Method and apparatus for removal of passive intermodulation in an antenna system

Abstract

A mechanism for PIM removal in an antenna system is provided. A method is performed by a controller of the antenna system. The method includes identifying which transmit radio chains of the antenna system cause signals received by a receiver radio chain of the antenna system to be affected by PIM during transmission using codebook-based beamforming. The transmit radio chains are identified based on which codeword in the codebook is used for beamforming. The method includes determining a correction signal by subjecting signals transmitted only by the identified transmit radio chains to a model of PIM. The method includes removing PIM from a signal received by the receiver radio chain by subtracting the correction signal from the signal received by the receiver radio chain.

Description

Technical Field

[0001] Embodiments presented herein relate to a method, a controller, a computer program and a computer program product for passive intermodulation removal in an antenna system. Background Art

[0002] In general, passive intermodulation (PIM) is a type of distortion generated by nonlinearities of passive components at a cell site, such as filters, duplexers, connectors, antennas, etc. PIM is categorized as internal or external, depending on the location of the component generating the PIM. For example, PIM generated by filters of a transmit (TX) radio chain in an antenna system at a cell site is referred to as internal PIM, while PIM generated by a metal fence on a building roof near a cell site is referred to as external PIM. PIM may cause the transmit power of a cell site to be backed off in order to avoid PIM affecting the receiver (RX) radio chain in the antenna system of the cell site, thereby impairing network performance.

[0003] One way to mitigate PIM for an active antenna system (AAS) is to scale up existing solutions for a classical antenna system comprising from 2 to 8 TX radio chains (and possibly RX radio chains as well) to an AAS comprising from 16 to 64 or more TX radio chains or more (and possibly RX radio chains as well). One drawback of this approach is the attendant computational cost. Implementing PIM mitigation functionality designed for conventional approaches in an AAS may be impractical.

[0004] Another way to mitigate PIM is to use the PIM eigencomponents in the uplink to steer the nulls in the downlink. However, this principle only attempts to avoid exciting PIM sources. Zero steering in the downlink comes with the cost of reduced capacity in terms of total power and spatial steering.

[0005] In addition, when polynomial modeling is used for PIM, the complexity of third-order nonlinear modeling increases as O(N) with respect to the number of antennas N. 3 ). For example, if six polynomial terms are needed to model the PIM of an antenna system with two TX antennas and two RX antennas, the number of terms increases to 40. In an antenna system with 4 TX antennas and 4 RX antennas, 40*4=160 polynomial terms need to be calculated and tracked each time the transmitter's beam direction is changed.

[0006] Therefore, there remains a need for improved PIM mitigation, particularly for AAS. Summary of the invention

[0007] It is an object of embodiments herein to provide efficient PIM mitigation that does not suffer from the above-mentioned problems, or at least wherein the above-identified problems are reduced or mitigated.

[0008] According to a first aspect, a method for PIM removal in an antenna system is proposed. The method is performed by a controller of the antenna system. The method comprises identifying which transmission radio chains of the antenna system cause signals received by a receiver radio chain of the antenna system to be affected by PIM during transmission using codebook-based beamforming. The transmission radio chains are identified based on which codeword in the codebook is used for beamforming. The method comprises determining a correction signal by subjecting signals transmitted only by the identified transmission radio chains to a model of PIM. The method comprises removing PIM from a signal received by a receiver radio chain by subtracting the correction signal from the signal received by the receiver radio chain.

[0009] According to a second aspect, a controller of an antenna system for PIM removal in an antenna system is proposed. The controller comprises a processing circuit. The processing circuit is configured to cause the controller to identify which transmission radio chains of the antenna system cause a signal received by a receiver radio chain of the antenna system to be affected by PIM during transmission using codebook-based beamforming. The transmission radio chains are identified based on which codeword in the codebook is used for beamforming. The processing circuit is configured to cause the controller to determine a correction signal by subjecting only the signals transmitted by the identified transmission radio chains to a model of PIM. The processing circuit is configured to cause the controller to remove PIM from a signal received by the receiver radio chain by subtracting the correction signal from the signal received by the receiver radio chain.

[0010] According to a third aspect, a controller of an antenna system for PIM removal in an antenna system is proposed. The controller comprises an identification module, the identification module being configured to identify which transmission radio chains of the antenna system cause a signal received by a receiver radio chain of the antenna system to be affected by PIM during transmission using codebook-based beamforming. The transmission radio chains are identified based on which codeword in the codebook is used for beamforming. The controller comprises a determination module, the determination module being configured to determine a correction signal by subjecting only signals transmitted by the identified transmission radio chains to a model of PIM. The controller comprises a removal module, the removal module being configured to remove PIM from a signal received by a receiver radio chain by subtracting the correction signal from the signal received by the receiver radio chain.

[0011] According to a fourth aspect, there is proposed a computer program for PIM removal in an antenna system, the computer program comprising computer program code which, when run on a controller of the antenna system, causes the controller to perform the method according to the first aspect.

[0012] According to a fifth aspect, a computer program product is proposed, the computer program product comprising the computer program according to the fourth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium may be a non-transitory computer readable storage medium.

[0013] Advantageously, these aspects provide efficient PIM mitigation.

[0014] Advantageously, the proposed PIM removal does not suffer from the above-mentioned problems.

[0015] Advantageously, these aspects enable fast tracking and convergence of the PIM model.

[0016] Advantageously, these aspects are based on PIM model coefficients calculated relative to beamforming codebook information.

[0017] Advantageously, these aspects result in a reduction in computational complexity compared to traditional mechanisms for PIM mitigation.

[0018] Further objectives, features and advantages of the attached embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

[0019] In general, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless otherwise explicitly defined herein. All references to "a / an / the element, device, component, part, module, step, etc." are to be openly interpreted as referring to at least one instance of the element, device, component, part, module, step, etc., unless otherwise explicitly stated. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The concepts of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0021] Figure 1 is a schematic diagram illustrating a communication network according to an embodiment;

[0022] Figure 2 schematically illustrates an antenna system according to an embodiment;

[0023] Figure 3 and 6 is a flow chart of a method according to an embodiment;

[0024] Figure 4 Schematically illustrates the selection of precoding decoding weights according to an embodiment;

[0025] Figure 5schematically illustrates PIM removal in a controller according to an embodiment;

[0026] Figure 7 is a schematic diagram showing functional units of a controller according to an embodiment;

[0027] Figure 8 is a schematic diagram showing functional modules of a controller according to an embodiment; and

[0028] Fig. 9 An example of a computer program product including a computer-readable storage medium according to an embodiment is shown. DETAILED DESCRIPTION

[0029] The concepts of the present invention will now be described more fully below with reference to the accompanying drawings, in which certain embodiments of the concepts of the present invention are shown. However, the concepts of the present invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be comprehensive and complete, and will fully convey the scope of the concepts of the present invention to those skilled in the art. Throughout this description, like numbers refer to like elements. Any steps or features illustrated by dashed lines should be considered optional.

[0030] Figure 1 is a schematic diagram illustrating a communication network 100 in which embodiments presented herein can be applied. The communication network 100 may be a third generation (3G) telecommunication network, a fourth generation (4G) telecommunication network or a fifth generation (5G) telecommunication network and supports any 3GPP telecommunication standard where applicable.

[0031] The communication network 100 comprises a radio network node 180 configured to provide network access to at least one terminal device 170a, 170b in a radio access network 110. The radio access network 110 is operatively connected to a core network 120. The core network 120 is in turn operatively connected to a service network 130, such as the Internet. The terminal devices 170a, 170b are thereby enabled to access services of the service network 130 and exchange data with the service network 130 via the radio access network node 180.

[0032] The radio access network node 180 comprises, is collocated with, integrated with, or is in operable communication with an antenna system 140. The antenna system 140 may be an active antenna system. The radio access network node 180 (via its antenna system 140) and the terminal devices 170a, 170b are configured to communicate with each other in beams 160a, 160b. The antenna system 140 is thereby configured for beamformed transmission.

[0033] The communication network 100 further comprises a controller 200. Further aspects of the controller 200 will be disclosed below.

[0034] Examples of radio access network nodes 180 are radio base stations, base transceiver stations, Node Bs, evolved Node Bs, gNode Bs, access points and access nodes, and backhaul nodes. Examples of terminal devices 170a, 170b are wireless devices, mobile stations, mobile phones, handsets, wireless local loop phones, user equipment (UE), smartphones, laptops, tablet computers, network-equipped sensors, network-equipped vehicles, and so-called IoT devices.

[0035] Figure 2 A more detailed schematic diagram Figure 1 The antenna system 140 includes two transmission radio chains 141 and two receiver radio chains 142. The original signals x0 and x1 are first weighted by the precoding coefficient w in the encoder part 143 to form the signals u0 and u1. The signals u0 and u1 can be used for PIM removal. The signals s0 and s1 at the receiver side are weighted by the decoding coefficient h in the decoder part 152. The signals s0 and s1 (and also y0 and / or y1) can be used for PIM removal. The values ​​of the precoding coefficient w and the decoding coefficient h are defined by which codeword is used for beamforming transmission. The precoding coefficients are generated by the precoding matrix using codebook information, i.e., the codebook index points to certain precoding coefficients with gain and phase information for beamforming in a specific direction.

[0036] Each transmit radio chain 141 in turn includes components such as a digital-to-analog (DAC) converter 144, a power amplifier (PA) 145, a bandpass filter (BPF) 146, and a single or dual polarized antenna element 147. Each receiver radio chain 142 in turn includes components such as a single or dual polarized antenna element 148, a BPF 149, a low noise amplifier (LNA) 150, and an analog-to-digital (ADC) converter 151. A skilled person will understand how the antenna system 140 can be expanded to include more than two transmit radio chains 141 and more than two receiver radio chains 142, such as in an AAS.

[0037] Figure 2An external PIM source 190 is also schematically illustrated. Thus, a signal transmitted from one or more of the transmit radio chains 141 may be reflected (and distorted) by the external PIM source 190 and received by at least one of the receiver radio chains 142, thereby causing PIM. Additionally or alternatively, PIM may be caused by any of the components 143-147 of the transmit radio chain 141. That is, in some embodiments, PIM is caused by a PIM source 190 external to the antenna system 140, while in other embodiments, PIM is caused by passive electronic components of at least one of the identified transmit radio chains 141.

[0038] As disclosed above, there remains a need for improved PIM mitigation, particularly for AAS.

[0039] In more detail, existing mechanisms for PIM elimination do not take advantage of the underlying legacy system information available to the AAS system in order to reduce the complexity and computational cost for removing PIM. Therefore, PIM model tracking as well as PIM modeling itself will become unnecessarily complex and computationally expensive.

[0040] Therefore, embodiments disclosed herein relate to a mechanism for PIM removal in the antenna system 140. In order to obtain such a mechanism, a controller 200 of the antenna system 140, a method performed by the controller 200, a computer program product including code, e.g. in the form of a computer program, which, when executed on the controller 200, causes the controller 200 to perform the method is provided.

[0041] Figure 3 is a flow chart illustrating an embodiment of a method for PIM removal in an antenna system 140. The method is performed by a controller 200 of the antenna system 140. The method is advantageously provided as a computer program 920.

[0042] Assume that signal transmission is to be performed using codebook-based beamforming. Beamforming involves precoding a signal using coefficients in a codebook specified by a codeword. The information of which codeword is used during the beamformed signal transmission is then used for PIM mitigation purposes. In particular, the controller 200 is configured to perform step S104:

[0043] S104: The controller 200 identifies which transmit radio chains 141 of the antenna system 140 cause signals received by the receiver radio chains 142 of the antenna system 140 to be affected by PIM during transmission using codebook based beamforming. These transmit radio chains 141 are identified based on which codeword in the codebook is used for beamforming.

[0044] Then, PIM is mitigated based only on the signals transmitted by the identified transmission radio chains 141. That is, information of the signals transmitted by any transmission radio chain 141 that has not been identified is not used for PIM mitigation. In particular, the controller 200 is configured to perform step S106:

[0045] S106 : The controller 200 determines a correction signal by subjecting the signal transmitted only by the identified transmit radio chain 141 to a model of the PIM.

[0046] The PIM is then removed from the signal by means of the determined correction signal. In particular, the controller 200 is configured to perform step S108:

[0047] S108 : The controller 200 removes the PIM from the signal received by the receiver radio chain 142 by subtracting the correction signal from the signal received by the receiver radio chain 142 .

[0048] Embodiments related to further details of PIM removal in the antenna system 140 as performed by the controller 200 of the antenna system 140 will now be disclosed.

[0049] In some aspects, PIM is removed from all receiver radio chains 142, while in other aspects, PIM is removed from only some of the receiver radio chains 142. In particular, according to an embodiment, PIM is removed from signals from only those of the receiver radio chains 142 that have been identified as being affected by PIM. Determining which of the receiver radio chains 142 need to have PIM removed from them may be done in a similar manner as which transmit radio chains 141 are identified as causing the receiver radio chains 142 to be affected by PIM.

[0050] There may be different ways to identify which transmit radio chains 141 cause PIM. In some aspects, the transmit radio chains 141 causing PIM are identified by means of a mapping between transmit radio chains 141 and codewords. Therefore, in some embodiments, the controller 200 is configured to perform (optional) step S102:

[0051] S102: The controller 200 determines a mapping between transmit radio chains 141 and codewords by identifying, for each codeword in the codebook, which of the transmit radio chains 141 cause any signal received by a receiver radio chain 142 of the antenna system 140 to be affected by PIM.

[0052] The mapping can then be used to identify the transmit radio chains 141 that cause PIM. That is, in some embodiments, the mapping is used to identify those of the transmit radio chains 141 that cause any of the aforementioned signals received by the receiver radio chain 142 to be affected by PIM during transmission using codebook based beamforming.

[0053] Figure 4 A precoding weight selector 400 is schematically illustrated, which selects the precoding coefficients w and decoding coefficients h based on which codeword c is used for beamforming. Each codebook entry or codeword used for precoding provides a pointer to one or more specific PIM aggressors (i.e., to one or more of the transmit radio chains 141). Thus, these PIM aggressors are transmit radio chains 141 that will cause one or more of the receiver radio chains 142 to be affected by PIM. Signals from other transmit radio chains 141 that will not cause PIM (e.g., for which the beam is not reflected by the external PIM source 190) will not be used for PIM mitigation purposes. In this way, the computational complexity of PIM removal can be reduced. The correction signal, denoted as x in the following pim :

[0054] x PIM =f c (U(M(c))|c),

[0055] Where U=[u0,u1,...,u K-1 ] represents the signal to the transmitter branch, c is the codebook index, M(c) is the index function, which generates a list of transmitter signals to be used for PIM modeling for the codebook index c, and U(M(c)) defines the signals u0, u1, ..., u selected by the index function M for modeling. K-1 The index function M selects k antenna branches that are important to the PIM model from among all K antenna branches (e.g., 4 out of 64 antenna branches), and then the function f c It is suitable to use only these inputs to optimally match and remove (or even eliminate) the PIM of the receiver branches. Function f c This may be applied separately for each receiver branch and each codebook index (but similar processing can be done if desired to eliminate PIM for only a selected few receiver branches). In another example, the index function M is different for each receiver branch.

[0056] Therefore, depending on which receiver radio chain 142 the PIM is to be removed from (ie, from s0 or s1), Figure 2 The modeled PIM signal x is subtracted from the received s0 or s1 pim(ie, the correction signal). As will be disclosed further below, the error from the subtraction can be minimized by adapting the model, resulting in a received output signal that is free of PIM.

[0057] When the signal is subjected to the model of the PIM in step S106, there may be different ways to process the signal. In some aspects, the processing involves filtering. In particular, in some embodiments, subjecting the signal to the model involves filtering the signal with estimated coefficients of the model that define the filter taps. There may be different examples of filters for such filtering. In some embodiments, the signal is filtered using a nonlinear filter.

[0058] As described above, in step S108, PIM is removed from the signal received by the receiver radio chain 142. Thus, removing PIM from the signal received by the receiver radio chain 142 produces a PIM-removed signal. In this regard, how much PIM is removed depends on the extent to which the correction signal captures PIM. In turn, the extent to which the correction signal captures PIM depends on the extent to which the model captures PIM.

[0059] In some aspects, a feedback mechanism is introduced, by means of which the model is adapted. In particular, in some embodiments, the controller 200 is configured to perform (optional) step S110:

[0060] S110 : The controller 200 adapts the estimated coefficients of the model based on how much PIM is still present in the PIM-removed signal.

[0061] There may be different ways of adapting the estimated coefficients. In some aspects, the estimated coefficients are selected according to a solution to an optimization problem in which the goal is to minimize the PIM. That is, in some embodiments, the controller 200 is configured to perform as part of step S110 (optional) step S110a:

[0062] S110a: The controller 200 determines which estimation coefficients produce the minimum PIM per codeword in the codebook for those of the transmitter radio chains that cause any signal received by the receiver radio chain 142 per codeword to be affected by PIM.

[0063] Reference here Figure 5 , Figure 5The PIM removal in the controller 200 according to an embodiment is schematically illustrated. Based on which codeword c is used for beamforming, those transmit radio chains of the antenna system that cause the signal received by the receiver radio chain of the antenna system to be affected by PIM are identified in the TX chain identification block 510. In general, the model coefficients are specific to each codeword. That is, in some embodiments, the model has a set of estimated coefficients per codeword. If a new codeword c' is used for beamforming, the new codeword c' is added to the codebook 520. The correction signal x is determined by subjecting the signal transmitted by only the identified transmit radio chains to the model 530 of PIM. pim Then, by subtracting the correction signal x from the signal s0 or s1 received by the receiver radio chain pim , removes the PIM from the signal s0 or s1 received by the receiver radio chain. The adaptation block 540 takes the PIM removed signal as input and determines which estimation coefficients produce the minimum PIM for the codeword used in the codebook. The adapted estimation coefficients are then provided to the model 530.

[0064] Figure 6 is a flowchart of a method for adapting a model according to an embodiment disclosed herein.

[0065] S201 : Identifying those transmission radio chains 141 of the antenna system 140 that cause a signal received by a receiver radio chain 142 of the antenna system 140 to be affected by PIM based on codebook information.

[0066] S202: Check whether PIM removal has been performed on the codeword used for beamforming, or whether the codeword indicates that beamforming is in a new direction. If beamforming is in a new direction, proceed to step S203, otherwise proceed to step S204.

[0067] S203: Add the new direction to the codebook.

[0068] S204: Determine a correction signal by subjecting only the signal transmitted by the identified transmit radio chain 141 to a model of PIM. PIM is modeled based on codebook information (such as which codeword is used for beamforming). That is, the estimated coefficients of the model are selected depending on which codeword is used for beamforming. If the beamforming is in a new direction (i.e., proceeding to step S203), default values ​​for the estimated coefficients may be used.

[0069] Then, steps S205, S206 and S208 are iteratively performed to estimate which estimated coefficients produce the minimum PIM for the codeword.

[0070] S205: Measure the error energy as the difference between the received signal and the corrected signal. This corresponds to the energy of the signal with the PIM removed.

[0071] S206: Check whether the minimum value of the error energy has been reached, and thus check whether the power of the PIM-removed signal has been minimized. If yes, proceed to step S207, and otherwise proceed to step S208.

[0072] S207: Storing the estimated coefficients of the model for later use when beamforming using the same codeword is performed.

[0073] S208: Adapt the estimated coefficients of the model based on how much PIM is still present in the PIM-removed signal. Then the process goes to step S204 again, where the estimated coefficients adapted in step S208 are used when determining the correction signal.

[0074] Ways of identifying which transmit radio chains 141 cause PIM have been disclosed. Also in this regard, there may be different ways of determining how many transmit radio chains 141 are to be considered by the model and thus identified. Aspects related thereto will now be disclosed.

[0075] In some aspects, how many transmit radio chains 141 are considered by the model depends on how many transmit radio chains 141 there are in total in the antenna system 140. For example, if the total number of transmit radio chains 141 is relatively high, the model may consider only a small portion of all transmit radio chains 141, while if the total number of transmit radio chains 141 is relatively low, the model may consider a large portion of all transmit radio chains 141 in order to maintain a reasonable level of complexity and computational cost at the controller 200. That is, in some embodiments, how many transmit radio chains 141 are identified in step S104 depends on how many transmit radio chains 141 there are in total.

[0076] In some aspects, how many transmit radio chains 141 are considered by the model depends on the amount of PIM caused. In particular, in some embodiments, how many transmit radio chains 141 are identified in step S104 depends on how many or how many receiver radio chains 142 are affected by PIM. In this regard, for relatively high levels of PIM, the model may consider more transmit radio chains 141 than for only relatively low levels of PIM. Thus, the design of the PIM model may be adapted to handle different levels of PIM in an efficient manner.

[0077] In some aspects, the model considers a fixed number of transmit radio chains 141. That is, in some embodiments, at step S104, at most a predetermined number of all transmit radios 141 are identified. This may enable efficient implementation of the PIM model, as it may be designed for a specific number of transmit radio chains 141.

[0078] Figure 7 The components of the controller 200 according to an embodiment are schematically illustrated in terms of a number of functional units. The processing circuit 210 is provided using any combination of one or more of the following: being able to execute a computer program product 910 (e.g., stored in the form of a storage medium 230) Fig. 9 The processing circuit 210 may be a suitable central processing unit (CPU), a multiprocessor, a microcontroller, a digital signal processor (DSP), etc., which executes software instructions in the processor 210 (as shown in FIG. 1 ). The processing circuit 210 may further be provided as at least one application specific integrated circuit (ASIC) or a field programmable gate array (FPGA).

[0079] In particular, the processing circuit 210 is configured to cause the controller 200 to perform a set of steps or operations as disclosed above. For example, the storage medium 230 may store a set of operations, and the processing circuit 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the controller 200 to perform the set of operations. The set of operations may be provided as a set of executable instructions.

[0080] Thus, the processing circuit 210 is thereby arranged to perform the method as disclosed herein. The storage medium 230 may also include a persistent storage device, which may be, for example, any single or combination of the following items: magnetic storage, optical storage, solid-state storage, or even remotely mounted storage. The controller 200 may further include a communication interface 220 configured at least for communicating with the antenna system 140. As such, the communication interface 220 may include one or more transmitters and receivers, which include analog and digital components. The processing circuit 210 controls the general operation of the controller 200, for example, by the following operations: sending data and control signals to the communication interface 220 and the storage medium 230, receiving data and reports from the communication interface 220, and retrieving data and instructions from the storage medium 230. Other components of the controller 200 and related functionality are omitted so as not to obscure the concepts presented herein.

[0081] Figure 8 Components of the controller 200 according to the embodiment are schematically illustrated in terms of a plurality of functional modules. Figure 8 The controller 200 includes a plurality of functional modules; an identification module 210b configured to execute step S104; a determination module 210c configured to execute step S106; and a removal module 210d configured to execute step S108. Figure 8The controller 200 may further include a plurality of optional functional modules, such as any of the following modules: a determination module 210a configured to perform step S210a, an adaptation module 210e configured to perform step S110, and a determination module 210f configured to perform step S11a. In general, each functional module 210a-210f may be implemented only with hardware in one embodiment, and with the help of software in another embodiment, i.e., the latter embodiment has computer program instructions stored on the storage medium 230, which, when executed on the processing circuit, causes the controller 200 to perform the above combined Figure 8 The corresponding steps mentioned. It should also be mentioned that even if the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way they are implemented in software depends on the programming language used. Preferably, one or more or all of the functional modules 210a-210f may be implemented by the processing circuit 210 (possibly in cooperation with the communication interface 220 and / or the storage medium 230). Thus, the processing circuit 210 may be configured to extract instructions as provided by the functional modules 210a-210f from the storage medium 230 and execute these instructions, thereby performing any steps as disclosed herein.

[0082] The controller 200 may be provided as a standalone device or as part of at least one additional device. For example, the controller 200 may be provided in a node of the radio access network and may be part of, integrated with, or collocated with the antenna system 140. Alternatively, the functionality of the controller 200 may be distributed between at least two devices or nodes. Thus, a first portion of instructions executed by the controller 200 may be executed in a first device, and a second portion of instructions executed by the controller 200 may be executed in a second device; the embodiments disclosed herein are not limited to any particular number of devices on which instructions executed by the controller 200 may be executed. Therefore, the methods according to the embodiments disclosed herein are suitable for execution by a controller 200 residing in a cloud computing environment. Therefore, although Figure 7 A single processing circuit 210 is illustrated in FIG, but the processing circuit 210 may be distributed among multiple devices or nodes. The same applies to Figure 8 Functional modules 210a-210j and Fig. 9 Computer program 920.

[0083] Fig. 9An example of a computer program product 910 comprising a computer-readable storage medium 930 is shown. On the computer-readable storage medium 930, a computer program 920 can be stored, which can cause the processing circuit 210 and entities and devices operatively coupled thereto (such as the communication interface 220 and the storage medium 230) to perform methods according to the embodiments described herein. The computer program 920 and / or the computer program product 910 can thus provide means for performing any steps as disclosed herein.

[0084] exist Fig. 9 In the example of , computer program product 910 is illustrated as an optical disc, such as a CD (Compact Disc) or a DVD (Digital Versatile Disc) or a Blu-ray Disc. Computer program product 910 may also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM) or an electrically erasable programmable read-only memory (EEPROM), and more particularly, as a non-volatile storage medium of a device in an external memory, such as a USB (Universal Serial Bus) memory or a flash memory, such as a compact flash memory. Thus, although computer program 920 is schematically shown here as a track on the depicted optical disc, computer program 920 can be stored in any manner suitable for computer program product 910.

[0085] The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept as defined by the appended patent claims.

Claims

1. A method for removing passive intermodulation (PIM) in an antenna system (140), the method being executed by a controller (200) of the antenna system (140), the method comprising: identifying (S104) which transmit radio chains (141) of the antenna system (140) cause a signal received by a receiver radio chain (142) of the antenna system (140) to be affected by PIM during transmission using codebook based beamforming, wherein the transmit radio chains (141) are identified based on which codeword in the codebook is used for the beamforming; determining (S106) a correction signal by subjecting signals transmitted only by the identified transmit radio chain (141) to a model of said PIM; and PIM is removed (S108) from the signal received by the receiver radio chain (142) by subtracting the correction signal from the signal received by the receiver radio chain (142).

2. The method of claim 1, wherein: Subjecting the signal to the model involves filtering the signal with estimated coefficients of the model defining filter taps.

3. The method of claim 2, wherein: The signal is filtered using a non-linear filter.

4. The method of claim 1, wherein: removing PIM from the signal received by the receiver radio chain (142) to produce a PIM removed signal, the method further comprising: The estimated coefficients of the model are adapted (S110) based on how much PIM is still present in the PIM removed signal.

5. The method of claim 4, further comprising: It is determined (S110a) which estimation coefficients produce minimum PIM per codeword in the codebook for those of the transmitting radio chains that cause any signal received by the receiver radio chain (142) per codeword to be affected by PIM.

6. The method according to any one of claims 1 to 5, wherein: The model has a set of estimated coefficients per codeword.

7. The method according to any one of claims 1 to 5, further comprising: A mapping between transmit radio chains (141) and codewords is determined (S102) by identifying, for each codeword in the codebook, which of the transmit radio chains (141) cause any signal received by the receiver radio chain (142) of the antenna system (140) to be affected by PIM.

8. The method of claim 7, wherein: The mapping is used to identify those of the transmit radio chains (141) that cause any signal received by the receiver radio chain (142) to be affected by PIM during transmission using codebook based beamforming.

9. The method according to any one of claims 1 to 5, wherein: How many transmission radio chains (141) are to be identified depends on how many transmission radio chains (141) there are in total.

10. The method according to any one of claims 1 to 5, wherein: How many transmit radio chains (141) are to be identified depends on how many or how few of the receiver radio chains (142) are affected by the PIM.

11. The method according to any one of claims 1 to 5, wherein: Up to a predetermined number of all transmitting radio chains (141) are identified.

12. The method according to any one of claims 1 to 5, wherein: PIM is removed from signals from only those of the receiver radio chains (142) that have been identified as being affected by the PIM.

13. The method according to any one of claims 1 to 5, wherein: The PIM is caused by a PIM source (190) external to the antenna system (140).

14. The method according to any one of claims 1 to 5, wherein: The PIM is caused by a passive electronic component of at least one of the identified transmit radio chains (141).

15. The method according to any one of claims 1 to 5, wherein: The antenna system (140) is an active antenna system.

16. A controller (200) of an antenna system (140) for passive intermodulation (PIM) removal in the antenna system (140), the controller (200) comprising a processing circuit (210), the processing circuit being configured to enable the controller (200): identifying, during transmission using codebook based beamforming, which transmit radio chains (141) of the antenna system (140) cause a signal received by a receiver radio chain (142) of the antenna system (140) to be affected by PIM, wherein the transmit radio chains (141) are identified based on which codeword in the codebook is used for the beamforming; determining a correction signal by subjecting signals transmitted only by the identified transmit radio chain (141) to a model of the PIM; and PIM is removed from the signal received by the receiver radio chain (142) by subtracting the correction signal from the signal received by the receiver radio chain (142).

17. The controller (200) of claim 16, further configured to perform the method of any one of claims 2 to 15.

18. A controller (200) of an antenna system (140) for removing passive intermodulation (PIM) in the antenna system (140), the controller (200) comprising: an identification module (210b) configured to identify, during transmission using codebook based beamforming, which transmit radio chains (141) of the antenna system (140) cause a signal received by a receiver radio chain (142) of the antenna system (140) to be affected by PIM, wherein the transmit radio chains (141) are identified based on which codeword in the codebook is used for the beamforming; a determination module (210c) configured to determine a correction signal by subjecting only signals transmitted by the identified transmission radio chain (141) to a model of the PIM; as well as A removal module (210d) is configured to remove PIM from the signal received by the receiver radio chain (142) by subtracting the correction signal from the signal received by the receiver radio chain (142).

19. The controller (200) of claim 18, further configured to perform the method of any one of claims 2 to 15.

20. A computer readable storage medium storing a computer program (920) for passive intermodulation (PIM) removal in an antenna system (140), the computer program comprising computer code, which, when executed on a processing circuit (210) of a controller (200) of the antenna system (140), causes the controller (200) to: identifying (S104) which transmit radio chains (141) of the antenna system (140) cause a signal received by a receiver radio chain (142) of the antenna system (140) to be affected by PIM during transmission using codebook based beamforming, wherein the transmit radio chains (141) are identified based on which codeword in the codebook is used for the beamforming; determining (S106) a correction signal by subjecting signals transmitted only by the identified transmit radio chain (141) to a model of said PIM; and PIM is removed (S108) from the signal received by the receiver radio chain (142) by subtracting the correction signal from the signal received by the receiver radio chain (142).

21. A computer program product (910) comprising a computer program (920) which, when executed on a processor, causes the processor to perform the method of any one of claims 1 to 15.

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