Abstract:
The present invention provides methods and apparatus for selectively notifying user equipments (UEs) of updated access barring statuses and for spreading access attempts by the UEs. The methods and apparatus disclosed in the present application reduce potential overloads in a radio access network caused by UEs simultaneously accessing the radio network, after being notified that access restrictions have been removed.

Description:
PRIORITY CLAIM 
     The present application claims priority to U.S. provisional application No. 61/592,348 filed on Jan. 30, 2012. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to access control techniques for reducing overload in an access network and, more specifically, to selectively paging a plurality of user equipments to notify the user equipments of changes in access control restrictions. 
     BACKGROUND 
     In a mobile network, a user equipment (UE) communicates with a core wireless network through a radio access network. In technical specifications provided by 3GPP, a UE is defined as a device providing user access to network services. It corresponds to a mobile station defined in GSM. Under certain circumstances, it is necessary to bar some UEs in a mobile network from accessing a radio access network, for example, to prevent overloading the network, to control congestion, to deal an emergency situation, or to handle a network failure such as Public Land Mobile Network (PLMN) failure, etc. 
     In an exemplary scenario in which the network discovers that there is congestion, e.g. in a random access channel, the network starts to restrict certain UEs that are configured with the access barring feature from performing a random access procedure so that the UE is effectively barred from accessing the network. The network broadcasts updated access barring information to indicate which UEs are barred from accessing the network. Since access barring information is often included in system information, the network notifies the UEs of the change in the system information by paging. Similarly, when the network congestion is alleviated, the network removes the access restrictions to allow the UEs to access the network again. The network modifies the access barring information in the system information and pages the UEs about the updated access barring information. 
     Different standards provide different access control mechanisms. In UTRAN standards, access control is achieved through access class barring (ACB). An access class is a random number allocated to each UE and is usually stored in the UE&#39;s SIM/USIM (Subscriber Identity Module/Universal Subscriber identity Module) card. There are ten normal access classes, ranging from 0 to 9. A special access class can range from 11 to 15. Access classes are used to identify which portion of the mobile terminals are allowed or disallowed to access the network at certain time. For example, access attempts by UEs belonging to class 0, 1, and 2 may be limited whereas UEs belong to classes 3-9 are allowed to access the network. For another example, access attempts by UEs belonging to a normal access class may be limited whereas access attempts by UEs belonging to a special access class may be allowed. In UTRAN standards, to inform the mobile terminals of the allowed/disallowed access classes, a bitmap indicating which access classes are barred and which are not may be broadcast by the network. 
     In E-UTRAN standards, the ACB mechanism is implemented using an access barring factor and an access barring time, both of which are broadcast in the system information (SI) when access class barring is in effect. 
     The updated bitmap or system information is broadcast to every UE. A paging message may be used to notify each device of the updated SI information. For example, when access restrictions have been removed, the affected UEs will be notified of the change via paging messages. In some standards, a UE may be further notified of an upcoming paging message via a paging indicator. 
     In the present application herein, the term “paging notification” may be used to refer to either a paging message or a paging indication or other equivalent notification signals that provide notifications to UEs of certain upcoming events. 
     If the UEs, upon being notified of the removal of access restrictions, attempt to access the network all at the same time, the access network may be overloaded by the number of access attempts. The overloading situation is more manifest in an access network in which a large number of Machine Type Communications (MTC) devices are deployed. 
     MTC devices are devices that generate delay-tolerant and/or low-priority traffic, therefore are more tolerant of access restrictions. MTC devices are often configured to support Extended Access Barring (EAB). EAB is a special access barring mechanism that allows a network operator to restrict access attempts originated from devices configured to support EAB. Network overloads caused by access attempts from MTC devices after EAB access restrictions have been removed are more common than network overloads caused by other ACB access restrictions, partly due to the large number of MTC devices deployed. 
     Improved methods and apparatus are needed for reducing overloads in access network caused by access attempts from UEs. 
     SUMMARY 
     The present invention provides methods and apparatus for selectively notifying UEs of updated access barring statuses and for spreading access attempts by the UEs. The methods and apparatus disclosed in the present application reduce potential overloads in a radio access network caused by UEs simultaneously accessing the radio network, after the UEs are notified that previous access restrictions have been removed for a particular access class or classes. 
     In an exemplary access control procedure, an access control message is broadcast to a plurality of UEs. Paging notifications are selectively sent to the plurality of UEs to notify the UEs of the broadcast access control message. When a UE receives a paging notification, it reads the broadcast access control message to retrieve the latest access control status. Selective paging prevents simultaneous access attempts from the UEs when the UEs are notified of the removal of access restrictions. 
     In some embodiments, paging notification may be transmitted on every paging occasion in every paging cycle. Selective paging may be achieved by assigning UEs to different paging occasions in one or more paging cycles. The number of paging cycles over which the assigned UEs are spread may be dependent on the number of UEs that are affected by the access control message. During an assigned paging occasion, a UE monitors a paging channel for paging notifications. 
     In some embodiments, selective paging may be achieved by transmitting paging notification in a subset of paging occasions during a paging cycle. For example, paging messages may be transmitted on every even paging occasion in one paging cycle and on every odd paging occasion in the next paging cycle. Such selective paging spreads access attempts from the affected UEs over multiple paging cycles. 
     In some embodiments, a timer may be implemented in a UE to ensure that, when a paging notification is missed or lost, the UE still receives updated access control message included in the broadcast access control message. Upon receiving a broadcast access control message, the UE starts the timer and waits for a paging notification. If the paging notification is received before the timer expires, the timer is stopped and the UE reads the broadcast access control message and acts accordingly, for example, initiating an access attempt to the network. If no paging notification is received when the timer expires, the UE reads the system information upon the expiration of the timer and acts accordingly. 
     Of course, the present invention is not limited to the features, advantages, and contexts summarized above, and those familiar with access control methods and techniques will recognize additional features and advantages upon reading the following detailed description and upon viewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary access network. 
         FIG. 2  illustrates an exemplary access control message. 
         FIG. 3  illustrates a flowchart of an exemplary access control process. 
         FIG. 4  illustrates an exemplary process of selectively sending paging messages to different groups of UEs. 
         FIG. 5  illustrates an exemplary paging procedure. 
         FIG. 6  is a flowchart illustrating the process of selectively sending paging notifications to a plurality of UEs. 
         FIG. 7  illustrates an exemplary embodiment of selectively paging a plurality of UEs. 
         FIG. 8  illustrates an exemplary apparatus configured to perform an access control process. 
         FIG. 9  is a flowchart illustrating an exemplary process implemented at a UE for updating access control information using a timer. 
         FIG. 10  illustrates an exemplary UE configured to received access control messages from the network and update access control information in accordance with the methods described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings,  FIG. 1  illustrates a mobile network  100  with a UTRAN radio access network. The mobile network  100  comprises a core network  102 , two NodeB&#39;s or base stations  103  and  104 , and a plurality of UEs,  106 ,  108 , . . .  114 , and  116 . The core network  102  may refer to a General Packet Radio Service (GPRS) core network implemented within a GSM switching subsystem. The core network  102  may include Gateway GPRS Support Nodes (GGSNs) and/or Serving GPRS Support Nodes (SGSNs) to handle IP data packets and to interface with the Internet. The NodeB  103  and  104  form a radio access network (RAN)  105  that provides a radio interface for UEs  106 ,  108 , . . . ,  116  to access the core network  102 . 
     When there are a large number of access attempts from the UEs trying to gain access to the mobile network  100 , the RAN  105  can become congested. Access control procedures are often implemented in the RAN  105  to prevent congestion. 
     Different communication standards provide different access control mechanisms. For example, as mentioned in the background section, the UTRAN standards implement an access control mechanism through access class barring. Each UE may be assigned an access class, e.g., a number between 0 and 9. The RAN network  105  determines which access classes are barred from accessing the network and broadcasts a bitmap to notify the plurality UEs, e.g.,  106 ,  108  . . . ,  114 , and  116  of the access class barring status. 
       FIG. 2  illustrates an exemplary ACB bitmap  200 . In the ACB bitmap  200 , there are a total of 10 bits. Each bit field in the bitmap corresponds to an access class as labeled. The value in each bit field indicates whether the access class corresponding to that bit field is barred from accessing the network. For example, for access class 4, the value in the corresponding bit field is 1, which indicates that accessing the network by a UE device of access class 4 is permitted. However, for access class 7, the value in the corresponding bit field is 0. That indicates a UE device of access class 7 is barred from accessing the network. 
     In E-UTRAN standards, access control information such as access barring factor and access barring time may be included in the system information broadcast to the UEs in the network. System information is also used to carry access control information related to Extended Access Barring (EAB). 
     In E-UTRAN standards, a UE receives broadcast system information that includes an access barring factor and an access barring time. To decide whether it is allowed to access the network, the UE first draws a random number and compares the random number with the broadcast access barring factor. For example, when the value of the random number drawn by the UE is lower than the access barring factor, the UE decides that its access attempt is barred this time. Otherwise the UE proceeds to initiate an access attempt. If barred, the UE determines an access barring period based on the access barring time broadcast in the system information. Methods for calculating the access barring period can be found in 3GPP TS 36.331. 
       FIG. 3  is a flowchart  300  illustrating an exemplary access control process implemented at a UE in accordance with E-UTRAN standards. The UE receives an access control message that includes an access barring factor and an access barring time (step  302 ). The UE generates a random number R (step  304 ) and then compares the random number R with the received access barring (AB) factor (step  306 ). If the random number R is less than or equal to the AB factor, the UEs determines that it is barred from accessing the network (step  312 ). If the random number R is larger than the AB factor, the UE reads the AB time (step  308 ) and determines when access of the network is allowed (step  310 ). 
     The ACB bitmap  200  depicted in  FIG. 2  and the broadcast system information referred to in  FIG. 3  may be generally referred to as access control messages in the present application. 
     In some implementations, UEs do not monitor broadcast system information or access control message. Instead, UEs rely on a paging notification to notify them of the availability of access control messages. In other implementations, a UE receives an access control message but does not immediately retrieve the information included in the access control message. The UE waits for a paging notification before reading the access control message. 
     A paging notification may be addressed to a specific UE or may be intended for every UE in the system. In the present application, a paging notification may refer to a paging message, or a paging indication that is used to notify a UE of an upcoming paging message, or other equivalent notifications used to notify a UE or UEs of certain events. 
     In LTE standards, three different types of paging indications are currently defined: 1) the normal system information (SI) modification indication, 2) the Earthquake and Tsunami Warning System (ETWS) notification indication, and 3) the Commercial Mobile Alert Service (CMAS) notification indication. 
     Upon receiving a normal SI modification indication informing a UE that the system information has been modified, the UE reads the related system information at the next modification boundary. With ETWS and CMAS notification indications, the UE reads the ETWS/CMAS system information immediately after receiving the ETWS/CMAS notification indication. 
     With respect to access barring information, a UE may be implemented to read the access barring information at the next modification boundary upon receiving a paging notification, similar to the implementation for normal SI modification indications. A UE may also be implemented to read the updated access barring information immediately after a paging notification, similar to the implementation for ETWS/CMAS messages. 
     To receive paging indications, all UEs are configured to listen to a paging channel or a paging indication channel at predefined paging occasions during their assigned paging cycles. 
       FIG. 4  depicts a typical paging channel divided into multiple paging frames or cycles. A paging cycle includes multiple paging occasions, t 0 , t 1  . . . , t N . Each paging occasion corresponds to the transmission of a paging indicator. A UE is often assigned one paging occasion within a paging cycle to monitor paging indicators. Typically there are several UEs listening to the same paging occasion. To inform the UEs of a broadcast access control message, the network may send paging indicators on all paging occasions during a paging cycle. 
     In one embodiment, the paging occasions assigned to the UEs are spread over one paging cycle. Therefore, the access attempts by the UEs will be spread over one paging cycle. For example, in  FIG. 5 , UEs  106 ,  108 , and  110  receive a paging notification  502  at paging occasion t 1  in paging cycle  1  (see  FIG. 4 ). After receiving the paging notification message, UEs  106 ,  108  and  110  read the updated system information. Upon discovering that the access restrictions have been removed, the three UEs initiate access attempts to the network. At a later time, a second paging notification  504  is received by a different group of UEs that belong to the paging occasion t 2  in paging cycle  1  (see  FIG. 4 ). After being notified, this group of UEs (UE  112 ,  114 , and  116 ) read the update system information and initiate network access attempts if the update system information indicates that access barring is no longer in effect. 
     The current maximum length for a paging cycle is 2.56 seconds. In some scenarios, if all UEs are assigned to one paging cycle, the length of the paging cycle may be insufficient, as the UEs assigned to the same paging cycle may be simultaneously accessing the network within 2.56 seconds. To prevent potential overloading of the access network, the paging occasions assigned to the UEs may be spread over multiple paging cycles. For example, the network can transmit a paging notification message in every even paging occasion during one paging cycle. Only the UEs assigned to the even paging occasions will receive a paging notification in this paging cycle. During the next paging cycle, the network transmits a paging message in the odd paging occasions, therefore only the UEs assigned to the odd paging occasions will receive a paging notification this time. 
       FIG. 6  describes a general access control procedure implemented at the NodeB  104  using a spread paging method. In  FIG. 6 , the NodeB  104  broadcasts an access control message to a plurality of user terminals for controlling access attempts to the radio access network by the plurality of user terminals (step  602 ). In some embodiments, the access control message may be a broadcast system information update. The NodeB then selectively sends paging indications to the plurality of user terminals in two or more paging cycles to notify the plurality of user terminals of the broadcast access control message (step  604 ). For example, the NodeB can send the paging notification in even paging occasions during one paging cycle and in odd paging occasions during the next paging cycle. In such case, approximately one-half of the UEs in the affected access classes will receive the notification in each paging cycle. 
       FIG. 7  illustrates an exemplary embodiment of selectively paging a plurality of UEs. In  FIG. 7 , the paging occasions within a paging cycle are divided into two or more subsets (step  702 ). The plurality of UEs are assigned to different paging occasions (step  704 ). Selective paging is done by sending paging notifications on the paging occasions belonging to a different subset during each paging cycle (step  706 ). For example, in a first paging cycle, the paging notifications are sent on the paging occasions of a first subset. In a second paging cycle, the paging notifications are sent on the paging occasions of a second subset. During the first paging cycle, only the UEs assigned to the paging occasions of the first subset receive paging notifications. In the second paging cycle, only the UEs assigned to the second subset of paging occasions receive paging notifications. 
     The structure of a NodeB configured to perform the process described in  FIGS. 6 and 7  is illustrated in  FIG. 8 . The NodeB  103  comprises a transceiver circuit  802  configured to communicate with an antenna  806  and a control circuit  804  configured for access control. The transceiver circuit  802  includes a transmitter  810  and a receiver  812  for transmitting and receiving signals. The control circuit  804  further comprises an access control processor  814  and a paging processor  816 . The access control processor  814  is configured to broadcast access control messages to the UEs for controlling access attempts by the UEs. The paging processor  816  is configured to selectively send paging indicators to the UEs to notify the UEs of broadcast access control messages. The control circuit  804  may be implemented by one or more processors, hardware, firmware, or a combination thereof. 
     In some embodiments, the UEs may be configured to read updated access barring information only after receiving a paging notification. To avoid the situation where the paging notification is missed or lost and the affected UEs are not able to update system information for a long period of time, a timer can be implemented and configured to expire after a predetermined period of time. For example, the predetermined period of time may be a typical time period between two system information updates. 
       FIG. 9  is a flowchart illustrating a process implemented at a UE for updating access control information using a timer. A UE first receives a broadcast access control message and updates access control information indicative of access control status based on the received message (step  902 ). The UE then sets a timer in response to the updating of the access control information ( 904 ) and operates according to the updated access control information ( 906 ). For example, the UE may initiate an access attempt if the access restriction is removed. 
     In step  908 , the UE checks if a trigger event has been detected. A trigger event is the first of either a reception of a paging indication or an expiration of the timer to occur. If a trigger event has not been detected, the UE awaits for a next trigger event. If a trigger event is detected, the UE updates the access control information in response to the predetermined trigger event when there is a change in system information (step  910 ). 
       FIG. 10  illustrates an exemplary UE  106  configured to perform the process described in  FIG. 9 . The UE  106  comprises a transceiver circuit  1002  connected to an antenna  1006  and a processing circuit  1004  comprises a message processor  1018  and a timer  1020 . The transceiver circuit  1002  further comprises a transmitter  1010  for transmitting signals and a receiver for receiving signals. The transceiver circuit  1002  is configured to send network messages related to access control to the processing circuit  1004  and to receive access requests from the processing circuit  1004 . The processing circuit  1004  comprises a message processor  1010  configured to process the received network messages related to access control. The message processor  1010  is also configured to prepare access requests to be sent by the transceiver circuit  1002  for accessing the network. The processing circuit  1004  further comprises a timer  1020 , which can be set by the processing circuit for scheduling the next update of access control information, in case the paging notification is lost. 
     The foregoing description uses UTRAN or ETRAN access control procedures as examples to illustrate the advantageous techniques disclosed herein. The present invention can be applied to other access control procedures by a person ordinarily skilled in the art, for example, service specific access control procedures. 
     Service specific access control is an access control procedure designed to restrict access attempts initiated by a particular service. For example, 3GPP TS 22.011 standard describes an access control mechanism referred to as Service Specific Access Control (SSAC) for certain multimedia telephony (MMTEL) functions. To initiate a MMTEL service, a UE sends a session request while in an idle-mode. SSAC uses an Evolved Packet System (EPS) to provide access control to MMTEL services. SSAC assigns a service probability factor and mean duration for each of MMTEL voice services and MMTEL video services. 
     For another example, Circuit Switched Fall Back (CSFB) standards also define an access control mechanism. The present invention is applicable to both SSAC and CSFB access control mechanisms, as well as other similar or equivalent access control mechanisms. 
     The foregoing description and the accompanying drawings represent non-limiting examples of the methods and apparatus taught herein. As such, the present invention is not limited by the foregoing description and accompanying drawings. Instead, the present invention is limited only by the following claims and their legal equivalents.