Methods, communications devices, and radio access nodes for calibration of coherent joint transmissions

Abstract

A method of operating a communications device operable to assist a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJTs) is provided. The method comprises receiving downlink reference signals from each of a subset of the radio access nodes, wherein the downlink reference signals received from each radio access node in the subset are not coded to compensate for a phase of a propagation channel between that radio access node and the communications device, and transmitting a calibration assistance report for use by each radio access node in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter determined at least in part based on the downlink reference signals and an indication of phase information determined from the downlink reference signals received within at least one subcarrier. Here, the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first of the subset of radio access nodes and a second of the subset of radio access nodes.

Description

[0001] METHODS, COMMUNICATIONS DEVICES, AND RADIO ACCESS NODES

[0002] BACKGROUND Field of Disclosure

[0003] The present disclosure relates to communications devices, radio access nodes of wireless communications networks, and methods of operating such communications devices and radio access nodes.

[0004] The present application claims the Paris Convention priority from United Kingdom patent application number GB2411844.0, filed on 9 August 2024, the contents of which are hereby incorporated by reference.

[0005] Description of Related Art

[0006] 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.

[0007] 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.

[0008] Current and future wireless communications networks are expected to routinely and efficiently support communications with an ever-increasing range of devices associated with a wider range of data traffic profiles and types than existing systems are optimised to support. For example, it is expected future wireless communications networks will be expected to efficiently support communications with devices including reduced complexity devices, machine type communication (MTC) devices, high resolution video displays, virtual reality headsets, Extended Reality (XR) and so on. Some of these different types of devices may be deployed in very large numbers, for example low complexity devices for supporting the “The Internet of Things”, and may typically be associated with the transmissions of relatively small amounts of data with relatively high latency tolerance. Other types of device, for example supporting high-definition video streaming, may be associated with transmissions of relatively large amounts of data with relatively low latency tolerance. Other types of device, for example used for autonomous vehicle communications and for other critical applications, may be characterised by data that should be transmitted through the network with low latency and high reliability. A single device type might also be associated with different traffic profiles / characteristics depending on the application(s) it is running. For example, different considerations 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).

[0009] 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.

[0010] SUMMARY OF THE DISCLOSURE

[0011] The present disclosure can help address or mitigate at least some of the issues discussed above.

[0012] Embodiments of the present technique can provide a method of operating a communications device operable to assist a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJTs). The method comprises receiving downlink reference signals from each of a subset of the radio access nodes, wherein the downlink reference signals received from each radio access node in the subset are not coded to compensate for a phase of a propagation channel between that radio access node and the communications device, and transmitting a calibration assistance report for use by each radio access node in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter determined at least in part based on the downlink reference signals and an indication of phase information determined from the downlink reference signals received within at least one subcarrier. Here, the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first of the subset of radio access nodes and a second of the subset of radio access nodes.

[0013] Such embodiments of the present technique, which, in addition to methods of operating communications devices, relate to methods of operating radio access nodes of wireless communications networks, to communications devices and radio access nodes, to circuitry for communications devices and radio access nodes, to wireless communications systems, to computer programs, and to computer-readable storage mediums, can allow for the more efficient and effective operation of communications devices and radio access nodes.

[0014] Respective aspects and features of the present disclosure are defined in the appended claims.

[0015] 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.

[0016] BRIEF DESCRIPTION OF THE DRAWINGS

[0017] 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:

[0018] 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;

[0019] Figure 2 schematically represents some aspects of a new radio access technology (RAT) wireless telecommunications system which may be configured to operate in accordance with certain embodiments of the present disclosure;

[0020] 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;

[0021] 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 is reproduced from [3], and schematically illustrates an example of multi-transmission and reception point (TRP) coherent joint transmission with non-ideal TRP synchronisation;

[0022] Figure 6 is a signalling diagram illustrating the reporting of TRP phase offset information by a user equipment (UE);

[0023] Figure 7 illustrates the observed phase at a UE for a TRP when receiving precoded and non-precoded channel state information reference signals (CSI-RS) from that TRP and another TRP;

[0024] Figure 8 shows a part schematic, part message flow diagram representation of an example wireless communications system comprising a communications device and an infrastructure equipment in accordance with embodiments of the present technique;

[0025] Figure 9 shows an example of how a UE may report TRP phase offset information for two TRPs when the TRPs provide indications of their power delay profdes (PDPs) to the UE in accordance with embodiments of the present technique;

[0026] Figure 10 shows an example of how a UE may report TRP phase offset information for two TRPs when the TRPs do not provide indications of their PDPs to the UE in accordance with embodiments of the present technique; and

[0027] Figure 11 shows a flow diagram illustrating an example process of communications in a communications system in accordance with embodiments of the present technique.

[0028] DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] Long Term Evolution Advanced Radio Access Technology (4G)

[0030] 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.

[0031] The network 6 includes a plurality of base stations 1 connected to a core network 2. Each base station provides a coverage area 3 (i.e. a cell) within which data can be communicated to and from communications devices 4. Although each base station 1 is shown in Figure 1 as a single entity, the skilled person will appreciate that some of the functions of the base station may be carried out by disparate, inter-connected elements, such as antennas (or antennae), remote radio heads, amplifiers, etc. Collectively, one or more base stations may form a radio access network.

[0032] 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 (UEs), user terminals, mobile radios, mobile terminals, terminal devices, wireless transmit and receive units (WTRUs), and so forth. Services provided by the core network 2 may include connectivity to the internet or to external telephony services. The core network 2 may further track the location of the communications devices 4 so that it can efficiently contact (i.e. page) the communications devices 4 for transmitting downlink data towards the communications devices 4. Base stations, which are an example of network infrastructure equipment, may also be referred to as transceiver stations, nodeBs, e-nodeBs, eNB, g-nodeBs, 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.

[0033] New Radio Access Technology (5G)

[0034] 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],

[0035] Massive Machine Type Communications (mMTC) is another example of a service which may be supported by NR-based communications networks. In addition, systems may be expected to support further enhancements related to Industrial Internet of Things (IIoT) in order to support services with new requirements of high availability, high reliability, low latency, and in some cases, high-accuracy positioning.

[0036] 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, forms a cell of the wireless communications network as represented by a circle 12. As such, wireless communications devices 14 which are within a radio communications range provided by the cells 12 can transmit and receive signals to and from the TRPs 10 via the wireless access interface. Each of the distributed units 41, 42 are connected to a central unit (CU) 40 (which may be referred to as a controlling node) via an interface 46. The central unit 40 is then connected to the core network 20 which may 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 25.

[0037] 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.

[0038] 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 of the first communication cell 12. This communications device 14 may thus exchange signalling with the first central unit 40 in the first communication cell 12 via one of the distributed units / TRPs 10 associated with the first communication cell 12.

[0039] 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.

[0040] 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 TRP10 of the kind shown in Figure 2 which is adapted to provide functionality in accordance with the principles described herein.

[0041] 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 TRP10 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 cell 12 formed by the TRP10. As shown in Figure 3, an example UE 14 is shown to include a corresponding transmiter 49, a receiver 48 and a controller 44 which is configured to control the transmiter 49 and the receiver 48 to transmit signals representing uplink data to the wireless communications network via the wireless access interface formed by the TRP10 and to receive downlink data as signals transmited by the transmiter 30 and received by the receiver 48 in accordance with the conventional operation.

[0042] The transmiters 30, 49 and the receivers 32, 48 (as well as other transmiters, receivers and transceivers described in relation to examples and embodiments of the present disclosure) may include radio frequency filters and amplifiers as well as signal processing components and devices in order to transmit and receive radio signals in accordance for example with the 5G / NR standard. The controllers 34, 44 (as well as other controllers described in relation to examples and embodiments of the present disclosure) may be, for example, a microprocessor, a CPU, or a dedicated chipset, etc., configured to carry out instructions which are stored on a computer readable medium, such as a non-volatile memory. The processing steps described herein may be carried out by, for example, a microprocessor in conjunction with a random access memory, operating according to instructions stored on a computer readable medium. The transmiters, the receivers and the controllers are schematically shown in Figure 3 as separate elements for ease of representation. However, it will be appreciated that the functionality of these elements can be provided in various different ways, for example using one or more suitably programmed programmable computer(s), or one or more suitably configured application-specific integrated circuit(s) / circuitry / chip(s) / chipset(s). As will be appreciated the infrastructure equipment / TRP / base station as well as the UE / communications device will in general comprise various other elements associated with its operating functionality.

[0043] As shown in Figure 3, the TRP 10 also includes a network interface 50 which connects to the DU 42 via a physical interface 16. The network interface 50 therefore provides a communication link for data and signalling traffic from the TRP 10 via the DU 42 and the CU 40 to the core network 20.

[0044] The interface 46 between the DU 42 and the CU 40 is known as the F 1 interface which can be a physical or a logical interface. The Fl interface 46 between CU and DU may operate in accordance with specifications 3GPP TS 38.470 and 3GPP TS 38.473, and may be formed from a fibre optic or other wired or wireless high bandwidth connection. In one example the connection 16 from the TRP10 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.

[0045] In order for a UE such as UE 4 or 14 to transmit uplink data to the network (e.g. on a Physical Uplink Control Channel (PUCCH) or a Physical Uplink Shared Channel (PUSCH)) to, for example, base station 1 or TRP 10, the UE must first ensure it is synchronised with the network on the uplink. Since a particular TRP or gNB expects to be receiving communications from many UEs, it needs to ensure that it shares a common timing understanding with each of these UEs (i.e. they are synchronised in terms of the starting times of frames and Orthogonal Frequency Division Multiplexing (OFDM) symbols). This is so that the eNB is able to schedule communication with each of them in a manner that avoids collisions and to ensure orthogonality of the uplink signals, such that inter-subcarrier interference is avoided or mitigated.

[0046] Although reference is made to 5G networks, the discussions in this specification apply equally to 6G networks (and beyond) where there is expected to be significantly higher throughput, lower latency and higher reliability utilising sub-THz frequencies. Multi-TRP Operation

[0047] In multiple Transmit / Receive Points (multi-TRPs) operation, a serving cell can schedule joint transmission of the same data to a user equipment (UE) from several TRPs, providing better coverage, reliability and / or data rates. In a setup where two TRPs simultaneously transmit data to a single UE but where TRP2 is further away from the UE than TRP1, the signal received at the UE from TRP2 is delayed in comparison with the signal received from TRP 1.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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 the transmission chain and the reception chain within a TRP will be discussed in more detail below.

[0054] Coherent Joint Transmission (CJT)

[0055] 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. When a TRP transmits a signal towards a UE, this signal is processed through the TRP’s Radio Frequency (RF) frontend, which introduces delays (that manifest themselves as a phase shift per subcarrier, where each subcarrier has its own individual phase shift) that cause the timing misalignment. As will be appreciated, such timing misalignments may change over time starting from initial factory calibration. Such changes in timing misalignment are mainly due to temperature dependency but may also be due to aging. An example of timing misalignments between TRPs is illustrated in Figure 5 (which is reproduced from [3]).

[0056] In Figure 5, a UE is transmitting a sounding reference signal (SRS) 516 in the uplink towards two TRPs (TRP1 and TRP2). After receiving the SRS 516, TRP1 and TRP2 each transmit a physical downlink control channel (PDSCH) 518 to the UE. Figure 5 illustrates a timing reference 502, a TRP1 timing 504, a TRP2 timing 506 and a UE timing 507. It will be appreciated from Figure 5 that the TRP1 timing 504 and the TRP2 timing 506 are misaligned with respect to each other.

[0057] As will be appreciated by a person skilled in the art, TRP1 has a transmission chain (e.g. a downlink (DL) 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 TRP1. In other words, TRPT'' TRP1TX. Therefore, TRP1 has non-ideal time synchronisation. As will be appreciated by a person skilled in the art, TRP2 has a transmission chain (e.g. a DL RF chain) and a reception chain (e.g. a UL RF chain).

[0058] There may be a difference between timings of the reception chain and transmission chain for TRP2. In other words, TRP2RXTRP2TX. Therefore, TRP2 has non-ideal time synchronisation.

[0059] 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, TRP1TXmay be a time taken for TRP1 to generate and transmit a signal using transmitter circuitry and TRP1K' 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.

[0060] Two TRPs, such as TRP 1 and TRP2, may have different delays (and thus different timing misalignments) , which implies that the phase shifts per subcarrier are not the same for these two TRPs. The TRPs are not able to fully compensate these delays. Thus, for each TRP, there is at least some form of residual delay that is unknown to the TRP and that may change over time. As a consequence, even if the TRPs know the propagation channels from themselves to the UE, they cannot force the two channels to arrive coherently at the UE as the delay-induced phase shifts are not the same for the two TRPs.

[0061] 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.

[0062] This is a matter that is heavily discussed currently by 3GPP, and different solutions are on the table, namely, option 1 and option 2. as summarised in Table I below. Table I: Compatibility between reporting formats and CSI-RS types (current 3GPP understanding)

[0063] There are two signal generation forms that should underlie the phase alignment, namely precoded channel state information reference signals (CSI-RS) and non-precoded CSI-RS. The meaning of this is that the TRPs should send CSI-RS towards the UE, and the UE should report information (according to either option 1 or option 2) sufficient for the TRPs to align their phases so that coherent reception is achieved at the UE. Option 1 and option 2 are described in greater detail below.

[0064] The current understanding of 3GPP is that option 1 cannot be used for non-precoded CSI-RS, and thus it is under discussion whether or not option 1 should be formally defined in the specifications altogether, as option 2 needs to be used anyway (to support non-precoded CSI-RS). However, it has been determined by the present inventors that the understanding that option 1 cannot be used for non-precoded CSI-RS is incorrect: option 1 can in fact, favourably, be used with non-precoded CSI-RS. It should also be noted that option 1 is more economic and efficient than option 2, as the UE is required to report fewer bits than in option 1 to allow the network to compensate for the effect of the residual delays.

[0065] To understand what option 1, option 2. and the terms precoded, and non-precoded mean, it is necessary to first introduce some notation.

[0066] Let the propagation channel (i.e., without any hardware-induced delays) from the two TRPs to the UE, per subcarrier k = 0 ... (K — 1), be denoted by = 1,2. The received signal (i.e. the received signal that comprises CSI-RS) at the UE from TRP n reads:

[0067] The term etTnfe+<^nis the hardware-induced phase due to TRP n while elTuEk+^UEis a similar effect due to the UE’s reception chain. These phases have the form of a slope Tnand an intercept (pn(which is the model adopted by 3GPP). Further, Hn(k) represents the communication channel between the UE and TRP n, which can be assumed to be same in the UL and DL directions. The variable An(k) is the transmitted signal from TRP n at subcarrier k. Finally, Nn(k) is noise with variance No.

[0068] The precoded and non-precoded signal generation forms are the following:

[0069] • Precoded - the transmitted signal An(k~) removes the phase due to An(Jf) = Pn(k) el Hn(k>, where Pnk) is a reference symbol known to the UE, which, without loss of generality, can be expressed as Pnk) = 1; and

[0070] • Non-precoded - in this form, the TRPs have not compensated for the channel phase. Therefore, AnQi) = Pn(kf Again, we may use Pnk) = 1.

[0071] As those skilled in the art would appreciate, the models and mathematic representations of the various signals and components defined herein, such as those modeling the precoded and non-precoded reference signals, are merely representative examples and are not intended to be either exact or limited. As those skilled in the art would understand, minor variations and differences will be present in reality due to a number of different factors, and these could change over time. To obtain ideal performance, it is not necessary to have " = r2= 0 (i.e. the timing misalignments for both TRPs are equal to 0) and 0X= (p2= 0 (i.e. the phase differences / offsets of both TRPs are equal to 0). With perfect time synchronization and ideal hardware for a TRP1 and a TRP2, one would have Tx= T2and = 2 • As noted above however, this is not the case in practice, and it is necessary to calibrate coherent joint transmissions. Therefore, to calibrate coherent joint transmissions, it is sufficient for a UE to acquire information relating to two quantities - a phase difference parameter 9 = 02— 0i and a time difference parameter ft = T2— TX- and provide these to the TRPs, in order to let the TRPs adjust their signals accordingly.

[0072] The two options in order to do this, as mentioned above, are:

[0073] • Option 1 - The UE should estimate the two quantities ft, 9 and report these back to the TRPs; and

[0074] • Option 2 - The UE should report a set of phase values Tn( / c) (for selected values - ideally a large set, but not necessarily all - of the subcarrier index k) and report these back to the TRPs. It is then the network’s task to deduce the two quantities ft, 9 from the information provided by the UE.

[0075] It has been proposed therefore, that for option 1, a UE should report estimates of 9 and ft. while for option 2, a UE should indicate a set of phase values to enable the network to deduce values of 9 and ft. Figure 6 shows a signalling diagram illustrating an example of the procedure for the reporting of TRP phase offset information by a UE. As noted above, according to the current 3GPP understanding, option 1 does not work with non-p recoded CSI-RS. That is, if non-p recoded CSI-RS are configured in the DL, the UE must prepare the set of phase offset reports according to option 2.

[0076] In step S601, the CSI-RS are configured for the UE by the network (for example, by TRP2 in the example of Figure 6), where these CSI-RS are configured to be precoded or non-precoded. Furthermore (and at least in part based on this configuration) the network also configures the UE to operate in accordance with either option 1 or option 2.

[0077] In steps S602 and S603, the UE transmits SRS1 and SRS2 respectively to TRP1 and TRP2.

[0078] Optionally, in step S604, the TRPs precode their CSI-RSs (CSI-RS 1 and CSI-RS2 respectively for TRP1 and TRP2) if so configured.

[0079] In steps S605 and S606, the UE receives CSI-RS 1 and CSI-RS2 from TRP1 and TRP2 respectively, where CSI-RS 1 and CSI-RS2 may or may not be precoded in accordance with the configuration.

[0080] In step S607, based on the transmitted SRSs and the received CSI-RSs, the UE calculates, if the CSI-RSs were precoded, the product of the complex conjugate of y and y2(i.e. y^y2)onmultiple subcarriers to derive the inter-TRP timing offset between TRP1 and TRP2 ( / ?) and the inter-TRP phase difference (0) between TRP1 and TRP2 according to option 1. However, if the CSI-RSs were not precoded, then the UE in step S607 prepares a set of phase values for a selected set subcarriers within which the CSI-RSs were received, according to option 2.

[0081] In step S608, the UE reports ft and 9 (option 1) or Tn( / c) (option 2) to the network (for example, to TRP2 in the example of Figure 6). In step S609, TRP1 and TRP2 synchronize their communications with respect to one another based on the reported ft and 9 or Tn( / c) where this set of sample values requires post-processing at the TRPs in the case of option 2 so as to determine the values of ft and 9).

[0082] The reason why option 1 does not, at first glance, work in combination with non-precoded CSI-RS is that the phase of the propagation channel Hn(k) destroys the linear structure of the phase of the quantityeiTnfc+n absence of noise, the phase of the received signals Tn( / c) appear random in nature.

[0083] Figure 7 illustrates the observed phase at a UE for a TRP when receiving precoded and non-precoded CSI-RS from that TRP and another TRP. As can be seen from the left-hand side of Figure 7, two TRPs 710, 720 may transmit CSI-RS 712, 722 to a UE 700 (non-simultaneously). The observed phase of these received CSI-RS 712, 722 at the UE 700, shown on the right-hand side of Figure 7, is the addition of four different components:

[0084] • the phase of the transmitted CSI-RS signals 712, 722;

[0085] • the hardware induced phase at TRP1 710 and TRP2 720;

[0086] • the phase in the propagation channel; and

[0087] • the hardware induced phase at the UE 700.

[0088] When the CSI-RS 712, 722 are non-precoded, as shown in plot 730, the channel phase dominates and appears to destroy the possibility of identifying the hardware phase of TRP1 710. However, when the CSI-RS 712, 722 are precoded, as shown in plot 740, the CSI-RS 712, 722 phase compensates the channel phase perfectly, so that the channel phase is not observable at the UE 700. That is, as noted above, the phase of the propagation channel destroys the linear structure of the non-precoded CSI-RS, and so the phase appears random.

[0089] Embodiments of the present technique propose and demonstrate a solution to this problem, thereby enabling the use of the more economic option 1 with non-precoded CSI-RS.

[0090] Low Overhead Phase Alignment Estimation for Non-Precoded CSI-RS in Multi-TRP Systems Figure 8 shows a part schematic, part message flow diagram representation of a wireless communications system comprising a communications device 800 (e.g. a UE 14) and a plurality of radio access nodes 810, 820 (e.g. TRPs 10) in accordance with at least some embodiments of the present technique. The communications device 800 is operable to assist radio access nodes of a wireless communications network to calibrate coherent joint transmissions (CJTs).

[0091] The communications device 800 may be configured to transmit signals to and / or receive signals from the wireless communications network, for example, to and from the radio access nodes 810, 820. Specifically, the communications device 800 may be configured to transmit data to and / or receive data from the wireless communications network (e.g. to / from radio access nodes 810, 820) via a wireless radio interface provided by the wireless communications network (e.g. a Uu interface between the communications device 800 and the Radio Access Network (RAN), which includes the radio access nodes 810, 820). The communications device 800 and the radio access nodes 810, 820 each comprise a transmitter (or transmitter circuitry) 801, 811, 821, a receiver (or receiver circuitry) 802, 812, 822, and a controller (or controller circuitry) 803, 813, 823. Each of the controllers 803, 813, 823 may be, for example, a microprocessor, a CPU, or a dedicated chipset, etc. The controllers 803, 813, 823 may also each be equipped with a memory unit (which is not shown in Figure 8). As those skilled in the art would appreciate the wireless communications system of Figure 8 may include a number of other communications devices and radio access nodes, which are not shown in the example of Figure 8 for the purposes of simplicity.

[0092] As shown in the example of Figure 8, the controller 803 of the communications device 800 is configured to control the transmitter 801 and the receiver 802 of the communications device 800 to receive 830 downlink reference signals from each of a subset of the radio access nodes 810, 820, wherein the downlink reference signals received 830 from each radio access node 810, 820 in the subset are not coded to compensate for a phase of a propagation channel between that radio access node 810, 820 and the communications device 800, and to transmit 840 (for example, to the first radio access node 810 that is currently serving the communications device 800, and furthermore, for example, upon determining that such received downlink reference signals 830 are not precoded to compensate for the phase of the propagation channel(s)) a calibration assistance report for use by each radio access node 810, 820 in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter determined at least in part based on the downlink reference signals and an indication of phase information (for example, sample phase values) determined from the downlink reference signals received 830 within at least one subcarrier (or sub-band). Here, the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first 810 of the subset of radio access nodes and a second 820 of the subset of radio access nodes.

[0093] These downlink reference signals may be, as described above, channel state information reference signals (CSI-RS), and these may be received 830 by the communications device 800 from the first and second radio access nodes 810, 820 in response to uplink reference signals (for example, sounding reference signals (SRS)) transmitted by the communications device 800 to the first and second radio access nodes 810, 820.

[0094] Essentially then, embodiments of the present technique, as exemplified by the example wireless communications system of Figure 8 for example, propose that a UE is able to report a small number of values relating to the phase of a set of TRPs (e.g. two or three values) even when these values are determined based on received CSI-RS that are not precoded. Such embodiments of the present technique therefore allow for the efficiency and economy of option 1 as described above, but without any requirement for CSI-RSs to be either precoded or non-precoded.

[0095] As described above, the task is to estimate the quantities ? = rx— r2and 9 = <p1— p2based on the received signals (i.e. CSI-RSs) at the UE:

[0096] Unfortunately, it is not possible for the UE to estimate the quantity 9 = <p1— p2when the CSI-RSs are not precoded. The technical reason for this is that the observed phase of the received signals at the UE will not be linear as described above, and the probability distributions for and Hn(k) are the same. However, this issue is dealt with in further detail below, enabling embodiments of the present technique to propose a solution that still is fully in line with option 1. In particular, it has the same signalling overheads.

[0097] The estimation of the value ? = rx— r2may be dealt with next. Here: d Hn(k) abide the same distribution.

[0098] If the values an, n = 1,2 are obtained, / 3 may be determined simply as a±— a2= . since the timing error TUEintroduced by the UE cancels out. An estimate of anis straightforward to produce. The loglikelihood of k = 0, ... , K — 1, given anreads: where and

[0099] Further, I is the K X K identity matrix, and RHHis the K X K correlation matrix of the subcarrier gains

[0100] Now, the covariance matrix RHHis given by the power delay profile (PDP) of the TRP being considered. This PDP cannot be estimated by the UE as it will then, inherently, include the TRP’s hardware impact which is what embodiments of the present technique are seeking to estimate in order to enable CJT. Loosely speaking, the basis of the estimation is that the UE observes that its received signal (i.e. the CSI- RS) does not conform, statistically, with the covariance of the channel. The statistical difference is the quantity that embodiments of the present technique seek to estimate.

[0101] If the covariance matrix is estimated by the UE, then it would conform perfectly with the received signal and the estimator cannot function. Thus, it is proposed in accordance with some arrangements of embodiments of the present technique that the PDP (which is in a one-to-one correspondence with the covariance matrix) is provided by the TRPs (one PDP per TRP). In other words the communications device may be configured to receive, from each of the subset of radio access nodes (i.e. TRPs), an indication of a power delay profile of that radio access node, the power delay profile for use by the communications device in determining a covariance matrix that is used to determine the estimate of the time difference parameter.

[0102] Here, when the PDPs are provided by the TRPs, these PDPs contain an indication of the effect of the reception chain of the TRPs as well, since the respective PDPs are estimated by recei ing SRS from the UE. This therefore enables the UE to determine the timing misalignment parameters of the TRPs in the manner described above with respect to Figure 5. In other words, the timing misalignment parameters may be determined by the communications device for each of the first and second radio access nodes based at least in part on a time difference between timing information of a transmission chain of each of the first radio access node and the second radio access node determined from the received downlink reference signals and timing information of a reception chain of each of the first radio access node and the second radio access node determined from the received power delay profiles.

[0103] An example of how a UE may report TRP phase offset information for two TRPs when the TRPs provide indications of their power delay profiles (PDPs) to the UE is shown by Figure 9. Here, in the example of Figure 9, each TRP may continuously estimate and update the indication of its PDP to the UE, based for example on SRSs being received by that TRP from the UE.

[0104] In steps S901 and S902, the UE transmits SRS to the TRPs (i.e. SRS1 and SRS2 are transmitted respectively to TRP1 and TRP2).

[0105] In response, the TRPs estimate (and / or update) their PDPs in response to these received SRSs, and transmit, in steps S903 and S904, indications of their PDPs to the UE.

[0106] In step S905, the CSI-RS are configured for the UE by the network (for example, by TRP2 in the example of Figure 9), where these CSI-RS are configured to be non-precoded. Here, in accordance with embodiments of the present technique (and at least in part based on this configuration) the network may also configure the UE at the same time to operate in accordance with embodiments of the present technique (rather than in accordance with the currently defined option 1 or option 2).

[0107] In steps S906 and S907, the TRPs transmit (non-precoded) CSI-RSs to the UE (i.e. CSI-RS1 and CSI- RS2 are transmitted respectively by TRP1 and TRP2).

[0108] In step S908, the UE estimates the value of ft based (at least in part) on the received CSI-RSs and based (at least in part) on the received indications of the TRPs’ PDPs.

[0109] In step S909, the UE transmits a calibration assistance report to the network (for example, to TRP2 in the example of Figure 9), where this calibration report comprises indications of the estimated value of ft and an indication of phase information (for example, sample phase values) determined from the received CSI- RSs.

[0110] When the PDPs are not provided by the TRPs, the effect of the reception chain of the TRPs does not influence the reported parameters. In this case, the TRPs suitably compensate the reported parameter values in a post-processing step. This is explained in further detail below.

[0111] It is noted that a reported PDP will contain an indication of the impact from the UE’s transmitter hardware as well as the TRP’s receiver hardware. However, this turns out to be irrelevant in respect of the purposes of embodiments of the present disclosure. The reason for this is that the hardware impact of the UE’s transmitter will be the same for both TRPs, and so signals still arrive coherently at the UE, as the hardware impact of the TRPs’ transmitters will be compensated for, during the data phase, by precoding, as the TRPs will compensate for the channel phase during the payload data phase in view of the values indicated by the UE in the calibration assistance report.

[0112] An estimate anof anmay now be straightforwardly produced by (numerical) optimization of -C({n( / c)}; an), i.e.:

[0113] Finally, a time difference parameter estimate P (which is an estimate of the value P) is produced by / ? = ay — a2.

[0114] Now, as stated above, the problem with the CSI-RSs not being precoded is that 9 cannot be estimated. To deal with this, and in accordance with arrangements of embodiments of the present technique, it suffices to report two (or a very small number of) sample values of Yn(k2), n = 1,2 for some arbitrary subcarrier k and as an example, k = k0can be chosen. In other words, the indication of phase information comprises an indication of a phase value (i.e. / ^(ko), Y2(0)}) of the downlink reference signals received from each of the subset of radio access nodes within the at least one subcarrier (i.e. k0) . Here, the subcarrier k0may be selected by the network and indicated to the UE, or the UE may itself select k0. If the UE performs the selection itself, then the UE will also need to indicate to the network that the reported sample values are for k0. such that the TRPs are able to acquire (via the SRSs previously transmitted by the UE) the channel at the same subcarrier k0in order to perform post-processing and properly estimate 9.

[0115] It should be noted that sample values can be reported for more than one subcarrier here to in order to better assist the network with estimating 9. whilst still reporting a far smaller number of samples than the set of phase values Tn( / c) required to be indicated in option 2. This set of subcarriers may be indicated to the UE by the network. Alternatively, the UE may select the set of subcarriers itself, and in this case - as noted above - the UE would need to indicate the selected subcarriers as well as the sample phase values.

[0116] Furthermore, the UE may report sample phase values for received signals within one or more sub-bands rather than subcarriers (either that it has selected itself or based on an indication from the network). This recognises that within a sub-band, which consists of a set of contiguous subcarriers, the channel can vary a little, and so the UE can improve its estimates of the sample phase values by averaging over some or all of the subcarriers in the sub-band.

[0117] The network may now compute Z = Y1( / c)y2*( ) (or, alternatively, Z = y2( / c)Y1*( / c)) which reads: where W is noise. Further manipulation yields:

[0118] However, since the UE has provided the value p. this can be used in place of p. Further, the network is aware of the phases of the two channels H (k) and H2(k). Therefore, it may readily estimate the quantity 9.

[0119] With option 2 as described above, 3GPP proposes that the UE should report sample values of the received signals, but for multiple subcarriers, i.e., {Fi( / c), T2( ): k G X} for some set X; this amounts to 21 X | numbers. With option 1, 3GPP proposes that a slope p and an intercept 9 is reported, and thus, two numbers are reported. Effectively, embodiments of the present technique propose a solution that is a mixture of the two, but where this solution is much closer in respect of overheads to option 1. Namely, such solutions propose that, when the UE reports the slope ft and an indication of phase information determined based on the received CSI-RS - for example, a pair of samples {Ii( / c0), E2( / c0)} - a total of three numbers.

[0120] As an alternative (which is more efficient), the UE may report the quantity Z instead of {yj( / c0), E2( / c0)}. In other words, the indication of phase information may comprise a single phase quantity, the single phase quantity being a product (i.e. (Zc) >2* ( / c)) of a first phase value of the downlink reference signals received from the first radio access node within the at least one subcarrier (or sub-band) and a complex conjugate of a second phase value of the downlink reference signals received from the second radio access node within the at least one subcarrier / sub-band (or, alternatively, the single phase quantity being a product (i.e. Y2 (k)) of the second phase value of the downlink reference signals received from the second radio access node within the at least one subcarrier (or sub-band) and a complex conjugate of the first phase value of the downlink reference signals received from the first radio access node within the at least one subcarrier (or sub-band)).

[0121] Whenever precoded CSI-RSs are used, then the intercept is reported by the UE instead of the pair {Yi (0), Y2(0)}, as per option 2 as detailed above. That is, upon determining that the CSI-RS are precoded to compensate for a phase of a propagation channel between the radio access node from which they are received and the communications device, the communications device may be configured to estimate and report both of the time difference parameter ft as described above and a phase difference parameter (i.e. the quantity 0) which can be understood as a phase difference between a phase misalignment parameter for the first radio access node and a phase misalignment parameter for the second radio access node, the phase misalignment parameter for each of the radio access nodes being dependent at least in part on a phase difference between a transmission chain and a reception chain of that radio access node.

[0122] As is explained above, it is not possible for the UE to estimate the quantity 9 = <p1— p2when the CSI- RSs are not precoded. However, in some arrangements of embodiments of the present technique, it is recognised that it is possible for the UE to estimate a quantity 9 that contains the desired phase offset

[0123] —2as wellasan additional quantity that depends on the channels H (k) and H2(k) . Here, 9 (which can be understood as being a phase offset parameter) can be computed by the UE as an intercept of the line that best fits the measured sample phase values of the received signals (within one or more subcarriers or sub-bands). The UE would effectively therefore be computing 9 = <p1— p2+ p — <p2, where (prand <p2can be understood as phase offsets for the signals received from TRP1 and TRP2 respectively associated with the mean delay of the channels H (k) and H2(k) respectively. In other words, the indication of phase information may comprise an indication of a phase difference parameter (i.e. 9) that indicates a phase difference between a phase misalignment parameter (i.e. 0X) for the first radio access node, a phase misalignment parameter (i.e. 02) for the second radio access node, a phase offset (i.e. (pr) of the downlink reference signals received from the first radio access node, and phase offset (i.e. <p2) of the downlink reference signals received from the second radio access node.

[0124] The network is then able to compute the difference (pr— <p2from the SRS previously transmitted by the UE, allowing the network to remove (p — (p2. and obtain an estimate of <p1— 02as desired. This therefore means that what the UE is effectively reporting in such arrangements is the same values as it would for option 1 as described above; estimates of ft and 9. As noted above, it may be that the TRPs are unable to provide the UE with their PDPs. In this case, the estimation technique described above cannot be performed by the UE as it requires knowledge of the PDPs of the TRPs. Embodiments of the present technique thus propose that the UE should determine the average slope of the phase of Yn(k) (across the index k) for both n = 1,2. This corresponds to the average delay of the received signals. In other words, the timing misalignment parameters may be determined by the communications device for each of the first and second radio access nodes based on an average delay of the downlink reference signals from the each of the first and second radio access nodes. The difference of said average slopes is reported to the TRPs. The TRPs, which know the propagation channels Hn(k), n = 1,2, can thus compute the average slope for the channels, and remove (subtract) the difference from the value indicated by the UE. What is left is an estimate of ft.

[0125] The average slope of the phase of a complex signal can x( / c), for example, be determined by (numerically) solving: arg

[0126] An example of how a UE may report TRP phase offset information for two TRPs when the TRPs do not provide indications of their PDPs to the UE is shown by Figure 10.

[0127] In step SI 001, the CSI-RS are configured for the UE by the network (for example, by TRP2 in the example of Figure 10), where these CSI-RS are configured to be non-precoded. Here, in accordance with embodiments of the present technique (and at least in part based on this configuration) the network may also configure the UE at the same time to operate in accordance with embodiments of the present technique (rather than in accordance with the currently defined option 1 or option 2).

[0128] In steps SI 002 and SI 003, the UE transmits SRS to the TRPs (i.e. SRS1 and SRS2 are transmitted respectively to TRP1 and TRP2).

[0129] In steps S1004 and S1005, the TRPs transmit (non-precoded) CSI-RSs to the UE (i.e. CSI-RS1 and CSI- RS2 are transmitted respectively by TRP1 and TRP2).

[0130] The UE is not able to estimate the value of ft in the same manner as in step S908 of Figure 9 as described above, because it does not know the PDPs of the TRPs and so cannot take these into consideration. Thus, in step SI 006, the UE determines an estimate of ft' . which is an estimate of ft that includes a bias due to the PDPs of the TRPs not being considered. This bias can later be removed at the TRPs.

[0131] In step S 1007, the UE transmits a calibration assistance report to the network (for example, to TRP2 in the example of Figure 10), where this calibration report comprises indications of the estimated value of ft' and an indication of phase information (for example, sample phase values) determined from the received CSI-RSs.

[0132] In step SI 008, each TRP may subtract the channel bias from ft' based on channel state information acquired via the SRS received from the UE. This then yields ft.

[0133] Figure 11 shows a flow diagram illustrating an example process of communications in a communications system in accordance with embodiments of the present technique. The process shown by Figure 11 is specifically a method of operating a communications device (e.g. UE) operable to assist a plurality of radio access nodes (e.g. TRPs) of a wireless communications network to calibrate coherent joint transmissions (CJTs).

[0134] The method begins in step SI 101. The method comprises, in step SI 102, receiving downlink reference signals from each of a subset of the radio access nodes, wherein the downlink reference signals received from each radio access node in the subset are not coded to compensate for a phase of a propagation channel between that radio access node and the communications device. In step S 1103, the process comprises transmitting a calibration assistance report for use by each radio access node in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter (e.g. ) determined at least in part based on the downlink reference signals and an indication of phase information (e.g. lj(k) and Y2(Jft) or Z = Y k)Y2k) or 9 = <p1— p2+ p — <p2) determined from the downlink reference signals received within at least one subcarrier (k) or within at least one subband and averaged across some or all of the subcarriers in the at least one sub-band. Here, the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first of the subset of radio access nodes and a second of the subset of radio access nodes. The process ends in step SI 104.

[0135] Those skilled in the art would appreciate that the method shown by Figure 11 and the procedures illustrated by Figures 9 and 10 may be adapted in accordance with embodiments of the present technique. For example, other intermediate steps may be included in such methods / procedures, or the steps may be performed in any logical order. Though embodiments of the present technique have been described largely by way of the example communications system shown in Figure 8, it would be clear to those skilled in the art that they could be equally applied to other systems to those described herein, provided that these are within the scope of the claims.

[0136] Those skilled in the art would further appreciate that such infrastructure equipment 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, provided that these are within the scope of the claims.

[0137] The following numbered paragraphs provide further example aspects and features of the present technique:

[0138] Paragraph 1. A method of operating a communications device operable to assist a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions, CJTs, the method comprising receiving downlink reference signals from each of a subset of the radio access nodes, wherein the downlink reference signals received from each radio access node in the subset are not coded to compensate for a phase of a propagation channel between that radio access node and the communications device, and transmitting a calibration assistance report for use by each radio access node in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter determined at least in part based on the downlink reference signals and an indication of phase information determined from the downlink reference signals received within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first of the subset of radio access nodes and a second of the subset of radio access nodes.

[0139] Paragraph 2. A method according to Paragraph 1, comprising receiving, from each of the subset of radio access nodes, an indication of a power delay profile of that radio access node, the power delay profile for use by the communications device in determining a covariance matrix that is used to determine the estimate of the time difference parameter.

[0140] Paragraph 3. A method according to Paragraph 2, wherein the timing misalignment parameters are determined by the communications device for each of the first and second radio access nodes based at least in part on a time difference between timing information of a transmission chain of each of the first radio access node and the second radio access node determined from the received downlink reference signals and timing information of a reception chain of each of the first radio access node and the second radio access node determined from the received power delay profiles.

[0141] Paragraph 4. A method according to Paragraph 1, wherein the timing misalignment parameters are determined by the communications device for each of the first and second radio access nodes based on an average delay of the downlink reference signals from the each of the first and second radio access nodes. Paragraph 5. A method according to any of Paragraphs 1 to 4, wherein the indication of phase information comprises an indication of a phase value of the downlink reference signals received from each of the subset of radio access nodes within the at least one subcarrier.

[0142] Paragraph 6. A method according to any of Paragraphs 1 to 4, wherein the indication of phase information comprises a single phase quantity, the single phase quantity being a product of a first phase value of the downlink reference signals received from the first radio access node within the at least one subcarrier and a complex conjugate of a second phase value of the downlink reference signals received from the second radio access node within the at least one subcarrier.

[0143] Paragraph 7. A method according to any of Paragraphs 1 to 4, wherein the indication of phase information comprises an indication of a phase difference parameter that indicates a phase difference between a phase misalignment parameter for the first radio access node, a phase misalignment parameter for the second radio access node, a phase offset of the downlink reference signals received from the first radio access node, and phase offset of the downlink reference signals received from the second radio access node.

[0144] Paragraph 8. A method according to any of Paragraphs 1 to 7, wherein the downlink reference signals are channel state information reference signals, CSI-RS.

[0145] Paragraph 9. A method according to any of Paragraphs 1 to 8, comprising transmitting, to each of the subset of radio access nodes, uplink reference signals, wherein the downlink reference signals are received in response to the transmitted uplink reference signals.

[0146] Paragraph 10. A method according to Paragraph 9, wherein the uplink reference signals are sounding reference signals, SRS.

[0147] Paragraph 11. A communications device operable to assist a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions, CJTs, 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 receive downlink reference signals from each of a subset of the radio access nodes, wherein the downlink reference signals received from each radio access node in the subset are not coded to compensate for a phase of a propagation channel between that radio access node and the communications device, and to transmit a calibration assistance report for use by each radio access node in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter determined at least in part based on the downlink reference signals and an indication of phase information determined from the downlink reference signals received within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first of the subset of radio access nodes and a second of the subset of radio access nodes.

[0148] Paragraph 12. Circuitry for a communications device operable to assist a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions, CJTs, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to receive downlink reference signals from each of a subset of the radio access nodes, wherein the downlink reference signals received from each radio access node in the subset are not coded to compensate for a phase of a propagation channel between that radio access node and the communications device, and to transmit a calibration assistance report for use by each radio access node in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter determined at least in part based on the downlink reference signals and an indication of phase information determined from the downlink reference signals received within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first of the subset of radio access nodes and a second of the subset of radio access nodes.

[0149] Paragraph 13. A method of operating a first radio access node of a wireless communications network operable to communicate with at least one communications device to calibrate coherent joint transmissions, CJTs, the method comprising transmitting downlink reference signals to the at least one communications device, wherein the downlink reference signals are not coded to compensate for a phase of a propagation channel between the first radio access node and the at least one communications device, and receiving, from the at least one communications device, a calibration assistance report for use by the first radio access node to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter and an indication of phase information of the downlink reference signals within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for the first radio access node and a second radio access node.

[0150] Paragraph 14. A method according to Paragraph 13, comprising transmitting, to the at least one communications device, an indication of a power delay profile of the first radio access node, the power delay profile for use by the at least one communications device in determining a covariance matrix that is used to determine the estimate of the time difference parameter. Paragraph 15. A method according to Paragraph 13 or Paragraph 14, wherein the indication of phase information comprises an indication of a phase value of the downlink reference signals transmitted by the first radio access node within the at least one subcarrier and an indication of a phase value of downlink reference signals received by the at least one communications device from the second radio access node in the at least one subcarrier.

[0151] Paragraph 16. A method according to Paragraph 13 or Paragraph 14, wherein the indication of phase information comprises a single phase quantity, the single phase quantity being a product of a first phase value of the downlink reference signals transmitted by the first radio access node within at least one subcarrier and a complex conjugate of a second phase value of downlink reference signals received by the at least one communications device from the second radio access node within the at least one subcarrier. Paragraph 17. A method according to Paragraph 13 or Paragraph 14, wherein the indication of phase information comprises an indication of a phase difference parameter that indicates a phase difference between a phase misalignment parameter for the first radio access node, a phase misalignment parameter for the second radio access node, a phase offset of the downlink reference signals transmitted to the at least one communications device by the first radio access node, and phase offset of the downlink reference signals transmitted to the at least one communications device by the second radio access node. Paragraph 18. A method according to any of Paragraphs 13 to 17, wherein the downlink reference signals are channel state information reference signals, CSI-RS.

[0152] Paragraph 19. A method according to any of Paragraphs 13 to 18, comprising receiving, from the at least one communications device, uplink reference signals, wherein the downlink reference signals are transmitted in response to the received uplink reference signals.

[0153] Paragraph 20. A method according to Paragraph 19, wherein the uplink reference signals are sounding reference signals, SRS.

[0154] Paragraph 21. A first radio access node of a wireless communications network operable to communicate with at least one communications device 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 transmit downlink reference signals to the at least one communications device, wherein the downlink reference signals are not coded to compensate for a phase of a propagation channel between the first radio access node and the at least one communications device, and to receive, from the at least one communications device, a calibration assistance report for use by the first radio access node to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter and an indication of phase information of the downlink reference signals within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for the first radio access node and a second radio access node. Paragraph 22. Circuitry for a first radio access node of a wireless communications network operable to communicate with at least one communications device to calibrate coherent joint transmissions, CJTs, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to transmit downlink reference signals to the at least one communications device, wherein the downlink reference signals are not coded to compensate for a phase of a propagation channel between the first radio access node and the at least one communications device, and to receive, from the at least one communications device, a calibration assistance report for use by the first radio access node to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter and an indication of phase information of the downlink reference signals within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for the first radio access node and a second radio access node.

[0155] Paragraph 23. A wireless communications system comprising a communications device according to Paragraph 11 and a first radio access node according to Paragraph 21.

[0156] Paragraph 24. A computer program comprising instructions which, when loaded onto a computer, cause the computer to perform a method according to any of Paragraphs 1 to 10 or Paragraphs 13 to 20.

[0157] Paragraph 25. A non-transitory computer-readable storage medium storing a computer program according to Paragraph 23.

[0158] 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.

[0159] 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.

[0160] 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

[0161] [1] Holma H. and Toskala A, “LTE for UMTS OFDMA and SC-FDMA based radio access”, John Wiley and Sons, 2009.

[0162] [2] TR 38.913, “3rdGeneration Partnership Project; Technical Specification Group Radio Access Network; Study on Scenarios and Requirements for Next Generation Access Technologies

[0163] (Release 14)”, 3GPP, vl4.3.0, August 2017.

[0164] [3] R1 -2403425, “CSI enhancements for >32 ports and UE -assisted CJT with non-ideal TRP synchronization”, Qualcomm Incorporated, April 2024.

Claims

CLAIMSWhat is claimed is:

1. A method of operating a communications device operable to assist a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions, CJTs, the method comprising receiving downlink reference signals from each of a subset of the radio access nodes, wherein the downlink reference signals received from each radio access node in the subset are not coded to compensate for a phase of a propagation channel between that radio access node and the communications device, and transmitting a calibration assistance report for use by each radio access node in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter determined at least in part based on the downlink reference signals and an indication of phase information determined from the downlink reference signals received within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first of the subset of radio access nodes and a second of the subset of radio access nodes.

2. A method according to Claim 1, comprising receiving, from each of the subset of radio access nodes, an indication of a power delay profile of that radio access node, the power delay profile for use by the communications device in determining a covariance matrix that is used to determine the estimate of the time difference parameter.

3. A method according to Claim 2, wherein the timing misalignment parameters are determined by the communications device for each of the first and second radio access nodes based at least in part on a time difference between timing information of a transmission chain of each of the first radio access node and the second radio access node determined from the received downlink reference signals and timing information of a reception chain of each of the first radio access node and the second radio access node determined from the received power delay profiles.

4. A method according to Claim 1, wherein the timing misalignment parameters are determined by the communications device for each of the first and second radio access nodes based on an average delay of the downlink reference signals from the each of the first and second radio access nodes.

5. A method according to Claim 1, wherein the indication of phase information comprises an indication of a phase value of the downlink reference signals received from each of the subset of radio access nodes within the at least one subcarrier.

6. A method according to Claim 1, wherein the indication of phase information comprises a single phase quantity, the single phase quantity being a product of a first phase value of the downlink reference signals received from the first radio access node within the at least one subcarrier and a complex conjugate of a second phase value of the downlink reference signals received from the second radio access node within the at least one subcarrier.

7. A method according to Claim 1, wherein the indication of phase information comprises an indication of a phase difference parameter that indicates a phase difference between a phase misalignment parameter for the first radio access node, a phase misalignment parameter for the second radio access node, a phase offset of the downlink reference signals received from the first radio access node, and phase offset of the downlink reference signals received from the second radio access node.

8. A method according to Claim 1, wherein the downlink reference signals are channel state information reference signals, CSI-RS.

9. A method according to Claim 1, comprising transmitting, to each of the subset of radio access nodes, uplink reference signals, wherein the downlink reference signals are received in response to the transmitted uplink reference signals.

10. A method according to Claim 9, wherein the uplink reference signals are sounding reference signals, SRS.

11. A communications device operable to assist a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions, CJTs, 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 receive downlink reference signals from each of a subset of the radio access nodes, wherein the downlink reference signals received from each radio access node in the subset are not coded to compensate for a phase of a propagation channel between that radio access node and the communications device, and to transmit a calibration assistance report for use by each radio access node in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter determined at least in part based on the downlink reference signals and an indication of phase information determined from the downlink reference signals received within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first of the subset of radio access nodes and a second of the subset of radio access nodes.

12. Circuitry for a communications device operable to assist a plurality of radio access nodes of a wireless communications network to calibrate coherent joint transmissions, CJTs, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to receive downlink reference signals from each of a subset of the radio access nodes, wherein the downlink reference signals received from each radio access node in the subset are not coded to compensate for a phase of a propagation channel between that radio access node and the communications device, and to transmit a calibration assistance report for use by each radio access node in the subset to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter determined at least in part based on the downlink reference signals and an indication of phase information determined from the downlink reference signals received within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for a first of the subset of radio access nodes and a second of the subset of radio access nodes.

13. A method of operating a first radio access node of a wireless communications network operable to communicate with at least one communications device to calibrate coherent joint transmissions, CJTs, the method comprising transmitting downlink reference signals to the at least one communications device, wherein the downlink reference signals are not coded to compensate for a phase of a propagation channel between the first radio access node and the at least one communications device, and receiving, from the at least one communications device, a calibration assistance report for use by the first radio access node to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter and an indication of phase information of the downlink reference signals within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for the first radio access node and a second radio access node.

14. A method according to Claim 13, comprising transmitting, to the at least one communications device, an indication of a power delay profile of the first radio access node, the power delay profile for use by the at least one communications device in determining a covariance matrix that is used to determine the estimate of the time difference parameter.

15. A method according to Claim 13, wherein the indication of phase information comprises an indication of a phase value of the downlink reference signals transmitted by the first radio access node within the at least one subcarrier and an indication of a phase value of downlink reference signals received by the at least one communications device from the second radio access node in the at least one subcarrier.

16. A method according to Claim 13, wherein the indication of phase information comprises a single phase quantity, the single phase quantity being a product of a first phase value of the downlink reference signals transmitted by the first radio access node within at least one subcarrier and a complex conjugate of a second phase value of downlink reference signals received by the at least one communications device from the second radio access node within the at least one subcarrier.

17. A method according to Claim 13, wherein the indication of phase information comprises an indication of a phase difference parameter that indicates a phase difference between a phase misalignment parameter for the first radio access node, a phase misalignment parameter for the second radio access node, a phase offset of the downlink reference signals transmitted to the at least one communications device by the first radio access node, and phase offset of the downlink reference signals transmitted to the at least one communications device by the second radio access node.

18. A method according to Claim 13, wherein the downlink reference signals are channel state information reference signals, CSI-RS.

19. A method according to Claim 13, comprising receiving, from the at least one communications device, uplink reference signals, wherein the downlink reference signals are transmitted in response to the received uplink reference signals.

20. A method according to Claim 19, wherein the uplink reference signals are sounding reference signals, SRS.

21. A first radio access node of a wireless communications network operable to communicate with at least one communications device 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 transmit downlink reference signals to the at least one communications device, wherein the downlink reference signals are not coded to compensate for a phase of a propagation channel between the first radio access node and the at least one communications device, and to receive, from the at least one communications device, a calibration assistance report for use by the first radio access node to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter and an indication of phase information of the downlink reference signals within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for the first radio access node and a second radio access node.

22. Circuitry for a first radio access node of a wireless communications network operable to communicate with at least one communications device to calibrate coherent joint transmissions, CJTs, the circuitry comprising transmitter circuitry configured to transmit signals, receiver circuitry configured to receive signals, and controller circuitry configured in combination with the transmitter circuitry and the receiver circuitry to transmit downlink reference signals to the at least one communications device, wherein the downlink reference signals are not coded to compensate for a phase of a propagation channel between the first radio access node and the at least one communications device, and to receive, from the at least one communications device, a calibration assistance report for use by the first radio access node to calibrate CJTs, wherein the calibration assistance report comprises an estimate of a time difference parameter and an indication of phase information of the downlink reference signals within at least one subcarrier, wherein the estimate of the time difference parameter is indicative of an estimate of a time difference between timing misalignment parameters for the first radio access node and a second radio access node.

23. A wireless communications system comprising a communications device according to Claim 11 and a first radio access node according to Claim 22.

24. A computer program comprising instructions which, when loaded onto a computer, cause the computer to perform a method according to Claim 1 or Claim 13.

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

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