Detecting antenna misalignment

Abstract

A method for determining whether a steerable directional antenna (2) is misaligned with a target (1') The antenna (2) has a main lobe (9), which defines a pointing axis (A) and has a double-curved portion, and a sidelobe (10) which has a locally single-curved portion. The method comprises: establishing a reference axis (B); moving a direction of the pointing axis (A) away from of the reference axis (B); obtaining values of a signal characteristic; estimating a dependence of the signal characteristic on a direction of the pointing axis (A); and determining whether the antenna (2) is misaligned with the target (1'). The dependence is estimated along an elliptical path enclosing the reference axis (B), and communication via the locally single-curved portion is indicated by the dependence having more than two extrema. The method can be used in, for example, telecommunication applications.

Description

[0001] DETECTING ANTENNA MISALIGNMENT

[0002] TECHNICAL FIELD

[0003] The present invention relates to a method for determining whether an antenna is misaligned with a target. The present invention further relates to an antenna arrangement, a computer program, a computer-readable medium and a computer program product, all of which implement the method.

[0004] BACKGROUND

[0005] A factor that typically affects the signal reception and transmission quality of a directional antenna is how well aligned the antenna is vis-a-vis the direction to the target with which it is communicating. Maintaining proper alignment between the antenna and its target is therefore important, but in many applications this can be challenging. The antennas of wireless point-to-point links of telecommunications networks, for instance, are often mounted on masts, and when the masts sway in the wind the antennas frequently become misaligned. Further, when a mast is unevenly heated by the sun, different sides of the mast may expand by different amounts so that the mast bends slightly but enough for its antenna not to point directly toward the other antenna of the link. The high-gain directional antennas that are often used in these links tend to be particularly easily misaligned due to their narrow and precisely oriented beams.

[0006] The antenna pattern of a directional antenna typically has a central main lobe surrounded by several sidelobes. The reception and transmission quality of such antennas is higher the more closely aligned the main lobe maximum is with the direction to the target. The quality may be particularly low if a sidelobe is pointing toward the target. There exist steerable antennas that are equipped with systems capable of detecting misalignment and adjusting the antenna direction to compensate for misalignment. These systems usually search for the direction in which the strength of a signal received by the antenna is maximized, and they are likely to find the main lobe maximum as long as the incoming signal is received somewhere on the main lobe. However, if a sidelobe receives the signal, they risk finding the sidelobe maximum instead, and the reception then gets "stuck” in the sidelobe. Detecting that an antenna has got stuck in sidelobe reception is less straightforward a task than might initially seem to be the case. For instance, it is usually not enough to rely solely on the received signal strength since a lower strength than expected may be due to other factors than misalignment, such as rain or snow causing temporary fading.

[0007] An example of method for antenna sidelobe identification is disclosed in CN 113391138 B. While this and other known methods may be adequate for many applications, there nevertheless exists a continuous need in the art for improved ways of detecting antenna misalignment. Simplicity and robustness are examples of aspects where improvements would be desirable, especially where such improvements can be accomplished in an economically feasible fashion.

[0008] SUMMARY

[0009] To better address one or more of the concerns discussed above, the present inventors have provided the invention as claimed herein. The invention is defined by the appended independent patent claims, with embodiments of the invention being set forth in the appended dependent patent claims, in the following description and in the drawings.

[0010] According to a first aspect of the invention, there is provided a method for determining whether a steerable directional antenna is misaligned with a target. The antenna is configured to communicate signals according to an antenna pattern comprising a main lobe and a sidelobe. The main lobe defines a pointing axis of the antenna and has a double-curved portion comprising a global maximum of the antenna pattern. The sidelobe has a locally single-curved portion comprising a local maximum of the antenna pattern. The method comprises: establishing a reference axis; moving a direction of the pointing axis to be in at least five different directions away from of the reference axis; and obtaining values of a signal characteristic. The values of the signal characteristic are associated with the pointing axis being oriented in the different directions. The method further comprises: estimating, based on the values of the signal characteristic, a dependence of the signal characteristic on the direction of the pointing axis; and determining, based on the dependence, whether the antenna is misaligned with the target. The dependence is estimated along an elliptical path enclosing the reference axis. Communication via the locally single-curved portion is indicated by the dependence having more than two extrema along the elliptical path.

[0011] The pointing axis and the reference axis are geometrical axes. The pointing axis is a principal axis of the directional antenna and points in the beam-maximum direction. The pointing axis may alternatively be referred to as the boresight axis.

[0012] The elliptical path is a geometrical path. It should be noted that a circle is considered herein to be a special case of an ellipse, so the elliptical path may for example be a circular path.

[0013] As used herein, the term "obtaining” is meant to be interpreted broadly. For example, a device or unit can obtain a value by determining the value itself or receiving the value from another device or unit.

[0014] The use of "based on” is herein meant to be open and inclusive, in that a determination, estimation, calculation, or other action "based on” one or more recited values, conditions or steps may be based on additional values, conditions or steps beyond those recited. Differently put, when an action is "based on” a value, condition or step, the action should be interpreted as based at least on that value, condition or step.

[0015] By an "extremum” is herein meant a maximum or a minimum.

[0016] The present invention is based on the realization that it is possible to determine if some types of antennas are misaligned with their targets by looking at how signals communicated, i.e. received or sent, by the antennas vary when these point in different directions. If the signal is communicated via a sidelobe, it will vary in a different way than if it is communicated via the main lobe, something which allows sidelobe communication to be distinguished from main lobe communication and, hence, misalignment to be detected.

[0017] More specifically, the inventors have realized that the number of extrema exhibited by a characteristic of the signal along an elliptical path in space can reflect whether the signal is communicated via a region of the antenna pattern that, at least approximately, may be described by a double-curved surface or by a single-curved surface. Thereby, one can determine from the number of extrema whether the antenna is communicating with its target via the main lobe or a sidelobe and, hence, whether or not the antenna and the target are misaligned.

[0018] The present invention allows for antenna misalignment to be detected in a straightforward and robust manner because, even if the signal sent or received by the antenna is relatively noisy or weak, the number of extrema are likely to be clearly identifiable. The inventive method can also be implemented without adding unnecessary complexity or cost. Having five or more values of the signal characteristic means that the Nyquist limit is reached. Increasing the number of values may result in a more accurate estimate of how the signal characteristic varies along the elliptical path.

[0019] The method may comprise performing a conical scan around the reference axis. The conical scan comprises the moving of the direction of the pointing axis and the obtaining of the values of the signal characteristic. During a conical scan, the pointing axis is moved so as to sweep out a circular, or approximately circular, cone around the reference axis. A circular cone is a cone that has a circular base. Conical scans can provide signal variations that are clear and straightforward to analyze, such as sinusoidal-like signal variations.

[0020] Each of the different directions may form an angle with the reference axis, which angles may be equal to or less than 20% of a half-power beamwidth of the antenna. Angles in this range are especially suitable for the invention. The angles formed by the different directions may have the same, or approximately the same, value, although this is not necessary. Their values may be different.

[0021] The antenna pattern may be circularly symmetric. The present invention is particularly suitable for antennas having such antenna patterns. It is noted that, in real-world applications, antennas do not generate antenna patterns that are perfectly circularly symmetric. Accordingly, an antenna pattern referred to herein as "circularly symmetric” may be only approximately circularly symmetric.

[0022] The method may comprise steps for directing the antenna toward the target. In further detail, if the antenna is determined to be misaligned with the target, the method may comprise: determining, based on respective positions of two extrema of the signal characteristic, a straight line, wherein a direction of the straight line estimates a direction connecting the main lobe and the sidelobe; moving the direction of the pointing axis along the straight line; obtaining values of a further signal characteristic, wherein the values of the further signal characteristic are associated with the direction of the pointing axis being at different positions along the straight line; estimating, based on the values of the further signal characteristic, a direction toward the target; and aligning the direction of the pointing axis with the direction toward the target.

[0023] The inventors hereof have realized that the estimated dependence of the signal characteristic may advantageously be used for re-aligning misaligned antennas. More specifically, it is possible to determine, from the estimated dependence, a straight line that passes through the main lobe. The direction in which the main lobe should point in order to be directed toward the target may then be found by searching along this line for the position where the signal fulfills a suitable criterion associated with the direction toward the target, such as where the signal reception or transmission is the strongest. The direction in which the main lobe should point can be found quickly this way because only a one-dimensional, not a two-dimensional, search is needed.

[0024] The direction connecting the main lobe and the sidelobe may be a radial direction defined by the sidelobe. This may be the case if the antenna pattern is circularly symmetric, for instance.

[0025] The direction toward the target may be estimated based on a position, along the straight line, where the further signal characteristic has an extremum. Because of measurement errors and other factors, the position, on which this estimate is based, is typically an approximation of where the true extremum is located.

[0026] The signal characteristic and / or the further signal characteristic may be a signal strength, a signal error, or a signal-to-interference-plus-noise ratio. The signal error may be an error rate, such as a bit error rate or a block error rate. The signal characteristic and the further signal characteristic may be of the same type or of different types.

[0027] The signal characteristic may be a characteristic of a signal sent by the target to the antenna. In such case, the values of the signal characteristic may be determined at, i.e. near, the antenna. For example, if the antenna is mounted high up on a mast, the values may be determined by a control unit, or some other type of device, located high up on the mast or in a cabinet close to the bottom of the mast. Alternatively, however, the values of the signal characteristic may be determined at a location remote from the antenna, such as by a remotely arranged computer or server that is configured to determine the values and to send them to a control unit, or some other type of device, near the antenna. Similarly, the further signal characteristic may be a characteristic of a signal sent by the target to the antenna.

[0028] The signal characteristic may be a characteristic of a signal sent by the antenna to the target. The obtaining of the values of the signal characteristic may comprise receiving, by the antenna, a signal indicating the values of the signal characteristic. In such case, the values of the signal characteristic may be determined at the target or elsewhere, such as by a remotely arranged computer or server. The signal indicating the values of the signal characteristic may be sent by the target to the antenna. Alternatively, this signal may be sent to the antenna via some other link or connection than that between the antenna and the target. Similarly, the further signal characteristic may be a characteristic of a signal sent by the antenna to the target, and the obtaining of the values of the further signal characteristic may comprise receiving, by the antenna, a signal indicating the values of the further signal characteristic.

[0029] The antenna may be connected to a transceiver configured to communicate signals using the antenna, and the method may comprise configuring, based on said dependence, an operation of the transceiver in order to compensate for an expected variation of the further signal characteristic during the moving of the direction of the pointing axis along the straight line.

[0030] Configuring the operation of the transceiver in this way may increase the likelihood that the search is "hitless, ” that is, that the transmission of information to or from the antenna is not interrupted. The expected variation of the further signal characteristic may be a decrease or an increase. For example, the signal strength typically goes down significantly during the search for the direction in which the main lobe should point because the signal is temporarily sent or received via nulls in the antenna pattern. It is possible to estimate, from the determined dependence of the signal characteristic, how much the signal strength will decrease, and the transceiver may then be configured accordingly so that the risk of transmission errors is reduced.

[0031] The method may comprise estimating, based on said dependence, a fading margin and / or a signal-to-noise ratio, and the configuring may be based on the estimated fading margin and / or the estimated signal-to-noise ratio.

[0032] The configuring may comprise at least one of: adjusting an output power of the antenna, adjusting a signal modulation scheme, and adjusting a signal coding scheme.

[0033] According to a second aspect of the invention, there is provided an antenna arrangement comprising a steerable directional antenna configured to communicate signals according to an antenna pattern comprising a main lobe and a sidelobe. The main lobe defines a pointing axis of the antenna and has a double-curved portion comprising a global maximum of the antenna pattern. The sidelobe has a locally single-curved portion comprising a local maximum of the antenna pattern. The antenna arrangement further comprises: a transceiver configured to communicate signals using the antenna; and a control unit operably connected to the antenna and to the transceiver. The control unit is configured to perform the method according to the first aspect of the invention.

[0034] According to a third aspect of the invention, there is provided a point-to-point radio link comprising a first antenna arrangement according to the second aspect of the invention. The first antenna arrangement is configured to be arranged at a first endpoint of the point-to-point radio link. The point-to-point radio link further comprises a second antenna arrangement configured to be arranged at a second endpoint of the point-to-point radio link. The first and second antenna arrangements are configured to communicate with each other using radio signals.

[0035] According to a fourth aspect of the invention, there is provided a computer program comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the first aspect of the invention.

[0036] According to a fifth aspect of the invention, there is provided a computer-readable medium comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the first aspect of the invention. The computer-readable medium may for example be a tangible, non-transient computer-readable medium. According to a sixth aspect of the invention, there is provided a computer program product comprising a computer program and a computer-readable medium on which the computer program is stored. The computer program comprises instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to the first aspect of the invention.

[0037] The second to sixth aspects of the invention can provide advantages similar to those discussed above in connection with the first aspects of the invention.

[0038] It is noted that embodiments of the invention relate to all possible combinations of features in the claims.

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] For exemplifying purposes, various embodiments of the present invention will now be described in more detail with reference to the accompanying drawings, in which:

[0041] Figure 1 shows a point-to-point radio link according to an example embodiment of the invention;

[0042] Figure 2 illustrates an antenna arrangement of the point-to-point radio link in Figure 1;

[0043] Figure 3 shows, in a perspective view, the antenna pattern of the antenna arrangement in Figure 2;

[0044] Figure 4 is a top view of the antenna pattern in Figure 3;

[0045] Figure 5 is a flowchart representation of a method according to an example embodiment of the invention, wherein the method relates to determining whether an antenna is aligned with a target;

[0046] Figures 6 and 7 show the point-to-point radio link in Figure 1 and illustrate main lobe communication and sidelobe communication, respectively;

[0047] Figures 8 and 9 show spatial signatures corresponding to main lobe communication and sidelobe communication, respectively;

[0048] Figure 10 is a flowchart representation of a method according to an example embodiment of the invention, wherein the method relates to directing an antenna toward a target;

[0049] Figure 11 is a top view of the antenna pattern in Figure 3;

[0050] Figures 12 and 13 show spatial signatures corresponding to main lobe communication and sidelobe communication, respectively;

[0051] Figure 14 shows a point-to-point radio link according to another example embodiment of the invention; and

[0052] Figure 15 shows a computer program product according to an example embodiment of the invention.

[0053] All of the figures are schematic, not to scale and generally only show parts that are necessary in order to elucidate the embodiments, wherein other parts may be omitted. The dimensions of some of the features may have been exaggerated for the sake of clarity.

[0054] DETAILED DESCRIPTION

[0055] The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. It should be noted that in the following the term exemplary is to be construed as serving as an example, instance or illustration.

[0056] Figure 1 illustrates an example of a point-to-point radio link 100, which in the following will be referred to as the link 100 for brevity. The link 100 is in this case included in a telecommunication network, such as a cellular communication network and in particular a 3rd Generation Partnership Project (3GPP) network. 3GPP networks include 2G, 3G, 4G, 5G and 6G networks. For instance, the point-to-point radio link 100 may be a link in a transport network of a cellular communication network, such as a fronthaul link or a backhaul link. The link 100 may be intended for some other type of network than a telecommunication network in a different example.

[0057] The link 100 comprises a first endpoint 101 and a second endpoint 102. The two endpoints 101, 102 may alternatively be referred to as nodes. The link 100 is configured for wireless communication between the endpoints 101, 102 using electromagnetic waves in the radio frequency range, such as radio frequencies below 300 GHz. In a typical application, the distance between the endpoints 101, 102 is somewhere in the range from tens of meters to several kilometers.

[0058] The link 100 includes a first antenna arrangement 1 and a second antenna arrangement T that are arranged at the first endpoint 101 and the second endpoint 102, respectively. The antenna arrangements 1, T are configured to send and receive radio signals to and from each other. Thus, the antenna arrangements 1, T are configured for bidirectional communication with each other. The second antenna arrangement T may be referred to as the target of the first antenna arrangement 1, and vice versa. The antenna arrangements 1, T are here mounted on respective masts 103, 104. One or both of the antenna arrangements 1, T may in a different example be mounted on some other type of structure, such as on the roof of a building or a radio tower.

[0059] Next, with reference to Figure 2 and with continued reference to Figure 1, the first antenna arrangement 1 will be described in more detail. The second antenna arrangement T is of the same type as the first antenna arrangement 1, but this is not necessarily the case in a different example. In particular, the second antenna arrangement T may in a different example lack the capability of detecting antenna misalignment and / or the capability of compensating therefor. The first antenna arrangement 1 has these capabilities, and they will be further discussed below.

[0060] Now, the first antenna arrangement 1 includes a directional antenna 2. For brevity, the first antenna arrangement 1 and the directional antenna 2 will in the following be referred to as the antenna arrangement 1 and the antenna 2, respectively. The antenna 2 is in this case a so-called pencil-beam antenna but may be some other type of directional antenna in a different example. The antenna 2 here has a reflector 3 which in this case is a parabolic reflector and which alternatively may be referred to as a dish. The antenna 2 further has a radiating element 4 for generating electromagnetic waves in the radio frequency spectrum. The radiating element 4 may alternatively be referred to as a feed. The antenna arrangement 1 also has a transceiver 5 for communicating, i.e. sending and receiving, signals via the antenna 2. The transceiver 5 is here configured to control the radiating element 4.

[0061] The antenna arrangement 1 includes in this case a mount 6 for attaching the antenna arrangement 1 to an external structure, which in this example is the above-mentioned mast 103. The antenna 2 is in this case movably attached to the mount 6. In more detail, the antenna 2 is here movably attached so as to allow the spatial direction of a pointing axis A of the antenna 2 to be varied. The pointing axis A is parallel to a main direction of radiation of the antenna 2.

[0062] The antenna arrangement 1 further includes a control unit 7 which is operably connected to the antenna 2 and to the transceiver 5. The control unit 7 here comprises one or more electric motors (not shown) for moving the antenna 2 and processing circuitry (not shown) connected to the transceiver 5 and to the one or more electric motors. The processing circuitry may for example be included in a general-purpose processor or a specialpurpose processor. Examples of a general-purpose processor and a special-purpose processor are a general- purpose central processing unit (CPU) and an application-specific integrated circuit (ASIC), respectively. The control unit 7 can control the operation of the antenna 2 and the transceiver 5 in various ways. For example, the control unit 7 can vary the direction of the pointing axis A by moving the antenna 2. Thus, the antenna 2 is in this case electrically steerable using the control unit 7. Put differently, the main direction of radiation of the antenna 2 is controllable to point in different directions using the control unit 7.

[0063] It is noted that, in a different example, the antenna arrangement 1 may be configured such that the direction of the pointing axis A can be varied without moving the entire antenna 2. The radiating element 4 may be movable, but not the reflector 3, for instance. Further, in an example where the radiating element 4 is an electrically steerable array or a switched array feed, it may be possible to vary the direction of the pointing axis without moving the entire antenna.

[0064] Moving on, the control unit 7 is here configured to determine the respective values of one or more signal characteristics, i.e. characteristics of the signals that are communicated by the antenna 2. Some examples of such characteristics include the power, the phase and the polarization of the signals. Further, the control unit 7 is in this case capable of determining spatial signatures, as will be further discussed below. A spatial signature represents a spatial dependence of a signal characteristic. Depending on the needs of the specific implementation, the control unit 7 may be configured to determine other quantities of interest as well.

[0065] It is noted that the control unit 7 may, as in this example, be constructed as a single unit and located near the antenna 2 high up on the mast 103. The control unit 7 may in a different example, however, be located elsewhere, such as in an electronics cabinet mounted on the mast 103 close to the ground. Further, the control unit 7 may alternatively be constructed as several separate units. In such case, the units may be arranged near each other or located in different places, such as near the top and bottom of the mast 103. Still further, it should be noted that although the control unit 7 and the transceiver 5 are here illustrated as separate units, they may form a single unit in a different example. Stated differently, the control unit 7 may be integrated with the transceiver 5.

[0066] Turning now to Figures 3 and 4, some aspects of the signal transmission and reception properties of the antenna 2 will be discussed. Continued reference will be made to Figures 1 and 2 throughout the discussion. The x and y axes represent spatial coordinates in Figure 3 and in Figure 4, and the z axis in Figure 3 represents power using a decibel scale.

[0067] Figures 3 and 4 graphically illustrates the exemplary antenna pattern 8 according to which the antenna 2 is configured to communicate, i.e. receive and send, signals. The antenna pattern 8 represents both the far-field radiation from the antenna 2 and the reception sensitivity of the antenna 2. Thus, the antenna pattern 8 illustrated in Figures 3 and 4 may be referred to as the radiation pattern as well as the receiving pattern of the antenna 2. The antenna pattern 8 includes a main lobe 9 located at the center. The main lobe 9 is the region of the antenna pattern 8 where the antenna 2 radiates the most power and where the reception sensitivity of the antenna 2 is the highest. The main lobe 9 is surrounded by a sidelobe 10. The sidelobe 10 represents a region of the antenna pattern 8 where the radiated power and the reception sensitivity are weaker than in the region of the main lobe 9. The sidelobe 10 and the main lobe 9 are separated by a null 11 . That is, the null 11 is located between the main lobe 9 and the sidelobe 10. The null 11 is a region associated with low, or very low, radiated power and reception sensitivity. As Figures 3 and 4 show, the antenna pattern 8 here has additional sidelobes which are farther way from the main lobe 9 and which represent regions of weaker radiation and reception sensitivity than the sidelobe 10 that is the one closest to the main lobe 9.

[0068] The antenna pattern 8 illustrated in Figure 3 is circularly symmetric around the pointing axis A. The pointing axis A passes through the center of the main lobe 9. The main lobe 9 here has an elongated shape extending along the pointing axis A. Figure 3 also shows that the sidelobe 10 in this example follows a circular path centered on the pointing axis A. The sidelobe 10 here has the shape of a circular ridge. The null 11 here forms a complete circle centered on the pointing axis A. In Figure 4, the null 11 and the sidelobe 10 are illustrated as concentric circular rings around the main lobe 9 which is illustrated as a circular region at the center.

[0069] It should be noted that Figures 3 and 4 show an idealized, perfectly circularly symmetric antenna pattern 8. In real-world applications, antennas patterns, which are referred to as circularly symmetric, are of course typically only approximately circularly symmetric due to manufacturing imperfections, polarization effects, disturbing objects near the antenna, etc.

[0070] Moving on, the main lobe 9 has a double-curved portion 9a, i.e. a portion that is curved in two directions. The double-curved portion 9a includes a global maximum of the antenna pattern 8 and a region around the global maximum. The global maximum is where the radiation strength and reception sensitivity are maximized. The global maximum is located at the top of the main lobe 9 in Figure 3. It is noted that the main lobe 9 of the exemplary antenna pattern 8 in Figure 3 is double curved everywhere, so the double-curved portion 9a may in this case be seen as the entire surface of the main lobe 9.

[0071] The sidelobe 10 has a locally single-curved portion 10a. The locally single-curved portion 10a includes a local maximum of the antenna pattern 8 and a region around the local maximum. In Figure 3, the local maximum is located at the top of the sidelobe 10. It is worth pointing out that the term single-curved herein means "curved in one direction” and that the sidelobe 10a of the exemplary antenna pattern 8 in Figure 3 is curved both at its top, where the local maximum is located, and in a circle around the main lobe 9. However, since the curvature around the main lobe 9 is relatively large, it may be disregarded when considering sufficiently small regions at the top of the sidelobe 10. Such regions are therefore to a good approximation curved in only one direction. Thus, the sidelobe 10 is here locally approximately single-curved.

[0072] Next, with reference to Figures 5 to 9 and continued reference to Figures 1 to 4, an example of a method for determining whether or not the antenna 2 is misaligned, i.e. incorrectly aligned, with its target will be discussed. The target is in this example the antenna of the second antenna arrangement T. According to this method, antenna misalignment is determined by distinguishing main lobe communication from sidelobe communication.

[0073] The method here comprises establishing S1 a reference axis B, moving S2 the spatial direction of the pointing axis A away from the reference axis B, and obtaining S3 values of a characteristic of a signal 12. In this example the antenna 2 receives the signal from the second antenna arrangement T. The characteristic of the signal 12 will in the following be referred to as the signal characteristic. In this case, the established S1 reference axis B coincides with an initial position of the pointing axis A, i.e. a position of the pointing axis A before it is moved S2. Further, in this example, the pointing axis A is moved S2 and the values of the signal characteristic are obtained S3 during a conical scan around the reference axis B. The control unit 7 may be configured to initiate the conical scan according to a predetermined schedule. Alternatively, or in addition, the control unit 7 may be configured to initiate the conical scan upon receiving a start signal from an operator. The operator can start the conical scan manually whenever a need therefor arises.

[0074] During the conical scan, the pointing axis A is moved around the reference axis B in a circular motion. More specifically, in this case, the control unit 7 controls the electric motors to move the antenna 2 so as to nutate around the reference axis B. The spatial direction of the pointing axis A is thereby moved so that the pointing axis A traces out a circular cone around the reference axis B. The pointing axis A here makes a substantially constant angle 9 with the reference axis B during the conical scan. The angle 9 is about 19% of the half-power beamwidth for the antenna 2, the half-power beamwidth being the angle between the two directions at which the radiated power is half the maximum radiated power. The angle 9 may be greater or smaller in a different example, such as about 5% or about 15%. The choice of angle 9 depends on various application-specific factors, such as the dimensions of the main lobe 9 and the sidelobe 19. These dimensions may be established from measurements or possibly found in the antenna manufacturer's product description. The larger the angle 9, the larger the movement of the pointing axis A. The movement should be sufficiently large for variations in the signal characteristic to be captured with sufficient accuracy. A high-noise signal typically requires the movement to be larger than a low-noise signal, but a large movement often results in a large drop in the signal characteristic during the scan, and thus a greater risk of transmission interruptions, than a small movement. Hence, choosing a suitable angle 9 involves compromise. It is worth noting that, in a different example, the values of the signal characteristic may be obtained while the pointing axis A is moved so as to sweep out a cone with a base that is not circular but elliptical or approximately elliptical.

[0075] In this example, the signal characteristic is the received signal level (RSL), which is a measure of the strength of the signal 12. Other signal characteristics that may alternatively be used include a received signal strength indicator (RSSI), a signal-to-interference-plus-noise ratio (SINR) and a signal error. The RSL and these other signal characteristics are examples of what may be referred to as signal quality measures.

[0076] The RSL is in this case determined continuously by the control unit 7 during the conical scan, but the RSL may be determined in a discontinuous manner in a different example. Thus, the control unit 7 here obtains S3 the signal characteristic by determining the RSL of the signal 12.

[0077] Next, a dependence of the RSL on the direction of the pointing axis A is estimated S4. In more detail, the obtained S3 values of the RSL are in this example used for determining a spatial signature which represents an estimate of how the RSL varies along a circular path around the reference axis B. It may be noted that, in practice, the RSL values usually include measurement errors and the direction of the pointing axis A is not moved in a perfectly circular motion during the conical scan, so the spatial signature typically describes the variation of the RSL approximately.

[0078] Now, there are two different situations to consider, depending on whether the antenna 2 is receiving the signal 12 via the main lobe 9 or via the sidelobe 10.

[0079] First, if the main lobe 9 is receiving the signal 12, moving S2 the direction of the spatial axis A along a circular path around the reference axis B causes the signal reception location, i.e. the location on the radiation pattern 8 where the signal 12 is received, to move along a path on the main lobe 9. The circle 13 on the main lobe 9 in Figure 4 represents such a path, and Figure 8 shows an example of a spatial signature 14 resulting from main lobe reception. In Figure 8, the y axis shows the RSL using a decibel scale, and the x axis covers the range from 0 to 360 degrees. The spatial signature 14 in Figure 8 has a sinusoidal-like shape and illustrates how the RSL approximately varies along a circle in space around the reference axis B.

[0080] Second, if the sidelobe 10 is receiving the signal 12, moving S2 the direction of the pointing axis A along a circular path around the reference axis B causes the signal reception location to move along a path on the sidelobe 10. The circle 15 on the sidelobe 10 in Figure 4 represents such a path, and Figure 9 shows an example of a spatial signature 16 resulting from sidelobe reception. In Figure 9, the y axis shows the RSL using a decibel scale, and the x axis covers the range from 0 to 360 degrees. The spatial signature 16 in Figure 9 has a sinusoidal-like shape and illustrates how the RSL approximately varies along a circle in space around the reference axis B.

[0081] It may be worth pointing out that in practice the signal 12 is not received at a single point in the antenna pattern 8 but rather at a region that has a certain size and that may not have a clear boundary. The signal reception location may be regarded as an approximate center of that region. The discussion herein assumes that the plane wave / straight single ray approximation holds, which is a very common assumption in the field of wireless communications.

[0082] Now, the spatial signature can be used for determining S5, here by the control unit 7, whether the signal 12 is received by the main lobe 9 or by the sidelobe 10. If the determined spatial signature has one maximum 14a and one minimum 14b, as the spatial signature 14 in Figure 8, then the control unit 7 determines that the doublecurved portion 9a of the main lobe 9 is receiving the signal 12. The maximum 14a and the minimum 14b correspond to the RSL at the locations indicated by the reference numerals 13a and 13b, respectively, in Figure 4. If, on the other hand, the determined spatial signature has two maxima 16a, 16b and two minima 16c, 16d, as the spatial signature 16 in Figure 9, then the control unit 7 determines that the locally single-curved portion 10a of the sidelobe 10 is receiving the signal 12. The maxima 16a, 16b correspond to the RSL at the locations indicated by the reference numerals 15a, 15b in Figure 4, and the minima 16c, 16d correspond to the RSL at the locations indicated by the reference numerals 15c, 15d in Figure 4.

[0083] Thus, in sum, communication via the double-curved portion is here indicated by the spatial dependence of the signal characteristic having two extrema along a circular path or, more generally, an elliptical path. Communication via the locally single-curved portion is here indicated by the spatial dependence of the signal characteristic having more than two extrema along a circular path or, more generally, an elliptical, path.

[0084] Next, the focus will be on a situation where it has been determined S5 that the signal 12 is received by the sidelobe 10. It may then be of interest to direct the antenna 2 toward the target, which in this case is the antenna of the second antenna arrangement T, and an exemplary way of doing this will now be described with reference to Figure 10 and with continued reference to Figures 1 to 9. It is thus assumed in the following that the spatial signature was found to be the one in Figure 9.

[0085] The method steps represented by the flowchart in Figure 10 comprises determining S6, based on the spatial signature 16, a straight line 17 that estimates a direction connecting the main lobe 9 and the sidelobe 10. The straight line 17 is in this case determined by the control unit 7. In more detail, the control unit 7 here determines (I) the locations 15c, 15d on the sidelobe 10 that correspond to the two minima 16c, 16d of the spatial signature 16 and (ii) a straight line 17 that passes through these two locations 15c, 15d. The straight line 17 determined in this manner is in this example parallel to a radial direction defined by the sidelobe 10. Of course, in practice, due to for example measurement errors and other inaccuracies when determining the spatial signature 16, the straight line 17 may be only approximately parallel to the radial direction, but it nevertheless typically provide a good estimate of the radial direction.

[0086] It is worth noting that there are other ways of determining the straight line 17. The straight line 17 could for example be determined as the line that is perpendicular to the line passing through the locations 15a, 15b on the sidelobe 10 that correspond to the two maxima 16a, 16b of the spatial signature 16. Further, the determination of the straight line 17 could be based on more information than just the positions of two extrema 16a, 16b, 16c, 16d of the spatial signature 16. For instance, the positions of three or four of the extrema 16a, 16b, 16c, 16d and / or other information extracted from the spatial signature 16 could also be included in the determination of the straight line 17.

[0087] As can be seen in Figure 4, the straight line 17 passes through the main lobe 9. Consequently, it is possible to find the direction in which the main lobe 9 should point by performing a search along the straight line 17, and this will be discussed below in more detail. Before such a search, however, the operation of the transceiver 5 may optionally be configured S7 to compensate for an expected reduction in signal communication quality during the search. By way of explanation, during the search, the signal reception location will move through nulls of the antenna pattern 8, something which is likely to cause the signal reception quality to decrease. The information contained in the spatial signature 16 may be used to estimate what the RSL, the fading margin and / or the signal- to-noise-ratio will be when the signal reception location moves through the nulls. The operation of the transceiver 5 may then be configured S7 accordingly, here by the control unit 7, with the aim of at least partially counteract the expected reduction in signal reception quality so that it probably does not decrease below an acceptable, threshold level. Configuring S7 the operation of the transceiver 5 may for instance include increasing the output power of the antenna 2 and, alternatively or in addition, using a more robust modulation and coding. The configuring S7 may comprise communicating information to the second antenna arrangement T about configuration changes, such as what power, modulation scheme and / or coding scheme to use.

[0088] Now, the search for finding the direction in which the main lobe 9 should point comprises moving S8 the direction of the pointing axis A along the straight line 17 and obtaining S9 values of a further characteristic of the signal 12. The further characteristic of the signal 12 will in the following be referred to as the further signal characteristic. The moving step S8 and the obtaining step S9 are here performed simultaneously, although in a different example these steps could be performed sequentially and repeatedly. In this case, the control unit 7 controls the electric motors to move the antenna 2. The control unit 7 determines, and thereby obtains S9, the values of the further signal characteristic. The further signal characteristic is here the RSL, but may in a different example be an SINR, a signal error or some other suitable characteristic of the signal 12. The further signal characteristic may, but does not have to, be of the same type as the above-discussed signal characteristic.

[0089] It should be noted that it is usually not known beforehand in which direction the main lobe 9 lies, so the search is typically conducted in both directions along the straight line 17. How far in either direction the search should be conducted varies depending on application-specific factors, such as the respective sizes of the main lobe 9, the sidelobe 10 and the null 11 . Such properties of the antenna pattern 8 can be determined by measurements, and they may also be listed in the antenna manufacturer's product documentation. It should also be noted that the presently described line search procedure for finding the desired main lobe direction works not only if the sidelobe closest to the main lobe is receiving the signal, as in the illustrated example, but also if the signal is received by a sidelobe farther away.

[0090] Moving on, the obtained S9 values of the further signal characteristic may be used for estimating S10 the direction toward the second antenna arrangement T. More specifically, in this case, the control unit 7 determines the position along the straight line 17 where the RSL is maximized or approximately maximized. Of course, due to measurement uncertainties, numerical errors and other factors, the determined position is usually not exactly where the RSL is maximized but only an estimate thereof. The determined position corresponds to the direction of incidence of the signal 12. It also corresponds to the direction toward the second antenna arrangement T, assuming the signal 12 has not undergone reflections on its way to the antenna 2. The control unit 7 then moves the pointing axis A to this position. Thereby, the pointing axis A is aligned S11 with the estimated direction toward the second antenna arrangement T. The main lobe 9 is now directed toward, or at least approximately toward, the second antenna arrangement T. It is noted that if the further signal characteristic had been a signal error instead of the RSL, the estimated direction of incidence would have been estimated based on where along the straight line 17 the signal error is minimized, or at least approximately minimized.

[0091] Turning to Figures 11, 12 and 13, an example of a method for determining a spatial signature without performing a conical scan will be discussed. The spatial signature is, according to this method, determined based on five values of the signal characteristic. The five values are obtained by moving the pointing axis A to point in five distinct spatial directions around the reference axis B and determining the signal characteristic for each of these directions. The pointing axis A may for example be moved up, down, left, right and in one additional direction relative to the reference axis B. The five directions may, but does not have to, be at approximately equal angles with the reference axis B. In Figure 11, the filled squares 18 correspond to the directions in which the signal characteristic is determined if the signal is received by the main lobe 9, and the filled triangles 19 correspond to the directions in which the signal characteristic is determined if the signal is received by the sidelobe 10.

[0092] After the five values have been obtained, the spatial signature is determined based thereon. As previously explained, the spatial signature represents an estimate of the dependence of the signal characteristic, which in this case is the RSL, on the direction of the pointing axis A. The spatial signature may be determined using, for instance, an interpolation technique or a regression technique, such as sinusoidal regression. Figures 12 and 13 depict two examples of spatial signatures 20, 21 which have been determined by fitting a sinusoidal regression model to five RSL values. The RSL is measured on the y axis using a decibel scale, and the x axis ranges from 0 to 360 degrees. The spatial signatures 20, 21 illustrate how the RSL approximately varies along respective circles in space around the reference axis B. The spatial signature 20 corresponding to main lobe reception is shown in Figure 12. This exemplary spatial signature 20 has two extrema, namely one maximum 20a and one minimum 20b. The spatial signature 21 corresponding to sidelobe reception is shown in Figure 13. This exemplary spatial signature 21 has four extrema, namely two maxima 21a, 21b and two minima 21c, 21 d.

[0093] It should be noted that the signal characteristic may of course be measured in more than five directions and that the pointing axis A may be moved in other ways with respect to the reference axis B than those mentioned above (up, down, etc.). Choosing directions that are fairly evenly distributed around the reference axis B may help to increase the accuracy with which the spatial signature estimates the dependence of the signal characteristic on the direction of the pointing axis A, but this is not necessary. The directions should allow a person skilled in the art to determine, at least approximately, how the signal characteristic varies along an elliptical path around the reference axis B. Further, the spatial signatures in Figures 12 and 13 represent the dependence of the signal characteristic along paths forming complete circles. However, the spatial signature may represent the dependence along only a portion of an ellipse or a circle, as long as the portion is long enough for it to be possible to reliably conclude how many extrema the spatial signature has over a complete ellipse or circle.

[0094] Next, the discussion turns to Figure 14. This figure shows an example of a point-to-point radio link 200 that is similar to the point-to-point link radio 100 in Figure 1 except for a few differences, some of which will be described below.

[0095] The point-to-point radio link 200 has a first antenna arrangement 1 and a second antenna arrangement T that are configured to communicate bidirectionally with each other. In order to determine whether it is correctly aligned with the direction toward the second antenna arrangement T, the first antenna arrangement 1 performs method steps similar to those discussed above with reference to Figure 5, a difference being that it obtains the values of the signal characteristic from the second antenna arrangement T. In further detail, the first antenna arrangement 1 moves its pointing axis and sends a first signal 22 to the second antenna arrangement T. The second antenna arrangement T receives the first signal 22 and, based thereon, determines the values of the signal characteristic. The second antenna arrangement T sends a second signal 23, which comprises information indicative of the determined values of the signal characteristic, to the first antenna arrangement 1 . The first antenna arrangement 1 receives the second signal 23 and, based on the information therein, determines the spatial signature.

[0096] If the spatial signature indicates antenna misalignment, the first antenna arrangement 1 may perform method steps similar to those discussed above with reference to Figure 10, a difference being that it obtains the values of the further signal characteristic from the second antenna arrangement T. More specifically, the first antenna arrangement 1 moves the direction of its pointing axis along the straight line and transmits a signal to the second antenna arrangement T. The second antenna arrangement T receives this signal and, based thereon, determines several values of the further signal characteristic. A third signal 24, which includes information indicative of the determined values of the further signal characteristic, is sent by the second antenna arrangement T to the first antenna arrangement 1. The first antenna arrangement 1 uses the information to estimate the correct direction toward second antenna arrangement T.

[0097] It is noted that, in a different example, the second antenna arrangement T may be configured to transmit the values of the signal characteristic and / or the values of the further signal characteristic to the first antenna arrangement 1, not via the point-to-point link 200, but via a different link or connection, such as via an operator of the point-to-point link radio 200.

[0098] Further, the values of the signal characteristic and / or the values of the further signal characteristic do in a different example not necessarily have to be determined by the second antenna arrangement T. Instead, the values may be determined by a remote device 25 arranged at a location away from the second antenna arrangement T, such as by a computer on the premises of the operator of the point-to-point link radio 200 or by a cloud-based computer. In such case, the second antenna arrangement T sends the necessary information to the remote device 25 which determines the values based on this information. The remote device 25 then transmits information indicative of the values to the first antenna arrangement 1, possibly via the second antenna arrangement T.

[0099] Still further, in a different example, the spatial signature may be determined, not by the first antenna arrangement 1, but by the second antenna arrangement T or the remote device 25, which then send, to the first antenna arrangement 1, information indicating whether there is misalignment and, if so, the direction in which the first antenna arrangement 1 should point.

[0100] Next, with reference to Figure 15, an example of a computer program product 300 will be discussed. The computer program product 300 comprises a computer-readable medium 301. The computer-readable medium 301 may for example comprise one or more of: a volatile memory, a non-volatile memory, a random-access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), a Universal Serial Bus (USB) memory, a flash memory, a solid-state drive, a hard disk drive, an optical disc, and cloud storage. The computer program product 300 further comprises a computer program 302 which is stored on the computer-readable medium 301 and which is executable by one or more processors, such as a general-purpose processor or a special-purpose processor. The computer program 302 is designed to cause, when executed, the one or more processors and thereto operatively connected devices to carry out methods described herein. As an example, the computer program product 300 may be a memory included in the control unit 7, and the computer program 302 may be designed to cause the method steps described in connection with Figure 5 and / or the method steps described in connection with Figure 10 to be performed.

[0101] The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the antenna arrangement 1 may comprise a radome which helps to protect the reflector 3 from debris and precipitation, such as rain and snow. As another example, the inventive methods presented herein can be used for determining whether an antenna, which is configured to communicate with a target in the form of a satellite, is directed toward the satellite and for re-directing the antenna toward the satellite if necessary. Such a satellite communication antenna may be mounted at a fixed, ground-based location or on a moving vehicle, such as a ship, an aircraft, a truck or some other land-based vehicle.

[0102] In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word "comprising” does not exclude the presence of other elements or steps than those listed in the claim. The word "a” or "an” preceding an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS1 . A method for determining whether a steerable directional antenna (2) is misaligned with a target (1'), wherein the antenna (2) is configured to communicate signals according to an antenna pattern (8) comprising a main lobe (9) and a sidelobe (10), the main lobe (9) defining a pointing axis (A) of the antenna (2) and having a double-curved portion (9a) comprising a global maximum of the antenna pattern (8), the sidelobe (10) having a locally single-curved portion (10a) comprising a local maximum of the antenna pattern (8), wherein the method comprises: establishing (S1) a reference axis (B); moving (S2) a direction of the pointing axis (A) to be in at least five different directions away from of the reference axis (B); obtaining (S3) values of a signal characteristic, wherein the values of the signal characteristic are associated with the pointing axis (A) being oriented in the different directions; estimating (S4), based on the values of the signal characteristic, a dependence of the signal characteristic on the direction of the pointing axis (A); and determining (S5), based on said dependence, whether the antenna (2) is misaligned with the target (1’), wherein said dependence is estimated along an elliptical path enclosing the reference axis (B), and wherein communication via the locally single-curved portion (10a) is indicated by said dependence having more than two extrema (16a, 16b, 16c, 16d; 21a, 21b, 21c, 21d) along the elliptical path.

2. The method according to claim 1, wherein the method comprises performing a conical scan around the reference axis (B), the conical scan comprising the moving (S2) of the direction of the pointing axis (A) and the obtaining (S3) of the values of the signal characteristic.

3. The method according to claim 1 or 2, wherein each of the different directions forms an angle (9) with the reference axis (B), and wherein the angles are equal to or less than 20% of a half-power beamwidth of the antenna (2).

4. The method according to any of the preceding claims, wherein the antenna pattern (8) is circularly symmetric.

5. The method according to any of the preceding claims, wherein the signal characteristic is one of: a signal strength, a signal error and a signal-to-interference-plus-noise ratio.

6. The method according to any of the preceding claims, wherein the signal characteristic is a characteristic of a signal (12) sent by the target (T) to the antenna (2).

7. The method according to any of claims 1 to 5, wherein the signal characteristic is a characteristic of a signal (22) sent by the antenna (2) to the target (1'), and wherein the obtaining (S3) of the values of the signal characteristic comprises receiving, by the antenna (2), a signal (23) indicating the values of the signal characteristic.

8. The method according to any of claims 1 to 7, wherein the method, if the antenna (2) is determined to be misaligned with the target (T), further comprises: determining (S6), based on respective positions of two extrema (16a, 16b, 16c, 16d; 21a, 21b, 21c, 21 d) of the signal characteristic, a straight line (17), wherein a direction of the straight line (17) estimates a direction connecting the main lobe (9) and the sidelobe (10); moving (S8) the direction of the pointing axis (A) along the straight line (17); obtaining (S9) values of a further signal characteristic, wherein the values of the further signal characteristic are associated with the direction of the pointing axis (A) being at different positions along the straight line (17); estimating (S10), based on the values of the further signal characteristic, a direction toward the target (T); and aligning (S11) the direction of the pointing axis (A) with the direction toward the target (T).

9. The method according to claim 8, wherein the direction connecting the main lobe (9) and the sidelobe (10) is a radial direction defined by the sidelobe (10).

10. The method according to claim 8 or 9, wherein the direction toward the target (T) is estimated based on a position, along the straight line (17), where the further signal characteristic has an extremum.11 . The method according to any one of claims 8 to 10, wherein the further signal characteristic is one of: a signal strength, a signal error and a signal-to-interference-plus-noise ratio.

12. The method according to any one of claims 8 to 11, wherein the further signal characteristic is a characteristic of a signal (12) sent by the target (T) to the antenna (2).

13. The method according to any one of claims 8 to 11, wherein the further signal characteristic is a characteristic of a signal (22) sent by the antenna (2) to the target (T), and wherein the obtaining (S9) of the values of the further signal characteristic comprises receiving, by the antenna (2), a signal (24) indicating the values of the further signal characteristic.

14. The method according to any one of claims 8 to 13, wherein the antenna (2) is connected to a transceiver (5) configured to communicate signals using the antenna (2), and wherein the method comprisesconfiguring (S7), based on said dependence, an operation of the transceiver (5) in order to compensate for an expected variation of the further signal characteristic during the moving (S8) of the direction of the pointing axis (A) along the straight line (17).

15. The method according to claim 14, wherein the method comprises estimating, based on said dependence, a fading margin and / or a signal-to-noise ratio, and wherein the configuring (S7) is based on the estimated fading margin and / or the estimated signal-to-noise ratio.

16. The method according to claim 14 or 15, wherein the configuring (S7) comprises at least one of: adjusting an output power of the antenna (2), adjusting a signal modulation scheme, and adjusting a signal coding scheme.

17. An antenna arrangement (1) comprising: a steerable directional antenna (2) configured to communicate signals according to an antenna pattern (8) comprising a main lobe (9) and a sidelobe (10), the main lobe (9) defining a pointing axis (A) of the antenna (2) and having a double-curved portion (9a) comprising a global maximum of the antenna pattern (8), the sidelobe (10) having a locally single-curved portion (10a) comprising a local maximum of the antenna pattern (8); a transceiver (5) configured to communicate signals using the antenna (2); and a control unit (7) operably connected to the antenna (2) and to the transceiver (5), the control unit (7) being configured to perform the method according to any one of claims 1 to 16.

18. A point-to-point radio link (100) comprising: a first antenna arrangement (1) according to claim 17, wherein the first antenna arrangement (1) is configured to be arranged at a first endpoint (101) of the point-to-point radio link (100); and a second antenna arrangement (T) configured to be arranged at a second endpoint (102) of the point-to-point radio link (100), wherein the first antenna arrangement (1) and the second antenna arrangement (T) are configured to communicate with each other using radio signals.

19. A computer program (302) comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to any one of claims 1 to 16.

20. A computer-readable medium (301) comprising instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to any one of claims 1 to 16.21 . A computer program product (300) comprising a computer program (302) and a computer- readable medium (301) on which the computer program (302) is stored, wherein the computer program (302)16comprises instructions which, when executed on at least one processor, cause the at least one processor to carry out the method according to any one of claims 1 to 16.17

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