Patent Publication Number: US-2017367004-A1

Title: Method and device for controlling congestion in mobile communication system

Description:
TECHNICAL FIELD 
     The present invention relates to a congestion control method and device in a mobile communication system. In particular, the present invention relates to a method and device for controlling congestion in consideration of applications and resources. 
     BACKGROUND ART 
     The mobile communication system has been developed for a user to communicate on the move. With the rapid advance of technologies, the mobile communication system has evolved to the level capable of providing high speed data communication service as well as voice telephony service. 
     Meanwhile, unlike voice services, data services are provided using resources determined based on the transmit data amount and channel condition. Accordingly, a wireless communication system, particularly a cellular communication system, is provided with a scheduler, which takes charge of resource allocation in consideration of the required resource amount, channel condition, data amount, etc. In most cellular communication systems, typically, the scheduler is located in base stations for radio resource management, and this is the case even in the Long-Term Evolution (LTE) system as one of the next generation mobile communication systems. 
     For such data services, it is necessary to guarantee service-specific qualities with different priorities. One of the service-specific quality control policies may be to allow subscribers paying much money to access more diverse services and to limit subscribers paying less money to specific services in an enhanced mobile communication system. In recent mobile communication systems, also the offer of the same service in different qualities has been used. 
     More recently, mobile communication operators have planned to deploy systems for providing their subscribers with multimedia services including voice and data services, the systems being characterized by classifying subscribers into different classes to provide the classes with services different in quantity and quality. In the mobile communication systems supporting multimedia services, it is preferable to guarantee Quality of Service (QoS) of all services as much as possible or, although it is difficult to guarantee the QoS for all services, to guarantee QoS for services with a high priority. 
     Meanwhile, if the number or ratio of users connected to the mobile communication system exceed a predetermined threshold capable of guaranteeing QoS for all the connected users with given resources, a congestion control technique is used to release the resources allocated to the users with a low priority among the connected users to maintain the number of users enjoying a guaranteed QoS at a certain level. The congestion control technique may also be used to resolve the congestion caused by overload on a bearer with a high priority in such a way of releasingthe resources allocated to the users with a low priority. 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     The present invention aims to provide an enhanced congestion control method and device for use in a mobile communication system. Also, the present invention aims to provide a method and device for controlling congestion in consideration of application characteristics and resources. 
     Solution to Problem 
     In accordance with an aspect of the present invention, a congestion control method of a mobile communication system includes identifying a congestion state, checking congestion control information associated with a plurality of users, determining at least one resource release target user based on Physical Resource Block (PRB) information included in the congestion control information, and releasing resources allocated to the at least one resource release target user. 
     In accordance with another aspect of the present invention, a congestion control device of a mobile communication system includes a communication unit which communicates with at least one network node and a control unit which controls identifying a congestion state, checking congestion control information associated with a plurality of users, determining at least one resource release target user based on Physical Resource Block (PRB) information included in the congestion control information, and releasing resources allocated to the at least one resource release target user. 
     The technical problems to be solved by the present invention are not restricted to the aforementioned, and those skilled in the art will clearly appreciate other technical problems not mentioned so far from the following description. 
     Advantageous Effects of Invention 
     The congestion control method and device of the present invention is advantageous in terms of being applicable to a mobile communication system. Also, the congestion control method and device of the present invention is advantageous in terms of controlling congestion efficiently based on application characteristics and resources. 
     Also, the congestion control method and device of the present invention is advantageous in terms of improving a service quality experience of a user by means of detecting the running of a video application and controlling congestion based on the video encoding rate of the application. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating LTE mobile communication system architecture; 
         FIG. 2  is a flowchart illustrating a QCI- or ARP-based congestion control method according to an embodiment of the present invention; 
         FIG. 3  is a diagram illustrating a configuration of a resource amount-based congestion control system according to an embodiment of the present invention; 
         FIG. 4  is a flowchart illustrating a congestion control method according to the second embodiment of the present invention; and 
         FIG. 5  is a block diagram illustrating a congestion controller according to an embodiment of the present invention. 
     
    
    
     MODE FOR THE INVENTION 
     Exemplary embodiments of the present invention are described with reference to the accompanying drawings in detail. The same reference numbers are used throughout the drawings to refer to the same or like parts. Detailed descriptions of well-known functions and structures incorporated herein may be omitted to avoid obscuring the subject matter of the present invention. The following description is made only of the parts necessary to help understand the operations according to various embodiments of the present invention and is not made of other parts to avoid obscuring the subject matter of the present invention. 
     The device and method proposed in an embodiment of the present invention is applicable to various communication systems such as a Long-Term Evolution (LTE) mobile communication system, an LTE-Advanced (LTE-A) mobile communication system, a High Speed Downlink Packet Access (HSDPA) mobile communication system, a High Speed Uplink Packet Access (HSUPA) mobile communication system, a 3 rd  Generation Project Partnership 2 (3GPP2) High Rate Packet Data (HRPD) mobile communication system, 3GPP2 Wideband Code Division Multiple Access (WCDMA) mobile communication system, a 3GPP2 Code Division Multiple Access (CDMA) mobile communication system, an Institute of Electrical and Electronics Engineers (IEEE) 802.16m communication system, an Evolved Packet System (EPS) , and a Mobile Internet Protocol (Mobile IP) systemThe following description is directed to the LTE system. 
       FIG. 1  is a diagram illustrating LTE mobile communication system architecture. 
     In reference to  FIG. 1 , a radio access network of the LTE mobile communication system includes a User Equipment (UE)  100 , a next generation base station (hereinafter, interchangeably referred to as evolved Node B, base station, RAN node, eNB, and node B)  105 , a Mobile Management Entity (MME)  110 , a Serving-Gateway (S-GW)  125 , a Packet Data Network Gateway (PDN-Gateway or P-GW)  130 , an Application Function (AF)  140 , and a Policy Control and Charging Rules Function (PCRF)  135 . The radio access network may include or connect to a Universal Terrestrial Radio Access Network (UTRAN)  180 , a GSM EDGE Radio Access Network (GERAN)  190 , a Serving GPRS Support Node (SGSN)  115 , and a Home Subscriber Server (HSS)  120 . 
     The UE  100  may connect to Internet Protocol (IP) services  150  of an external network, e.g., operator network, via the eNB  105 , the S-GW  125 , and the P-GW  130 . The AF  140  is an entity for exchanging application information with the user at an application level. The PCRF  135  is an entity for controlling Quality of Service (QoS) policies of a user. The PCRF  135  may provide a Policy and Charging Control (PCC) rule to the P-GW  130 . 
     The eNB  105  is a Radio Access Network (RAN) node, which is equivalent to Radio Network Controller (RNC) of the UTRAN  180  and Base Station Controller (BSC) of the GERAN  190 . The eNB  105  is connected to the UE  100  through a radio channel and functions similarly to the legacy RNC/BSC. The eNB  105  performs radio resource management (such as radio bearer control, radio admission control, dynamic radio resource allocation, load management, and inter-cell interference control) as well as providing the user with a radio interface. 
     In LTE, because all user traffic including real-time services such as Voice over IP (VoIP) is served through shared channels, it is necessary to perform scheduling based on status information collected from the UE  100 . The eNB  105  takes charge of this function. 
     The S-GW  125  provides data bearers. The S-GW  125  establishes and releases a data bearer under control of the MME  110 . The S-GW  125  terminates an Evolved-UTRAN (E-UTRAN) and Evolved Packet Core (EPC). The S-GW  125  is an anchoring point for inter-eNB handover and inter-3GPP system handover. 
     The MME  110  performs various control functions. There may be a plurality of eNBs connected to the MME  110 . The MME  110  is an E-UTRAN control plane entity which communicates with the HSS  120  for user authentication and user profile download and takes charge of EPS mobility management and EPS session management for the UE  100  through Non Access Stratum (NAS) signaling. The HSS  120  is a central database storing user profiles and provides the MME  110  with user authentication information and the user profilesThe P-GW  130  provides connectivity from the UE  100  to a Packet Data Network (PDN) and performs packet filtering. The P-GW allocates an IP address to the UE  100  and works as a mobility anchoring point for handover between 3GPP and non-3GPP systems. The P-GW  130  may control the bearers for a service flow selected for congestion control of the congestion control device. Adjusting bearers may include that the P-GW  130  releases a bearer or decreases data rate on the bearer. 
     The P-GW receives a PCC rule from the PCRF  135  and takes charge of per-UE billing function. The PCRF  135  is a policy and charging control entity, which takes charge of policy control determination and charging control function. The PCC rule generated by the PCRF  135  is sent to the P-GW  130 . 
     Typically, a User Plane (UP) link is established between the UE  100  and the P-GW  130  via the eNB  105  and the S-GW  125 . In particular, the radio channel established between the UE  100  and the eNB  105  is significantly resource constraint. 
     A description is made of the bearers and QoS for use in the LTE system hereinafter. Although the description is directed to video traffic, the type of data traffic is not limited to video traffic. An LTE network delivers all Internet video traffic, with the exception of video conference traffic of the operator, over a default bearer in a best effort service. That is, it is possible to deliver the traffic within the default bearer regardless of the characteristics of the aforementioned video traffic. For example, the eNB fairly schedules the traffic within the default bearer. However, different traffic patterns may require different types of bearer QoS characteristics. The different types of bearer QoS characteristics configured depending on the video traffic pattern may be used efficiently for transporting video traffic to a UE through a mobile network. 
     In an LTE network, the bearer QoS is identified with parameters such as QoS Class Identifier (QCI), Guaranteed Bit Rate (GBR), Maximum Bit Rate (MBR), and Allocation and Retention Priority (ARP). Among these parameters, GBR and MBR are configured for a GBR bearer regardless of the characteristics of the aforementioned default bearer. The ARP indicates a priority of allocation and retention for allocating and maintaining a bearer in the LTE network. The ARP is used to determine whether to establish a bearer in want of resources or accept a change request and to select a bearer to drop in handover. The ARP value is neither used in a data transmission process (e.g., scheduling and rate control) of the eNB nor transmitted to the user. The QCI is used for packet forwarding on a bearer. For example, the QCI includes scheduling weights, admission thresholds, queue management thresholds, and link layer protocol configuration. In more detail, the QCI includes resource type, packet delay budget, and packet error loss rate. 
     If overload occurs in a mobile communication system, this may cause congestion. Such a congestion may occur when the number or ratio of users connected to the mobile communication system exceed a predetermined threshold capable of guaranteeing the service QoS for all the connected users with given resources. Such a congestion may incur a delay in communication between a UE and an eNB. In order to resolve the congestion, a congestion control is performed in the mobile communication system. For example, a congestion control device may control an access attempt of at least one UE. This means that the congestion control device may release the connection to the UE or stop providing a service. The congestion control device may release the resources allocated to the user with the lowest priority among the connected users to maintain a predetermined number of users being satisfied with the QoS. Also, when it is to necessary to resolve the congestion caused by overload on a bearer with a high priority, the congestion control technique for releasing the sources allocated to the user with a low priority may be used. It may be possible to preempt a user with a low priority for congestion control. This may be called a preemption technique. It may also be possible to perform the congestion control in such a way of terminating the process still in progress for providing a service to a certain UE. 
     It may be possible to use QCI or ARP as a metric for selecting a user for release of the resources allocated thereto in congestion control.  FIG. 2  is a flowchart illustrating a QCI- or ARP-based congestion control method according to an embodiment of the present invention. A description is made of the operation of a congestion controller with reference to  FIG. 2 . The congestion controller may be one of core network nodes. In an embodiment of the present invention, the terms “congestion control target user”, “congestion control target application”, and “congestion control target service” may be used interchangeably with a similar meaning. The expressions “releasing resources allocated to a user” and “releasing a resource allocated to a user” may be used with a similar meaning. 
     In reference to  FIG. 2 , if congestion occurs in the network, the congestion controller operates as follows. The congestion controller may measure a congestion level at step S 200 . The congestion controller may also determine ARP or QCI corresponding to the measured congestion level. The ARP or QCI value may be determined according to the congestion level because the resource release is from a large number of users for a high congestion level and from a small number of users for a low congestion level. 
     The congestion controller may determine at step S 205  whether a service being provided to at least one user is a congestion control-enabled service. A congestion control-enabled service means a service which allows release of the resources allocated therefor when a predetermined threshold condition is fulfilled. Such a service may also be called a preemption-enabled service. If it is determined at step S 205  that the corresponding service is not a congestion control-enabled service, the procedure goes to step S 210 . If it is determined that the corresponding service is a congestion control-enabled service, the procedure goes to step S 220 . 
     At step S 210 , the congestion controller may skip releasing the resources allocated for the corresponding service because the corresponding service is not a congestion control-enabled service. At step S 220 , the congestion controller may determine whether a predetermined condition is fulfilled and, if so, release the resources allocated for the corresponding service. 
     At step S 220 , the congestion controller determines whether the congestion level of at least one service fulfills an ARP threshold condition related to the corresponding service. For example, if the ARP of the corresponding service is equal to or greater than a threshold ARP value, it may be determined that the threshold condition is fulfilled. If the threshold condition is fulfilled, the procedure goes to step S 225 . If the threshold condition is not fulfilled, the procedure goes to step S 210 . 
     At step S 225 , the congestion controller determines whether the congestion level of at least one service fulfils a QCI threshold condition related to the corresponding service. For example, if the QCI of the corresponding service is equal to or greater than a threshold QCI value, it may be determined that the threshold condition is fulfilled. If the threshold condition is fulfilled, the procedure goes to step S 230 . If the threshold condition is not fulfilled, the procedure goes to step S 210 . 
     Steps S 220  and S 225  may be performed in reverse order or selectively omitted so as to be followed by step S 230 . 
     At step S 230 , the congestion controller may perform preemption on the service fulfilling the above conditions. That is, the congestion controller may release the resources allocated for the service fulfilling the above conditions. The congestion controller may release the resources allocated to multiple services fulfilling the above conditions. It may be possible to escape from the congestion state by releasing the resources allocated for the services fulfilling a predetermined condition. The above procedure may be repeated until the congestion state is resolved or the congestion level drops below a predetermined level. 
     A congestion state may be resolved in this way. The embodiment of  FIG. 2  is directed to the QCI- and/or ARP-based preemption method. In the embodiment of  FIG. 2 , how to select a preemption target with priority among the services fulfilling the conditions of steps S 220  and S 225  is not described. This means that the preemption may lower the quality experience of a user who is enjoying a high quality experience. Also, releasing the resources allocated for a certain service may have little substantial effect on congestion resolution because no consideration is taken of how much effect the resource release from the user to be preempted has on quality improvement of other users. 
     The second embodiment of the present invention is directed to a method for substantially improving the quality experience by overcoming the shortcomings of the embodiment of  FIG. 2 . Although the description of the second embodiment is made separately from the embodiment of  FIG. 2  for convenience of explanation, it may also be possible to incorporate the method of  FIG. 2  into the second embodiment. The second embodiment of the present invention proposes a method for identifying a user, a UE, or a service for releasing the resources allocated thereto in a congestion situation based on the application characteristics and resource usage amounts. 
     For example, it may be possible to detect the running of a video application and the video encoding rate of the video application and release the resources allocated to the users being served at an unsatisfactory level of quality experience so as to protect against a lowering of the quality experience of other users. It may also be possible to release the bearers consuming a large amount of Physical Resource Block (PRB) to improve the quality of the service to other users. Although the description is directed to a case where video applications are resource release targets, the resource release targets are not limited to the video applications in an embodiment of the present invention. Video applications, and applications or services for which required resource amounts can be acquired may be the preemption target services or applications in the present invention. In the case of a video application, it may be possible to acquire the information on the resource amount required for providing the corresponding service. 
       FIG. 3  is a diagram illustrating a configuration of a resource amount-based congestion control system according to an embodiment of the present invention. 
     The congestion control system  300  may include a packet analysis unit  310 , a scheduler  320 , a bearer selection unit  330 , and a bearer management unit  350 . The packet analysis unit  310  may check for the characteristics and encoding rate of an application. It is assumed that the application is a video application. In this case, the packet analysis unit  310  may detect the video application and the encoding rate of the video application. The packet analysis unit  310  may calculate a number of PRBs required for supporting the video encoding rate based on the video encoding rate, per-user channel quality, and a per-PRB bit amount. The information on the application, the encoding rate, the channel quality and/or the number of required PRBs may be transferred to the bearer selection unit  330 . 
     The scheduler  320  may allocate bearers to the bearer selection unit  330  for providing a service. The scheduler  320  may also check for bearer PRB usage amount per user and report it to the bearer selection unit  330 . 
     The bearer selection unit  330  may generate a candidate resource release target list. The bearer selection unit  330  may generate the candidate list based on the information received from the packet analysis unit  310  and the scheduler  320 . The bearer selection unit  330  may compare a resource usage amount and a resource requirement amount to select the users whose resource requirement amount is less than the resource usage mount as the candidate resource release (preemption) targets. The candidate list may be generated for a plurality of users. The resource usage amount may be a per-user PRB usage amount, and the resource requirement amount may be a per-user PRB requirement amount. 
     The bearer selection unit  330  may select at least one of the users that does not meet the required resources for the service as the resource release target user. For example, the bearer selection unit  330  may select a user with a low priority and the highest PRB requirement amount as the resource release target user from the candidate list. The bearer selection unit  330  may also select a user with a low priority and the highest PRB usage amount as the resource release target user from the candidate list. It may also be possible to apply the PRB requirement amount and PRB usage amount in sequence. 
     The bearer selection unit  330  may transfer the information on the user selected as the resource release target to the bearer management unit  340 . For example, it may be possible to transmit a release target bearer identifier. The information on the user may be the information on the service or bearer designated for the corresponding user. 
     The bearer management unit  340  may manage bearers for the selected users based on the information received from the bearer selection unit  340 . For example, it may be possible to release the bearer allocated to the user. Releasing a bearer may be releasing the resources allocated to the user. The bearer management unit  340  may release the bearer established with an Evolved Packet Core (EPC). The bearer management unit  340  may notify the scheduler  320  of the released bearer. The scheduler  320  may notify the bearer selection unit  330  of the release of the bearer for the corresponding user. 
     The bearer selection unit  330  may perform part of the operations of the packet analysis unit  310  and/or part of the operations of the bearer management unit  340 , the bearer selection unit  330  may perform part of the operations of the scheduler  320 . The bearer selection unit  330  may include at least one of the packet analysis unit  310 , the scheduler  320 , and the bearer management unit  340 . The bearer selection unit  330  may be a congestion controller. The congestion controller may be included in the PCRF or implemented as a separate entity. In the case of being implemented as a separate entity, the congestion controller may be connected electrically or via communication to the PCRF . 
     In the embodiment of  FIG. 2 , there is no provision of a method for determining a priority order of the users fulfilling the threshold conditions as resource release targets. In the second embodiment, however, it may be possible to determine the priority order of the candidate resource release target users based on the PRB requirement amount or PRB usage amount and release the resources based on the priority order. In the second embodiment, it may be possible to check for the use of a service (e.g., video application) of which resource requirement can be checked and the video encoding rate in a congestion situation and release the resources allocated to the users experiencing an unsatisfactory quality experience. Releasing the resources allocated to the users experiencing an unsatisfactory quality experience may have less effect on degradation of the quality experience. It may also be possible to predict the contribution of a resource release to the entire congestion control and make a control decision based thereon in consideration of the PRB usage amount or PRB requirement amount, thereby improving the communication quality experience. In the second embodiment, the process of determining resource release target users for congestion control and handling the allocated resources may be applied to the process of selecting bearer adjustment target service flows and adjusting the bearers corresponding to the selected service flows. Here, the bearer adjustment may be performed by the P-GW under the control of the congestion controller. The bearer adjustment may include releasing the corresponding bearer and adjusting data rate (data rate reduction) on the bearer. 
       FIG. 4  is a flowchart illustrating a congestion control method according to the second embodiment of the present invention. 
     In reference to  FIG. 4 , the congestion controller may detect that the system is in a congestion situation at step S 410 . For example, the congestion controller may measure the congestion level. If the measured congestion level is equal to or greater than a predetermined level, the congestion controller may determine this as a congestion situation and start a congestion control operation. The congestion control may be performed differently according to the congestion level. If the congestion level is high, it is necessary to release the resources allocated to plural users or a user with a large amount of resources. If the congestion level is low, it may be sufficient to release the resources allocated to a relatively small number of users or a user with a small amount of resources. 
     The congestion controller may check congestion control information at step S 420 . The congestion control information includes the information necessary for selecting the users for releasing the resources or bearers allocated thereto. This information may include per-user or per-service application characteristic information (e.g., video application), encoding rate information, required resource amount information (required PRB), and resource use information (using PRB). The congestion controller may check the encoding rate of the application. The congestion controller may also check for the number of PRBs required for supporting the encoding rate of the corresponding application based on the encoding rate of the application, channel quality of the user, or the per-PRB bit amount. The congestion control information may be acquired in such a way of performing measurement or calculation at the congestion controller or receiving from another entity. 
     The congestion controller may identify the resource release targets based on the congestion control information at step S 430 . It may also be possible to check a candidate resource release target list for multiple users or services. The information on the resource release target or the candidate resource release target list may be received from another entity or generated by the congestion controller based on the congestion control information. The congestion controller may select the candidate resource release targets based on the resource usage amount and resource requirement amount information. The congestion controller may select a user (service or service flow) whose resource usage amount is less than a resource requirement amount as the candidate resource release (preemption) target user. It may also be possible to generate a candidate resource release target list including plural users. The resource requirement amount may be determined based on the encoding rate of the corresponding application, the channel quality, or the per-PRB bit amount. 
     There are resources required for executing normally a specific application. For example, the encoding rate of a video application may be set to 2 mbps or 4 mbps depending on the quality of the video application. If the information on the size and time of the video application is implicitly provided, it may also be possible to calculate the encoding rate based on the time and size information. Meanwhile, the data rate supportable with the given PRBs may be limited depending on the communication environment of the user. If the communication environment is good, it may be possible to support the service using a small number of PRBs, each PRB having a large number of bits. If the communication environment is bad, it may be necessary to use a plurality of PRBs for supporting the service normally because the per-PRB bit amount is small. The per-PRB bit amount information or the per-PRB available bit amount information may be the Modulation and Coding Scheme (MCS) or Modulation order Product code Rate (MPR) for use in transmission from an eNB to a UE. The eNB may determine or calculate the per-PRB available bit amount based on the Channel Quality Indicator (CQI) indicating per-user channel condition reported by the UE. It eNB is possibleto determine a high MCS level for a stable communication environment and a low MCS level for an unstable communication environment, the communication environment being determined based on the channel status information received from the UE. Also, It is possible to increase the MPR upon receipt of an ACK and decrease the MPR upon receipt of a NACK. Increasing the MPR may be equivalent to increasing the MCS level, and decreasing the MPR may be equivalent to deceasing the MCS level. 
     The encoding rates of applications different in type and running for different users may be different from each other, the numbers of available bits per PRB may vary depending on the communication environment, and the numbers of PRBs necessary for normally providing the services may be different. 
     Table 1 shows a number of required PRBs and a number of in-use PRBs per application. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Number of 
                 Number of 
                 Number of additionally 
               
               
                 Application 
                 required PRBs 
                 in-use PRBs 
                 required PRBs 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Application 1 
                 10 
                 5 
                 5 
               
               
                 Application 2 
                 4 
                 2 
                 2 
               
               
                 Application 3 
                 8 
                 6 
                 2 
               
               
                 Application 4 
                 4 
                 5 
                 — 
               
               
                 Application 5 
                 6 
                 6 
                 — 
               
               
                   
               
            
           
         
       
     
     In Table 1, the number of required PRBs is the information determined based on the encoding rate of an application and a number of available bits per BRP. The number of in-use PRBs is the number of PRBs in use by the currently running application. 
     It is assumed that the congestion controller is managing 5 applications in a congestion situation. Applications 1, 2, and 3 are in a situation where the number of required PRBs is greater than the number of in-use PRBs. This means that the number of allocated PRBs is not enough for providing the corresponding service normally and there are no PRB resources allocated for maintaining the encoding rate. Applications 4 and 5 are in a situation where the number of in-use PRGs is greater than the number of required PRBs so as to provide the corresponding service normally. Accordingly, the congestion controller may select one of applications 1, 2, and 3 as a candidate resource release (preemption) target. In the case that a candidate resource release target list is generated, applications 1, 2, and 3 may be included in the list. 
     In the second embodiment of the present invention, applications 4 and 5 being normally served are not selected as candidate resource release targets. It is not preferable to release the resources allocated to the user or application being normally served for the purpose of congestion control. It is most efficient, in view of a user&#39;s quality experience, to release the resources allocated to part of applications 1, 2, and 3, which cannot guarantee normal services. In the case of considering only the QCI or ARP, the resource release target is selected based on the QCI level regardless of the normality of the current service, and this may increase the probability of selecting applications 4 and 5 being normally serviced as the resource release target, resulting in reduction of the quality experience of the corresponding user. The second embodiment of the present invention proposes a method for solving this problem. 
     The congestion controller may select a resource release target at step S 440 . The resource release target may be selected in consideration of information on priority, required PRB amount, and in-use PRB amount. The priority may include QCI or ARP information. It may be possible to select an application with a low priority in association with the QCI or ARB as the resource release target among application included in the candidate resource releases target list. For more detail on the QCI- and ARP-based resource release target selection procedure, refer to the description made with reference to  FIG. 2 . 
     A description is made hereinafter of the method for selecting a resource release target based on the required PRB amount and/or in-use PRB amount. It may be possible to select a resource release target based on the required PRB amount. For example, an application with the largest required PRB amount may be selected as a resource release target. In Table 1, application 1 requires the largest amount of resources. Accordingly, it may be possible to release the resources allocated to application 1. 
     The congestion controller may select a resource release target based on the number of PRBs to be allocated for normal service additionally. In order for the application requiring a large number of additional PRBs to operate normally, it may be necessary to release a large number of resources allocated to another application, incurring the risk of reduction of the quality experience of a user. Accordingly, it may be possible to select the application requiring a large number of additional PRBs as a resource release target. In Table 1, application 1 requires 5 additional PRBs for normal service, and each of applications 2 and 3 requires 2 additional PRBs for normal service. Accordingly, the congestion controller may select application 1 as a resource release target. 
     It may also be possible to select a resource release target based on the the amount of in-use PRBs. Using the metric of the number of in-use PRBs is advantageous in terms of congestion control because it is easy to acquire the information on the number of PRBs capable of being secured through resource release. In Table 1, application 1 has 5 in-use PRBs, application 2 has 2 in-use PRBs, and application 3 has 6 in-use PRBs. This means that it is preferable to release the resources allocated to applications 1 or 3 rather than releasing the resources allocated to application 2 in terms of the number of PRBs capable of being secured. In the case of using the metric of the amount of in-use PRBs, it may be possible to consider the impact of releasing the resources allocated to an application on another application. In this case, it may be possible to use the information on the number of PRBs additionally required for normal service per application. The number of additionally required PRBs for normal service per application may be determined based on the number of required PRBs and the number of in-use PRBs. In the example of Table 1, it may be possible for applications 2 and 3 to provide their services normally by releasing the resources allocated to application 1 because the 5 PRBs secured by releasing the resources allocated to application 1 are sufficient in number for applications 2 and 3 that each require 2 additional PRBs. In the case of releasing the resources allocated to application 2 or 3 , however, at least one application still lacks PRBs. Accordingly, if resource release is to be considered, it is most efficient to release the resources allocated to application 1. 
     The congestion controller may transmit the information on at least one application or user selected as the resource release target. The congestion controller may transmit the information to an entity responsible for managing bearers. The information on the application may be the information indicating the resource, bearer, or service allocated to the application. The information indicating the bearer may include a bearer ID, and the information indicating the service may include a service ID. 
     Next, the congestion controller may determine whether the congestion state has been resolved. If the congestion state has been resolved, the procedure may end. If the congestion state has not been resolved, the steps S 410  to S 450  may be repeated. 
       FIG. 5  is a block diagram illustrating a congestion controller according to an embodiment of the present invention. 
     In reference to  FIG. 5 , the congestion controller  500  may include a communication unit  510  and a control unit  530 . The communication unit may transmit and/or receive signals to and/or from at least one network node. The control unit  530  may control the overall operation of the congestion controller  500 . The control unit  530  may perform a congestion control operation in a congestion situation. In an embodiment of the present invention, the congestion controller may be included in a PCRF. Also, the congestion controller may be an external entity connected to the PCRF. 
     According to an embodiment, the control unit  530  may identify a congestion state, check for the congestion control information per user, determine at least one resource release target user based on the PRB-related information included in the congestion control information, and control to release the resources allocated to the resource release target user. The PRB-related information may include information on the required PRBs per user and in-use PRBs per user. The information on the required PRBs may be determined based on the encoding rate of the service provided to the user and the per-PRB available bit amount information. The resource release may include releasing part of bearers allocated to the user. 
     The per-PRB available bit amount information may include a Modulation and Coding Scheme (MCS) or Modulation order Product code Rate (MPR), which is determined based on the channel status information reported by the user. The channel status information may be a Channel Quality Indicator (CQI). 
     The control unit  530  may control to identify a candidate resource release target user based on the information on the required PRBs and in-use PRBs. Here, the candidate resource release target user may be a user having the in-use PRBs less in number than the required PRBs. The control unit  530  may control to determine, among the candidate resource release target users, the user with the lowest priority determined based on QCI as the resource release target user. 
     The control unit  530  may also control to determine, among the users, the user with the largest number of in-use PRBs as the resource release target user. The control unit  530  may also control to determine, among the users, the user with the greatest difference between the number of required PRBs and the in-use PRBs as the resource release target user. 
     The control unit  530  may also control to determine the resource release target user based on the number of in-use PRBs of a predetermined user and the number of additional PRBs required for multiple users. 
     If the congestion control state is not resolved after allocating additional resources to the determined user, the control unit  530  may control to determine an additional resource release target user and control releasing the resources allocated to the additional resource release target user. 
     The control unit  530  may control to select a service flow related to a resource release target and release the resources allocated for the selected service flow. That is, the control unit  530  may select a service flow related to the resource release target and adjust the service flow for congestion control as well as determine a resource release target user and release the resources allocated to the resource release target user. 
     The control unit  530  may control a P-GW to block resource transmission on the bearer related to the selected service flow. The control unit  530  may also control the P-GW to reduce data rate on the bearer related to the selected service flow. 
     Although it is exemplarily depicted in  FIG. 5  for convenience of explanation, the congestion controller is not limited to the exemplary configuration. The control unit may include a plurality of entities. The operations of the congestion controller are not limited by those described with reference to  FIG. 5 . The congestion controller may perform the congestion control operations disclosed in the embodiments of the present invention as described with reference to  FIGS. 1 to 5 . 
     Although various embodiments of the present disclosure have been described using specific terms, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense in order to help understand the present invention. It is obvious to those skilled in the art that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention.