EP4197113A1

EP4197113A1 - Multi-antenna wireless transmitter and method with transmission power control

Info

Publication number: EP4197113A1
Application number: EP20820825.6A
Authority: EP
Inventors: Avi WEITZMAN; Doron Ezri; Michael Kadichevitz; Ezer Melzer
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2020-12-04
Filing date: 2020-12-04
Publication date: 2023-06-21
Also published as: WO2022117204A1

Abstract

A wireless transmitter (610) configured to communicate with a wireless receiver (620) is disclosed. The wireless transmitter (610) comprises an array of antennas (611 a-n), wherein each antenna (611a-n) is arranged at a respective position of the array and comprises one or more antenna properties, including a radiation pattern. Moreover, the wireless transmitter (610) comprises a processing circuitry (613) configured to operate the array of antennas (611a-n) with a precoding configuration for a current transmission of the wireless transmitter (610). The processing circuitry (613) is further configured to adjust a transmit power for the current transmission of the wireless transmitter (610) and to determine the transmit power back-off for the current transmission based on information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas (611an).

Description

Multi-antenna wireless transmitter and method with transmission power control

TECHNICAL FIELD

The present disclosure relates to wireless communications. More specifically, the present disclosure relates to a multi-antenna wireless transmitter, such as a multi-antenna base station, and method with transmission power control.

BACKGROUND

IEEE-802.11 -based WLANs have become popular at an unprecedented rate. WLAN supports a variety of data transfer modes including (but not only) file transfer, emails, web browsing and real-time applications such as audio and video applications. For efficiently supporting high throughputs, the evolving IEEE 802.11 standards specify several transmission (TX) schemes that can be used by a wireless transmitter. Particularly useful for increasing the link throughput are TX schemes which deploy multiple TX antennas (some, but not all, also requiring multiple RX antennas on the receiver side, i.e. the wireless receiver), which are so called MIMO modes. Multiple TX antennas, often with each antenna being accompanied by a dedicated TX processing chain including a Power Amplifier (PA), can be utilized in different advantageous ways, such as spatial TX diversity for improving the link reliability and performance, beamforming (BF), i.e. focusing the radiated power in the direction(s) of target receiver(s) (and/or suppressing it in undesirable directions, for reducing unwanted interference to non-targeted receivers), and/or spatial multiplexing (SM), i.e. sending multiple data streams simultaneously over the same timefrequency resources, either to the same receiver or to different ones.

Regardless of the deployed TX scheme, the wireless transmitter generally must comply with various (regulatory or standardized) constraints, like putting an upper bound on the total radiation power and its directivity. The latter, specifically, is characterized by the so- called Equivalent/Effective Isotropically Radiated Power (EIRP), i.e., the hypothetical power (usually measured in dBm) that would have to be radiated by a single isotropic antenna to give the same ("equivalent") signal strength as produced by the actual TX antenna array in the direction of its strongest beam. Such restrictions, particularly the one limiting the EIRP, may work against the very nature of the mechanism by which the multiantenna TX scheme attempts to provide gain for the link performance. Specifically, in the BF mode the wireless transmitter attempts to focus the total allowed radiated power onto a target receiver, in order to increase the SNR it experiences and thus improve the link performance. However, due to the EIRP constraint the wireless transmitter must take some output power back-off (OBO), which in turn reduces the RX SNR. Thus, a multiantenna wireless transmitter may be faced with the problem that activating a BF/precoding MIMO mode with a certain OBO, needed for complying with the EIRP requirement, might actually not provide any gain to the link performance.

Moreover, the design (and thus implementation) of the IEEE 802.11 WLAN standards calls for a separation of the responsibilities over different system functions between the so-called MAC and PHY layers in a wireless transmitter in compliance therewith.

By common practice, the MAC layer of a wireless transmitter is responsible for setting the TX power (or, equivalently, the PA OBO) at the analog-RF level, as well as for selecting the MIMO mode activated in the next frame to be transmitted (in particular, the number of spatial streams and whether or not BF (or precoding) is invoked is decided by the MAC layer). As already described above, the selected TX power level should be such that the total radiated power as well as EIRP (and occasionally EVM) constraints are complied with.

The PHY layer, following the MAC’S instruction regarding the selected mode of operation, prepares the baseband (power-normalized) signals to be eventually mapped (post-PA) onto the TX antennas of the wireless transmitter. In other words, once the MAC layer selects usage of precoding, it is the responsibility of the PHY layer to set the specific (per- subcarrier) precoding weights, which may be computed based on Channel State Information (CSI), e.g. compressed 7-matrix feedback provided earlier by the target wireless receiver(s).

Consequently, the technical problem which arises - especially in the context of a wireless transmitter in compliance with the 802.11 WLAN standard - is to what power the MAC layer should adjust, i.e. control the TX power for a given MIMO TX mode. As already described above, for optimizing the link performance the TX power should be as large as possible, while at the same time not violating the total power constraints and in particular the EIRP constraints. SUMMARY

It is an objective of the present disclosure to provide a multi-antenna wireless transmitter and method with improved transmission power control (TPC).

The foregoing and other objectives are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

According to a first aspect a wireless transmitter is configured to communicate with a wireless receiver. The wireless transmitter comprises an array of antennas, wherein each antenna is arranged at a respective position of the array and comprises one or more antenna properties, including a directionally and polarization dependent radiation pattern.

The wireless transmitter comprises a processing circuitry configured to operate the array of antennas with an adjustable precoding configuration or setting (herein also referred to as beamforming configuration or setting) for adjusting the relative gains and phases of digital signals mapped onto the antennas of the array of antennas for a current transmission of the wireless transmitter.

The processing circuitry is further configured to adjust, i.e. control a transmit power for the current transmission of the wireless transmitter to an allowed upper transmit power limit reduced by a transmit power back-off for the current transmission. The allowed upper transmit power limit may be an upper transmit power limit defined by a standard, in particular the IEEE 802.11 WLAN standard and/or by regulatory bodies such as the FCC or ETSI.

The processing circuitry is further configured to determine the transmit power back-off for the current transmission based on information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas.

Thus, a wireless transmitter is provided allowing for an improved control of the transmission power.

In a further possible implementation form of the first aspect, the wireless transmitter further comprises one or more power amplifiers for driving the array of antennas, wherein the transmit power back-off is associated with an output power back-off of the one or more power amplifiers. In a further possible implementation form of the first aspect, the wireless transmitter further comprises a memory, wherein the memory is configured to store information about the one or more antenna properties of the antennas of the array of antennas, including a representation, in particular a quantized representation of the radiation pattern of one or more antennas of the array of antennas, for instance, in the form of a look-up-table.

In a further possible implementation form of the first aspect, the one or more antenna properties of each antenna of the array of antennas comprise a directionally dependent first radiation pattern for a first polarization direction and a second radiation pattern for a second polarization direction orthogonal to the first polarization direction.

In a further possible implementation form of the first aspect, the processing circuitry is further configured to determine the transmit power back-off for the current transmission using a generalized steering vector <p) defined by the following equation: wherein: p denotes a polarization index,

K denotes the wave number, denotes the elevation or zenith angle, p denotes the azimuth angle, r denotes a unit direction vector corresponding to the elevation angle and the azimuth angle p, i.e. a unit direction vector with the three-dimensional Cartesian components (sin 0 cos <p , sin 0 sin <p , cos 0) ,

Xj denotes the position vector of the /-th antenna of the array of N antennas, and g^(p) (9, p) denotes a normalized radiation pattern of the /-th antenna of the array of antennas as a function of direction.

In a further possible implementation form of the first aspect, the processing circuitry is further configured to determine the transmit power back-off for the current transmission based on a maximum g™^x of a total gain g_arr(r) of the array of antennas over all directions r.

In a further possible implementation form of the first aspect, the processing circuitry is further configured to determine the total gain g_arr(r) of the array of antennas as a norm of an expectation value of an array factor of the array of antennas, wherein the processing circuitry is configured to determine the expectation value over an ensemble of excitation signals representing a signal for the current transmission. For instance, in commonly used modes of operation of a WLAN 802.11 transmitter, the excitation signals are multi-carrier (OFDM or OFDMA modulated) signals, where each frequency subcarrier carries a stream (or possibly multiple spatial streams) of independent and identically distributed QAM symbols with the expectation value of the total power of the QAM symbols normalized to a constant value for all subcarriers, and wherein the (possibly multiple) spatial streams undergo linear mapping onto the multiple TX chains via BF/precoding.

In a further possible implementation form of the first aspect, the processing circuitry is further configured to determine the total gain of the array of antennas based on the following equation: wherein:

K denotes the number of frequency subcarriers used for the current transmission, N_ss(k) denotes the number of spatial streams mapped onto the -th frequency subcarrier for the current transmission, denote two relatively orthogonal polarization directions, and denotes vectors of precoding weights defined by the precoding configuration of the -th frequency subcarrier for the current transmission.

In a further possible implementation form of the first aspect, the processing circuitry is further configured to determine the transmit power back-off for the current transmission as the total gain maximum times an adjustment factor.

In a further possible implementation form of the first aspect, the processing circuitry is further configured to determine the transmit power back-off for the current transmission by reducing the total gain maximum times the adjustment factor by a further factor depending on the number of antennas of the array of antennas.

In a further possible implementation form of the first aspect, the precoding configuration is defined by one or more precoding matrices (or vectors) and/or one or more precoding weights.

In a further possible implementation form of the first aspect, the processing circuitry is configured to determine the precoding configuration for the current transmission based on channel state information about the communication channel between the wireless transmitter and a wireless receiver. In a further possible implementation form of the first aspect, the processing circuitry comprises a PHY layer portion and a MAC layer portion, wherein the PHY layer portion is configured to operate the array of antennas with the precoding configuration for the current transmission of the wireless transmitter, and wherein the MAC layer portion is configured to adjust the transmit power for the current transmission of the wireless transmitter to the allowed upper transmit power limit reduced by the transmit power backoff for the current transmission. The MAC layer portion is further configured to determine the transmit power back-off for the current transmission based on the information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas.

According to a second aspect a method of operating a wireless transmitter configured to communicate with a wireless receiver is provided. The wireless transmitter comprises an array of antennas, wherein each antenna is arranged at a respective position of the array and comprises one or more antenna properties, including a directionally and polarization dependent radiation pattern, and a processing circuitry. The method according to the second aspect comprises the following steps by the processing circuitry: operating the array of antennas with an adjustable precoding configuration or setting (herein also referred to as beamforming configuration or setting) for a current transmission of the wireless transmitter; determining a transmit power back-off for the current transmission based on information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas; and adjusting, i.e. controlling a transmit power for the current transmission of the wireless transmitter to an allowed upper transmit power limit reduced by the transmit power backoff for the current transmission.

In a further possible implementation form of the second aspect, the wireless transmitter further comprises one or more power amplifiers for driving the array of antennas and wherein the transmit power back-off corresponds to an output power back-off of the one or more power amplifiers.

In a further possible implementation form of the second aspect, the wireless transmitter further comprises a memory, wherein the method further comprises retrieving from the memory information about the one or more antenna properties of the antennas of the array of antennas, including a representation, in particular a quantized representation, of the radiation pattern of one or more antennas of the array of antennas, for instance, in the form of a look-up-table.

In a further possible implementation form of the second aspect, the one or more antenna properties of each antenna of the array of antennas comprises a directionally dependent first radiation pattern for a first polarization direction and a second radiation pattern for a second polarization direction orthogonal to the first polarization direction.

In a further possible implementation form of the second aspect, the method comprises determining the transmit power back-off for the current transmission using a generalized steering vector ^ defined by the following equation: wherein: denotes a polarization index, denotes the wave number, denotes the elevation or zenith angle, denotes the azimuth angle, denotes a unit direction vector corresponding to the elevation angle and the azimuth angle p ( unit direction vector having three-dimensional Cartesian components ,

Xj denotes the position vector of the /-th antenna of the array of N antennas, and denotes a normalized radiation pattern of the /-th antenna of the array of antennas as a function of direction.

In a further possible implementation form of the second aspect, the method comprises determining the transmit power back-off for the current transmission based on a maximum of a total gain of the array of antennas over all directions r.

In a further possible implementation form of the second aspect, the method comprises determining the total gain of the array of antennas as a norm of an expectation value of an array factor of the array of antennas, wherein the method further comprises determining the expectation value over an ensemble of excitation signals representing a signal for the current transmission.

In a further possible implementation form of the second aspect, the method comprises determining the total gain of the array of antennas based on the following equation: wherein: denotes the number of frequency subcarriers used for the current transmission, denotes the number of spatial streams mapped onto the -th frequency subcarrier for the current transmission, denote two relatively orthogonal polarization directions, and denotes vectors of precoding weights defined by the precoding configuration of the frequency subcarrier for the current transmission.

In a further possible implementation form of the second aspect, the method comprises determining the transmit power back-off for the current transmission as the total gain maximum times an adjustment factor.

In a further possible implementation form of the second aspect, the method comprises determining the transmit power back-off for the current transmission by reducing the total gain maximum times the adjustment factor by a further factor depending on the number of antennas of the array of antennas.

In a further possible implementation form of the second aspect, wherein the precoding configuration is defined by one or more precoding matrices (or vectors) and/or one or more precoding weights.

In a further possible implementation form of the second aspect, the method comprises determining the precoding configuration for the current transmission based on channel state information about the communication channel between the wireless transmitter and a wireless receiver.

According to a third aspect a computer program product is provided, comprising program code which causes a computer or a processor to perform the method according to the second aspect, when the program code is executed by the computer or the processor.

Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments of the present disclosure are described in more detail with reference to the attached figures and drawings, in which:

Fig. 1 shows an example of the azimuthal radiated power distribution for a wireless transmitter;

Fig. 2 shows a further example of the azimuthal radiated power distribution for a wireless transmitter;

Fig. 3 shows a further example of the azimuthal radiated power distribution for a wireless transmitter;

Fig. 4 shows a further example of the azimuthal radiated power distribution for a wireless transmitter;

Fig. 5 shows a further example of the azimuthal radiated power distribution for a wireless transmitter;

Fig. 6a shows a schematic diagram of a wireless communication system, including a wireless transmitter according to an embodiment;

Fig. 6b shows a schematic diagram illustrating further details of a wireless transmitter according to an embodiment;

Fig. 7 shows a diagram illustrating the definition of a steering angle;

Fig. 8 shows an example of the azimuthal radiated power distribution of a single antenna element of a wireless transmitter; and

Fig. 9 is a flow diagram illustrating a wireless transmission method according to an embodiment.

In the following, identical reference signs refer to identical or at least functionally equivalent features. DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments of the present disclosure or specific aspects in which embodiments of the present disclosure may be used. It is understood that embodiments of the present disclosure may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.

For instance, it is to be understood that a disclosure in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise.

Before describing different embodiments in more detail, in the following some technical background as well as terminology concerning wireless transmitters, in particular wireless transmitters in accordance with the IEEE 802.11 WLAN standard will be introduced.

For the sake of ensuring that the max EIRP constraint described above is not violated, in a conventional wireless transmitter, when selecting a precoding mode for the next transmission, the MAC adds some crude extra OBO (depending essentially only on the number of TX antennas, denoted #Tx, and their gains, or rather the maximal gain thereof). More specifically, when operating in a precoding MIMO mode, the MAC layer of a conventional wireless transmitter reduces the (per PA/antenna) TX power TxPower [dBm], setting it according to the following equation 1: Equation 1 : TxPower = MaxTotEirpPower — 10 log₁₀(#Tx) — BfOBO .

The subtraction of 10 log₁₀(#Tx) from MaxTotEirpPower in the equation above expresses an equal division of the allowed max total power between the #Tx PAs deployed, and BfOBO is an extra power backoff taken due to antenna gains and the activation of precoding. The conventional approach is to set BfOBO = MaxGain, where MaxGain = MaxAntGain + MaxBfGain, where MaxAntGain denotes the maximal antenna gain (out of the #Tx elements in the TX antenna array). Usually, MaxBfGain is set to a fixed value, e.g. equal to 10 log₁₀(#T%) - 8. Usually, the parameter 8 (set, for instance, within the range 0~2 dB when #Tx = 8) is used as a kind of margin taken from the theoretical, i.e. ideal max BF gain 10 log₁₀(#T%) achievable via perfect constructive superposition of fully-correlated phased-aligned signals emanating from an array of isotropic antennas (each antenna providing no gain by itself). Thus, as will be appreciated, a conventional wireless transmitter will often choose a value for BfOBO that highly overestimates the actual total EIRP gain (which we will henceforth denote as DynBfGain) achieved in practice by the activation of the specific precoding weights and a given utilized antenna array. Consequently, since often DynBfGain < MaxGain, the transmission power is unnecessarily set too low in a conventional wireless transmitter, leading paradoxically to a situation where employing precoding may incur a link performance loss rather than a link performance gain, e.g. relative to using the default (open loop MIMO operation mode of 802.11) Cyclic Shift Diversity (CSD) TX scheme which entails no BF gain. This often sub-optimal performance of a conventional wireless transmitter will be further illustrated in the sequel, based on two-dimensional (2D) radiation patterns (in azimuth only; illustrated in figures 1 to 5) for a plurality of examples for a conventional wireless transmitter employing beamforming with an antenna array with 2 Tx-antennas (with an exemplary 0.5A spacing between the two antennas).

Figure 1 illustrates the distribution of the radiated power from a wireless transmitter equipped with two isotropically radiating antennas (AntGain = 0 dB) of the same polarization. As can be taken from figure 1, SVD-based BF in a 1-tap channel (i.e. flat in frequency, e.g. IEEE 802.11 TGn A-NLOS channel model) leads to the same BF vector in all OFDM subcarriers, which in turn yields DynBfGain = MaxGain = 0 + 101og₁₀2 = 3 dB, which is the maximal theoretically expected value (in the target direction of departure towards the wireless receiver, no matter where it is located). Thus, in this case/scenario the naive (theoretical) max BF gain can indeed be achieved "in practice", and thus a conventional wireless transmitter which sets its PAs’ OBO based on the max BF gain would not lead to unnecessary reduction in the receiver’s SNR, thus not degrading the link performance.

Figure 2 illustrates another distribution of the radiated power from a wireless transmitter equipped with two isotropically radiating antennas of the same polarization. As can be taken from figure 2, SVD-based BF in a multipath channel (i.e. frequency selective, e.g. IEEE 802.11 TGn B-NLOS channel model) leads to BF vectors which vary in frequency between different OFDM subcarriers, which in turn yields DynBfGain < MaxGain = 0 + 3 = 3 dB. Thus, in this case/scenario, the naively expected max BF gain is usually not reached, and a conventional wireless transmitter which sets its PAs’ OBO based on the max BF gain would lead to unnecessary degradation in the receiver’s SNR and the link performance.

Figure 3 illustrates the distribution of the radiated power from a wireless transmitter equipped with two non-isotropically radiating antennas of the same polarization (for both antennas AntGain = 2.1 dB, but with boresights pointing at different directions). As can be taken from figure 3, SVD-based BF, even in a flat channel (like IEEE 802.11 TGn A-NLOS channel model), yields DynBfGain < MaxGain = 2.1 + 3 = 5.1 dB. Thus, also in this case/scenario, the naively expected max gain BF is usually not attained, and thus a conventional wireless transmitter would lead to unnecessary loss in the link performance.

Figure 4 illustrates the distribution of the radiated power from a wireless transmitter equipped with two isotropically radiating antennas of orthogonal polarization (i.e. a crosspolarized pair of antennas). As can be taken from figure 4, BF of any kind cannot lead to any increase in EIRP, since beams carrying orthogonally polarized signals never combine coherently. Thus, DynBfGain = 0, which is always 3 dB below the naively expected max BF gain MaxGain, and thus a conventional wireless transmitter which sets its PAs’ OBO based on the latter MaxGain would lead to degradation in the receiver’s SNR by at least 3 dB, and hence would lose link performance for no good reason.

Figure 5 illustrates the distribution of the radiated power from a wireless transmitter equipped with two isotropically radiating antennas of the same polarization when two precoded spatial streams are being transmitted. As can be taken from figure 5, when the number of spatial streams is equal to the number of TX antennas, precoding cannot lead to any increase in EIRP, since beam components carrying uncorrelated signals do not combine coherently. Thus, DynBfGain = 0, the naively expected max BF gain MaxGain is not reached, and a conventional wireless transmitter would lose link performance due to setting the PA OBO unnecessarily too high. Figure 6a is a schematic diagram of a wireless communication system 600, including a wireless transmitter 610 according to an embodiment. The wireless transmitter is configured to communicate with one or more wireless receivers 620. In an embodiment, the wireless transmitter 610 may be a wireless base station 610 configured to communicate with one or more wireless terminals 620, such as mobile phones, tablet computers, laptop computers and the like. In an embodiment, the wireless transmitter 610 may be configured to communicate with the one or more wireless terminals 620 in accordance with a standard, in particular the IEEE 802.11 WLAN standard.

The wireless transmitter 610 comprises an array of antennas 611a-n. By way of example, in the embodiment shown in figure 6a the array comprises two antennas 611a, 611b, while in figure 7 an array with more than two antennas 611a-n is illustrated. Each antenna 611a-n of the array is arranged at a respective physical position, i.e. location of the array (which is usually fixed for at least a current transmission session) and comprises, i.e. is defined by one or more antenna properties, in particular a directionally and polarization dependent radiation pattern of the respective antenna 611a-n. In an embodiment, the one or more antenna properties of each antenna of the array of antennas 611a-n may comprise a first radiation pattern for a first polarization direction and a second radiation pattern for a second polarization direction orthogonal to the first polarization direction.

As will be described in more detail below, by means of the array of antennas 611 a-n the wireless transmitter 610 is configured to communicate with the one or more wireless receivers 620 using a precoding or beamforming communication scheme, as defined, for instance, by a standard, in particular the IEEE 802.11 WLAN standard.

As can be taken from figure 6a, the wireless transmitter 610 further comprises a processing circuitry 613 configured to operate the array of antennas 611 a-n with an adjustable precoding configuration or setting (herein also referred to as beamforming configuration or setting) for adjusting the relative gains and phases of digital signals mapped onto the antennas of the array of antennas 611 a-n for a current transmission of the wireless transmitter 610. As illustrated in figure 6a, the wireless transmitter 610 may further comprise one or more power amplifiers 615 for each one of the antennas 611 a-n for amplifying the respective transmission signal for each antenna 611 a-n. In an embodiment, the transmit power back-off corresponds to an output power back-off of the one or more power amplifiers 615.

Moreover, as illustrated in figure 6a, the wireless transmitter 610 may further comprise a memory 617, wherein the memory 617 is configured to store information about the one or more antenna properties of the antennas of the array of antennas 611 a-n, including a representation, in particular a quantized representation of the radiation pattern of one or more antennas of the array of antennas 611 a-n, for instance, in the form of a look-up- table. Further embodiments of the memory 617 will be described below.

The processing circuitry 613 of the wireless transmitter 610 is further configured to adjust, i.e. control a transmit power for the current transmission of the wireless transmitter 610 to an allowed upper transmit power limit reduced by a transmit power back-off for the current transmission. The allowed upper transmit power limit may be an upper transmit power limit defined by a standard, in particular the IEEE 802.11 WLAN standard.

Moreover, as will be described in more detail below, the processing circuitry 613 of the wireless transmitter 610 is further configured to determine the transmit power back-off for the current transmission based on information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas 611 a-n.

Thus, embodiments disclosed herein are based on the idea that when preparing for a next transmission - for which a precoding mode has been selected - the transmit power backoff (also referred to as OBO, i.e. output power back-off and denoted by DynBfOBO) is controlled on the basis of the expected DynBfGain. As already described above and as will be described in more detail below, the processing circuitry 613 of the wireless transmitter 610 may be configured to dynamically (i.e. for each new transmission with a different precoding setting) determine DynBfGain and, thus, the transmit power back-off DynBfOBO based on two main inputs, namely (i) the information about the one or more antenna properties of the antennas of the array of antennas 611 a-n stored in the memory 617 (such as the relative locations of the antennas 611 a-n, their gains and radiation patterns, including polarization properties) and (ii) information about the precoding configuration for the current transmission. In an embodiment, the information about the precoding configuration may comprise the actual precoders P to be applied by the wireless transmitter 610 upon transmission. In case a multi-carrier waveform (such as OFDM-based) is being deployed, a precoder P may be used for each subcarrier. Herein the general term "precoder" is used as referring either to BF weight vectors (used in the case of transmission of a single spatial stream) or to precoding matrices (used in the multi-stream case). Having computed DynBfGain, the processing circuitry 613 of the wireless transmitter 610 according to an embodiment may set the required DynBfOBO for complying with the total TX power and EIRP (and possibly EVM), leading to

Equation 2: TxPower = MaxTotEirp Power — 10 log₁₀(#Tx) — DynBfOBO.

In an embodiment, the processing circuitry 613 of the wireless transmitter 610 may choose DynBfOBO to be equal to the determined DynBfGain. In further embodiments, the processing circuitry 613 of the wireless transmitter 610 may be configured to determine DynBfOBO based on DynBfGain in a different way. For instance, in an embodiment the processing circuitry 613 of the wireless transmitter 610 may be configured to determine DynBfOBO as DynBfGain + 8, with a fixed 8 > 0, such as 8 = 1.5 dB. In a further embodiment, the processing circuitry 613 of the wireless transmitter 610 may be configured to determine DynBfOBO as DynBfGain multiplied by a factor larger than 1. As will be appreciated, the processing circuitry 613 of the wireless transmitter 610 may be configured to determine DynBfOBO based on DynBfGain using a function DynBfOBO = f (DynBfGain) such that DynBfOBO > DynBfGain.

Though bounded by the difference MaxGain - DynBfGain, the increase in transmission power provided by embodiments disclosed herein turns out to be significant in many cases/scenarios, especially for large TX arrays (namely when #Tx, i.e. the number of antennas 611a-n becomes large). The resulting potential increase in TX power offered by embodiments disclosed herein translates into an increase in the target RX SNR, which can be exploited in various ways to improve the system performance with respect, for instance, to robustness, reliability, throughput, capacity, coverage, and/or the quality of service experienced by a user.

As illustrated in figure 6b, the processing circuitry 613 may implement a back-off algorithm 613b for determining an optimal transmission power back-off. In the embodiment shown in figure 6b the processing circuitry 613 of the wireless transmitter 610 comprises a PHY layer portion 613a and a MAC layer portion 613b, 613c, wherein the PHY layer portion 613a is configured to operate the array of antennas 611a-n with the precoding configuration for the current transmission of the wireless transmitter 610. The MAC layer portion 613c is configured to adjust the transmit power for the current transmission of the wireless transmitter 610 to the allowed upper transmit power limit reduced by the transmit power back-off for the current transmission. The MAC layer portion 613b implements the back-off algorithm 613b and is, thus, configured to determine the transmit power back-off for the current transmission based on the information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas 611a-n.

More specifically, in an embodiment, the processing circuitry 613, in particular the back-off algorithm 613b may be configured to dynamically compute (i.e. for every transmission opportunity) the expected actual BF gain DynBfGain that would result in from the activation of specific precoder(s) by the PHY layer 613a (for both Single-User and Multiuser scenarios), when deployed with the given array of antennas 611a-n (described by the information stored in the memory 617, for instance, in a database 617a about the position and the one or more antenna properties of each antenna of the array of antennas 611a-n, in particular information about the radiation pattern of each antenna 611a-n). As illustrated in figure 6b, in an embodiment, the output of the back-off algorithm 613b implemented by the processing circuitry 613 is provided to the MAC layer portion 613c implemented by the processing circuitry 613, allowing the MAC layer portion 613c to make "better-informed" optimal decisions regarding the scheduling and especially TX Power Control (TPC) of future transmissions.

In the following some further embodiments of the processing circuitry 613 of the wireless transmitter 610 for implementing the back-off algorithm 613b for determining the transmit power back-off for the current transmission will be described in more detail.

In an embodiment, the processing circuitry 613 makes use of a Generalized Steering Vector (GSV) of the array of antennas 611 a-n, which will be defined in the following. Given an antenna array of x antennas 611 a-n positioned in 3D space at points specified by the coordinates (as illustrated in figure 7), the /V-dimensional steering vector a(K) is usually defined as:

Equation 3 where denotes the 3-dimensional wave-vector, pointing in an arbitrary direction of departure (DoD) r (normalized such that ||r|| = 1) with the wave-number

(where A = wavelength, f = frequency, c = speed of light).

In an embodiment, the processing circuitry 613, in particular the back-off algorithm 613b makes use of a generalized version of the steering vector defined by equation 3 above. This GSV further takes into account the radiation patterns {g i^p)(r), ...,g^(r)) of the plurality of antennas 611 a-n of the antenna array, where r is typically parametrized using polar (elevation and azimuth) coordinates namely in the form , and indicates the polarization direction; customarily, or namely given in terms of unit vectors in the two (orthogonal) directions, tangential to the corresponding polar coordinates.

More specifically, in an embodiment, the processing circuitry 613, in particular the back-off algorithm 613b for determining the transmit power back-off for the current transmission makes use of the defined by the following equation:

Equation 4:

As will be appreciated, this /V-dimensional vector (parametrized by K,p and (0, <p)) may be considered as the LOS MISO channel (up to an insignificant overall complex scaling factor) from the TX antenna array to a(n imaginary) p-polarized RX antenna probe positioned in the direction r(0, <p) away from the array (in the far field region).

In an embodiment, the radiation pattern g g p of each antenna 611a-n may be a dimensionless nonnegative function, representing the relative amplitude gain (in a given direction r) with respect to that of an ideal isotropic (omni, purely p-polarized) antenna, i.e. an antenna whose radiation pattern is given As such, in an embodiment, the radiation pattern of each antenna 611a-n may be normalized in the following way:

Equation 5: where

In an embodiment, the convention with respect to the 3D coordinate system may be chosen such that the z-axis s pointing upwards. For an embodiment, where the wireless transmitter 610 is configured as a wireless base station or access point 610 to be mounted on the ceiling of a room, this implies that the desirable "boresight" direction(s) of maximal antenna gain should point at 0 in the range -115-155° with uniform coverage of the azimuth angle <p. An example for the three azimuthal patterns of a horizontally-polarized (^-dominant) antenna, all evaluated at 0=147°, are shown (in dB scale) in Figure 7.

Based on the ^ defined above (which, in an embodiment, may be computed by the processing circuitry 613 once per a given transmitter/antenna-array and per carrier frequency the processing circuitry 613 may be configured to compute in a next stage a coherent radiation pattern resulting by superposition from the precoding in the following way.

Denoting by the /V-dimensional column vectors of the precoding matrix P(/c), which may be normalized by convention according to = N for all k (where is the number of spatial streams [being 1 in the BF case] to be transmitted over the /c^th subcarrier, and enumerates the subcarriers undergoing precoding within the channel bandwidth BWf the processing circuitry 613 may be configured to determine the total array gain ) in an arbitrary direction - and subsequently the resulting EIRP gain and th® final DynBfGain - on the basis of the following equations:

In other words, in an embodiment, the processing circuitry 613 of the wireless transmitter 610 is configured to determine the resulting EIRP gain ^as th® maximum of the total array gain over all directions As will be appreciated, in the expression for the array gain the coherent summation is implicit by the scalar/inner product between the vectors involved, namely w and a, in contrast to the incoherent combining implied by the explicit sums over polarizations, spatial streams, and frequency subcarriers. Equation 6 generally holds for BVF « f_c. In an embodiment, the processing circuitry 613 may be configured to make use of an equation that is valid for more general cases as well, which takes into account the dispersive nature of the GSV. For instance, in an embodiment, the processing circuitry 613 of the wireless transmitter 610 may be configured to use a variant of Equation 6, where the expression is replaced by and f denotes the frequency of the subcarrier. In a more general variant of Equation 6, the processing circuitry 613 is configured to determine the total gain of the array of antennas 611a-n as a norm of an expectation value of an Array Factor (AF) of the array of antennas 611a-n, wherein the processing circuitry 613 is configured to determine the expectation value over an ensemble of excitation signals representing a signal for the current transmission. In an embodiment, the ensemble of excitation signals may be a multicarrier per-subcarrier- precoded multi-stream OFDM[A] (as used e.g. in Wi-Fi or LTE), where the QAM symbols q_s(k) per subcarrier and per stream are all uncorrelated (i.i.d.) and normalized, namely obeying the following statistics (expectation value):

As already described above and illustrated in figure 6b, the back-off algorithm 613b implemented by the processing circuitry 613 has two main inputs ingredients for computing the desired output, i.e. DynBfGain and, thus, the transmission power back-off.

As already described above, in an embodiment, the information about the one or more antenna properties of the antennas of the array of antennas 611 a-n, including a representation of the radiation pattern of the antennas 611 a-n, may be stored in the database 617a operated in the memory 617 of the wireless transmitter 610. In an embodiment, the information may be represented in the database 617a by its raw data, consisting of the information per antenna 611 a-n of the array shown in the following table:

In the table above, each coordinate vector consists of a triplet of distances along the axes of some (Cartesian) coordinate system for 3-dimensional space, wherein the distances are measured from an origin which is placed somewhere in the vicinity of the antenna array. In order to save storage space in the database 617a, the origin can be conveniently chosen to be at the center of one of the antennas 611 a-n of the array, e.g. the first one , and then the positions, i.e. coordinates of the other antennas of the array are measured with respect thereto. The (nonzero) coordinates can be stored as triplets of real numbers, or some binary representation thereof, encoding the distances from the origin to some desirable accuracy (according to some finite resolution) in certain absolute units (like millimeters or inches), or possibly using other predefined units, such as the wavelength corresponding to some radio carrier frequency

The antenna patterns may be represented and stored as Look-Up Tables (LUTs) of possibly different formats in the database 617a, which are designed to enable some desired resolution (and thus level of accuracy) in the angular (spatial), gain and polarization “spaces”.

The 2-dimensional angular space is represented by the argument r, expressing all possible DoDs from the origin (in the same coordinate system used to express the coordinates x_t). In an embodiment, this space of all directions may be sampled using a finite discrete set of , e.g. by quantizing the elevation and azimuth angles (9, <p) and storing only the antenna gains evaluated for these angle pairs ( _d, p_d) ( , , , ), y g _d . The quantization resolution does neither need to be necessarily homogeneous (same for nor uniform. Furthermore, regions in angular space where the antenna gain is negligible, namely where , may be sparsely sampled (if at all), with negligible impact on the accuracy of the algorithm’s final output For instance, for representing antenna patterns of the antennas 611a-n of the wireless transmitter 610 implemented as a ceiling-mounted Wi-Fi AP 610, which radiates primarily in a downward direction, and when the coordinate system is such that the z-axis points in the direction of the ceiling, a useful quantization scheme is specified by letting the elevation angle 9 take the discrete values

2A0, ...,180° (in degrees), where 9₀ is some lowest value (e.g. 100°) and AS is some step size (e.g. 5°). Furthermore, in an embodiment, for each of the above 9 values the azimuth angle may take the values n ■ <p( ), where for 0,1, ..., |360°/A<p(e)J may be chosen, whereas for a single value for p, say may be chosen. In the example described above, where 9 100° AS 5° the total size of the set of discretized DoDs turns out to be D = 726.

The gain axis, along which the antenna pattern values are measured, may also be quantized. One way to achieve this axis quantization is to use a logarithmic scale (dB), so that the digitally stored values encode the discrete real values Q_&G = round where AG is some gain step and the round(') function rounds its argument to the nearest integer value. Regarding the “binary space” of polarizations, in case the antennas 611a-n of the array are designed such that each antenna has some dominant polarization (e.g. Horizontal = ^-dominant, with g^ f) g^ ) for DoDs r for which , or Vertical = dominant with for DoDs r for which , a possible efficient implementation is to store and use for computations just a single pattern for each antenna 611 a-n. For instance, in an embodiment, the database 617a may store just the dominantpolarization pattern or alternatively just the total polarization pattern g accompanied by a single-bit variable (per element) indicating the element’s dominant polarization, e.g.

In a further embodiment, the information about the location and the one or more antenna properties of the antennas of the array of antennas 611 a-n may be realized in terms of the associated with the array, which can be pre-computed (whether offline, or online by the processing circuitry 613) according to Equation 4 above based on the raw data/information described for the previous embodiments. More specifically, for any carrier frequency f_c desired to be used, the pair (due to the two polarization options p complex /V-dimensional vector sets can be pre-computed once (recall that K_C = and note that a^(0, <p) can be reused also for other carrier frequencies without incurring large inaccuracies as long as The angular directions for fc which the vectors a are computed and/or stored may be quantized in the same way as described for the previous embodiments above, leading to the finite discrete sets of vectors where d = 1,2, ...,£>. Likewise, the values of the (real and imaginary) components of the vectors may be quantized in a similar manner like the previously described quantization of the antenna patterns

As already described above, the information about the precoding configuration for the current transmission may comprise the set of precoding matrices P(/c) (degenerating into column vectors in the special case of single-stream BF), to be deployed per subcarrier. Often, in practice, when a OFDM- (or OFDMA-)based waveform is being used, the selected precoder P(/c) varies slowly along the frequency axis, represented here by the subcarrier index k. More specifically, the number of streams of P(/c)) is kept unchanged within frequency subbands (or Rlls, in the IEEE 802.11 terminology). Consequently, the averaging over subcarriers carried out in Equation 6 may be simplified by taking a diluted version thereof with an insignificant compromise in the accuracy of the final result. This dilution may be implemented by the processing circuitry 613 by replacing the operation in Equation where the subset of subcarrier indices {/c_1; k₂, ... , k_K>} c {1,2, ... , K induces a representative sampling, precoding-wise, of the full set of indices. In this way the full input {P(/c) | k = may be reduced to As an example, assuming the enumeration of the subcarriers is monotonic in frequency, namely the subcarrier dilution may be implemented by the processing circuitry via a simple decimation

As another example, especially relevant in the context of the wireless transmitter 610 implemented in accordance with the IEEE 802.11 WLAN standard, a wireless receiver 620 (i.e. "beamformee") sends a CSI report containing preferred precoding to the wireless receiver 610 (i.e. the "beamformer"). The preferred precoding feedback may be sent in the form of compressed V-matrices (e.g. based on Singular Value Decomposition (SVD) of the MIMO channel matrix estimated by the beamformee), which are provided one per group of N_g consecutive subcarriers, where, for instance, N_g = 1,2 or 4. The wireless transmitter 610 then may decompress the V-matrices, and generate from them a set of precoders P(/c). In the case of SU-MIMO precoding, whether in OFDM or OFDMA mode of operation, the precoders P(/c) are often constructed directly from columns of the fed back y-matrices (decompressed and possibly interpolated along the subcarrier indices). Therefore the corresponding input to the back-off algorithm 613b implemented by the processing circuitry 613 (as illustrated in figure 6b) may be diluted/decimated according to the parameter N_g, and in fact the compressed V-matrices may be viewed as representing succinctly the precoders while the decompression operation can be viewed as a preprocessing step which is part of the back-off algorithm 613b. If MU-MIMO precoding is deployed, at least in part of the spectrum, then the precoders P(/c) in the corresponding subcarriers may be computed jointly based on the reported V-matrices from several wireless receivers 620 (i.e. "beamformees").

Once the two inputs described above have been generated, the back-off algorithm 613b implemented by the processing circuitry 613 of the wireless transmitter 610 may determine the transmission power back off based on Equation 6 above. More specifically, given 1. the sets of (possibly quantized) complex /V-dimensional vectors and

2. the set of (possibly quantized or compressed) complex N with their /V-dimensional columns where I induces the desired dilution of the subcarrier indices; the back-off algorithm 613b implemented by the processing circuitry 613 of the wireless transmitter 610 may perform the following steps based on Equation 6:

0 (Initialization): S

1b1a2 (Accumulate):

1c (Maximize):

2 (End): DynBfGain [dB] = 20 log₁₀ S'EIRP(P)

As already described above, the processing circuitry 613 is further configured to use DynBfGain for determining DynBfOBO and subsequently controlling the final TX output power TxPower of the wireless transmitter 610.

Figure 9 is a flow diagram illustrating a method 900 for operating the wireless transmitter 610 configured to communicate with the one or more wireless receivers 620. As already described above, the wireless transmitter 610 comprises the array of antennas 611a-n, wherein each antenna is arranged at a respective position of the array and comprises one or more antenna properties, including a radiation pattern. The method 900 comprises the following steps performed by the processing circuitry 613 of the wireless transmitter 610: operating 901 the array of antennas 611a-n with a precoding configuration for a current transmission of the wireless transmitter 610; determining 903 a transmit power back-off for the current transmission based on information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas 611 a-n; and adjusting, i.e. controlling 905 a transmit power for the current transmission of the wireless transmitter 610 to an allowed upper transmit power limit reduced by the transmit power back-off for the current transmission.

Further features of the method 900 result directly from the structure and/or functionality of the wireless transmitter 610 as well as its different embodiments described above.

The person skilled in the art will understand that the "blocks" ("units") of the various figures (method and apparatus) represent or describe functionalities of embodiments of the present disclosure (rather than necessarily individual "units" in hardware or software) and thus describe equally functions or features of apparatus embodiments as well as method embodiments (unit = step).

In the several embodiments provided in the present application, it should be understood that the disclosed system, apparatus, and method may be implemented in other manners. For example, the described embodiment of an apparatus is merely exemplary. For example, the unit division is merely logical function division and may be another division in an actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented by using some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms.

The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one position, or may be distributed on a plurality of network units. Some or all of the units may be selected according to actual needs to achieve the objectives of the solutions of the embodiments.

In addition, functional units in the embodiments of the invention may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units are integrated into one unit.

Claims

1. A wireless transmitter (610) configured to communicate with a wireless receiver (620), wherein the wireless transmitter (610) comprises: an array of antennas (611a-n), wherein each antenna (611a-n) is arranged at a respective position of the array and comprises one or more antenna properties, including a radiation pattern; and a processing circuitry (613) configured to operate the array of antennas (611a-n) with a precoding configuration for a current transmission of the wireless transmitter (510), and wherein the processing circuitry (613) is further configured to adjust a transmit power for the current transmission of the wireless transmitter (610) to an allowed upper transmit power limit reduced by a transmit power back-off for the current transmission, wherein the processing circuitry (613) is further configured to determine the transmit power back-off for the current transmission based on information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas (611 a-n).

2. The wireless transmitter (610) of claim 1 , wherein the wireless transmitter (610) further comprises one or more power amplifiers (615) and wherein the transmit power back-off corresponds to an output power back-off of the one or more power amplifiers (615).

3. The wireless transmitter (610) of claim 1 or 2, wherein the wireless transmitter (610) further comprises a memory (617), wherein the memory (617) is configured to store information about the one or more antenna properties of the antennas of the array of antennas (611 a-n), including a representation of the radiation pattern of one or more antennas of the array of antennas (611 a-n).

4. The wireless transmitter (610) of any one of the preceding claims, wherein the one or more antenna properties of each antenna of the array of antennas (611 a-n) comprise a first radiation pattern for a first polarization direction and a second radiation pattern for a second polarization direction.

5. The wireless transmitter (610) of any one of the preceding claims, wherein the processing circuitry (613) is further configured to determine the transmit power back-off for the current transmission using a generalized steering vector based on the following equation: wherein: p denotes a polarization index,

K denotes the wave number, θ denotes the elevation angle, φ denotes the azimuth angle, r denotes a unit direction vector corresponding to the elevation angle θ and the azimuth angle φ , x_i denotes the position vector of the /-th antenna of the array of N antennas (611a-n), and g_i ^(p) (θ, φ ) denotes a normalized radiation pattern of the /-th antenna of the array of antennas (611a-n).

6. The wireless transmitter (610) of claim 5, wherein the processing circuitry (613) is further configured to determine the transmit power back-off for the current transmission based on a maximum of a total gain of the array of antennas (611a-n) over all directions r.

7. The wireless transmitter (610) of claim 6, wherein the processing circuitry (613) is further configured to determine the total gain of the array of antennas (611a-n) as a norm of an expectation value of an array factor of the array of antennas (611a-n), wherein the processing circuitry (613) is configured to determine the expectation value over an ensemble of excitation signals representing a signal for the current transmission.

8. The wireless transmitter (610) of claim 7, wherein the processing circuitry (613) is further configured to determine the total gain of the array of antennas (611a-n) based on the following equation: wherein: denotes the number of frequency subcarriers used for the current transmission, denotes the number of spatial streams mapped onto the k-th frequency subcarrier for the current transmission, denote two relatively orthogonal polarization directions, and denotes vectors of precoding weights defined by the precoding configuration of the k-th frequency subcarrier for the current transmission.

9. The wireless transmitter (610) of any one of claims 6 to 8, wherein the processing circuitry (613) is further configured to determine the transmit power back-off for the current transmission as the total gain maximum times an adjustment factor.

10. The wireless transmitter (610) of claim 9, wherein the processing circuitry (613) is further configured to determine the transmit power back-off for the current transmission by reducing the total gain maximum times the adjustment factor by a further factor depending on the number of antennas of the array of antennas (611 a-n).

11. The wireless transmitter (610) of any one of the preceding claims, wherein the precoding configuration is defined by one or more precoding matrices and/or one or more precoding weights.

12. The wireless transmitter (610) of any one of the preceding claims, wherein the processing circuitry (613) is configured to determine the precoding configuration for the current transmission based on channel state information about the communication channel between the wireless transmitter (610) and a wireless receiver (620).

13. The wireless transmitter (610) of any one of the preceding claims, wherein the wireless transmitter (610) is configured to operate in accordance with the IEEE 802.11 WLAN standard.

14. The wireless transmitter (610) of any one of the preceding claims, wherein the processing circuitry (613) comprises a PHY layer portion (613a) and a MAC layer portion (613b, 613c), wherein the PHY layer portion (613a) is configured to operate the array of antennas (611 a-n) with the precoding configuration for the current transmission of the wireless transmitter (610), and wherein the MAC layer portion (613b, 613c) is configured to adjust the transmit power for the current transmission of the wireless transmitter (610) to the allowed upper transmit power limit reduced by the transmit power back-off for the current transmission, wherein the MAC layer portion (613b) is further configured to determine the transmit power back-off for the current transmission based on the information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas (611 a-n).

15. A method (900) of operating a wireless transmitter (610) configured to communicate with a wireless receiver (620), wherein the wireless transmitter (610) comprises an array of antennas (611 a-n), wherein each antenna is arranged at a respective position of the array and comprises one or more antenna properties, including a radiation pattern, and a processing circuitry (613), wherein the method (900) comprises the following steps by the processing circuitry (613): operating (901) the array of antennas (611a-n) with a precoding configuration for a current transmission of the wireless transmitter (610); determining (903) a transmit power back-off for the current transmission based on information about the precoding configuration for the current transmission and based on the position and the one or more antenna properties of each antenna of the array of antennas (611a-n); and adjusting (905) a transmit power for the current transmission of the wireless transmitter (610) to an allowed upper transmit power limit reduced by the transmit power back-off for the current transmission.

16. The method (900) of claim 15, wherein the wireless transmitter (610) further comprises one or more power amplifiers (615) and wherein the transmit power back-off corresponds to an output power back-off of the one or more power amplifiers (615).

17. The method (900) of claim 15 or 16, wherein the wireless transmitter (610) further comprises a memory (617), wherein the method (900) further comprises retrieving from the memory (617) information about the one or more antenna properties of the antennas of the array of antennas (611a-n), including a representation of the radiation pattern of one or more antennas of the array of antennas (611 a-n).

18. The method (900) of any one of claims 15 to 17, wherein the one or more antenna properties of each antenna of the array of antennas (611 a-n) comprises a first radiation pattern for a first polarization direction and a second radiation pattern for a second polarization direction.

19. The method (900) of any one of claims 15 to 18, wherein the method (900) comprises determining the transmit power back-off for the current transmission using a generalized steering vector based on the following equation: wherein: p denotes a polarization index,

K denotes the wave number, denotes the elevation angle, p denotes the azimuth angle, denotes a unit direction vector corresponding to the elevation angle θ and the azimuth angle denotes the position vector of the i-th antenna of the array of ^ antennas (611a-n), and denotes a normalized radiation pattern of the i-th antenna of the array of antennas (611a-n).

20. The method (900) of claim 19, wherein the method (900) comprises determining the transmit power back-off for the current transmission based on a maximum of a total gain of the array of antennas (611a-n) over all dir ections ^.

21. The method (900) of claim 20, wherein the method (900) comprises determining the total gain of the array of antennas (611a-n) as a norm of an expectation value of an array factor of the array of antennas (611a-n), wherein the method (900) further comprises determining the expectation value over an ensemble of excitation signals representing a signal for the current transmission.

22. The method (900) of claim 21, wherein the method (900) comprises determining the total gain ( ) of the array of antennas (611a-n) based on the following equation: wherein: ^ denotes the number of frequency subcarriers used for the current transmission, denotes the number of spatial streams mapped onto the k-th frequency subcarrier for the current transmission, , denote two relatively orthogonal polarization directions, and denotes vectors of precoding weights defined by the precoding configuration of the frequency subcarrier for the current transmission.

23. The method (900) of any one of claims 20 to 22, wherein the method (900) comprises determining the transmit power back-off for the current transmission as the total gain maximum times an adjustment factor.

24. The method (900) of claim 23, wherein the method (900) comprises determining the transmit power back-off for the current transmission by reducing the total gain maximum times the adjustment factor by a further factor depending on the number of antennas of the array of antennas (611a-n).

25. The method (900) of any one of claims 15 to 24, wherein the precoding configuration is defined by one or more precoding matrices and/or one or more precoding weights.

26. The method (900) of any one of claims 15 to 25, wherein the method (900) comprises determining the precoding configuration for the current transmission based on channel state information about the communication channel between the wireless transmitter (610) and a wireless receiver (620).

27. A computer program product comprising program code which causes a computer or a processor to perform the method (900) according to any one of claims 15 to 26 when the program code is executed by the computer or the processor.