WO2020165429A1

WO2020165429A1 - Time stamp correction information determiner, transceiver for use in a positioning system, calculator for determining a time-of-flight, system and methods

Info

Publication number: WO2020165429A1
Application number: PCT/EP2020/053943
Authority: WO
Inventors: Juri Sidorenko; Norbert Scherer-Negenborn; Michael Arens
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2019-02-14
Filing date: 2020-02-14
Publication date: 2020-08-20
Also published as: DE102019202010B3

Abstract

A time stamp correction information determiner for determining a time stamp power correction information on the basis of transmission time stamp information and reception time stamp information associated with at least three signals. The time stamp correction information determiner is configured to determine the time stamp power correction information based on a deviation between a first time interval between a transmission of two signals having different signal power levels, wherein the first time interval is described by a difference of transmission time stamps related to a first clock, and a second time interval between a reception of the two signals having different signal power levels, wherein the second time interval is described by a difference of reception time stamps related to a second clock, using a clock drift correction. The clock drift correction is based on transmission time stamp information and reception time stamp information of at least two signals having same signal power levels.

Description

Time Stamp Correction Information Determiner, Transceiver for Use in a Positioning System, Calculator for Determining a Time-of-Flight, System and

Methods

Description

Technical Field

Embodiments according to the invention relate to a time stamp correction information determiner, a transceiver for use in a positioning system, a calculator for determining a time-of-flight, a system and methods.

Background of the Invention

In the last century autonomous systems become omnipresent in near most every field of the industry and it is expected that spending on robotics reach 67 Billion US Dollar by 2025, compared to 11 billion in 2005 [3] One of the most important tasks in robotics is the interaction between the robot and its environment. This task can only be accomplishment if the location of the robot with respect to the environment is known. Visual sensors are very common for localization [5, 18]. In some cases it is required to estimate the position in non line of sight conditions. Radio frequency (RF) based sensors are able to operate in this conditions, but the outcome depends highly on the measurement principle like RSSI [16], fingerprinting [20], FMCW [13], UWB [21] and the used technique like angle of arrival [6], time of arrival [4] or time difference of arrival [19]. Indoor positioning is in general a challenge for RF based localization systems. Reflections could cause interference with the main signal. In contrast to narrow band signals are ultra-wideband (UWB) signals more robust to fading [9, 15]. A common UWB system is the Decawave UWB transceiver [14]. It is low cost and provides centimeter precision. The accuracy and precision of this chip is effected by three factors. Antenna offset, clock drift and the signal power [11 , 2J. The herein described invention deals with the signal power error and the clock drift correction. The signal power error is specific for the Decawave UWB transceiver and effects the accuracy of the position significantly. The general approach to estimate signal power dependency is to use ground truth data, provided by additional measurement equipment [1] The clock drift error is caused by different frequencies of the transceiver clocks. The general approach for Decawave UWB clock drift correction is to use the integrator of the phase locked loop (PLL) [8, 17, 12, 7] The general approach for the clock drift correction is not suitable, due to the fact that the PLL is also effected by the signal power.

In practice it is not possible to manufacture exact same clock generators, this leads to different clock frequencies for every transceiver. The clock drift correction represents, for example, just the difference between clock frequencies but not current time values.

The general approach for clock drift correction is to use the phase locked loop (PLL) integrator [8, 17, 12, 7] Figure 15 shows an example of frequency demodulation by a phase locked loop. The voltage controlled oscillator (VCO) is set to the mid position and the loop is in lock at the frequency of the carrier wave. Modulations on the carrier would cause the VCO frequency to follow the incoming signal, changes of the voltage correspond to the applied modulations. The difference between the received carrier frequency (VE) and the internal loop frequency (VI) can be observed in the integrator of the loop filter.

It is known that the time stamp of the DW1000 is effected by the signal power. Increase in signal power cause to smaller time stamps and vice versa.

Figure 19 illustrates the reported distance error with respect to the received signal power. In other words Fig. 19 is a diagram illustrating the effect of range bias on the reported distance. At a certain signal strength the range bias effect should be zero, in Figure 19 it appears between -80 and -75 dBm. The correction curve is effected by the system design elements as such as printed circuit board (PCB), antenna gain, pulse repetition frequency (PRF) etc. The general approach for correction curve estimation is to compare ground truth distance measurements with ground truth distances. This method has two disadvantages. First of all additional measurement equipment is necessary. Secondly, every created curve applies for two stations but not for every station individually.

Therefore, it is desired to get a concept which makes a better compromise between a more accurate clock drift correction and a reduction of additional hardware for an estimation of a signal power correction curve. Optionally it is desired to have an alternative for a method for the clock drift correction and a method for the signal power correction compared to general methods. This is achieved by the subject-matter of the independent claims of the present application.

Further embodiments according to the invention are defined by the subject-matter of the dependent claims of the present application.

Summary of the Invention

An embodiment according to this invention is related to a time stamp correction information determiner for determining a time stamp power correction information on the basis of transmission time stamp information and reception time stamp information associated with at least three signals. The time stamp power correction information is, for example, a power correction curve or an information describing a power correction curve or a databank comprising one, two or a plurality of time stamp power correction values, wherein each time stamp power correction value is associated with a signal power level of a signal, for example, of a signal received by a station, for which the time stamp power correction information is determined. The transmission time stamp information describes, for example, when a signal is transmitted and the reception time stamp information describes, for example, when a signal is received. Thus, for example, each of the at least three signals comprises a transmission time stamp information and a corresponding reception time stamp information. The time stamp correction information determiner is configured to determine the time stamp power correction information based on a deviation between a first time interval between a transmission of two signals having different signal power levels and a second time interval between a reception of the two signals having different signal power levels. The first time interval is described by a difference of transmission time stamps related to a first clock and the second time interval is described by a difference of reception time stamps related to a second clock. According to an embodiment, the first time interval is reference to a first clock and the second time interval is referenced to a second clock. The first clock and the second clock can comprise different or same features and functionalities. The time stamp correction information determiner is configured to determine the time stamp power correction information using a clock drift correction which is based on transmission time stamp information and reception time stamp information of at least two signals having same signal power levels.

Thus, for example, two of the at least three signals have the same signal power level and one of the three signals has a signal power level differing from the signal power level of the two signals having same signal power levels. Alternatively, the time stamp correction information determiner can be configured for determining a time stamp power correction information on the basis of transmission time stamp information and reception time stamp information associated with more signals, wherein at least two signals have same signal power levels and at least two signals have different signal power levels like, for example, four signals with two signals having a first signal power level, a signal having a second signal power level and a signal having a third signal power level. For example, the determined time stamp power correction information based on three signals can comprise two time stamp power correction values because two signal power levels are analyzed by the time stamp correction information determiner, wherein the time stamp power correction value related to the signal power level of the two signals having same signal power level is, for example, zero. Thus, for example, the determined time stamp power correction information based on four signals can comprise three time stamp power correction values or two time stamp power correction values because either three signal power levels or two signal power levels are analyzed by the time stamp correction information determiner.

This invention is based on the idea that it is efficient to use at least three signals, for example, only three signals, to determine the time stamp power correction information. The time stamp correction information determiner, for example, receives the transmission time stamp information and the reception time stamp information from two transceivers, between which the at least three signals are transmitted. Thus, no additional hardware is, for example, needed to determine the time stamp power correction information by the time stamp correction information determiner. Furthermore, the used clock drift correction is, for example, independent from the signal power level and results in a high accuracy. Since the determination of the time stamp power correction information is based on the deviation between the first time interval and the second time interval corresponding to time stamps of two signals having different signal power levels a usage of a clock drift correction independent from the signal power increases the accuracy of the time stamp correction information determiner for determining the time stamp power correction information. The determined time stamp power correction information is thus highly accurate and deterministic. Furthermore, the time stamp correction information determiner provides, for example, individual time stamp power correction information for every station receiving signals, whereby it is possible with the time stamp power correction information to correct, for example, time stamps, for example, reception time stamps, determined by an individual station independent of the station transmitting the signal, because, for example, only time differences are analyzed, whereby the influence of the transmitting station is reduced or canceled.

According to an embodiment, the time stamp power correction information comprises correction values for a time stamp, for example, for a reception time stamp, of a received signal and/or correction values for time differences, for example, a time difference between a transmission time stamp and a reception time stamp of a received signal and/or correction values for a distance, for example, a distance between the station transmitting a signal and a station receiving the signal and/or a relationship between a measured signal power and a real signal power of an analyzed signal. This allows very accurate distance estimations for methods based on the signal power level.

According to an embodiment, the time stamp correction information determiner is configured to obtain a first transmission time stamp information describing when a reference transceiver transmits a first signal using a first signal power level. In other words the first transmission time stamp information describes, for example, with respect to a clock of the reference transceiver when the reference transceiver transmits the first signal. The time stamp correction information determiner is configured to obtain a first reception time stamp information describing when a second transceiver receives the first signal. In other words the first reception time stamp information describes, for example, with respect to a clock of the second transceiver when a second transceiver receives the first signal. The time stamp correction information determiner can be configured to obtain a second transmission time stamp information describing, for example, with respect to a clock of the reference transceiver, when the reference transceiver transmits a second signal using a second power level. The time stamp correction information determiner can be configured to obtain a second reception time stamp information describing, for example, with respect to a clock of the second transceiver, when the second transceiver receives the second signal. Furthermore, the time stamp correction information determiner can be configured to obtain a third transmission time stamp information describing, for example, with respect to a clock of the reference transceiver, when the reference transceiver transmits a third signal using the first power level. The time stamp correction information determiner can be configured to obtain a third reception time stamp information describing, with respect to a clock of the second transceiver, when the second transceiver receives the third signal. The time stamp correction information determiner can be configured to determine the first time interval based on a deviation between the first transmission time stamp information and the second transmission time stamp information and to determine the second time interval based on a deviation between the first reception time stamp information and the second reception time stamp information.

The deviation between the first transmission time stamp information and the second transmission time stamp information is optionally clock-drift-corrected and/or the deviation between the first reception time stamp information and the second reception time stamp information is optionally clock drift corrected. The second power level (second signal power level) differs, for example, from the first power level (first signal power level). The first transmission time stamp information, the second transmission time stamp information and the third transmission time stamp information comprise, for example, transmission time stamps related to the first clock, associated with the clock of the reference transceiver. The first reception time stamp information, the second reception time stamp information and the third reception time stamp information comprise, for example, time stamps corresponding to the second clock, associated with the clock of the second transceiver. If the time stamp correction information determiner is part of the reference transceiver, the first transmission time stamp information, the second transmission time stamp information and the third transmission time stamp information can be determined by itself, else the first transmission time stamp information, the second transmission time stamp information and/or the third transmission time stamp information are, for example, received from the reference transceiver. If the time stamp correction information determiner is part of the second transceiver, the first reception time stamp information, the second reception time stamp information and/or the third reception time stamp information can be determined by itself, else the first reception time stamp information, the second reception time stamp information and/or the third reception time stamp information are, for example, received from the second transceiver.

According to an embodiment, the first reception time stamp information, the second reception time stamp information and the third reception time stamp information are associated with the second clock associated with the second transceiver and the first transmission time stamp information, the second transmission time stamp information and the third transmission time stamp information are associated with the first clock associated with the reference transceiver. The time stamp correction information determiner is configured to apply the clock drift correction to the deviation between the first transmission time stamp information and the second transmission time stamp information. Furthermore or alternatively, the time stamp correction information determiner is configured to apply the clock drift correction to the deviation between the first reception time stamp information and the second reception time stamp information. According to an embodiment, it is efficient to only correct the deviation between the transmission time stamp information or the deviation between the reception time stamp information by the clock drift correction, because the clock drift correction can, for example, transform a time stamp corresponding to the first clock into a time stamp corresponding to the second clock. If, for example, the second clock runs faster than the first clock a time of the reception time stamps, corresponding to the second clock, can be reduced to be comparable with the transmission time stamps corresponding to the first clock or a time of the transmission time stamps can be changed to a later time, to be comparable with the reception time stamps corresponding to the second clock. Thus, for example, the time stamps corresponding to the first clock are adapted to correspond to a time of the second clock or the time stamps of the second clock are adapted to correspond to a time of the first clock, so all transmission time stamps and reception time stamps correspond to one clock.

According to an embodiment, the time stamp correction information determiner is configured to use the clock drift correction to correct the first time interval and/or the second time interval based on a deviation between the first clock and the second clock. The first clock runs, for example, faster or slower than the second clock, whereby the deviation is caused. Since the first time interval and the second time interval correspond to different clocks, with this feature the two time intervals, e.g., the first time interval and/or the second time interval, are synchronized, to be able to compare time stamps corresponding to a first clock with time stamps corresponding to the second clock.

According to an embodiment, the time stamp correction information determiner is configured to use the clock drift correction to correct the transmission time stamp information and/or the reception time stamp information based on a deviation between the first clock and the second clock. The transmission time stamp information comprises, for example, the first transmission time stamp information and the second transmission time stamp information and the reception time stamp information comprises, for example, the first reception time stamp information and the second reception time stamp information. Thus, the first time interval and/or the second time interval are corrected by the clock drift correction, which corresponds to a synchronization of the first clock and the second clock.

According to an embodiment, the time stamp correction information determiner is configured to determine the clock drift correction using time-interpolation or time- extrapolation of a deviation between a first difference and a second difference. The first difference is a difference of transmission time stamps associated with a transmission of two signals having same power levels. The second difference is a difference of reception time stamps associated with a reception of the two signals having same signal power levels. The two signals, on which the first difference is based and the two signals on which the second difference is based are, for example, the same two signals. The first difference can relate to the first clock and the second difference can relate to the second clock. The deviation between the first difference and the second difference is, for example, caused by a deviation between the first clock and the second clock. If the first clock and the second clock would run simultaneously, i.e., synchronized, the first difference would equal the second difference, since no clock drift or nearly no clock drift occurs. If the clock drift correction is determined using the time-interpolation, then, for example, the clock drift correction is determined for a signal, which transmission is timed between the transmission of the two signals having same signal power levels and if the clock drift correction is determined using the time-extrapolation, the clock drift correction is determined for a signal, whose transmission is timed after the transmission of the two signals having same signal power levels. By the time-interpolation or the time- extrapolation a very exact clock drift correction can be determined, whereby a high accuracy in determining the time stamp power correction information can be achieved.

According to an embodiment, the time stamp correction information determiner is configured to determine the clock drift correction based on the first difference between the first transmission time stamp information and the third transmission time stamp information and based on the second difference between the first reception time stamp information and the third reception time stamp information. Thus, the clock drift correction is, for example, based on the first signal and the third signal having same signal power levels. Thus, by time-interpolation the second transmission time stamp and/or the second reception time stamp of the second signal can be corrected, to determine a time stamp power correction information with high accuracy.

According to an embodiment, the time stamp correction information determiner is configured to determine the clock drift correction CDC based on a deviation Ci,₃ between a first difference AT™ of transmission time stamps associated with a transmission of two signals having same signal power levels and a second difference AT of reception time stamps associated with a reception of the two signals having same signal power levels according to

wherein AT™ represents a difference of transmission time stamps of a transmission of the two signals having different signal power levels. The deviation Ci,3 is, for example, determined according to Ci,3 = AT™ - AT™. The first difference AT is, for example, determined according to AT™ = T™ - T™, wherein T™ represents the first transmission time stamp information and T™ represents the third transmission time stamp information. The second difference AT™ is, for example, determined according to AT™ = T™ - T™, wherein T™ represents a first reception time stamp information and the T™ represents the third reception time stamp information. The difference AT™ is, for example, determined according to AT™ = T™ - T™, wherein T™ represents the first transmission time stamp information and T™ represents the second transmission time stamp information. The term Ci,3 / AT™ corresponds, for example, to a time-interpolation to provide a clock drift correction CDC for the second signal represented by AT™. With the clock drift correction it is not only possible to correct time differences like AT™ but also to correct exact time stamps.

According to an embodiment, the time stamp correction information determiner is configured to determine a deviation between a difference of transmission time stamps of a transmission of the two signals having different signal power levels and a difference of reception time stamps of a reception of the two signals having different signal power levels. The deviation is caused by a clock drift and by the different signal power levels. Both errors, the clock drift and the signal power level influence, have, for example, still an effect on the deviation, because none of them is already corrected by, for example, the clock drift correction or the time stamp power correction information. The time stamp correction information determiner is configured to at least partially remove a contribution caused by the clock drift from said deviation to thereby obtain a clock drift corrected version of the deviation. The time stamp correction information determiner is, for example, configured to provide the clock drift correct version as the time stamp power correction information or to determine the time stamp power correction information from the clock drift corrected version. This feature is based on the idea that the deviation is caused by the clock drift and by the different signal power levels and by correcting the clock drift with the clock-drift-correction the influence of the different signal power levels can be determined. According to an embodiment, the time stamp correction information determiner is configured to determine the time stamp power correction information C ₂, which is, for example, associated with a power level, according to

The value Ci,₂ represents the deviation between the first time interval described by a difference AT_{X 2} of transmission time stamps of two signals having different signal power levels related to a first clock and the second time interval described by a difference of reception time stamps of the two signals having different signal power levels related to a second clock. The value Ci_,3 represents a deviation between a first difference DT™ of transmission time stamps associated with a transmission of the two signals having same signal power levels related to the first clock and a second difference AT of reception time stamps associated with a reception of the two signals having same signal power levels related to the second clock. Thus the deviation Ci_,2 between the first time interval and the second time interval is clock drift corrected, resulting in a deviation, for example, only comprising an influence of a signal power level, which can approximate the time stamp power correction information

very accurate. Thus, the deviation Ci_,2 between the first time interval and the second time interval is, for example, clock drift corrected by the last term -¾ AT_{X 2} of the equation. The term represents, for example, a contribution

^^Tl,3

of a clock drift, which is removed from the deviation Ci,2 to receive the time stamp power correction information C_X' ₂·

The time stamp power correction information C{ ₂ is, for example, associated with a time period, which can be added to a reception time stamp to get a new power corrected reception time stamp. The time period can be a positive time period resulting in a new later reception time stamp, or the time period can be a negative time period, resulting a new earlier reception time stamp. At a predetermined signal power level, corresponding, for example, to a signal power level used for determining the clock drift correction, for example, the signal power level of the two signals having same signal power levels, the time stamp power correction information is zero. If a signal is received, with a signal power level higher than the predetermined signal power level, the time stamp power correction information is, for example, associated with a negative time period. If a signal is received with a smaller signal power level than the predetermined signal power level, the time stamp power correction information is, for example, associated with a positive time period. According to an embodiment, the time stamp power correction information is associated with a distance. This is based on the idea that the time differences can be converted into a distance passed by a signal in the time differences. Corresponding to the time differences, the distance can also comprise a positive distance and/or a negative distance if a signal, whose reception time stamp has to be power corrected, has a signal power level higher than the predetermined signal power level, the signal is faster and thus the time stamp power correction information is, for example, associated with a negative distance. If the signal, whose reception time stamp has to be power corrected, has a signal power level smaller than the predetermined signal power level, the signal is slower and thus the time stamp power correction information is, for example, associated with a positive distance. The time stamp power correction information comprises, for example, the time difference and/or the distance.

According to an embodiment, the time stamp correction information determiner is configured to determine the time stamp power correction information for more than two different signal power levels. Thus, the time stamp correction information determiner is, for example, configured to determine a time stamp power correction curve, from which correction values corresponding to a signal power level can be read, or a time stamp power correction information comprising time stamp power correction values for at least two different signal power levels. Thus, the time stamp correction information determiner is, for example, configured to provide the time stamp power correction information for at least two different signal power levels, whereby for signals having one of the at least two different signal power levels a very accurate analysis can be achieved with the time stamp power correction information.

An embodiment of the invention is related to a transceiver for use in a positioning system, for example, a reference transceiver. The transceiver is configured to transmit at least three signals during a calibration phase. The at least three signals comprise, for example, different transmission time stamps. At least two of the at least three transmitted signals comprise same signal power levels and at least two of the at least three transmitted signals comprise different signal power levels. This means, for example, that a first signal has a first signal power level and a second signal has the first signal power level and a third signal has a second signal power level, wherein the first signal power level differs from the second signal power level. Thus, two of the three signals, e.g., the first signal and the second signal, comprise same signal power levels associated with the first signal power level and at least two of the three signals, e.g., the first signal and the third signal or the second signal and the third signal, comprise different signal power levels. The transceiver is configured to provide transmission time stamps, describing times at which the at least three signals are transmitted, for a determination of a time stamp power correction information.

Thus, the herein proposed transceiver can be used by the time stamp correction information determiner. According to an embodiment, the time stamp correction information determiner is configured to obtain the transmission time stamps from the transceiver. According to an embodiment, the transceiver comprises a time stamp correction information determiner. Thus, the time stamp correction information determiner is, for example, configured to determine the transmission time stamps by itself. With this feature, the transceiver is, for example, configured to provide the transmission time stamps corrected by the time stamp correction information determiner using the time stamp power correction information.

An embodiment of the invention is related to a transceiver for use in a positioning system, for example, a second transceiver. The transceiver is configured to receive at least three signals during a calibration phase. The three signals are, for example, received at different times. At least two of the at least three received signals comprise same signal power levels and at least two of the at least three received signals comprise different signal power levels. This means, for example, that a first signal has a first signal power level and a second signal has the first signal power level and a third signal has a second signal power level, wherein the first signal power level differs from the second signal power level. Thus, two of the three signals, e.g., the first signal and the second signal, comprise same signal power levels associated with the first signal power level and at least two of the three signals, e.g., the first signal and the third signal or the second signal and the third signal, comprise different signal power levels.

Optionally, the transceiver is configured to provide a power level information describing powers of the at least three transmitted signals. Alternatively, the transceiver is, for example, configured to obtain the power level information describing the powers of the at least three transmitted signals from a transceiver receiving the at least three signals.

The transceiver is configured to provide reception time stamps, describing times at which the at least three signals are received, for a determination of a time stamp power correction information. Optionally, the transceiver is configured to provide a power level information describing powers of the at least three received signals. Alternatively, the transceiver is, for example, configured to obtain the power level information describing the powers of the at least three received signals from a transceiver transmitting the at least three signals.

An embodiment according to this invention is related to a calculator for determining a time-of-flight of one or more signals (e.g., in a preferred embodiment, of two or more signals) and/or a distance passed by the one or more signals (e.g., in a preferred embodiment, of the two or more signals) on the basis of transmission time stamp information and reception time stamp information associated with two or more signals. In other words, for example, at least two signals (e.g., transmitted between the same two transceiver) are used by the calculator, to calculate the time-of-flight of one of the two signals (it can be inferred, that the other signal of the two signals is associated with the same time-of-flight). The calculator is configured to obtain a time stamp power correction information and a clock-drift correction. The calculator is configured to determine the time- of-flight of the one or more signals (e.g., in a preferred embodiment, of the two or more signals) and/or the distance passed by the one or more signals (e.g., in a preferred embodiment, of the two or more signals) based on a transmission time stamp and a reception time stamp of a first signal transmitted from a first transceiver to a second transceiver, e.g., of the first signal transmitted by the first transceiver and received by the second transceiver, and a transmission time stamp and a reception time stamp of a second signal transmitted from the second transceiver to the first transceiver, e.g., of the second signal transmitted by the second transceiver and received by the first transceiver. Furthermore, the calculator is configured to use the time stamp power correction information and the clock-drift correction for determining the time-of-flight of the one or more signals (e.g., in a preferred embodiment, of the two or more signals) and/or the distance passed by the one or more signals (e.g., in a preferred embodiment, of the two or more signals). Since the first transceiver and the second transceiver always comprise, for example, the same distance to each other, the first signal and the second signal comprise the same time-of-flight. Thus, the calculator is, for example, configured to determine the time-of-flight of the first signal and/or the distance passed by the first signal between the first transceiver and the second transceiver and to associate the determined time-of-flight and/or the determined distance of the first signal with the time-of-flight and/or the distance of the second signal. According to an embodiment, the calculator is configured to obtain the time stamp power correction information and/or the clock-drift correction from a time stamp correction information determiner as described herein. Thus, for example, the calculator comprises the time stamp correction information determiner or the calculator has access to an external time stamp correction information determiner, wherein the calculator is, for example, configured to transmit relevant information, like transmission time stamp information and/or reception time stamp information to the time stamp correction information determiner and/or to receive corrected transmission time stamp information and/or corrected reception time stamp information and/or the time stamp power correction information and/or the clock-drift correction, to use them by itself to determine the time-of- flight of a signal and/or the distance passed by the signal very accurate.

According to an embodiment, the calculator is configured to determine the clock-drift correction using transmission time stamp information and reception time stamp information associated with a third signal. The calculator is, for example, configured to use transmission time stamp information and reception time stamp information of two signals, having same signal power levels, transmitted from the first transceiver to the second transceiver and/or of two signals, having same signal power levels, transmitted from the second transceiver to the first transceiver. The third signal has, for example, the same signal power level as one of the two or more signals, for which the time-of-flight and/or the distance passed by the signal is determined.

According to an embodiment the calculator is configured to calculate the time-of-flight TO A of the two signals transmitted between the first transceiver and the second transceiver according to

TO A = 0.5

The value T ₂ can represent a difference between a reception time stamp information of a second signal received by the first transceiver and transmitted, for example, by the second transceiver and a transmission time stamp information of a first signal transmitted by the first transceiver and, for example, received by the second transceiver. The difference ATz₂ can be calculated according to DT ₂ = T₂ - T*, wherein Tf represents the transmission time stamp information of the first signal at the first transceiver and wherein T₂ represents the reception time stamp information of the second signal at the first transceiver. The value AT[₂ can represent a difference between a transmission time stamp information of the second signal transmitted by the second transceiver and a reception time stamp information of the first signal received by the second transceiver. The difference AT[₂ can be calculated according to AT[₂ - j - T[, wherein T[ represent the reception time stamp information of the first signal at the second transceiver and wherein T₂ is the transmission time stamp information of the second signal at the second transceiver. The value CDC represents a clock drift correction factor for correcting a clock drift between a first clock associated with the first transceiver and a second clock associated with the second transceiver. The clock drift correction factor CDC, for example, corrects time stamps, time differences, time intervals and/or time stamp power correction information (DT ₂ and Ei) associated with the second clock. Thus, in the equation for time- of-flight TOA all time stamps and/or time differences and/or time stamp power correction information associated with the second clock are corrected by the clock drift correction factor CDC such that all time stamps and/or time differences and/or time stamp power correction information in the equation of the time-of-flight TOA are associated with the first clock and are thus, for example, represented in same time units, as if the first clock and the second clock would be synchronized. Thus, it is possible to determine the time-of- flight TOA by the calculator very accurately. The value E represents a first time stamp power correction information associated with a first signal. Ei is, for example, obtained from the time stamp correction information determiner. Ei is, for example, associated with a timing error of the reception time stamp information of the first signal received by the second transceiver, related to a signal power level of the first signal. The value E₂ represents a second time stamp power correction information associated with a second signal. E₂ is, for example, obtained from the time stamp correction information determiner. E₂ is, for example, associated with a timing error of the reception time stamp information of the second signal received by the first transceiver, relating to a signal power level of the second signal. The value Z represents a constant offset. Z represents, for example, a zero line for the signal power level and/or the antenna offset. According to an embodiment, Z can also be equal to zero.

According to an embodiment, the clock-drift correction CDC is determined according to CDC = using an obtained deviation Ci_,3 between a difference AT*₃ of transmission time stamps and a difference D¾ of reception time stamps of two signals transmitted from one of the transceivers to another one of the transceivers and having same signal power levels. Thus, for example, the two signals are transmitted by the first transceiver and received by the second transceiver or are transmitted by the second transceiver and received by the first transceiver. The determination of CDC is, e.g., based on the transmission time stamp information and the reception time stamp information of the first signal and the third signal having same signal power levels. According to an embodiment the clock-drift correction CDC is determined according to CDC = = ^ATl~f ^Tl'³ =

, wherein T₃ ^b is the transmission time stamp information of the third signal

(^T3 ^~Tΐ )

at the reference transceiver, wherein T-f is the transmission time stamp information of the first signal at the reference transceiver, wherein Tx is the reception time stamp information of the third signal at the second transceiver and wherein

is the reception time stamp information of the first signal at the second transceiver. Thus, the clock-drift correction CDC represents, for example, a time-interpolation of a time difference between the first and the third signal to a time difference between the first and the second signal, wherein the second signal is timed between the first and the third signal. To determine the clock drift correction CDC it is, for example, advantageous if the two signals used for determining the clock-drift correction have same signal power levels. Thus, the signal power level has no or nearly no influence on the clock-drift correction.

An embodiment according to this invention is related to a transceiver, e.g., the reference transceiver or the second transceiver, comprising a time stamp correction information determiner according to one of the embodiments described herein and/or a calculator according to one of the embodiments described herein. Thus it is possible, that the transceiver is configured to correct time stamps by itself using the time stamp correction information determiner. Optionally the transceiver is configured to calculate a time-of-flight of a signal transmitted by the transceiver and received by a different station and/or to calculate a time-of-flight of a signal received by the transceiver and transmitted by the different station.

An embodiment according to this invention is related to a system comprising two transceiver, a time stamp correction information determiner according to one of the embodiments described herein and a calculator according to one of the embodiments described herein. Thus a system is realized to determine, for example, a position of one of the two transceiver using the time stamp correction information determiner and/or the calculator, when the position of the other transceiver of the two transceiver is known. It is advantageous, that the system can be calibrated and corrected by itself using, for example, the time stamp correction information determiner.

An embodiment according to this invention is related to a method for determining a time stamp power correction information on the basis of transmission time stamp information and reception time stamp information associated with at least three signals. The method comprises determining the time stamp power correction information based on a deviation between a first time interval between a transmission of two signals having different signal power levels and a second time interval between a reception of the two signals having different signal power levels. The first time interval is described by a difference of transmission time stamps related to a first clock and the second time interval is described by a difference of reception time stamps related to a second clock. The method uses a clock drift correction which is based on transmission time stamp information and reception time stamp information of at least two signals having same signal power levels to determine the time stamp power correction information based on the deviation.

An embodiment according to this invention is related to a method comprising transmitting at least three signals during a calibration phase. At least two of the at least three transmitted signals comprise same signal power levels, and at least two of the at least three transmitted signals comprise different signal power levels. Furthermore the method comprises providing transmission time stamps, describing times at which the at least three signals are transmitted, for determining a time stamp power correction information. Optionally the method comprises providing a power level information describing powers of the at least three transmitted signals/for example, at the transmission time stamp.

An embodiment according to this invention is related to a method comprising receiving at least three signals during a calibration phase. At least two of the at least three received signals comprise same signal power levels, and at least two of the at least three received signals comprise different signal power levels. Furthermore the method comprises providing transmission time stamps, describing times at which the at least three signals are received and optionally a power level information describing powers of the at least three received signals, for example, at the reception time stamp, for determining a time stamp power correction information.

An embodiment according to this invention is related to a method for determining a time- of-flight of one or more signals (e.g., in a preferred embodiment, of two or more signals) and/or a distance passed by the one or more signals (e.g., in a preferred embodiment, of the two or more signals) on the basis of transmission time stamp information and reception time stamp information associated with two or more signals. In other words, for example, at least two signals (e.g., transmitted between the same two transceiver) are used, to calculate the time-of-flight of one of the two signals (it can be inferred, that the other signal of the two signals is associated with the same time-of-flight). The method comprises obtaining a time stamp power correction information and a clock-drift correction. Furthermore the method comprises determining the time-of-flight of the one or more signals (e.g., in a preferred embodiment, of the two or more signals) and/or the distance passed by the one or more signals (e.g., in a preferred embodiment, of the two or more signals) based on a transmission time stamp and a reception time stamp of a first signal transmitted from a first transceiver to a second transceiver (e.g. transmitted by the first transceiver and received by the second transceiver) and a transmission time stamp and a reception time stamp of a second signal transmitted from the second transceiver to the first transceiver (e.g. transmitted by the second transceiver and received by the first transceiver) using the time stamp power correction information and the clock-drift correction.

An embodiment according to this invention is related to a computer program having a program code for performing, when running on a computer, a method according to one of the embodiments described herein.

The method as described above are, for example, based on the same considerations as the above-described time stamp correction information determiner, transceiver and/or calculator. The methods can, by the way, be completed with all features and functionalities, which are also described with regard to the time stamp correction information determiner, transceiver and/or calculator.

Brief Description of the Drawings

The drawings are not necessarily to scale, emphasis instead generally being placed upon illustration of the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which:

Fig. 1 shows a schematic view of a time stamp correction information determiner according to an embodiment of the present invention;

Fig. 2 shows a schematic view of a transceiver configured to transmit at least three signals during a calibration phase according to an embodiment of the present invention; Fig. 3 shows a schematic view of a transceiver configured to receive at least three signals during a calibration phase according to an embodiment of the present invention;

Fig. 4 shows a schematic view of a calculator according to an embodiment of the present invention;

Fig. 5 shows a schematic view of two transceivers according to an embodiment of the present invention;

Fig. 6 shows a schematic view of two transceivers in their calibration phase according to an embodiment of the present invention;

Fig. 7 A shows a schematic diagram of signal power levels of three signals, having same signal power levels, for determining a clock-drift correction according to an embodiment of the present invention;

Fig. 7B shows a schematic diagram of a clock-drift error, used for a clock-drift correction according to an embodiment of the present invention;

Fig. 7C shows a schematic diagram of results of a clock-drift correction according to an embodiment of the present invention;

Fig. 8A shows a schematic diagram of signal power levels of three signals, wherein two signals have same signal power levels and a signal power level of a third signal changes in steps according to an embodiment of the present invention;

Fig. 8B shows a schematic diagram of a time stamp power correction information, determined based on the three signals show in Fig. 8A, according to an embodiment of the present invention;

Fig. 9A shows a schematic diagram of signal power levels of three signals, wherein two of the three signals have same signal power levels and the signal power level of a third signal differs from the two signals having same signal power level, by a continuous change of its signal power level according to an embodiment of the present invention;

Fig. 9B shows a schematic diagram of a time stamp power correction information, determined based on the three signals shown in Fig. 9A, according to an embodiment of the present invention;

Fig. 9C shows a schematic diagram of signal power levels of three signals with a short update time according to an embodiment of the present invention;

Fig. 10A shows a schematic diagram of a measured signal power level versus a real signal power level according to an embodiment of the present invention;

Fig. 10B shows a schematic diagram of a signal power level correction curve representing the time stamp power correction information according to an embodiment of the present invention;

Fig. 10C shows a schematic diagram of a measured signal power level versus a real signal power level associated with several restarts according to an embodiment of the present invention;

Fig. 10D shows a schematic diagram of a signal power level correction curve representing a time stamp power correction information associated with several restarts according to an embodiment of the present invention;

Fig. 1 1 shows a schematic view of two signals sent between two transceivers, to be analyzed by a calculator according to an embodiment of the present invention; Fig. 12 shows a schematic view of three signals sent between two transceivers, to be analyzed by a calculator according to an embodiment of the present invention;

Fig. 13 shows a schematic diagram of test results of a calculator according to an embodiment of the present invention; Fig. 14 shows a schematic diagram of a channel impulse response of a Decawave

1000 chip;

Fig. 15 shows a schematic view of a phased locked loop (PLL);

Fig. 16A shows a schematic diagram of an integrator of PLL;

Fig. 16B shows a schematic diagram of a filtered integrator of a PLL four times restarted;

Fig. 17 shows a schematic diagram of a temperature crystal oscillator warmup of a

Decawave 1000 chip;

Fig. 18A shows a schematic diagram of a filtered received signal power of a general receiver;

Fig. 18B shows a schematic diagram of a filtered integrator of a general receiver of a

PLL corresponding to the filtered received signal power shown in Fig. 18A;

Fig. 19 shows a schematic diagram illustrating an effect of range bias on a reported distance; and

Fig. 20 shows a schematic diagram of an estimated signal power level of a received signal with respect to an actual signal power level of the received signal.

Detailed Description of the Embodiments

Equal or equivalent elements or elements with equal or equivalent functionality are denoted in the following description by equal or equivalent reference numerals even if occurring in different figures.

In the following description, a plurality of details is set forth to provide a more throughout explanation of embodiments of the present invention. However, it will be apparent to those skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form rather than in detail in order to avoid obscuring embodiments of the present invention. In addition, features of the different embodiments described herein after may be combined with each other, unless specifically noted otherwise.

Herein, the following notation can be used: A time stamp can be indicated as T^x, wherein i represents the corresponding signal and x corresponds to the station determining the time stamp. The index x can be TX (transmission transceiver), RX (reception transceiver), R (reference station), T (tag), etc. The index i can be in the range of 1 to n, wherein n is a positive integer of at least 2. Thus, for example, the time stamp T[^x represents a transmission time stamp corresponding to a first signal determined by a transmission transceiver. A difference between two time stamps T„ -

can be indicated as

wherein the index N and the index M represent values of the index i in the range of 1 to n and the two time stamps T and

represent times determined by the same station, see index x. A clock drift with respect to the time stamps of a signal N and a signal M, for example, transmission time stamps and reception time stamps of both signals, can be indicated by CN,M. A time stamp power correction information can comprise a time stamp error due to the signal power level, which can be indicated as Ej, wherein the index i represents the corresponding signal with, for example, a defined signal power level. Furthermore, an antenna delay and a signal power correction offset can be indicated by Z.

Fig. 1 shows a schematic view of a time stamp correction information determiner 100 for determining a time stamp power correction information 1 10 on the basis of transmission time stamp information 120 and reception time stamp information 130 associated with at least three signals.

The time stamp power correction information 110 is, for example, a power correction curve or an information describing a power correction curve. The power correction curve defines, for example, for a signal with a defined signal power level a power correction value for correcting a reception time stamp of this signal. The time stamp power correction information 1 10 can alternatively be a power correction value corresponding to a signal power level or a databank comprising a plurality of power correction values corresponding to different signal power levels. The time stamp power correction information 110 is, for example, related to a station receiving the at least three signals. Thus, the time stamp power correction information 110 is, for example, independent of the station transmitting the at least three signals. The transmission time stamp information 120, for example, comprises transmission time stamps of the at least three signals and/or transmission time stamp information of all signals of the at least three signals. The reception time stamp information 130 comprises, for example, reception time stamps of the at least three signals and/or reception time stamp information of all signals of the at least three signals. The at least three signals are, for example, transmitted at different transmission time stamps and/or received at different reception time stamps. Thus, the time stamp correction information determiner 100 can be configured to analyze the transmission time stamp information 120 and the reception time stamp information 130 to determine the time stamp power correction information 110.

According to an embodiment, the time stamp correction information determiner 100 is configured to obtain the transmission time stamp information 120 and/or the reception time stamp information 130. This means, for example, that if the time stamp correction information determiner 100 is part of a station transmitting the at least three signals, the transmission time stamp information 120 can be determined by itself and the reception time stamp information 130 can be received from a station receiving the at least three signals. If the time stamp correction information determiner 100 is part of a station receiving the at least three signals, the time stamp correction information determiner 100 can be configured to receive the transmission time stamp information 120 from the station transmitting the at least three signals and to determine the reception time stamp information 130 by itself. Alternatively, if the time stamp correction information determiner 10 is not part of the station transmitting the at least three signals and the station receiving the at least three signals, the time stamp correction information determiner 100 is, for example, configured to receive the transmission time stamp information 120 and the reception time stamp information 130 from the corresponding stations providing the information.

According to an embodiment, the transmission time stamp information 120 and the reception time stamp information 130 are associated with the at least three signals transmitted between a reference transceiver and a second transceiver, wherein the reference transceiver, for example, transmits the at least three signals and the second transceiver receives the at least three signals.

According to an embodiment, the transmission time stamp information 120 comprises a first transmission time stamp information, a second transmission time stamp information and a third transmission time stamp information and the reception time stamp information 130 comprises a first reception time stamp information, a second reception time stamp information and a third reception time stamp information. The first transmission time stamp information describes, for example, with respect to a clock of the reference transceiver when the reference transceiver transmits a first signal using a first signal power level. The second transmission time stamp information describes, for example, with respect to the clock of the reference transceiver, when the reference transceiver transmits a second signal using a second signal power level and the third transmission time stamp information describes, for example, with respect to the clock of the reference transceiver, when the reference transceiver transmits a third signal using the first signal power level. The first reception time stamp information describes, for example, with respect to a clock of the second transceiver, when the second transceiver receives the first signal. The second reception time stamp information describes, for example, with respect to the clock of the second transceiver, when the second transceiver receives the second signal and the third reception time stamp information describes, for example, with respect to the clock of the second transceiver, when the second transceiver receives the third signal.

The time stamp correction information determiner 100 can be configured to determine the time stamp power correction information 1 10 based on a deviation 140 between a first time interval 150 between a transmission of two signals having different signal power levels which is, for example, referenced to a first clock, and a second time interval 160 between a reception of the two signals having different signal power levels which is, for example, referenced to a second clock. The first time interval 150 can be described by a difference of transmission time stamps related to the first clock, wherein the transmission time stamps can be obtained from the transmission time stamp information 120. The second time interval 160 can be described by a difference of reception time stamps related to the second clock, wherein the reception time stamps can be obtained from the reception time stamp information 130.

According to an embodiment, the time stamp correction information determiner 100 is configured to determine the first time interval 150 based on a deviation between the first transmission time stamp information and the second transmission time stamp information. Optionally, the deviation between the first and the second transmission time stamp information is clock-drift-corrected. Additionally, the time stamp correction information determiner is, according to an embodiment, configured to determine the second time interval 160 based on a deviation between the first reception time stamp information and the second reception time stamp information. Optionally, the deviation between the first and the second reception time stamp information is clock-drift-corrected.

The time stamp correction information determiner 100 is configured to determine the time stamp power correction information 110 using a clock drift correction 170 which is based on the transmission time stamp information 120 and the reception time stamp information 130 of at least two signals having same signal power levels.

According to an embodiment, the time stamp correction information determiner 100 is configured to obtain the clock drift correction from an external device or to determine it by itself. If the time stamp correction information determiner 100 is configured to determine the clock drift correction 170 by itself, the time stamp correction information determiner 100 can use the transmission time stamp information 120 and the reception time stamp information 130, wherein only transmission time stamps and reception time stamps of at least two signals having same signal power levels are, for example, used. Thus, the time stamp correction information determiner 100 is, for example, configured to select the transmission time stamp information and the reception time stamp information of the at least two signals having same signal power levels out of the transmission time stamp information 120 and the reception time stamp information 130 associated with at least three signals.

According to an embodiment, the clock of the reference transceiver represents the first clock and the clock of the second transceiver represents the second clock. Thus, the first reception time stamp information, the second reception time stamp information and the third reception time stamp information are associated with the first clock associated with the reference transceiver and the first transmission time stamp information, the second transmission time stamp information and the third transmission time stamp information are associated with the second clock associated with the second transceiver. The time stamp correction information determiner 100 is, for example, configured to apply the clock drift correction 170 to the deviation between the first transmission time stamp information and the second transmission time stamp information, wherein the determination of the first time interval 150 is, for example, based on this deviation.

Alternatively or additionally, the time stamp correction information determiner 100 is configured to apply the clock drift correction 170 to the deviation between the first reception time stamp information and the second reception time stamp information, wherein the determination of the second time interval 160 is, for example, based on this deviation. Thus, the time stamp correction information determiner 100 is, for example, configured to use the clock drift correction 170 to correct the first time interval 150 and/or the second time interval 160 based on a deviation between the first clock and the second clock.

According to an embodiment, the time stamp correction information determiner 100 is configured to use the clock drift correction 170 to correct the transmission time stamp information 120 and/or the reception time stamp information 130 based on the deviation between the first clock and the second clock.

The clock drift is, for example, associated with at least two clocks running with different velocities. Thus, for example, the first clock runs faster than the second clock or the second clock runs faster than the first clock. Because of this clock drift, time stamps corresponding to the first clock, like the transmission time stamp information 120, is not comparable to time stamps related to the second clock, like the reception time stamp information 130. With the clock drift correction 170 either the transmission time stamp information 120 corresponding to the first clock can be transformed to transmission time stamp information related to the second clock or the reception time stamp information 130 related to the second clock can be transformed to reception time stamp information related to the first clock. Thus, the time stamp correction information determiner 100 can be configured to analyze the transmission time stamp information 120 and the reception time stamp information 130 relating to one common clock, using the clock drift correction 170, for determining the time stamp power correction information 110.

According to an embodiment, the time stamp correction information determiner 100 is configured to determine the clock drift correction 170 using time-interpolation or time- extrapolation of a deviation between a first difference of transmission time stamps associated with a transmission of two signals having same signal power levels by one station and a second difference of reception time stamps associated with a reception of the two signals having same signal power levels by another station. The first difference is, for example, related to the first clock and the second difference is, for example, related to the second clock, wherein the deviation between the first difference and the second difference is caused by a deviation between the first clock and the second clock. If no deviation between the first clock and the second clock occurs, the clock drift correction equals, for example, zero. According to an embodiment, the time stamp correction information determiner 100 is configured to determine the clock drift correction 170 based on the first difference between the first transmission time stamp information and the third transmission time stamp information and based on the second difference between the first reception time stamp information and the third reception time stamp information.

According to an embodiment the time stamp correction information determiner is configured to determine the clock drift correction CDC based on a deviation Ci,₃ between a first difference AT™ of transmission time stamps associated with a transmission of two signals having same signal power levels and a second difference AT of reception time stamps associated with a reception of the two signals having same signal power levels according to

The value AT * is associated with a difference of transmission time stamps of a transmission of the two signals having different signal power levels.

According to an embodiment, the time stamp correction information determiner 100 is configured to determine a deviation, for example, the deviation 140, between a difference of transmission time stamps of a transmission of the two signals having different signal power levels, represented, for example, by the first time interval 150, and a difference of reception time stamps of a reception of the two signals having different signal power levels, represented, for example, by the second time interval 160. This deviation is, for example, caused by a clock drift and by the different signal power levels of the two signals. Thus, this deviation is, for example, an error value comprising the clock-drift error and the signal-power-level-error. To determine the signal power level error, the time stamp correction information determiner 100 is, for example, configured to at least partially remove a contribution caused by the clock drift, for example, the clock drift error, from said deviation, to thereby obtain a clock drift corrected version of the deviation, representing, for example, the signal power level error. The time stamp correction information determiner 100 is configured to provide the clock drift corrected version as the time stamp power correction information 110 or to determine the time stamp power correction information from the clock drift corrected version. According to an embodiment, the time stamp correction information determiner is configured to determine the time stamp power correction information C{ ₂, which is associated with a power level, according to

The value Ci,2 represents a deviation between the first time interval described by a difference DG™ of transmission time stamps related to the first clock and the second time interval described by a difference of reception time stamps related to the second clock. The value Ci,3 represents a deviation between a first difference DG™ of transmission time stamps associated with a transmission of the two signals having same signal power levels and a second difference AT™ of reception time stamps associated with a reception of the two signals having same signal power levels.

According to an embodiment, the time stamp correction information determiner 100 is configured to determine the time stamp power correction information 110 for more than two different signal power levels. Thus, the time stamp correction information determiner is, for example, configured to obtain the transmission time stamp information 120 and the reception time stamp information 130 for, for example, 4 signals, 10 signals, 20 signals, 100 signals or any other number of signals, preferably an even multiple of three, since then two of a group of three signals have same signal power levels and one of the group of the three signals, timed between the two signals having same signal power levels, comprises a signal power level, for which the time stamp power correction information is to be determined. Thus, for example, for each group of three signals, the time stamp correction information determiner 100 is, for example, configured to determine a very exact time stamp power correction information, since for each group of three signals the actual clock-drift-correction 170 can be determined and no predetermined clock drift correction is needed. Alternatively, it is also possible to use a predetermined clock drift correction 170 to determine the time stamp power correction information 110 for a plurality of signal power levels and only to determine a new clock drift correction, when the first clock and the second clock have drifted too much apart.

For example, on the basis of transmission time stamp information 120 and reception time stamp information 130 associated with four signals, the time stamp power correction information can comprise two time stamp power corrections for two different signal power levels. According to this embodiment, two of the four signals have a first signal power level, for example, a first signal and a fourth signal, a second signal has a second signal power level and a third signal has a third signal power level. With these four signals, the time stamp correction information determiner 100 is, for example, configured to determine the clock drift correction based on transmission time stamp information and reception time stamp information of the first signal and the fourth signal. The determination of the time stamp power correction information 1 10 is then, for example, based on a first deviation 140 between a first time interval between a transmission of the first signal and the second signal and a second time interval between a reception of the first signal and the second signal. Furthermore, the determination of the time stamp power correction information is based on a second deviation 140 between a first time interval between a transmission of the first signal and the third signal and a second time interval between a reception of the first signal and the third signal. Because of the first deviation 140 and the second deviation 140, the time stamp power correction information 110 comprises a power correction information for the first signal power level, for the second signal power level and/or for the third signal power level, wherein the time stamp power correction for the first signal power level is zero.

According to an embodiment the transceivers from which the time stamp correction information determiner obtains the transmission time stamp information and the reception time stamp information can be Decawave UWB (ultra-wideband) transceiver. Thus with the invention a Decawave UWB clock drift correction and power self-calibration can be realized. For the self-calibration the transceiver performing the self-calibration, for example, comprises the time stamp correction information determiner or is configured to get access to an external time stamp correction information determiner.

According to an embodiment time of arrival, localization, positioning, navigation, two way ranging, TWR, decawave, TOA and self-calibration are keywords.

Fig. 2 shows a schematic view of a transceiver 200 for use in a positioning system. The transceiver 200 is configured to transmit at least three signals 210, 220 and 230 during a calibration phase. Two of the at least three transmitted signals, for example, the first signal 210 and the third signal 230, comprise same signal power levels and at least two of the at least three transmitted signals, for example, the first signal 210 and the second signal 220 or the third signal 230 and the second signal 220, comprise different signal power levels. The transceiver 200 is configured to provide transmission time stamps 120, describing times at which the at least three signals 210, 220 and 230 are transmitted, for a determination of a time stamp power correction information. The transceiver 200 comprises a first clock 240 for determining the transmission time stamps. According to an embodiment, the transmission time stamps of the three signals determined by the transceiver 200 can represent a transmission time stamp information.

Optionally, the transceiver 200 is configured to provide a power level information 250 describing signal power levels of the at least three transmitted signals 210, 220 and 230. According to an embodiment, the power level information 250 can be provided together with the transmission time stamps 120 in a combined signal.

According to an embodiment, the transceiver 200 comprises a time stamp correction information determiner, for example, like the time stamp correction information determiner 100 illustrated in Fig. 1.

According to an embodiment of the invention, the time stamp correction information determiner is configured to use the power level information 250 to associate the time stamp power correction information with the corresponding power level information 250.

Fig. 3 shows a transceiver 200 which can comprise features and functionalities as with regard to the transceiver 200 illustrated in Fig. 2. The transceiver 200 in Fig. 3 differs from the transceiver 200 in Fig. 2 in the following:

The transceiver 200 is configured to receive at least three signals 210, 220 and 230 during a calibration phase. The transceiver 200 is configured to provide reception time stamps 130, describing times at which the at least three signals 210, 220 and 230 are received, for a determination of a time stamp power correction information. The reception time stamps 130 can be determined using the clock 240, which can be referenced herein as a second clock.

Optionally, the transceiver 200 is also configured to determine a power level information 250 describing powers of the at least three received signals 210, 220 and 230. Furthermore, the transceiver 200 can be configured to comprise a time stamp correction information determiner as described regarding Fig. 1 and/or Fig. 2.

The transceiver 200 as described in Fig. 2 represents, for example, the reference transceiver as described regarding Fig. 1 and the transceiver 200 in Fig. 3 represents, for example, the second transceiver as described regarding Fig. 1. According to an embodiment, the signals analyzed herein and, for example, transmitted by a transceiver as described with respect to Fig. 2 and received by a transceiver as described with respect to Fig. 3, are ultra-wideband signals, using extremely large frequency ranges with a bandwidth of, for example, at least 500 MHz or at least 20% of an arithmetic mean of lower and upper limit frequencies of a used frequency band.

Fig. 4 shows a calculator 300 for determining a time-of-flight 310a of one or more signals and/or a distance 310b passed by the one or more signals on the basis of transmission time stamp information 120 and reception time stamp information 130 associated with two or more signals. The two or more signals comprise the one or more signals, from which the time-of-flight 310a and/or the distance 310b is determined. The calculator 300 is configured to obtain a time stamp power correction information 110 and a clock-drift correction 170, wherein the calculator 300 either comprises a time stamp correction information determiner to determine the time stamp power correction information and the clock-drift correction by itself, or the calculator 300 is configured to receive the time stamp power correction information 110 and the clock-drift correction 170 from an external time stamp correction information determiner. For sake of convenience, the time-of-flight 310a of the one or more signals and/or the distance 310b passed by the one or more signals is in the following designated as an output. The calculator 300 is configured to determine the output based on a transmission time stamp and a reception time stamp, extracted from the transmission time stamp information 120 and the reception time stamp information 130, of a first signal transmitted from a first transceiver to a second transceiver and a transmission time stamp and a reception time stamp, for example, extracted from the transmission time stamp information 120 and the reception time stamp information 130, of a second signal transmitted from the second transceiver to the first transceiver. According to an embodiment, the first transceiver can also be designated as a reference transceiver. The calculator 300 is configured to use the time stamp power correction information 110 and the clock drift correction 170 to determine the output 310a/310b.

According to an embodiment, the calculator 300 is configured to determine the clock-drift correction 170 using the transmission time stamp information 120 and the reception time stamp information 130 associated with a third signal. According to an embodiment, the calculator 300 is configured to use transmission time stamp information 120 and reception time stamp information 130 of two signals having same signal power levels transmitted from the first transceiver to the second transceiver or of two signals having same signal power levels transmitted from the second transceiver to the first transceiver. Thus, the calculator 300 obtains, for example, transmission time stamp information 120 and reception time stamp information 130 of at least three signals, wherein two of the three signals are, for example, transmitted from the first transceiver and one of the three signals is transmitted from the second transceiver or one signal is transmitted from the first transceiver and two signals are transmitted by the second transceiver.

According to an embodiment the calculator is configured to use features and/or functionalities described with respect to the time stamp correction information determiner regarding the determination of the clock-drift-correction. Thus the calculator can be configured to calculate the clock-drift-correction by itself.

According to an embodiment the calculator is configured to calculate the time-of-flight TOA 310a of the two signals transmitted between the first transceiver and the second transceiver according to

TOA = 0.5

The value D7¾ can represent a difference between a reception time stamp information 130 of a second signal received by the first transceiver and transmitted, for example, by the second transceiver and a transmission time stamp information 120 of a first signal transmitted by the first transceiver and, for example, received by the second transceiver. The value D7¾ can represent a difference between a transmission time stamp information 120 of the second signal transmitted by the second transceiver and a reception time stamp information 130 of the first signal received by the second transceiver. The value CDC represents a clock drift correction factor, determined based on the clock-drift correction 170, for correcting a clock drift between a first clock associated with the first transceiver and a second clock associated with the second transceiver. The value Ei represents a first time stamp power correction information 110 associated with a first signal. The value E₂ represents a second time stamp power correction information 1 10 associated with a second signal and the value Z represents a constant offset. Z represents, for example, a zero line for the signal power level and/or the antenna offset. According to an embodiment, Z can also be equal to zero.

According to an embodiment, the clock-drift correction CDC 170 is determined according to CDC using an obtained deviation Ci_,3 between a difference DG/₃ of transmission

time stamps and a difference AT ₃ of reception time stamps of two signals transmitted from one of the transceivers to another one of the transceivers and having same signal power levels. The transmission time stamps can be obtained from the transmission time stamp information 120 and the reception time stamps can be determined from the reception time stamp information 130.

Fig. 5 shows a measurement setup, which can be used for a proposed method for a clock drift correction, which is, for example, independent from a signal power level and can be seen as an alternative to known clock drift corrections. All herein proposed measurements and calibrations can be carried out with Decawave EVK1000 boards, as illustrated in Fig. 5. The station with the identification (ID) two is, for example, the transmitting station (TX) 200₂, which can comprise features and functionalities as described with regard to the transceiver 200 in Fig. 2. The receiving station (RX) 200i has the identification one and can comprise features and functionalities as described with regard to the transceiver 200 in Fig. 3. Receiving signal power, representing a signal power level of received signals by the receiving station 200i, as well as time stamps, which represent, for example, a transmission time stamp information and/or a reception time stamp information, can be obtained by reading a register provided by the transceivers 200i and 200₂ [2,1] The transmission time stamp information is, for example, provided by the transmitting station 2002 and the reception time stamp information is, for example, provided by the receiving station 200i.

According to an embodiment the transmitting station 200₂ and/or the receiving station 200i can comprise a time stamp correction information determiner and/or a calculator as described herein to determine a clock-drift correction, a time stamp power correction information, a TOA of a signal transmitted by the transmitting station 200₂ and received by the receiving station 200i and/or a distance between the transmitting station 200₂ and the receiving station 200i.

Also this embodiment is described with Decawave transceivers it is clear that the time stamp correction information determiner and the calculator are configured to analyze signals transmitted and received by other transceiver, transmitter and/or receiver.

According to an embodiment, general settings for the hardware setup as shown in Fig. 5 can be as follows: The transmitting station 200₂ and/or the receiving station 200i comprises, for example, as configuration a channel 2 or a channel 5, a center frequency of below 960 MHz, in the range of 3.1 GHz to 10.6 GHz, like 3993.6 MHz, or in the range of 22 GHz to 29 GHz, with a bandwidth of at least 480 MHz, like 499.2 MHz, or of at least 500 MHz with a pulse repetition frequency of, for example, 64 MHz or 16 MHz, a preamble length of 128 or 1024 and/or a data rate of 6.81 Mbps or 110 Kbps.

Decawave transceivers as used herein, according to an embodiment, are based on the ultra-wideband (UWB) technology and are compliant with the IEEE802.15.4-2011 standard [10]. They support six frequency bands with center frequencies from 3.5 GHz to 6.5 GHz and data rates up to 6.8 Mb/s. The bandwidth varies with the selected center frequencies from 500 up to 1000 MHz. With higher bandwidth the send impulse is getting sharper. The timestamps for the positioning are provided by an estimation of the channel impulse response (CIR) 1. The CIR estimation is obtained by correlating a known preamble sequence against the received signal and accumulating the result over a period of time.

In contrast to narrow band signals is the UWB technology more resistant to multipath fading. Reflections would cause an additional peak in the impulse response. The probability that two peaks interfere with each other is small. The sampling of the impulse response is performed by an internal 64 GHz chip with 15 ps event timing precision (4,496 mm). Due to the general regulations the transmit power density is limited to -41.3 dBm/MHz. These regulations are due to the high bandwidth occupied by the UWB transceiver. The maximum permissible power level is averaged over 1 ms period, hence the power can be increased for shorter message durations. Experiments described herein are, for example, carried out with the Decawave EVK1000 but the invention is not limited hereto. This board manly consist of a DW1000 chip and a STM32 ARM processor. Fig. 14 shows a Channel impulse response of the DW1000.

Fig. 6 shows a schematic view of a proposed approach to determine a time stamp power correction information and/or a clock drift correction. The proposed approach can represent an alternative clock drift correction. The transmitting station (TX) 2OO₂ is, for example, sending three signals P1 210, P2 220, and P3 230 at transmission time stamps T[^x 120I (T1 ), Tj^x 1202 (T2) and Tx^c 120₃ (T3), which represent, for example, a transmission time stamp information, and a receiver 200i is, for example, configured to receive the three signals 210, 220 and 230 at the reception time stamps T ^x 130i (T1), T™ 130₂ (T2) and T™ 130₃ (T3), which can represent a reception time stamp information. The transmitter 200₂ can comprise features and/or functionalities as described with regard to the transceiver 200 in Fig. 2 and the receiver 200i can comprise feature and/or functionalities as described with regard to the transceiver 200 in Fig. 3.

A clock 240i of the transmitter 200₂ and a dock 240₂ of a receiver 200i are, for example, not synchronous. The clock 240i of the transmitter 200₂ can be indicated as a first clock and the clock 240₂ of the receiver 200i can be indicated as a second clock. If the clocks 240i, 2402 have no drift, then both clocks 240i, 240_å should have the same frequency and the difference between D T_{l 2} - T2 - T1 should be the same for the transmitter 2OO2 and the receiver 200i if the first signal 210 and the second signal 220 have same signal power levels, otherwise AT ¹ T™. The same applies for DT_{1 3}. If the clock of the receiver 200i (RX) is faster than the clock 240i of the transmitter station TX 2OO2, then AT™ > AT™ and the clock drift error equates C_{1 2} = AT f - AT™ A ΊO_Ί and/or C_{1 3} = AT^x - AT™ 170₂.

According to an embodiment the receiver 200i (RX) and/or the transmitter station 2OO2 (TX) can comprise the herein described time stamp correction information determiner and/or the herein described calculator to determine a clock-drift correction based on the clock drift error C_{1 2} 170i and or the clock drift error C_{1 2} 170₂. Alternatively the herein described time stamp correction information determiner and/or the herein described calculator can represent external devices or one external device comprising both the time stamp correction information determiner and the calculator, wherein the external devices, for example, obtain the transmission time stamp information T™ \ 20-_\ (T1), T™ 120₂ (T2) and T™ I2O3 (T3) from the transmitter station 200₂ (TX) and obtain the reception time stamp information Tz^c 130i (T1), Tf^x 130₂ (T2) and T§^x 130₃ (T3) from the receiver 200i (RX).

Commonly, a frequency difference between two clocks was presented by an integrator of PLL. After the warm-up time the clocks, for example, the clock 240i and/or the clock 240₂, reached their final frequency. The clock drift error would now increase linearly. For short measurement periods the linear clock drift error could also be assumed during the oscillator warm-up for example, based on the herein proposed approach according to an embodiment of the present invention. According to an embodiment, shown in Fig. 6, a time stamp correction information determiner for determining a time stamp power correction information on the basis of transmission time stamp information and reception time stamp information associated with the at least three signals 210, 220 and 230 is configured to obtain, for example, the transmission time stamp information comprising the first time stamp T[^x 120i, the second transmission time stamp T ^x 120_å and the transmission time stamp Tx^c 120₃ and to obtain the reception time stamp information comprising the first reception time stamp T 130i, the second reception time stamp T 130₂ and a third reception time stamp information Tx^c 130₃. The time stamp correction information determiner is configured to determine the time stamp power correction information using a clock drift correction which is based on transmission time stamp information, comprising the first transmission time stamp T™ 120i and the third transmission time stamp Tx^c, and reception time stamp information, comprising the first reception time stamp T ^x 130i and the third reception time stamp Tx^c 130₃, of at least two signals, for example, the first signal 210 and the third signal 230, having same signal power levels. The first transmission time stamp information T[^x, the second transmission time stamp information T * and the third transmission time stamp information Tx^c are associated with the second clock associated with the second transceiver and the first reception time stamp information T ^x, the second reception time stamp information T ^x and the third reception time stamp information Tx^c are associated with the second clock 240₂

Thus the transmission time stamp information and the reception time stamp information of the first signal 210 and the third signal 230 having same signal power levels are, for example, used to determine the clock-drift-correction and the transmission time stamp information and the reception time stamp information of the first signal 210 and the second signal 220 having different signal power levels are, for example, used to determine the time stamp power correction information.

If the first signal 210 and the third signal 230 have same signal power levels and the second signal 220 has a different signal power level the time stamp correction information determiner is configured to determine the clock drift correction to correct time stamps related to the second signal using time-interpolation— ¾ of a deviation C_{1 3}/170i between a first difference AT[^X1122₂ of transmission time stamps T ^c, T[^x associated with a transmission of the two signals 210, 230 having same signal power levels and a second difference DT ^/132₂ of reception time stamps Tx^c, T ^x associated with a reception of the two signals 210, 230 having same signal power levels. The main idea is, that the clock drift error c_{l 3} = AT - T™ 170i can be used to correct the timestamp T™ 120₂ with simple linear interpolation.

If the first signal 210 and the second signal 220 have same signal power levels and the third signal 220 has a different signal power level the time stamp correction information is configured to determine the clock drift correction to correct time stamps related to the second signal using time-extrapolation — ¾ of a deviation C1 2/170£ between a first difference AT™ /M21 of transmission time stamps T™, T™ associated with a transmission of the two signals 210, 220 having same signal power levels and a second difference AT™/† 32i of reception time stamps T™, T™ associated with a reception of the two signals 210, 220 having same signal power levels.

According to an embodiment the time stamp correction information determiner is configured to determine the clock drift correction CDC based on a deviation C_{I ,3}/170_I between a first difference AT™/ 122₂ of transmission time stamps T™, T™ associated with a transmission of two signals 210, 230 having same signal power levels and a second difference AT™/ 132₂ of reception time stamps associated with a reception of the two signals having same signal power levels according to

CDC = - ^Cl,3

TX ^a A T^Il™,2’

^AT1„3

wherein AT™ is associated with a difference 122i of transmission time stamps of a transmission of the two signals having different signal power levels.

According to an embodiment the time stamp correction information determiner is configured to apply the clock drift correction to the deviation 122i between the first transmission time stamp information T™ and the second transmission time stamp information T™ to determine the time stamp power correction information. Additionally or alternatively the time stamp correction information determiner is configured to apply the clock drift correction to the deviation 132i between the first reception time stamp information T™ and the second reception time stamp information T™ to determine the time stamp power correction information.

According to an embodiment the time stamp correction information determiner is configured to determine the time stamp power correction information based on a deviation between a first time interval 122i between a transmission of two signals 210, 220 having different signal power levels and a second time interval 122₂ between a reception of the two signals 210, 220 having different signal power levels using the clock drift correction which is based on transmission time stamp information T™, T [^x and reception time stamp information Tx^c, T ^c of at least two signals 210, 230 having same signal power levels. The first time interval 122i represents, for example, an optionally clock-drift-corrected deviation between the first transmission time stamp information T[^x and the second transmission time stamp information T ^x and the second time interval 132i represents, for example, an optionally clock-drift-corrected deviation between the first reception time stamp information T^^x and the second reception time stamp information T₂ ^X.

According to an embodiment the deviation C_{I ,} /17C>₂ is caused by a clock drift and by the different signal power levels. Thus the time stamp correction information determiner can be configured to at least partially remove a contribution caused by the clock

drift from said deviation C_{I 2}/170₂, to thereby obtain a clock drift corrected version C’i_,2 of the deviation C_{I ,2}/170₂. Furthermore the time stamp correction information determiner can be configured to provide the clock drift corrected version C\₂ as the time stamp power correction information or to determine the time stamp power correction information from the clock drift corrected version C’i_,2.

According to an embodiment the clock drift corrected version C[ ₂, which is, for example, associated with a power level, can be calculated according to C[ ₂ C

In the following Fig. 7A, three messages P1 210, P2 220 and P3 230 with constant signal power levels have been sent. If also a time stamp power correction information is determined, a fourth signal with different signal power level is sent or alternatively the signal power level of one of the three signals 210, 220 and/or 230 is changed. The delay between every message was, for example, about two milliseconds. The values are, for example, already filtered, hence every point consists, for example, of a mean of 4000 measurements. According to an embodiment, Fig. 7A shows a signal power level (signal strength) against measurements. Also the signal power level of the three messages 210, 220 and 230 comprises fluctuations, this fluctuations are so small, that the three signals 210, 220 and 230 can be seen as three signals with the same constant signal power level. Fig. 7B shows a clock drift error 170₂ C_{l 2} = DTz$ - DG™, i.e. an error due to clock drift. In other words it shows the clock drift error 170₂ versus measurements, corresponding to the three signals 210, 220 and 230 shown in Fig. 7a. Due to a long delay, the drift is about one meter. It is clear that for two different clocks, the clock drift error can differ from the clock drift error shown in Fig. 7B.

In the next step, the clock drift error C_{l 2} 170₂ can be corrected with linear interpolation of C_{1 3} according to C[ ₂ = indicated as 110 in Fig. 7C. In other words

Fig. 7C shows results of the clock drift correction C’i_,2. The correction requires, for example, just three messages and the remaining offset is, for example, about -1.9160e-05 m. It can also be seen that the linear interpolation is also suitable for the warm-up phase. Fig. 7C makes it clear that the clock drift correction is very accurate and efficient. The implementation of the present clock drift correction can be used for a two way ranging as described with respect to Fig. 11 and/or Fig. 12.

Regarding Figs. 7 A to 7C, an alternative approach for the clock drift correction with three messages (P1 210, P2 220 and P3 230) was shown. The following method, shown in Fig. 8A to Fig. 10D, is based on this concept, but additionally the TX station 200₂ (see Fig. 6) changes the signal strength, the signal power level, of the second message (P2 220). Fig. 8A shows how the signal strength of the first 210 and the third 230 message remain constant (P1 , P3) and only the signal strength for the second signal (P2, 220) is, for example, decreasing after about 10000 measurements. Also, Fig. 8A and Fig. 8B show a determination of a time stamp power correction information based on 1000 measurements per signal power level, it is also possible to decrease or increase the signal power level of one of the three signals after 10 measurements, 100 measurements, 10000 measurements, etc. and/or any different number of measurements per signal power level can be realized.

Every measurement point, shown in Fig. 8A to Fig. 10D, is, for example, a result of the mean of 2000 signals. Thus, in one measurement, 2000 signals are, for example, analyzed. Also, the determination of the time stamp power correction information is herein described with regard to an analysis of 2000 signals per measurement, it is also possible to use three signals per measurement, to use the results of the mean of 10 signals, the results of the mean of 100 signals, the results of the mean of 1000 signals, etc. or any different number of signals. According to an embodiment the tests were carried out with a cable connection of 10 cm and the transmitter decreased the signal gain with a stepsize of three dB. This is only an example and embodiments are not limited hereto. In Fig. 7C it is shown that after the clock drift correction, the remaining error of C^’1 ,2 equation (1) is close to zero. With decreasing signal strength of the second message it can be observed that the error C’1 ,2 1 10 is increasing, see Fig. 8B, hence it is possible to create a dependency between the measured signal strength, see, for example, the signal power level change of the second message 220 in Fig. 8A, and the time stamp power correction information 110 in Fig. 8B.

According to an embodiment Fig. 8A shows the signal strength with cable and Fig. 8B shows the timestamp error with cable.

Figs. 8A and 8B show a proposed approach for determining the time stamp power correction information 110, which can also be called signal power correction, by a time stamp correction information determiner according to an embodiment of the present invention.

According to an embodiment Fig. 8A shows the signal power levels of the three messages 210, 220 and 230 in a range of -77.4 dBm to -76.2 dBm in steps of 0.2 dBm against the measurements in a range of 0 to 50000 measurements in steps of 10000 measurements.

According to an embodiment Fig. 8B shows a resulting power level timestamp error in a range of -0.02 m to 0.1 m in steps of 0.02 m against the measurements in a range of 0 to 50000 measurements in steps of 10000 measurements.

In the following test scenario the power calibration is, for example, repeated with an antenna and a distance of 1.5m between the RX and TX stations. The gain stepsize was, for example, reduced to 0.5 dB. Figure 9B shows the results of the filtered signal power calibration curve 110 based on the signals 210, 220 and 230 shown in Fig. 9A. The main difference to the curve provided by Decawave 11 and our is that the zgero line is unknown. This line marks at which signal power the time stamp error is zero. The stepsize of the transmitting signal power gain decreasing was, for example, constant, yet the measured signal power curve 110 for the second message 220 is decreasing nonlinear. This is, for example, due to the fact that the measured signal power does not equates the correct signal power for high signal strength. According to an embodiment Fig. 9A shows the signal strength with antenna and Fig. 9B shows the timestamp error with antenna.

According to an embodiment Fig. 9A shows the signal power levels of the three messages 210, 220 and 230 in a range of -88 dBm to -79 dBm in steps of 1 dBm against the measurements in a range of 0 to 35 measurements in steps of 5 measurements.

According to an embodiment Fig. 9B shows a resulting power level timestamp error in a range of 0 m to 0.1 m in steps of 0.02 m against the measurements in a range of 0 to 35 measurements in steps of 5 measurements.

It is, for example, necessary to pay attention to the timings between the messages 210, 220 and 230. With short delays between the messages it is possible that they effect each other. This effect can be seen by the gap between P1 210 and P3 230 in Figure 9C. Fig. 9C shows a signal strength for a short update time.

It was mentioned before that, for example, just for small signal power levels the measured signal strength equates the correct signal power level. Therefore, it is possible to use the very first measurements with small signal strengths to estimate a slope. Figure 10A shows an estimated line based on the estimated slope. The results equate the one obtained by Decawave, with the difference that in our case no additional measured equipment is required and it can be obtained individually for every station. Figure 10B illustrate the correction curve 110 with respect to the signal power. In other words Fig. 10A and Fig. 10B show final results of a herein proposed power correction, wherein Fig. 10A shows a measured signal power versus a real signal power and wherein Fig. 10B shows a correction curve representing, for example, a time stamp power correction information.

According to an embodiment Fig. 10A shows an estimated RX level in a range of -86 dBm to -70 dBm in steps of 2 dBm against an actual RX level in a range of -86 dBm to -70 dBm in steps of 2 dBm.

According to an embodiment Fig. 10B shows a resulting power level timestamp error in a range of 0 m to 0.12 m in steps of 0.02 m against a received signal level in a range of -86 dBm to -70 dBm in steps of 2 dBm. Even for the same hardware design it is possible that the shape of the correction curve 1 10 differs. In Fig. 10C and in Fig. 10D final results of a power correction curve 1 10i to 110₆ are obtained from another station. The calibration has been repeated, for example, six times, it can be seen that the curves shape is deterministic but different to the previous station, compare the curves 1 10i to 110e of Fig. 10D with the curve 110 of Fig. 10B. Therefore, it makes sense to repeat the calibration for every station individually. In other words Fig. 10C and Fig. 10D show final results with several restarts for a different station than shown in Fig. 10A and Fig. 10B. According to an embodiment Fig. 10C shows an estimated RX level in a range of -82 dBm to -74 dBm in steps of 1 dBm against an actual RX level in a range of -82 dBm to -74 dBm in steps of 1 dBm.

According to an embodiment Fig. 10D shows a resulting power level timestamp error in a range of 0 m to 0.08 m in steps of 0.01 m against a received signal level in a range of -82 dBm to -74 dBm in steps of 1 dBm.

The following section describes how the inventive clock drift and signal power correction can be used for precise two way ranging (TWR). Figure 11 shows the concept for the TWR. The initial message 210 is send by a reference station 200i at Tf 120i and received by the tag 200₂ at T[ 130i. The timestamp T[ 130i is, for example, effected by the signal power, which cause to an error E1 1 10i. After some delay, due to internal processing the tag 200₂ sends a response message 220 at T 120₂. The reference station 200i gets the response from the tag 200₂ and saves the time stamp T 130₂, which is effected by the signal power error E2 110₂. In this example the delay due to the antenna offset is, for example, not considered.

The time of flight (TOA) between the reference station and the tag can be estimated a herein described calculator using the following formula:

The values E1 1 10i and E2 110₂ can be obtained by the calculator from a signal power correction curve, as described in embodiments above (see, for example, one of the embodiments described in one of the Figures 1 to 10). It should be taken into account that the signal power effects the tag 200₂ and reference station 200i differently. Due to the signal power is, for example, the time difference D7^₂ increasing and the difference D7¾ decreasing. A zero line for the signal power and the antenna offset are both unknown but, for example, constant, hence both values can be represented by the variable Z.

The calculator can be configured to receive the transmission time stamp 120i and 120₂ and the reception time stamps 13Qi and 130₂ from the respective transceiver, for example, from the reference station 200i and from the tag 200₂. According to an embodiment the reference station 200i and/or the tag 200₂ comprise the calculator, whereby the calculator can be configured to at least partially determine the transmission time stamp 120i and 120₂ and the reception time stamps 130i and 130₂ by itself.

In the previous embodiments it is shown that the clock drift can be corrected by three messages. Figure 12 shows how this principle can be adapted for the two way ranging (TWR). According to an embodiment the method shown in Fig. 12 can be performed by an inventive calculator, for example, using an inventive time stamp correction information determiner. According to an embodiment the method shown in Fig. 12 can comprise features and functionalities as described with respect to Fig. 1 1 , wherein in Fig. 12 additionally a third message 230 is transmitted, wherein T₃ 120₃ describes when the third signal 230 is transmitted by the reference station 200i and wherein T₃ 130₃ describes when the third signal 230 is received by the tag 200₂. The, for example, last message 230 can be used to obtain a clock drift error C_{1 3} = AT*₃ - D7¾, whereby the clock-drift correction can be calculated based on the clock drift error C_{1 3}. It can be seen that the signal power (see the time stamp power correction information E1 110i) has no or nearly no effect on the time stamp difference

The final time of flight (TOA) equation with the clock drift correction becomes, for example:

TOA = 0.5

The results of a TWR test, determined by a calculator according to an embodiment, with signal power correction and clock drift correction are, for example, illustrated by the line 310a/310b in Fig. 13, wherein the line 310a/310b represents, for example, a TOA of a signal and/or a distance passed by a signal. The line 312 stands for a laser distance measurements (Ground Truth). Fig. 13 shows the results for, e.g., 11 distances between a transceiver and a tag. The 1 1 distances extend, for example, from 3.515m to 0.562m. Every point results, for example, from a mean of 2000 measurements. The standard deviation between both curves is, for example, 1.5 cm. The nethermost constant offset shows that the signal power and clock drift correction are both sufficient. The antenna area was, for example, 4-3 cm², therefore it was not possible to obtain ground truth data with a precision higher than few centimeters.

Herein a new method for signal power and clock drift correction is presented. It was shown that the obtained curves for the signal power correction can be highly accurate, deterministic and provide individual results for every station. Additionally to the signal power correction curve estimation, it was also possible to obtain the relationship between the measured signal power and the real signal power. This allows better distance estimations for methods based on the signal strength. The presented clock drift correction is in contrast to the general approach independent from the signal power and promises results with centimeter accuracy. The description with regard to Fig. 11 , Fig. 12 and/or Fig. 13 shows how the signal power and clock drift correction are both fused together to provide a highly accurate two way ranging.

A position estimation by a herein described calculator based on Decawave UWB depends, for example, mainly on three factors. Antenna offset, clock drift and signal power. This invention deals with the last two factors. A general approach for the clock drift correction uses the Phase Locked loop (PLL) integrator. We show that the PLL is subject to the signal power and is therefore less suitable for the clock drift correction. The general approach for the signal power correction curve estimation requires additional measurement equipment. The invention presents a new method how the signal power correction curve can be obtained without additional hardware and the clock drift correction without the PLL integrator. Both correction methods can be fused together to improve the two way ranging (TWR).

In Fig. 16a and Fig. 16b an integrator outcome used for a common determination of a clock-drift-correction by PLL is presented. The test scenario is based on measurements obtained every 50ms between two stationary transceivers. The difference between two frequencies is about six parts per million (PPM). It took up to 15 minutes before the final condition was reached.

The tests have been repeated in Fig. 16b four times with another two stationary stations compared the two stations for the test results shown in Fig. 16a. Figure 16b shows the filtered results of the obtained curves provided by a 500 point moving average filter. It can be seen that the curve progression is deterministic. Decawave indicates that the logarithmic increase of the integrator at the beginning is due to the warm-up at turn-on of the room temperature crystal oscillator (RTXO) [1]. This oscillator follows from the combination of a quartz crystal and the circuitry within the DW1000 based design. Fig. 17 shows a DW1000 temperature crystal oscillator warm up [1]·

In the following test scenario the effect of signal power on the integrator used for a common determination of a clock-drift correction is investigated, see Fig. 18a and Fig. 18b. Transmitter and receiver station were both stationary. Figure 18a shows a measured signal strength at the receiving station. After about 4600 measurements the transmitter dropped the signal power. The integrator of the receiver, shown in Fig. 18b, jumped after the signal power change to a new level 6. This indicates that distance changes between the transmitter and receiver would affect the integrator and therefore the clock drift correction. In other words Fig. 18a shows a filtered received signal power and Fig. 18b shows a filtered integrator of the PLL.

The reason for this dependency could be the analog phase detectors of the PLL in which the loop gain KD is a function of amplitude. This would cause to that the amplitude affects the error signal v_e (t) = K_D [<P_0ut(t) - 0_in(t)] and therefore the pull-in time.

Figure 20 shows a relationship between a measured and a correct signal strength for different PRF (pulse repetition frequency) for a common determination of a power-level- correction. It can be observed that the measured signal power is only correct for measurements smaller than -85 dBm. The knowledge about the difference between the measured and correct signal strength can be used for additional measurement techniques like the received signal strength indicator (RSSI) for distance estimation. In other words Fig. 20 shows an estimated RX level with respect to an actual RX level [2]

Although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. The inventive methods can be stored on a digital storage medium or can be transmitted on a transmission medium such as a wireless transmission medium or a wired transmission medium such as the Internet.

Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, for example a floppy disk, a DVD, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed.

Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.

Generally, embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may for example be stored on a machine readable carrier.

Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein.

A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may for example be configured to be transferred via a data communication connection, for example via the Internet.

A further embodiment comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein.

A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein.

In some embodiments, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are preferably performed by any hardware apparatus.

The above described embodiments are merely illustrative for the principles of the present invention. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein.

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Claims

1. A time stamp correction information determiner (100) for determining a time stamp power correction information (110, 110i to 110e) on the basis of transmission time stamp information (120, 120i to 120₃) and reception time stamp information (130, 130i to130₃) associated with at least three signals (210, 220, 230), wherein the time stamp correction information determiner (100) is configured to determine the time stamp power correction information (1 10, 110i to 110e) based on a deviation (140, c_{l 2}) between

a first time interval (150, 122i) between a transmission of two signals (210, 220) having different signal power levels, wherein the first time interval (150, 122i) is described by a difference of transmission time stamps (120i, 120_å) related to a first clock (240, 240i), and

a second time interval (160, 132i) between a reception of the two signals (210, 220) having different signal power levels, wherein the second time interval (160, 132i) is described by a difference of reception time stamps (130i, 130₂) related to a second clock (240, 240₂),

using a clock drift correction (170, 170i, 170₂) which is based on transmission time stamp information (120, 120i to 120₃) and reception time stamp information (130, 130i to130₃) of at least two signals (210, 220, 230) having same signal power levels.

2. The time stamp correction information determiner (100) according to claim 1 , wherein the time stamp correction information determiner (100) is configured to obtain a first transmission time stamp information (120i) describing when a reference transceiver (200, 200i) transmits a first signal (210) using a first signal power level, wherein the time stamp correction information determiner (100) is configured to obtain a first reception time stamp information (130i) describing when a second transceiver (200, 200₂) receives the first signal (210), wherein the time stamp correction information determiner (100) is configured to obtain a second transmission time stamp information (120₂) describing when the reference transceiver (200, 200i) transmits a second signal (220) using a second signal power level, wherein the time stamp correction information determiner (100) is configured to obtain a second reception time stamp information (130₂) describing when the second transceiver (200, 200 ) receives the second signal (220), wherein the time stamp correction information determiner (100) is configured to obtain a third transmission time stamp information (120₃) describing when the reference transceiver (200, 20Qi) transmits a third signal (230) using the first signal power level, wherein the time stamp correction information determiner (100) is configured to obtain a third reception time stamp information (130₃) describing when the second transceiver (200, 200₂) receives the third signal (230), wherein the time stamp correction information determiner (100) is configured to determine the first time interval (150, 122i) based on a deviation (150, 122i) between the first transmission time stamp information (120i) and the second transmission time stamp information (120₂); and wherein the time stamp correction information determiner (100) is configured to determine the second time interval (160, 132i) based on a deviation (160, 132i) between the first reception time stamp information (130i) and the second reception time stamp information (130₂).

3. The time stamp correction information determiner (100) according to claim 2, wherein the first reception time stamp information (130i), the second reception time stamp information (130₂) and the third reception time stamp information (130₃) are associated with the second clock (240, 240₂) associated with the second transceiver (200, 200₂) and wherein the first transmission time stamp information (120i), the second transmission time stamp information (120₂) and the third transmission time stamp information (120₃) are associated with the first clock (240, 240i) associated with the reference transceiver (200, 200i), and wherein the time stamp correction information determiner (100) is configured to apply the clock drift correction (170, 170i, 170₂) to the deviation (150, 122i) between the first transmission time stamp information (120i) and the second transmission time stamp information (I2O2), and/or the time stamp correction information determiner (100) is configured to apply the clock drift correction (170, 170i, 170₂) to the deviation (160, 132i) between the first reception time stamp information (130i) and the second reception time stamp information (130₂).

4. The time stamp correction information determiner (100) according to one of the claims 1 to 3, wherein the time stamp correction information determiner (100) is configured to use the clock drift correction (170, 170i, 170₂) to correct the first time interval (150, 122i) and/or the second time interval (160, 132i) based on a deviation between the first clock (240, 240i) and the second clock (240, 240₂).

5. The time stamp correction information determiner (100) according to one of the claims 1 to 4, wherein the time stamp correction information determiner (100) is configured to use the clock drift correction (170, 170i, 170₂) to correct the transmission time stamp information (120, 120i to 120₃) and/or the reception time stamp information (130, 130i to130₃) based on a deviation between the first clock (240, 240i) and the second clock (240, 24O2).

6. The time stamp correction information determiner (100) according to one of the claims 2 to 5, wherein the time stamp correction information determiner (100) is configured to determine the clock drift correction (170, 170i, 170_å) using time- interpolation or time-extrapolation of a deviation (170i, 1 7O2) between a first difference (1 222) of transmission time stamps (120i, 120₃) associated with a transmission of two signals (210, 230) having same signal power levels and a second difference (132₂) of reception time stamps (130i, 130₃) associated with a reception of the two signals (210, 230) having same signal power levels, wherein the first difference (122₂) is related to the first clock (240, 240i) and the second difference (132₂) is related to the second clock (240, 240₂).

7. The time stamp correction information determiner (100) according to claim 6, wherein the time stamp correction information determiner (100) is configured to determine the clock drift correction (170, 170i, 170₂) based on the first difference (122₂) between the first transmission time stamp information (120i) and the third transmission time stamp information (120₃) and based on the second difference (132₂) between the first reception time stamp information (130i) and the third reception time stamp information (130₃).

8. The time stamp correction information determiner (100) according to one of the claims 1 to 7, wherein the time stamp correction information determiner (100) is configured to determine the clock drift correction (170, 170i, 170₂) CDC based on a deviation Ci,₃ (170i) between a first difference (122₂) AT™ of transmission time stamps (120i, 120₃) associated with a transmission of two signals (210, 230) having same signal power levels and a second difference (132₂) AT™ of reception time stamps (130i, 130₃) associated with a reception of the two signals (210, 230) having same signal power levels according to

CDC = - Cl, 3

TX

Ml m

wherein T™ is associated with a difference (150, 122i) of transmission time stamps (120i, 120₂) of a transmission of the two signals (210, 220) having different signal power levels.

9. The time stamp correction information determiner (100) according to one of the claims 1 to 8, wherein the time stamp correction information determiner (100) is configured to determine a deviation (140, C_{l 2}) between a difference (150, 122i) of transmission time stamps (120i, 120₂) of a transmission of the two signals (210, 220) having different signal power levels and a difference (160, 132₂) of reception time stamps (130i, 130₃) of a reception of the two signals (210, 220) having different signal power levels, which is caused by a clock drift and by the different signal power levels; and to at least partially remove a contribution caused by the clock drift from said deviation (140, C_{l 2}), to thereby obtain a clock drift corrected version (110, 110i to 110Q) of the deviation (140, C_1>2); and wherein the time stamp correction information determiner (100) is configured to provide the clock drift corrected version (110, 110i to 110e) as the time stamp power correction information (1 10, 110i to 1 10b) or to determine the time stamp power correction information (1 10, 110i to 1 10e) from the clock drift corrected version (1 10, 110i to 110₆).

10. The time stamp correction information determiner (100) according to one of the claims 1 to 9, wherein the time stamp correction information determiner (100) is configured to determine the time stamp power correction information (1 10, 110i to 110e) C[ ₂ according to

wherein Ci,2 represents the deviation (140, C_{l 2}) between the first time interval (150, 122i) described by a difference (122i) DG™ of transmission time stamps (120i, I2O2) related to a first clock (240, 240i) and the second time interval (160, 132i) described by a difference (132i) of reception time stamps (130i, 13O₂) related to a second clock (240, 240_å);

wherein Ci,3 represents a deviation (170i) between

a first difference (1222) DG™ of transmission time stamps (120i, 120₃) associated with a transmission of the two signals (210, 230) having same signal power levels and

a second difference (132₂) ATfJ of reception time stamps (130i, 13O₃) associated with a reception of the two signals (210, 230) having same signal power levels.

1 1. The time stamp correction information determiner (100) according to one of the claims 1 to 10, wherein the time stamp correction information determiner (100) is configured to determine the time stamp power correction information (110, 110i to 110b) for more than two different signal power levels.

12. A transceiver (200, 200i) for use in a positioning system,

wherein the transceiver (200, 200i) is configured to transmit at least three signals (210, 220, 230) during a calibration phase,

wherein at least two (210, 230) of the at least three transmitted signals (210, 220, 230) comprise same signal power levels, and

wherein at least two (210, 220) of the at least three transmitted signals (210, 220, 230) comprise different signal power levels, and

wherein the transceiver (200, 200i) is configured to provide transmission time stamps (120, 120i to 120₃), describing times at which the at least three signals (210, 220, 230) are transmitted, for a determination of a time stamp power correction information (110, 110i to 110₆).

13. The transceiver (200, 200i) according to claim 12, wherein the transceiver (200,

200₁) comprises a time stamp correction information determiner (100) according to one of the claims 1 to 11.

14. A transceiver (200, 2OO₂) for use in a positioning system,

wherein the transceiver (200, 200₂) is configured to receive at least three signals (210, 220, 230) during a calibration phase,

wherein at least two (210, 220) of the at least three received signals (210, 220, 230) comprise same signal power levels, and

wherein at least two (210, 230) of the at least three received signals (210, 220, 230) comprise different signal power levels, and

wherein the transceiver (200, 200₂) is configured to provide reception time stamps (130, 130i to I3O3), describing times at which the at least three signals (210, 220, 230) are received, for a determination of a time stamp power correction information (110, 110i to 110s).

15. The transceiver (200, 200₂) according to claim 14, wherein the transceiver (200,

2002) comprises a time stamp correction information determiner (100) according to one of the claims 1 to 11.

16. A calculator (300) for determining a time-of-flight (310a) of one or more signals (210, 220, 230) and/or a distance (310b) passed by the one or more signals (210, 220, 230) on the basis of transmission time stamp information (120, 120i to 120₃) and reception time stamp information (130, 130i to130₃) associated with two or more signals (210, 220, 230), wherein the calculator (300) is configured to obtain a time stamp power correction information (110, 110i to 110_Q) and a clock drift correction (170, 170i, 170₂); and wherein the calculator (300) is configured to determine the time-of-flight (310a) of the one or more signals and/or the distance (310b) passed by the one or more signals based on

a transmission time stamp (120i) and a reception time stamp (130i) of a first signal (210) transmitted from a first transceiver (200, 200i) to a second transceiver (200, 20O₂) and a transmission time stamp (120 ) and a reception time stamp (130 ) of a second signal (220) transmitted from the second transceiver (200, 2OO₂) to the first transceiver (200, 200i), using the time stamp power correction information (1 10, 110i to 110₆) and the clock drift correction (170, 170i, 170₂).

17. The calculator (300) according to claim 16, wherein the calculator (300) is configured to obtain the time stamp power correction information (110, 110i to 1 10s) and/or the clock drift correction (170, 170i , 170₂) from a time stamp correction information determiner (100) according to one of the claims 1 to 11.

18. The calculator (300) according to claim 16 or claim 17, wherein the calculator (300) is configured to determine the clock drift correction (170, 170i , 170₂) using transmission time stamp information (120₃) and reception time stamp information (130₃) associated with a third signal (230).

19. The calculator (300) according to one of the claims 16 to 18, wherein the calculator (300) is configured to calculate the time-of-flight (310a) TOA of the two signals (210, 220) transmitted between the first transceiver (200, 200i) and the second transceiver (200, 2OO₂) according to

TOA = 0.5

wherein D7¾ represents a difference (132i) between a reception time stamp information (130₂) of the second signal (220) received by the first transceiver (200, 200i) and

a transmission time stamp information (120i) of the first signal (210) transmitted by the first transceiver (200, 200i); wherein D¾ represents a difference (122i) between

a transmission time stamp information (120₂) of the second signal (220) transmitted by the second transceiver (200, 200₂) and

a reception time stamp information (130i) of the first signal (210) received by the second transceiver (200, 200₂); wherein CDC represents a clock drift correction (170, 170i, 170₂) factor for correcting a clock drift between a first clock (240, 240i) associated with the first transceiver (200, 200i) and a second clock (240, 240₂) associated with the second transceiver (200, 200₂); wherein Ei represents a first time stamp power correction information (1 10, 1 10i to 110e) associated with a first signal (210); wherein E₂ represents a second time stamp power correction information (110, 110i to 110s) associated with a second signal (220); and wherein Z represents a constant offset.

20. The calculator (300) according to claim 19, wherein the clock drift correction (170,

170-1, 170₂) CDC is determined according to CDC =

using an obtained

AT_{1 3}

deviation C_{1 3} between a difference (122₂) D7¾ of transmission time stamps (120i, I2O3) and a difference (132₂) DT_c ^t ₃ of reception time stamps (130i, I 3O3) of two signals (210, 230) transmitted from one of the transceivers (200, 200i, 200₂) to another one of the transceivers (200, 200i, 200₂) and having same signal power levels.

21. A transceiver (200, 200i, 200₂) comprising a time stamp correction information determiner (100) according to one of the claims 1 to 11 and/or a calculator (300) according to one of the claims 16 to 20.

22. A system comprising two transceiver (200, 200i, 2OO2), a time stamp correction information determiner (100) according to one of the claims 1 to 11 and a calculator (300) according to one of the claims 16 to 20.

23. A method for determining a time stamp power correction information on the basis of transmission time stamp information and reception time stamp information associated with at least three signals, comprising: determining the time stamp power correction information based on a deviation between

a first time interval between a transmission of two signals having different signal power levels, wherein the first time interval is described by a difference of transmission time stamps related to a first clock, and a second time interval between a reception of the two signals having different signal power levels, wherein the second time interval is described by a difference of reception time stamps related to a second clock, using a clock drift correction which is based on transmission time stamp information and reception time stamp information of at least two signals having same signal power levels.

24. A method comprising: transmiting at least three signals during a calibration phase,

wherein at least two of the at least three transmitted signals comprise same signal power levels, and

wherein at least two of the at least three transmitted signals comprise different signal power levels, and

providing transmission time stamps, describing times at which the at least three signals are transmitted, for determining a time stamp power correction information.

25. A method comprising: receiving at least three signals during a calibration phase,

wherein at least two of the at least three received signals comprise same signal power levels, and wherein at least two of the at least three received signals comprise different signal power levels, and

providing transmission time stamps, describing times at which the at least three signals are received, for determining a time stamp power correction information.

26. A method for determining a time-of-flight of one or more signals and/or a distance passed by the one or more signals on the basis of transmission time stamp information and reception time stamp information associated with two or more signals comprising: obtaining a time stamp power correction information and a clock-drift correction; and determining the time-of-flight of the one or more signals and/or the distance passed by the one or more signals based on

a transmission time stamp and a reception time stamp of a first signal transmitted from a first transceiver to a second transceiver and a transmission time stamp and a reception time stamp of a second signal transmitted from the second transceiver to the first transceiver, using the time stamp power correction information and the clock-drift correction.

27. A computer program having a program code for performing, when running on a computer, a method according to one of the claims 23 to 26.