor device minus a time of receipt, T2, of the first wireless signal at the second anchor device; calculate a time difference of arrival, TDOA, between receipt of the first wireless signal at the first wireless device and receipt of the first wireless signal at the second wireless device based at least in part on the first time difference, the second time difference and a known time of flight of a wireless signal between the first anchor device and the second anchor device.
16. A wireless communication device comprising the processing circuitry of claim 15.
17. A wireless communication system comprising: a tag device to transmit a first wireless signal; a first anchor device to receive the first wireless signal at a first time, T1 and to receive a second wireless signal at a fourth time, T4; a second anchor device to transmit, before or after the transmission of the first wireless signal, a second wireless signal at a third time, T3 and to receive the first wireless signal at a second time, T2; and processing circuitry to: determine a first time difference, D1, corresponding to the fourth time T4 minus the first time Ti; determine a second time difference, D2, corresponding to the third time T3 minus the second time T2; and calculate a TDOA between the receipt of the first wireless signal at the first anchor device at the first time,T1, and receipt of the first wireless signal at the second anchor device at the second time, T2, based, at least in part, on the first time difference D1, the second time difference D2, and a known time of flight, TOF, between the first anchor device and the second anchor device.
18. The wireless communication system of claim 17, wherein the processing circuitry is located in at least one of the first anchor device and the second anchor device.
19. The wireless communication system of claim 17, comprising a location server and wherein the processing circuitry is provided in at least one of the first anchor device, the second anchor device and the location server.