Abstract:
A handover method and apparatus in a communication system are provided. The method includes an MS sending to a serving BS a bandwidth allocating request message for sending a MOB_MSHO-REQ message requesting a handover; activating a timer set to a bandwidth allocation waiting time in relation to the bandwidth allocating request message; stopping the timer upon receipt of a bandwidth allocation message from the serving BS before the timer expires; and determining to perform a handover to a target BS if the MS does not receive the bandwidth allocation message from the serving BS before the timer expires.

Description:
PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119(a) to a Korean Patent Application filed in the Korean Intellectual Property Office on Mar. 31, 2006 and assigned Serial No. 2006-29934, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to handover in a communication system, and in particular, to a method and system for performing a handover in a Mobile Station (MS), when the MS is not allocated an uplink bandwidth in a communication system.  
         [0004]     2. Description of the Related Art  
         [0005]     Provisioning of services with different Quality of Service (QoS) requirements at high rates to users is an active research area in a 4 th  Generation (4G) communication system. Studies are under way on provisioning of high-speed services in the 4G communication system by ensuring mobility and QoS to a Broadband Wireless Access (BWA) communication system such as a Wireless Local Area Network (WLAN) and a Wireless Metropolitan Area Network (WMAN). The 4G communication system uses Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) for physical channels in order to support a broadband transmission network.  
         [0006]     In a typical cellular wireless communication system, each Base Station (BS) covers its service area called cell and provides services to MSs within the cell. When an MS moves from the serving cell to a neighbor cell, it performs a handover from the serving BS to a target BS covering the neighbor cell.  
         [0007]      FIG. 1  is a diagram illustrating a signal flow for a handover process in a conventional communication system. In  FIG. 1 , a handover operation among an MS  100 , a serving BS  130 , and a target BS  160  will be described below.  
         [0008]     Furthermore, in  FIG. 1 , the MS  100  carries out a communication service with the serving BS  130  by setting up a call. The MS  100  receives reference signals, for example, pilot signals from neighbor BSs, measures the Carrier-to-Interference and Noise Ratios (CINRs) of the pilot signals, and makes a decision as to whether to change the serving BS  130 , that is, whether to perform a handover.  
         [0009]     If the MS  100  determines to change the serving BS from the current serving BS  130  to another BS, it sends a Mobile Station HandOver Request (MOB_MSHO-REQ) message to the serving BS  130  in step  111 . The MOB_MSHO-REQ message is a Medium Access Control (MAC) layer message carrying neighbor BS information that the MS  100  has measured to perform a handover to another BS.  
         [0010]     The serving BS  130  replies to the MS  100  with a Base Station HandOver Response (MOB_BSHO-RSP) message in step  113 . The MOB_BSHO-RSP message is a MAC layer message including recommended BS information.  
         [0011]     The MS  100  acquires handover information regarding neighbor BSs from the MOB_BSHO-RSP message. While the MS  100  initiates the handover, as illustrated in  FIG. 1 , the serving BS  130  may initiate the handover without the MS&#39;s request, for the reason of load sharing or other reasons. Hence, while not shown, the serving BS  130  may send a Base Station HandOver Request (MOB_BSHO-REQ) message to the MS  100 . The MOB_BSHO-REQ message is a MAC layer message by which the serving BS  130  requests a handover to the MS  100 .  
         [0012]     Handover information sent between the MS  100  and the serving BS  130  by the MOB_MSHO-REQ message, the MOB_BSHO-RSP message, and the MOB_BSHO-REQ message contains service level prediction information, handover process optimization information, HandOver Identification (HO-ID) information, HO_authorization_policy_support information, and the like.  
         [0013]     Upon receipt of the MOB_BSHO-RSP message, the MS  100  sends a HandOver Indication (MOB_HO-IND) message to the serving BS, notifying that it will perform a handover to the target BS  160  in step  115 . Then, the MS  100  releases the call from the serving BS  130 .  
         [0014]     The MS  100  performs network re-entry to the target BS  160  as a new serving BS. The network re-entry is done for ranging, re-negotiation, re-authentication, and re-registration between the MS  100  and the target BS  160 .  
         [0015]     The target BS  160  sends a ranging information element, for example, a Fast Ranging Information Element (Fast_Ranging_IE) to the MS  100 , for fast handover in step  117 . As the Fast_Ranging_IE is delivered in an UpLink (UL) MAP, a Code Division Multiple Access (CDMA) code ranging procedure may not be performed.  
         [0016]     As the target BS  160  sends the Fast_Ranging_IE to the MS  100 , it allocates an UpLink BandWidth (UL BW) in which the MS  100  will send a RaNGing-REQuest (RNG-REQ) message. Then, the MS  100  sends the RNG-REQ message in the allocated UL BW to the target BS  160 , for ranging in step  119 .  
         [0017]     However, since the above messages are exchanged between the MS  100  and the BSs  130  and  160  at a cell boundary with weak signals or in a handover region, the messages have a high transmission/reception failure probability. Specifically when the MS  130  fails to receive a message, for instance, the MOB_BSHO-RSP message from the serving BS  130  or is not allocated a UL BW by failing to receive the UL MAP from the target BS  160 , the handover is failed. Even though the MS  100  normally receives the MOB_BSHO-RSP message, if it fails to be allocated a UL BW by which to send the MOB_HO-IND message to the serving BS  130 , the handover is failed. When the MS  100  does not receive the MOB_BSHO-RSP message or is not allocated a UL BW from the serving BS, it cannot perform the handover to the target BS  160  normally.  
       SUMMARY OF THE INVENTION  
       [0018]     An aspect of exemplary embodiments of the present invention is to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, one aspect of the present invention is to provide a method and apparatus for performing a handover in a communication system.  
         [0019]     Another aspect of the present invention is to provide a method and apparatus for performing a handover in case of a failed transmission/reception of a handover message in a communication system.  
         [0020]     A further aspect of the present invention is to provide a method and apparatus for performing a handover without a handover indication message in a communication system.  
         [0021]     Still another aspect of the present invention is to provide a method and apparatus for performing a handover in an MS, when the MS is not allocated a UL BW in a communication system.  
         [0022]     Yet another aspect of the present invention is to provide a method and apparatus for performing a handover to prevent a deadlock caused by an abnormal implementation of a handover process in a communication system.  
         [0023]     According to one aspect of the present invention, there is provided a handover method of an MS in a communication system having the MS, a serving BS for providing a service to the MS, and a target BS to which the MS moves by a handover. The method includes the MS sending to the serving BS a bandwidth allocating request message for sending a MOB_MSHO-REQ message requesting a handover; activating a timer set to a bandwidth allocation waiting time in relation to the bandwidth allocating request message; stopping the timer upon receipt of a bandwidth allocation message from the serving BS before the timer expires; and the target BS determining to perform a handover if the MS does not receive the bandwidth allocation message from the serving BS before the timer expires.  
         [0024]     According to another aspect of the present invention, there is provided a handover method of an MS in a communication system having the MS, a serving BS for providing a service to the MS, and a target BS to which the MS moves by a handover. The method includes the MS sending to the serving BS a bandwidth allocating request message for sending a MOB_HO-IND message indicating that the MS will perform a handover, if the MS determines to perform the handover; activating a timer set to a bandwidth allocation waiting time in relation to the bandwidth allocating request message; stopping the timer upon receipt of a bandwidth allocation message from the serving BS before the timer expires; and performing the handover to the target BS if the MS does not receive the bandwidth allocation message from the serving BS before the timer expires.  
         [0025]     According to another aspect of the present invention, there is provided a handover apparatus in a communication system having an MS, a serving BS for providing a service to the MS, and a target BS to which the MS moves by a handover. The apparatus includes the MS sending to the serving BS a bandwidth allocating request message for sending a MOB_MSHO-REQ message requesting a handover, activating a timer set to a bandwidth allocation waiting time in relation to the bandwidth allocating request message, stopping the timer upon receipt of a bandwidth allocation message from the serving BS before the timer expires, and the target BS determining to perform a handover if the MS does not receive the bandwidth allocation message from the serving BS before the timer expires.  
         [0026]     According to still another aspect of the present invention, there is provided a handover apparatus in a communication system having an MS, a serving BS for providing a service to the MS, and a target BS to which the MS moves by a handover. The apparatus includes the MS sending to the serving BS a bandwidth allocating request message for sending a MOB_HO-IND message indicating that the MS will perform a handover, if the MS determines to perform the handover, activating a timer set to a bandwidth allocation waiting time in relation to the bandwidth allocating request message, stopping the timer upon receipt of a bandwidth allocation message from the serving BS before the timer expires, and performing the handover to the target BS if the MS does not receive the bandwidth allocation message from the serving BS until the timer expires. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     The above and other objects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0028]      FIG. 1  is a flow diagram illustrating a signal flow for a handover process in a conventional communication system;  
         [0029]      FIG. 2  is a flow diagram illustrating a signal flow between an MS and a serving BS in a handover process in a communication system according to an exemplary embodiment of the present invention; and  
         [0030]      FIGS. 3A, 3B  and  3 C are flowcharts illustrating an MS operation in the handover process in the communication system according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]     The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of exemplary embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.  
         [0032]     Exemplary embodiments of the present invention provide a handover method for an MS when the MS is not allocated an UL BW during a handover process between a transmitter and a receiver, i.e. between a BS and the MS in a wireless communication system. A conventional handover process is described with reference to  FIG. 1 .  
         [0033]      FIG. 2  is a flow diagram illustrating a signal flow for an MS operation in a handover process in a communication system according to an exemplary embodiment of the present invention.  
         [0034]     In  FIG. 2 , in a handover process among an MS  200 , a serving BS  230 , and a target BS  260 , the MS  200  first performs a service with the serving BS  230  by setting up a call. The MS  200  receives reference signals, for example, pilot signals from neighbor BSs, measures the CINRs of the pilot signals, and makes a decision as to whether to change the serving BS  230 , that is, whether to perform a handover.  
         [0035]     If the MS  200  determines to change the serving BS from the current serving BS  230  to another BS, the MS  200  sends to the serving BS  230  a BW allocation request message requesting a UL BW for sending a MOB_MSHO-REQ message in step  211 .  
         [0036]     In step  213 , the serving BS  230  replies to the MS  200  with a BW allocation message of allocating BW. This step is branched into step  213 - 1  in case of successful reception at the MS  200  and step  213 - 2  in case of a failed reception at the MS  200 .  
         [0037]     If the MS  200  receives the BW allocation message successfully in step  213 - 1 , a handover is performed in a conventional method. Conversely, if the MS  200  fails to receive the BW allocation message in step  213 - 2 , the MS  200  performs a handover operation using a timer set to a predetermined time value or based on a predetermined number of transmissions, as described below with reference to  FIGS. 3A, 3B  and  3 C.  
         [0038]     In case of the successful reception of the BW allocation message in step  213 - 1 , the MS  200  sends the MOB_MSHO-REQ message to the serving BS  230  in step  215 . The MOB_MSHO-REQ message is a MAC layer message carrying neighbor BS information that the MS  100  has calculated to perform a handover to another BS.  
         [0039]     The serving BS  230  replies to the MS  100  with a MOB_BSHO-RSP message in step  217 . The MOB_BSHO-RSP message is a MAC layer message including recommended BS information.  
         [0040]     The MS  200  may receive the MOB_BSHO-SRP message successfully in step  217 - 1  and may fail to receive the MOB_BSHO-SRP message in step  217 - 2 . The MS  200  performs a normal handover operation in the former case and the handover operation using a timer or a predetermined number of transmissions as described later in  FIGS. 3A, 3B  and  3 C.  
         [0041]     If the MS  200  receives the MOB_BSHO-RSP message successfully in step  217 - 1 , the MS  200  acquires handover information regarding neighbor BSs from the MOB_BSHO-RSP message.  
         [0042]     The MS  200  receives handover information through the MOB_BSHO-RSP message from the serving BS  230 . The handover information contains service level prediction information, handover process optimization information, HO-ID information, HO_authorization_policy_support information, and the like.  
         [0043]     Upon receipt of the MOB_BSHO-RSP message, the MS  200  sends a MOB_HO-IND message to the serving BS, notifying that the MS  200  will perform a handover to the target BS  160  in step  223 . Then, the MS  200  releases the call from the serving BS  230 .  
         [0044]     For the transmission of the MOB_HO-IND message, the MS  200  sends a BW allocation request message to the serving BS  230  in step  219 . In step  221 , the serving BS  230  replies to the MS  200  with a BW allocation message. This step is branched into step  221 - 1  in case of successful a reception at the MS  200  and step  221 - 2  in case of a failed reception at the MS  200 .  
         [0045]     If the MS  200  receives the BW allocation message successfully in step  221 - 1 , the handover is performed normally as done conventionally. On the other hand, if the MS  200  fails to receive the BW allocation message in step  221 - 2 , the MS  200  performs the handover operation using a timer or based on a predetermined number of transmissions as shown in  FIGS. 3A, 3B  and  3 C.  
         [0046]     Then, the MS  200  performs network re-entry to the target BS  260  as a new serving BS. The network re-entry is done for ranging, re-negotiation, re-authentication, and re-registration between the MS  200  and the target BS  260 .  
         [0047]     The target BS  260  sends a ranging information element, for example, Fast_Ranging_IE to the MS  200 , for fast handover in step  225 . As the Fast_Ranging_IE is delivered in a UL MAP, a CDMA code ranging procedure may not be performed.  
         [0048]     As the target BS  260  sends the Fast_Ranging_IE to the MS  200 , the target BS  260  allocates an UL BW in which the MS  200  will send an RNG-REQ message. Then, the MS  200  sends the RNG-REQ message in the allocated UL BW to the target BS  260 , for ranging in step  227 .  
         [0049]     Below is a description of a process of the MS in a broadband wireless communication system according to an exemplary embodiment of the present invention.  
         [0050]      FIGS. 3A, 3B  and  3 C are flowcharts illustrating an MS operation in the handover process in the communication system according to an exemplary embodiment of the present invention.  
         [0051]     In  FIG. 3A , the MS sends to the serving BS a BW allocation request message requesting a UL BW in which the MS will send a MOB_MSHO-REQ message in step  311 .  
         [0052]     The MS activates a timer T BW-MSHOREQ  set to a predetermined time value in step  313 . The predetermined time value is a time period for which the MS waits after sending the BW allocation message.  
         [0053]     In step  315 , the MS monitors allocation of an UL BW for sending the MOB_MSHO-REQ message. If the UL BW has been allocated from the serving BS, the MS proceeds to step  317  and otherwise, the MS jumps to step  335 .  
         [0054]     The MS stops the timer T BW-MSHOREQ  because of the successful allocation of the UL BW in step  317 . Then the handover process starts with the MOB-MSHO-REQ message.  
         [0055]     The MS sends the MOB_MSHO-REQ message in the allocated UL BW to the serving BS in step  319  and activates a timer T 41  in step  321 . In step  323 , the MS monitors reception of a MOB-BSHO-RSP message from the serving BS. If the MS fails to receive the MOB-BSHO-RSP message, the MS goes to step  325 .  
         [0056]     Upon receipt of the MOB-BSHO-RSP message, the MS stops the timer T 41  in step  331  and goes to step  329 .  
         [0057]     In step  325 , the MS monitors timeout of the timer T 41 . Upon timeout, the MS goes to step  327 . If the timer T 41  is still running, the MS returns to step  323 .  
         [0058]     In step  327 , the MS compares the number of retransmissions of the MOB_MSHO-REQ message with a predetermined retransmission number. If the retransmission number is less than the predetermined retransmission number, the MS returns to step  311 . If, however, the retransmission number is greater than or equal to the predetermined retransmission number, the MS proceeds to step  329 .  
         [0059]     In step  335 , the MS monitors timeout of the timer T BW-MSHOREQ . Upon timeout of the timer T BW-MSHOREQ , the MS goes to step  337 . If the timer T BW-MSHOREQ  is still running, the MS returns to step  315 .  
         [0060]     In step  337 , the MS compares the number of retransmissions of the BW allocation request message for the MOB-MSHO-REQ message with a predetermined retransmission number. If the retransmission number is less than the predetermined retransmission number, the MS returns to step  311 . If the retransmission number is greater than or equal to the predetermined retransmission number, the MS proceeds to step  329 .  
         [0061]     The MS decides on a handover in step  329  and then proceeds to step  341 .  
         [0062]     In  FIG. 3B , the MS searches for a target BS for the handover in step  341 . If the target BS is detected, the MS goes to step  343 . In the absence of the target BS, the MS goes to step  347 .  
         [0063]     The MS sends to the serving BS a BW request message requesting a UL BW by which the MS will send a MOB_HO-IND message with an IND-TYPE set to Serving BS Release in step  343  and goes to step  345 .  
         [0064]     In step  345 , the MS activates a timer T BW-HOIND  set to a predetermined time value. Then the MS proceeds to step  367 . The predetermined time value is a time period for which the MS waits after sending the BW allocation message.  
         [0065]     In step  347 , the MS determines whether any candidate BS for the handover exists. In the presence of any candidate BS, the MS goes to step  349 . In the absence of any candidate BS, the MS goes to step  357 .  
         [0066]     The MS cancels the handover and sends to the serving BS a BW allocation request message requesting a UL BW in which the MS will send a MOB_HO-IND message with the IND-TYPE set to HO-Cancel to maintain communications with the serving BS in step  349 .  
         [0067]     Then the MS monitor allocation of the UL BW for the MOB_HO-IND message in step  351 . Upon the UL BW allocation, the MS goes to step  353 . If the UL BW has not been allocated, the MS returns to step  311 . The MS sends the MOB_HO-IND message to the serving BS in step  353  and returns to step  311 .  
         [0068]     In step  357 , the MS sends to the serving BS a BW allocation request message requesting a UL BW in which the MS will send a MOB_HO-IND message with IND-TYPE set to HO-Reject to obtain information about a new candidate BS for the handover in step  357 .  
         [0069]     Then the MS monitor allocation of the UL BW for the MOB_HO-IND message in step  359 . Upon the UL BW allocation, the MS goes to step  361 . If the UL BW has not been allocated, the MS returns to step  311 .  
         [0070]     The MS sends the MOB_HO-IND message to the serving BS in step  361  and activates a timer T 42  in step  363 . Then the MS awaits reception of a MOB_BSHO-RSP message or a MOB_BSHO-REQ message in step  365 .  
         [0071]     In  FIG. 3C , the MS monitors allocation of the UL BW for sending the MOB_HO-IND message in step  367 .  
         [0072]     Upon the UL BW allocation, the MS goes to step  369 . If the UL BW has not been allocated from the serving BS, the MS goes to step  373 .  
         [0073]     The MS stops the timer T BW-HOIND  in step  369  and sends the MOB_HO-IND message to the serving BS in step  371 . Then the MS goes to step  375 .  
         [0074]     In step  373 , the MS monitors timeout of the timer T BW-HOIND . Upon timeout of the timer T BW-HOIND , the MS goes to step  375 . If the timer T BW-HOIND  is still running, the MS returns to step  367 .  
         [0075]     In step  375 , the MS decides on cell switching to the target BS, that is, determines to change the serving BS. Thus, the MS changes the serving BS from the current serving BS to the target BS by network entry to the target BS in step  377 .  
         [0076]     The time values to which the timer is set and the predetermined transmission numbers are variable depending on system situation or system characteristics. Hence, a detailed description of the time values and the transmission numbers is not provided herein.  
         [0077]     The present invention is characterized in that it flexibly copes with obstacles to reliable implementation of the handover process by use of a timer set to a time value for which the MS waits for BW allocation to send the MOB_MSHO-REQ message or the MOB_HO-IND message. The present invention achieves technical advantages in cases where the serving BS fails to receive the MOB_HO-IND message, the target BS uses a serving BS ID set in an RNG-REQ message and can receive information about the serving BS over a backbone network.  
         [0078]     As described above, during a handover process between an MS and a BS in a communication system, if a message is lost or a UL BW for message transmission is not allocated to the MS, the MS performs the handover process using a BW allocation timer. Therefore, the handover is carried out flexibly and a service is seamlessly provided. Furthermore, the handover can be provided without transmission/reception of handover-associated messages, especially a handover indication message.  
         [0079]     While the invention has been shown and described with reference to certain exemplary embodiments of the present invention 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 present invention as defined by the appended claims and their equivalents.