Method, apparatus and computer program

Abstract

An access node receives, from user equipment (UE), first channel state information (CSI) 506 associated with a channel between the access node and the UE, and sends, to the UE, a calibration reference signal configuration 510. The access node receives, from the UE via the channel, based on the calibration reference signal configuration, one or more calibration reference signals 512 and determines second CSI associated with the channel 514 based on the one or more calibration reference signals. The access node determines, based on the first CSI and the second CSI, assistance data for waveform correction 516 and sends, to the UE, the assistance data 518. The UE adjusts one or more transmission parameters 520 based on the assistance data. The access node may also adjust one or more reception parameters 522 based on the assistance data. One or more of the calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude associated with the channel.

Description

A communication network can be seen as a facility that enables communications between two or more communication devices or provides communication devices access to a data network. A mobile or wireless communication network is one example of a communication network. A communication device may be provided with a service by an application server. Such communication networks operate in accordance with standards such as those provided by 3GPP (Third Generation Partnership Project) or ETSI (European Telecommunications Standards Institute). Examples of standards provided by 3GPP are the so-called 3GPP standards for cellular technology generations, such as 3GPP standards for 4G technology and 3GPP standards for 5G technology. SUMMARY Some example embodiments of this disclosure will be described with respect to certain aspects. These aspects are not intended to indicate key or essential features of the various example embodiments of this disclosure, nor are they intended to be used to limit the scope of thereof. Other features, aspects, and elements will be apparent to a person skilled in the art in view of this disclosure. For example, it should be appreciated that further aspects may be provided by the combination of any two or more of the various aspects described below. According to an aspect, there is provided an access node comprising means for: receiving, from a user equipment, first channel state information associated with a channel between the access node and the user equipment; sending, to the user equipment, a calibration reference signal configuration; receiving, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals; determining second channel state information associated with the channel based on the one or more calibration reference signals; determining, based on the first channel state information and the second channel state information, assistance data for waveform correction; and sending, to the user equipment, the assistance data. At least one of the one or more calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude associated with the channel. The means may be further for: determining, based on the first channel state information, the calibration reference signal configuration. The calibration reference signal configuration may indicate at least one of: a transmission power of the one or more calibration reference signals; one or more carriers of the one or more calibration reference signals; a bandwidth and / or bandwidth part of the one or more calibration reference signals; a number of symbols comprised in the one or more calibration reference signals; a constellation of symbols comprised in the one or more calibration reference signals; or beam information for the user equipment to use when transmitting the one or more calibration reference signals. The beam information may indicate a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized. The means may be further for: sending, to the user equipment, one or more downlink reference signals, wherein the first channel state information may be based on the one or more downlink reference signals. Determining the assistance data may comprise: determining, based on the first channel state information, a channel impulse response of the channel; removing the channel impulse response from received one or more calibration reference signals to estimate the one or more calibration reference signals as transmitted by the user equipment; determining an expected one or more calibration reference signals based on the calibration reference signal configuration sent to the user equipment; determining the user equipment’s power amplifier response based on the estimated one or more calibration reference signals as transmitted by the user equipment and the expected one or more calibration reference signals; and determining the assistance data based on the determined user equipment’s power amplifier response. The means may be further for: adjusting one or more reception parameters based on the assistance data. According to an aspect there is provided a user equipment comprising means for: sending, to an access node, first channel state information associated with a channel between the access node and the user equipment; receiving, from an access node, a calibration reference signal configuration; based on the calibration reference signal configuration, sending one or more calibration reference signals to the access node via the channel; receiving, from the access node, assistance data for waveform correction; and adjusting one or more transmission parameters based on the assistance data. The means may be further for: receiving, from the access node, one or more downlink reference signals; and determining, based on the one or more downlink reference signals, the first channel state information. At least one of the one or more calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude of the channel. The calibration reference signal configuration may indicate at least one of: a transmission power of the one or more calibration reference signals; one or more carriers of the one or more calibration reference signals; a bandwidth and / or bandwidth part of the one or more calibration reference signals; a number of symbols comprised in the one or more calibration reference signals; a constellation of symbols comprised in the one or more calibration reference signals; or beam information for the user equipment to use when transmitting the one or more calibration reference signals. The beam information may indicate a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized, and wherein sending the one or more calibration reference signals may comprise sending the one or more calibration reference signals using the indicated sequence of beams. According to an aspect, there is provided an access node comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the access node at least to: receive, from a user equipment, first channel state information associated with a channel between the access node and the user equipment; send, to the user equipment, a calibration reference signal configuration; receive, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals; determine second channel state information associated with the channel based on the one or more calibration reference signals; determine, based on the first channel state information and the second channel state information, assistance data for waveform correction; and send, to the user equipment, the assistance data. At least one of the one or more calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude associated with the channel. The at least one processor may be configured to cause the access node to: determine, based on the first channel state information, the calibration reference signal configuration. The calibration reference signal configuration may indicate at least one of: a transmission power of the one or more calibration reference signals; one or more carriers of the one or more calibration reference signals; a bandwidth and / or bandwidth part of the one or more calibration reference signals; a number of symbols comprised in the one or more calibration reference signals; a constellation of symbols comprised in the one or more calibration reference signals; or beam information for the user equipment to use when transmitting the one or more calibration reference signals. The beam information may indicate a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized. The at least one processor may be configured to cause the access node to: send, to the user equipment, one or more downlink reference signals, wherein the first channel state information may be based on the one or more downlink reference signals. The at least one processor may be configured to cause the access node to: determine, based on the first channel state information, a channel impulse response of the channel; remove the channel impulse response from received one or more calibration reference signals to estimate the one or more calibration reference signals as transmitted by the user equipment; determine an expected one or more calibration reference signals based on the calibration reference signal configuration sent to the user equipment; determine the user equipment’s power amplifier response based on the estimated one or more calibration reference signals as transmitted by the user equipment and the expected one or more calibration reference signals; and determine the assistance data based on the determined user equipment’s power amplifier response. The at least one processor may be configured to cause the access node to: adjust one or more reception parameters based on the assistance data. According to an aspect, there is provided a user equipment comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the user equipment at least to: send, to an access node, first channel state information associated with a channel between the access node and the user equipment; receive, from an access node, a calibration reference signal configuration; based on the calibration reference signal configuration, send one or more calibration reference signals to the access node via the channel; receive, from the access node, assistance data for waveform correction; and adjust one or more transmission parameters based on the assistance data. The at least one processor may be configured to cause the user equipment to: receive, from the access node, one or more downlink reference signals; and determine, based on the one or more downlink reference signals, the first channel state information. At least one of the one or more calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude of the channel. The calibration reference signal configuration may indicate at least one of: a transmission power of the one or more calibration reference signals; one or more carriers of the one or more calibration reference signals; a bandwidth and / or bandwidth part of the one or more calibration reference signals; a number of symbols comprised in the one or more calibration reference signals; a constellation of symbols comprised in the one or more calibration reference signals; or beam information for the user equipment to use when transmitting the one or more calibration reference signals. The beam information may indicate a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized, and wherein the at least one processor may be configured to cause the user equipment to send the one or more calibration reference signals using the indicated sequence of beams. According to an aspect, there is provided a method performed by an access node, the method comprising: receiving, from a user equipment, first channel state information associated with a channel between the access node and the user equipment; sending, to the user equipment, a calibration reference signal configuration; receiving, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals; determining second channel state information associated with the channel based on the one or more calibration reference signals; determining, based on the first channel state information and the second channel state information, assistance data for waveform correction; and sending, to the user equipment, the assistance data. At least one of the one or more calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude associated with the channel. The method may further comprise: determining, based on the first channel state information, the calibration reference signal configuration. The calibration reference signal configuration may indicate at least one of: a transmission power of the one or more calibration reference signals; one or more carriers of the one or more calibration reference signals; a bandwidth and / or bandwidth part of the one or more calibration reference signals; a number of symbols comprised in the one or more calibration reference signals; a constellation of symbols comprised in the one or more calibration reference signals; or beam information for the user equipment to use when transmitting the one or more calibration reference signals. The beam information may indicate a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized. The method may further comprise: sending, to the user equipment, one or more downlink reference signals, wherein the first channel state information may be based on the one or more downlink reference signals. Determining the assistance data may comprise: determining, based on the first channel state information, a channel impulse response of the channel; removing the channel impulse response from received one or more calibration reference signals to estimate the one or more calibration reference signals as transmitted by the user equipment; determining an expected one or more calibration reference signals based on the calibration reference signal configuration sent to the user equipment; determining the user equipment’s power amplifier response based on the estimated one or more calibration reference signals as transmitted by the user equipment and the expected one or more calibration reference signals; and determining the assistance data based on the determined user equipment’s power amplifier response. The means may be further for: adjusting one or more reception parameters based on the assistance data. According to an aspect there is provided a method performed by a user equipment, the method comprising: sending, to an access node, first channel state information associated with a channel between the access node and the user equipment; receiving, from an access node, a calibration reference signal configuration; based on the calibration reference signal configuration, sending one or more calibration reference signals to the access node via the channel; receiving, from the access node, assistance data for waveform correction; and adjusting one or more transmission parameters based on the assistance data. The method may further comprise: receiving, from the access node, one or more downlink reference signals; and determining, based on the one or more downlink reference signals, the first channel state information. At least one of the one or more calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude of the channel. The calibration reference signal configuration may indicate at least one of: a transmission power of the one or more calibration reference signals; one or more carriers of the one or more calibration reference signals; a bandwidth and / or bandwidth part of the one or more calibration reference signals; a number of symbols comprised in the one or more calibration reference signals; a constellation of symbols comprised in the one or more calibration reference signals; or beam information for the user equipment to use when transmitting the one or more calibration reference signals. The beam information may indicate a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized, and wherein sending the one or more calibration reference signals may comprise sending the one or more calibration reference signals using the indicated sequence of beams. According to an aspect, there is provided a computer readable medium comprising instructions which, when executed by an access node, cause the access node to perform at least the following: receiving, from a user equipment, first channel state information associated with a channel between the access node and the user equipment; sending, to the user equipment, a calibration reference signal configuration; receiving, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals; determining second channel state information associated with the channel based on the one or more calibration reference signals; determining, based on the first channel state information and the second channel state information, assistance data for waveform correction; and sending, to the user equipment, the assistance data. At least one of the one or more calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude associated with the channel. The instructions, when executed by the access node, may cause the access node to further perform: determining, based on the first channel state information, the calibration reference signal configuration. The calibration reference signal configuration may indicate at least one of: a transmission power of the one or more calibration reference signals; one or more carriers of the one or more calibration reference signals; a bandwidth and / or bandwidth part of the one or more calibration reference signals; a number of symbols comprised in the one or more calibration reference signals; a constellation of symbols comprised in the one or more calibration reference signals; or beam information for the user equipment to use when transmitting the one or more calibration reference signals. The beam information may indicate a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized. The instructions, when executed by the access node, may cause the access node to further perform: sending, to the user equipment, one or more downlink reference signals, wherein the first channel state information may be based on the one or more downlink reference signals. Determining the assistance data may comprise: determining, based on the first channel state information, a channel impulse response of the channel; removing the channel impulse response from received one or more calibration reference signals to estimate the one or more calibration reference signals as transmitted by the user equipment; determining an expected one or more calibration reference signals based on the calibration reference signal configuration sent to the user equipment; determining the user equipment’s power amplifier response based on the estimated one or more calibration reference signals as transmitted by the user equipment and the expected one or more calibration reference signals; and determining the assistance data based on the determined user equipment’s power amplifier response. The instructions, when executed by the access node, may cause the access node to further perform: adjusting one or more reception parameters based on the assistance data. According to an aspect, there is provided a computer readable medium comprising instructions which, when executed by a user equipment, cause the user equipment to perform at least the following: sending, to an access node, first channel state information associated with a channel between the access node and the user equipment; receiving, from an access node, a calibration reference signal configuration; based on the calibration reference signal configuration, sending one or more calibration reference signals to the access node via the channel; receiving, from the access node, assistance data for waveform correction; and adjusting one or more transmission parameters based on the assistance data. The instructions, when executed by the user equipment, may cause the user equipment to further perform: receiving, from the access node, one or more downlink reference signals; and determining, based on the one or more downlink reference signals, the first channel state information. At least one of the one or more calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude of the channel. The calibration reference signal configuration may indicate at least one of: a transmission power of the one or more calibration reference signals; one or more carriers of the one or more calibration reference signals; a bandwidth and / or bandwidth part of the one or more calibration reference signals; a number of symbols comprised in the one or more calibration reference signals; a constellation of symbols comprised in the one or more calibration reference signals; or beam information for the user equipment to use when transmitting the one or more calibration reference signals. The beam information may indicate a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized, and wherein sending the one or more calibration reference signals may comprise sending the one or more calibration reference signals using the indicated sequence of beams. According to an aspect, there is provided a non-transitory computer readable medium comprising program instructions that, when executed by an apparatus, cause the apparatus to perform at least the method according to any of the preceding aspects. In the above, many different aspects have been described. As previously noted, it should be appreciated that further aspects may be provided by the combination of any two or more of the aspects described above. Other features, aspects, and elements will become apparent in view of the following. DESCRIPTION OF FIGURES Some example embodiments will now be described, by way of non-limiting and illustrative example only, with reference to the accompanying Figures (FIGs.) in which: FIG. 1 shows a representation of a 5th generation communication system; FIG. 2 shows a representation of an apparatus for the communication system of FIG. 1 according to some example embodiments; FIG. 3 shows a representation of an apparatus according to some example embodiments; FIG. 4 shows methods according to some examples; FIG. 5 shows a signalling exchange according to some examples; FIG. 6 shows a schematic representation of an apparatus according to some examples; FIG. 7 shows a method for waveform correction according to some examples; FIG. 8 shows results comparing measured out of band emissions against values predicted according to some examples; and FIG. 9 shows a method according to some examples. DETAILED DESCRIPTION In the following various example embodiments are explained with reference to communication devices capable of communication with a communication system. Before explaining in detail the various example embodiments of this disclosure, a 5th generation communication system (5GS), an access network and a core network (5GC) thereof, and communication devices are briefly explained with reference to FIG. 1, 2 and 3. It should be understood that aspects of the present disclosure may be applied not only to 5G systems, but may be applied to other systems (such as but not limited to 6th generation communication systems, 6G systems). FIG. 1 shows a schematic representation of a 5G communication system (5GS). The 5GS may comprise a user equipment (UE) or Terminal 100, an access network, such as a 5G radio access network (5G-RAN) 101 or next generation radio access network (NG-RAN), a 5G core network 102, and one or more application functions 103. An application function 103 may be deployed in the 5GS as trusted application function or may be deployed or host on one or more application servers of the data network (DN) 104. Such application functions are untrusted application functions. The 5GS connects the UE to a data network the access network and the 5GC 102 (e.g., a UPF of the 5GC). The 5G-RAN 101 may comprise one or more radio access nodes, such as a gNodeB (gNB). A gNB may include one or more gNodeB (gNB) distributed units (DUs) connected to one or more gNodeB (gNB) centralized units (CUs). The 5GC may comprise the following network functions: Network Slice Selection Function (NSSF); Network Exposure Function (NEF) 105; Network Repository Function (NRF); Policy Control Function (PCF); Unified Data Management (UDM) 106; Application Function (AF) 103; Authentication Server Function (AUSF) 107; an Access and Mobility Management Function (AMF) 108; Session Management Function (SMF) 109; and a user plane function (UPF) 110. FIG. 1 also shows the various interfaces (N1, N2 etc.) that may be implemented between the various elements of the system. FIG. 2 illustrates an example of a control apparatus 200 for controlling a function of the access network (e.g., a 5G-RAN or the NG-RAN illustrated in FIG. 1) of FIG. 1. The control apparatus 200 may comprise at least one random access memory (RAM) 211a, at least on read only memory (ROM) 211b, at least one processor 212, 213 and a network interface 214. The at least one processor 212, 213 may be coupled to the RAM 211a and the ROM 211b. The at least one processor 212, 213 may be configured to execute an appropriate software code 215. Execution of the software code 215 may, for example, may cause the apparatus to perform operations for controlling a function of the access network. The software code 215 may be stored in the ROM 211b. The control apparatus 200 may be interconnected with another control apparatus 200 for controlling another function of the 5G-RAN or the NG-RAN. In some embodiments, each function of the 5G-RAN or the NG-RAN is deployed or hosted on a control apparatus 200. In alternative embodiments, two or more functions of the 5G-RAN or the NG-RAN may share a control apparatus. FIG. 3 illustrates an example of a communication device 300, such as the UE of FIG. 1. The communication device 300 may be provided by any device capable of sending and receiving radio signals. Non-limiting examples of a communication device 300 comprise a user equipment, a mobile station (MS) or mobile device, such as a mobile phone or what is known as a ’smart phone’, a computer provided with a wireless interface card or other wireless interface facility (e.g., USB dongle), a personal data assistant (PDA) or a tablet provided with wireless communication capabilities, a machine-type communications (MTC) device, an Internet of things (loT) type communication device or any combinations of these or the like. The communication device 300 may comprise a transceiver for transmitting and / or receiving, for example, wireless signals carrying communications, for example radio signals. The communications may be one or more of voice, electronic mail (email), text messages, multimedia data, machine data and so on. The communication device 300 may receive wireless signals (e.g., radio signals) over an air or radio interface 307 via appropriate apparatus for receiving and may transmit wireless signals via appropriate apparatus for transmitting radio signals. In FIG. 3 transceiver is designated schematically by block 306. The transceiver 306 may comprise, for example, a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device and may comprise one or more antenna elements. The antenna arrangement may be a multi-input multi output (MIMO) antenna. The communication device 300 may be provided with at least one processor 301, at least one memory ROM 302a, at least one RAM 302b and other possible components 303 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access networks (e.g., the 5G-RAN or NG-RAN illustrated in FIG. 1) and other communication devices. The at least one processor 301 is coupled to the RAM 302b and the ROM 302a. The at least one processor 301 may be configured to execute an appropriate software code 308. The software code 308 may, for example, allow to perform one or more operations of the communication device. The software code 308 may be stored in the ROM 302a. The processor, the ROM, and the RAM, the transceiver and other circuitry of the communication device (e.g., a modem) can be provided on a circuit board, in chipsets, or in a system on chip. The circuit board, chipsets or system on chip is denoted by reference 304. The communication device 300 may optionally have a user interface, such as keypad 305, a touch sensitive screen or a pad, combinations thereof or the like. Optionally one or more of a display, a speaker and a microphone may be provided depending on the type of communication device. In some examples, when performing transmissions the UE may generate a signal X(t). The signal X(t) may be input into a power amplifier (PA) at the UE, which amplifies the input signal and generates an output signal A*X(t) at a higher power than the original signal. The amplified signal A * X(t) may then be transmitted by the UE. The PA may operate in a linear region and a non-linear region. In the linear region, the output signal (A * X(t)) is linearly-proportional to the input signal (X(t)). In the non-linear region, the output signal (A*X(t)) is not linearly-proportional to the input signal (X(t)), but is instead modified by the non-linear behaviour of the PA. For example, if the non-linear behaviour of the PA is h(t), then the output signal in the non-linear region A * h(t) * X(t). This may mean that, when the PA is operating in the non-linear region, the signal becomes distorted (due to h(t)) and unwanted or undesirable effects may occur. The distortion and / or unwanted effects may be characterized by different performance metrics (e.g., as per3GPP TS38.101, section 6.5.2.4). For example, the non-linearity in the transmitted signal may introduce out of band emissions, which are on adjacent channels outside the assigned channel bandwidth. The out of band emissions may be characterized by an adjacent channel leakage ratio (ACLR). The ACLR may define a ratio of the filtered mean power centred on the assigned channel frequency to the filtered mean power centred on an adjacent channel frequency at a nominal channel spacing, where the filtered mean power of the assigned channel frequency and the filtered mean power centred on the adjacent channel frequency are measured with rectangular filters, for example according to the bandwidths shown below in Table 1. Channel bandwidth (MHz) 5,10,15,20,25,30,35,40,45,50 60,70,80,90,100 REF_SCS (kHz) 15 30 NR ACLR measurement bandwidth (MHz) M BW= REF_SCS*(12*NRB+1 ) / 1000 Table 1 - example ACLR measurement bandwidths The ACLR may therefore indicate the extent to which the bandwidth of the transmitted signal leaks into the neighbouring channels. The non-linearity in the transmitted signal may also distort the transmitted signal to the extent that the constellation points are erroneous. The difference between the transmitted signal and the input signal (which may also be referred to as a reference signal) may be referred to as the error vector. The magnitude of the error vector, the error vector magnitude (EVM), may be calculated and compared to a threshold value. If the EVM is greater than the threshold then the signal may be distorted to the extent that it is no longer understandable at the receiver. Different parameters and / or modulation schemes may have different acceptable EVM requirements, for example as shown below in Table 2. Parameter Unit Average EVM Level Pi / 2-BPSK % 30 QPSK % 17.5 16 QAM % 12.5 64 QAM % 8 256 QAM % 3.5 Table 2 - examples of different acceptable EVM requirements The EVM may therefore indicate how much the transmitted signal deviates from the input signal. Operating the PA in the non-linear region may therefore introduce unwanted effects in the transmitted signal. However there may be situations in which the UE needs to operate the PA in the non-linear region - for example, the UE may need to boost the transmission power of the signal when performing coverage extension operations, or the UE may be operating in a high power added efficiency (PAE) mode, both of which may require the PA to operate in the non-linear region. It may be possible to compensate for the non-linear behaviour of the PA using waveform correction techniques - for example pre-distortion and / or post-distortion techniques. For example, if the non-linear behaviour of the PA h(t) is known, then the input signal X(t) may be pre-distorted at the UE by (t) before being input to the PA to compensate for the distortion, such that h(t) and (t) cancel each other out and the output from the PA is A * X(ty Similarly, for post-distortion techniques if the receiver knows the non-linear behaviour of the UE’s PA, then the receiver may be able to receive and isolate the transmitted signal A * h(t) * X(t) and remove the non-linear behaviour component h(t) to obtain the intended signal. In some examples, machine learning (ML) tools may be implemented to compensate for the UE’s PA non-linearity, for example performing waveform correction (e.g., using pre-distortion or post-distortion techniques). Such ML tools may require labelled data for model training purposes. It may be beneficial to know the non-linear behaviour of the UE’s PA, so that waveform correction can be implemented. However the non-linear behaviour of the PA may be specific to the hardware of the UE, and may change over the lifetime of the UE. Additionally, UE vendors may not want to disclose the specifics of the UE’s PA, which may mean that labelled data for ML purposes may not be readily available. Furthermore, many different variables, such as PA bandwidth, carrier frequencies, antenna responses etc. may influence the compensation techniques required, meaning that a robust solution to waveform correction is challenging to achieve. Some examples of the present disclosure may address one or more of these issues. Some examples may provide methods and apparatuses for waveform correction at the UE and / or access node. Some examples may provide a signalling procedure between an access node (e.g., gNB) and UE for determining assistance data for waveform correction. Some examples may therefore enhance the UE’s capability for operating in the UE’s PA non-linear region. Reference is made to FIG. 4, which shows methods according to some examples. With reference to FIG. 4a, a method is shown that may be performed by an access node (e.g., gNB) of a network, such as but not limited to a 5G or 6G network. At 400, the method comprises receiving, from a user equipment, first channel state information associated with a channel between the access node and the user equipment. At 402 the method comprises sending, to the user equipment, a calibration reference signal configuration. At 404 the method comprises receiving, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals. At 406 the method comprises determining second channel state information associated with the channel based on the one or more calibration reference signals. At 408 the method comprises determining, based on the first channel state information and the second channel state information, assistance data for waveform correction. At 410 the method comprises sending, to the user equipment, the assistance data. With reference to FIG. 4b, a method is shown that may be performed by the UE. At 412, the method comprises sending, to an access node, first channel state information associated with a channel between the access node and the user equipment. At 414 the method comprises receiving, from an access node, a calibration reference signal configuration. At 416 the method comprises, based on the calibration reference signal configuration, sending one or more calibration reference signals to the access node via the channel. At 418, the method comprises receiving, from the access node, assistance data for waveform correction. At 420, the method comprises adjusting one or more transmission parameters based on the assistance data. Reference is made to FIG. 5, which shows a signalling exchange according to some examples. At 500, the UE may send, to the access node, capability information indicating the UE’s capability with respect to a number of transmission beams and a beam overlapping factor. In some examples the UE may send the capability information in response to receiving a request from the access node. At 502 the access node may send, to the UE, one or more downlink reference signals. The one or more downlink reference signals may be one or more channel state information (CSI) reference signals (RSs). At 504, the UE may determine first CSI based on the received one or more downlink RS. At 506, the UE sends the first CSI to the access node. At 508, the access node may determine a calibration reference signal configuration. In some examples, the access node may determine the calibration reference signal configuration based on the first CSI (received at 504) and / or UE capability information (received at 500). The calibration reference signal configuration may indicate one or more of: • One or more carriers of the one or more calibration reference signals; • A transmission power of the one or more calibration reference signals; • A bandwidth and / or bandwidth part of the one or more calibration reference signals; • A number of symbols comprised in the one or more calibration reference signals; • A constellation of symbols comprised in the one or more calibration reference signals; or • Beam information for the user equipment to use when transmitting the one or more calibration reference signals - e.g., a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized. At 510 the access node may send the calibration reference signal configuration to the UE. At 512, the UE may send, to the access node, based on the calibration reference signal configuration, one or more calibration reference signals. In some examples at least one of the one or more calibration reference signals (as indicated by the calibration reference signal configuration) may violate an allowed performance metric associated with the channel. For example, the at least one of the one or more calibration reference signals may violate an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude associated with the channel. At 514, the access node may determine second CSI based on the received one or more calibration reference signals. At 516, the access node may determine assistance data for waveform correction based on the first CSI and the second CSI. The assistance data for waveform correction may be for correcting the effect of the non-linear response of the UE’s PA on the waveform. For example, the assistance data may be used for performing pre-distortion and / or post-distortion processing. For example, the access node may determine a channel impulse response (CIR) based on the first CSI. The CIR may represent any distortion to a signal transmitted via the channel according to channel-specific conditions, i.e. it is not dependent on the UE’s PA response. The received one or more calibration signals (i.e. the one or more calibration reference signals as observed by the access node) may be a convolution of the one or more calibration reference signals as transmitted by the UE and the channel impulse response. For example: ^observed ^transmitted * C1R Where Xobserved is the calibration reference signal observed by the access node, Xtransmitted is the calibration reference signal as transmitted by the UE, and * denotes convolution. The access node may remove the CIR from the received one or more calibration signals to estimate the one or more calibration reference signals as transmitted by the UE. That is to say, the access node may isolate and remove the CIR’s effect on the observed signal to determine the signal transmitted from the UE. The calibration reference signal transmitted by the UE may be a convolution of the actual calibration reference signal to be transmitted by the UE (that is, the signal input into the UE’s PA) and the UE’s PA response. For example: ^transmitted Xactua[ * PAgE Where Xactuai is the actual calibration reference signal to be transmitted by the UE and PAUE is the UE’s PA response. The access node may determine the actual calibration reference signal to be transmitted by the UE based on the calibration reference signal configuration. The access node may then determine the UE’s PA response based on the estimated calibration reference signal as transmitted by the UE (Xtransmitted) and the actual calibration reference signal to be transmitted by the UE (Xactuai)- The access node may then determine the assistance data for waveform correction based on the determined UE’s PA response. It should be understood that the specific example provided above is just one possible mechanism for determining the assistance data, and that in some examples the access node may implement a different mechanism to determine the assistance data for waveform correction based on the first CSI and the second CSI other than the specific example provided above. The access node may repeat this for each of the one or more calibration reference signals to determine the UE’s PA response. For example, each of the one or more calibration reference signals may have a different transmission characteristic (e.g., carrier frequency) as indicated by the calibration reference signal configuration, and thus by repeating the calculation for each transmission characteristic the access node may determine the UE’s PA response across the different characteristics (e.g., across the different carrier frequencies). In some examples, a ML model may be implemented on the network-side to determine the UE’s PA response, for example using the techniques described previously. The ML model implemented on the network-side may in some examples be implemented at the access node or may be implemented elsewhere in the network and be in communication with the access node. For example, the ML model may send a request to the access node to initiate the determination of the UE’s PA response, and in response to receiving the request the access node may determine and send the calibration reference signal configuration. The access node may receive the one or more calibration reference signals, demodulate the symbols within the signals, and send those symbols to the ML model. The ML model may determine the UE’s PA response and send the determined UE’s PA response to the access node. The access node may send the UE’s PA response, received from the ML model, as part of the assistance data sent to the UE. When the ML model determines that the UE’s PA model is outdated, the ML model may send an update request to the access node, which may obtain updated calibration reference signals using the abovementioned techniques, and the ML model may update the UE’s PA model based on the symbols obtained by the access node from the updated calibration reference signals. At 518, the access node may send the assistance data to the UE. The assistance data may comprise information for waveform correction. For example the assistance data may comprise the UE’s PA response as determined by the access node (which the UE may utilize, for example as an input into a UE-side ML model for determining pre-distortion corrections to account for the UE’s PA non-linearity), or parameters for performing pre-distortion processing at the UE to correct for the waveform distortion caused by the UE’s PA non-linearity. At 520, the UE may adjust one or more transmission parameters for subsequently transmitted signals based on the assistance data. For example, the UE may apply pre-distortion correction techniques to subsequent signals based on the assistance data. At 522, instead of or in addition to sending the assistance data to the UE at 520, the access node may adjust one or more reception parameters for subsequently received signals based on the assistance data. For example the access node may implement post-distortion techniques (e.g., a convolutional deep learning receiver for digital post-distortion) based on the assistance data to mitigate effects of the waveform distortion in subsequent signals caused by the UE’s PA non-linearity. In some examples the procedure described above may be repeated periodically. This may help ensure that the determined UE PA response is kept up-to-date, and that the waveform correction applied (at the UE and / or at the access node) is accurately calibrated over time. In some examples the one or more calibration reference signals may be allocated for transmission in a dedicated slot or slots prior to uplink data slots. The periodicity of the dedicated slot or slots may be adjusted as required to ensure accurate calibration. In some examples a neural network (e.g., a Liquid Time Constant (LTC) network) may be used to process limited in-band time signals from a measured PA response to generate both in-band and out-of-band distortion predictions. Advantageously, examples may provide a significant reduction in computational complexity and the elimination of extensive feedback loops typically used in other methods. FIG. 7 shows an example method implemented ata transmitter (e.g., UE) and a receiver (e.g., access node) for waveform correction. In the example shown in FIG. 7, a signal x(t) is generated at the transmitter and is power-amplified and sent to the receiver over the channel with response h. The signal x(t) may for example be a calibration reference signal as described earlier. The receiver (e.g. the access node) samples the signal, and the in-band signal samples collected in the complex vector y_inband are input into a first prediction block which returns a vector of predicted receiver out of band (OOB) emissions y_OOB. The vector y_OOB may be comprised in the assistance data described previously. The assistance data (comprising vector y_OOB) may be then sent to the transmitter, which may then use the vector y_OOB to predict the transmitter OOB emissions x_OOB. Additionally or alternatively, as shown in FIG. 7, the transmitter may use feedback from the power amplifier to predict the transmitter OOB emissions x_OOB. The predicted x_OOB may be used in the transmitter digital pre-distortion block to cancel the actual OOB emission which the PA would produce at its output. That is to say, the transmitter may adjust one or more transmission parameters, such as one or more parameters of the digital pre-distortion block, based on the feedback received from the power amplifier and / or the assistance data (and more specifically in some examples,based on vector x_OOB determined based on vector y_OOB comprised in the assistance data) received from the receiver. As mentioned above, in some examples the creation of the OOB prediction for power amplifiers may involve the use of a neural network, which may be a form of an Ordinary Differential Equation (ODE) network. Such ODE networks may be described by a system of linear ODE equations. For example, a LTC network may implement an ODE of the form: dx(t) dt fl - + / (x(£), I(t), t, 0) x(t) + f(x(tl I(t), t, 9)A _T Where t is the time constant of the network determining the speed and coupling sensitivity of the ODE, f() is the neural network parameterized by the network parameters, x(t) is the hidden state at time t, I(t)is the input at time t, 9,A are the network parameters. The neural network may model the behaviour of the power amplifier. For example the Liquid Time Constant network may utilize 8 neurons that can all be used with time dependent connections that are liquid between each other - in other words the neural network can learn the time dependency between the in-band and the out-of-band emissions (that normally is found using oversampling of the data) by limiting the receiver-side or the transmitter-side internal DPD receiver to only sample the in-band spectrum and utilize the LTC network to predict the OOB. FIG. 8 shows results utilizing limited training data, and shows that even with very limited data the OOB emission may be accurately predicted. In some examples the algorithm can instead of being a neural network become a closed form neural network where instead of neurons being able to adapt throughout time and continue learning can become fixed. This may be implemented using Closed form solution of a Euler solver and thus can reduce the computational complexity of the neural network. As explained previously, in some examples Digital Pre- and Post-Distortion (DPD) based methods may be implemented in both the transmitter (e.g., UE) and the receiver (e.g., access node). For example, as explained previously in relation to FIG. 7, the Digital Pre-Distortion (DPD) processing at the transmitter side may be based on feedback from the power amplifier at the transmitter, and / or based on feedback from the receiver. In some examples the transmitter may comprise a neural network that is trained based on an approximate gradient approach, while the receiver may comprise a neural network that is trained based on the true gradient. In some examples ACLR reduction may be achieved at the transmitter through dedicated digital signal processing and neural network integration. In the receiver, a similar setup may be used for EVM reduction. This separation may allow for targeted optimization of distortion compensation. Reference is made to FIG. 9, which shows an example method implemented by the transmitter (e.g., UE) and the receiver (e.g., access node). Here it is assumed that the UE transmits an information bit vector b to the access node using an uplink PUSCH transmission. The UE may OFDM-modulate the vector b into an OFDM symbol m by sequentially performing coding, scrambling, QAM modulation, IFFT and CP addition, etc. These operations are well known, and they can be generically described by a non-trainable function fTX that generates the OFDM symbol m = fTx(b). The resulting symbol may then be passed to a trainable transmitter (PA-DPD TX) that performs the digital pre-distortion to compensate for the PA response. The trainable transmitter, {fw), may map the signal (m) to a pre-distorted signal (%) in (R2n), where (6 T) represents the trainable transmitter’s parameters: x = feT (m). The signal x may then be input to the PA with response hPA and may be sent over the unknown wireless propagation channel with response h. The access node’s receiver obtains a signal y which can be approximated as the convolution between x and the combined channel response (hPA *h):y~ (hPA * h* x). This receiver may comprise a trainable receiver (PA-DPoD RX) and a non-trainable OFDM receiver. The trainable receiver may be described by the function ( / sr) , where {OR} represents the trainable receiver parameters. The receiver may receive the signal y in ( / ?2W)and perform postdistortion operations to produce a signal u = f eR (y). The trainable receiver may further compensate for the nonlinearities caused by the memory PA response. The post-distorted signal u may be passed to the non-trainable OFDM receiver, which may output a probability vector over the transmitted bits (6): pb = fRX{u). Prior to UL transmission, the UE and access node trainable parts need to be jointly trained. However, to jointly train the trainable transmitter and the trainable receiver (e.g., derive the functions fQT , fR) using a single loss function and training using stochastic gradient descent may not be straightforward since it may require that the combined channel response (or more specifically the gradient of the instantaneous combined channel response) to be known. To circumvent the unknown channel problem, in some examples a hybrid approach may be implemented, in which the trainable receiver of the access node is trained on a first loss function, using supervised learning approach, while the trainable transmitter of the UE is derived using a second loss function (obtained using a relaxation of the first loss function) and reinforcement learning. The combined loss function for both Tx and Rx side (L{0T, 0r )) may be given by the sum: L>combined (0r , 0k ) — ocLce {9r , 9t} + PLaclr{9t ) Where a, p, are weighting coefficients that balance the contributions of the cross-entropy (CE) loss and the ACLR emission loss where the cross entropy loss may be given by the integral: Lce (Qt , 0r ) = Em [l{feR (y),m)p (y | f&r (m)) dy] Here,(Z(p, m) = - log(pm)) is the categorical cross-entropy, and(p(y | %)) represents the unknown combined channel response {hPA * h). The loss function of the ACLR emission loss may be found as: Where S is the batch size, log is the logarithmic scale of the power, Ns is the number of OFDM symbols per slot, the ACLR contains the indices for the set of unused subcarriers at band edges, MACLR is the total number of unused subcarriers, and Hklq is the frequency domain representation of hPA(n) in the oversampled slot format. The estimation of the gradient loss function for the receiver may be given as: V0R L = Em,y [V0R l(feR (y), m)] This gradient can be estimated using samples, as (p(y|x)) does not need to be differentiable. V8bL= 1^78Bi(feB(y‘).,n‘) Where S is the batch size, m is the ith training sample, y is the corresponding received signal. The estimation of the gradient loss function for the transmitter may be more complex due to the unknown or non-differentiable nature of (p(y|x)). Some examples may relax the channel input (x) to a random variable and then estimate the gradient using an alternate distribution as: ^otL= Em^>y[l(f0 (y),m)\70 f0 (m) ■ V7log7T^(x)] + V0tLaclr(9t) The resulting approximation gradient for the transmitter is then: The training may alternate between updating the receiver and the transmitter, where the training may be based on the gradients described above. The training process may be repeated until a predetermined stop criterion is met, for example a fixed number of iterations have been performed, or until no improvement in the loss is achieved. Examples have been described whereby waveform correction can be implemented at the UE and / or the access node. The waveform correction is based on assistance data derived from CSI derived from downlink RS and CSI derived from uplink RS transmissions in a manner which is specific to the UE and does not require specific hardware details to be disclosed. By determining the assistance data for waveform correction in the manner described in the various examples, the effects of the UE’s PA non-linearity may be mitigated, which may enable or enhance UE functionality, such as improving coverage extension or PAE techniques. In some examples there is provided an access node comprising means for: receiving, from a user equipment, first channel state information associated with a channel between the access node and the user equipment; sending, to the user equipment, a calibration reference signal configuration; receiving, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals; determining second channel state information associated with the channel based on the one or more calibration reference signals; determining, based on the first channel state information and the second channel state information, assistance data for waveform correction; and sending, to the user equipment, the assistance data. In some examples there is provided an access node comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the access node at least to: receive, from a user equipment, first channel state information associated with a channel between the access node and the user equipment; send, to the user equipment, a calibration reference signal configuration; receive, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals; determine second channel state information associated with the channel based on the one or more calibration reference signals; determine, based on the first channel state information and the second channel state information, assistance data for waveform correction; and send, to the user equipment, the assistance data. In some examples there is provided a user equipment comprising means for: sending, to an access node, first channel state information associated with a channel between the access node and the user equipment; receiving, from an access node, a calibration reference signal configuration; based on the calibration reference signal configuration, sending one or more calibration reference signals to the access node via the channel; receiving, from the access node, assistance data for waveform correction; and adjusting one or more transmission parameters based on the assistance data. In some examples there is provided a user equipment comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the user equipment at least to: send, to an access node, first channel state information associated with a channel between the access node and the user equipment; receive, from an access node, a calibration reference signal configuration; based on the calibration reference signal configuration, send one or more calibration reference signals to the access node via the channel; receive, from the access node, assistance data for waveform correction; and adjust one or more transmission parameters based on the assistance data. While examples have been described with respect to method(s) performed by a UE and / or an access node, it should be understood that in other examples different entities (e.g., other than a UE and / or access node) may implement the described methods. For instance, a first node may perform the method(s) attributed to the UE and a second node may perform the method(s) attributed to the access node as described previously FIG. 6 shows a schematic representation of non-volatile memory media 600a (e.g., computer disc (CD) or digital versatile disc (DVD)) and 600b (e.g. universal serial bus (USB) memory stick) storing instructions and / or parameters 602 which when executed by a processor allow the processor to perform one or more of the steps of the method of FIG. 4 or 5. It is understood that references in the above to various network functions (e.g., to an AMF, an SMF, TNF etc.) may be implemented by apparatus that perform at least some of the functionality associated with those network functions. Further, an apparatus configured to implement a network function may further be configured to implement a virtual network function instance of that network function. It should be understood that the apparatuses may comprise or be coupled to other units or modules etc., such as radio parts or radio heads, used in or for transmission and / or reception. Although the apparatuses have been described as one entity, different modules and memory may be implemented in one or more physical or logical entities. It is noted that whilst some example embodiments have been described in relation to 5G networks, similar example embodiments can be applied in relation to other networks and communication systems. Therefore, although certain example embodiments were described above by way of example with reference to certain example architectures for wireless networks, technologies and standards, further example embodiments may be applied to any other suitable forms of communication systems than those illustrated and described herein. It is also noted herein that there are several variations and modifications which may be made to the various example embodiments described herein without departing from the scope of this disclosure. As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements. As used herein, the expression “and / or” includes any and all combinations of the listed terms, including at least any one of the elements, at least any two or more of the elements, or at least all of the elements. As used herein, the term “or” refers to a non-exclusive “or” unless otherwise indicated (e.g., use of “or else” or “or in the alternative”). As used herein, unless stated explicitly, performing a step “in response to A” does not indicate that the step is performed immediately after “A” occurs and one or more intervening steps may be included. Analogously, performing a step or functionality “based on A” does not indicate that the step or functionality is performed solely based on “A” as one or more additional conditions may be included. In general, the various embodiments may be implemented in hardware or special purpose circuitry, software, logic or any combination thereof. Some aspects of the disclosure may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the disclosure is not limited thereto. While various aspects of the disclosure may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting and illustrative examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof. As used herein, the term “circuitry” may refer to one or more or all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and / or digital circuitry) and (b) combinations of hardware circuits and software, such as (as applicable): (i) a combination of analog and / or digital hardware circuit(s) with software / firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that utilizes software (e.g., firmware) for operation, but the software may not be present when it is not utilized for operation.” This definition of circuitry applies to all uses of this term herein, including in any claims. As a further example, as used herein, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device. The embodiments of this disclosure may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Computer software or program, also called program product, including software routines, applets and / or macros, may be stored in any apparatus-readable data storage medium and they comprise program instructions to perform particular tasks. A computer program product may comprise one or more computerexecutable components which, when the program is run, are configured to carry out embodiments. The one or more computer-executable components may be at least one software code or portions of it. Further in this regard it should be noted that any blocks of the logic flow as in the FIGs. may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media, such as hard disk or floppy disks, and optical media, such as DVD and the data variants thereof, CD. The physical media is a non-transitory media. The term “non-transitory,” as used herein, is a limitation of the medium itself (e.g., tangible, not a signal) as opposed to a limitation on data storage persistency (e.g., RAM vs. ROM). The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may comprise one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), FPGA, gate level circuits and processors based on multi core processor architecture, as non-limiting examples. Various example embodiments of the disclosure may be practiced in various components, such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate. The scope of protection sought for various example embodiments of the disclosure is set out by the independent claims. The example embodiments and features thereof, if any, described in this disclosure that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various example embodiments of the disclosure. The foregoing description has provided, by way of non-limiting and illustrative examples, a full and informative description of the various example embodiments of this disclosure. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of this disclosure, when read in conjunction with the drawings and the claims. However, all such and similar modifications of the teachings will still fall within the various example embodiments of this disclosure. By way of non-limiting and illustrative example, there is a further example embodiment comprising a combination of one or more example embodiments with any of the other example embodiments previously discussed.

Claims

1. An access node comprising means for:receiving, from a user equipment, first channel state information associated with a channel between the access node and the user equipment;sending, to the user equipment, a calibration reference signal configuration;receiving, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals ;determining second channel state information associated with the channel based on the one or more calibration reference signals;determining, based on the first channel state information and the second channel state information, assistance data for waveform correction; andsending, to the user equipment, the assistance data.

2. The access node of claim , wherein at least one of the one or more calibration reference signals violates an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude associated with the channel.

3. The access node of claim 1 or 2, wherein the means is further for: determining, based on the first channel state information, the calibration reference signal configuration.

4. The access node of any preceding claim, wherein the calibration reference signal configuration indicates at least one of:a transmission power of the one or more calibration reference signals;one or more carriers of the one or more calibration reference signals;a bandwidth and / or bandwidth part of the one or more calibration reference signals;a number of symbols comprised in the one or more calibration reference signals;a constellation of symbols comprised in the one or more calibration reference signals; orbeam information for the user equipment to use when transmitting the one or more calibration reference signals.

5. The access node of claim 4, wherein the beam information indicates a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized.

6. The access node of any preceding claim, wherein the means is further for: sending, to the user equipment, one or more downlink reference signals, wherein the first channel state information is based on the one or more downlink reference signals.

7. The access node of any preceding claim, wherein determining the assistance data comprises:determining, based on the first channel state information, a channel impulse response of the channel;removing the channel impulse response from received one or more calibration reference signals to estimate the one or more calibration reference signals as transmitted by the user equipment;determining an expected one or more calibration reference signals based on the calibration reference signal configuration sent to the user equipment;determining the user equipment’s power amplifier response based on the estimated one or more calibration reference signals as transmitted by the user equipment and the expected one or more calibration reference signals; anddetermining the assistance data based on the determined user equipment’s power amplifier response.

8. The access node of any preceding claim, wherein the means is further for: adjusting one or more reception parameters based on the assistance data.

9. A user equipment comprising means for:sending, to an access node, first channel state information associated with a channel between the access node and the user equipment;receiving, from an access node, a calibration reference signal configuration;based on the calibration reference signal configuration, sending one or more calibration reference signals to the access node via the channel;receiving, from the access node, assistance data for waveform correction; and adjusting one or more transmission parameters based on the assistance data.

10. The user equipment of claim 9, wherein the means is further for: receiving, from the access node, one or more downlink reference signals; and determining, based on the one or more downlink reference signals, the first channel state information.

11. The user equipment of claim 9 or 10, wherein at least one of the one or more calibration reference signals violates an allowed adjacent channel leakage ratio and / or an allowed error vector magnitude of the channel.

12. The user equipment of any of claims 9 to 11, wherein the calibration reference signal configuration indicates at least one of:a transmission power of the one or more calibration reference signals;one or more carriers of the one or more calibration reference signals;a bandwidth and / or bandwidth part of the one or more calibration reference signals;a number of symbols comprised in the one or more calibration reference signals;a constellation of symbols comprised in the one or more calibration reference signals; orbeam information for the user equipment to use when transmitting the one or more calibration reference signals.

13. The user equipment of claim 12, wherein the beam information indicates a sequence of beams for the user equipment to use when transmitting the one or more calibration reference signals and a given time point at which each of the beams is to be utilized, and wherein sending the one or more calibration reference signals comprises sending the one or more calibration reference signals using the indicated sequence of beams.

14. An access node comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the access node at least to:receive, from a user equipment, first channel state information associated with a channel between the access node and the user equipment;send, to the user equipment, a calibration reference signal configuration;receive, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals;determine second channel state information associated with the channel based on the one or more calibration reference signals;determine, based on the first channel state information and the second channel state information, assistance data for waveform correction; andsend, to the user equipment, the assistance data.

15. A user equipment comprising at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the user equipment at least to:send, to an access node, first channel state information associated with a channel between the access node and the user equipment;receive, from an access node, a calibration reference signal configuration;based on the calibration reference signal configuration, send one or more calibration reference signals to the access node via the channel;receive, from the access node, assistance data for waveform correction; and adjust one or more transmission parameters based on the assistance data.

16. A method performed by an access node, the method comprising: receiving, from a user equipment, first channel state information associated with a channel between the access node and the user equipment;sending, to the user equipment, a calibration reference signal configuration;receiving, from the user equipment via the channel, based on the calibration reference signal configuration, one or more calibration reference signals ;determining second channel state information associated with the channel based on the one or more calibration reference signals;determining, based on the first channel state information and the second channel state information, assistance data for waveform correction ; andsending, to the user equipment, the assistance data17. A method performed by a user equipment, the method comprising:sending, to an access node, first channel state information associated with a channel between the access node and the user equipment;receiving, from an access node, a calibration reference signal configuration;based on the calibration reference signal configuration, sending one or more calibration reference signals to the access node via the channel;receiving, from the access node, assistance data for waveform correction; and adjusting one or more transmission parameters based on the assistance data.34

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