Technical Field

The present disclosure generally relates to a technique for communicating data using an antenna array. More specifically, methods and devices are provided for sending and receiving the data.

Background

The modernization of antenna technologies in practice is moving forward in a high pace, which enables the use of more advance antenna setups and techniques in order to increase throughput and robustness in a mobile radio network. One antenna technique includes shaping beams (beamforming) generated by an antenna array. With the use of so-called reconfigurable antenna systems (RAS), which are already available in practice, beamforming is made possible. The gains that can be achieved with such techniques are promising and seem to be of great importance to achieve the goals of future radio networks.

However, the gains that are theoretically expected with advanced beamforming do not come for free, as several new problems open up. The more antenna elements a base station uses, the more signaling overhead is needed by conventional beamforming techniques. This is a major restriction that is especially present when channel reciprocity cannot be exploited. Therefore, there is a need to develop new techniques, which can harvest the potential gains of beamforming using many antennas without causing a large control signaling overhead, which in some cases is the limiting factor for practical realization.

One beamforming technique that has the potential of providing high beamforming gains with low control signaling overhead is differential beamforming (DBF). This technique makes the beamforming narrower (which is also referred to as "zoom-in") in a step-wise manner using a fixed amount of control signaling in both downlink and uplink. In each zoom-in step, the reference signals are beamformed to cover a smaller area with higher intensity in the covered area. Due to the probability of making a wrong step in the beginning of the DBF

procedure, there are also mechanisms for widening the beamformed reference signals (which is also referred to as "zoom-out").

However, when the propagation from the base station to the user equipment causes significant angular spreading of downlink beams directed towards the user equipment, the conventional DBF process selects the relatively strongest beam among the next narrower beams, so that more and more of the radiation power is scattered out of the intended direction towards the user equipment. As a result, the zoom-in step does not improve antenna gain and may even worsen signal quality at the user equipment.

Summary

Accordingly, there is a need for a technique that efficiently improves the gain of a radio link from an antenna array to a user equipment.

As to one aspect, a method of sending data from an antenna array to a user equipment is provided. The method comprises or triggers a step of assigning a plurality of beams to the user equipment, wherein the assigned beams differ in a radiation characteristic of the antenna array; a step of sending a configuration message to the user equipment, the configuration message causing the user equipment to send a report if the assigned beams fulfill a criterion; a step of receiving the report from the user equipment; and a step of sending the data to the user equipment based on the report.

By assigning the plurality of beams to the user equipment, the user equipment can receive at least some of the different beams originating from the different radiation characteristics and apply the criterion for determining the report. At the data sending side, a transmission format for sending the data and/or a subset of the assigned beams for sending the data may be determined based on the report.

By way of example, a positive report for two or more of the assigned beams may indicate that the different radiation characteristics have been scattered and/or reflected along their paths to the user equipment such that each of the two or more beams contributes to radio reception at the user equipment. Scattering and reflecting typically increase the angular spread of the beam, which is disadvantageous for a conventional radio reception based on a single beam, e.g., due to channel fading and mobility-induced channel volatility. Based on the report, the sending side can detect the angular spread, so that sending the data based on the report can avoid or reduce such disadvantages, e.g., by maintaining or increasing a width of the radiation characteristic in response to the report and/or by combining the two or more of the assigned beams based on the report.

The "radiation characteristic" may encompass any pattern or angular distribution of electromagnetic radiation power. The radiation characteristic may result from any combination or subcombination of antenna arrays and/or antenna elements. E.g., the same radiation pattern generated by two different antenna arrays and/or different combinations of antenna elements may be considered as different radiation characteristics.

The assigned beams may differ in a direction of the beam and/or a lateral offset of the beam. A main lobe of the radiation characteristic (e.g., a maximum of the main lobe) may define the direction and/or the lateral offset. The direction may be defined by a maximum of a Poynting vector (e.g., on a sphere enclosing the antenna array) for the corresponding beam.

A base station of a radio access network may perform the method. The antenna array may be located at and/or controlled by the base station. The base station may include a plurality of antenna arrays, e.g., for different cells or sectors of the radio access network. The base station may perform the method with respect to each of a plurality of user equipments and/or each of the plurality of antenna arrays.

The configuration message sent to the user equipment may be indicative of the beams assigned to the user equipment and/or beams derived (e.g., by linear combination) from the assigned beams. Alternatively or in addition, the report received from the user equipment may be indicative of one or more beams derived based on the assigned beams. The user equipment (e.g., the report) may specify one, two or more linear combinations (e.g., by means of complex weights or preceding factors defined by reference to a codebook entry) of the assigned beams.

The assigned beams or the derived beams may be different in terms of the radiation characteristic of the antenna array. The configuration message may specify the criterion for triggering the report. The criterion may relate to a significant angular spread (SAS) for at least two of the assigned or derived beams. The criterion may require that the at least two assigned (or derived) beams be received with similar power, e.g., within a margin of 10% to 20% of the greatest received power among the assigned (or derived) beams.

The reception of the report may be indicative of the presence of SAS. Alternatively or in addition, the report may expressly indicate whether or not SAS is present.

The report may be indicative of those beams among the assigned beams that fulfill the criterion. The criterion may require a minimum power of the beams as received at the user equipment or a minimum channel gain.

The configuration message may specify a minimum number of beams that have to fulfill the criterion for triggering the report. The report may be sent, if at least two of the assigned beams fulfill the criterion. The data may be sent to the user equipment through the at least two beams based on the report.

The report may be indicative of a number of those assigned beams that fulfill the criterion. Alternatively or in addition, the report may be indicative of those assigned beams that fulfill the criterion. E.g., the report may include an identifier of those beams.

The report may further be indicative of a significance of those assigned beams that fulfill the criterion. The significance may be a function of at least one of the received power or gain of the corresponding beam as measured at the user equipment and a channel correlation between the indicated beams as measured at the user equipment. The criterion may requires that the observed correlation does not exceed a maximum correlation between channels defined by the at least two beams, as received at the user equipment.

A plurality of reference signals may be sent to the user equipment through the assigned beams. Each of the assigned beams may send a different one of the plurality of reference signals. The step of sending the reference signals may be performed after the step of sending the configuration message and/or before the step of receiving the report.

The reference signals may be sent through the assigned beams sequentially. For example, one reference signal is sent through its corresponding beam in one

Transmission Time Interval (ΤΤΊ). The other assigned beams may be muted in the ΤΤΊ. Alternatively or in addition, the reference signals sent through different beams may be orthogonal, e.g., by means of orthogonal cover codes. The orthogonal reference signals may be sent simultaneously. E.g., more than one reference signal may be sent within one ΊΓΠ.

The step of sending the reference signals may include sending an identifier associated with each of the assigned beams. The report may be indicative of a subset of the assigned beams by including the corresponding beam identifiers.

The criterion may depend on the reference signal received power, RSRP, of the beams. The criterion may include comparing the RSRP of the assigned beams. The configuration message may cause the user equipment to measure the RSRP for each of the assigned beams.

The criterion may require that a ratio between the greatest RSRP and the second- greatest RSRP among the measured RSRP be below a threshold value. Alternatively or in addition, the criterion may require that the measured RSRP of each reported beam is greater than a threshold value. The threshold value may be determined based on the greatest RSRP among the measured RSRP.

The threshold value and/or a function for determining the threshold value may be specified by the configuration message. The threshold value may be a function of the greatest RSRP. The threshold value may be smaller than the greatest RSRP. E.g., the threshold value may be 50%, 75% or 90% of the greatest RSRP.

The configuration message may cause the user equipment to determine channel coefficients for each of the beams. The threshold value for the >th beam among the assigned beams may be determined based on a correlation between the channel coefficients for the >th beam and the channel coefficients for the beam with the greatest RSRP.

The antenna array may include a plurality of antenna elements. The radiation characteristic of each of the assigned beams may be generated using at least two of the antenna elements.

The beams may differ in terms of a relative phase and/or the antenna elements used for generating the radiation characteristic. Each of the assigned beams may be associated with one direction. The direction may be defined by the main lobe of the corresponding beam and/or a maximum of a Poynting vector (e.g., on a sphere enclosing the antenna array) for the corresponding beam.

The beams may be sent by means of the antenna array including a plurality of antenna elements. Each of the beams may be sent with phase differences between at least some of the antenna elements. The phase differences may define the direction of the corresponding one of the beams. A subset of the antenna elements and/or the different amplitudes used for sending the beam may define the lateral offset of the corresponding beam.

The antenna array may be an arrangement of the antenna elements in one dimension. Alternatively, the antenna array may be an arrangement of the antenna elements according to a two-dimensional lattice.

The base station may perform Differential Beamforming (DBF). DBF is described, inter alia, in Chapter 5 of the thesis "Differential Beam-Forming Using Two- Dimensional Antenna Arrays in an LTE FDD System" by F. Stenmark, Linkoping University, 2015. The method may be triggered, if the DBF fails to converge, e.g., within a predefined time period and/or a predefined number of steps. The assigned beams may result from the DBF procedure. The DBF procedure may be terminated in response to the report.

Sending the data to the user equipment may include sending the data to the user equipment through a combination of at least two beams indicated in the report. The same data may be sent simultaneously through each of the at least two beams indicated in the report.

The data may be sent through the at least two indicated beams with different phases and/or different amplitudes. The phases and/or the amplitudes may be determined based on the report. The report may be indicative of a measured phase difference and/or a measured gain for each of those beams that fulfill the criterion. The different amplitudes used for sending the data through the at least two beams may be determined based on the reported gains.

The different phases used for sending the data through the at least two beams may be determined based on the reported phase difference. For example, the report may be further indicative of at least one phase shift for the at least two of the beams indicated in the report. The data may be sent through each of the at least two indicated beams using the at least one phase shift. Alternatively, the base station measures time of arrival (TOA) differences at the antenna array, e.g., when receiving the report, and determines the different phases for sending the data through the at least two beams based on the measured TOA differences.

As to another aspect, a method of receiving data from an antenna array at a user equipment is provided. The method comprises or triggers a step of receiving a configuration message at the user equipment, the configuration message causing the user equipment to send a report if beams assigned to the user equipment fulfill a criterion; a step of receiving a plurality of the assigned beams at the user equipment, wherein the assigned beams differ in a radiation characteristic of the antenna array; a step of sending the report from the user equipment; and a step of receiving the data at the user equipment in response to the report.

The other method aspect may further include any feature or any step of (or any feature or step corresponding to a feature or a step of) the one method aspect.

As to a still further aspect, a computer program product is provided. The computer program product comprises program code portions for performing any one of the steps of the method aspects disclosed herein when the computer program product is executed by one or more computing devices. The computer program product may be stored on a computer-readable recording medium. The computer program product may also be provided for download via a data network, e.g., the radio access network and/or the Internet.

As to a further aspect, a device for sending data from an antenna array to a user equipment is provided. The device is configured to perform or trigger assigning a plurality of beams to the user equipment, wherein the assigned beams differ in a radiation characteristic of the antenna array; sending a configuration message to the user equipment, the configuration message causing the user equipment to send a report if the assigned beams fulfill a criterion; receiving the report from the user equipment; and sending the data to the user equipment based on the report.

As to a further aspect, a device for receiving data from an antenna array at a user equipment is provided. The device is configured to perform or trigger receiving a configuration message at the user equipment, the configuration message causing the user equipment to send a report if beams assigned to the user equipment fulfill a criterion; receiving a plurality of the assigned beams at the user equipment, wherein the assigned beams differ in a radiation characteristic of the antenna array; sending the report from the user equipment; and receiving the data at the user equipment in response to the report.

As to a further aspect, a base station is provided. The base station is configured for radio communication by means of an antenna array. The base station may be configured to perform the one method aspect. Alternatively or in addition, the base station comprises a beam assignment module for assigning a plurality of beams to the user equipment, wherein the assigned beams differ in a radiation characteristic of the antenna array; a configuration send module for sending a configuration message to the user equipment, the configuration message causing the user equipment to send a report if the assigned beams fulfill a criterion; a report reception module for receiving the report from the user equipment; and a data send module for sending the data to the user equipment based on the report.

As to a still further aspect, a user equipment is provided. The user equipment is configured for radio communication with an antenna array. The user equipment may be configured to perform the other method aspect. Alternatively or in addition, the user equipment comprises a configuration reception module for receiving a

configuration message at the user equipment, the configuration message causing the user equipment to send a report if beams assigned to the user equipment fulfill a criterion; a beam reception module for receiving a plurality of the assigned beams at the user equipment, wherein the assigned beams differ in a radiation characteristic of the antenna array; a report send module for sending the report from the user equipment; and a data reception module for receiving the data at the user

equipment in response to the report.

The devices, the base station and/or the user equipment may further include any feature disclosed in the context of the method aspects. Particularly, any one of the modules, or a dedicated module or unit, may be adapted to perform one or more of the steps of any one of the method aspects.

Brief Description of the Drawings

Further details of embodiments of the technique are described with reference to the enclosed drawings, wherein: Fig. 1 schematically illustrates a radio access network including at least one antenna array;

Fig. 2 shows a schematic block diagram of an embodiment of a device for sending data from an antenna array to a user equipment, which is implementable at the antenna array in Fig. 1;

Fig. 3 shows a schematic block diagram of an embodiment of a device for receiving data at a user equipment from an antenna array, which is implementable at the user equipment in Fig. 1;

Fig. 4 shows a flowchart for a method of sending data from an antenna array to a user equipment, which is implementable by the device of Fig- 2;

Fig. 5 shows a flowchart for a method of receiving data at a user

equipment from an antenna array, which is implementable by the device of Fig. 3;

Figs. 6 to 8 schematically illustrate different radiation characteristic; Figs. 9 and 10 schematically illustrate a conventional beamforming technique that fails to converge in certain situations;

Fig. 11 schematically illustrates a beam configuration combining multiple significant beams; Fig. 12 schematically illustrates a beam configuration resulting from an

implementation of the technique; Fig. 13 schematically illustrates a temporal sequence of tracking a mobile user equipment; Fig. 14 shows a schematic block diagram for an embodiment of a base

station; and Fig. 15 shows a schematic block diagram for an embodiment of a user equipment. Detailed Description

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as a specific network environment in order to provide a thorough understanding of the technique disclosed herein. It will be apparent to one skilled in the art that the technique may be practiced in other embodiments that depart from these specific details. Moreover, while the following embodiments are primarily described for a Long Term Evolution (LTE) implementation, it is readily apparent that the technique described herein may also be implemented in any other wireless communication network, including Fifth Generation or Next Generation networks, a Wireless Local Area Network (WLAN) according to the standard family IEEE 802.11 (e.g., IEEE 802.11a, g, n or ac) and/or a Worldwide Interoperability for Microwave Access (WiMAX) according to the standard family IEEE 802.16.

Moreover, those skilled in the art will appreciate that the functions, steps and modules explained herein may be implemented using software functioning in conjunction with a programmed microprocessor, an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Digital Signal Processor (DSP) or a general purpose computer, e.g., including an Advanced RISC Machine (ARM). It will also be appreciated that, while the following embodiments are primarily described in context with methods and devices, the invention may also be embodied in a computer program product as well as in a system comprising a computer processor and memory coupled to the processor, wherein the memory is encoded with one or more programs that may perform the functions, steps and implement the modules disclosed herein.

Fig. 1 schematically illustrates a radio access network 100 as an exemplary environment for implementing the technique. The radio access network 100 includes one or more base stations 102 providing radio access to at least one user equipment 104.

The base station 102 comprises one or more antenna arrays 110. Each antenna array 110 includes a plurality of regularly arranged antenna elements 112.

The base station 102 further comprises a device 200 for sending data from the antenna array 110 to the user equipment 104, e.g., when in a connected state with the base station 102 of a radio link control protocol. The user equipment 104 comprises a device 300 for receiving the data at the user equipment 104 from the antenna array 110.

The device 200 is operatively coupled with the antenna array to send any signal (including, e.g., control signaling and/or data) in a beam 120. The device 200 defines a radiation characteristic 122 of the antenna array 110 for sending the beam 120. The radiation characteristic 122 defines a direction and/or a lateral or transversal offset position (that is briefly referred to as the offset) relative to the antenna array 110.

In one implementation, the device 200 controls each of the antenna elements 112 individually for generating the beam 120 according to the radiation characteristic 122. In another implementation, the device 200 defines parameters (e.g., the direction and the offset) of the radiation characteristic 122. Control logic at the antenna array 110 derives individual output signals for the antenna elements 112 from the signal and the parameters provided by the device 200.

For example, a main lobe of the radiation characteristic 122 may define the direction and the offset. The radiation characteristic 122 may further include side lobes 124. The side lobes 124 carry less radiation power than the main lobe. The side lobes 124 may be ignored for defining the direction and the offset.

The base station 102 may further comprise an interface 130 to a backhaul network. The backhaul network connects the base stations 102, e.g., for a cellular topology of the radio access network.

One or more receive antennas at the user equipment are coupled to the device 300. While the device 200 sends the data by means of the antenna array 110 with a defined radiation characteristic 122, it is not necessary that the device 300 has comparable directional sensitivity or that the device 300 has any directional sensitivity.

Fig. 2 shows a schematic block diagram of an embodiment of the device 200 for sending data from an antenna array 110 to a user equipment 104. The device 200 comprises a beam assignment module 202 for assigning a plurality of beams 120 to the user equipment 104. In the case of multiple user equipments 104 being concurrently in the connected state, disjoint sets of beams 120 are assigned to each of the user equipments 104. Each of the assigned beams 120 has a different radiation characteristic 122 that is generated by means of the antenna array 110.

The device 200 further comprises a configuration send module 204 for sending a configuration message to the user equipment 104. The configuration message includes a command controlling the user equipment 104 to send a report, if the assigned beams 120 fulfill a criterion. Alternatively or in addition, the configuration message may command the user equipment to send the report, additionally or exclusively, as to one or more of the derived beams. The derived beams may be specified, e.g., for DBF, by the device 200 (e.g., in the configuration message) or the device 300 (e.g., in the report).

A report reception module 206 of the device 200 receives the report from the user equipment 104. Thus, the device 200 acquires knowledge of the criterion being fulfilled for at least some of the assigned beams. A data send module 208 sends the data to the user equipment 104 using the antenna array 110 in a configuration that depends on the report.

Fig. 3 shows a schematic block diagram of an embodiment of the device 300 for receiving data from an antenna array 110 at a user equipment 104. The device 300 comprises a configuration reception module 302 for receiving a configuration message at the user equipment 104. The configuration message includes a command that controls the device 300 to trigger sending a report, if beams 120 assigned to the user equipment 104 fulfill a criterion. The criterion may be predefined (e.g., in a communication standard), specified by the configuration message, or combination of both.

A beam reception module 304 of the device 300 receives a plurality of the assigned beams 120 at the user equipment 104. The assigned beams 120 differ in a radiation characteristic at the antenna array. A report send module 306 of the device 300 sends the report from the user equipment 104. A data reception module 308 of the device 300 receives the data at the user equipment 104 in response to the report.

Figs. 4 and 5 show flowcharts for methods 400 and 500 for operating the devices 200 and 300, respectively. Steps 402, 404, 406 and 408 of the method 400 may be performed by the modules 202, 204, 206 and 208, respectively. Steps 502, 504, 506 and 508 of the method 500 may be performed by the modules 302, 304, 306 and 308, respectively. Certain resource elements of each beam 120 include a reference signal. The resource elements may correspond to predefined positions in a time-frequency grid. The resource elements for the reference signals may be defined by a protocol for the physical layer of the radio communication between the base station 102 and the user equipment 104. Each of the assigned beams 120 has a different reference signal that is unique at least for a given combination of the base station 102 and the user equipment 104.

Based on the reference signals, the device 300 at the user equipment 104 is able to distinguish the assigned (or derived) beams 120 as they are received. Furthermore, the device 300 is able to determine a Reference Signal Received Power (RSRP), a Reference Signal Received Quality (RSRQ), a Signal to Noise and Interference Ratio (SNIR), a channel gain and/or a channel estimate (e.g., channel coefficients) for each of the assigned (or derived) beams 120. Furthermore, the device 300 is able to determine a channel correlation between any two of the assigned (or derived) beams 120.

An unconditional reporting can be implemented standard transparent, e.g., in LTE by the use of several Channel State Information (CSI) processes. Each CSI process is set up with certain directions of a CSI reference signal. The user equipment provides feedback per process.

In an LTE implementation of the technique requiring a conditional report depending on the beam criterion, a virtual antenna port may be defined for each of the assigned beams 120. In same or another LTE implementation, an LTE CSI process may be may be performed for each of the assigned beams 120. The reference signals may be implemented by LTE CSI reference signals. The criterion-driven report may be backward compatible with a CSI report (e.g., for rank 1).

In the step 504 or 506, the device 300 computes a significance value for at least two of the assigned beams 120. Based on at least one of the RSRP, the RSRQ, the SNIR and the channel estimate determined by the device 300 for each of at least a subset of the assigned beams 120, the device 300 may compute the significance value for at least two of the assigned beams 120 in the subset. The significance value of an assigned beam 120 is an increasing function of the channel gain or the RSRP of the corresponding assigned beam 120. Alternatively or in addition, the significance value of an assigned beam 120 is a decreasing function of its channel correlation with another assigned beam 120, e.g., with the assigned beam 120 having the greatest channel gain or the greatest RSRP. An assigned beam 120, the significance value of which exceeds a threshold value, may be referred to as a significant beam. The assigned beam 120 having the greatest channel gain or RSRP may be significant by definition.

The criterion for sending the report may require that at least two of the assigned beams 120 are significant. At least one of the function for computing the significance value, the threshold value and a minimum number of significant beams required for sending the report may be specified by the configuration message.

For example, the report may identify when a Differential Beamforming (DBF) process performed by the base station 102 encounters high angular spread towards the user equipment 104, so that the DBF process may be terminated at the step 406 or 408. The detection of high angular spread may be achieved by triggering the report, if multiple beams in the set of beams assigned with the user equipment 104 have quality estimates that exceed the threshold, e.g. compared to the strongest beam among the assigned beams 120.

The assigned beams 120 may be narrow in the sense that more than half of the number of antenna elements 112 are active for generating each of the assigned beams 120. The device 300 may further estimate and report phase differences. Estimating phase differences between the assigned beams 120, as received at the user equipment 104, and including the estimated phase differences in the report to the device 200 in the steps 406 and 506 allows enhancing the antenna gain of the antenna array 110 towards the user equipment 104. In the step 408, the reported phase differences are compensated by combining the reported beams with phase shifts that correspond to the reported phase differences. In order to minimize the resource-intensive reports including the phase differences, the report does not include the phase differences, if an entropy of a distribution of the phase differences exceeds an entropy threshold. The high entropy indicates a random distribution of the phase differences, so that combining the reported beams without compensating phase shifts also improves the enhances the antenna gain towards the user equipment 104.

Figs. 6 and 7 illustrate the different radiation characteristics 122 for simple implementations of the assigned beams 120. Figs. 6 and 7 schematically illustrate a cross-sectional views perpendicular to a plane defined by the antenna array 110. The two beams 120 illustrated in Fig. 6 differ in their directions by an angle 600. The angle of each of the assigned beams 120 may be defined relative to a normal of the plane of the antenna array 110.

The two beams 120 illustrated in Fig. 7 differ in their lateral position by an offset 700. The offset of each of the assigned beams 120 may be defined relative to a (e.g., geometrical) center position of the antenna array 110.

The assigned beams 120 may include beams with more than two different directions, more than two different offsets and/or different combinations of the direction and the offset.

While the radiation characteristics schematically illustrated in Figs. 6 and 7 can be generated by means of a one-dimensional antenna array 110, the antenna elements 112 may be arranged in a two-dimensional structure.

Fig. 8 schematically illustrates a two-dimensional antenna array 110. The antenna elements are arranged in a hexagonal lattice. The direction 600 of any assigned beam 120, as defined by the main lobe of the characteristic 122, includes two degrees of freedom, e.g., a horizontal angle 602 and a vertical angle 604 (which may also be referred to as tilt). Antenna elements 112 may also be arranged in other lattice structure and/or on a curved surface of the antenna array 110.

The technique may be implemented for detection and actuation upon situations of significant angular spread (SAS), as is schematically illustrated in Figs. 9 to 13.

Applying conventional beamforming techniques in an SAS situation may lead to signal degradation, as is schematically illustrated in Figs. 9 and 10.

Conventional differential beamforming (DBF) compares two processes at different zoom levels with each other to decide whether the narrow beam 120 A including a first reference signal is better than a wider beam 120 including a second reference signal, as is schematically illustrated in Fig. 9. If the narrow beam 120A including the first reference signal is better, a further zoom-in step is performed. If, instead, the wider reference signal is better, a zoom-out step is performed. The zoom-in step can be realized, as schematically illustrated in Fig. 10, by keeping the narrow beam 120A including the first reference signal and creating a new set of narrower beams within the kept narrow beam 120 A, such as the narrower beam 120 in Fig. 10, each of which includes a different third reference signal. In the zoom-out step, the wide beam including a reference signal is kept, and an even wider beam is introduced.

In situations as depicted in Fig. 9, the conventional DBF procedure zooms in and results in a situation as depicted in Fig. 10. However, in the situation depicted in Fig. 10, the conventional report indicates that the beam 120A is better suited than the narrower beam 120, no matter where the narrower beam 120 is directed due to angular spread. The conventional DBF procedure thus performs a zoom-out step.

The base station 102 may detect the SAS situation, if the same zoom-in/zoom-out step has been repeated N times, wherein N is a configurable or fixed threshold value. Alternatively, the SAS situation is detected, if the steps are repeated on too wide zoom-levels indicating that the conventional DBF process is not converging. For example, SAS may be detected, if the narrow beam is a threshold number of zoom- steps wider than the narrowest beam available for the antenna array. In some embodiment, N consecutive zoom-in and zoom-out iterations are required. In other embodiments, N zoom-in and zoom-out iterations are required within a certain time window.

The technique can be implemented to detect and terminate the non-converging DBF, in order to stop the radio resource consumption of the conventional control signaling. Furthermore, a multi-beam configuration can be established in the step 408 that is adapted to the SAS situation, whereas conventional beamforming techniques using only directional information, such as DBF and traditional DFT precoded beams, do not harvest the beamforming gain a large antenna array can provide.

A first aspect of the technique includes an SAS configuration. In some embodiments, the base station 102 (e.g., the device 200) configures the user equipment (UE) 104 in the step 404 to be "SAS aware", i.e., the criterion triggers sending the report, if the assigned (or derived) beams 120 suffer an SAS (which is also referred to as SAS report). Based on different measurements, the user equipment 104 processes and decides whether to trigger the SAS report. The configuration message sets

parameters at the user equipment 104, e.g., for the step 504.

The configuration message specifies the set of assigned beams 120 to measure over. Each of the assigned beams 120 may be measured according based on its reference signal. Alternatively, at least some of the assigned beams 120 include a set of reference signals and the configuration message defines how to combine them (e.g., using weights of a codebook entry).

Alternatively or in addition, the configuration message specifies a type of the SAS triggering (e.g., a model and/or a functional dependency for the criterion). For example, the configuration message specifies at least one of the threshold value for the criterion, a function for determining the significance value for each of the assigned beams 120 and a minimum number of significant beams. Setting the threshold value allows controlling a sensitivity of the SAS trigger.

Alternatively or in addition, the configuration message specifies a frequency at which the user equipment 104 checks the SAS trigger, i.e., the criterion. The user equipment 104 checks (e.g., between the step 504 and 506) whether the SAS report should be triggered or not, according to the configured trigger model, at the specified frequency.

Alternatively or in addition, the configuration message specifies an amount of information that the user equipment 104 has to report, once the SAS criterion is fulfilled.

In one embodiment, some or all parameters set by the configuration message are coded jointly or implicitly with other parameters, such as transmission mode or pilot configurations. In same or another embodiment, detection of the SAS is used to reduce signaling overhead of the user equipment 104, e.g., by terminating the DBF procedure. Terminating the DBF procedure may include sending the data in the step 408 through the antenna array 110 using the best of the assigned beams 120 or a best beam from a previous iteration of the DBF procedure. In one implementation, the frequency of reporting and/or measuring on a wide beam of the previous iteration of the DBF procedure is reduced. In another implementation, the reference signal of the wide beam is not sent any more.

The set of beams to measure over may in many embodiments be the same set of beams measured for other purposes, e.g. CSI reporting. Any of the assigned or derived beams, as disclosed herein, may also be referred to as an estimated beam, since the user equipment may estimate (and distinguish) each of the assigned or derived beams based on a reference signal. A second aspect of the technique, which is combinable with the first aspect, includes an SAS trigger. In some embodiments, the base station 102 (e.g., the device 200) is asked to trigger an SAS report, if the channel gain or the RSRP of at least one non- optimal beam is within a certain threshold compared to the channel gain or the RSRP of the optimal beam, e.g., during the DBF process. In one implementation, reporting on the one or more sub-optimal beams with beam identifier J is triggered, if the following criterion is met:

RSRP _/ - RSRP, < X dB . (1)

Herein, RSRP _/ is an estimate of the received power of the optimal beam (that is the "strongest" beam that defines the beam identifier /of the -th beam). Furthermore, RSRP/ is an estimate of the received power of any one of the at least one other beam (i.e., one of the sub-optimal beams Jth with beam identifier y ^' so that pi). X is the dimensionless threshold value of the criterion.

While the beam identifier /is fixed by the maximum of RSRP* over k _f the index J in the inequality (1) is a running index. Hence, the computational complexity is linear in the number of beams.

The running index, j, can be further restricted. In some embodiments, the criterion is only applied to a subset of the beams 120 assigned by the device 200 or for a subset of the beams 120 evaluated by the device 300. The subset can be signaled from the base station 102, e.g., using the device 200, or may be predetermined in a communication protocol (e.g., a communication standard). For example, among the assigned beams 120, those beams 120 with different polarizations only (but identical radiation characteristics 122) are not compared.

Alternatively or in addition, those assigned beams 120 that are neighboring beams in terms of the angle 600 or the angles 602 and 604 (for the direction) and/or the offset 700 (for the lateral position) at the antenna array 110 are not compared (and not reported). In a further advanced implementation, the neighboring beams are not compared (and not reported), if their channel estimates are correlated.

In a still further advanced implementation, the neighboring beams, or all of the assigned beams 120, are compared using beam-specific threshold values. In some embodiments, a report is only triggered, if the criterion is fulfilled for multiple sub- optimal beams. In other embodiments, the user equipment 104 computes a cross-coupling of the beams to decide whether there are more than one significant beam that are sufficiently decoupled. More specifically, the user equipment 104 compares each beam-pair according to different configured threshold values X,y. Denoting the received signal power (or signal strength) of the strongest beam /by RSRP,> the criterion for triggering the SAS report requires

RSRP _/ - RSRP/ < X _/j dB . (2)

Herein, /is the identifier of the optimal beam, e.g., defined by the maximum RSRP among the assigned beams 120. Each of the sub-optimal beams with identifiers J is compared with the optimal beam according to the inequality (2).

In an exemplary LTE implementation, the RSRP may be the CSI RSRP. In a variant of the inequalities (1) or (2), the RSRP is replaced by the gain, H H*, of the channel defined by the corresponding beam. Herein, H* is the channel estimate using the reference signal associated with the k-t beam.

Above inequalities (1) and (2) for the criterion are advantageously checked using a logarithmic scale (e.g., dBm) for the RSRP. Alternatively, using a linear scale for the RSRP, the inequalities correspond to q < linRSRP, / linRSRP/ < 1, wherein q = 10 ^"x/1 ° and q = 10 ^"x ^ ¹⁰ , respectively.

For a base station 102 that performs the DBF process, the RSRP is determined (and optionally reported) by the user equipment 104 for the assigned beams 120. In this case, the available RSRP values are also used for assessing the cross coupling between the assigned beams 120 for the criterion according to the inequalities (1) or (2).

In a variant, the criterion for the cross coupling (e.g., according to the inequalities (1) or (2)) is assessed, wherein a cross correlation between the corresponding two channel coefficients H _/ and H ₇ of the two beams determines the threshold values X, The cross correlation may be computed over time. For the base station 102 performing DBF, the cross coupling criterion is particularly useful when determining whether to zoom-in or not, since the user equipment 104 has to report over multiple narrow beams (i.e., the basis for a zoom-in hypothesis). Hence, when evaluating the assigned beams 120 (i.e., the zoom-in options), each of the assigned beams 120 for a zoom-in operation may be evaluated against the cross coupling criterion of inequality (1) or (2).

Given that a signal is received strongest in the -th beam, the expected difference in the received power in the >th beam may be determined based on the cross correlation between the beams /and j. Alternatively or in addition, the expected difference in the received power in the >th beam may be estimated based on a spatial overlap of the beam paths.

A strong signal in the two beams /and /with high cross coupling (e.g. large beam- overlap) does not indicate large angular spread. A high received power value in the two beams /and /with low cross coupling (e.g., low overlap) may indicate two distinct propagation paths and, hence, angular spread.

Any of the above embodiments for the criterion may further be implemented by sending the report in the step 506 only if the criterion is fulfilled for a certain number of measurement instances and/or during a certain time period. The number and/or the time period may be configured by the access network (e.g., in the configuration message) or fixed in a standard.

A third aspect, which is combinable with the first aspect or the second aspect, includes the SAS report. The report may feed back in the step 506 a Boolean SAS trigger output. That is, the report indicates whether there is SAS or not. Optionally, the report further indicates the number of significant beams (e.g., up to a maximum number that is configured).

Based on a criterion (e.g., a trigger model), which may be different from that described for the SAS trigger above, the user equipment 104 is able to compare any pair of beams it senses in the step 504 and generates an SAS report.

Furthermore, the assigned beams 120 may be virtually constructed by the device 300 at the user equipment 104 using a set of at least two reference signals, e.g., the output of a 2-TX codebook on 2 antenna ports. Alternatively or in addition, the device 300 may compute the eigenvectors and eigenvalues of the matrix H _a ^* H _b (i.e., the channel estimates) for the assigned beams. The device 300 may report in the step 506 virtual beams (which may also be referred to as derived beams) constructed from those eigenvectors (e.g., as the complex weights for combining the assigned beams) having an eigenvalue greater than the threshold value.

Depending on the configuration message, the report may be indicative of identifiers of the significant beams. Optionally, the significance value is reported in association with the beam identifier of the "significant beams".

In an advanced implementation, the report includes a phase relation between the significant beams. A comprehensive report would, for instance, include a Channel Quality Information (CQI) for each of the assigned beams 120 and a phase shift (e.g., via a Pre-coding Matrix Indicator, PMI) of each of the sub-optimal beams 120 relative to the strongest beam 120. Based on the reported phase shifts, the device 200 uses a combined beam pattern in the step 408.

As depicted in Fig. 11, the combination is constructed from the significant beams 120 A in order for the signal to add up coherently at the user equipment 104 after the signal has propagated over a plurality of signal paths 900 including different phase shifts 902 (e.g., dispersive obstacles, scattering obstacles or reflecting obstacle). The multiple significant beams 120A thus constructively add up along converging path segments 904.

In some embodiments, detection of SAS is used to initiate an alternative

beamforming process that mitigates the angular spread of the assigned beams 120. In one aspect of these embodiments, the user equipment 104 is configured to report a measurement combined over multiple of the (e.g., narrow) assigned beams 120, enabling a collection of energy from multiple paths 900. Another embodiment incorporates branching out the DBF procedure and using precoding information over several such branches of the procedure in order to combine the weights and, even more importantly, the different phase offsets from the different "angular

contributions" according to the beams. In some embodiments, the multiple directions are stored for future usage, e.g., although only one narrow beam is selected for further zoom-in procedure.

In an embodiment using a report that includes the observed RSRP values for the assigned beams 120, the device 200 at the base station 102 may decide whether there is SAS or not. For example, as illustrated in Fig. 12, the user equipment 104 reports RSPR values for 8 assigned beams 120. Out of these 8 reported beams, e.g. 3 of the reported beams 120 A have significant (and roughly equally significant) RSRP values. This report implies a significant amount of the signal energy propagates through more than just one signal path and that there is SAS. This information is used by the device 200 at the base station 102 to form beams that send signal energy in all of these 3 significant directions.

Since no phase relation between the beams 120 A is reported, the device 200 cannot actively ensure that the signal contributions from the different beams 120A

coherently accumulate at the user equipment 104. However, due to randomness in the channel fading, one may expect that the signals do not cancel out coherently.

In the case of a report including the observed RSRP values and their phase offsets (which may also be referred to as angular relations) for the significant beams 120A, the device 200 at the base station 102 can transmit along the 3 significant beams with proper phases on each beam 120A. The signal transmitted with beam 1 has phase offset cpi, beam 2 has phase offset q>2, and beam 3 has phase offset q>3. Then, if that information (i.e., the phase offsets cpi, φ _2/ ψ3) is included in the SAS report, the device 200 at the base station 102 is able to compensate for these phases.

Hence, with high probability, the signal contributions accumulate constructively at the user equipment 104.

Depending on the reporting rate or frequency of the device 300, the device 200 may track a mobile user equipment, as is schematically illustrated in Fig. 13 for a sequence of 3 points in time at ti, and t3.

Fig. 14 shows a block diagram for an embodiment of the base station 102. The base station 102 comprises an interface 1402 to an antenna array. The base station 102 includes a processor 1404 coupled to the interface 1402 and memory 1406 including the modules 202 to 208 for performing the method 400.

Fig. 15 shows a block diagram for an embodiment of the user equipment 104. The user equipment 104 comprises an interface 1502 to a mobile antenna. The user equipment 104 includes a processor 1504 coupled to the interface 1502 and memory 1506 including the modules 302 to 308 for performing the method 500.

As has become apparent from above description of exemplary embodiments, a node of a radio access network, e.g., at a base station, can determine an angular spread towards a user equipment. The base station may, thereby, utilize that information to reduce a signaling overhead and/or to further improve an antenna directivity gain towards the user equipment. The technique is applicable for determining a robust radio link when differential beamforming does not converge to the robust radio link.

The technique may be implemented to enable a beamforming that spreads the signal energy in such a way that multiple beams diverging at the sending side coherently accumulation at a particular spatial point in the case the signal has propagated over a plurality of different signal paths.

The technique can further be implemented to capturing options of different transmission directions to enable a prediction of the different branches of a beam- forming hypothesis.

Many advantages of the present invention will be fully understood from the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the units and devices without departing from the scope of the invention and/or without sacrificing all of its advantages. Since the invention can be varied in many ways, it will be recognized that the invention should be limited only by the scope of the following claims.