Method of operating a load-modulated linearizer and arrangement therefor.

Abstract

A method of operating a load-modulated linearizer is described for linearizing the output signal of a nonlinear power amplifier. The load-modulated linearizer comprises a first and second hybrid coupler and at least two control amplifiers, connected to different ports of the hybrid couplers. The at least two control power amplifiers are configured for providing an identical signal response in use. The first hybrid coupler comprises a control signal input and the second hybrid coupler comprises a linearization port for providing the linearized output signal. For performing the linearization the method comprises generating, by a signal generator, the control signal. This generating comprises receiving, at an input port of the signal generator, a main input signal to be amplified by the nonlinear power amplifier; and generating, by the signal generator, a mimicked output signal wherein the mimicked output signal is a representation of the output signal of the nonlinear power amplifier. The generating of the control signal further comprises generating, by the signal generator and based on the mimicked output signal, said control signal for the at least two control power amplifiers.

Description

[0001]Title: Method of operating a load-modulated linearizer and arrangement therefor. Field of the invention The present invention is directed at a method of operating a load- modulated linearizer for linearizing the output signal of a nonlinear power amplifier, for removing by said linearizing a distortion signal component from said output signal of a nonlinear power amplifier, wherein the load-modulated linearizer comprises a first hybrid coupler, a second hybrid coupler and at least two control power amplifiers, wherein the at least two control power amplifiers are connected to different ports of the first and second hybrid coupler such that each of the control power amplifiers is connected between the first and the second hybrid coupler, and wherein the at least two control power amplifiers are configured for providing an identical signal response in use; wherein the first hybrid coupler comprises a control signal input for receiving a control signal to be provided via the first hybrid coupler to each of the control power amplifiers, and wherein the second hybrid coupler comprises a linearization port which is connectable to a signal output of the nonlinear power amplifier for receiving the output signal therefrom, wherein the output signal comprises a main signal component and said distortion signal component, and wherein the second hybrid coupler further comprises a linearized output port for providing the linearized output signal from which at least a part of the distortion signal component has been removed. The present invention further relates to an arrangement for use in the above method. Background Although the present document includes references to various other documents, no admission is made that any reference constitutes prior art. The discussion of references refers to their content as presented therein, and does not acknowledge nor confirm the accuracy or pertinency thereof. It will be understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art in any country. The rapidly increasing demand for wireless devices and high data-rates drives the development of more complex digital modulation techniques in which the signals make efficient use of the available bandwidth. These complex signals should be transmitted with satisfactory signal quality while the efficiency of the wireless devices for a given output power is as high as possible. A high efficiency lowers power consumption expenses and mitigates the need for a bigger battery and strong thermal management, and a high output power increases the range of transmission. The ongoing adoption of 5G and the current development of 6G both promise increasingly faster download speeds, low latency, and reduced congestion on mobile networks. The development of 6G, in particular, will support new emerging technologies such as virtual reality (VR), augmented reality (AR), the brave new worlds of the Metaverse, and artificial intelligence (AI). These new ambitions present novel and exciting engineering challenges, in particular for the transmitter (TX) systems. Whether space-borne, terrestrial or mobile, the next generation TX systems must achieve large bandwidths, deliver high power, remain linear, maintain as high efficiency as possible, and - of course - remain economical. Thus, there is a clear need to develop new amplification and linearisation systems to address the ever-increasing commercial and research demands for the reliable transmission of data. High efficiency in power amplifier (PA) systems comes at the cost of increased intermodulation products (IMD), resulting in high in-band and, often more limiting, out-of-band (OOB) distortion. This necessitates a compromise in the form of increased output back-off (OBO) power levels and consequently significantly lower power added efficiency (PAE). Developing ways of mitigating this OBO is an active field of study with the Doherty amplifiers used in telecom being a well-known example. The most widely used linearization process for telecommunication applications is digital predistortion (DPD), which is most effective when the PA is in sufficient OBO and less so at or above the P1dB compression point where PA efficiency tends to be highest. Active scanning array systems using DPD can incur additional power efficiency penalties due to how computational costs scale against array gain. Within the context of simultaneous multi-beam transmit array systems for radar, where multiple narrowband tones are amplified by each PA, the main challenge of linearization is the very high AM / AM distortion as the PAs operate in high compression levels where they are either most efficient or generate maximum output power. This presents a unique challenge for existing DPD systems as they become unable to correct for the high AM / AM distortion products despite the narrow-band nature of the radar signals. The resulting IMD and harmonic products radiate in various directions depending on the beam-pointing directions of the main tones, which may cause unwanted jamming of adjacent systems. Another such example where multiple narrowband tones are amplified by each PA is provided by orthogonal frequency division multiplexing (OFDM) for communication purposes. A relatively new power amplifier architecture is the load modulated balanced PA (LMBA). The LMBA is able to modulate the impedance seen by a pair of RF power transistors in a quadrature balanced configuration, by varying the amplitude and phase of an external control signal. This enables power and efficiency to be optimized dynamically at specific power back-off levels and frequencies. Unlike the Doherty PA, the load seen by the active devices can be modulated upwards or downwards, both resistively and reactively, with minimal loss of power combination efficiency. The LMBA is considered to enable any specific amplifier characteristic to be controlled dynamically over wide signal amplitude and frequency ranges. Linearization techniques are commonly used to extend the linear range of the PA. These techniques are usually applied in combination with efficiency enhancement PA topologies. As earlier referred to, a widely used linearization technique is digital predistortion (DPD), in which the input of the PA is predistorted in magnitude and phase to compensate for the non-linear effects of the PA. The amount of predistortion power supplied by DPD for correcting AM-AM distortion is proportional to the difference between the ideal and realistic PA behaviour. Consequently, close to compression levels, a large portion of the PA’s input power consists of the predistortion components, restricting the gain headroom for the main input signal. Correction is not possible for peak excursions beyond the saturated output power level. This reveals that for correcting AM-AM distortion DPD is most effective in OBO, where the PA efficiency is low. On the other hand, correcting AM-PM distortion with DPD only requires a phase rotation of the input signal and is therefore not restricted to OBO. Depending on the PA implementation, DPD may require advanced behavioural models of the amplifier, which can be implemented e.g. as a look-up-table (LUT), Volterra series or memory polynomial. Memory polynomial based behavioural models can be seen as a simplified version of a Volterra series, which have a high computational complexity, which can greatly deteriorate the overall system efficiency. Alternatively, the PA can be linearized at its output, as done with feedforward linearization. In this technique first the scaled difference between PA input and output is used to estimate its distortion, and then this difference is inverted and added to the PA output. The main limitation of this technique is that it requires as much control power as the output generated by the main PA. This imposes a very demanding requirement on the control PA, which must not only remain linear, but also deliver nearly as much power as the main PA to be linearized. In the realistic case when the power combination involves different power magnitudes, power is split between the load and the combiner’s isolated port, and thus, also lowering the overall efficiency. Summary of the invention It is an object of the present invention to provide a new linearization method and arrangement for linearizing the output signal of a nonlinear power amplifier (PA; also referred to as ‘main PA’), which enables to linearize both in-band and out-of-band AM / AM distortion at high PA compression levels, and which is highly efficient in power usage. The present document thereby relates to operation of a new type of power amplifier architecture which is provided by the load-modulated linearizer (LML) to actively absorb individual in-band and out-of-band intermodulation distortion (IMD) components at the output of a power amplifier operating in high compression. In particular, the present invention relates to a method of operating a load- modulated linearizer for linearizing the output signal of a nonlinear power amplifier, for removing by said linearizing a distortion signal component from said output signal of a nonlinear power amplifier. The load-modulated linearizer comprises a first hybrid coupler, a second hybrid coupler and at least two control power amplifiers. The at least two control power amplifiers are connected to different ports of the first and second hybrid coupler such that each of the control power amplifiers is connected between the first and the second hybrid coupler. The at least two control power amplifiers are configured for providing an identical signal response in use. The first hybrid coupler comprises a control signal input for receiving a control signal to be provided via the first hybrid coupler to each of the control power amplifiers. The second hybrid coupler comprises a linearization port which is connectable to a signal output of the nonlinear power amplifier for receiving the output signal therefrom. The output signal comprises a main signal component and said distortion signal component, and the second hybrid coupler further comprises a linearized output port for providing the linearized output signal from which at least a part of the distortion signal components have been removed or reduced. For performing the linearization the method comprises generating, by a signal generator, the control signal and providing the control signal to the control signal input for controlling the control power amplifiers to provide a linearization signal to the second hybrid coupler. This generating comprises receiving, at an input port of the signal generator, a main input signal to be amplified by the nonlinear power amplifier; and generating, by the signal generator, a mimicked output signal wherein the mimicked output signal is a representation of the output signal of the nonlinear power amplifier. The generating of the control signal further comprises generating, by the signal generator and based on the mimicked output signal, said control signal for the at least two control power amplifiers. The proposed operating method generates a control signal by applying a feed forward scheme. In this feed forward scheme, the output signal of the PA in response to the main input signal is mimicked, and thereafter used in order to determine a control signal that causes the LML to effectively amplify the desired ideal output signal and to absorb the undesired distortion components therein. Thereby, the output of the power amplifier can be effectively linearized by the LML. In this manner, the system is able to correct AM / AM distortion by absorbing the unwanted IMD components using active load-modulation. The LML reverses the conventional roles of the main PA and the control PAs in the original LMBA design, retains the same bandwidth and power conservation properties and achieves linearization at a very low power and complexity penalty. The LML is a hybrid linearization system which can either be applied to the output of any nonlinear power amplifier, or be co-designed as a complete, state-of- the-art power amplifier solution. The LML is device agnostic and can be implemented using any technology. For example, the LML may be applied to linearize the output of many types of PAs, such as Si-PAs, SiGe-PAs, GaN-PAs, Doherty type PAs, class-A, class-B, class-AB, class-C, class-D, transistors, tubes, accelerators, operational amplifiers, or others, broadly described as power amplifiers (PAs). The key feature of the LML is its ability to operate at very strong compression levels, maintain linearity, high efficiency, and good bandwidth. The LML is a system in which the LMBA architecture is modified by reversing the roles of the balanced (high power) and control (low power) devices. By using control signals to actively modulate the impedances seen by the two balanced devices, the LML is capable of absorbing unwanted intermodulation distortion products, while simultaneously also reinforcing the main tones in a lossless manner, resulting in highly efficient performance. It belongs to the class of feed-forward linearizers as the cancellation of the unwanted tones is achieved at the output of the PA instead of its input (or within it), as with digital predistortion. Additionally, the LML achieves this linearization using a completely different mechanism from conventional feed-forward architectures, thus avoiding their drawbacks and limitations. The LML can linearize amplifiers operating at strong compression levels where other linearization techniques such as digital predistortion (DPD), which is currently the most broadly used, cannot operate. As such, the LML offers new opportunities for radar, satcom and telecom applications. In some embodiments, the step of generating the control signal based on the mimicked output signal comprises: generating, by the signal generator and based on the mimicked output signal, an estimated linearization signal and scaling the estimated linearization signal by based on a control gain factor, the control gain factor being equal to a gain factor of each of the at least two control power amplifiers to obtain said control signal. Properly dimensioning the linearization signal to take into account the gain factor of the (identically responding) control power amplifiers (control PAs) is desired to reduce power usage and prevent introduction of new distortion components. In some embodiments thereof, the scaling of the estimated linearization signal is performed by attenuating the estimated linearization signal with an attenuation factor equal to a mathematical inverse of the control gain factor. In other or further embodiments, the mimicked output signal is generated, by the signal generator, such as to include a mimicked main output signal component, wherein the mimicked main output signal component is generated by amplification of the main input signal with a main gain factor, the main gain factor being equal to a gain factor of the nonlinear power amplifier. Here, the ideal output signal may be obtained by mimicking the ideal behavior (linear gain) of the amplifier. In yet further embodiments, the mimicked output signal is generated, by the signal generator, such as to include a mimicked distortion signal component, wherein the mimicked distortion signal component is estimated based on the main input signal by applying thereto a digital model of a nonlinear behavior of the nonlinear power amplifier. Advantageously, even, both is done in order to obtain a very good estimate of the real output signal of the non-linear power amplifier. By modelling both the ideal and non- ideal behavior of the power amplifier, prior to generating the control signal on the basis of this mimicked output signal, a control signal is obtained that optimally performs the desired function of operating the LML to amplify the desired ideal output signal and to absorb the undesired distortion components therein. In some embodiments, the signal generator is a digital signal generator (DSG), such as an arbitrary waveform generator. A digital signal generator is able to perform the above steps well, in order to generate a well estimated mimicked output signal. Furthermore, the DSG provides a flexible implementation that may be tuned easily to any changing circumstances. However, implementation of the proposed concept is not limited to applying a DSG. In other or further embodiments, the signal generator (or parts thereof) may be provided by an analog signal generator. In accordance with a second aspect, there is provided an arrangement including a load-modulated linearizer and a nonlinear power amplifier, wherein the load-modulated linearizer for linearizing the output signal of a nonlinear power amplifier, for removing by said linearizing a distortion signal component from said output signal of the a nonlinear power amplifier, wherein the load-modulated linearizer comprises a first hybrid coupler, a second hybrid coupler and at least two control power amplifiers, wherein the at least two control power amplifiers are connected to different ports of the first and second hybrid coupler such that each of the control power amplifiers is connected between the first and the second hybrid coupler, and wherein the first hybrid coupler comprises a control signal input for receiving a control signal to be provided via the first hybrid coupler to each of the control power amplifiers, and wherein the second hybrid coupler comprises a linearization port and a linearized output port, wherein the linearization port is connected to a signal output of the nonlinear power amplifier for receiving the output signal, and wherein the linearized output port in use provides the linearized output signal from which at least a part of the distortion signal component has been removed; wherein the arrangement further comprises a signal generator for generating the control signal and providing the control signal to the control signal input for controlling the control power amplifiers to provide a linearization signal to the second hybrid coupler, wherein the signal generator comprises an input port for receiving a main input signal to be amplified by the nonlinear power amplifier; and wherein the signal generator is configured for: generating a mimicked output signal wherein the mimicked output signal is a representation of the output signal of the nonlinear power amplifier; and for generating, based on the mimicked output signal, said control signal for the at least two control power amplifiers. The above arrangement applies a load-modulated linearizer. The arrangement including the LML is designed and operated to absorb AM-AM distortion from a PA in compression using control signals generated by auxiliary amplifiers. The required output power of these auxiliary amplifiers is significantly less than the output power of the main PA, making the LML potentially highly efficient. In some embodiments, for generating the control signal, the signal generator is configured for generating, based on the mimicked output signal, an estimated linearization signal, wherein the signal generator further comprises an attenuator or amplifier for scaling the estimated linearization signal based on a control gain factor, wherein the control gain factor is equal to a gain factor of each of the at least two control power amplifiers to obtain said control signal. In some of these embodiments, the attenuator is configured for scaling the estimated linearization signal by an attenuation factor equal to a mathematical inverse of the control gain factor. In some embodiments, the signal generator is configured for generating the mimicked output signal such as to include therein a mimicked main output signal component, wherein the signal generator comprises amplification means for amplifying the main input signal with a main gain factor for generating the mimicked output signal, the main gain factor being equal to a gain factor of the nonlinear power amplifier. Furthermore, in some embodiments, the signal generator is configured for generating the mimicked output signal such as to include a mimicked distortion signal component, wherein the signal generator comprises a digital modelling element for modelling a nonlinear behavior of the nonlinear power amplifier for estimating the mimicked distortion signal component based on the main input signal. In particular, in some implementations, the signal generator is a digital signal generator, such as an arbitrary waveform generator. Yet in other or further implementations, the signal generator or parts thereof is or are provided by an analog signal generator. In a third aspect, there is provided a computer program product comprising instructions for loading into a memory of a digital signal generator, for enabling a processor of the digital signal generator when processing said instructions, to perform a method according to the first aspect. Brief description of the drawings The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is not to be regarded as describing the only embodiments falling under the scope. The scope of the invention is defined in the claims, and the description is to be regarded as illustrative without being restrictive on the invention. In the drawings: Figure 1 schematically illustrates the conceptual performance of a power amplifier; Figure 2 schematically illustrates a circuit including a main power amplifier connected to a load modulated linearizer in accordance with the invention; Figure 3 shows an LML arrangement in accordance with the invention; Figure 4 is a schematic representation of an experimental setup in accordance with the invention. Detailed description Terminology used for describing particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and / or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise. The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference to schematic and / or cross-section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise. The rapidly increasing demand for wireless devices and high data-rates drives the development of more complex digital modulation techniques in which the signals make efficient use of the available bandwidth. These complex signals should be transmitted with satisfactory signal quality while the efficiency of the wireless devices for a given output power is as high as possible. A high efficiency lowers power consumption expenses and mitigates the need for a bigger battery and strong thermal management, and a high output power increases the range of transmission. The power amplifier (PA) is the most power demanding component in a transmitter front-end. When it is most efficient, it behaves in a non-linear way and produces strong distortion including harmonics and intermodulation distortion (IMD) tones degrading the signal quality (EVM) and failing regulatory requirements (ACLR, out-of-band distortion, harmonics). This fundamental trade-off of the PA is visualized in fig. 1, which shows that a conventional PA by itself cannot produce a satisfactory signal quality when it is most efficient. Figure 1 illustrates the conceptual performance of a power amplifier, with on the horizontal axis 5 the input power Pin, on the left hand vertical axis 6 the output power Pout, and on the right hand vertical axis 7 the efficiency as a percentage. The line 10 is indicative of the PA’s real behavior, whereas the dashed line 17 indicates its ideal behavior. Further indicated are the output power back-off (OBO) regime 14 and the compression mode regime 15. From figure 1, it is clear that the efficiency loss 11 increases when the PA operates near maximum output power. Distortion components are produced when the realistic PA behavior 10 deviates from the ideal PA behavior 17. To meet signal quality constraints the maximum power 19 is limited, resulting in a loss in efficiency 11. The distortions (e.g. harmonics, clock spurs, IMD signals and other disturbances) of a PA operated at high peak power, i.e. in compression 15, exist both in-band (IB) and out-of-band (OOB) and can be in the form of amplitude to amplitude (AM-AM) and amplitude to phase (AM-PM) distortion. Out-of-band distortion interferes with other spectral users, and in-band distortion interferes with the transmitted signal itself and hence can cause bit errors in the transmitted signal. The maximum allowed IB and OOB distortion is implicitly or explicitly defined by governing authorities, and either one may be limiting depending on the PA design and telecommunication standards. As a compromise, the PA is often operated in output back-off (OBO), where the efficiency 11 can be significantly lower than when operating in compression 15. Hence, PA topologies which enhance the efficiency at OBO have been deployed, of which the Doherty, envelope-tracking PA and the load-modulated balanced amplifier (LMBA) are commonly known. Linearization techniques are commonly used to extend the linear range of the PA. These techniques are usually applied in combination with efficiency enhancement PA topologies. This document relates to the PA linearizer concept referred to as the load- modulated linearizer. The LML is designed to absorb AM-AM distortion from a PA in compression using control signals generated by auxiliary amplifiers. The required output power of these auxiliary amplifiers is significantly less than the output power of the main PA, making the LML potentially highly efficient. The LML absorbs unwanted IMD tones at the output of the PA. In contrast to feedforward linearization, the LML exploits active load modulation for removing unwanted distortion. Like DPD, the LML requires sufficiently accurate behavioral models, but the major difference with DPD is that DPD adds predistortion before the PA, while the LML performs the linearization after the PA. This limits DPD for AM- AM distortion absorption to OBO 14 while the LML can operate across the whole operational range, including in compression 15. However, correcting AM-PM distortion with the LML is power related and can only be corrected at a high power cost. Thus, combining the two techniques together results in a system capable of efficiently compensating both AM-AM and AM-PM distortion at very high compression levels. The ability to remove distortion products while a PA operates in strong compression, where it is most efficient, is of great interest both for radar and telecommunication applications. For example, in multi-frequency continuous waveform radar and in simultaneous multi-beam radar systems, the LML could be employed to absorb the strong IMD tones. Additionally, having PAs in base stations which can operate at greater power levels and efficiencies will simultaneously increase the coverage range and reduce the cooling requirements. The LML can absorb unwanted out-of-band (OOB) distortions without affecting the desired main tones. Additionally, proper operation also allows the LML to just as easily absorb in-band unwanted distortions. Figure 2 schematically illustrates a circuit 20 including a main power amplifier 30 connected to a load modulated linearizer 21. It consists of two control amplifiers 23 and 24, PAC, with identical gains Gctrl and output impedances Zout. The two control power amplifiers 23 and 24 are configured for providing an identical signal response in use. Thus, they do not need to be identical (although this is of course a possible implementation) but they must respond with an identical output signal to an identical signal at their inputs. The two control amplifiers 23 and 24, PAC, are connected to ports 2 and 3 of a first 90◦hybrid coupler 25. The two control Pas 23 and 24 are excited in quadrature such that their outputs sum to port 4 of a second 90◦hybrid coupler 26, which is connected to a matched load Z0, representing a matched antenna 29 at the output of LML 21. Port 1 of the second hybrid coupler 26 is connected to the output of the main PA 30 to be linearized. The main PA 30 operates in strong compression 15, i.e. above P1dB, and producing a set of desired and undesired distortion currents. In figure 2, the desired output power of PAM is indicated as PM, whereas the distortion power is indicated as PD. Their sum, PM+PD is the total output power provided by PAM 30. We occasionally refer to the main PA 30 shortly as PAM, and it has an available gain of Gmain. The output impedance of PAM is Z0, same as the characteristic impedance of the second 90◦hybrid coupler 26. The LML 21 requires significantly less control power compared to the main PA 30 output power even for very high amounts of distortion. The LML 21 thus consists of three active devices capable of delivering electromagnetic power. These power amplifiers PAC 23 and 24 and PAM 30 could be of any desired type, such as transistors, tubes, accelerators, operational amplifiers, or others, broadly described as power amplifiers (PAs). The LML 21 consists of two control power amplifiers PAC 23 and 24, which are excited in quadrature by use of two 90 degree (90◦) hybrid couplers 25 and 26 which have an input impedance (e.g.50 Ohm; rectangular blocks). An isolation resistor is connected to port 1 of the input hybrid coupler or first 90◦hybrid coupler 25. The two control PAs 23 and 24 have very low output impedance (no output matching networks present) and their outputs are directly connected to the output hybrid coupler or second 90◦hybrid coupler 26 at ports 2 and 3. The main amplifier 30 to be linearized, PAM, operates in compression 15 (fig.1) producing a sum total of desired main power, PM, and a sum total of unwanted distortion power, PD, both spread across the bandwidth of interest. The PAM 30 is connected to the isolated port (1) of the second 90◦hybrid coupler 26 (i.e. quadrature balanced amplifier) which serves as control devices and performs active load- modulation at all relevant frequencies. A set of control signals 32 (CTRL) are delivered via port 4 of the first 90◦hybrid coupler 25 to the control amplifiers 23 and 24 and are used to actively load- modulate the input impedances at ports 2 and 3 of the second 90◦hybrid coupler 26 (90◦quadrature hybrid coupler) such that the desired signals of the main amplifier are reflected towards the output of the system at port 4 of the second 90◦hybrid coupler 26. Simultaneously, the undesired distortion signals are absorbed into the control devices 23 and 24. The exact amplitude and phase of the control signals 32 CTRL are based on estimates of the nonlinear behavior of the main amplifier 30. Such estimates can be created in either a digital or analog manner. This mechanism functions for signals that do not overlap spectrally (i.e. out-of-band) as well as those that are fully or at least partially spectrally overlapping (i.e. in-band). The LML 21 performs two tasks simultaneously across the operational bandwidth. First, it uses a set of control currents to losslessly couple all the desired main currents to the output port 1 of the second 90◦hybrid coupler 26; and second, it uses another set of control currents to absorb all unwanted distortion currents. Both tasks are performed for all relevant currents across the bandwidth of operation. The phase components of the control currents are relative to the main and distortion currents, respectively, and are used to control the load-modulation mechanism of the LML 21. Figure 3 shows the proposed architecture of the LML arrangement 40, including an LML 21. The arrangement 40 is designed in a feedforward configuration, due to its simplicity, in which a digital model of the main PA 30 is used to estimate the PM and PD power given some bandwidth-limited input signal. The output impedance Zout is fixed and designed for the expected average PM and PD powers. This feedforward architecture requires a sufficiently accurate model of the main PA 30 and control PAs 23 and 24, since there is no feedback to correct model inaccuracies, as would be in an adaptive architecture implementation. Although not implemented in the illustrated embodiment, active feedback may well be implemented as a further improvement. For example, in signal processing such feedback may be used to update the model parameters applied in the feed forward architecture, e.g. the model parameters of the main PA model 54 or the expected linear main PA gain 52.This may for example be achieved using a LUT, LMS-filter, neural network or any other suitable means. In the embodiment shown, to keep the overall system complexity low for explanatory purposes, no feedback has been implemented. Figure 3 provides a block diagram of the feedforward architecture for creating the control signals 32 in the LML 21. It shows the LML 21 and nonlinear main PA 30 in the RF domain, and the control sequence 50 in digital baseband (although neither the concept nor this embodiment is limited to this) consisting of the main PA model 54, the expected linear main PA gain 52 (Gmain), and attenuation 60 to correct for the control PA gain (Gcon). The main PA model 54 and the expected linear main PA gain 52 respectively provide for a realistic or real expected power output Pout and an ideal power output PM. Subtracting these at 65 will provide the estimated disturbance power PD. This is used in linearizer elements 56 and 58 to respectively provide a first set of control signals 57 (or first partial linearization signal) suitable for controlling the control PAs 23 and 24 of the LML 21 to absorb all unwanted distortion currents; and a second set of control signals 59 (or second partial linearization signal) suitable for controlling the control PAs 23 and 24 of the LML 21 to couple all the desired main currents to the output port 1 of the second 90◦hybrid coupler 26. In 67, these partial signals are summed to obtain the total control signal PT, which is attenuated in element 60 to correct for the control gain of the control power amplifiers 23 and 24 in the LML 21. The scaled control signal is provided at second output port 70 of the arbitrary waveform generator 80. The input signal 72 to be transmitted Pin is further provided via first output port 71 to the main amplifier 30. The below provides an implementation example that is helpful for understanding the inventive concept, and should not be interpreted as limiting on the claimed invention in any way. The scope is determined by the appended claims. The below relates to an experimental setup that was used to study the behavior of the LML. It must be understood, though, that any suitable type of control PAs 23 and 24 may be applied in order to implement an LML 21 as described herein. For example, the control PAs 23 and 24 of LML 21 may be constructed from two commercially available Mini-Circuits PAs of type ZX60-2411BM-S+, 0.8 − 2.4GHz, having a gain of 13dB and input-referred P1dB of 10dBm. The main PA 30 that was applied for testing was a ZKL-2R7+, 0.01 − 2.7GHz, having a high gain of 24dB but low input-referred P1dB of -6dBm. Naturally, the LML 21 may be applied in order to linearize the behavior of any other arbitrary main PA 30, and the described setup is only an example. The same is true for the two control PAs 23 and 24. Although it is important that the two control PAs 23 and 24 applied in the LML 21 provide an identical output signal to an identical signal at their input, the type amplifiers used for the control PAs may be freely chosen by the skilled person. Naturally, the operational bandwidth of the control PAs 23 and 24 preferably covers the bandwidth of the transmission signal 72, and preferably more in order to absorb both the IB and OOB disturbance and linearize the desired output of the main PA 30. This combination of control PAs 23 and 24 and main PA 30 in the experimental setup ensures that the control PAs 23 and 24 do not introduce their own distortion products as the main PA 30 operates in compression. The control PAs 23 and 24 and main PA 30 are by design unconditionally stable. The 50Ω output impedance of the control PAs 23 and 24 is transformed to the desired Zout using custom-made PCB-based quarter-wavelength transformers (λ / 4 TFs) 82 and 83 (see fig.4). Multiple transformers have been realized with five discrete measured Zout values ranging from 2.07Ω up to 5.78Ω, each of them optimal for a different normalized distortion power contribution. Note that in a regular PA design, the typical output impedance of the transistors is already quite low, obviating the need for (most) matching components. The control PAs 23 and 24 and the λ / 4 TFs 82 and 83 are connected in between the hybrid couplers 25 and 26. The hybrid couplers 25 and 26 applied in this example are provided by two Mini Circuits power splitter / combiners of type ZX10Q-2- 27+, 2 way-90°, 50Ω, 1.7 - 2.7 GHz. In the balanced pair, cables and connectors are selected to reduce gain and phase imbalances as much as possible. Outside of the balanced pair, sub miniature version A (SMA) type cables are used to connect the components to the instruments. In the experimental setup, the input signal 72 of the main PA 30 and the control signal 32 are generated on an arbitrary waveform generator 80. The arbitrary waveform generator 80 is and M8190A type AWG of Agilent with a MATLAB (R2022a type) interface. The LML output signal at output port 73 is analyzed simultaneously on both a spectrum analyzer 87 (SA, Siglent SVA 1032X) and a digital signal analyzer 86 (DSA, Agilent Infiniium DSA-Z-204A) with a commercially available power splitter 85 of type ZN2PD2-63-S+. Figure 4 is a schematic representation of the experimental setup described above. Here, the output 73 of LML 21 is connected, via splitter element 85, to a digital signal analyzer 86 and a spectrum analyzer 87. Control signals 32 are provided by the arbitrary waveform generator 80, which also provides the transmission signal to the main power amplifier 30. Figure 4 merely shows the main components and signals of the experimental setup for the LML 21. As stated above and repeated herewith, this experimental setup described and illustrated is merely provided for explanatory purposes and is by no means intended to be limiting on the invention. As may be appreciated, it is undoable to list all the different parts and circuits that may be applied in combination to provide an LML 21 falling within the scope of the claimed concept. The main PA 30 and control PAs 23 and 24 are modelled by the AWG 80 in the described experimental setup using a look-up-table (LUT) containing AM-AM and AM-PM data, while all other circuit components are modelled using S-parameter data. Since the LML 21 can only efficiently compensate AM-AM distortion, a digital pre- rotation of the I and Q signals is applied to the input data based on the inverse expected AM-PM LUT to compensate for any AM-PM distortion of the main PA 30 and control PAs 23 and 24. This does not affect the LML’s efficiency, as it is a purely digital manipulation. The architecture of figure 3 is implemented together with the digital models in MATLAB / Simulink to estimate the required control signals 32. These control signals 32 are generated on the same AWG 80 with a synchronized clock and trigger with the modulated input signal 72 of main PA 30. These conditions are relevant since the phase and amplitude between the input signal 72 and control signal 32 preferably is accurately controlled for optimal distortion reduction. In the present document, the concept has been explained with reference to a load-modulated linearizer that includes two hybrid couplers: a first hybrid coupler, a second hybrid coupler and at least two control power amplifiers. The first hybrid coupler comprises a control signal input for receiving a control signal to be provided to each of the control power amplifiers. The second hybrid coupler comprises a linearization port which is connectable to a signal output of the nonlinear power amplifier for receiving the output signal therefrom. In all of the above implementation examples of the described concept, the first and the second hybrid couplers have been described to be 90ohybrid couplers. The first hybrid coupler does not necessarily have to be provided by a 90ohybrid coupler. In fact, any component which can divide a signal in two signals with a 90 degree relative phase offset will be effective, and the first hybrid coupler as recited in the appended claims may thus be replaced by such a different component. The present invention has been described in terms of some specific embodiments thereof. It will be appreciated that the embodiments shown in the drawings and described herein are intended for illustrational purposes only and are not by any manner or means intended to be restrictive on the invention. It is believed that the operation and construction of the present invention will be apparent from the foregoing description and drawings appended thereto. It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which should be considered within the scope of the appended claims. Also kinematic inversions are considered inherently disclosed and to be within the scope of the invention. Moreover, any of the components and elements of the various embodiments disclosed may be combined or may be incorporated in other embodiments where considered necessary, desired or preferred, without departing from the scope of the invention as defined in the claims. In the claims, any reference signs shall not be construed as limiting the claim. The term 'comprising' and ‘including’ when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus the expression ‘comprising’ as used herein does not exclude the presence of other elements or steps in addition to those listed in any claim. Expressions such as "consisting of", when used in this description or the appended claims, should be construed not as an exhaustive enumeration but rather in an inclusive sense of "at least consisting of". Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. Features that are not specifically or explicitly described or claimed may be additionally included in the structure of the invention within its scope. Any of the claimed or disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise, without departing from the claimed invention. Expressions such as: "means for ...” should be read as: "component configured for ..." or "member constructed to ..." and should be construed to include equivalents for the structures disclosed. The use of expressions like: "critical", "preferred", "especially preferred" etc. is not intended to limit the invention. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the spirit and scope of the invention, as is determined by the claims. The invention may be practiced otherwise then as specifically described herein, and is only limited by the appended claims.

Claims

Claims 1. Method of operating a load-modulated linearizer for linearizing the output signal of a nonlinear power amplifier, for removing by said linearizing a distortion signal component from said output signal of the nonlinear power amplifier, wherein the load-modulated linearizer comprises a first hybrid coupler, a second hybrid coupler and at least two control power amplifiers, wherein the at least two control power amplifiers are connected to different ports of the first and second hybrid coupler such that each of the control power amplifiers is connected between the first and the second hybrid coupler, and wherein the at least two control power amplifiers are configured for providing an identical signal response in use; wherein the first hybrid coupler comprises a control signal input for receiving a control signal to be provided via the first hybrid coupler to each of the control power amplifiers, and wherein the second hybrid coupler comprises a linearization port which is connectable to a signal output of the nonlinear power amplifier for receiving the output signal therefrom, wherein the output signal comprises a main signal component and said distortion signal component, and wherein the second hybrid coupler further comprises a linearized output port for providing the linearized output signal from which at least a part of the distortion signal component has been removed; wherein for performing the linearization the method comprises generating, by a signal generator, the control signal and providing the control signal to the control signal input for controlling the control power amplifiers to provide a linearization signal to the second hybrid coupler, wherein said generating comprises: receiving, at an input port of the signal generator, a main input signal to be amplified by the nonlinear power amplifier; generating, by the signal generator, a mimicked output signal wherein the mimicked output signal is a representation of the output signal of the nonlinear power amplifier; and generating, by the signal generator and based on the mimicked output signal, said control signal for the at least two control power amplifiers.

2. Method according to claim 1, wherein the step of generating the control signal based on the mimicked output signal comprises: generating, by the signal generator and based on the mimicked output signal, an estimated linearization signal and scaling the estimated linearization signal by based on a control gain factor, the control gain factor being equal to a gain factor of each of the at least two control power amplifiers to obtain said control signal.

3. Method according to claim 2, wherein the scaling of the estimated linearization signal is performed by attenuating the estimated linearization signal with an attenuation factor equal to a mathematical inverse of the control gain factor.

4. Method according to any one or more of the preceding claims, wherein the mimicked output signal is generated, by the signal generator, such as to include a mimicked main output signal component, wherein the mimicked main output signal component is generated by amplification of the main input signal with a main gain factor, the main gain factor being equal to a gain factor of the nonlinear power amplifier.

5. Method according to any one or more of the preceding claims, wherein the mimicked output signal is generated, by the signal generator, such as to include a mimicked distortion signal component, wherein the mimicked distortion signal component is estimated based on the main input signal by applying thereto a digital model of a nonlinear behavior of the nonlinear power amplifier.

6. Method according to any one or more of the preceding claims, wherein the signal generator is a digital signal generator, such as an arbitrary waveform generator.

7. Method according to any one or more of the preceding claims, wherein the signal generator is an analog signal generator.

8. Arrangement including a load-modulated linearizer and a nonlinear power amplifier, wherein the load-modulated linearizer for linearizing the output signal of anonlinear power amplifier, for removing by said linearizing a distortion signal component from said output signal of the nonlinear power amplifier, wherein the load-modulated linearizer comprises a first hybrid coupler, a second hybrid coupler and at least two control power amplifiers, wherein the at least two control power amplifiers are connected to different ports of the first and second hybrid coupler such that each of the control power amplifiers is connected between the first and the second hybrid coupler, wherein the at least two control power amplifiers are configured for providing an identical signal response in use; and wherein the first hybrid coupler comprises a control signal input for receiving a control signal to be provided via the first hybrid coupler to each of the control power amplifiers, and wherein the second hybrid coupler comprises a linearization port and a linearized output port, wherein the linearization port is connected to a signal output of the nonlinear power amplifier for receiving the output signal, and wherein the linearized output port in use provides the linearized output signal from which at least a part of the distortion signal component has been removed; wherein the arrangement further comprises a signal generator for generating the control signal and providing the control signal to the control signal input for controlling the control power amplifiers to provide a linearization signal to the second hybrid coupler, wherein the signal generator comprises an input port for receiving a main input signal to be amplified by the nonlinear power amplifier; and wherein the signal generator is configured for: generating a mimicked output signal wherein the mimicked output signal is a representation of the output signal of the nonlinear power amplifier; and for generating, based on the mimicked output signal, said control signal for the at least two control power amplifiers.

9. Arrangement according to claim 8, wherein for generating the control signal, the signal generator is configured for generating, based on the mimicked output signal, an estimated linearization signal, wherein the signal generator further comprises an attenuator for scaling the estimated linearization signal based on a control gain factor, wherein the control gain factor is equal to a gain factor of each of the at least two control power amplifiers to obtain said control signal.

10. Arrangement according to claim 9, wherein the attenuator is configured for scaling the estimated linearization signal by an attenuation factor equal to a mathematical inverse of the control gain factor.

11. Arrangement according to any one or more of claims 8-10, wherein the signal generator is configured for generating the mimicked output signal such as to include therein a mimicked main output signal component, wherein the signal generator comprises amplification means for amplifying the main input signal with a main gain factor for generating the mimicked output signal, the main gain factor being equal to a gain factor of the nonlinear power amplifier.

12. Arrangement according to any one or more of claims 8-11, wherein the signal generator is configured for generating the mimicked output signal such as to include a mimicked distortion signal component, wherein the signal generator comprises a digital modelling element for modelling a nonlinear behavior of the nonlinear power amplifier for estimating the mimicked distortion signal component based on the main input signal.

13. Arrangement according to any one or more of claims 8-12, wherein the signal generator is a digital signal generator, such as an arbitrary waveform generator.

14. Arrangement according to any one or more of claims 8-13, wherein the signal generator is an analog signal generator.

15. Computer program product comprising instructions for loading into a memory of a digital signal generator, for enabling a processor of the digital signal generator when processing said instructions, to perform a method according to any one or more of claims 1-7.

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