WO2023073036A1 - Method for efficiently selecting a coverage level on recovery from out of service in cellular systems - Google Patents
Method for efficiently selecting a coverage level on recovery from out of service in cellular systems Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W48/00—Access restriction; Network selection; Access point selection
- H04W48/18—Selecting a network or a communication service
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W48/00—Access restriction; Network selection; Access point selection
- H04W48/16—Discovering, processing access restriction or access information
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W88/00—Devices specially adapted for wireless communication networks, e.g. terminals, base stations or access point devices
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Abstract
The invention relates to a method for efficiently selecting a coverage level on recovery from out of service in cellular systems performed by UE. The objective of the invention to present a method, which allows a UE after lack of coverage to get again service to a PLMN as fast as possible, will be solved by repeating a PLMN search within a retry PLMN search time interval, which is the sum of an EARFCN scan duration, a Band scan duration and a sleep time duration, whereas the UE selects between three scan modes comprising a last-registered EARFCN scan mode, a previous- registered EARFCN scan mode and a Band scan mode, the UE selects said scan modes in this order, whereas the UE starts with the last-registered EARFCN scan mode and selects an extreme coverage level (ECL2) for PLMN scanning within the EARFCN scan duration, if no PLMN is found, the UE enters the previous-registered EARFCN scan mode and first recalculates a remaining EARFCN scan duration of the EARFCN scan duration and selects randomly a coverage level and a registered EARFCN from a registered EARFCN list for PLMN scanning, if no PLMN is found within the remaining EARFCN scan duration, the UE enters the Band scan mode and calculates the Band scan duration and a coverage level budget for each coverage level and performs a Band scan if the coverage level budget is enough for at least one Band scan, if no PLMN is found, the UE increments the retry PLMN search time interval by a retry PLMN scan factor and passes through the procedure again until an accepted PLMN is found.
Description
Method for efficiently selecting a coverage level on recovery from out of service in cellular systems The invention dis closes a method for efficiently selecting a coverage level on recovery from out of service in cellular systems performed by a user equipment (UE ) .
To simplify the readability of the document the following abbreviations will be used in the text :
AS Acces s Stratum
CIoT Cellular Internet of Things
DRX Dis continuous Reception
DL Downlink
EARFCN E-UTRA Absolute Radio Frequency Channel Number
ECL Enhanced Coverage Level eDRX Extended Discontinuous Reception
EHPLMN Equivalent Home PLMN
EPS Evolved Packet System
E-UTRA Evolved Universal Terrestrial Radio Acces s
GPRS General Packet Radio Service
HPLMN Home PLMN
ID Identifier loT Internet of Things
MME Mobility Management Entity
MS Mobile Station
NAS Non Acces s Stratum
NB-IoT Narrow Band Internet of Things
PDU Protocol Data Unit
PGW Packet Data Network Gateway
PLMN public land mobile network
PTW Paging Time Window
RRC Radio Resource Control
RF Radio Frequency
RPLMN Recovery Registered public land mobile network
RPS Retry PLMN Scan Time
RSRP Reference Signal Received Power
SCE F Service Capability Exposure Function
SGW Serving Gateway
SINR signal-to-interference -plus- noise- ratio
SMF Session Management Function
UE User Equipment
UL Uplink
VPLMN Visited PLMN
In addition to the above abbreviations, document 3GPP TR 21.905 V16.0.0 (2019-06) "Vocabulary for 3GPP Specifications" contains further relevant abbreviations and definitions.
In order to illustrate the advantages of the method according to the invention, the prior art with respect to PLMN search procedure will first be discussed, with reference to 3GPP TS 36.304 version 16.2.0 "User Equipment (UE) procedures in idle mode" and 3GPP TS 23.122 version 16.7.0 "Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode".
A known PLMN search procedure is as follows :
At switch on the UE, or following recovery from lack of coverage, a UE performs a PLMN selection procedure in order to find a suitable cell and perform registration.
A UE maintains a PLMN list, which includes the registered PLMN code and the list of equivalent PLMNs that are provided by the network. The UE shall choose one of these PLMNs.
All these PLMNs are considered as equivalent i. e. allowed for service, for PLMN selection procedure in NAS or cell selection, re-selection and handover in AS.
If the registered PLMN is available among these PLMNs, the UE prioritizes it over other PLMNs for PLMN selection.
If no PLMNs are available, the UE enters out of service and repeats the PLMN search procedure until a PLMN becomes available. If a PLMN is available, upon selecting either registered PLMN or equivalent PLMN, the UE stops the PLMN search procedure.
If the UE selected a PLMN different from the stored registered PLMN, and registration is successfully completed, then this PLMN is updated as the registered PLMN .
When selecting a PLMN different from registered PLMN, the UE priories the PLMN with highest signal quality as specified in details in the standard 3GPP TS 36 . 304 version 16 . 2 . 0 "User Equipment (UE ) procedures in idle mode" section 5 . 2 . 3 Cell Selection Process .
Once the UE has selected a PLMN, the cell selection procedure shall be performed in order to select a suitable cell of that PLMN to camp on .
Upon successfully selecting a suitable cell , the UE indicates the same to NAS . If the UE is not registered to the network already, then NAS initiates the initial registration procedure . Upon successfully completing registration procedure , the UE enters REGISTERED state . The EARFCN on which the UE complete this registration procedure successfully is referred to as its latest- registered EARFCN .
Upon successfully selecting or reselecting a suitable cell , a reregistration may be required due to several factors , including but not limited to the cell belonging a tracking area , which is different from the currently registered tracking area . The currently registered EARFCN is now referred to as previous- registered EARFCN . The EARFCN on which the re-registration ( tracking area update ) is completed successfully is considered as the latest-registered EARFCN .
The PLMN search provides some restrictions :
If the UE only supports control plane CIoT EPS optimization, and the UE camps on an E-UTRA cell which is not a NB-IoT cell , the UE shall not consider PLMNs which do not advertise support of EPS
services with control plane CIoT EPS optimization. Control Plane CIoT EPS optimization is used to reduce the total number of control plane messages when handling a short data transaction. User data or SMS messages is conveyed to the IOT services via MME by encapsulating them in NAS messages. There are two paths through which, user data or SMS can be transmitted on the Control Plane CIoT EPS optimization: 1)UL data are transferred from the eNB (CIoT RAN) » the MME >> the Serving Gateway (SGW) >> the Packet Data Network Gateway (PGW) >> IOT Services or 2) UL data are transferred from the eNB (CIoT RAN) » the MME >> the Service Capability Exposure Function (SCEF) (only for non-IP data packets) >> IOT Services .
If the UE only supports control plane CIoT EPS optimization, and the UE camps on an E-UTRA cell connected to EPS, which is not a NB-IoT cell, the UE shall not consider PLMNs, which do not advertise support of EPS services with control plane CIoT EPS optimization. The Control Plane CIoT EPS optimization is used to exchange user data between the UE and the SME as payload of a NAS message in both uplink and downlink directions, avoiding the establishment of a user plane connection for the PDU Session. If the UE supports CIoT EPS optimizations, the UE shall not consider the PLMN/access technology combinations for which the UE preferred CIoT network behavior is not advertised as supported by the PLMN/access technology combination. For example, if the UE is configured for User Plane optimization as preferred and EPS does not advertise User Plane optimization, the UE shall not consider the PLMN.
Considering a Roaming (VPLMN) Scenario:
If the UE is in a VPLMN (Visited PLMN) , the UE shall periodically attempt to obtain service on its HPLMN or one of its EHPLMNs or a higher priority PLMN.
If the HPLMN (if the EHPLMN list is not present or is empty) or an
EHPLMN ( if the list is present ) or a higher priority PLMN is not found, the UE shall remain on the VPLMN .
A known Cell Selection procedure is as follows :
The NAS sends the PLMN ID to be selected, to the RRC layer . The UE shall scan all RE channels in the E-UTRA bands according to its capabilities to find available PLMNs . On each carrier , the UE shall search for the strongest cell and read its system information, in order to find out which PLMN ( s ) the cell belongs to . The PLMN ID on the selected cell is reported back to NAS .
The UE shall scan all RE channels in the E-UTRA bands according to its capabilities to find a suitable cell . On each carrier frequency, the UE needs only search for the strongest cell . Once a suitable cell is found this cell shall be selected . In this type of cell selection, no prior information about the cell or frequency is required . This procedure is also referred to as an Initial Cell Selection .
The UE may store several information related to a cell , which may include , but not limited to carrier frequencies , information on cell parameters from previously received measurement control information elements / from previously detected cells etc . The UE may optimize PLMN search by using this stored information . Once a suitable cell is found this cell shall be selected . This procedure is also referred to as a stored information cell selection . If no suitable cell is found using this stored information, the Initial Cell Selection procedure shall be started .
Upon selecting a PLMN, the NAS instructs AS to perform cell selection on a suitable cell belonging to the selected PLMN . A selected cell is considered to be suitable if the cell satisfies the cell selection criteria as provisioned in the specification 3GPP TS 36 . 304 .
When no suitable cell is found on the selected PLMN, and the UE is unable to obtain normal service from a PLMN, the UE enters out of service .
If a registration is not possible on recovery from lack of coverage due to the registered PLMN being unavailable, a UE is allowed to continue looking for the registered PLMN for an implementation dependent time.
The UE scans all RE channels in the requisite E-UTRA bands to find available PLMNs . On each carrier, the UE searches for the strongest cell and reads its system information, in order to find out which PLMN(s) the cell belongs to.
All the PLMNs that the found cell belongs to are reported to the NAS if the cell's measured RSRP value is greater than or equal to -110 dBm. If the measured RSRP is lower, but the UE is able to read the PLMN IDs, these are also reported to the NAS.
A Loss of Service is provisioned in 3GPP TS 36.133 as follows:
If the UE is not configured with eDRX IDLE cycle and the serving NB-IoT cell does not fulfill the cell selection S criteria (see in details in the standard 3GPP TS 36.304 version 16.2.0 "User Equipment (UE) procedures in idle mode" section 5.2.3.2a Cell Selection Criterion for NB-IoT) for a pre-defined number of consecutive DRX cycles, the UE shall initiate the measurements of all neighbor cells indicated by the serving NB-IoT cell.
If the UE is configured with eDRX IDLE cycle and the serving NB- loT cell does not fulfill the cell selection S criteria for a predefined Nserv NB-NC (as specified in details in the standard 3GPP TS 36.133 version 16.7.0 "Requirements for support of radio source management" section 4.6.2.1 Measurement and evaluation of serving NB-IoT cell for UE category NB1 in normal coverage) consecutive DRX cycles within a single PTW, the UE shall initiate the
measurements of all neighbor cells indicated by the serving NB-IoT cell .
If the UE in RRC IDLE cycle has not found any new suitable cell based on searches and measurements using the intra-f requency and inter-frequency information indicated in the system information for a duration of 40 seconds , the UE enters out of service and initiates PLMN search procedure .
If no PLMNs are available , the UE enters out of service and repeats the PLMN search procedure until a PLMN becomes available .
Coverage Level
NB-IoT UEs can achieve up to 20 dB coverage extension compared with legacy cellular technologies like GPRS and LTE , while limiting the maximum UE transmit power to 23 dBm . Multiple coverage classes are supported to adapt to different path and penetration losses experienced by different UEs . This allows MSs with better than worst case coupling loss to benefit from improved battery life and lower latency and benefits the network in terms of improved capacity .
By the means of smaller system bandwidth and higher number of repetitions on the physical layer, the coverage of legacy technology ( called "normal coverage" ) is enhanced to extended ( +10dB ) and extreme coverage ( +20dB ) as illustrated in figure 1 . The coverage levels have a direct impact on the PLMN search duration and, thus , also on the power consumption of the modem IC of the UE . The signal of base stations in normal coverage can be detected within order of magnitude lower time as the signal quality in terms of SINR is also higher . For extreme coverage , on the other hand, the signal of the cell is more than 15dB below the noise floor , which requires substantial accumulation within the time direction leading to long receive operations . As a rule of thumb , every 3dB coverage extension will double the respective PLMN search time and, thus , the PLMN search time can be expected
to increase by a factor of 100 when going from normal to extreme coverage .
Message sequences and basic procedures between the NAS , the RRC layer and Lower Layers involved in the PLMN Search Request Procedure is illustrated in figure 2 .
A detailed procedure for PLMN selection and cell selection according to the current state of the art is illustrated in figure 3 ( its reference is 3GPP TS 36 . 304 version 16 . 2 . 0 Release 16 , Figure 5 . 2 . 2 -2 ) and figure 4 ( its reference is 3GPP TS 23 . 122 version 16 . 7 . 0 Release 16 , Figure 2a ) .
PLMN selection after recovery from lack of coverage
A detailed strategy of the PLMN selection in the UE are not specified in 3GPP , but left to UE implementation . The standard distinguishes between PLMN selection after terminal switch-on and after recovery from lack of coverage . After switch-on, the target is to get service as fast as possible .
For the recovery from lack of coverage , however, there is the uncertainty when coverage can be regained . The lack of coverage situation might last from several 10s of seconds up to days and weeks . The standard allows for searching the RPLMN for an implementation dependent time . The RPLMN search is generally less power hungry compared to an initial PLMN search across all RF bands . Thus , the PLMN search strategy for recovery from lack of coverage determines a tradeoff between energy consumption of the modem IC vs . time to recover the service . The modem-proprietary PLMN search strategy determines the type of PLMN search to be executed within each search round as well as the repetition interval in case of PLMN selection failures .
One of the most prominent features of NB-IoT is the ability of adaption of the coverage level . Thereby, NB-IoT extends the coverage by 20dB coupling loss beyond state of the art 2G and 4G
networks . By the means of smaller system bandwidth and higher number of repetitions on the physical layer , the coverage of legacy technology ( called "normal coverage" ) is enhanced to extended coverage ( +10dB ) and extreme coverage ( +20dB ) .
The coverage levels have a direct impact on the PLMN search duration and, thus , also on the power consumption of the modem IC of the UE . The signal of base stations in normal coverage can be detected within order of magnitude lower time as the signal quality in terms of SINR is also higher . For extreme coverage , on the other hand, the signal of the cell is more than 15dB below the noise floor , which requires substantial accumulation within the time direction leading to long receive operations . As a rule of thumb , every 3dB coverage extension will double the respective PLMN search time and, thus , the PLMN search time can be expected to increase by a factor of 100 when going from normal to extreme coverage .
The NB-IoT coverage level is another dimension within the modemproprietary PLMN selection strategy, which furthermore affects the aforementioned tradeoff between energy consumption vs . recovery time . The question now arises as to how the NB-IoT Coverage Levels within PLMN Selection for recovery from lack of coverage can be used efficiently . As it will be illustrated below by two use cases , there is no dominant NB-IoT coverage level selection strategy that is optimal in all scenarios .
Use case 1 : Passing a Tunnel at Vehicular Speed (Temporary Lack of Coverage )
As illustrated in figure 5 , a vehicle in normal coverage enters a tunnel for a short time . Within the tunnel , there is no NB-IoT coverage . After leaving the tunnel after some seconds or minutes , however , the modem might regain service on the same cell in normal coverage again . The time and energy to regain the service depends on the PLMN search strategy when recovering from lack of coverage .
When attempting to perform cell selection at extreme coverage level too early in the PLMN search for recovery from out of coverage , the UE is busy for a longer duration, searching for weak PLMN on different EARFCNs . Therefore , once the DL signal recovers on the most recent serving cell , the UE may not detect it for a significant amount of time , due to ongoing search of extreme coverage . This scenario is illustrated in figure 6 .
Use Case 2 : Entering an Underground Parking Garage ( Permanent change of NB-IoT Coverage Level )
In figure 7 , a different scenario is illustrated where a vehicle enters an underground parking garage . Before entering the garage , the UE on the vehicle was having excellent signal quality within normal coverage . When entering the garage , however , the signal quality drops permanently so that an escalation of the coverage to extreme coverage is required .
Opposite to the use case above , the best strategy of the UE is the early escalation into extreme coverage as shown in figure 8 .
Nevertheless , the good recovery time performance of the aggressive coverage level escalation strategy comes at the expense of high- energy consumption when the modem remains out of coverage for a longer period of time .
It is therefore an obj ective of the invention to present a method, which allows a UE after lack of coverage to get again service to a PLMN as fast as possible . The method should provide a PLMN search strategy for recovery from lack of coverage that considers a tradeoff between energy consumption of the modem IC of the UE versus time to recover the service . As it is not possible to achieve both, low energy consumption and low recovery time for the problem of selecting the coverage level within the PLMN search during the recovery from lack of coverage , a compromise between both competitive targets needs to be found .
The obj ective of the present invention will be solved by a method
according to independent claim 1 . According to the inventive method for efficiently selecting a coverage level on recovery from out of service in cellular systems performed by a user equipment , UE , a public land mobile network, PLMN, search is repeated within a retry PLMN search time interval , which is the sum of an E-UTRA Absolute Radio Frequency Channel Number , EARFCN, scan duration, a Band scan duration and a sleep time duration, whereas the UE selects between three scan modes comprising a last-registered EARFCN scan mode , a previous-registered EARFCN scan mode and a band scan mode , the UE selects said scan modes in this order, whereas the UE starts with the last-registered EARFCN scan mode ( state#2 ) and selects an extreme coverage level ( ECL2 ) for PLMN scanning within the EARFCN scan duration, if no PLMN is found, the UE enters the previous-registered EARFCN scan mode ( state#5 ) and first recalculates a remaining EARFCN scan duration of the EARFCN scan duration and selects randomly a coverage level and a registered EARFCN from a registered EARFCN list for PLMN scanning, if no PLMN is found within the remaining EARFCN scan duration, the UE enters the Band scan mode ( state#6 ) and calculates the Band scan duration and a coverage level budget for each coverage level and performs a Band scan if the coverage level budget is enough for at least one Band scan, if no PLMN is found, the UE increments the retry PLMN search time interval by a retry PLMN scan factor and passes through the procedure again until an accepted PLMN is found .
The retry PLMN search time interval or the retry PLMN scan time interval is the interval to repeat the PLMN search .
The sum of the EARFCN scan duration and the Band scan duration shall be less than or equal to the retry PLMN search time interval . In case of the sum is less than the PLMN search time interval , the remaining time is a sleep time duration for the modem IC of the UE and accordingly least power is consumed during this time .
The inventive method differentiates between three scans modes : a last-registered EARFCN scan mode , a previous-registered EARFCN scan mode and a Band scan mode . On one dimension, each scan mode is operated in different time slots . The duration of each time slot is decided/calculated according to the corresponding/conf igured duty cycle . There are two duty cycles defined : one for EARFCN scan and one for Band scan . Each duty cycle is configured over the retry PLMN scan time . The retry PLMN search time or retry PLMN search time interval is increased by a configured retry PLMN search factor after finishing a corresponding Band scan time slot , accordingly, hence the retry PLMN search time interval is increased with said configured PLMN search factor . The last-registered EARFCN scan and the previous- registered EARFCN scan is operated within the EARFCN scan time slot . The Band scan is operated within the Band scan time slot .
On another dimension, each scan mode selects a coverage level , taken into consideration the allowed scan time , which is the EARFCN scan duration or Band scan duration .
The inventive method or algorithm gives the last-registered EARFCN scan mode more search opportunities than the others , hence the previous-registered EARFCN scan mode and the Band scan mode . This is achieved by selecting an extreme coverage level ( ECL2 ) with higher scan time .
Furthermore , the inventive algorithm gives the previous-registered EARFCN scan in the EARFCN scan mode higher search opportunities than the Band scan in the Band scan mode . This is achieved by randomly selecting a scan coverage level and a registered EARFCN from a registered EARFCN list for PLMN scanning through a scan time that is more frequent repeated than in the Band scan mode .
Finally, for each coverage level , the algorithm starts the Band scan mode by considering a coverage level budget . A coverage level budget is the allowed duration of a time slot offered by the algorithm to operate Band scan for the selected coverage level and
it is accumulated to the next iteration in case of not totally consumed by Band scans .
During or in the allowed EARFCN scan duration ( defined according to a configured duty cycle ) , the method or algorithm starts with the last-registered EARFCN scan mode and selects an extreme coverage level ( ECL2 ) for the PLMN scanning . This allows the UE to have different time opportunities to rescan for the last- registered PLMN and not give up quickly as explained in the use case #1 mentioned above .
After the last-registered EARFCN scan mode is considered and if no PLMN is found, the algorithm enters the previous-registered EARFCN scan mode and first checks the remaining allowed EARFCN scan time of the allowed EARFCN scan duration/duty cycle to evaluate the possibility for further EARFCN and hence PLMN scanning . If EARFCN scan time is still left , the algorithm selects randomly a coverage level and selects randomly previously registered EARFCNs from a registered EARFCN list that can be scanned within the remaining EARFCN scan time slot . The selection of the coverage level could be another function of the algorithm. If no enough EARFCN scan time of the EARFCN scan time slot is left , the algorithm escalates to the Band scan mode .
After the EARFCN scan duration is elapsed and no accepted PLMN was found, the algorithm starts the Band scan mode . For each coverage level , there is a coverage level budget . The accumulated coverage level budget is the Band scan duration . Consequently, the algorithm checks for each coverage level if at least one Band scan can fit , if so the algorithm considers it and decrements the executed Band scan time from the coverage level budget and from the remaining time of the Band scan time slot . Otherwise , the algorithms waits for a new retry PLMN scan .
The algorithm or method is terminated once an accepted PLMN is found . It is re-initiated in case of a new PLMN search scan .
As it is not possible to achieve both low energy consumption and low recovery time at the same time for the problem of selecting the coverage level within the PLMN search during the recovery from lack of coverage , any possible solution needs to compromise between both competing targets . The invention discloses an efficient method how to select the coverage levels within the considered PLMN search procedure or scan that allows to fine- adj ust the tradeoff , e . g . according to the requirements of the underlying application .
The proposed method balances energy consumption and recovery time using ( long-term) duty cycles for PLMN search activities , which are at the same time also limiting the long-term average energy consumption during the considered PLMN search procedure . Depending on its preferences , the application is able to adj ust both responsiveness and the energy consumption according to the configuration options as described in different variants in the next paragraphs .
In a variant of the inventive method, at the beginning of the PLMN search, parameters like the retry PLMN search time interval , the retry PLMN search factor and the coverage level budget for each coverage level are configured and initialized .
At the beginning, the initial retry PLMN search time interval is set to a small value , which is configurable , and increased gradually by the retry PLMN search factor . This factor can be configured by the user of the UE according to its requirements . The retry PLMN search timer counting the retry PLMN search time interval keeps running until the algorithm finds an accepted PLMN .
In another variant of the inventive method, an EARFCN scan duty cycle is configurable according to the application needs and is realized as percentage of the retry PLMN search time interval .
The EARFCN scan duration is the allowed time for EARFCN scanning in the last-registered EARFCN scan mode and the previous-
registered EARFCN scan mode .
In a further variant of the inventive method, a Band scan duty cycle is configurable according to the application needs and is realized as percentage of the retry PLMN search time interval .
The Band scan duration is the allowed time for band scanning in the Band scan mode .
In another further variant of the inventive method, at the beginning of the last-registered EARFCN scan mode , the UE calculates an allowed EARFCN scan duration and further calculates an estimated EARFCN scan time and checks if the estimated EARFCN scan time fits in the EARFCN scan duration, if it fits , the UE passes a PLMN scan request to a Radio Resource Control , RRC, layer , if it does not fit , the UE enters a waiting state until a PLMN scan timer is expired .
The allowed EARFCN scan duration is a product of the retry PLMN scan time interval and the EARFCN scan duty cycle . The coverage level for this state is constant , which is the Extreme Coverage Level ( ECL2 ) and in this state , only the last registered EARFCN is scanned .
In the last-registered EARFCN scan mode an estimated EARFCN scan time is calculated as well and it is checked if said estimated EARFCN scan time can fit within the allowed EARFCN scan time/duration/duty cycle . If it can fit , then a PLMN scan request , considering the selected EARFCN and coverage level , is sent to the RRC , and then the algorithm enters into a waiting state waiting for the response of the RRC . If it cannot fit , then the algorithm enters the waiting state , where the modem of the UE enters the sleeping mode , waiting for retry PLMN timer to expire and no PLMN scanning is running .
In a variant of the inventive method, at the beginning of the previous-registered EARFCN scan mode and after recalculating the
remaining EARFCN scan duration, the UE checks if the remaining EARFCN scan duration is greater than zero , if not , the UE enters the waiting state until the PLMN scan timer is expired, if yes , the UE tries to find a coverage level that fits into the remaining EARFCN scan duration, and if the UE finds a coverage level that fits , the UE passes a PLMN scan request to the RRC , otherwise the UE enters the Band scan mode .
In the previous-registered EARFCN scan mode , the algorithm recalculates the remaining allowed registered EARFCN scan time . Accordingly, if there is a remaining allowed time so the algorithm selects randomly a coverage level that can fit the remain allowed time and then select randomly registered EARFCNs from a registered EARFCN list , excluding the last registered EARFCN . It selects a number of registered EARFCNs that can fit the remaining allowed time . Subsequently, a PLMN scan request is sent to RRC layer using the selected random coverage level and random list of registered EARFCN and then the modem of the UE enters the waiting state waiting for the response of the RRC .
In case that there is no enough remaining allowed time that can at least scan one registered EARFCN of the registered EARFCN list , the algorithm enters the Band scan mode , which is starting the Band scan .
In another variant of the inventive method, in the Band scan mode the UE calculates the band scan duration and accumulates it to the coverage level budget for each coverage level , whereas the UE checks for each coverage level a maximum number of bands that fits the corresponding coverage level and adds it to a Band scan request , if the Band scan request is empty the UE enters the waiting state until the PLMN scan timer is expired, otherwise the UE passes the band scan request to the RRC and waits for the response of the RRC .
The Band scan mode performs an expensive scanning operation, because the Band scan consumes long time in scanning . So a special
mechanism is used in this mode to avoid continuous scanning for bands . When the algorithm / the modem of the UE enters the Band scan mode , the allowed Band scan time is calculated, which is accumulated to a budget for each coverage level which is called coverage level budget . The algorithm decides when the accumulated budget is enough for at least one Band scan . This mechanism protects from dominating of the band scanning in the PLMN scanning procedure .
The allowed Band scan time is calculated as a product of the retry PLMN scan time interval and the Band duty cycle . After that , the algorithm accumulates it to the coverage level budget . Then, for each coverage level , the algorithm checks the maximum number of bands that can fit the corresponding coverage level and adds it to a Band scan request . In case that the Band scan request has bands to be scanned, then the request is passed to the RRC layer and the UE enters the waiting state waiting for the response of the RRC, otherwise , the UE enters sleeping mode until the PLMN scan timer is expired (Wait Retry PLMN Scan Time State ) .
In case where the UE waits for the response of the RRC this is called Waiting scan Result by RRC .
In a further variant of the inventive method, if the UE enters the waiting state , the UE goes into a sleep mode until a retry PLMN scan timer, counting the retry PLMN time interval , expires , then the retry PLMN scan time interval is multiplied by the retry PLMN scan factor and the UE restarts the retry PLMN timer and the UE enters the last-registered EARFCN scan mode again .
In another further variant of the inventive method, if the EARFCN scan request or the Band scan request is sent to the RRC layer, the RRC layer responds if a PLMN is found and the UE stops the retry PLMN scan timer and exits the method, otherwise the UE enters the previous-registered EARFCN scan mode and continues for more scan opportunity .
The proposed inventive method how to select the coverage levels within the considered PLMN search procedure that allows to fine- adjust the tradeoff between a low energy consumption and a low recovery time, e. g. according to the requirements of the underlying application will be explained in more detail using exemplary embodiments. Depending on its preferences, a UE will be able with the proposed method to balance energy consumption and recovery time by changing the duty cycles for PLMN search activities .
The appended drawings show
Fig. 1 Illustration of coverage level ranges;
Fig. 2 PLMN Search Request Procedure;
Fig. 3 RRC IDLE Cell Selection and Reselection for NB-IoT;
Fig. 4 PLMN Selection State diagram (automatic mode) ;
Fig. 5 Use case 1: Passing a Tunnel at Vehicular Speed;
Fig. 6 Time Line of Use Case 1 with Aggressive Coverage Level
Escalation;
Fig. 7 Use Case 2: Entering an Underground Parking Garage;
Fig. 8 Time Line of Use Case 2 with Aggressive Coverage Level Escalation;
Fig. 9 Inventive central idea for selecting the Coverage Level in each scan mode;
Fig. 10 Time line of Use Case 1 using the proposed method;
Fig. 11 Time line of Use Case 2 using the proposed method;
Fig. 12 A state machine of the algorithm (state #1 to state #7) according to the invention;
Fig . 13 Detailed flow chart of state #1 ;
Fig . 14 Detailed flow chart of state #2 - Last-registered EARFCN scan mode ;
Fig . 15 Detailed flow chart of state #3 - a "Waiting Scan Result by RRC" state waiting for the RRC Response ;
Fig . 16 Detailed flow chart of state #5 - previous-registered EARFCN scan mode ;
Fig . 17 Detailed flow chart of state #6 - Band scan mode ,
Fig . 18 Detailed flow chart of state #7 - "Wait for Retry PLMN
Scan Time" state , the UE enters sleeping mode while waiting expiration of retry PLMN scan timer ;
Fig . 19 EARFCN scan duty cycle and Band scan duty cycle spread selecting different Coverage level vs . Time .
Figure 9 illustrates briefly how the inventive algorithm decides which coverage level shall be selected . The retry PLMN search time duration/interval 1 is the sum of an EARFCN scan duration 2 , a Band scan duration 3 and a sleep time duration 4 .
The EARFCN scan duration is the allowed time for EARFCN scanning . It is a product of the retry PLMN scan time interval and the EARFCN scan duty cycle . The EARFCN scan duty cycle is realized/conf igured as percentage of the retry PLMN search time interval 1 . The EARFCN scan duty cycle is configurable according to the application needs .
In the allowed EARFCN scan duration 2 , the algorithm starts 11 with the last-registered EARFCN scan mode 5 , 12 and selects an extreme coverage level ( ECL2 ) 8 for the scanning . This allows the UE to have different time opportunities to rescan for the last registered PLMN and not give up quickly as explained in the Use Case #1 above . After the last-registered EARFCN scan 5 , 12 is
considered, the algorithm checks the remaining allowed EARFCN scan time to evaluate the possibility for further EARFCN scanning ( the UE enters previous-registered EARFCN scan mode 6 , 15 ) . If scan time is left , the algorithm selects randomly a coverage level 9 , this selection can be another function of the algorithm, and selects randomly previously registered EARFCNs that can be scanned within the remaining EARFCN scan duration . Otherwise , the algorithm scans only the last registered EARFCN and keeps waiting for a new retry PLMN search/scan 17 .
If there is no accepted PLMN detected in the EARFCN scan modes 5 , 12 , 6 , 15 , the algorithm starts the Band scan mode 7 , 16 . For each coverage level , there is a coverage level budget . The accumulated coverage level budget is the remaining Band scan duration . Consequently, the algorithm checks for each coverage level if at least one Band scan can fit , if so the algorithm considers it and decrements the executed Band scan time from the coverage level budget .
Otherwise , if no accepted PLMN is detected after the Band scan 7 , the UE enters Wait for Retry PLN Scan Time State ( sleep mode ) 17 until the retry PLMN scan timer is expired and a new retry PLMN scan starts 11 .
The algorithm is terminated once an accepted PLMN is found 14 . It is re-initiated in case of a new retry PLMN scan 11 .
The proposed method balances the energy consumption and the recovery time using ( long-term) duty cycles for PLMN search activities , which are at the same time also limiting the long-term average energy consumption during the considered PLMN search procedure . Depending on its preferences , the application is able to adj ust both responsiveness and the energy consumption by changing the duty cycle .
The escalation into extreme coverage scales with the time the modem of a UE is without coverage . In order words : When the modem
has recently lost service, the time (and, thus, power) budget available for PLMN search is small. The budget is the product of a fixed duty cycle, determined according to the application needs, and the time the modem is out of service. Due to the small budget, also the available coverage levels for PLMN search are limited, e. g. to normal coverage (ECL 0) . This point is illustrated in figure 10 using the tunnel Use Case #1 introduced in figures 5 and 6. The recovery time is greatly reduced compared to figure 6 due to the more conservative escalation into extreme coverage and, at the same time, also the energy consumption is reduced as shown by the increased sleep time in-between the PLMN search repetitions.
When considering Use Case #2, the underground parking garage, the recovery time increases when using the proposed method as shown in figure 11 whereas the energy consumption almost remains constant. Thus, the proposed method achieves a different tradeoff between recovery time vs. energy consumption that can be controlled by the duty cycle parameter. In contrast, the aggressive coverage level escalation can only achieve one particular tradeoff that is having maximal power consumption.
Figures 12 to 18 explain in more details the inventive algorithm. Figure 12 shows a state machine illustrating the inventive method, each states labelled by a number. Figures 13 to 18 explain how each of the states #2: Last-registered EARFCN scan 12, #5: previous-registered EARFCN Scan 15, and #6: Band scan 16 is selecting the proper coverage level and show how the other states, like #1: start algorithm 11, #4: exit 14, #7: Wait for retry scan timer is expired 17, #3: Wait scan result by RRC layer 13, work.
Figure 13 shows the main actions taken in state#l 11 (Start Algorithm) . It is the initial trigger of the algorithm, which indicates a PLMN scan request is required. In this state#l all algorithm parameters are initialized and reset. The parameters that are reset in this state are the retry PLMN search timer to its configured initial value, the coverage level budget for each
coverage level , which is used in Band scan mode , to zeros .
Finally, the retry PPLMN Scan Timer is started and the algorithm enters state#2 .
Figure 14 shows state#2 12 flow chart . First , the allowed EARFCN Scan time is calculated as a product of retry PLMN scan time and EARFCN duty cycle . The EARFCN duty cycle is a constant value that is configured by the application . Then, a constant coverage level is selected for this state#l , which is the Extreme Coverage Level ( ECL2 ) , and this state scans only the last registered EARFCN . Consequently, in this state , the estimated EARFCN scan time is calculated and it is checked if it can fit within the allowed EARFCN scan time previously calculated . If it can fit , then a PLMN scan request , considering the selected EARFCN and coverage level , is sent to the RRC layer , and the algorithm enters state#3 13 . If it cannot fit , then the algorithm enters state#? 17 while sleeping until expiring of Retry PLMN scan timer .
Figure 15 illustrates state#3 13 . The algorithm in this state gets the response from RRC layer according to the request sent from state#2 12 , #5 15 or #6 16 . The algorithm checks the response if there is a PLMN selected as it is explained above . If there is a PLMN selected, the algorithm stops the retry PLMN scan timer and exits the algorithm ( state#4 ) 14 , otherwise , the modem enters state#5 15 ( the previous-registered EARFCN scan mode ) and continues for more scan opportunity .
Figure 16 illustrates a detailed design for state#5 15 . The algorithm recalculates the remaining allowed registered EARFCN scan time . Accordingly, if there is a remaining allowed time , the algorithm selects randomly a coverage level that can fit the remaining allowed time and then selects randomly registered EARFCNs from a registered EARFCN list ( excluding the last registered EARFCN ) . It selects number of registered EARFCNs that can fit in the remaining allowed registered EARFCN scan time . Subsequently, a PLMN scan request is sent to the RRC layer using
the selected random coverage level and random list of registered EARFCN and the algorithm enters state#3 13 waiting for the response of the RRC layer . In case that there is not enough remaining allowed time that can at least scan one registered EARFCN, the algorithm enters state#6 16 , which is starting the Band scan .
Figure 17 shows the detailed design for state#6 16 . The Band scan is an expensive scanning operation as already explained above . This is because Band scan consumes long time in scanning . So a special mechanism is used in this state to avoid continuous scanning for bands . When the algorithm enters the Band scan mode ( state#6 ) , the algorithm calculates the allowed band scan time and accumulates it to budget for each coverage level which is called coverage level budget . So that the Band scan is decided when the accumulated budget is enough for at least one Band scan . This mechanism protects from dominating of the band scanning in the PLMN scanning procedure . The allowed Band scan time is calculated as a product of the retry PLMN scan time and a Band scan duty cycle , which is a constant value according to the requirements of the underlying application . After that , the algorithm accumulates it to the coverage level budget . Then, for each coverage level , the algorithm checks the maximum number of bands that can fit the corresponding coverage level and adds it to a Band scan request . In case that a Band scan request has bands to be scanned, then the request is passed to RRC layer and the algorithm enters state#3 13 waiting for a response from the RRC layer, otherwise , the algorithm enters state#? 17 waiting for expiring the retry PLMN scan timer .
Figure 18 illustrates state#? 17 . In this state , the modem is sleeping until the retry PLMN scan timer is expired . After the expiration, the value of the retry PLMN scan timer is recalculated where the new value is the multiply of old value by retry PLMN scan factor , which is a configurable value . The timer is restarted accordingly . At the end, the algorithm moves to state#2 12 to
start a new scanning opportunity, starting with the last- registered EARFCN scan mode.
Figure 19 shows an example of a PLMN search using the inventive method for efficiently selecting a coverage level on recovery from out of service in cellular systems performed by a user equipment. Figure 19 shows the UE behavior regarding coverage level and scan mode relative to time dimension.
First: Explanation of the example configuration:
- ED (EARFCN Scan Duty Cycle) = 1% of the Retry PLMN Scan Time.
- BD (Band Scan Duty Cycle) = 4% of the Retry PLMN Scan Time.
- RPS (Retry PLMN Scan Time) initialized by 16 sec.
- Retry PLMN Scan Time Factor = 1.5
- EARFCN Scan Time for Normal Coverage Level (ECLO ) =0.016 sec.
- EARFCN Scan Time for Extended Coverage Level (ECL1 ) =0.06 sec.
- EARFCN Scan Time for Extreme Coverage Level (ECL3 ) =0.36 sec.
- Average Band Scan Time for Normal Coverage Level ( ECLO ) =5.6 sec.
- Average Band Scan Time for Extended Coverage Level (ECL1 ) =21 sec.
- Average Band Scan Time for Extreme Coverage Level (ECL3) =126 sec.
- Number of Registered EARFCN = 4.
- Assume the random selection function at state#5 in ascending order so it selects first ECLO then ECL1 then ECL2. This assumption for simplifying the drawing of the blocks.
Second: Definition of the Colors and Blocks
• Block #1 (Green) represents Last-registered EARFCN scan using extreme coverage level (ECL2) .
• Block #2 (Sky Blue) represents only one previous-registered EARFCN scan using normal coverage level (ECLO) .
• Block #3 (Normal Blue) represents only one previous-registered EARFCN scan using extended coverage level (ECL1) .
• Block #4 (Dark Blue) represents only one previous-registered EARFCN scan using extreme coverage level (ECL2) .
• Block #5 (Violet) represents only one Band scan using normal
coverage level (ECLO) .
• Block #6 (Light Violet) represents only one Band scan using extended coverage level (ECL1) .
• Block #7 (Magenta) represents only one Band scan using extreme coverage level (ECL2) .
Finally: Example Description
Figure 19 shows the different coverage level selection by the inventive method across the scanning time as well as the frequency of the scanning type (EARFCN or Band Scan) , where the example illustrates the following:
1. The frequency of repeating block#l (Green) in each RPS .
2. The escalation for another scanning type rather than block#l, such as a. in the third RPS, the algorithm decides to escalate to block#2 and block#3 b. in the eighth RPS, the algorithm decides to escalate for higher levels like block#5 and block#6.
3. The incrimination factor of the RPS by time, which affects the ED and BD periods as well as the remaining sleeping time.
4. The consideration of scanning different EARFCNs and bands for the same block, for example, in the seventh RPS: a. The algorithm decides to scan block#2 three times each one with different previous-registered EARFCN. b. The algorithm decides to scan block#3 four times each one with different previous-registered EARFCN. c. The algorithm decides to scan block#4 two times each one with different previous-registered EARFCN. d. The algorithm decides to scan block#5 one time with only one band selected.
Method for efficiently selecting a coverage level on recovery from out of service in cellular systems
List of Reference Signs
1 Retry PLMN Search Time interval
2 EARFCN Scan duration
3 Band Scan duration
4 sleep time
5 last-registered EARFCN scan mode
6 previous-registered EARFCN scan mode
7 Band scan mode
8 Selecting an extreme coverage level
9 Selecting randomly a coverage level
10 selecting a coverage level within a coverage level budget
11 state #1 : start algorithm
12 state #2 : last-registered EARFCN scan mode
13 state #3 : wait scan result by RRC
14 state #4 : exit , when PLMN is found
15 state #5 : previous-registered EARFCN scan mode
16 state #6 : Band scan mode
17 state #7 : Waiting state for Retry PLM scan timer to expire
Claims
1. A method for efficiently selecting a coverage level on recovery from out of service in cellular systems performed by a user equipment, UE, wherein a public land mobile network, PLMN, search is repeated within a retry PLMN search time interval (1) , which is the sum of a E-UTRA Absolute Radio Frequency Channel Number, EARFCN, scan duration (2) , a Band scan duration (3) and a sleep time duration (4) , whereas the UE selects between three scan modes comprising a last-registered EARFCN scan mode (5, 12) , a previous- registered EARFCN scan mode (6, 15) and a Band scan mode (7, 16) , the UE selects said scan modes in this order, whereas the UE starts with the last-registered EARFCN scan mode (5, 12) and selects an extreme coverage level (8) , ECL2 , for PLMN scanning within the EARFCN scan duration (2) , if no PLMN is found, the UE enters the previous-registered EARFCN scan mode (6, 15) and first recalculates a remaining EARFCN scan duration of the EARFCN scan duration (2) and selects randomly a coverage level (9) and a registered EARFCN from a registered EARFCN list for PLMN scanning, if no PLMN is found within the remaining EARFCN scan duration, the UE enters the Band scan mode (7, 16) and calculates a Band scan duration (3) and a coverage level budget for each coverage level and performs a band scan if the coverage level budget is enough for at least one Band scan, if no PLMN is found, the UE increments the retry PLMN search time interval (1) by a retry PLMN scan factor and passes through the procedure again until an accepted PLMN is found (14) .
2. The method according to claim 1, wherein at the beginning of the PLMN search (11) , parameters like the retry PLMN search time interval, the retry PLMN search factor and the coverage
level budget for each coverage level are configured and initialized . The method according to claim 1, wherein an EARFCN scan duty cycle is configurable and is realized as percentage of the retry PLMN search time interval (1) . The method according to claim 1, wherein a Band scan duty cycle is configurable and is realized as percentage of the retry PLMN search time interval (1) . The method according to claim 1, wherein at the beginning of the last-registered EARFCN scan mode (5, 12) , the UE calculates an allowed EARFCN scan duration and further calculates an estimated EARFCN scan time and checks if the estimated EARFCN scan time fits in the EARFCN scan duration (2) , if it fits, the UE passes a PLMN scan request to a Radio Resource Control, RRC, layer, if it does not fit, the UE enters a waiting state (17) . The method according to claim 1, wherein at the beginning of the previous-registered EARFCN scan mode (6, 15) and after recalculating the remaining EARFCN scan duration, the UE checks if the remaining EARFCN scan duration is greater than zero, if not, the UE enters the waiting state (17) , if yes, the UE tries to find a coverage level that fits into the remaining EARFCN scan duration, and if the UE finds a coverage level that fits, the UE passes a PLMN scan request to a Radio Resource Control, RRC, layer otherwise the UE enters the Band scan mode (7, 16) . The method according to claim 1, wherein in the Band scan mode (7, 16) the UE calculates the Band scan duration (3) and accumulates it to the coverage level budget for each coverage level, whereas the UE checks for each coverage level a maximum number of bands that fits the corresponding coverage level and adds it to a Band scan request, if the
Band scan request is empty the UE enters the waiting state (17) , otherwise the UE passes the Band scan request to a Radio Resource Control, RRC, layer (13) . The method according to one of the former claims, wherein if the UE enters the waiting state (17) , the UE goes into a sleep mode (4) until a retry PLMN scan timer, counting the retry PLMN time interval, expires, then the retry PLMN scan time interval (1) is multiplied by the retry PLMN scan factor and the UE restarts the retry PLMN timer and the UE enters the last-registered EARFCN scan mode (5, 12) again. The method according to one of the former claims, wherein if the EARFCN scan request or the Band scan request is sent to the RRC layer, the RRC layer responds if a PLMN is found and the UE stops the retry PLMN scan timer and exits the method (14) , otherwise the UE enters the previous-registered EARFCN scan mode (6, 15) and continues for more scan opportunity.
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