FIELD

The invention relates to a method and a device for determining time of arrival (TOA) values of received wireless signals. The invention further relates to devices and systems for geo Io cation.

BACKGROUND

In recent years, the use of geo location systems such as the Global Positioning System (GPS) has become ubiquitous. However, GPS technology has limitations. It does not work, or not very well, inside buildings, in some urban locales, or when a GPS receiver is surrounded by foliage. In addition, the precision of currently available GPS technology is limited to approximately 1 meter resolution. Geolocation using ultra-wideband (UWB) technology overcomes some of these limitations. UWB technology in combination with algorithms based on measurement of

Time of Arrival (TOA) or Time Difference of Arrival (TDOA) can be applied in geolocation systems to reach centimeter level accuracy, since the relatively large bandwidth of UWB pulse signals allows a fine time resolution and accurate estimation of the time of flight along the direct path from the transmitter. See e.g., S. Gezici, Zhi Tian, G.B. Giannakis, H. Kobayashi, A.F. Molisch, H.V. Poor, and Z. Sahinoglu,

"Localization via ultra- wideband radios: a look at positioning aspects for future sensor networks", IEEE Signal Processing Magazine, vol. 22, no. 4, pp. 70-84, JuI. 2005. Figure 1 schematically shows a prior art UWB geolocation system. Such a system is for example disclosed in US6054950 and will be discussed here briefly, in particular concerning the use of TOA measurements.

The system in Figure 1 comprises a receiver 1 located at coordinates (xl,yl,zl), and transmitters 2, 3, and 4. In a known embodiment, the receiver 1 and transmitters 2, 3, and 4 are UWB devices, and the transmitters 2, 3, and 4 send out pulsed signals. Care is taken to make sure that the transmitters 2, 3, and 4 send out pulses one at a time, and that the receiver 1 "knows" which transmitter 2, 3, or 4 is broadcasting at which moment. This can be achieved by pre-configuring each device accordingly, or by enabling additional communications between the various devices, or in another manner. In addition, care must be taken to make sure the receiver 1 knows at which coordinates (x2,y2,z2; x3,y3,z3; x4,y4,z4) each transmitter 2, 3, and 4 is located. These data may also be pre-configured, communicated, or otherwise made known to the receiver 1.

The receiver 1 will receive an input signal from the transmitters 2, 3, and 4 and record arrival times of pulses in that input signal.

This signal transmitted by the transmitters 2, 3, and 4 will consist of a series of pulses. Due to the wideband nature of UWB technology, these pulses can have a relatively short time duration, in the order of tens or hundreds of picoseconds. Due to reflections, scattering, and other disturbances the input signal as received by the receiver can be a series of relatively broad pulses with many peaks. Each received pulse contains the contributions of many replicas of a transmitted pulse that arrive via different paths, so-called multipath components, and thus each received pulse can have a duration in the tens or hundreds of nanoseconds. It is common to define the arrival time of the first main peak in the received pulse as the TOA. In the following, with "received" or "input" pulse is meant the combination of all multipath contributions, as it is received at the receiver 1.

When an input signal pulse is received, the receiver 1 will use its knowledge of the transmitters to determine which transmitter sent the pulse and at which time it was sent. From the TOA of that pulse, here defined as the time difference between sending and receiving the pulse, and the known speed of light the receiver 1 can calculate its distance from the transmitter. E.g., if transmitter 2 sends a pulse that is recorded by receiver 1, receiver 1 can then calculate distance dl2 to transmitter 2. The similar procedure for transmitters 3 and 4 will give values for distances dl3 and dl4, respectively.

Information from three transmitters is enough to determine the coordinate values (xl, yl, zl). Adding more transmitters will make the system more robust, and give an indication of the error in the measured location (xl, yl, zl), or can be used to solve an unknown reference time in TDOA systems. Sources of error can be for example electromagnetic interference from other electromagnetic sources, or multiple reflections and scatterings of the signals making TOA determinations more difficult. However, while the problems of TOA based UWB ranging have been widely addressed in literature and most of the main theoretical issues have been investigated, the proposed solutions are still difficult to implement in practical systems. In particular, complex signal processing and very high sampling rates, in the order of several GHz, usually required for these signals, are today the main obstacles for the realization of cheap and affordable UWB positioning systems.

A prior art alternative to using costly high frequency sampling means is presented by estimators based on energy detection (ED). These can be implemented simply at sub-Nyquist sampling rates, see for example I. Guvenc and Z. Sahinoglu, "Threshold-based TOA estimation for impulse radio UWB systems", Proc. IEEE Int. Conf. on Ultra- Wideband (ICU), Zurich, Switzerland, September 2005, pp 420-425.

In ED based estimators, the energy in small time intervals ("bins") is determined. A drawback is that TOA precision is reduced due to the bin size and that integrator banks are needed, which are also costly. In "Low Complexity Frequency Domain TOA Estimation for IR-UWB

Communications" by Monica Navarro et ai, presented at the 2006 IEEE 64th vehicular technology conference, a number of TOA estimation approaches, including frequency domain TOA estimation procedures are presented. The article further presents a frequency domain TOA estimation procedure using a bank of orthogonal analog filters for a frequency-domain sampling of the input signal.

Electromagnetic signal detectors comprising TOA based UWB ranging technology find a wide variety of applications. Said detectors can be used in geo location modules of mobile electronic equipment, such as mobile communication devices, mobile phones, mobile or desktop computers. They can also be used in measurement devices, for example altimeters (such as an altimeter in an airplane) or liquid level sensors.

It is an object of this invention to provide a method and unit for determining the TOA of wireless signals in a manner that overcomes or reduces some or all of the drawbacks from the prior art.

SUMMARY OF THE INVENTION

The object is achieved by a device for determining a time-of-arrival of an input signal, comprising a first derived signal generation device, a second derived signal generation device, a first sampler device, a second sampler device, and a calculation device; the device for determining time-of-arrival of the input signal being arranged for receiving the input signal; the first derived signal generation device being arranged to generate a first time dependent signal with a first time dependence from the received input signal and being connected to an input of the first sampler device; the second derived signal generation device being arranged to generate a second time dependent signal with a second time dependence from the received input signal and being connected to an input of the second sampler device, the first time dependence of the first time dependent signal being a first predetermined decrease rate, and the first time dependence of the first time dependent signal being different from the second time dependence of the second time dependent signal; the first and second sampler device being arranged for sampling at least once the first time dependent signal and the second time dependent signal, respectively; the calculation device being arranged for receiving the at least once sampled first time dependent signal and the at least once sampled second time dependent signal and for determining a value of the time-of-arrival from the received at least once sampled first time dependent signal and at least once sampled second time dependent signal.

In an embodiment, the second time dependence is a second predetermined decrease rate.

In an embodiment, the first time dependent signal is a first exponential decay function with a first time constant (τl). In an embodiment, the second time dependent signal is a second exponential decay function with a second time constant {τl).

In an embodiment, the device as described above further comprises a controller device, wherein the controller device is arranged for receiving the input signal and the controller device is arranged for controlling, based on the received input signal, the generation of the first time dependent signal and the second time dependent signal from the received input signal.

In an embodiment, the controller device is arranged to establish from the received input signal if a first condition of the input signal is fulfilled, the first condition being a test if a value of the input signal increasing and exceeding a threshold value; if the first condition is fulfilled the controller device being arranged for enabling the first derived signal generation device to generate the first time dependent signal and the second derived signal generation device to generate the second time dependent signal.

In an embodiment, the controller device is arranged to establish from the received input signal if a second condition of the input signal is fulfilled, the second condition being a test if a value of the input signal is decreasing in time after reaching a maximum value; if the second condition is fulfilled the controller device being arranged for disabling the first derived signal generation device to generate the first time dependent signal and the second derived signal generation device to generate the second time dependent signal.

In an embodiment, the controller device is arranged for receiving either the first or the second time dependent signal and the controller device is arranged to establish from either the first or the second time dependent signal if a second condition of the input signal is fulfilled, the second condition being a test if a value of the input signal is decreasing in time after reaching a maximum value; if the second condition is fulfilled the controller device being arranged for disabling the first derived signal generation device to generate the first time dependent signal and the second derived signal generation device to generate the second time dependent signal.

In an embodiment, the device as described above further compries a clock, wherein the controller device is arranged for controlling the sampling of the first and second time dependent signals by a clock signal from the clock, the sampling being enabled if the second condition is fulfilled.

In an embodiment, the device further comprises a logic element arranged for receiving the input signal and for passing the input signal to the first and second derived signal generation device; the first derived signal generation device comprises a first envelope detector block comprising a first diode, a first capacitor, a first resistor, and a first switch element , and the second derived signal generation device comprises a second envelope detector block comprising a second diode, a second capacitor, a second resistor, and a second switch element ; the first diode having an anode connected to an output of the logic element for receiving the input signal, a cathode of the first diode being connected to one terminal of the first capacitor, to one terminal of the first resistor and to one terminal of the first switch element, the other terminals of the first capacitor, the first resistor and the first switch element each being connected to ground; the second diode having an anode connected to the output of the logic element for receiving the input signal, a cathode of the second diode being connected to one terminal of the second capacitor, to one terminal of the second resistor and to one terminal of the second switch element, the other terminals of the second capacitor, the second resistor and the second switch element each being connected to ground, wherein a product of a resistance of the first resistor and a capacitance of the first capacitor is proportional to the first time constant and a product of a resistance of the second resistor and a capacitance of the second capacitor is proportional to the second time constant.

In an embodiment, the device comprises a third capacitor ; the cathode of the first diode having a connection with an input terminal of the first sampling device and with a terminal of the third capacitor; the cathode of the second diode having a connection with an input terminal of the second sampling device .

In an embodiment, the device for determining a time-of-arrival of an input signal comprises an operational amplifier ; in the connection between the cathode of the first diode with the input terminal of the first sampling device and with the third capacitor a positive input (+) of the operational amplifier being connected to the cathode of the first diode, an output of the operational amplifier being connected to the negative input (-) of the operational amplifier and the output of the operational amplifier being connected to the input terminal of the first sampling device and the input of the third capacitor.

In an embodiment, the device for determining a time-of-arrival of an input signal comprises a third switch element and a signal slope detection block comprising the third capacitor, a third resistor and a further logic element ; the output of the operational amplifier further being connected to a terminal of third capacitor, the other terminal of the third capacitor being connected to an input of the further logic element and to a terminal of third resistor, the other terminal of the third resistor being connected to ground; the third switch element having one terminal connected to the anode of first diode and to the anode of the second diode, the other terminal of the third switch element being connected to ground; the further logic element being arranged for controlling the third switch element based on a detection of a peak of the input signal by the slope detection block.

In an embodiment, the device further comprises a coarse detection circuit for determining a repetition pattern of the input signal, a repetition time of the repetition pattern being used for coarse estimation of the time-of-arrival.

In an embodiment, the input signal is selected from a group comprising radio signals, acoustic signals and optical signals.

Further, the object is achieved by a method for determining a time-of-arrival of an input signal, comprising: receiving the input signal; generating a first time dependent signal with a first time dependence from the received input signal, the first time dependence of the first time dependent signal being a first predetermined decrease rate; generating a second time dependent signal with a second time dependence from the received input signal, the first time dependence of the first time dependent signal being different from the second time dependence of the second time dependent signal; sampling at least once the first time dependent signal and the second time dependent signal; determining a value of the time-of-arrival from the at least once sampled first time dependent signal and at least once sampled second time dependent signal.

In an embodiment, the method comprises controlling, based on the received input signal, the generation of the first time dependent signal and the second time dependent signal from the received input signal.

In an embodiment, the method further comprises establishing from the received input signal if a first condition of the input signal is fulfilled, the first condition being a test if a value of the input signal increasing and exceeding a threshold value, if the first condition is fulfilled, enabling the generation of the first time dependent signal and the second time dependent signal.

In an embodiment, the method further comprises establishing from the received input signal if a second condition of the input signal is fulfilled, the second condition being a test if a value of the input signal is decreasing in time after reaching a maximum value; if the second condition is fulfilled, the method comprising disabling the generation of the first time dependent signal and the second time dependent signal.

In an alternative embodiment, the method further comprises establishing from either the first or the second time dependent signal if a second condition of the input signal is fulfilled, the second condition being a test if a value of the input signal is decreasing in time after reaching a maximum value; if the second condition is fulfilled, the method comprising disabling the generation of the first time dependent signal and the second time dependent signal.

In an embodiment, the method comprises controlling of the sampling of the first and second time dependent signals, the sampling being enabled if the second condition is fulfilled.

In an embodiment, determining the value of the time-of-arrival from the at least once sampled first time dependent signals and second time dependent signals comprises solving an equation , where t is the time at which first and second time dependent signal samples are taken, T is point of time-of-arrival TOA, Si is the value of the first time dependent signal sample, S ₂ is the value of the second time dependent signal sample, and τl is the first time constant.

In an alternative embodiment, determining the value of the time-of-arrival from the at least once sampled first time dependent signals and second time dependent signals comprises solving a set of equations

S _1)I eχp((^ - r)/τ ₁ ) = S _2)I , where U is the time at which first and second time dependent signal sample i are taken, S ₁ ,! is the value of the first time dependent signal sample i at time t _l5 and S ₂ ,i is the value of the second time dependent signal sample i at time t _l5 T is point of time-of-arrival TOA, and τl is the first time constant.

In yet another alternative embodiment, determining the value of the time-of-arrival from the at least once sampled first time dependent signals and second time dependent signals comprises solving a set of equations S ₁ , exp((f, - T) ZT ₁ ) = S _2>1 exp((f, - T) IT ₂ ), where I ₁ is the time at which first and second time dependent signal sample i are taken, S ₁ ,! is the value of the first time dependent signal sample i at time t _l5 and S ₂ ,i is the value of the second time dependent signal sample i at time t _l5 T is point of time-of-arrival TOA, τl is the first time constant, and τ2 is the second time constant.

In an embodiment, the method further comprises determining a repetition pattern of the input signal, a repetition time of the repetition pattern being used for coarse estimation of the time-of-arrival, the determination of the repetition pattern preceding the generation of the first time dependent signal and of the second time dependent signal.

Further the invention relates to an electromagnetic signal detector, comprising:

- a Time Of Arrival determining unit comprising a device for determining a time-of- arrival of an input signal as described above, - an electromagnetic signal receiver arranged to provide an input signal to the Time Of Arrival determining unit.

Additionally the present invention relates to a system for determining the position of a remote object, comprising:

- a plurality of electromagnetic signal transmitters each arranged for transmitting electromagnetic signals,

- a electromagnetic signal detector as described above, the electromagnetic signal detector being arranged for receiving the electromagnetic signals as input signal from each of the electromagnetic signal transmitters, the electromagnetic signal detector being associated with the remote object. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail below with reference to a few drawings in which illustrative embodiments of the invention are shown. It will be appreciated by the person skilled in the art that other alternative and equivalent embodiments of the invention can be conceived and reduced to practice without departing from the true spirit of the invention, the scope of the invention being limited only by the appended claims.

Figure 1 schematically shows a prior art UWB geolocation system; figure 2 schematically shows a TOA determination unit according to an embodiment of the invention;

Figure 3 schematically shows an electrical circuit embodiment of the unit according to an embodiment of the invention;

Figure 4 schematically shows an electromagnetic detector according to an embodiment of the invention; Figure 5A-B schematically shows a single UWB pulse, with first and second derived, or time dependent, signals according to an embodiment of the invention;

Figure 6 schematically shows a train of UWB pulses and corresponding energy density measurements;

Figure 7 schematically shows a single UWB pulse with TOA estimates according to an embodiment of the invention;

Figure 8 schematically shows a flow diagram for the TOA determination method according to an embodiment of the invention.

The figures are not necessarily drawn to scale. In the figures identical components are denoted by the same reference numerals.

DETAILED DESCRIPTION

Figure 2 schematically shows a device 78 for determining TOA of an input signal pulse according to an embodiment of the invention.

The device 78 comprises a controller device 73, a first derived signal generation device 71, a second derived signal generation device 72, a first sampler device 74, a second sampler device 75, and a calculation device 76.

An input signal 70 is received on an input 79 which is connected to controller device 73. The controller device 73 is arranged for controlling the first derived signal generation device 71, the second derived signal generation device 72, the first sampler device 74, the second sampler device 75, and the calculation device 76 depending on a condition of the input signal 70.

Further, the input 79 is shared with the first derived signal generation device 71 and the second derived signal generation device 72 for receiving the input signal 70

In an embodiment, the controller device 73 can be described as being in one of three states:

"zero": awaiting an input signal pulse,

"tracking": receiving an input signal pulse, and awaiting the maximum of the pulse that signifies the TOA,

"decreasing": the period after the TOA.

The reasons for the names of the states will become clear in the following description of the drawings.

In the "tracking" state the derived signal generation devices 71 and 72 will each generate a derived signal which increases in value in proportion to an increasing input signal strength, generally tracking the value of the input signal. Below, the derived signal may also be referred to as time dependent signal. An output of the first derived signal generation device 71 is connected to an input of the first sampler device 74 and an output of the second derived signal generation device 72 is connected to an input of the second sampler device 75.

The generated signals of the first 71 and second 72 derived signal generation devices are made available to a first sampler device 74 and a second sampler device 75, respectively.

In the "decreasing" state, the first 74 and second 75 sampler devices will sample derived signal strength values of the first 71 and second 72 derived signal devices, thus recording first and second signal samples, respectively.

The first and second sampler devices 74 and 75 each have an output which is connected to an input of the calculation device 76. Thus, first and second signal samples from the first and second sampler devices are fed to the calculation device 76. The calculation device 76 is arranged for calculating the TOA 77 from the first and second signal samples taken by the sampler devices 74 and 75. This calculation may be done according to the principles described in connection with Figure 5B.

In another embodiment, the controller 73 is arranged for controlling the first derived signal generation device 71, the second derived signal generation device 72, the first sampler device 74, the second sampler device 75, and the calculation device 76 depending on conditions of the input signal and the first and/or the second derived signals from the first and second derived signal generation device, respectively. In this embodiment, the input signal 70 and the signal from the first and/or the second derived signal generation devices 71 and 72 is available to the controller device 73 for controlling. In the "tracking" state, when the signals of the derived signal generation devices track the input signal, these first and second derived signals may be used by the controller device instead of the input signal.

Figure 3 schematically shows an electrical circuit as an embodiment according to the invention. The electrical circuit as device for determining time-of-arrival comprises a logic element 81.

It further comprises a first envelope detector block Rl comprising a first diode 87, a first capacitor 88, a first resistor 89, and a first switch element 90, and a second envelope detector block R2 comprising a second diode 82, a second capacitor 83, a second resistor 84, and a second switch element 85.

In the first envelope detector block Rl, the first diode 87 has an anode connected to an output of the logic element 81. The cathode of the first diode 87 is connected to one terminal of the first capacitor 88, one terminal of the first resistor 89 and one terminal of the first switch element 90. The other terminals of the first capacitor 88, the first resistor 89 and the first switch element 90 are each connected to ground.

In the second envelope detector block R2, the second diode 82 has an anode connected to an output of the logic element 81. The cathode of the second diode 82 is connected to one terminal of the second capacitor 83, one terminal of the second resistor 84 and one terminal of the second switch element 85. The other terminals of the second capacitor 83, the second resistor 84 and the second switch element 85 are each connected to ground. The electrical circuit further comprises a first sampling device 91 and a second sampling device 86. The electrical circuit also comprises a timer 98, and a signal slope detection block comprising a third capacitor 95, a third resistor 96 and a further logic element 97. In the embodiment shown in Figure 3 the electrical circuit comprises an operational amplifier 94 as buffer amplifier.

A positive input (+) of the operational amplifier 94 is connected to the cathode of the first diode 87. The output of the operational amplifier 94 is connected to the negative input (-) of the operational amplifier. The first sampling device 91 is connected with an input to the output of the operational amplifier 94, the second sampling device 86 is connected with an input to the cathode of second diode 82. Further, both the first and second sampling devices are connected to a clock 92.

Also, the output of the operational amplifier 94 is connected to a terminal of capacitor 95. The other terminal of the capacitor 95 is connected to an input of logic element 97 and to a terminal of third resistor 96. The other terminal of the third resistor 96 is connected to ground.

The logic element 97 is arranged for controlling a third switch element 99 which has one terminal connected to the anode of first diode 87 and the anode of the second diode 82. The other terminal of the third switch element 99 is connected to ground.

Referring to Figure 2, the first derived signal generation device 71 is implemented in the circuit of Figure 3 as the first envelope detector block Rl circuit with first capacitor 88, first resistor 89, and with first diode 87 preventing current flowing back out of the first envelope detector block Rl . Similarly, the second signal generation device 72 is implemented as the second envelope detector block R2, with second capacitor 83, second resistor 84, and second diode 82.

The input signal 70 is available as an electrical voltage at point 80. Logic element 81 conducts current if the voltage at 80 exceeds a threshold value. Beyond block 81, the first and second envelope detector blocks Rl, R2 are placed in parallel. As long as the voltage exceeds the threshold value, and third switch element 99 does not connect the signal to ground, first and second capacitors 88 and 83 will become increasingly charged. In an embodiment, the TOA determination unit comprising the electrical circuit of Figure 7 can then be said to be in the state "tracking".

In an embodiment, a first time constant τl of the first envelope detector block Rl obtained as the product of the resistance of first resistor 89 and capacitance of first capacitor 88 is of the order of half the sampling time of the analog-to-digital converter. For example, if the signal is sampled with a sampling frequency 50 MHz, a suitable value for τl can be 10 ns. A second time constant τ2 of the second envelope detector block R2 obtained as the product of the resistance of second resistor 84 and capacitance of second capacitor 83 is significantly larger than the first time constant τl. The operational amplifier 94, which acts as a buffer amplifier, is arranged to prevent the first envelope detector block (Rl) and the signal slope detection block from loading each other.

As such, in the absence of an input voltage, a voltage at the input of first sampling device 91 will be determined by first resistor 89 and first capacitor 88, whereas a voltage at the input of the second sampling device 86 will be determined by the second resistor 84 and second capacitor 83.

An output of the operational amplifier 94 is connected to a resistor-capacitor (RC) circuit 95, 96.

The RC circuit 95, 96 formed by capacitor 95 and resistor 96 is used as a differentiator. In combination with a sign detector or logic element 97, a peak detector is formed. When the differentiated signal obtained from the output of the amplifier 94 goes from positive to negative, a peak in the received input signal is detected and a peak received signal is output to the third switch element 99.

After a peak has been detected and the peak received signal is output to the third switch element 99, the third switch element 99 will connect the input signal 70 to ground, thus stopping the flow of current to the first and second envelope detector block Rl, R2 . The first and second diodes 87 and 82 will each prevent that the first and second envelope detector block, respectively, discharges to ground via the third switch element 99. The voltage at the input side of the sampling devices 91 and 86 will now decrease exponentially, with time constants τl and τ2, respectively. In an embodiment, the TOA determination unit comprising the electrical circuit of Figure 3 can then be said to be in the state "decreasing". In an embodiment, time constant τ2 can be approximated as infinity, so that the voltage at the input side of the second sampling device 86 is substantially constant. Alternatively, however, this is not required. As long as τl and τ2 are different, the method can be used to determine TOA as will be illustrated below. Connecting the RC circuit 95, 96 which is used as a differentiator to the output of the envelope detector block with the lowest time constant, in this case Rl, is advantageous since the exponential decay of the voltage at the output side of Rl ensures that the differentiated signal remains negative, and thus the third switch element will continue connecting the input signal 70 to ground. It is noted that from this signal it is easier to detect the peak, since it will decrease after reaching the peak. The duration of short-circuiting the input to ground is preferably determined by a predetermined timing circuit.

The sampling devices 91 and 86 operate on a clock 92. On an output 93, the values from the sampling devices will be available for further processing in a calculation device (not shown in Figure 3) to calculate the TOA value. The sample values may be first stored in a memory connected to the output 93. The details of the TOA calculation based on the recorded sample values will be discussed in more detail with reference to Figure 5B.

In the embodiment of the invention shown in Figure 3, a timer 98 sends a reset signal after a predetermined time interval to the first and second switch elements 90, 85. The first and second switch elements 90 and 85 are arranged when receiving the reset signal, to connect the first and second capacitors 89 and 84 to ground, thus providing a reset of the first and second envelope detector circuits.

It is noted that resetting the circuit can be done by using other means than timer 98. The circuit of Figure 3 can be implemented as an Integrated Circuit (IC).

It is further noted that the operational amplifier 94 as buffer amplifier is optional and may be omitted and be replaced by wired connections between the cathode of the first diode 87 and the first sampling device 91 and the third capacitor 95.

Figure 4 schematically shows an electromagnetic receiver 100 according the invention. The electromagnetic receiver 100 comprises a signal receiver 101. In an embodiment the signal receiver 101 is an UWB receiver.

The electromagnetic receiver 100 further comprises a TOA determination unit 102, as is embodied by a device for determining time-of-arrival of an input signal according to the present invention, for example as represented by the device shown in Figure 2.

The electromagnetic receiver 100 also comprises a location calculator device 103. An output of the signal receiver 101 is connected to an input of the TOA determination unit 102. An output of the TOA determination unit 102 is connected to an input of the location calculator device 103.

In an embodiment, the signal receiver 101 receives the input signal 70 and presents the received input signal to the TOA determination unit 102, which in turn provides TOA values to the location calculator device 103.

In an embodiment, the TOA determination unit 102 is further arranged for performing a coarse TOA estimate. In a further embodiment, the TOA determination unit 102 is arranged for coarse TOA determination following the approach outlined with reference to Figure 6. In a further embodiment, the TOA determination unit 102 is arranged for performing statistical processing of the determined TOA values.

In a further embodiment, the TOA determination unit 102 is arranged for performing statistical processing of the determined TOA values following the approach outlined with reference to Figure 7. In an alternative embodiment, the location calculator device 103 is arranged for performing statistical processing of the determined TOA values and/or of the calculated positions.

It is noted that the TOA determination unit 102 may comprise a calculation device to calculate TOA values from the measured samples (obtained as explained with reference to Figure 2). However, it may also be advantageous to place a single calculation device such as a microprocessor coupled to a memory, in the location calculator device 103 for calculation of both TOA values and positions from TOA values. Likewise, the control functions discussed in connection with Figure 2 may be handled by dedicated hardware components, or by a programmable device such as a processor.

In an embodiment, the electromagnetic receiver 100 is arranged and configured for use in a geolocation system. For example the geolocation system may be arranged as the one schematically shown in Figure 1. It is noted that the invention allows that the electromagnetic radiation used complies with governmental guidelines, such as for example Federal Communications Commission (FCC) guidelines.

In an embodiment, frequency ranges and bandwidths of UWB standards, such as for example a (future) IEEE standard, are used in devices and systems according to the invention. However, the invention is not limited to use by means of only UWB standards. In particular, use of signals with bandwidths larger than those common in UWB (typically 500 MHz) may be advantageous, for example due to increased location precision and/or improved signal-to-noise ratios. Another application can involve the use of light pulses. In this embodiment the device for determining a time-of-arrival of an input signal, is arranged for using light pulses (i.e. an optical signal) as input signal, in which a photo sensitive device is arranged as input device for receiving the input signal.

The light pulses may have a wavelength in the infrared range or the visible range of the spectrum.

In some embodiments, the light pulses may have a wavelength in the ultraviolet range of the spectrum.

In an embodiment the device for determining a time-of-arrival of an input signal, is arranged for using acoustic signal as input signal, in which a microphone or hydrophone is arranged as input device for receiving the input signal.

The method for determining TOA values according to the invention will now be illustrated in more detail.

Figure 5A schematically shows a received input signal pulse 70 using UWB signals. On the horizontal axis, time is plotted. On the vertical axis, received input signal strength is indicated.

A typical time duration for the emitted UWB pulses is tens or hundreds of picoseconds (ps), whereas the received, multipath, signal as depicted in Figure 5A, composed of contributions from the same original pulse arriving via different paths, can be tens to hundreds of nanoseconds (ns) in width. The received signal is a multipath signal since each emitted UWB pulse may follow a number of paths due to reflections of the pulse signal with relevant obstacles between the emitter and the receiver.

Figure 5B shows the same input signal as Figure 5A, but now representations of the first 31 and the second 32 derived, or time dependent, signal are overlaid. As indicated earlier, what was transmitted as a relatively narrow pulse signal can due to reflections, scattering, and other disturbances, be received by the receiver as a relatively broad signal with many peaks. It is common to define the arrival time of the first main peak in the received pulse as the TOA. In Figure 5B, the TOA is indicated by point in time 43, and the corresponding peak value by 41. The threshold value, the exceeding of which at time 42 causes the change from "zero" to "tracking" state, is indicated by 44. The threshold value is used as a tuning parameter. A relatively too high threshold value will cause low peak values to be missed, whereas a relatively too low threshold value may trigger measurements by noise rather than the main peak.

After the peak that occurs at point in time 43, the slope of the input signal 70 turns negative, and the state will go from "tracking" to "decreasing". The first and second derived signals, which have been overlapping up to point in time 43, will now start to diverge, since the first and second derived signal generators 71 and 72 differ in such a way that the first derived signal has a different decrease rate than the second derived signal. In an embodiment, the first and second derived signal are each controlled to decrease at a respective predetermined rate. In a further embodiment, the first and second derived signals are each controlled to decay exponentially with a first time constant τl and a second time constant τ2, respectively. Expressed in a mathematical formula, the first derived signal value will be proportional to ) , where exp indicates the exponential function, t is the time, and T the TOA value. The second derived signal value will be proportional to exp(-(f -r)/τ ₂ ) . It is advantageous to set the first time constant τl to a value that is in the order of half the sampling time of the analog-to-digital converter, i.e. about 10 nanoseconds for a sampling frequency of 50 MHz, whereas the second time constant τ2 is set to a much larger value. This will cause the second derived signal value to be approximately constant in the time frame of the pulse width, whereas the first derived signal value significantly decreases in the same time frame.

A first 37 and a second 33 sample value are taken of the first and the second derived signal, respectively. In an embodiment, the sampling may be done at a sampling rate which is low compared to the rate at which the input signal fluctuates. Such a strategy may provide cost savings. However, due to the low sampling rate, it is unlikely that at exactly the TOA point in time 43 the first and second samples 33 and 37 will be recorded. In such a case, the invention advantageously provides that from the known decay rate of the first derived signal and the constant second derived signal, respectively, the TOA point in time 43 may be calculated by solving the equation [1]

S ₁ cx _V ((t - T)lτ ₁ ) = S ₂ , [1] where t is the time at which samples 37 and 33 are taken, T is TOA, Si is the value of sample 37, S ₂ is the value of sample 33, and τl is the first time constant τl.

In an embodiment, the first 31 and second 32 derived signals are sampled multiple times, for example giving samples 37, 38, 39, and 40 of the first derived signal, and 33, 34, 35, and 36 of the second derived signal. TOA can then be determined by solving the system of equations [2] where U is the time at which sample i is taken, Si ₁₁ is the value of sample i at time U of the first derived signal, and S ₂ ,i is the value of sample i at time U of the second derived signal. The advantage is that in this system of equations [2] the effects of the sampling error, schematically represented by error bars in Figure 5B, can be reduced. Also the effects of clock jitter in the sampler control, giving uncertainty in the times U at which samples are taken, can be reduced.

The skilled in the art will appreciate that by solving the system of equations with a first and a second set of samples, a TOA value is obtained, optionally also including an estimate of the TOA error. A further advantage is that solving a system of equations allows solving for τl as well as for TOA. In this manner, the system can be made robust against fluctuations in the time constant τl. For example, in certain embodiments of the invention the decay function may be implemented using electrical resistance and capacitor elements, in which the time constant may be generally temperature dependent. In an embodiment, the time constant τ2 is not taken to be effectively infinite.

The equations [3] then become of the form

S ₁ , exp((f, - T) ZT ₁ ) = S _2>1 exp((f, - T) IT ₂ ), _[3] In an embodiment, the time constants τl and τ2 values are checked by introducing a periodic calibration based on a known input voltage. Again, multiple samples can be used to reduce the quantization error.

In a further embodiment, the first and second sampling devices used to record samples 33 and 37, or sets 33, 34, 35, 36 and 37, 38, 39, 40 are only active in the state "decreasing", in other words after TOA, to save power. This is advantageous in low power devices, for example, battery powered mobile devices to increase battery lifetime.

In an alternative embodiment, the first 31 and second 32 derived signals are not sampled synchronously. For example, a single sampling device combined with an input multiplexer, the input multiplexer in an alternating manner connecting an input of the single sampling device to the output of the first and second derived signal generation devices, can sample value 33 and 38. Assuming that the not sampled value 34 is approximately equal to the sampled value 33, equation [1] can still be used to solve for TOA with Si having the value of sample 38 and S ₂ the value of sample 33.

Alternatively, multiple samples may be taken in an alternating manner with a single sampling device, for example resulting in a first sample set Si ₁₁ containing samples 37 and 39, and a second sample set S ₂ ,i containing samples 34 and 36. The intersection of the first curve formed by plotting the first sample set values against sampling time and the second curve formed by plotting the second sample set values against sampling time will also give TOA. Figure 6 schematically illustrates a coarse selection method according to an embodiment of the invention.

With a plurality of transmitters sending pulses 50, 51, and 52, in a properly configured geolocation system pulses will be received one after another in a relatively regular sequence.

It is advantageous that the device for determining TOA of an input signal pulse is capable to coarsely determine when to expect a pulse, especially if the threshold mechanism as explained in reference to Figure 3, Figure 5B, and Figure 8 is employed.

Ideally, for reception of each of the received multipath pulses 50, 51, and 52 from the respective transmitters, the system receives a reset signal and is therefore brought into the "zero" state at or around points in time 56, 57, and 58.

In an embodiment, a coarse energy detection scheme is used with a binning method to measure the energy in a number of relatively broad time bins. With broad is meant typically one tenth of the time between successive input pulses. For example, if input pulses arrive each 800 ns, the bin width would be 80 ns.

The energy measured in the bins can be represented by a histogram, see 53, 54, and 55 in Figure 6. These bins can be implemented, as a skilled person will know, for example using banks of integrators. The histograms constructed from the bin measurements are relatively too imprecise to derive the TOA from, but advantageously, the histograms can be used to predict suitable reset times 56, 57, and 58 at which the reset signal is to be generated and entered in the system. Thus a coarse TOA estimate is made, to be refined by the method according to the invention. The time axis in Figure 6 is not necessarily drawn to scale. For example, the received pulses may arrive at 800 ns intervals, with each received pulse having a width of approximately 100 ns. With bins of 80 ns wide, this would means that eight bins would likely contain only noise, while two, at most three, bins would contain contributions from the received pulse. Reset times 56, 57, and 58 may then be picked by selecting a suitable time prior to the central time of the bin with the highest value.

Alternatively, the method provides a prediction of suitable reset times based on a fixed time interval after the previous determined TOA. In a further embodiment, the method provides a prediction of suitable reset time based on the input pulse width as fixed time interval. The reset signal can be generated by an external controller (not shown).

Figure 7 schematically shows the effects of statistical processing of the determined TOA values. Curve 60 represents one of the received input pulses. For the previously received pulses, a number of TOA values 61, 62, 63, 64, and 65 are determined. For example, value 61 is an so-called outlier, likely due to an erroneous triggering of the system. Value 65 is also an outlier. Values 62, 63, and 64 are near the actual TOA value. From a statistical processing of the determined values, including possibly elimination of outliers, a final estimate 66 of the TOA can be made.

Optionally the statistical processing may provide an estimate of the error. Figure 8 schematically shows a flow diagram 200 for a TOA determination method according to an embodiment of the invention. In the embodiment, the device for determining TOA of an input signal pulse is embodied as a state machine having at least three specific states. In a first state "zero", the expected input signal pulse has not been received yet. In a second state "tracking", the input signal strength is increasing, and the time at which the next maximum in the signal occurs will be taken to be the TOA point in time. In a third state "decreasing", the abovementioned maximum in the signal has occurred.

In a first stage of the flow diagram a selection 20 is made based on the current state.

If the selection 20 shows that the state is "zero", then in a next stage 21 an inspection is carried out if the input signal exceeds the predetermined threshold value. If this is not the case, the state remains unchanged. If the inspection 21 evaluates that the input signal exceeds the predetermined threshold value, then the state changes from 'zero' and is set to "tracking" 22.

Next, the flow then passes back to the initial selection point 20. If at the selection stage 20 the state is "tracking", then in a next stage 23 a second inspection is carried out if the slope of the input signal's derivative is negative. If the second inspection 23 evaluates that the slope is negative, then in next stage 24 the state will be set to "decreasing".

Next, the flow again passes back to the initial selection point 20. If at the selection stage 20 the state is "decreasing", then in a next stage 25 a third inspection is carried out if a reset signal has been received. If a reset signal has been received, then in a next stage 26, the state is reset to "zero" and the system is cleared.

Clearing the system may involve resetting derived signal generation devices, clearing timers, etc, generally bringing the system back in the condition it was in before the input signal exceeded the threshold value 21.

Next, the flow passes back to the initial selection point 20. The reset signal can be generated by an external controller performing a coarse selection as described above with reference to Figure 6. It can also be automatically triggered after a predetermined period of time in the "decreasing" state. It will be clear to a person skilled in the art that a signal processing flow as described above can be implemented in an electrical circuit using standard analog and/or digital components, or in an integrated circuit, or in a computer program for a suitable processing unit coupled to such electrical circuit or such integrated circuit. Such a computer program that comprises instructions for the processing unit, allows the processing unit, after being loaded in a memory coupled to the processing unit, to carry out an embodiment of the method according to the invention. Alternatively, the processing unit may be embodied as a system on chip which comprises the electrical or integrated circuit on chip. It should be noted that the abovementioned embodiments and examples illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The invention, or at least parts thereof, can be implemented in hardware and/or software. The hardware may comprise digital or digital and analog components.