Random access procedures for ltm execution

Abstract

The invention relates to LTM procedures. One claim specifies that when and random access (RA) procedure preamble re-transmission is ordered by a physical downlink control channel (PDCCH) the PREAMBLE_POWER_RAMPING_COUNTER is incremented by one. Another claim specifies that a UE configured with an LTM candidate cell configuration performing LTM cell switching terminates the RA procedure by sending a preamble to the network. Computing of Random Access Radio Temporary Network Identifiers (RA_RNTI) and various LTM termination / completion triggers are mentioned.

Description

The present invention relates to L1 / L2-Triggered Mobility (LTM) in a telecommunication network. It provides low latency, low complexity and no user plane data loss. Embodiments also consider the impacts on legacy behaviours, such that embodiments incorporating a new LTM cell switch feature described herein does not conflict with them. According to the present invention there is provided an apparatus and method as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and the description which follows. According to a first aspect of the present invention, there is provided a method of transmitting a Random Access preamble in a telecommunication system, wherein if the Random Access procedure is initiated by a Physical Downlink Control Channel, PDCCH, order for an LTM candidate cell as preamble re-transmission, then a variable, PREAMBLE_POWER_RAMPING_COUNTER, is incremented by 1. In an embodiment, except for contention-free Random Access Preamble for beam failure recovery request and contention-free Random Access Preamble triggered by a PDCCH order for an LTM candidate cell, Radio Network Temporary Identifier for Random Access Response, RA-RNTI, associated with the Physical Random Access Channel, PRACH, occasion in which the Random Access Preamble is transmitted, is computed. In an embodiment, if the Random Access Procedure is triggered by a PDCCH order for an LTM candidate cell, then the Random Access procedure is considered to be completed. According to a second aspect of the present invention, there is provided a method for performing a LTM cell switch by a User Equipment, UE, communicatively coupled to a telecommunication network, configured with an LTM candidate cell configuration, the method comprising terminating a Random Access procedure by sending a preamble to the telecommunication network. In an embodiment, if the random access procedure fails, the UE receives a PDCCH instruction from the network indicating whether the preamble is a first transmission or a retransmission, with the UE increasing a power ramping variable accordingly. In an embodiment, the UE completes the LTM cell switch procedure by sending a RRCReconfigurationComplete message to the candidate cell. In an embodiment, in a RACH-based LTM cell switch the LTM cell switch is completed when the RACH is successfully completed. In an embodiment, in a RACH-less based LTM cell switch, the LTM call switch is successfully completed when the UE determines that the network has successfully received its first uplink data. In an embodiment, there is further provided the step ofchecking HARQ ACK or RLC ACK for the first uplink data. According to a third aspect of the present invention, there is provided apparatus arranged to perform the method of any preceding aspect. Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example only, to the accompanying diagrammatic drawings in which: Figure 1 shows a signalling procedure for LTM; and Figure 2 shows a flowchart illustrating an embodiment of the invention. LTM is a procedure in which a base station (gNB) receives L1 measurement reports from User Equipments (UEs), and on their basis, the gNB changes UEs’ serving cell(s) by a cell switch command through a Medium Access Control (MAC) Control Element (CE), which indicates an LTM candidate cell configuration that the gNB previously prepared and provided to the UE through Radio Resource Control (RRC) signalling. Then, a cell switch is triggered by selecting the indicated LTM candidate cell configuration as the target configuration by the gNB. An LTM candidate cell configuration can only be added, modified and released by network via RRC signalling. The LTM procedure can be used to reduce the mobility. The following principles apply to LTM: Each LTM candidate cell configuration can be provided as delta configuration on top of a reference configuration, which is used to form a complete candidate cell configuration. The reference configuration can be managed separately, and a UE stores the reference configuration as a separate configuration. The LTM candidate cell configuration can be configured in RRCReconfiguration message via SRB1 (e.g. Signaling Radio Bearer), i.e. it can be configured after SRB1 establishment. - When a complete candidate cell configuration is applied, it replaces the current UE configuration at the time of cell switch. Although the reconfiguration procedure makes replacement, it doesn’t necessarily reset MAC, RLC or PDCP layer. - User plane is continued without reset to support lossless delivery of user palne data (e.g. intra-DU LTM), if it is configured in RRC signaling, with the target to avoid data loss and the additional delay of data recovery. Specifically, indicators for RLC re-establishment or MAC reset (or Partial MAC reset) or PDCP re-establishment or PDCP data recovery or SDU discard can be included in RRCReconfiguration message as listed below, which can be included with LTM candidate cell configuration together: a indicator for MAC reset or Partial MAC reset included in RRCReconfiguration message (e.g. in Cell group (or Cell) configuration) a   indicator   for RLC   re-establishment   (e.g. reestablishRLC)   included   in RRCReconfiguration message (e.g. in Cell group (or Cell) configuration) a  indicator  for PDCP  re-establishment  (e.g.  reestablishPDCP)  included  in RRCReconfiguration message (e.g. in radio bearer configuration, i.e. RadioBearerConfig IE ) a indicator for PDCP data recovery (e.g. recoverPDCP) included in RRCReconfiguration message (e.g. in radio bearer configuration, i.e. RadioBearerConfig IE ) a indicator for SDU discard (e.g. discardOnPDCP) for SRBs (e.g. SRB1 or SRB3) included in RRCReconfiguration message (e.g. in radio bearer configuration, i.e. RadioBearerConfig IE ), which can be called PDCP SDU discard. The above indicators can be included in RRCReconfiguration message (e.g. in Cell group configuration or LTM candidate cell configuration) including candidate cell configurations for LTM. Upon the reception of the RRCReconfiguration message, UE can store the cell configuration and the indicators and does not apply them to UE configuration. When UE successfully completes it after triggering the LTM procedure (or cell switch) by a cell switch command through a MAC CE indicating the target cell(s) (e.g. identifier(s)), beam index, or, TA (Timing Advance) value, UE can apply the LTM candidate cell configuration and the indicators corresponding to the target cell (or indicated cell in MAC CE). The following conditions are considered as successful completion of the LTM procedure (i.e. cell switch): For RACH-based LTM procedure (cell switch), the UE considers that LTM execution procedure is successfully completed when the RACH is successfully completed. For RACH-less LTM procedure (cell switch), the UE considers that LTM execution procedure is successfully complete when the UE determines the NW has successfully received its first UL data. When the above condition for successful completion of LTM cell switch is met (or the LTM cell candidate configuration is complete, i.e. if the LTM cell candidate configuration is indicated to be applied by a indicator), UE can apply the LTM candidate cell configuration and the indicators corresponding to the target cell (or indicated cell in MAC CE) to UE configuration. This approach can avoid UE’s early application and reverting it back when it fails, which eases UE implementation. Moreover, the UE cannot know the time when the network sends MAC CE indicating LTM cell switch to UE. For example, when the above condition is met, UE performs MAC reset (or partial MAC reset) if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. When the condition is met, the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate MAC reset (or partial MAC reset) to MAC layer, if configured. For example, when the above condition is met, UE performs RLC re-establishment if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. When the condition is met, the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate RLC re-establishment to RLC layer, if configured. For example, when the above condition is met, UE performs PDCP re-establishment if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. When the condition is met, the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate PDCP re-establishment to PDCP layer, if configured. For example, when the above condition is met, UE performs PDCP data recovery if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. PDCP data recovery can be configured only for a PDCP entity associated with AM RLC entities (RLC entity with AM (Acknowledged Mode) mode). When the condition is met, the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate PDCP data recovery to PDCP layer, if configured. For example, when the above condition is met, UE performs SDU discard in PDCP entity (i.e. PDCP SDU discard) if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. SDU discard can be configured only for a PDCP entity of SRBs associated with AM RLC entities (RLC entity with AM (Acknowledged Mode) mode). When the condition is met, the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate SDU discard to PDCP layer, if configured. For SRBs, when upper layers (e.g. RRC layer) request a PDCP SDU discard, the PDCP entity shall discard all stored PDCP SDUs and PDCP PDUs. It is beneficial to discard old RRC messages of SRBs to prevent unnecessary (re-)transmission to the target cell. The PDCP SDU discard for SRBs (e.g. SRB1 or SRB3) can be triggered and performed when the LTM cell switch procedure fails (e.g. the supervisor timer for LTM cell switch is expired), in order to avoid unnecessary (retransmission of RRC message (e.g. RRC Reconfiguration Complete message for the target cell UE failed to LTM cell switch to). The RLC re-establishment for SRBs (e.g. SRB1 or SRB3) can be triggered and performed when the LTM cell switch procedure fails (e.g. the supervisor timer for LTM cell switch is expired), in order to avoid unnecessary (re-)transmission of RRC message (e.g. RRC Reconfiguration Complete message for the target cell UE failed to LTM cell switch to) In another embodiment, upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), UE can apply the LTM candidate cell configuration and the indicators corresponding to the target cell (or indicated cell in MAC CE) to UE configuration. This approach can avoid UE’s early application and revert it back when it fails, which eases UE implementation. This approach can avoid UE’s early application. As the UE cannot know the time when the network sends MAC CE indicating LTM cell switch, UE can follow this approach to apply the configuration timely. The reception of MAC CE indicating LTM cell switch can imply LTM cell switch execution. For example, upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), UE performs MAC reset (or partial MAC reset) if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. Upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate MAC reset (or partial MAC reset) to MAC layer, if configured. For example, upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), UE performs RLC re-establishment if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. Upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate RLC reestablishment to RLC layer, if configured. For example, upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), UE performs PDCP re-establishment if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. Upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate PDCP reestablishment to PDCP layer, if configured. For example, upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), UE performs PDCP data recovery if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. PDCP data recovery can be configured only for a PDCP entity associated with AM RLC entities (RLC entity with AM (Acknowledged Mode) mode). Upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate PDCP data recovery to PDCP layer, if configured. For example, upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), UE performs SDU discard in PDCP entity (i.e. PDCP SDU discard) if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. SDU discard can be configured only for a PDCP entity of SRBs associated with AM RLC entities (RLC entity with AM (Acknowledged Mode) mode). Upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate SDU discard to PDCP layer, if configured. For SRBs, when upper layers (e.g. RRC layer) request a PDCP SDU discard, the PDCP entity shall discard all stored PDCP SDUs and PDCP PDUs. It is beneficial to discard old RRC messages of SRBs to prevent unnecessary (re-)transmission to the target cell. The PDCP SDU discard for SRBs (e.g. SRB1 or SRB3) can be triggered and performed when the LTM cell switch procedure fails (e.g. the supervisor timer for LTM cell switch is expired), in order to avoid unnecessary (re-)transmission of RRC message (e.g. RRC Reconfiguration Complete message for the target cell UE failed to LTM cell switch to). The RLC reestablishment for SRBs (e.g. SRB1 or SRB3) can be triggered and performed when the LTM cell switch procedure fails (e.g. the supervisor timer for LTM cell switch is expired), in order to avoid unnecessary (re-)transmission of RRC message (e.g. RRC Reconfiguration Complete message for the target cell UE failed to LTM cell switch to) In another embodiment, upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution) or upon the reception of RRCReconfiguration message including the indicators(or the LTM cell candidate configuration is complete, i.e. if the LTM cell candidate configuration is indicated to be applied by a indicator), UE can apply the LTM candidate cell configuration (e.g. complete LTM cell configuration) and the indicators corresponding to the target cell (or indicated cell in MAC CE) to UE configuration. This approach can be efficiently performed by the network. For example, the network sends MAC CE indicating LTM cell switch and RRCReconfiguration message including indicators together (e.g. at a time or in the same MAC PDU) to make UE perform the following actions. For example, upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution) or upon the reception of RRCReconfiguration message including the indicators, UE performs MAC reset (or partial MAC reset) if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. Upon the reception of MAC CE indicating LTM cell switch (or LTM cell switch execution), the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate MAC reset (or partial MAC reset) to MAC layer, if configured. For example, upon the reception of RRCReconfiguration message including the indicators, UE performs RLC re-establishment if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. Upon the reception of RRCReconfiguration message including the indicators, the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate RLC re-establishment to RLC layer, if configured. For example, upon the reception of RRCReconfiguration message including the indicators, UE performs PDCP re-establishment if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. Upon the reception of RRCReconfiguration message including the indicators, the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate PDCP re-establishment to PDCP layer, if configured. For example, upon the reception of RRCReconfiguration message including the indicators, UE performs PDCP data recovery if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. PDCP data recovery can be configured only for a PDCP entity associated with AM RLC entities (RLC entity with AM (Acknowledged Mode) mode). Upon the reception of RRCReconfiguration message including the indicators, the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate PDCP data recovery to PDCP layer, if configured. For example, upon the reception of RRCReconfiguration message including the indicators UE performs SDU discard in PDCP entity (i.e. PDCP SDU discard) if the indicator is configured in the stored configuration (e.g. LTM candidate cell configuration for LTM) corresponding to the target cell (or indicated cell in MAC CE or successfully switched cell), which can be done by applying the complete LTM cell configuration. SDU discard can be configured only for a PDCP entity of SRBs associated with AM RLC entities (RLC entity with AM (Acknowledged Mode) mode). Upon the reception of RRCReconfiguration message including the indicators, the MAC layer can indicate the successful completion of LTM cell switch to the RRC layer. The RRC layer can indicate SDU discard to PDCP layer, if configured. For SRBs, when upper layers (e.g. RRC layer) request a PDCP SDU discard, the PDCP entity shall discard all stored PDCP SDUs and PDCP PDUs. It is beneficial to discard old RRC messages of SRBs to prevent unnecessary (re-)transmission to the target cell. The PDCP SDU discard for SRBs (e.g. SRB1 or SRB3) can be triggered and performed when the LTM cell switch procedure fails (e.g. the supervisor timer for LTM cell switch is expired), in order to avoid unnecessary (re-)transmission of RRC message (e.g. RRC Reconfiguration Complete message for the target cell UE failed to LTM cell switch to). The RLC reestablishment for SRBs (e.g. SRB1 or SRB3) can be triggered and performed when the LTM cell switch procedure fails (e.g. the supervisor timer for LTM cell switch is expired), in order to avoid unnecessary (re-)transmission of RRC message (e.g. RRC Reconfiguration Complete message for the target cell UE failed to LTM cell switch to) NOTE: This delayed application of configuration is totally different from the legacy behaviour because UE performs MAC reset / RLC / PDCP re-establishment, if configured, upon the reception of RRCReconfiguration in legacy procedure. In the above, the stored LTM candidate cell configuration can be regarded as reference configuration, which can be applied at a specific time as proposed. Security is not updated in LTM. In another embodiment, the network decides whether to update the security based on the type of mobility (e.g. to which cell UE is indicated to perform cell switch). For example, the security configuration for security update is not included in the LTM candidate cell configuraton (RRCReconfiguration) for the case that this candidate cell belongs to intra-gNB-DU or intra-gNB-CU. However, the security configuration for security update is included in the LTM candidate cell configuraton (RRCReconfiguration) for the case that this candidate cell belongs to inter-gNB-DU (i.e. inter-gNB-DU mobility case) When the condition for successful completion of LTM cell switch is met after triggering the LTM procedure (or cell switch) by a cell switch command through a MAC CE, UE applies and updates the security configuration to the current configuration, if configured, Subsequent LTM between LTM candidate cell configurations (i.e., UE does not release other LTM candidate cell configurations after LTM is triggered) can be performed without RRC reconfiguration. LTM supports both intra-gNB-DU and intra-gNB-CU inter-gNB-DU mobility. LTM also supports inter-frequency mobility, including mobility to inter-frequency cell that is not a current serving cell. The following scenarios are supported: PCell change in non-CA scenario, PCell change without SCell change in CA scenario, PCell change with SCell change(s) in CA scenario, including the following cases: a) The target PCell / target SCell(s) is not a current serving cell (CA-to-CA scenario with PCell change) b) The target PCell is a current SCell c) The target SCell is the current PCell. Dual connectivity scenario, at least for the PSCell change without MN involvement case, i.e. intra-SN. When UE is configured with dual connectivity (i.e. SCG(Secondary Cell Group) and MCG(Master Cell Group)), For SCG (or LTM procedure for SCG), the LTM candidate cell configuration of SCG can be configured in RRCReconfiguration message via SRB3, i.e. it can be configured after SRB3 establishment. For SCG (or LTM procedure for SCG), the LTM candidate cell configuration of SCG cannot be configured via SRB1. For MCG (or LTM procedure for MCG), the LTM candidate cell configuration of MCG can be configured in RRCReconfiguration message via SRB1, i.e. it can be configured after SRB1 establishment. Cell switch trigger is conveyed in a MAC CE, which contains at least a candidate configuration index together with beam indication. UE may perform CBRA or CFRA at cell switch. UE may also skip random access procedure (i.e. RACH-less solution) if UE doesn’t need to acquire TA for the target cell during cell switch.. The overall procedure for LTM is shown in Figure 1. Subsequent LTM is done by repeating the early synchronization, LTM execution, and LTM completion steps without releasing other LTM candidate cell configurations after each LTM completion. The procedure for LTM, as shown in Fig 1 is as follows: 1. The UE sends a MeasurementReport message to the gNB. The gNB decides to use LTM and initiates candidate cell(s) preparation. 2. The gNB transmits an RRCReconfiguration message to the UE including the LTM candidate cell configurations of one or multiple candidate cells. 3. The UE stores the LTM candidate cell configurations and transmits a RRCReconfigurationComplete message to the gNB. 4a / 4b. The UE may perform DL synchronization and TA acquisition with candidate cell(s) before receiving the cell switch command. Synchronization for candidate cell(s) before cell switch command is supported, at least based on SSB. In an embodiment of this invention, TA acquisition of candidate cell(s) before LTM cell switch command is supported, is at least based on Physical Downlink Control Channel (PDCCH) ordered RACH, where the PDCCH order is only triggered by source cell. The source cell can trigger UE’s RACH (Random Access Procedure) toward a candidate cell by PDCCH order to acquire TA(Timing Advance or Timing Advance value) for the candidate cell, which only performs preamble transmission and does not expect the reception of RAR (Random Access Response) to ease network implementation and UE implementation. Specifically, the preamble transmission during this RACH(Random Access procedure) for TA acquisition (i.e. early RACH) can be considered as this Random Access procedure is successfully completed. To reduce the processing complexity, UE does not have to calculate RA-RNTI (RNTI(Radio Network Temporary Identifier) for Random Access Response) before / when the preamble is transmitted, unlike normal Random Access procedure (RACH). To be more specific, UE transmits preamble to a candidate cell as indicated by PDCCH order. The network (or DU(Distributed Unit) or the candidate cell) calculates the TA (Timing Advance). The source cell / DU can get the calculated TA from the candidate cell / DU. By doing this RACH(Random Access procedure) for TA acquisition (i.e. early RACH), the network can have the TA values for the candidate cells and knows whether these TAs are still valid or not, e.g., by maintaining a network side timer (i.e. TAT(timeAlignmentTimer) for each TA value or each candidate cell). In this way, the source cell / DU gets to know the value and the validity of candidate cell TA. The source cell / DU needs to know whether a candidate cell TA is still valid because the source cell / DU needs to determine whether it can initiate a RACH-less solution for LTM cell switch and then determine whether it needs to include a beam indication (e.g.TCI state) and TA information in the LTM MAC CE. Therefore, the network can indicate a valid TA to the UE or indicate whether a TA is still valid in LTM MAC CE. The UE may not need to maintain a TA timer for candidate cells, which simplifies UE implementation. Upon the reception of the TA information indicated in LTM MAC CE, the UE can apply the TA value and start the TA timer for the target LTM candidate cell upon LTM execution (i.e. LTM cell switch) and UE can perform LTM cell switch without Random access procedure (i.e. with RACH-less solution) if TAT for the target LTM candidate cell is running (i.e. TA value is vaild) or if Beam failure is not detected for the target LTM candidate cell, which means that UE can monitor PDCCH from the target LTM candidate cell or UE can use configured grants the first UL data transmission to the target cell for RACH-less LTM execution (LTM cell switch). 5. The UE performs L1 measurements on the configured candidate cell(s), and transmits lower-layer measurement reports to the gNB. Note that the order of DL / UL sync (step 4a / 4b) and L1 measurement (step 5) may be reversed. 6. The gNB decides to execute cell switch to a target cell, and transmits a MAC CE triggering cell switch by including the candidate configuration index of the target cell. The UE switches to the configuration of the target cell. 7. The UE performs random access procedure towards the target cell, if cell switch needs to include performing random access procedure. 8. The UE indicates successful completion of the cell switch towards the target cell. The UE can perform the steps 4-8 multiple times for subsequent LTM cell switch based on the configuration provided in step 2. The following relates to User Plane (UP) handling. In LTM, whether the UE performs partial or full MAC reset, re-establishes RLC, performs data recovery with PDCP during cell switch is explicitly controlled by the network through RRC signalling. MAC / RLC re-establishment / PDCP data recovery / PDCP re-establishment (when configured) RF retuning (e.g. needed for inter-frequency), baseband retuning When a Random Access procedure is initiated, UE selects a set of Random Access resources and initialises the following parameters for the Random Access procedure according to the values configured by RRC for the selected set of Random Access resources: preambleReceivedTargetPower initial Random Access Preamble power for 4-step RA type; powerRampingStep: the power-ramping factor; msgA-PreamblePowerRampingStep: the power ramping factor for MSGA preamble; ra-Preamblelndex'. Random Access Preamble; ra-PreambleStartlndex: the starting index of Random Access Preamble(s) for on-demand SI request; startPreambleForThisPartition: the first preamble associated with the set of Random Access Resources applicable to the Random Access procedure; preambleTransMax: the maximum number of Random Access Preamble transmission; The following UE variables are used for the Random Access procedure: PREAMBLEJNDEX; PREAMBLE_TRANSMISSION_COUNTER; PREAMBLE- TRANSMISSION_COUNTER_LTM; PREAMBLE_POWER_RAMPING_COUNTER; PREAMBLE_POWER_RAMPING_COUNTER_LTM; PREAMBLE_POWER_RAMPING_STEP; PREAMBLE_RECEIVED_ TARGET_POWER; POWER_OFFSET_2STEP_RA; MSGA_PREAMBLE_POWER_RAMPING_STEP. In embodiments of this invention, a RACH-less solution is supported (i.e. LTM cell switch without Random Access procedure) when UE performs LTM procedure (e.g. LTM execution) by the first MAC CE (i.e. LTM triggering MAC CE). In RACH-less procedure, the UE needs a valid TA to send the first UL message during LTM execution procedure (i.e. LTM cell switch). To provide the TA with early RACH procedure (i.e. PDCCH-ordered Random Access procedure before the first MAC CE), PDCCH-ordered Random Access procedure without RAR(Random Access Response) is provided. When the Random Access procedure for TA acquisition of LTM candidate cell(s) is triggered / indicated by PDCCH order (e.g. by an indication), UE performs Random Access procedure, i.e. UE transmits the preamble to PRACH (Physical Random Access Channel) resource of the indicated LTM candidate cell(s) and complete the Random Access procedure, i.e. the preamble transmission during this Random Access procedure for TA acquisition (i.e. early RACH) can be considered as this Random Access procedure is successfully completed. The preamble or the PRACH resources can be indicated by PDCCH order or (pre-)configured by RRC message (e.g. RRCReconfiguration message). To reduce the processing complexity, the UE does not calculate RA-RNTI (RNTI(Radio Network Temporary Identifier) for Random Access Response) before / when the preamble is transmitted, unlike normal Random Access procedure (RACH). To enable this functionality, one of the following options can be implemented: Option 1: In this option, the network can avoid another Random access procedure during the Random Access procedures for TA acquisition (i.e. before the successful completion of TA acquisition). The first preamble transmission for TA acquisition may not be successful and the network can request preamble retransmission for TA acquisition. In this case, the UE increments the first variable (PREAMBLE_POWER_RAMPING_COUNTER'), calculates the preamble received target power, and retransmits the preamble with a higher power than the first preamble. For example, if the Random Access procedure is not initiated by the PDCCH order for an LTM candidate cell as preamble re-transmission (or if the normal Random Access procedure is initiated or if the Random Access procedure is initiated for LTM execution (i.e. LTM cell switch)), UE sets the first variable to 1. In other words, if the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell as preamble transmission, i.e. first transmission (or if the normal Random Access procedure is initiated or if the Random Access procedure is initiated for LTM execution (i.e. LTM cell switch)), UE sets the first variable to 1. If the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell as preamble re-transmission, UE does not set the first variable to 1. If the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell as preamble re-transmission, UE increments the first variable by 1. Based on this, UE can calculate the preamble received target power, and retransmit the preamble with the higher power than the first preamble. To achieve this, the following procedure can be implemented : Random Access procedure initialization. When the Random Access procedure is initiated on a Serving Cell or to an LTM candidate cell (or if the Random Access procedure is initiated for LTM execution (i.e. LTM cell switch) or if the Random Access procedure is initiated for TA acquisition for an LTM candidate cell), the MAC entity shall: 1>   flush the Msg3 buffer; 1>   flush the MSGA buffer; 1 > set the PREAMBLE_TRANSMISSION_COUNTER to 1; 1> set the PREAMBLE_POWER_RAMPING_COUNTER to 1, if the Random Access procedure is not initiated by the PDCCH order for an LTM candidate cell (i.e. for TA acquisition of the cell) as preamble re-transmission ; Random Access Preamble transmission The MAC entity shall, for each Random Access Preamble: 1 > if PREAMBLE_TRANSMISSION_COUNTER is greater than one; and 1> if the notification of suspending power ramping counter has not been received from lower layers; and 1> if LBT failure indication was not received from lower layers for the last Random Access Preamble transmission; and 1> if SSB or CSI-RS selected is not changed from the selection in the last Random Access Preamble transmission: 2> increment PREAMBLE_POWER_RAMPING_COUNTER by 1. 1> if the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell (i.e. for TA acquisition of the cell) as preamble re-transmission: 2> increment PREAMBLE_POWER_RAMPING_COUNTER by 1. 1 > select the value of DELTA_PREAMBLE ; 1> set PREAMBLE_RECEIVED_TARGET_POWER to preambleReceivedTargetPower + DELTA_PREAMBLE + (PREAMBLE_POWER_RAMPING_COUNTER      1) x PREAMBLE_POWER_RAMPING_STEP + POWER_OFFSET_2STEP_RA; 1> except for contention-free Random Access Preamble for beam failure recovery request and Random Access Preamble for an LTM candiate cell by the PDCCH-ordered Random Access procedure (i.e. for TA acquisition of the cell), compute the RA-RNTI associated with the PRACH occasion in which the Random Access Preamble is transmitted; (UE doesn’t have to compute the RA-RNTI associated with the PRACH occasion in which the Random Access Preamble is transmitted if Random Access Preamble is transmitted by PDCCH order for TA acquisition of the LTM candidate cell because the reception of RAR is not expected. However, UE computes the RA-RNTI associated with the PRACH occasion in which the Random Access Preamble is transmitted, for normal Random Access procedure or Random Access procedure for LTM execution (i.e. LTM cell switch). 1> instruct the physical layer to transmit the Random Access Preamble using the selected PRACH occasion, corresponding RA-RNTI (if available), PREAMBLE_INDEX, and PREAMBLE_RECEIVED_ TARGET_POWER. 1> if the Random Access Procedure is triggered by a PDCCH order for a LTM candidate cell (i.e. for TA acquisition of the cell): (The preamble transmission during this Random Access procedure for TA acquisition (i.e. early RACH) can be considered as this Random Access procedure is successfully completed) 2> consider this Random Access procedure successfully completed (or consider this Random Access Response reception successful). Option 2: In this option, the network can allow another Random Access procedure during the Random Access procedures for TA acquisition (i.e. before the successful completion of TA acquisition)          by          introducing          the          second          variable (PREAMBLE_POWER_RAMPING_COUNTER_LTM). It can be also be extended to support TA acquisition for multiple LTM candidates by having the second variable per LTM candidate cell. The first preamble transmission for TA acquisition may not be successful and the network can request preamble retransmission for TA acquisition. In this case, the UE increments the second variable, calculates the preamble received target power, and retransmit the preamble with the higher power than the first preamble. For example, if the Random Access procedure is not initiated by the PDCCH order for an LTM candidate cell as preamble re-transmission, UE sets the second variable to 1. In other words, if the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell as preamble transmission, UE sets the second variable to 1. If the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell as preamble re-transmission, UE does not set the second variable to 1. If the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell as preamble re-transmission, UE increments the second variable by 1. Based on this, UE can calculate the preamble received target power, and retransmit the preamble with the higher power than the first preamble. However, if the normal Random Access procedure is initiated or if the Random Access procedure is initiated for LTM execution (i.e. LTM cell switch), UE sets the first variable to 1. In other words, if the normal Random Access procedure is initiated or if the Random Access procedure is initiated for LTM execution (i.e. LTM cell switch), UE sets the first variable to 1. If the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell as preamble re-transmission, UE does not set the first variable to 1. if PREAMBLE_TRANSMISSION_COUNTER is greater than one; and if the notification of suspending power ramping counter has not been received from lower layers; and if LBT failure indication was not received from lower layers for the last Random Access Preamble transmission and if SSB or CSI-RS selected is not changed from the selection in the last Random Access Preamble transmission, UE increments the first variable by 1. Based on this, UE can calculate the preamble received target power, and retransmit the preamble with the higher power than the first preamble. To achieve this, the following procedure can be implemented (In another embodiment, the first variable can be reused as the second variable): Random Access procedure initialization When the Random Access procedure is initiated on a Serving Cell (or when the Random Access procedure is initiated for LTM execution (i.e. LTM cell switch) (and if the Random Access procedure is not initiated on a Serving Cell towards an LTM candidate cell (for TA acquisition of the LTM candidate cell by a PDCCH order including indications), the MAC entity shall: 1>   flush the Msg3 buffer; 1>   flush the MSGA buffer; 1 > set the PREAMBLE_TRANSMISSION_COUNTER to 1; 1 > set the PREAMBLE_PO WER_RAMPING_COUNTER to 1; 1 > set the PREAMBLE_BACKOFF to 0 ms; When the Random Access procedure is initiated on a Serving Cell towards an LTM candidate cell (for TA acquisition of the LTM candidate cell by a PDCCH order including indications, e.g. LTM candidate cell identity, TA acquisition, preamble, PRACH resource, etc), the MAC entity shall: 1>   flush the Msg3 buffer; 1>   flush the MSGA buffer; 1 > set the PREAMBLE_TRANSMISSION_COUNTER_LTM to 1; 1> set the PREAMBLE_POWER_RAMPING_COUNTER_LTM to 1 (or set the PREAMBLE_POWER_RAMPING_COUNTER_LTM for the LTM candidate cell to 1), if the Random Access procedure is not initiated by the PDCCH order for an LTM candidate cell (i.e. for TA acquisition of the cell) as preamble re-transmission ; (PREAMBLE_POWER_RAMPING_COUNTER_LTM can be set to 1 if the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell (i.e. for TA acquisition of the cell) as preamble transmission (i.e. first transmission) or if LTM execution is successfully completed or upon the reception of the first MAC CE (i.e. LTM triggered MAC CE). Random Access Preamble transmission The MAC entity shall, for each Random Access Preamble: 1 > if PREAMBLE_TRANSMISSION_COUNTER is greater than one; and 1> if the notification of suspending power ramping counter has not been received from lower layers; and 1> if LBT failure indication was not received from lower layers for the last Random Access Preamble transmission; and 1> if SSB or CSI-RS selected is not changed from the selection in the last Random Access Preamble transmission: 2> increment PREAMBLE_POWER_RAMPING_COUNTER by 1. 1> if the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell (i.e. for TA acquisition of the cell) as preamble re-transmission: 2> increment PREAMBLE_POWER_RAMPING_COUNTER_LTM by 1 (or increment PREAMBLE_POWER_RAMPING_COUNTER_LTM for the LTM candidate cell by 1. 1 > select the value of DELTA_PREAMBLE ; 1> set PREAMBLE_RECEIVED_TARGET_POWER to preambleReceivedTargetPower + DELTA_PREAMBLE + (PREAMBLE_POWER_RAMPING_COUNTER_LTM - 1) x PREAMBLE_POWER_RAMPING_STEP + POWER_OFFSET_2STEP_RA, if the Random Access procedure is initiated by the PDCCH order for an LTM candidate cell (i.e. for TA acquisition of the cell); 1> set PREAMBLE_RECEIVED_TARGET_POWER to preambleReceivedTargetPower + DELTA_PREAMBLE + (PREAMBLE_POWER_RAMPING_COUNTER      1) x PREAMBLE_POWER_RAMPING_STEP + POWER_OFFSET_2STEP_RA if the Random Access procedure is not initiated by the PDCCH order for an LTM candidate cell (i.e. for TA acquisition of the cell); 1> except for contention-free Random Access Preamble for beam failure recovery request and Random Access Preamble for an LTM candiate cell by the PDCCH-ordered Random Access procedure (i.e. for TA acquisition of the cell), compute the RA-RNTI associated with the PRACH occasion in which the Random Access Preamble is transmitted; (UE doesn’t have to compute the RA-RNTI associated with the PRACH occasion in which the Random Access Preamble is transmitted if Random Access Preamble is transmitted by PDCCH order for TA acquisition of the LTM candidate cell because the reception of RAR is not expected. However, UE computes the RA-RNTI associated with the PRACH occasion in which the Random Access Preamble is transmitted, for normal Random Access procedure or Random Access procedure for LTM execution (i.e. LTM cell switch). 1> instruct the physical layer to transmit the Random Access Preamble using the selected PRACH occasion, corresponding RA-RNTI (if available), PREAMBLE_INDEX, and PREAMBLE_RECEIVED_ TARGET_POWER. 1> if the Random Access Procedure is triggered by a PDCCH order for a LTM candidate cell (i.e. for TA acquisition of the cell): (The preamble transmission during this Random Access procedure for TA acquisition (i.e. early RACH) can be considered as this Random Access procedure is successfully completed) 2> consider this Random Access procedure successfully completed (or consider this Random Access Response reception successful); Completion of Random Access procedure. It would be beneficial to have cross-layer interaction between MAC layer and RRC layer to make RRC layer perform RRC-specific behaviours (e.g. stop the supervisor timer for LTM execution procedure). Upon completion of the Random Access procedure, the MAC entity shall: 1> discard any explicitly signalled contention-free Random Access Resources for 2-step RA type and 4-step RA type except the 4-step RA type contention-free Random Access Resources for beam failure recovery request, if any; 1> flush the HARQ buffer used for transmission of the MAC PDU in the Msg3 buffer and the MSGA buffer. Upon successful completion of the Random Access procedure initiated for DAPS handover, the target MAC entity shall: 1> indicate the successful completion of the Random Access procedure to the upper layers. Upon successful completion of the Random Access procedure initiated for LTM execution (or LTM cell switch or LTM execution procedure or by the reception of the first MAC CE), the MAC entity shall: 1> indicate the successful completion of the Random Access procedure (or the LTM execution procedure) to the upper layers. Upon successful completion of the LTM execution procedure initiated for LTM execution (or LTM cell switch or by the reception of the first MAC CE), the MAC entity shall: 1> indicate the successful completion of the LTM execution procedure to the upper layers. For RACH-based LTM execution procedure (i.e. LTM execution procedure with Random Access procedure), the UE considers that LTM execution procedure is successfully completed when the RACH is successfully completed. For RACH-less LTM execution procedure (i.e. LTM execution procedure without Random Access procedure), the UE considers that LTM execution procedure is successfully completed when the UE determines that the network has successfully received its first UL data (e.g. by checking HARQ ACK or RLC ACK for the first UL data). For the sake of completeness, Figure 2 shows a flowchart illustrating an embodiment of the invention. At S101, a random access procedure is initiated. At S102, a Physical Downlink Control Channel, PDCCH, order is provided for an LTM candidate cell as preamble re transmission. At S103, a variable, PREAMBLE_POWER_RAMPING_COUNTER, is incremented by 1. At least some of the example embodiments described herein may be constructed, partially or wholly, using dedicated special-purpose hardware. Terms such as ‘component’, ‘module’ or ‘unit’ used herein may include, but are not limited to, a hardware device, such as circuitry in the form of discrete or integrated components, a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC), which performs certain tasks or provides the associated functionality. In some embodiments, the described elements may be configured to reside on a tangible, persistent, addressable storage medium and may be configured to execute on one or more processors. These functional elements may in some embodiments include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. Although the example embodiments have been described with reference to the components, modules and units discussed herein, such functional elements may be combined into fewer elements or separated into additional elements. Various combinations of optional features have been described herein, and it will be appreciated that described features may be combined in any suitable combination. In particular, the features of any one example embodiment may be combined with features of any other embodiment, as appropriate, except where such combinations are mutually exclusive. Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of others. Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, 5 or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A method of transmitting a Random Access preamble in a telecommunication system, wherein if the Random Access procedure is initiated by a Physical Downlink Control Channel, PDCCH, order for an LTM candidate cell as preamble re-transmission, then a variable, PREAMBLE_POWER_RAMPING_COUNTER, is incremented by 1.

2. The method of claim 1 wherein except for contention-free Random Access Preamble for beam failure recovery request and contention-free Random Access Preamble triggered by a PDCCH order for an LTM candidate cell, Radio Network Temporary Identifier for Random Access Response, RA-RNTI, associated with the Physical Random Access Channel, PRACH, occasion in which the Random Access Preamble is transmitted, is computed.

3. The method of claims 1 or 2 wherein if the Random Access Procedure is triggered by a PDCCH order for an LTM candidate cell, then the Random Access procedure is considered to be completed.

4. A method for performing a LTM cell switch by a User Equipment, UE, communicatively coupled to a telecommunication network, configured with an LTM candidate cell configuration, the method comprising terminating a Random Access procedure by sending a preamble to the telecommunication network.

5. The method of claim 4 wherein if the random access procedure fails, the UE receives a PDCCH instruction from the network indicating whether the preamble is a first transmission or a retransmission, with the UE increasing a power ramping variable accordingly.

6. The method of any of claims 4 or 5 wherein the UE completes the LTM cell switch procedure by sending a RRCReconfigurationComplete message to the candidate cell.

7. The method of claim 6 wherein in a RACH-based LTM cell switch the LTM cell switch is completed when the RACH is successfully completed.

8. The method of claim 6 wherein in a RACH-less based LTM cell switch, the LTM call switch is successfully completed when the UE determines that the network has successfully received its first uplink data.

9. The method of claim 8 comprising checking HARQ ACK or RLC ACK for the first uplink data.

10. Apparatus arranged to perform the method of any preceding claim.

Images