Abstract:
The present invention relates to a method and system for performing a handoff in a mobile telecommunications system. Particularly, the present invention provides a method of performing an access handoff in which a mobile station (MS) receives an extended channel assignment from a new base station without reception of overhead messages from the new base station, thereby allowing a traffic channel to be quickly assigned and providing a faster call connection for a user.

Description:
PRIORITY  
         [0001]    This application claims priority to an application entitled “Method and System for Performing Fast Access Handoff in Mobile Telecommunications System”, filed in the Korean Industrial Property Office on Aug. 8, 2001 and assigned Serial No. 2001-47641, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a handoff in a mobile telecommunications system, and more particularly, to a method and system for performing a fast access handoff in a mobile telecommunications system in which a mobile station is directly assigned a channel without receiving overhead messages, as in the case for performing an access handoff with requiring no access information, thereby providing a faster call connection for a user and preventing the user from suffering inconvenience in making a call.  
           [0004]    2. Description of the Related Art  
           [0005]    Generally, as shown in FIG. 1, a handoff in a mobile telecommunications system is a procedure where a base station (BS) providing service to a mobile station (MS)  112  changes from a current base station  101  to a new base station  103  while maintaining a wireless link between the MS  112  and the base station  101  or  103  and not disconnecting the link while the MS  112  moves into a new cell  123  outside the boundary of a current cell  121 . The above handoff may include, for example, a soft handoff, CDMA to CDMA hard handoff, CDMA to analog handoff, softer handoff, idle handoff, etc. The procedure of performing this handoff is disclosed in publications associated with mobile communication and supporting IS-95B. Therefore, a detailed description will be omitted. If an MS transmits or receives an outgoing call or incoming call and moves over a boundary of its current cell before the call is set up, the MS receives originating messages from BSs, analyzes pilot offsets and pilot strength, as shown in FIG. 2, of the originating messages, and predicts its moving path. Then, one of the BSs allocates its resources and transmits an acknowledgement message (BS-ACK Message) or channel assignment message to the MS, thereby raising the call setup rate. This is known as an access handoff method. In the construction of the MS as shown in FIG. 3, an application program  213  is run by a message control processor  201  to process the messages from the BSs for implementing the access handoff method.  
           [0006]    There are two technologies for implementing the access handoff method, namely, an access handoff and an access probe handoff.  
           [0007]    The access handoff is classified into an in-BS access handoff, inter-BS access handoff and combination of the in-BS and inter-BS access handoff. The in-BS access handoff is a technique of assigning an extended channel in a BS to set up a call. The inter-BS access handoff is a technique of assigning channels to two or more BSs. The access probe handoff occurs while an MS attempts access in a page response substate. In this handoff, if an MS transmits an origination call to a BS and receives no BS_ACK message from the BS, it then re-transmits the origination call to the BS. The access probe handoff is performed in the same manner as the access handoff with the exception that the BS transmits the BS_ACK message to the MS upon receiving the origination call retransmitted from the MS during its call setup operation. An example of the access handoff method a BS requires fundamental information of a system. In the conventional access handoff method as shown in FIG. 4, an MS must receive overhead messages, as signaling messages for system information, access information and environment information, upon making a transition to an idle state. Though not shown specifically, the overhead messages may be, for example, a system parameters message, access parameters message, extended system parameters message, neighbor list message, extended neighbor list message, general neighbor list message and CDMA channel list message. The MS may further receive a private neighbor list message, user zone identification message, extended CDMA channel list message, global service redirection message or an extended global service redirection message, if necessary. Of course, it is not always necessary for the MS to receive the global service redirection message, CDMA channel list message, extended global service redirection message and extended CDMA channel list message so as to perform an access handoff. However, as shown in FIG. 4, the MS has to receive overhead messages from a new BS (NBS) in every case and it takes at least 1.28 seconds to receive all of the above overhead messages. For this reason, the conventional access handoff method causes a delay in the call setup being inconvenient for a user when making a call. Further, with growth of demand for MSs, the number of BSs and the number of the channels assignable to each BS has also grown, and therefore there is a problem that handoffs frequently occur even within a short time.  
         SUMMARY OF THE INVENTION  
         [0008]    It is, therefore, an object of the present invention to provide a method and system that allow a mobile station to be assigned a channel while receiving no overhead messages in performing an access handoff with no access requirements, thereby providing a faster call connection for a user.  
           [0009]    It is another object of the present invention to provide a method for eliminating a delay in traffic channel assignment in performing an access handoff in a mobile telecommunication system.  
           [0010]    In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a method for performing a fast access handoff of a mobile station in a mobile telecommunications system, including the steps of a) allowing the mobile station to detect a strength of a pilot signal while moving to a boundary between a current base station and a new base station, the pilot signal being generated by the new base station; b) allowing the mobile station to check information and environmental conditions of the new base station generating the pilot signal in step a) to determine whether the new base station is adjacent to the current base station; c) if the new base station is determined to be adjacent to the current base station in step b), allowing the mobile station to recognize that the new base station belongs to an active set and to store the information of the new base station; d) if the information is stored in step c), allowing the mobile station to receive an extended channel assignment message from the new base station without receiving overhead messages from the new base station such that the mobile station is assigned a traffic channel based on the extended channel assignment message; and e) if the traffic channel is assigned in step d), allowing the mobile station to determine whether information of a system having the current base station has been changed, and allowing the mobile station to receive new information of the system if the information of the system has been changed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0012]    [0012]FIG. 1 is a view illustrating an example of a conventional operation of a mobile station (MS) in a mobile communication network;  
         [0013]    [0013]FIG. 2 is a graph showing variations in strength of pilot signals, which an MS receives from base stations, based on sudden variations in field intensity;  
         [0014]    [0014]FIG. 3 is a block diagram schematically showing the construction of the MS of FIG. 1;  
         [0015]    [0015]FIG. 4 is a sequence diagram showing a procedure of performing a conventional access handoff;  
         [0016]    [0016]FIG. 5 is a sequence diagram illustrating a procedure of performing a fast access handoff according to the present invention;  
         [0017]    [0017]FIG. 6 is a flowchart illustrating a procedure of performing a fast access handoff according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein will be omitted since they would obscure the invention in unnecessary detail.  
         [0019]    [0019]FIG. 5 is a sequence diagram illustrating a procedure of performing a fast access handoff according to the present invention. As illustrated in FIG. 5, the MS is configured to, during a transition from a first base station, or a current base station (BS 1 =OBS), to a second base station, or a new base station (BS 2 =NBS), directly receive an extended channel assignment message (ECAM)  414  from the BS 2  without reception of overhead messages from the BS 2  such that it is assigned a traffic channel  415  from the BS 1  and receives a traffic system parameters message  416  from the BS 1 . By doing so, it is possible to perform a fast access handoff reducing the delay time due to the reception of the overhead messages, unlike the example of FIG. 4.  
         [0020]    [0020]FIG. 6 is a flowchart illustrating a procedure of performing a fast access handoff according to the preferred embodiment of the present invention.  
         [0021]    Before a detailed description is given of the present invention, a brief description will be given of a layer  2  (L 2 ), layer  3  (L 3 ) and searcher, all of which are associated with the present invention and included in the application program  213  of FIG. 3.  
         [0022]    The L 2  is a determination module which determines whether there is an error in a processed message, and transmits an acknowledgement message if there is no error in the processed message. The L 2  sorts out duplicate messages and performs an address matching operation of determining whether a destination address of a termination call, that arrived at the MS, matches an address of the MS.  
         [0023]    The L 3  is a control module which controls the entire operation of the MS as a main controller. For example, the L 3  receives the values of signaling messages to be transmitted from corresponding tasks, fills the signaling messages with the received values and then transmits the resulting messages. The L 3  further sends received messages to corresponding tasks.  
         [0024]    The searcher is a detection module which searches for a system and acquires it. The searcher further performs an operation of detecting pilot signals from neighbor BSs during a handoff.  
         [0025]    Referring back to FIG. 6, assuming that an MS is currently located near a boundary between cells of a first BS (BS=OBS) and a second BS (BS 2 =NBS), and an access handoff between the BSs is just occurring, the MS is in an initialization mode at a current position in a cell of the BS 1  (Step  6   a ). Then, the MS receives overhead messages  400  from the BS 1 . When the MS receives all the overhead messages  400 , the MS makes a transition to an overhead reception completion (OVHD-MSG-OK) state. At this time, the MS enters an idle state (Step  6   b ). When a “send” key of the MS is pushed, an origination message  411  is transmitted from the MS to the BS 1 . Then, the MS makes a transition from the idle state to an access state (Step  6   c ). Thereafter, if the BS 1  normally receives the origination message  411  from the MS, the BS 1  then generates a BS acknowledgement order (BS-ACK-ORDER) signal  412  and the MS receives the BS-ACK-ORDER signal  412  from the BS 1  (Step  6   d ). An access handoff can occur under the present status that the MS is in the access state and has received the BS-ACK-ORDER signal  412 . The MS detects the strength of a pilot signal generated by BS 2 . The MS can more easily detect the pilot strength if it is larger than the pilot strength generated by the BS 1  (Step  6   e ).  
         [0026]    Then, the MS checks information of the BS 2  in order to determine whether the BS 2  is located in a current neighbor set of the MS and examines all of the environmental conditions of the BS 2  (Step  6   f ). If the information is invalid, namely, the BS 2  is not located in the neighbor set, the MS makes a transition to the initialization state. However, if the information is valid, namely, the BS 2  is located in the neighbor set, then the control module (L 3 ) recognizes that the BS 2  belongs to an active set and stores the valid information of BS 2 . On the other hand, the determination module (L 2 ) recognizes that not the BS 2 , but the BS 1 , still belongs to the active set and, therefore, does not receive overhead messages from the BS 2 . Namely, the MS ignores all of the overhead messages from the BS 2 , since it recognizes that there is no change of a BS (or determines that a handoff did not occur) in the OVHD-MSG-OK state (Step  6   g ). Thereafter, the MS waits for the ECAM  414  to be received from the BS 2  (Step  6   h ). If the ECAM is determined to be received (Step  6   i ), then BS 1  assigns the MS a traffic channel (Step  6   j ). After the traffic channel is assigned, the BS 1  determines whether BS information has been changed (Step  6   k ). If BS information has been changed, then the MS receives new information of a system (ITSPM=In Traffic System Parameters Message) from the BS 1  and if not, it does not receive the new information. The BS 1  transmits the ITSPM to the MS if BS 1  information and BS 2  information are not the same. The reason for reception of the ITSPM is that the MS must have information of the BS 2  to perform a handoff in a speech state. Therefore, the MS receives and processes the ITSPM (Step  61 ). Then, the MS establishes a call connection to the BS 2  over the traffic channel (Step  6   m ).  
         [0027]    As apparent from the above description, the present invention provides a method and system that allow an MS to be assigned a channel while receiving no overhead messages in performing an access handoff with no access requirements and that provide a fast call connection for a user, thereby eliminating a user&#39;s complaint caused by delays in traffic channel assignment.  
         [0028]    Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.