Improvements in and relating to rach-less timing advance management

Abstract

A UE is configured for conditional RACH-less handover by a network. In another aspect, whether to configure the UE for RACH-less handover is determined. A RACH-less handover command is sent to the UE

Description

Random Access Channel, RACH, is an essential part of wireless communication systems, including Fifth Generation, 5G, (or New Radio, NR), Fourth Generation, 4G, (or Long-Term Evolution, LTE) and even Third Generation, 3G. It plays a significant role in establishing an initial connection (Initial Access) between a device and a network. It also significant in handing over a User Equipment, UE, from one cell to another. RACH-less is a handover method introduced in Evolved Universal Terrestrial Radio Access Network, E-UTRAN, Release 14. It enables a UE to skip performing random access during a handover. This is to improve the delay of handovers in some certain cases. RACH-less can be performed in certain cases related to the Timing Advance (TA): • When TA to the target cell is equal to zero • When TA to the target cell is equal to the TA of the source cell Figures 1a illustrates RACH-less handover where the UE is configured with uplink grants; and Figure 1b illustrates the case where the UE is configured to monitor PDCCH of target cell. RACH-less can be considered to comprise two different methods: Consecutive Uplink Grants are configured to be used by the UE to send the first message after the handover, which is the RRCReconfigurationComplete and optionally any data. For this case, the UE is configurated with an uplink grant (ul-Grant), the scheduling interval (ul-Schedlnterval) as well as how many occasions that are configured (numberOfConfUL-Processes) The UE is scheduled to synchronize with a cell without sending any messages to the target cell. Instead, the UE starts monitoring PDCCH for assignments from the target cell. The assignments may be for downlink PDSCH transmissions or UL grants for the UE to send PUSCH. The following fields are configured with RACH-less: • targetTA • numberOfConfUL-Processes • ul-Sched I nterval • ul-StartSubframe • ul-Grant For the target TA, there are a number of options of what to apply in the target cell for the timing advance. The UE can either set Nta=0 or Nta equal to another serving cell that the UE is already connected to, for instance the PCell or the SCell. Not only was RACH-less introduced for the purpose of handovers, but also for the establishment of a secondary eNB, i.e a SeNB. The establishment of a SeNB is normally done via random access procedure. Carrier Aggregation, CA, is key technique for modern cellular connectivity that was introduced for LTE in Rel-10. It was made native with the first 5G NR release and is an important technique to ensure higher rates. Carrier aggregation, as the name implies, allows for the aggregation of multiple carriers for a single base station, gNB. In addition to the main cell where the UE is RRC-connected to, i.e the PCell or PSCell, the gNB also configures secondary cells, SCells. Each carrier is sometimes referred to as a Component Carrier. Dual Connectivity, DC, is another important technique that was introduced for LTE in Rel-12 and made native to 5G NR. It is implemented in a number of different ways, such as E-UTRAN-NR DC, NR-E-UTRAN DC and NR DC. Dual connectivity allows a UE to simultaneously connect to a Main Node, MN, and Secondary Node, SN, i.e two different gNBs. In NR, the MN will have an associated Master Cell Group (MCG) and the SN will have an associated Secondary Cell Group (SCG), allowing for Dual Connectivity and Carrier Aggregation to co-exist. A cell group refers to either an MCG or an SCG. The Primary Cell (PCell) refers to the non-secondary cell of the MCG, and the Primary SCG Cell (PSCell) refers to the non-secondary cell of the SCG. Special Cell (SpCell) refers to either the PCell or the PSCell. These gNBs may or may not be collocated and each node may have multiple SCells, i.e carriers. In 5G NR a common use case is that the MN / MCG is at Frequency Range 1, FR1 and SN / SCG is at FR2. Compared to Carrier Aggregation, in Dual Connectivity the MN and SN will act a lot more independently whereby the MN and SN can configure the UE, for instance, with separate measurements at each Cell Group. SCG addition, also known as SN addition, SCG establishment or DC establishment is the procedure used to establish Dual Connectivity by adding another SCG. The procedure to add the SCG from the perspective of the UE can be seen in Figure 2. The steps set out in Figure 2 are: Step 1: First the decision to attempt to establish an SCG is made. Step 2: The MN initiates the SCG addition by sending the S-Node Addition Request to the SN. If the SN is able to accommodate the UE and the SCG role, the SN generates the SCG RRC configuration and replies with S-Node Addition Request Acknowledge, containing RRC configuration of the SCG that the UE shall apply. Step 3: RRCReconfiguration is sent to the UE to configure the SCG. For NR SCG addition, the mrdc-SecondaryCellGroupConfig is set to nr-SCG and the gNB includes the field CellGroupConfig. Step 4: RRCReconfigurationComplete may be generated before or after having performed the random access to SN. Step 5: Perform random access to the SgNB. The RRC message SCGFailurelnformation is used in order to indicate to a MN that the UE has experienced a failure on the SCG link. This can, for instance, be due to radio link failure, failure to synchronize on the SCG or SCG configuration failure. The SCGFailurelnformation contains a field FailureReportSCG, which may contain the failure type, possible UE measurements, location info, previously visited cells etc. In connection with EN-DC SCG addition, Figure 3 shows the procedure to add the NR SCG from E-UTRAN MN from the perspective of the UE. The steps set out in Figure 3 are: Step 1: First the decision to attempt to establish an SCG is made. Step 2: The MN initiates the SCG addition by sending the SGNB Addition Request to the SN. If the SN is able to accommodate the UE and the SCG role, the SN generates the SCG RRC configuration and replies with SGNB Addition Request Acknowledge, containing RRC configuration of the SCG that the UE shall apply. Step 3: RRCConnectionReconfiguration is sent to the UE to configure the SCG. For NR SCG addition, the nr-SecondaryCellGroupConfig with a transparent NR RRC container, containing the configuration to use in the NR SCG. Step 4: RRCReconfigurationComplete may be generated before or after having performed the random access to SN. Step 5: Perform random access to the SgNB. It is instructive to understand more about how Timing Advance functions in NR and LTE. In LTE, the full timing advance of a UE, TTAis defined as: TfA = (^TA + ^TA,offset + ^TA^dj0" + ^TAadj) Where the individual components are: • Ts = basic time unit 1 / (15000 x 2048) = 1 / 30720000 • Nta = TA x 16 • ^TA™n + ^TAEadj are Non-Terrestrial Networks, NTN, specific In NR, Tta, which is the full and absolute Timing Advance of a UE, is defined as: Tta = (^TA + ^TA,offset + ^TA^dj0" + ^TAadj)^ Where the individual components are: • Tc is the basic time unit for NR = 1 / (480x103 x 4096) = 1 / 196608 0000 • The / VTA, which is the MAC-signalled timing advance is calculated as: o / VTA = Ta ■ 16 ■ 64 / 2^, which is how to calculate the timing advance when receiving the TA in Random Access response o / VTA NEW = NTAold + (Ta - 31) ■ 16 ■ 64 / 2^, calculated when receiving the TAin a Timing Advance MAC CE • ft is the subcarrier spacing configuration, which can take the values [0, 1, 2, 3, 4, 5, 6] which corresponds to the Subcarrier Spacing (SCS) [15, 30, 60, 120, 240, 480, 960] kHz. In general, the higher the frequency, the higher the subcarrier spacing. • M”’" + <Eadj are NTN SPeCifiC What can be seen from the above is that in NR the granularity of / VTAvalues changes with the subcarrier spacing. For instance for SCS = 15 kHz, the values of the timing advance changes that the UE applies are the values Nta = [...-4096, -3072, -2048, -1024, 0, 1024, 2048, ...]. For SCS = 60 kHz, the UE applies the values Nta = [..., -1024, -768, -512, -256, 0, 256, 512, 768, 1024, ...]. A difference between LTE and NR is that in LTE, the SCS is always the same and that in LTE, the basic time unit is much smaller. When performing RACH-less handovers between different cells or using RACH-less for establishment of new cells in LTE, the UE can either apply the same Timing Advance in the target cell as the source cell, or use Timing Advance equal to zero. Applying the same timing advance is specified as reusing the same WTA. In NR, the adjustments of the Timing Advance changes as the subcarrier spacing changes (which is often configured to be correlated to the frequency). This may cause issues when either performing RACH-less handovers between different cells at different frequency or establishing another cell or Radio Access Technology, RAT, of another frequency. For LTE, the option of using RACH-less to perform SN addition was introduced in Rel-14 in addition to being introduced for mobility purposes. Now that RACH-less is being introduced for NR for mobility purposes, it would also be beneficial to introduce RACH-less for SN addition. However, the number and variety of Dual Connectivity scenarios in 5G are greater than compared to what was the case in LTE. As an example, the following are different: • The range of frequency bands of NR are a lot wider compared to LTE, affecting how Dual Connectivity functions; • In addition to NR-DC, where both legs in DC are NR, there are also EN-DC and NE DC scenarios. As an example of an issue, LTE RACH-less has some options on how to deal with the Timing Advance of the target cell / target SN. One option is that Nta=0 is used in the target cell, Nta of one of the already established cells, such as the PCell or any SCells is used in the target cell. However, if the Nta of an LTE cell is used in NR, there is a need to ensure that the Nta can truly be applied in an NR cell. 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 operating a telecommunication network to perform RACH-less handover between a source cell and a target cell, comprising the steps of: a) determining whether to configure an operably connected User Equipment, UE, for RACH-less handover according to a predetermined condition; b) if so determined, sending a handover command to the UE to perform RACH-less handover, wherein the predetermined condition is one of: (i) the target cell is not on a different Sub-Carrier Spacing, SCS, compared to source cell; (ii) the target cell is not on a different frequency compared to source cell; (Hi) the target cell is on a different frequency compared to source cell and the timing advance is configured to be zero; (iv) the target cell is on a different SCS compared to source cell and the timing advance is configured to be zero; and (v) the target cell is on a different SCS compared to source cell and the timing advance is adjusted according to the SCS in the target and source cell. In an embodiment, the handover command comprises a configuration of the timing advance comprising one of: • the timing advance is configured to be zero if the target cell is on a different SCS compared to the source cell; • the timing advance is configured to be zero if the target cell is on a different frequency compared to source cell; and • the timing advance is configured to be the same as that of the target cell if the SCS of the target cell is adjusted according to a difference in the target cell SCS and the source cell SCS In an embodiment, if the predetermined condition is (v) the target cell is on a different SCS compared to source cell and the timing advance is adjusted according to the SCS in the target and source cell, then the adjustment is made by either rounding down to the nearest Nta or rounding up to the nearest Nta. In an embodiment, if the timing advance is adjusted by rounding down, then a flooring function is utilised such that a minimum value is assigned. In an embodiment, if the timing advance is adjusted by rounding up, then a ceiling function is utilised such that a maximum value is assigned. In an embodiment, if the predetermined condition is (v) the target cell is on a different SCS compared to source cell and the timing advance is adjusted according to the SCS in the target and source cell, then In an embodiment, rounding is performed if one or more of: • the target cell SCS is smaller than the source cell SCS; • the target cell frequency is smaller than the source cell frequency; • the handover is an inter-frequency handover. In an embodiment, in the case that target cell frequency is smaller than the source cell frequency, the handover is from Frequency Range 2, FR2, to FR1. In an embodiment, an explicit instruction to perform rounding is included in a RACH-less configuration message. In an embodiment, in certain cases, regardless of any determination, a RACH-less handover may not be performed, such certain cases including inter-SCS or inter-frequency handovers. In an embodiment, inter-SCS, inter-frequency RACH-less handovers or RACH-less establishment, may only be performed if Timing Advance=0 is configured. According to a second aspect of the present invention, there is provided method of operating a User Equipment, UE, arranged for RACH-less handover, wherein the UE has been configured for conditional RACH-less handover by an operably connected telecommunication network. In an embodiment, the conditional RACH-less handover is performed on the basis of one of: (i) a target cell is not on a different Sub-Carrier Spacing, SCS, compared to a source cell; (ii) a target cell is not on a different frequency compared to a source cell; (Hi) a target cell is on a different frequency compared to a source cell and the timing advance is configured to be zero; (iv) a target cell is on a different SCS compared to a source cell and the timing advance is configured to be zero; and (v) a target cell is on a different SCS compared to source cell and the timing advance is adjusted according to the SCS in the target and source cell. According to a third aspect of the present invention, there is provided telecommunication network arranged to perform the method of the first aspect. According to a fourth aspect of the present invention, there is provided User Equipment arranged to perform the method of the second aspect. A key idea in an embodiment of this invention revolves around methods for managing the Timing advance during a RACH-less handover or a RACH-less cell establishment when it is signalled that the UE shall use the timing advance of the source cell in the target cell. Embodiments describe the RACH-less handover using what is known as the configured or the pre-configured uplink grant, but this is only one option. The other option of configuring the UE to monitor PDCCH and then the target eNB sends PDCCHs containing UL grant is also applicable. Embodiments are mostly described using gNBs (4G E-UTRAN), but may equally apply to eNBs or NG-RAN (5G NR). Embodiments may also apply to a eMTC / LTE-M UE, or 5G NR UE, or E-UTRAN UE. When performing a RACH-less handover or RACH-less establishment, the source cell may a PCell, or a PSCell, or a SpCell. Similarly, the target cell may become the PCell, or a PSCell or a SpCell. 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 1a shows a RACH-less handover with uplink grants configured; Figure 1b shows a RACH-less handover where the UE is configured to monitor PDCCH of target cell; Figure 2 shows SCG addition procedures; Figure 3 shows EN-DC SCG addition procedures; Figure 4a shows timing advance from a cell of SCS=30 kHz used in a cell of SCS=15 kHz and a timing advance command is later used; Figure 4b shows rounding down the TA using in SCS=30 kHz to an a value that would be used in SCS=15 kHz; Figure 5a shows SCG establishment using NTA translation for EN-DC; and Figure 5b shows SCG establishment using NTA translation for NE-DC. In order to perform a RACH-less handover or RACH-less SgNB addition, there needs to be some management of the Timing Advance. For instance, when performing RACH-less handover from a first cell to a second cell with lower SCS, the Nta that is used in the first cell may not be suitable in the second cell. This is because the Nta may be in-between the configurable Nta values for the second cells. For instance, the Nta value from the first cell with SCS = 60 kHZ can be 3 x 16 x 64 / 23 = 384, while in the second cell, granularity of the Nta are in multiples of 1024. Thus, in one embodiment of the invention, when performing inter-SCS RACH-less handovers, inter-frequency RACH-less handover or RACH-less establishment, the Nta used from the first cell is rounded up or rounded down to a value that is consistent with the specific SCS of the cell. If rounded down, a flooring function is used, similar to the following: • WTA Target= Floor (WTA Source / 16 ■ 64 / 2^-) x 16 ■ 64 / 2^- If rounded up, a ceiling function is used, similar to the following: • WTA Target= Ceil (WTA_source / 16 ■ 64 / 2^-) x 16 ■ 64 / 2^- In the above, the WTA Target is the Nta to apply in the target cell when calculating the full timing advance. The WTASource is the value that is used in the source cell. In the above, the condition to apply may be one or a combination of the below: • The target cell SCS is smaller than the source cell SCS • The target cell frequency is smaller than the source cell frequency o For instance it can be FR2 to FR1 • Condition may be that it is an inter-frequency handover. The above can be applied by implicit indication or explicit indication. By implicit indication, the above is applied when one of the above conditions are fulfilled, i.e. when the UE is indicated to apply source TA in target cell during RACH-less handover and the target cell has a lower SCS than the source cell. By explicit indication, a bit in the RACH-less configuration may indicate that rounding shall be performed. In a refinement, two bits are used, with 00 indicating no rounding, 01 indicating rounding up, 10 indicating rounding down, and 11 being unused. In one embodiment of the invention, there are restrictions on when RACH-less handovers may be performed. For instance, for inter-SCS, inter-frequency (such as between FR1 and FR2) RACH-less handovers may not be performed. For instance, for inter-SCS or inter-frequency RACH-less handovers or RACH-less establishment, it may only be performed if TA=0 is configured. This can be seen in Example 1. In a further refinement, RACH-less handover to a lower SCS can only be performed using TA=0 or, RACH-less handover or RACH-less establishment may only be performed when the SCS values are adjacent, for instance 120kHz -> 60 kHz, 60 kHz -> 30 kHz, 30 kHz -> 15 kHz. In another refinement, RACH-less handover or RACH-less establishment may only be performed when the difference between JVTASource and WTA Target is below a certain pre-defined or configurable threshold. In one embodiment of the invention, the same methods can be used when performing a handover from a first cell, where the UE is configured to operate on the first cell with a Bandwidth Part (BWP) with a specific SCS, to a second cell with a different SCS from the configuration in first cell. This means that even though the first cell and second cell may operate with the same SCS, the UE may be dedicatedly configured with BWP that has a different SCS from any of the two cells. In other words, the change of the SCS may occur even though the common configuration suggests that the cells have the same SCS. Some of the above may, for instance, mean that RACH-less handover or SCG establishment may only be performed within the same Frequency Range (FR1->FR1, FR2->FR2, FR2-1->FR2-1, etc.). In the above, there may be a need to round to the nearest multiple of the Timing Advance, i.e 16 -64 / 2^, where fz is the SCS number. This can be seen in Figure 4a, which shows timing advance from a cell of SCS=30 kHz used in a cell of SCS=15 kHz and a timing advance command is later used. In another embodiment of the invention, it can be configurable whether the UE shall round up / down to the nearest multiple of the Timing Advance. This can be seen in Figure 4b, where it is rounded down. More particularly, the TA is rounded down from a value used in SCS=30 kHz to a value that would be used in SCS=15 kHz. The following relates to EN-DC and NE-DC RACH-less SCG establishment. As the absolute timing in advance is defined based on Nta in both NR and LTE, but the steps involved in establishing the Nta are different in each case, one feature of embodiments of this invention, relates to a translation of the Nta when performing EN-DC and NE-DC RACH-less SCG establishment. In one embodiment of the invention, when performing EN-DC RACH-less SCG establishment and the MN eNB signals that the target TA should depend on an already serving cell, there is a translation from LTE to NR Nta: Nta_nr = Nta_lte x Ts / Tc = Nta_lte x 64 Thus, the Nta to apply in a target NR cell from the source E-UTRAN cell, is the target E-UTRAN Nta multiplied by the factor that is the difference between the LTE and NR basic timing unit. The condition to apply the above translation may for instance be that one or more of the following conditions shall be true: • Target cell or SN is NR • Source cell or MN is E-UTRAN • A part of nr-SecondaryCellGroupConfig in LTE RRC message RRCConnectionReconfiguration. Similarly, when performing NE-DC RACH-less SCG establishment and the MN gNB signals that target TA should depend on an already serving cell, there is a translation from NR to LTE Nta: Nta_lte = Nta_nr x Tc / Ts = Nta_lte I 64 The condition to apply the above translation may, for instance, be that one or more of the following conditions shall be true: • Target cell or SN is E-UTRAN • Source cell or MN is NR • Signalled as part of rach-SkipSCG • A part of eutran-scg in NR RRC message RRCReconfiguration. The above will be applied regardless of the serving cell that the network signals that the UE use the timing advance from (i.e PCell or any SCell). These two examples can be seen later in this application as “Example #2” and “Example #3” as well as in Figures 5a and 5b, which show SCG establishment using NTA translation for EN-DC, and SCG establishment using NTA translation for NE-DC, respectively. As the number of scenarios for RACH-less SCG establishment are many, in some cases, there can be reasons why RACH-less SCG establishment may not be performed. Thus, there may be conditions when RACH-less can be performed or not. Or in other cases, these restrictions may be related to UE capabilities. In one embodiment of this invention, when a failure occurs when trying to establish an SCG and using RACH-less, this is reported in the SCGFailurelnformation. This can for instance be a flag in the SCGFailurelnformation RRC message that RACH-less was used. It can also be indicated the Nta that was used in for RACH-less. In the following part of the description, implementation examples are present in ASN.1 format. Example #1 Example based on TS38.331 V18.0.0 - CellGroupConfig The CellGroupConfig IE is used to configure a master cell group (MCG) or secondary cell group (SCG). A cell group comprises of one MAC entity, a set of logical channels with associated RLC entities and of a primary cell (SpCell) and one or more secondary cells (SCells). For an NCR-MT, the CellGroupConfig IE is also used to provide the configuration of side control information for the NCR-Fwd access link. CellGroupConfig ::= EE OTENCE { cellGroupId CellGroupId, rlc-BearerToAddModList EEOuENCE (EjEE(1..maxLC-ID)) CE' RLC-BearerConfig rlc-BearerToReleaseList SEERjERCE (3I2E(1..maxLC-ID)) OE LogicalChannelldentity mac-CelIGroupConfig MAC-CellGroupConfig OPTIAAE, •••• physicalCellGroupConfig PhysicalCellGroupConfig CPHORAE, •• a SpCellConfig SpCellConfig OP'AOuAE, -- in sCellToAddModList SEOAEEA (SICE (1..maxNrofSCells)) 01’ SCellConfig sCellToReleaseList SEQUENCE (SHE (1. .maxNrofSCells) ) OE SCelllndex reportUplinkTxDirectCurrent {true} . OMITTED . SpCellConfig ::= EEC servCellIndex reconfigurationWithSync rlf-TimersAndConstants rlmlnSyncOutOfSyncThreshold spCellConfigDedicated Aim { ServCell Index OPT Reconf igurationWithSync OPT i ORAL, SetupRelease { RLF-TimersAndConstants } ENAEEATEE {nl} ServingCellConfig lowMobilityEvaluationConnected-rl7 SEQEERCE { s-SearchDeltaP-Connected-rl7 ENUMERATED {dB3, dB6, dB9, dB12, dB15, spare3, spare2, sparel}, t-SearchDeltaP-Connected-rl7 ERRMERA'?EE {s5, slO, s20, s30, s60, sl20, sl80, s240, s300, spare?, spare6, spared, spare4, spare3, spare2, sparel} ) goodservingCellEvaluationRLM-r17 goodservingCellEvaluationBFD-r17 deactivatedSCG-Config-r17 GoodServingCellEvaluation-rl7 ODTIORAl, GoodServingCellEvaluation-rl7 OPT i ORAL, SetupRelease { DeactivatedSCG-Config-rl7 } ReconfigurationWithSync ::= spCellConfigCommon ServingCellConfigCommon OC-TIO'd?.:., •••• RM c newUE-Identity RNTI-Value, t304 ERUME'7A'?ED {ms50, mslOO, msl50, ms200, ms500, mslOOO, ms2000 mslOOOO}, rach-ConfigDedicated uplink RACH-ConfigDedicated, supplementaryUplink RACH-ConfigDedicated smtc SSB-MTC daps-UplinkPowerConfig-rl6 DAPS-UplinkPowerConfig-rl6 sl-PathSwitchConfig-rl7 SL-PathSwitchConfig-rl7 rach-LessHO-rl8 RACH-LessHO-rl8 . . . OMITTED . . . RACH-LessHO-rl8 ::= 3EO”E?’CE { targetNTA-rl8 EkuMEPRIED {zero, source} tci-StateID-rl8 TCI-Stateld dg-beam-rl8 SSB-Index . OMITTED . RACH-LessHO field descriptions dg-beam This field indicates a beam that the UE should use in the target cell to monitor PDCCH for initial uplink transmission, see TS 38.321 [3], targetNTA This field refers to the timing adjustment, see TS 38.213

[13] and TS 38.321 [3], indicating the Nta value which the UE shall use for the target PTAG of handover. Only value source is configured by the network in case source cell is a mobile IAB cell. If the SCS of the target cell is smaller than the source cell, the UE rounds Nta value applied is Floor (N_TA_source / (16 * 64 / 2u)) * 16 * 64 / 2u where u is the target cell SCS. tci-StatelD This field indicates a beam that the UE should use in the target cell to monitor PDCCH for initial uplink transmission. This field is always present in case this cell is a mobile IAB cell. ReconfigurationWithSync field descriptions rach-ConfigDedicated Random access configuration to be used for the reconfiguration with sync (e.g. handover). The UE performs the RA according to these parameters in the firstActiveUplinkBWP (see UplinkConfig). smtc The SSB periodicity / offset / duration configuration of target cell for NR PSCell change and NR PCell change. The network sets the periodicityAndOffset to indicate the same periodicity as ssb-periodicityServingCell in spCellConfigCommon or sets to the same periodicity as ssb-Periodicity-r17 in nonCellDefiningSSB-r17 if the first active DL BWP included in this RRC message is configured with nonCellDefiningSSB-r17. For case of NR PCell change, the smtc is based on the timing reference of (source) PCell. For case of NR PSCell change, it is based on the timing reference of source PSCell. If both this field and targetCellSMTC-SCG are absent, the UE uses the SMTC in the measObjectNR having the same SSB frequency and subcarrier spacing, as configured before the reception of the RRC message. If the first active DL BWP included in this RRC message is configured with nonCellDefiningSSB-r17, this field corresponds to the NCD-SSB indicated by nonCellDefiningSSB-r17, otherwise, this field corresponds to the CD-SSB indicated by absoluteFrequencySSB in frequencylnfoDL. Example #2 5 --------------------------Example based on TS38.331 V18.0.0 - CellGroupConfig The CellGroupConfig IE is used to configure a master cell group (MCG) or secondary cell group (SCG). A cell group comprises of one MAC entity, a set of logical channels with associated RLC entities and of a primary cell (SpCell) and one or more secondary cells (SCells). For an NCR-MT, the CellGroupConfig IE is also used to provide the configuration of side control information for the NCR-Fwd access link. CellGroupConfig information element CellGroupConfig ::= SEOOENCE { cellGroupId rlc-BearerToAddModList rlc-BearerToReleaseList mac-CelIGroupConfig physicalCellGroupConfig SpCellConfig sCellToAddModList sCellToReleaseList CellGroupId, EE.OuE.uCE. (1-111 (1. .maxLC-ID) ) 1-11 RLC-BearerConfig SBQPENCo (1.:11(1. .maxLC-ID) ) 1-1 LogicalChannelldentity MAC-CellGroupConfig (.-111(.-11]., •••• lead 1 PhysicalCellGroupConfig 01110111-, -- ?■” SpCellConfig OllICuA;,, -- 1 SE-OLENCE (.1(11 (1..maxNrofSCells)) (.-1 SCellConfig SuQL'Oh'CG (S ; 1E. (1. .maxNrof SCells) ) OP' SCelllndex reportUplinkTxDirectCurrent ENUMERATED- {true} . . . OMITTED . . . SpCellConfig ::= Srl servCellIndex reconfigurationWithSync rlf-TimersAndConstants rlmlnSyncOutOfSyncThreshold spCellConfigDedicated KNOT { ServCellIndex OPT Reconf igurationWithSync CPU ONA(., SetupRelease { RLF-TimersAndConstants } ENuMEPRTED {nl} ServingCellConfig lowMobilityEvaluationConnected-rl7 SEO1ANCE { s-SearchDeltaP-Connected-rl7 ENUMERATED {dB3, dB6, dB9, dB12, dB15, spare3, spare2, sparel}, t-SearchDeltaP-Connected-rl7 E'AvenateO {s5, slO, s20, s30, s60, sl20, sl80, s240, s300, spare?, spare6, spared, spare4, spare3, spare2, sparel} } OPTIONAL, Neeo goodServingCellEvaluationRLM-rl7 GoodServingCellEvaluation-rl7 -- au; R goodServingCellEvaluationBFD-rl7 GoodServingCellEvaluation-rl7 , ••- P deactivatedSCG-Config-rl7 SetupRelease { DeactivatedSCG-Config-rl7 } ReconfigurationWithSync ::= spCellConfigCommon newUE-Identity t304 mslOOOO}, rach-ConfigDedicated uplink supplementaryUplink } 3EQ'JE>’CE { ServingCellConfigCommon OPTIONAL, • • Aa X RNTI-Value, bAUME.RATED {ms50, mslOO, msl50, ms200, ms500, mslOOO, ms2000, CuOICo { RACH-ConfigDedicated, RACH-ConfigDedicated smtc SSB-MTC daps-UplinkPowerConfig-rl6 DAPS-UplinkPowerConfig-rl6 sl-PathSwitchConfig-rl7 SL-PathSwitchConfig-rl7 rach-LessHO-rl8 RACH-LessHO-rl8 . . . OMITTED . . . RACH-LessHO-rl8 : : = targetNTA-rl8 tci-StateID-rl8 dg-beam-rl8 TAI { Eunyyizyyun {zero, source} TCI-Stateld SSB-Index . . . OMITTED . . . RACH-LessHO field descriptions dg-beam This field indicates a beam that the UE should use in the target cell to monitor PDCCH for initial uplink transmission, see TS 38.321 [3], targetNTA This field refers to the timing adjustment, see TS 38.213

[13] and TS 38.321 [3], indicating the Nta value which the UE shall use for the target PTAG of handover. Only value source is configured by the network in case source cell is a mobile IAB cell. If signalled as part of nr-SecondaryCellGroupConfig, see TS 36.331

[10] , for establishment of NR SCG, the Nta applied in NR SCG Nta..new — Nta_oici x 64, whGTG the Nta_oici is the Nta from E-UTRAN PCgII. tci-StatelD This field indicates a beam that the UE should use in the target cell to monitor PDCCH for initial uplink transmission. This field is always present in case this cell is a mobile IAB cell. ReconfigurationWithSync field descriptions rach-ConfigDedicated Random access configuration to be used for the reconfiguration with sync (e.g. handover). The UE performs the RA according to these parameters in the firstActiveUplinkBWP (see UplinkConfig). smtc The SSB periodicity / offset / duration configuration of target cell for NR PSCell change and NR PCell change. The network sets the periodicityAndOffset to indicate the same periodicity as ssb-periodicityServingCell in spCellConfigCommon or sets to the same periodicity as ssb-Periodicity-r17 in nonCellDefiningSSB-r17 if the first active DL BWP included in this RRC message is configured with nonCellDefiningSSB-r17. For case of NR PCell change, the smtc is based on the timing reference of (source) PCell. For case of NR PSCell change, it is based on the timing reference of source PSCell. If both this field and targetCellSMTC-SCG are absent, the UE uses the SMTC in the measObjectNR having the same SSB frequency and subcarrier spacing, as configured before the reception of the RRC message. If the first active DL BWP included in this RRC message is configured with nonCellDefiningSSB-r17, this field corresponds to the NCD-SSB indicated by nonCellDefiningSSB-r17, otherwise, this field corresponds to the CD-SSB indicated by absoluteFrequencySSB in frequencylnfoDL. Example #3 --------------------------TS36.331 V18.0.0-------------------------- - MobilityControlInfo The IE MobilityControlInfo includes parameters relevant for network controlled mobility to / within E-UTRA. MobilityControlInfo information element -- ASN1START MobilityControlInfo ::= SEQUENCE ( targetPhysCellld PhysCellld, carrierFreq CarrierFreqEUTRA OPTIONAL,-- Cond HO-toEUTRA2 carrierBandwidth CarrierBandwidthEUTRA OPTIONAL,-- Cond HO-toEUTRA additionalSpectrumEmission AdditionalSpectrumEmission OPTIONAL,-- Cond HO- toEUTRA t304 ENUMERATED { newUE-Identity radioResourceConfigCommon rach-ConfigDedicated ms50, mslOO, msl50, ms200, ms500, mslOOO, ms2000, mslO000-V1310}, C-RNTI, RadioResourceConfigCommon, RACH-ConfigDedicated OPTIONAL,-- Need OP [ [ carrierFreg-v9eO CarrierFregEUTRA-v9eO OPTIONAL -- Need ON ] ] , [[ drb-ContinueROHC-rll ENUMERATED {true} OPTIONAL -- Cond HO ] ] , [[ mobilityControlInfoV2X-rl4 MobilityControlInfoV2X-rl4 OPTIONAL,-- Need ON handoverWithoutWT-Change-rl4 ENUMERATED {keepLWA-Config, sendEndMarker} OPTIONAL,-- Cond HO makeBeforeBreak-rl4 rach-Skip-rl4 sameSFN-Indication-rl4 ] ] , [ [ mib-Repetitionstatus-rl4 schedulinglnfoSIBl-BR-rl4 ] ] , [ [ daps-Config-rl6 ENUMERATED {true} OPTIONAL,-- Need OR RACH-Skip-rl4 OPTIONAL,-- Need OR ENUMERATED {true} OPTIONAL -- Cond HO-SFNsynced BOOLEAN INTEGER (0..31) DAPS-Config-rl6 OPTIONAL,-- Need OR OPTIONAL -- Cond HO-SFNsynced OPTIONAL -- Cond NotFullConfigHO [ [ gnss-PositionFixDurationReporting-rl8 ENUMERATED {true} OPTIONAL -- Need OR . . . OMITTED . . . RACH-Skip-rl4 ::= targetTA-rl4 ta0-rl4 mcg-PTAG-rl4 scg-PTAG-rl4 mcg-STAG-rl4 scg-STAG-rl4 } , ul-Configlnfo-rl4 numberOfConfUL-Processes-ul-SchedInterval-rl4 ul-StartSubframe-rl4 ul-Grant-r!4 SEQUENCE { CHOICE { NULL, NULL, NULL, STAG-Id-rll, STAG-Id-rll SEQUENCE { 14 INTEGER (1. .8) , ENUMERATED {sf2, sf5, sflO}, INTEGER (0..9), BIT STRING (SIZE (16)) OPTIONAL -- Need OR -- ASN1STOP i MobilityControlInfo field descriptions i rach-Skip I This field indicates whether random access procedure for the target PCell is skipped. i rach-SkipSCG i This field indicates whether random access procedure for the target PSCell is skipped. I MobilityControlInfo field descriptions i targetTA I This field refers to the timing adjustment indication, see TS 36.213

[23] , indicating the Nta value which the I UE shall use for the target PTAG of handover or the target PSTAG of SCG change. If signalled as part i of rach-SkipSCG and MCG is NR, the Nta applied in E-UTRAN SCG is NiA_new = Nta_oici / 64, where i the Nta_oici is the Nta from NR MCG. taO corresponds to Nta=0. mcg-PTAG corresponds to the latest Nta I value of the PTAG associated with MCG. scg-PTAG corresponds to the latest Nta value of the PTAG I associated with SCG. mcg-STAG corresponds to the latest Nta value of a MCG STAG indicated by the i STAG-ld. scg-STAG corresponds to the latest Nta value of a SCG STAG indicated by the STAG-ld. 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, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A method of operating a telecommunication network to perform RACH-less handover between a source cell and a target cell, comprising the steps of:a) determining whether to configure an operably connected User Equipment, UE, for RACH-less handover according to a predetermined condition;b) if so determined, sending a handover command to the UE to perform RACH-less handover, wherein the predetermined condition is one of:(i) the target cell is not on a different Sub-Carrier Spacing, SCS, compared to source cell;(ii) the target cell is not on a different frequency compared to source cell;(Hi) the target cell is on a different frequency compared to source cell and the timing advance is configured to be zero;(iv) the target cell is on a different SCS compared to source cell and the timing advance is configured to be zero; and(v) the target cell is on a different SCS compared to source cell and the timing advance is adjusted according to the SCS in the target and source cell.

2. The method of claim 1 wherein the handover command comprises a configuration of the timing advance comprising one of:• the timing advance is configured to be zero if the target cell is on a different SCS compared to the source cell;• the timing advance is configured to be zero if the target cell is on a different frequency compared to source cell; and• the timing advance is configured to be the same as that of the target cell if the SCS of the target cell is adjusted according to a difference in the target cell SCS and the source cell SCS3. The method of claim 1 or 2, wherein if the predetermined condition is (v) the target cell is on a different SCS compared to source cell and the timing advance is adjusted according tothe SCS in the target and source cell, then the adjustment is made by either rounding down to the nearest Nta or rounding up to the nearest Nta.

4. The method of claim 3 wherein if the timing advance is adjusted by rounding down, then a flooring function is utilised such that a minimum value is assigned.

5. The method of claim 3 wherein if the timing advance is adjusted by rounding up, then a ceiling function is utilised such that a maximum value is assigned.

6. The method of any preceding claim wherein if the predetermined condition is (v) the target cell is on a different SCS compared to source cell and the timing advance is adjusted according to the SCS in the target and source cell, then7. The method of any of claims 3 to 5 wherein rounding is performed if one or more of:• the target cell SCS is smaller than the source cell SCS;• the target cell frequency is smaller than the source cell frequency;• the handover is an inter-frequency handover.

8. The method of claim 7 wherein in the case that target cell frequency is smaller than the source cell frequency, the handover is from Frequency Range 2, FR2, to FR1.

9. The method of any of claims 3 to 5 wherein an explicit instruction to perform rounding is included in a RACH-less configuration message.

10. The method of any preceding claim wherein, in certain cases, regardless of any determination, a RACH-less handover may not be performed, such certain cases including inter-SCS or inter-frequency handovers.

11. The method of claim 10 wherein inter-SCS, inter-frequency RACH-less handovers or RACH-less establishment, may only be performed if Timing Advance=0 is configured.

12. A method of operating a User Equipment, UE, arranged for RACH-less handover, wherein the UE has been configured for conditional RACH-less handover by an operably connected telecommunication network.

13. The method of claim 12 wherein the conditional RACH-less handover is performed on the basis of one of:(i) a target cell is not on a different Sub-Carrier Spacing, SCS, compared to a source cell;(ii) a target cell is not on a different frequency compared to a source cell;5(Hi) a target cell is on a different frequency compared to a source cell and the timing advance is configured to be zero;(iv) a target cell is on a different SCS compared to a source cell and the timing 10 advance is configured to be zero; and(v) a target cell is on a different SCS compared to source cell and the timing advance is adjusted according to the SCS in the target and source cell.15 14. A telecommunication network arranged to perform the method of any of claims 1 to 11.

15. A User Equipment arranged to perform the method of any of claims 12 or 13.

Images