Radio Frequency Distance Determination

Abstract

A method for determining the distance between a radio frequency device (2) and a target (18) is disclosed, wherein the radio frequency device (2) receives a radio frequency signal (16) from the target (18). The method includes: determining a time-domain channel response from the received radio frequency signal (16); determining the amplitude of the maximum peak in the time-domain channel response; determining the amplitude of the second earlier peak in the time-domain channel response; comparing the second peak amplitude with a threshold based on the maximum peak amplitude; if the second peak amplitude is less than the threshold, then identifying the maximum peak as the shortest path peak; if the second peak amplitude is less than the threshold, then identifying the second peak as the shortest path peak; and calculating the distance between the radio frequency device (2) and the target (18) based on the time corresponding to the shortest path peak.

Description

Technical Field

[0001] The present invention relates to a method for determining the distance between a radio frequency device and a target. Background Art

[0002] It is often useful to estimate the distance between two communicating radio frequency nodes (e.g., between two RF transceivers, or between an RF transceiver and a passive RF reflector). For example, estimating the distance between two radio frequency devices can be used to trigger proximity-based actions on one or both devices (e.g., triggering an alarm if the two devices get too close or too far apart). This distance can be determined by analyzing the radio frequency signals propagated between the two nodes.

[0003] Traditional methods for distance estimation using RF signals involve phase-based ranging techniques, where a radio frequency device transmits a radio signal that returns from a target and is then received at the radio frequency device. The target can be a passive target, such as a wall, or an active target, such as a second device arranged to receive and retransmit incoming signals. The RF device measures the phase difference φ between the transmitted and received signals, which depends on the distance d between the device and the target according to the following formula:

[0004]

[0005] where c is the speed of light and f is the frequency of the transmitted signal. By transmitting multiple signals with different frequencies (e.g., by sequentially hopping between a series of carrier frequencies), the distance between the transmitter and the target can be determined by examining the phase difference as a function of the frequencies of the multiple signals:

[0006]

[0007] where φ i is the phase difference between the transmitted signal with frequency f i and the returned signal. Thus, the distance d between the transmitter and the target can be determined by plotting the phase difference against the frequency and measuring the gradient This technique is known as Multi Carrier Phase Difference (MCPD) ranging.

[0008] However, this method relies on the assumption that the received signal follows the shortest path between the transmitter and the target. However, in a real-world environment, RF signals follow many different paths between the transmitter and the target (and then back again) (e.g., bouncing off walls, floors, and ceilings). Although the longer distances that the signals travel on these alternative paths tend to attenuate their strength relative to the signals following the direct (shortest) path, they can still cause uncertainty in the phase difference measurement, thereby reducing the accuracy of the determined distance. Additionally, signals traveling along different paths can interfere, causing the received signal strength to increase and decrease with frequency (multipath effect), thereby further reducing the accuracy of distance measurements using traditional MCPD methods. Another method may be needed. Summary of the Invention

[0009] In a first aspect, the present invention provides a method for determining the distance between a radio frequency device and a target, the method comprising:

[0010] The radio frequency device receives a radio frequency signal from the target;

[0011] Determine the time-domain channel response from the received radio frequency signal;

[0012] Determine the amplitude of the maximum peak in the time-domain channel response;

[0013] Determine the amplitude of the second-earliest peak in the time-domain channel response;

[0014] Compare the second peak amplitude with a threshold based on the maximum peak amplitude;

[0015] If the second peak amplitude is less than the threshold, then identify the maximum peak as the shortest path peak;

[0016] If the second peak amplitude is greater than the threshold, then identify the second peak as the shortest path peak; and

[0017] Calculate the distance between the radio frequency device and the target based on the time corresponding to the shortest path peak.

[0018] Thus, those skilled in the art will recognize that even if, for example, the multipath interference effect reduces the amplitude of the peak corresponding to the shortest round-trip path, or if two or more peaks generated by multiple reflection paths combine to provide higher energy than the true signal associated with the shortest round-trip path, the distance between the radio frequency device and the target can still be accurately determined. Thus, the applicant recognizes that the maximum peak may not always be correct, and thus according to the present invention, if a second smaller peak that appears earlier (i.e., corresponds to a shorter round-trip path) in the time-domain channel response meets the threshold requirements, it can still be identified as the shortest path peak.

[0019] Although the time domain channel response (TDCR) may also include many other peaks that do not correspond to different physical paths (e.g., peaks that occur due to noise or due to the inherent sidelobes of the peaks in a bandwidth-limited TDCR), by making the threshold based on the maximum peak amplitude, these can be ignored to allow identification of shorter true shortest path peaks (if any).

[0020] In some embodiments, the method includes only a one-way transmission of a signal, such as where a radio frequency signal received from a target is not based on a signal previously transmitted from a first radio frequency device and timing or phase information is obtained in another way.

[0021] However, the method may also include a two-way transmission, so in a set of embodiments, the radio frequency signal received from the target is a second radio frequency signal that is based on a first radio frequency signal previously transmitted by the radio frequency device to the target (i.e., the radio frequency signal is a returned radio frequency signal). In other words, the method may include the radio frequency device transmitting a first radio frequency signal and receiving a second radio frequency signal from the target that is based on the first radio frequency signal.

[0022] Accordingly, the present invention extends to a method of determining the distance between a radio frequency device and a target, the method comprising:

[0023] The radio frequency device transmits a first radio frequency signal;

[0024] The radio frequency device receives from the target a second radio frequency signal that is based on the first radio frequency signal;

[0025] Determine the time domain channel response based on the second radio frequency signal;

[0026] Determine the amplitude of the maximum peak in the time domain channel response;

[0027] Determine the amplitude of the second earliest peak in the time domain channel response;

[0028] Compare the second peak amplitude with a threshold based on the maximum peak amplitude;

[0029] If the second peak amplitude is less than the threshold, then identify the maximum peak as the shortest path peak;

[0030] If the second peak amplitude is greater than the threshold, then identify the second peak as the shortest path peak; and

[0031] Calculate the distance between the radio frequency device and the target based on the time corresponding to the shortest path peak.

[0032] It should be understood that the reference to a radio frequency signal refers to the radio frequency signal received by the radio frequency device and / or the first radio frequency signal transmitted by the radio frequency device (based on the received (second) radio frequency signal).

[0033] In a set of embodiments, the radio frequency signal (i.e., the radio frequency signal received and / or transmitted by the radio frequency device) includes multiple frequencies, for example, including several different frequency components (where several frequencies are transmitted simultaneously), and / or a series of radio frequency signals having different carrier frequencies (i.e., in different frequency channels), for example, transmitted according to a frequency hopping protocol.

[0034] The different frequencies of the radio frequency signal (i.e., the radio frequency signal received and / or transmitted by the radio frequency device) may be substantially evenly distributed over the bandwidth of the radio frequency signal (the bandwidth is the frequency range between the highest and lowest frequencies of the radio frequency signal). For example, the target and / or the radio frequency device may transmit radio frequency signals in multiple adjacent or nearly adjacent frequency channels (e.g., the channel spacing is 1 MHz or 2 MHz). The target and / or the radio frequency device may rapidly and continuously transmit multiple signals with different frequencies, for example, changing the frequency up to 1600 times per second.

[0035] The radio frequency signal may include a bandwidth of at least 10 MHz, at least 50 MHz, or even up to 100 MHz or higher. In a set of embodiments, the radio frequency signal includes a series of radio frequency signals with a bandwidth of approximately 74 MHz.

[0036] The radio frequency signal (i.e., the radio frequency signal received and / or transmitted by the radio frequency device) may include a modulated signal, i.e., including information encoded in the carrier. In such embodiments, the bandwidth of the radio frequency signal includes the bandwidth of the carrier. For example, the radio frequency signal may include a signal transmitted according to a frequency hopping protocol using carrier frequency channels spaced 1 MHz or 2 MHz apart between 2404 and 2478 MHz. Not all frequency channels within the bandwidth may be used.

[0037] The target may include a second radio frequency device that generates and transmits radio frequency signals.

[0038] However, as described above, in a set of embodiments, the radio frequency signal received from the target is a second radio frequency signal based on a first radio frequency signal transmitted by the radio frequency device to the target. In some such embodiments, the target may include a passive reflector, such as a reflective surface (e.g., a wall), such that the second radio frequency signal includes a reflection of the first radio frequency signal. In other such embodiments, the target may include an active transmitter, in which case the second radio frequency signal includes a retransmitted signal based on the first radio frequency signal, e.g., including the same frequency and / or phase (e.g., by synchronizing a local oscillator with the received first RF signal). In such embodiments, the retransmitted second radio frequency signal may have the same phase as the incoming signal or a known phase offset from the incoming signal. The second radio frequency signal may include an amplified version of the first RF signal. Alternatively, the second radio frequency signal may contain only information about the first radio frequency signal (e.g., identifying characteristics of the first radio frequency signal, such as amplitude or phase, when the first radio frequency signal is received at the target).

[0039] The time domain channel response (TDCR) can be determined using the frequency domain channel response (FDCR) of the received radio frequency signal, i.e., a representation including the relative amplitudes and phases of the signals in different frequency channels. For example, the incoming radio frequency signal can be Fourier transformed (e.g., using the fast Fourier transform method) to produce the FDCR. The FDCR can include the output of a multi-carrier phase ranging (MCPD) method applied to the radio frequency signal. Then, the FDCR can be inverse Fourier transformed to produce the TDCR, e.g., using the inverse fast Fourier transform (IFFT) method.

[0040] The time domain channel response can include a discrete time domain channel response, which includes a plurality of discrete time exponents (e.g., at least 100, 500, 1000, 2000 or more time exponents). In one embodiment, the time domain channel response includes 2048 time points, e.g., determined using a 2048-point IFFT.

[0041] In a set of embodiments, the threshold corresponds to an amplitude greater than the amplitude of the sidelobe of the maximum peak. In a bandwidth-limited TDCR, each path may appear as a sine function, having a main peak and several adjacent sidelobe peaks, and the relative amplitude of the sidelobe peaks is determined by the shape of the sine function. The first sidelobe peak (i.e., the secondary peak) in a perfect sine function has an amplitude of -13.3 dB relative to the main peak. In some embodiments, the threshold includes an amplitude greater than -13.3 dB (about 5%) relative to the amplitude of the maximum peak. In a set of embodiments, the threshold includes an amplitude of -10 dB (10%) relative to the amplitude of the maximum peak (e.g., taking into account noise and other defects).

[0042] The distance between the RF device and the target can be calculated by determining the distance traveled by the RF signal within the time t corresponding to the peak of the shortest path (i.e., the location where the shortest path is in the time-domain channel response). sp In other words, the distance d between the RF device and the target can be determined according to the following formula:

[0043] d = t sp ·c, (3)

[0044] Alternatively, in the case of using a discrete time-domain channel response:

[0045]

[0046] where n sp is the time point where the peak of the shortest path is located, N is the total number of time points, and Δf is the minimum interval of frequencies in the received signal.

[0047] However, in some embodiments, the time of the peak of the shortest path may not accurately represent the shortest path between the RF device and the target. For example, if the signal takes an alternative path that is only slightly longer than the shortest path and its amplitude is comparable to that of the signal following the shortest path, then the peak of the shortest path appearing in the time-domain channel response may actually represent two (or more) overlapping peaks, where only one actually corresponds to the shortest path. For example, in a bandwidth-limited TDCR with a bandwidth BW, the Rayleigh criterion (representing the minimum time interval between two paths that can be resolved separately in the TDCR) is equal to Peaks closer than this may overlap, thus blurring their respective positions. The Rayleigh criterion corresponds to the expected time interval between a peak and its nearest local minimum. In such cases, using the time of the peak of the shortest path to determine the distance may be inaccurate.

[0048] Therefore, in a set of embodiments, it further includes:

[0049] Identify the nearest local minimum in the time-domain channel response that is earlier than the shortest-path peak;

[0050] Determine the time interval between the nearest local minimum and the shortest-path peak;

[0051] Compare the time interval with the expected time interval based on the bandwidth of the received RF signal; and

[0052] If the time interval is greater than the expected time interval, then calculate the time corresponding to the shortest-path peak as the time corresponding to the nearest local minimum plus the expected time interval.

[0053] In other words, the method may include checking whether the nearest local minimum before the shortest-path peak is located at the time expected for a single peak in the bandwidth-limited time-domain channel response, or whether it is located at another position indicating that the shortest-path peak is actually composed of two (or more) overlapping peaks. If the interval between the nearest local minimum and the shortest-path peak is longer than expected, then a corrected time for distance determination is calculated by adding the expected time interval to the time of the local minimum (since this is where the corresponding peak of the minimum should be located), which is closer to the actual time of the shortest path between the RF device and the target.

[0054] The expected time interval may include the Rayleigh criterion of the TDCR, equal to In the discrete time-domain channel response, the expected time-exponential interval may be equal to:

[0055]

[0056] Thus, in some such embodiments, the time corresponding to the shortest-path peak to be used for determining the distance between the RF device and the target can be calculated as:

[0057]

[0058] Such that the distance d between the RF device and the target (using Equation 4) is given by:

[0059]

[0060] The nearest local minimum in the time-domain channel response that is earlier than the shortest-path peak may include the nearest time (e.g., the nearest time index in the discrete TDCR) in the TDCR where the amplitude is less than the amplitudes at the immediately preceding and immediately following times (i.e., the strict mathematical definition of a local minimum). For the discrete TDCR, this can be expressed as the nearest time in the TDCR as follows:

[0061] h mag [n locmin -1]>h mag[n locmin < h mag [n locmin + 1], (8)

[0062] where h mag [n] is the TDCR amplitude at time index n.

[0063] However, in some cases, noise and / or other defects can create spurious small variations in the TDCR, including spurious local minima. In such cases, using a strict mathematical definition of local minima can lead to errors. In one set of embodiments, the nearest local minimum can be identified from a smoothed version of the TDCR (e.g., a moving average of the TDCR) to reduce the impact of random noise. Additionally or alternatively, if the local minimum in the TDCR is not significant enough, i.e., if the depth of the local minimum in the TDCR is less than a threshold (e.g., a threshold defined relative to the amplitude of the maximum peak), then the local minimum can be ignored.

[0064] In some embodiments, the nearest local minimum in the time-domain channel response that is earlier than the shortest-path peak can be defined by a heuristic definition that takes into account noise and other defects. For example, the following criteria can be used to identify the nearest local minimum:

[0065] a) h mag [n locmin < h mag [n sp · T1, where T1 < 1.0, e.g., T1 = 0.5;

[0066] b) h mag [n locmin < h mag [n locmin - 1] · T2, where T2 > 1.0, e.g., T2 = 1.1;

[0067] c) h mag [n locmin < h mag [n sp · T3, where T3 >> 1.0, e.g., T3 = 10.0.

[0068] where the nearest local minimum is the nearest earlier time in the TDCR that satisfies both condition a) and b) or satisfies condition c).

[0069] This distance between the RF device and the target can be determined by the RF device itself, e.g., by a processor of the RF device. However, in some embodiments, one or more steps can be performed on another device (e.g., by a processor of an active reflector, or by a remote server).

[0070] According to a second aspect, the present invention provides a radio frequency transceiver device, the radio frequency transceiver device being arranged to:

[0071] Receive a radio frequency signal from a target;

[0072] Determine a time-domain channel response from the radio frequency signal;

[0073] Determine the amplitude of the maximum peak in the time-domain channel response;

[0074] Determine the amplitude of the second earlier peak in the time-domain channel response;

[0075] Compare the second peak amplitude with a threshold based on the maximum peak amplitude;

[0076] If the second peak amplitude is less than the threshold, then identify the maximum peak as the shortest path peak;

[0077] If the second peak amplitude is greater than the threshold, then identify the second peak as the shortest path peak; and

[0078] Calculate the distance between the radio frequency device and the target based on the time corresponding to the shortest path peak.

[0079] This aspect of the present invention extends to a radio frequency transceiver device, the radio frequency transceiver device being arranged to:

[0080] Transmit a first radio signal;

[0081] Receive a second radio frequency signal from a target based on the first radio frequency signal;

[0082] Determine a time-domain channel response from the second radio frequency signal;

[0083] Determine the amplitude of the maximum peak in the time-domain channel response;

[0084] Determine the amplitude of the second earlier peak in the time-domain channel response;

[0085] Compare the second peak amplitude with a threshold based on the maximum peak amplitude;

[0086] If the second peak amplitude is less than the threshold, then identify the maximum peak as the shortest path peak;

[0087] If the second peak amplitude is greater than the threshold, then identify the second peak as the shortest path peak; and

[0088] Calculate the distance between the radio frequency device and the target based on the time corresponding to the shortest path peak.

[0089] Features of any aspect or embodiment described herein may, where appropriate, be applied to any other aspect or embodiment described herein. When referring to different embodiments, it should be understood that these embodiments are not necessarily different but may overlap. BRIEF DESCRIPTION OF THE DRAWINGS

[0090] One or more embodiments will now be described by way of example only and with reference to the drawings, in which:

[0091] Figure 1 is a schematic diagram of an arrangement for an embodiment of the present invention;

[0092] Figure 2 is a schematic diagram of another arrangement for an embodiment of the present invention;

[0093] Figure 3 is a flowchart illustrating a method according to an embodiment of the present invention;

[0094] Figure 4 shows the simulated time-domain channel response of a signal transmitted between an RF device and a target without multipath effects;

[0095] Figure 5 illustrates the time-domain channel response of a signal transmitted between an RF device and a target with multipath effects;

[0096] Figures 6 to 8 illustrates the time-domain channel response containing two incoming paths of equal intensity with various different spacings;

[0097] Figure 9 is a flowchart illustrating the steps of a method according to an embodiment of the present invention;

[0098] Figure 10 is an enlarged view of the time-domain channel response;

[0099] Figure 11 is a simulated time-domain channel response for another embodiment of the present invention;

[0100] Figure 12 and Figure 13 are simulated results illustrating the improved distance estimation performance of an embodiment of the present invention; and

[0101] Figure 14 illustrates the phase difference of an example of two-way ranging. DETAILED DESCRIPTION

[0102] Figure 1 Shows a radio frequency (RF) device 2 including a transmitter 4, a receiver 8, and a processor 14. The RF device 2 is located at a distance r from a passive reflector 18.

[0103] In use, the transmitter 4 transmits a series of radio frequency signals 16 having different carrier frequencies (e.g., following a predetermined frequency hopping pattern). The radio frequency signals 16 are reflected by the reflector 18 and received by the receiver 8. The received radio frequency signals 16 can be used to estimate the distance r between the RF device 2 and the reflector 1, as described in more detail below with reference to Figures 3 to 8 which is described in more detail.

[0104] Figure 2 Another arrangement is shown, characterized in that the first RF device 102 is spaced from the second RF device 122 by a distance r. The first RF device 102 includes a transmitter 104, a receiver 108, and a processor 112. The second RF device 122 also includes a transmitter 124, a receiver 128, and a processor 132.

[0105] In use, the transmitter 104 of the first RF device 102 transmits a series of radio frequency signals 146 having different carrier frequencies (e.g., following a predetermined frequency hopping pattern). The radio frequency signals 146 are received by the receiver 128 of the second RF device 122 and then retransmitted by the transmitter 124 (i.e., acting as an active reflector). For example, the second RF device 122 may include a local oscillator locked to the frequency of the received radio frequency signal 146 (and thus locked to the same frequency as the local oscillator of the first RF device 102). The retransmitted (i.e., reflected) signal 146 is then received by the receiver 108 of the first RF device 102.

[0106] For Figure 1 the arrangement shown, the received radio frequency signals 146 can be used to estimate the distance r between the first device 102 and the second RF device 122, as described in more detail below with reference to Figures 3 to 8 which is described in more detail.

[0107] Reference will be made to Figures 3 to 5 to describe a method of determining the distance r between an RF device (e.g., Figure 1 the RF device 2 in Figure 2 or the first RF device 102 in Figure 1 and a target (e.g., Figure 2 the passive reflector 18 in Figures 6 to 10 or the second RF device 122 acting as an active reflector in

[0108] In this example, in step 150, the RF device transmits a series of RF signals (i.e., Figure 1 and Figure 2 the signals 16, 146 in

[0109] These signals are returned by the target and received back at the RF device in step 152. The received signals have a frequency-dependent amplitude and phase, which depend (among other things) on the distance r between the RF device and the target.

[0110] The received signals are analyzed (e.g., by processors 14, 112) to produce a frequency-domain channel response (FDCR), H(jω), which is a representation of the relative amplitude and phase of the received signals over different frequency channels. The FDCR can be produced by performing a Fourier transform on the time series of the received signals (e.g., using Fast Fourier Transform (FFT) techniques). The FDCR can be produced by performing a Multi-Carrier Phase Distancing (MCPD) procedure on the received signals. This enables one-way and two-way ranging.

[0111] Two-way ranging

[0112] In one example of two-way ranging (2WR), the local oscillators (LOs) of two RF devices (a starter and a reflector) are locked. Assume the LOs are not phase-locked, so there is a time offset ΔT, which results in a channel-dependent phase offset θ LO = ω LO Δ T . We can describe the instantaneous carrier phase of the starter's LO as ω LO t, and that of the reflector as ω LO (t + Δ T ).

[0113] The starter first sends a tone ω LO t, and the receiver gets ω LO t - ω LO r / c, then down-converts to baseband and measures the phase (step 1). In step 2, the reflector then sends ω LO (t + ΔT), and the starter measures the received baseband phase. It should be noted that during steps 1 and 2, the LOs remain on, but each party changes roles between the two. Then the devices can exchange the measured phase values (ψ I , ψ R ) via a data channel to perform distance measurement.

[0114] The 1st step Launcher TX RF Reflector RX RF Reflector RX BB <![CDATA[ω LO t]]> <![CDATA[ω LO t - ω LO r / c]]> <![CDATA[ψ R =-ω LO Δ T -ω LO r / c]]> The 2nd step Reflector TX RF Launcher RX RF Launcher RX BB <![CDATA[ω LO (t + Δ T )]]> <![CDATA[ω LO (t + Δ T ) - ω LO r / c]]> <![CDATA[ψ I = ω LO Δ T - ω LO r / c]]> The 3rd step <![CDATA[The reflector transmits ψ R > <![CDATA[The launcher calculates ψ = ψ R + ψ I > <![CDATA[-ω LO Δ T -2πr / c]]> <![CDATA[ψ = -2ω LO r / c]]>

[0115] Table 1 2WR Phase Measurement on Single Tone

[0116] This is shown in Table 1 and Figure 14 . We can see that if we sum the received phases, we get a function of the distance and the time offset between the devices is cancelled. Conversely, if we take the difference of the received phases, we get a function of the time offset between the devices.

[0117] We see that the phase measurements of the initiator and receiver (ψ I , ψ R ) are equivalently offset by the distance but offset in opposite directions due to the LO phase offset between the devices.

[0118] ψ I = ω LO Δ T - ω LO r / c

[0119] ψ R = -ω LO Δ T - ω LO r / c

[0120] So we get

[0121] φ dist = ψ I + ψ R = -2ω LO r / c

[0122] φ ΔT = ψ I - ψ R = -2ω LO Δ T

[0123] We can measure the distance and the time offset from this. In fact, due to the internal delay of each device, φ dist will be perturbed by an additional phase offset. As long as these remain constant during the scan band, they will have no effect.

[0124] To generate the FDCR, i.e., the channel response We assume that the I and Q values of the reflector and the initiator are given by

[0125]

[0126] where f k is the k-th channel.

[0127] Assuming that the physical channel between the initiator and the reflector is symmetric, the measured phase depends on the physical communication channel and the relative difference in the RF carrier phase between the devices, i.e., θ REFL (f k ) = θ CH (f k ) + Δθ LO (f k ) and θ INIT (f k ) = θ CH (f k ) - Δθ LO (f k ), where θ CH (f k ) is the phase delay of the channel, and Δθ LO (f k ) is the relative difference in the RF carrier phase between the devices.

[0128] For example, an estimate of the square of the actual channel response is as follows

[0129]

[0130] From an estimate of the actual channel transfer function H(f k ) can be constructed. Unfortunately, and are both valid solutions. For example, to determine the correct solution, it can be assumed that the Euclidean distance between consecutive values in is minimized, or other methods can be used based on the phase coherence assumption of the PLL between different frequencies.

[0131] One-way ranging (1WR)

[0132] In 1WR, all events occur on a timing grid such that Δ T remains constant throughout the channel scan.

[0133]

[0134] Table 2 1WR in multipath So we need to estimate Δ T . This can be done using a set of 2WR measurements as explained above.

[0135]

[0136]

[0137] Therefore,

[0138]

[0139] Therefore, we can measure Δ by looking at the slope of the frequency. to measure Δ T . In the presence of multipath, this slope remains a straight line. One-way ranging methods may require highly accurate frequency correction to relate the LO and CLK frequencies. This fine frequency measurement can be accomplished by measuring the same frequency over many tones propagated over a large amount of time.

[0140] Now back to Figures 3 to 5 the two-way method illustrated. In step 154, once the FDCR is generated, an inverse fast Fourier transform (IFFT) technique with 2048 points is used to perform an inverse Fourier transform to generate the time-domain channel response (TDCR), |h(t)|.

[0141] Figure 4 Figure 202 shows the TDCR in a simulated scenario where the RF device is separated from the target by 25 m and there is no multipath effect. At time index n peak = 342, the maximum peak 204 in the TDCR is found. Since there is no multipath effect, the time index of the maximum peak 204 can be used to calculate the distance between the RF device and the target according to the following formula:

[0142]

[0143] This distance differs from the true distance by only 0.05 m (this error is caused only by the resolution of the IFFT).

[0144] However, in real-world scenarios, multipath effects can cause the strength of the received signal to increase and decrease depending on the frequency. In some such scenarios, the maximum peak in the TDCR may not actually represent the shortest path between the two radio nodes (i.e., the maximum peak may not be the "shortest path peak").

[0145] Figure 5 Figure 302 shows a more realistic TDCR for a simulated scenario where the RF device is separated from the target by 11.5 m and there is a real multipath effect. In step 156, the amplitude of the maximum peak 304 is determined to be approximately 0.014 and at the time index n largestpeak ~400, the index corresponding to a distance of approximately 29 m (i.e., differing from the true distance by 17.5 m).

[0146] Therefore, in the case of significant multipath effects, simply using the maximum peak 304 may not be particularly accurate. To improve the accuracy of distance estimation, in step 158, the next highest peak 306 with an earlier time index in the TDCR is selected and analyzed to determine whether it corresponds to a shorter path between the RF device and the target.

[0147] Since the TDCR 302 is derived from a band-limited FDCR (i.e., limited by the bandwidth of the transmitted RF signal), the peaks in the TDCR 302 corresponding to the paths between the RF device and the target appear as a sine function, where the first sidelobe peak has an amplitude of approximately -13.3 dB compared to the main peak (i.e., approximately 4.7% of the main peak amplitude). Therefore, the first sidelobe peak in the TDCR 302 of the maximum peak 304 is expected to have an amplitude of approximately 0.0007.

[0148] Therefore, to determine whether the earlier peak 306 in the TDCR 302 actually corresponds to a shorter path between the RF device and the target than the path corresponding to the maximum peak 304, or whether it is just a sidelobe of the maximum peak 304, a threshold test is used in step 160, where the earlier peak 306 is compared with a threshold 308 based on the amplitude of the maximum peak 304. The threshold 308 has an amplitude of -10.0 dB (i.e., 10% or approximately 0.0014) compared to the maximum peak 304, i.e., greater than the expected amplitude of the first sidelobe -13.3 dB (twice as large). If the amplitude of the earlier peak 306 is below the threshold, then the maximum peak is identified as the shortest path peak in step 162. However, as can be seen from Figure 5 it can be seen that the amplitude of the earlier peak 306 is greater than the threshold 308, so it is identified as the shortest path peak in step 164.

[0149] Once the shortest path peak 306 is determined, it can be used to determine the distance r between the RF device and the target with higher accuracy than using only the maximum peak 304. The distance between the RF device and the target can be determined in step 166 by combining equation (4) and using only the time index of the shortest path peak. For Figure 5 the illustrated example, the shortest path peak 306 is located at a time index n of approximately 170 sp which corresponds to a distance of approximately 12.5 m, i.e., an error of only 1 m.

[0150] However, the accuracy of distance estimation may still need to be improved. Since the TDCR 302 is band-limited, the individual peaks corresponding to paths of similar lengths may overlap, such that a single peak in the TDCR 302 may not actually correspond to a single path. In this embodiment, where the bandwidth BW of the transmitted signal is 74 MHz, the Rayleigh criterion (representing the minimum time interval between two paths that can be accurately resolved separately in the TDCR) is equal to corresponding to a distance of approximately 4.05 m.

[0151] Figures 6 to 8 Illustrates the uncertainty introduced when two paths are closely spaced. Figure 6 Shows the TDCR 600 (where the x-axis has been converted to distance for clarity) in the case where two paths 602, 604 of equal intensity are spaced 2 m apart, i.e., approximately half of the Rayleigh distance of 4.05 m (all other parameters of the TDCR are the same as above). Only one peak 606 located at 3 m can be seen in the TDCR. Figure 7 Shows the TDCR 700 in the case where two paths 702, 704 are spaced 3 m apart, i.e., approximately 0.75 of the Rayleigh distance. Two separate peaks can be seen in this TDCR, but they are still located at positions approximately 1 m away from the true path distances. Figure 8 Shows the TDCR 800 in the case where two paths 802, 804 are spaced 4 m apart, i.e., approximately equal to the Rayleigh distance. In this example, the peaks 802, 804 are still approximately 70 cm away from the true path distances.

[0152] Therefore, to further improve the accuracy of distance determination, the shortest path peak 306 is further analyzed to determine whether it is actually the product of two different paths. Now reference will be made to Figure 9 the flowchart of Figure 10 and Figure 10 to explain this process, Figure 5 which is an enlarged view of the TDCR shown.

[0153] In step 350, the nearest local minimum 310 earlier (i.e., at a lower n value) than the shortest path peak 306 in the TDCR of Figure 10 is identified. Then, in step 352, the time interval Δn between the shortest path peak 306 and the nearest local minimum 310 is determined. In step 354, this time interval is compared with the interval Δn BW that would be expected to be observed if the nearest local minimum 310 were merely the first zero of the shortest path peak 306 (i.e., the first zero of the sine function describing the peak in the TDCR).

[0154] In this example, the expected interval Δn BW(Calculated using Equation 5) is 55 time index units. If the time interval is less than the expected interval, then the time of the shortest path peak 306 in step 356 is used to calculate the distance between the RF device and the target. However, in this instance, the local minimum 310 is at n = 100, such that the time interval Δn measured between the shortest path peak 306 and the nearest local minimum 310 is Δn = 70. The time interval Δn is greater than the expected time interval Δn BW , which indicates that the shortest path peak 306 actually consists of two overlapping peaks. To determine the time index of the earlier of these two peaks (i.e., corresponding to the shortest path), in step 358 the expected time interval Δn BW is simply added to the time index of the local minimum 310 (since this is where the corresponding peak of the minimum should be) to produce a corrected time index n Corrected = 155, which is used to calculate a distance of approximately 11.4 m (using Equation 7), within 0.1 m of the actual distance.

[0155] Figure 11 Illustrated is another simulated TDCR 900 resulting from a situation where the RF device is spaced 8.828 m from the target, in which the above method is implemented. The TDCR 900 includes a maximum peak 902 at an index corresponding to approximately 13.184 m (i.e., an error of 4.356 m). The magnitude of the earlier peak 904 is compared to a threshold based on the maximum peak 902, but is below this threshold, such that the maximum peak 902 is identified as the shortest path peak.

[0156] Then the nearest local minimum 906 earlier than the shortest path peak 902 is identified, and the time interval between the local minimum 906 and the shortest path peak 902 is compared to the expected time interval based on the bandwidth of the signal transmitted between the RF device and the target. The time interval exceeds the expected time interval, so the corrected time is determined by adding the expected time interval to the time of the local minimum. This corrected time is used to produce a final distance estimate of 8.496 m (i.e., an error of only 0.332 m).

[0157] Figure 12 Illustrated are the simulation results of the cumulative distribution function (CDF) for the absolute distance estimation error using the prior art method and the method according to an embodiment of the present invention. The simulation is performed using MCPD data generated from a Matlab model.

[0158] The lower line 1002 shows the distance estimation error using the prior art phase slope (MCPD) method. The middle line 1004 shows the case when the TDCR-based method used employs the shortest path peak threshold test (i.e., as described above with reference to Figures 3 to 5The distance estimation error when (... as described above). The upper line 1006 shows the distance estimation error when using further local minimum analysis (i.e., as described above with reference to Figures 6 to 10 ... as described above). It can be seen that the embodiments of the present invention produce more accurate distances in the vast majority of simulations. Compared with approximately 65% of the prior art methods, only approximately 20% of the simulations result in an error of more than 1 m.

[0159] Figure 13 illustrates the same simulation results as Figure 12 ... but only for those simulations where the local minimum analysis 1006 results in a correction of the time used for distance estimation (i.e., the case where the nearest local minimum in the TDCR is greater than the expected time interval). In these cases, approximately 85% of the simulations improved using the further local minimum analysis 1006 result in an error of less than 2 m, compared with approximately 50% of the simulations using only the shortest path peak threshold test 1004, and only 25% of the simulations using the prior art MCPC method 1002.

[0160] Although the present invention has been described in detail in connection with a limited number of embodiments, it should be readily understood that the present invention is not limited to such disclosed embodiments. On the contrary, the present invention can be modified to incorporate any number of variations, changes, substitutions, or equivalent arrangements that were not previously described but are equivalent to the scope of the present invention. Additionally, although various embodiments of the present invention have been described, it should be understood that aspects of the present invention may only include some of the described embodiments. Therefore, the present invention should not be considered limited by the foregoing description, but only by the scope of the appended claims.

Claims

1. A method for determining the distance between a radio frequency device and a target, the method comprising: The radio frequency device receives a radio frequency signal from the target; Determine the time-domain channel response from the received radio frequency signal; Determine the amplitude of the maximum peak in the time-domain channel response; Determine the amplitude of the second earlier peak in the time-domain channel response; Compare the second peak amplitude with a threshold based on the maximum peak amplitude; If the second peak amplitude is less than the threshold, then identify the maximum peak as the shortest path peak; If the second peak amplitude is greater than the threshold, then identify the second peak as the shortest path peak; And Calculate the distance between the radio frequency device and the target based on the time corresponding to the shortest path peak; Wherein, the method further comprises: Identify the nearest local minimum in the time-domain channel response earlier than the shortest path peak; Determine the time interval between the nearest local minimum and the shortest path peak; Compare the time interval with an expected time interval based on the bandwidth of the received radio frequency signal; and If the time interval is greater than the expected time interval, then calculate the time corresponding to the shortest path peak as the time corresponding to the nearest local minimum plus the expected time interval.

2. The method according to claim 1, wherein the radio frequency signal received from the target comprises a second radio frequency signal based on a first radio frequency signal previously transmitted by the radio frequency device to the target.

3. The method according to claim 1 or 2, wherein the radio frequency signal comprises a plurality of frequencies.

4. The method according to claim 3, wherein the radio frequency signal comprises a series of radio frequency signals with different carrier frequencies.

5. The method according to claim 3, wherein the radio frequency signal comprises a bandwidth of at least 10 MHz.

6. The method according to claim 1 or 2, wherein the target comprises a second radio frequency device that generates and transmits the radio frequency signal.

7. The method according to claim 1 or 2, wherein the time-domain channel response is determined using the frequency-domain channel response of the received radio frequency signal.

8. The method according to claim 7, wherein the frequency-domain channel response comprises the output of a multi-carrier phase ranging method applied to the radio frequency signal.

9. The method according to claim 1 or 2, wherein the threshold corresponds to an amplitude greater than the sidelobe amplitude of the maximum peak.

10. The method according to claim 1 or 2, comprising calculating the distance between the radio frequency device and the target by determining the distance traveled by the radio frequency signal within the time corresponding to the shortest path peak.

11. The method according to claim 1, wherein the expected time interval comprises the Rayleigh criterion of the time-domain channel response.

12. A radio frequency transceiver device, the radio frequency transceiver device being arranged to: Receive a radio frequency signal from a target; Determine the time-domain channel response from the received radio frequency signal; Determine the amplitude of the maximum peak in the time-domain channel response; Determine the amplitude of the second earlier peak in the time-domain channel response; Compare the second peak amplitude with a threshold based on the maximum peak amplitude; If the second peak amplitude is less than the threshold, then identify the maximum peak as the shortest path peak; If the second peak amplitude is greater than the threshold, then identify the second peak as the shortest path peak; and Calculate the distance between the RF device and the target based on the time corresponding to the shortest path peak; wherein the RF transceiver device is further arranged to: Identify the nearest local minimum in the time domain channel response that is earlier than the shortest path peak; Determine the time interval between the nearest local minimum and the shortest path peak; Compare the time interval with an expected time interval based on the bandwidth of the received RF signal; and If the time interval is greater than the expected time interval, then calculate the time corresponding to the shortest path peak as the time corresponding to the nearest local minimum plus the expected time interval.

13. A method for determining the distance between an RF device and a target, the method comprising: The RF device transmits a first RF signal; The RF device receives a second RF signal based on the first RF signal from the target; Determine the time domain channel response according to the second RF signal; Determine the amplitude of the maximum peak in the time domain channel response; Determine the amplitude of the second earlier peak in the time domain channel response; Compare the second peak amplitude with a threshold based on the maximum peak amplitude; If the second peak amplitude is less than the threshold, then identify the maximum peak as the shortest path peak; If the second peak amplitude is greater than the threshold, then identify the second peak as the shortest path peak; and Calculate the distance between the RF device and the target based on the time corresponding to the shortest path peak; wherein the method further comprises: Identify the nearest local minimum in the time domain channel response that is earlier than the shortest path peak; Determine the time interval between the nearest local minimum and the shortest path peak; Compare the time interval with an expected time interval based on the bandwidth of the second RF signal; and If the time interval is greater than the expected time interval, then calculate the time corresponding to the shortest path peak as the time corresponding to the nearest local minimum plus the expected time interval.

14. An RF transceiver device, the RF transceiver device being arranged to: Transmit a first RF signal; Receive a second RF signal based on the first RF signal from a target; Determine the time domain channel response from the second RF signal; Determine the amplitude of the maximum peak in the time domain channel response; Determine the amplitude of the second earlier peak in the time domain channel response; Compare the second peak amplitude with a threshold based on the maximum peak amplitude; If the second peak amplitude is less than the threshold, then identify the maximum peak as the shortest path peak; If the second peak amplitude is greater than the threshold, then identify the second peak as the shortest path peak; and Calculate the distance between the RF device and the target based on the time corresponding to the shortest path peak; wherein the RF transceiver device is arranged to: Identify the nearest local minimum in the time domain channel response earlier than the shortest path peak; Determine the time interval between the nearest local minimum and the shortest path peak; Compare the time interval with an expected time interval based on the bandwidth of the second RF signal; and If the time interval is greater than the expected time interval, then calculate the time corresponding to the shortest path peak as the time corresponding to the nearest local minimum plus the expected time interval.

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