WO2008152410A1

WO2008152410A1 - Method and apparatus for determining the speed and orientation of networked mobile stations

Info

Publication number: WO2008152410A1
Application number: PCT/GB2008/002058
Authority: WO
Inventors: Zhuoqun Li; Lingfen Sun; Emmanuel C. Ifeachor
Original assignee: University Of Plymouth
Priority date: 2007-06-15
Filing date: 2008-06-13
Publication date: 2008-12-18
Also published as: GB2450476B; GB0711534D0; GB2450476A

Abstract

A method of estimating the speed of movement of a mobile device of a wireless communication system comprises the steps of determining the distance of the mobile device from a reference device at a first time instant; determining the distance of the mobile device from the reference device at a second time instant; calculating the distance travelled by the mobile device between the first time instant and the second time instant; and estimating the speed of movement of the mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant. The distances from the reference device to other mobile devices in the system may then be determined using relative distance measurements at successive time instants. An apparatus for performing the method and a wireless communications network comprising the apparatus are also provided.

Description

METHOD AND APPARATUS FOR DETERMINING THE SPEED AND ORIENTATION OF NETWORKED MOBILE STATIONS

The present invention relates to a method of determining the speed and orientation of mobile stations in a wireless network and to an apparatus for carrying out the same. In a further aspect, the invention relates to a network comprising the apparatus.

Estimation of the moving speed and orientation of mobile devices is important in traditional cellular mobile communications as well as the emerging wireless mobile ad-hoc networking. For example, information relating to the speed and direction of travel of mobile devices can be used for predicting the hand-over time of a mobile device to a base station in the cellular mobile networks. This is required when a mobile device is moving from the area of coverage of one base station (commonly referred to as a 'cell') to that of another base station. In order to avoid an interruption in communication with the mobile device, it is important that appropriate handover technology is employed to transfer communicate from one base station to the other. A fast handover is required in order for the network to have an overall high efficiency and quality of service.

The information relating to the speed and direction of travel of a mobile device can be also utilized to predict link availability in ad-hoc networks.

It is known to use a localisation system, such as a Global Position System

(GPS) to determine the speed and direction of travel of a mobile device. This information may be deduced from a change of location of the device as detected by the GPS. A two-tier mobility model to represent the realistic movements of devices or nodes in a mobile ad-hoc network is disclosed by Zaidi, Z.R., et al., 'A two-tier Representation of Node Mobility in Ad Hoc Networks', Proc. of IEEE International Conference on Sensor and Ad Hoc Communications and Networks (SECON), October 2004. Such a system, for example reliant on a GPS, is limited in its use, in particular in situations where the GPS signal is too weak to be relied upon, for example when the mobile device is at an interior location or at a location where in the line of sight with the GPS satellite^' is obscured.

In addition, the hardware and software required for a mobile device to receive and process GPS signals require significant amounts of energy, which places a considerable drain on the often limited batteries carried in mobile devices.

If the model or the properties of signal propagation can be successfully identified, the movement of mobile devices or stations can be also estimated using the statistics of the received signal. For example, in known methods, the Doppler frequency is utilized in for mobile velocity estimation. Thus, methods of determining the speed and location of a mobile device or station, for example for use in handover tasks, is disclosed by Sampath, A. and Holtzmann, J. M., 'Estimation of Maximum Doppler Frequency for Handoff Decisions', Proc. of IEEE Vehicular Technology Conference, May 1993, pages 859 to 862, and by Narasimhan, R., 'Estimation of Mobile Speed and Average Received Power in Wireless Systems using Best Basis Methods', IEEE Transactions on Communications, Vol. 49, No. 12, pages 2172 to 2183, December 2001.

US 6,879,920 discloses a method and system for estimating the movement speed of a mobile unit. The method comprises receiving a first signal from the mobile unit and calculating an envelope of the first signal. The envelope is squared to generate a second signal. A third signal is obtained by multiplying the second signal by a modulated carrier including a frequency, from which the movement speed of the mobile unit is estimated. The movement speed is estimated using the Doppler frequency.

US 6,987,971 discloses a process and device for estimating the speed of movement of a mobile terminal, in particular a cellular mobile telephone. The method includes calculating a normalised auto-covariance of the instantaneous power of the signal received by the mobile terminal or by the base station. The normalised auto- covariance may be obtained by a calculation of the signal power, a low-pass filtering of the signal so as to retain the Doppler information, and subtracting the average signal power value. However, the Doppler frequency of a signal from a moving mobile device cannot be easily determined. Further, the accuracy of the Doppler frequency measurement is vulnerable to additive channel noise, making methods reliant on the Doppler frequency very unreliable when the mobile device is broadcasting from a region that generates or induces significant levels of noise in the signal. This has been recognised in the art. Thus, Xiao, C. et al., 'Mobile Speed Estimation for TDMA-Based Hierarchical Cellular Systems', IEEE Transactions on Vehicular Technology, Vol. 50, No. 4, pages 981 to 991 , July 2001 , propose a method using the normalised autocorrelation values of the received signal in a six-step procedure to provide coarse estimates of the speed of travel of mobile units and overcome the adverse influence of signal noise. The method utilises the normalised autocorrelation values of the received signal to provide coarse estimates of the speed of mobiles, for example slow, medium, or fast. However, the method cannot produce estimates of exact values of the speed of movement of mobile units.

A common feature of these known methods is that they require precise information of the characteristics/statistics of the wireless channel and the received signal, which prevent their applications in scenarios where the exact properties of the wireless channel are unknown or varying with time, such as occur when the signal to noise ratio is high.

There remains a need for a novel method and an apparatus for estimating relative velocities between mobile stations in a way that is less dependent on the signal properties. Such a method must be robust and efficient enough for use in predicting accurate handover events from one base station to another. In addition, the method must not be adversely affected by signal noise.

It has now been found that the speed and direction of movement of a mobile device or terminal in a network may be accurately determined using inter-station ranging information that is changing over time, that is, the changes in the distance between the stations in a network due to the relative movement of one with respect to the other. According to a first aspect of the present invention there is provided a method of estimating the speed of movement of a mobile device of a wireless communication system, the method comprising the steps of: a) determining the distance of the mobile device from a reference device at a first time instant; b) determining the distance of the mobile device from the reference device at a second time instant; c) calculating the distance travelled by the mobile device between the first time instant and the second time instant; and d) estimating the speed of movement of the mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant.

It has been found that the method of the present invention is a simple yet robust means of estimating the speed of movement of a mobile device within a wireless communication system or network. In particular, the method is particularly robust when handling signals from mobile devices with significant noise and can provide increased accuracy in such cases, compared with known methods.

The reference device may be a stationary device, for example a base station in the wireless communication system. In this case, the method of the present invention will provide an estimate of the actual speed of movement of the mobile device. Alternatively, the reference device may be a mobile device. In the latter case, the method of the present invention will provide an estimate of the relative speed between the two moving devices. The relative speeds of movement may be used to calculate the absolute speeds of movement, provided that the speed of movement of at least one of the mobile devices relative to a stationary device, such as a base station, is determined.

The distances between the mobile device and the reference device may be determined using techniques known in the art. Examples of suitable known techniques include received signal strength (RSSI), time-of-arrival (ToA) or time- difference-of-arrival (TDoA). Such techniques are well known for measuring the distances between devices in a wireless communication system. The distance travelled by the mobile device between the first time instant and the second time instant may be determined or calculated using any suitable technique. In the method of the present invention, a particularly preferred technique is to use the perpendicular location. In this respect, the term 'perpendicular location' as used herein is a reference to the location or point along the path of travel of the mobile device that is closest to the reference device, at which point the line extending between the mobile device and the reference device extends perpendicular to the path of travel of the mobile device.

The position of the mobile device relative to the perpendicular location may be determined by taking three measurements of the distance between the reference device and the mobile device. The mobile device will have passed the perpendicular location when the three consecutive measurements are such that the second distance measured is less than both the first and third distances measured. Thus, for a mobile device having its distance d_tn from a reference device measured at time tn, the distances measured at three successive time instants tn, tn+1, and tn+2, must satisfy the following relationship if the mobile device passes the perpendicular location between time instants tn and tn+2:

If the mobile device is moving in a path relative to the reference device such that it is either before or after the perpendicular location, this will also be indicated by the three consecutive measurements. Thus, if the mobile device has yet to reach the perpendicular location, the following relationship will apply to the three consecutive distance measurements:

Similarly, if the mobile device has passed the perpendicular location, the following relationship will apply to the three consecutive distance measurements:

dtn < dtn+1 < dtπ+2 In one embodiment of the method of the present invention, one of the first or second time instants is selected such that the mobile device is at or very close to the perpendicular location. In this respect, the closer the mobile device is to the perpendicular location at the second time instant, the more accurate the estimate of the speed of movement of the mobile device. In this way, the distance travelled by the mobile device between the first time instant and the second time instant may be determined using simple mathematics and Pythagoras theorem.

In one preferred embodiment, in step (b) a determination is made whether the mobile device has passed the perpendicular location. In the case that the mobile device has not passed the perpendicular location, it is preferred in this embodiment that step (b), and optionally step (a) are repeated until the perpendicular location has been passed by the second time instant.

In an alternative embodiment, the first time instant is before the mobile device has reached the perpendicular location and the second time instant is after the mobile device has reached the perpendicular location. In this case, the perpendicular location is determined by interpolation, as is the time instant at which the mobile device is at the perpendicular location. In a further alternative embodiment, both the first and second time instants are either before or after the mobile device has reached the perpendicular location, the perpendicular location being determined by extrapolation.

In the method of the present invention, it is assumed that the mobile device is travelling in a straight line and at a constant speed. In this respect, if the time that elapses between the first and second time instants is sufficiently short, this assumption is sufficiently accurate to provide a good estimation of the speed of movement of the mobile device. The time that is allowed to elapse between the first and second time instants may be from 0.1 to 10 seconds, preferably from 0.2 to 5 seconds, more preferably from 0.5 to 2 seconds. Elapsed times of about 1 second are suitable and provide accurate estimations of the speed of movement of the mobile device in all but those cases where the mobile device is moving at very high speeds and/or in a path that deviates significantly from a straight line. The time that is allowed to elapse between the first and second time instants may be varied, according to the position and trajectory of the mobile device with respect to the reference device. Thus, for example, a mobile device that is close to the perimeter of the area of coverage of the reference station on a trajectory that is close to tangential to the perimeter will pass the perpendicular location within a short space of time of coming into range of the reference station. In such a case, it may be preferable to use an elapsed time that is short, of the order of 1 second or less. In contrast, a mobile station that enters the area of coverage and is on a path that takes it close to the reference station will take a relatively long time to reach the perpendicular location. In such a case, a longer elapsed time may be used, for example of the order of several seconds.

The method of the present invention may be employed as described hereinbefore in a wireless communication system that comprises a reference device and a single mobile device. The method may also be used as described above to estimate the movements of each mobile device in so-called sparse node networks comprising a plurality of mobile devices, that is systems in which the number of mobile devices is relatively low and/or the mobile devices are widely distributed across the area of coverage of the reference station. Alternatively, the method may be employed to estimate the speed of movement of each of the mobile devices in a densely populated network. While the method steps as described above may be applied to each and every mobile device in the dense node network individually, it is a further advantage of the present invention that the method can be applied to estimate the speed of movement of one mobile device, in particular a device that has passed the perpendicular location for that device, and the results used to estimate the speed of movement of other mobile devices in the system without needing to establish their position relative to their respective perpendicular locations.

In particular, the method of the present invention may be applied to estimate the speed of movement of a mobile device in a communication network or system, in which the mobile device has passed its perpendicular location, and the results used to calculate directly the speed of movement of a second or further mobile devices in the same system. In one embodiment, the method of estimating the speed of movement of a first and second mobile device in the communication system includes a step (a) which comprises: (i) determining the distance of a first mobile device from a reference device at a first time instant; and

(ii) determining the distance of a second mobile device from the reference device at the first time instant; and a step (b) which comprises: (i) determining the distance of the first mobile device from the reference device at a second time instant; and

(ii) determining the distance of the second mobile device from the reference device at the second time instant.

Once the speed of movement of the first mobile device has been determined, for example as described hereinbefore using the perpendicular location of that device, the speed of movement of the second mobile device may be readily determined using simple mathematical algorithms and trigonometry, as will be described in greater detail below.

As noted above, the method of the present invention may be employed to estimate the speed of movement of a single mobile device with respect to a reference device in a wireless communications system using the perpendicular location of the mobile device. Accordingly, in a further aspect, the present invention provides a method of estimating the speed of movement of a mobile device of a wireless communication system, the method comprising the steps of: a) determining the distance of the mobile device from a reference device at a first time instant; b) determining the distance of the mobile device from the reference device at a second time instant; c) determining if the mobile device has passed the perpendicular location at the second time instant; if the mobile device has not passed the perpendicular location, repeating step (b) until the perpendicular location has been reached or passed; d) calculating the distance travelled by the mobile device between the first time instant and the second time instant; and e) estimating the speed of movement of the mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant.

As also noted above, the present invention allows the speeds of movement of a plurality of mobile devices relative to a given reference device to be determined, in particular once the speed of movement of one device has been determined using the aforementioned method. Thus, in a further aspect, the present invention provides a method of estimating the speed of movement of a plurality of mobile devices of a wireless communication system, the method comprising the steps of: a) determining the distance of a first mobile device from a reference device at a first time instant; b) determining the distance of the first mobile device from the reference device at a second time instant; c) determining if the first mobile device has passed the perpendicular location at the second time instant; if the mobile device has not passed the perpendicular location, either repeating step (b) until the perpendicular location has been reached or passed or carrying out steps (a) and (b) in respect of further mobile devices until a device having passed the perpendicular location for that device by the second time instant; d) calculating the distance travelled by the first mobile device between the first time instant and the second time instant; e) estimating the speed of movement of the first mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant; f) determining the distance of both the first and second mobile devices from the reference device at a third time instant; e) determining the distance of both the first and second mobile devices from the reference device at a fourth time instant; f) calculating the distance travelled by the first mobile device between the third time instant and the fourth time instant from the estimated speed of movement of the first mobile device; g) calculating the distance travelled by the second mobile device between the third time instant and the fourth time instant using the distance calculated in step (f); and h) calculating the speed of movement of the second mobile device from the distance calculated in step (g) and the time elapsed between the third time instant and the fourth time instant.

The method of this aspect uses the estimated speed of movement of a first mobile device to calculate the speed of movement of a second. If the first device is determined to have passed its perpendicular location before the second time instant, or has already had its speed of movement estimated, the first and third time instants may be the same and the second and fourth time instants the same.

In a further aspect, the present invention provides an apparatus for estimating the speed of movement of a mobile device of a wireless communication system, the apparatus comprising: a) means for determining the distance of the mobile device from a reference device at a plurality of time instants; b) means for calculating the distance travelled by the mobile device between a first and a second time instant; and c) means for estimating the speed of movement of the mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant.

The apparatus may comprise or consist of any suitable processor, coupled to a wireless communication system or network. The apparatus may be located in the reference device and/or one or more mobile devices of the system. It is a particular advantage of the present invention that the method relies upon the signal transmitted between devices in the system and can thus be carried out using simple processor means. This allows the apparatus of the invention to be incorporated into mobile devices, such as mobile telephones or the like, without the apparatus requiring a significant amount of energy from the battery of the mobile device. This is in contrast to known apparatus, for example those that rely upon GPS to achieve the same results. As noted, the reference device may be stationary or may be a mobile device within the communications system.

The means for determining the distance of the mobile device from the reference device may use any known technique. Preferred means use received signal strength (RSSI)₁ time-of-arrival (ToA) or time-difference-of-arrival (TDoA) and are known in the art.

As described hereinbefore, a particularly preferred method of calculating the distance travelled by the mobile device between successive time instants is to use the perpendicular location for the mobile device. Accordingly, it is preferred that the means for calculating the distance travelled by the mobile device between the first time instant and the second time instant uses the perpendicular location for the mobile device. The apparatus most preferably further comprises means for determining the perpendicular location of the mobile device. As described above, the means for determining the perpendicular location of the mobile device may be arranged to derive the perpendicular location by extrapolation and/or interpolation from a series of distance measurements.

In one preferred embodiment, the apparatus comprises means for triggering the means for determining the distance of the mobile device from a reference device at a plurality of time instants to repeat its tasks if the means for determining the perpendicular location of the mobile device determines that the mobile device has not passed the perpendicular location. In this way, the estimation of the speed of movement of the mobile device may be carried out once the mobile device has reached or passed the perpendicular location.

As noted previously, it may be advantageous to adjust the elapsed time between consecutive measurements of the distance between the mobile device and the reference device, in particular according to the position of the mobile device relative to the reference device. Accordingly, the apparatus may comprise means to adjust the time period that elapses between successive determinations of the distance between the mobile device and the reference device. Such means are preferably responsive to data relating to the position of the mobile device relative to the reference device.

The apparatus may be adapted to estimate the speed of movement of one or more further mobile devices, in particular using the estimate of the speed of movement of the first mobile device. Accordingly, the apparatus may comprise means to estimate the speed of movement of a second mobile device, the said means using the data produced by means (c).

One embodiment of an apparatus for estimating the speed of movement of a mobile device of a wireless communication system comprises: a) means for determining the distance of the mobile device from a reference device at a plurality of time instants; b) means for determining if the mobile device has passed the perpendicular location at a given time instant; c) means for triggering means (a) if the mobile device has not passed the perpendicular location until the perpendicular location has been reached or passed; d) means for calculating the distance travelled by the mobile device between a first time instant and a second time instant; and e) means for estimating the speed of movement of the mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant.

One preferred embodiment of an apparatus for estimating the speed of movement of a plurality of mobile devices of a wireless communication system comprises: a) means for determining the distance of a first mobile device from a reference device at a plurality of time instants; b) means for determining if the first mobile device has passed the perpendicular location at a given time instant; c) means for triggering means (a) if the mobile device has not passed the perpendicular location; d) means for calculating the distance travelled by the first mobile device between a first time instant and a second time instant; e) means for estimating the speed of movement of the first mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant; f) means for calculating the distance travelled by the first mobile device between a third time instant and a fourth time instant from the estimated speed of movement of the first mobile device; g) means for calculating the distance travelled by the second mobile device between the third time instant and the fourth time instant using the distance calculated by means (f); and h) means for calculating the speed of movement of the second mobile device from'the distance calculated in means (g) and the time elapsed between the third time instant and the fourth time instant.

In a further aspect, the present invention provides a wireless communication system or network comprising an apparatus as hereinbefore described. The wireless communication system may comprise a reference device and one or more mobile devices. In particular, the wireless communication system may be either sparsely populated or densely populated with mobile devices.

Embodiments of the present invention will now be described, by way of example only, having reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of an ad-hoc mobile communications network having a reference device and a mobile device, the mobile device being shown in motion;

Figure 2 is a further diagram of the mobile network of Figure 1 illustrating the calculation of the distances between the mobile device and the reference device;

Figure 3 is a diagrammatic representation of a dense mobile communications network having a reference device and two mobile devices, the mobile devices being shown in motion; Figure 4 is a further diagram of the mobile network of Figure 3 illustrating the calculation of the distances between the mobile device and the reference device;

Figures 5a to 5d are diagrammatic representations of the different possible movement patterns of the mobile devices of the network of Figure 4;

Figure 6 is a block diagram representation of an apparatus according to one embodiment of the present invention;

Figure 7 is a graph of results obtained from a simulation of a sparse mobile communications network with clean signal data using the method of the present invention;

Figure 8 is a graph of results obtained from simulation of sparse mobile communications network with noisy signal data using the method of the present invention;

Referring to Figure 1 , there is shown a mobile ad-hoc communication network comprising a reference device 100 and a mobile device 101. The network represented in Figure 1 is an example of a sparse network, that is one have relatively few mobile units or units dispersed over a wide area. The reference device 100 may be a base station, for example, with the mobile device 101 being any mobile communications device, such as a mobile telephone, portable computer or the like. The mobile device 101 is moving relative to the reference device 100 in a straight line indicated by arrows extending from the left to the right, as viewed in Figure 1. The mobile device 101 is shown in four locations at time instants I₁, t_n.i, t_π, and t_n+1. The distance between the mobile device 101 and the reference device 100 at each of these time instants are shown by the lines 111 , 112, 113 and 114, respectively. The perpendicular location of the mobile device 101 is at time instant t_n, at which instant the mobile device 101 passes closest to the reference device 100 and the line 113 between the two devices is perpendicular to the path being followed by the mobile device 101.

Referring to Figure 2, the network of Figure 1 is represented to illustrate the method of calculation of the speed of the mobile device 101 using a method of the present invention and is annotated with the variables used in the formulae described below.

In Figure 2, the mobile device j has entered the area of coverage of the reference device i, indicated by the dotted line, and is again travelling from left to right as viewed in the figure. It will be assumed that the trajectory of device j is a straight line. The distance d_e is the estimate of the distance of the device j at the time that the device enters the area of coverage of the reference device i. The estimate of distance d_e may be obtained using techniques known in the art, for example ToA; RSSI or TDoA. In Figure 2, d_p-i, d_p, and d_p+1 are the distance estimates sampled at consecutive time instants t_p - Δt, t_p and t_p + Δt. The distance dp is used to approximate the distance between the mobile device j and the reference device i when the mobile device j is at the perpendicular location and satisfies the relationship:

dp^i > dp A d_p+χ > d_{p ^}

This relationship assumes that there is no effect from measurement noise and that the noise levels in adjacent time slots are the same or very similar.

Taking Mj(t) to be the movement of the mobile device j relative to the reference device i during the time t with a relative velocity Vy, then it follows that

M_j(Δt) = Vi_jΔt Accordingly, applying Pythagoras' theorem to the right triangle in Figure 2, the relative velocity Vy is estimated as follows:

(i)

where ^^-1' P' j_s the error component of the distance measurements d_p.₁ and d_p resulting from the noise present in the signal. The relative velocity v_β may also be estimated from a longer time scale using the following relationship:

(H)

When the determination of the distances between the mobile device j and the _ref_erence device i is accurate, in particular when there is little or no noise in the signal, it is preferred to use relationship (II) above, as the velocity is calculated over a longer time period t_e to t_p. However, when the signal contains significant noise, for example when the mobile device j is at a location with no line of sight (NLOS) to the reference device, it is preferred to use relationship (I) above. Referring to Figure 3, there is shown a representation of a dense communications network. The network includes a reference device 200 and two mobile devices 201 and 202, both of which are moving relative to the reference device and each other. The distances between each mobile device 201 and 202 and the reference device 200 and between each other at time t-Δt are shown by lines 211 , 212 and 213 respectively. Similarly, the distances between each mobile device 201 and 202 and the reference device 200 and between each other at time t are shown by lines 221 , 222 and 223 respectively. In time Δt the mobile device 201 has moved the distance represented by line 231 and the mobile device 202 has moved the distance represented by line 232.

Referring to Figure 4, the network of Figure 2 is represented to illustrate the method of calculation of the speed of the mobile devices 201 and 202 using a method of the present invention and is annotated with the variables used in the formulae described below.

Referring to Figure 4, the reference device i is communicating with two mobile devices j and k, both moving relative to the reference device. The positions of the two mobile devices j and k at times t-Δt and t are shown in Figure 4, with the mobile device j having travelled a distance of M_j and the mobile device' k having travelled a distance of M_k in this elapsed time Δt.

The speed of movement of the mobile device j is known, having been estimated, for example, using the method described above and illustrated in Figure 2. Using this estimated speed of movement of the device j, the speed of movement of the mobile device k may be determined, as follows.

During time Δt, the distance between the mobile device j and the reference device i changes from K₀ to Ki. Similarly, in the same elapsed time, the distance from the reference device i to the mobile device k changes from J₀ to J₁. The change in the angle between the lines joining the reference device to both of the mobile devices is from Φj to O₁', as shown in Figure 4. The included angle of K₀ and K₁ is Φ_M, and the included angle of J₀ and J₁ is Φ_x, as shown in Figure 4. In the elapsed time Δt, there are four different relative patterns of movement that may arise, depending upon the relative velocities of the mobile devices j and k. The four possible patterns are represented in Figures 5a to 5d. The four relative movement patterns shown in Figures 5a to 5d give rise to four different relationships between the included angles O₁, Φι', Φ_M, and Φ_x, as follows:

(III) where Φj" is the included angle of J₀ and K₁.

The relative speed Vy of the mobile device j is known, and is used to determine Φ_M, as follows:

^ΨM = ^C0S 2KoK₁ (IV)

In relationship (IV), the values of K₀ and K₁ are determined by measurement of the distances between the mobile device j at times t-Δt and t, using the techniques mentioned above and known in the art. M_j is the distance travelled by the mobile device j in the time Δt and is calculated as M_j = Vy Δt.

The above method assumes that there is little or no noise in the signals. If significant noise is present, the relationship (III) is as follows:

-f Si) - (φi' + ei') - (φ_M + ψi > φ'_i

Ψκ

+ S₁) - (φ. + ε-) 4- (φ_M +

ψi < Ψι

(V)

where Cj₁C₁' and e_M are the error components in angles Φj, Φ_\ and Φ_M arising as a result of the noisy signal. It can be seen that the accuracy of the estimation of Φ_x is largely determined by the error e_M in the estimation of the speed of movement of the mobile device j, in the case that the values of C_\ and ei' are similar or the same. In this respect, it is to be noted that the method described above and illustrated in Figure 2 provides a robust way of estimating speed of movement of the mobile device j, hence keeping the error CM to a minimum.

It is difficult to determine Φj" and without a value for this variable, the solutions to relationships (III) and (V) will provide two estimates of Φ_x. The true estimate of Φ_x may be determined using a second set of data, for example generated using a further mobile device having a known speed of movement, or by repeating the method with the mobile device j after a further elapsed time Δt. The true value for Φ_x will be the value that is repeated in both sets of data.

The relative distance M_k moved by the mobile device k in the elapsed time Δt may be determined using the following relationship:

The relative speed of movement of the mobile device k is calculated as follows: Vik At

If required, an estimated orientation θ_ik of the mobile device k relative to the reference device i is found as follows:

Turning to Figure 6, there is shown a block diagram representation of one embodiment of an apparatus for estimating the relative speed of a reference device and a mobile device in a mobile ad-hoc network.

The relative speed/orientation estimation apparatus 400 is provided in a mobile station in a wireless ad-hoc network. The apparatus 400 has the following structure. A ranging unit 401 is provided for carrying out periodical ranging to neighbouring mobile stations. The ranging determines the distance between the mobile station and a neighbouring station using existing technologies to analyse the signals received from the neighbouring station, such as ToA. A ranging information cache 402 is provided, which is electrically connected to the ranging unit 401 for temporary storage of the measured ranging information. A ranging information analyzer 403 is provided, which is electrically connected to the ranging information cache 402 for retrieving ranging information associated to a mobile station whose speed/orientation is to be estimated and for making the decision on which method of the ranging-based method for relative speed estimation should be utilized. In particular, the analyzer will determine if the relative speed of one of the neighbouring mobile stations is known, in which case the general method outlined above and shown in Figures 4 and 5 is employed. If no relative speed data relating to a neighbouring mobile station are known, the analyzer will first initiate the general method described above and shown in Figure 2. An estimator A unit 404 is provided, which is electrically connected to the ranging information analyzer 403 for receipt of ranging information from the ranging information analyzer 403 in order to estimate the relative speed and orientation of a neighbouring mobile station using the ranging-based method for relative speed estimation in accordance with the principle of the present invention and as illustrated in Figure 2.

An estimator B unit 405 is also provided, which is electrically connected to the ranging information analyzer 403 for receipt of ranging information from the ranging information analyzer 403 and for receipt of a speed estimate of another neighboring mobile station in order to estimate the relative speed and orientation of a neighboring mobile station using the ranging-based method for relative speed estimation in accordance with the principle of the present invention as illustrated in Figures 4 and 5, in the case that the relative speed of at least one neighbouring mobile station is known.

A timer 406 is provided, which is electronically connected to the ranging unit 401 and the ranging information analyzer 403 for feeding timing information to the ranging unit 401 for periodical ranging and to the ranging information analyzer 403 for periodical speed/orientation estimation.

Operations of the above relative speed and orientation apparatus 400 will now be described.

The timer 406 constantly generates timing information, which is then fed into the ranging unit 401 and the ranging information analyzer 403. The ranging unit 401 periodically performs ranging to every mobile station in the radio coverage area and stores the measured ranging information in the ranging information cache 402. The ranging information analyzer 403 periodically retrieves ranging information associated with a mobile station whose relative speed/orientation is to be estimated and analyzes the retrieved ranging information in accordance with the principles of the present invention in order to determine which ranging-based method for estimating relative speed should be used. The estimator A unit 404 calculates the relative speed of a neighbouring mobile station on the basis of the cached ranging information to the station utilizing the ranging-based method for relative speed estimation in the case that the relative speed of movement of a first mobile station is to be determined. The estimator B unit 405 calculate the relative speed of a neighbouring mobile station on the basis of the cached ranging information to the station and that to another mobile station in the radio coverage utilizing the ranging- based method for relative speed estimation using the known data relating to the speed of movement of at least one of the neighbouring mobile stations.

Examples

To evaluate the performance of the methods of the present invention, an ns-2 simulator was employed to simulate a network scenario. The network scenario was a mobile ad hoc network consisting of 40 nodes or mobile devices moving within a square area 1400 m by 1400 m, to represent a sparse network topology, and within a square area 800 m by 800 m, to represent a dense topology. In both cases, the radius of the area of coverage of each node was set at 150 m.

The mobility of nodes within the network was modelled using Bonnmotion ('Bonnmotion: a mobility scenario generation and analysis tool', ex. University of Bonn, Germany). The simulation covered a wide range of nodal mobilities, characterised by the Random Waypoint Model (RWP) (Camp, T. et al., 'A survey of mobility models for ad hoc network research', Wireless Communication & Mobile Computing (WCMC), Vol. 2, No. 5, pages 483 to 502, 2002). For the RWP model, the pause time was kept at 0 to produce continuous movement of the nodes, The movement speed of the mobile nodes was uniformly distributed between a minimum speed of 3.5 m/s and a maximum speed that ranged from 5 to 40 m/s.

In the simulation, each node collected all distance estimates within its one- hop vicinity, after which each node carried out a range-based relative velocity estimate for each of its passing neighbours using the method of the present invention, as illustrated in Figure 2, for a duration of 500 seconds starting after 1250 seconds of warm-up period to avoid speed decay. In the figures, each data point corresponds to a mean of 30 repeated experiments with different random seeds. The results of the relative speed estimates were compared with results obtained using the same simulation and a known velocity estimation method (Link Prediction Algorithm (LPA)₁ Qin, L. and Kunz, T., 'Increasing packet delivery ratio in DSR by link prediction', International Conference of Hawaii System Science, Pages 300 to 309, January 2003).

Figures 7 to 10 contain graphs of the estimated and actual inter-node relative velocities over a range of maximum node speeds of from 5 to 40 m/s.

Figure 7 shows a graph of the results obtained using perfect signal data, with no noise component in the case of a sparse network topology. The results show the method of the present invention in two modes. RVEs (t_p-t_e) is the method illustrated in Figure 2 and using the larger elapsed time of t_p - t_e, while RVEs (Δt) is the method illustrated in Figure 2 using the smaller elapsed time interval Δt. The results are plotted together with the actual relative velocities of the simulation and the results obtained using the known LPA method.

As can be seen, the methods of the present invention provided velocity estimates very close to the actual values. It will be noted that with the noise-free signal data, the method RVEs (t_p-t_e) performed marginally better than the method RVEs (Δt).

Figure 8 shows a graph of the corresponding results to those shown in Figure 7 in the case of a signal with noise. It can be seen that the known LPA method struggled to provide an accurate estimate of the node velocities with a noisy signal. In contrast, the methods of the present invention again provided estimates of node velocity that closely matched the actual values in the simulation. It will be noted that with the noisy signal data, the method RVEs (Δt) performed marginally better than the method RVEs (t_p-t_e).

Figure 9 shows a graph of the results obtained using perfect signal data, with no noise component in the case of a dense network topology. The results show the method of the present invention in one mode. RVEd is the method illustrated in Figures 4 and 5, estimating the speed of movement of mobile nodes on the basis of the known estimated speed of a neighbouring node. The results are plotted together with the actual relative velocities of the simulation and the results obtained using the known LPA method.

As can be seen from Figure 9, the method of the present invention provided velocity estimates very close to the actual values.

Figure 10 shows a graph of the corresponding results to those shown in Figure 9 in the case of a signal with noise. It can be seen that the known LPA method again struggled to provide an accurate estimate of the node velocities with a noisy signal. In contrast, the method of the present invention again provided estimates of node velocity that closely matched the actual values in the simulation.

In summary, the results of the simulation demonstrate that the methods of the present invention for estimating the relative speed of mobile devices in a network can provide accurate values even in the case where the signal contains significant noise.

Claims

1. A method of estimating the speed of movement of a mobile device of a wireless communication system, the method comprising the steps of: a) determining the distance of the mobile device from a reference device at a first time instant; b) determining the distance of the mobile device from the reference device at a second time instant; c) calculating the distance travelled by the mobile device between the first time instant and the second time instant; and d) estimating the speed of movement of the mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant.

2. The method according to claim 1 , wherein the reference device is stationary.

3. The method according to claim 1 , wherein the reference device is a mobile device, the method estimating the relative speed of movement of the two mobile devices.

4. The method according to any preceding claim, wherein the distance of the mobile device from the reference device is determined using received signal strength (RSSI), time-of-arrival (ToA) or time-difference-of-arrival (TDoA).

5. The method according to any preceding claim, wherein the distance travelled by the mobile device between the first time instant and the second time instant is determined using the perpendicular location.

6. The method according to claim 5, wherein one of the first or second time instants is selected so as to be the time that the mobile device is at the perpendicular location or close thereto.

7. The method according to claim 5, wherein the first time instant is before the mobile device has reached the perpendicular location and the second time instant is after the mobile device has reached the perpendicular location, the perpendicular location being determined by interpolation.

8. The method according to claim 5, wherein both the first and second time instants are either before or after the mobile device has reached the perpendicular location, the perpendicular location being determined by extrapolation.

9. The method according to any of claims 1 to 6, wherein in step (b), a determination is made whether the mobile device has passed the perpendicular location.

10. The method according to claim 9, wherein, if the mobile device has not passed the perpendicular location, steps (a) and (b) are repeated until at the second time instant the mobile device is past the perpendicular location.

11. The method according to any preceding claim, wherein the time elapsed between step (a) and step (b) is dependent upon the position of the mobile device with respect to the reference device.

12. The method according to claim 11 , wherein the time elapsed between step (a) and step (b) is increased as the distance between the reference device and the perpendicular location decreases.

13. The method according to any preceding claim, wherein the system comprises a reference device and a plurality of mobile devices.

14. The method according to claim 13, wherein the speed of movement of a first mobile device is estimated, the estimated speed being used to estimate the speed of a second mobile device.

15. The method according to claim 14, wherein the first mobile device is past the perpendicular location for the first mobile device.

16. The method according to any of claims 13 to 15, wherein step (a) comprises: (i) determining the distance of a first mobile device from a reference device at a first time instant; and

(ii) determining the distance of a second mobile device from the reference device at the first time instant; and step (b) comprises: (i) determining the distance of the first mobile device from the reference device at a second time instant; and

17. A method of estimating the speed of movement of a mobile device of a wireless communication system, the method comprising the steps of: a) determining the distance of the mobile device from a reference device at a first time instant; b) determining the distance of the mobile device from the reference device at a second time instant; c) determining if the mobile device has passed the perpendicular location at the second time instant; if the mobile device has not passed the perpendicular location, repeating step (b) until the perpendicular location has been reached or passed; d) calculating the distance travelled by the mobile device between the first time instant and the second time instant; and e) estimating the speed of movement of the mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant.

18. A method of estimating the speed of movement of a plurality of mobile devices of a wireless communication system, the method comprising the steps of: a) determining the distance of a first mobile device from a reference device at a first time instant; b) determining the distance of the first mobile device from the reference device at a second time instant; c) determining if the first mobile device has passed the perpendicular location at the second time instant; if the mobile device has not passed the perpendicular location, either repeating step (b) until the perpendicular location has been reached or passed or carrying out steps (a) and (b) in respect of further mobile devices until a device having passed the perpendicular location for that device by the second time instant; d) calculating the distance travelled by the first mobile device between the first time instant and the second time instant; e) estimating the speed of movement of the first mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant;

T) determining the distance of both the first and second mobile devices from the reference device at a third time instant; e) determining the distance of both the first and second mobile devices from the reference device at a fourth time instant; T) calculating the distance travelled by the first mobile device between the third time instant and the fourth time instant from the estimated speed of movement of the first mobile device; g) calculating the distance travelled by the second mobile device between the third time instant and the fourth time instant using the distance calculated in step (T); and h) calculating the speed of movement of the second mobile device from the distance calculated in step (g) and the time elapsed between the third time instant and the fourth time instant.

19. An apparatus for estimating the speed of movement of a mobile device of a

_, wireless communication system, the apparatus comprising: a) means for determining the distance of the mobile device from a reference device at a plurality of time instants; b) means for calculating the distance travelled by the mobile device between a first and a second time instant; and c) means for estimating the speed of movement of the mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant.

20. The apparatus according to claim 19, wherein the reference device is stationary.

21. The apparatus according to claim 19, wherein the reference device is a

. mobile device.

22. The apparatus according to any of claims 19 to 21 , wherein the means for determining the distance of the mobile device from the reference device uses received signal strength (RSSI), time-of-arrival (ToA) or time-difference-of- arrival (TDoA).

23. The apparatus according to any of claims 19 to 22, wherein the means for calculating the distance travelled by the mobile device between the first time instant and the second time instant uses the perpendicular location for the mobile device.

24. The apparatus according to claim 23, further comprising means for determining the perpendicular location of the mobile device.

25. The apparatus according to claim 24, wherein the means for determining the perpendicular location of the mobile device is able to derive the perpendicular location by extrapolation and/or interpolation.

26. The apparatus according to claim 24, further comprising means for triggering the means for determining the distance of the mobile device from a reference device at a plurality of time instants to repeat its tasks if the means for determining the perpendicular location of the mobile device determines that the mobile device has not passed the perpendicular location.

27. The apparatus according to any of claims 19 to 26, further comprising means to adjust the time period that elapses between successive determinations of the distance between the mobile device and the reference device.

28. The apparatus according to claim 27, wherein the said means are responsive to data relating to the position of the mobile device relative to the. reference device.

29. The apparatus according to any of claims 19 to 28, further comprising means to estimate the speed of movement of a second mobile device, the said means using the data produced by means (c).

30. An apparatus for estimating the speed of movement of a mobile device of a wireless communication system, the apparatus comprising: a) means for determining the distance of the mobile device from a reference device at a plurality of time instants; b) means for determining if the mobile device has passed the perpendicular location at a given time instant; c) means for triggering means (a) if the mobile device has not passed the perpendicular location until the perpendicular location has been reached or passed; d) means for calculating the distance travelled by the mobile device between a first time instant and a second time instant; and e) means for estimating the speed of movement of the mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant.

31. A wireless communication network comprising an apparatus according to any of claims 19 to 30.

32. The wireless communication network according to claim 31 , wherein the network comprises a plurality of mobile devices.

33. An apparatus for estimating the speed of movement of a plurality of mobile devices of a wireless communication system, the apparatus comprising: a) means for determining the distance of a first mobile device from a reference device at a plurality of time instants; b) means for determining if the first mobile device has passed the perpendicular location at a given time instant; c) means for triggering means (a) if the mobile device has not passed the perpendicular location; d) means for calculating the distance travelled by the first mobile device between a first time instant and a second time instant; e) means for estimating the speed of movement of the first mobile device from the calculated distance and the time elapsed between the first time instant and the second time instant; f) means for calculating the distance travelled by the first mobile device between a third time instant and a fourth time instant from the estimated speed of movement of the first mobile device; g) means for calculating the distance travelled by the second mobile device between the third time instant and the fourth time instant using the distance calculated by means (f); and h) means for calculating the speed of movement of the second mobile device from the distance calculated in means (g) and the time elapsed between the third time instant and the fourth time instant.

34. An apparatus for estimating the speed of movement of a mobile device of a wireless communication system substantially as hereinbefore described having reference to the accompanying figures.

35. A wireless communication system comprising an apparatus according to either of claims 33 or 34.