Patent Description:
In communications systems, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications systems is deployed.

For example, future generation wireless communications systems are expected to provide ubiquitous high data-rate network coverage. Currently emerging standards, such as the 3rd Generation Partnership Project (3GPP) Long Term Evolutional Advanced ( LTE-Advanced), are targeted to support up to <NUM> Gbps in the downlink (i.e., from the network nodes to the wireless devices) and <NUM> Mbps in the uplink (i.e., from the wireless devices to the network nodes). In general terms, achieving such data rates requires an efficient use of the available resources and typically requires large frequency bandwidths to be used which may be more available at carrier frequencies higher than at about the <NUM> frequency bandwidth commonly used in existing wireless communication systems. Further if time division duplex (TDD) access is used reciprocity between properties of the radio channel for downlink transmission (i.e. transmission from serving network node to served wireless device) and properties of the radio channel for uplink transmission (i.e. transmission from served wireless device to serving network node) can be utilized.

If high frequencies are used network coverage could be a challenge. For uplink transmission, in certain scenarios transmit beam forming could be necessary to achieve sufficient network coverage.

In general terms, methods for selecting beam and setting beam weights (or pre-coder) transmit beam forming are based on either closed-loop approaches or open-loop approaches. Closed-loop approaches for the uplink are based on uplink measurements that are reported back from the network node to the wireless device. Open-loop approaches are based on utilizing the uplink/downlink reciprocity. Closed-loop approaches cost radio resources for feedback. Closed-loop approaches often require sounding reference signals to be transmitted in uplink to get reliable uplink measurements. Closed-loop approaches also suffer from reporting delay. This reporting delay can be significant if the wireless device is moving.

Open-loop approaches require that the same antennas are used in the wireless device for both transmission and reception. However, device implementations are sometimes done with separate transmit and receive antennas, as this can e.g. remove the need for components such as splitters , combiners, duplex filters, etc. If there are a larger number of receive antennas this cannot easily improve the beam forming accuracy.

Hence, there is still a need for an improved beam selection.

Document <CIT> describes a method of measuring distance between two communication units.

Document <CIT> describes a method of receiving wireless transmission of a real radio system from at least one base station as a function of reception direction.

Document <CIT> describes an estimation apparatus of a signal angle of arrival which improves the data transmission in a high-speed movement environment.

An object of embodiments herein is to provide efficient beam direction selection. Furthermore, the embodiments of the invention are those defined by the claims. Moreover, examples and embodiments, which are not covered by the claims are presented not as embodiments of the invention, but as background art or examples useful for understanding the invention.

According to a first aspect there is presented a method for selecting beam direction for a radio communications device. The method is performed by the radio communications device. The method comprises obtaining radio channel estimates of a radio channel on which radio waves have been transmitted between the radio communications device and another radio communications device at an angle of arrival and departure. The method comprises determining a Doppler shift from the radio channel estimates. The method comprises estimating at least one of the angle of arrival and departure of the radio waves based on the Doppler shift. The method comprises selecting a beam direction for a signal to be transmitted between the radio communications device and this another radio communications device over the radio channel according to the estimated angle of arrival or departure.

According to a second aspect there is presented a radio communications device for selecting beam direction for the radio communications device. The radio communications device comprises processing circuitry. The processing circuitry is configured to cause the radio communications device to obtain radio channel estimates of a radio channel on which radio waves have been transmitted between the radio communications device and another radio communications device at an angle of arrival and departure. The processing circuitry is configured to cause the radio communications device to determine a Doppler shift from the radio channel estimates. The processing circuitry is configured to cause the radio communications device to estimate at least one of the angle of arrival and departure of the radio waves based on the Doppler shift. The processing circuitry is configured to cause the radio communications device to select a beam direction for a signal to be transmitted between the radio communications device and this another radio communications device over the radio channel according to the estimated angle of arrival or departure.

According to a third aspect there is presented a radio communications device for selecting beam direction for the radio communications device. The radio communications device comprises processing circuitry and a computer program product. The computer program product stores instructions that, when executed by the processing circuitry, causes the radio communications device to perform operations, or steps. The operations, or steps, cause the radio communications device to obtain radio channel estimates of a radio channel on which radio waves have been transmitted between the radio communications device and another radio communications device at an angle of arrival and departure. The operations, or steps, cause the radio communications device to determine a Doppler shift from the radio channel estimates. The operations, or steps, cause the radio communications device to estimate at least one of the angle of arrival and departure of the radio waves based on the Doppler shift. The operations, or steps, cause the radio communications device to select a beam direction for a signal to be transmitted between the radio communications device and this another radio communications device over the radio channel according to the estimated angle of arrival or departure.

According to a fourth aspect there is presented a radio communications device for selecting beam direction for the radio communications device. The radio communications device comprises an obtain module configured to obtain radio channel estimates of a radio channel on which radio waves have been transmitted between the radio communications device and another radio communications device at an angle of arrival and departure. The radio communications device comprises a determine module configured to determine a Doppler shift from the radio channel estimates. The radio communications device comprises an estimate module configured to estimate at least one of the angle of arrival and departure of the radio waves based on the Doppler shift. The radio communications device comprise a select module (210d) configured to select a beam direction for a signal to be transmitted between the radio communications device and this another radio communications device over the radio channel according to the estimated angle of arrival or departure.

According to a fifth aspect there is presented a computer program for selecting beam direction for a radio communications device, the computer program comprising computer program code which, when run on the radio communications device, causes the radio communications device to perform a method according to the first aspect.

According to a sixth aspect there is presented a computer program product comprising a computer program according to the fifth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium.

Advantageously this method, these radio communications devices, this computer program, and this computer program product provide efficient beam direction selection. One or more beams could then be selected from the selected beam, thus resulting in efficient beam selection.

Advantageously this method, these radio communications devices, this computer program, and this computer program product provide an efficient open-loop approach that is applicable to TDD as well as frequency division duplex (FDD).

Advantageously this method, these radio communications devices, this computer program, and this computer program product provide an efficient open-loop approach that is applicable even when transmit antennas and receive antennas at the radio communications device are different in number or configuration.

Advantageously this method, these radio communications devices, this computer program, and this computer program product can be applied in combination with existing beam forming methods to improve performance.

Advantageously this method, these radio communications devices, this computer program, and this computer program product is more efficient than open-loop approaches for moving radio communications devices.

It is to be noted that any feature of the first, second, third, fourth, fifth and sixth aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of the first aspect may equally apply to the second, third, fourth, fifth and/or sixth aspect, respectively, and vice versa.

<FIG> is a schematic diagram illustrating a communications system <NUM> where embodiments presented herein can be applied. The communications system <NUM> comprises a first radio communications device <NUM> and a second radio communications device <NUM>. The radio communications devices <NUM>, <NUM> are configured to communicate with each other over a radio channel.

One of the radio communications devices <NUM>, <NUM> (for example, but not necessary, radio communications device <NUM>) could be part of a wireless device, such as a portable wireless device, mobile station, mobile phone, handset, wireless local loop phone, user equipment (UE), smartphone, laptop computer, tablet computer, wireless modem, or network equipped sensor. The other of the radio communications devices <NUM>, <NUM> (for example, but not necessary, radio communications device <NUM>) could be part of a network node, such as a radio access network node, radio base station, base transceiver station, node B, evolved node B, access point, or access node.

The embodiments disclosed herein relate to mechanisms for selecting beam direction for the radio communications device <NUM>. In order to obtain such mechanisms there is provided a radio communications device <NUM>, a method performed by the radio communications device <NUM>, a computer program product comprising code, for example in the form of a computer program, that when run on a radio communications device <NUM>, causes the radio communications device <NUM> to perform the method.

In particular, the herein disclosed mechanisms for selecting beam direction for the radio communications device <NUM> are based on determining Doppler shift. An initial reference is therefore made to <FIG> and <FIG> before proceeding further with the description of the embodiments.

<FIG> schematically illustrates how to obtain a Doppler spectrum according to an embodiment. In general terms the Doppler spectrum can be estimated by calculating a frequency transform, such as the fast Fourier transform (FFT), of radio channel estimates over a relatively short period in the time domain. In more detail, <FIG> schematically indicates a time-frequency diagram of the radio channel on which radio waves have been transmitted between the radio communications device <NUM> and the radio communications device <NUM>. The FFT is determined for the time-frequency representation over time in a window of length w time units, resulting in the Doppler spectrum-frequency representation in <FIG>. The Doppler spectrum-frequency representation is averaged over frequency, resulting in the average Doppler spectrum representation of <FIG>. Alternatively, the Doppler spectrum in <FIG> may represent a single frequency of the Doppler spectrum-frequency representation in <FIG>. This procedure is repeated for multiple short periods of time resulting in the time varying Doppler spectrum of <FIG>. Hence, multiple Doppler shifts could be determined from a short-term frequency transform of a time series of the radio channel estimates.

<FIG> shows an example time varying Doppler spectrum averaged over a <NUM> bandwidth for <NUM> short period segments, where each short period segment is <NUM> seconds long, resulting in a total measurement route of <NUM> seconds. The time varying Doppler spectrum represents radio channel estimates of the radio channel on which radio waves have been transmitted between the radio communications device <NUM> and the radio communications device <NUM>. During the first <NUM> seconds the radio communications device <NUM> is stationary (with respect to the radio communications device <NUM> and the surrounding environment) and the Doppler spread of the radio communications device <NUM> is close to <NUM>. In this respect, multiple Doppler shifts correspond to the Doppler spread whereas one such Doppler shift corresponds to the Doppler speed. For example, assume that one Doppler shift has a frequency value denoted fd and that the wavelength of the radio waves is λ, then the Doppler speed Vd can be determined as Vd = fd · λ.

Another term for Doppler speed is radial velocity of the radio communications device <NUM>. In this respect, the Doppler speed is the radial velocity relative to the transmitter of the radio waves or relative to any mirrored version of the transmitter caused by reflections of the radio waves. The Doppler speed is the speed represented by the strongest Doppler shift in the Doppler spectrum. In general terms, the radial velocity varies as a function of the angle α between the line of sight (assuming that no reflected radio waves are stronger than the radio waves received along the line of sight) and the speed of the radio communications device <NUM>. In the following the angle α will be denoted angle of arrival (AoA) or angle of departure (AoD). With reference back to <FIG>, assuming that the speed of the radio communications device <NUM> is Vr, then the radial velocity, defining the Doppler speed Vd, can be determined according to Eq. (<NUM>): <MAT>.

With reference again to <FIG>, in the timer interval between <NUM> and <NUM> seconds, the radio communications device <NUM> moves with a constant speed, <NUM>/s, which is seen as a spread <NUM> in one Doppler shift between about - <NUM> and +<NUM>/s. This spread corresponds to the speed Vr of the radio communications device <NUM>. In the timer interval between <NUM> to <NUM> seconds the radio communications device <NUM> moves away from the radio communications device <NUM>. The strongest path (corresponding to the dark part <NUM> in <FIG>) is in this case the line-of-sight path and defines the Doppler speed Vd (which thus is negative). The Doppler speed of this strongest path is increasingly negative corresponding to that the angle to the radio communications device 300is decreasing relatively to the direction of the movement of the radio communications device <NUM>. At <NUM> seconds, the radio communications device <NUM> turns back and moves towards the radio communications device <NUM> and the strongest line-of-sight path then has a positive Doppler speed. Regardless of whether the radio waves have been transmitted or received by the radio communications device <NUM>, the strongest downlink path could also be the best path for uplink transmission, and vice versa. Thus if a beam direction is selected for uplink transmission it could be in the same direction as the seen strongest line-of-site path in <FIG>. Particular details of how to select beam direction for the radio communications device <NUM> will be disclosed next.

<FIG> are flow charts illustrating embodiments of methods for selecting beam direction for the radio communications device <NUM>. The methods are performed by the radio communications device <NUM>. The methods are advantageously provided as computer programs <NUM>.

Reference is now made to <FIG> illustrating a method for selecting beam direction for the radio communications device <NUM> as performed by the radio communications device <NUM> according to an embodiment.

As disclosed above, the herein disclosed mechanisms for selecting beam direction for the radio communications device <NUM> are based on determining Doppler shift. The Doppler shift is based on radio channel estimates. Hence the radio communications device <NUM> is configured to perform step S102:
S102: The radio communications device <NUM> obtains radio channel estimates of a radio channel. Radio waves have been transmitted between the radio communications device <NUM> and the radio communications device <NUM> on this radio channel. The radio waves have been transmitted between the radio communications device <NUM> and the radio communications device <NUM> at an angle of arrival and departure. In this respect, the radio waves are generally transmitted in all directions (depending on properties of the transmitter of the radio waves), but only those transmitted in certain of these directions will reach the receiver. One or more of these directions correspond to the angle of arrival and departure. Further, the radio waves could either be transmitted from the radio communications device <NUM> to the radio communications device <NUM> or from the radio communications device <NUM> to the radio communications device <NUM>. Still further, while the radio communications device <NUM> obtains the radio channel estimates, measurements of the radio channel yielding the radio channel estimates could be performed either by the radio communications device <NUM> or the radio communications device <NUM>, independently of whether the radio waves were transmitted from the radio communications device <NUM> to the radio communications device <NUM> or from the radio communications device <NUM> to the radio communications device <NUM>. Hence, the radio communications device <NUM> could obtain the radio channel estimates either by performing channel measurements, or by receiving the radio channel estimates from the radio communications device <NUM>.

Once the radio channel estimates have been obtained the radio communications device <NUM> could determine the Doppler shift. Hence the radio communications device <NUM> is configured to perform step S104:
S104: The radio communications device <NUM> determines a Doppler shift from the radio channel estimates. In some aspects determining the Doppler shift involves the radio communications device <NUM> to perform operations, or steps, as described with reference to <FIG> above.

The Doppler shift is by the radio communications device <NUM> used to determine an angle α of arrival or departure of the radio waves. Hence the radio communications device <NUM> is configured to perform step S106:
S106: The radio communications device <NUM> estimates at least one of the angle α of arrival and departure of the radio waves based on the Doppler shift. Embodiments of different ways for the radio communications device <NUM> to estimates the angle α of arrival or departure of the radio waves will be provided below.

The angle α of arrival or departure is by the radio communications device <NUM> used to select a beam direction. Hence the radio communications device <NUM> is configured to perform step S108:
S108: The radio communications device <NUM> selects a beam direction for a signal to be transmitted between the radio communications device <NUM> and the radio communications device <NUM> over the radio channel according to the estimated angle α of arrival or departure.

In this respect the signal to be transmitted can be transmitted either by the radio communications device <NUM> or by the other radio communications device <NUM>. Hence, the radio communications device <NUM> could be configured to select either a beam direction for a signal to be transmitted by the radio communications device <NUM> or a beam direction for a signal to be transmitted by the radio communications device <NUM>.

Embodiments relating to further details of selecting beam direction for the radio communications device <NUM> will now be disclosed.

The angle α of arrival or departure could be defined in relation to the direction of travel of the radio communications device <NUM>. This direction of travel could be either absolute or relative the radio communications device <NUM>.

As disclosed above, the radio channel estimates are obtained for a radio channel on which radio waves have been transmitted between the radio communications device <NUM> and the radio communications device <NUM>. Hence, the radio waves could have been transmitted by either the radio communications device <NUM> and received by the radio communications device <NUM>, or transmitted by the radio communications device <NUM> and received by the radio communications device <NUM>. Hence, the method could be implemented in the thus transmitting radio communications device (for example in a wireless device when the radio waves are transmitted in an uplink transmission) or the thus receiving radio communications device (for example in a network node when the radio waves are transmitted in an uplink transmission). Similarly, the estimation of the radio channel may be performed on the direct or the reverse link transmissions. In any case, the Doppler shifts for both uplink and downlink transmissions will typically reflect the angle of arrival in relation to the moving radio communications device of the link (typically the wireless device).

There could be different examples of radio waves transmitted between the radio communications device <NUM> and the radio communications device <NUM>. In general terms, the radio waves represent signal waveforms. The signal waveforms in turn represent signals being transmitted between the radio communications device <NUM> and the radio communications device <NUM>. Examples of such signals are control signals and data signals. The signals could comprise reference symbols or other types of symbols for which the radio channel estimates of the radio channel could be obtained.

Reference is now made to <FIG> illustrating methods for selecting beam direction for the radio communications device <NUM> as performed by the radio communications device <NUM> according to further embodiments. It is assumed that steps S102, S104, S106, and S108 are performed as described above with reference to <FIG> and a thus repeated description thereof is therefore omitted.

In some aspects the selected beam direction defines one or more beams for transmitting or receiving the signal. Hence, according to an embodiment the radio communications device <NUM> is configured to perform step S110:
S110: The radio communications device <NUM> selects at least one beam for transmitting or receiving the signal. The at least one beam is based on the selected beam direction. For example, assuming that the radio communications device <NUM> has at least two beam for transmitting or receiving the signal, then that one of the at least two beams that points in a direction closest to the selected beam direction (i.e., the beam direction as selected in step S108) could be selected in step S110. Alternatively, in some aspects at least two beams are selected in step S110. This could be the case where no single beam points in the selected beam direction. Particularly, according to an embodiment at least two beams are selected, and transmission power is distributed between the at least two beams according to the estimated angle α of arrival or departure. This will allow the signal to be transmitted in several directions with a power distribution between them defined by a relative measured strength of the radio channel estimates.

Further, assuming that the radio waves are received or transmitted by at least two antenna elements of the radio communications device <NUM>, the angle α of arrival or departure could then be estimated based on combined radio channel estimates of the radio waves received or transmitted by the at least two antenna elements. Hence, the Doppler shift could be estimated for more than one antenna element and the angle α of arrival or departure could then be estimated based on combined radio channel estimates for all antenna elements receiving or transmitting the radio waves.

Each beam could correspond to one or more antenna element of the radio communications device <NUM>. Hence, according to an embodiment, selecting the at least one beam results in at least one antenna element being selected at the radio communications device <NUM>.

There are different types of beam forming that can be applied at the radio communications device <NUM> in order to form the one or more beam for transmitting or receiving the signal. Examples for beam forming the at least one beam include, but are not limited to, grid of beam selection, pre-coding and selection of transmit antenna element based on an antenna pattern.

There may be different ways to estimate the angle α of arrival or departure. Different embodiments relating thereto will now be described in turn.

As described above, <FIG> shows the relation between the angle α of arrival or departure, the speed Vr of the radio communications device <NUM> and the Doppler speed Vd of the radio communications device <NUM>. In general terms, Doppler speed will be different for each propagation path. In other words, there can be multiple Doppler speeds for a single radio communications device <NUM>. The Doppler speed is thus more a characteristic of the radio waves than of the radio communications device <NUM>, although the Doppler speed is scaled by the (physical) speed Vr of the radio communications device <NUM>. If the speed Vr of the radio communications device <NUM> and the Doppler speed Vd of the radio communications device <NUM> are known, the angle α of arrival or departure can be determined according to Eq. (<NUM>): <MAT>.

Hence, according to an embodiment the radio communications device <NUM> is configured to perform steps S10a, S106b, S106c in order to estimate the angle α of arrival or departure:.

There could then be different ways to determine the speed Vr of the radio communications device <NUM>. According to a first embodiment the speed Vr is determined from a global positioning system (GPS) or other positioning measurements. According to a second embodiment the speed Vr is estimated from the Doppler spread as described above. In general terms, multiple Doppler shifts are needed in order to determine the the Doppler spread. Hence, according to an embodiment multiple Doppler shifts are determined from the radio channel estimates, where the multiple Doppler shifts define the Doppler spread of the radio channel estimates. The speed Vr could then be determined based on the Doppler spread.

There could be different ways to determine the Doppler speed Vd. According to some aspects the Doppler speed Vd for the strongest path is selected. That is, each of the multiple Doppler shifts could correspond to a path along which the radio waves are transmitted. It is assumed that the strongest path corresponds to the strongest Doppler shift. Hence, according to an embodiment the radial velocity (which defines the Doppler speed Vd) is based on a strongest one of the multiple Doppler shifts. Further, a candidate angle of arrival or departure could be estimated for each of the at least two of the multiple Doppler shifts, and the angle α of arrival or departure could be estimated based on the candidate angles of arrival or departure. Alternatively, in some aspects more than one strongest path can be identified including the relative strength of the paths. Hence, according to an embodiment the radial velocity is based on relative strengths of at least two of the multiple Doppler shifts. The herein disclosed method for selecting beam direction is thus not limited to selecting a single direction but can be expanded approaching eigenvalue beamforming.

The orientation of the antenna array of the radio communications device <NUM> may not be aligned with the direction of travel of the radio communications device <NUM>, which could be needed to take into account for when selecting the beam direction based on the angle α of arrival or departure. The radio communications device <NUM> could therefore be assumed to have a structured antenna configuration, such as a linear array of antenna elements, to allow easily determined relations between beam direction and antenna element phase shifts for beam forming. In relation thereto it in some embodiments therefore is assumed that the relation between the angle of arrival and the antenna array orientation is known. Such a relation is already available when using the GPS and by means of existing sensors in common radio communications devices <NUM>, such as so-called smartphones. The ambiguity in direction gives two alternative directions in some environments, such as a city environment where reflections mainly will appear from the sides and not from above or below. In more detail, an angle relative to the movement of the radio communications device <NUM> reduces the ambiguity in direction to two ambiguous alternatives instead of a "cone" of directions in three dimensions. One method to resolve the ambiguity in direction is to test both these alternatives, but there are also methods to resolve the ambiguity that involve utilizing e.g. movement of the radio communications device <NUM> in multiple directions over time, i.e. by testing which of the ambiguous direction that stays the same when changing the direction of movement in order to resolve the ambiguity in direction.

<FIG> schematically illustrates, in terms of a number of functional units, the components of a radio communications device <NUM> according to an embodiment. Processing circuitry <NUM> is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product <NUM> (as in <FIG>), e.g. in the form of a storage medium <NUM>. The processing circuitry <NUM> may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, the processing circuitry <NUM> is configured to cause the radio communications device <NUM> to perform a set of operations, or steps, S102-S110, as disclosed above. For example, the storage medium <NUM> may store the set of operations, and the processing circuitry <NUM> may be configured to retrieve the set of operations from the storage medium <NUM> to cause the radio communications device <NUM> to perform the set of operations.

Thus the processing circuitry <NUM> is thereby arranged to execute methods as herein disclosed. The radio communications device <NUM> may further comprise a communications interface <NUM> at least configured for communications with at least one other radio communications device <NUM>. As such the communications interface <NUM> may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry <NUM> controls the general operation of the radio communications device <NUM> e.g. by sending data and control signals to the communications interface <NUM> and the storage medium <NUM>, by receiving data and reports from the communications interface <NUM>, and by retrieving data and instructions from the storage medium <NUM>. Other components, as well as the related functionality, of the radio communications device <NUM> are omitted in order not to obscure the concepts presented herein.

<FIG> schematically illustrates, in terms of a number of functional modules, the components of a radio communications device <NUM> according to an embodiment. The radio communications device <NUM> of <FIG> comprises a number of functional modules; an obtain module 210a configured to perform step S102, a determine module 210b configured to perform step S104, an estimate module 210c configured to perform step S106, and a select module 210d configured to perform step S108. The radio communications device <NUM> of <FIG> may further comprises a number of optional functional modules, such as any of a select module 210e configured to perform step S110, a determine module 210f configured to perform step S106a, a determine module <NUM> configured to perform step S106b, and an estimate module <NUM> configured to perform step S106c. In general terms, each functional module 210a-<NUM> may in one embodiment be implemented only in hardware or and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium <NUM> which when run on the processing circuitry makes the radio communications device <NUM> perform the corresponding steps mentioned above in conjunction with <FIG>. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 210a-<NUM> may be implemented by the processing circuitry <NUM>, possibly in cooperation with functional units <NUM> and/or <NUM>. The processing circuitry <NUM> may thus be configured to from the storage medium <NUM> fetch instructions as provided by a functional module 210a-<NUM> and to execute these instructions, thereby performing any steps as disclosed herein.

Claim 1:
A method for selecting a beam direction for a radio communications device (<NUM>), the method being performed by the radio communications device (<NUM>), the method comprising:
obtaining (S102) radio channel estimates of a radio channel on which radio waves have been transmitted between the radio communications device (<NUM>) and another radio communications device (<NUM>) at an angle of arrival and an angle of departure;
determining (S104) multiple Doppler shifts from the radio channel estimates, the multiple Doppler shifts defining a Doppler spread of the radio channel estimates;
estimating (S106) the angle of arrival and the angle of departure of the radio waves based on the multiple Doppler shifts, wherein the estimation comprises:
determining (S106a) speed, Vr, of the radio communications device (<NUM>) from the Doppler spread;
determining (S106b) a radial velocity based on the Doppler spread, the radial velocity defining a Doppler speed, Vd; and
estimating (S106c) the angle α of arrival and the angle of departure as <MAT> and
selecting (S108) the beam direction for a signal to be transmitted between the radio communications device (<NUM>) and said another radio communications device (<NUM>) over the radio channel according to the estimated angle of arrival and the estimated angle of departure.