Mobility in wireless communication systems

Abstract

A user equipment configured to apply lower layer, that is layer 1 or layer 2, triggered 5 mobility LTM and configured to receive at least one LTM candidate cell configuration including a transmission configuration indicator TCI state of the at least one candidate cell; and in response to uplink synchronization acquisition comprising of a reference signal associated with said TCI state of said candidate cell, starting at least one timer for measuring at least one of the following: a first time period or a second time period. The 10 apparatus is caused to consider timing synchronisation of the TCI state to be valid during the first time period and the TCI state to be known during the second time period.

Description

TECHNOLOGICAL FIELD Various example embodiments relate to wireless communication systems, and more particularly, to mobility within such wireless communication systems. BACKGROUND As user equipment (UE) moves through a wireless communication system, it may move through cells that comprise regions of radio coverage that are supported by one or more network access node. Layer 1 or layer 2, sometimes termed lower layer, triggered mobility LTM is being increasingly used to reduce the latency, overhead and interruption time associated with a UE moving between the different cells. It would be desirable to be able to further reduce the latency of the cell switch procedure. BRIEF SUMMARY The scope of protection sought for various example embodiments of the invention is set out by the independent claims. The example embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention. According to various, but not necessarily all, example embodiments of the invention there is provided an apparatus configured to apply lower layer, that is layer 1 or layer 2, triggered mobility LTM, said apparatus comprising: at least one processor; and at least one memory storing instructions that when executed by the at least one processor cause the apparatus at least to perform: receiving a configuration including at least one LTM candidate cell configuration including one or more transmission configuration indicator, TCI, state of the at least one candidate cell; and in response to uplink synchronization acquisition comprising acquisition of a reference signal associated with said TCI state of said candidate cell, starting at least one timer for measuring at least one of the following: a first time period or a second time period; said apparatus being caused to consider timing synchronisation of said TCI state to be valid during said first time period; and said apparatus being caused to consider said TCI state being known during said second time period. When switching between cells using LTM, there is latency associated with the switch due to processing the cell switch signals and the need to do fine time / frequency tracking of the target cell so that the apparatus may have valid timing synchronisation information forthat cell. This fine time / frequency tracking may be done in advance, but it may only be valid for a certain period. The time period during which an apparatus such as a user equipment is deemed to have valid timing synchronisation information for a TCI state and for that TCI state to be known may be triggered by the apparatus acquiring uplink synchronisation information which may include acquisition of a reference signal, which may be within a synchronisation system block SSB, associated with the TCI state of the candidate cell. Where the apparatus has valid information regarding the timing synchronisation for the candidate cell then at cell switch it does not need to do fine frequency / timing tracking prior to switch but can switch with reduced latency. Conventionally this timing synchronisation information has been known in response to a TCI state activation command, but it was recognised that other procedures may trigger this information to be known and valid and render the fine time / frequency tracking prior to the cell switch not to be required. These other procedures include uplink synchronisation acquisition and thus, embodiments provide one or more timers to be triggered in response to uplink synchronization acquisition by the apparatus, These one or more timers allow the apparatus to track when it has timing synchronisation information and when it does not and thus, when fine time / frequency tracking will need to be performed prior to cell switch. In some example embodiments, said at least one timer comprises a first timer configured to measure said first time period. In some example embodiments, said apparatus is further caused to: in response to said uplink synchronization acquisition, to determine whether said TCI state is known, and start said first timer for said first time period when said TCI state is determined to be known, and not start said first timer for said first time period when said TCI state is determined not to be known. In some example embodiments, said apparatus is further caused to: in response to receipt of said uplink synchronization order, determine whether said TCI state is active, and start said first timer when said TCI state is determined to be active, and not start said first timer when said TCI state is determined not to be active. In some cases the TCI state being known when the uplink synchronisation is acquired may be a prerequisite for the at least one timer to be started. In some example embodiments, said at least one timer comprises a second timer configured to measure said second time period. In some example embodiments, said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether said TCI state is known, and start said second timer for said second time period when said TCI state is known, and not start said second timer for said second time period when said TCI state is determined to not be known. In some example embodiments, said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether said TCI state is active, and start said second timer when said TCI state is determined to be active, and not start said second timer when said TCI state is determined not to be active. In some cases the TCI state being active when the uplink synchronisation is acquired may be a prerequisite for the at least one timer to be started. In other cases the TCI state having previously been active may be a prerequisite for the at least one timer to be started. In some example embodiments, said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether timing synchronisation of said TCI state is valid, and restart said first timer when said TCI state is determined to have valid timing synchronization, and not restart said first timer when said TCI state is determined to be not active or determined not have valid timing synchronisation. In some example embodiments, said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether timing synchronisation of said TCI state is valid, and start said second timer when said TCI state is determined to have valid timing synchronization, and not start said second timer when said TCI state is determined to be not active or determined not to have valid timing synchronisation. In some cases the timing synchronisation of said TCI state being valid when the uplink synchronisation is acquired may be a prerequisite for the at least one timer to be started. In some example embodiments, said apparatus comprises a first and a second timer, said first timer being configured to measure said first time period and said second timer being configured to measure said second time period. In some embodiments, said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether said TCI state is active, and when said TCI state is active to restart said first timer and to start said second timer, and when said TCI state is determined not to be active to not start said first timer. Although, the two timers may be triggered whenever uplink synchronisation is acquired, in some embodiments the apparatus may be configured to determine whether the TCI state is active and to re-start the first timer and start the second timer where it is and not start the first timer where it is not. In other words, the timer synchronisation is not deemed to be valid and the TCI state is not deemed to be active if the TCI state is not currently active when the acquisition of the uplink synchronisation occurs. In other embodiments, said apparatus is further caused to: in response to said preamble transmission caused by receipt of said uplink synchronization order, determine whether said TCI state has previously been active, and restart said first timer and said second timer when said TCI state is determined to have been previously active and not start said first timer when said TCI state is determined not to have been previously active. In other embodiments, it may be that previous acquisition of the TCI state is sufficient for the two timers to be started, however if it has not previously been acquired then the first timer may not be started. In some example embodiments, said uplink synchronisation acquisition is in response to a preamble transmission caused by receipt of an uplink synchronization order including said RS associated with a TCI state of a candidate cell. The RS may be a SSB. In some embodiments, said uplink synchronisation order comprises a PDCCH (physical downlink control channel) order. In some example embodiments in response to receipt of said uplink synchronisation order, said apparatus is caused to: transmit said preamble to said candidate cell and start said first and second timers. In some embodiments, said uplink synchronisation acquisition comprises said apparatus being caused to estimate at least some timing synchronisation information including a timing advance based on said received reference signal. The uplink synchronisation acquisition may be performed autonomously by the apparatus in response to receiving a reference signal RS such as an SSB that allows the apparatus to estimate the timing advance of a candidate cell. In some embodiments, the apparatus is further caused to perform in response to receipt of a cell switch command indicating said candidate cell as said target cell, determining whether said at least one timer indicates said user equipment has valid timing synchronisation information for said cell, and where so switching to said cell and using said timing synchronisation information for performing communications; and where not switching to said cell and acquiring timing synchronisation information for said cell, prior to performing communications. The communications may be DL / UL reception / transmission on an indicated beam of the target cell. In some embodiments, said second time period is equal to or greater than said first time period. The apparatus may be a user equipment, such as any mobile end or terminal device that may be capable of wireless communication. By way of example rather than limitation, user equipment UE may also be referred to as a communication device, a terminal device, a Mobile Station (MS). The user equipment may include, but is not limited to, a mobile phone, a cellular phone, a smart phone, a tablet, a wearable terminal device, a personal digital assistant (PDA), vehicle-mounted wireless terminal devices, smart devices etc. According to various, but not necessarily all, example embodiments of the invention there is provided a method performed on an apparatus configured to apply lower layer, that is layer 1 or layer 2, triggered mobility LTM, said method comprising: receiving a configuration including at least one LTM candidate cell configuration including one or more transmission configuration indicator, TCI, state of the at least one candidate cell; and in response to uplink synchronization acquisition comprising acquisition of a reference signal associated with said TCI state of said candidate cell, starting at least one timer for measuring at least one of the following: a first time period or a second time period; said first time period providing a time period during which timing synchronisation of said TCI state is deemed to be valid; and said second time period providing a time period during which timing synchronisation of said TCI state is deemed to be known. In some example embodiments, said method comprises: in response to said uplink synchronization acquisition, determining whether said TCI state is known, and starting said first timer for said first time period when said TCI state is determined to be known, and not starting said first timer for said first time period when said TCI state is determined not to be known. In some example embodiments, said method comprises: in response to receipt of said uplink synchronization order, determining whether said TCI state is active, and starting said first timer when said TCI state is determined to be active, and not starting said first timer when said TCI state is determined not to be active. In some example embodiments, said method comprises: in response to said uplink synchronization acquisition, determining whether said TCI state is known, and starting said second timer for said second time period when said TCI state is known, and not starting said second timer for said second time period when said TCI state is determined to not be known. In some example embodiments, said method comprises: in response to said uplink synchronization acquisition, determining whether said TCI state is active, and starting said second timer when said TCI state is determined to be active, and not starting said second timer when said TCI state is determined not to be active. In some example embodiments, said method comprises: in response to said uplink synchronization acquisition, determining whether timing synchronisation of said TCI state is valid, and restarting said first timer when said TCI state is determined to have valid timing synchronization, and not restarting said first timer when said TCI state is determined to be not active or determined not have valid timing synchronisation. In some example embodiments, said method comprises: in response to said uplink synchronization acquisition, determining whether timing synchronisation of said TCI state is valid, and starting said second timer when said TCI state is determined to have valid timing synchronization, and not starting said second timer when said TCI state is determined to be not active or determined not to have valid timing synchronisation. In some embodiments, said method comprises: in response to said uplink synchronization acquisition, determining whether said TCI state is active, and when said TCI state is active restarting said first timer and to start said second timer, and when said TCI state is determined not to be active not starting said first timer. In some example embodiments, said uplink synchronisation acquisition comprises transmitting a preamble transmission in response to receipt of an uplink synchronization order including said RS associated with a TCI state of a candidate cell. The RS may be a SSB. In some example embodiments in response to receipt of said uplink synchronisation order, said method comprises transmitting said preamble to said candidate cell and starting said first and second timers. In some example embodiments, said uplink synchronisation acquisition comprises estimating at least some timing synchronisation information including a timing advance based on said received reference signal. The uplink synchronisation acquisition may be performed autonomously by the apparatus in response to receiving a reference signal RS such as an SSB that allows the apparatus to estimate the timing advance of a candidate cell. In some example embodiments, said method comprises: in response to receipt of a cell switch command indicating said candidate cell as said target cell, determining whether said at least one timer indicates said user equipment has valid timing synchronisation information for said cell, and where so switching to said cell and using said timing synchronisation information for performing communications; and where not switching to said cell and acquiring timing synchronisation information for said cell, prior to performing communications. According to various, but not necessarily all, example embodiments of the invention there is provided a computer program comprising instructions which, when executed by an apparatus, cause the apparatus to perform the method set out above According to various, but not necessarily all, example embodiments of the invention there is provided a computer program comprising instructions stored thereon for performing the method set out above. According to various, but not necessarily all, example embodiments of the invention there is provided a non-transitory computer readable medium comprising program instructions stored thereon for performing the method set out above. According to various, but not necessarily all, example embodiments of the invention there is provided an apparatus configured to apply lower layer, that is layer 1 or layer 2, triggered mobility LTM, said apparatus comprising: means for receiving a configuration including at least one LTM candidate cell configuration including one or more transmission configuration indicator, TCI, state of the at least one candidate cell; at least one means for timing at least one of the following: a first time period or a second time period; means for starting said at least one means for timing in response to uplink synchronization acquisition comprising acquisition of a reference signal associated with said TCI state of said candidate cell; wherein said apparatus is configured to consider timing synchronisation of said TCI state to be valid during said first time period; and said TCI state to be known during said second time period. The means may perform the optional features set out in relation to the apparatus mentioned above. The processor, memory, and example algorithms, encoded as instructions, program, or code, may be the means for providing or causing the performance of the operation. According to various, but not necessarily all, example embodiments of the invention there is provided an apparatus configured to apply lower layer, that is layer 1 or layer 2, triggered mobility LTM, said apparatus comprising: circuitry configured to receive a configuration including at least one LTM candidate cell configuration including one or more transmission configuration indicator, TCI, state of the at least one candidate cell; Circuitry configured to time at least one of the following: a first time period or a second time period; circuitry configured to start said circuitry configured to time in response to uplink synchronization acquisition comprising acquisition of a reference signal associated with said TCI state of said candidate cell; wherein said apparatus is configured to consider timing synchronisation of said TCI state to be valid during said first time period; and said TCI state to be known during said second time period. The circuitry may be configured perform the optional features set out in relation to the apparatus mentioned above. Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. BRIEF DESCRIPTION Some example embodiments will now be described with reference to the accompanying drawings in which: FIG. 1 shows a signalling diagram for early transmission configuration indicator TCI state activation; FIG. 2 shows a signalling diagram for early timing advance TA acquisition; FIG. 3 shows signalling procedure for LTM; FIG. 4 shows components of LTM latency; FIG. 5 shows TCI state being active / known based on L1 measurements and report; FIG. 6 shows TCI state active / known based on early TCI state activation; FIG. 7 shows TCI state becoming known and active in response to transmission of a preamble following a PDCCH order according to an embodiment; FIG. 8 shows a PDCCH order being received following a TCI state being previously but not currently active; FIG. 9 shows a case according to an embodiment where a preamble transmission triggers the two timers, the preamble transmission being in response to a PDCCH order received during a TCI active state; FIG. 10 shows a UE determined TA acquisition according to an embodiment; FIG. 11 shows a further example of a UE based TA according to a further embodiment; FIG. 12 shows a UE based TA acquisition where the TA is determined when the TCI state is active according to an embodiment; FIGs 13A - D show a signalling diagram illustrating a LTM cell switch according to an embodiment; and Fig. 14 shows network nodes and user equipment according to an embodiment. DETAILED DESCRIPTION Before discussing the example embodiments in any more detail, first an overview will be provided. When performing a LTM cell switch there are various steps with a known delay that the UE performs in response to a cell switch command. These steps may include time tracking such that communication between the UE and cell are synchronised. Time tracking may not be required where the UE already has timing synchronisation information for the target cell. The network node needs to know the delay required by the UE to perform the LTM switch, so it needs to know if the time tracking delay is zero or another value. If it is zero then latency is reduced. Embodiments seek to reduce latency as well as provide a known delay between a cell switch command and communications with the target cell starting. Embodiments, recognise that there are circumstances (examples of which are described in detail later in the description) in response to which the UE may be deemed to have valid timing synchronisation information for a certain target cell. This may be because the UE is aware of the timing synchronisation due to either having performed some TA acquisition itself in response to receipt of a reference signal such as an SSB for that target cell during LTM configuration, or due to receipt of a reference signal such as an SSB in a PDCCH order for example. Some background to the process of LTM switching and the associated delays is provided below. Lower layer or L1 / L2 triggered mobility LTM is a procedure in which a network node gNB receives (L1 layer 1) measurement report(s) from a UE, and on their basis the gNB changes UE serving cell (PCell or PSCell) by a cell switch command signalled via a MAC CE. The cell switch command indicates to the UE to switch to a new cell. The new cell is a LTM candidate cell, which may be a neighboring cell or one of UE’s current serving cells (e.g. SCell), and a configuration for that cell has previously been prepared by the gNB and provided to the UE through RRC signalling. The UE switches to the target configuration according to the cell switch command content. The LTM procedure can be used to reduce the cell switch delay and the mobility latency. Early TCI state activation (Early DL synchronization) If the above is supported by the UE and configured by the network, it is possible to activate transmission configuration indicator TCI state(s) of one or multiple (non-serving or serving) cells that are configured as LTM candidate cells. For instance, the TCI states of one or more LTM candidate cells can be activated in advance before any of those cells become the serving PCell or PSCell cell (via the LTM cell switch command). This early TCI activation allows the UE to be DL synchronized with those cells associated with an activated TCI state, thereby facilitating a faster cell switch to one of those cells when cell switch is triggered. If the TCI state of the candidate cell is not activated before the cell switch, the TCI state activation happens with the cell switch command. An additional delay is then added to the switch delay as the UE needs time for fine time tracking in the candidate cell. In TS 38.300 early TCI state activation procedure is as shown in Fig. 1. Early TA acquisition (Early UL synchronization) When supported by the UE and configured by the network, it is possible to initiate UL TA timing advance acquisition (called early TA) procedure of one or multiple cells that are different from the current serving cells. If the cell has the same NTA (network estimated TA), as the current serving cells or NTA=0 early TA acquisition procedure may not be required. The network may request the UE to perform early TA acquisition of a candidate cell before a cell switch: 1) The early TA acquisition procedure is triggered by PDCCH order from the serving cell as specified in clause 9.2.6 of TS 38.133 (network estimated TA), or 2) Realized through UE-based TA measurement as configured by RRC (UE autonomous TA estimation). In the former case, the gNB to which the candidate cell belongs calculates the TA value based on the received preamble from the UE and sends the result to the gNB to which the serving cell belongs. The serving cell sends the TA value to the UE in the LTM cell switch command (MAC CE) when triggering LTM cell switch. In the latter case, the UE performs TA measurement for the candidate cells after being configured by RRC but the exact time the UE performs TA measurement is up to UE implementation. The UE applies the TA value measured by itself. In both cases the UE performs RACH-less LTM upon receiving the cell switch command. The network may also send a TA value in the LTM cell switch command MAC CE without network or UE early TA acquisition. Early TA acquisition procedure based on PDCCH order is described in TS 38.300 and shown in Fig. 2 and described below. 1. The gNB to which Cell A belongs provides the TA acquisition configuration to the UE within the RRCReconfiguration message. The TA acquisition configuration includes all RRC configuration information required to send a Random Access Preamble to Cell B so that the gNB to which Cell B belongs can calculate a TA value to be used by the UE, e.g., in case an LTM cell switch procedure is executed to Cell B. The TA acquisition configuration may include information for one or multiple cells to which the TA acquisition procedure may be executed by the UE. 2. The UE replies with the RRCReconfigurationComplete message. 3. The gNB to which Cell A belongs sends a PDCCH order message to the UE in order to initiate a TA acquisition procedure with Cell B. [The PDCCH order message includes the information required to sends a Random Access Preamble to Cell B.] 4. The UE sends a Random Access Preamble to Cell B so that the gNB to which Cell B belongs can estimate a TA value to be used by the UE, e.g., if an LTM cell switch procedure is triggered to Cell B. Note X: The gNB to which Cell A belongs may indicate the retransmission of preamble for TA acquisition in case no TA is obtained. 5. The gNB to which Cell A belongs provides the TA value calculated by the gNB to which Cell B belongs during the TA acquisition procedure, e.g. in LTM cell switch command MAC CE which initiate cell switch procedure to Cell B. Depending on the availability of a valid TA value, the UE performs either a RACH-less LTM or RACH-based LTM cell switch. If TA value is available at LTM switch: • If the TA value is provided in the cell switch command, the UE applies the TA value as provided by the network and performs RACH less LTM switch. • In the case where UE-based TA measurement is configured, and no TA value is provided in the cell switch command, the UE applies the TA value by itself if available. • In the case where UE-based TA measurement is configured, and a TA value is provided in the cell switch command, the UE applies the TA value provided in the LTM switch command. If TA value is not available at LTM switch: • In the case where UE-based TA measurement is configured, and no TA value is provided in the cell switch command, if UE has not acquired any valid TA value, the UE performs RACH-based LTM cell switch. • In the case where UE-based TA measurement is not configured, if no TA value is provided in the cell switch command, the UE performs RACH based LTM switch. Hence, the whole LTM procedure becomes as shown in Fig. 3, where step 7 depends on the availability of TA in the LTM cell switch command (or possible UE estimated TA value): TCI state activation for LTM candidate cell The requirements below apply for a UE configured with one or more TCI state configurations on LTM candidate cell. UE shall complete the activation of TCI state within the delay defined in this clause after receiving a MAC CE indicating LTM candidate cell TCI state activation before cell switch command as specified in clause 6.1.3.75 of TS 38.321. Known conditions for TCI state The candidate TCI state in the TCI state activation list is known if the following conditions are met: During the period from the last transmission of the RS resource used for the L1-RSRP measurement reporting for the target downlink TCI state to the completion of active downlink TCI state activation, where the RS resource for L1-RSRP measurement is the RS in target downlink TCI state or QCLed quasi co-located to the target downlink TCI state The MAC CE command indicating the activation of downlink TCI state is received within 1280 ms upon the last transmission of the RS resource for beam reporting or measurement The UE has sent at least 1 valid L1-RSRP report for the target downlink TCI state before the MAC CE command is received. The target downlink TCI state remains detectable during the downlink TCI state activation period The SSB associated with the downlink TCI state remain detectable during the downlink TCI activation period SNR of the downlink TCI state >-3dB PDCCH ordered Random Access for LTM The below applies to a UE capable of early TA acquisition configured to trigger RACH procedure toward target LTM candidate cell before receiving cell switch command MAC-CE. PDCCH ordered Random Access delay T_SSB is the time for T / F tracking T_SSB = 0 if the following conditions are met. The target LTM neighbour cell is on an FR1 carrier whose TCI state associated with SSB indicated in the PDCCH-order is activated and the time gap between receiving the MAC-CE activating the target TCI state and PDCCH order is larger than a predetermined time, and - L1-RSRP measurement period of candidate cell is not larger than 160ms or The time between receiving the MAC-CE activating the target TCI state and PDCCH order is not larger than 160ms Otherwise: T_SSB is the time to first SSB transmission after PDCCH-order RACH command is decoded by the UE when SSB is within active BWP + 2ms. T_SSB is the time to first SSB transmission overlapping with MGL after PDCCH-order RACH command is decoded by the UE when SSB is outside active BWP + 2ms Signalling procedure for LTM LTM delays HO handover delays for LTM depends on multiple factors as discussed next. One of the delay aspects to consider is the time needed for the UE to acquire TA for UL transmission in the new cell after the LTM switch (Tltm-iu &Trar in Fig 4). HO delay for L1 / L2-based inter-cell mobility is the time from UE receiving the cell switch command to the UE performing the first DL / UL reception / transmission on the indicated beam of the target cell. LTM interruption time contains all the other parts of the LTM cell switch delay except the time for decoding the cell switch command MAC-CE and related HARQ / ACK response (Tcmd). For Rel-18, RAN2 assumed the latency of the mobility procedure is characterized by the terms illustrated in Fig. 4. LTM Cell Switch delay LTM cell switch delay Dltm is the delay from the end of the last TTI containing the MAC-CE command for cell switch until the time the UE transmits the first UL message on the target cell. When the target cell and the target joint UL / DL TCI state or separate UL and DL TCI states in the MAC-CE LTM cell switch command are known, the LTM cell switch delay is defined as: Dltm — Tcmd TuTM-interrupt Where: Tcmd equals to Tharq+ 3ms, where Tharq is the timing between cell switch command and acknowledgement as specified in TS 38.213. TiTM-interrupt is as stated in section 6.3.1.2.1. The target cell in the LTM cell switch command is known if the following conditions are met: During the last 5 seconds before the reception of the cell switch command: The UE has sent a valid L1 or L3 measurement report for the target cell, and One of the SSBs measured from the NR target cell configured for measurement remains detectable according to the cell identification conditions specified in clause 9.2 for intra-frequency cell and in clause 9.3 for inter-frequency cell of TS 38.133, One of the SSBs measured from the NR target cell configured for measurement remains detectable according to the cell identification conditions specified in clause 9.2 for intra-frequency cell and in clause 9.3 for inter-frequency cell of TS 38.133, Otherwise, the cell is unknown. The target joint DL / UL TCI state or separate DL and UL TCI states in the LTM cell switch command are known if the following conditions are met: [- The target TCI state in the cell switch command is activated not more than TBD ms before the reception of the cell switch command and SNR of the SSB associated to TCI state >-3dB; or] [- The target TCI state in cell switch command is activated before receiving the cell switch command and the SSB associated to target TCI state is available at least once every TBD ms after the TCI state activation command is received and SNR of the SSB associated to TCI state >-3dB; or] During the period from the last transmission of the RS resource used for the L1-RSRP measurement reporting for the target DL / UL TCI state to the completion of LTM cell switch, where the RS resource for L1-RSRP measurement is the RS in target DL / UL TCI state or QCLed to the target DL / UL TCI state LTM cell switch command is received within 1280 ms upon the last transmission of the RS resource for beam reporting or measurement The UE has sent at least 1 L1-RSRP report for the target DL / UL TCI state before the LTM cell switch command The target DL / UL TCI state remains detectable during the LTM cell switching period The SSB associated with the target DL / UL TCI state remain detectable during the cell switching period SNR of the TCI state >-3dB Otherwise, the target joint DL / UL TCI state or separate DL and UL TCI state is unknown. Interruption time The interruption time TL™-interrupt is the time between the end of the last TTI containing the MAC-CE command for LTM cell switch until the time the UE transmits the first UL message on the target cell, excluding Tcmd stated in section 6.3.1.2. Tltm-interrupt — TLTM-RRC-processing TLTM-processing + Tfirst-RS + Trs-proc + Tltm-iu ms, Where: TLTM-RRe-processing is the time for ASN.1 decoding and validity / compliance check for the RRC configuration of the LTM target cell indicated in the LTM cell switch command. T LTM-RRC-processing “ 0, if the UE supports [earlyDecodingAndValidityCheck] capability, and at least one of the following conditions is met: The number of candidate cells in the LTM candidate cell configuration does not exceed [number of candidate cells for early ASN. 1 decoding and validity check], UE has received LTM candidate cell TCI state activation command for the target cell at least Tharq+ 13 ms before the LTM cell switch command, and the number of candidate cells with TCI state(s) in LTM candidate cell active TCI state list does not exceed [number of candidate cells for early ASN. 1 decoding and validity check], UE has received PDCCH order for early RACH for the target cell. Otherwise TLTM-RRe-processing = 10 ms. TLTM-processing is the time for UE processing, consisting of applying the target cell parameters and L1 / L2 change. If the UE supports [faster LTM processing] capability, the value of TLTM-processing equals to [faster intra-FR processing delay (10 ms or 15 ms)] for FR1 to FR1 and FR2 to FR2 LTM cell switch. [faster inter-FR processing delay (20 ms or 30 ms)] for FR1 to FR2 and FR2 to FR1 LTM cell switch. Otherwise, the value of TurM-processing equals to 20 ms for FR1 to FR1 and FR2 to FR2 LTM cell switch. 40 ms for FR1 to FR2 and FR2 to FR1 LTM cell switch. Tfirst-Rs is the time for fine time tracking and acquiring full timing information of the target cell. Trs-pfoc is the time for SSB processing. Tfirst-Rs = 0 and TRs-Proc= 0 under the following conditions: The target TCI state indicated in the LTM cell switch command is in the serving cell active TCI state list, or The target TCI state in the cell switch command is in the LTM candidate cell active TCI state list, and the time gap between receiving the LTM candidate cell TCI state activation MAC-CE and the cell switch command is at least as stated in section [TBD], and the time gap between receiving the LTM candidate cell TCI state activation MAC-CE and the cell switch command is not more than [160 ms], or the SSB associated to target TCI state is transmitted every 160 ms after the LTM candidate cell TCI state activation MAC-CE is received. Otherwise, Tfirst-Rs is the time to the first SSB transmission on the target cell [after Tomd]. Trs-proc “ 2 ms. Tltm-iu is the interruption uncertainty during LTM cell switch. For RACH-based LTM cell switch, Tltm-iu is the interruption uncertainty in acquiring the first available PRACH occasion in the new cell. Tltm-iu can be up to the summation of SSB to PRACH occasion association period and 10 ms. SSB to PRACH occasion associated period is defined in the table 8.1-1 of TS 38.213. For RACH-less LTM cell switch, Tltm-iu Js the uncertainty on transmitting the first uplink transmission on the target cell. There is a difference when the TCI state is known and when the TCI state is active. The following summarizes the main points for PDCCH Order scenario (for transmitting preamble in candidate cell): TCI state is active based on (early) TCI state activation if: TCI state is on the active TCI state list. TCI state being active means that during PDCCH ordered preamble transmission or cell switch the UE does not need time to do fine time / frequency tracking (TSSB = 0 for PDCCH ordered RACH, and Tfirst-RS = 0 for cell switch delay). However, for PDCCH order some addition conditions have been set up: The time from receiving the TCI state activation MAC-CE and the PDCCH Order is no more than A ms (value A under discussion), and The measurement period for the RS (SSB) associated to the TCI state no longer than 160 ms, or if the time between receiving the TCI state activation MAC-CE and the PDCCH order is not longer than [160 ms] ago. TCI state is known if: At early TCI state activation: There has been a L1-RSRP report for the RS associated to the TCI state The RS (SSB) associated to the TCI state is transmitted no longer time ago than up to 1280 ms before TCI state activation MAC-CE. RS remains detectable At LTM cell switch: There has been a L1-RSRP report for the RS associated to the TCI state The RS (SSB) associated to the TCI state is transmitted no longer time ago than up to 1280 ms before TCI state activation MAC-CE. RS remains detectable TCI state was activated up to [TBD] ms ago, or measurement period for the RS associated to the TCI state is not longer than [TBD] ms. Target TCI state being known is a prerequisite for cell switch delay requirements to apply. Hence, if TCI state is unknown, cell switch delay is unknown to the network. From the above it can be concluded that the TCI state may be known either based on L1 measurements or TCI state activation. When the TCI state is known based on TCI state activation, the UE may not be required to report L1 or L3 measurements before LTM cell switch, except for ensuring that the target cell is known. For the PDCCH Order, when L1 measurements are reported, the requirement to ensure short Tssb depends on active and known TCI state can be illustrated as in Fig. 5. Fig. 5 shows timings when TCI state is active / known based on L1 measurements (and report). For the case when measurement period for the SSB associated to the TCI state is larger than 160 ms longer Tssb can be expected. The portion following the activation order and before TCI state becoming active is the TCI state activation delay. Fig. 6 shows TCI state active / known based on early TCI state activation. For the case when L1 measurement period for the RS associated to an activated TCI state is not larger than 160 ms, the TCI state remains active until cell switch command. A problem with the current RAN4 conditions for PDCCH Order that triggers TA acquisition and an active and known TCI state is that when the L1- measurement period is larger than 160 ms, the network has very little time after TCI state activation to send the cell switch command, because the TCI state stops being active [160 ms] after receiving the TCI state activation MAC-CE. In Fig. 5 and Fig. 6 this is the “TCI state active” box, whose duration is actually less than 160 ms, considering that after receiving the TCI state activation MAC-CE, the UE needs time to activate the TCI state, which is the TCI state activation delay in the figure. Furthermore, between TCI state activation and cell switch the network may need to fit PDCCH ordered RACH within the TCI active period in the figure. The duration of PDCCH ordered RACH may be tens of milliseconds. If the network cannot fit all this within the 160 ms period after sending the TCI state activation MAC-CE, early TCI state activation does not make sense, because the UE will anyways need one SSB to do fine T / F tracking during the cell switch (Tfirst-Rs + 0). Furthermore, if it is considered that there is no L1 report and TCI state activation is the only way to make the TCI state known, if cell switch command is not sent within the 160 ms, TCI state becomes unknown and cell switch delay requirements stop applying. Hence, using LTM becomes very unreliable as the cell switch delay is not known. A way to get around the 160 ms limitation with a measurement period larger than 160 ms would be to keep sending TCI state activation command every 160 ms, which provides a high signalling overhead. The duration of 160 ms might also be extended, but this may not be possible considering all UE implementations. Hence, other ways to extend the duration that the TCI state remains active need to be considered. Embodiments, provide for the case where in response to uplink synchronisation acquisition by a user equipment at least one timer is started the at least one timer indicating at least one of a first time period during which the timing synchronisation information held by the user equipment can be considered valid, and a second time period during which the TCI state may be considered to be known. In some embodiments the user equipment may have two timers to measure the first and second time periods and each may be configured to be started in response to uplink synchronisation acquisition. In many scenarios this provides an additional or extended time period during which the user equipment may switch cell without needing to perform fine timing synchronisation. TCI state remains / becomes active / known based on PDCCH order In embodiments, where the network sends a PDCCH order for a target cell where the RS used for the timing reference is an RS that is a QCL reference for a TCI state in the LTM candidate cell configuration, the apparatus receiving the PDCCH Order has the same response as it would to receiving a TCI state activation command, that is for the next X ms from the time where the UE receives the PDCCH order the TCI state is considered active, and If during the next X ms the network sends • another TCI state activation command for the same TCI state, or • another PDCCH order for the RS associated to the TCI state, or • cell switch command with this TCI state as the target TCI state, the UE is not allowed / does not require time to do fine T / F tracking for the TCI state during: • the TCI state activation delay, or • PDCCH ordered preamble transmission, or • cell switch delay. Otherwise, if the time from PDCCH order is longer than X ms and no other condition for UE having the fine T / F synchronization of the target TCI state is fulfilled, the UE is allowed time for fine T / F tracking during: • TCI state activation, or • PDCCH ordered preamble transmission, or • LTM cell switch. The above may apply either: 1. Always when receiving PDCCH order for an LTM candidate cell (preamble transmission starts the timers; or 2. When TCI state for this candidate cell was activated before the PDCCH order (and may still be active or not i.e. preamble transmission extends or restarts the timer); or 3. When the TCI state is still active at the time of receiving the PDCCH order or at the time of preamble transmission (preamble transmission extends the timer) Where, “timer” means the time where the UE is expected to have valid fine T / F tracking for the candidate cell SSB. I.e. Tfirst-RS and TRS_proc in cell switch delay will be equal to zero if cell switch command arrives when the timer is running. In one alternative the ‘timer’ is started once the apparatus has received (and potentially processed) the candidate reference signal (e.g. SSB) used for time / frequency tracking. Under the same conditions as given above, the TCI state may be considered known based on PDCCH order for the following Y ms after receiving the PDCCH order, where Y may equal to X or larger than X. Here, the ‘known’ refer to the conditions for known TCI state defined in TS 38.133, and the same options 1.-3. may apply. TCI state becomes active / known based on UE based TA estimation In another embodiment, if the UE is configured to do UE based TA estimation for an LTM candidate cell. If the UE is expected to have estimated the TA based on an RS that is a QCL source for a TCI state configured for this candidate cell, the TCI state is considered known. Additionally, if the network has activated a TCI state whose QCL reference is the RS used by the UE for acquiring TA through UE based TA estimation, the TCI state activation status associated with the RS used for UE based TA estimation is considered active. When the TCI state is active, the UE is not allowed / does not require time for fine T / F tracking when receiving another TCI state activation MAC-CE for this TCI state, PDCCH order with SSB associated to this TCI state, or cell switch with this TCI state as the target TCI state. Valid fine time / frequency tracking fora candidate TCI state based on PDCCH order When the UE is expected to maintain a TCI state active for only M ms after receiving TCI state activation MAC-CE, if not more than M ms has passed since TCI state activation i.e. the TCI is still considered active: Based on current requirements: PDCCH ordered RACH delay is defined with TSSB=0 i.e. the UE is not given time for fine T / F tracking. Hence UE is expected to have the fine timing of this TCI state. Embodiments propose that from the time of receiving the PDCCH order, UE responds to it in the same way as it would to receiving TCI state activation and the ‘TCI state activation’ condition remain valid for (an additional) X ms. The reason being that the UE is expected to have fine T / F synchronization at the time of a successful preamble transmission based on PDCCH order. From requirement point of view, the above may apply either: 1. Always when receiving PDCCH order for preamble transmission towards an LTM candidate cell i.e. no matter if the TCI state (of the LTM candidate) was activated or not with MAC-CE before receiving the PDCCH order; or 2. When TCI state for this candidate LTM cell was activated before receiving the PDCCH order, no matter if the TCI state is still active with valid time / frequency synchronization at the time of receiving the PDCCH order or not; or 3. If the TCI state is active with valid time / frequency synchronization at the time of receiving the PDCCH order or at the time of preamble transmission. I.e. the time since TCI state activation is not longer than defined by TS 38.133 (e.g. 160 ms) and PDCCH ordered preamble transmission extends the time the TCI state is considered active and UE has valid time / frequency synchronization of the TCI state. Case 1 without TCI state activation before receiving PDCCH order is illustrated in Fig 7. Here the UE is expected to have the fine T / F synchronization for a duration of X ms after the PDCCH ordered preamble transmission for the RS (SSB) that was given in the PDCCH order. Hence, if cell switch command is sent during the X ms and the target TCI state in the cell switch command is associated to the RS (SSB) that was used for fine T / F tracking for the PDCCH order preamble transmission, the UE is not allowed fine T / F tracking during the cell switch (Tfirst-RS = 0 and TRS-proc = 0). Furthermore, the TCI state associated to the RS (SSB) given in PDCCH order is considered known for Y ms after preamble transmission. X and Y may have same or different value. In Case 1 this applies no matter if the target TCI state in the cell switch command was activated before PDCCH order or not. In one example the UE is assumed having fine time / frequency tracking of the target cell starting from when the UE has received (and possibly processed) the RS used for fine / frequency tracking in the target cell (e.g. SSB). Hence, in fig. 7 it would for example be based on an SSB received between receiving the PDCCH Order and transmitting the preamble. Case 2 is illustrated in Fig. 8 and shows the case when TCI state was activated before PDCCH order, but it is no longer active (or known) at the time of PDCCH order. At the time of PDCCH ordered RACH preamble transmission, the TCI state becomes known again for Y ms, and the UE is expected to have the fine T / F synchronization with this cell for X ms. In one example the UE is assumed to have fine time / frequency tracking of the target cell starting from when the UE has received (and possibly processed) the RS used for fine / frequency tracking in the target cell (e.g. SSB). Hence, in fig. 8 it would for example be based on an SSB received between receiving the PDCCH Order and transmitting the preamble. Case 3 is illustrated in Fig 9 with the case that PDCCH order arrives during the time TCI state is still active based on earlier TCI state activation command. Here the time TCI state remains active is extended by X ms. The time TCI state remains known is also be extended by Y ms. In Case 3 if PDCCH order would arrive when the TCI state is not active anymore, PDCCH order would not activate the TCI state, and the TCI state would be considered deactivated if cell switch is sent after the PDCCH order. Hence, UE may be allowed time for fine T / F tracking during the cell switch. Furthermore, in this case PDCCH order may make the TCI state a. known but not active, or b. not known or active. With any of the options in Figs 7-9, the network has more flexibility with the time it can send cell switch command than under the existing requirements that force the network to send cell switch command within 160 ms from sending the TCI state activation command. TCI state active based on UE based TA estimation Current status: In Rel-18, RAN2 has left it up to UE implementation how and based on which signals the UE based TA estimation is performed. RAN4 has not defined requirements for UE based TA estimation, but when the requirements are defined, it is expected that RAN4 defines which signals the UE is expected to use (e.g. SSB). Release 18 only supports SSBs as a source. Furthermore, RAN4 is expected to define when (within which delay and under which scenario) the UE is expected to have performed UE based TA estimation for a candidate cell after receiving the LTM candidate cell configuration. None of this is specified yet, but these are essential parts to define for the feature to work. It is proposed that when the UE is configured with UE based TA estimation and is expected to have the TA estimated (under any conditions to be defined by RAN4) at the time of receiving: • early TCI state activation command or • cell switch command, if the target TCI state has QCL relationship to the RS UE has used for UE based TA estimation, the TCI state activation status is considered fulfilled. The TCI state is also considered known. In other words, after (e.g., x number of RS samples or delay) a time it takes for UE to estimate TA accurately enough for the neigbouring cell, the TCI state activation status becomes also fulfilled, and / or the TCI state becomes known. Furthermore, if at the time of TCI state activation or cell switch command the target TCI state is already on the active TCI state list, and the UE is expected to have done UE based TA estimation using the RS associated to the target TCI state, the TCI state activation status is considered fulfilled (and active). When TCI state activation status is considered fulfilled, the UE is not given / does not require time for fine T / F tracking after receiving TCI state activation MAC-CE or cell switch command. Additionally, if the network sends a PDCCH order for a cell for which it configured the UE to do UE based TA estimation, and the UE is expected to have estimated the TA successfully, if the SSB in the PDCCH order is the same SSB as the UE has used for UE based TA estimation, the UE is not allowed / does not require time for fine T / F tracking during PDCCH ordered RACH procedure (Tssb = 0). Similar to PDCCH ordered RACH based options, three cases can be considered. 1. UE is expected to have fine T / F synchronization with the candidate cell RS (SSB) to which it has done UE based TA estimation always when UE based TA estimation is expected to be done (for X ms). A TCI state associated to the RS (SSB) is also considered known (for Y ms). 2. UE is only expected to have fine T / F synchronization based on UE based TA estimation if TCI state was activated for the candidate cell, no matter if TCI state is active / known at the time of UE based TA estimation completion. • l.e. UE based TA estimation may happen before or after TCI state activation but TCI state becomes known and UE is expected to have fine T / F synchronization only if TCI state was activated for this candidate cell before. 3. Only if TCI state is active / known at the time of UE based TA estimation completion, UE based TA estimation extends the time the TCI state is active for X ms and the time TCI state is known for Y ms. Case 1 is shown in Fig 10, where at the time of UE based TA estimation completion timers for fine T / F synchronization (X) and for TCI state being known (Y) are started. The value Case 2 is shown in Fig 11. UE based TA estimation starts timers X and Y but only because TCI state was activated before UE based TA estimation was completed. If UE based TA estimation would be done before TCI state activation, X and Y would not be started before TCI state is activated. Case 3 is shown in Fig 12, where UE based TA estimation completion extends the time TCI state remains active for X ms from the time UE based TA estimation is completed, and the time TCI state is known for Y ms from the time UE based TA estimation is completed. If TCI state would not be known at the time of UE based TA completion, Y would not be started. If the TCI state would not be active at the time of UE based TA completion, X would not be started. Fig. 13A - D shows a signalling diagram illustrating steps performed in lower layer triggered mobility according to an embodiment. This Figure provides one example using the PDCCH order for acquiring the UL synchronization in the LTM candidate cell. In general the idea is agnostic to the exact method of how to acquire the UL synchronization in the candidate cell. Fig. 13A starts with Step 2: UE 10 is in connected mode. Step 3: network has configured the UE with a measurement configuration including one or more carriers to measure. At some point the UE sends measurement report to network via serving network node gNB20, the report including measurements of one or more cells which could include for example potential LTM candidate cell(s). Step 4: network node 20 prepares one or more LTM candidate cells to be configured to the UE. Step 5: network node 20 has configured the UE with one or more LTM candidate cells. Step 6: UE replies with confirmation message. Moving to Fig. 13B Box 7: Early synchronization procedure is described. Here we use the PDCCH Order as one non-limiting example. Another example would the UE autonomous UL synchronization acquisition procedure (not defined in details and no requirements exists). Step 8: UE acquires DL synchronization on the LTM candidate cell (of network node gNB1 22). Step 9: UE measures the LTM candidate cell (here using the SSB1 as example). UE performs one or more measurements (samples). Step 10: UE reports the measurement results to the network. In this example L1-RSRP reporting is used. However, also other reporting means could be used such as L3 measurement reporting. The results sent to network includes measurement results for the candidate LTM cell and here for SSB1. Step 11: UE starts timer T1 (used for tracking time since measurement results were sent to the network). Step 12: Network activates the TCI state. In this case it is the TCI state which is associated with the SSB1 (from the measurement report). This step is optional in light of the general idea. Hence, not mandatory for the method to be applicable. Step 13: UE start timer T2 (used for tracking the time since the TCI state was activated). Box 14: UL synchronization with the candidate LTM cell (the following steps show the conventional procedure based on PDCCH Order). Step 15: network requests the UE to send preamble to the LTM candidate cell using the PDCCH Order. The PDCCH Order is in this case targeted the reported LTM candidate cell for which the UE reported measurement results for earlier. PDCCH Order in this example use SSB1. Step 16: UE check timer condition T1 is less than 160ms. Step 17: UE check timer condition T2 is less than a predetermined time ms. Box 18: in this case both T1 and T2 conditions are fulfilled. Step 19: UE retunes to the LTM candidate cell. Step 20: the UE transmits the preamble towards the LTM candidate cell. Step 21: UE retunes back to the serving cell. Box 22: in this case either T1 or T2 conditions or both are not fulfilled. Step 23: UE retunes to the LTM candidate cell. Step 24: UE receives one or more references signals (RSs) from the LTM candidate cell for acquisition of time / frequency tracking. Step 25: the UE transmits the preamble towards the LTM candidate cell. Step 26: UE retunes back to the serving cell. At this point the UE has transmitted one or more preambles to the network (steps in boxes 18 and 22 may be repeated if needed). And hence the UE will have acquired the time / frequency tracking of the LTM candidate cell. Box 27: LTM cell switch according to existing procedure. Step 28: network sends LTM switch command to the UE (target cell being the cell containing the SSB1 used in the earlier steps). Box 29: LTM switch delay and interruption (actual UE cell switch where UE cannot be scheduled and cannot transmit). Moving to Fig 13C Steps 31-34: Delays as described above. Hereafter the UE is ready in the new source cell (former LTM candidate cell). Box 35: shows an alternative UL synchronization procedure with the candidate LTM cell based on PDCCH Order according to an embodiment. Step 36: network requests the UE to send preamble to the LTM candidate cell using the PDCCH Order. The PDCCH Order is in this case targeted to the reported LTM candidate cell for which the UE reported measurement results for earlier. PDCCH Order in this example uses SSB1. Step 37: UE check timer condition T1 is less 160ms. Step 38: UE check timer condition T2 is less predetermined time ms. Box 39: in this case both T1 and T2 conditions are fulfilled. Step 40: UE retunes to the LTM candidate cell. Step 41: the UE transmits the preamble towards the LTM candidate cell. Step 42: UE retunes back to the serving cell. Box 43: in this case either T1 or T2 conditions or both are not fulfilled. Step 44: UE retunes to the LTM candidate cell. Step 45: UE receives one or more references signals (RSs) from the LTM candidate cell for acquisition of time / frequency tracking. Step 46: the UE transmits the preamble towards the LTM candidate cell. Step 47: UE retune back to the serving cell. At this point the UE has transmitted one or more preambles to the network (steps in boxes 41 and 46 may be repeated if needed). And hence the UE will have acquired the time / frequency tracking of the LTM candidate cell. Step 48: • UE will start or restart timers T1 and / or T2. In some embodiments, both the timers will be started / restarted as the preamble transmission will render the timing synchronisation for the TCI state valid and the TCI state known for the respective time periods. In other embodiments, it may be that the current TCI state being either active or known affects whether the timers are started or not, so one or more of the timers may be (re)started only when the TCI state is known on preamble transmission and / or when the TCI state is active on preamble transmission. Box 49: LTM cell switch is received . Step 50: network sends LTM switch command to the UE (target being the cell containing the SSB1 used in the earlier steps). Step 51: UE acknowledges the LTM cell switch. Box 52: LTM switch delay and interruption (actual UE cell switch where UE cannot be scheduled and cannot transmit). Steps 53-55: Delays as described in the text above. Box 56: UE has valid time / frequency information of the LTM target. Hence, the UE has for example acquired valid TA autonomously or be use of preamble transmission (TA is in this case estimated by the network (LTM candidate cell)). Step: 57: UE has valid timing information of the candidate LTM cell. Step 58: There is no need for time / frequency acquisition in the target cell after cell switch and network can assume the related delay to be zero. Box 59: If UE does not have valid time / frequency information of the LTM target. Step 60: UE needs time / frequency acquisition in the target cell after cell switch and network can assume the related timers indicating the expected delay to have a non-zero value. Step 61: delay indicates the delay. Hereafter the UE is ready in the new source cell (former LTM candidate cell). Fig. 14 shows a UE 10, a serving network node 20 and a candidate network node or target node 22 according to an embodiment. UE 10 comprises a processor 18 a data store 12 for storing candidate cell information received from the network node 20 and timers 14 and 17. There is also transmitting and receiving circuitry 16 for transmitting and receiving messages comprising information and data to the network. Network node 20 comprises a data store 22, transmitting and receiving circuitry 26 for transmitting and receiving messages with the user equipment, processing circuitry 24 and one or more timers 25. Details of known and active TCI state for cell switch delay (Tt^t-Rs) are provided below. Those where the TCI state is considered active or known based on a PDCCH order may be implemented as shown below in italics. Those that are based on UE based TA estimation conditions may be implemented as shown underlined. Any further conditions may be introduced on top of these conditions depending on how RAN4 defines UE based TA estimation requirements. Known TCI state condition includes: The target joint DL / UL TCI state or separate DL and UL TCI states in the LTM cell switch command are known if the following conditions are met: The target TCI state in the cell switch command is activated not more than a certain time before the reception of the cell switch command and SNR of the SSB associated to TCI state >-3dB; or The target TCI state in cell switch command is activated before receiving the cell switch command and the SSB associated to target TCI state is available at least once every certain time after the TCI state activation command is received and SNR of the SSB associated to TCI state >-3dB; and the UE has completed RACH preamble transmission to the target cell based on PDCCH order with the SSB associated to the target TCI state not more than a predetermined time before the reception of the cell switch command', or UE has estimated TA for the target cell based on an SSB associated to the target TCI state: or During the period from the last transmission of the RS resource used for the L1-RSRP measurement reporting for the target DL7UL TCI state to the completion of LTM cell switch, where the RS resource for L1-RSRP measurement is the RS in target DL / UL TCI state or QCLed to the target DL / UL TCI state LTM cell switch command is received within 1280 ms upon the last transmission of the RS resource for beam reporting or measurement The UE has sent at least 1 L1-RSRP report for the target DL / UL TCI state before the LTM cell switch command, and the UE has completed RACH preamble transmission to the target cell based on a PDCCH order with the SSB associated to the target TCI state not more than [Y ms] before the reception of the cell switch command, or UE has estimated TA for the target cell based on an SSB associated to the target TCI state The target DL / UL TCI state remains detectable during the LTM cell switching period The SSB associated with the target DL / UL TCI state remain detectable during the cell switching period SNR of the TCI state >-3dB Otherwise, the target joint DL / UL TCI state or separate DL and UL TCI state is unknown. Condition for TCI state being active or valid i.e. Tfirst-Rs = 0, as noted previously includes Alternative 1: Tf^t-Rs is the time for fine time tracking and acquiring full timing information of the target cell. TRs-proc is the time for SSB processing. Tfirst-Rs — 0 and Trs-pwc- 0 under the following conditions: The target TCI state indicated in the LTM cell switch command is in the serving cell active TCI state list, or The target TCI state in the cell switch command is in the LTM candidate cell active TCI state list, and the UE has estimated TA for the target cell based on an SSB associated to the target TCI state, or the time gap between receiving the LTM candidate cell TCI state activation MAC-CE and the cell switch command is at least a predetermined time and the time gap between receiving the LTM candidate cell TCI state activation MAC-CE and the cell switch command is not more than 160 ms, or the time gap between completion of RACH preamble transmission on the target cell based on a PDCCH order for the target cell with an SSB that has a QCL relationship to the target TCI state is not more than X ms, or the SSB associated to target TCI state is transmitted every 160 ms after the LTM candidate cell TCI state activation MAC-CE is received, or Otherwise, Tfirst-Rs is the time to the first SSB transmission on the target cell [after Tcmd], Trs-proc “ 2 ms. Condition for Ta^-rs = 0 (and potentially TCI state being active) Alternative 2: Tfirst-Rs is the time for fine time tracking and acquiring full timing information of the target cell. Trs-ptoc is the time for SSB processing. Tfirst-Rs = 0 and TRs-Proc= 0 under the following conditions: The target TCI state indicated in the LTM cell switch command is in the serving cell active TCI state list, or the time gap between completion of RACH preamble transmission based on a PDCCH order for the target cell with an SSB that has a QCL relationship to the target TCI state is not more than [X ms], or the UE has estimated TA for the target cell based on an SSB associated to the target TCI state, or The target TCI state in the cell switch command is in the LTM candidate cell active TCI state list, and the time gap between receiving the LTM candidate cell TCI state activation MAC-CE and the cell switch command is at least a predetermined time, and the time gap between receiving the LTM candidate cell TCI state activation MAC-CE and the cell switch command is not more than [160 ms], or the SSB associated to target TCI state is transmitted every 160 ms after the LTM candidate cell TCI state activation MAC-CE is received, or Otherwise, Tfirst-Rs is the time to the first SSB transmission on the target cell [after Tcmd], Trs-pfoc — 2 ms. A person of skill in the art would readily recognize that steps of various above-described methods can be performed by programmed computers. Herein, some embodiments are also intended to cover program storage devices, e.g., digital data storage media, which are machine or computer readable and encode machine-executable or computerexecutable programs of instructions, wherein said instructions perform some or all of the steps of said above-described methods. The program storage devices may be, e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic tapes, hard drives, or optically readable digital data storage media. The embodiments are also intended to cover computers programmed to perform said steps of the above-described methods. The tern non-transitory as used herein, is a limitation of the medium itself (i.e., tangible, not a signal) as opposed to a limitation on data storage persistency (e.g. RAM vs ROM). As used in this application, the term “circuitry” may refer to one or more or all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and / or digital circuitry) and (b) combinations of hardware circuits and software, such as (as applicable): (i) a combination of analog and / or digital hardware circuit(s) with software / firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation. This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device. As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more elements are joined by “and” or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements. The ordering of method steps set out above may not be critical or fixed and the exact ordering of the steps may be varied as appropriate. Although example embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon. Abbreviation Meaning CA Carrier Aggregation DL Downlink DRX Discontinuous reception FFS For further study FR1 Frequency range 1 FR1 / FR2 Frequency range 1 / 2 gNB gNodeB L1 / L2 / L3 Layer 1 / 2 / 3 LTM L1-L2 triggered mobility MAC Medium access control (sublayer of Layer 2 in RAN) MAC-CE Medium access control control element NR New radio NW Network PCell Primary Cell PDCCH Physical Downlink Control CHannel PDCCH Physical downlink control channel PRACH Physical Random Access Channel PSCell Primary secondary cell QCL Quasi co-Iocation RACH Random access channel RRC Radio Resource control RRC Radio resource control SCell Secondary Cell SSB Synchronization signal block T / F Time / Frequency TA Timing advance TCI Transmission configuration indicator UE User Equipment UL Uplink

Claims

1. An apparatus configured to apply lower layer, that is layer 1 or layer 2, triggered mobility LTM, said apparatus comprising:at least one processor; andat least one memory storing instructions that when executed by the at least one processor cause the apparatus at least to perform:receiving a configuration including at least one LTM candidate cell configuration including one or more transmission configuration indicator, TCI, state of the at least one candidate cell; andin response to uplink synchronization acquisition comprising acquisition of a reference signal associated with said TCI state of said candidate cell, starting at least one timer for measuring at least one of the following: a first time period or a second time period;said apparatus being caused to consider timing synchronisation of said TCI state to be valid during said first time period; andsaid apparatus being caused to consider said TCI state being known during said second time period.

2. An apparatus according to claim 1, wherein said at least one timer comprises a first timer configured to measure said first time period.

3. An apparatus according to claim 2, wherein said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether said TCI state is known, andstart said first timer for said first time period when said TCI state is determined to be known, andnot start said first timer for said first time period when said TCI state is determined not to be known.

4. An apparatus according to claim 2 or 3, wherein said apparatus is further caused to: in response to receipt of said uplink synchronization order, determine whether said TCI state is active, andstart said first timer when said TCI state is determined to be active, andnot start said first timer when said TCI state is determined not to be active.

5. An apparatus according to any preceding claim, wherein said at least one timer comprises a second timer configured to measure said second time period.

6. An apparatus according to claim 5, wherein said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether said TCI state is known, andstart said second timer for said second time period when said TCI state is known, andnot start said second timer for said second time period when said TCI state is determined to not be known.

7. An apparatus according to claim 5 or 6, wherein said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether said TCI state is active, andstart said second timer when said TCI state is determined to be active, and not start said second timer when said TCI state is determined not to be active.

8. An apparatus according to claim 2 to 4, wherein said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether timing synchronisation of said TCI state is valid, andrestart said first timer when said TCI state is determined to have valid timing synchronization, andnot restart said first timer when said TCI state is determined to be not active or determined not have valid timing synchronisation.

9. An apparatus according to any one of claims 5 to 7, wherein said apparatus is further caused to: in response to said uplink synchronization acquisition, determine whether timing synchronisation of said TCI state is valid, andstart said second timer when said TCI state is determined to have valid timing synchronization, andnot start said second timer when said TCI state is determined to be not active or determined not have valid timing synchronisation.

10. An apparatus according to any preceding claim, wherein said apparatus comprises a first and a second timer, said first timer being configured to measure said first time period and said second timer being configured to measure said second time period.

11. An apparatus according to any preceding claim, wherein said uplink synchronisation acquisition is in response to a preamble transmission caused by receipt of an uplink synchronization order including said reference signal associated with said TCI state of a candidate cell.

12. An apparatus according to claim 11, wherein said uplink synchronisation order comprises a PDCCH order.

13. An apparatus according to any one of claims 1 to 10, wherein said uplink synchronisation acquisition comprises said apparatus being caused to estimate at least some timing synchronisation information including a timing advance based on said received reference signal.

14. An apparatus according to any preceding claim, said apparatus being further caused to perform:in response to receipt of a cell switch command indicating said candidate cell as said target cell, determining whether said at least one timer indicates said user equipment has valid timing synchronisation information for said cell, and where so switching to said cell and using said timing synchronisation information for performing communications; and where notswitching to said cell and acquiring timing synchronisation information for said cell, prior to performing communications.

15. A method performed on an apparatus configured to apply lower layer, that is layer1 or layer 2, triggered mobility LTM, said method comprising:receiving a configuration including at least one LTM candidate cell configuration including one or more transmission configuration indicator, TCI, state of the at least one candidate cell; andin response to uplink synchronization acquisition comprising acquisition of a reference signal associated with said TCI state of said candidate cell, starting at least one timer for measuring at least one of the following: a first time period or a second time period;said first time period providing a time period during which timing synchronisation of said TCI state is deemed to be valid; andsaid second time period providing a time period during which timing synchronisation of said TCI state is deemed to be known.

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