Method and apparatus for synchonization in non-terrestrial networks

Abstract

A wireless communication system including a serving communication unit (base station, gNB, NTN cell) supporting communications for a plurality of wireless communication units (User device, UE). The ba

Description

The technical field relates generally to implementing techniques to support synchronization in non-terrestrial networks (NTNs). Background In recent years, there has been a rapid development in communications technologies that are compliant with third generation partnership project (3GPP™) standards. A 4th generation (4G) wireless communication standard (sometimes referred to as long term evolution (LTE™) was designed to support mobile internet and higher speeds for activities, such as video streaming and gaming. The 3GPP™ standards then developed a fifth generation (5G) of mobile wireless communications, which provides a step change in the delivery of better and faster communications, for example powering businesses, improving communications within homes and spearheading advances such as driverless cars. A sixth generation (6G) wireless communication standard is currently under development, as the planned successor to 5G, and will likely be significantly faster. Like its predecessors, 6G networks will likely be broadband cellular networks, in which the service area is divided into small geographical areas called cells. 6G networks are expected to be even more diverse than their predecessors and are likely to support applications beyond current mobile use scenarios, such as virtual and augmented reality (VR / AR), ubiquitous instant communications, pervasive intelligence and the Internet of Things (loT). It is expected that mobile network operators will adopt flexible decentralized business models for 6G, with local spectrum licensing, spectrum sharing, infrastructure sharing, and intelligent automated management underpinned by mobile edge computing, artificial intelligence (AI), short-packet communication and blockchain technologies. Non-Terrestrial Networks (NTNs) is an emerging area in 3GPP™ with Release 17 defining a solution to enable 5G New Radio (NR) and NG-RAN to support Non-Terrestrial Networks. It addressed solution for Transparent payload for both Geostationary and non-Geostationary network scenarios, with the UE having GNSS capability and the satellite beams being both earth-fixed or earth-moving. The aim is to provide 5G cellular coverage using space borne and / or air borne platforms, where traditional ground-based networks have difficulty in providing coverage and / or capacity. loT NTN was a 3GPP study and work item in 3GPP release 17 to provide Non-Terrestrial Network access for E-UTRAN loT devices (NB-IoT and LTE-M / eMTC) [RP-202689], And NR NTN was a work item in Rei-17 to specify adaptation to allow NR to function over NTN [RP-211557], Non-Terrestrial Network access may be through Lower Earth Orbit (LEO), Medium Earth Orbit (MEO) and Geostationary Orbit (GEO), as well as through High-Altitude Platform Systems (HAPS). Following the Work items in Release 17 there were work items to enhance NR NTN [RP-220953] and loT NTN [RP-220979] in Release 18. NR NTN phase 3 [RP-234078] is a 3GPP Work Item in 3GPP Release 19 aiming to enhance NR NTN with a range of enhancements that include: Downlink coverage enhancements; Uplink capacity and throughput enhancements by using Orthogonal Coverage Codes; MBS broadcast over NTN; Introduction of regenerative payload; Redcap and NTN enhancements; Terrestrial E-UTRAN to NR NTN mobility. 5G and 5G new radio (5G NR) include three RRC states; RRC connected mode, RRC inactive mode and RRC idle mode, where RRC inactive is a newly introduced mode. RRC idle and RRC inactive operation are to a large part similar and a differentiation is to a large part in the procedures used to go to RRC connected mode. 5G NR also features several new improvements that allow for higher throughput, lower latency and extreme flexibility. Some of these new improvements include: (i) Beam-based procedures. Here, the 5G NR device will take into account the beams in a cell in several procedures to better accommodate the advancements in MIMO and beamforming seen over the last decade. (ii) Ultra-lean carriers: This improvement provides a reduction in the number of “always-on” signals where the network may broadcast reference signals a lot more infrequently compared to previous generations, and allow a network to reduce the amount of system information broadcasted. (iii) More efficient state transitions: Here, a new RRC state is introduced, RRCIN ACTIVE. In RRC INACTIVE, the UE performs similar actions as in RRCIDLE, e.g., measuring and performing the cell reselection procedure to ensure that the UE is camping on the best cell. The network will save the UE context in the gNB and the UE will save the RRC configuration. This ensures that the state transition from RRC INACUVE and RRCCONNECTED can be completed in a much smaller number of steps compared to moving from RRC IDLE to RRC CONNECTED. In a network where there are a lot of state transitions, this can reduce latency and improve capacity as there is a lot less need for control signals to occupy capacity and resources. Referring to FIG. 1, a known simplified cellular architecture diagram 100 illustrates a first nonterrestrial base station / satellite 102 supporting communications within a coverage area 104, including communication support for a wireless communication unit, sometimes referred to as a terminal device, such as a user equipment UE 106. In 4G as well as 5G, the UE 106 is able to support traditional Human Type Communications (HTC) or the new emerging Machine Type Communications (MTC). The UE 106 is considered to be active when communicating and in the operational state technically known as radio resource control (RRC) connected. 4G or LTE™, which Internet of Things (loT) (NB-IoT and LTE-M) is based on, has two RRC states; RRC connected mode and RRC idle mode. The known simplified cellular architecture diagram 100 comprises a connection 120 that connects the first non-terrestrial base station / satellite 102 and a second terrestrial base station 108 via a gateway 114. For RRC- idle state UEs 106, a cell re-selection process may be warranted, if the signal strength from the current serving cell (e.g., non-terrestrial base station 102) deteriorates as the UE transitions from base station 102 to 108. Similarly, a cell re-selection process is performed for ‘RRC idle state UEs when the UE nears the cell edge of the current cell 104 that it is camped on, and is able to receive a signal from the neighbour base station 108. For loT, this is a known 4G or E-UTRAN cell reselection process, which is also driven by signal strength measurements of the base stations carried out by the UE. Thus, the 3GPP™ LTE™ (and NR) cell reselection, e.g., the act of camping on another cell, is an autonomous decision based on radio signal measurements performed by UEs 106 based on signals received from serving base station 102 and one or more neighbour base station(s) 112, sometimes referred to as fifth generation Node Bs (gNB) or eNBs and configured thresholds related to such radio signal measurements. In RRC connected mode, a threshold A dB may be set to avoid ping-pong type handover near the cell border, as the instantaneous signal strengths can vary dynamically. The serving gNB 102 can instruct each individual UE 106 to provide these measurement reports and the measurement frequency can be adapted depending on whether a particular UE is nearing a cell edge, for example. Also, cell re-selection processes happen on an individual UE basis, based on the measurements of the current camped-on and neighbour base stations (gNBs). In RRC idle mode / state, thresholds are also used to govern the cell reselections, but they are not used by comparing one cell with the serving cell as is done in RRC connected mode. In cell reselection in idle mode, the cells will, for instance, compute a ranking based on parameters such as thresholds and the measured signal strength. In the ‘RRC- idle’ state, the current camped-on base station (gNB) 102 is able to instruct the UEs 106 on an individual basis in order to carry out these measurements and the UE 106 themselves will initiate and carry out the cell re-selection process. One of the key aspects of orthogonal frequency division multiple access (OFDMA) wireless communication systems is to ensure that a UE is well-synchronized before accessing the system. For this, both 4G and 5G rely on random access to ensure that the UE is well-synchronized. During the random access procedure the UE receives a timing advance command for the UE to compensate for its propagation delay with a cell. The frequency offset is generally compensated by the UE adjusting its frequency according to the downlink frequency. This method works for networks that have a cell radius of up to around 10 kilometers, and is illustrated, with respect to a 5GNR in the message sequence chart 200 of FIG. 2 that illustrates a known synchronization procedure between a UE 210 and an eNB 220. Within 5G NR, and in order to move to RRC CONNECTED, a UE 210 must first synchronize and connect to a cell / eNB 220 at 230, which it does through a 4-step random access procedure. The 4-step random access procedure was introduced for the first release of 5G NR and includes a Msgl 240 and a Msg2 250. Msgl 240 consists of a preamble sent on the Random Access Channel (RACH), which signals a number from 1 to 64 identifying the UE 210. Msg2 250 is the Random Access Response, which contains a timing advance to synchronize the UE 210, as well as an uplink grant to send Msg3 260. Msg3 260 contains an RRC message and Msg4 270 contains the reply to the first RRC message as well as a contention resolution MAC CE to resolve any contention. The UE 210 then enters the connected mode at 280. For a 2-step random access a MsgA consists of both the preamble and the first RRC message. MsgB consists of the random access response to synchronize the UE, which is the reply to the first RRC message as well as contention resolution. To move to RRCCONNECTED mode from RRCIDLE, the RRC Setup procedure is triggered. RRC Setup procedures establishes an SRB1 connection along with basic radio configurations. This means that the RRC message RRCSetupRequest will be included in Msg3 260 and the RRC message RRCSetup may be included in Msg4 270. After the RRC Setup procedures the network may also have to acquire capabilities, establish AS security before user plane data can be transmitted. To move to RRC CONNECTED from RRC INACUVE, the RRC Resume procedure is triggered. These procedures allow for the re-establishment of the full RRC connection, as well as resuming the AS security. This means that the RRC message RRCResumeRequest will be included in Msg3 260 and the RRC message RRCResume may be included in Msg4 270. It should be noted that MAC random access procedures are often independent of the RRC procedures, which means that the random access procedures may in general be the same for RRC Setup, RRC Resume, RRC Re-establishment and RRC reconfiguration with sync. One way to enter RRC idle or RRC inactive mode is by the network releasing the UE through the RRC release procedures. The RRC release procedures are initiated when the UE receives a RRCRelease message from the gNB. The RRCRelease message sent from the gNB may in turn have been triggered by the either the gNB or the AMF. This can for instance be due to any of the following reasons: Load balancing; Re-direction (both in RRC idle and RRC inactive) to other frequencies or RATs; UE context release triggered by the AMF (CN); Suspend indication to send the UE to RRC inactive; or Failure to retrieve UE context when UE resumes RRC connection from RRC inactive. During the NTN Release 16 work item, it was discussed how a 5G UE is to synchronize in an NTN cell, considering how synchronization in a 5G terrestrial network is performed. The issue of synchronization in a non-terrestrial network can be said to have 3 components: (i) distance between UE and the satellite is several times larger than a terrestrial network from 300 km to 35000 km, which means that if the same methods as used for terrestrial networks, the timing advance would potentially have to be very large. The UE may be able to synchronize the downlink, but the uplink cannot so easily synchronized; (ii) the distance between UE and the gNB changes several times faster than in a terrestrial network, leading to the constant need of adjusting the timing advance; and (iii) the rapid movement of the satellite causes very high doppler frequency offset in the range of 10s of kHz, which the UE needs to track. This is compared to a maximum of 100s of Hz of doppler frequency offset in a terrestrial scenario. The two options, as illustrated in FIG. 2, were either: (i) that the UE 210 uses UE location acquired via GNSS to synchronize in time and frequency, or (ii) that UE 210 is synchronized in a similar manner to a terrestrial network, but increasing the size of the Timing Advance (TA). Using GNSS to synchronize has the drawback that the system becomes reliant on GNSS and that the network has to trust that the UE 210 is capable of self-synchronizing. Increasing the size of Timing Advance would for instance mean that the RACH in 290 would become a lot more complicated as the network would be required to detect very large timing offset 294, which 4G and 5G RACH is not designed to do. Referring now to FIG. 3, and early on in the NTN standardization phase it was agreed that the UE should rely on GNSS and the satellite / gNB location in order to synchronize. The reason for this is that this allows for the physical layer design of the random access procedures to be reused to a great degree, thereby allowing faster implementations. FIG. 3 illustrates a schematic diagram 300 of a known pre-computed TA approach to NTN synchronization with a first non-terrestrial base station / satellite 302 supporting communications within a coverage area, including communication support for a wireless communication unit, sometimes referred to as a terminal device, such as a user equipment UE 306. The known schematic diagram 300 comprises a connection 320 that connects the first non-terrestrial base station / satellite 302 and a second terrestrial base station 308 via a gateway 314. The UE 306 has an access link 310 to the first nonterrestrial base station / satellite 302 and acquires the UE location via GNSS, the location of the satellite 302 and the delay from satellite to gNB on the ground, as shown at 312. The doppler frequency offset is pre-compensated by using the UE location and the location of the satellite. The timing advance is pre-compensated by using the UE location, the location of the satellite as well as the delay from the satellite to the gNB, as shown at 322, since the Release 17 NTN operates as transparent payload. The location of the satellite can be computed based on the ephemeris of the satellite. Ephemeris may be either a PVT vector, which is a vector that gives the position and the velocity of an object, or it may be an information element that containing parameters that define an orbit of an object. This is broadcasted in the system information of the cell along with the common TA, which is used to compute the timing advance between the satellite and the gNB on the ground in a transparent payload setting. System information is information that is broadcasted by a cell for a wide range of purposes. System information is divided into a set of System Information Blocks (SIB). Some system information is required for a UE to access a cell. Without having acquired these system information blocks the UE may not be allowed to access a cell. In example of such a SIB is SIB1, which contains access information, for instance the PLMN of the cell, the cell identity, the tracking area code as well as cell selection information. SIB1 also contains the serving cell radio configuration. Another set of SIBs contain information on other frequencies and RATs for the purpose of idle and inactive mode cell reselection as well as related parameters. These are for instance in SIB2-SIB5. As NTN has a number of NTN-specific information elements that are only required when accessing an NTN cell, and also due to the rather large information elements, it was agreed that new system information block (SIB) was needed. In NR NTN, SIB 19 contains the required information to access an NTN cell, with the SIB 19 field descriptions shown in Table 1 below. Table 1: SIB19 field descriptions ; distanceThresh i Distance from the serving cell reference location and is used in location-based measurement initiation in RRC_IDLE ; i and RRC_INACTIVE, as defined in TS 38.304

[20] , Each step represents 50m. This field is only present in an NTN i cell. i i movingReferenceLocaiion i Reference location of the serving cell of an NTN Earth moving system at a time reference. It is used in location- i based measurement initiation in RRC_IDLE and RRC_INACTIVE, as defined in TS 38.304

[20] , The time reference i ; of this field is indicated by epochTime in ntn-Config of the serving cell. This field is excluded when determining i changes in system information, i.e., changes to movingReferenceLocation should neither result in system i information change notifications nor in a modification of valueTag in SIB1. This field is only present in an NTN cell. i ntn-Config i Provides parameters needed for the UE to access NR via NTN access such as Ephemeris data, common TA i parameters, k_offset, validity duration for UL sync information and epoch. In a TN cell, this field is only present in i ntn-NeighCellConfigList and ntn-NeighCellConfigListExt. i i nin-NeighCeHConfigLisi, ntn-NeighCeliConfigUsiExi i Provides a list of NTN neighbour cells including their ntn-Config, carrier frequency and PhysCellld. This set includes i i all elements of ntn-NeighCellConfigList and all elements of ntn-NeighCellConfigListExt. If ntn-Config is absent for an i ; entry in ntn-NeighCellConfigListExt, the ntn-Config provided in the entry at the same position in ntn- i NeighCellConfigList applies. Network provides ntn-Config for the first entry of ntn-NeighCellConfigList. If the ntn-i Config is absent for any other entry in ntn-NeighCellConfigList, the ntn-Config provided in the previous entry in ntn-i NeighCellConfigList applies. i i referenceLocaiion i Reference location of the serving cell provided via NTN quasi-Earth fixed system and is used in location-based ; measurement initiation in RRC_IDLE and RRC_INACTIVE, as defined in TS 38.304

[20] , This field is only present i in an NTN cell. i saiSwiichWiihReSync i Provides parameters for the target satellite required to perform satellite switch with re-synchronization. This field is i only present in an NTN cell and its presence indicates that satellite switch without PCI change is supported in the i cell. i i i-Service i Indicates the time information on when a cell provided via NTN system is going to stop serving the area it is i currently covering. This field applies for both service link switches in NTN quasi-Earth fixed system and feeder link i switches for both NTN quasi-Earth fixed and Earth moving system. The field indicates a time in multiples of 10 ms ; after 00:00:00 on Gregorian calendar date 1 January, 1900 (midnight between Sunday, December 31,1899 and i Monday, January 1, 1900). The exact stop time is between the time indicated by the value of this field minus 1 and i the time indicated by the value of this field. The reference point for t-Service is the uplink time synchronization i reference point of the cell. This field is only present in an NTN cell. satSwitchWithReSync field descriptions i ssb-TimeOffset i Indicates the time offset between the SSB from source and target satellite at the uplink time synchronization i reference point. It is given in number of subframes. i t-ServiceStart i Indicates the time information on when the target satellite is going to start serving the area currently covered by the i serving satellite. The field indicates a time in multiples of 10 ms after 00:00:00 on Gregorian calendar date 1st i January 1900 (midnight between Sunday, December 31,1899, and Monday, January 1, 1900). The exact start time i ; is between the time indicated by the value of this field minus 1 and the time indicated by the value of this field. The system information contains the following: (i) Serving cell Ephemeris elements, which allows a UE to calculate the satellite position for doppler and time pre-compensation. This can be of three formats: a. PVT format - which describes a (X,Y,Z) position as well as a speed vector (vX, vY, vZ). The position elements (X, Y, Z respectively) each occupy 26 bits and the speed elements (vX, vY, vZ) each occupy 18 bits. Altogether they occupy 132 bits; b. Orbital parameters (also referred to as Keplerian format) provides parameters that indicate how a celestial body / satellite moves in space, which is then used to infer the satellite position and allows for accurate synchronization and prediction of future NTN payload position. Altogether they occupy 164 bits; and c. TLE ephemeris elements (only used for Discontinuous Coverage in loT NTN). Altogether they occupy 189 bits. (ii) Timing Advance (TA) common parameters - this provides the common timing advance parameters that are introduced to compensate for the feeder link delays. The signalling consists of (in total taking up 57 bits): a. Absolute TA common, taking up 23 bits; b. Drift of the TA common - how the TA common drifts, e.g., the first derivative, taking up 19 bits; and c. Variation of the TA common - how the TA common varies, e.g., the second derivative of the TA common, taking up 15 bits. (iii) Synchronization validity duration - used to define how long the ephemeris and TA common is valid. (iv) Epoch time: when the synchronization validity duration should start. (v) K-Offset: scheduling offset for timing relationship in NTN. (vi) K-Mac: Scheduling offset used when the downlink and uplink frame timing is not aligned. (vii) NR NTN specific information also include (as part of 3 8.3 31): a. T-Service (signalled in SIB3 in loT NTN). b. Reference location and distance threshold - used for location-based measurement initiation in RRC IDLE and RRC Connected mode. c. Neighbour cell ephemeris, which is used for idle mode measurements. Thus, whenever UE 306 is connected to an eNB or gNB, the UE 306 needs to read the system information. There is furthermore a timer (T317 in loT NTN and T430 in 5GNTN) associated with the ephemeris element that is started everytime the system information containing the ephemeris (SIB31 in loT NTN and SIB 19 in NR NTN) is read. Referring back to FIG. 1, as the ephemeris constantly changes due to the movement of the NTN payload, there is a need to make sure that the UE is correctly synchronized. Thus, whenever an UE connects to an eNB, the UE needs to read the system information. In the lower part of FIG. 1, a timing diagram illustrates the ephemeris synchronization operation, where in a) SIB31 functions as normal in 150; b) where the UE fails to read SIB31 in 160 during T318, which then expires and triggers Radio Link Failure (RLF); and c) when an NR NTN UE acquires SIB 19 at 180 before an end of T430 timer. As illustrated, a timer T317 152 associated with the ephemeris element that is started every time SIB31 is read. At expiry of T317 152, the UE is no longer considered synchronized and it will have to re-acquire SIB31 in order to stay synchronized. In NR NTN, the UE 106 shall ensure that it has a recent ephemeris (SIB 19 in NR) by reading the SIB in time by UE implementation. In loT NTN, since an loT UE (LTE-M and NB-IoT UE) is not expected to be able to acquire system information in connected mode, the UE 106 tunes away and is likely unreachable while reading SIB31. If the loT NTN UE is unable to read the SIB31 within a second timer T318 154 with a configured duration, the UE performs RLF at 170 in a similar manner to other cases where RLF is performed. The T317 timer is different compared to a normal timer in RRC does not commence at having received the SIB31. This is because the ephemeris has an epoch time, which is the reference point in time of when the ephemeris is defined. Thus, the T317 152 is started from the epoch time, which may be in the past or in the future relative to having received SIB31. This means that in a UE implementation, the timer may be started with a different value with what was signalled according to what was signalled in the field ul-SyncValidityDuration in SIB31. In NTN, using the position of the UE 106 in certain procedures is a lot more useful compared to a terrestrial network owing to the very large cells. Similarly, for quasi earth-fixed cells, where the satellite moves but the cell illuminated on the ground changes with time, using time to initiate or stop certain procedures can be very useful. The T317 and T430 timers are different compared to a normal timer in RRC as it is not started at having received the LTE SIB31 or NR SIB 19. This is because the ephemeris has an epoch time, which is the reference point in time of when the ephemeris is defined. Thus the T317 is started from the epoch time, which may be in the past or in the future relative to have received SIB31. This means that in a UE implementation, the timer may be started with a different value with what was signaled according to what was signaled in the field ul-SyncValidityDuration in SIB31. In both loT NTN and NR NTN, when a handover is performed to another cell, the handover command will contain ephemeris information so that the UE may synchronize without having to acquire the system information from the target cell. FIG. 4 illustrates known message sequence chart procedures 400, 450 relating to another part of synchronizing, which is acquiring the GNSS position. In a first procedure 400, illustrates the expected network-controlled operation where the network / eNB 408 releases the UE 406, (e.g., moved to RRC Idle mode) to ensure that eNB knows the state of the UE 406; and a second procedure 450 illustrates the UE 406 releases itself autonomously according to the [36.331 V17.3.0] specification. It is specified as a requirement that the UE 406 shall have or obtain, at 410, a recent and precise enough GNSS position before the UE 406 attempts to connect to a cell or eNB 408. For instance, the UE 406 is required to have determined its own position to be used for time and frequencysynchronization before any RRC procedure is started. In NR NTN, the UE 406 is considered capable of acquiring GNSS position at 410 while in connected mode. In loT NTN, the UE is not considered to be as capable of a device and cannot operate in connected mode and acquire GNSS at the same time. If the GNSS is deemed to be invalid and the UE is in connected mode, the UE shall move to idle mode to ensure that it does not cause interference, and it also reduces the requirement for a network to communicate with a UE 406 that might have poor synchronization. The UE 406 establishes a connection at 420 with the eNB 408 and the UE 406 sends the eNB 408 a GNSS validity duration message at 430. At 440, the UE enters a connected mode. In the first procedure 400, the eNB releases the UE due to the GNSS validity duration being low at 442. In the second procedure 450, the UE leaves the connected mode at 444. Referring now to FIG. 5, for loT NTN Release 18, it was realized that the UE may in certain conditions not need to have a recent GNSS position in order to remain synchronized if the UE has previously acquired the GNSS position. FIG. 5 illustrates a known message sequence chart 500 where a UE 506 does not need to have a recent GNSS position in order to remain synchronized. It was specified that the conditions for this is that the UE 506 is in connected mode at 530 and have previously acquired the GNSS position at 510 (and established a connection with the eNB 508 at 520) either before entering connected mode (as illustrated), or when the UE is in connected mode, and the network has configured the feature. At this point, it is assumed that the GNSS is valid at 540. A timer (timer T390) is started at 550 when the UE GNSS position is invalid or out of date, which gives how long the UE may stay in connected mode. A newly introduced MAC CE, the UL Transmission Extension Update MAC CE, may be used to re-start the timer. Once the timer expires and the UE 506 has not acquired a new GNSS position, the UE 506 will leave connected mode autonomously and move to RRC idle mode at 560. In light of the above, the inventors have recognised and appreciated that in the first release of NTN, it was specified for both NR NTN and loT NTN that a UE shall have a valid GNSS position in order to synchronize to a cell. The UE uses GNSS to acquire its position and acquires the position of the satellite via ephemeris and some more assistance information in order to synchronize to a non-terrestrial network. This had clear benefits at the time, as it allowed the basic parts of terrestrial 5G NR and loT specifications to remain the same in NTN. However, there are serious drawbacks to this approach. The inventors have recognised and appreciated that one drawback is that if GNSS is unavailable or jammed, then a UE would not be able to connect or remain connected to an NTN cell. Furthermore, acquiring GNSS is time-consuming and power-consuming, which is not suitable in many cases. However, it is further recognized by the inventors that performing random access using GNSS and without GNSS may have its benefits and that it should be possible to imagine a case where it would be possible to use both methods in a cell in order to synchronize. Thus, a need exists for improved devices and methods that may define a framework to allow for multiple synchronization methods. Summary A wireless communication system a wireless communication unit and a method of synchronization for a wireless communication unit are described. The wireless communication system, wireless communication unit and method of synchronization for a wireless communication unit enable the wireless communication unit to perform random access and synchronization in a cell without necessarily having GNSS. In a first aspect a wireless communication unit, having communications supported by at least one serving communication unit is described. The wireless communication unit comprises: a transceiver; and a processor, operably coupled to the transceiver and arranged to: identify that the wireless communication unit is in an unsynchronized state with the at least one serving communication unit; and in response thereto, perform a procedure that transitions the wireless communication unit from the unsynchronized state to a semi-synchronized state with the at least one serving communication unit; and perform a subsequent procedure that transitions the wireless communication unit from the semi-synchronized state to a fully synchronized state with the at least one serving communication unit. In some examples, the processor being arranged to identify that the wireless communication unit is in the unsynchronized state with the at least one serving communication unit may comprise the processor being arranged to identify at least one of: the wireless communication unit does not have access to a geographical location of the wireless communication unit; the wireless communication unit has access to an inaccurate geographical location of the wireless communication unit; the wireless communication unit does not have access to a spatial or geographical location of the at least one serving communication unit; the wireless communication unit has access to an inaccurate spatial location or geographical location of the at least one serving communication unit. In some examples, in response to the processor identifying that the wireless communication unit is in the unsynchronized state, the processor may be arranged to perform at least one of the following: prevent the wireless communication unit from performing random access to connect to a communication cell; enter a radio resource control, RRC, connected mode that allows the wireless communication unit to perform random access to connect to a communication cell; prevent the wireless communication unit from establishing an RRC connection with a communication cell; prevent the wireless communication unit from establishing carrier aggregation to multiple cells or multiple nodes; prevent the wireless communication unit from establishing Dual Connectivity, DC to multiple cells or multiple nodes; prevent the wireless communication unit from performing a cell selection or cell reselection operation. In some examples, in response to the processor identifying that the wireless communication unit is in the unsynchronized state, the processor may be arranged to perform at least one of the following: prevent the wireless communication unit from establishing a fifth generation-sixth generation, 5G-6G carrier aggregation to multiple cells or multiple nodes; prevent the wireless communication unit from establishing a fifth generation-sixth generation, 5G-6G DC to multiple cells or multiple nodes; prevent the wireless communication unit from establishing a fifth generation-sixth generation, 5G-6G, Dual Stack. In some examples, in response to the processor identifying that the wireless communication unit is in the unsynchronized state, the processor may be arranged to perform at least one of the following: prevent the wireless communication unit from performing random access to connect to a communication cell; or enter a radio resource control, RRC, connected mode that allows the wireless communication unit from performing random access to connect to a communication cell, wherein the communication cell is one of a Non-Terrestrial Network, NTN, communication cell, an air-to-ground, ATG, communication cell. In some examples, in response to the processor identifying that the wireless communication unit is in the unsynchronized state, the processor may be arranged to perform a random access procedure that transitions the wireless communication unit from the unsynchronized state to a semi-synchronized state whereby a successful random access procedure synchronizes the wireless communication unit to a communication cell supported by the at least one serving communication unit. In some examples, in response to the processor identifying that the wireless communication unit is in the unsynchronized state, the processor may be arranged to establish a radio resource control, RRC, connection with the at least one serving communication unit or re-establish a radio resource control, RRC, connection with the at least one serving communication unit in response to a change of RRC state of the wireless communication unit, that transitions the wireless communication unit from the unsynchronized state to a semi-synchronized state whereby a successful RRC connection synchronizes the wireless communication unit to a communication cell supported by the at least one serving communication unit. In some examples, in response to the processor identifying that the wireless communication unit is in the semi-synchronized state and operating in a radio resource control, RRC, connected mode, the processor may be arranged to perform at least one of the following: prevent the wireless communication unit from establishing access stratum, AS, security; prevent the wireless communication unit from establishing data radio bearers to carry user plane data; prevent the wireless communication unit from establishing carrier aggregation to multiple cells or multiple nodes; prevent the wireless communication unit from establishing Dual Connectivity, DC to multiple cells or multiple nodes; prevent the wireless communication unit from performing a cell selection or cell reselection operation. In some examples, in response to the processor identifying that the wireless communication unit is in the semi-synchronized state, the processor may be arranged to perform at least one of the following: prevent the wireless communication unit from establishing a fifth generation-sixth generation, 5G-6G carrier aggregation to multiple cells or multiple nodes; prevent the wireless communication unit from establishing a fifth generation-sixth generation, 5G-6G DC to multiple cells or multiple nodes; prevent the wireless communication unit from establishing a fifth generation-sixth generation, 5G-6G, Dual Stack. In some examples, in response to the processor identifying that the wireless communication unit is in the semi-synchronized state and operating in a radio resource control, RRC, connected mode, the processor may be arranged to perform at least one of the following: prevent the wireless communication unit from initiating a RRC Resume procedure; prevent the wireless communication unit from initiating a handover to a second communication cell without first synchronizing with a first communication cell; prevent the wireless communication unit from performing neighbour cell measurements. In some examples, in response to the processor identifying that the wireless communication unit is in the semi-synchronized state and has acquired and applied at least one of: assistance information, a location of the wireless communication unit, the processor may be arranged to transition the wireless communication unit, or may receive an instruction from the at least one serving communication unit to transition, from the semi-synchronized state to a fully synchronized state. In some examples, the processor may be arranged to acquire assistance information from at least one of the following: via system information or secured system information broadcast from the at least one serving communication unit; first acquire a less accurate assistance information that is accurate enough to enter semi-synchronized state, and then the processor transitions to an RRCCONNECTED mode and acquires more accurate assistance information that enables the wireless communication unit to enter the fully synchronized state; a dedicated transmission of assistance sent by the at least one serving communication unit. In some examples, the receiver and the processor may be arranged to acquire the location of the wireless communication unit when in an RRCCONNECTED mode from at least one of the following: Network-assistance Global Navigation Satellite System, GNSS, information; Observed Time Difference of Arrival, OTDOA, positioning information; Motion sensor positioning information; Multi-cell roundtrip time, Multi-RTT, positioning information; Downlink Time Difference of Arrival, DL TDOA information; Downlink Angle of Departure, DL AoD information. In some examples, in response to the processor identifying that the wireless communication unit is in the fully synchronized state, the processor may be arranged to enter into a semisynchronized state in response to at least one of the following: identification of a failure-related condition of the wireless communication unit; detection of one of: a downlink channel of the wireless communication unit being out of synchronization, a physical downlink control channel, PDCCH, signal strength or signal quality being lower than a threshold; identification of at least one of: assistance information of at least one serving communication unit no longer being accurate, a wireless communication unit position no longer being accurate; a trigger of a reestablishment of an RRC connection; prevent the wireless communication unit from establishing Dual Connectivity, DC to multiple cells or multiple nodes; prevent the wireless communication unit from performing a cell selection or cell reselection operation; during or after a handover from another cell has been performed; a transition of the wireless communication unit from RRC connected mode to RRC idle mode or to RRC inactive mode. In some examples, in response to the processor identifying that the wireless communication unit is in the semi-synchronized state, the processor may be arranged to transition the wireless communication unit to an unsynchronized state in response to the processor identifying at least one of the following: a failure-related condition of the wireless communication unit; an expiration of a timer associated with the semi-synchronized state; a location of the wireless communication unit or the assistance information received by the wireless communication unit being out of date; a requirement to perform public land mobile network, PLMN, selection; a selection of a non-terrestrial network after previously having camped on a terrestrial network. In some examples, the failure-related condition may comprise at least one of: a radio link failure, RLF, a communication cell handover failure, a trigger to perform a re-establishment of a radio resource control, RRC, connection, identification of a start of a timer related to a RLF. In some examples, the at least one serving communication unit comprises a first serving communication unit arranged to support communications in a first communication cell and a second serving communication unit arranged to support communications in a second communication cell, wherein the wireless communication unit is in a fully synchronized state with a first serving communication unit and the wireless communication unit is in a semisynchronized state with a second serving communication unit, wherein the processor may be arranged to: acquire assistance information from the second serving communication unit or geographical location information of the wireless communication unit from the first serving communication unit; and transition the wireless communication unit from the semi-synchronized state to a fully synchronized state with the second serving communication unit. In some examples, the processor and the transceiver of the wireless communication unit may be arranged to indicate a state transition of the wireless communication unit to the at least one serving communication unit. In some examples, the processor and the transceiver of the wireless communication unit may be arranged to indicate a state transition of the wireless communication unit to the at least one serving communication unit in one of: a radio resource control, RRC, message; a medium access control, MAC, control element, CE. In some examples, the processor and the transceiver of the wireless communication unit may be arranged to indicate a state transition of the wireless communication unit to the at least one serving communication unit in an indication that includes a new state of the wireless communication unit and a previous state of the wireless communication unit. In some examples, a framework is defined to allow for multiple synchronization methods. In some examples, methods for a wireless communication unit, such as a UE, to synchronize to a cell are described and may include: a synchronization state that is not a fully synchronized state or non-synchronized state where in one or more of the following conditions are fulfilled: a UE does not have access to its location or only has access to an inaccurate geographical location; a UE does not have access to the location of the network node or only has access to an inaccurate geographical location. In this example, the UE enters the said synchronization state from a synchronization state where the UE is not synchronized. In some examples, when entering the said synchronization state from a state where the UE is not synchronized, the wireless communication unit and methods may include successfully performing random access. In some examples, when entering the said synchronization state from a state where the UE is not synchronized, the wireless communication unit and methods may include establishing an RRC connection with the cell. In some examples, when entering the said synchronization state from a state where the UE is not synchronized, the wireless communication unit and methods may include a UE having access to its location; and / or a UE having access to a location of its network node. In some examples, when entering the said synchronization state from a state where the UE is fully synchronized, the wireless communication unit and methods may include where either the UE location or location of the network node is no longer accurate, available or up-to-date. In some examples, when entering the said synchronization state from a state where the UE is fully synchronized, the wireless communication unit and methods may be as a result of one of more failure-related actions, for example, a failure of the radio link (e.g., RLF), a handover failure, a re-establishment of the RRC connection triggered. In some examples, when entering the said synchronization state from a state where the UE is fully synchronized, the wireless communication unit and methods may occur due to the RRC state of the UE changing. In a second aspect, a method of synchronization for a wireless communication unit supported by at least one serving communication unit in a wireless communication system is described. The method at the wireless communication unit comprises: identifying that the wireless communication unit is in an unsynchronized state with the at least one serving communication unit; and in response thereto: performing a procedure that transitions the wireless communication unit from the unsynchronized state to a semi-synchronized state with the at least one serving communication unit; and performing a subsequent procedure that transitions the wireless communication unit from the semi-synchronized state to a fully synchronized state with the at least one serving communication unit. Brief Description of the Drawings Further details, aspects and embodiments will be described, by way of example only, with reference to the drawings. In the drawings, similar reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. FIG. 1 illustrates a known simplified non-terrestrial cellular architecture with a known NR NTN and loT NTN operation. FIG. 2 illustrates a known synchronization procedure. FIG. 3 illustrates a schematic diagram of a known pre-computed TA approach to NTN synchronization. FIG. 4 illustrates a known message sequence chart procedure relating to another part of synchronizing that is acquiring the loT NTN GNSS position. In a) is the expected network-controlled operation where the network releases the UE to ensure that eNB knows the state of the UE and in b) the UE releases itself autonomously according to the specification. FIG. 5 illustrates a known message sequence chart where a UE does not need to have a recent GNSS position in order to remain synchronized and once a timer expires, the UE will move to RRC idle. FIG. 6 illustrates a framework and high-level procedure to synchronize an NTN cell for using multiple approaches and movement between different states to synchronize to an NTN cell, in accordance with some example embodiments. FIG. 7 illustrates a simplified example of a message sequence chart when a network supplies a UE with more accurate NTN assistance information and potentially the UE performing more accurate position to determine the UE position, in accordance with some example embodiments. FIG. 8 illustrates a 3GPP™ 5G communication system with NTN base stations, adapted in accordance with some example embodiments. FIG. 9 illustrates one example of using a RRC reconfiguration message, for instance received in a semi-synchronized state, whereby the UE performs the appropriate actions in order to enter the synchronized state, in accordance with some example embodiments. FIG. 10 illustrates a block diagram of an NTN base station communicating with a UE, adapted in accordance with some example embodiments. FIG. 11 illustrates one example of a message sequence chart whereby a UE is considered semisynchronized after having entered RRC connected and then is released and a UE is considered to still be in a semi-synchronized state after leaving RRC connected, in accordance with some example embodiments. FIG. 12 illustrates a simplified message sequence chart of a UE entering a non-synchronized state from synchronized or semi-synchronized state, in accordance with some example embodiments. FIG. 13 illustrates a simplified message sequence chart of a UE entering a synchronized state from a semi-synchronized state, in accordance with some example embodiments. FIG. 14 illustrates a simplified message sequence chart of a verification of a UE position, in accordance with some example embodiments. FIG. 15 illustrates a simplified message sequence chart of a synchronization state during a handover, in accordance with some example embodiments. FIG. 16 illustrates a simplified message sequence chart of a synchronization state MAC CE, in accordance with some example embodiments. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and / or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various example embodiments. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments. It will be further appreciated that certain actions and / or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein. Detailed Description The inventors have recognised and appreciated that a wireless communication unit, such as a UE, may benefit from being able to transition between a non-synchronized state and a semisynchronized state, before transitioning to a fully synchronized state, and vice versa. It is also envisaged that the approaches described below apply equally to eNBs, gNBs, NG-RAN or NG-eNB. Hence, it is envisaged that the terms are to be considered interchangeably. For instance, in some cases the below description may be for an eNB, but it may also apply for NG-eNB (eNB is connected to EPC, while NG-eNB is connected to 5GC). In some cases below, there may also be methods explicitly for an NG-eNB, en-gNB, gNB, or NG-RAN. It is also envisaged that an E-UTRAN (cell) is (in a loose sense) the base station of an eMTC / LTE-M cell. Thus, the term ‘E-UTRAN’ should be considered, in one example, an ToT NTN base station’. In 3GPP™ so far, only loT is supported for LTE NTN and not ordinary non-loT LTE NTN. It should be noted that examples herein described are not limited to loT NTN E-UTRAN as it is envisaged that “ordinary” non-IoT LTE NTN are also supported. It is also envisaged that the approaches described herein are also applicable if non-Standalone NTN is supported, e.g., EN-DC (E-UTRAN-NR Dual Connectivity) is supported in the future. Example embodiments are described with reference to ‘radio access networks’, which term encompasses and is considered to be equivalent to and interchangeable with communication cells, namely the facilitation of communications within a cell that may access other parts of the communication system as a whole. Although some examples target loT NTN, it is envisaged that the concepts herein described may also be relevant for 5G NR. Referring now to FIG. 6, FIG. 6 illustrates a framework and high-level procedure 600 to synchronize a wireless communication unit, such as a UE, to an NTN cell that accommodates a movement of the wireless communication unit between different states to synchronize to an NTN cell, in accordance with some example embodiments. As illustrated, the wireless communication unit is able to transition between a non-synchronized state 610 and a semi-synchronized state 620. Furthermore, the wireless communication unit is able to transition between the semisynchronized state 620 and a fully synchronized state 630. Thus, in order to transition between the non-synchronized state 610 and the fully synchronized state 630, the wireless communication unit must first transition to the semi-synchronized state 620. Referring now to FIG. 7, FIG. 7 illustrates a simplified example of a message sequence chart 700 of a scenario when a network supplies a UE 710 with more accurate NTN assistance information and potentially the UE 710 performing more accurate position detection in order to determine the UE’s position, for example the UE 710 may become synchronized with a cell group B, in accordance with some example embodiments. In this example, the network includes a main gNB 720 supporting communications in cell group ‘A’ and a second gNB 730 supporting communications in cell group ‘B’. At 740, the UE 710 is in an RRC connected state. At 742, the UE is in a fully synchronized state with the main gNB 720 and cell group ‘A’. At 744, the UE is in a semi-synchronized state with the second gNB 730 and cell group ‘B’. At 746, the UE 710 acquires NTN assistance information on cell group ‘B’ from the second gNB 730 or the position of the UE 710 from the main gNB 720. Following this, the UE is able to be in a synchronized state with cell group ‘B’ at 748. Referring now to FIG. 8, part of a wireless communication system 800 is shown in outline, in accordance with one example embodiment. In this example embodiment, the wireless communication system 800 is compliant with, and contains network elements capable of operating over, a 4th generation (4G), a 5th generation (5G) or 6th generation (6G) wireless communication system, which are currently under discussion in the third Generation Partnership Project (3GPP™). The wireless communication system 800 architecture consists of radio access network (RAN) and core network (CN) elements (not shown), with the core network elements being coupled to external networks (named Packet Data Networks (PDNs)), such as the Internet or a corporate network. As illustrated, the CN is operably connected to two NodeBs (eNB 810 and gNB 812), with respective, coverage areas (or cells) 880, 885. A plurality of wireless communication units 825 communicate with the serving eNB 810 or gNB 812. In accordance with example embodiments, at least one eNB 810, gNB 812 and at least one UE 710 (amongst other elements) have been adapted to support the concepts hereinafter described. In this example, the main components of the RAN include a TN eNB 810 and an NTN eNB 812, in a form of a satellite, which perform many standard base station functions and are connected to the CN via an SI interface / feeder link and to the wireless communication units 825 via a Uu interface. A wireless communication system will typically have a large number of such infrastructure elements, including a number of terrestrial base stations where, for clarity purposes, only a limited number are shown in FIG. 8. Each of the TN eNB 810 and the NTN gNB 812 are able to control and manage the radio resource related functions for a plurality of wireless communication units 825. Each of the wireless communication units 825 comprise a transceiver unit operably coupled to signal processor (with one wireless communication unit illustrated in such detail for clarity purposes only). The system comprises many other wireless communication units 825 and eNBs 810 and gNBs 812, which for clarity purposes are not shown. In accordance with examples herein described, a wireless communication system comprises a first base station, such as NTN eNB 810, supporting a first network and a second base station, such as gNB wireless base station 812 supporting a second terrestrial network, and a plurality of wireless communication units / UEs 710. In this example, at least one of the first network and second network is a non-terrestrial network, NTN, the first base station 810 comprising: a transceiver; and a processor, operably coupled to the transceiver and arranged to communicate with at least a first wireless communication unit, UE 710, of the plurality of wireless communication units, to facilitate synchronization. FIG. 9 illustrates one example of a message sequence chart 900 in using a RRC reconfiguration message, for instance received in a semi-synchronized state, whereby the UE 710 performs the appropriate actions in order to enter the synchronized state, in accordance with some example embodiments. In this example, the network includes a NTN gNB 812 supporting communications in a cell. At 940, the UE 710 is in an RRC connected and semi-synchronized state. At 942, the UE 710 receives a RRCReconfiguration message, e.g., a command to enter a fully synchronized state. At 944, the NTN gNB 812 and UE 710 communicate to acquire the UE location and at 946, the NTN gNB 812 and UE 710 communicate to acquire a satellite position or associated assistance information. At 948, the UE 710 sends a RRCReconfiguration message that indicates that the UE 710 has entered into a fully synchronized state. Referring now to FIG. 10, more detailed block diagrams of an eNB or gNB TN or NTN wireless base station (equivalent in functionality details to eNB or gNB TN or NTN base station 812 in FIG. 8) and a wireless communication unit (such as a UE 710) are illustrated, where the respective communications units have been adapted in accordance with some example embodiments. The eNB or gNB TN or NTN wireless base station 812 contains an antenna 1002, for receiving transmissions, coupled to an antenna switch or duplexer 1004 that provides isolation between receive and transmit chains within the eNB or gNB TN or NTN wireless base station 812. One or more receiver chains, as known in the art, include receiver front-end circuitry 1006 (effectively providing reception, filtering and intermediate or base-band frequency conversion). The receiver front-end circuitry 1006 is coupled to a signal processor 1008 (generally realized by a digital signal processor (DSP)). A skilled artisan will appreciate that the level of integration of receiver circuits or components may be, in some instances, implementation-dependent. The controller 1014 maintains overall operational control of the eNB or gNB TN or NTN wireless base station 812. The controller 1014 is also coupled to the receiver front-end circuitry 1006 and the signal processor 1008. In some examples, the controller 1014 is also coupled to a frequency generation circuit 1017 and a memory device 1016 that selectively stores operating regimes, such as decoding / encoding functions, synchronization patterns, code sequences, and the like. A timer 1018 is operably coupled to the controller 1014 to control the timing of operations (e.g., transmission or reception of time-dependent signals) within the eNB or gNB TN or NTN wireless base station 812. As regards the transmit chain, this essentially includes an input interface 1020, coupled in series through transmitter / modulation circuitry 1022 and a power amplifier 1024 to the antenna 1002, antenna array, or plurality of antennas. The transmitter / modulation circuitry 1022 and the power amplifier 1024 are operationally responsive to the controller 1014. The signal processor 1008 in the transmit chain may be implemented as distinct from the signal processor in the receive chain. Alternatively, a single processor may be used to implement a processing of both transmit and receive signals, as shown in FIG. 10. Clearly, the various components within the eNB or gNB TN or NTN wireless base station 812 can be realized in discrete or integrated component form, with an ultimate structure therefore being an application-specific or design selection. The processor 1008 and transceiver (e.g., transmitter / modulation circuitry 1022 and receiver front-end circuitry 1006) of the eNB or gNB TN or NTN wireless base station 812 are configured to facilitate synchronization of a served wireless communication unit, such as a UE, to or from another eNB or gNB TN or NTN wireless base station, according to at least one of the operations in accordance with the approach described in one of FIG. 11, FIG. 12 or FIG. 13. The eNB or gNB TN or NTN wireless base station 812 is configured to determine whether (or not) the UE 710 shall measure TN or NTN cells or frequencies, in accordance with some example embodiments. The processor 1008 and transmitter / modulation circuitry 1022 are configured to facilitate synchronization according to at least one of the operations in accordance with the approach described in one of FIG. 11, FIG. 12, FIG. 13, FIG. 14 or FIG. 15 or any of the examples described below. FIG. 10 also shows a high-level block diagram of the wireless communication unit (a user equipment (UE) in 3GPP parlance) 710 contains an antenna 1052, for receiving transmissions, coupled to an antenna switch or duplexer 1054 that provides isolation between receive and transmit chains within the wireless communication unit 710. One or more receiver chains, as known in the art, include receiver front-end circuitry 1056 (effectively providing reception, filtering and intermediate or base-band frequency conversion). The receiver front-end circuitry 1056 is coupled to a signal processor 1058 (generally realized by a digital signal processor (DSP)). A skilled artisan will appreciate that the level of integration of receiver circuits or components may be, in some instances, implementation-dependent. The controller 1064 maintains overall operational control of the wireless communication unit 710. The controller 1064 is also coupled to the receiver front-end circuitry 1056 and the signal processor 1058. In some examples, the controller 1064 is also coupled to a frequency generation circuit 1067 and a memory device 1066 that selectively stores operating regimes, such as decoding / encoding functions, synchronization patterns, code sequences, and the like. A timer 1068 is operably coupled to the controller 1064 to control the timing of operations (e.g., transmission or reception of time-dependent signals) within the wireless communication unit 710. As regards the transmit chain, this essentially includes an input interface 1070, coupled in series through transmitter / modulation circuitry 1072 and a power amplifier 1074 to the antenna 1052, antenna array, or plurality of antennas. The transmitter / modulation circuitry 1072 and the power amplifier 1074 are operationally responsive to the controller 1064. The signal processor 1058 in the transmit chain may be implemented as distinct from the signal processor in the receive chain. Alternatively, a single processor may be used to implement a processing of both transmit and receive signals, as shown in FIG. 10. Clearly, the various components within the wireless communication unit 710 can be realized in discrete or integrated component form, with an ultimate structure therefore being an application-specific or design selection. The processor 1058 and transceiver (e.g., transmitter / modulation circuitry 1072 and receiver front-end circuitry 1056) of the wireless communication unit 710 are configured to communicate with the eNB or gNB TN or NTN wireless base station 812 on a first frequency that is set by frequency generation circuit 1067. In accordance with some examples, the processor 1058 and transceiver (e.g., transmitter / modulation circuitry 1072 and receiver front-end circuitry 1056) of the wireless communication unit 710 are configured to facilitate synchronization of the wireless communication unit, such as a UE, to or from an alternative eNB or gNB TN or NTN wireless base station, according to at least one of the operations in accordance with the approach described in one of FIG. 11, FIG. 12, FIG. 13, FIG. 14 or FIG. 15 or any of the examples described below. In some examples, it is envisaged that the ephemeris is not only applicable for satellite payloads, but can also apply to other platforms such as High Altitude Platform Systems (HAPS), or to other types of networks such as Air-To-Ground networks. Thus, any mention of “satellite ephemeris” herein described is intended to not only apply to satellites but also other platforms and / or payloads above the earth surface. In some examples, it is envisaged that the expression “Terrestrial network”, when used herein, is intended to not only relate to a land mobile network (e.g., IMT), but may also encompass other terrestrial telecommunication services. This, for instance, may include Air-To-Ground networks or High Altitude Platform Systems (HAPS). On the other hand, in some examples, it is envisaged that the expression may also be applied to such Air-To-Ground networks or High Altitude Platforms. Also, when examples mention NTN and / or satellites, it is intended to additionally encompass such other platforms / payloads above the earth surface. In some examples, it is envisaged that the wording “RRC connected”, “connected mode” or “RRCCONNECTED” herein described is intended to be used interchangeably. Similarly, “idle mode”, “RRC idle” or ’’RRCIDLE” herein described is intended to be used interchangeably. Sometimes, when methods related to “idle mode” are mentioned, this may also encompass “inactive mode”, “RRC inactive” or “RRC INACTIVE” herein described as the actions performed in those two states in general are the same. In some examples herein described, it is envisaged that terms of 5G NR, all proposals, embodiments, and examples may also apply for eNBs, NG-eNBs (eNBs connected via 5GC) or 6G NBs. Furthermore, in some examples herein described, it is envisaged that all related, newly defined and / or existing: RRC signaling and / or messages, X2, Xn, SI, NG, and / or Fl signaling and messages, and / or related network entities (e.g., MME, AMF, other). In some examples, it is envisaged that concepts herein described related to 4G and 5G and potential equivalents in a 6G system may apply equally and include: (i) 4G and 5G RRC connected state: UE having established a connection with a RAN, e.g., a cell, gNB or similar identity. (ii) Cell: This may also be a different concept in a 6G system. For instance, in a cell-less case a UE may attach, connect and be associated with a beam or other similar identity. (iii) RRC idle: UE not in a RRC connected state, e.g., not having established a connection. RRC idle also means that UE will be camping on a cell or similar identity and then performing measurements and evaluating to find a better cell or similar identity. (iv) 5G RRC inactive: UE in a state similar to RRC idle where the UE stores the RRC configuration and resumes the RRC connection using the configuration. The network also stores the UE context and uses it to restore the UE connection. (v) 5G RRC procedures (RRC Setup, RRC Resume, RRC Re-establishment, RRC Reconfiguration): Any procedures that aim to establish a connection with a cell, a gNB or similar identity. For instance, a procedure that aims to establish a 5G-6G Dual Connectivity setting with a 5G and 6G cell. (vi) Random access: The process of synchronizing the MAC layer via sending a preamble and receiving a message that synchronizes the uplink, as well as following messages to resolve any contention. (vii) Radio Link Failure: Failure of the radio link, which may be a failure based on measured radio signals, or based on operation in the cell, such as a number of retransmissions, random access failures, the radio beams failing etc. After the radio link failure the UE may try to reselect to another cell and re-establish the RRC connection. (viii) Handover: Performing active mobility to another cell, gNB or similar identity. May also be termed a ‘reconfiguration with synch’. (ix) Releasing RRC connection: The UE is released via a messages such as RRC Release that releases the RRC connection the UE has to one or more cells. This may also include the UE. In some examples, it is envisaged that the notion of ‘out-of-date’, ‘outdated’, ‘invalid’ or ‘not valid’ may be used. Similarly, the notion of ‘valid’ may also be used. This can be defined, for instance, based on how old the specific information or data value is, or how long time ago that the information was acquired. A timer may be associated with this specific information or data value, which governs whether it is out-of-date, outdated or invalid (or conversely whether it is valid). The length of this timer or duration may be configured by the UE, the network, or it may be determined for instance based on the scenario. A network may for instance allow or configure certain UEs, for instance stationary UEs serving a Fixed Wireless Access scenario, to always have its UE position be valid. A network may also change the method for determining whether the information or data value is valid or not. This could for instance mean that if UE is allowed or configured to determine whether the information or data value is out-of-date, outdated or invalid, then if the network detects issues, then the network may reconfigure the UE to only use a timing-related invalidity determination. The notion of data being valid or invalid may also be based on the accuracy of said data. For instance, if there is an accuracy value along with the estimation of for instance the UE or the network position, then this can be used to determine whether the UE or network position is valid or not. The validity or invalidity may also be determined by the UE based on any type of sensors or information, for instance using inertial navigation methods. Any other determination of the data being valid or invalid may potentially be used. In some examples, it is envisaged that the UE position mentioned below may be derived based on GNSS, which may be based on GPS, Galileo, Beidou or GLONASS. In some examples, it is envisaged that the UE position mentioned below may also be based on a 3 GPP-based position determination such as Downlink Angle of Departure (DL AoD) or Downlink Time Difference of Arrival (DL TDOA) or any other suitable technique to determine the UE position or an estimate of the UE position. Synchronization states In some examples, it is envisaged that in order to better combine the different methods and how these methods are applied for synchronizing to a cell, separate synchronization states are defined. In some examples, it is envisaged that the synchronization state may be related to a specific cell. In some examples, it is envisaged that this may be relevant in a carrier aggregation state where the UE is connected to multiple serving cells. The synchronization state for a specific cell may also imply that the synchronization state is for a cell that may also be a neighbouring cell. In some examples, it is envisaged that the synchronization state can be related to a specific cell group, or a specific RAT. In some examples, it is envisaged that this may be relevant in a case where the UE 710 needs to acquire the satellite assistance information of a cell in order to synchronize, which means that the synchronization status may always be cell-specific. This may be also relevant in a dual connectivity setting, or in an inter-RAT aggregation case. As an example, the UE 710 may be connected to two cell groups, one at a lower frequency, cell group A, and another at a higher frequency, cell group B. The UE 710 is synchronized with cell group A as it has accurate enough UE 710 position and NTN assistance information, but it may not yet be considered synchronized with cell group B as the requirements for synchronized state is higher. If the network supplies the UE 710 with more accurate NTN assistance information and potentially the UE 710 performing more accurate position to determine the UE 710 position, the UE 710 may become synchronized with cell group B. This can be seen in FIG. 9. Non-synchronized state In some examples, it is envisaged that the non-synchronized state can loosely be defined as a state where the UE 710 lacks enough synchronization to be in another state. In some examples, it is envisaged that a UE 710 may be defined to be in or to be moved to the non-synchronized state if one or more of the following are true: (i) No UE 710 position, or no accurate enough or valid UE position; (ii) No NTN assistance information, accurate enough or valid NTN assistance information (a) The NTN assistance information may comprise satellite position, or network node position or network node ephemeris. It may also comprise other related synchronization parameters such as common Timing advance (TA) and the epoch time. Here, if the network is not an NTN network, the information may comprise network node position or similar; (b) For instance, if the UE 710 does not have a long-term or short-term NTN assistance information available: • Short-term ephemeris may for instance be a PVT ephemeris, which is the Position Velocity Time ephemeris. This is a vector that indicates the position and the velocity; • Short-term ephemeris may be any type of short term orbital parameters; • Long-term ephemeris may be any type of long-term ephemeris consisting of a large amount of parameters, such as the TLE orbital parameter. This can be used to roughly estimate satellite positions for several days or even weeks. (c) If the UE 710 does not have an outdated ephemeris, which can be a long-term or short-term ephemeris. In some examples, it is envisaged that when a UE 710 is in a non-synchronized state it may not be allowed to do certain actions. These actions may be: (i) Connect or attempt to connect to a cell: a. In other words, this may mean that the UE 710 may not be allowed to perform random access to a cell. b. These cells may be cells for a specific scenario, such as NTN, ATG or special terrestrial cells, for example 5G Air-to-Ground (ATG) technology leverages 5G cellular infrastructure to provide high-speed, low-latency internet access to aircraft in flight, which allows for improved in-flight connectivity for passengers and enhanced communication capabilities for pilots and crew. 5G ATG networks are designed to offer wider coverage, higher speeds, and greater capacity than previous ATG systems, while also addressing potential interference issues. c. The UE 710 may not be allowed to enter RRC connected, but may be allowed to perform random access. d. It may also mean that the UE 710 may not be allowed to establish an RRC connection with a cell. e. It may not perform these actions if the UE 710 is not synchronized with a specific cell. (ii) Not allowed to transmit certain messages. (iii) Cannot establish carrier aggregation or Dual connectivity, or any form or setup where the UE is established to multiple cells or multiple nodes. For instance, any type of 5G-6G aggregation: 5G-6GDC or 5G-6GDual Stack. (iv) Perform certain cell selection and cell reselection operation In some examples, it is envisaged that these options may be configurable by the network. In other words, the network may configure what the UE 710 is allowed or is not allowed to do in a nonsynchronized state. For instance, if the UE 710 is not allowed to connect to a cell when in a nonsynchronized state. In some examples, it is envisaged that a UE 710 in a non-synchronized state is applicable to a certain RRC states. As an example, the UE 710 may only be non-synchronized in any RRC states where it is not directly connected, such as the RRC idle and RRC inactive - which implies that a UE 710 in non-synchronized state cannot be in RRC connected. Similarly, RRC idle and RRC inactive may imply that the UE 710 is in a non-synchronized state. As another example, the UE 710 in a non-synchronized state can only be in RRC idle, or the RRC idle state implies that the UE 710 is in a non-synchronized state. In some examples, it is envisaged that the UE 710 in a nonsynchronized state may also be in RRC connected. Semi-synchronized state In some examples, it is envisaged that the non-synchronized state may be loosely defined as a state where the UE 710 is not fully synchronized in order to perform full timing advance precompensation. In some examples, it is envisaged that a UE 710 may be defined to be in, or be moved to the semi-synchronized state if one or more of the following are true: (i) Whether UE 710 has no UE 710 position or not an accurate enough UE 710 position: (a) This can be a more accurate position compared to the non-synchronized state (b) The UE 710 may have access to an inaccurate UE 710 position, but may also have access to an accurate satellite position, network node position or network node ephemeris. (c) The UE 710 may have no UE 710 position at all (ii) Whether UE 710 has no NTN assistance information or accurate NTN assistance information: (a) The NTN assistance information may comprise satellite position, or network node position or network node ephemeris. It may also comprise other related synchronization parameters such as common TA and the epoch time. (b)For instance, if the UE 710 only has a long-term but not a short-term ephemeris available. (c)If the UE 710 does not have an outdated ephemeris, which can be a long-term or short-term ephemeris. (d)As an example, if the UE 710 has access to long-term ephemeris that is valid, but the short-term ephemeris expires, the UE 710 may be in semi-synchronized state. (e)If the UE 710 has access to a specific type of ephemeris, for instance any type of ephemeris that gives a set of orbital parameters. (iii) Whether the UE 710 has been configured or indicated by the network to enter the state. (iv) UE 710 having successfully performed random access without self-precompensating and have entered RRC connected. (v) The UE 710 is not actively performing self-precompensation, but potentially relying on closed loop timing precompensation by receiving Timing Advance commands. (vi) Have not entered the synchronized state during a very long time. This time may be configurable by the network. This can be useful if for instance the UE-acquired UE 710 position is not considered very accurate and there is a need to ensure that even if the UE 710 has acquired the UE 710 position, there is a need to verify that the UE 710 has accurate UE 710 position before entering a synchronized state. (vii) The position of the UE 710 has not been verified by the network. This can be useful as a safety precaution, especially if the UE 710 is not allowed to establish AS security. In some examples, this may mean that the UE 710 can be in the semi-synchronized state before the network indicates that the UE 710 position has been verified by the network. In this case the UE 710 may first indicate its UE 710 position to the network and stay in semi-synchronized state. When the UE 710 receives an indication that its position has been verified, then the UE 710 may for instance be allowed or be configured to enter the synchronized state. In some examples, it is envisaged that the combination of the above may embodied in the following examples: (i) UE 710 is considered in semi-synchronized state if UE 710 does not have access to an accurate UE position, but has access to an accurate NTN assistance information, e.g., accurate ephemeris along with TA common and epoch time; (ii) UE 710 is considered in semi-synchronized state if UE 710 does have access to accurate UE 710 position, but does not have an accurate enough NTN assistance information, but only outdated or long-term ephemeris; (iii) UE 710 is considered in semi-synchronized state if the UE 710 has successfully completed random access procedures and / or entered RRC connected without having access to either accurate or inaccurate UE 710 position, accurate or inaccurate NTN assistance information. In some examples, it is envisaged that when a UE 710 is in a semi-synchronized state it may not be allowed to perform certain actions. These actions may include: (i) When in RRC connected the UE 710 may not be allowed to transmit certain messages, or request certain procedures: Not allowed to establish AS security (ii)When in RRC connected the UE 710 may not establish carrier aggregation, Dual connectivity, multi-connectivity or any form or setup where the UE 710 is established to multiple cells or multiple nodes, for instance any type of 5G-6G aggregation: 5G-6G DC or 5G-6G Dual Stack. (iii) The UE 710 may not be allowed to initiate certain RRC procedures: a. For instance, the UE 710 may not be allowed to initiate RRC Resume. In other words, the condition for initiating RRC resume would be that the UE 710 is not in the semisynchronized state. b. Network may not be allowed to handover the UE 710 from one cell to another, or UE 710 may not be allowed to initiate a handover to another cell without indicating to the UE 710 to first synchronize. c. A UE 710 may not be allowed to perform neighbour cell measurements, or network may not be allowed to configure the UE 710 to perform neighbour cell measurements (iv) Not allowed to establish AS security: This can be useful if there is a need for the UEs location, or UEs coarse location to be established before any AS security is established. (v) Not allowed to transmit user plane data: This can be defined as the UE 710 is not allowed to establish Data Radio Bearers to carry user plane data In some examples, it is envisaged that if the UE 710 is not allowed to perform or initiate an action in semi-synchronized state, this may imply that the UE 710 is only allowed to initiate the action in a state where it is more stringently synchronized. For instance, the UE 710 is only allowed to perform or initiate the above-mentioned actions when the UE 710 is in a fully synchronized state. A UE 710 not being allowed to perform or initiate an action may also imply that the network is not allowed to initiate the procedure. For instance, the network may not be allowed to initiate a procedure to establish carrier aggregation or dual connectivity when the UE 710 is in a semisynchronized state. This can also be that the network ensures that the UE 710 is in a synchronized state before initiating carrier aggregation or dual connectivity. For instance, the network may ensure that the UE 710 is in a synchronized state by configuring or commanding the UE 710 to enter the synchronized state from semi-synchronized state. To ensure this, it may be required that the UE 710 reports or indicates its synchronization state, or the network may have to assume a certain state depending on the procedures. In some examples, it is envisaged that the UE may be allowed to perform or initiate the following actions: (i) Allowed to connect to an NTN cell. (ii) Allowed to initiate certain RRC procedures. For instance, when the UE is in a semisynchronized state the UE may be allowed to setup an RRC connection via RRC Setup, or reestablish the RRC connection via RRCRe-establishment. (iii) Allowed to transmit control plane data. It is envisaged that this can for instance be useful if a cell wants to allow cells to perform registration or tracking area updates to an NTN cell without having the need to fully synchronize. If there is a need to transmit user plane data, the UE may be required to move to fully synchronized state. (iv) Allowed to initiate emergency calls. (v) Establish AS security. (vi) Allowed to transmit at lower throughput or with lower amount of data. It is envisaged that the UE can be limited to lower Quality of Service (QoS) when in a semi-synchronized state. It is also envisaged that the UE can initiate procedures that aim to deliver a small amount of data. Such procedures can for instance be Small Data Transmission, Early Data transmission or Preconfigured Uplink. Here, a Small Data transmission is a 5G NR enhancement that allows a UE to transmit data from RRC inactive state. In some examples, it is envisaged that if the UE 710 is allowed to perform or initiate an action in semi-synchronized state, this may imply that the UE 710 is not allowed to initiate the action in a state where it is less than fully synchronized. For instance, the condition may be that the UE 710 is not allowed to perform or initiate the action when the UE 710 is in a non-synchronized state. The UE 710 being allowed to perform or initiate an action may also imply that the network is allowed to initiate the corresponding action. Furthermore, a semi-synchronized state may also imply certain RRC state. For instance, the UE 710 can only be in semi-synchronized state if the UE 710 is in RRC inactive and RRC connected. Synchronized state In some examples, it is envisaged that a UE 710 may be in the synchronized state if one or more of the following are true: (i) Accurate or not outdated UE 710 position: it is envisaged that this can be accurate up to some pre-defined threshold. (ii) Accurate or not outdated NTN assistance information: it is envisaged that this can be accurate or up-to-date up to some pre-defined threshold. (iii) A UE 710 is autonomously performing pre-compensation of the timing advance: it is envisaged that this can be, for instance, that the UE 710 is actively performing precompensation of the timing advance, for instance that the UE 710 pre-compensates the timing advance every X msec, for instance every 10 milliseconds. Alternatively or additionally, it is envisaged that this can for instance be that the UE 710 is performing pre-compensation on a specific cell or for a specific cell group or MAC entity. (iv) Having been signaled to be in a synchronized state, for instance by the network. (v) A UE 710 is in RRC connected state: it is envisaged that this could potentially imply that the UE 710 is not in a synchronized state if the UE 710 is not in an RRC connected state. In some examples, it is envisaged that a UE 710 may be allowed to perform or initiate the following actions: (i) Allowed to connect to an NTN cell. (ii) Allowed to initiate all RRC procedures. (iii) Allowed to transmit control plane and user plane data. (iv) Allowed access to any type of QoS. (v) Allowed to establish carrier aggregation, Dual connectivity, multi-connectivity or any form or setup where the UE 710 is established to multiple cells or multiple nodes, for instance any type of 5G-6G aggregation: 5G-6GDC or 5G-6GDual Stack. In some examples, it is envisaged that if the UE 710 is allowed to perform or initiate an action in a fully synchronized state, this may imply that the conditions for performing certain procedures is that the UE 710 is in the synchronized state. The UE 710 being allowed to perform or initiate an action may also imply that the network is allowed to initiate the procedure, or that the network ensures that the UE 710 is in a fully synchronized state before initiating the procedure. State transitions In some examples, it is envisaged that the state transition or the UE 710 having entered a certain state may be indicated by the UE 710 to the network. In some examples, it is envisaged that this may be indicated via an RRC message. Given that such an indication is likely only a short message, it is envisaged that it may also be indicated via a MAC-level indication such as MAC CE. If it is unclear whether the UE 710 has entered the semi-synchronized or the synchronized state when connecting via random access, the UE 710 may indicate its state via an indication, for instance via the MAC layer in a MAC CE. In some examples, it is envisaged that the indication of the state transition of the UE 710 having entered a certain state may include the new state and the previous state. An example of this indication may be as part of a MAC CE, which can be seen in Specification Example #1. In some alternative examples, this indication may also be included in a Msg3 or its related RRC message, for instance the RRCSetupRequest, RRCResumeRequest or RRCReestablishmentRequest. It may also be included in a Msg5 or its related RRC message, e.g., the Complete message, which can be RRCSetupComplete, RRCResumeComplete, RRCReestablishmentComplete or even RRCReconfigurationComplete. An example of this can be seen in Specification Example #2. Similarly, in some examples, it is envisaged that the network may signal to the UE 710 to enter a specific state. When the UE 710 is in connected mode, such signalling may comprise of RRC signalling, such as an RRC reconfiguration message. Thus, in some examples, it is envisaged that the network can send an RRC reconfiguration message that triggers the UE 710 to enter a specific state. Such signalling may also comprise MAC signalling, which in some examples it may be envisioned that the network sends a MAC CE that triggers the UE 710 to enter a specific state. In some examples, it is envisaged that the state transition may be on a per-UE basis, or it may be for a specific cell, specific cell group or main / secondary node. If the state transition is on a per cell or cell group basis, this means that the movement of the synchronization state can be largely independent per cell. In some examples, it is envisaged that it may be signalled in a handover command the expected synchronization state that the UE 710 shall be in before connecting to the target cell. This may mean that if the UE 710 is not currently in the indicated synchronization, the UE 710 may have to perform the related actions to enter the synchronization state with a target cell. If the synchronization state is indicated in a handover command to be a lower synchronization state than what the UE 710 can achieve, then the synchronization state in the target cell may be indicated once the UE 710 has connected. A UE 710 may always indicate synchronization state to a target cell after or during the handover, subject to target cell configuration. This is important to ensure that the UE 710 is synchronized with a target cell, else there is a risk that the UE 710 may be handed over to a target cell without clarity on how synchronization is to be achieved. For instance, if the target cell assumes that the synchronization will be achieved via self-precompensation of the timing advance (UE 710 may in other words be the in synchronized state), then it is important that a UE 710 that connects to the cell is in the correct state. Entering a synchronized state from other states In some examples, it is envisaged that the UE 710 may enter a fully synchronized state if the following has occurred: (i) The UE 710 has acquired and applied accurate NTN assistance information and / or UE 710 position: (a) The NTN assistance information may be acquired in a broadcasted fashion. This can be acquired via system information, or via secured system information. Here, as an example, the UE 710 may first acquire a less accurate NTN assistance information that is accurate enough to enter semi synchronized state, and then in RRCCONNECTED acquire a more accurate NTN assistance information that can allow the UE 710 to enter fully synchronized state. (b) The NTN assistance information may be acquired in a dedicated fashion. This can be requested by the UE 710 if the UE 710 wants to enter the fully synchronized state from semi-synchronized state. Here, there can be conditions when a UE 710 shall request to enter synchronized state / request for NTN assistance information. This condition may be that user plane data has arrived in the buffer, which requires that the UE 710 establishes DRBs and AS security. This condition may also be that a timer has expired whereby the UE has to request the NTN assistance information, for instance if the UE 710 has entered the semi-synchronized state that starts a timer. Alternatively, the UE 710 has been attempting to acquire a broadcasted NTN assistance information for a time, which may be configurable, but does not succeed, which triggers the UE 710 to request the NTN assistance information in a dedicated fashion, upon which the UE may for instance receive a dedicated NTN assistance sent by the network. This is less efficient compared to acquiring the broadcasted NTN assistance information, but at least ensures that the UE can get the required assistance information to enter the synchronized state. (c) The UE 710 may acquire a UE 710 position in connected mode, for instance via any positioning protocol such as LPP. These can for instance be: i. Network-assistance GNSS methods; ii. Observed Time Difference of Arrival (OTDOA) positioning; iii. Motion sensor positioning; iv. Multi-RTT (a multi-cell roundtrip time positioning method in 5G networks) positioning; v. Downlink Time Difference of Arrival (DL TDOA); vi. Downlink Angle of Departure (DL AoD). (d) When the UE 710 has for instance acquired one of the above, there may be one or more conditions that confirms whether the UE 710 has met all other conditions, and if all of these conditions are fulfilled, then the UE 710 may move to the fully synchronized state. (ii) The network has indicated that the UE 710 shall enter synchronized state (a) If for instance the network detects that the timing and frequency offset of the transmissions are sufficiently accurate, the network may indicate that the UE 710 is sufficiently accurate to enter synchronized state. (b) This can be indicated during the random access procedure, for instance in a Msg2 (Random access Response message), in a Msg4 message. For instance if the UE 710 receives a Message 4, which completes the random access procedure, the UE 710 may be configured to enter the synchronized state, for instance if all other conditions are fulfilled. It may be indicated in Msg2 or the Msg4 whether the UE 710 shall enter the synchronized state, which can be a flag in a MAC CE or an RRC message. If the UE 710 does not receive this response, the UE 710 may be configured to enter another synchronization state and operate accordingly. Here, if the UE 710 fails random access and is also indicated to enter another synchronization state other than the synchronization state that the UE 710 was attempting to perform random access with, when the UE 710 re-attempts random access, it may attempt random access with another method, such as without self-precompensating. (iii) If the UE 710 position is verified by the network (a) In this case the UE 710 may have sent its UE 710 position or UE 710 positioning measurements to the network in order for the network (may be the gNB, the AMF / MME, the LPP node or any other location service server) to verify the UE position. Once the UE receives an indication that the UE 710 position is verified, then the UE enters the synchronized state. (b) The indication that the UE 710 position is verified may be received from a gNB, from the core network via AMF, or via a positioning server via LPP, such as LMF or similar. (c) This may be one of other conditions, and whether this condition is required may be configured by the network. (d) This may add further assurances that a UE 710 will not cause interference on the network, e.g., by first verifying the UE position before it is used for self-precompensation, (e) The UE 710 may be configured to only be considered to have its UE 710 position verified by the network for a configured amount of time, and once the time has passed the UE 710 will no longer attempt to self-precompensate or consider itself to be in the synchronized state. In some examples, it is envisaged that the network may indicate that the UE 710 shall enter the synchronized state. This may then require the UE 710 to perform appropriate actions to enter the synchronized state, such as acquiring the UE 710 location, in order to acquire the satellite ephemeris or any other associated assistance information. Depending on the missing parts to ensure that the UE 710 can fulfil the conditions to be in synchronized state, the UE 710 may only acquire UE 710 position and may only acquire satellite position or other assistance information. It is envisaged that this can, for instance, be performed via an RRC reconfiguration message. If this RRC reconfiguration message for instance is received in a semi-synchronized state, the UE 710 may perform the appropriate actions in order to enter the synchronized state. If the RRC reconfiguration message is used, once the UE 710 has completed its actions, such as acquiring the UE 710 location, acquiring satellite position and other associated assistance information, the UE 710 may reply with an RRCReconfigurationComplete to indicate success. This indicates to the network that the UE 710 is now in a fully synchronized state. This can be seen in FIG. 9. In some examples, it is envisaged that the UE 710 may perform random access when entering a synchronized state from a semi-synchronized state. It is envisaged that this can be a connected mode random access. If the UE 710 is indicated by the network to enter the synchronized state, then the UE 710 may be configured to perform random access. This can for instance be useful in order for the UE 710 to re-synchronize if the UE 710 is self-precompensating for the Timing Advance. The UE 710 may for instance be configured with specific random access resources, such as for instance a contention-free random access preamble in this transition. The network may also trigger a random access to occur if the UE 710 sends a control message, such as a PDCCH that the UE 710 shall perform random access. Enter a semi-synchronized state In some examples, it is envisaged that the UE 710 may enter semi-synchronized state from a non-synchronized state: (i) If the UE has successfully performed random access, a further condition may be that UE has successfully performed random access and has not been rejected. The random access type that has been performed may be a specific type of random access that is specific for the case of non-synchronized UE, for instance non-GNSS NTN random access type. a. During or after successful random access procedure (a) The UE may be indicated during the random access procedure to have entered semi-synchronized state. For instance if the UE successfully receives a Msg2 (Random Access Response message) after having transmitted Msgl (the RACH preamble). This may also be indicated in the Msg2 message that the UE shall enter the semi-synchronized state. (b) The Random Access Response may thus contain a field indicating what synchronization state that the UE should assume, either during the random access procedure, after the random access procedure or during following random access attempts. (c) This can be considered a fallback indicator, or the indication may be part of a fallback indication. b. Successfully entered RRC connected (a) Semi-synchronized may be implied by the UE entering RRC connected, or it may be started once the UE enters RRC connected. If the UE is considered semi-synchronized after having entered RRC connected and then is released or the UE leaves to RRC idle or RRC inactive, the UE may or may not still be considered to be in semi-synchronized state. In particular if semi-synchronized has to do with the state being configured with a timer, then when released the UE may still be in semi-synchronized after leaving RRC connected. This can be seen in FIG. 11. c. The UE may be considered to have entered the semi-synchronized state when the UE has acquired: (a) UE position, which may be accurate or outdated. This can be accurate up to some pre-defined threshold. This threshold may be less accurate than the threshold for accuracy for the synchronized state. (b) NTN assistance information, which may be accurate or outdated. This can be accurate or up-to-date up to some pre-defined threshold. This may be a long-term ephemeris or long term NTN assistance information. Referring now to FIG. 11, FIG. 11 illustrates one example of a message sequence chart 1100 whereby a UE 710 is considered semi-synchronized after having entered RRC connected and then is released and a UE 710 is considered to still be in a semi-synchronized state after leaving RRC connected, in accordance with some examples. In this example, the network includes a NTN gNB 812 supporting communications in a cell. At 1140, the UE 710 is in an RRC idle and non-synchronized state. At 1142, the UE 710 receives a random access and RRC establishment message, e.g., a command to enter a semi-synchronized state. At 1144, the UE 710 enters a RRC connected and semi-synchronized state. At 1146, the UE 710 starts a semi-synchronized state timer and at 1148, the NTN gNB 812 releases the UE 710. At 1150, the UE 710 leaves the RRC idle or RRC inactive state. In one example, it is envisaged that the UE 710 may enter semi-synchronized from synchronized state, in one of the following scenarios: (a) Radio link failure related: a. Radio Link Failures are triggered during certain failures when the UE 710 is in RRC connected mode. b. If the timers related to radio link failures start, for instance if timer T310 start, the UE 710 will move to a semi-synchronized state. Here, a T310 is a timer that starts upon detection of a number of consecutive “out-of-sync” indication, in other words that the UE 710 detects the downlink being out of sync by detecting the PDCCH signal strength or signal quality being lower than a threshold. When the T310 timer expires the UE 710 a radio link failure is triggered. Thus, the UE 710 may be moved to semi-synchronized before the radio link failure occurs as the start of the T310 timer may indicate issues occurring that may persist even if the timer T310 is stopped due to regaining synchronization by detecting a number of in-sync indications. (ii) If either NTN assistance information and / or UE 710 position is no longer accurate or out-of-date. (iii) If certain procedures are triggered: (a) If the re-establishment of an RRC connection is triggered. The RRC re-establishment procedure by the UE 710 may be triggered or initiated if there is either a RLF declared, if there is a handover failure or if there is a failure to comply with a message to configure the UE 710. (b) If a handover failure occurs. (iv) During or after a handover from another cell has occurred, or completed: (a) This can for instance be useful if it is likely that the UE 710 will not be well-synchronized after having performed a handover to the cell from another cell. (b) When a handover command indicating handover to an NTN cell is received, the UE 710 may initialize its synchronization status to semi-synchronized just before the handover command occurs. This can for instance be a RRCReconfiguration message or similar. (v) If UE 710 moves to certain RRC states: (a) If UE 710 moves from RRC connected to RRC idle or from RRC connected to RRC inactive. (b) If the UE 710 is released or re-directed from a cell to either RRC idle or RRC inactive and still has accurate and up-to-date NTN assistance information and UE 710 position. This can for instance be indicated in the release message. Such indication may be that the UE 710 shall remain in the semi-synchronized state. In one example, it is envisaged that the network may configure whether (or not) the semisynchronized state should be allowed to be used. In one example, it is envisaged that this may also be defined as the network indicating whether (or not) random access without self-precompensating or connecting to the cell without self-precompensating is allowed. If the semisynchronized state is not allowed, this means that some of the state transitions that would normally make the UE 710 transition from a fully synchronized to a semi-synchronized state would change to the UE 710 transitioning directly to a non-synchronized state. Similarly, for the state transitions from a semi-synchronized state to a fully synchronized state, the UE 710 in a non-synchronized state may have to perform methods to go directly to a synchronized state. It is envisaged that such a configuration may be configured in a broadcasted configuration. This configuration may for instance be associated with a specific use cases, such as for emergency purposes. In one example, it is envisaged that the semi-synchronized state may be considered a ‘temporary state’ in which the UE 710 should attempt to move to synchronized state. For instance, when the UE 710 enters the semi-synchronized state from the non-synchronized state, the UE 710 may start a related timer. During this time the UE 710 may be required to attempt to perform actions in order to enter the fully synchronized state. If the UE 710 fails, e.g., the timer expires, the UE 710 may be required to enter a non-synchronized state, which may for instance trigger the UE 710 to enter a RRC idle mode. As an example of the above procedure, the UE performs random access from a non-synchronized state to enter a semi-synchronized state. This starts a configurable timer where the UE 710 has to attempt to enter the synchronized state. Enter a non-synchronized state In some examples, it is envisaged that the UE 710 may enter a non-synchronized state from a fully synchronized or a semi-synchronized state due to any of the following: (i) If a radio link failure (RLF) occurs: i.Certain types of radio link failures - these can for instance be for more “serious” radio link failures compared to entering semi-synchronized state; ii.If a radio link failure occurs and the UE does not have an NTN assistance information or UE position that is accurate or out-of-date. (ii) If either NTN assistance information and / or UE position is no longer accurate or out of date. In this example, the UE 710 may enter the non-synchronized state if both NTN assistance information and the UE position is no longer accurate or out-of-date. For instance a UE 710 may enter non-synchronized state from synchronized state if both NTN assistance information and / or UE position is no longer accurate or out-of-date. Part of this can be seen in FIG. 12. (iii) If UE moves to certain RRC states: (a) If UE 710 moves from RRC connected to RRC idle, from RRC connected to RRC inactive or RRC inactive to RRC idle; (b) If the UE 710 is released or re-directed from a cell to either RRC idle or RRC inactive. This can, for instance, be indicated in the release message and can apply when moving from both semi-synchronized and synchronized state. In other examples, it is envisaged that the UE 710 may enter a non-synchronized state from semi-synchronized state: (i) If there is a radio link failure (RLF) or any other type of radio-related failures: (a) These can be specific radio link failures, such as detecting that the radio link signal is weak, or not being able to transmit in the uplink, or losing downlink synchronization. (b) This may only be relevant when the UE is in a connected state. In other examples, it is envisaged that if the semi-synchronized state is associated with a timer, e.g., the semi-synchronized state is temporary, and the timer expires, then the UE 710 may move to the non-synchronized state. Referring now to FIG. 12, FIG. 12 illustrates a simplified flowchart 1200 of a UE entering a nonsynchronized state from a synchronized or semi-synchronized state, in accordance with some example embodiments. In this example, the UE is in an RRC connected and fully synchronized state at 1210. The UE stays in this state until one of either the UE location or the NTN assistance information is out of date. At this point, the UE may enter non-synchronized state at 1230 from a semi-synchronized state at 1220 (or from no synchronization state). In one example, the UE may have recently selected a specific network or “sub”-network, e.g., the PLMN, a tracking area, a frequency or a new RAT. In some cases if the UE, for instance via idle or inactive mode operation, selects a type of network such as a new PLMN or a new tracking area or a new frequency, it can be assumed that the network does not have any knowledge of the ephemeris of the specific location. Alternatively, if a prior knowledge exists, it can be assumed that the information is outdated. In one example, when the UE performs PLMN selection, e.g., selecting between different PLMNs, the UE can be considered to be in or to enter a non-synchronized state. In one example, the UE may have recently selected or performed mobility to an NTN cell, frequency or PLMN. As an example, if the UE selects a non-terrestrial network after camping having camped on a terrestrial network, the UE can be considered to be in a non-synchronized state. This can for instance be during the cell reselection procedures. As another example, if the synchronization is specific to a specific cell, then when a UE re-selects a cell, for instance via a cell reselection procedure, then the UE enters non-synchronized state. In this procedure, the UE first camps on a specific cell and maintains a semi-synchronized state with this cell. Then the UE performs the cell re-selection algorithm where it compares the signal qualities and the priorities of cells and then reselects to another NTN cell. At this stage the UE changes the synchronization state to non-synchronized. In one example if the UE may select a terrestrial network, for example if the UE had previously been connected to a non-terrestrial network. Here, there may be a need to reset the synchronization status when connecting to a terrestrial network. In one example, it is envisaged that the synchronization states and the associated parts may only be applicable in a non-terrestrial network (satellite). In another example, it is envisaged that the synchronization states and the associated invention may be applicable to terrestrial networks with elevated platforms (such as air-to-ground and HAPS). An example of terrestrial networks where this may be relevant would be in networks where the coverage is very large, such as cells mounted on very tall towers. In these cases, where the distance to the UE may be very large, the synchronization methods in this invention may be highly useful. In another example, it is envisaged that if a UE does not support a certain state that the network has configured, such a cell may be considered to be barred for access for a UE. For instance if the network only supports a UE to be fully synchronized, which may mean or imply that the network does not support the semi-synchronized state, to be able to connect to the cell, and the UE does not support such capabilities, the UE may be barred from accessing, camping or reselecting to such a cell. The UE may also be barred from accessing the cell, while not barred from camping on the cell in such a case. As such, the UE may have to first acquire the required data (UE position, network node position, system information such as NTN assistance information) in order to fully synchronize to a cell before attempting access. During this time the UE may be considered barred from accessing a cell. This is only a temporary barring until the UE has acquired all of the required conditions to enter fully synchronized state. In another envisaged example, the network may indicate that it does not support the fully synchronized state. This may mean that the UE does not need to self-precompensate the timing advance, or perform any of the other actions or fulfill any of the related requirements to enter the synchronized state. This could be, for instance, implied if the network indicates that it supports the semi-synchronized state. This could be useful if the network may not want to rely on its UEs self-precompensating, or if the network knows that it may be difficult to acquire the UE location. In another envisaged example, if the UE does not support the semi-synchronized state, and then network does not support the synchronized state, then the UE may be considered barred from accessing the cell. A network, which may be a non-terrestrial network, may be required to indicate what synchronization states (or modes) that it supports. In another envisaged example, the UE may indicate its capability to support different types of states or its related methods. For instance, the UE can indicate its capability for self-precompensation, or its capability for the synchronized state. The UE may indicate its capability for the semi-synchronized state, e.g., to be able to enter connected mode without self-precompensation. This capability may have several types of differentiation. In one aspect of the invention, the UE indicates its support for the different synchronization states or its related methods, for different frequencies, or different types of frequency bands or frequency ranges. For instance the UE may support self-precompensation, which may be the synchronized state, in Frequency Range 1 (FR1). The UE may in this example support FR2, where it may support the semi-synchronized, which may in other words be that the UE is able to enter connected mode without self-precompensating. Examples of synchronization procedures The following message sequence charts provide some example procedures. Referring now to FIG. 13, a simplified message sequence chart 1300 of a UE 710 communicating with a NTN gNB 812 and entering a synchronized state from a semi-synchronized state, is illustrated in accordance with a first example embodiment. At 1330, the UE 710 is in an RRC idle (or equivalent) state, and is in non-synchronized state with NTN gNB 812. The cell supported by NTN gNB 812 indicates the access method that is allowed, e.g., in this scenario, a random access without self-precompensating to the cell is allowed. At 1350, in a first operation, the UE 710 successfully performs random access with NTN gNB 812 without self-precompensating. A second operation starts at 1360 and the UE 710 enters a RRC connected and a semi-synchronized state at 1362. At 1364, the network maintains synchronization via closed loop methods, e.g., the network (NTN gNB 812) uses closed-loop methods to pre-compensate for the timing advance (TA), though Timing Advance commands or any other closed-loop methods. The UE 710 can now perform control plane set up procedures at 1366, such as registration, capability exchange, security setup, etc. A third operation starts at 1370 and the UE 710 acquires the satellite position and its UE position. The UE 710 may for instance acquire the satellite position at 1372, say, via dedicated ephemeris sent by the network, or broadcasted ephemeris. The UE 710 can acquire its position via positioning methods, for instance via GNSS or network-based methods at 1374. In some examples, this can be done via any type of positioning services such as LPP or other protocols. A fourth operation starts at 1380 and the UE 710 enters synchronized state at 1382 and starts to autonomously (self) pre-compensate the timing advance at 1384. Here, in one example, the UE 710 indicates to the network that it has entered a fully synchronized state and signals this to the NTN gNB 812, where upon the NTN gNB 812 stops utilizing closed-loop methods. The network (e.g., the NTN gNB 812) may now start transmitting data or may configure the UE 710 with carrier aggregation or dual connectivity, where the secondary cell or cell group is on a higher frequency. Similarly, in a second example of FIG. 13, where the UE may enter a semi-synchronized state at 1362 for an emergency call, the UE 710 is in an RRC idle (or equivalent) state, and is in nonsynchronized state with NTN gNB 812 at 1330. In this second example, the UE 710 again successfully performs random access without self-precompensating and enters a RRC connected state and is moved to a semi-synchronized state at 1362. Again, at 1364, the NTN gNB 812 may use closed-loop methods to pre-compensate for the timing advance and at 1366 the UE 710 establishes the necessary procedures for emergency calls and performs the emergency call. The UE 710 may then be released after the emergency call. Similarly, in a third example of FIG. 13, where the UE may be barred due to requirements of its synchronized state. Again, at 1330, the UE 710 is in an RRC idle (or equivalent) state, and is in non-synchronized state with NTN gNB 812. In this example, the UE 710 may have recently selected the cell from camping on another NTN cell, or may have recently selected the cell when camping on a terrestrial cell. At 1350, in this example, the UE 710 may acquire the system information that indicates that random access without self-compensating is not allowed to the cell. This may, for instance in this example, mean that the semi-synchronized state is not allowed in the cell. Thus, the cell is considered to be barred for the UE 710. In this scenario, the UE 710 acquires the required UE position and NTN assistance information at 1370. Following acquiring this information, the cell is no longer considered barred for the UE 710. The UE 710 is then able to self-precompensate and perform random access to the cell at 1350. If successful the UE enters RRC connected and synchronized state. Similarly, in a fourth example of FIG. 13, where the UE may enter a semi-synchronized state due to a radio link failure the UE 710 may be in a RRC connected and is in synchronized state and thus performs self-precompensation and has access to both UE location and NTN assistance information at 1384. Following this, a radio link failure may be triggered, which causes the UE 710 to enter semi-synchronized state at 1362. Thus, the UE 710 performs the Radio link failure and RRC re-establishment procedure at 1350, which causes the UE 710 to perform cell selection. The UE 710 selects the same cell and RRC Re-establishment is triggered. The UE 710 performs RRC re-establishment to the cell in semi-synchronized state. The UE 710 subsequently acquires the necessary information and the conditions to re-enter the synchronized state at 1384. Referring now to FIG. 14, a simplified message sequence chart 1400 of a verification of a position of a UE 710 communicating with a NTN gNB 812 and a Core Network element or a positioning server 1410, is illustrated in accordance with some example embodiments. At 1430, the UE 710 is in an RRC idle (or equivalent) state, and is in non-synchronized state with NTN gNB 812. The cell supported by NTN gNB 812 may indicate the random access method that is allowed. At 1440, in a first operation, the UE 710 successfully performs random access with NTN gNB 812 RRC establishment and without self-precompensating and enters a RRC connected and semi-synchronized state at 1452. With this, the network (NTN gNB 812) uses closed-loop methods to pre-compensate for the timing advance, though Timing Advance commands at 1454 or any other closed-loop methods and can utilise set up procedures at 1456. At 1460, the UE 710 sends the UE location to the network (NTN gNB 812) or the UE 710 sends the UE location to the Core Network element or positioning server 1410 at 1464 or the NTN gNB 812 forwards the UE location to the Core Network element or positioning server 1410 at 1462. At 1470, the network (NTN gNB 812 or Core Network element or positioning server 1410) verifies the UE location, which may be, for instance, verified by a core network element performing or requiring another network element to perform its own measurements of UE signals and computing the UE location. A UE location may also be verified by requiring a UE to perform some measurements that are reported, which is then used to compute the UE location. In some examples, the network (e.g., NTN gNB 812) may indicate to the UE 710 that the UE 710 position has been verified, or the Core Network or positioning server 1410 indicates to the UE 710 via the NTN gNB 812 that the UE 710 position has been verified. At 1490, the UE 710 enters the synchronized state (if the UE 710 has also acquired the network node position, for instance via NTN assistance information) and starts self-precompensating. Referring now to FIG. 15, a simplified message sequence chart 1500 illustrates a synchronization state during a handover of a UE 710 from a source gNB 1520 to a target gNB 1522, in accordance with some example embodiments. At 1540, the UE 710 is in a RRC connected and a semi-synchronized state (with the source gNB 1520). At 1542, the UE 710 receives a handover command from the source cell (which may have partly been generated at the target cell). The handover command indicates that the required synchronization state in the target cell (supported by the target gNB 1522) is fully synchronized. At 1544, the UE 710 then performs one or more related actions in order to enter the fully synchronized state with the target gNB 1522. This may encompass performing a UE position acquisition, or performing a more precise UE position acquisition, etc. At 1546, 1548 the UE 710 performs handover to the target cell / target gNB 1522 and once completed the UE 710 enters fully synchronized state with the target gNB 1522. At 1550, the UE 710 optionally indicates the synchronization state to the target gNB 1522. Specific examples are now described with regard to a 5G implementation. -------------------- 38.321 Example-------------------- In this first example, a specification example based on 38.321 MAC can be seen. In this example a MAC CE with two fields are defined. One field indicates previous synchronization state and one field indicates the current synchronization state. 6.1.3.10 Synchronization state MAC CE 1600, as illustrated in FIG. 16, in accordance with some example embodiments. The Synchronization state MAC CE 1600 of one octet is identified by a MAC subheader with LCID as specified in Table X. It has a fixed size and consist of a single octet. The Synchronization state MAC CE is defined, for a UE or a MAC entity. P: This first P field 1614 indicates the previous synchronization state. The value 0 indicates that the previous synchronization state is non-synchronized state, the value 1 indicates that the previous synchronization state is semi-synchronized state and the value 2 indicates that the previous synchronization state is synchronized state. - C: This second C field 1612 indicates the current synchronization state. The value 0 indicates that the current synchronization state is non-synchronized state, the value 1 indicates that the current synchronization state is semi-synchronized state and the value 2 indicates that the current synchronization state is synchronized state. R: Reserved bit 1610, set to 0. -------------------- 38.331 Example based on V18.2.0-------------------- In this second example, a specification example based on 38.331 RRC can be seen. In this example, a synchronization state field is introduced in RRCResumeRequest, which indicates whether UE is in synchronized or semi-synchronized state. RRCResumeRequest message jsssssssssSORSsOSRSOSSSsssssssssssssjsOO llllllllllllllllllllllll J Sisi: i: i: i iiiiiiiiiO&O^O&OO^i^i^i^i^i^i^i^i^i^i^i^i^i^i^i^i^i^i^i^i^i^i^^ iiiiiiiiiiiiiiiiiii^^i^Il^^^OiBiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii^ :i:i:i:i:i:i:i:i:i:^^Sii4i^S:Ci^ySi^:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:SsSy^ :i:i:i:i:i:i;i:i:i:iSyriClhiSSiWii0i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i:i^ OsOOstSb RRCResumeRequest-IEs field descriptions resumeCause Provides the resume cause for the RRC connection resume request as provided by the upper layers or RRC. The network is not expected to reject an RRCResumeRequest due to unknown cause value being used by the UE. resumeidentity UE identity to facilitate UE context retrieval at gNB. resum eMAC-l Authentication token to facilitate UE authentication at gNB. The 16 least significant bits of the MAC-I calculated using the AS security configuration as specified in 5.3.13.3. synchstate Indicates the synchronization state of the UE. -------------------- 38.331 Example based on V18.2.0-------------------- In particular, it is envisaged that the aforementioned inventive concept can be applied by a semiconductor manufacturer to any integrated circuit comprising a signal processor configured to perform any of the aforementioned operations. Furthermore, the inventive concept can be applied to any circuit that is able to configure, process, encode and / or decode signals for wireless distribution. It is further envisaged that, for example, a semiconductor manufacturer may employ the inventive concept in a design of a stand-alone device, such as a digital signal processor, or application-specific integrated circuit (ASIC) and / or any other sub-system element. It will be appreciated that, for clarity purposes, the above description has described example embodiments with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors, for example with respect to the signal processor may be used without detracting from the concepts described herein. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controller. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality, rather than indicative of a strict logical or physical structure or organization. Aspects may be implemented in any suitable form including hardware, software, firmware or any combination of these. Example embodiments may optionally be implemented, at least partly, as computer software running on one or more data processors and / or digital signal processors or configurable module components such as FPGA devices. Thus, the elements and components of an 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. Although the concepts have been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in other examples. In the claims, the term ‘comprising’ does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by, for example, a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and / or advantageous. Also, the inclusion of a feature in one category of claims does not imply a limitation to this category, but rather indicates that the feature is equally applicable to other claim categories, as appropriate. Thus, examples have been described that provide improved synchronization of communication units (such as UEs) with terrestrial base stations or base stations such as NTN airborne gNBs, eNBs, to NTN base stations, such as satellite base stations. In accordance with examples herein described, a number of approaches are provided to enable the network to request or ensure that the UE performs measurements on terrestrial frequencies if indicated to do so, wherein the aforementioned disadvantages with prior art arrangements have been substantially alleviated. Abbreviations / Definitions In the present disclosure, the following acronyms / definitions are used. 3 GPP 6G 5G 5GC 5GS 3rd Generation Partnership Project 6th Generation 5th Generation 5G Core 5G System ACK AM Acknowledge Acknowledged Mode AMF Access and Mobility management Function AS Access Stratum 5 BL Bandwidth-reduced Low-complexity CA Carrier Aggregation CCCH Common Control Channel CDMA Code Division Multiple Access CE Coverage Enhancement 10 CIoT Cellular loT CN Core Network C-RNU Cell RNTI CS Circuit Switched DC Dual Connectivity 15 DCCH Dedicated Control Channel DRB Data Radio Bearer EDGE Enhanced Data rates for Global Evolution EDT Early Data Transmission eMTC enhanced Machine Type Communication 20 EN E-UTRAN NR eNB Base Station EPC Evolved Packet Core EPS Evolved Packet System E-UTRA Evolved Universal Terrestrial Radio Access 25 E-UTRAN Evolved Universal Terrestrial Radio Access Network GEO Geosynchronous Equatorial Orbit GERAN GSM EDGE Radio Access Network gNB 5G Base Station GNSS Global Navigation Satellite System 30 GSM Groupe Special Mobile HAPS High Altitude Platform Station HARQ Hybrid Automatic Repeat Request ID Identity / Identifi cation IE Information Element 35 loT Internet of Things LEO Lower Earth Orbit LTE Long Term Evolution LTE-M LTE Machine Type Communication MAC MCG Medium Access Control Master Cell Group MEO Medium Earth Orbit MME Mobility Management Entity 5 NAS Non Access Stratum NB Narrow Band BS Base Station NG Next Generation NR New Radio 10 NTN Non-Terrestrial Network PCell Primary Cell PDCP Packet Data Convergence Protocol PDU Protocol Data Unit PSCell Primary and Secondary Cells 15 RAN Radio Access Network RAT Radio Access Technology RB Radio Bearer RLC Radio Link Control RLF Radio Link Failure 20 RNTI Radio Network Temporary Identifier ROHC Robust Header Compression RRC Radio Resource Control SI Interface between RAN and CN SAP Service Access Point 25 SCG Secondary Cell Group SIB System Information Block SRB Signalling Radio Bearer S-TMSI Short TMSI TAU Tracking Area Update 30 TM Transparent Mode TMSI Temporary Mobile Subscriber Identity TN Terrestrial Network TS Technical Specification Txxx Timer xxx 35 UE User Equipment UP User Plane X2 / Xn Interface between RAN nodes.

Claims

1. A wireless communication unit (710) having communications supported by at least one serving communication unit (720), wherein the wireless communication unit (710) comprises:a transceiver; anda processor, operably coupled to the transceiver and arranged to:identify that the wireless communication unit (710) is in an unsynchronized state (610) with the at least one serving communication unit (720); and in response theretoperform a procedure that transitions the wireless communication unit (710) from the unsynchronized state (610) to a semi-synchronized state (620) with the at least one serving communication unit (720); andperform a subsequent procedure that transitions the wireless communication unit (710) from the semi-synchronized state (620) to a fully synchronized state (630) with the at least one serving communication unit (720).

2. The wireless communication unit (710) of Claim 1 wherein the processor being arranged to identify that the wireless communication unit (710) is in the unsynchronized state (610) with the at least one serving communication unit (720) comprises the processor being arranged to identify at least one of:the wireless communication unit (710) does not have access to a geographical location of the wireless communication unit (710);the wireless communication unit (710) has access to an inaccurate geographical location of the wireless communication unit (710);the wireless communication unit (710) does not have access to a spatial or geographical location of the at least one serving communication unit (720);the wireless communication unit (710) has access to an inaccurate spatial location or geographical location of the at least one serving communication unit (720).

3. The wireless communication unit (710) of Claim 1 or Claim 2, wherein, in response to the processor identifying that the wireless communication unit (710) is in the unsynchronized state(610), the processor is arranged to perform at least one of the following:prevent the wireless communication unit (710) from performing random access to connect to a communication cell;enter a radio resource control, RRC, connected mode that allows the wireless communication unit (710) to perform random access to connect to a communication cell;prevent the wireless communication unit (710) from establishing an RRC connection with a communication cell;prevent the wireless communication unit (710) from establishing carrier aggregation to multiple cells or multiple nodes;prevent the wireless communication unit (710) from establishing Dual Connectivity, DC to multiple cells or multiple nodes;prevent the wireless communication unit (710) from performing a cell selection or cell reselection operation.

4. The wireless communication unit (710) of Claim 3, wherein, in response to the processor identifying that the wireless communication unit (710) is in the unsynchronized state (610), the processor is arranged to perform at least one of the following:prevent the wireless communication unit (710) from establishing a fifth generation-sixth generation, 5G-6G carrier aggregation to multiple cells or multiple nodes;prevent the wireless communication unit (710) from establishing a fifth generation-sixth generation, 5G-6G DC to multiple cells or multiple nodes;prevent the wireless communication unit (710) from establishing a fifth generation-sixth generation, 5G-6G, Dual Stack.

5. The wireless communication unit (710) of Claim 3, wherein, in response to the processor identifying that the wireless communication unit (710) is in the unsynchronized state (610), the processor is arranged to perform at least one of the following:prevent the wireless communication unit (710) from performing random access to connect to a communication cell; or enter a radio resource control, RRC, connected mode that allows thewireless communication unit (710) to perform random access to connect to a communication cell, wherein the communication cell is one of a Non-Terrestrial Network, NTN, communication cell, an air-to-ground, ATG, communication cell.

6. The wireless communication unit (710) of any preceding Claim, wherein in response to the processor identifying that the wireless communication unit (710) is in the unsynchronized state, the processor is arranged to perform a random access procedure that transitions the wireless communication unit (710) from the unsynchronized state to a semi-synchronized state whereby a successful random access procedure synchronizes the wireless communication unit (710) to a communication cell supported by the at least one serving communication unit (720).

7. The wireless communication unit (710) of any of preceding Claims 1 to 5, wherein in response to the processor identifying that the wireless communication unit (710) is in the unsynchronized state, the processor is arranged to establish a radio resource control, RRC, connection with the at least one serving communication unit (720) or re-establish a radio resource control, RRC, connection with the at least one serving communication unit (720) in response to a change of RRC state of the wireless communication unit (710), that transitions the wireless communication unit (710) from the unsynchronized state to a semi-synchronized state whereby a successful RRC connection synchronizes the wireless communication unit (710) to a communication cell supported by the at least one serving communication unit (720).

8. The wireless communication unit (710) of Claim 1 or Claim 2, wherein, in response to the processor identifying that the wireless communication unit (710) is in the semi-synchronized state (620) and operating in a radio resource control, RRC, connected mode, the processor is arranged to perform at least one of the following:prevent the wireless communication unit (710) from establishing access stratum, AS, security;prevent the wireless communication unit (710) from establishing data radio bearers to carry user plane data;prevent the wireless communication unit (710) from establishing carrier aggregation to multiple cells or multiple nodes;prevent the wireless communication unit (710) from establishing Dual Connectivity, DC to multiple cells or multiple nodes;prevent the wireless communication unit (710) from performing a cell selection or cell reselection operation.

9. The wireless communication unit (710) of Claim 8, wherein, in response to the processor identifying that the wireless communication unit (710) is in the semi-synchronized state (620), the processor is arranged to perform at least one of the following:prevent the wireless communication unit (710) from establishing a fifth generation-sixth generation, 5G-6G carrier aggregation to multiple cells or multiple nodes;prevent the wireless communication unit (710) from establishing a fifth generation-sixth generation, 5G-6G DC to multiple cells or multiple nodes;prevent the wireless communication unit (710) from establishing a fifth generation-sixth generation, 5G-6G, Dual Stack.

10. The wireless communication unit (710) of Claim 1 or Claim 2, wherein, in response to the processor identifying that the wireless communication unit (710) is in the semi-synchronized state (620) and operating in a radio resource control, RRC, connected mode, the processor is arranged to perform at least one of the following:prevent the wireless communication unit (710) from initiating a RRC Resume procedure;prevent the wireless communication unit (710) from initiating a handover to a second communication cell without first synchronizing with a first communication cell;prevent the wireless communication unit (710) from performing neighbour cell measurements.

11. The wireless communication unit (710) of Claim 1 or Claim 2, wherein in response to the processor identifying that the wireless communication unit (710) is in the semi-synchronizedstate and has acquired and applied at least one of: assistance information, a geographical or spatial location of the wireless communication unit (710), the processor is arranged to transition the wireless communication unit (710), or receives an instruction from the at least one serving communication unit (720) to transition, from the semi-synchronized state to a fully synchronized state.

12. The wireless communication unit (710) of Claim 11, wherein the processor is arranged to acquire assistance information from at least one of the following:via system information or secured system information broadcast from the at least one serving communication unit (720);first acquire a less accurate assistance information that is accurate enough to enter semisynchronized state, and then the processor transitions to an RRCCONNECTED mode and acquires more accurate assistance information that enables the wireless communication unit (710) to enter the fully synchronized state;a dedicated transmission of assistance sent by the at least one serving communication unit (720).

13. The wireless communication unit (710) of Claim 11, wherein the receiver and the processorare arranged to acquire the geographical or spatial location of the wireless communication unit (710) when in an RRC CONNECTED mode from at least one of the following:Network-assistance Global Navigation Satellite System, GNSS, information;Observed Time Difference of Arrival, OTDOA, positioning information;Motion sensor positioning information;Multi-cell roundtrip time, Multi-RTT, positioning information;Downlink Time Difference of Arrival, DL TDOA information; Downlink Angle of Departure, DL AoD information.

14. The wireless communication unit (710) of Claim 1 or Claim 2, wherein, in response to the processor identifying that the wireless communication unit (710) is in the fully synchronized state (630), the processor is arranged to enter into a semi-synchronized state in response to atleast one of the following:identification of a failure-related condition of the wireless communication unit (710);detection of one of: a downlink channel of the wireless communication unit (710) being out of synchronization, a physical downlink control channel, PDCCH, signal strength or signal quality being lower than a threshold;identification of at least one of:assistance information of at least one serving communication unit no longer being accurate, a wireless communication unit (710) position no longer being accurate;a trigger of a re-establishment of an RRC connection;prevent the wireless communication unit (710) from establishing Dual Connectivity, DC to multiple cells or multiple nodes;prevent the wireless communication unit (710) from performing a cell selection or cell reselection operation.during or after a handover from another cell has been performed,a transition of the wireless communication unit (710) from RRC connected mode to RRC idle mode or to RRC inactive mode.

15. The wireless communication unit (710) of Claim 1 or Claim 2, wherein in response to the processor identifying that the wireless communication unit (710) is in the semi-synchronized state, the processor is arranged to transition the wireless communication unit (710) to an unsynchronized state in response to the processor identifying at least one of the following:a failure-related condition of the wireless communication unit (710);an expiration of a timer associated with the semi-synchronized state;a geographical or spatial location of the wireless communication unit (710) or the assistance information received by the wireless communication unit (710) being out of date;a requirement to perform public land mobile network, PLMN, selection;a selection of a non-terrestrial network after previously having camped on a terrestrial network.

16. The wireless communication system of Claim 14 or Claim 15, wherein the failure-related condition comprises at least one of: a radio link failure, RLF, a communication cell handover failure, a trigger to perform a re-establishment of a radio resource control, RRC, connection, identification of a start of a timer related to a RLF.

17. The wireless communication unit (710) of any preceding Claim, wherein the at least one serving communication unit comprises a first serving communication unit (720) arranged to support communications in a first communication cell and a second serving communication unit (730) arranged to support communications in a second communication cell, wherein the wireless communication unit (710) is in a fully synchronized state with a first serving communication unit (720) and the wireless communication unit (710) is in a semi-synchronized state with a second serving communication unit (730), wherein the processor is arranged to:acquire assistance information from the second serving communication unit (730) or geographical or spatial location information of the wireless communication unit (710) from the first serving communication unit (720); andtransition the wireless communication unit from the semi-synchronized state (620) to a fully synchronized state (630) with the second serving communication unit (730).

18. The wireless communication unit (710) of any preceding Claim wherein the processor and the transceiver of the wireless communication unit (710) are arranged to indicate a state transition of the wireless communication unit (710) to the at least one serving communication unit (720).

19. The wireless communication unit (710) of Claim 18 wherein the processor and the transceiver of the wireless communication unit (710) are arranged to indicate a state transition of the wireless communication unit (710) to the at least one serving communication unit (720) in one of: a radio resource control, RRC, message; a medium access control, MAC, control element, CE.

20. The wireless communication unit (710) of Claim 18 or Claim 19 wherein the processor andthe transceiver of the wireless communication unit (710) are arranged to indicate a state transition of the wireless communication unit (710) to the at least one serving communication unit (720) in an indication that includes a new state of the wireless communication unit (710) and a previous state of the wireless communication unit (710).

21. A method (600) of synchronization for a wireless communication unit (710) supported by at least one serving communication unit (720) in a wireless communication system, the method at the wireless communication unit comprising:identifying that the wireless communication unit (710) is in an unsynchronized state (610) with the at least one serving communication unit (720); and in response thereto:performing a procedure that transitions the wireless communication unit (710) from the unsynchronized state (610) to a semi-synchronized state (620) with the at least one serving communication unit (720); andperforming a subsequent procedure that transitions the wireless communication unit (710) from the semi-synchronized state (620) to a fully synchronized state (630) with the at least one serving communication unit (720).T +44(0)30 0300 2000A

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