TECHNICAL FIELD

The present disclosure relates to geo-location of wireless devices, and in particular to a method and system for the geo-location of wireless local area network (WLAN) devices.

BACKGROUND

Initially, it is noted that IEEE Standard 802.11-2016 is used as the base reference for disclosures used herein, the entire contents of which are incorporated herein by reference. The IEEE 802.11-2016 Standard is commonly referred to as “Wi-Fi” and is referred to as such herein.

Location of wireless devices can be determined by various methods. These methods may be classified as active, passive and combined active and passive. In an active location scheme, a device that is determining the location or range, the measuring device, transmits certain packets, referred to as “ranging packets”, to a device being located, i.e., the target device. A common method is to measure the time of arrival (TOA) of a response packet from the target device and compare that to a time of departure (TOD) of the ranging packet that was transmitted by the measuring device so as to determine the round trip time (RTT).

In an active location scheme, the TOD may be measured for a ranging packet that is transmitted from the measuring station addressed to the target station. The TOA of the response from the target station at the measuring station is then also measured. If the turnaround time for the target station to receive the packet from the measuring station and to start to transmit the response is known, or is known to be a constant, then the time difference at the measuring station between the TOA and the TOD, minus the turnaround time at the target station will be directly proportional to twice the distance of the target station from the measuring station. For example, if the target station is a wireless device based upon IEEE 802.11 technology, and if the packet transmitted from the measuring station to the target station is a data packet, the response from the target station will normally be an acknowledgement (ACK) packet. If the packet transmitted from the measuring station to the target station is a control packet, for example, a request-to-send (RTS) packet, then the response from the target station will normally be a clear-to-send (CTS) packet. In these two examples, the turnaround time at the target station is defined in the IEEE 802.11 standard as the short interframe spacing (SIFS), which is a preset value. Hence, the time delay (td) between the measuring station and the target station, may be determined from the calculation td=(TOA−TOD−SIFS)/2, and the distance between the measuring station and the target station is then td*c, where c is the speed of light. This method of estimating the distance to a target station by measuring the TOD and TOA and accounting for the turnaround time is known.

FIG. 1is a diagram of a typical location system100which includes three measuring stations110a,110band110c(referred to collectively herein as “measuring stations” or “measuring receivers”110). The target station120may be a wireless device, such as, for example, an Access Point (AP) that is to be located by the three measuring stations110. The distance of the target station120from measuring station110ais D1130. The distance of the target station120from measuring station110bis D2140. The distance of the target station120from measuring station110cis D3,150. The round trip time, RTT1, determined from the calculation RTT=(TOA−TOD−SIFS), is measured for transmissions from measuring station110aand this can used to calculate the distance D1130using the formula D1=RTT1*c/2 where c is the speed of light. Similarly, RTT2and RTT3measurements result in the determination of distances D2140and D3150. The methods for calculating the location of target station120using the distances D1130, D2140and D3150are well known.

FIG. 2is a diagram of a location system where an airborne measuring station110is used. The airborne measuring station110is depicted being flown in a circular orbit200around a target station120. The distance of the target station120from the measuring station110, when the measuring station110is at position A201, is D4210. The distance of the target station120from the measuring station110, when the measuring station110is at position B202, is D5211. The distance of the target station120from the measuring station110, when the measuring station110is at position C203, is D6212. Unless the target station120is positioned at the center of the orbit200, the distances D4210, D5211, and D6212will not be equal. An operator may identify an area of interest220within which the target station120may be expected to be positioned, and hence may point a directional antenna in the airborne station110in the direction of that area220.

A reception window may be defined as the start and end times after the end of the transmission of each ranging signal. A reception window restricts the time that the measuring station120listens for the response signal. When using certain methods to detect the response signals, it is possible to falsely detect spurious signals as the response signal, and the use of a reception window may reduce the number of false detections and improve the overall performance.

The timing of the reception window may be related to distance of an area of interest220within which the target station120may be expected to be positioned. The values for the reception window may be entered by an operator based upon advanced knowledge of an area of interest220related to the expected range to the target device120, but, as discussed above with reference toFIG. 2, in the general sense, the range to the target device120may not be constant and may vary as the airborne measuring station110orbits the target station120. Furthermore, as the measuring station110is airborne at a certain altitude, the slant ranges from the airborne measuring station to the area of interest220are required which would require updated information on the altitude of the airborne measuring station110.

FIG. 3is an exemplary plot of the gain of a typical directional antenna. The antenna vertical gain plot310is displayed in dBs against direction. The plot is shown in polar form with the direction angle320and the maximum gain axis330. In this example, the maximum gain 300 is shown at the direction 90 degrees. In this example at about 110 degrees340the gain is at −3 db compared to the maximum gain 300 at 90 degrees. Similarly, at about 70 degrees350the gain is also at −3 db compared to the maximum gain 300 at 90 degrees. Hence the vertical beam width for this example antenna is about 40 degrees. Also, in this example, at about 122 degrees345the gain is at −12 db compared to the maximum gain 300 at 90 degrees and, at about 58 degrees355the gain is also at −12 db compared to the maximum gain 300 at 90 degrees. Hence the vertical −12 dB beam width for this example antenna is about 64 degrees. Such a directional antenna may be used in the airborne measuring station110. Furthermore, such a directional antenna may be mounted in a gimballed system such that it may be continuously pointed at the area of interest220.

A classic method to represent the relative gain of a directional antenna with angle, based upon the number of elements in the array, is the following equation:
AntGain=sin(nx)/nsin(x)  (1)

where n is the number of elements, and x is the angle from the center or boresight, where x=0 at the boresight of the antenna. The solving of equation (1) for any particular beam width can be expressed in terms of Euler's equation and binomial coefficients but, in order to simplify the calculations, it is common to use an approximation of De Moivre's theorem, as per the following equation:
AntGain=cosN(x)  (2)

where x is the angle from the center or boresight, and N can be calculated for any given beam width.

Using equation (2), at boresight x=0, AntGain=1, and the 3 dB beam width, at angles±Φ, is determined when AntGain=0.5, hence:
cosNΦ=0.5
OrNLog(cos Φ)=Log(0.5)
N=Log(0.5)/Log(cos Φ)  (3)

where the antenna beam width is 20. For example, for a beam width of ±10 degrees, Φ=10, N calculates to be 45.28.

Table 1 shows the values for N for half beam widths, Φ, varying from 10 to 45 degrees.

TABLE 1BW/2, ΦN1045.281519.992011.14257.05304.82353.47402.60452.00

SUMMARY

The present disclosure advantageously provides a method, an apparatus and a measuring station for the geo-location of wireless local area network (WLAN) devices.

In one aspect of the disclosure, a method for determining reception window timing using a measuring station is provided. The measuring station includes a directional antenna having a boresight and transmits signals to a target station. In addition, the measuring station receives response signals from the target station corresponding to the transmitted signals. The method includes receiving an antenna beam width 2Φ, receiving an antenna tilt angle θ, and receiving an altitude A, where A is an altitude of the measuring station above ground level. A far projection angle Δf is determined based at least upon a far signal strength at ground level. The far signal strength is a far predetermined fraction of a maximum signal strength at ground level, and the far predetermined fraction is based at least in part upon the beam width 2Φ and tilt angle θ. A near projection angle Δn is determined based at least upon a near signal strength at ground level. The near signal strength is a near predetermined fraction of the maximum signal strength at ground level, and the near predetermined fraction is based upon the beam width 2Φ and tilt angle θ. In addition, the method includes determining a far projection range, Rf, corresponding to the far projection angle Δf and based at least upon the values of Δf and A. A near projection range, Rn, corresponding to the near projection angle Δn and based at least upon the values of Δn and A is determined. An end time of a reception window, Tt, is determined based at least upon the value of the far projection range, Rf. The reception window is a window of time in which a response from the target station is expected to be received. The method further includes determining a start time of the reception window based at least upon the value of the near projection range, Rn.

In some embodiments of this aspect, the far projection angle Δf is determined from a formula given by (sin2(Δf)cosN(ψ))/(sin2(θ+ψmax)cosN(ψmax))=Fraction, where (ψ) is a deviation in degrees from the boresight of the directional antenna, (ψmax) is a deviation from the boresight of the directional antenna that corresponds to a maximum gain at the ground level, (θ+ψ) being a pitch angle, (θ+ψmax) is the pitch angle of the maximum antenna gain at the ground level, cosN(x) is an antenna gain at angle x, x is an angle from one of a center of the directional antenna and the boresight of the directional antenna, and Fraction is a predetermined fraction.

In some embodiments of this aspect, the near projection angle Δn is determined from a formula given by (sin2(Δn)cosN(ψ))/(sin2(θ+ψmax)cosN(ψmax))=Fraction, where (ψ) is a deviation in degrees from the boresight of the directional antenna, (ψmax) is a deviation from the boresight of the directional antenna that corresponds to a maximum gain at the ground level, (θ+ψ) is a pitch angle, (θ+ψmax) is the pitch angle of the maximum antenna gain at the ground level, cosN(x) is an antenna gain at angle x, x is an angle from one of a center of the directional antenna and the boresight of the directional antenna, and Fraction is a predetermined fraction.

In some embodiments of this aspect, the near predetermined fraction of the maximum signal strength at the ground level is 1/15.8, and the far predetermined fraction of the maximum signal strength at the ground level is 1/15.8. In another embodiment of this aspect, the far projection angle Δf is determined as Δf=(−14.83+1.08Φ−0.013Φ2)+(1.11−0.061Φ+0.00085Φ2)θ+(0.00069+0.00028Φ−0.0000057Φ2)θ2, where 2Φ is a beam width of the directional antenna. In some embodiments of this aspect, the near projection angle Δn is determined as Δn=(−6.35+3.48Φ−0.034Φ2)+(1.04−0.019Φ+0.0004Φ2)θ+(0.000156+0.000062Φ−0.0000027Φ2)θ2, where 2Φ is a beam width of the directional antenna.

In some embodiments of this aspect, the far projection range Rf is determined as:

Rf={Δf=0⁢°,RfmaxAsin⁢⁢(Δf)>Rfmax,Rfmax,Asin⁢⁢(Δf)⁢⁢where⁢⁢Δf={Δf<0⁢°,0⁢°Δf<θ,θ,

Rfmax being a maximum value for Rf derived from the time between successive transmitted ranging packets.

In some embodiments of this aspect, the near projection range Rn is determined as:

Rn={Δn>90⁢°,AAsin⁢⁢(Δn).

In some embodiments of this aspect, the reception window end time Tt is determined as:

Tt={Δf=0⁢°,Ttmax2⁢Rfc+tSIFS>Ttmax,Ttmax,2⁢Rfc+tSIFS
Ttmaxis a maximum value for Tt derived from a time between successive transmitted ranging packets, and tSIFSbeing a short interframe spacing time.

In some embodiments of this aspect, the maximum value for the reception window end time, Ttmaxis determined as Ttmax=(Tp−tp−tr), where Tp is the time between transmitted request packets, tp is the duration of the request packet, and tr is the duration of the response packet. In some embodiments of this aspect, a maximum value of the far projection range, Rfmax, is determined as Rfmax=c(Ttmax−tSIFS)/2, where c=speed of light, Ttmaxis a maximum value for Tt, and tSIFSbeing the short interframe spacing time. In some embodiments of this aspect, a reception window start time Ts, is determined as:

Ts={Δn>90⁢°,2⁢Ac+tSIFS2⁢Rnc+tSIFS,

where Rn=A/sin (Δn), c=speed of light, and tSIFSbeing the short interframe spacing time.

In another aspect of the disclosure, an apparatus for determining reception window timing using a measuring station is provided. The measuring station includes a directional antenna having a boresight and transmits signals to a target station. The measuring station receives response signals from the target station corresponding to the transmitted signals. The apparatus includes processing circuitry in communication with the apparatus. The processing circuitry is configured to receive an antenna beam width 2Φ, receive an antenna tilt angle θ, and receive an altitude A, where A is the altitude of the measuring station above ground level. The processing circuitry is further configured to determine a far projection angle Δf based at least upon a far signal strength at ground level, where the far signal strength is a far predetermined fraction of a maximum signal strength at ground level, and the far predetermined fraction is based at least in part upon the beam width 2Φ and tilt angle θ. In addition, the processing circuitry is configured to determine a near projection angle Δn based at least upon a near signal strength at ground level, where the near signal strength is a near predetermined fraction of the maximum signal strength at ground level, and the near predetermined fraction is based upon the beam width 2Φ and tilt angle θ. The processing circuitry is further configured to determine a far projection range, Rf, corresponding to the far projection angle Δf and based upon the values of Δf and A, and determine a near projection range, Rn, corresponding to the near projection angle Δn and based at least upon the values of Δn and A. Additionally, the processing circuitry is configured to determine an end time of a reception window, Tt, based at least upon the value of the far projection range, Rf, where the reception window is a window of time in which a response from the target station is expected to be received. The processing circuitry is further configured to and determine a start time of the reception window based at least upon the value of the near projection range, Rn.

In some embodiments of this aspect, the far projection angle Δf is determined from the formula (sin2(Δf)cosN(ψ))/(sin2(θ+ψmax)cosN(ψmax))=Fraction, where (ψ) is a deviation in degrees from the boresight of the directional antenna, (ψmax) is a deviation from the boresight of the directional antenna that corresponds to a maximum gain at the ground level, (θ+ψ) is a pitch angle, (θ+ψmax) is the pitch angle of the maximum antenna gain at the ground level, cosN(x) is an antenna gain at angle x, x being an angle from one of a center of the directional antenna and the boresight of the directional antenna, and Fraction is a predetermined fraction.

In some embodiments of this aspect, the near projection angle Δn is determined from the formula (sin2(Δn)cosN(ψ))/(sin2(θ+ψmax)cosN(ψmax))=Fraction, where (ψ) is a deviation in degrees from the boresight of the directional antenna, (ψmax) is a deviation from the boresight of the directional antenna that corresponds to a maximum gain at the ground level, (θ+ψ) is a pitch angle, (θ+ψmax) is the pitch angle of the maximum antenna gain at the ground level, cosN(x) is an antenna gain at angle x, x is an angle from one of a center of the directional antenna and the boresight of the directional antenna, and Fraction is a predetermined fraction.

In some embodiments of this aspect, the near predetermined fraction of the maximum signal strength at the ground level is 1/15.8, and the far predetermined fraction of the maximum signal strength at the ground level is 1/15.8. In some embodiments of this aspect, the far projection angle Δf is determined as Δf=(−14.83+1.08Φ−0.013Φ2)+(1.11−0.061Φ+0.00085Φ2)θ+(0.00069+0.00028Φ−0.0000057Φ2)θ2, 2Φ being a beam width of the directional antenna. In some embodiments of this aspect, the near projection angle Δn is determined as Δn=(−6.35+3.48Φ−0.034Φ2)+(1.04−0.019Φ+0.0004Φ2)θ+(0.000156+0.000062Φ−0.0000027Φ2)θ2, 2Φ is a beam width of the directional antenna.

In some embodiments of this aspect, the far projection range Rf is determined as:

Rf={Δf=0⁢°,Rf⁢⁢maxAsin⁡(Δf)>Rf⁢⁢max,Rf⁢⁢max,Asin⁡(Δf)⁢⁢where⁢⁢Δf={Δf<0⁢°,0⁢°Δf<θ,θ,
and Rfmax is a maximum value for Rf derived from the time between successive transmitted ranging packets.

In some embodiments of this aspect, the near projection range Rn is determined as:

Rn={Δn>90⁢°,AAsin⁡(Δn).

In some embodiments of this aspect, the reception window end time Tt, is determined as:

Tt={Δf=0⁢°,Tt⁢⁢max2⁢Rfc+tSIFS>Tt⁢⁢max,Tt⁢⁢max,2⁢Rfc+tSIFS⁢⁢where⁢⁢Δf={Δf<0⁢°,0⁢°Δf<θ,θ,⁢Rf=A/sin⁡(Δ⁢⁢f),⁢c=speed⁢⁢of⁢⁢light,
Ttmaxis a maximum value for Tt derived from a time between successive transmitted ranging packets, and tSIFSis a short interframe spacing time.

In some embodiments of this aspect, the reception window start time Ts, is determined as:

Ts={Δn>90⁢°,2⁢Ac+tSIFS2⁢Rnc+tSIFS,

where Rn=A/sin (Δn), c=speed of light, and tSIFSis the short interframe spacing time.

In yet another aspect of the disclosure, a measuring station for determining reception window timing is provided. The measuring station includes a directional antenna in communication with a transmitter receiver. The measuring station includes the transmitter receiver that is configured to transmit radio frequency (RF) signals, including RF signals to a target station, and receive RF signals, including RF signals from a target station corresponding to the transmitted RF signals. The measuring station further includes a processing circuitry configured to receive an antenna beam width 2Φ, receive an antenna tilt angle θ, and receive an altitude A, where A is the altitude of the measuring station above ground level. The processing circuitry is further configured to determine a far projection angle Δf based at least upon a far signal strength at ground level, where the far signal strength is a far predetermined fraction of a maximum signal strength at ground level, and the far predetermined fraction is based at least in part upon the beam width 2Φ and tilt angle θ. In addition, the processing circuitry is configured to determine a near projection angle Δn based at least upon a near signal strength at ground level, where the near signal strength is a near predetermined fraction of the maximum signal strength at ground level, and the near predetermined fraction is based upon the beam width 2Φ and tilt angle θ. The processing circuitry is further configured to determine a far projection range, Rf, corresponding to the far projection angle Δf and based at least upon the values of Δf and A, and determine a near projection range, Rn, corresponding to the near projection angle Δn and based at least upon the values of Δn and A. Additionally, the processing circuitry is configured to determine an end time of a reception window, Tt, based at least upon the value of the far projection range, Rf, where the reception window is a window of time in which a response from the target station is expected to be received. The processing circuitry is further configured to determine a start time of the reception window based at least upon the value of the near projection range, Rn.

