Cell reselection for a user equipment

Abstract

There is provided techniques for cell reselection in a wireless communication network. A method is performed by a user equipment. The method comprises performing a cell reselection evaluation procedure for a second cell whilst camping on a first cell in the wireless communication network and whilst operating in a first power mode. The method comprises either performing the cell reselection to the second cell in the wireless communication network whilst operating in the first power mode when cell information of the second cell is in a first set of cell information, or performing the cell reselection to the second cell whilst operating in a second power mode when cell information of the second cell is in a second set of cell information.

Description

[0001] CELL RESELECTION FOR A USER EQUIPMENT

[0002] TECHNICAL FIELD

[0003] Embodiments presented herein relate to a method, a user equipment, a computer program, and a computer program product for cell reselection in a wireless communication network.

[0004] BACKGROUND

[0005] In fifth generation telecommunication systems (5GS), Radio Resource Control (RRC) modes define the state of the connection between the user equipment (UE) and the network, optimizing balance between battery life and connectivity. In the RRC Inactive mode, the UE has a connection context stored both in the network and the device, allowing for quick resumption of data transfer without a full signaling procedure. This optimizes battery life while ensuring responsiveness. In the RRC Idle mode, the UE has no dedicated network resources, minimizing battery usage. The UE performs monitoring for paging messages but must perform a full signaling process to transition to an active state for data transfer. In the RRC Active mode the UE has a direct, active connection with the network, enabling immediate data transmission. This state is necessary for ongoing data exchange but consumes more power due to the constant connection and signaling.

[0006] In further detail, in the RRC Idle mode and the RRC Inactive mode, the UE is associated with a camping cell whose quality it monitors to ensure that it maintains a sufficient link quality to contact the network using a random access channel (RACH) procedure or be contacted via paging. For the quality monitoring, the UE performs regular measurements of received signals, e.g., synchronization signal blocks (SSBs), from the current camped cell. The UE may also perform measurements on neighboring cells based on measurement rules and may reselect which cell to camp on based on cell reselection criteria. The UE performs cell selection and reselection using the selection- or reselection-related parameters from system information (SIB). Typically, the UE selects the cell with the highest available link quality for camping.

[0007] According to the third generation partnership project (3GPP), when performing cell evaluation measurements, the UE shall evaluate the parameters Srxlev and Squal, where:

[0008] Srxlev = Qrxlevmeas - (Qrxlevmin + Qrxlevminoffset )- Pcompensation - Qoffsettemp, and

[0009] Squal = Qqualmeas - (Qqualmin + Qqualminoffset) - Qoffsettemp

[0010] The Qrxlevmeas and Qqualmeas are measured values in terms of reference signal received power (RSRP) and reference signal received quality (RSRQ), while the other parameters (Qrxlevmin, Qrxlevminoffset, Pcompensation, Qoffsettemp, Qqualmin, Qqualminoffset, and Qoffsettemp) are either determined from specifications or so-called cell-reselection information parameters, which for a cell is provided in the SIBs denoted SIB1 (serving cell), SIB2 / 3 (intra-frequency), SIB4 (inter-frequency), SIB5 (inter-RAT EUTRAN, where RAT is short for radio access technologies, and EUTRAN is short for Evolved Universal Terrestrial Radio Access Network).

[0011] Further, a UE needs to know the content of SIB1 for cell access rules. The UE will via SIB1 receive parameter for cell access, such as tracking area code, radio access network (RAN) area code, Public Land Mobile Network (PLMN) identity and barring information, or cell reserved information). In other words, for a reselection candidate cell, the UE needs to read SIB1 for basic cell camping and access: Tracking Area (TA), PLMN, barring information and for cell selection information parameters. In the simplest case, if the new camping cell has the same TA, the UE changes its camping cell transparently to the network, only changing the internal reference cell and SSB parameter information. If the TA changes, the UE performs a RACH procedure and informs the network about the new TA where the UE can be reached via RRC signaling.

[0012] The above principles are also valid for UEs in RRC Inactive mode. The procedures for cell reselection are very similar for UE is RRC Idle mode and RRC Inactive mode. A UE in RRC Inactive mode will monitor a RAN area code and perform a RAN area update procedure if the RAN area code is different in a selected cell. The present disclosure is applicable to UEs in both RRC Idle mode and RRC Inactive mode. For illustration, but without loss of generality, wording and procedures typically applicable to the RRC Idle mode case will be used hereinafter.

[0013] Cell reselection in 5GS poses challenges to power preservation, significantly impacting device battery life. In 5GS, UEs frequently scan the network for optimal cells to ensure the best possible connection, which is crucial for maintaining high-speed data transmission and reliable connectivity. However, this continuous monitoring and reselection process can lead to increased power consumption and therefore poses significant challenges to power efficiency, necessitating advancements in power-saving technologies and strategies to mitigate its impact on device battery life.

[0014] SUMMARY

[0015] An object of embodiments herein is to provide cell reselection procedures that address the above-noted shortcomings of existing cell reselection procedures.

[0016] A UE may be in RRC Idle mode, where the network can reach the UE by means of paging. Besides the general paging signal transmission during paging occasions (PO) for UEs in RRC Idle mode, signaling enhancements to enable further energy efficient paging monitoring in cellular wireless networks have been introduced. In e.g., the 3GPP standards for Long-Term Evolution (LTE) machine-type communication (MTC) narrowband Intemet-of-Things (NB-IoT) technologies, a wake-up signal (WUS) has been introduced for UEs in RRC Idle mode to allow UEs to save energy for the purpose of paging monitoring. Rather than to decode the paging channel data for determining if a paging request is received, a wake-up signal is specified to indicate if there is a valid page in the next paging occasion to the UE or not. Hence, the UE wakes up for WUS monitoring with same periodicity as for the ordinary PO. Further, in 3GPP standards for the New Radio (NR) air interface, there is a “Paging Early Indication” for UEs in RRC Idle mode. This is a signal that the network sends before a UE’s paging occasion to indicate if the UE should wake up and actually read the paging channel or not. Further, the network might send an indication just before a UEs discontinuous (DRX) on-Duration timer is about to start to inform the UE whether the UE shall actually wake up or if the UE can continue to sleep.

[0017] Still further, a separate low-power receiver, such as a Wake-Up Radio (WUR) unit, or other type of low- power mode in the main receiver may be used to detect a WUS ahead of the PO, with a time offset that allows activating the main receiver in time for PO reception. Since the main energy cost for a WUR- equipped UE is associated with the transition energy when waking up its main receiver, it is generally not desirable to wake up the main receiver for mobility measurements.

[0018] Camping (i.e., idle mode monitoring in a selected cell) and candidate-cell SSB monitoring may be viable using the low-power mode that can operate at 2-3 orders of magnitude lower power than the main power mode due to simpler circuitry and lower fidelity criteria. But performing cell reselection generally also requires reading the target cell system information (e.g., SIB 1) to obtain the TA information and camping permission information (in terms of PLMN and barring status)), which is not assumed feasible to do using the low-power mode. Waking up the main receiver, or at least activating the main power mode (in contrast to the low-power mode), for each cell reselection instance, on the other hand, constitutes a considerable fraction of UE energy consumption since every wake-up carries a large sleep-to-active state transition energy cost.

[0019] A particular object is therefore to facilitate energy-efficient cell reselection for a UE capable of operating in different power modes by reducing this fraction of UE energy consumption.

[0020] According to a first aspect there is presented a method for cell reselection in a wireless communication network. The method is performed by a UE. The method comprises performing a cell reselection evaluation procedure for a second cell whilst camping on a first cell in the wireless communication network and whilst operating in a first power mode. The method comprises either performing the cell reselection to the second cell in the wireless communication network whilst operating in the first power mode when cell information of the second cell is in a first set of cell information, or performing the cell re selection to the second cell whilst operating in a second power mode when cell information of the second cell is in a second set of cell information. A higher level of power is used by the UE in the second power mode than in the first power mode. The first set of cell information and the second set of cell information are mutually exclusive with respect to each other.

[0021] According to a second aspect there is presented a UE for cell reselection in a wireless communication network. The UE comprises processing circuitry. The processing circuitry is configured to cause the UE to perform a cell reselection evaluation procedure for a second cell whilst camping on a first cell in the wireless communication network and whilst operating in a first power mode. The processing circuitry is configured to cause the UE to either perform the cell reselection to the second cell in the wireless communication network whilst operating in the first power mode when cell information of the second cell is in a first set of cell information, or perform the cell reselection to the second cell whilst operating in a second power mode when cell information of the second cell is in a second set of cell information. A higher level of power is used by the UE in the second power mode than in the first power mode. The first set of cell information and the second set of cell information are mutually exclusive with respect to each other.

[0022] According to a third aspect there is presented a UE for cell reselection in a wireless communication network. The UE comprises an evaluate module configured to perform a cell reselection evaluation procedure for a second cell whilst camping on a first cell in the wireless communication network and whilst operating in a first power mode. The UE comprises a reselect module configured to either perform the cell reselection to the second cell in the wireless communication network whilst operating in the first power mode when cell information of the second cell is in a first set of cell information, or perform the cell reselection to the second cell whilst operating in a second power mode when cell information of the second cell is in a second set of cell information. A higher level of power is used by the UE in the second power mode than in the first power mode. The first set of cell information and the second set of cell information are mutually exclusive with respect to each other.

[0023] According to a fourth aspect there is presented a computer program for cell reselection in a wireless communication network. The computer program comprises computer code which, when run on processing circuitry of a UE, causes the UE to perform actions. One action comprises the UE to perform a cell reselection evaluation procedure for a second cell whilst camping on a first cell in the wireless communication network and whilst operating in a first power mode. One action comprises the UE to either perform the cell reselection to the second cell in the wireless communication network whilst operating in the first power mode when cell information of the second cell is in a first set of cell information, or perform the cell reselection to the second cell whilst operating in a second power mode when cell information of the second cell is in a second set of cell information. A higher level of power is used by the UE in the second power mode than in the first power mode. The first set of cell information and the second set of cell information are mutually exclusive with respect to each other.

[0024] According to a fifth aspect there is presented a computer program product comprising a computer program according to the fourth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium.

[0025] Advantageously, these aspects do not suffer from shortcomings of existing cell reselection procedure.

[0026] Advantageously, these aspects enable energy-efficient cell reselection for a UE capable of operating in different power modes. Advantageously, these aspects enable the low-power mode to be used by the UE during the cell reselection process for most cell reselection instances. This is since most reselection instances occur within the same tracking area (as part of the first set of cell information).

[0027] Advantageously, these aspects enable the UE energy consumption to be reduced whilst increasing the LP- WUS PS gains.

[0028] Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

[0029] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

[0030] BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

[0032] Fig. 1 is a schematic diagram illustrating a wireless communication network according to embodiments;

[0033] Figs. 2 and 4 are flowcharts of methods according to embodiments;

[0034] Fig. 3 is a block diagram of UEs according to embodiments;

[0035] Fig. 5 is a schematic diagram showing structural units of a UE according to an embodiment;

[0036] Fig. 6 is a schematic diagram showing functional modules of a UE according to an embodiment; and

[0037] Fig. 7 shows one example of a computer program product comprising computer readable storage medium according to an embodiment.

[0038] DETAILED DESCRIPTION

[0039] The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional. Fig. 1 is a schematic diagram illustrating a wireless communication network 100 where embodiments presented herein can be applied. The wireless communication network 100 comprises access network nodes 110a, 110b, 110c, 1 lOd that provide coverage, and thus network access, in cells 120a, 120b, 120c, 120d. Each access network node 110a: 1 lOd could be any of a radio base station, base transceiver station, node B (NB), evolved node B (eNB), gNB, transmission and reception point (TRP), access point (AP), integrated access and backhaul (IAB) node, etc. The illustration of the wireless communication network 100 in Fig. 1 is only schematic. For example, although illustrated to have the shape of hexagons, the cells 120a: 120d can be of other shapes as well. Further, although illustrated as one access network node 110a: 1 lOd providing coverage in one respective cell 120a: 120d, one and the same access network node 110a: 1 lOd could be configured to provide coverage in more than one cell 120a: 120d.

[0040] A UE 130 is illustrated as being located in cell 120a and thus being served by access network node 110a. The UE 130 could be any of a portable wireless device, mobile station, mobile phone, handset, wireless local loop phone, smartphone, laptop computer, tablet computer, network equipped vehicle, network equipped gaming control, Intemet-of-Things (loT) device, etc. It is assumed that the UE 130 is moving in any of the directions 140a, 140b, 140c, and hence that the UE 130 will leave cell 120a and enter one of cells 120b: 120d. This requires a cell reselection to be performed for the UE 130. However, as noted above, there is a need for improved cell reselection procedures, and a particular object is to facilitate energy-efficient cell reselection for a UE capable of operating in different power modes.

[0041] The embodiments disclosed herein therefore relate to techniques for cell reselection in a wireless communication network 100. In order to obtain such techniques, there is provided a UE 130, a method performed by the UE 130, a computer program product comprising code, for example in the form of a computer program, that when run on a UE 130, causes the UE 130 to perform the method.

[0042] In some aspects, the cell reselection can be performed using different power modes in the radio of the UE 130, depending on what type of cell information of the new cell that is known to the UE 130. In the examples of Fig. 1, it is therefore for illustrative purposes assumed that cell 120b represents a cell for which the cell information is in a first set of cell information, cell 120c represents a cell for which the cell information is in a second set of cell information, and cell 120d represents a cell for which the cell information is in a third set of cell information. Different examples of cell information will be disclosed below.

[0043] Fig. 2 is a flowchart illustrating embodiments of methods for cell reselection in a wireless communication network 100. The methods are performed by the UE 130. The methods are advantageously provided as computer programs 720.

[0044] The UE 130 maintains a list of known Physical Cell IDs (PCIs) in its current geographical area, and for each PCI a set of critical system information elements required for camping and cell reselection. The UE 130 uses a first power mode for camping and candidate cell measurement and reselection evaluation, as in step SI 02.

[0045] S102: The UE 130 performs a cell reselection evaluation procedure for a second cell 120b: 120d whilst camping on a first cell 120a in the wireless communication network 100 and whilst operating in a first power mode.

[0046] S106: The UE 130 performs cell reselection either according to step S106-2 or step S106-4.

[0047] If, for example, cell reselection to a known PCI in the current TA is triggered, the reselection is performed using the first power mode, and the stored critical system information elements required for camping and cell reselection are used. This is for example the case if the cell reselection is to cell 120b.

[0048] S 106-2: The UE 130 performs the cell reselection to the second cell 120b: 120d in the wireless communication network 100 whilst operating in the first power mode when cell information of the second cell 120b: 120d is in a first set of cell information.

[0049] If, for example, cell reselection to a known PCI in a new TA is triggered, the reselection is performed using a second power mode. The network is informed about the TA change. The UE 130 may reread and update the critical system information elements required for camping and cell reselection. This is for example the case if the cell reselection is to cell 120c or cell 120d.

[0050] S 106-4: The UE 130 performs the cell reselection to the second cell 120b: 120d whilst operating in the second power mode when cell information of the second cell 120b: 120d is in a second set of cell information.

[0051] A higher level of power is used by the UE 130 in the second power mode than in the first power mode. The first set of cell information and the second set of cell information are mutually exclusive with respect to each other.

[0052] It follows that the power modes refer to the power consumption of the radio in the UE 130. Therefore, the first power mode might be referred to as a first radio power mode, and the second power mode might be referred to as a second radio power mode. Further aspects of the different power modes will be disclosed below.

[0053] In this way, since critical system information elements required for camping and cell reselection (e.g., barring status, TA association and related cell selection, or reselection parameters) in practice never change (or change extremely infrequently), the UE 130 can assume that these remain valid and store these parameters.

[0054] Embodiments relating to further details of cell reselection in a wireless communication network 100 as performed by the UE 130 will now be disclosed with continued reference to Fig. 1. In some aspects, it is verified that the UE 130 is not of a UE category, or has a location, that depends on delivery of a critical service, such as Earthquake and Tsunami Warning System (ETWS) messages etc. Therefore, in some embodiments, the UE 130 is configured to perform (optional) step S104 before performing the cell reselection in step S106.

[0055] S104: The UE 130 verifies, before performing the cell reselection, that the UE 130 is operating in a non- critical operation mode.

[0056] Further aspects of the cell information will be disclosed next, starting with the first set of cell information and continuing with the second set of cell information.

[0057] In some embodiments, the first set of cell information comprises a set of cell identifiers for which system information is accessible to the UE 130 and that has same area identifier as the first cell 120a. For example, the first set of cell information might comprise a list of previously seen PCIs and previously obtained SIBs for which the TA is the same as for the current cell. In other words, that the system information is accessible to the UE 130 implies that the UE 130 has previously seen the cell, obtained and stored the relevant system information elements, and these system information elements are still considered valid.

[0058] In some embodiments, the second set of cell information comprises a set of cell identifiers for which system information is accessible to the UE 130 but that has different area identifier than the first cell 120a. For example, the second set of cell information might comprise a list of previously seen PCIs and previously obtained SIBs for which the TA is not the same as for the current cell. As above, that the system information is accessible to the UE 130 implies that the UE 130 has previously seen the cell, obtained and stored the relevant system information elements, and these system information elements are still considered valid

[0059] Further, in case the cell identifier of the second cell is unknown, the second power mode is used for the UE 130 read critical system information elements as required for camping and cell reselection. That is, in some embodiments, the cell reselection further is performed to the second cell 120b: 120d whilst the UE 130 is operating in the second power mode. This is the case when the cell information of the second cell 120b: 120d is in a third set of cell information. This is for example the case if the cell reselection is to cell 120d. The third set of cell information is mutually exclusive with respect to the first set of cell information and the second set of cell information. The UE 130 is configured to perform (optional) step S108 when the cell information of the second cell 120b: 120d is in the third set of cell information.

[0060] S 108: The UE 130 reads system information of the second cell 120b: 120d whilst operating in the second power mode.

[0061] The third set of cell information comprises a set of cell identifiers for which system information is not accessible to the UE 130. The UE 130 might then store the system information for the new cell identifier. Hence, in some embodiments, the UE 130 is configured to perform (optional) step SI 10.

[0062] S 110: The UE 130 stores the system information in the data storage of the UE 130.

[0063] The stored system information elements might be associated with a validity timer. Then, upon expiration of the timer, system information elements might be removed from the data storage and hence be made inaccessible to the UE 130 again. That is, in some embodiments, the system information is associated with a validity timer that starts when the system information has been stored, and the system information is removed from the data storage upon expiration of the validity timer.

[0064] In some examples, the UE 130 reads, or updates, relevant SIB information of currently known candidate cells regularly, e.g., using the second power mode at predetermined intervals (e.g., minutes or hours), or when the second power mode is used for other reasons, such as when the UE 130 is in active state being utilized for data communication.

[0065] There can be different examples of critical system information for the purposes of performing cell reselection. In some examples, the system information for each cell identifier comprises at least one of: an area identifier, a mobile network identifier, cell barring information. In some examples, the system information is a SIB, such as any of SIB1 to SIB5. Further, cell reselection measurement and evaluation rule parameters may be obtained e.g., in SIB2 / 3 (in terms of intra-frequency information), SIB4 (in terms of inter-frequency information), and SIB5 (in terms of inter-RAT EUTRAN information). In some embodiments, the cell reselection is performed whilst the UE 130 is in RRC Idle mode, and / or the area identifier is a TA index. In other embodiments, the cell reselection is performed whilst the UE 130 is in RRC Inactive mode, and / or the area identifier is a RAN area code. Hence, for a UE 130 operating in RRC Idle mode, TA update signaling can be used as an example of signaling relevant for the area identifier. Likewise, for a UE 130 operating in RRC Inactive mode, the UE 130 will monitor a RAN area code and perform a RAN area update procedure if the RAN area code is different in the selected cell, and hence where the area identifier is a RAN area code.

[0066] Further critical parameters may be further be split into two sets. The first set is needed to monitor paging and perform measurements for cell reselection - parameters that are likely constant over time - and it needs to be stored for each known cell. The second set contains parameters that are used to access the cell (when entering RRC Connected mode). The latter may also be stored, or preferably they may be read using the second mode each time when they are needed, e.g., after being paged or when there is data in the UE 130 and the UE 130 wants to enter the RRC Connected mode. This ensures that the UE 130 will not try to transmit using outdated parameters (since RACH parameters may be updated more frequently by the network), and it does not require any additional use of the second power mode since the second power mode will in any case be use for the access procedure. Parameters that are needed for the UE 130 to enter RRC Connected mode (e.g., RACH configurations) can be read using the second power mode when needed. For example, the UE 130 might read SIB1 of the candidate cell to obtain these parameters when the UE informs the network of a TA change or when the UE 130 wants to enter RRC Connected mode for other reasons, such as being paged or having data to transmit. This ensures that the UE 130 always uses up-to-date parameters when it transmits. To avoid long-term unaligned camping (e.g., camping cell that is no longer permitted), the UE 130 may use the second power mode infrequently, or whenever the second power mode needs to be used for other reasons, to read the related system information.

[0067] Further aspects of the different power modes will be disclosed next with reference to Fig. 3. In Fig. 3 is shown block diagrams of UEs 400a, 400b, 400c, 400d with different examples of implementations of the first power mode and the second power mode with respect to baseband units and radios. In some aspects, each of the power modes is associated with its own receiver, or receiver module.

[0068] In some embodiments, the UE 130 comprises a main receiver (MR) radio 310a, and the MR radio 310a is switched off in the first power mode and switched on in the second power mode. This is the example for UEs 300a, 300b. In some embodiments, the UE 130 comprises an MR radio 310a and an auxiliary receiver (AR) radio 310b, where operating in the first power mode comprises using the AR radio 310b, and operating in the second power mode comprises using the MR radio 310a. This is the example for UEs 300a, 300b. In some examples, the AR radio 310b is a low power (LP) Wake Up Radio (WUR) unit. In UE 300a, the AR radio 310b is separated from the MR radio 310a and the baseband unit 320. UE 300a thus has a (separate) AR radio 310b with both radio reception capabilities and signal detection / demodulation capabilities. UE 300b comprises an AR baseband unit 320a and an AR radio 310b that are separated from the MR radio 310a and the MR baseband unit 320a. This UE 300b thus has one (separate) AR radio 310b with radio reception capabilities and a (separate) AR baseband unit 320a with signal detection / demodulation capabilities.

[0069] In other aspects, the same hardware is used for all different power modes, but where the hardware is configured to operate in different power modes. This is the example for UE 300d and partly for UE 300a. In UE 300a, the AR radio 310b and the MR radio 310a share a common baseband unit 320, and in UE 300d there is a common baseband unit 320 and a common radio unit 310 that are both configured for operation both in the first power mode and the second power mode. In UE 300c the AR baseband unit 320b is separated from the MR baseband unit 320a but they utilize the same radio 310. The radio 310 is thus common for the AR baseband unit 320b and the MR baseband unit 320a. In UE 300d the functionality of the second power mode is completely integrated with the first power mode. The AR implementations in UEs 300a, 300b, 300c are thus using at least partly dedicated hardware for low power operation. Such dedicated hardware might be implemented to operate with very low energy consumption. Depending on the design constraints for its low power operation, the dedicated hardware might not be capable to detect or correctly receive (such as decode, demodulate, etc.) signals when the UE is located at the cell edge. In that case, the UE 400a:400c might utilize its main receiver hardware for channel monitoring in poor channel conditions. In some examples, the AR might, as in UE 400c, use at least partly common hardware components. In some examples, the AR might, as in UE 400d, operate as a functionality fully within the common hardware, i.e., the transceiver used for regular UE operation. When using such a common hardware, the AR might still operate in different configurations or modes, where some hardware or software parameters are adapted depending on the usage of the AR. As example, the radio 310 might have one or more specific operation modes for energy efficient cell reselection operation, where such efficient modes might imply that only strong signals can be detected in case sensitivity, linearity, and / or other requirements can be relaxed.

[0070] One particular embodiment for cell reselection in a wireless communication network 100 as performed by the UE 130 according to at least some of the above disclosed embodiments, aspects, and examples, will now be disclosed with reference to the flowchart of Fig. 4.

[0071] In step S202, the UE 130 uses the first power mode to perform camping and neighbor cell measurements using the first power mode, e.g., based on the primary synchronization signal (PSS) or the secondary synchronization signal (SSS) in the SSB or the Low Power Synchronization Signal (LP-SS). Based on the SSB content, the UE 130 may determine the PCI of the cell, and based on the LP-SS, another type of cell identifier may be determined. The link quality as estimated using the first power mode may be calibrated with respect to the corresponding estimate using the second power mode, and reselection criteria regarding camping and candidate cell qualities can be configured for the first power mode. If one or more criteria are fulfilled for a candidate cell, the cell may be considered a reselection candidate.

[0072] Either step S204 or step S210 is then entered.

[0073] In step S204, the UE 130 recognizes the candidate cell as previously seen, and for which valid SIB1 information is available. The recognition may be based on e.g., the PCI value and a validity timer from previous SIB1 acquisition. The recognition may also be based on a checksum, or hash value, based on SIB 1 coded bits as captured using the first power mode, without requiring decoding using the second power mode. The UE 130 may then retrieve the TA and barring information for the cell, stored in the UE 130. The UE 130 may also retrieve other candidate cell measurement and evaluation parameters, to evaluate whether reselection to the candidate cell should be performed.

[0074] In case step S204 was entered, either step S206 or step S208 is then entered.

[0075] In step S206, if the TA of the candidate cell is the same as the current camping cell, and if the candidate cell is not barred for the UE 130, the UE 130 performs reselection to the candidate cell using the first power mode. The UE 130 will subsequently consider that cell to be its camping cell and perform any candidate cell evaluations with respect to that cell, using the stored critical system information elements required for camping and cell reselection. The camping cell related PCI parameters are also updated accordingly.

[0076] In step S208, if in contrast to step S206, the TA of the new cell differs from the current camping cell, the UE 130, in addition to performing the reselection, will need to inform the network about the new TA where it should be paged. The UE 130 uses the second power mode when accessing the cell and when signaling to the network its new TA index. In addition, the UE 130 may read parameters required to access the cell (e.g., the RACH configuration), using the second power mode, before preforming the cell access. After the TA change reporting, the UE 130 might again use the first power mode.

[0077] In step S210, in contrast to step S204, the UE 130 finds that the PCI was not previously known, i.e., that the cell has not been previously visited and the critical system information elements required for camping and cell reselection are not known. The UE 130 then uses the second power mode to read critical system information elements required for camping and cell reselection, such as SIB1 and other relevant SIBs. The UE 130 may then use the second power mode to extract critical system information elements required for camping and cell reselection (e.g., TA, selection criteria and barring information fields) and store these critical system information elements, labeled with the PCI or another cell ID, for future use.

[0078] In case step S210 was entered, step S212 is then entered.

[0079] In step S212, if the cell is not barred, the UE 130 performs the cell reselection and configure its first power mode with the new camping cell information. If the TA is different from the previous camping cell, the UE 130 uses the second power mode to signal to the network its new TA index. After that, the UE 130 might again use the first power mode.

[0080] In summary, the herein disclosed embodiments provide an implementation approach for minimizing the usage of the second power mode for performing cell reselection when the UE 130 is in RRC Idle mode or RRC Inactive mode. The cell quality measurements and reselection evaluation may be performed in the first power mode, and the UE uses critical system information elements, required for the reselection process, stored from previous instances where these cells were found. Obtaining this information would otherwise every time require the use of the second power mode, possibly necessitating the wakeup of the MR, assuming that all critical system information elements (e.g., SIB1) cannot be read using the first power mode.

[0081] At least some of the herein disclosed embodiments rely on the practical and robust observation that critical system information elements required for camping and cell reselection (e.g., PLMN information, barring status, cell reselection information parameters and TA allocation) of individual cells in a practical network deployment change very rarely, if ever.

[0082] Fig. 5 schematically illustrates, in terms of a number of structural units, the components of a UE 500 according to an embodiment. Processing circuitry 510 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 710 (as in Fig. 7), e.g., in the form of a storage medium 530. The processing circuitry 510 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

[0083] Particularly, the processing circuitry 510 is configured to cause the UE 500 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 530 may store the set of operations, and the processing circuitry 510 may be configured to retrieve the set of operations from the storage medium 530 to cause the UE 500 to perform the set of operations. The set of operations may be provided as a set of executable instructions.

[0084] Thus the processing circuitry 510 is thereby arranged to execute methods as herein disclosed. The storage medium 530 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The UE 500 may further comprise a communications (comm.) interface 520 at least configured for communications with the access network nodes 110a: 1 lOd in the wireless communication network 100. As such the communications interface 520 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 510 controls the general operation of the UE 500 e.g., by sending data and control signals to the communications interface 520 and the storage medium 530, by receiving data and reports from the communications interface 520, and by retrieving data and instructions from the storage medium 530. Other components, as well as the related functionality, of the UE 500 are omitted in order not to obscure the concepts presented herein.

[0085] Fig. 6 schematically illustrates, in terms of a number of functional modules, the components of a UE 600 according to an embodiment. The UE 600 of Fig. 6 comprises a number of functional modules; an evaluate module 610 configured to perform step S102, and a reselect module 630 configured to perform step SI 06 (including step SI 06-2 and step SI 06-4). The UE 600 of Fig. 6 may further comprise a number of optional functional modules, such as any of an obtain module 620 configured to perform step SI 04, a read module 640 configured to perform step S108, and a store module 650 configured to perform step S110.

[0086] In general terms, each functional module 610:650 may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium 530 which when run on the processing circuitry makes the UE 600 perform the corresponding steps mentioned above in conjunction with Fig 6. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 610:650 may be implemented by the processing circuitry 510, possibly in cooperation with the communications interface 520 and / or the storage medium 530. The processing circuitry 510 may thus be configured to from the storage medium 530 fetch instructions as provided by a functional module 610:650 and to execute these instructions, thereby performing any steps as disclosed herein.

[0087] Fig. 7 shows one example of a computer program product 710 comprising computer readable storage medium 730. On this computer readable storage medium 730, a computer program 720 can be stored, which computer program 720 can cause the processing circuitry 510 and thereto operatively coupled entities and devices, such as the communications interface 520 and the storage medium 530, to execute methods according to embodiments described herein. The computer program 720 and / or computer program product 710 may thus provide means for performing any steps as herein disclosed.

[0088] In the example of Fig. 7, the computer program product 710 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 710 could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 720 is here schematically shown as a track on the depicted optical disk, the computer program 720 can be stored in any way which is suitable for the computer program product 710.

[0089] The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

CLAIMS1. A method for cell reselection in a wireless communication network (100), wherein the method is performed by a user equipment (130, 400, 500, 600), and wherein the method comprises: performing (S102) a cell reselection evaluation procedure for a second cell (120b: 120d) whilst camping on a first cell (120a) in the wireless communication network (100) and whilst operating in a first power mode; and either performing (S 106-2) the cell reselection to the second cell (120b: 120d) in the wireless communication network (100) whilst operating in the first power mode when cell information of the second cell (120b: 120d) is in a first set of cell information; or performing (S 106-4) the cell reselection to the second cell (120b: 120d) whilst operating in a second power mode when cell information of the second cell (120b: 120d) is in a second set of cell information, wherein a higher level of power is used by the user equipment (130, 400, 500, 600) in the second power mode than in the first power mode, and wherein the first set of cell information and the second set of cell information are mutually exclusive with respect to each other.

2. The method according to claim 1, wherein the first set of cell information comprises a set of cell identifiers for which system information is accessible to the user equipment (130, 400, 500, 600) and that has same area identifier as the first cell (120a).

3. The method according to claim 1 or 2, wherein the second set of cell information comprises a set of cell identifiers for which system information is accessible to the user equipment (130, 400, 500, 600) but that has different area identifier than the first cell (120a).

4. The method according to any preceding claim, wherein the user equipment (130, 400, 500, 600) comprises a main receiver, MR, radio (310a), and wherein the MR radio (310a) is switched off in the first power mode and switched on in the second power mode.

5. The method according to any preceding claim, wherein the user equipment (130, 400, 500, 600) comprises a main receiver, MR, radio (310a), and an auxiliary receiver, AR, radio (310b), wherein operating in the first power mode comprises using the AR radio (310b), and operating in the second power mode comprises using the MR radio (310a).

6. The method according to claim 5, wherein the AR radio (310b) is a low power, LP, Wake Up Radio, WUR, unit.

7. The method according to any preceding claim, wherein the cell reselection further is performed to the second cell (120b: 120d) whilst operating in the second power mode when the cell information of thesecond cell (120b: 120d) is in a third set of cell information, wherein the third set of cell information is mutually exclusive with respect to the first set of cell information and the second set of cell information, and wherein, when the cell information of the second cell (120b: 120d) is in the third set of cell information, the method further comprises: reading (S 108) system information of the second cell (120b: 120d) whilst operating in the second power mode.

8. The method according to claim 7, wherein the third set of cell information comprises a set of cell identifiers for which system information is not accessible to the user equipment (130, 400, 500, 600).

9. The method according to claim 8, wherein the method further comprises: storing (SI 10) the system information in the data storage of the user equipment (130, 400, 500, 600).

10. The method according to claim 9, wherein the system information is associated with a validity timer that starts when the system information has been stored, and wherein the system information is removed from the data storage upon expiration of the validity timer.

11. The method according to any preceding claim, wherein the system information for each cell identifier comprises at least one of: an area identifier, a mobile network identifier, cell barring information.

12. The method according to any preceding claim, wherein the system information is a system information block, SIB, such as any of SIB1 to SIB5.

13. The method according to any preceding claim, wherein the cell reselection is performed whilst the user equipment (130, 400, 500, 600) is in radio resource control, RRC, idle mode.

14. The method according to any of claims 2, 3, or 11, in combination with claim 13, wherein the area identifier is a tracking area, TA, index.

15. The method according to any of claims 1 to 12, wherein the cell reselection is performed whilst the user equipment (130, 400, 500, 600) is in radio resource control, RRC, inactive mode.

16. The method according to any of claims 2, 3, or 11, in combination with claim 15, wherein the area identifier is a radio access network, RAN, area code.

17. The method according to claim 1, wherein the method further comprises: verifying (S104), before performing the cell reselection, that the user equipment (130, 400, 500, 600) is operating in a non-critical operation mode.

18. A user equipment (500) for cell reselection in a wireless communication network (100), the user equipment (500) comprising processing circuitry (510), the processing circuitry being configured to cause the user equipment (500) to: perform a cell reselection evaluation procedure for a second cell (120b: 120d) whilst camping on a first cell (120a) in the wireless communication network (100) and whilst operating in a first power mode; and either perform the cell reselection to the second cell (120b: 120d) in the wireless communication network (100) whilst operating in the first power mode when cell information of the second cell (120b: 120d) is in a first set of cell information; or perform the cell reselection to the second cell (120b: 120d) whilst operating in a second power mode when cell information of the second cell (120b: 120d) is in a second set of cell information, wherein a higher level of power is used by the user equipment (500) in the second power mode than in the first power mode, and wherein the first set of cell information and the second set of cell information are mutually exclusive with respect to each other.

19. A user equipment (600) for cell reselection in a wireless communication network (100), the user equipment (600) comprising: an evaluate module (610) configured to perform a cell reselection evaluation procedure for a second cell (120b: 120d) whilst camping on a first cell (120a) in the wireless communication network (100) and whilst operating in a first power mode; and a reselect module (630) configured to either: perform the cell reselection to the second cell (120b: 120d) in the wireless communication network (100) whilst operating in the first power mode when cell information of the second cell (120b: 120d) is in a first set of cell information; or perform the cell reselection to the second cell (120b: 120d) whilst operating in a second power mode when cell information of the second cell (120b: 120d) is in a second set of cell information, wherein a higher level of power is used by the user equipment (600) in the second power mode than in the first power mode, and wherein the first set of cell information and the second set of cell information are mutually exclusive with respect to each other.

20. The user equipment (500, 600) according to claim 18 or 19, further being configured to perform the method according to any of claims 2 to 17.

21. A computer program (720) for cell reselection in a wireless communication network (100), the computer program comprising computer code which, when run on processing circuitry (510) of a user equipment (500), causes the user equipment (500) to: perform (S102) a cell reselection evaluation procedure for a second cell (120b: 120d) whilst camping on a first cell (120a) in the wireless communication network (100) and whilst operating in a first power mode; and either perform (S106-2) the cell reselection to the second cell (120b: 120d) in the wireless communication network (100) whilst operating in the first power mode when cell information of the second cell (120b: 120d) is in a first set of cell information; or perform (S 106-4) the cell reselection to the second cell (120b: 120d) whilst operating in a second power mode when cell information of the second cell (120b: 120d) is in a second set of cell information, wherein a higher level of power is used by the user equipment (500) in the second power mode than in the first power mode, and wherein the first set of cell information and the second set of cell information are mutually exclusive with respect to each other.

22. A computer program product (710) comprising a computer program (720) according to claim 21, and a computer readable storage medium (730) on which the computer program is stored.

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