Method and arrangement for estimating the distance between at least two antenna units - Patents.com

Abstract

A method for estimating a distance between at least a first antenna unit and a second antenna unit by bidirectional transmission of at least two signals having different circular polarizations between the antenna units, where phase information is determined and used to determine at least a first phase sum and a second phase sum, which are utilized to determine at least one distance indicator indicative of a distance between the first antenna unit and the second antenna unit.

Description

[Technical field]

[0001] The present invention relates generally to wireless communications and localization. More specifically, the present invention relates to estimating a distance between at least a first antenna unit and a second antenna unit by bidirectional transmission of at least two signals having different circular polarizations between the antenna units. FIELD OF THEINVENTION

[0002] Systems and methods involving signal transmission, such as radio signals, from a first (transmitting) device to a second (receiving) device are used in many applications, such as tracking and various other purposes where distance measurements are utilized. In some systems, it may be possible to assess the distance between two devices by alternately using the first and second devices as transmitter and receiver. In this case, the phase of one or more transmitted signals received by the receiving device may be determined and used for distance measurements. However, such measurements have many complications and may be tedious or infeasible to obtain accurate results regarding the distance between the devices.

[0003] One problem is related to signal reflections. Reflections of the transmitted signal from other objects that reach the receiving device may be indistinguishable from the signal received directly from the transmitting device. When determining the phase of the received signal relative to the local oscillator of the receiving device and determining the distance or phase length from the determined phase, the determined distance or phase length may be affected by the reflected signal and may be erroneous. Distinguishing between the original transmitted signal and the reflected signal is particularly difficult when the reflection occurs in a direction close to that of the original signal. In such cases, the reflected signal has almost the same delay as the original signal, so that in spatial or temporal signal processing, the reflected signal becomes practically indistinguishable from the original signal. However, in applications where extremely high distance measurement accuracy is required, such distortions of the signal phase are detrimental.

[0004] To reduce or eliminate the effects of reflections, polarized signals can be used for transmission. It has been found that circularly polarized signals are particularly advantageous in that they can reduce the effects of reflected signals from metal surfaces, which are the most problematic reflective surfaces when linearly polarized signals are used. Reflections from metal surfaces (or other surfaces with a similar reflection coefficient) essentially change the polarization of the signal, so that if the receiving antenna is configured to receive essentially only signals with the circular polarization of the original signal, the reflected signal will not be received, or will at least be strongly attenuated. Circularly polarized signals can also reduce the effects of reflections from other types of surfaces, even if the polarization of the reflected signal is not perfectly circular.

[0005] However, the use of circularly polarized signals also has some problems. Rotation along the Line-of-Sight (LOS) axis between the antenna units affects the phase measurement and therefore the determination of the distance between the antenna units. For example, in geodetic GNSS, the problem of rotation along the LOS axis between the transmitter and receiver is attempted to be solved by using a joint solution of the rotation angle and the position coordinates, and differentiation with respect to a nearby reference station. However, in this case, a large number of satellites are required to solve the problem, making this solution impractical for terrestrial systems where only a few antenna units are available to track the position of a moving wireless device. Also, reference antennas may be impractical for terrestrial navigation. Summary of the Invention

[0006] It is an object of the present invention to alleviate at least some of the problems in the prior art. According to one aspect of the present invention, there is provided a method for estimating a distance between at least a first antenna unit and a second antenna unit, the method comprising at least: transmitting a first signal having a first circular polarization via the first antenna unit; receiving the first signal with the second antenna unit; determining first phase information indicative of a phase of the received first signal relative to a local oscillator of a radio unit to which the second antenna unit is associated; transmitting, via the second antenna unit, a first response signal substantially corresponding to the first signal; receiving the first response signal at the first antenna unit; determining first response phase information indicative of a phase of the received first response signal relative to a local oscillator of a radio unit with which the first antenna unit is associated; determining a first phase sum indicative of a sum of the first phase information and the first response phase information; transmitting a second signal having a second circular polarization opposite to the first circular polarization via one of the first antenna unit or the second antenna unit; receiving the second signal at the other of the first antenna unit or the second antenna unit; determining second phase information indicative of a phase of the received second signal relative to a local oscillator of a radio unit associated with an antenna unit through which the second signal is received; transmitting a second response signal substantially corresponding to the second signal through the other of the first antenna unit or the second antenna unit that is not transmitting the second signal; receiving the second response signal at an antenna unit that transmitted the second signal; determining second response phase information indicative of a phase of the received second response signal relative to a local oscillator of a radio unit associated with an antenna unit through which the second response signal is received; determining a second phase sum indicative of a sum of the second phase information and the second response phase information; determining at least one distance indicator indicative of a distance between the first antenna unit and the second antenna unit based on at least the first phase sum and the second phase sum; Includes.

[0007] According to another aspect, the invention also relates to a configuration as defined in independent claim 13.

[0008] The invention allows for the determination of at least one distance indicator, indicative of the distance between the antenna units, with high accuracy based on the determined phase information. The accuracy of the determined distance indicator may be high enough so that the distance between a first antenna unit and a second antenna unit may be evaluated or determined with sub-millimeter accuracy. However, the absolute distance measurement accuracy depends on the accuracy of the phase measurement at the carrier frequency converted to wavelength in meters. The invention concerns the determination of phase information and the subsequent operation via one or more distance indicators, whereby the absolute distance can be determined with high accuracy. However, the determination of the absolute distance is not a requirement of the invention as specified in the claims. The various methods that may be used to obtain such absolute distance measurements will not be discussed in detail here.

[0009] The present invention allows for highly accurate measurements using cost-effective antenna units. This solution allows for the use of circularly polarized antenna units and circularly polarized signals to reduce the effects of reflections on the phase measurements and to compensate for rotations between antenna units and possibly changes in the antenna phase center, which are a function of the signal direction. Without these corrections and compensations made possible by the present invention, if centimeter wave radios are used for distance measurements, the accuracy of the determined distance would be at best at the centimeter level.

[0010] The invention may be useful in any application in which one or more distances are assessed or monitored. The arrangement according to the invention may be used, for example, in a system for tracking an object, or other position tracking, indoor positioning system, etc. Tracking of an object may be accomplished, for example, by tracking a change in distance between a first antenna unit associated with a reference position and a second antenna unit associated with or coupled to the object.

[0011] The antenna units that can be used may be of lower quality than those used in, for example, geodetic GNSS measurements. Such antenna units are selected to eliminate the problem of phase center fluctuations, and such high quality antenna units are expensive. However, the present invention allows the use of simpler and cheaper circularly polarized antenna units. Thus, the arrangement according to the present invention is inexpensive to implement and use.

[0012] According to the invention, in determining the distance indicator between the antenna units (or at least their reference points), the effect of the rotation between the first and second antenna units along the LOS axis can be compensated for or removed (possibly in the subsequently determined distance). This leads to a more accurate assessment of the distance than without compensation. It may be understood that with respect to the distance between the first and second antenna units, the distance can be determined or evaluated through a distance indicator that can be used to determine a phase length indicative of the distance between the antenna units. The evaluation of the distance does not necessarily mean that the absolute distance is determined, but the distance may be evaluated through knowledge of the phase length that gives a known fractional part of the wavelength of the total distance between the antenna units, as will be understood by those skilled in the art. The determination of the absolute complete distance between the antenna units may then involve further calculations, for example of integer ambiguities, which will not be described herein.

[0013] It may also be possible to assess the distance between the antenna units using only the first and second antenna units, i.e. only between the antenna units assessing the link distance or baseline distance may be sufficient to assess the distance (or at least the distance indication) with a satisfactory accuracy. There may be no need to consider a separate reference antenna unit or a joint solution (where multiple satellites or antenna units are observed and a solution that fits the data obtained therefrom is determined). In some embodiments, calibration data is obtained for the standard antenna pattern of the antenna units. However, this data may be previously determined data and only data for antenna units that are similar in structure to the first and / or second antenna units may be required.

[0014] The at least one distance indication may comprise a first distance variable indicative of a sum of a first phase sum and a second phase sum. By using this first distance variable, it may be possible to exclude rotation between the first and second antenna units along the LOS axis in determining the distance indication. The distance indication and the (possibly determined) distance may then be determined completely independent of the rotation.

[0015] An orientation of the signal data with respect to a coordinate system of each of the first and second antenna units may be obtained, and the orientation of the signal data may be utilized in determining the at least one distance indication. In particular in some embodiments, the orientation data may be utilized in determining the at least one distance indication by obtaining antenna phase pattern calibration data and interpolating antenna phase responses.

[0016] The transmitted signal may be received at at least three different antenna elements provided in the first antenna unit and / or the second antenna unit to determine the direction of the signal data.

[0017] An embodiment of the method may include determining self-measured data indicative of a phase of a self-measured signal for the first polarization and the second polarization received at a transmitting antenna unit during transmission of a signal, the self-measured data may be used in determining at least one distance indication.

[0018] The at least one distance indicator may include a rotation variable indicative of a difference between the first and second phase sums. The method may then, in some embodiments, include determining a rotation angle between the first and second antenna units based on the rotation variable. Thus, it is also possible to explicitly determine the rotation angle. The rotation variable may be determined at least once and then used in tracking the rotation angle between the first and second antenna units. Subsequent first signals and first response signals may be transmitted to determine subsequent first phase sums for tracking the distance between the first and second antenna units. Thus, it is possible to track distance indicators at a high rate with a single polarization measurement without transmission of a signal having a second polarization, and even if transmission of a signal also including the second polarization is performed, it may only be at a low rate.

[0019] Each of the at least first and second antenna units may transmit at least one signal within a predefined time slot, preferably in a predefined sequence. In some embodiments, the first antenna unit may be a master unit and the remaining at least the second antenna units may be slave units. The master unit may be configured to transmit the first signal, where the master unit may be configured to check, before transmission of the first signal in each measurement cycle, whether a radio channel is free for transmission, to transmit at least the first signal if the radio channel is free, and not to perform the transmission if the radio channel is not free.

[0020] In an embodiment having master and slave antenna units, the slave unit may be configured, prior to transmitting a signal in a given measurement cycle, to determine whether a previous antenna unit in a given sequence of antenna units transmitted a signal in the given measurement cycle, and if so, to transmit the slave unit's signal, whereas if it determines that the previous antenna unit did not transmit a signal, to not transmit the slave unit's signal.

[0021] In some embodiments, the first signal may also be received by at least a third antenna unit. The third antenna unit may also be configured to transmit a first response signal, which may be received by at least the first antenna unit and / or the second antenna unit, and in some embodiments, by all other antenna units that did not transmit the first response signal. The second signal may also be received by at least the third antenna unit, which may also be configured to transmit a second response signal. This second response signal may be received by at least the first antenna unit and / or the second antenna unit. In this way, at least two sets of antenna units may be obtained, which perform bidirectional transmission of at least a signal with a first polarization and a signal with a second polarization. For each set of antenna units, respective phase information, a first phase sum and a second phase sum may be determined. Thus, for each set of antenna units, at least one distance indicator indicative of the distance between the antenna units may be determined and at least two distances between the antenna units may be evaluated.

[0022] The novel features which are believed to be characteristic of the invention are set forth with particularity in the appended claims, but the invention itself, both its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the following description of certain illustrative embodiments read in conjunction with the accompanying drawings.

[0023] As will be appreciated by those skilled in the art, the discussions presented above with respect to the various method embodiments can be flexibly applied mutatis mutandis to the apparatus embodiments, and vice versa. [Brief description of the drawings]

[0024] The invention will now be explained in more detail with reference to exemplary embodiments according to the accompanying drawings. [Figure 1] 1 illustrates a schematic diagram of an exemplary configuration according to an embodiment of the present invention; [Diagram 2] 4 illustrates a further exemplary configuration according to an embodiment of the present invention. [Diagram 3] 1 illustrates a first antenna unit and a second antenna unit according to an embodiment of the present invention, also showing a circularly polarized signal. [Figure 4] The orientation of the first and second antenna units along the connecting lines and the electrical vector reference direction (as unit vector e) of both antenna units and their LCP and RCP feeds are shown. [Diagram 5] A first wireless unit, a first antenna unit, a second wireless unit, and a second antenna unit are shown. [Figure 6] 1 illustrates an exemplary radio unit and antenna unit that may be utilized in embodiments of the present invention. [Figure 7] 1 shows the allocation of time slots in a measurement cycle. [Figure 8] 1 is a flowchart of a method according to an embodiment of the present invention. [Figure 9] 4 is a message sequence chart for a method according to an embodiment of the present invention. Detailed explanation

[0025] Figure 1 shows a schematic diagram of an arrangement 100 according to an embodiment of the invention. The arrangement comprises at least a first antenna unit (AU) 104 and a second antenna unit 106, which are associated with a first radio unit 108 and a second radio unit 110, respectively. The antenna units 104, 106 may be located within the radio units 108, 110 or may be coupled to the radio units, for example via cables. The antenna units may comprise one or more antennas and are not shown in detail in Figure 1. An exemplary antenna unit is depicted in more detail in Figure 3.

[0026] The configuration 100 may also have other numbers of antenna units and radio units, such as a third radio unit and a fourth radio unit. Each pair of antenna units transmits and receives one or more signals between the antenna units. The antenna units can be considered to be separated by a baseline or distance D, which generally refers herein to the distance between the antenna units or at least their reference points, such as the geometric center on the antenna unit plane relative to the antenna element location, e.g., a point on the antenna unit that can be simplistically considered to be the point at which the signal is transmitted or received. In some configurations, the distance between each pair of antenna units can be evaluated.

[0027] The radio units 108, 110 are coupled to at least one processing unit 102. The processing unit 102 may be a controller unit external to the radio units 108, 110, may be implemented as a microprocessor unit, or may be provided as part of a larger computing device such as a personal computer. However, in some embodiments, the processing unit 102 may be located within the radio units 108, 110 or may be considered part of the radio units 108, 110.

[0028] The processing unit 102 may be configured to control the radio units and / or antenna units of the arrangement 100. The processing unit 102 may receive data from the antenna units 104, 106 or the radio units 108, 110 and may utilize the received data in any information decisions made. The processing unit 102 may also have or have access to one or more databases or memory units containing additional data. Such additional data may include, for example, complex (amplitude and phase) antenna gain patterns and antenna phase pattern calibration data for one or more antenna unit types.

[0029] Additionally or alternatively, the processing unit 102 may be configured to receive data from antenna units and / or radio units included in the arrangement 100 in a wired (e.g. Ethernet) or wireless (e.g. WLAN) manner. Figure 2 shows an embodiment of the arrangement 100 in which the processing unit 102 is wirelessly coupled to the radio units 108, 110. The processing unit 102 may be associated with a processor antenna unit 112.

[0030] The processing unit 102 and wireless units 108, 110 may be powered using, for example, Power-over-Ethernet (PoE), direct mains power, batteries, solar panels, or mechanical generators (such as wind turbine blades).

[0031] In some embodiments, in addition to the processing device 102, which may be, for example, a locally equipped processing device, a processing device located at a remote location may be utilized in the configuration 100. Or, the processing device 102 may be implemented as a remote processing device without the need for a local processing device. The remote processing device may receive any of the obtained data and, for example, perform at least a portion of the data determination performed by the configuration 100. The remote processing device may refer to a processing device that may be accessed via cloud computing. Or, the remote processing device may refer to, for example, a virtual processor configured at multiple locations and configured to perform the processing presented herein through parallel processing means.

[0032] The first antenna unit 104 is configured to transmit a first signal having at least a first circular polarization. The first circular polarization may be, for example, Left Circular Polarization (LCP). The first signal and the subsequent transmitted signals may be radio frequency signals (RF signals). The signals may be unmodulated RF signals (i.e. sine waves), a comb of RF sine waves over a predefined frequency range, or an RF signal modulated by any known (complex) sequence.

[0033] The frequency of the first signal and subsequent signals may be, for example, less than 10 GHz. However, the configuration 100 is not limited to any frequency range. For example, 60 GHz may be utilized. For higher frequencies, the quality of the local oscillator (LO) of the radio unit may be a factor to be considered. For example, to utilize a frequency of 60 GHz, an oscillator of, for example, 5 PPB may provide sufficient quality.

[0034] The frequencies used for at least the first signal, the first response signal, the second signal, and the second response signal should essentially match each other. If the arrangement is configured to transmit subsequent signals (to determine subsequent phase information indicative of the distance between the antenna units at subsequent times), the signal frequencies may be the same as those used for the first signal, for example, or may vary in frequency.

[0035] The duration of the first signal (and subsequent signals transmitted by any of the radio or antenna units of the arrangement 100) may be between 10 μs and 10000 μs, depending on, for example, the distance between the antenna or radio units, the time interval between measurement cycles, and / or the quality of the local oscillators that the radio units 108, 110 have path losses, etc. The duration of the signal may be, for example, around 100 μs.

[0036] It should be noted that in some systems the time variation of the signal over the measurement sequence, i.e. the phase rotation, can be corrected. Such phase rotation can be caused by local oscillator frequency differences or by the movement of the antenna unit (Doppler). Estimation and correction of local oscillator frequency differences and Doppler can be performed using additional signals (e.g. signals repeated in time), but the techniques are well known and are outside the scope of this invention.

[0037] A first signal is received at the second radio unit 110 through the second antenna unit 106. At least first phase information associated with the first signal is determined based on the received first signal. The first phase information indicates a phase of the received first signal relative to a local oscillator of the second radio unit 110.

[0038] To be precise, the signal frequency is usually higher than the local oscillator frequency, and the phase measurement is often done in digital baseband, for example using a Fast Fourier Transform (FFT), which is essentially equivalent to measuring the phase with respect to a local oscillator, which for simplicity is considered to run at the signal frequency.

[0039] If the configuration 100 has further radio units and antenna units, such as a third radio unit associated with a third antenna unit, the first signal may also be received at the third radio unit (through the third antenna unit) and the first phase information may also be determined at the third (and other) radio units.

[0040] Configurations having two or more antenna units may be utilized to determine relative positions between the antenna units. Relative distances may be determined multiple times to track or monitor changes between the distances. For example, relative distances may be determined multiple times to track a three-dimensional shape that includes three or more antenna units. Embodiments of the configurations may be used, for example, for Terrestrial Positioning.

[0041] The second radio unit 110 is configured to transmit at least a first response signal through the second antenna unit 106. The first response signal may be equivalent to or essentially match the first signal, at least in frequency and polarization.

[0042] The first response signal is received at the first radio unit 108 via the first antenna unit 104. Based on the received first response signal, at least first response phase information is determined. The first response phase information indicates a phase of the received first response signal relative to a local oscillator of the first radio unit 108.

[0043] The first phase information and the first response phase information are then used (by the processing unit 102) to determine at least a first phase sum indicative of a sum of the first phase information and the first response phase information.

[0044] Furthermore, if the configuration comprises further radio units and antenna units, for example a third antenna unit may also be configured to transmit a signal corresponding to the first response signal. This signal may be received at least by the first antenna unit, and preferably by all other antenna units that did not transmit the first response signal, and in each of the antenna units that received the first response signal, first response phase information for the received first response signal may be determined. Of course, the third antenna unit may receive the first response signal transmitted by the second antenna unit 106 and determine the respective phase information. Thus, a plurality of first phase sums may be determined.

[0045] The first radio unit 108 may also be configured to transmit at least a second signal having a second circular polarization through the first antenna unit 104. The second circular polarization may be different from the first circular polarization, for example, Right Hand Circular Polarization (RCP).

[0046] The second signal may then be received at the second radio unit 110 via the second antenna unit 106. Based on the received second signal, at least second phase information associated with the second signal is determined, the second phase information indicating a phase of the received second signal relative to a local oscillator of the second radio unit 110.

[0047] The second radio unit 110 may be configured to transmit at least a second response signal through the second antenna unit 106. The second response signal may be identical to the second signal or may essentially match the second signal at least in frequency and polarization.

[0048] The second response signal is received at the first radio unit 108 through the first antenna unit 104. At least second response phase information is determined based on the received second response signal. The second response phase information indicates a phase of the received second response signal relative to a local oscillator of the first radio unit 108.

[0049] A second signal is transmitted by the second antenna unit and received by the first antenna unit. A second response signal is transmitted by the first antenna unit and received by the second antenna unit. If the transmission sequence (which antenna unit transmits which signal) is known, the time dependence due to Doppler shift may be corrected.

[0050] The second phase information and the second response phase information are then used to determine at least a second phase sum indicative of a sum of the second phase information and the second response phase information.

[0051] Based on the first phase sum and the second phase sum, at least one distance metric may be determined that indicates the distance D between the first antenna unit 104 and the second antenna unit 106 (or at least the distance between their reference points). At least some of the candidate distance metric or metric may be independent of the rotation angle between the LOS axes of the antenna units, as will be introduced later.

[0052] Thus, the arrangement 100 comprises at least one pair of antenna units for transmitting and receiving at least two signals and their response signals to each other, the bidirectional transmission allowing the determination of bidirectional phase information and its sum.

[0053] In an embodiment of the arrangement 100 including further radio and antenna units, such as a third radio unit and an associated third antenna unit, the further antenna unit may be configured to receive at least the second signal transmitted by the second antenna unit 108 and possibly also a second response signal, determine respective second phase information, and transmit a second response signal. A plurality of second phase sums may then be determined, and a plurality of distance indicators may be determined (for each assessed distance, based on the plurality of first phase sums and the second phase sums).

[0054] Any antenna unit in the configuration may be configured to receive any or all of the signals transmitted by the other antenna units in the configuration. In some embodiments, each transmitting antenna unit may broadcast its signal in a predefined time slot and the other antenna units may receive the signal.

[0055] Thus, the arrangement may include a plurality of antenna units, and may include a set of antenna units configured to perform bidirectional transmission of first and second signals having a first polarization and first and second response signals having a second polarization. Bidirectional phase information may be determined for each received transmission signal to obtain a first phase sum and a second phase sum for each set of antenna units that transmitted and received the first and second signals having the first polarization and transmitted and received the first and second response signals having the second polarization. In this manner, a plurality of distance indicators may be determined, each of which indicates a distance between antenna units in a particular set of antenna units.

[0056] In some embodiments, each of the transmitting antenna units may transmit at least one signal within a given time slot, and further, the first antenna unit may be a master unit and the second antenna unit (and the rest) may be slave units. The master unit may be an antenna unit configured to transmit the first signal. The master unit may be configured to check whether the wireless channel is free for transmission before transmitting the first signal, and may be configured to transmit at least the first signal if the channel is free and not perform the transmission of the first signal if the channel is not free.

[0057] The master unit may check whether the radio channel is free before transmitting the first signal and, if yes, may continue the measurement cycle, which may be reserved by the arrangement for at least one measurement cycle. That is, the arrangement may advantageously utilize radio bands / channels that require Listen Before Talk (LBT) functionality. If it determines that the radio channel is not free, it may not transmit the first signal and may abort or cancel the measurement cycle itself without transmitting a signal. The master unit or the first antenna unit may then wait a predefined time between measurement cycles. Then, in the next measurement cycle, it may check again whether the radio band is free and, if so, perform the transmission of the first signal to start the measurement cycle.

[0058] In some embodiments having a master antenna unit and one or more slave antenna units, before transmitting a signal in a given measurement cycle, each slave unit may determine whether the previous antenna unit in a given sequence of antenna units transmitted a signal in the measurement cycle, and if yes, the slave unit may be configured to transmit its signal, whereas if it determines that the previous antenna unit did not transmit a signal, i.e., a valid measurement signal was not received, it may be configured to not transmit the signal (i.e., to wait for a complete measurement cycle).

[0059] In an embodiment of the configuration, at least a first antenna unit may transmit a time synchronization signal, which is received by at least the remaining radio antenna units, before transmitting the first signal. By time synchronization, the antenna units may perform the transmission of the signals in a synchronized manner, taking into account time slots related to the predetermined order in which the signals should be transmitted. Such a synchronized transmission of the signals may be performed, especially in embodiments in which the time between measurement cycles in subsequent measurement cycles is relatively long, such as one minute or more.

[0060] The information may be processed and steps performed in a different order than suggested here, for example the first signal may be an RCP signal and the second signal may be an LPC signal.

[0061] In some embodiments, the first and second signals may be transmitted before the first and second response signals.

[0062] In other embodiments of the invention, the first and second signals may be transmitted simultaneously (and thus in the same time slot). The first and second response signals may additionally or alternatively be transmitted simultaneously. The simultaneous transmission of multiple signals, each having left or right circular polarization, utilizes coded transmission.

[0063] 3A illustrates an exemplary first antenna unit 104 and a second antenna unit 106 that may be included in the configuration 100 according to an embodiment of the present invention. Here, both antenna units 104, 106 each have four antenna elements. The first antenna unit 104 includes a first antenna element 114, a second antenna element 116, a third antenna element 118, and a fourth antenna element 120. The second antenna unit 106 includes a first antenna element 122, a second antenna element 124, a third antenna element 126, and a fourth antenna element 128. In some embodiments, the first and second antenna units may each have a different number of antenna elements. The antenna units 104, 106 may only have one dual-polarized antenna element, in conjunction with an antenna element that has high phase center stability at least with respect to the signal direction.

[0064] If the antenna elements do not have high phase performance, the antenna units 104, 106 preferably have at least three antenna elements 114, 116, 118, 120, 122, 124, 126, respectively. In this case, the direction of arrival of the signal may be determined. In particular, it is preferable that the determination of the direction of arrival of the signal is performed when the phase centers of the antenna elements are not constant.

[0065] When the antenna unit 104, 106 comprises multiple antenna elements 114, 116, 118, 120, 122, 124, 126, all antenna elements of the antenna unit preferably receive signals transmitted by the other antenna unit in the pair of antenna units consisting of the antenna units 104 and 106. Some or all of the multiple antenna elements of an antenna unit may transmit, for example, the first signal and / or the second signal. However, it may be more efficient if all antenna elements are used for receiving signals but only one antenna element is used for transmission.

[0066] 3A, for the first antenna unit 104, the first antenna element 114 may be a transmit / receive (TX / RX) antenna element, and the second antenna element 116, the third antenna element 118, and the fourth antenna element 120 may be receive antenna elements. For the second antenna unit 106, the first antenna element 122 may be a transmit / receive (TX / RX) antenna element, and the second antenna element 124, the third antenna element 126, and the fourth antenna element 128 may be receive (RX) antenna elements.

[0067] The first antenna unit 104 and the second antenna unit 106 may each have at least one RCP feed and one LCP feed. Also, each antenna element may have either a single polarization feed or a multi-polarization feed. The TX / RX elements 114, 122 preferably have feeds for both polarizations.

[0068] 3A also shows a reference point P1 of the first antenna unit and a reference point P2 of the second antenna unit. The measurements performed may be simplified as being made at the reference points. The reference points P1 and P2 may be considered to be connected by a connecting line X (LOS axis), the length of which corresponds to the distance D between the antenna units.

[0069] The LOS axis X forms an angle β with the plane in which the antenna unit lies, for example the plane x1,y1 of the first antenna unit 104. The projection of the LOS axis X onto the plane x1,y1 forms an angle α with the x1 axis. These angles α and β can be obtained by a direction of arrival (DOA) measurement. The axis z1 denotes the normal to the surface of the first antenna unit 104.

[0070] The rotation of the antenna unit about the LOS axis X cannot be determined or estimated by the DOA method, but when using circularly polarized signals, this rotation affects the determination of the phase information and must be taken into account in order to obtain more accurate phase information.

[0071] FIG. 3B shows, as an example, a circularly polarized signal emitted by a first antenna 104 with a first antenna element 114 (not shown). This signal may be received by a second antenna unit 106 with a first antenna element 122. For simplicity, only the first antenna elements 114, 122 are considered. FIG. 3B shows the transmission of a circularly polarized signal in order to illustrate the effect of a rotating antenna unit on the signal and the determined phase. As will be understood by those skilled in the art, for example, the rotation of the first antenna unit 104 relative to the second antenna unit 106 affects the phase of the received signal at the second antenna unit 106. This affects the determined phase information and the determined distance indicator, and thus the evaluated distance D, even though the distance D between the antenna units remains the same.

[0072] Figure 4 shows the orientation of the first and second antenna units along the line of sight (LOS) axis, with the line of sight perpendicular to the page. These orientations are expressed as unit vectors normal to the first antenna unit. TIFF2024521318000002.tif2656 and the unit vector of the normal to the second antenna unit The electrical vector reference directions of both antenna units and their LCP and RCP feeds are also unit vectors. It is shown as TIFF2024521318000004.tif2656. For example, TIFF2024521318000005.tif2656 corresponds to the direction of the unit vector of the LCP feed of the first antenna unit 104. In what follows, the signs of the terms are when the first antenna unit 104 is below the second antenna unit 106 along the page normal.

[0073] 5 illustrates a first radio unit 104, a first antenna unit 104, a second radio unit 110, and a second antenna unit 106. In this example, the first antenna unit 104 includes antenna elements corresponding to the first antenna element 114 and the fourth antenna element 120 of FIG. 3. The second antenna unit 106 includes antenna elements corresponding to the first antenna element 122 and the fourth antenna element 128. It is assumed that the TX / RX switch of FIG. 4 is in the "TX" position.

[0074] In the following, the first antenna element 114 of the first antenna unit transmits the first and second signals, which are received by the fourth antenna element 128 of the second antenna unit, and the first antenna element 122 of the second antenna unit transmits the first and second response signals, which are received by the fourth antenna element 120 of the first antenna unit. If the antenna units 104, 106 have more than one antenna element, similar considerations apply to the other antenna elements. The individually determined, for example, phase information and / or distance indicators may be combined to obtain information (such as distance indicators) corresponding to information about the corresponding reference points of the antenna units 104, 106.

[0075] The first antenna unit 104 may transmit a first signal, for example in LCP polarization, in its timeslot (time T1) through the first antenna element 114. The phase of the transmitted signal received by the second antenna unit 106 through the fourth antenna element 128, i.e., the first phase information to be determined, is as follows:

[0076] TIFF2024521318000006.tif26170

[0077] Where: TIFF2024521318000007.tif2656 and TIFF2024521318000008.tif2656 indicates the transmission time T 1 Φ is the phase of the local oscillators of the first wireless unit 108 and the second wireless unit 110 in the above-mentioned case. Note that the subscripts represent antenna units, not antenna elements. 12 is the geometric phase corresponding to the distance or the baseline or the connecting geometric line between the transmitting antenna element 114 of the first antenna unit 104 and the receiving antenna element 128 of the second antenna unit 106. TIFF2024521318000009.tif2656 is the phase length of the transmission branch (transmitter) corresponding to the first antenna unit 104, TIFF2024521318000010.tif2656 is the phase length of the receive branch (receiver) corresponding to the second antenna unit 106. (Here, phase length refers to the phase shift that occurs in a signal traversing a certain distance.) Ψ LCP is the electrical vector reference plane (X and TIFF2024521318000011.tif2656) and the electrical vector reference plane (X and The angle between the surfaces (the surfaces formed by TIFF2024521318000012.tif2656).

[0078] The phase lengths of the transmitting and receiving branches, e.g. TIFF2024521318000013.tif2656 and TIFF2024521318000014.tif2656 includes phase lengths due to the physical lengths of the transmit and receive branches of the antenna unit and associated radio unit, and possibly cable lengths. For example, as can be seen in FIG. TIFF2024521318000015.tif2656 corresponds to the length of the transmit branch of the first radio unit 108, from the digital-to-analog converter (DAC) to the transmit antenna 114. It should be noted that the phase length between the DAC and the mixer corresponds to a much lower frequency (baseband frequency) than the phase length between the mixer and the antenna. Also, the phase length from the local oscillator to the mixer must be taken into account. All these effects are taken into account here. Assume it is contained in the term TIFF2024521318000016.tif2656.

[0079] Following the second antenna unit 106 receiving the first signal, the second antenna unit 106 transmits a first response signal by the first antenna element 122. This first response signal is transmitted in the time slot (time T 2 ) may be received by the fourth antenna element 120 of the first antenna unit. The first response phase information determined or measured relative to the received first antenna unit (at the fourth antenna element 120), assuming that the second antenna unit 106 transmitted a response signal with zero phase relative to its local clock / oscillator, is:

[0080] TIFF2024521318000017.tif26170

[0081] Where: TIFF2024521318000018.tif2656 and TIFF2024521318000019.tif2656 is the transmission time T 2 108 and 110. For simplicity, in the following processing, the phase relationship between the wireless nodes 108 and 110 is We assume that TIFF2024521318000020.tif2656 remains the same between transmissions. Normally, this does not happen, since wireless nodes have independently running clocks. But with a reasonably good clock, the rate of change of this phase relationship is constant in time and can be easily determined by repeated measurements, and therefore easily corrected for. Φ 21 is the geometric phase corresponding to the distance between the transmitting antenna, here the first antenna element 122 of the second antenna unit, and the receiving antenna, here the fourth antenna element 120 of the first antenna unit. TIFF2024521318000021.tif2656 and TIFF2024521318000022.tif2656 are the transmit and receive branch phase lengths corresponding to the second antenna unit 106 and the first antenna unit 104, respectively. LCP is as defined above. We assume here that the rotation between antenna units 104 and 106 does not change significantly between transmissions. This is a valid assumption since typical times between transmissions are on the order of 100 microseconds.

[0082] Then, a second signal having RCP polarization is transmitted from the first antenna element 114 of the first antenna unit (at time T 3 ) and received by the fourth antenna element 128 of the second antenna unit. The second phase information that is determined is then: TIFF2024521318000023.tif26170

[0083] Here, Ψ RCP As can be seen from Figure 4,LCP The second response signal is transmitted by the first antenna element 122 of the second antenna unit. Then, at time T 4 In that time slot, the signal is received by the fourth antenna element 120 of the first antenna unit. The second response phase information that is determined is:

[0084] TIFF2024521318000024.tif26170

[0085] The topological terms of the branch, e.g. TIFF2024521318000025.tif2656 may be removed by self-measured or self-calibrated data indicating the phase of the self-measured signal received at the transmitting antenna unit during the transmission of the signal. For example, when the first antenna element 114 of the first antenna unit transmits a first signal, the fourth antenna element 120 of the first antenna unit may determine a first self-measured phase as follows: TIFF2024521318000026.tif26170

[0086] When the first antenna 104 unit transmits the second signal, the self-measured phase information may be determined as follows: TIFF2024521318000027.tif26170

[0087] Where: TIFF2024521318000028.tif2656 is the phase of the signal transfer function between the first antenna element 114 (transmitting antenna element) of the first antenna unit and the fourth antenna element 120 (receiving antenna element) of the first antenna unit. A stable phase term such as TIFF2024521318000029.tif2656 may be measured for one antenna unit, for example in an anechoic chamber, This measurement data may be used to remove the effect of the stable phase term on the self-calibrated phase data such as TIFF2024521318000030.tif2656 (assuming that it is approximately the same for all similar antenna units). TIFF2024521318000031.tif2656 may be omitted or compensated for in further calculations. The second antenna unit 106 may determine self-measured phase information in a similar manner.

[0088] The first phase sum S1 is determined as follows.

[0089] TIFF2024521318000032.tif26170

[0090] On the other hand, the second phase sum S2 is as follows.

[0091] TIFF2024521318000033.tif26170

[0092] In particular, if the transmit and receive branches are stable, their lengths are known, and it is known that they have been compensated for somewhere, then the phase sum can be calculated without self-measured phase information.

[0093] However, if the configuration has multiple antenna units, each transmitting antenna unit may determine its own self-calibrated / self-measured phase information.

[0094] As can be seen from Figure 4,

[0095] TIFF2024521318000034.tif26170 and

[0096] The file is TIFF2024521318000035.tif26170.

[0097] where Ω is the rotation angle along the LOS axis between the first antenna unit 104 and the second antenna unit 106; TIFF2024521318000036.tif2656 and TIFF2024521318000037.tif2656 shows the z-axis and electrical reference vector, measured along the LOS axis (signal direction) for antenna unit i and its respective polarized feed. TIFF2024521318000038.tif2656 and The angle between TIFF2024521318000039.tif2656. TIFF2024521318000040.tif2656 and TIFF2024521318000041.tif2656 can also be understood as the phase response of each polarized feed in a given direction.

[0098] As a distance indicator, a first distance variable VD may be determined based on the first and second phase sums S1, S2, for example as their sum.

[0099] TIFF2024521318000042.tif51170

[0100] Assuming that there is no significant change in rotation during transmission, we can see that the rotation Ω between the antenna units is cancelled. This is a safe assumption for tracking a typical object, e.g. a forklift, considering that the time between transmissions is on the order of 100 microseconds. Thus, the geometric phase length between the reference points of the first and second antenna units, 1 / 2(Φ 12 +Φ 21 ) may be determined from a sum of self-calibrated phase measurements, e.g., it can be easily derived from equation (11) using the first distance variable VD:

[0101] Remaining feed-specific phase response parameters TIFF2024521318000043.tif2656 and TIFF2024521318000044.tif2656 depends on the signal direction. If the antenna units have good phase performance, i.e., the antenna phase center is essentially constant independent of the signal direction, then for antenna unit i, we can write

[0102] TIFF2024521318000045.tif26170

[0103] As a result, Φ 12 Such a phase length can be derived directly from the (self-calibrated) phase sums S1, S2, as can be seen from equation (13).

[0104] However, if the approximation (12) does not hold, the phase response of the antenna feed must be measured, for example in an anechoic chamber, to obtain antenna phase pattern calibration data. TIFF2024521318000046.tif2656 and It is possible to measure the .tif2024521318000047.tif2656. Such calibration data may be obtained by measurement, from an external source, or stored in a database. Assuming that the produced antenna units are sufficiently similar to each other, which is quite common in mass production, these antennas may be assumed to have essentially similar patterns at their feeds. If the signal direction (α,β) is known in the antenna unit coordinate system, then TIFF2024521318000048.tif2656 and TIFF2024521318000049.tif2656 may be interpolated from stored antenna phase pattern calibration data and from (preferably self-calibrated) phase sum measurements to obtain 1 / 2(Φ 12 +Φ 21 ), may be used as follows:

[0105] TIFF2024521318000050.tif26170

[0106] Some examples of determining the direction of a signal (DOA) include the following: 1) Applying a direction-of-arrival algorithm (such as beamforming or subspace-based methods such as MUSIC) to signals received by multiple receive antennas. 2) Track the location and orientation of the antenna unit. Or a combination of these.

[0107] The interpolation of the antenna phase response for a signal direction determined from the antenna phase pattern calibration data may be modeled based on well-known techniques, such as modeled based on vector spherical harmonics or based on a 2D-FFT-based decomposition calculated from the measured data.

[0108] In yet another embodiment, it is also possible to explicitly determine the rotation angle Ω. This can be done by determining a rotation variable VR as a distance measure. The rotation variable VR can be determined by forming the difference of the phase sums as follows: TIFF2024521318000051.tif26170

[0109] The Ψ term, which depends on the signal direction, can be found as described above, and the (self-calibrated) phase sum determines the rotation angle Ω between the antenna units.

[0110] TIFF2024521318000052.tif26170

[0111] The advantage of explicitly determining Ω is that it can be tracked by estimation algorithms such as Kalman estimation or particle estimation. The estimate can be used to remove rotation effects in single-polarization measurements.

[0112] TIFF2024521318000053.tif51170

[0113] With this configuration, it is possible to track a rapidly changing geometric phase length with a single polarization measurement by transmitting only a first signal having a first polarization and a first response signal to determine a first phase sum without transmitting a signal having a second polarization. The first phase sum may be determined at a selected first time interval (e.g., 70 to 300 times, 100 to 250 times, or 150 to 200 times per second), from which the geometric phase length may be determined as follows:

[0114] TIFF2024521318000054.tif26170

[0115] The slowly varying rotation angle Ω may be tracked with an estimator using less frequent dual polarization measurements, where signals having first and second polarizations (first signal, first response signal, second signal, and second response signal) may be transmitted at a selected second time interval (e.g., between 1-30 times per second or between 1-10 times per second).

[0116] Figure 6 illustrates exemplary radio units 108, 110 and antenna units 104, 106 having four antenna elements that may be utilized in embodiments of the present invention. The radio units 108, 110 of Figure 6 may be utilized in embodiments in which self-calibration and / or signal direction measurement is performed.

[0117] 7A and 7B show how time slots may be allocated in a measurement cycle for transmitting and receiving signals and possibly communicating data in the configuration 100. A measurement cycle may represent a set of signals to be transmitted or may represent a time duration during which signals are transmitted one after the other such that the time between transmissions is less than a threshold. A first measurement cycle may include, for example, transmitting (and receiving) a first signal, a first response signal, a second signal, and a second response signal. In some embodiments, a second measurement cycle may be performed. The second measurement cycle may, for example, be equivalent to the first measurement cycle.

[0118] One measurement cycle may include at least one measurement frame (having N measurement slots). During the measurement frame, at least the first antenna unit 104 and the second antenna unit 106 may transmit their respective signals separately in their assigned time slots. The measurement cycle of FIG. 7A is applicable to the configuration 100 having N radio units, and the distance between each antenna unit may be evaluated. Each antenna unit may transmit its respective signal in its respective time slot.

[0119] The transmissions may be implemented such that transmissions occur in subsequent time slots such that no empty time slots are left between transmissions. The transmissions and time slots may be balanced such that the time interval between the end of a transmission and the start of the subsequent time slot in which the subsequent antenna unit begins transmitting is less than or equal to a selected maximum time interval. The time interval between the end of a transmission and the start of a subsequent transmission may be at least less than 50 μs, preferably less than 20 μs, e.g. less than 16 μs, between transmissions occurring within the same measurement frame.

[0120] However, between measurement frames there may be long time intervals between the end of one transmission and the start of the following transmission, in which case the master unit may be configured to check at the start of each measurement frame whether the transmission channel is free.

[0121] The provision of a compact transmission signal can be advantageously used in combination with, for example, a Wi-Fi network: in the present invention, a radio channel for transmission needs to be reserved only once per measurement cycle. This feature may enable compatibility with networks like Wi-Fi.

[0122] If no transmission takes place in the following timeslot, the measurement cycle may take a longer, unknown amount of time to complete. This is because one measurement cycle cannot be effectively performed as one transmission in a radio channel where the channel must be contended only once, as defined, for example, in ETSI EN 301 893 (the standard specification regulating Wi-Fi transmissions at 5 GHz). Since each transmitting antenna unit must contend the channel separately during transmission, the measurement sequence may become quite long if the channel is occupied by other users between transmissions.

[0123] FIG. 7B shows how time slots are allocated in a measurement cycle that also employs at least one communication frame (having one or more communication slots). During the communication frame, signals, measurement / determination data, or other data may be transmitted to the processing device 102. At least one data communication may be transmitted and may be multiplexed in the time or frequency domain with the measurement signal transmitted by the wireless unit. The at least one data communication may include at least the determined phase information. Additionally or alternatively, the data communication may include any other information. In this way, the configuration 100 can function simultaneously as a measurement system and a communication network.

[0124] The required time synchronization accuracy is preferably better than one-quarter of the duration of the possible guard time between subsequent signals to prevent overlapping transmissions.

[0125] FIG. 8 illustrates a flow chart of an embodiment of the present invention. At 802, at least a first signal having a first polarization is transmitted by a first antenna unit (AU) 104. At 804, the first signal is received by at least a second antenna unit (AU) 106, and at least a first phase information is determined. The first signal transmitted by the first antenna unit 104 may also be received at any other antenna unit of the configuration 100. Then, first phase information regarding the phase of the received first signal relative to the local oscillator of the receiving antenna unit may be determined, respectively. At least one first response signal is transmitted by at least the second antenna unit 106 at 806. The first response signal essentially corresponds to the first signal. The first response signal may also be transmitted through any third or other antenna unit of the configuration, each antenna unit preferably transmitting a signal in its own predefined time slot, preferably in a predefined order. At 808, the first response signal is received by at least the first antenna unit 104 and first response phase information is determined. Any signal transmitted may be received at the antenna unit that did not transmit it and phase information determined, respectively. If self-calibration information is also determined, the signal may also be received at the receive antenna of the antenna unit that transmitted it.

[0126] At least one first phase sum is determined 810 for at least the first antenna unit 104 and the second antenna unit 106. The first phase sum may be a sum of the first phase information and the first response phase information. The first phase sum may be determined for any or all of the pairs of antenna units that have transmitted bidirectional signals between each other.

[0127] At 812, a second signal having at least a second polarization is transmitted to the antenna unit. The second signal may be transmitted by the first antenna unit 104, the second antenna unit 106, or any other AU of the configuration. At 814, the second signal is received by at least one other antenna unit that did not transmit the second signal, and at least second phase information is determined. The second signal transmitted by any antenna unit may also be received at any other antenna unit of the configuration 100. Then, second phase information regarding the phase of the received second signal relative to the local oscillator of the receiving antenna unit may be determined, respectively. At least one second response signal is transmitted by at least one of the other antenna units that did not transmit the second signal, at 816. The second response signal essentially corresponds to the second signal. The second response signal may also be transmitted through another antenna unit (not transmitting the second signal). These antenna units transmit the second response signal, preferably in their own predefined time slots, preferably in a predefined order. At 818, the second response signal is received by at least one of the antenna units to determine second response phase information.

[0128] At least one second phase sum is determined 820 for at least the first antenna unit 104 and the second antenna unit 106. The second phase sum may be a sum of the second phase information and the second response phase information. The second phase sum may be determined for any or all pairs of antenna units that have transmitted bidirectional signals between each other.

[0129] At least one distance indicator is determined at 822, the distance indicator being indicative of at least a distance between the first antenna unit 104 and the second antenna unit 106. At least one distance indicator may be determined for each pair of antenna units that have transmitted bidirectional signals therebetween on first and second polarizations, i.e., signals including at least a first signal, a first response signal, a second signal, and a second response signal.

[0130] FIG. 9 illustrates a message sequence chart for at least a portion of a method according to an embodiment of the present invention. FIG. 9 illustrates signal and data transmissions that may be performed for an arrangement and method including N antenna units 104, 106. Each antenna unit is associated with a radio unit 108, 110, and a radio unit may be associated with one or more antenna units. For example, the first antenna unit 104 and the second antenna unit 106 may be associated with or considered to be included in the same radio unit. In the embodiment of FIG. 9, the first antenna unit 104 transmits a first signal at time T1. The first signal may be received by at least some of the other antenna units of the arrangement 100. FIG. 9 illustrates that the first signal is received by the second antenna unit 106, the third antenna unit 902, and the Nth antenna unit 904.

[0131] The second antenna unit 106 may be configured to transmit a first response signal at time T2, which may be received by at least some or all of the other antenna units of the configuration 100. The third antenna unit 902 may also be configured to transmit a first response signal at time T3, which may be received by at least some of the other antenna units of the configuration. Figure 9 also shows that antenna unit N transmits a first response signal at time TN, which is received by at least the first antenna unit 104, the second antenna unit 106, and the third antenna unit 902.

[0132] At time TN+1, the first antenna unit 104 may be configured to transmit a second signal. In other embodiments of the invention, the second signal may be transmitted by one of the other antenna units in the configuration. A second response signal is shown being transmitted by each of the second antenna unit (time TN+2), the third antenna unit (time TN+3), and the Nth antenna unit (time T2N).

[0133] Each of the radio units 108, 110 (the first radio unit 108, the second radio unit 110, and / or any further radio units of the arrangement 100) may determine at least a first phase information, a first response phase information, a second phase information, and / or a second response phase information. Figure 9 shows that the determined phase information is transmitted to the processing unit 102 at time T2N+1. The processing unit 102 may then determine at least a first and a second phase sum associated with each pair of antenna units. It may also determine at least one distance indication.

[0134] Although the present invention has been described above with reference to the above embodiments and some advantages of the present invention have been shown, the present invention is not limited to these embodiments, but includes all possible embodiments within the spirit and scope of the inventive concept and the scope of the following claims.

[0135] Unless expressly stated otherwise, the features recited in the dependent claims are mutually freely combinable.

Claims

1. 1. A method for estimating a distance between at least a first antenna unit and a second antenna unit, comprising: transmitting a first signal having a first circular polarization via the first antenna unit; receiving the first signal with the second antenna unit; determining first phase information indicative of a phase of the received first signal relative to a local oscillator of a radio unit with which the second antenna unit is associated; transmitting, via the second antenna unit, a first response signal that substantially corresponds to the first signal; receiving the first response signal at the first antenna unit; determining first response phase information indicative of a phase of the received first response signal relative to a local oscillator of a radio unit with which the first antenna unit is associated; determining a first phase sum indicative of a sum of the first phase information and the first response phase information; transmitting a second signal through one of the first antenna unit or the second antenna unit, the second signal having a second circular polarization opposite to the first circular polarization; receiving the second signal at the other of the first antenna unit or the second antenna unit; determining second phase information indicative of a phase of the received second signal relative to a local oscillator of a radio unit associated with an antenna unit that received the second signal; transmitting a second response signal essentially corresponding to the second signal through the other of the first antenna unit or the second antenna unit that is not transmitting the second signal; receiving the second response signal at an antenna unit that transmitted the second signal; determining second response phase information indicative of a phase of the received second response signal relative to a local oscillator of a radio unit associated with an antenna unit that received the second response signal; determining a second phase sum indicative of a sum of the second phase information and the second response phase information; determining at least one distance indicator indicative of a distance between the first antenna unit and the second antenna unit based on at least the first phase sum and the second phase sum; A method comprising:

2. The method of claim 1 , wherein the at least one distance metric comprises a first distance variable indicative of a sum of a first phase sum and a second phase sum.

3. obtaining a direction of signal data with respect to a coordinate system of each of the first antenna unit and the second antenna unit; utilizing a direction of the signal data in determining the at least one distance metric; The method of claim 1 , comprising:

4. 4. The method of claim 3, wherein to determine the direction of the signal data, a transmitted signal is received by at least three different antenna elements provided in the first antenna unit and / or the second antenna unit.

5. the phase information, the phase sum and / or the distance indicator are determined with respect to a reference point of the first antenna unit and / or the second antenna unit, The reference point is a geometric center of the positions of two or more antenna elements of each antenna unit and is a geometric center on the antenna unit plane. The method of claim 1.

6. 2. The method of claim 1, comprising: determining self-measured data indicative of a phase of a self-measured signal for the first circular polarization and the second circular polarization received at a transmitting antenna unit during transmission of a signal; and using the self-measured data in determining at least one distance indicator.

7. The method of claim 1 , wherein the at least one distance metric comprises a rotation variable indicative of a difference between the first phase sum and the second phase sum.

8. 8. The method of claim 7, wherein the rotation variable is determined at least once and thereafter utilized in tracking a rotation angle between the first antenna unit and the second antenna unit, the method including transmitting a subsequent first signal and a subsequent first response signal to determine a subsequent first phase sum, and the rotation variable is used to compensate for the rotation angle between the first antenna unit and the second antenna unit to track a distance between the first antenna unit and the second antenna unit.

9. 2. The method of claim 1, wherein each of the at least first and second antenna units transmits at least one signal in a predetermined time slot and in a predetermined sequence.

10. 10. The method of claim 9, wherein the first antenna unit is a master unit and the remaining at least the second antenna units are slave units, the master unit is configured to transmit the first signal, and the master unit is configured, before transmitting the first signal in each measurement cycle, to check whether a radio channel is free for transmission, to transmit at least the first signal if the radio channel is free, and not to perform the transmission if the radio channel is not free.

11. 11. The method of claim 10, wherein the slave unit is configured to, before transmitting a signal in a predetermined measurement cycle, determine whether a previous antenna unit in a predetermined order of antenna units has transmitted a signal in the predetermined measurement cycle, and if it is determined that it has transmitted, transmit the slave unit's signal, and if it is determined that the previous antenna unit has not transmitted a signal, not transmit the slave unit's signal.

12. The method includes evaluating at least two distances between a plurality of antenna units, the evaluating comprising at least: receiving the first signal with at least a third antenna unit; transmitting a first response signal via the third antenna unit and receiving the first response signal at least at the first antenna unit; receiving the second signal with at least the third antenna unit; transmitting a second response signal via the third antenna unit and receiving the second response signal at least at the first antenna unit; and obtaining at least two sets of antenna units that perform bidirectional transmission of at least a signal having the first circular polarization and a signal having the second circular polarization by determining phase information for each of the sets of antenna units; determining a first phase sum and a second phase sum for each pair of antenna units; - determining, for each pair of antenna units, at least one distance indicator indicative of the distance between the antenna units; The method of claim 1 , comprising:

13. The method of claim 3, wherein utilizing the direction of the signal data in determining the at least one distance indicator includes obtaining antenna phase pattern calibration data and interpolating an antenna phase response.

14. The method of claim 7, comprising determining a rotation angle between the first antenna unit and the second antenna unit based on the rotation variable.

15. A system comprising a processing means and a memory means, wherein the memory means stores program instructions which, when executed by the processing means, are configured to cause the system to perform a method according to any one of claims 1 to 14.

16. A computer program comprising program instructions configured, when executed by a processing means of a system, to cause the system to perform a method according to any one of claims 1 to 14.

17. 1. An arrangement for estimating a distance between at least a first antenna unit and a second antenna unit, the arrangement comprising at least a first antenna unit associated with a first radio unit, a second antenna unit associated with a second radio unit, and a processor, the arrangement comprising: transmitting a first signal having a first circular polarization via the first antenna unit; receiving the first signal with the second antenna unit; determining first phase information indicative of a phase of the received first signal relative to a local oscillator of a radio unit with which the second antenna unit is associated; transmitting, via the second antenna unit, a first response signal that substantially corresponds to the first signal; receiving the first response signal at the first antenna unit; determining first response phase information indicative of a phase of the received first response signal relative to a local oscillator of a radio unit with which the first antenna unit is associated; determining a first phase sum indicative of a sum of the first phase information and the first response phase information; transmitting a second signal having a second circular polarization opposite to the first circular polarization through one of the first antenna unit or the second antenna unit; receiving the second signal at the other of the first antenna unit or the second antenna unit; determining second phase information indicative of a phase of the received second signal relative to a local oscillator of a radio unit associated with an antenna unit that received the second signal; transmitting a second response signal essentially corresponding to the second signal through the other of the first antenna unit or the second antenna unit that is not transmitting the second signal; receiving the second response signal at an antenna unit that transmitted the second signal; determining second response phase information indicative of a phase of the received second response signal relative to a local oscillator of a radio unit associated with an antenna unit that received the second response signal; determining a second phase sum indicative of a sum of the second phase information and the second response phase information; determining at least one distance indicator indicative of a distance between the first antenna unit and the second antenna unit based on at least the first phase sum and the second phase sum; A configuration that is configured to perform.

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