Patent Description:
In general terms, passive intermodulation (PIM) is a type of distortion generated by nonlinearity of passive components, such as filters, duplexers, connectors, antennas and so forth at a cell site. Depending on the location of the component that generates the PIM, the PIM is categorized as either internal or external. For example, PIM generated by the filters of the transmission (TX) radio chains in the antenna system at the cell site is called internal PIM whereas PIM generated by a metal fence on the roof top of a building in vicinity of the cell site is called external PIM. PIM might cause the transmission power of the cell site to be backed off in order to avoid PIM to affect the receiver (RX) radio chains in the antenna system of the cell site, thus compromising the network performance.

One way to mitigate PIM for active antenna systems (AASs) is to scale up existing solutions for classic antenna systems comprising from <NUM> to <NUM> TX radio chains (and equally may RX radio chains) to the AASs comprising from <NUM> to <NUM> or more TX radio chains or more (and equally may RX radio chains). One drawback of this approach is the computational cost that comes with it. Implementing a PIM cancellation function designed for traditional approaches in an AAS might be impractical.

Another way to mitigate PIM is to use the PIM eigen components in the uplink to steer the nulls in the downlink. However, this principle only tries to avoid exciting the PIM source. Null steering in the downlink comes with a cost of reduced capacity in terms of total power and spatial steering.

Further, in case of polynomial modeling of the PIM, the complexity of the third order non-linear modeling increases as O(N<NUM>) with respect to the number of antennas N. For example, if six polynomial terms are needed to model the PIM for an antenna system with two TX antennas and two RX antennas, the number of terms will increase to <NUM>. In an antenna system with four TX antennas and <NUM> RX antennas, <NUM> = <NUM> polynomial terms need to be computed and tracked for each time the beam direction of the transmitter is changed.

Hence, there is still a need for an improved PIM mitigation, especially for AASs.

Document <CIT> relates to a method, a controller, a computer program, and a computer program product for passive intermodulation removal in an antenna system.

An object of embodiments herein is to provide efficient PIM mitigation, not suffering from the issues noted above, or at least where the above identified issues are reduced or mitigated.

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

According to a second aspect there is presented a controller of an antenna system for PIM removal in the antenna system. The controller comprises processing circuitry. The processing circuitry is configured to cause the controller to identify, during transmission using codebook based beamforming, which transmission radio chains of the antenna system that cause a signal received by receiver radio chains of the antenna system to be impacted by PIM. These transmission radio chains are identified based on which codeword in the codebook is used for the beamforming. The processing circuitry is configured to cause the controller to determine a correction signal by subjecting the signals only as transmitted by the identified transmission radio chains to a model of the PIM. The processing circuitry is configured to cause the controller to remove PIM from the signal received by the receiver radio chains by subtracting the correction signal from the signal received by the receiver radio chains.

According to a third aspect there is presented a controller of an antenna system for PIM removal in the antenna system. The controller comprises an identify module configured to identify, during transmission using codebook based beamforming, which transmission radio chains of the antenna system that cause a signal received by receiver radio chains of the antenna system to be impacted by PIM. These transmission radio chains are identified based on which codeword in the codebook is used for the beamforming. The controller comprises a determine module configured to determine a correction signal by subjecting the signals only as transmitted by the identified transmission radio chains to a model of the PIM. The controller comprises a remove module configured to remove PIM from the signal received by the receiver radio chains by subtracting the correction signal from the signal received by the receiver radio chains.

According to a fourth aspect there is presented 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 a method according to the first aspect.

According to a fifth aspect there is presented a computer program product comprising a 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 could be a non-transitory computer readable storage medium.

Advantageously, these aspects provide efficient PIM mitigation.

Advantageously, the proposed PIM removal does not suffer from the issues noted above.

Advantageously, these aspects enable fast tracking and convergence of the PIM model
Advantageously, these aspects are based on the PIM model coefficients being computed in relation to the beamforming codebook information
Advantageously, these aspects result in a reduction in computational complexity compared to traditional mechanisms for PIM mitigation.

<FIG> is a schematic diagram illustrating a communication network <NUM> where embodiments presented herein can be applied. The communication network <NUM> could be a third generation (<NUM>) telecommunications network, a fourth generation (<NUM>) telecommunications network, or a fifth (<NUM>) telecommunications network and support any 3GPP telecommunications standard, where applicable.

The communication network <NUM> comprises a radio network node <NUM> configured to provide network access to at least one terminal device 170a, 170b in a radio access network <NUM>. The radio access network <NUM> is operatively connected to a core network <NUM>. The core network <NUM> is in turn operatively connected to a service network <NUM>, such as the Internet. The terminal devices 170a, 170b is thereby enabled to, via the radio access network node <NUM>, access services of, and exchange data with, the service network <NUM>.

The radio access network node <NUM> comprises, is collocated with, is integrated with, or is in operational communications with, an antenna system <NUM>. The antenna system <NUM> might be an active antenna system. The radio access network node <NUM> (via its antenna system <NUM>) and the terminal devices 170a, 170b are configured to communicate with each other in beams 160a, 160b. The antenna system <NUM> is thus configured for beamformed transmission.

The communication network <NUM> further comprises a controller <NUM>. Further aspects of the controller <NUM> will be disclosed below.

Examples of radio access network nodes <NUM> are radio base stations, base transceiver stations, Node Bs, evolved Node Bs, g Node 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, laptop computers, tablet computers, network equipped sensors, network equipped vehicles, and so-called Internet of Things devices.

<FIG> schematically illustrates the antenna system <NUM> of <FIG> in more detail. The antenna system <NUM> comprises two transmission radio chains <NUM> and two receiver radio chains <NUM>. The original signals x<NUM> and x<NUM> are first weighted by precoding coefficients w in an encoder part <NUM> to form signals u<NUM> and u<NUM>. The signals u<NUM> and u<NUM> can be used for PIM removal. The signals s<NUM> and s<NUM> at the receiver side are weighted by decoding coefficients h in a decoder part <NUM>. The signals s<NUM> and s<NUM> (and also y<NUM> and/or y<NUM>) can be used for PIM removal. The values of the precoding coefficients w and the decoding coefficients h are defined by which codeword is used for the beamformed transmission. The precoding coefficients are generated by the precoding matrix with the codebook information, i.e. the codebook index is pointed to certain precoding coefficients which have gain and phase information for beamforming in a specific direction.

Each transmission radio chain <NUM> in turn comprises components, such as a digital-to-analogue (DAC) converter <NUM>, a power amplifier (PA) <NUM>, a bandpass filter (BPF) <NUM>, and a single or double polarized antenna element <NUM>. Each receiver radio chain <NUM> in turn comprises components, such as a single or double polarized antenna element <NUM>, a BPF <NUM>, a low noise amplifier (LNA) <NUM>, and an analogue-to-digital (ADC) converter <NUM>. The skilled person would understand how the antenna system <NUM> could be extended to comprise more than two transmission radio chains <NUM> and more than two receiver radio chains <NUM>, such as in an AAS.

<FIG> also schematically illustrates an external PIM source <NUM>. Signals transmitted from one or more of the transmission radio chains <NUM> might thus be (distorted and) reflected by the external PIM source <NUM> and received by at least one of the receiver radio chains <NUM>, thus causing PIM. Additionally or alternatively, PIM might be caused by any of the components <NUM>-<NUM> of the transmission radio chain <NUM>. That is, in some embodiments, the PIM is caused by a PIM source <NUM> external to the antenna system <NUM> whereas in other embodiments, the PIM is caused by a passive electric component of at least one of the identified transmission radio chains <NUM>.

As disclosed above there is still a need for an improved PIM mitigation, especially for AASs.

In further detail, existing mechanisms for PIM cancellation do not take advantage of the potential inherited system information that can be leveraged upon for an AAS system in order to reduce complexity and computational cost for removing PIM. As a consequence, PIM model tracking, as well as the PIM modeling itself, would be unnecessary complex and computationally costly.

The embodiments disclosed herein therefore relate to mechanisms for PIM removal in an antenna system <NUM>. In order to obtain such mechanisms, there is provided a controller <NUM> of the antenna system <NUM>, a method performed by the controller <NUM>, a computer program product comprising code, for example in the form of a computer program, that when run on a controller <NUM>, causes the controller <NUM> to perform the method.

<FIG> is a flowchart illustrating embodiments of methods for PIM removal in an antenna system <NUM>. The methods are performed by the controller <NUM> of the antenna system <NUM>. The methods are advantageously provided as computer programs <NUM>.

It is assumed that signal transmission is to be performed using codebook based beamforming. The beamforming is involves precoding the signal using coefficients in the codebook as specified by a codeword. The information of which codeword was used during the beamformed signal transmission is then utilized for PIM mitigation purposes. In particular, the controller <NUM> is configured to perform step S104:
S104: The controller <NUM> identifies, during transmission using codebook based beamforming, which transmission radio chains <NUM> of the antenna system <NUM> that cause a signal received by receiver radio chains <NUM> of the antenna system <NUM> to be impacted by PIM. These transmission radio chains <NUM> are identified based on which codeword in the codebook is used for the beamforming.

The PIM is then mitigated based only on the signals transmitted by the identified transmission radio chains <NUM>. That is, information of the signals as transmitted by any transmission radio chain <NUM> not having been identified is not utilized for the PIM mitigation. In particular, the controller <NUM> is configured to perform step S106:
S106: The controller <NUM> determines a correction signal by subjecting the signals only as transmitted by the identified transmission radio chains <NUM> to a model of the PIM.

PIM is then removed from the signal by means of the determined correction signal. In particular, the controller <NUM> is configured to perform step S108:
S108: The controller <NUM> removes PIM from the signal received by the receiver radio chains <NUM> by subtracting the correction signal from the signal received by the receiver radio chains <NUM>.

Embodiments relating to further details of PIM removal in an antenna system <NUM> as performed by the controller <NUM> of the antenna system <NUM> will now be disclosed.

In some aspects, PIM is removed from all receiver radio chains <NUM> whereas in other aspects PIM is removed from only some of the receiver radio chains <NUM>. Particularly, according to an embodiment, PIM is removed from the signal only from those of the receiver radio chains <NUM> having been identified as being impacted by the PIM. From which of the receiver radio chains <NUM> PIM needs to be removed can be determined in similar manner as which transmission radio chains <NUM> are identified as causing the receiver radio chains <NUM> to be impacted by PIM.

There may be different ways to identify which transmission radio chains <NUM> that cause the PIM. In some aspects, the transmission radio chains <NUM> that cause the PIM are identified by means of a mapping between transmission radio chains <NUM> and codewords. Therefore, in some embodiments, the controller <NUM> is configured to perform (optional) step S102:
S102: The controller <NUM> determines a mapping between transmission radio chains <NUM> and codewords by identifying which of the transmission radio chains <NUM> that, for each codeword in the codebook, cause any signal as received by the receiver radio chains <NUM> of the antenna system <NUM> to be impacted by PIM.

This mapping can then be used to identify the transmission radio chains <NUM> that cause the PIM. That is, in some embodiments, this mapping is used to identify those of the transmission radio chains <NUM> that cause the aforementioned any signal received by the receiver radio chains <NUM> to be impacted by PIM during transmission using codebook based beamforming.

<FIG> schematically illustrates a precoding weight selector <NUM> that selects precoding coefficients w and decoding coefficients h based on which codeword c is used for the beamforming. Each codebook entry, or codeword, used for the precoding provides a pointer to one or more specific PIM aggressors (i.e., to one or more of the transmission radio chains <NUM>). These PIM aggressors are thus the transmission radio chains <NUM> which will cause one or more of the receiver radio chains <NUM> to be impacted by PIM. The signals from other transmission radio chains <NUM> that will not cause PIM (for example for which the beam is not reflected by the external PIM source <NUM>) will not be used for PIM mitigation purposes. As such, the computation complexity of the PIM removal can be reduced. The correction signal,hereinafter denoted xpim, : <MAT> where U = [u<NUM>, u<NUM>,. , uK-<NUM>] denote the signals to the transmitter branches, c is the codebook index, M(c) is an index function that produces a list of transmitter signals to be used for PIM modelling for a codebook index c, U(M(c)) defines the subset of the signals u<NUM>, u<NUM>,. , uK-<NUM> to be used for modelling, as selected by the index function M. The index function M selects k out of all K antenna branches (for example, <NUM> out of <NUM> antenna branches) as important for the PIM model, and the function fc is then adapted to match and remove (or even cancel) PIM of a receiver branch optimally using only these inputs. The function fc might be separately adapted for each receiver branch and each codebook index (but a similar process could be made to only cancel PIM for a selected few receiver branches if required). In further examples, the index function M is different for each receiver branch.

The modelled PIM signal xpim (i.e., the correction signal) will hence be subtracted from the received so or s<NUM> in <FIG> depending on from which receiver radio chain <NUM> PIM is to be removed (i.e., from s<NUM> or s<NUM>). As will be further disclosed below, the error from the subtraction can be minimized by adapting the model, resulting in a PIM-free received output signal.

There could be different ways to process the signal when the signal is subjected to the model of the PIM in step S106. In some aspects, the processing involves filtering. In particular, in some embodiments, subjecting the signal to the model involves filtering the signal with estimation coefficients of the model defining filter taps. There could be different examples of filters used for this filtering. In some embodiments, the signal is filtered using a non-linear filter.

As disclosed above, in step S108, PIM is removed from the signal received by the receiver radio chains <NUM>. Removing PIM from the signal received by the receiver radio chains <NUM> thus yields a PIM removed signal. In this respect, how much PIM that is removed depends on how well the correction signal captures the PIM. In turn, how well the correction signal captures the PIM depends on how well the model captures the PIM.

In some aspects, a feedback mechanism is introduced by means of which the model is adapted. In particular, in some embodiments, the controller <NUM> is configured to perform (optional) step S110:
S110: The controller <NUM> adapts estimation coefficients of the model based on how much PIM is still present in the PIM removed signal.

There could be different ways in which the estimation coefficients are adapted. In some aspects, the estimation coefficients are selected according as the solution to an optimization problem where the object is to minimize the PIM. That is, in some embodiments, the controller <NUM> is configured to perform (optional) step S110a as part of step S110:
S110a: The controller <NUM> determines which estimation coefficients that per each codeword in the codebook yield minimum PIM for those of the transmitter radio chains that per each codeword cause any signal received by the receiver radio chains <NUM> to be impacted by PIM.

Reference is here made to <FIG> which schematically illustrates PIM removal in a controller <NUM> according to an embodiment. Based on which codeword c is used for the beamforming, those transmission radio chains of the antenna system that cause a signal received by receiver radio chains of the antenna system to be impacted by PIM are identified in TX chains identification block <NUM>. In general terms, the model coefficients are specific for each codeword. That is, in some embodiments, the model has one set of estimation coefficients per each codeword. If a new codeword c' is used for the beamforming, the new codeword c'is added to the codebook <NUM>. A correction signal xpim is determined by subjecting the signals only as transmitted by the identified transmission radio chains to the model <NUM> of the PIM. The PIM is then removed from the signal s<NUM> or s<NUM> as received by the receiver radio chains by subtracting the correction signal xpim from the signal s<NUM> or s<NUM> as received by the receiver radio chains. The adaptation block <NUM> takes as input the PIM removed signal and determines which estimation coefficients that for the used codeword in the codebook yield minimum PIM. The estimation coefficients as adapted are then provided to the model <NUM>.

<FIG> is a flowchart of a method for adapting the model in accordance with embodiments disclosed herein.

S201: Those transmission radio chains <NUM> of the antenna system <NUM> that cause a signal received by receiver radio chains <NUM> of the antenna system <NUM> to be impacted by PIM are identified based on codebook information.

S202: It is checked whether PIM removal has been performed for the codeword used for the beamforming or if the codeword indicates that the beamforming is in a new direction. If the beamforming is in a new direction step S203 is entered, and else step S204 is entered.

S203: The new direction is added to the codebook.

S204: A correction signal is determined by subjecting the signals only as transmitted by the identified transmission radio chains <NUM> to a model of the PIM. The PIM is modelled based on codebook information, such as which codeword was used for the beamforming. That is, the estimation coefficients of the model are selected dependent on which codeword was used for the beamforming. If the beamforming is in new direction (i.e., step S203 was entered) use default values of the estimation coefficients could be used.

Which estimation coefficients that for the codeword yield minimum PIM are then estimated by steps S205, S206, and S208 being iteratively performed
S205: The error energy is measured as a difference between the received signal and the correction signal. This corresponds to the energy of the PIM removed signal.

S206: It is checked whether a minimum of the error energy has been reached, and hence if the power of the PIM removed signal has been minimized. If yes, step S207 is entered, and else step S208 is entered.

S207: The estimation coefficients of the model are stored for later use when beamforming using the same codeword is performed.

S208: The estimation coefficients of the model are adapted based on how much PIM is still present in the PIM removed signal. Step S204 is then entered again where the estimation coefficients as adapted in step S208 are used when determining the correction signal.

Ways to identify which transmission radio chains <NUM> that cause the PIM have been disclosed. Further in this respect, there could be different ways to determine how many transmission radio chains <NUM> that are to be considered by the model and thus that are to be identified. Aspects relating thereto will now be disclosed.

In some aspects, how many transmission radio chains <NUM> that are to be considered by the model depends on how many transmission radio chains <NUM> there are in total in the antenna system <NUM>. For example, if the total number of transmission radio chains <NUM> is comparatively high, only a small fraction of all the transmission radio chains <NUM> might be considered by the model whereas if the total number of transmission radio chains <NUM> is comparatively low, a large fraction of all the transmission radio chains <NUM> might be considered by the model in order to keep a reasonable level of complexity and computational cost at the controller <NUM>. That is, in some embodiments, how many transmission radio chains <NUM> that are to be identified in step S104 is dependent on how many transmission radio chains <NUM> there are in total.

In some aspects, how many transmission radio chains <NUM> that are to be considered by the model depends on the amount of PIM caused. In particular, in some embodiments, how many transmission radio chains <NUM> that are to be identified in step S104 is dependent on how much, or how many of, the receiver radio chains <NUM> are impacted by the PIM. In this respect, for a comparatively high level of PIM, more transmission radio chains <NUM> might be considered by the model than for only a comparatively low level of PIM. The design of the PIM model could thereby be adapted to handle different levels of PIM in an efficient manner.

In some aspects, a fixed amount of transmission radio chains <NUM> are considered by the model. That is, in some embodiments, at most a predefined number of all transmission radio chains <NUM> are identified in in step S104. This might enable an efficient implementation of the PIM model since it could be designed for one particular number of transmission radio chains <NUM>.

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

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

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

<FIG> schematically illustrates, in terms of a number of functional modules, the components of a controller <NUM> according to an embodiment. The controller <NUM> of <FIG> comprises a number of functional modules; an identify module 210b configured to perform step S104, a determine module 210c configured to perform step S106, and a remove module 210d configured to perform step S108. The controller <NUM> of <FIG> may further comprise a number of optional functional modules, such as any of a determine module 210a configured to perform step S210a, an adapt module 210e configured to perform step S110, and a determine module 210f configured to perform step Sna. In general terms, each functional module 210a-210f may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium <NUM> which when run on the processing circuitry makes the controller <NUM> perform the corresponding steps mentioned above in conjunction with <FIG>. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 210a-210f may be implemented by the processing circuitry <NUM>, possibly in cooperation with the communications interface <NUM> and/or the storage medium <NUM>. The processing circuitry <NUM> may thus be configured to from the storage medium <NUM> fetch instructions as provided by a functional module 210a-210f and to execute these instructions, thereby performing any steps as disclosed herein.

The controller <NUM> may be provided as a standalone device or as a part of at least one further device. For example, the controller <NUM> may be provided in a node of the radio access network and might be part of, integrated with, or collocated with, the antenna system <NUM>. Alternatively, functionality of the controller <NUM> may be distributed between at least two devices, or nodes. Thus, a first portion of the instructions performed by the controller <NUM> may be executed in a first device, and a second portion of the of the instructions performed by the controller <NUM> may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the controller <NUM> may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a controller <NUM> residing in a cloud computational environment. Therefore, although a single processing circuitry <NUM> is illustrated in <FIG> the processing circuitry <NUM> may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 210a-210f of <FIG> and the computer program <NUM> of <FIG>.

Claim 1:
A method for passive intermodulation, PIM, removal in an antenna system (<NUM>), the method being performed by a controller (<NUM>) of the antenna system (<NUM>), the method comprising:
identifying (S104), during transmission using codebook based beamforming, which transmission radio chains (<NUM>) of the antenna system (<NUM>) that cause a signal received by receiver radio chains (<NUM>) of the antenna system (<NUM>) to be impacted by PIM, wherein these transmission radio chains (<NUM>) are identified based on which codeword in the codebook is used for the beamforming;
determining (S106) a correction signal by subjecting the signals only as transmitted by the identified transmission radio chains (<NUM>) to a model of the PIM; and
removing (S108) PIM from the signal received by the receiver radio chains (<NUM>) by subtracting the correction signal from the signal received by the receiver radio chains (<NUM>).