Method for analog beamforming and transmitter for wireless communication network

Abstract

A method for analog beamforming performed by a transmitter (110) of a wireless communication network (100) is disclosed. The transmitter (110) comprises a plurality of antenna branches (114, 115, 116), each antenna branch comprising an antenna element (111, 112, 113). The method comprises, for each antenna branch (114, 115, 116), obtaining a first and a second signal of an analog radio signal, the first and the second signal being separated from the analog radio signal and the analog radio signal being identical at each antenna branch; and obtaining information indicating a branch-specific phase shift angle and a branch-specific amplitude, the branch-specific phase shift angle and the branch-specific amplitude being determined from information identifying a radiation pattern, the radiation pattern comprising at least two directions for wireless transmission to at least one receiver (120). The method further includes phase shifting the first signal according to a first phase shift angle and phase shifting the second signal according to a second phase shift angle, the first and second phase shift angles being selected so that when the first and second signals are combined, the combined signal has a branch-specific phase shift angle and a branch-specific amplitude indicated by the obtained information; combining the phase-shifted first and second signals into a combined signal; and wirelessly transmitting the combined signal via the antenna units (111, 112, 113).

Description

Technical Field

[0001] The present disclosure generally relates to a method and a transmitter for analog beam steering of wireless signals to be transmitted from a transmitter to a receiver in a wireless communication network. The present disclosure further relates to a computer program and a carrier wave corresponding to the above method and transmitter. Background Art

[0002] For the fifth-generation (5G) wireless communication network radio technology known as New Radio (NR), transmitters (e.g., base stations) have advanced (also known as active or adaptive) antenna systems (AAS) operating at high frequencies (i.e., short wavelengths, such as the millimeter wave (mmW) band). Such AASs typically perform beamforming, also known as beam steering, by analog time-domain phase shifting of each antenna branch. The carrier bandwidth can be hundreds of megahertz, and an AAS can contain, for example, 256 or even more antenna branches.

[0003] Beamforming means directing the signal generated by wirelessly transmitting from the antenna elements of the transmitter in the requested geographical direction toward the receiver. Full-frequency-domain beamforming (BF), such as that used in the low-frequency bands of both Long Term Evolution (LTE) (i.e., 4G wireless communication networks) and NR, is more flexible than time-domain BF. However, since each antenna branch requires its own Fast Fourier Transform (FFT) / Inverse FFT (IFFT) calculation unit, full-frequency-domain BF is too complex for the mmW band given the large number of antenna branches. In contrast to frequency-domain BF, time-domain BF does not require a separate FFT / IFFT for each antenna branch because the FFT / IFFT is performed jointly for all antenna branches. As a result, the complexity of the transmitter is significantly reduced.

[0004] It is possible to further reduce the complexity of time-domain BF by performing analog BF rather than digital BF. This is true because the wide bandwidth and large number of antenna branches of an AAS require high clock frequencies and many parallel operations in each antenna branch, such as filtering, analog-to-digital, and digital-to-analog conversion (if digital processing is used). This can be avoided with analog BF, because in analog BF these operations are performed jointly for all branches. Therefore, for AAS operating at high frequencies and with a large number of antenna branches, the use of time-domain analog BF is beneficial.

[0005] In time-domain analog BF, the same signal is distributed in the time domain to all antenna branches of a transmitter. By adjusting only the phase of the signal at each antenna branch (hereinafter referred to as phase-only BF), a single "pencil beam"—i.e., a narrow, sharp beam with relatively high amplitude—can be created by wirelessly transmitting the generated signal (resulting from the simultaneous transmission of the signals from each antenna branch). A pencil beam is suitable for plane waves, with a linear phase progression across the antenna element. This pencil beam is very convenient when only one receiver receives the generated wirelessly transmitted signal. When a more complex beam is required to transmit the generated signal, for example, due to multiple receivers located in different directions from the transmitter, phase-only BF can provide multiple pencil beams directed toward the receivers, but at the expense of high sidelobes and a low signal-to-noise ratio (SNR). Therefore, to achieve good performance, i.e., a beam directed toward a receiver to receive the signal with low sidelobes and a high SNR, both amplitude and phase control at the antenna branches is required. However, when using analog beamforming in, for example, the mmW band, amplitude control via attenuators or variable gain amplifiers has proven to be insufficiently accurate and is rarely used. On the other hand, phase shifting can be done with high precision and is therefore the preferred beamforming method for the mmW band. However, as shown above, due to the problem of high sidelobes of the shaped beam, phase-only BF is usually limited to simple beams, such as a pencil beam. For wideband carriers, such a beam may waste a lot of capacity because only one user (receiving via its wireless device) may be scheduled at each moment, even if the bit rate requirement of this user is small. Furthermore, in a rich scattering environment where the signal from a single user arrives at the base station from multiple directions, the SNR may be significantly reduced if a single pencil beam is used. Therefore, it is desirable to use more complex beam shapes, referred to as shaped beams from here on.

[0006] Phase-only BF uses constant (maximum) weights on all antenna branches and can therefore produce higher BF gain in terms of received or transmitted signal power compared to time-domain analog BF using both amplitude and phase control, but the higher gain at the receiver will also amplify noise and interference at the antenna branches (if the amplitude of the received signal is low), which leads to the above-mentioned SNR degradation. In addition, as mentioned and as Figure 1 As shown in , if a shaped beam is created using phase-only BF, undesirable sidelobes may become very strong compared to the case where both amplitude and phase beamforming are used. Such sidelobes are undesirable because they will increase interference from other users in the uplink and interference to other users in the downlink, which will result in a decrease in the signal-to-interference-plus-noise ratio (SINR) and system throughput. Figure 1As can be seen in Figure 2, the sidelobes for the pencil beam case are lower compared to the phase-only BF that steers the user signal in three directions, but are still much higher than the BF using both amplitude and phase. Furthermore, for the pencil beam BF, although the desired signal energy in this example has three different directions (see Figure 1 A single pointing direction must also be selected based on the user signal direction in the source block. If the three directions correspond to different users, the pencil beam cannot be used to communicate with more than one user in a given time interval, which could waste capacity if the user does not have enough data to fill all resource blocks.

[0007] As indicated above, there is a need for a method and transmitter that can perform time-domain analog beamforming, wherein simultaneous beams can be formed into multiple directions while maintaining a high SNR. Summary of the Invention

[0008] It is an object of the present invention to solve at least some of the problems and issues outlined above.These objects and other objects may be achieved by using a method and a transmitter as defined in the appended independent claims.

[0009] According to one aspect, a method for analog beamforming is provided, the method being performed by a transmitter of a wireless communication network. The transmitter includes a plurality of antenna branches, each antenna branch including an antenna element. The method comprises, for each antenna branch, obtaining first and second signals of an analog radio signal, the first and second signals being separated from the analog radio signal and being identical at each antenna branch; and obtaining information indicating a branch-specific phase shift angle and a branch-specific amplitude, the branch-specific phase shift angle and the branch-specific amplitude being determined from information identifying a radiation pattern, the radiation pattern including at least two directions for wireless transmission to at least one receiver. The method further comprises phase shifting the first signal according to a first phase shift angle and phase shifting the second signal according to a second phase shift angle, the first and second phase shift angles being selected such that, when the first and second signals are combined, the combined signal has the branch-specific phase shift angle and the branch-specific amplitude indicated by the obtained information; combining the phase-shifted first and second signals into a combined signal; and wirelessly transmitting the combined signal via the antenna element.

[0010] According to another aspect, a transmitter operable in a wireless communication network is provided, the transmitter configured for analog beamforming. The transmitter includes a plurality of antenna branches, each antenna branch including an antenna element. The transmitter also includes processing circuitry and a memory. The memory contains instructions executable by the processing circuitry, whereby the transmitter is operable to obtain, for each antenna branch, first and second signals of an analog radio signal, the first and second signals being separated from the analog radio signal, the analog radio signal being identical at each antenna branch; and information indicating a branch-specific phase shift angle and a branch-specific amplitude, the branch-specific phase shift angle and the branch-specific amplitude being determined from information identifying a radiation pattern, the radiation pattern including at least two directions for wireless transmission to at least one receiver. The transmitter is further operable to, for each antenna branch, phase shift the first signal according to a first phase shift angle and phase shift the second signal according to a second phase shift angle, the first and second phase shift angles being selected such that when the first and second signals are combined, the combined signal has a branch-specific phase shift angle and a branch-specific amplitude indicated by the obtained information; combine the phase-shifted first and second signals into a combined signal; and wirelessly transmit the combined signal through the antenna unit.

[0011] According to other aspects, a computer program and a carrier wave are also provided, details of which will be described in the claims and the detailed description.

[0012] Other possible features and benefits of this solution will become apparent from the detailed description below. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The solution will now be described in more detail by means of exemplary embodiments and with reference to the accompanying drawings, in which:

[0014] Figure 1 are polar plots of normalized beamforming gain versus direction for an example where the receiver is at three different directions (-5°, +18°, and +30°) for three different simulated beamforming methods.

[0015] Figure 2 is a schematic block diagram illustrating a wireless communication network in which the present invention may be used.

[0016] Figure 3 is a schematic diagram of a wireless communication network in which the present invention may be used, the network including base stations and wireless devices.

[0017] Figure 4 is a flow chart illustrating a method performed by a transmitter according to a possible embodiment.

[0018] Figure 5is a schematic block diagram illustrating another wireless communication network in which the present invention may be used.

[0019] Figure 6 is a schematic block diagram of a receiver in which the present invention may be used.

[0020] Figure 7 is a schematic block diagram of a variable amplifier and phase shifter for analog beamforming according to the prior art.

[0021] Figure 8 is a schematic block diagram of a portion of a transmitter according to an embodiment.

[0022] Figure 8 is a block diagram illustrating more detailed nodes according to other possible embodiments.

[0023] Figure 9 is a polar coordinate system which shows how the beam weights are decomposed into two constant magnitude values.

[0024] Figure 10 is a polar plot showing an example in which three different analog beamforming methods are used to transmit to five receivers simultaneously.

[0025] Figure 11 is another polar plot showing an example of simultaneously transmitting to multiple receivers using three different analog beamforming methods.

[0026] Figure 12 is a flowchart of a method according to an embodiment.

[0027] Figure 13 is another flow chart of a method according to another embodiment.

[0028] Figure 14 is a schematic block diagram of a more detailed transmitter according to an embodiment. DETAILED DESCRIPTION

[0029] Figure 2A wireless communication network 100 is shown in which the present invention may be used. The wireless communication network includes or is adapted to communicate wirelessly with a receiver 120. Transmitter 110 has multiple antenna elements 111, 112, and 113. An analog time-domain signal arrives at line 118. Line 118 is divided into multiple antenna branches 114, 115, and 116, each of which terminates at at least one of the multiple antenna elements 111, 112, and 113. The antenna elements of different branches 114, 115, and 116 are independently steerable. However, if more than one antenna element is present on an antenna branch, the antenna elements on the same branch are not independently steerable from one another. The analog time-domain signal arriving at line 118 is split into the multiple antenna branches 114, 115, and 116, and the analog signal is wirelessly transmitted from each antenna element 111, 112, and 113 toward receiver 120. In other words, the same analog signal is transmitted to each antenna branch 114, 115, and 116. In such a transmitter, analog beamforming can be used at the individual antenna branches to steer the wirelessly transmitted combined signal.In this disclosure, an inventive method for phase shifting analog signals in different antenna branches to achieve time domain analog beamforming in both amplitude and phase will be shown.

[0030] Figure 3 An example of a wireless communication network 100 is shown in which the present invention may be used. The network 100 comprises a radio access network node 130 in or adapted to communicate wirelessly with a wireless communication device 140. Figure 2 The transmitter 110 may be a radio access network node 130, and Figure 2 The receiver 120 may be a wireless communication device 140. Alternatively, Figure 2 The transmitter 110 may be a wireless communication device 140 , and the receiver 120 may be a radio access network node 130 .

[0031] Figure 2 and Figure 3 The wireless communication network 100 may be any type of wireless communication network that can provide radio access to wireless devices. Examples of such wireless communication networks are Long Term Evolution (LTE), Advanced LTE, Wireless Local Area Networks (WLAN), fifth generation wireless communication networks based on technologies such as New Radio (NR), and any possible future sixth generation wireless communication networks.

[0032] The RAN node 130 may be any type of network node that, alone or in conjunction with another network node, provides wireless access to the wireless device 140. Examples of a radio access network node 130 are a base station (BS), a radio BS, a base transceiver station, a BS controller, a network controller, a Node B (NB), an evolved Node B (eNB), a gNodeB (gNB), a multi-cell / multicast coordination entity, a relay node, an access point (AP), a radio AP, a remote radio unit (RRU), a remote radio head (RRH), and a multi-standard BS (MSR BS).

[0033] The wireless device 140 may be any type of device capable of wirelessly communicating with the RAN node 130 using radio signals. For example, the wireless device 140 may be a user equipment (UE), a machine-type UE, or a UE capable of machine-to-machine (M2M) communication, a sensor, a tablet, a mobile terminal, a smartphone, a laptop embedded equipment (LEE), a laptop mounted equipment (LME), a USB dongle, a customer premises equipment (CPE), etc.

[0034] Combine Figure 2 , Figure 4 A method for analog beamforming performed by a transmitter 110 of a wireless communication network 100 is described. The transmitter 110 comprises a plurality of antenna branches 114, 115, 116, each antenna branch comprising an antenna element 111, 112, 113. The method comprises, for each antenna branch 114, 115, 116, obtaining 206 a first and a second analog radio signal, the first and the second signals being separated from the analog radio signal and the analog radio signal being identical at each antenna branch; and obtaining 210 information indicative of a branch-specific phase shift angle and a branch-specific amplitude, the branch-specific phase shift angle and the branch-specific amplitude being determined from information identifying a radiation pattern, the radiation pattern comprising at least two directions for wireless transmission to at least one receiver 120. The method further includes phase shifting 214 the first signal according to a first phase shift angle and phase shifting 214 the second signal according to a second phase shift angle, the first and second phase shift angles being selected so that when the first and second signals are combined, the combined signal has a branch-specific phase shift angle and a branch-specific amplitude indicated by the obtained information; combining 216 the phase-shifted first and second signals into a combined signal; and wirelessly transmitting 218 the combined signal via the antenna units 111, 112, 113.

[0035] To achieve phase and amplitude beamforming, the first and second phase shift angles are different angles. When the first and second signals have the same amplitude, the first and second phase shift angles are positioned so that they are at the same angular distance from the combined signal, but on opposite sides of the combined signal. In other words, the phase angle of the combined signal is the average of the phase angles of the first and second signals, and the amount by which one of the phase angles of the first and second signals is greater than the phase angle of the combined signal is the same as the amount by which the other phase angle of the first and second signals is less than the phase angle of the combined signal. By spreading the phase-shifted first and second signals more or less apart from each other, the combined signal will achieve different amplitudes. Furthermore, this spreading is performed so that when the first and second signals are combined, the combined signal will achieve branch-specific phase shift angles. In other words, the angular difference between the first and second signals (i.e., the difference between the first and second phase shift angles) varies depending on the desired branch-specific amplitude. Thus, the amplitude of the combined signal can be selected based on information about the branch-specific amplitudes. Thus, compared to prior art analog phase-only beamforming, amplitude and phase beamforming can be implemented with little additional equipment in the transmitter, i.e., one signal splitter, two phase shifters instead of one, and one signal combiner in each antenna branch, or even just one additional signal splitter to separate the incoming analog radio signal. Moreover, with this phase and amplitude beamforming, sidelobes will be significantly reduced compared to phase-only beamforming, resulting in significantly less interference. Furthermore, multiple users (i.e., receivers, e.g., wireless devices) can be scheduled simultaneously (i.e., within the same symbol) in different directions. This can be used so that a first user uses one part of the spectrum and a second user uses another part of the spectrum, even if the same information is being sent to both users. Furthermore, the above-described approach using only phase shifters offers higher accuracy than using a separate amplifier to control the amplitude.

[0036] Transmitter 110 may be a base station 130 or part of a base station, while at least one receiver 120 may be a wireless device 140 or part of a wireless device. Alternatively, transmitter 110 may be a wireless device 140 or part of a wireless device, while at least one receiver 120 may be a base station 130 or part of a base station. Antenna branches are signal branches, i.e., one or more current conducting wires, each of which receives an analog version of the radio signal to be transmitted. Each antenna branch leads to one or more antenna elements, where the analog radio signal is wirelessly transmitted. The antenna branches are electrically arranged in parallel. Different versions of the analog radio signal can be processed differently on different antenna branches. An antenna element is the portion of the antenna from which signals are transmitted and received. Each antenna branch may have only one antenna element, or a branch may have more than one antenna element, such as a subarray of antenna elements. Antenna branches are individually controllable. That is, the antenna elements of different antenna branches are individually controllable, while if more than one antenna element is present in the same antenna branch, the antenna elements of that same antenna branch may not necessarily be individually controllable. This means that the antenna elements of one antenna branch can be controlled differently from the antenna elements of another antenna branch. The first and second signals can have the same amplitude, which corresponds to half the signal power of the analog radio signal reaching one antenna branch. According to another embodiment, the first and second signals can have different amplitudes. In this case, the amplitude of the combined signal can still be adjusted, but the available adjustment range will be smaller than using the same amplitude. The received radio signal can be separated at each antenna branch, or the analog radio signal can be separated centrally, and the first and second signals of the analog radio signal can be fed to each antenna branch. Information indicating the branch-specific phase shift angle and amplitude can be obtained at the time magnitude per symbol or per time slot. In other words, the branch-specific phase shift angle and amplitude are used within a time period of approximately one symbol or one time slot, and then they can be changed again.

[0037] The information identifying the radiation pattern of at least two directions for wireless transmission to at least one receiver can be determined by the transmitter based on channel state information. Alternatively, the transmitter only receives information about the transmission to be performed in at least two directions. In addition, the at least two directions refer to the main transmission lobe rather than the side lobes. Based directly on the channel state information or the transmission direction information, individual beam weights are calculated for each antenna branch, or alternatively, the individual beam weights are determined from a lookup table with predetermined beam weights specific to the transmitter in order to achieve the at least two transmission directions mentioned. The individual beam weights for each antenna branch are selected so that the combined wireless transmission signal identification from all antenna branches has the desired radiation pattern for at least two directions of wireless transmission. The individual beam weights are selected so that they indicate a branch-specific phase shift angle and a branch-specific amplitude. In other words, the individual beam weights include a branch-specific phase shift angle and amplitude.

[0038] According to embodiments, splitting the signal of a single antenna branch into two signals (also called sub-branches) and having a phase shifter in each sub-branch can also be used to create a radiation pattern with only a single direction, although in this case the advantages compared to a single phase shifter design are smaller. Nevertheless, the possibility of amplitude adjustment in a dual phase shifter arrangement can still be used (e.g., for amplitude tapering) to reduce sidelobes.

[0039] Figure 5 A wireless communication network is shown having a distributed base station system 300 comprising a baseband unit (BBU) 310 and a radio unit (RU) 320 interconnected via a fronthaul connection 340. The RU 320 has multiple antenna branches, each comprising an antenna unit 321, 322, 323. The RU 320 is configured to transmit and receive wireless signals to and from wireless devices 331, 332, 333. According to an embodiment, the BBU 310 is connected to other RAN nodes and a core network 350.

[0040] according to Figure 4 An embodiment of the method shown in Figure 5 The RU 320 is the receiver 110, and one or more of the wireless devices 331, 332, 333 is at least one receiver 120. In addition, the RU 320 is configured to perform Figure 4 The method described in .

[0041] In the following, several embodiments are provided, which show how to divide the Figure 4Different alternatives to the steps of the method are provided. According to a first embodiment, obtaining 210 information indicating branch-specific phase shift angles and branch-specific amplitudes includes RU 320 performing the following steps: obtaining an estimate of a wireless communication channel between RU 320 and one or more of wireless devices 331, 332, and 333; determining branch-specific beam weights for each antenna branch based on the estimate of the wireless communication channel; and calculating first and second phase shift angles for each antenna branch based on the calculated branch-specific beam weights. In this embodiment, the RU may perform the wireless communication channel estimate itself based on channel state information (CSI) received from one or more wireless devices, or the RU may obtain the wireless communication channel estimate by receiving the wireless communication channel estimate from a BBU. This approach would require significant computation in the RU, but would require little or no transmission of information indicating branch-specific phase shift settings and branch-specific amplitude settings over the fronthaul connection, thereby conserving fronthaul connection capacity. The wireless communication channel estimate may be channel state information obtained from measurements performed by the RU on reference signals sent from one or more wireless devices to the RU, or from measurements of reference signals sent from the RU to the one or more wireless devices. In the latter, one or more wireless devices send the performed measurements to the RU and possibly further to the BBU, possibly as CSI.

[0042] According to another embodiment, obtaining 210 information indicating branch-specific phase shift angles and branch-specific amplitudes includes the RU 320 performing the following steps: receiving an estimated beam space representation of a wireless communication channel between the RU 320 and one or more wireless devices 331, 332, 333 from the BBU 310 via the fronthaul connection 340; determining a branch-specific beam weight for each antenna branch based on the received beam space representation, and calculating a first phase shift angle and a second phase shift angle for each antenna branch based on the determined branch-specific beam weight. The beam space representation requires a beam index identifying each active beam and an optional beam weight for each active beam, which can be complex. Alternatively, the beam space representation requires a bit mask indicating the active beams and an optional beam weight for each active beam according to the bit mask. For a small number of active beams (i.e., pencil beams), transmitting the beam index may be most efficient, while for a large number of active beams, transmitting the bit mask may be most efficient. Alternatively, separate beam weights are sent for all possible beams and the beam weights are set to 0 for all inactive beams, but this alternative would result in a lot of unnecessary data being sent. In other words, in contrast to the antenna branch space where each antenna branch provides a value, a beam space representation may refer to a combination of pencil beams or referred to as a plurality of beams with properties. By using the beam space representation, a set of so-called pencil beams (i.e. several individual beams from, for example, a codebook) are combined to determine the beam weights per branch. The beam space representations are calculated by the BBU or any other node or combination of nodes at a higher level in the network and sent to the RU via the fronthaul connection. Compared to the above case, the calculations that have to be performed in the RU may be less, depending on the beamforming method used in the RU, but the beam space representation needs to be sent in some form via the fronthaul connection, which takes up some fronthaul connection capacity. However, when the beam space representation is implemented as codebook indications, only these codebook indications need to be sent via the fronthaul connection, which does not require much fronthaul capacity consideration.

[0043] According to another embodiment, obtaining 210 information indicating branch-specific phase shift angles and branch-specific amplitudes includes the RU 320 performing the following steps: receiving beam properties from the BBU 310 via the fronthaul connection 340, the beam properties being determined from an estimate of a wireless communication channel between the RU (320) and one or more wireless devices 331, 332, 333; determining branch-specific beam weights based on the received beam properties, and calculating a first phase shift angle and a second phase shift angle based on the determined branch-specific beam weights. The beam properties may be a list of pointing directions and beam widths in azimuth and elevation directions. This determination in the RU may include a table lookup in the RU to find the beam weights for each antenna branch for each beam. This is efficient in terms of fronthaul connection usage and computation in the RU, particularly when the shaped combined beam can be described as a combination of a small number of wide beams or beams with specific characteristics. Furthermore, an advantage over a beamspace representation is that the resolution of, for example, the beam width and the information sent over the fronthaul connection do not depend on the codebook size.

[0044] According to another embodiment, obtaining 210 information indicating a branch-specific phase shift angle and a branch-specific amplitude includes the RU 320 performing the following steps: receiving branch-specific beam weights from the BBU 310 via the fronthaul connection 340, and calculating the first phase shift angle and the second phase shift angle based on the received branch-specific beam weights. With this embodiment, fewer calculations are required in the RU compared to when the beam attributes are received from the BBU, but the fronthaul bit rate will be higher. However, particularly for complex beam shapes, transmitting the beam weights may be more efficient than transmitting the beam attributes.

[0045] According to another embodiment, obtaining 210 information indicating branch-specific phase shift angles and branch-specific amplitudes includes RU 320 performing the following steps: receiving the first and second phase shift angles for each branch from BBU 310 via fronthaul connection 340, or receiving the branch-specific phase shift angle for each branch and the separation angle between the first and second phase shifted signals from BBU 310 via fronthaul connection 340, and calculating the first and second phase shift angles for each branch based on the received branch-specific phase shift angles and separation angles. Compared to sending beam weights via the fronthaul connection, this embodiment has even fewer calculations in the RU and a higher fronthaul bit rate.

[0046] Return to Figure 4 and Figure 2 But also back to Figure 5In the embodiment shown in , when the antenna elements 114, 115, and 116 of multiple antenna branches 111, 112, and 113 are arranged in an array, the following embodiment applies: For a first antenna element 114, 115, and 116 of the multiple antenna branches 111, 112, and 113 that is arranged in the region of the physical edge of the array, the branch-specific amplitude is attenuated such that the closer the first antenna element is to the physical edge of the array, the more the branch-specific amplitude is attenuated. Furthermore, a phase shift 214 is performed based on the attenuated branch-specific amplitude. This feature is known as amplitude tapering. This is typically static over time, i.e., the same amplitude tapering is performed over time. The function of the amplitude tapering is to attenuate the determined branch-specific amplitude, with the attenuation increasing the closer the antenna element is to the physical edge of the array. This results in a larger main lobe and lower side lobes. The amplitude tapering decreases the further the antenna element is from the physical edge of the array. The region of the physical edge may include a certain percentage of the antenna elements in the array. For example, it can be assumed that 10-90% of the antenna elements are located in the region of the physical edge of the array. Consequently, the amplitude of this 10-90% is attenuated, with the attenuation being greater the closer the individual antenna elements are to the actual physical edge of the array.

[0047] according to Figure 4 In yet another embodiment shown in FIG, the method further includes obtaining 211 information about a desired amplitude control ratio and, based on the desired amplitude control ratio, adjusting 212 the first and second phase shift angles for each antenna branch such that: for full amplitude control, the first and second phase shift angles are maintained; for no amplitude control, the first and second phase shift angles are identical to the branch-specific phase shift angles; and for half amplitude control, the angular difference between the first and second phase shift angles is reduced. No amplitude control means phase-only beamforming. Full amplitude control means full amplitude and phase beamforming. By adapting the angular phase shifts of the first and second signals, as described above, a trade-off between phase-only beamforming and full amplitude and phase beamforming can be achieved in a simple manner. This trade-off may be of interest, for example, when higher sidelobe levels are acceptable and it is important to be able to reach a great distance with the signal. An example of this interest is when the receiver is far from the transmitter, so having a high amplitude is important. If the transmitter is a base station and the receiver is a wireless device, this means the wireless device is located at or near a cell border. Specifically, if the receiving wireless device is located at a cell border and there are no other wireless devices in the cell or the other wireless devices are in directions or distances where sidelobe interference is not a big problem, reducing the amplitude control (i.e., keeping it more or less closer to full amplitude than the amplitude value obtained by using full amplitude and phase beamforming) can be beneficial.

[0048] As mentioned, beamforming can be implemented in different ways. Figure 6 An example of prior art single-layer analog uplink beamforming is shown, in which the signals received at the antenna elements of each of the N antenna branches 361, 362, 363 are processed separately for each antenna branch by amplification in branch-specific amplifiers 371, 372, 373 and application of separate beam weights in beam weight applicators 381, 382, ​​383. The signals on the separate branches are then combined in a combiner 390, and the combined analog signal is then converted from analog to digital form in an A / D converter 395 for further processing. Similar processing is performed in the downlink, but in the opposite direction. In the downlink, the A / D converter would be a D / A converter, and the combiner would be a splitter. In the present disclosure, an antenna branch (also called an antenna port) is the smallest unit (e.g., a single antenna element) that can be controlled in phase and / or amplitude, or a subarray of antenna elements with a fixed phase and amplitude distribution within the subarray. To support a second user layer, for example by using two different polarizations, the structure in the figure would be duplicated.

[0049] The beam weights are typically determined based on some type of channel state information or selected from a codebook, for example as part of beam scanning. In order to create shaped beams with low sidelobes in undesired directions (e.g., complex multi-beam patterns or wide beams), it is necessary to apply separate weights to the signal for each antenna branch, preferably where both the magnitude and phase of the weights can be varied, as described above.

[0050] For analog beamforming, one might try to apply complex weights by using a combination of variable gain / attenuation and phase shift, but when Figure 7 As shown in , it is difficult to achieve sufficient accuracy when using a variable amplifier 402 and a phase shifter 404 to change the analog gain or attenuation in one antenna branch. In addition to amplitude inaccuracies, changes in analog gain or attenuation can also introduce phase errors. Therefore, prior art analog beamforming systems typically use only phase shifting, that is, changing the phase without changing the amplitude, which limits the types of beams that can be achieved. Amplitude tapering on the array (which means that the amplitude is lower for elements closer to the physical edge of the transmitter or receiver) is sometimes used to control sidelobes, but this is usually static in order to achieve the desired accuracy. One problem with static tapering is that it will reduce the BF gain and widen the beam, even for the case with a single narrow beam. In addition, it is mainly effective for sidelobes caused by discontinuities in weighting at the edges of the antenna panel (for example, between the left and right or upper and lower edges).

[0051] According to an embodiment of the invention, both amplitude and phase variations are implemented so that each antenna branch adds two differently phase-shifted or differently time-delayed copies of the same antenna signal. Figure 8 As shown in Figure 8 The analog beamforming in the transmitter (i.e., downlink, when the transmitter is a base station) using a variable phase shifter is shown. The same principle applies to the receiver, i.e., uplink, when the receiver is a base station. For analog beamforming, the present invention allows for higher precision than using a variable amplifier to control the amplitude. For the transmitter, Figure 8 A D / A converter (DAC) 408 is shown, which converts the digital signal into an analog format. The analog signal is fed to each antenna branch 414, 415, 416 as a branch-specific signal x by, for example, a splitter (not shown) arranged in a common line (i.e. the same line as the DAC 408). n (t), where n = 1, 2, ..., N represents the number of antenna branches. A splitter (not shown) further splits the analog signal into branch-specific signals that are fed to each antenna branch. Furthermore, a splitter 424, 425, 426 in each antenna branch is used to separate the branch-specific signal in each antenna branch into a first signal and a second signal for each branch. For each first and second signal of each branch, a separate phase shifter 431, 432, 433, 434, 435, 436 is used, and the first and second signals are individually phase-shifted and are separate for each branch according to the phase and amplitude requested for each branch. Figure 8 The individual first and second phase shift angles are denoted by θ n1 and θ n2 , where n=1, 2, ..., N are used for the first and second signals, respectively. It should be noted that the first and second phase shift angles θ n1 and θ n2 is a separate value for each antenna branch and is typically different between antenna branches. The individually phase-shifted first and second signals are then combined by combiners 444, 445, 446 in each branch to form a combined signal y for each branch. n (t), n=1,2,…,N. The combined signal y of each branch n (t), n = 1, 2, ..., N are in turn fed to their corresponding one or more antenna elements 411, 412, 413, where the combined signal of each branch is simultaneously transmitted wirelessly to the receiver. The splitters 424, 425, 426 and combiners 444, 445, 446 shown may include, for example, branch line couplers or Wilkinson power dividers.

[0052] In the following, we will show that the desired beamforming in both phase and amplitude can be achieved by adding two phase-shifted versions of the input signal per branch. The phase shifts and other values ​​calculated below are for each antenna branch (also known as port n), but the subscript n is omitted from the phase shifts and other variables to make the notation less cluttered.

[0053]

[0054] where v1 and v2 are constant-magnitude complex values ​​with Figure 8 The corresponding phase shifts θ1, θ2 are obtained by: Thus, the principle can be demonstrated by showing that the sum of the two constant amplitude vectors is equal to the desired beam weight (expressed as the desired branch-specific phase shift angle and amplitude).

[0055] First, the desired complex weight w for antenna n is n is converted to polar form:

[0056] a=|w n |≤A,φ=∠w n

[0057] Where a is the amplitude, A is the maximum amplitude value, and Φ is the phase. Furthermore, we define the following value, corresponding to half the angle of separation between our two constant amplitude complex values:

[0058] ψ=cos -1 (a / A)

[0059] The complex beam weight is now described by the beam weight angle φ and the angle ψ, which controls the amplitude. Now, the two phase control signals (phase shift angle pair, referred to as the first and second phase angles) for antenna branch n can be calculated as:

[0060] θ1=φ-ψ

[0061] θ2=φ+ψ

[0062] The complex values ​​of these two constant magnitudes become:

[0063]

[0064]

[0065] By adding these two constant magnitude complex values ​​and evaluating the expression, we obtain:

[0066]

[0067] This completes the proof. This means that we can apply any desired complex beam weight by adding two phase-shifted versions of the same signal, whose absolute value does not exceed A, see Figure 9 . Figure 9 The complex amplitude and phase beam weights w are shown n How can be decomposed into two constant amplitude complex values ​​v1 and v2, each with amplitude A / 2, as long as the maximum amplitude of the beam weights is not larger than A. The maximum amplitude value A may be different for different antenna branches, e.g. if static amplitude tapering is applied.

[0068] By adjusting only the separation angle 2Ψ (e.g., by applying a function or scaling the separation angle with a value between 0 and 1, where 0 means only phase BF and 1 means full amplitude + phase BF), it is possible to smoothly change between phase BF only and amplitude + phase BF. In some cases, this may be desirable to trade off BF gain versus sidelobe level. Similar control is also possible, for example, by applying a function to the amplitude of the beam weights.

[0069] Most of the above operations are common in digital signal processing, such as converting to polar coordinate format. The inverse cosine function, or arccos function, can be implemented, for example, via a one-dimensional table lookup. Note that the phase control signal changes less frequently than the data signal. Typically, the beam weights are changed every time slot—for example, every 14 OFDM symbols for 5G NR, or at most every OFDM symbol—meaning the lifetime of the phase control signal is typically thousands of data samples. Therefore, when implementing the present invention, the overall complexity of the transmitter is not expected to increase significantly.

[0070] We have shown that the determination of the phase offset or constant amplitude complex vector can be done based on the complex beam weights, but starting from the desired beam weights is not mandatory; other inputs such as channel state information or codebook-based information can also be used. For example, a well-known uplink beamforming scheme is maximum ratio combining (MRC), where the beam weights are the complex conjugate of the channel estimate. Therefore, if a channel estimate is available and MRC is desired, it is easy to modify the above formula to calculate the first and second phase shift angles θ based on the channel estimate instead of the beam weights. n1 and θ n2 The same principle applies to the downlink with Maximum Ratio Transmission (MRT).

[0071] The calculation of the two phase offsets from the channel state information or beam weights can be done in the transmitter, or done in another node and sent to the transmitter. One embodiment for doing this is a distributed base station system 300, e.g. Figure 5. In such a distributed base station system, the calculation of data such as beam weights or phase angles can then be completed in the BBU 310 and sent to the RU 320 via a fronthaul connection (FH). If the first and second phase shift angles are sent via the FH, the bit rate may need to be reduced. If the calculation is completed in the RU 320, an efficient representation with reduced memory requirements may still be of interest.

[0072] There are several options for what beam-related information to send over the FH or what beam-related information to store in the memory / storage of the RU 320. In addition to the impact on the FH bit rate and storage requirements, different options will also have different impacts on the amount of processing required in the RU 320. The following list gives examples of options for what to send over the FH:

[0073] - Channel state information, where the RU can use beamforming algorithms, such as MRT / MRC, zero forcing (ZF), minimum mean square error (MMSE), to determine the beam weight w for each antenna port n .

[0074] - Beam space representation, where multiple pencil beams may need to be combined to determine the required w n .

[0075] - Beam properties, where complex beam shapes can be described by a set of one or more pointing directions and beamwidths.

[0076] -Beam weight for each antenna port (w n ), which is common for massive multiple-input multiple-output (MIMO) RUs in low-band (i.e., non-mmW) frequencies.

[0077] - a phase shift angle pair, namely the first and second phase shift angles used in the present invention, or the beam weight w from each antenna port n The calculated middle value.

[0078] Of the first three options above, the most straightforward approach is for the RU to first determine (i.e., calculate or use a lookup table) the beam weight w for each antenna port based on information received from the BBU via the FH. n , and then calculate the required phase shift angle pair for each antenna branch.

[0079] According to the first option, i.e., obtaining channel state information (CSI) from the RU, the following applies. CSI can be obtained, for example, from measurements of reference signals in the uplink (i.e., by the base station) and / or downlink (i.e., by the wireless device). CSI may include:

[0080] o A channel path gain matrix describing the complex path gains between the base station antenna branches and one or more wireless device antennas.

[0081] o A precoding matrix indication sent from one or more wireless devices.

[0082] Based on the CSI, the RU can calculate the beam weight w for each antenna port using methods known in the prior art. n For example, using the scaled conjugate of the complex channel path gain, as in Maximum Ratio Combining / Transmission (MRC / MRT), using Zero Forcing (ZF), or using MMSE beamforming.

[0083] According to the second option, i.e., according to receiving a beamspace representation via FH, the following applies. The beamspace representation is typically based on transforming the beamweights per antenna port into a space where most of the signal energy is concentrated into fewer coefficients than the number of antenna ports. This can be used to compress the beamweights and is typically codebook-based.

[0084] A common method for transforming the beam weights is the Discrete Fourier Transform (DFT), where the codebook is a set of DFT basis vectors. For example, if the DFT is oversampled, the number of entries in the codebook can be greater than the number of antenna ports. This oversampling is done to achieve better resolution in the beam pointing direction (i.e., the scan angle). Singular value decomposition (SVD) has better energy compression properties than DFT, but it is data dependent, which makes it impractical to transmit beam spatial information because in addition to the coefficients, the codebook, which may be a large matrix, will have to be sent. Other transforms (in addition to DFT, with or without oversampling) are also possible, including transforms based on pre-computed codebooks. The codebook is not limited to orthogonal beams.

[0085] Using the beam space representation, a set of simple “pencil beams” or generally several single beams from a codebook are combined to compute the beam weights w for each antenna port n For DFT-based beamforming, the beam weight w for each antenna port is n The calculation can be done directly based on the inverse transform of the beam spatial coefficients (i.e., complex beam scaling values), where the coefficients for unused beams are padded with zeros before the inverse transform. If only a few DFT-based beams are active, it can be more efficient to directly apply the so-called Goertzel algorithm to each beam spatial coefficient and add the results for each antenna port. This eliminates the need for zero padding and avoids the need to calculate the full-scale transform.

[0086] For table-based codebooks, the beam weights for each antenna port are determined for each active beam separately, and then a linear combination is performed using these coefficients to obtain the total beam weight w for each antenna port. nIn turn, the RU may continue by calculating the first and second phase shift angles based on the beam weights for each antenna port.

[0087] Different options are possible for how to represent the beam spatial information via the FH: According to one option, a single index into the codebook is sent via the FH. The single index can be based on the DFT basis vectors or pre-computed, for example, using constrained optimization techniques to approximate the desired beam using phase-only beamforming. This is used, for example, to generate a single pencil beam in prior art analog beamforming.

[0088] According to another option, an indication of the active beams from the codebook is sent via the FH (using a bitmap or an indexed list of active beams). This option can be used if it is not necessary to have different phases or amplitudes for the codebook beams when combining them (i.e., each beam has a coefficient of 1). c When different codebook beams are available, the bitmap to support any combination of simultaneous beams requires N on the FH. c bits, excluding any overhead. If the number of codebook beams N b If the number of indexes is limited, it is more efficient to use a list of indices to describe the beamforming. Each index requires log2(N c ) bits, so if N b ×log2(N c ) <N c Or equivalently N b <N c / log2(N c ), the index list is more efficient. For example, in N c =256, the intersection point is located at N b = 32 simultaneous beams. For mmW beamforming using the present invention, the number of simultaneous beams is expected to be small compared to the number of antennas, and so the list can be more efficient in practice.

[0089] According to yet another option, an indication of the active beams from the codebook is sent via the FH (as in the previous option, but including scaling values ​​for each codebook beam). The scaling values ​​can be real or complex and can be used to change the gain and / or phase for each codebook beam to better control the resulting overall beam. The scaling values ​​can be in a Cartesian format (e.g., 8 bits each for real and imaginary parts) or in a polar format, preferably with more bits used for phase than for amplitude.

[0090] Implementing a set of active beams without any scaling (beam spatial coefficient = 1) can work well if the beams are well separated and intended for different users (i.e., transmitted in different resource blocks). However, if a set of active beams is for a single user, or if wide beams are required, then simply implementing the beams without any gain / phase adjustment is usually not enough, as the result can be destructive interference when combining the different beams. When transmitting / receiving signals from / to a single user in different directions, it is important to be able to control the phase on the different beams to avoid destructive interference. Additionally, some gain scaling is required to obtain the best SINR. An example could be Figure 1 , where the three beams directed to three users have different gains, where more gain is applied in the direction where the signal is stronger to avoid enhancing the noise.

[0091] Figure 10 An attempt is shown to create a single wide beam by activating five adjacent codebooks (i.e. pencil beams in a 16-element array without DFT oversampling and without any scaling, which is the same gain and phase for all five pencil beams). Figure 10 As can be seen from the solid shape in , the individual beams are clearly visible, but the sidelobe levels are quite high. For reference, a single pencil beam from the same codebook is also shown, which is shown as a dashed line. This single pencil beam is directed to the user in the middle. It can be seen that the solid shape does not look like a single wide beam due to the large ripples in the antenna gain. It can be used to communicate with up to 5 different users on different resource blocks, or to send a common channel to all users, but since the beam is very narrow, high user mobility and / or beam pointing errors may be a problem, where a properly wide beam would be more desirable. Compared to the pencil beam marked with a dashed line, it can be observed that the problem is not that the five individual beams are too narrow, but that there is destructive interference when the beams are added. By adjusting the (complex) scaling of each individual beam, a more ideal shape of the wide beam can be obtained.

[0092] Here, it was empirically determined that a linear phase reduction in steps of 1.83 radians for each codebook beam would make the wide beam shape more desirable, since destructive interference would then occur midway between each pair of beams. Additionally, fine-tuning can be accomplished by gain scaling, in this case by reducing the amplitude of the outermost beam by 10%. The results are shown in Figure 11 As shown in , the same five beams are active, but now with appropriate scaling. The main lobe is now flatter and has high gain, while the side lobe levels are significantly reduced, see Figure 11Here, we also compare it to phase-only beamforming (dashed line), where the prior art approach to achieving a wide beam is to enable only a subset of antenna elements, in this case the first three of the 16 antennas, since round(16 / 5)=3. Phase-only beamforming results in higher sidelobe levels, lower beamforming gain, and less flat gain in the mainlobe. The amplitude and phase BF using gain scaling (solid line) is also compared to a single pencil beam, marked with a dashed line.

[0093] In this case, complex scaling is used on the active beams to achieve the desired shape. However, this does not mean that the complex scaling (gain / phase) must be sent over the FH interface. For wide beams as described above, it was observed that the same scaling ratio (i.e., a 1.83 radian phase step from one beam component to the next, with a slight amplitude reduction at the edge beams) achieved good results over a wide scan angle range for wide beams. This means that when only the indication of the active beams from the codebook is sent over the FH interface, at least some wide beams can be created and the scaling factor determined in the RU, for example, as a constant value or based on a lookup table. For other cases, it may still be beneficial to include beam scaling over the FH interface.

[0094] exist Figure 12 Reference Figure 5An example method for analog beamforming when beamspace information is transmitted over a FH connection between a first node and a second node is shown. The first node may be a BBU 310 or a digital unit (DU), and the second node may be a RU 320 or a similar type of unit. The method begins with the first node determining 502 a set of one or more users, i.e., wireless devices, to be frequency reused in the same time interval. Thereafter, the first node determines 504 a radiation pattern covering the set of users based on, for example, channel state information. The first node then encodes 506 the radiation pattern in a beamspace representation that includes an indication of active beams and possible scaling factors for each beam, and transmits 508 the indication of the active beams over the FH connection, each active beam having optional real or complex coefficients. Furthermore, upon receiving the indication of the active beams, the second node determines 510 complex beam weights for each antenna port (i.e., antenna branch), e.g., by performing an inverse discrete Fourier transform (IDFT) or using a lookup table. Thereafter, the second node calculates 512 a phase shift angle pair for each antenna branch, i.e., the first and second phase shift angles. As input to this calculation, the second node may determine 511 or receive information about the BF amplitude control (i.e., the desired ratio of amplitude control, from 0 (i.e., phase control only) to 1 (i.e., full amplitude and phase control). Thereafter, the second node applies 514 the calculated phase shift angle pair to the first and second separated signals for each antenna branch and transmits data during a given time interval. If the second node operates as a receiver, it receives data during the given time interval.

[0095] According to yet another option, beam properties or an indication of beam properties are sent to the RU via the FH connection. The beam properties may include, for example, a list of pointing directions and beam widths in azimuth and elevation, and possibly also real or complex scaling values ​​for each beam. This may include a lookup table in the RU to find the beam weights for each antenna branch for each beam. The beam property representation can be effective when the shaped beam can be described as a small number of wide beams or a combination of beams with specific characteristics. Furthermore, an advantage over a spatial representation of the beam is that the resolution of, for example, the beam width and the information sent over the FH connection / interface do not depend on the codebook size.

[0096] Additionally, the RU can approximate the requested beam properties by converting to the closest beam space (e.g., codebook, representation, and continuing as in the previous option). Another option is to convert the properties directly into beam weights per antenna port for each beam and perform a linear combination to obtain the total beam weight w per antenna port. nFor example, the Fourier transform relationship can be used to create a wide beam by approximating the beam with a rectangle in the antenna gain versus spatial angle domain and computing the corresponding (truncated) sinc (i.e., sin(x) / x) function in antenna port / element space. Pointing direction changes are accomplished by varying the phase slope across the antenna element.

[0097] According to yet another option, the beam weights for each antenna port (i.e., branch) are sent to the RU via the FH connection. Here, the beam weights for each antenna branch w n It can be calculated in the first node (e.g., DU or BBU), in a dedicated beamforming processing unit, in the fronthaul gateway, or as a beamforming processing function in the cloud and sent to the RU, typically in Cartesian format (i.e., real and imaginary parts) or polar format (i.e., phase φ and amplitude a). The main advantage is that fewer calculations are required in the RU compared to when the beam attributes are sent. For simple shaped beams, the FH bit rate will be higher than when the beam attributes are sent, but for complex beam shapes consisting of a large number of pencil beams with different scales, sending the beam weights for each antenna branch can be more efficient.

[0098] exist Figure 13 An example method for analog beamforming when beam weights are transmitted over an FH connection between a first node and a second node is shown in The first node may be a BBU or a digital unit (DU), and the second node may be a RU or a similar type of unit.

[0099] The method begins with a first node determining 532 one or more users, i.e., wireless devices, to be frequency multiplexed within the same time interval (e.g., time slot or symbol). The first node then determines 534 a desired radiation pattern for each user by determining, for each user, complex coefficients in beam space for each beam in the desired radiation pattern based on channel state information. Furthermore, for each beam, the complex coefficients for all users are summed 536 together. The resulting coefficients for each used beam are then transmitted 538 via the FH, preferably along with data to be wirelessly transmitted to the user. Furthermore, upon receiving the resulting beam coefficients via the FH, the second node, for each beam, looks up 540 the complex coefficients for each antenna branch in a lookup table using the beam number as an index into the table, and multiplies each found antenna branch coefficient by the obtained beam coefficient for that beam to obtain the resulting antenna branch complex coefficients for that beam for each antenna branch. Furthermore, for each antenna branch, the antenna coefficients obtained for all used beams are summed 542 together, resulting in a single complex coefficient for each antenna branch. Furthermore, for each antenna branch, a phase shift angle pair corresponding to the obtained antenna coefficients, i.e., a first and a second phase shift angle, is calculated 544. Thereafter, the second node applies 546 the calculated phase shift angle pair to the first and second separated signals per antenna branch and transmits data during a given time interval. If the second node operates as a receiver, it receives data during the given time interval. Furthermore, the amplitude may need to be adjusted by the same factor to keep the total radiated power within the allowed level and ensure that the power required on any branch is within technical limitations.

[0100] According to yet another option, the phase shift value or the intermediate phase shift value is sent to the RU via the FH connection. The following options have similar advantages and disadvantages to sending the beam weight for each antenna branch, i.e., high FH bit rate in the case of a large number of antenna branches, but further, the complexity of the RU is reduced compared to sending the beam weight for each antenna port by moving the phase shift value calculation from the RU to the first node (e.g., BBU). The disadvantage is of course that these two options cannot be used with traditional RUs (i.e., RUs not using the present invention), which means that two different options must be maintained when mixing traditional RUs and RUs using the present invention in a wireless communication network.

[0101] In the first option, the intermediate phase values ​​φ and ψ for each antenna branch are determined by the first node and sent via the FH connection. Here, φ has a uniform distribution and ψ is a separation angle or amplitude control angle with a non-uniform distribution. Here, ψ can benefit from non-uniform quantization, for example by companding (i.e., compression and expansion to allow fewer bits in the quantizer) combined with uniform quantization, or by non-uniform quantization, for example, using Max-Lloyd calculation of the quantizer interval and codeword. The quantization performance should be similar to quantizing the beam weights for each antenna branch using a suitable non-uniform quantizer. Furthermore, the intermediate phase values ​​are converted into a phase shift angle pair for each antenna branch, i.e., a first and a second phase shift angle, in the RU.

[0102] In the second option, the first node determines the phase shift angle pair θ1 and θ2 for each antenna branch and sends it via the FH connection. This minimizes complexity in the RU. The two angles of each phase shift angle pair have a uniform distribution, but for a given φ, the conditional distribution of the two angles is non-uniform, meaning this representation may not be as good as the previous one in terms of quantization error.

[0103] Figure 14 A transmitter 110 configured for analog beamforming and operable in a wireless communication network 100 is shown. The transmitter 110 includes a plurality of antenna branches 114, 115, 116, each antenna branch including an antenna element 111, 112, 113. The transmitter 110 also includes processing circuitry 603 and memory 604. The memory contains instructions executable by the processing circuitry, whereby the transmitter 110 is operable to obtain, for each antenna branch 114, 115, 116, a first and a second signal of an analog radio signal, the first and the second signal being separated from the analog radio signal and being identical at each antenna branch; and to obtain information indicating a branch-specific phase shift angle and a branch-specific amplitude, the branch-specific phase shift angle and the branch-specific amplitude being determined from information identifying a radiation pattern, the radiation pattern including at least two directions for wireless transmission to at least one receiver 120. The transmitter 110 is further operable to, for each antenna branch 114, 115, 116, phase shift the first signal according to a first phase shift angle and phase shift the second signal according to a second phase shift angle, the first and second phase shift angles being selected such that when the first and second signals are combined, the combined signal has a branch-specific phase shift angle and a branch-specific amplitude indicated by the obtained information; combine the phase-shifted first and second signals into a combined signal; and wirelessly transmit the combined signal via the antenna units 111, 112, 113.

[0104] According to one embodiment, the transmitter 110 is a RU 320 operable in a distributed base station system 300, which further includes a BBU 310 interconnected with the RU 320 via a fronthaul connection 340. Furthermore, the at least one receiver 120 is one or more wireless devices 331,332,333.

[0105] According to another embodiment, the transmitter 110 is operable to obtain information indicating a branch-specific phase shift angle and a branch-specific amplitude by performing the following operations through the RU 320: obtaining an estimate of a wireless communication channel between the RU (320) and one or more wireless devices 331, 332, 333; determining a branch-specific beam weight for each antenna branch based on the estimate of the wireless communication channel; and calculating a first phase shift angle and a second phase shift angle for each antenna branch based on the determined branch-specific beam weight.

[0106] According to another embodiment, the transmitter 110 is operable to obtain information indicating a branch-specific phase shift angle and a branch-specific amplitude by performing the following operations through the RU 320: receiving an estimated beam space representation of a wireless communication channel between the RU 320 and one or more wireless devices 331, 332, 333 from the BBU 310 through the fronthaul connection 340; determining a branch-specific beam weight for each antenna branch based on the received beam space representation, and calculating a first phase shift angle and a second phase shift angle for each antenna branch based on the determined branch-specific beam weight.

[0107] According to another embodiment, the transmitter 110 may be operable to obtain information indicating a branch-specific phase shift angle and a branch-specific amplitude by performing the following operations through the RU 320: receiving beam properties from the BBU 310 through the fronthaul connection 340, the beam properties being determined from an estimate of a wireless communication channel between the RU 320 and one or more wireless devices; determining a beam weight for a specific branch based on the received beam properties; and calculating a first phase shift angle and a second phase shift angle based on the determined branch-specific beam weight.

[0108] According to another embodiment, the transmitter 110 may be operable to obtain information indicating the branch-specific phase shift angle and the branch-specific amplitude by performing the following operations through the RU 320: receiving the branch-specific beam weight from the BBU 310 through the fronthaul connection 340, and calculating the first phase shift angle and the second phase shift angle based on the received branch-specific beam weight.

[0109] According to another embodiment, the transmitter 110 may be operable to obtain information indicating the branch-specific phase shift angle and the branch-specific amplitude by performing the following operations through the RU 320: receiving the first phase shift angle and the second phase shift angle for each branch from the BBU 310 through the fronthaul connection 340, or receiving the branch-specific phase shift angle for each branch and the separation angle between the first and second phase shifted signals from the BBU 310 through the fronthaul connection, and calculating the first and second phase shift angles for each branch based on the received branch-specific phase shift angle and the separation angle.

[0110] According to yet another embodiment, the antenna elements 114, 115, 116 of the plurality of antenna branches 111, 112, 113 are arranged in an array. Furthermore, for a first antenna element of the antenna elements 114, 115, 116 of the plurality of antenna branches 111, 112, 113 that is arranged in the region of a physical edge of the array 110, the transmitter 110 is operable to attenuate a branch-specific amplitude such that the closer the first antenna element is to the physical edge of the array, the more the branch-specific amplitude is attenuated, and wherein the transmitter is operable to perform a phase shift based on the attenuated branch-specific amplitude.

[0111] According to yet another embodiment, the transmitter 110 is further operable to: obtain information about a desired amplitude control ratio; and according to the desired amplitude control ratio, adjust, for each antenna branch, the first phase shift angle and the second phase shift angle such that: for full amplitude control, the first and second phase shift angles are maintained, for no amplitude control, the first and second phase shift angles are the same as the branch-specific phase shift angles, and for half amplitude control, the angular difference between the first and second phase shift angles is reduced.

[0112] According to other embodiments, transmitter 110 may further include a communication unit 602, which can be considered to include conventional components for wireless communication with at least one receiver 120, such as a transceiver for wirelessly transmitting and receiving signals in a communication network. If transmitter 110 is located in a RAN node, communication unit 602 may also include conventional components for communicating with other RAN nodes in wireless communication network 100. If transmitter 110 is located in a RU, communication unit 602 may also include conventional components for communicating with its BBU. Instructions executable by processing circuit 603 may be provided as a computer program 605 stored in memory 604, for example. Processing circuit 603 and memory 604 may be arranged in sub-device 601. Sub-device 601 may be a microprocessor and, as appropriate, software and storage devices, and thus may be a programmable logic device (PLD) or other electronic component / processing circuit configured to perform the above-mentioned methods. Processing circuit 603 may include one or more programmable processors, application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or a combination of these devices suitable for executing instructions.

[0113] Computer program 605 can be configured such that, when its instructions are executed in the processing circuitry, these instructions cause transmitter 110 to perform the steps described in any of the described embodiments of transmitter 110 and its methods. Computer program 605 can be carried by a computer program product connectable to processing circuitry 603. The computer program product can be memory 604, or at least be located in the memory. Memory 604 can be implemented as, for example, RAM (random access memory), ROM (read-only memory), or EEPROM (electrically erasable programmable ROM). In some embodiments, a carrier wave can contain computer program 605. The carrier wave can be an electronic signal, an optical signal, an electromagnetic signal, a magnetic signal, an electrical signal, a radio signal, a microwave signal, or a computer-readable storage medium. The computer-readable storage medium can be, for example, a CD, a DVD, or a flash memory, from which the program can be downloaded into memory 604. Alternatively, the computer program can be stored on a server or any other entity accessible to transmitter 110 via communication unit 602. Computer program 605 can, in turn, be downloaded from the server into memory 604.

[0114] Although most of the disclosure relates to a transmitter and methods in a transmitter, the present invention and its embodiments are also suitable for use in a receiver. Those skilled in the art will appreciate that a "transmitter" may actually be a transceiver (i.e., having both transmit and receive capabilities), and a "receiver" may also be a transceiver. However, to make it easier for readers to distinguish between the nodes while reading, they will be referred to as a transmitter and a receiver in this document.

[0115] In a general sense, the desired phase and amplitude at the antenna elements of an antenna branch can be achieved by feeding the antenna branch with a signal that is a combination of two identical signals, except that the phase of each signal can be controlled individually per signal and per branch, possibly with the exception of a fixed difference in amplitude. By controlling the phase of these two signals according to the mathematical explanation given in this disclosure, the phase and amplitude of the signal fed to the branch can be controlled. Similarly, for reception, the signal wirelessly received on the antenna branch can be separated into two signals, each undergoing a controlled phase shift and then combined to achieve the desired phase shift and gain / attenuation of the signal. The described phase control can be used to control the radiation pattern of the AAS.

[0116] The phases of the signals to be combined are kept constant for a time sufficient to transmit useful information. The rate of change of the phase control of the two signals from one setting to the other is less than the bandwidth of the signals, typically less than 10% of the bandwidth, and preferably less than 1% or 0.1% of the bandwidth, to allow useful information to be transmitted without causing signal distortion. Therefore, the information content of the two signals to be combined and the combined signal is identical. Only the phase and / or amplitude vary, but the shape of the signal curves is the same. It is advantageous, but not absolutely necessary, for the two signals to have the same amplitude. A difference in amplitude will reduce the achievable amplitude range of the combined signal. The mathematical explanation presented is also applicable to the case of different amplitudes using standard methods for adding vectors or complex numbers. The two signals are typically created by separating the common signal to be transmitted or the received signal. In other words, in a method for controlling the phase and optionally the amplitude of a signal to be transmitted on or received from an antenna unit, two signals having the same shape and undergoing individually controlled phase shifts are combined, wherein the phase shift is set so that the combined signal has the desired amplitude and phase and the rate of change of the setting of the phase shift is lower than the bandwidth of the signal.

[0117] Although the above description contains a number of characteristics, these characteristics should not be construed as limiting the scope of the concepts described herein, but rather merely providing illustrations of some exemplary embodiments of the concepts described. It will be understood that the scope of the presently described concepts fully encompasses other embodiments that may become apparent to those skilled in the art, and does not thereby limit the scope of the presently described concepts. Unless expressly stated otherwise, references to singular elements do not mean "one and only one", but rather "one or more". All structural and functional equivalents to the elements of the above-described embodiments known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be included herein. Furthermore, a device or method does not have to address every problem sought to be addressed by the presently described concepts for it to be hereby included herein. In the exemplary figures, a broken / dashed line generally indicates that the features within the broken line are optional.

Claims

1. A method for analog beamforming performed by a transmitter of a wireless communication network, the transmitter comprising a plurality of antenna branches, each antenna branch comprising an antenna element, the method comprising, for each antenna branch: obtaining a first signal and a second signal of an analog radio signal, wherein the first signal and the second signal are separated from the analog radio signal and the analog radio signal is the same at each of the antenna branches; obtaining information indicative of a branch-specific phase shift angle and a branch-specific amplitude, the branch-specific phase shift angle and the branch-specific amplitude being determined from information identifying a radiation pattern, the radiation pattern comprising at least two directions for wireless transmission to at least one receiver; phase shifting the first signal according to a first phase shift angle and phase shifting the second signal according to a second phase shift angle, the first phase shift angle and the second phase shift angle being selected such that when the first signal and the second signal are combined, the combined signal has the branch-specific phase shift angle and the branch-specific amplitude indicated by the obtained information; combining the phase-shifted first signal and the phase-shifted second signal into a combined signal; wirelessly transmitting the combined signal via the antenna unit; obtaining information regarding a desired amplitude control ratio; and According to the desired amplitude control ratio, the first phase shift angle and the second phase shift angle are adjusted for each antenna branch so that: for full amplitude control, the first phase shift angle and the second phase shift angle are maintained; for no amplitude control, the first phase shift angle and the second phase shift angle are the same as the branch-specific phase shift angle; and for half amplitude control, the angular difference between the first phase shift angle and the second phase shift angle is reduced.

2. The method according to claim 1, wherein The transmitter is a radio unit RU of a distributed base station system, the distributed base station system further comprising a baseband unit BBU interconnected with the RU via a fronthaul connection, and the at least one receiver is one or more wireless devices.

3. The method according to claim 2, wherein: Obtaining information indicating a branch-specific phase shift angle and a branch-specific amplitude includes the RU: obtaining an estimate of a wireless communication channel between the RU and the one or more wireless devices; determining branch-specific beam weights for each antenna branch based on the estimate of the wireless communication channel; as well as The first phase shift angle and the second phase shift angle are calculated for each antenna branch based on the determined branch-specific beam weights.

4. The method according to claim 2, wherein: Obtaining information indicating a branch-specific phase shift angle and a branch-specific amplitude includes the RU: receiving, from the BBU over the fronthaul connection, an estimated beamspace representation of a wireless communication channel between the RU and the one or more wireless devices; determining branch-specific beam weights for each antenna branch based on the received beam space representation; as well as The first phase shift angle and the second phase shift angle are calculated for each antenna branch based on the determined branch-specific beam weights.

5. The method according to claim 2, wherein: Obtaining information indicating a branch-specific phase shift angle and a branch-specific amplitude includes the RU: receiving beam properties from the BBU over the fronthaul connection, the beam properties determined from an estimate of a wireless communication channel between the RU and the one or more wireless devices; determining a beam weight for a particular branch based on the received beam attributes; as well as The first phase shift angle and the second phase shift angle are calculated based on the determined branch-specific beam weights.

6. The method according to claim 2, wherein: Obtaining information indicating a branch-specific phase shift angle and a branch-specific amplitude includes the RU: receiving branch-specific beam weights from the BBU via the fronthaul connection; and The first phase shift angle and the second phase shift angle are calculated based on the received branch-specific beam weights.

7. The method according to claim 2, wherein: Obtaining information indicating a branch-specific phase shift angle and a branch-specific amplitude includes the RU: receiving the first phase shift angle and the second phase shift angle for each branch from the BBU via the fronthaul connection, or The branch-specific phase shift angle for each branch and a separation angle between the phase-shifted first signal and the phase-shifted second signal are received from the BBU through the fronthaul connection, and the first phase shift angle and the second phase shift angle for each branch are calculated based on the received branch-specific phase shift angle and the separation angle.

8. A method according to any one of the preceding claims, wherein The antenna elements of the multiple antenna branches are arranged in an array, and for a first antenna element among the antenna elements of the multiple antenna branches that is arranged in an area of ​​a physical edge of the array, the branch-specific amplitude is attenuated so that the closer the first antenna element is to the physical edge of the array, the more the branch-specific amplitude is attenuated, and wherein the phase shift is performed based on the attenuated branch-specific amplitude.

9. A transmitter operable in a wireless communication network, the transmitter configured for analog beamforming, the transmitter comprising a plurality of antenna branches, each antenna branch comprising an antenna element, the transmitter comprising processing circuitry and a memory, the memory containing instructions executable by the processing circuitry, whereby the transmitter is operable to, for each antenna branch: obtaining a first signal and a second signal of an analog radio signal, wherein the first signal and the second signal are separated from the analog radio signal and the analog radio signal is the same at each of the antenna branches; obtaining information indicative of a branch-specific phase shift angle and a branch-specific amplitude, the branch-specific phase shift angle and the branch-specific amplitude being determined from information identifying a radiation pattern, the radiation pattern comprising at least two directions for wireless transmission to at least one receiver; phase shifting the first signal according to a first phase shift angle and phase shifting the second signal according to a second phase shift angle, the first phase shift angle and the second phase shift angle being selected such that when the first signal and the second signal are combined, the combined signal has the branch-specific phase shift angle and the branch-specific amplitude indicated by the obtained information; combining the phase-shifted first signal and the phase-shifted second signal into a combined signal; wirelessly transmitting the combined signal via the antenna unit; obtaining information regarding a desired amplitude control ratio; and According to the desired amplitude control ratio, the first phase shift angle and the second phase shift angle are adjusted for each antenna branch so that: for full amplitude control, the first phase shift angle and the second phase shift angle are maintained; for no amplitude control, the first phase shift angle and the second phase shift angle are the same as the branch-specific phase shift angle; and for half amplitude control, the angular difference between the first phase shift angle and the second phase shift angle is reduced.

10. The transmitter according to claim 9, wherein The transmitter is a radio unit RU operable in a distributed base station system further comprising a baseband unit BBU interconnected with the RU via a fronthaul connection, and the at least one receiver is one or more wireless devices.

11. The transmitter of claim 10, operable to obtain information indicating a branch-specific phase shift angle and a branch-specific amplitude by the RU performing the following operations: obtaining an estimate of a wireless communication channel between the RU and the one or more wireless devices; determining branch-specific beam weights for each antenna branch based on the estimate of the wireless communication channel; as well as The first phase shift angle and the second phase shift angle are calculated for each antenna branch based on the determined branch-specific beam weights.

12. The transmitter of claim 10, operable to obtain information indicating a branch-specific phase shift angle and a branch-specific amplitude by the RU performing the following operations: receiving, from the BBU over the fronthaul connection, an estimated beamspace representation of a wireless communication channel between the RU and the one or more wireless devices; determining branch-specific beam weights for each antenna branch based on the received beam space representation; as well as The first phase shift angle and the second phase shift angle are calculated for each antenna branch based on the determined branch-specific beam weights.

13. The transmitter of claim 10, operable to obtain information indicating a branch-specific phase shift angle and a branch-specific amplitude by the RU performing the following operations: receiving beam properties from the BBU over the fronthaul connection, the beam properties determined from an estimate of a wireless communication channel between the RU and the one or more wireless devices; determining a beam weight for a particular branch based on the received beam attributes; as well as The first phase shift angle and the second phase shift angle are calculated based on the determined branch-specific beam weights.

14. The transmitter of claim 10, operable to obtain information indicating a branch-specific phase shift angle and a branch-specific amplitude by the RU performing the following operations: receiving branch-specific beam weights from the BBU via the fronthaul connection; and The first phase shift angle and the second phase shift angle are calculated based on the received branch-specific beam weights.

15. The transmitter of claim 10, operable to obtain information indicating a branch-specific phase shift angle and a branch-specific amplitude by the RU performing the following operations: receiving the first phase shift angle and the second phase shift angle for each branch from the BBU via the fronthaul connection, or The branch-specific phase shift angle for each branch and a separation angle between the phase-shifted first signal and the phase-shifted second signal are received from the BBU through the fronthaul connection, and the first phase shift angle and the second phase shift angle for each branch are calculated based on the received branch-specific phase shift angle and the separation angle.

16. The transmitter according to any one of claims 9 to 15, wherein: The antenna elements of the multiple antenna branches are arranged in an array, and for a first antenna element among the antenna elements of the multiple antenna branches that is arranged in an area of ​​a physical edge of the array, the transmitter is operable to attenuate the branch-specific amplitude so that the closer the first antenna element is to the physical edge of the array, the more the branch-specific amplitude is attenuated, and wherein the transmitter is operable to perform the phase shift based on the attenuated branch-specific amplitude.

17. A computer program product comprising instructions that, when executed by at least one processing circuit of a transmitter of a wireless communication network configured for emulating beamforming, the transmitter comprising a plurality of antenna branches, each antenna branch comprising an antenna element, cause the transmitter to perform the following steps for each antenna branch: obtaining a first signal and a second signal of an analog radio signal, wherein the first signal and the second signal are separated from the analog radio signal and the analog radio signal is the same at each of the antenna branches; obtaining information indicative of a branch-specific phase shift angle and a branch-specific amplitude, the branch-specific phase shift angle and the branch-specific amplitude being determined from information identifying a radiation pattern, the radiation pattern comprising at least two directions for wireless transmission to at least one receiver; phase shifting the first signal according to a first phase shift angle and phase shifting the second signal according to a second phase shift angle, the first phase shift angle and the second phase shift angle being selected such that when the first signal and the second signal are combined, the combined signal has the branch-specific phase shift angle and the branch-specific amplitude indicated by the obtained information; combining the phase-shifted first signal and the phase-shifted second signal into a combined signal; wirelessly transmitting the combined signal via the antenna unit; obtaining information regarding a desired amplitude control ratio; and According to the desired amplitude control ratio, the first phase shift angle and the second phase shift angle are adjusted for each antenna branch so that: for full amplitude control, the first phase shift angle and the second phase shift angle are maintained; for no amplitude control, the first phase shift angle and the second phase shift angle are the same as the branch-specific phase shift angle; and for half amplitude control, the angular difference between the first phase shift angle and the second phase shift angle is reduced.

18. A computer-readable storage medium having stored thereon a computer program, the computer program, when executed by at least one processing circuit of a transmitter of a wireless communication network configured for emulating beamforming, the transmitter comprising a plurality of antenna branches, each antenna branch comprising an antenna element, causing the transmitter to perform the following steps for each antenna branch: obtaining a first signal and a second signal of an analog radio signal, wherein the first signal and the second signal are separated from the analog radio signal and the analog radio signal is the same at each of the antenna branches; obtaining information indicative of a branch-specific phase shift angle and a branch-specific amplitude, the branch-specific phase shift angle and the branch-specific amplitude being determined from information identifying a radiation pattern, the radiation pattern comprising at least two directions for wireless transmission to at least one receiver; phase shifting the first signal according to a first phase shift angle and phase shifting the second signal according to a second phase shift angle, the first phase shift angle and the second phase shift angle being selected such that when the first signal and the second signal are combined, the combined signal has the branch-specific phase shift angle and the branch-specific amplitude indicated by the obtained information; combining the phase-shifted first signal and the phase-shifted second signal into a combined signal; wirelessly transmitting the combined signal via the antenna unit; obtaining information regarding a desired amplitude control ratio; and According to the desired amplitude control ratio, the first phase shift angle and the second phase shift angle are adjusted for each antenna branch so that: for full amplitude control, the first phase shift angle and the second phase shift angle are maintained; for no amplitude control, the first phase shift angle and the second phase shift angle are the same as the branch-specific phase shift angle; and for half amplitude control, the angular difference between the first phase shift angle and the second phase shift angle is reduced.

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