WO2019078778A1

WO2019078778A1 - Distance measurements in wireless networks

Info

Publication number: WO2019078778A1
Application number: PCT/SE2018/051064
Authority: WO
Inventors: Hjalmar WENNERSTRÖM
Original assignee: Terranet Ab
Priority date: 2017-10-20
Filing date: 2018-10-18
Publication date: 2019-04-25

Abstract

A wireless communication device (10A) determines a distance to a second device (10B) based on timestamps received from the second device (10B) during a measurement procedure, e.g. an FTM procedure. The measurement procedure comprises: causing the second device (10B) to transmit, in accordance with a current setting of control parameters, one or more bursts (B) of signals; receiving the signals by the transceiver; and computing, based on timestamps associated with the signals, at least one distance value (D, D). The wireless communication device (10A) is further configured to update the current setting by analyzing timestamps obtained either during the measurement procedure or during a dedicated estimation procedure, in which the communication device (10A) causes the second device (10B) to transmit, in accordance with a test setting of the control parameters, at least one burst (B) of signals. The control parameters may include number of bursts (B), burst duration (BD), burst period (BP), and number of signals per burst (B).

Description

DISTANCE MEASUREMENTS IN WIRELESS NETWORKS

Technical Field

The present invention relates generally to the field of wireless communication and, more particularly, to techniques for determination of a measurement value corresponding to a distance between two communication devices based on timestamps measured by and transmitted wirelessly between the communication devices.

Embodiments of the invention are particularly, but not exclusively, applicable to FTM procedures.

Background Art

Various positioning techniques may be employed for determining the position of a wireless communication device, also denoted station herein, in relation to another station or in absolute coordinates. For example, positioning techniques may utilize the so-called Fine Timing Measurement (FTM) protocol for determining a value

corresponding to the distance between the stations, and calculate the position of one of the stations based on the distance value by use of any well-known positioning technique, e.g. based on trilateration, multilateration, etc. The FTM protocol is well- known and standardized in accordance with the IEEE 802.1 lmc specification which is part of the IEEE 802.11-2016 standard. The FTM protocol provides a procedure for measuring and exchanging timestamps by wireless communication between two stations (nodes). Generally, one of the stations (initiator or requester) transmits a setting of control parameters to the other station (responder) and thereby causes the responder to generate one or more bursts of FTM signals (frames) in accordance with this setting. The initiator, in turn, generates response signals (frames) to the FTM signals, and records timestamps for arrival of FTM signals and transmission of response signals. The responder records timestamps for transmission of FTM signals and arrival of response signals and includes corresponding timestamps in the FTM signals. By this FTM procedure, timestamps are exchanged between the stations, from the responder to the initiator, so as to allow the initiator to compute the above-mentioned distance value for each burst of FTM signals. Such a distance value may e.g. be a Round Trip Time (RTT), a Time of Flight (ToF), a distance, etc.

There are other standardized protocols that enable measurement and exchange of timestamps through bursts of wireless signals set by control parameters. One such protocol is defined by IEEE 802.1 lv and offers a lower time resolution than the FTM standard. There may be further wireless standards as well as proprietary protocols that allow for exchange of timestamps in corresponding manner. Irrespective of protocol, it is vital to ensure that the resulting distance values are sufficiently accurate and precise and represent relevant changes in distance between the stations.

In wireless communication systems, it is a general desire to reduce the required processing power, and thus power consumption, of the included stations. It is also desirable to ensure that the available communication channels in the wireless communication system are used as efficiently as possible.

In this context, US2017/0156031 discloses FTM signal exchange between two wireless devices. One of the wireless devices determines its latest time of movement by use of a built-in movement sensor and includes the time of movement in a discovery frame for receipt by the other wireless device, which then may compare the received time of movement with a time of movement stored in memory and decide not to initiate an FTM session if the comparison indicates no movement or slight movement. While this technique of disabling the FTM procedure by postponing the FTM session might save processing power, it does not ensure that the FTM session, when eventually performed, results in distance values that are sufficiently relevant and/or accurate and/or precise.

US2017/0142608 proposes that one of the stations involved in an exchange of FTM messages during an FTM session may transmit one or more messages to alter at least one parameter defining the FTM session for a remaining portion of the FTM session. The at least one parameter may increase the extent of the FTM session, or change the number of bursts, the number of frames per burst, the minimum time between transmission of FTM messages in a burst, the burst period or the burst duration.

Brief Summary

It is an objective of the invention to at least partly overcome one or more limitations of the prior art.

Another objective is to enable reduced power consumption in stations that exchange timestamps through bursts of wireless signals, preferably without compromising the relevance and/or accuracy and/or precision of the resulting distance values.

A further objective is to enable efficient of use of available communication channels in a wireless communication system with such stations.

One or more of these objectives, as well as further objectives that may appear from the description below, are at least partly achieved by communication devices, methods of operating a communication device, and a computer-readable medium according to the independent claims, embodiments thereof being defined by the dependent claims. In a first aspect, there is provided a communication device with a transceiver for wireless communication and further comprising logic and circuitry configured to perform a measurement procedure. The measurement procedure comprises: causing a second communication device to transmit, in accordance with a current setting of control parameters, one or more bursts of signals; receiving the signals by the transceiver, and computing, based on timestamps associated with the signals, at least one measurement value corresponding to a distance between the communication device and the second communication device. The logic and circuitry is further configured to perform an estimation procedure. The estimation procedure comprises: performing at least one test session by causing the second communication device to transmit, in accordance with a test setting of the control parameters, at least one burst of signals; receiving, by the transceiver, the signals transmitted during the respective test session; performing an analysis of timestamps associated with the signals transmitted during the respective test session; and updating the current setting of the control parameters based on the analysis.

In a second aspect, there is provided a communication device with a transceiver for wireless communication and further comprising logic and circuitry configured to perform a measurement procedure. The measurement procedure comprises: causing a second communication device to transmit, in accordance with a current setting of control parameters, one or more bursts of signals; receiving the signals by the transceiver; and computing, based on timestamps associated with the signals, at least one measurement value corresponding to a distance between the communication device and the second communication device. The logic and circuitry is further configured to update the current setting of the control parameters based on the timestamps.

In a third aspect, there is provided a method of operating a communication device with a transceiver for wireless communication. The method comprises operating the communication device to perform a measurement procedure. The measurement procedure comprises: causing a second communication device to transmit, in accordance with a current setting of control parameters, one or more bursts of signals; receiving the signals by the transceiver; and computing, based on timestamps associated with the signals, at least one measurement value corresponding to a distance between the communication device and the second communication device. The method further comprises operating the communication device to perform an estimation procedure. The estimation procedure comprises: performing at least one test session by causing the second communication device to transmit, in accordance with a test setting of the control parameters, at least one burst of signals; receiving, by the transceiver, the signals transmitted during the respective test session; performing an analysis of timestamps associated with the signals transmitted during the respective test session; and updating the current setting of the control parameters based on the analysis.

In a fourth aspect, there is provided a method of operating a communication device with a transceiver for wireless communication. The method comprises: causing a second communication device to transmit, in accordance with a current setting of control parameters, one or more bursts of signals; receiving the signals by the transceiver; and computing, based on timestamps associated with the signals, at least one measurement value corresponding to a distance between the communication device and the second communication device. The method further comprises updating the current setting of the control parameters based on the timestamps.

In a fifth aspect, there is provided a computer-readable medium comprising program instructions which, when executed by a processor, cause the processor to perform the method of the third or fourth aspects. The computer-readable medium may be a tangible (non-transitory) product (e.g. magnetic medium, optical disk, read-only memory, flash memory, etc) or a propagating signal.

Other objectives, as well as features, aspects and advantages of embodiments of the present invention will appear from the following detailed description, from the attached claims as well as from the drawings.

Brief Description of Drawings

The accompanying schematic drawings illustrate example embodiments of the invention.

FIG. 1 is a network diagram illustrating an exemplary network environment with two stations.

FIG. 2 is a timing diagram for timestamp measurement and signal exchange between the stations in FIG. 1 during an FTM session.

FIG. 3 is a timing diagram for measurements made during a burst of FTM frames.

FIG. 4 is a diagram to exemplify bursts of signals transmitted during an FTM session and associated control parameters and measurement values.

FIG. 5 is a flow chart of an overall method of operating an initiator station in accordance with an embodiment.

FIG. 6 is a flow chart of an embodiment of an estimation procedure in the method of FIG. 5.

FIGS 7-9 are flow charts of embodiments of sub-procedures in the estimation procedure of FIG. 6.

FIGS 10-11 are flow charts of FTM procedures in accordance with embodiments. FIG. 12 is a block diagram for a wireless station. Detailed Description of Example Embodiments

Embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all, embodiments of the invention are shown. Like reference signs refer to like elements throughout.

Also, it will be understood that, where possible, any of the advantages, features, functions, devices or operational aspects of any of the embodiments of the present invention described or contemplated herein may be included in any of the other embodiments of the present invention described or contemplated herein. In addition, where possible, any terms expressed in the singular form herein are meant to also include the plural form, and vice versa, unless explicitly stated otherwise. As used herein, "at least one" shall mean "one or more" and these phrases are intended to be interchangeable. Accordingly, the terms "a" and "an" shall mean "at least one" or "one or more," even though the phrase "one or more" or "at least one" is also used herein. As used herein, except where the context requires otherwise owing to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, that is, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It will furthermore be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing the scope of the present invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

Well-known functions or constructions may not be described in detail for brevity and/or clarity. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

In the following description, embodiments of the invention will be exemplified with reference to the Fine Timing Measurement (FTM) protocol in accordance with the IEEE 802.11 standard. The term "FTM protocol" is intended to include the current protocol IEEE 802.11mc, as well as future IEEE 802.11 protocols that are based on the same principle of timestamp exchange. It should be noted that the embodiments are not limited to the FTM protocol, but may be implemented in accordance with any current or future standardized or proprietary protocol that enables timestamp exchange, by use of any suitable wireless communication technology, including WW AN, WLAN and WPAN technologies, such as WiFi, WiMAX, UWB, Bluetooth and Zigbee, including technologies for wireless mesh networking.

FIG. 1 is a network diagram illustrating an exemplary network environment suitable for FTM exchanges. The wireless network 10 in FIG. 1 includes two communica- tion devices 10A, 10B, denoted nodes or stations in the following, which communicate in accordance with the IEEE 802.11 standard. The respective station 10A, 10B may be a stationary or non-stationary device of any structure. Examples of such devices include mobile phones, tablets, computers, access points, wearables, headsets, vehicles, IoT devices, etc.

In the following examples, station 10A operates as initiator station ("initiator") for the FTM exchange, and station 10B operates as responder station ("responder"). The initiator 10A thus initiates an FTM session with the responder 10B. As will be described below, an FTM session involves an exchange of timestamps between the stations 10A, 10B to allow the initiator 10A to determine at least one distance value. The distance value is indicative of the distance between the stations 10A, 10B and may be given in either time units or length units. For example, the distance value may be a distance (range), a time-of-flight (ToF), or a round-trip time (RTT).

The distance value may be used for any conceivable purpose by the initiator 10A. In one example, the initiator 10A may determine, based on one or more distance values, a position in relative coordinates or geo-coordinates. To this end, the initiator 10A may, but need not, perform FTM exchanges with more than one responder 10B, and process the resulting distance values, e.g. by trilateration and/or by time-difference-of-arrival (TDOA) techniques, to determine the position of the initiator 10A or the position of the respective responder 10B, as is well-known in the art.

Reference is now made to FIG. 2, which is a timing diagram that exemplifies operations and signal exchanges between the initiator 10A and the responder 10B during an FTM session in accordance with the FTM standard. As shown, the initiator 10A may transmit an initial FTM request (iFTMR) frame to request an FTM session with the responder 10B. The iFTMR frame may include parameters such as category, public action, trigger, LCI measurement request, location civic measurement request and one or more FTM parameters. The FTM parameters, denoted "control parameters" in the following, define the desired number and timing of the measurements to be performed during the FTM session, as will be further exemplified below. The responder 10B may acknowledge receipt of the iFTMR frame with an acknowledgement (ACK) frame. Alternatively, the responder 10B may refuse the request or return a message indicating a modification of one or more control parameters ("negotiation phase"). Under the current FTM standard, further messages (frames) may be exchanged between the initiator 10A and the responder 10B before the FTM session is started. During the FTM session, the responder 10B transmits a sequence of FTM frames in accordance with the control parameters, and the initiator 10A responds with an acknowledgement (ACK) frame to the respective FTM frame, where at least a subset of the FTM frames contains timestamps that are recorded by the responder 10B. Specifically, in the illustrated example, the responder 10B sends a first FTM frame, FTMl, and records the time of departure (ToD) for FTMl by timestamp tl(l). The initiator 10A records the time of arrival (To A) for FTMl by timestamp t2(l), and sends ACK in response to FTMl. The initiator 10A records ToD for ACK by timestamp t3(l). The responder 10B records ToA for ACK by timestamp t4(l), and includes the timestamps tl(l) and t4(l) in a second FTM frame, FTM2, which is received by the initiator 10A. Thereby, the initiator 10A is able to calculate a sample value for the first measurement. The sample value is indicative of the distance between the stations 10A, 10B and may be given in length units or time units. Examples of sample values include RTT, ToF and range. For example, RTT may be calculated as t4(l)-t3(l)+t2(l)-tl(l), ToF may be calculated as RTT/2, and the range may be calculated as ToF*c, where c is the propagation speed of the wireless signals (e.g. the speed of light).

The FTM session is defined in terms of bursts of FTM frames. This is schematically illustrated in the timing diagram of FIG. 3, which shows a burst B comprising N measurements, designated by FTM_1, FTM_2, FTM_N. The respective measure- ment comprises an exchange of an FTM frame and an ACK frame as shown in FIG. 2. During each measurement, the timestamps presented in FIG. 2 are measured by the stations 10A, 10B, whereupon ToD of the FTM frame and ToA of the ACK frame are sent with the next FTM frame (of the next measurement). The time delay between FTM frames, designated by AFTM in FIG. 3, may be directly or indirectly defined by the above-mentioned control parameters. The FTM standard also sets a minimum value for the time delay AFTM.

According to the FTM standard, each of the stations 10A, 10B may terminate an ongoing FTM session at any time, by transmitting a frame with a dedicated parameter value.

FIG. 4 is a timing diagram to schematically illustrate an FTM session S between the stations 10A, 10B. The FTM session S comprises a number of consecutive bursts B, where each burst comprises a number of consecutive measurements FTM_1, FTM_2, FTM_N. In the illustrated example, the responder 10B sends N FTM frames during each burst B. Each burst has a burst duration BD. The bursts B are transmitted with a burst period BP, which is the time difference between initiations of consecutive bursts B. The above-mentioned control parameters for the FTM session may comprise the number of bursts (designated by #B in the following), the burst period BP, the burst duration BD, and the number of FTM frames for each burst (designated by #FTM in the following).

According to the current FTM standard, the control parameter #B may be given by an exponent of an exponential function with base 2, with the exponent being set in the range of 0-14. The burst period BP may be set to multiples of 100 ms, given by a multiplicative factor in the range of 1-65535. Thus, BP may be as small as 100 ms and as large as approx. 1.8 hours. The burst duration BD may be set to certain discrete values in the range of 250 μ8 - 128 ms, given by a control value in the range of 0-15. In the current FTM standard, BD may be set to one of the following: 250 μ8, 500 μ8, 1 ms, 2 ms, 4 ms, 8 ms, 16 ms, 32 ms, 64 ms and 128 ms. It may also be possible to set an infinite BD. The number of FTM frames per burst, #FTM, may be set to an integer in the range of 1-31.

When the FTM session S is completed, the initiator 10A may request a new FTM session with the responder 10B by sending an iFTMR frame in accordance with FIG. 2.

As noted above, the initiator 10A may compute a sample value for each measurement, FTM_1, FTM2, FTM_N. These sample values are denoted "frame distances" in the following, since they are computed from timestamps that are measured based on a respective FTM frame. In FIG. 4, frame distances are designated by D. Typically, the initiator 10A computes an aggregated measurement value for each burst B, designated by D in FIG. 4 and denoted "burst distance" in the following. The burst distance D may be computed by an aggregation of the frame distances D measured for the burst B, e.g. by averaging. Alternatively, as well known in the art, the computation of frame distances D may be omitted and the burst distance D may be computed as a function of the timestamps that are measured during the burst B.

Embodiments of the invention are based on the insight that it may be undesirable to use a nominal setting of the control parameters, since the nominal setting must be defined to ensure that proper distance values are generated by the FTM procedure in a worst-case scenario. For example, consider a use case in which the initiator 10A is a handheld tool which determines its distance to a responder 10B, e.g. a fixed access point. Here, the nominal setting may be defined to fulfill a requirement specification while the handheld tool is in use, typically while the handheld tool is moved around a maximum speed. Such a requirement specification may, e.g., define a required sampling rate of burst distances and a required accuracy and precision of the burst distances. The initiator 10A will then operate with the nominal settings irrespective of the actual movement of the handheld tool, leading to a waste of energy and bandwidth, and possibly congestion in a wireless network that includes several handheld tools.

Embodiments of the invention provide an automatic mechanism for adjusting an FTM procedure between two wireless stations so as to free up the resources of the involved stations as much as possible, e.g. to save energy and/or bandwidth, and to resolve or reduce contention issues in the wireless network, while ensuring that the resulting burst distances fulfill the requirement specification. Specifically, embodiments of the invention provide a technique of adjusting the setting of the control parameters so as to achieve the foregoing effect.

Embodiments of the inventions are also based on an insight that the control parameters may be separated by their effect on the resulting burst distances. For example, #B may be seen to determine for how long the burst distances should be measured, BP may be seen to determine the sampling rate of the burst distances, BD may be seen to determine the accuracy of the burst distances, and #FTM may be seen to determine the precision of the burst distances. The terms accuracy and precision are used in their ordinary meaning, where accuracy refers to systematic errors for samples and precision refers to variability among samples. A lack of accuracy may be caused by large changes in distance between stations during a burst B, and a lack of precision indicates a variabi- lity in the measurements. According to this insight, BD may be reduced with increasing relative speed between the stations 10A, 10B to maintain a required accuracy, and #FTM may be increased with increasing uncertainty of the timestamps, e.g. caused by deteriorating quality in the wireless communication between the stations. The deteriorating quality may be due to signal interference, transceiver problems or other communi- cation issues.

Embodiments of the invention are also based on the insight that the timestamps that are acquired by the initiator 10A during an FTM procedure contain information about the relative speed of the stations, i.e. the actual change in distance over time, and that it is possible to infer from the timestamps how a current setting of the control parameters may be changed to optimize the use of resources while still fulfilling the requirement specification.

Embodiments of the invention are also based on the insight that the timestamps may be analyzed to infer a deteriorating quality in the wireless communication between the stations and to determine how the setting of the control parameters may be changed to counteract such deteriorating quality.

Thus, in accordance with embodiments of the invention, the timestamps acquired during an FTM procedure are intermittently analyzed, whereupon the current setting of the control parameters is updated based on the analysis.

Reverting to the use case of a handheld tool as described above, it is realized that an initiator 10A that is configured in accordance with embodiments of the invention may modify the control parameters of the FTM procedure so as to reduce the number of measurements per unit time with decreasing relative speed, as indicated by the time- stamps. For example, when the handheld tool is placed to rest, the number of measure- ments per unit time may be significantly reduced, thereby saving both energy and bandwidth.

FIG. 5 is a flow chart of a method of operating an initiator 10A in accordance with an embodiment of a first inventive concept. According to the first inventive concept, the initiator 10A is capable of performing both a regular FTM procedure ("measurement procedure") 100 and an estimation procedure 102. The measurement procedure 100 is performed as described in the foregoing, and allows the initiator 10A to compute at least one burst distance based on the timestamps received with the one or more bursts B that are transmitted by the responder 10B in accordance with a current setting of the control parameters. In the estimation procedure 102, the initiator 10A initiates at least one dedicated FTM session ("test session") for at least one test setting of the control parameters, analyses the resulting timestamps, and updates the current setting of the control parameters based on the analysis. By the test session(s), the initiator 10A gains access to timestamps that are generated for test setting(s) that may be tailored to enable a well-controlled update of the control parameters.

In the example of FIG. 5, the initiator 10A performs the measurement procedure 100 and the estimation procedure 102 in sequence, i.e. at different times. This may facilitate the design and operation of the stations 10A, 10B, and both procedures 100, 102 may be performed on the same communication channel. However, it is conceivable to perform the procedures 100, 102 in parallel on different communication channels.

In the example of FIG. 5, the initiator 10A performs the measurement procedure 100 until a detection step 101 detects a predefined event, which causes the initiator 10A to switch to the estimation procedure 102. The predefined event may be generated by a process executed by the initiator 10A in parallel to the measurement procedure 100. In one example, the predefined event is generated to indicate expiration of a timer. Such an event may cause the initiator 10A to switch to the estimation procedure 102 at given intervals, e.g. in terms of time, number of bursts, number of FTM sessions, or any other time-related parameter. In another example, the predefined event is generated to indicate an error condition among the distance values that are generated during the measure - ment procedure 100. The error condition may be indicated when one or more burst distances are unreasonable and/or when the change between burst distances is unreasonable. In another example, the predefined event is generated based on an output signal from a movement sensor in the initiator 10A (cf. 26 in FIG. 12). For example, the predefined event may be generated when the output signal indicates that the initiator 10A starts to move and/or ceases to move.

FIG. 6 illustrates an embodiment of the estimation procedure 102. In this embodiment, the estimation procedure 102 comprises sub-procedures 102A, 102B, 102C which are performed in sequence by the initiator 10A. Each sub-procedure is designed to generate a value of a respective control parameter, with sub-procedure 102A generating #FTM, sub-procedure 102B generating BD, and sub-procedure 102C generating BP. Each sub-procedure 102A, 102B, 102C is operated in accordance with a respective set of test settings 103 A, 103B, 103C. As indicated in FIG. 5, #FTM in test settings 103B, 103C for sub-procedures 102B, 102C is set by sub-procedure 102A, and BD in test setting(s) 103C for sub-procedure 102C is set by sub-procedure 102B. Thus, the control parameter value generated by one sub-procedure is applied by subsequent sub- procedure(s). It should be understood that the estimation procedure 102 may be controlled to perform only one or any combination of the sub-procedures 102A-102C, and that the included sub-procedures 102A-102C may differ over time, e.g. based on the type of event that triggered the estimation procedure 102 in step 101 (FIG. 5). The control parameter value that would have been generated by an excluded sub-procedure may be set to a predefined value or its latest value in the measurement procedure 100.

FIG. 7 illustrates an embodiment of the sub-procedure 102A for setting the control parameter #FTM. The purpose of sub-procedure 102A is to determine the smallest number of measurements needed in each burst B to achieve a required precision for the resulting burst distances D. The test setting is selected to minimize the impact of any movement of the stations 10A, 10B on the frame distances that are generated for each burst B. Thus, in sub-procedure 102A, the variability of the frame distances reflects other factors that influence the timestamps, such as signal interference, transceiver jitter, etc. In the illustrated embodiment, sub-procedure 102A comprises steps 110-112 to define the test setting. Step 110 sets BP and #B to predefined values, e.g. small values to speed up the sub-procedure 102A. Step 111 sets #FTM, preferably to a large value, such as at least 10. The value of #FTM determines the amount of statistics available for the subsequent analysis step 114 (below). Given BP and #FTM, step 112 then sets BD to ensure that there is effectively no change in the distance between the stations 10A, 10B during the burst. Thus, BD may set to be below a predefined duration limit, BD_MIN1. In one implementation, step 112 may set BD as small as possible while ensuring that the time delay AFTM (FIG. 4) is above its mini- mum value according to the FTM standard. For example, if the minimum value for the time delay is 100 μ8 and #FTM is set to 10, BD may be set to 1 ms. Step 113 then initiates a test session with the test setting given by steps 110-112. The test session may involve a signal exchange as shown in FIG. 2. Step 113 involves in a rapid succession of measurements, resulting in a sequence of frame distances D. Step 114 analyzes the ensemble of D values with respect to the variability with increasing number of samples (i.e. D values). For example, step 114 may compute a discrete function that represents the change in variability with increasing number of D values. Step 115 then sets #FTM to the smallest number of D values that yields, according to the discrete function, a variability that falls below a predefined variability threshold. The variability threshold may be set to represent a required precision of the D values. Steps 114-115 are based on the presumption that the statistical distribution of the D values is such that variability will generally decrease with increasing number of samples.

As used herein, the "variability" for a set of data samples may be given by any function VAR that represents the variation among the data samples. For example, the variability function VAR may generate a variance, a standard deviation, a (normalized) sum of differences between consecutive data samples ("total variation"), a (normalized) sum of differences between the data samples and their mean ("mean deviation"), an interquartile range, etc.

FIG. 8 illustrates an embodiment of the sub-procedure 102B for setting the control parameter BD. The purpose of sub-procedure 102B is to identify the longest BD for which the measurements are similar. Therefore, sub-procedure 102B uses test sessions which differ by BD, and thereby by AFTM (FIG. 4), to assess the impact of the move- ment of the stations 10A, 10B on the variability of the frame distances. In the illustrated embodiment, sub-procedure 102B comprises steps 121-123 to define an initial test setting. Step 121 sets #FTM and #B, where #FTM may be given by sub-procedure 102A and #B may be set to a small value to speed up sub-procedure 102B. Step 122 sets BD to a predefined value, BD_MIN2, and step 123 sets BP as a function of BD (indicated by f(BD) in FIG. 8). In a preferred embodiment, which further speeds up sub-procedure 102B, BP is set as small as possible while being larger than or equal to BD. For example, if BD is set to 250 μ8, BP would be set to 100 ms. Step 124 then performs a test session with the test setting given by steps 121-123, for which the initiator 10A computes a set of D values. Step 125 then sets a reference value REF to represent the variability of the D values. Thus, steps 121-125 may be seen to determine a baseline for the variability of the D values. Sub-procedure 102B then repeatedly performs steps 126- 129 to initiate test sessions with increasing BD until the variability exceeds a variability threshold. Each such test session may involve a signal exchange as shown in FIG. 2. Steps 126-127 updates the test setting, by step 126 increasing BD, e.g. to the next value in accordance with the FTM standard, and by step 127 setting BP as in step 123. The control parameters #FTM and #B are suitably unchanged. Step 128 then initiates a test session with the test setting given by steps 126-127, resulting in a set of D values. Step 129 computes the variability of the D values and compares the variability to a variability threshold, which is set as a function of REF from step 125. Thereby, the sub- procedure 102B is unaffected by changes in the baseline variability, which is unrelated to the relative movement between the stations 10A, 10B. In the illustrated example, the variability threshold is given by a sum of REF and a predefined offset value, OFF. In an alternative embodiment, the variability threshold is given by a predefined and fixed value. If the variability is below the variability threshold, step 129 proceeds to step 126. Otherwise, step 129 proceeds to step 130 which sets BD based on the burst durations used in the preceding test sessions. For example, step 130 may be set BD to the largest BD that yielded a variability below the variability threshold.

Sub-procedure 102B is designed to test different burst durations, assess the variability of the resulting D values and select the longest burst duration that yields an acceptable variability. Reverting to FIG. 4, it should be realized that a change of BD with fixed #FTM results in a change of AFTM. Thus, as BD increases so does AFTM and thereby the impact of relative movement between the stations 10A, 10B on the variability among the D values. It is realized that the sub-procedure 102B sets BD so as to limit the impact of relative movement on the resulting D values, and thereby ensure an adequate accuracy of the burst values D .

It should be noted that sub-procedure 102B may test different burst durations in other ways than by gradually increasing BD (step 126). In one alternative, BD may be gradually decreased from a maximum value until the variability falls below the variability threshold.

FIG. 9 illustrates an embodiment of the sub-procedure 102C for setting the control parameter BP. The purpose of sub-procedure 102C is to identify the longest BP for which there is little movement between the stations 10A, 10B, e.g. as represented by the burst distances D . The underlying rationale is that if there is little change in the distance between stations 10A, 10B, the sampling rate of the D values may be decreased without significant loss of information. In the illustrated embodiment, sub-procedure 102C comprises steps 131-132 to define the test setting. Step 131 sets #FTM, BD and #B, where #FTM and BD may be given by sub-procedures 102A, 102B, and #B is set to a predefined value, which is at least 2 (burst exponent 1). Step 132 sets BP to a predefined minimum value, BP_MIN. Step 133 then initiates a test session with the test setting given by steps 131-132, resulting in a set of D values. The test session may involve a signal exchange as shown in FIG. 2. Step 134 computes the variability of the D values and compares the variability to a variability threshold THl, which may be predefined and fixed. In a variant, if frame distances D are computed during the respective test session, step 134 may instead compute the variability of the D values. If the variability is below THl, step 134 proceeds to step 135, which updates the test setting with an increased BP. For example, BP may be increased by a fixed value (e.g. 100 ms) or multiplied by a predefined factor (e.g. 2). Step 135 then proceeds to step 133 to initiate a test session with the updated test setting. If the variability exceeds THl, step 134 proceeds to step 136, which sets BP based on the burst periods used in the preceding test sessions. For example, step 130 may be set BP set to the largest BP that yielded a variability below THl. It should be noted that the sub-procedure 102C may test different burst periods in other ways than by gradually increasing BP (step 135). In one alternative, BP may be gradually decreased from a maximum value until the variability falls below TH1.

FIG. 10 illustrates an embodiment of the measurement procedure 100, in which the control parameter #B is evaluated for update between consecutive FTM sessions. The underlying rationale is that if there is little change in the distance between stations 10A, 10B during an FTM session, the number of bursts may be increased without significant loss of information. By increasing the number of bursts, the extent of the FTM session is increased. This will reduce the need for initial signal exchange between the stations 10A, 10B and free resources in both of the stations 10A, 10B. In the illustrated embodiment, the measurement procedure 100 comprises a step 141 of defining a current setting of the control parameters, by setting initial values of the control parameters. Such initial values may be predefined or set based on previous values of the control parameters. Step 142 then initiates an FTM session with the current setting. Step 143, which may be performed in parallel with step 142, computes D values based on the timestamps that are obtained during the FTM session. Step 144 computes the variability of the D values and compares the variability to a variability threshold TH2, which may be predefined and fixed. In a variant, if frame values D are generated during the respective FTM session, step 144 may instead compute the variability of the D values. If the variability is below TH2, which indicates that there is little movement between stations 10A, 10B, step 144 proceeds to step 145 which updates the current setting with an increased #B. For example, #B may increased by increasing the burst exponent by a fixed value (e.g. 1). Step 145 then proceeds to step 142 to initiate an FTM session with the updated setting. If the variability exceeds TH2, step 144 proceeds to initiate another FTM session with the current setting.

FIG. 11 is a flow chart of a method of operating an initiator station 10A in accordance with an embodiment of a second inventive concept. According to the second inventive concept, the initiator 10A does not perform the estimation procedure 102 as described in the foregoing, but instead assesses the need to update the control para- meters based on the burst values D (and/or frame values D) that are generated during the measurement procedure 100. In the illustrated embodiment, the measurement procedure 100 comprises steps 151-153 which corresponds to steps 141-143 as described above. Then, step 154 analyses the D values that are generated for the current FTM session, and possibly D values generated during one or more preceding FTM sessions, and updates, if deemed necessary, the current setting of the control parameters based on the D values. In one embodiment, step 154 computes one or more velocity values based on the D values and sets BD and/or BP based on the velocity value(s). In one example, step 154 estimates the relative speed RS between the stations 10A, 10B at some time instant or over a certain time period, and sets BD so as to at least achieve a predefined accuracy of the D values. Assuming that the predefined accuracy, PA, is given in length units, an upper limit for BD may be given by RS/PA. For example, if PA is 0.1 m and RS is estimated to 4 m/s, then BD may be set to a value below 40 ms, e.g. 32 ms in accordance with the FTM standard. In a further example, step 154 determines BP based on RS and a predefined update distance, UD, which is the desired maximum change in distance between consecutive D values. An upper limit for BP may be given by BD + UD/RD, where BD may be determined based on RS, as described above. Thus, assuming that RS is estimated to 4 m/s, BD is set to 32 ms and UD is 0.4 m, then BP may set to a value below 132 ms, e.g. 100 ms in accordance with the FTM standard.

FIG. 12 is a block diagram of an exemplary communication station 20 according to some embodiments. The station 20 may e.g. be the same as the station 10A or station 10B in FIG. 1. The station 20 comprises a controller 22, a memory 23, a transceiver 24, an antenna 25 and a motion sensor 26. The controller or control unit 22 is responsible for the overall operation of the station 20 and may be implemented by any commercially available CPU ("Central Processing Unit"), DSP ("Digital Signal Processor"), microprocessor or other electronic programmable logic device. The controller 22 may be implemented using instructions that enable hardware functionality, e.g. executable computer program instructions that may be stored on the memory 23. The controller 22 may be configured to read the instructions from the memory 23 and execute these instructions to control the operation of the station 20. The memory 23 may be implemented using any commonly known technology for computer-readable memories such as ROM, RAM, SRAM, DRAM, CMOS, FLASH, DDR, SDRAM or some other memory technology. The transceiver 24 is configured for communication in accordance with any wireless communication standard. The antenna 25 may include any suitable configuration, structure and/or arrangement of one or more antenna elements, components, units, assemblies and/or arrays. The motion sensor 26 is any type of device configured to detect, and possibly quantify, motion of the station 20. In a non-limiting example, the motion sensor 26 may include one or more of a gyroscope, a magneto- meter, an acceleration sensor, an inertial sensor, a speedometer, a GPS detector, an ultrasonic motion detector, a camera-based motion detector, and a radar detector. The operation of the station 20 may be controlled by a combination of circuitry and logic, where the circuitry may comprise the processor 22 and the memory 23, as well as further hardware, and the logic may be at least partly provided as executable program instructions. The program instructions may be provided to the station on a computer- readable medium, which may be a tangible (non-transitory) product (e.g. magnetic medium, optical disk, read-only memory, flash memory, etc) or a transitory product, such as a propagating signal. In some embodiments, the initiator 10A and/or the responder 10B may apply specific constraints when determining the current setting and/or the test setting(s). The constraints may be application specific, in that they differ depending on the intended use case for the respective station. The constraints may, e.g., define lower and upper limits for any one of the control parameters, define the sub-procedure(s) to be included in the estimation procedure 102, define the switching criterion of step 101 in FIG. 5, or define input values, limits or thresholds for any one of the above-described procedures 100, 102 and sub-procedures 102A, 102B, 102C.

In some embodiments, the initiator 10A may store (e.g. in memory 23) one or more previous values of a control parameter, i.e. values that have been included in the current setting for one or more previous FTM sessions. The initiator 10A may use the previous value(s) when determining a current value of the control parameter. For example, the step 130 may compute the current value of BD as a function of the BD value given by steps 121-129 and one or more previous BD values, e.g. as a (weighted) average. This would introduce a certain inertia into the updating of the control parameter.

In some embodiments, the initiator 10A uses the previous values(s) of the control parameter to set initial values of a sub-procedure. This might speed up the sub- procedure. For example, steps 126-129 in sub-procedure 102B may be modified to start the test sessions at or near the most recent BD value. Similarly, steps 132-135 in sub- procedure 102B may be modified to start the test session at or near the most recent BP value.

In some embodiment, the initiator 10A may store (e.g. in memory 23) one or more parallel values of a control parameter, i.e. values that have been determined for one or more other stations than station 10B in the wireless network. The initiator 10A may, e.g., use the parallel values(s) of the control parameter to set initial values of a sub- procedure.

The description given above relates to various general and specific embodiments, but the scope of the invention is limited only by the appended claims.

Claims

1. A communication device with a transceiver (24) for wireless communication and further comprising logic and circuitry (22, 23) configured to perform a

measurement procedure (100) comprising:

causing a second communication device (10B) to transmit, in accordance with a current setting of control parameters, one or more bursts (B) of signals,

receiving the signals by the transceiver (24), and

computing, based on timestamps associated with the signals, at least one measurement value (D, D) corresponding to a distance between the communication device and the second communication device (10B),

said logic and circuitry (22, 23) being further configured to perform an estimation procedure (102) comprising:

performing at least one test session by causing the second communication device (10B) to transmit, in accordance with a test setting of the control parameters, at least one burst (B) of signals,

receiving, by the transceiver (24), the signals transmitted during the respective test session,

performing an analysis of timestamps associated with the signals transmitted during the respective test session, and

updating the current setting of the control parameters based on the analysis.

2. The communication device of claim 1, which is configured to perform the measurement procedure (100) and the estimation procedure (102) in sequence.

3. The communication device of claim 2, which is configured to switch from measurement procedure (100) to the estimation procedure (102) upon detection of predefined event.

4. The communication device of claim 3, wherein the predefined event is associated with one or more of: an expiration of a timer, an error detected as a function of said at least one measurement value computed during the measurement procedure (100), a movement of the communication device as indicated by a movement sensor (26) in the communication device.

5. The communication device of any preceding claim, wherein the estimation procedure (102) comprises a first sub-procedure (102A; 102B) for determining a value of a first control parameter, and a second sub-procedure (102B; 102C) for determining a value of a second control parameter, wherein the first communication device is configured to perform the first sub-procedure (102A; 102B) and the second sub- procedure (102B; 102C) in sequence and to set the first control parameter to said value during the second sub-procedure (102B; 102C).

6. The communication device of any preceding claim, wherein said control parameters comprise one or more of: a count (#B) of the bursts (B) to be transmitted by the second communication device (10B), a burst duration (BD) of the respective burst (B), a burst period (BP) representing a time difference between initiations of consecutive bursts (B), and a count (#FTM) of the signals to be transmitted by the second communication device (10B) within the respective burst (B).

7. The communication device of claim 6, wherein the estimation procedure (102) comprises a sub-procedure (102A) for setting the control parameter that defines the count (#FTM) of the signals to be transmitted by the second communication device (10B) within the respective burst (B), wherein said sub-procedure (102A) comprises a test session which is performed in accordance with a test setting that sets the burst duration (BD) to a predefined minimum value, wherein said sub-procedure (102A) further comprises:

computing sample values (D) for individual signals received during the test session, the sample values (D) being indicative of the distance between the

communication device and the second communication device (10B),

analyzing changes in variability among the sample values with increasing number of samples value (D), and

setting the control parameter to the smallest number of sample values (D) for which the variability is below a predefined variability threshold.

8. The communication device of claim 6 or 7, wherein the estimation procedure comprises a sub-procedure (102B) for setting the control parameter that defines the burst duration (BD), wherein said sub-procedure (102B) comprises test sessions that are performed in accordance with test settings that differ by the burst duration (BD), each burst duration (BD) resulting in a different time delay (AFTM) between the signals transmitted by the second communication device (10B) within the respective burst (B), wherein said sub-procedure (102B) further comprises:

computing sample values (D) for individual signals received during the respective test session,

computing a variability of the sample values (D) for the respective test session, identifying the test session with the longest burst duration for which the variability is below a variability threshold, and

setting the control parameter as a function of the longest burst duration.

9. The communication device of claim 8, wherein one of the test sessions is performed with a predefined minimum burst duration, and wherein the variability threshold is set as a function of the variability of the sample values (D) computed for said one of the test sessions.

10. The communication device of claim 8 or 9, wherein the test settings define the burst period (BP) as small as possible but larger than or equal to the burst duration (BD).

11. The communication device of any one of claims 6- 10, wherein the estimation procedure comprises a sub-procedure (102C) for setting the control parameter that defines the burst period (BP), wherein said sub-procedure (102C) comprises test sessions that are performed in accordance with test settings that differ by the burst period (BP), wherein said sub-procedure (102C) further comprises:

computing measurement values (D, D) based on the signals received during the respective test session,

computing a variability for the measurement values (D, D) of the respective test session, and

identifying the test session with the longest burst period for which the variability is below a variability threshold, and

setting the control parameter as a function of the longest burst period.

12. The communication device of claim 11, wherein each of the measurement values is computed as an aggregate value for the signals within the respective burst (B) of signals.

13. The communication device of any one of claims 6- 12, which is configured to perform a sequence of measurement sessions during the measurement procedure (100), wherein each measurement session is performed in accordance the current setting of the control parameters and results in a set of measurement values (D, D) that correspond to the distance between the communication device and the second communication device (10B), and wherein each measurement session further comprises:

computing a variability for the measurement values (D, D) of the measurement session, and updating the current setting of the control parameters by increasing the count (#B) of bursts (B) if the variability is below a predefined variability threshold.

14. The communication device of any preceding claim, which is configured to operate in accordance with a Fine Timing Measurement (FTM) Protocol, and wherein the burst (B) of signals is a burst of FTM frames.

15. A communication device with a transceiver (24) for wireless communication and further comprising logic and circuitry (22, 23) configured to perform a

measurement procedure (100) comprising:

receiving the signals by the transceiver (24), and

said logic and circuitry (22, 23) being further configured to update the current setting of the control parameters based on the timestamps.

16. The communication device of claim 15, which is configured to update the current setting of the control parameters by computing at least one velocity value based on the timestamps, and modifying at least one of a burst duration (BD) of the respective burst (B), and a burst period (BP) representing a time difference between initiations of consecutive bursts (B), based on the at least one velocity value.

17. A method of operating a communication device with a transceiver (24) for wireless communication, said method comprising operating the communication device to perform a measurement procedure (100) comprising:

causing a second communication device (10B) to transmit, in accordance with a current setting of control parameters, one or more bursts (B) of signals;

receiving the signals by the transceiver (24); and

computing, based on timestamps associated with the signals, at least one measurement value (D, D) corresponding to a distance between the communication device and the second communication device (10B);

said method further comprising operating the communication device to perform an estimation procedure (102) comprising: performing at least one test session by causing the second communication device (10B) to transmit, in accordance with a test setting of the control parameters, at least one burst (B) of signals;

receiving, by the transceiver (24), the signals transmitted during the respective test session;

performing an analysis of timestamps associated with the signals transmitted during the respective test session; and

updating the current setting of the control parameters based on the analysis.

18. A method of operating a communication device with a transceiver (24) for wireless communication, said method comprising:

receiving the signals by the transceiver (24); and

said method further comprising updating the current setting of the control parameters based on the timestamps.

19. A computer-readable medium comprising program instructions which, when executed by a control unit (22), cause the control unit (22) to perform the method of claim 17 or 18.