Methods, communications devices, and nodes

Abstract

A communications device operates to assist a first set of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJT). The communications device transmits

Description

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. 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 ty pes 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 schematically illustrates a wireless communications network with multiple UEs transmitting mutually incompatible reports; Figure 8A and Figure 8B are flow diagrams illustrating a method for assisting radio access nodes to calibrate coherent joint transmissions in accordance with example embodiments; Figure 9 is a signalling diagram illustrating a method for assisting radio access nodes to calibrate coherent joint transmissions in accordance with example embodiments; Figure 10 is a signalling diagram illustrating a method for assisting radio access nodes to calibrate coherent joint transmissions in accordance with example embodiments; Figure 11 schematically illustrates the behaviour of estimators under conditions of high SNR and low SNR. 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 I 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 tire 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 acentral 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 dow nlink 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. Tire 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 computers), 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 netw ork 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 Fl 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 Fl interface 46 from the DU 42 to the CU 40. Multi-TRP Operation In multiple Transniit / 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 140 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 via 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 in within a TRP. Timing misalignments between die 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 synchronized with each other. 3GPP is currently discussing coherent joint transmission with non-ideal TRP synchronization. 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 is 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, TRP1Rx #= TRP1Tx. 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, TRP2Rx #= TRP2Tx.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 through the receiver circuity of the TRP once it arrives at an antenna of the TRP. Therefore, for example, T RP1Tx may be a time taken for TRP 1 to generate and transmit a signal using transmitter circuitry and TRPlRx 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 I of [3] an analysis of the situation m Figure 5 is provided. The results of that analysis are reproduced below: At subcarrier k. • the effective channel from TRP1 to the UE is y± = \HTRP1(k')\2 exp(j2nk / \fT1 + j^~)exp(j2nkh{TUE + J¢UE) • the effective channel from TRP2 to the UE is y2 — \HTRP2(k)\2 exp(j2nkhft2 + 702) expQ2nkAfTUE + where • Maximum ratio transmission (MRT) type precoding has been adopted at both TRPs • Af is the subcarrier spacing • 'Q is the time difference between TRW’s reception and transmission chains, i.e., Te = TRPSRx — TRPtTx • is a phase difference, or offset, between TRW’s reception and transmission chains (constant across frequency of TRW’s reception and transmission chains) • \HTRPf(k') \ is the channel magnitude at subcarrier k of TRW . • tue is the time difference between the UE’s transmission and reception chains. • (f)UE is the phase difference, or offset, between the UE’s transmission and reception chains. With perfect time synchronization and ideal hardware for a TRP 1 and a TRP 2, one would have Tt = t2 and = 02 ■ Thus, the combined channel from the two TRPs towards the UE reads E(k) = (\HTRP1(k)\2 + \HTRP2(k)^^ + exp(j2nk^fTUE + ]<pUE)- Equation 1 To obtain ideal performance, it is not necessary to have = t2 = 0 and <p1 = <p2 = 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 Tq = r2 0 and <p1 = <p2 0 shows up as a rotation of the channel, but does not impact the SNR. Therefore, to calibrate coherent joint transmissions, it is sufficient to estimate the differences <pTRP2toi = 02 — 01 and TTRP2toi = t2 — Ti, provide these to the TRPs, and let the TRPs adjust their signals accordingly. It has been proposed that a UE should report estimates on the misalignments between pairs of TRPs. However, as networks grow, e.g., cell-free systems, there may be mismatches among the reports of different UEs. However, in many cases, the estimates may be far off, and it is therefore not easy to combine reports from several UEs. An example of a proposal illustrating a UE reporting misalignments between pairs of UEs 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 CSI-RS from TRP 2. In step S608, based on the SRS and the received CSI-RSs, tire UE calculates the product of the complex conjugate of y} and y2 (i.e. y^y2) on multiple subcarriers to derive the mter-TRP timing offset between TRP 1 and TRP 2 (TTRP2t01) and the inter-TRP phase difference (<l>TRP2toi) between TRP1 and TRP 2. In step S610, the UE reports TTRP2t01 and (pTRPitoi to TRP 2. In step S612, TRP 2 synchronises its communications with respect to TRP 1 based on TrRP2toi and <pTRP2tol- The method illustrated in Figure 6 works well for the case of L TRPs and a single UE (further information on studies for this case can be found [3] and [4]). However, for larger networks, such as cell-free systems, the network may get reports from multiple UEs. Several of these reports may relate to the same TRPs, and may not be mutually compatible. This appears for example in bad channel conditions. An example is provided in Figure 7. In this example, there are three UEs, A, B, and C. Each UE may only receive signals from two TRPs as shown in Figure 7. However, due to noise and other impairments, the estimations are not ideal. Clearly, if TRP 2 is 100ns earlier (in its time-synchronization) than TRP 1, and TRP 3 is 100ns earlier than TRP 2, then it follows that TRP 1 must be 200ns after TRP 3. However, this contradicts the report from UE C which states that TRP1 is 150ns after TRP 3. Hence, the network knows that there is an inconsistency. The time differences being referred to in Figure 7 are timing differences caused by misalignment between the transmission chain and the reception chain within each of the TRPs. In such cases, the network may assume that the error is spread evenly over the reports. One reasonable outcome is that the network would regard UE A’s report of 100ns as 83.66.. ns, UE B’s report as 83.66.. .ns, and UE C’s report as 166.66...ns. By doing so, die adjusted reports are mutually compatible, and the situation is apparently resolved. However, this may lead to highly inaccurate results because, for example, the SNR at UE C may be much worse than at A and B, with almost the entire error actually at UE C’s side. Accordingly, there is a need for improved methods, communications devices, and nodes for assisting radio access nodes to calibrate coherent joint transmissions (CJT). In view of the above, there is provided a method of operating a communications device to assist at least a first set of a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJT). The method comprises transmitting a calibration assistance report. Tire calibration assistance report comprises information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the first set. The time difference parameter for at least each radio access node in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node. The phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and tire reception chain of the radio access node. By providing information for determining one or more confidence metrics in the calibration assistance report, a radio access node, or another node (such as a CU or DU), receiving a plurality of calibration assistance reports may combine the calibration assistance reports to determine revised estimates of one or more of the estimated time difference parameter and estimated phase difference parameter by taking into account the confidence metrics. Therefore, more accurate time and phase parameters may be determined and thus more accurate calibration of coherent joint transmissions can be provided. As will be appreciated, an example of calibrating coherent joint transmissions is radio access nodes synchronising their transmissions and / or receptions in time and / or phase based on the revised estimates of the time difference parameter and the phase difference parameter. In some embodiments, the radio access node which determines the revised estimate may synchronise its own transmissions and / or receptions by adjusting transmission and / or reception times based on the revised estimates. Alternatively, or additionally, the radio access node may transmit the revised estimates to one or more of the other radio access nodes in the first set for those radio access nodes to synchronise their transmissions and / or receptions based on the revised estimates. In embodiments when another node (such as a CU or DU) determines the revised estimates, the node may transmit the revised estimates to one or more of the radio access nodes in the first set for the radio access nodes to synchronise their transmissions and / or receptions based on the revised estimates. In some embodiments, the calibration assistance report may be referred to as a synchronisation report. In some embodiments, the calibration assistance report may be referred to as a calibration report. References herein to at least a “subset” of the first set may be referring to all of the radio access nodes in the first set, or fewer than all of the radio access nodes in the first set. In some embodiments, the time difference parameter may comprise a time offset slope parameter and a and a time offset intercept parameter. In some embodiments, the phase difference parameter may comprise a phase offset slope parameter and a phase offset intercept parameter. The first set of the plurality of radio access nodes may comprise two or more radio access nodes which are assisted in calibrating CJTs by the communications device. In some embodiments, the first set of radio access nodes are the only radio access nodes of the plurality of nodes which are assisted in calibrating CJTs by the communications device. In some embodiments, the plurality of radio access nodes comprise radio access nodes, other than the radio access nodes in the first set, which are also assisted by the communications device in calibrating CJTs. A method of assisting at least a first set of a plurality of radio access nodes of a wireless communications network to CJTs in accordance with example embodiments is illustrated in Figures 8A and 8B. The method starts in step S800. In step S801, the communications device may transmit capability information. The capability information may indicate that the communications device is capable of configuring and transmitting a calibration assistance report. The capability information may comprise an indication of whether tire communications device can transmit calibration assistance reports of a first type, of a second type or both. The capability information may comprise other assistance information for assisting the first set of TRPs to calibrate CJTs which is not already included in the calibration assistance report. In step S802, the communications device may receive, from one or more of the radio access nodes of the first set, configuration information for reporting a calibration assistance report. The configuration information may comprise, for example, an indication of information which should be included in the calibration assistance report or an indication of conditions (such as an SNR threshold) for use by the communications device to determine what information to include in the calibration assistance report. Alternatively, or additionally, the configuration information may indicate to the communications device which uplink signals to transmit in step S804. In step S803, the communications device may receive an activation signal from one or more of the radio access nodes of the first set. The activation signal may instruct the communications device to transmit the calibration assistance report. Although not shown in Figure 8A, in some embodiments, a deactivation signal may be transmitted to the communications device after the communications device has transmitted the calibration assistance report to instruct the communications device not to transmit the calibration assistance report again. In step S804, the communications device may transmit one or more uplink reference signals such as sounding reference signals (SRSs). As will be appreciated, a reference signal may be a signal which is usable to determine channel quality. In step S806, the communications device may receive one or more downlink reference signals from each of the radio access nodes in the first set. The received downlink reference signals may be CSI-RSs, for example. In some embodiments the downlink reference signals are precoded based on the uplink reference signals by the radio access nodes which receive the uplink signals. The downlink reference signals may be precoded based on a property of a propagation channel obtained from the uplink reference signals, for example. As explained in more detail below, the method comprises transmitting a calibration assistance report, which may comprise information derived from the reference signals transmitted by the communications device in step S804 and the reference signals received by the communications device in step S806, The information comprised in the calibration assistance report comprises at least information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each the radio access nodes in the first set (or a subset of the first set). The calibration assistance report may be one of a plurality of types of calibration assistance report, where the content of difference types of calibration assistance report is different as will be described in more detail below. In step S810, the communications device may determine whether to transmit a calibration assistance report of a first type or a calibration assistance report of a second type. For example, the communications device may determine to transmit the first type of calibration assistance report based on an SNR of the communications device. For example, the communications device may use a rule, or implicit signalling, based on the SNR of the communications device to determine whether to transmit the first type of calibration assistance report. For example, the communications device may determine to transmit the first type of calibration assistance report in response to a signal to noise ratio (SNR) of the communications device being above a first pre-defined threshold, or in response to receiving a signal from the wireless communications network (e.g. from one of the radio access node set) instructing the communications device to transmit the first type of calibration assistance report. Such an instruction may be included in the configuration information in step S802. On the other hand, the communications device may transmit the calibration assistance report of the second type based on an SNR of the communications device. For example, the communications device may use a rule, or implicit signalling, based on the SNR of the communications device to determine whether to transmit the second type of calibration assistance report. For example, the communications device may determine to transmit the second type of calibration assistance report in response to the SNR of the communications device being below a second pre-defined threshold (which may be the same as, or different from, the first threshold), or in response to the communications device receiving an instruction from the wireless communications network (e.g. from one of the radio access node set) instructing the communications device to transmit the second type of calibration assistance report. Such an instruction may be included in the configuration information in step S802. Although two types of calibration assistance report have been discussed, there may be one or more other types of calibration assistance report. After determining whether to transmit the first type or second type of calibration assistance report, the method proceeds to step S812 in which the communications device determines the information for detenuining one or more confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for each of the radio access nodes in the first set (or the subset of the first set). In some embodiments, if the communications device determines to transmit the first type of calibration assistance report, the method proceeds to step S812a. In step S812a, the communications device may estimate one or more of the time difference parameter and the phase difference parameter for each of the radio access nodes in the first set (or the subset of the first set). The estimation of the time and phase difference parameters may be based on the one or more uplink reference signals transmitted in step S804 and the one or more downlink reference signals received in step S806. For example, the communications device may perform the process described in step S608 of Figure 6. In step S812b, the communications device may determine a confidence metric associated with one or more of the time difference parameter and the phase difference parameter for each of the radio access nodes in the first set. The determined confidence metric associated with one or more of the time and phase difference parameters for each of the radio access nodes in the first set (or the subset of the first set) may be based on the one or more downlink reference signals received m step S806. The confidence metric of the time difference parameter may be a variance of the time difference parameter and the confidence metric of the phase difference parameter may be a variance of the phase difference parameter. An example of the variance of the time difference parameter is K(n) in Equation 4 below. An example of the variance of the phase difference parameter is (n) in Equation 4 below. In step S818a, the communications device may transmit the first type of calibration assistance report. The first type of calibration assistance report comprises one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for each of the radio access nodes in the first set (or the subset of the first set) and comprises one or more of the determined confidence metric of the time difference parameter and the determined confidence metric of the phase difference parameter for each of the radio access nodes in the first set (or the subset of the first set). In some embodiments the first type of calibration assistance report comprises the estimate of the time difference parameter, the estimate of the phase difference parameter, the determined confidence metric associated with the time difference parameter and the determined confidence metric associated with the phase difference parameter. If the communications device determines to transmit the second type of calibration assistance report, the method proceeds to step S812c after step S810. In step S812c, the communications device may determine, for each of the radio access nodes in the first set (or the subset of the first set), a set of values for evaluating a likelihood function for the radio access node. An example of a likelihood function for a radio access node is 4(t + tue, <p + <pUE: shown in Equation 10 below. The likelihood function provides a confidence metric associated with the time difference parameter and / or the phase difference parameter. The set of values may be obtained by evaluating a function of at least the time difference between the transmission chain and the reception chain of the radio access node. Alternatively, the set of values may be coefficients of the function when the function is expressed as a sum of the coefficients multiplied by a basis. An example of the function is / (t) as defined in Equation 11 below. An example of a set of values obtaining by evaluating the function are values of An example of coefficients of the function are values of ar in Equation 13 below and an example of the basis is ? / ;rin Equation 13 below. After step S812c, the method proceeds to step S818b. In step S818b, the communications device may transmit the second type of calibration assistance report, rhe second type of calibration assistance report comprises, for each radio access node of the first set (or the subset of the first set), the set of values for evaluating the likelihood function for the radio access node. The transmission of a calibration assistance report as discussed herein may comprise one or more separate transmissions. For example, in some embodiments, the communications device may transmit estimates of the time difference parameters and / or the phase difference parameters in one transmission and transmit the confidence metrics in another transmission. Such transmissions may be regarded as collectively forming a calibration assistance report. In some such embodiments, the confidence metrics may be transmitted less often than the estimates of the time difference parameters and / or the phase difference parameters. In some embodiments, the communications device may only transmit the confidence metrics to the radio access nodes in response to a request from the radio access nodes. In some embodiments, the communications device may transmit estimates of the time difference parameters and / or the phase difference parameters and transmit the confidence metrics in the same transmission. In some embodiments, the calibration assistance report is one or more of a Radio Resource Control (RRC) signal, a Medium Access Control (MAC) signal In step S820, a first radio access node of the first set receives calibration assistance reports from a plurality of communications devices including the calibration assistance report explained with reference to Figure 8A. In step S822, the first radio access node determines a revised estimate of one or more of the time difference parameter and the phase difference parameter for each of the radio access nodes in the first set (or the subset of the first set) based at least in part on the determined confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for each radio access node in the first set (or the subset of the first set). In other words, the first radio access nodes combines the information in the received calibration assistance reports to determine the revised estimates. In some embodiments (not shown), the first radio access node does not determine the revised estimates but forwards the plurality of calibration assistance reports to another node (such as a DU or CU) of the wireless communications network to determine the revised estimates. In some embodiments, the determination of the revised estimate of the time difference parameter and the phase difference parameter for each of the radio access nodes in the first set (or the subset of the first set) comprises numerically optimising Equation 14 below. In step S824, the radio access nodes in the first set calibrate coherent joint transmissions (CJTs) based on one or more of the revised estimate of the time difference parameter and the revised estimate of the phase difference parameter. For example, one or more of the radio access nodes in the first set may adjust for misalignments in time and phase offsets. For example, when there are two radio access nodes in the first set, one of the radio access nodes may determine the revised estimates and adjust for the misalignments. Alternatively, or additionally, the radio access node determining the revised estimates may transmit the estimates to the other radio access node for the other radio access node to adjust for the misalignments. In another example, where a CU or DU determines the revised estimates, the CU or DU may transmit the revised estimates to one or both of the two radio access nodes for the one or both of the radio access nodes to adjust for the misalignments. The method ends in step S826. It will be appreciated that the order of the steps in Figures 8A and 8B is one example, and the order of the steps may be interchanged in any logical order. Accordingly, via the method described in Figures 8 A and 8B, the communications device can assist at least a first set of a plurality of radio access nodes of a wireless communications network to calibrate CJTs. The communications device assists the radio access nodes in the first set to calibrate CJTs by providing a radio access node with a calibration report assistance which is used by the radio access node, or another node of the wireless communications network which receives the calibration assistance report from the radio access node, to determine revised estimates of one or more of the time difference and phase difference parameter by taking into account the confidence metrics in the calibration assistance report. In some embodiments, the time difference parameter for each radio access node is a parameter (ttrp) indicating the time difference between the transmission chain and the reception chain of the radio access node. In some embodiments, the phase difference parameter for each radio access node is a parameter (<|)trp) indicating the phase difference between the transmission chain and the reception chain for the radio access node. In some embodiments, the time difference parameter for each radio access node in the first set is further dependent on a time difference between a transmission chain is further dependent on a phase difference between the transmission chain and the reception chain of the communications device. For example, the timing difference parameter may be r which is a sum of ttrp and the time difference between the transmission chain and the reception chain of the communications device (tue). The phase difference parameter may be 1 which is a sum of (<|)trp) and the phase difference between the transmission chain and the reception chain of the communications device (<|)ue). In some embodiments, the time difference parameter for each radio access node is a time difference (P) between r and the time difference between a transmission chain and a reception chain for another one of the radio access nodes. In some embodiments, the phase difference parameter for each radio access node is a phase difference (0) between <p and the phase difference between a transmission chain and a reception chain of the other one of the radio access nodes. In such embodiments, where p represents die time difference parameter and 0 represents the phase difference parameter, p and 0 may not depend on tue or |ue because the contributions of tue and ((>ue cancel out when determining p and 0. Ulis, for example, is because the same communications device determines p and 0 using the same antenna port on die UE. In embodiments where the time parameter is dependent on the time difference between the transmission chain and reception chain of the communications device, the function of the time difference between die transmission chain and reception chain of the radio access node referred to above may also be a function of the time difference between the transmission chain and the reception chain of the communications device. In some embodiments, the function may be based on, or depend on, observations of the communications device on the downlink reference signals received in step S806. In embodiments where the communications device determines to transmit the first type of calibration assistance report for the first set of radio access nodes, the communications device may transmit the second type of calibration assistance report for a second set of radio access nodes in the wireless communications network. The following description will make reference to the behavior of estimators. An overview of the behavior of estimators is provided in the Appendix. For ease of explanation, the following description will 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. Unless otherwise stated or clear from the context, references to a report below should be understood as referring to a calibration assistance report. 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. High SNR Cases In accordance with example embodiments, an SNR of a UE may be high (for example, above a predefined threshold). Example embodiments applicable when a UE has a high SNR will be described below. Example embodiments will be described from a perspective of a single UE (referred to as UE1) receiving signals from a set of TRPs and transmitting a calibration assistance report. This is for ease of explanation only and it will be appreciated that this description applies for a plurality of UEs receiving signals from a set of TRPs and each of the UEs transmitting a calibration assistance report. Assume UE1 can receive signals from a set of N TRPs. Based on the teachings of [3] and [4], the received signals at the UE read: yi,k = |H1(k)|2expQ27T / cAfT1 + j^) exp(j2nk&{z2 + exp(j2nk^TUE1 + J^uei)+wlk, V2,k = \H2W\2 exp(j2nk^fT2 + exp(j2nk^fT2 + ;02) exp(j2nk^fTUE1 + j<pUE1) + w2ik, yN,k = \HN(k)\2 exp(j2nk^N + J¢N') exp(j2irkkfT2 + ]<p2~) exp(]2nk^UE1 + ]<pUE1) + wNik. Equation 2 In Equation 2, noise variables wnk are independent over the variable n. Further assume that noise density is absorbed into channel gains, so that all w-variables are complex standard Gaussian. As the SNR is high, it is known from the asymptotic property of maximum likelihood (ML) that the ML estimates of Tn, <pn behaves as A Ui + ^uei ' \ .071 + 0u£i. , I 1 (t„ + TUE1, $>n + 0UE1) Equation 3 where / (r„ + tue1, ^)n + Quei) is the Fisher information matrix. The Fisher information is represented as: (2^)2^¾^4 KTn + ?UE1> fin + 0UE1) — rK(n) o " o ^(n)J’ Equation 4 Now we will consider the offsets TTRP2toi and (pTRP2toi reported in [3], Introduce notation pn = ^TRPntoi = Tn — Ti and 0n = <pTRPntoi = — 0i>n >1- From the transformation theorem, and the independence of t„ and (pn, it follows that / (Mi.....Pn) = K(l)Hr + diag(K(2),, K W) ,...,0N) = VS(1)11T + diag^CZ), ...MN)}, Equation 5 where 1 is an all-one column vector. To combine the calibration assistance report from UE1 with the calibration assistance reports from other UEs, the UEs may either estimations of the r, ^-variables or the p, 6-variables to the network. As will be appreciated, UE1 does not estimate r„ or directly, but instead measures r„ + tue1 or (pn + <pUE1. Therefore, the reported r, ^-variables being referred to here are Tn + Tuei and (pn + <puEi- As will be appreciated, the contributions from tue1 and <pUE1 in pn and 0n respectively cancel out. The UE may report one or more of: t and <p (or one or more of P and 0) for each of the TRPs. In other words in some embodiments, the UE does not transmit both of r and 0 (or both of p and 0) for each of the TRPs. It should be noted that the Fisher information of the p, 0-variables are fully described by variances of the t, 0-variables. Therefore, the UEs should report the variances of the t, $-variables rather than the covariance among the p, 0-variables. The UE may report one or more of: a variance of t and a variance of <p for each of the TRPs. In other words in some embodiments, the UE does not transmit both of the variance of r and the variance of 0 for each of the TRPs. An example embodiment where a UE has high SNR is described in the following bullets: • The network indicates to the UE that there are M available TRPs to which the UE should transmit one or more SRSs; • The UE receives one or more reference signals from the M available TRPs; • The UE indicates to the network that it can receive high SNR from N' of the M TRPs; • The network indicates to the UE that it requires reports on N of the N’ TRPs (if N <Nr , the UE may also indicates which TRPs the network requires reports on); • The network indicates to the UE if it should report the r, ^-variables or the p, 0-variables (a report on the t, (^-variables is preferable if N <M) • The network indicates to the UE that it requires report of the variances {14 (n), (n), 1 <n <N}. • The UE transmits a report comprising the t, (^-variables (or the 0-vanables) and the variances {V^n^V^n),! <n<N}\ • The network combines the report with reports from other UEs to obtain a revised estimate of the t, ^-variables (or the p, 0-variables). Low SNR Cases In accordance with example embodiments, an SNR of a UE may low. Example embodiments applicable when a UE has a low SNR will be described below. Example embodiments will be described from a perspective of a single UE (referred to as UE1) receiving signals from a set of TRPs and transmitting a calibration assistance report. This is for ease of explanation only and it will be appreciated that this description applies for a plurality of UEs receiving signals from a set of TRPs and each of the UEs transmitting a calibration assistance report. UE1 receives signals from N TRPs. At least a subset of these are received at low SNR. In other words, UE1 determines that the asymptotic property of ML does not apply. Hie steps performed by UE1 with respect to each of the subset of TRPs is the same. Therefore, for ease of explanation, the steps performed by UE1 with respect to one of the subset of TRPs (a “first” TRP) will be described below. The received signal at UE1 from the first TRP is represented by yk = I^WI2 exp( / 2n±AfT + j<p) exp(j2nkhfTUE1 + j(pUEP) + wk Equation 6 where all subscripts have been removed relative to Equation 2, save for the one indexing subcarriers, for ease of explanation. As the SNR is low, it is not sufficient to report estimates (such as estimates of t, $-variables (or the p,6-variables)) and variances (such as {K(n), ^(n), 1 <n <Al}), as this does not capture the “substantial error behavior” discussed in the Appendix. Hie present application proposes two other alternative solutions: (i) report an entire likelihood function, or (ii) report possible locations of other plausible parameter values and their probability of occurrence (see Appendix). Solution (ii) is difficult because the locations of the plausible parameter values are not easily determined. Therefore we proceed with solution (i) in die following: Given [yk], the log-likelihood of {t + rUE, (p + (pUE} reads (save for additive constants) A(T + Tyfi,0 + 0uB;{yfe}) = 2Re exp(-j(0 + ¢^)) yk\H(k~)\2 exp(-j2nk^{(r + rUE)) k YjyiP2-k k Let us define ykl2-yW)l4 Equation 8 and / 0 + Tub) — ^ykIW)l2 exp(-j2nkkf(T + tue)) k Equation 9 which allows us to write A(t + tue, 0 + <pUE; {yfe}) = 2Re{ / (T + tue) exp(-j($ + 0UB))} + a0. Equation 10 In accordance with example embodiments, the likelihood function A(r + tue, <p + <puE; {y^}) may be transferred to the network. However, as a0 is a constant with respect to unknowns, it may be discarded. The conceptually simplest way of doing said transfer is to all values {yk \H(k) |2}. By doing so, one allows the network to create the function {yk}) (save for the irrelevant a0). However, with N subcarriers, we must then transfer N values. Since N is a large number, this signaling is inefficient. Using principles developed in standard textbooks such as [5], it can be shown that it is not necessary to transfer N values to reproduce the likelihood function in Equation 10. Instead, it is sufficient to transfer 1 + 2zmax^fN complex coefficients, where rmax is the largest assumed value of |t + tue\. Thus, it is proposed to transfer the function / (r) by reporting 1 + 2rmaxAf / V values to the network. Whenever 2rmaxAf « 1, the gain over a full report is substantial. Using principles developed in standard textbooks such as [5], it follows that function f(-) is 1 + 2rmaxAflV -dimensional (whenever H(k) is slowly changing). Let R be an integer no less than 1 + ZTmaxAfN. There exists a family of basis functions ipr(%) such that R QTl^r(f) , 1^1 — ^max' r=l The coefficients ar are found from the inner products Tmax J iMOAOdr. —Tmax Equation 12 The UEs do not need to be aware of the basis functions to produce the expansion coefficients. For example, let x1; x2,..., xR be a collection of real numbers, evenly spaced out over the interval (—Tmax, rmax). Then, to the precision of the approximation, we have Equation 13 Inspecting Equation 13, it will be appreciated that it is possible to recover {ar} from a set of R function values / (xr) by outright matrix inversion., provided the matrix [ipr (xs)] is known. The UE is aware of the basis ipr(j) and can therefore recover {ar}. Therefore, it suffices if the UE computes the function / "(r) at the R points xp ...,xR. Typical values of rmax are about 100ns. With a subcarrier spacing of Af = 30kHz and 1024 subcarriers (i.e., a total bandwidth of about 30MHz), we obtain R « 6. Thus, the savings over a full report is substantial. An example embodiment where a UE has low SNR is described in the following bullets: • The network indicates to the UE that there are M available TRPs to which the UE should transmit one or more SRSs; • The UE receives one or more reference signals from the M available TRPs; • The UE indicates to the network that it receives low SNR from Q of the M TRPs (in some embodiments Q= M — N' where, as explained above, N' is the number of TRPs from which the UE can receive high SNR); • The network indicates to the UE that it should transfer the function / (t) for a subset of these, say, for L of the Q TRPs. • For each of the L indicated TRPs, the network indicates a set xp x2, ...,xR. This may be the same set for all L TRPs. • The UE computes the values f(xr), 1 <r <R and reports these to the network. The following bullet is an alternative to the last bullet stating that the UE computes the values / (xr), 1 <r <R and reports these to the network: • The network indicates to the UE the values of (xr) (for example by referring the UE to a certain table of a specification). The network then requests the UE to determine and report the values ..., • Combining Calibration Assistance Reports In accordance with example embodiments, when the reports have arrived at the network, the network combines the reports to produce revised estimates of Tn, (pn, n >1, or, alternatively, of pn, 9n,n> 1. Assume P UEs in the network, M TRPs, and that UE p has reported high-SNR reports for TRPs ... and low-SNR reports for £p(l), ...,£p(Lp). If there are only high-SNR reports, then Lp = 0; similarly, Hp = 0 indicates that there are only low-SNR reports from UE p. The following explanation will assume that the time difference and phase difference parameters in the high-SNR reports are variables Tn, <pn, but it will be appreciated that the variables of pn, 9n may be used instead. A high-SNR report on fn from UE m may be seen as an observation of + TUEm. That is, tn = Tn + TyEm + 9n and — <pn + <puE,m + where the noise terms have variances according to the and k^G) variables. Assume that that the Hp high-SNR reports from UE p are denoted by fp(s), 0p(s), 1 <s <Hp. Likewise, let the associated reported variances by UE p be denoted by VTp(s) and V(pp(s). For the low-SNR reports, we introduce notation fp t(f) and aQpt, where 1 <t <Lp. to denote the reports from UE p on TRP £p(t) (the UE p may report coefficients that are sufficient for re-creating fPit(f)). The overall log-likelihood function may be expressed as A(Ti, ..., <pM,TUE,1> ■■■’TUE,P>Que,p> — >Que,p) p r / Hp r 12 , p \ Zl y Pp(s) — TMp(s) — TUE,Wp(s)| |0p(s) — — 0UEXp(s)| \ 1i^) v^s) ) p=i[\s=i / / Lp \' + I 2Re [fp,t^pV + *UE,£p(t)) exp - jQuE^t))} I ■ \t=l / Equation 14 In some embodiments, the network generates a final, revised estimate of the time difference and phase difference parameters by numerically optimising the overall likelihood equation. However, this is an example and the network may generate a final, revised estimate based on the reports received by the UEs by using another method of numerical optimization, or by using a factor graph for example. In some embodiments, the generation of the final, revised estimate of the time and phase difference parameters comprises numerically optimising the likelihood function using equation 15 and then discarding all estimated values associated with the UEs (i.e., all variables with a subscript "TIE”). Examples of techniques which may be used to perform the numerical optimisation include interior-point methods, gradient-descent, and factor-graph based methods. ^1,-, 01, , 0M, ^UE,1> ■■■> ^UE,P> QuE,P> — >QuE.P = argmax A(tv ..., tm, ^,..., (pM, tuea, ..., tUE:P, $UE:P,..., <pliEiP) In some embodiments, the network may interpret a high SNR report as meaning that the true values of the time and phase difference parameters must lie in the neighborhood of the reported one. That is, the network only has to search close to the reported high-SNR values of the time and phase parameters. In some embodiments, the network may not utilise low SNR reports because the true In some embodiments, the network observes that some of the high-SNR reports disagree to a too large extent, then the network may request low-SNR reports from one or more selected TRPs (for example on outliers). In some embodiments, a UE may indicate to the network that it is unable to produce a reliable estimate of the time difference and / or phase difference parameter. In response, the network may indicate to the UE to report a low SNR report as discussed herein. Example Use Cases Figure 9 illustrates an example use case in accordance with example embodiments. Figure 9 illustrates communications between a UE 902, a network node (NN) 904 and a set of TRPs 906 comprising a first TRP1. In some embodiments, the NN 904 may be one of the TRPs in the set of TRPs 906. In some embodiments, the NN may be a central processing unit (CPU) separated from, and possibly located geographically far away from, the set of TRPs 906. In step 907, the NN 904 transmits an activation signal to the UE instructing the UE to report one or more confidence metrics for the estimates of the time difference parameter and the phase difference parameter for each of the set of TRPs 906. The UE calculates the confidence metrics and includes the confidence parameters in the report transmitted to the NN 904 in step 914. In some embodiments, the activation signal indicates to the UE to report a variance of the timing difference parameters and the phase difference parameters for the set of TRPs 906. In some embodiments, the activation signal comprises an SNR threshold and an instruction to the UE to report the variances if the SNR of the UE is above the threshold. In some embodiments, the activation signal indicates to the UE to report values of likelihood functions for a set {r^..., zLp} oftiming difference parameters and / or a set {(p!,..., <pLp} of phase difference parameters for the set of TRPs 906, where the UE may be regarded as UEp in this example. In embodiments where the activation signal comprises an SNR threshold, the activation signal may comprise an instruction to report the likelihood functions of the SNR of the UE is below the threshold. In some embodiments, the activation signal may indicate to the UE to report both the variance of the timing difference parameters and the phase difference parameters and the values of the likelihood functions for the set of TRPs 906. This may occur when, for example, the set of TRPs 906 are nonoverlapping TRPs. In step 908, the UE 902 transmits one or more uplink reference signals (such as SRSs) to the set of TRPs 906. In step 910, each of the set of TRPs 906 transmit one or more precoded downlink reference signals (such as channel state information reference signals (CSI-RSs)). In step 912, the UE estimates time difference parameters and phase difference parameters for each of the TRPs based on the received downlink signals. In step 914, the UE transmits a report comprising an indication of the estimated time difference parameters and phase difference parameters for each of the TRPs to the NN 904. In some embodiments, the UE may include a measured SNR of the received CSI-RS for each of the set of TRPs 906 in the report. In step 920, the network node 904 and the set of TRPs 906 use the received estimated time difference parameters and phase difference parameters, and the received confidence metrics, to compensate transmission / reception timing misalignments and phase offsets. Figure 10 illustrates an example use case in accordance with example embodiments. Figure 10 is based on Figure 9 so only the differences between Figures 9 and 10 will be explained for brevity. After step 920, the network may determine that it requires a likelihood function of one or more of the TRPs 906 and the UE 902 (and possible one or more additional UEs). Therefore, in step S922, the NN 904 instructs the UE 902 to report a likelihood function for one or more of the TRPs 906. In accordance with example embodiments, there is provided another method of operating a communications device to assist at least a first set of a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJT). Hie method comprises transmitting information indicative of an estimate of a time difference parameter for each radio access node in the first set. The time difference parameter for each radio access node in the first set depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node. The method comprises transmitting information indicative of an estimate of a phase difference parameter for each radio access node in the first set. The phase difference parameter for each radio access node in the first set depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node. The method comprises transmitting infonnation for determining one or more confidence metrics associated with at least one of the estimate of the time difference parameter and the estimate of the phase difference parameter for each radio access node in the first set. 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 to assist at least a first set of a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJT), the method comprising transmitting a calibration assistance report, the calibration assistance report comprising information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the first set, wherein the time difference parameter for at least each radio access node in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node. Paragraph 2. A method according to paragraph 1, wherein the method comprises estimating one or more of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset, and including, in the calibration assistance report, one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each of the radio access nodes in the subset. Paragraph 3. A method according to paragraph 2, wherein the information for determining the one or more confidence metrics comprises one or more of: a confidence metric assoc iated with the time difference parameter for at least each of the radio access nodes in the subset determined by the communications device and a confidence metric associated with the phase difference parameter for at least each of the radio access nodes in the subset determined by the communications device. Paragraph 4. A method according to paragraph 3, wherein the confidence metric of the time difference parameter is indicative of a variance of the time difference parameter and the confidence metric of the phase difference parameter is a variance of the phase difference parameter. Paragraph 5. A method according to paragraph 3 or paragraph 4, wherein the calibration assistance report comprising one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each of the radio access nodes in the subset and comprising one or more of the determined confidence metric associated with the time difference parameter and the determined confidence metric associated with the phase difference parameter for at least each of the radio access nodes in the subset is a calibration assistance report of a first type, wherein the transmitting the calibration assistance report comprises transmitting the calibration assistance report of the first type. Paragraph 6. A method according to paragraph 5, wherein the transmitting the calibration assistance report of the first type comprises transmitting the calibration assistance report of the first type in response to a signal to noise ratio (SNR) of the communications device being above a first pre-defined threshold. Paragraph 7. A method according to paragraph 5, wherein transmitting the calibration assistance report of the first type comprises receiving a signal from the wireless communications network instructing the communications device to transmit the calibration assistance report of the first type f, and in response, transmitting the calibration signal of the first type. Paragraph 8. A method according to paragraph 1, wherein information for determining the one or more confidence metrics associated with one or more of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset comprises, for each of the radio access nodes in the subset, an indication of a set of values for evaluating a likelihood function for the radio access node, the set of values being obtained by evaluating a function of at least the time difference between the transmission chain and the reception chain of the radio access node, or the set of values being coefficients of the function when tire function is expressed as a sum of the coefficients multiplied by a basis, the likelihood function providing a confidence metric. Paragraph 9. A method according to paragraph 8, wherein the calibration assistance report comprising the indication of the set of values for at least each of the radio access nodes in the subset is a calibration assistance report of a second type, wherein the transmitting the calibration assistance report comprises transmitting the calibration assistance report of the second type. Paragraph 10. A method according to paragraph 9, wherein transmitting the calibration assistance report of the second type comprises transmitting the calibration assistance report of the second type in response to a signal to noise ratio (SNR) of the communications device being below a second pre-defined threshold. Paragraph 11. A method according to paragraph 8, wherein the transmitting the calibration assistance report of the second type comprises receiving a signal from the wireless communications network instructing the communications device to transmit the calibration assistance report of the second type , and in response, transmitting the calibration signal of the second type. Paragraph 12. A method according to any preceding paragraph comprising transmitting one or more reference signals, receiving one or more reference signals from each of the radio access nodes in the first set, forming the calibration assistance report based on measurements performed on the received reference signals. Paragraph 13. A method according to any preceding paragraph, wherein the calibration assistance report is transmitted in response to an activation signal received by the communications device. Paragraph 14. A method according to any preceding paragraph, wherein the time difference parameter for at least each radio access node in the subset is a parameter (t) indicating the time difference between the transmission chain and the reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset is a parameter (<]>) indicating the phase difference between the transmission chain and the reception chain for the radio access node. Paragraph 15. A method according to any of paragraphs 1 to 13, wherein the time difference parameter for at least each radio access node in the subset is a time difference (P) between (i) the time difference between the transmission chain and the reception chain of the radio access node, and (ii) the time difference between a transmission chain and a reception chain for another one of the radio access nodes, wherein the phase difference parameter for each radio access node is a phase difference (9) between (iii) the phase difference between the transmission chain and the reception chain for the radio access node, and (iv) the phase difference between a transmission chain and a reception chain of the other one of the radio access nodes. Paragraph 16. A method according to any preceding paragraph, wherein the time difference parameter for at least each radio access node in die subset is further dependent on a time difference between a transmission chain and a reception chain of the communications device and the phase difference parameter for at least each radio access node in the subset is further dependent on a phase difference between the transmission chain and the reception chain of the communications device. Paragraph 17. A metiiod of operating a first radio access node of a wireless communications network to communicate with a plurality of communications devices to calibrate coherent joint transmissions (CJTs), the method comprising receiving a plurality of calibration assistance reports from the plurality of communications devices respectively, the first radio access node belonging to the first set, wherein each of the calibration assistance reports comprise information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the first set, wherein the time difference parameter for at least each radio access nodes in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node. Paragraph 18. A method according to paragraph 17, wherein the one or more confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset are determined based on each calibration assistance report, and a revised estimate of one or more of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset is determined based at least in part on the determined confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset. Paragraph 19. A method according to paragraph 17, wherein the method comprises forwarding the plurality of calibration assi stance reports to another node of the wireless communications network. Paragraph 20. A method according to any of paragraphs 17 to 19, comprising transmitting, to the plurality of communications devices, a signal for use by the plurality of communications devices to determine whether to transmit the calibration assistance report as a calibration assistance report of a first type or a calibration assistance report of a second type, wherein a calibration assistance report of the first type is a calibration assistance report comprising, for at least each of the radio access nodes in the subset, one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each of the radio access nodes in the subset, and one or more of a confidence metric of the time difference parameter determined by the communications device for at least each of the radio access nodes in the subset and a confidence metric of the phase difference parameter determined by the communications device for at least each of the radio access nodes in the subset, and a calibration assistance report of the second type comprises, for at least each of the radio access nodes in the subset, an indication of a set of values for evaluating a likelihood function for the radio access node, the set of values being obtained by the communications device evaluating a function of at least the time difference between the transmission chain and the reception chain of the radio access node, or the set of values being coefficients of the function when the function is expressed as a sum of the coefficients multiplied by a basis, the likelihood function providing a confidence metric. Paragraph 21. A method according to paragraph 20, wherein the signal for use by the plurality of communications devices to determine whether to transmit the calibration assistance report as a calibration assistance report of a first type or a calibration assistance report of a second type comprises an indication of a signal to noise (SNR) threshold, an indication drat a calibration assistance report of the first type should be transmitted if an SNR if the communications device is above the SNR threshold, and an indication that a calibration assistance report of the second type should be transmitted if the SNR of the communications device is below the threshold. Paragraph 22. A method according to paragraph 20 or paragraph 21, wherein the signal for use by the plurality of communications devices to determine whether to transmit the calibration assistance report as a calibration assistance report of a first type or a calibration assistance report of a second type comprises an indication of a set of points at which to evaluate the function to obtain to set of values; or an indication of the basis when the function is expressed as a sum of the coefficients multiplied by the basis and an indication of which coefficients are to be included in the calibration assistance report. Paragraph 23. A method of operating a node of a wireless communications network, the method comprising receiving a plurality' of calibration assistance reports for assisting at least a first set of a plurality of radio access nodes of the wireless communications network to calibrate coherent joint transmissions (CJT), wherein each of the calibration assistance reports comprise information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the first set, wherein the time difference parameter for at least each radio access node in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node, wherein the one or more confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset are determined based on each calibration assistance report, and a revised estimate of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset is determined based at least in part on the determined confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset. Paragraph 24. A method according to paragraph 23, wherein the node is a distributed unit (DU). Paragraph 25. A method according to paragraph 24, wherein the node is a central unit (CU). Paragraph 26. A communications device operable to assist at least a first set of a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJT), the 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 a calibration assistance report, the calibration assistance report comprising information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the subset, wherein the time difference parameter for at least each radio access node in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node. Paragraph 27. A first radio access node of a w ireless communications netw ork operable to communicate with a plurality of communications devices to calibrate coherent joint transmissions (CJTs), 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 receive a plurality of calibration assistance reports from the plurality of communications devices respectively, the first radio access node belonging to the first set, wherein each of the calibration assistance reports comprise information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for at least each of the radio access nodes in the subset, wherein the time difference parameter for at least each radio access nodes in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node. Paragraph 28. A node of a wireless communications network, the 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 receive a plurality of calibration assistance reports for assisting at least a first set of a plurality of radio access nodes of the wireless communications network to calibrate coherent joint transmissions (CJT), wherein each of the calibration assistance reports comprise information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the first set, wherein the time difference parameter for at least each radio access node in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node, wherein the one or more confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset are determined based on each calibration assistance report, and a revised estimate of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset is determined based at least in part on the determined confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset. 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, v 14.3.0, August 2017. [3] R1-2403425. “CSI enhancements for >32 ports and UE-assisted CJT with non-ideal TRP synchronization”, Qualcomm. [4] RI-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 Theoiy1968. APPENDIX Estimator Behaviour Consider a problem of estimating a parameter 9 from a vector y. To do this, typical approaches minimize a function / (0; y); this function should be read as a function over 9. but where the functional form depends on y. That is, once y is fixed, the function only depends on 9. In maximum likelihood estimation, said function is the negative (logarithm) of the conditional probability p(y|0). Examples of the function p(y\9) are shown in Figure 11. Figure 11 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 11, a constant value of the unknown 9 has been assumed; this is labeled 0true. With the given 0trUe- four examples of the observation y are generated and the f(9; y) is computed. These examples 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, y has been generated at high observation SNR while, in tire middle two sub-figures, y has been generated at low SNR. As will be appreciated from the two sub-figures in the top row, the minimum of / (0; y), which is denoted by 9 and is the estimator output, occurs close to the true value 0true. However, due to some noise, the estimate 9 does not coincide with 0trUe 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 0O for which the function value f(90; y) is rather small. However, at high SNR, the estimator is able to tell 0O and the true value 0true apart, and it does not get confused by 0O. 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 tire minimum of / (0;y) occurs in the vicinity of 0O. Tirus, 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. Tire 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 11), 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 11), 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 p a large error occurs, but with probability 1 — p a small one around the true value occurs”. The value of p reduces as one moves further and further towards SNR-regime B.

Claims

1. A method of operating a communications device to assist at least a first set of a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJT), the method comprisingtransmitting a calibration assistance report, tire calibration assistance report comprising information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the first set, whereinthe time difference parameter for at least each radio access node in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node.

2. A method according to claim I, wherein the method comprisesestimating one or more of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset, andincluding, in the calibration assistance report, one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each of the radio access nodes in the subset.

3. A method according to claim 2, whereinthe information for determining the one or more confidence metrics comprises one or more of: a confidence metric associated with the time difference parameter for at least each of the radio access nodes in the subset determined by the communications device and a confidence metric associated with the phase difference parameter for at least each of the radio access nodes in the subset determined by the communications device.

4. A method according to claim 3, wherein the confidence metric of the time difference parameter is indicative of a variance of the time difference parameter and the confidence metric of the phase difference parameter is a variance of the phase difference parameter.

5. A method according to claim 3 or claim 4, wherein the calibration assistance report comprising one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each of the radio access nodes in the subset and comprising one or more of the determined confidence metric associated with the time difference parameter and the determined confidence metric associated with the phase difference parameter for at least each of the radio access nodes in the subset is a calibration assistance report of a first type, wherein the transmitting the calibration assistance report comprisestransmitting the calibration assistance report of the first type.

6. A method according to claim 5, wherein the transmitting the calibration assistance report of the first type comprisestransmitting the calibration assistance report of the first type in response to a signal to noise ratio (SNR) of the communications device being above a first pre-defined threshold.

7. A method according to claim 5, wherein transmitting the calibration assistance report of the first type comprisesreceiving a signal from the wireless communications network instructing tire communications device to transmit the calibration assistance report of the first type f, and in response,transmitting the calibration signal of the first type.

8. A method according to claim 1, wherein the information for determining the one or more confidence metrics associated with one or more of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset comprises, for each of the radio access nodes in the subset,an indication of a set of values for evaluating a likelihood function for the radio access node, the set of values being obtained by evaluating a function of at least the time difference between the transmission chain and the reception chain of the radio access node, or the set of values being coefficients of the function when the function is expressed as a sum of the coefficients multiplied by a basis, the likelihood function providing a confidence metric.

9. A method according to claim 8, wherein the calibration assistance report comprising the indication of the set of values for at least each of the radio access nodes in the subset is a calibration assistance report of a second type, wherein the transmitting the calibration assistance report comprises transmitting the calibration assistance report of the second type.

10. A method according to claim 9, wherein transmitting the calibration assistance report of the second type comprisestransmitting the calibration assistance report of the second type in response to a signal to noise ratio (SNR) of the communications device being below a second pre-defined threshold.

11. A method according to claim 8, wherein the transmitting the calibration assistance report of the second type comprisesreceiving a signal from the wireless communications network instructing the communications device to transmit the calibration assistance report of the second type, and in response,transmitting the calibration signal of the second type.

12. A method according to any preceding claim comprisingtransmitting one or more reference signals,receiving one or more reference signals from each of the radio access nodes in the first set, forming the calibration assistance report based on measurements performed on the received reference signals.

13. A method of operating a first radio access node of a wireless communications network to communicate with a plurality of communications devices to calibrate coherent joint transmissions (CJTs), the method comprisingreceiving a plurality of calibration assistance reports from the plurality of communications devices respectively, the first radio access node belonging to the first set, wherein each of the calibration assistance reports compriseinformation for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the first set, whereinthe time difference parameter for at least each radio access nodes in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, andthe phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node.

14. A method according to claim 13, whereinthe one or more confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset are determined based on each calibration assistance report, anda revised estimate of one or more of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset is determined based at least in part on the determined confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset.

15. A method according to claim 13, wherein the method comprisesforwarding the plurality of calibration assistance reports to another node of the wireless communications network.

16. A method according to any of claims 13 to 15, comprisingtransmitting, to the plurality of communications devices, a signal for use by the plurality of communications devices to determine whether to transmit the calibration assistance report as a calibration assistance report of a first type or a calibration assistance report of a second type, whereina calibration assistance report of the first type is a calibration assistance report comprising, for at least each of the radio access nodes in the subset,one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each of the radio access nodes in the subset, andone or more of a confidence metric of the time difference parameter determined by the communications device for at least each of tine radio access nodes in the subset and a confidence metric of the phase difference parameter determined by the communications device for at least each of the radio access nodes in the subset, anda calibration assistance report of the second type comprises, for at least each of the radio access nodes in the subset,an indication of a set of values for evaluating a likelihood function for the radio access node, the set of values being obtained by the communications device evaluating a function of at least the time difference between the transmission chain and the reception chain of the radio access node, or the set of values being coefficients of the function when the function is expressed as a sum of the coefficients multiplied by a basis, the likelihood function providing a confidence metric.

17. A method according to claim 16, wherein the signal for use by the plurality of communications devices to determine whether to transmit the calibration assistance report as a calibration assistance report of a first type or a calibration assistance report of a second type comprisesan indication of a signal to noise (SNR) threshold,an indication that a calibration assistance report of the first type should be transmitted if an SNR if the communications device is above the SNR threshold, andan indication that a calibration assistance report of the second type should be transmitted if the SNR of the communications device is below die threshold.

18. A method according to claim 16 or claim 17, wherein the signal for use by the plurality of communications devices to determine whether to transmit the calibration assistance report as a calibration assistance report of a first type or a calibration assistance report of a second type comprisesan indication of a set of points at which to evaluate the function to obtain to set of values; oran indication of the basis when the function is expressed as a sum of tire coefficients multiplied by the basis and an indication of which coefficients are to be included in the calibration assistance report.

19. A method of operating a node of a wireless communications network, the method comprisingreceiving a plurality of calibration assistance reports for assisting at least a first set of a plurality of radio access nodes of the wireless communications network to calibrate coherent joint transmissions (CJT), wherein each of the calibration assistance reports compriseinformation for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the first set, whereinthe time difference parameter for at least each radio access node in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node, whereinthe one or more confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset are determined based on each calibration assistance report, anda revised estimate of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset is determined based at least in part on the determined confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset.

20. A method according to claim 19, wherein the node is a distributed unit (DU) or a central unit (CU).

21. A communications device operable to assist at least a first set of a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJT), the communications device comprisinga transmitter configured to transmit signals,a receiver configured to receive signals, anda controller configured in combination with the transmitter and the receiver totransmit a calibration assistance report, the calibration assistance report comprising information for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the subset, whereinthe time difference parameter for at least each radio access node in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node.

22. A first radio access node of a wireless communications network operable to communicate with a plurality of communications devices to calibrate coherent joint transmissions (CJTs), the first radio access node comprisinga transmitter configured to transmit signals,a receiver configured to receive signals, anda controller configured in combination with the transmitter and the receiver toreceive a plurality of calibration assistance reports from the plurality of communications devices respectively, the first radio access node belonging to the first set, wherein each of the calibration assistance reports compriseinformation for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for at least each of the radio access nodes in the subset, whereinthe time difference parameter for at least each radio access nodes in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for at least each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node.

23. A node of a wireless communications network, the node comprisinga transmitter configured to transmit signals,a receiver configured to receive signals, anda controller configured in combination with the transmitter and the receiver toreceive a plurality of calibration assistance reports for assisting at least a first set of a plurality of radio access nodes of the wireless communications network to calibrate coherent joint transmissions (CJT), wherein each of the calibration assistance reports compriseinformation for determining one or more confidence metrics associated with one or more of an estimate of a time difference parameter and an estimate of a phase difference parameter for each of at least a subset of the radio access nodes in the first set, whereinthe time difference parameter for at least each radio access node in the subset depends at least in part on a time difference between a transmission chain and a reception chain of the radio access node, and the phase difference parameter for each radio access node in the subset depends at least in part on a phase difference between the transmission chain and the reception chain of the radio access node, whereinthe one or more confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset are determined based on each calibration assistance report, anda revised estimate of the time difference parameter and the phase difference parameter for at least each of the radio access nodes in the subset is determined based at least in part on the determined confidence metrics associated with one or more of the estimate of the time difference parameter and the estimate of the phase difference parameter for at least each radio access node in the subset.

24. A computer program which, when the program is executed by a computer, cause the computer to perform the method of any of claims 1 to 23.

25. A non-transitory computer-readable storage medium storing a computer program according to claim 24.

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