WO2015036020A1

WO2015036020A1 - Antenna device and method for controlling focusing of an antenna beam of an antenna array

Info

Publication number: WO2015036020A1
Application number: PCT/EP2013/068829
Authority: WO
Inventors: Karel Sotek; Songhai Wang
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2013-09-11
Filing date: 2013-09-11
Publication date: 2015-03-19
Also published as: CN105519009A; CN105519009B

Abstract

The invention relates to an antenna device (600), comprising: a controller (602) being configured to control a focusing (605) of an antenna beam of an antenna array towards a user equipment, the focusing (605) being based on a feedback loop (601) with respect to a pilot beam, the pilot beam comprising a set of predetermined antenna beams of the antenna array, and being based on a distance (603) between the antenna array and the user equipment.

Description

Antenna device and method for controlling focusing of an antenna beam of an antenna array

TECHNICAL FIELD

The present disclosure relates to an antenna device and a method for controlling a focusing of an antenna beam of an antenna array and to a radio base transceiver station, BTS. The disclosure further relates to BTS antenna beam forming in a cellular network. In particular, the present disclosure relates to increased precision of antenna beam forming.

BACKGROUND

Broad acceptance and extensive usage of mobile broadband services put enormous pressure on available cellular network radio resources. Expensive auctioned spectrum must be used efficiently. BTS antenna beam forming is an advanced radio interface technology allowing for efficient usage of radio spectrum. Precise steering the antenna beam direction towards the mobile user equipment, UE, allows for dense reuse of radio frequencies in a cellular network increasing the capacity and reducing the cost of mobile broadband services. Contrary to legacy cellular network where the allocated spectrum is equally used within one cell of the network, antenna beam forming focuses the spectrum portion used by a user towards this particular user. This reduces the interference with other cell users and allows for denser reuse of the spectrum. Legacy parameters for beam steering include angle-of-arrival estimation and Pre-coding Matrix Index, PMI, reporting in Long Term Evolution, LTE, closed-loop Multiple-Input Multiple-Output, MIMO, operation. A general problem in a cellular network is multipath propagation between BTS and UE.

Radio waves from UE are reflected on buildings and arrive at BTS from different directions in the uplink case. The same is valid for the opposite direction from BTS to UE in the downlink case. Each reflection is highly frequency selective and thus the conditions for reflections in uplink and downlink are different in Frequency Division Duplex, FDD, systems, where uplink and downlink use different frequencies. This is the reason why downlink beam steering by estimating the uplink angle-of-arrival estimation is delivering sub-optimum performance.

Figure 1 shows the principle of different antenna beams determined by different PMI values. This figure illustrates the closed loop PMI reporting. The BTS 101 generates a first beam 102, a second beam 104 and a third beam 106 directed to a UE 103 within a radio cell 108. The PMI can be implemented according to a 3GPP TS standard. In this specific example, the PMI is implemented according to 3GPP TS 36.331 standard. Different antenna downtilt angles a, of the three beams 102, 104, 106 correspond to different PMI, values. By receiving periodic UE reports on strongest PMI, value, PMI2 in this example, the BTS knows the optimum a, downtilt value, a2 in this example according to the second beam 104, to be used for payload data transmission. The UE 103 is reporting the strongest antenna main-lobe direction 104 via so-called PMI reporting. The beam selection is based on the UE feedback loop.

Thus, the basic algorithm for estimating the antenna downtilt value runs as follows: The BTS periodically transmits a combination of all available antenna downtilt values where each of these beams is coded by its dedicated PMI code (value). These PMI codes are decoded in the UE and the strongest PMI is reported back to BTS. The BTS uses the downtilt value corresponding to the reported PMI, for example a2, when PMI2 is reported.

Imperfections in antenna beam form as beam side lobes cause occasionally erroneous PMI reports. The UE is served erroneously by a side lobe while the main lobe generates strong interference with other UEs in the cellular network. More specifically, antenna side lobes do radiate unwanted signals which can be erroneously detected by UE, leading to erroneous PMI report and a selection.

Figure 2 shows an example of erroneous PMI4 reporting. The BTS 201 generates a main lobe 204 and two side lobes 202, 206 directed to a UE 203 within a radio cell 208. The PMI value with respect to the main lobe 204 is reported although the side lobe beam 206 is more precisely directed to the UE 203. That is, antenna beam angles, which are better covered by side lobes of different antenna main lobes, are erroneously reported. The antenna main lobe 204 with PMI4 and downtilt angle a4, according to the representation of Fig. 1 , has two side lobes 202, 206. The lower side lobe 206 is radiating into a direction where there is no stronger antenna main lobe with its different PMI value. The proper beam downtilt should be a2 coded with PMI2. Detecting the PMI4 leads the BTS 201 to transmit with a4 instead of using a2. As a result, the BTS 201 transmits into a different direction than towards the UE 203. The legacy implementation of antenna beam steering is based upon one to one mapping of UE PMI reports received by BTS to antenna beam angle a. Disadvantage of this method is the inaccuracy of selected beam angle caused by antenna side lobes, channel noise and measurement inaccuracy.

SUMMARY

It is the object of the invention to provide a technique for enhancing the steering precision of an antenna beam of an antenna array.

This object is achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

The invention is based on the finding that a technique for enhancing the steering precision of an antenna beam of an antenna array can be achieved by determining the distance between the BTS and UE by measuring the propagation delay and its statistics.

In Cellular Base Stations including antenna array allowing for beam direction steering, the beam direction is determined by UE PMI reporting. Each reported PMI value corresponds to a beam angle to be used for next transmission. To increase the precision of the beam angle, BTS may perform an additional measurement of propagation time between BTS and UE. This time can be translated to BTS-UE distance. A simple trigonometric function may be used to compare the a value, i.e. the downtilt angle, and propagation time t. The BTS may trace the estimated t and a values for consistency with stationary or

continuously moving UE conditions. Non-conform values may be discarded, other values may be input values for algorithm processing and beam control.

The invention includes the processing of other parameters being available in the mobile network as timing difference between UE and BTS. This timing difference is also called roundtrip time, and is estimated as a time the radio signal needs to propagate from BTS to UE and back. The roundtrip time corresponds to the distance between the BTS and the UE. As this distance can only smoothly change (no discontinuous hops for example), monitoring this distance is efficient for guiding and plausibility checking of beam selection. The BTS parameter equivalent to roundtrip time is called Timing Advance. The Timing Advance parameter estimated by BTS in combination with PMI reports may be used in order to select and trace the BTS beam angle direction towards a UE.

By that, the precision of BTS antenna beam forming in a cellular network can be increased.

In order to describe the invention in detail, the following terms, abbreviations and notations will be used:

BTS: base transceiver station

UE: user equipment

LTE: long term evolution

PMI: pre-coding matrix index

Tx: transmission

Rx: reception

According to a first aspect, the invention relates to an antenna device, comprising: a controller being configured to control a focusing of an antenna beam of an antenna array towards a user equipment, the focusing being based on a feedback loop with respect to a pilot beam, the pilot beam comprising a set of predetermined antenna beams of the antenna array, and being based on a distance between the antenna array and the user equipment.

When the focusing of the antenna beam is additionally based on a second parameter such as a distance between the antenna array and the user equipment, the steering precision of the antenna beam can be enhanced.

In a first implementation form of the antenna device according to the first aspect, the controller is configured to determine the distance based on a propagation time measurement.

A propagation time measurement can be easily implemented, e.g. by time stamp measurement. In a second implementation form of the antenna device according to the first

implementation form of the first aspect, the propagation time measurement is based on a round-trip delay measurement between the antenna array and the user equipment. The round-trip delay can be easily measured, e.g. by adding a time stamp to a signal transmitted from BTS to UE and back.

In a third implementation form of the device according to the first implementation form or according to the second implementation form of the first aspect, the propagation time measurement is based on a known processing time inside the user equipment.

A propagation time measurement based on a known processing time inside the user equipment is precise. In a fourth implementation form of the antenna device according to any of the first to the third implementation forms of the first aspect, the propagation time measurement is based on one of a timing advance measurement and a time stamp measurement, in signaling or payload traffic. Timing advance and time stamp measurements can be efficiently implemented with respect to computational complexity. Signaling or payload traffic can be used for adding a time base.

In a fifth implementation form of the antenna device according to any of the second to the fourth implementation forms of the first aspect, the controller is configured to determine a beam angle of the antenna beam based on reports of the user equipment.

The beam angle can be efficiently and accurately determined by using reports of the user equipment. By additionally using the propagation time measurement, accuracy can be further increased. The reports of the user equipment can be efficiently implemented reporting both, beam angle and propagation time.

In a sixth implementation form of the antenna device according to the fifth implementation form of the first aspect, the controller is configured to control the focusing of the antenna beam by combining the reported beam angle and the measured propagation time, by using trigonometric functions with respect to the reported beam angle.

By combining the reported beam angle and the measured propagation time, accuracy of the beam steering can be increased. The trigonometric functions with respect to the reported beam angle allow to derive a relation between beam angle and propagation time, e.g. such as tangent of the beam angle corresponds to the relation of BTS height and distance between BTS and UE. In a seventh implementation form of the antenna device according to the fifth

implementation form or according to the sixth implementation form of the first aspect, the controller is configured to control the focusing based on statistics and/or changes of the reported beam angle and the measured propagation time. When using statistics and/or changes of the reported beam angle and the measured propagation time, the accuracy of the beam controlling can be increased, in particular for the cases where a height of the BTS is not known.

In an eighth implementation form of the antenna device according to the sixth

implementation form or according to the seventh implementation form of the first aspect, the controller is configured to detect failures of the feedback loop by tracing the reported beam angle and the measured propagation time.

Tracing the reported beam angle and the measured propagation time can show unsteady characteristics in the beam angles which is an indication for failures. Tracing can be easily performed, e.g. by using a memory.

In a ninth implementation form of the antenna device according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the controller is configured to code the set of predetermined antenna beams by using pre- coding matrices.

Coding the set of predetermined antenna beams by using pre-coding matrices can allow to easily steer the antenna beams and to apply PMI reporting on the UE side. In a tenth implementation form of the antenna device according to the first aspect as such or according to any of the preceding implementation forms of the first aspect, the feedback loop is based on pre-coding matrix index, PMI, reporting, as defined according to a 3GPP TS standard, in particular as defined according to 3GPP TS 36.331 standard.

When using a feedback loop that is based on PMI reporting, the device can be implemented in any standard BTS, e.g. in a BTS according to the 3GPP TS 36.331 standard. According to a second aspect, the invention relates to a radio base transceiver station, comprising: an antenna array being capable of vertical beam forming; a set of radio transceivers being configured to drive the antenna array with transceiver signals; a baseband module being configured to process radio baseband signals based on the transceiver signals, wherein the baseband module comprises the antenna device according to the first aspect or the antenna device according to any of the first to the tenth implementation forms of the first aspect, and being configured to control focusing of an antenna beam of the antenna array towards a user equipment.

A radio base transceiver station additionally controlling the focusing of its antenna beam based on a second parameter such as a distance between the antenna array of the radio base station and the user equipment can allow to enhance the steering precision of its antenna beam.

According to a third aspect, the invention relates to a method for controlling focusing of an antenna beam of an antenna array towards a user equipment, the method comprising: controlling the focusing of the antenna beam based on a feedback loop with respect to a pilot beam, the pilot beam comprising a set of predetermined antenna beams of the antenna array, and being based on a distance between the antenna array and the user equipment.

When the focusing of the antenna beam is additionally based on a second parameter such as a distance between the antenna array and the user equipment, the steering precision of the antenna beam can be enhanced. In a first implementation form of the method according to the third aspect, the method comprises: determining a beam angle of the antenna beam based on PMI reporting; determining the distance based on a propagation time measurement between the antenna array and the user equipment; and adjusting the focusing of the antenna beam based on a difference between the reported beam angle and a beam angle corresponding to the measured propagation time.

By adjusting the focusing of the antenna beam based on a difference between the reported beam angle and a beam angle corresponding to the measured propagation time, the precision of the focusing can be improved.

In a second implementation form of the method according to the first implementation form of the third aspect, the method comprises: evaluating statistics and/or changes of the reported beam angle and the measured propagation time in order to obtain the difference.

By evaluating statistics and/or changes of the reported beam angle and the measured propagation time, the focusing can be performed without knowledge of the exact dimensions of the BTS.

BRIEF DESCRIPTION OF DRAWINGS

Further embodiments of the invention will be described with respect to the following figures, in which:

FIG. 1 shows a schematic beam diagram 100 of a conventional base station 101 for illustrating the principle of different antenna beams determined by different PMI values;

FIG. 2 shows a schematic beam diagram 200 of a conventional base station 201 for illustrating the situation of erroneous PMI reporting by UE erroneously detecting antenna side lobe;

FIG. 3 shows a schematic beam diagram 300 of a BTS 301 controlling a focusing of an antenna beam of an antenna array according to an embodiment of the invention; FIG. 4 shows a block diagram of a radio base station 400 according to an embodiment of the invention;

FIG. 5 shows a schematic diagram 500 illustrating the propagation time estimation between a BTS 501 and a UE 503, according to an embodiment of the invention;

FIG. 6 shows a block diagram of a device 600 for controlling a focusing of an antenna beam of an antenna array, according to an embodiment of the invention; and FIG. 7 shows a block diagram of a method 700 for controlling a focusing of an antenna beam of an antenna array, according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS In the following, PMI reporting is described. PMI reporting is a LTE closed-loop control mechanism where UE reports to BTS the best downlink propagation path. For vertical beam forming application, the BTS repeatedly transmits a pilot beam including the mixture of all available downtilt beams, each coded by different PMI. Using UE reports of best PMI, the BTS selects most suitable downtilt beam for payload data transmission. PMI reporting is described for example in the standard 3GPP TS 36.21 1 . Depending on a codebook index and a number of spatial layers, table 6.3.4.2.3.-1 specifies different exemplary pre-coding parameter sets for spatial multiplexing. By using that pre-coding parameter sets, the antenna array may produce beam-steering. PMI reporting can be used for reporting the effects of the used parameter sets at the UE and the UE can report a quality indicator with respect to the PMI parameter set to BTS.

Figure 3 shows a schematic beam diagram 300 of a BTS 301 controlling a focusing of an antenna beam 305 of an antenna array, according to an embodiment. The cellular BTS 301 includes an antenna array allowing for beam direction steering. The beam direction is determined by UE PMI reporting. Each reported PMI value corresponds to a beam angle to be used for next transmission. To increase the precision of the beam angle 302, BTS 301 performs an additional measurement of propagation time 310 between BTS 301 and UE 303. This time is translated to BTS-UE distance. A simple trigonometric function may be used to compare the a value 302 and propagation time t 310. The BTS 301 traces the estimated t and a values 310, 302 for consistency with stationary or continuously moving UE conditions. Non-conform values are discarded. Other values are input values for algorithm processing and beam control.

The above-mentioned example of failure in beam steering as described in Fig. 2 can be avoided by processing other known parameters specific to the particular UE. Such parameters are for example propagation time measurement between UE 303 and BTS 301 , and the propagation losses between BTS 301 and UE 303. As the propagation losses do rapidly change due to obstacles such as walls and buildings, the propagation time 310 measurement is the most suitable method to measure and continuously observe the distance between the BTS 301 and the UE 303. As the UE 303 is either stationary or continuously moving within a cell 308, the angle a 302 and the propagation time t 310 are also continuously changing. In the above examples illustrated with respect to Figs. 1 and 2, the UE 303 moving towards the BTS 301 follows the sequence PMI 1 -2-3 while an erroneous sequence would be PMI 1 -4-3. This erroneous sequence can be discovered by tracing the t, which is continuously decreasing as the UE 303 is moving towards the BTS 301 . Tracing and post-processing, for example averaging, the estimated t thus increases the precision of used a value 302.

The BTS antenna beam forming is an advanced radio interface technology allowing for efficient usage of radio spectrum. Precise steering the antenna beam direction towards the mobile UE 303 allows for dense reuse of radio frequencies in a cellular network, increasing the capacity and reducing the cost of mobile broadband services. Steering precision of down-tilt angle a 302 is enhanced by determining the distance between the BTS 301 and UE 303 by measuring the propagation delay t, 310, and its statistics.

Figure 3 also shows the basic principle of vertical antenna beam forming. This figure shows the relationship between antenna beam angle a 302 and BTS-UE distance determined by a propagation time t, 310, estimation. The BTS 301 can focus its antenna beam towards the served UE 303. This is represented by the vertical downtilt a 302. Using this downtilt, the BTS antenna beam is precisely pointing towards the UE 303. A second parameter can be used for supervising of the antenna downtilt and to improve its precision. This parameter may be the distance between UE 303 and BTS 301 , which correspond to propagation time t, 310, estimated by the BTS 301. Figure 4 shows a block diagram of a radio base station 400 according to an embodiment. The radio base transceiver station 400 includes an antenna array 401 being capable of vertical beam forming, a set of radio transceivers 403 being configured to drive the antenna array 401 with transceiver signals 402, and a baseband module 407 being configured to process radio baseband signals 406 based on the transceiver signals 402. The baseband module 407 includes an antenna device 600 as described below with respect to Fig. 6, being configured to control focusing of an antenna beam of the antenna array 401 towards a user equipment. The baseband module 407 provides the baseband signals IQ 406, which may be weighted or multiplied by respective coefficients K, 408 in a weighting unit 405, to the radio transceivers 403. Figure 4 shows the principle of PMI encoding in BTS. The digital baseband module 407 is encoding the user data 410 to IQ signals 406 for transmission. The IQ signals 406 are passing a multiplication matrix X which is multiplying these signals 406 with a coefficient set K, 408 (including individual amplitude, phase and delay) generating weighted IQ signals 404. Each PMI corresponds to a set of K, coefficients. After multiplication, the so generated weighted IQ signals 404 are modulated for air interface transmission via antenna set 401 towards the UE.

Figure 5 shows a schematic diagram 500 illustrating the propagation time estimation between a BTS 501 and a UE 503, according to an embodiment. The BTS 501 transmits a reference time stamp, BTS Tx, which is received after propagation time t, 510, as UE Rx. The UE processes internally the time stamp signal and retransmit it as UE Tx. After propagation time delay t, 514, the signal arrives back as BTS Rx. Knowing the UE processing delay 512, the BTS 501 can calculate the propagation time t, 510, 514. The BTS 501 can convert the propagation time t, 510, 514, into BTS-UE site distance. A trigonometric function can be used to compare both a and BTS-UE distance. As the BTS antenna height and landscape profile between BTS 501 and UE 503 are not known, it is in praxis sufficient to trace and compare statistics and changes of both a and t values.

Figure 6 shows a block diagram of an antenna device 600 for controlling a focusing of an antenna beam of an antenna array, according to an embodiment. The antenna device 600 may be implemented in the baseband module 407 of the BTS 400, as described above with respect to figure 4. The antenna device 600 may be implemented in a BTS 301 , as described above with respect to figure 3. The antenna device 600 includes a controller 602 being configured to control a focusing 605 of an antenna beam of an antenna array towards a user equipment, the focusing 605 being based on a feedback loop 601 with respect to a pilot beam, the pilot beam comprising a set of predetermined antenna beams of the antenna array, and being based on a distance 603 between the antenna array and the user equipment.

According to an embodiment, the controller 602 can be configured to determine the distance 603 based on a propagation time measurement. According to an embodiment, the propagation time measurement can be based on a round-trip delay measurement between the antenna array and the user equipment. According to an embodiment, the propagation time measurement can be based on a known processing time inside the user equipment. According to an embodiment, the propagation time measurement can be based on one of a timing advance measurement and a time stamp measurement, in signaling or payload traffic. According to an embodiment, the controller 602 can be configured to determine a beam angle of the antenna beam based on reports of the user equipment. According to an embodiment, the controller 602 can be configured to control the focusing 605 of the antenna beam by combining the reported beam angle and the measured propagation time, by using trigonometric functions with respect to the reported beam angle. According to an embodiment, the controller 602 can be configured to control the focusing 605 based on statistics and/or changes of the reported beam angle and the measured propagation time. According to an embodiment, the controller 602 can be configured to detect failures of the feedback loop 601 by tracing the reported beam angle and the measured propagation time. According to an embodiment, the controller 602 can be configured to code the set of predetermined antenna beams by using pre-coding matrices. According to an embodiment, the feedback loop 601 can be based on pre- coding matrix index reporting, as defined according to a 3GPP TS standard, in particular as defined according to 3GPP TS 36.331 standard.

Figure 7 shows a block diagram of a method 700 for controlling a focusing of an antenna beam of an antenna array, according to an embodiment. The method 700 includes:

controlling 702 the focusing 705 of an antenna beam based on a feedback loop 701 with respect to a pilot beam, the pilot beam comprising a set of predetermined antenna beams of the antenna array, and being based on a distance 703 between the antenna array and the user equipment. According to an embodiment, the method 700 comprises: determining a beam angle of the antenna beam based on PMI reporting, determining the distance 703 based on a propagation time measurement between the antenna array and the user equipment, and adjusting the focusing 705 of the antenna beam based on a difference between the reported beam angle and a beam angle corresponding to the measured propagation time. According to an embodiment, the method 700 comprises: evaluating statistics and/or changes of the reported beam angle and the measured propagation time in order to obtain the difference. The method 700 may be processed in the antenna device 600 as described above with respect to Fig. 6. The method 700 may be processed in a radio base transceiver station 400 as described above with respect to Fig. 4. The method 700 may be processed in a BTS 301 as described above with respect to Fig. 3. The invention provides a radio transceiver base station as shown in Fig. 4 above, comprising an antenna array being capable of vertical beam forming, a set of radio transceivers driving the antenna array, and a baseband module processing the radio baseband signals. The baseband module may implement the PMI algorithm in both downlink and uplink according to 3GPP TS36.331 standard. This is also illustrated in the example above with respect to Fig. 4. A set of PMI values can be periodically transmitted in a so called pilot beam. The UE can determine the best PMI value and report it back to BTS. The BTS can perform propagation time t measurement between BTS and UE as illustrated in Fig. 5 above. The measured time can be optionally post-processed to enhance the precision of its value. This processing can for example use averaging, weighting or other functions suppressing errors due to noise or value sampling

inaccuracy. The BTS can combine the reported PMI values with its calculated propagation time t. Simple trigonometric functions or value statistics can be used to compare the PMI reported downtilt value a with the BTS-UE distance represented by measured propagation time t. The BTS can optionally trace calculated t and a values. As the UE is either stationary or continuously moving within the BTS area, the traced history and each new value of t and a should correspond to the stationary or continuously moving UE. Each new sample not matching this criterion is either discarded or its weighting factor is reduced. The propagation time measurement by BTS can be implemented as Timing Advance measurement implemented in an LTE system. Other implementations are also possible as dedicated time stamps in signaling or payload traffic. From the foregoing, it will be apparent to those skilled in the art that a variety of methods, systems, computer programs on recording media, and the like, are provided. The present disclosure can also support a computer program product including computer executable code or computer executable instructions that, when executed, causes at least one computer to execute the performing and computing steps described herein.

The methods and devices described in the present application may be implemented as software in a Digital Signal Processor, DSP, in a micro-controller or in any other side- processor or as hardware circuit within an application specific integrated circuit, ASIC.

The invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations thereof, e.g. in available hardware of conventional mobile devices or in new hardware dedicated for processing the methods described herein.

Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the present invention has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described herein.

Claims

CLAIMS:

1. An antenna device (600), comprising: a controller (602) being configured to control a focusing (605) of an antenna beam of an antenna array towards a user equipment, the focusing (605) being based on a feedback loop (601 ) with respect to a pilot beam, the pilot beam comprising a set of predetermined antenna beams of the antenna array, and being based on a distance (603) between the antenna array and the user equipment.

2. The antenna device (600) of claim 1 , wherein the controller (602) is configured to determine the distance (603) based on a propagation time measurement.

3. The antenna device (600) of claim 2, wherein the propagation time measurement is based on a round-trip delay measurement between the antenna array and the user equipment.

4. The antenna device (600) of claim 2 or claim 3, wherein the propagation time measurement is based on a known processing time inside the user equipment.

5. The antenna device (600) of one of claims 2 to 4, wherein the propagation time measurement is based on one of a timing advance measurement and a time stamp measurement, in signaling or payload traffic.

6. The antenna device (600) of one of claims 3 to 5, wherein the controller (602) is configured to determine a beam angle of the antenna beam based on reports of the user equipment.

7. The antenna device (600) of claim 6, wherein the controller (602) is configured to control the focusing (605) of the antenna beam by combining the reported beam angle and the measured propagation time, by using trigonometric functions with respect to the reported beam angle.

8. The antenna device (600) of claim 6 or claim 7, wherein the controller (602) is configured to control the focusing (605) based on statistics and/or changes of the reported beam angle and the measured propagation time.

9. The antenna device (600) of claim 7 or 8, wherein the controller (602) is configured to detect failures of the feedback loop (601 ) by tracing the reported beam angle and the measured propagation time.

10. The antenna device (600) of one of the preceding claims, wherein the controller (602) is configured to code the set of predetermined antenna beams by using pre-coding matrices.

1 1. The antenna device (600) of one of the preceding claims, wherein the feedback loop (601 ) is based on pre-coding matrix index, PMI, reporting, as defined according to a 3GPP TS standard, in particular as defined according to 3GPP TS 36.331 standard.

12. A radio base transceiver station (400; 301 ), comprising: an antenna array (401 ) being capable of vertical beam forming; a set of radio transceivers (403) being configured to drive the antenna array (401 ) with transceiver signals (402); a baseband module (407) being configured to process radio baseband signals (406) based on the transceiver signals (402), wherein the baseband module (407) comprises the antenna device (600) of one of the claims 1 to 1 1 , and being configured to control focusing of an antenna beam of the antenna array (401 ) towards a user equipment.

13. A method (700) for controlling focusing (705) of an antenna beam of an antenna array towards a user equipment, the method (700) comprising: controlling (702) the focusing (705) of the antenna beam based on a feedback loop (701 ) with respect to a pilot beam, the pilot beam comprising a set of predetermined antenna beams of the antenna array, and being based on a distance (703) between the antenna array and the user equipment.

14. The method (700) of claim 13, comprising: determining a beam angle of the antenna beam based on pre-coding matrix index reporting; determining the distance (703) based on a propagation time measurement between the antenna array and the user equipment; and adjusting the focusing (705) of the antenna beam based on a difference between the reported beam angle and a beam angle corresponding to the measured propagation time.

15. The method of claim 14, comprising: evaluating statistics and/or changes of the reported beam angle and the measured propagation time in order to obtain the difference.