Methods, communications devices, and nodes

Abstract

A method of operating a communications device is provided. The method comprises transmitting an invalid calibration assistance report in response to one or more invalidity trigger conditions being met. The invalid calibration assistance report indicates that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJTs). The one or more invalidity trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter.

Description

[0001] METHODS, COMMUNICATIONS DEVICES, AND NODES BACKGROUND Field of Disclosure The present disclosure relates to communications devices and nodes of wireless communications networks and methods of operating such communications devices and nodes. The present application claims Paris Convention priority from GB patent application number: 2411719.4, filed on 8 August 2024, the contents of which are hereby incorporated by reference in their entirety. Description of Related Art The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention. Previous generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture, are able to support a wider range of services than simple voice and messaging services offered by previous generations of mobile telecommunication systems. For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as mobile video streaming and mobile video conferencing that would previously only have been available via a fixed line data connection. The demand to deploy such networks is therefore strong and the coverage area of these networks, i.e. geographic locations where access to the networks is possible, is expected to continue to increase rapidly. Current and future wireless communications networks are expected to routinely and efficiently support communications with an ever-increasing range of devices associated with a wider range of data traffic profiles and types than existing systems are optimised to support. For example, it is expected future wireless communications networks will be expected to efficiently support communications with devices including reduced complexity devices, machine type communication (MTC) devices, high resolution video displays, virtual reality headsets, eXtended Reality (XR) and so on. Some of these different types of devices may be deployed in very large numbers, for example low complexity devices for supporting the “The Internet of Things”, and may typically be associated with the transmissions of relatively small amounts of data with relatively high latency tolerance. Other types of device, for example supporting high-definition video streaming, may be associated with transmissions of relatively large amounts of data with relatively low latency tolerance. Other types of device, for example used for autonomous vehicle communications and for other critical applications, may be characterised by data that should be transmitted through the network with low latency and high reliability. A single device type might also be associated with different traffic profiles / characteristics depending on the application(s) it is running. For example, different consideration may apply for efficiently supporting data exchange with a smartphone when it is running a video streaming application (high downlink data) as compared to when it is running an Internet browsing application (sporadic uplink and downlink data) or being used for voice communications by an emergency responder in an emergency scenario (data subject to stringent reliability and latency requirements). In view of this there is expected to be a desire for current wireless communications networks, for example those which may be referred to as 5G or new radio (NR) systems / new radio access technology (RAT) systems, or indeed future 6G wireless communications, as well as future iterations / releases of existing systems, to efficiently support connectivity for a wide range of devices associated with different applications and different characteristic data traffic profiles and requirements. The desire to support these new use-cases and scenarios gives rise to new challenges for efficiently handling communications in wireless communications systems that need to be addressed. SUMMARY OF THE DISCLOSURE The present disclosure can help address or mitigate at least some of the issues discussed above. Respective aspects and features of the present disclosure are defined in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein like reference numerals designate identical or corresponding parts throughout the several views, and wherein: Figure 1 schematically represents some aspects of an LTE-type wireless telecommunication system which may be configured to operate in accordance with certain embodiments of the present disclosure; Figure 2 schematically represents some aspects of an NR-type wireless telecommunications system which may be configured to operate in accordance with certain embodiments of the present disclosure; Figure 3 is a schematic block diagram of an example infrastructure equipment and communications device which may be configured to operate in accordance with certain embodiments of the present disclosure; Figure 4 is a diagram illustrating an example wireless communication system comprising example first and second network nodes and an example communications device; Figure 5 schematically illustrates an example of multi-TRP coherent joint transmission with non-ideal TRP synchronisation; Figure 6 schematically illustrates a proposed solution to non-ideal TRP synchronisation; Figure 7 is a flow diagram illustrating a method of operating a communications device in accordance with example embodiments; Figure 8 is a signalling diagram illustrating an example of the transmission of an invalid calibration assistance report in accordance with example embodiments; Figure 9 schematically illustrates a contour plot of a Euclidean distance function in accordance with example embodiments; Figure 10 is a flow diagram illustrating a method of operating a radio access node in accordance with example embodiments; Figure 11 is a signalling diagram illustrating an example of the transmission of an indication that a communications device must transmit an estimate of a time difference parameter and / or a phase difference parameter in accordance with example embodiments; Figure 12 schematically illustrates the behavior of estimators under conditions of high SNR and low SNR in accordance with example embodiments; Figure 13 schematically illustrates a contour plot of a Euclidean distance function in accordance with example embodiments; and Figure 14 schematically illustrates a plot of the probability of large errors in parameter estimates. DETAILED DESCRIPTION OF THE EMBODIMENTS Long Term Evolution Advanced Radio Access Technology (4G) Figure 1 provides a schematic diagram illustrating some basic functionality of a mobile telecommunications network / system 6 operating generally in accordance with LTE principles, but which may also support other radio access technologies, and which may be adapted to implement embodiments of the disclosure as described herein. Various elements of Figure 1 and certain aspects of their respective modes of operation are well-known and defined in the relevant standards administered by the 3GPP (RTM) body, and also described in many books on the subject, for example, Holma H. and Toskala A [1]. It will be appreciated that operational aspects of the telecommunications networks discussed herein which are not specifically described (for example in relation to specific communication protocols and physical channels for communicating between different elements) may be implemented in accordance with any known techniques, for example according to the relevant standards and known proposed modifications and additions to the relevant standards. The network 6 includes a plurality of base stations 1 connected to a core network 2, which may be for example an Evolved Packet Core (EPC). Each base station provides a coverage area 3 (i.e., a cell) within which data can be communicated to and from communications devices 4. Although each base station 1 is shown in Figure 1 as a single entity, the skilled person will appreciate that some of the functions of the base station may be carried out by disparate, inter-connected elements, such as antennas (or antennae), remote radio heads, amplifiers, etc. Collectively, one or more base stations may form a radio access network. Data is transmitted from base stations 1 to communications devices 4 within their respective coverage areas 3 via a radio downlink (DL). Data is transmitted from communications devices 4 to the base stations 1 via a radio uplink (UL). The core network 2 routes data to and from the communications devices 4 via the respective base stations 1 and provides functions such as authentication, mobility management, charging and so on. Communications devices may also be referred to as mobile stations, user equipment (UE), user terminal, mobile radio, communications device, and so forth. Services provided by the core network 2 may include connectivity to the internet or to external telephony services. The core network 2 may further track the location of the communications devices 4 so that it can efficiently contact (i.e., page) the communications devices 4 for transmitting downlink data towards the communications devices 4. Base stations, which are an example of infrastructure equipment of a wireless communications network, may also be referred to as transceiver stations, nodeBs, eNodeBs, eNB, gNodeBs, gNB and so forth. In this regard different terminology is often associated with different generations of wireless telecommunications systems for elements providing broadly comparable functionality. However, certain embodiments of the disclosure may be equally implemented in different generations of wireless telecommunications systems, and for simplicity certain terminology may be used regardless of the underlying network architecture. That is to say, the use of a specific term in relation to certain example implementations is not intended to indicate these implementations are limited to a certain generation of network that may be most associated with that particular terminology. New Radio Access Technology (5G) Systems incorporating NR technology are expected to support different services (or types of services), which may be characterised by different requirements for latency, data rate and / or reliability. For example, Enhanced Mobile Broadband (eMBB) services are characterised by high capacity with a requirement to support up to 20 Gb / s. The requirements for Ultra Reliable and Low Latency Communications (URLLC) services are for one transmission of a 32 byte packet to be transmitted from the radio protocol layer 2 / 3 SDU ingress point to the radio protocol layer 2 / 3 SDU egress point of the radio interface within 1 ms with a reliability of 1 – 10-5 (99.999 %) or higher (99.9999%) [2]. An example configuration of a wireless communications network which uses some of the terminology proposed for and used in NR and 5G is shown in Figure 2. In Figure 2 a plurality of transmission and reception points (TRPs) 10 are connected to distributed control units (DUs) 41, 42 by a connection interface represented as a line 16. Each of the TRPs 10 is arranged to transmit and receive signals via a wireless access interface within a radio frequency bandwidth available to the wireless communications network. Thus, within a range for performing radio communications via the wireless access interface, each of the TRPs 10, has a coverage area 12 where the aggregate of the coverage areas under the control of the DU forms a cell. As such, wireless communications devices 14 which are within a radio communications range provided by the coverage areas 12 can transmit and receive signals to and from the TRPs 10 via the wireless access interface. Each of the distributed units 41, 42 are connected to a central unit (CU) 40 (which may be referred to as a controlling node) via an interface 46. The central unit 40 is then connected to the core network 20 (which may be for example referred to as 5GC) which may contain all other functions required to transmit data for communicating to and from the wireless communications devices and the core network 20 may be connected to other networks 30. The elements of the wireless access network shown in Figure 2 may operate in a similar way to corresponding elements of an LTE network as described with regard to the example of Figure 1. It will be appreciated that operational aspects of the telecommunications network represented in Figure 2, and of other networks discussed herein in accordance with embodiments of the disclosure, which are not specifically described (for example in relation to specific communication protocols and physical channels for communicating between different elements) may be implemented in accordance with any known techniques, for example according to currently used approaches for implementing such operational aspects of wireless telecommunications systems, e.g. in accordance with the relevant standards. The TRPs 10 of Figure 2 may in part have a corresponding functionality to a base station or eNodeB of an LTE network. Similarly, the communications devices 14 may have a functionality corresponding to the UE devices 4 known for operation with an LTE network. It will be appreciated therefore that operational aspects of a new RAT network (for example in relation to specific communication protocols and physical channels for communicating between different elements) may be different to those known from LTE or other known mobile telecommunications standards. However, it will also be appreciated that each of the core network component, base stations and communications devices of a new RAT network will be functionally similar to, respectively, the core network component, base stations and communications devices of an LTE wireless communications network. In terms of broad top-level functionality, the core network 20 connected to the new RAT telecommunications system represented in Figure 2 may be broadly considered to correspond with the core network 2 represented in Figure 1, and the respective central units 40 and their associated distributed units / TRPs 10 may be broadly considered to provide functionality corresponding to the base stations 1 of Figure 1. The term network infrastructure equipment / access node may be used to encompass these elements and more conventional base station type elements of wireless telecommunications systems. Depending on the application at hand the responsibility for scheduling transmissions which are scheduled on the radio interface between the respective distributed units and the communications devices may lie with the controlling node / central unit and / or the distributed units / TRPs. A communications device 14 is represented in Figure 2 within the coverage area 12. This communications device 14 may thus exchange signalling with the central unit 40 in the coverage area 12 via one of the distributed units / TRPs 10 associated with the coverage area 12. It will further be appreciated that Figure 2 represents merely one example of a proposed architecture for a new RAT based telecommunications system in which approaches in accordance with the principles described herein may be adopted, and the functionality disclosed herein may also be applied in respect of wireless telecommunications systems having different architectures. Thus, certain embodiments of the disclosure as discussed herein may be implemented in wireless telecommunication systems / networks according to various different architectures, such as the example architectures shown in Figures 1 and 2. It will thus be appreciated the specific wireless telecommunications architecture in any given implementation is not of primary significance to the principles described herein. In this regard, certain embodiments of the disclosure may be described generally in the context of communications between network infrastructure equipment / access nodes and a communications device, wherein the specific nature of the network infrastructure equipment / access node and the communications device will depend on the network infrastructure for the implementation at hand. For example, in some scenarios the network infrastructure equipment / access node may comprise a base station, such as an LTE-type base station 1 as shown in Figure 1 which is adapted to provide functionality in accordance with the principles described herein, and in other examples the network infrastructure equipment may comprise a control unit / controlling node 40 and / or a TRP 10 of the kind shown in Figure 2 which is adapted to provide functionality in accordance with the principles described herein. A more detailed diagram of some of the components of the network shown in Figure 2 is provided by Figure 3. In Figure 3, a TRP 10 as shown in Figure 2 comprises, as a simplified representation, a wireless transmitter 30, a wireless receiver 32 and a controller or controlling processor 34 which may operate to control the transmitter 30 and the wireless receiver 32 to transmit and receive radio signals to one or more UEs 14 within a coverage area 12 formed by the TRP 10. As shown in Figure 3, an example UE 14 is shown to include a corresponding transmitter 49, a receiver 48 and a controller 44 which is configured to control the transmitter 49 and the receiver 48 to transmit signals representing uplink data to the wireless communications network via the wireless access interface formed by the TRP 10 and to receive downlink data as signals transmitted by the transmitter 30 and received by the receiver 48 in accordance with the conventional operation. The transmitters 30, 49 and the receivers 32, 48 (as well as other transmitters, receivers and transceivers described in relation to examples and embodiments of the present disclosure) may include radio frequency filters and amplifiers as well as signal processing components and devices in order to transmit and receive radio signals in accordance for example with the 5G / NR standard. The controllers 34, 44 (as well as other controllers described in relation to examples and embodiments of the present disclosure) may be, for example, a microprocessor, a CPU, or a dedicated chipset, etc., configured to carry out instructions which are stored on a computer readable medium, such as a non-volatile memory. The processing steps described herein may be carried out by, for example, a microprocessor in conjunction with a random access memory, operating according to instructions stored on a computer readable medium. The transmitters, the receivers and the controllers are schematically shown in Figure 3 as separate elements for ease of representation. However, it will be appreciated that the functionality of these elements can be provided in various different ways, for example using one or more suitably programmed programmable computer(s), or one or more suitably configured application-specific integrated circuit(s) / circuitry / chip(s) / chipset(s). As will be appreciated the infrastructure equipment / TRP / base station as well as the UE / communications device will in general comprise various other elements associated with its operating functionality. As shown in Figure 3, the TRP 10 also includes a network interface 50 which connects to the DU 42 via a physical interface 16. The network interface 50 therefore provides a communication link for data and signalling traffic from the TRP 10 via the DU 42 and the CU 40 to the core network 20. The interface 46 between the DU 42 and the CU 40 is known as the F1 interface which can be a physical or a logical interface. In one example the connection 16 from the TRP 10 to the DU 42 is via fibre optic. The connection between a TRP 10 and the core network 20 can be generally referred to as a backhaul, which comprises the interface 16 from the network interface 50 of the TRP 10 to the DU 42 and the F1 interface 46 from the DU 42 to the CU 40. Multi-TRP Operation In multiple Transmit / Receive Points (multi-TRPs) operation, a serving cell can schedule joint transmission of the same data to a user equipment (UE) from several TRPs, providing better coverage, reliability and / or data rates. In a setup where two TRPs simultaneously transmit data to a single UE but where TRP2 is further away from the UE than TRP1, the signal received at the UE from TRP2 is delayed in comparison with the signal received from TRP1. Figure 4 is a diagram illustrating an example wireless communication system 400 comprising an example communications device 414, an example first node being a first TRP 406, and an example second node being a second TRP 408 according to this disclosure. The communications device 414 may be configured to communicate with the first and second TRPs 406 and 408 via wireless links 410 and 412, respectively. The example wireless communication system 1 of Figure 4 may comprise a core network, CN, node 402, such as a network node operating in the core network, such as in the Evolved Packet Core Network, EPC, and / or a 5G Core Network, 5GC. Examples of CN nodes in EPC include a Mobility Management Entity, MME. The CN node 402 may provide information such as instructions to, and receive information from, TRPs 406 and 408 via wireless links 14. The CN node 402 may provide information such as instructions to, and receive information from, the communications device 414 and TRPs 406 and 408. As illustrated in Figure 4, a transmission path of wireless link 410 is much shorter than a transmission path of wireless link 412, leading to a difference in time-of-flight, TOF, between signals transmitted via wireless link 412 and signals transmitted via wireless link 410. As will be appreciated, the TOF depends on the distance between the TRPs 406, 408 and the communications device 414. The TOF difference may for example result in the delay in the time at which communications device 414 receives transmissions from the second TRP 408 when compared with the time at which communications device 414 receives transmissions from the first TRP 406. In other words, the communications device may receive transmissions from the second TRP 408 later than transmissions from the first TRP 406 due to the TOF difference. The difference in TOF may cause a difference in a time of arrival (ToA) between the signals transmitted via wireless link 412 and signals transmitted via wireless link 410 arriving at the communications device 414. As will be appreciated, the ToA depends on the distance between the TRPs 406, 408 and the communications device 414 and also depends on a time at which the TRPs 406, 408 transmit the signals eia wireless links 410, 412. The difference in ToA leads to a ToA offset between transmissions between the communications device 414 and the first TRP 406, and transmissions between the communications device 414 and the second TRP 408. Methods have been proposed to correct for ToF and ToA offsets, for example, by adjusting the start time of transmissions from the TRPs 406, 408. However, such methods do not account for timing misalignments between a transmission chain and a reception chain within a TRP. Timing misalignments between the transmission chain and the reception chain within a TRP will be discussed in more detail below. Coherent Joint Transmission (CJT) In multi-TRP systems, it is important that the TRPs are time and phase synchronised with each other. 3GPP is currently discussing coherent joint transmission with non-ideal TRP synchronisation. In the context of this disclosure, TRPs with non-ideal synchronisation means there is a timing misalignment between the transmission chain and reception chain of the TRP. As will be appreciated, such timing misalignments may change over time starting from initial factory calibration. Such changes in timing misalignment are mainly due to temperature dependency but may also be due to aging. An example of timing misalignments between TRPs is illustrated in Figure 5 ([3]). In Figure 5, a UE is transmitting a sounding reference signal (SRS) 516 in the uplink towards two TRPs (TRP 1 and TRP 2). After receiving the SRS 516, TRP 1 and TRP 2 each transmit a physical downlink control channel (PDSCH) 518 to the UE. Figure 5 illustrates a timing reference 502, a TRP 1 timing 504, a TRP 2 timing 506 and a UE timing 507. It will be appreciated from Figure 5 that the TRP 1 timing 504 and the TRP 2 timing 506 are misaligned with respect to each other. As will be appreciated by a person skilled in the art, TRP1 has a transmission chain (e.g. a downlink (DL) Radio Frequency (RF) chain) and a reception chain (e.g. an uplink (UL) RF chain). There may be a difference between timings of the reception chain and transmission chain for TRP 1. In other words, Therefore, TRP 1 has non-ideal time synchronisation. As will be appreciated by a person skilled in the art, TRP 2 has a transmission chain (e.g. a DL RF chain) and a reception chain (e.g. a UL RF chain). There may be a difference between timings of the reception chain and transmission chain for TRP2. In other words, . Therefore, TRP 2 has non-ideal time synchronisation. A transmission chain may comprise circuitry involved in transmitting a wireless signal whereas a reception chain may comprise circuitry involved in receiving a wireless signal. As will be appreciated, the differences in timing between transmission and reception chains in the same TRP may arise due to imperfections in transmitter and / or receiver circuitry of a TRP. The time difference for a TRP may be, for example, a time difference between the time it takes the TRP to generate and transmit a signal using its transmitter circuitry and the time it takes for the signal to pass th h he receiver circuity of the TRP once it arrives at an antenna of the TRP. Therefore, for exampl e may be a time taken for TRP 1 to generate and transmit a signal using transmitter circuitry and may be the time taken for a signal to pass through the receiver circuitry of TRP 1 once the signal arrives at the antenna of TRP 1. Figure 5 also illustrates two bar-plots 508, 510 showing the impulse responses between the UE and the two TRPs. In particular, bar plot 510 illustrates a power delay profile (PDP) measured by the network via the UE and bar plot 508 illustrates a PDP observed by the UE on the DL. In the UL, the TRPs believe that the impulse response from the UE towards the set of two TRPs is as shown in plot 510. Assuming that the two TRPs attempt to transmit the PDSCH 518 in the DL exactly one OFDM symbol later, the signal arrives at the UE as shown in plot 508. As will be appreciated, the TRPs may not actually transmit the DL exactly one OFDM symbol later because the UL RF chain and the DL RF chain for a TRP are different as explained above. In Appendix 1 of [3] an analysis of the situation in Figure 5 is provided. The results of that analysis are reproduced below: At subcarrier ^, ^ the effective channel from TRP1 to the UE is ^^ = |^^^^^(^)|^exp(^2^^Δ^^^ + ^ where ^ Maximum ratio transmission (MRT) type precoding has been adopted at both TRPs ^ ^ − ^ ^ ^ ^ With perfect time synchronisation and ideal hardware for a TRP 1 and a TRP 2, one would ha and ^ Thus, the combined channel from the two TRPs towards the UE reads Equation 1 To obtain ideal performance, it is not necessary to have ^ = ^ = 0 and ^ = ^ = 0. In the combined channel stated above, the two channel contributions have been coherently superimposed, which means that the SNR has been maximized. The fact that ^ = ^ ≠ 0 and ^ = ^ ≠ 0 shows up as a rotation of the channel, but does not impact the SNR. Therefo t lib te coherent joint transmissions, it i sufficient to estimate a phase difference parameter ^ = ^^ − ^^ and a time difference parame ter = ^ ^ ^, provide these to the TRPs, and let the TRPs adjust their signals accordingly. It has been proposed that a UE should report estimates of θ and β. An example of a proposal illustrating a UE reporting estimates of θ and β is shown in Figure 6. In step S602, the UE transmits a sounding reference signal (SRS) to TRP 1 and TRP 2. In step S604, the UE receives a precoded channel state information reference signal (CSI-RS) from TRP 1. In step S606, the UE receives a precoded CSI-RS from TRP 2. In step S608, based on the received CSI-RSs, the UE calculates the product of the complex conjugate of ^∗^and ^^(i.e. ^^^^) on multiple subcarriers to derive the inter-TRP timing offset between TRP 1 and TRP 2 (^) and the inter-TRP phase difference (^) between TRP1 and TRP 2. In step S610, the UE transmits a calibration assistance report to TRP. In this example, the calibration assistance report reports θ and β to TRP 2. In step S612, TRP1 and TRP2 calibrate coherent joint transmissions (CJTS) based on ^and ^ in the calibration assistance report. For example, TRP 2 synchronises its communications with respect to TRP 1 based on θ and β. Although Figure 6 describes an example in which a UE reports estimates of θ and β, the present inventors have recognised that, if the estimates of θ and β are low accuracy estimates, then TRP 1 and TRP 2 may fail to synchronise correctly. Accordingly, TRP 1 and TRP 2 may waste power and communications resources attempting to synchronise based on low accuracy estimates. There is therefore a need for improved communications devices, radio access nodes and methods which can improve power and communications resource utilisation efficiency. Although Figure 6 illustrates an example where a time difference parameter, ^, and a phase difference parameter, θ, are reported in a calibration assistance report (First Reporting Option), it will be appreciated that the calibration assistance report may comprise time and phase parameters other than θ and β which assist a plurality of radio access nodes (such as TRPs) to calibrate CJTs . Therefore, a calibration assistance report may generally be understood as a report comprising an estimate of one or more of a time parameter and phase parameter for assisting a plurality of radio access nodes (such as TRPs) to calibrate CJTs. In some examples, the calibration assistance report may comprise a plurality (i.e. two or more) phase parameters and no time parameter (Second Reporting Option). In such cases, the plurality of phase parameters may be estimated at a number of subbands into which the measurement bandwidth is divided. In such cases, on receiving the report, the network may interpolate to obtain a phase parameter for any missing subband. In other examples (Third Reporting Option), the calibration assistance report may comprise one phase parameter and no time parameter. Furthermore, although Figure 6 describes an example where precoded CSI-RSs are used, it is also possible to use non-precoded CSI-RSs. In such cases, since the uplink is not leveraged when transmitting the CSI-RSs, the time parameter and one or more phase parameters will depend on a timing of the transmission chain of TRP1 and / or TRP2 and, possibly, a timing of the reception chain of the UE, but will not depend on the timing of the reception chain of TRP1 and / or TRP2 and the timing of the transmission chain of the UE. Precoded or non-precoded CSI-RSs may be used in any of the First, Second or Third Reporting Options as discussed above. Invalid Calibration Assistance Report In view of the above, there is provided a method of operating a communications device as illustrated in Figure 7. The method starts in step S1. In step S2, the method comprises transmitting an invalid calibration assistance report in response to one or more invalidity trigger conditions being met. The invalid calibration assistance report indicates that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJTs), the plurality of radio access nodes comprising a first radio access node and a second radio access node. The one or more invalidity trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter. The indication that the communications device is not be permitted to transmit an estimate of one or more of the time parameter and the phase parameter may be alternatively referred to as an indication that the one or more invalidity trigger conditions are not met. For example, a network node (such as a radio access node) which receives the indication may determine, based on the indication that the one or more invalidity trigger conditions are not met, that the communications device is not permitted to transmit an estimate of one or more of the time parameter and the phase parameter. The communications device may be regarded as being “not permitted” to transmit an estimate of one or more of the time parameter and the phase parameter in the sense that the communications device is not allowed to transmit the estimate of one or more of the time parameter and the phase parameter according to the invalidity conditions. A calibration assistance report may generally be understood as a report comprising an estimate of one or more of a time parameter and phase parameter for assisting a plurality of radio access nodes (such as TRPs) to calibrate CJTs. By contrast, an invalid calibration assistance report is a report indicating that the communications device is not permitted to transmit an estimate of one or more of a time parameter and phase parameter for assisting a plurality of radio access nodes. Accordingly, the invalid calibration assistance report may indicate that the communications device is not permitted to transmit some or all of the information comprised in a calibration assistance report. In some embodiments, the time parameter for assisting the plurality of radio access nodes to calibrate CJTs is a time difference parameter which is a time difference between a timing misalignment parameter for the first radio access node and a timing misalignment parameter for the second radio access node. In some embodiments, the timing misalignment parameter for each of the radio access nodes is dependent at least in part on a time difference between a transmission chain and a reception chain of the radio access node. In some embodiments, the phase parameter for assisting the plurality of radio access nodes to calibrate CJTs is a phase difference parameter which is a phase difference between a phase misalignment parameter for the first radio access node and a phase misalignment parameter for the second radio access node. In some embodiments, the phase misalignment parameter for each of the radio access nodes is dependent at least in part on a phase difference between a transmission chain and a reception chain of the radio access node. An example of th ti diff nce parameter is: = ^ ^ and an example of the phase difference parameter is: ^ = ^ ^. An example of the timing misalignment parameter for the first radio ss node is ^^ and an example of the timing misalignment parameter for the second radio access node i s ^^. An example of the phase misalignment parameter for the first rad ess node is ^^. An example of the phase misalignment parameter for the second radio access node is ^^. In some embodiments, the time difference parameter is further dependent on a time difference (e.g. ^UE) between a transmission chain and a reception chain of the communications device, and the phase difference parameter is further dependent on a phase difference (e.g. (|>UE) between the transmission chain and the reception chain of the communications device.

[0002] In some embodiments, the time difference parameter and the phase difference parameter may not depend on TUE or (|)UE because the contributions of TUE and (|)UE cancel out when determining the time difference parameter and the phase difference parameter. This, for example, is because the same communications device determines the time difference parameter and the phase difference parameter using the same antenna port on the communications device.

[0003] In some embodiments, the time parameter may depend on a timing of a transmission chain of the first radio access node and / or the second radio access node and, possibly, also on a timing of a reception chain of the communications device. In such embodiments, the time parameter may not depend on a timing of a reception chain of the first radio access node / and or the second radio access node and may not depend on a timing of a transmission chain of the communications device. Such embodiments may apply when the communications device estimates the time parameter using non-precoded CSI-RS signals, for example.

[0004] In some embodiments, the phase parameter may depend on a phase of a transmission chain of the first radio access node and / or the second radio access node and, possibly, also on a phase of a reception chain of the communications device. In such embodiments, the time parameter may not depend on a phase of a reception chain of the first radio access node / and or the second radio access node and may not depend on a phase of a transmission chain of the communications device. Such embodiments may apply when the communications device estimates the phase parameter using non-precoded CSI-RS signals, for example.

[0005] In some embodiments, the invalid calibration assistance report may indicate that the communications device is not permitted to transmit an estimate of: only the time parameter (in which case the communications device may transmit an estimate of the phase parameter); only the phase parameter (in which case the communications device may transmit an estimate of the time parameter); or both the time parameter and the phase parameter (in which case neither an estimate of the time parameter nor an estimate of the phase parameter can be transmitted). In some embodiments, the indication that the communications device is not permitted to transmit an estimate of a phase parameter means the communications device cannot transmit an estimate of some or all of one or more phase parameters used for assisting the plurality of radio access nodes in calibrating CJTs - for example, in the case where two or more phase parameters would normally be reported (such as in the Second Reporting Option discussed above), the indication may indicate that one or more (or all) of the phase parameters cannot be reported.

[0006] In some embodiments, the method comprises receiving an indication of the one or more invalidity trigger conditions from the wireless communications network (e.g. from the first radio access node or the second radio access node). The indication of the one or more invalidity trigger conditions may comprise one or more thresholds for use to determine whether the one or more trigger conditions are met, for example. Examples of such thresholds, as will be explained in more detail below include: δθ, Pθ, δβ, Pβ, Vθ, Vβ, γlarge, γβ , Yθ, Tβ, γβ, Tθ, and Yθ -

[0007] In some embodiments, the communications device is pre-configured with the one or more invalidity trigger conditions because, for example, the one or more invalidity trigger conditions are fixed in the specifications.

[0008] In some embodiments, the invalid calibration assistance report comprises a codepoint that indicates “invalid”. In other words, the codepoint indicates that the communications device is not permitted to transmit one or more of the time parameter and the phase parameter. For example, a calibration assistance report may comprise a code with N bits, which therefore contains 2^codepoints. A subset of the code points (e.g.2^) may indicate a valid estimate of the time parameter (or the phase parameter) and the remaining code point may indicate “invalid” for the estimate of the other of the time parameter and the phase parameter. Accordingly, the invalid calibration assistance report for the one parameter is transmitted as part of a calibration assistance report for the other parameter. As mentioned above, the communications device transmits the invalid calibration assistance report “in response to” one or more invalidity trigger conditions being met. In some embodiments, one invalidity trigger conditions is met. In some embodiments, a plurality of the invalidity trigger conditions are met. In some embodiments, all of the invalidity trigger conditions are met. The phrase “in response to” should be interpreted as meaning that the invalid calibration assistance report is transmitted as a reaction to one or more invalidity trigger conditions being met. This may alternatively be phrased as the invalid calibration assistance report is transmitted “in the instance that” one or more invalidity trigger conditions are met. For ease of explanation, the following description may refer to communications between UEs and TRPs. It should be appreciated that the present disclosure applies more generally to communications between communications devices and radio access nodes of a wireless communications network. For ease of explanation, example embodiments may be described in terms of a time difference parameter (such as ^) and a phase difference parameter (such as ^). It should be appreciated that the present disclosure applies more generally to a time parameter and phase parameter for assisting the plurality of radio access nodes to calibrate CJTs. Unless otherwise stated or clear from the context, references to a network performing an operation should be understood as one of radio access nodes (such as a TRP) or another node of the network (such as a CU or DU) performing the operation. Furthermore, “calibrating CJTs” may alternatively be referred to as synchronising CJTs. In step S3, the method ends. By transmitting an invalid calibration assistance report in response to one or more invalidity trigger conditions being met, a radio access node receiving the report can be informed that that communications device is not permitted to transmit an estimate of one or more of the time parameter and the phase parameter. Therefore, the radio access node receiving the report may determine not to synchronise with another radio access node. Accordingly, power and communications wastage due to a failed, or inaccurate, synchronisation resulting from inaccurate estimates of the time parameter and / or time parameter can be avoided. Figure 8 is a signalling diagram illustrating an example of the transmission of an invalid calibration assistance report in accordance with example embodiments. In step S802, TRP2 transmits an indication of the one or more invalidity trigger conditions to the UE. In this example, the indication of the one or more invalidity trigger conditions comprises one or more thresholds for use to determine whether or not the invalidity trigger conditions are met. In step S803, the UE transmits a first sounding reference signal (SRS1) to TRP2. In step S804, the UE transmits a second SRS (SRS 2) to TRP1. In step S806, TRP2 transmits a first precoded channel state information reference signal (CSI-RS 1) to the UE. In step S808, TRP1 transmits a second precoded CSI-RS (CSI-RS 2) to the UE. In step S810, based CSI-RS 1 and CSI-RS 2, the UE calculates the product of the complex conjugate of ^ ∗ ^ and ^^ (i.e. ^^^^) on multiple subcarriers to derive the inter-TRP timing offset between TRP 1 and TRP 2 (^) and the inter-TRP phase difference (^) between TRP1 and TRP 2. In step S812, the UE determines an accuracy with which it can estimate ^ and ^. Then, using the one or more thresholds received in step S802 and the determined accuracy with which it can estimate ^ and ^, the UE determines whether or not one or more of the invalidity trigger conditions are met. In step S814, if the UE determines that one or more of the invalidity trigger conditions are met, the UE transmits an invalid calibration assistance report to TRP 2. In step S816, if the UE determines that one or more of the invalidity trigger conditions are not met, the UE transmits an estimate of ^ and an estimate of ^ to TRP 2. Invalidity Trigger Conditions The following description will make reference to the behavior of estimators. An overview of the behavior of estimators is provided in the Appendix 1. At high SNR (for example, above a pre-defined threshold), an estimate of a parameter is typically close to a true value of the parameter. In the following description, the estimate parameter will be referred to as ^^(i.e. an estimate of the phase difference parameter) and the true value of the phase difference parameter will be referred to as ^, though it will be appreciated that the following description applies to any parameter. The noise ^^ − ^ is well approximated by a zero-mean Gaussian distribution with the reciprocal of the Fisher information as variance. By contrast, at low SNR, there may be a substantial Gaussian noise and bias present in ^^. Thus, we may express the estimate ^^ as shown in Equation 2: Equation 2 where ^ represents the Gaussian noise and ^ represents the bias. The bias, ^, is zero with a probability of 1 − ^^^^^^. Therefore, ^^^^^^ is the probability that ^ is not zero. As can be seen in Figure 12 (described in detail in Appendix 1), ^ ∈ {0, ^ − ^^} and, as SNR increases, the probability ^^^^^^ decreases. Similarly, as SNR increases, the variance of ^ reduces. Accordingly, if ^^^^^^ is not negligible (or not below a predetermined threshold), the estimate ^^ may be far away from ^ and therefore may be regarded as an invalid estimate. Even if ^^^^^^ is negligible (or below the predetermined threshold), the estimate ^^ may still be regarded as invalid if ^ is too large (e.g. above a predetermined threshold). Of course, as will be appreciated, Equation 2 applies equally to an estimate of ^. Accordingly, in accordance with example embodiments, invalidity trigger conditions are specified which are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter. If the one or more of the invalidity trigger conditions are met, the communications device transmits an invalid calibration assistance report indicating that the communications device is not permitted to transmit one or more of the estimate of the time parameter and the phase parameter (because, for example, the accuracy with which the communications device can estimate the time difference and / or phase parameter is not sufficient for a valid estimate). The invalidity trigger conditions may comprise semi-stringent and / or stringent invalidity trigger conditions as will be explained in more detail below. Further details regarding the mathematical basis for the invalidity trigger conditions are provided in Appendix 2. Semi-stringent conditions Assume that a maximal delay of |^ℓ| ≤ ^^^^. As will be appreciated by a person skilled in the art, this implies that |^| ≤ 2^^^^ and 0 ≤ ^ ≤ 2^. In some embodiments, the semi-stringent conditions may comprise one or both of conditions A and B below: where ^^ and ^^ are probability thresholds, ^^ and ^^ are deviation thresholds, and ^^ and ^^ are variance thresholds. As will be appreciated by a person skilled in the art, condition A imposes a constraint on ^. In other words, condition A limits the probability that ^ will be non-zero. A radio access node may provide an indication of conditions A and / or B to the communications device, for example, by transmitting an indication of the probability thresholds ^^ and ^^, an indication of the deviation thresholds ^^ and ^^, and / or an indication of the variance thresholds ^^ and ^^. However, even if the probability that ^ will be non-zero is small, it is advantageous to also constrain other sources of errors in the estimates (such as the Gaussian noise). This is done via condition B. The above description applies equally if ^^^^^^ is compared with ^^ +^^ or max(^^ , ^^). ^^^^^^ may be referred to as a probability that a bias (e.g. ^) in the estimate of ^ and / or ^ is non-zero. Stringent Conditions Based on the received one or more signals received from TRP 1, {^^(^)} and one or more signals received from {^^(^)}, the UE may estimate the channel gain |^^^^^(^)|^between the UE and TRP1 and the channel gain|^^^^^(^)|^between the UE and TRP2. The UE then forms the Euclidean distance function shown in Equation 3: ^ The function ^^(Δ^ , Δ^) is the Euclidean distance, normalized by the noise density, between two parameter pairs (^, ^), (^^, ^^) where ^^ = ^ + Δ ^ ^ and ^ = ^ + Δ^ . The UE determines all local minimums of ^^(Δ^ , Δ^), except for the one at ^Δ^ , Δ^^ = (0,0) which is a global minimum. Let ^∗denote the function value of the smallest local minimum (except for the one at (0,0)), let Δ∗^ be the associated value of Δ^ when ^^(Δ^ , Δ^) = ^∗, and let Δ∗^ be the associated value of Δ^ when ^^(Δ^ , Δ^) =^∗. Therefore, (Δ^∗, Δ∗^ ) may be referred to as the “minimiser”. Then, a good approximation for the probability of a bias in ^ and / or ^. ^^^^^^, is shown in Equation 5: Equation 5 Whenever ^^^^^^ is non-negligible, there is a good probability of a bias in ^ and / or ^. The size of the bias is determined by how close the local minimum is to (0,0) - the bias is small if the local minimum occurs close to (0,0). In some embodiments, D* may represent the function value of the smallest local minimum outside the region ^ centered at the origin. In such cases, ^^^^^^ represents the probability of a large bias in ^ and / or ^ . The approximation of ^^^^^^ shown in Equation 5 offers a good balance between accuracy and simplicity. Whenever ^^^^^^ is small (e.g. below a pre-defined threshold such as 10-xwhere X is 2), the estimation error is computed as Equation 6 Combined Invalid Calibration Assistance Report In some embodiments, the UE transmits a combined invalid calibration assistance report indicating that the UE is not permitted to transmit an estimate of ^ and ^. In such embodiments, the UE may transmit the invalid calibration assistance report if one or more of conditions C, D and E are met: C ^^^^^^ ≥ ^^^^^^ D E In some embodiments, a TRP may transmit an estimate of the set of thresholds {^^ , ^^ , ^^^^^^} to the UE. As mentioned above, in some embodiments, D* used in the calculation of ^^^^^^ may represent the function value of the smallest local minimum outside the region ^ centered at the origin. In such cases, ^^^^^^ represents the probability of a large bias in ^ and / or ^. This region specifies what may be considered a “large error”. An example of ^ is ^ = {Δ^ , Δ^ : ^Δ^^ < ^^ , |Δ^| < ^^, for some values ^^ , ^^ that are indicated by the network or predetermined. Separate Reports In some embodiments, the UE transmits a first invalid calibration assistance report indicating that the UE is not permitted to transmit an estimate of ^ and a second invalid calibration assistance report indicating that the UE is not permitted to transmit an estimate of ^. In such embodiments, the UE may transmit the first invalid calibration assistance report if one or more of invalidity trigger conditions F and G are met. F G In such embodiments, the UE may transmit the second invalid calibration assistance report if one or more of invalidity trigger conditions H and I are met. As mentioned above, in some embodiments, D* used in the calculation of ^^^^^^ may represent the function value of the smallest local minimum outside the region ^ centered at the origin. In such cases, ^^^^^^represents the probability of a large bias in ^ and / or ^. H I . ≥ ^ Figure 9 illustrates an example contour plot of ^^(Δ^ , Δ^). A global minimum 902 is shown at (0,0) which is irrelevant for calibration assistance reporting. Therefore, the global minimum 902 can be ignored. In the example shown in Figure 9, a local minimum 906 is shown near the bottom of the plot and another local minimum 904 is shown in the upper right corner of the plot. Assuming that the value of the local minimum 906 has a function value much smaller than the local minimum 904, and is close in function value to the global minimum 902, then the joint estimate (^^ , ^^) will be close to (^ + Δ^ , ^ + Δ^) where (Δ^ , Δ^) are the coordinates of the local minimum 906. In this case, the estimate ^^ is not too erroneous and is not “invalid” whereas ^^ is invalid in this case. However, if the local minimum 904 is the smallest local minimum, then both ^^ and ^^ are invalid. In some embodiments, where a combined invalid calibration assistance report is transmitted, the report may be transmitted in either of these cases i.e. in the case where only ^^ is invalid or in the case where ^^ and ^^ are both invalid. In some embodiments, ^^^^^^ may be defined as the maximum of the following equation. In Equation 7, ^ is a cumulative density function of a standard normal random variable (i.e., a normal random variable with mean zero and standard deviation one). Equation 7 may be applied to parameter values (i.e. points of the form (Δ^ , Δ^)) outside a circle centered at the origin with a predetermined radius. Equation 7 has the advantage of therefore taking further values than local minimums into account. Communications device must transmit estimates of ^ and / or ^ In some embodiments, the radio access node may determine that it requires an estimate of a time and / or phase parameter (such as ^ and / or ^) from the communications device. In such embodiments, the radio access node may transmit an indication that the communications device must transmit an estimate of ^ and / or ^. In other words, the radio access node may prohibit the communications device from transmitting an invalid calibration assistance report for ^ and / or ^. In some embodiments, the radio access node may determine an accuracy with which ^ and / or ^ can be estimated. In such embodiments, the radio access node may determine that one or more of the invalidity trigger conditions are not met. For example, the radio access node may determine that the communications device should be able to transmit estimates of ^ and / or ^ with a desired accuracy. Figure 10 is a flow diagram illustrating an example of operating a first radio access node of a wireless communications network in accordance with example embodiments. The method starts in step S11. In step S12, the method comprises transmitting, to a communications device an indication that the communications device must transmit an estimate of one or more of a time parameter and a phase parameter for assisting the first radio access node and a second radio access node of the wireless communications network to calibrate coherent joint transmissions (CJTs). In some embodiments, the indication that the communications device must transmit one or more of the time parameter and the phase parameter is transmitted in response to one or more invalidity conditions not being met. The one or more invalidity trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter. In some embodiments, the time parameter for assisting the plurality of radio access nodes to calibrate CJTs is a time difference parameter which is a time difference between a timing misalignment parameter for the first radio access node and a timing misalignment parameter for the second radio access node. In some embodiments, the timing misalignment parameter for each of the radio access nodes is dependent at least in part on a time difference between a transmission chain and a reception chain of the radio access node. In some embodiments, the phase parameter for assisting the plurality of radio access nodes to calibrate CJTs is a phase difference parameter which is phase difference between a phase misalignment parameter for the first radio access node and a phase misalignment parameter for the second radio access node. In some embodiments, the phase misalignment parameter for each of the radio access nodes is dependent at least in part on a phase difference between a transmission chain and a reception chain of the radio access node. In some embodiments, the first radio access node may determine whether or not the one or more invalidity trigger conditions are met. In some embodiments, the first radio access node may estimate one or more of the time parameter and the phase parameter. It will be appreciated that “one or more invalidity trigger conditions being met” may alternatively be referred to as “one or more validity trigger conditions being met”, where the validity trigger conditions are the opposite of the invalidity trigger conditions. The method ends in step S13. By transmitting an indication that the communications device must transmit an estimate of one or more of the time parameter and the phase parameter, radio access node can ensure that the communications device transmits the required estimates. In embodiments where the indication is transmitted in response to one or more invalidity conditions being met, the communications device may not have to determine whether invalidity trigger conditions have been met. Therefore, in some embodiments, the radio access node can ensure that the estimates transmitted by the communications device are of an acceptable accuracy. Figure 11 is a signalling diagram illustrating an example of the transmission of an indication that a communications device must transmit an estimate of a time difference parameter and / or a phase difference parameter in accordance with example embodiments. In step S1002, the UE transmits a first SRS (SRS 1) to TRP 2. In step S1003, the UE transmits a second SRS (SRS 2) to TRP 1. In step S1004, TRP 1 transmits an estimate of a channel gain between TRP 1 and the UE (such as an estimate of|^^^^^(^)|^) to TRP 2. TRP 1 may determine the estimate of the channel gain by measuring the channel gain between TRP 1 and the UE, for example. In step S1005, TRP 2 uses the estimate of the channel gain between TRP 1 and the UE and an estimate of a channel gain between TRP2 and the UE (such as an estimate of |^^^^^(^)|^) to determine an accuracy with which the UE can determine an estimate of ^ and ^. Then, based on the determined accuracy, TRP2 determines whether or not one or more of the invalidity trigger conditions are met. In this example, TRP2 determines that one or more of the invalidity trigger conditions are not met. Alternatively a network node such as a CU or a DU may determine that that one or more of the invalidity trigger conditions have been met and transmit an indication to the TRP2 or TRP1 that one or more of the invalidity trigger conditions have been met. In some embodiments, step S1005 can be bypassed. In such embodiments, the network (e.g TRP1, TRP2, a CU or a DU) determines that an invalid calibration assistance report from UE is needed. In step S1006, in response to determining that one or more of the invalidity trigger conditions are not met, TRP 2 transmits an indication to the UE that the UE must transmit an estimate ^ and ^. In other words, TRP2 transmits an indication that the UE is not permitted to transmit an invalid calibration assistance report. In some embodiments, the indication indicates that the UE must not transmit invalid calibration assistance report from that point onwards. In embodiments where Step S1005 is bypassed, the network may transmit the indication to the UE that the UE must transmit an estimate ^ and / or ^ directly upon the setting up to the multi-TRP CJT to the UE. In step S1008, TRP 2 transmits a first precoded CSI-RS to the UE. In step S1010, TRP 1 transmits a second precoded CSI-RS to the UE. In step S1012, based on CSI-RS 1 and CSI-RS 2, the UE calculates the product of the complex conjugate of ^ ∗ ^ and ^^ (i.e. ^^^^) on multiple subcarriers to derive the inter-TRP timing offset between TRP 1 and TRP 2 (^) and the inter-TRP phase difference (^) between TRP1 and TRP 2. In step 1014, the UE transmits an estimate of ^ and an estimate of ^ to TRP 2. Although embodiments have been discussed herein where the invalid calibration assistance report indicates that the communications device is not permitted to transmit an estimate of one or more of a time difference parameter and a phase difference parameter, example embodiments are equally applicable to a case in which the invalid calibration assistance report indicates that the communications device is not permitted to transmit an estimate of one or more of the timing misalignment parameter for the first radio access node, the timing misalignment parameter for the second radio access node, the phase misalignment parameter for the first radio access node, and the phase misalignment parameter for the second radio access node. In some embodiments, the invalid calibration assistance report may be transmitted in response to one or more invalidity trigger conditions associated with an accuracy with which the communications device can estimate one or more of the timing misalignment parameter for the first and second radio access node and the phase misalignment parameter for the first and second radio access node. In some embodiments, a radio access node may transmit, in response to one or more invalidity trigger conditions not being met, an indication that the communications device must transmit an estimate of one or more of the timing misalignment parameter for the first radio access node, the timing misalignment parameter for the second radio access node, the phase misalignment parameter for the first radio access node, and the phase misalignment parameter for the second radio access node. In this case, the one or more invalidity trigger conditions associated with an accuracy with which the communications device can estimate one or more of the timing misalignment parameter for the first and second radio access node and the phase misalignment parameter for the first and second radio access node. Those skilled in the art would further appreciate that such infrastructure equipment, nodes and / or communications devices as herein defined may be further defined in accordance with the various arrangements and embodiments discussed in the preceding paragraphs. It would be further appreciated by those skilled in the art that such infrastructure equipment and communications devices as herein defined and described may form part of communications systems other than those defined by the present disclosure. The following numbered paragraphs provide further example aspects and features of the present technique: Paragraph 1. A method of operating a communications device, the method comprising transmitting an invalid calibration assistance report in response to one or more invalidity trigger conditions being met, the invalid calibration assistance report indicating that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJTs), the plurality of radio access nodes comprising a first radio access node and a second radio access node, wherein the one or more invalidity trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter. Paragraph 2. A method according to paragraph 1, wherein the time parameter is a time difference parameter which is a time difference between a timing misalignment parameter for the first radio access node and a timing misalignment parameter for the second radio access node, and the phase parameter is a phase difference parameter which is a phase difference between a phase misalignment parameter for the first radio access node and a phase misalignment parameter for the second radio access node. Paragraph 3. A method according to paragraph 2, wherein the timing misalignment parameter for each of the radio access nodes is dependent at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase misalignment parameter for each of the radio access nodes is dependent at least in part on a phase difference between a transmission chain and a reception chain of the radio access node. Paragraph 4. A method according to any preceding paragraph, comprising receiving an indication of the one or more invalidity trigger conditions from the first radio access node or the second radio access node. Paragraph 5. A method according to any preceding paragraph, wherein the one or more invalidity trigger conditions comprise one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the time parameter. Paragraph 6. A method according to paragraph 5, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the time parameter comprise a condition requiring that a probability that a modulus of a difference between an estimated value of the time parameter (e.g. ^^) and a true value of the time parameter (e.g. ^) is greater than a pre-defined deviation (e.g. ^^) is less than a threshold probability(e.g. ^^). Paragraph 7. A method according to paragraph 5 or paragraph 6 wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the time parameter comprise a condition requiring that a variance of an estimated value of the time parameter is less than a threshold variance (e.g. ^^). Paragraph 8. A method according to any of paragraphs 5 to 7, wherein the accuracy with which the communications device can estimate the time parameter is determined based on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node. Paragraph 9. A method according to paragraph 8, wherein the estimate of the channel gain between the communications device and the first radio access node is determined based on a reference signal received by the communications device from the first radio access node and the estimate of the channel gain between the communications device and the second radio access node is determined based on a reference signal received by the communications device from the second radio access node. Paragraph 10. A method according to paragraph 8 or paragraph 9, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the time parameter comprise a condition that an estimation error of an estimate of the time parameter is greater than or equal to an estimation error threshold (e.g. ^^), wherein the estimation error is determined based on the estimate of the channel gain between the communications device and the first radio access node and the estimate of the channel gain between the communications device and the second radio access node. Paragraph 11. A method according to any preceding paragraph, wherein the one or more invalidity trigger conditions comprise one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the phase parameter. Paragraph 12. A method according to paragraph 11, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the phase parameter comprise a condition requiring that a probability that a modulus of a difference between an estimated value of the phase parameter (e.g. ^^) and a true value of the phase parameter (e.g. ^) is greater than a pre- defined deviation (e.g. ^^) is less than a threshold probability (e.g. ^^). Paragraph 13. A method according to paragraph 11 or paragraph 12, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the phase parameter comprise a condition requiring that a variance of an estimated value of the phase parameter is less than a threshold variance (e.g. ^^). Paragraph 14. A method according to any of paragraphs 11 to 13, wherein the accuracy with which the communications device can estimate the phase parameter is determined based on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node. Paragraph 15. A method according to paragraph 14, wherein the estimate of the channel gain between the communications device and the first radio access node is determined based on a reference signal received by the communications device from the first radio access node and the estimate of the channel gain between the communications device and the second radio access node is determined based on a reference signal received by the communications device from the second radio access node. Paragraph 16. A method according to any of paragraphs 11 to 15, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the phase parameter comprise a condition that an estimation error of an estimate of the phase parameter is greater than or equal to a pre-defined estimation error threshold (e.g. ^^), wherein the estimation error is determined based on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node. Paragraph 17. A method according to any preceding paragraph, wherein the one or more invalidity trigger conditions comprise a condition that a probability of a bias in the estimate of the time parameter and / or the estimate of the phase parameter is non-zero is greater than or equal to a probability threshold, wherein the probability of the bias in the estimate of the time parameter and / or the phase parameter is determined based on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node. Paragraph 18. A method according to paragraph 17, wherein the probability of the bias in the estimate of the time parameter and / or the phase parameter is determined based on a Euclidean distance function, wherein the Euclidean distance function is a function of a difference between a pair of time parameters and a difference between a pair of phase parameters, the Euclidean distance function being dependent on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node. Paragraph 19. A method according to paragraph 18, wherein the probability of the bias in the estimate of the time parameter and / or the estimate of the phase parameter is determined based on a value of the smallest local minimum of the Euclidean distance function which is not a global minimum of the Euclidean distance function. Paragraph 20. A method according to any preceding paragraph, wherein the transmitting the invalid calibration assistance report in response to one or more invalidity trigger conditions being met comprises transmitting a first invalid calibration assistance report in response to determining that one or more of the invalidity trigger conditions which are associated with the accuracy with which the communications device can determine the time parameter are met, the first invalid calibration assistance report indicating that the communications device is not permitted to transmit the estimate of the time parameter, and transmitting a second invalid calibration assistance report in response to determining that one or more of the invalidity trigger conditions which are associated with the accuracy with which the communications device can determine the phase parameter are met, the second invalid calibration assistance report indicating that the communications device is not permitted to transmit the estimate of the phase parameter. Paragraph 21. A method according to any of paragraphs 1 to 19, wherein the transmitting the invalid calibration assistance report in response to one or more invalidity trigger conditions being met comprises transmitting the invalid calibration assistance report in response to determining that one or more of the invalidity trigger conditions which are associated with the accuracy with which the communications device can determine the time parameter are met and / or determining that one or more of the invalidity trigger conditions which are associated with the accuracy with which the communications device can determine the phase parameter are met, the calibration assistance report indicating that the communications device is not permitted to transmit an estimate of the time parameter and is not permitted to transmit an estimate of the phase parameter. Paragraph 22. A method of operating a first radio access node of a wireless communications network, the method comprising transmitting, one or more invalidity trigger conditions to a communications device for determining whether or not to transmit an invalid calibration assistance report, the invalid calibration assistance report indicating that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting the first radio access node and a second radio access node of the wireless communications network to calibrate coherent joint transmissions (CJTs), and receiving the invalid calibration assistance report from the communications device in response to one or more of the invalidity trigger conditions being met, wherein the one or more trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter. Paragraph 23. A method of operating a first radio access node of a wireless communications network, the method comprising transmitting, to a communications device, an indication that the communications device must transmit an estimate of one or more of a time parameter and a phase parameter for assisting the first radio access node and a second radio access node of the wireless communications network to calibrate coherent joint transmissions (CJTs). Paragraph 24. A method according to paragraph 23, wherein the indication that the communications device must transmit an estimate of one or more of a time parameter and a phase parameter is transmitted in response to one or more invalidity trigger conditions not being met, wherein the one or more invalidity trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter. Paragraph 25. A method according to paragraph 24, wherein the method comprises receiving, from the second radio access node, an indication of an estimate of a channel gain between the second radio access node and the communications device, wherein the accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter is determined based on an estimate of a channel gain between the between the first radio access node and the communications device and the estimate of the channel gain between the second radio access node and the communications device. Paragraph 26. A communications device comprising a transmitter configured to transmit signals, a receiver configured to receive signals, and a controller configured in combination with the transmitter and the receiver to transmit an invalid calibration assistance report in response to one or more invalidity trigger conditions being met, the invalid calibration assistance report indicating that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJTs), the plurality of radio access nodes comprising a first radio access node and a second radio access node, wherein the one or more invalidity trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter. Paragraph 27. A first radio access node for a wireless communications network, the first radio access node comprising a transmitter configured to transmit signals, a receiver configured to receive signals, and a controller configured in combination with the transmitter and the receiver to transmit, one or more invalidity trigger conditions to a communications device for determining whether or not to transmit an invalid calibration assistance report, the invalid calibration assistance report indicating that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting the first radio access node and a second radio access node of the wireless communications network to calibrate coherent joint transmissions (CJTs), and receive the invalid calibration assistance report from the communications device in response to one or more of the invalidity trigger conditions being met, wherein the one or more trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter. Paragraph 28. A first radio access node for a wireless communications network, the first radio access node comprising a transmitter configured to transmit signals, a receiver configured to receive signals, and a controller configured in combination with the transmitter and the receiver to transmit, to a communications device, an indication that the communications device must transmit an estimate of one or more of a time parameter and a phase parameter for assisting the first radio access node and a second radio access node wireless communications network to calibrate coherent joint transmissions (CJTs). Paragraph 29. A computer program which, when the program is executed by a computer, cause the computer to perform the method of any of paragraphs 1 to 25. Paragraph 30. A non-transitory computer-readable storage medium storing a computer program according to paragraph 29. It will be appreciated that the above description for clarity has described embodiments with reference to different functional units, circuitry and / or processors. However, it will be apparent that any suitable distribution of functionality between different functional units, circuitry and / or processors may be used without detracting from the embodiments. Described embodiments may be implemented in any suitable form including hardware, software, firmware or any combination of these. Described embodiments may optionally be implemented at least partly as computer software running on one or more data processors and / or digital signal processors. The elements and components of any embodiment may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the disclosed embodiments may be implemented in a single unit or may be physically and functionally distributed between different units, circuitry and / or processors. Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognise that various features of the described embodiments may be combined in any manner suitable to implement the technique. References [1] Holma H. and Toskala A, “LTE for UMTS OFDMA and SC-FDMA based radio access”, John Wiley and Sons, 2009. [2] TR 38.913, “Study on Scenarios and Requirements for Next Generation Access Technologies (Release 14)”, 3rd Generation Partnership Project, v14.3.0, August 2017. [3] R1-2403425, “CSI enhancements for >32 ports and UE-assisted CJT with non-ideal TRP synchronization”, Qualcomm. [4] R1-2403476, “CSI enhancements for large antenna arrays and CJT”, Ericsson. [5] H. Van Trees, “Detection, Estimation, and Modulation Theory, Part I: Detection, Estimation, and Filtering Theory,” 1968.APPENDIX 1Estimator Behaviour Consider a problem of estimating a parameter ^ from a vector ^. To do this, typical approaches minimize a function ^(^; ^); this function should be read as a function over ^, but where the functional form depends on ^. That is, once ^ is fixed, the function only depends on ^. In maximum likelihood estimation, said function is the negative (logarithm) of the conditional probability ^(^|^). Examples of the function ^(^|^) are shown in Figure 12. Figure 12 illustrates five sub-figures. The two sub-figures on the top row are under conditions of high SNR, the two sub-figures in the middle row are under conditions of low SNR and the sub-figure on the bottom row represents estimator performance. In Figure 12, a constant value of the unknown ^ has been assumed; this is labeled ^^^^^. With the given ^^^^^, four examples of the observation ^ are generated and the ^(^; ^) is computed. These examples are in the two sub-figures in the top row and the two-sub figures in the middle row. In the two sub-figures in the top row, ^ has been generated at high observation SNR while, in the middle two sub-figures, ^ has been generated at low SNR. As will be appreciated from the two sub-figures in the top row, the minimum of ^(^; ^), which is denoted by ^^ and is the estimator output, occurs close to the true value ^^^^^ . However, due to some noise, the estimate ^^ does not coincide with ^^^^^ and the difference is the resulting error. Furthermore, as will be appreciated from the two sub-figures in the top row, there is another value ^^ for which the function value ^(^^; ^) is rather small. However, at high SNR, the estimator is able to tell ^^ and the true value ^^^^^ apart, and it does not get confused by ^^. However, at low SNR (represented in the two sub figures in the middle row) the received signal is substantially more noisy. In the leftmost sub-figure in the middle row, the signal is so noisy so that the minimum of ^(^; ^) occurs in the vicinity of ^^. Thus, the resulting error is substantial. In the rightmost sub-figure of the middle row, there is also low SNR, but in this signal realization the major error is avoided. As will be appreciated, in many estimation problems there are some values that appear much more likely than others; these typically include values in the vicinity of the true value, but also some selected ones far away. At high SNR, the estimator manages to discard values far away from the true value – this results in an error that is small. However, at low SNRs, the estimator occasionally picks an estimate that is far away, producing a very large error. The resulting performance of the estimator is shown in the sub-figure in the bottom row. The dashed curve is the resulting error behavior in situations where the estimator has a-priori information about the vicinity of the true value, i.e., it can discard far-away, but otherwise likely, values at the get-go. This curve is the Cramer-Rao lower bound. In SNR regime B (illustrated in the sub figure in the bottom row of Figure 12), the estimator never gets confused from far-away, but otherwise likely, values, and therefore follows the Cramer-Rao bound tightly. But in SNR-regime A (illustrated in the sub figure in the bottom row of Figure 12), it sometimes gets confused. Then there are major errors, producing a very large gap to the bound. Thus, in SNR-regime A, the error can be described as “with probability ^ a large error occurs, but with probability 1 − ^ a small one around the true value occurs”. The value of ^ reduces as one moves further and further towards SNR-regime B. APPENDIX 2A UE transmits an SRS towards two TRPs (TRP1 and TRP2). Each TRP applies MRT-type precodingand transmits a CSI towards the UE; the TRPs do this in orthogonal channels (either time or interlaced in frequency). The two signals received at the UE can be represented by the following equations: ^(^) ^ (^) ^^2^Δ ^^ ^ (^^ ) + ^ (^)where |^ℓ| ≤ ^^^^, 0 ≤ ^ℓ < 2^ are unknown, ^ℓ(^) is complex Gaussian with variance ^^ and independent over both indices, and ^ is the subcarrier index. The functions ^ℓ(^) are the effective channels at subcarrier ^ from TRP ℓ to the UE and are assumed real-valued (the assumption part here stems from an assumption of perfect CSI at the TRPs). These functions depend on what type of precoding that is applied at the TRPs. For example, the TRPs may apply power balancing across the subcarriers in which case ^ℓ(^) = 1, ∀^. Outright MRT produces ^ℓ(^) = |^^^^ℓ(^)|^ where ^^^^ℓ(^) is the true channel between TRP ℓ and the UE at subcarrier ^. (In both cases a wideband power normalization would be applied). As mentioned previously, the UE or can estimate the time difference parameter ^ = ^^ − ^^ and the phase difference parameter ^ = ^^ − ^^. Such estimation can be made on the basis of the statistics. Consider Equations 9 and 10: The first term in Equation 10 is useful signal, while the remaining three terms act as noise. Assume that this noise is both independent from the useful signal as well as complex Gaussian distributed. Thus, we may write: Where and where ^(^) ^ (^)^ (^) We may manipulate this into a situation in which the noise has uniform density across subcarriers, i.e., ^^(^) = ^^(^) exp^ ^2^Δ ^^^ exp( ^^) + ^^(^)where and This is an instance of a linear model in (assumed) complex Gaussian noise. Thus, only the Euclidean distance structure plays a role for performance evaluation. The Euclidean distance between a pair (^, ^) and another pair (^^, ^^) reads quat o 7 Introducing notation ^^ = ^ + Δ ^ ^ and ^ = ^ + Δ^ produces Equation 18 which is a function only of Δ ^ ^ and Δ^ wherefore, we may write ^ (Δ^ , Δ^) instead of ^^^^, ^, ^ + Δ^ , ^ + Δ^^. Note that ^Δ^^ ≤ 4^^^^. Using standard arguments as will be appreciated by a person skilled in the art in digital communications (such as replacing the union bound with its largest component and also dealing with continuous alphabets), the probability of a large error can be shown to be related to the smallest value of ^^(Δ^ , Δ^) outside a region centered at origin (0,0). How “large” the error is depends on the size of the region. The pairwise error probability of confusing the true pair (^, ^) with the pair (^^, ^^) is ^(^(Δ^ , Δ^) / 4). However, taking multiplicities into account produces the following error term Numerical result We next demonstrate the accuracy of the approximation for ^^^^^^. We consider a case with 512 subcarrirers, and a flat channel, i.e., ^^(^) = ^^(^) = 1, ∀^. W assume Δ^ = 15000 and that ^^^^ = 100ns. In Figure 13 contour plot of the function ^^(Δ^ , Δ^) at ^^ = 10 is shown. As shown in Figure 13, there is a global minimum 1302 at (0,0). Furthermore, there are two local minimums 1304, 1306 (this is a consequence of the assumption ^^(^) = ^^(^) = 1, ∀^) that are equal in value. These occur for ^Δ^^ = 5^^^^ / 3. As there are two equal local minimums 1304, 1306, we may approximate ^^^^^^ as Figure 14 schematically illustrates a plot of ^^^^^^ approximated according to Equation 19 (approximated line 1402) against 1 / ^^ and a plot of ^^^^^^ determined using simulation results (simulated line 1404) against 1 / ^^. The simulated line 1404 gives the probability ( 0.7 o 0.7) while the approximated line 1402 is the approximation of ^^^^^^. As will be appreciated from Figure 14, the approximated line 1402 corresponds well with the simulated line 1404. Accordingly, the approximation accurately predicts the probability of large errors. The reason why the simulated line 1404 overshoots at high SNR is that further local minimums than just the smallest ones actually contribute to the error performance. At low SNR, the approximation in Equation 19 is less accurate because the quantity ^^^^ − ^^ is large and dominant.

Claims

CLAIMS 1. A method of operating a communications device, the method comprising transmitting an invalid calibration assistance report in response to one or more invalidity trigger conditions being met, the invalid calibration assistance report indicating that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJTs), the plurality of radio access nodes comprising a first radio access node and a second radio access node, wherein the one or more invalidity trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter.

2. A method according to claim 1, wherein the time parameter is a time difference parameter which is a time difference between a timing misalignment parameter for the first radio access node and a timing misalignment parameter for the second radio access node, and the phase parameter is a phase difference parameter which is a phase difference between a phase misalignment parameter for the first radio access node and a phase misalignment parameter for the second radio access node.

3. A method according to claim 2, wherein the timing misalignment parameter for each of the radio access nodes is dependent at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase misalignment parameter for each of the radio access nodes is dependent at least in part on a phase difference between a transmission chain and a reception chain of the radio access node.

4. A method according to claim 1, comprising receiving an indication of the one or more invalidity trigger conditions from the first radio access node or the second radio access node.

5. A method according to claim 1, wherein the one or more invalidity trigger conditions comprise one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the time parameter.

6. A method according to claim 5, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the time parameter comprise a condition requiring that a probability that a modulus of a difference between an estimated value of the time parameter and a true value of the time parameter is greater than a pre-defined deviation is less than a threshold probability.

7. A method according to claim 5, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the time parameter comprise a condition requiring that a variance of an estimated value of the time parameter is less than a threshold variance.

8. A method according to claim 5, wherein the accuracy with which the communications device can estimate the time parameter is determined based on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node.

9. A method according to claim 8, wherein the estimate of the channel gain between the communications device and the first radio access node is determined based on a reference signal received by the communications device from the first radio access node and the estimate of the channel gain between the communications device and the second radio access node is determined based on a reference signal received by the communications device from the second radio access node.

10. A method according to claim 8, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the time parameter comprise a condition that an estimation error of an estimate of the time parameter is greater than or equal to an estimation error threshold, wherein the estimation error is determined based on the estimate of the channel gain between the communications device and the first radio access node and the estimate of the channel gain between the communications device and the second radio access node.

11. A method according to claim 1, wherein the one or more invalidity trigger conditions comprise one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the phase parameter.

12. A method according to claim 11, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the phase parameter comprise a condition requiring that a probability that a modulus of a difference between an estimated value of the phase parameter and a true value of the phase parameter is greater than a pre-defined deviation is less than a threshold probability.

13. A method according to claim 11, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the phase parameter comprise a condition requiring that a variance of an estimated value of the phase parameter is less than a threshold variance.

14. A method according to claim 11, wherein the accuracy with which the communications device can estimate the phase parameter is determined based on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node.

15. A method according to claim 14, wherein the estimate of the channel gain between the communications device and the first radio access node is determined based on a reference signal received by the communications device from the first radio access node and the estimate of the channel gain between the communications device and the second radio access node is determined based on a reference signal received by the communications device from the second radio access node.

16. A method according to claim 11, wherein the one or more invalidity trigger conditions associated with the accuracy with which the communications device can estimate the phase parameter comprise a condition that an estimation error of an estimate of the phase parameter is greater than or equal to a pre-defined estimation error threshold, wherein the estimation error is determined based on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node.

17. A method according to claim 1, wherein the one or more invalidity trigger conditions comprisea condition that a probability of a bias in the estimate of the time parameter and / or the estimate of the phase parameter is non-zero is greater than or equal to a probability threshold, wherein the probability of the bias in the estimate of the time parameter and / or the phase parameter is determined based on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node.

18. A method according to claim 17, wherein the probability of the bias in the estimate of the time parameter and / or the phase parameter is determined based on a Euclidean distance function, wherein the Euclidean distance function is a function of a difference between a pair of time parameters and a difference between a pair of phase parameters, the Euclidean distance function being dependent on an estimate of the channel gain between the communications device and the first radio access node and an estimate of the channel gain between the communications device and the second radio access node.

19. A method according to claim 18, wherein the probability of the bias in the estimate of the time parameter and / or the estimate of the phase parameter is determined based on a value of the smallest local minimum of the Euclidean distance function which is not a global minimum of the Euclidean distance function.

20. A method according to claim 1, wherein the transmitting the invalid calibration assistance report in response to one or more invalidity trigger conditions being met comprises transmitting a first invalid calibration assistance report in response to determining that one or more of the invalidity trigger conditions which are associated with the accuracy with which the communications device can determine the time parameter are met, the first invalid calibration assistance report indicating that the communications device is not permitted to transmit the estimate of the time parameter, and transmitting a second invalid calibration assistance report in response to determining that one or more of the invalidity trigger conditions which are associated with the accuracy with which the communications device can determine the phase parameter are met, the second invalid calibration assistance report indicating that the communications device is not permitted to transmit the estimate of the phase parameter.

21. A method according to claim 1, wherein the transmitting the invalid calibration assistance report in response to one or more invalidity trigger conditions being met comprises transmitting the invalid calibration assistance report in response to determining that one or more of the invalidity trigger conditions which are associated with the accuracy with which the communications device can determine the time parameter are met and / or determining that one or more of the invalidity trigger conditions which are associated with the accuracy with which the communications device can determine the phase parameter are met, the calibration assistance report indicating that the communications device is not permitted to transmit an estimate of the time parameter and is not permitted to transmit an estimate of the phase parameter.

22. A method of operating a first radio access node of a wireless communications network, the method comprising transmitting, one or more invalidity trigger conditions to a communications device for determining whether or not to transmit an invalid calibration assistance report, the invalid calibration assistance report indicating that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting the first radio access node and a second radio access node of the wireless communications network to calibrate coherent joint transmissions (CJTs), andreceiving the invalid calibration assistance report from the communications device in response to one or more of the invalidity trigger conditions being met, wherein the one or more trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter.

23. A method of operating a first radio access node of a wireless communications network, the method comprising transmitting, to a communications device, an indication that the communications device must transmit an estimate of one or more of a time parameter and a phase parameter for assisting the first radio access node and a second radio access node of the wireless communications network to calibrate coherent joint transmissions (CJTs).

24. A method according to claim 23, wherein the indication that the communications device must transmit an estimate of one or more of a time parameter and a phase parameter is transmitted in response to one or more invalidity trigger conditions not being met, wherein the one or more invalidity trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter.

25. A method according to claim 24, wherein the method comprises receiving, from the second radio access node, an indication of an estimate of a channel gain between the second radio access node and the communications device, wherein the accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter is determined based on an estimate of a channel gain between the between the first radio access node and the communications device and the estimate of the channel gain between the second radio access node and the communications device.

26. A communications device comprising a transmitter configured to transmit signals, a receiver configured to receive signals, and a controller configured in combination with the transmitter and the receiver to transmit an invalid calibration assistance report in response to one or more invalidity trigger conditions being met, the invalid calibration assistance report indicating that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJTs), the plurality of radio access nodes comprising a first radio access node and a second radio access node, wherein the one or more invalidity trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter.

27. A first radio access node for a wireless communications network, the first radio access node comprising a transmitter configured to transmit signals, a receiver configured to receive signals, and a controller configured in combination with the transmitter and the receiver to transmit, one or more invalidity trigger conditions to a communications device for determining whether or not to transmit an invalid calibration assistance report, the invalid calibration assistance report indicating that the communications device is not permitted to transmit an estimate of one or more of a time parameter and a phase parameter for assisting the first radio access node and a second radio access node of the wireless communications network to calibrate coherent joint transmissions (CJTs), andreceive the invalid calibration assistance report from the communications device in response to one or more of the invalidity trigger conditions being met, wherein the one or more trigger conditions are associated with an accuracy with which the communications device can estimate one or more of the time parameter and the phase parameter.

28. A first radio access node for a wireless communications network, the first radio access node comprising a transmitter configured to transmit signals, a receiver configured to receive signals, and a controller configured in combination with the transmitter and the receiver to transmit, to a communications device, an indication that the communications device must transmit an estimate of one or more of a time parameter and a phase parameter for assisting the first radio access node and a second radio access node wireless communications network to calibrate coherent joint transmissions (CJTs).

29. A computer program which, when the program is executed by a computer, cause the computer to perform the method of claim 1.

30. A non-transitory computer-readable storage medium storing a computer program according to claim 29.

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