Abstract:
A call admission method considering call characteristics and the service level of a subscriber terminal in a CDMA mobile communication system is provided. When a new call is requested in a situation where it cannot be admitted, a call being serviced with a lower service level than the new call is held for admission of the new call. If the held service can be resumed under some circumstances, it is resumed.

Description:
PRIORITY 
   This application claims priority to an application entitled “Call Admission Method in CDMA Mobile Communication System” filed in the Korean Industrial Property Office on Jul. 18, 2000 and assigned Serial No. 2000-41127, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a call admission method in a CDMA mobile communication system, and in particular, to a call admission method considering call characteristics and the service class of a subscriber terminal. 
   2. Description of the Related Art 
   Today&#39;s rapid growth in the mobile communication industry increases the demand for a mobile communication system that is capable of providing multimedia services including transmission of data and pictures as well as typical voice services. This mobile communication system is called the future mobile communication system. This future mobile communication system generally operates in CDMA (Code Division Multiple Access). The CDMA systems are deployed in a synchronous scheme or in an asynchronous scheme. Europe and Japan employ the asynchronous scheme, whereas the U.S. employs the synchronous scheme. Therefore, different standardization works are under way in the U.S. and Europe. The European future mobile communication system is UMTS (Universal Mobile Telecommunication Systems) or W-CDMA (Wideband Code Division Multiple Access). 
   A variety of regulations should be defined for multimedia services, besides voice call services, in the on-going standardization work for the future mobile communication system. Among them, an efficient call admission scheme is a major issue. 
   In general, the CDMA mobile communication system has limited radio resources available for a UTRAN (UMTS Terrestrial Access Network) to assign to UEs (Unit Equipments). To properly assign the limited resources for traffic generated from the UEs, a call admission method should be exploited. 
   The W-CDMA mobile communication system can afford to provide multimedia services efficiently that support QoS (Quality of Service) for each transport channel. However, it cannot implement a reliable call control for unexpectedly generated traffic. The unexpected traffic includes a frequent handoff and a call setup request for a special use from a UE. Moreover, an efficient, immediate call process cannot be provided when overload is imposed on the UTRAN due to excessive call processing requests. 
   This situation impedes implementation of fast and reliable services in the W-CDMA mobile communication system. What is worse, the ever-increasing packet data service makes it more difficult to ensure QoS through call admission control and efficient bandwidth management. 
   Accordingly, there is a pressing need for a call admission control which considers the characteristics of a requested call and the service class of a subscriber terminal. For this purpose, a novel RRC (Radio Resource Control) message and its related operation must be offered. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a call processing method which ensures QoS by efficient call admission in a CDMA mobile communication system. 
   It is another object of the present invention to provide a call admission method at an RRC layer to ensure QoS in a CDMA mobile communication system. 
   It is a further object of the present invention to provide a novel RRC message that ensures QoS in a CDMA mobile communication system. 
   The foregoing and other objects of the present invention are achieved by providing a call admission method in a CDMA mobile communication system. To admit calls from a plurality of UEs (Unit Equipments) within a cell with limited radio resources, a UTRAN temporarily releases radio resources assigned to at least one call being serviced with a service level lower than a new call from a UE if the new call cannot be admitted with available radio resources. Then, the UTRAN assigns radio resources to admit the new call with the available radio resources and the released radio resources. 
   If a UE receives a waiting message requesting the UE to hold its service during the service in progress, the UE releases the assigned radio resources at an action time set in the waiting message and holds the service temporarily. Upon receipt of a reconnect message requesting the UE to resume the held service from the UTRAN, the UE is assigned to the radio resources from the UTRAN at an action time set in the reconnect message and resumes the held service. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a schematic view of a CDMA mobile communication system to which the present invention is applied; 
       FIG. 2  is a diagram showing transmission of messages between a UTRAN and a UE during call setup according to an embodiment of the present invention; 
       FIG. 3  is a flowchart illustrating a control operation for call admission in the UTRAN in a CDMA mobile communication system according to the embodiment of the present invention; and 
       FIG. 4  is a flowchart illustrating a control operation for call admission in the UE in the CDMA mobile communication system according to the embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention will be described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
   Radio resources used hereinafter refer to radio channels that a UTRAN can assign to UEs for wireless communication. That is, the radio resources can be radio channels or frequency bandwidth available to the UTRAN. 
     FIG. 1  is a schematic view of a CDMA mobile communication system to which the present invention is applied. 
   Referring to  FIG. 1 , a UTRAN  110  manages radio resources to provide services requested by UEs  120 - 1  to  120 -n (hereinafter,  120 ). The UTRAN  110  also acknowledges a call connection request and sets up a call by negotiation in which radio resources are assigned according to the characteristic of the requested call. The call connection and bandwidth assignment to ensure QoS occur within limited radio resources. 
     FIG. 2  is a signal flow diagram illustrating exchange of messages between the UTRAN  110  and a UE  120  for a call setup according to an embodiment of the present invention. 
   Referring to  FIG. 2 , for the UE  120  to request a call initially, a higher layer, i.e., an RRC layer, must transmit a CM (Call Management) service request to the UTRAN  110 . That is, the UE  120  transmits an RRC connection request message to the UTRAN  110  in an idle state to request a call setup in step  210 . Because a traffic channel, that is, a dedicated channel is not connected between the UE  120  and the UTRAN  110 , the UE  120  transmits the RRC connection request message on a random access channel (RACH) or a common control channel (CCCH). In step  220 , the UTRAN  110  transmits an RRC connection setup message in response to the RRC connection request message. Upon receipt of the RRC connection setup message, the UE  120  transitions from the idle state to a UTRAN connected state. Then, the UE  120  transmits an RRC connection setup complete message in step  230 . Hence, the pending CM service request is transmitted by an initial direct transfer message and the call setup is completed. 
     FIG. 3  is a flowchart illustrating a UTRAN operation for acknowledging a call connection request during the above-described call setup, that is, a control operation for call admission in the UTRAN in a CDMA mobile communication system according to the embodiment of the present invention. According to the procedure of  FIG. 3 , the UTRAN checks radio resources in use and available radio resources, compares the available radio resources with radio resources required for a requested call, and acknowledges the new call setup, holding a call being serviced according to the comparison result. 
   To perform the procedure shown in  FIG. 3 , an RRC message (waiting message) for holding a UE from being serviced and another RRC message (reconnect message) indicating resumption of the held service should be configured. 
   Upon receipt of an RRC waiting message, the UE discontinues data transmission. Since the UE has not released RRC connection, it can still be in a UTRAN connected state. If the UE maintains a traffic channel, it can be in a CELL_DCH state. A transport channel reconfiguration message can be used as the RRC waiting message in this RRC state. Or a new message may be defined to transmit a waiting command to the UE as long as it includes common information elements required to instruct the UE to wait. 
   Upon receipt of the RRC reconnect message, the UE can resume the discontinued data transmission. In resuming the data transmission, the UE can utilize the same physical channel layer-related parameters, service options, and associated RAB (Radio Access Bearer) parameters as set when receiving the RRC waiting message. The RRC reconnect message may be a transport channel reconfiguration message or may be defined as a new message as long as it includes common information elements required to instruct the UE to reconnect. 
     FIG. 4  is a flowchart illustrating a control operation for service holding or service reconnection for call admission in the UE of the CDMA mobile communication system according to the embodiment of the present invention. 
   The procedure of  FIG. 4  includes the steps of processing an RRC waiting message indicating temporal discontinuation of data transmission upon receipt of the RRC waiting message during a service in progress and processing an RRC reconnect message upon receipt of the RRC reconnect message in the service holding state. 
   If the UE receives the RRC waiting message during a service in progress, it discontinues data transmission/reception and enters a holding state. In the holding state, the UE is not allowed to transmit a reconnection request message until it receives an RRC reconnect message. To receive the RRC reconnect message, the UE should be in the CELL_DCH state of the UTRAN connected state. If the UE was transitioned to a state without a dedicated channel, the RRC state of the UE can be a CELL_FACH state. Since the UE can monitor a scheduled common channel FACH in the CELL_FACH state, it can receive the RRC reconnect message from the UTRAN. 
   The call admission method of the present invention will now be described in detail referring to  FIGS. 3 and 4 . 
   In  FIG. 3 , the UTRAN  110  checks whether a call setup request has been received from a new UE  120  in step  301 . The call setup request is issued by an RRC connection request message on an RACH or a CCCH. Since it does not know accurately radio resources available to the UTRAN  110 , the UE  120  requests radio resource requirements for data traffic to be transmitted by the RRC connection request message. 
   In step  302 , the UTRAN  110  judges the priority level of the UE  120  by determining whether the UE  120  is at a premium level offering a special service or a relatively high quality service or at an emergency state for an emergency call and then determines a service priority for the UE  120 . Here, the UE-based priority level is determined regardless of the class of a requested service and requested service options. The UE-based priority level can be set using a predetermined access class (AC) for the UE or notified to the UTRAN by the RRC connection request. 
   In step  303 , the UTRAN  110  determines an application-based access priority for the UE  120 . Application services are divided largely into a voice service and a packet data service. The packet data services are in turn divided into a high data rate packet data service, a picture service, or a packetized voice service. The application-based access priority depends on the characteristic of a requested application service, regardless of the characteristic of the UE  120 . 
   In step  304 , the UTRAN  110  determines a final service level taking the UE-based priority level and the application-based access priority level into account, referring to a service level database. While the UTRAN  110  may set service level criteria in many ways, the UE characteristic and the application service quality must be considered to efficiently control QoS. Particularly, to efficiently assign and manage bandwidth serviced by the UTRAN  110 , a service level must be determined after taking the above characteristics into full consideration. 
   The UTRAN  110  calculates radio resources required to satisfy the determined service level and checks the volume of available radio resources in step  305 . 
   In step  306 , the UTRAN  110  determines whether to acknowledge or deny the call setup request based on the volume of the available radio resources. If the available radio resources are enough to satisfy the service level, the UTRAN  110  acknowledges the call setup request. Then, the UTRAN  110  goes to step  314  and performs a call setup. 
   On the other hand, if the UTRAN  110  determines that the call setup request is to be denied in step  306 , it considers the determination again based on the service level determined in step  304 . 
   In step  307 , the UTRAN  110  checks the service levels of calls being serviced within the UTRAN  110 . The service levels of the calls in progress were also determined in the same manner in steps  301  to  304 . 
   The UTRAN  110  determines whether there exists a call that is being serviced but can be held, by comparing the service levels of the calls being serviced with that of the requested call in step  308 . A service can be held if it has a lower service level than the requested call. 
   If the service levels of the calls being serviced are higher than the service level of the requested call, the UTRAN  110  denies the call setup request in step  315 . 
   If the service level of at least one call being serviced is lower than that of the requested call, the UTRAN  110  proceeds to step  309 . If a plurality of calls being serviced have lower service levels than the requested call, the UTRAAN  110  selects one of them to be held. The prefered way of determining a call to be held is to check the number of service holdings for each of the calls and select the call with the smallest number of service holdings. 
   In step  309 , the UTRAN  110  transmits an RRC waiting message requesting the selected UE to hold its service. As stated above, the UE receiving the RRC waiting message has a lower service level than the UE requesting the call. That is, to establish a call with a higher service level and its QoS, radio resources in use are released temporarily. The RRC waiting message has information elements shown below in Table 1. Table 1 lists information elements required to implement the present invention. 
   Upon receipt of the RRC waiting message, the corresponding UE holds the service temporarily according to the procedure shown in  FIG. 4 . The UE still maintains parameters for establishing a physical channel and service option information representing the characteristics of an application service when holding the service. The UE discontinues data transmission until it receives an RRC reconnect message indicting resumption of the call from the UTRAN  110 . This will be described below with reference to  FIG. 4 . 
   In step  310 , the UTRAN  110  acknowledges the call setup request by assigning the radio resources of the UE of which the service is held to the UE requesting the call. To perform step  310 , the UTRAN  110  can transmit the RRC waiting message to two or more UEs. It implies that in the case where the available radio resources and radio resources secured by holding a call being serviced are not enough for the requested call, a plurality of calls being serviced can be held. Step  310  is performed in the same manner as step  314 . 
   The UTRAN  110  updates a bandwidth management database in step  311 . The UTRAN  110  resets the volume of the available radio resources, taking into account the radio resources assigned to the new call and the radio resources saved from the held call. Step  311  is essential to efficient bandwidth management along with call admission and maintenance of calls in service. While various bandwidth and database management techniques may be employed, it is important to consider the priority of the UE, the characteristic of a call (e.g., premium call, emergency call, handoff call, or reconnect call), and service characteristics including delay time and loss. Concentration of RRC waiting messages on a specific call, that is, a specific UE must be prevented. To do so, the bandwidth management database should include indexes indicating the number of occurrences of the RRC waiting messages for each UE and the UTRAN  110  should refer to the indexes in transmitting an RRC waiting message. 
   In step  312 , the UTRAN  110  determines whether the held service can be resumed. This is possible when available radio resources are secured after an arbitrary call is released. 
   If the held call can be reconnected, the UTRAN  110  transmits an RRC reconnect message to the corresponding UE in step  313 . Table 2 below lists information elements for the RRC reconnect message. 
   Now, receipt of an RRC waiting message and an RRC reconnect message and processing the messages in a UE according to the embodiment of the present invention will be described in detail with reference to  FIG. 4 . 
   In  FIG. 4 , the UE  120  receives a message on a dedicated channel in a traffic state, that is, during a call in progress in step  401 . In step  402 , the UE  120  determines whether the received message is an RRC waiting message by analyzing its message type. If the message is not an RRC waiting message, the UE  120  determines whether it is an RRC reconnect message in step  404 . 
   If the received message is an RRC waiting message in step  402 , the UE  110  goes to  403 . If the received message is an RRC reconnect message in step  404 , the UE  120  resumes data transmission in step  406 . If the received message is not an RRC waiting message nor an RRC reconnect message in steps  402  and  404 , the UE  120  processes the received message, considering that it is not related to bandwidth management and control but a typical message receivable on a traffic channel in step  405 . 
   In step  403 , the UE  120  holds the service in progress in response to the RRC waiting message. Both the UTRAN  110  and the UE  120  discontinue data transmission/reception simultaneously at an action time set in the RRC waiting message. The simultaneous discontinuation of data transmission/reception is possible only if the UTRAN  110  is synchronized to the UE  120 . Without timing synchronization between them, the action time is different between the UTRAN  110  and the UE  120 . This causes asynchronous discontinuation of data transmission/reception. Therefore, the UTRAN  110  transmits an SFN (System Frame Number) to the UE  120  every predetermined intervals (e.g., 10 ms) for synchronization between the UTRAN  110  and the UE  120 . To rapidly reconnect the held service, the UE  120  maintains physical channel-related parameters and service options even if it discontinues data transmission. The use and fields of the RRC waiting message will be described in detail below referring to Table 1. 
   In step  405 , the UE  120  performs an operation as indicated by the received message, considering that it is a typical message receivable on a traffic channel. On the other hand, the UE  120  resumes the held service in step  406 . The UTRAN  110  and the UE  120  resume data transmission/reception simultaneously at an action time set in the RRC reconnect message as in the RRC waiting message. Assignment of radio resources will be requested to resume the held service. Therefore, the UE  120  is assigned to radio resources from the UTRAN  110  according to the maintained information, upon receipt of the RRC reconnect message. The use and fields of the RRC reconnect message will be described below in detail referring to Table 2. 
   As described above, the UTRAN can force the UE to hold its service in progress and to resume the held service if available radio resources are secured. 
   Meanwhile, the RRC waiting message and the RRC reconnect message should be defined to implement the present invention. Table 1 and Table 2 list only fields of the RRC waiting message and the RRC reconnect message required to perform the functions according to the present invention. The fields of the present invention can be inserted into a typical traffic channel message, or the function of the traffic channel message can be incorporated into the message configured according to the present invention. 
   
     
       
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
                 
                 
               Type and 
                 
             
             
               Information element 
               Need 
               Multi 
               reference 
               Semantic description 
             
             
                 
             
           
           
             
               Waiting information 
               MP 
                 
                 
                 
             
             
               &gt;Duration time 
               MP 
                 
                 
               Waiting time for packet 
             
             
                 
                 
                 
                 
               retransmission 
             
             
               &gt;Reason 
               MP 
                 
                 
               Waiting reason 
             
             
               &gt;Action time 
               MP 
                 
                 
               Start time for 
             
             
                 
                 
                 
                 
               discontinuation of packet 
             
             
                 
                 
                 
                 
               transmission 
             
             
                 
             
           
        
       
     
   
   In Table 1, upon receipt of a message from the UTRAN  110 , the UE  120  analyses Message Type (not shown) to determine what type the received message is. Here, Waiting Information is written in Message Type. General Message fields (not shown) are common to typical traffic channel messages. The General Message fields are not related directly to the present invention. Duration is a field indicating the duration time of non-data transmission in the case of an RRC waiting message. This field enables the UE  120  to resume message transmission without the need of receiving an RRC reconnect message from the UTRAN  110 . Duration is added to the RRC waiting message only if Duration Flag (not shown) is set to 1. An operator sets Duration Flag to determine transmission of the RRC reconnect message. Duration can be set when the duration of a call in service can be estimated, the call is released at an expected time, and the resulting available radio resources are enough for the radio resource requirement of the UE  120  for which the held service is to be resumed. Reason notifies the UE  120  why the service is to be held. According to Reason, the UE  120  can perform an appropriate operation. Use Time (not shown) is a field indicating whether Action Time is included or not in the RRC waiting message. If Use Time is set to 1, Action Time is included and the RRC waiting message takes effect at a time set in Action Time. The use of Action Time aims at reduction of data loss during transmission/reception. The UTRAN  110  and the UE  120  discontinue data transmission/reception at an action time simultaneously. Duration Flag and Use Time can be set as separate fields in the RRC waiting message or using one bit respectively from Duration and Action Time. 
   
     
       
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
                 
                 
               Type and 
                 
             
             
               Information element 
               Need 
               Multi 
               reference 
               Semantic description 
             
             
                 
             
           
           
             
               Reconnection 
               MP 
                 
                 
                 
             
             
               information 
             
             
               &gt;Duration time 
               MP 
                 
                 
               Waiting time for packet 
             
             
                 
                 
                 
                 
               retransmission 
             
             
               &gt;Action time 
               MP 
                 
                 
               Start time for 
             
             
                 
                 
                 
                 
               retransmission 
             
             
                 
             
           
        
       
     
   
   In Table 2, upon receipt of a message from the UTRAN  110 , the UE  120  analyses Message Type (not shown) to determine what type the received message is. Here, Reconnection Information is written in Message Type. General Message fields (not shown) are common to typical traffic channel messages. The General Message fields are not related directly to the present invention. Duration is a field used when the UE  120  cannot resume the data transmission even after the time set in Waiting Duration of the RRC waiting message or a time set in a previously received RRC reconnect message. If data transmission is possible to be resumed after the time set in Duration, there is no need for receiving another RRC reconnect message. Duration is added to the RRC reconnect message only if Duration Flag (not shown) is set to 1. Duration can be used when the available radio resources are enough for the radio resource requirement of the UE  120  for which the held service is to be resumed. Use Time (not shown) is a field indicating whether Action Time is included or not in the RRC reconnect message. If Use Time is set to 1, Action Time is included and the RRC reconnect message takes effect at a time set in Action Time. The use of Action Time aims at reduction of data loss during resumed data transmission/reception. The UTRAN  110  and the UE  120  resume data transmission/reception at an action time simultaneously. 
   In Table 2, Duration Flag and Use Time can be set as separate fields in the RRC reconnect message or using one bit respectively from Duration and Action Time. 
   The message structures shown in Table 1 and Table 2 are mere examples to implement the present invention. Therefore, new messages can be defined or existing messages can be utilized with the use of the information elements defined in Table 1 and Table 2. 
   In accordance with the present invention as described above, a new call is admitted by holding a call being serviced with a lower service level than the new call. Accordingly, radio resources are efficiently utilized and assigned. 
   While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.