Abstract:
A handover method and apparatus in a communication system are provided. A serving Base Station (BS) alternately receives a target BS indicator and a downlink (DL) channel status from a Mobile Station (MS) at intervals of predetermined frames in a BS switching period for handover of the MS. The serving BS determines whether to continue the handover according to a signal received in a frame corresponding to a second reception of the target BS indicator. The method and apparatus provide a detailed definition of operation parameters for a switching time between the serving BS and a target BS.

Description:
PRIORITY 
     This application claims the benefit under 35 U.S.C. §119(a) of a Korean Patent Application filed in the Korean Intellectual Property Office on Dec. 27, 2006 and assigned Serial No. 2006-135208, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for handover in a communication system. More particularly, the present invention relates to a method and apparatus for Fast Base Station Switching (FBSS) handover in a communication system. 
     2. Description of the Related Art 
     In a communication system, a terminal is provided signal and traffic resources by a serving Base Station (BS). When the terminal moves out of range of the serving BS and into range of a target BS, a hard handover is used to release the signal and traffic resources from the serving BS and allocate the resources of the target BS. However, this hard handover process causes a considerable time delay. If the hard handover occurs during the provision of a service supporting real-time Quality of Service (QoS), for example Voice over Internet Protocol (VoIP), many restrictions due to the time delay may occur. 
     Recently, consideration has been given to the Fast Base Station Switching (FBSS) handover scheme because it is capable of performing a hard handover quickly. The FBSS handover scheme is a technology for performing handover in synchronization with a Mobile Station (MS) and a BS at a switching time from a serving BS to a target BS. Specifically, an MS that has previously been allocated signal and traffic resources for the candidate BSs to which it will move during handover, transmits information on the handover target BS to the serving BS over a Channel Quality Indicator Channel (CQICH) over which the MS periodically reports a status of Downlink (DL) radio resources. However, a need exists in the FBSS handover scheme for a detailed definition of operation parameters for the switching time between the serving BS and the target BS and for a scheme for scheduling traffic data in the serving BS via a relay station. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an operation parameters set to minimize the handover time in the FBSS handover scheme. 
     Another aspect of the present invention is to provide an optimized call processing flow between an MS, a relay station and a BS for execution of FBSS handover. 
     Another aspect of the present invention is to provide a call processing scheme for calculating the optimized BS switching time during FBSS handover and the corresponding transmission/reception scheduling time of traffic data, to minimize the drop (or interruption) of traffic data during handover. 
     According to one aspect of the present invention, a handover method of a serving Base Station (BS) in a communication system is provided. The handover method includes alternately receiving a target BS indicator and a downlink (DL) channel status from a Mobile Station (MS) at intervals of predetermined frames in a BS switching period for handover of the MS and determining whether to continue the handover according to a signal received in a frame where the target BS indicator is received in a second time. 
     According to another aspect of the present invention, a handover apparatus in a communication system is provided. The handover apparatus includes a serving Base Station (BS) for alternately receiving a target BS indicator and a downlink (DL) channel status from a Mobile Station (MS) at intervals of predetermined frames in a BS switching period for handover of the MS and for determining whether to continue the handover according to a signal received in a frame where the target BS indicator is received in a second time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1A  illustrates a scheduling procedure in an Anchor Switching Report (ASR) period according to an exemplary embodiment of the present invention; 
         FIG. 1B  illustrates another scheduling procedure in an ASR period according to an exemplary embodiment of the present invention; 
         FIG. 2  illustrates an implementation where a serving BS continues a handover procedure according to an exemplary embodiment of the present invention; 
         FIG. 3  illustrates a transmission stop/restart procedure for DL data in a BS switching period by a serving BS according to an exemplary embodiment of the present invention; 
         FIG. 4  illustrates an implementation where a serving BS cancels a handover procedure according to an exemplary embodiment of the present invention; 
         FIG. 5  illustrates a call processing flow for FBSS handover according to an exemplary embodiment of the present invention; 
         FIG. 6  illustrates a call processing flow for FBSS handover according to an exemplary embodiment of the present invention; and 
         FIG. 7  illustrates a configuration of a communication system according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. 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. 
     Exemplary embodiments of the present invention provide a scheduling scheme for minimizing an interrupted period of traffic data in an Anchor Switching Report (ASR) period indicative of the time for which an MS switches its BS during handover and a call processing flow for the FBSS handover. 
       FIGS. 1A and 1B  illustrate scheduling procedures for an ASR period according to an exemplary embodiment of the present invention. 
     Referring to  FIGS. 1A and 1B , ASR periods  100  and  110  are periods for which an MS is connected to a target BS after interrupting the connection from a serving BS, to perform FBSS handover. That is, the ASR period is a period for which an ASR slot, over which the MS reports a target BS indicator and a status of a radio DL channel, is repeated at intervals of preset frames. Here, the ASR slot is performed at intervals of preset P frames, and is set such that it should alternately transmit the target BS indicator and the channel status in interleaving order. The ASR period is set as a product of the total number M of frames, or a slot length until the next ASR period, and a repetition coefficient L of the M (i.e., ASR=M×L), and for this period, the MS attempts BS switching. 
     In an exemplary implementation illustrated in  FIG. 1A , the ASR period starts from a frame  102  whose Modular (M) value is 0, and the elements {M,L} for determining the ASR period have a value of {16,1}. That is, if M=16 and L=1, 16 frames are set as the ASR period, or BS switching period. In the ASR period of 16 frames, the MS alternately reports an indicator for the target BS and an indicator for the channel status at an interval of every 4 frames. Accordingly, a modem of the serving BS alternately detects each of the indicator for the target BS and the indicator for the channel status report two times in the ASR period. Therefore, even if the modem misses the target BS indicator from the MS one time, it still has an opportunity to normally detect the BS switching period. 
     An increase in the values of M and L excessively increases the BS switching period. Conversely, a decrease in the values of M and L reduces the number of reports on the target BS indicator and the channel status detected in the serving BS, thus causing a decrease in the detection accuracy. 
     A detailed description will now be made of an exemplary BS switching procedure between the MS, the serving BS and the target BS during FBSS handover. 
     In  FIGS. 1A and 1B , ASR periods  100  and  110  each indicate the time for which the MS switches from the serving BS to the target BS, C A  is an indicator indicating a status report for a radio DL channel, and I B  is an indicator indicating the target BS. The indicator C A  alternates with the indicator I B  at intervals of P frames, where P is a predetermined value and is, for example, 4 (P=4). 
       FIG. 1A  illustrates an exemplary scheduling procedure for use during a good Uplink (UL) radio channel state. Upon receiving I B  from the MS in the first ASR slot  104 , or M=0 frame, of the ASR period  100 , the modem of the serving BS recognizes the start of BS switching for FBSS handover. 
     Thereafter, the serving BS determines whether to perform the BS switching according to the value received in the second ASR slot  106 . In the ASR slot  106 , the serving BS can receive four types of signals, each of which is explained below. Upon receipt of the first or second signal, the serving BS continues the ongoing handover procedure according to an exemplary embodiment of the present invention. Upon receipt of the third or fourth signal, the serving BS cancels the ongoing handover procedure according to another exemplary embodiment of the present invention. 
     In the ASR slot  106 , the serving BS can receive the following four types of signals. 
     First, upon receiving I B  in the ASR slot  106 , the serving BS continues the ongoing handover procedure. 
     Second, upon failure to detect any signal in the ASR slot  106 , the serving BS continues the ongoing handover procedure. In so doing, the serving BS determines that, although the MS has transmitted the target BS indicator, the serving BS has failed to normally detect it. 
     Third, the serving BS receives a signal C A  in the ASR slot  106  indicating that the MS has cancelled the current handover. In an alternative exemplary implementation, even though the MS desires to cancel the ongoing handover for the ASR period  100 , if the corresponding ASR slot is the time for which the MS should transmit the target BS indicator, the MS does transmit the corresponding target BS indicator. However, after expiration of the ASR period  100 , when the MS should report a status of the radio DL channel to the serving BS, the MS does not report the status. In this case, therefore, the serving BS performs a procedure for canceling the ongoing handover due to the rule violation. 
     Fourth, upon receiving an unknown or wrong indicator I C  in the ASR slot  106 , the serving BS performs a procedure for canceling the ongoing handover. In doing so, the serving BS determines that the data transmitted by the MS is damaged due to the poor state of the radio UL channel. 
       FIG. 1B  illustrates an exemplary scheduling procedure for a poor UL radio channel state wherein the modem of the serving BS has failed to normally detect the I B  transmitted from the MS in the first ASR slot  112 , or M=0 frame, of the ASR period  110 . Upon receiving the I B  transmitted by the MS after 8 frames, i.e., in the second ASR slot  114 , the serving BS performs a modular operation on the second ASR slot  114  and determines whether the result is 0. If the M value  116  for the second ASR slot  114  is not 0, the serving BS recognizes that the BS switching actually started in the frame  112 . In doing so, the serving BS determines that even though the I B  was transmitted from the MS to the serving BS in the frame  112 , the I B  was missing. Herein, the ongoing handover procedure is carried out. 
       FIG. 2  illustrates an implementation where a serving BS continues the ongoing handover procedure according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , the I B  signal  212  transmitted by an MS  210  is normally received at a first ASR slot  222  of a serving BS  220  in an ASR period  200 . Thereafter, in one case  224   a , the serving BS  220  normally receives the I B    214   a  transmitted by the MS  210  in a second ASR slot. In another case  224   b , the serving BS  220  receives no signal. Thereafter, upon receiving the target BS indicator transmitted by the MS  210 , in the next ASR slot  225  after the second ASR slot  224   a , the serving BS  220  recognizes that the MS  210  has started the frame transmission to the target BS and sends an “Anchor Switch Indication” for delivering Start Frame to the Target BS. 
     At the time  226  that the MS  210  switches to the target BS for handover, the serving BS  220  stops the transmission of its DL data and starts DL data transmission in the target BS via a relay station. That is, the stopping of the DL data transmission in the serving BS  220  occurs in the next frame  226  of the reception time of the second C A  of the ASR period  200 , and the restart of the DL data transmission in the target BS is performed in the second frame after expiration of the ASR period  200 , because of the time delay between the MS and the BS. A detailed description thereof will be made below. 
       FIG. 3  illustrates a transmission stop/restart procedure for DL data in a BS switching period by a serving BS according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , after normally receiving I B  in the second ASR slot  302 , or M≠0 frame, of an ASR period  300 , a serving BS transmits time information indicating an ‘expected occurrence of BS switching’ to the relay station in a next frame  304 . This is to prepare for a situation in which a Data Transmission Stop message transmitted from the serving BS to the relay station is missing before the MS performs BS switching. That is, the serving BS transfers the detailed time information required for switching from the relay station to the target BS, along with the ‘expected occurrence of BS switching’ time information, before the switching occurs. This process, while illustrated in  FIG. 3 , is explained in more detail below with reference to  FIG. 5 . Then the target BS, recognizing that the MS will perform handover thereto, starts transmission of corresponding data at time  312  which is the time that the MS receives a message for restarting the transmission of traffic data from the relay station. The restarting of the transmission of traffic data corresponds to the time  308  at which the MS actually performs BS switching. 
     Because a radio transmission delay from the serving BS to the MS actually occurs for about two frames, the DL data transmission is stopped at a next frame  306  after the time that a second C A  of the ASR period  300  should be received. Therefore, the MS performs switching to the corresponding target BS after the serving BS has processed the DL traffic. That is, because the DL data transmission is stopped at frame  306  and because of the transmission delay of about 2 frames, the transmission of DL data may not be complete until the immediately previous frame  308  of the time that the MS performs switching to the target BS. 
     Because the delay time between the BS and the MS occurs for about two frames as stated above, CQICH_Alloc_IE is transmitted in a second frame  310  after the BS switching. Thereafter, the MS needs two more frames for re-acquiring synchronization with the target BS and processing DL data, so it suffers from a total delay of about four frames  314 . After the BS switching, if the target BS transmits CQICH_Alloc_IE( ) in the second frame  310 , the MS suffers from a delay of two frames until it receives the CQICH_Alloc_IE( ). Considering the case where the MS can receive the CQICH_Alloc_IE within the delay time  314 , the target BS, after the BS switching, continuously transmits the CQICH_Alloc_IE( ) to the MS for three frames from the second frame  310 , and starts the transmission of DL data in the from of Quadrature Phase Shift Keying (QPSK) ½ during transmission of the CQICH_Alloc_IE( ) in the last third frame  312 . 
     After switching to the target BS in the frame  316 , the MS receives CQICH_Alloc_IE( ) from the fourth frame  318 , given after a 2-frame delay from the frame  310  at which the target BS started the transmission of the CQICH_Alloc_IE, until the sixth frame  320 , and starts the reception of DL data in the sixth frame  320 . 
     Because an Adaptive Modulation Codec (AMC) function cannot be enabled before the MS transmits a report on the CQICH, the target BS starts the first modulation for the DL data in the form of QPSK ½ to start the transmission in the default form. Therefore, the target BS needs 3.2 seconds until it first receives a report on the CQICH from the MS. Here, for the CQICH report, about 640 frames and a fixed value of QPSK ½ are used. 
     As described above, a traffic delay of a total of about 6 frames, or 30 ms, occurs for the period in which the serving BS stops the transmission of DL data and the transmission of the DL data is restarted after BS switching to the target BS. This value is much lower than the maximum traffic delay of 100 ms for VoIP, which is one of the stable real-time QoS services. 
       FIG. 4  illustrates an implementation where a serving BS cancels the ongoing handover procedure according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , a serving BS receives I B  from an MS in a first ASR slot of an ASR period  400 , and then receives, in a second ASR slot, C A    302  rather than the expected I B  or receives an unknown or wrong I C    304 . In this case, the serving BS sends Anchor_BS_Switch_IE( ) for commanding a cancellation of the ongoing handover, to the MS in a next frame  306 . 
     As described above, in an exemplary embodiment of the present invention, the handover cancel procedure can be simply realized by merely sending the Anchor_BS_Switch_IE( ) from the serving BS to the MS without the need to transmit a particular message to the target BS. 
       FIG. 5  illustrates a call processing flow for FBSS handover according to an exemplary embodiment of the present invention. Here, the FBSS handover occurs between an MS  500 , a serving BS  510  that includes a modem  512  and a call processing block  514 , a relay station  515 , and a target BS  520  that includes a call processing block  522  and a modem  524 . In an exemplary embodiment, the FBSS handover starts from the time that the serving BS  510  detects a target BS indicator from the MS  500  at a second ASR slot in an ASR period. 
     Referring to  FIG. 5 , in step  526 , the MS  500  sends a CQICH including an indicator of the target BS  520  for the FBSS handover in the second ASR slot. Upon receipt of the CQICH, the modem  512  sends a BS Switching Indication message with an action code=‘0’ to the call processing block  514  in step  528 . The BS Switching Indication message is delivered in the initial process of performing handover in the serving BS  510 , but the action code is set to distinguish the features based on the situations. For example, if the target BS  520  first receives a CQICH report from the MS  500 , the action code is set to ‘2’. 
     In step  530 , the call processing block  514  sends, to the relay station  515 , a BS Switching Indication message to the target BS  520 . The BS Switching Indication message includes the detailed information on the time required for switching from the serving BS  510  to the target BS  520 , i.e., includes a time offset for a BS switching start time. 
     In step  532 , the relay station  515  starts a timer that operates until a BS switching period of the MS  500  expires. The timer is provided for stopping the transmission of DL data in the serving BS  510  for the BS switching period, and for preparing for the missing (or loss) of a DL data transmission indication command to the target BS  520 . That is, when the timer expires and the DL data transmission of the serving BS  510  is stopped, the relay station  515  instructs the target BS  520  to start the DL data transmission. 
     At a transmission stop time  534  for the DL data, the modem  512  stops the transmission of the DL data, and sends a DL Data Transmission Stop message to the call processing block  514  in step  536 . The DL Data Transmission Stop message includes information on the DL data packet number that the serving BS  510  has last transmitted. The DL data packet number is provided to allow the MS  500  to use the intact Traffic Encryption Key (TEK) value for the traffic data, which was used in the serving BS  510 , even after the MS  500  performs handover to the target BS  520 . 
     In step  538 , the call processing block  514  sends a Data Transmission Stop Confirm message to the relay station  515  to indicate the stop of the data transmission by the serving BS  510 . The Data Transmission Stop Confirm message includes a packet number for the final UL/DL transmission data. 
     In step  540 , the relay station  515  sends a DL Data Transmission Start Indication message to the call processing block  522  to instruct the transmission restart for the DL data. The DL Data Transmission Start Indication message includes the DL packet number that the serving BS  510  has last transmitted. 
     In step  542 , if the call processing block  522  sends a DL Data Transmission Restart Command message including the last packet number to the modem  524 , the modem  524  restarts the transmission of the DL data. Upon receiving DL Data Transmission Restart Confirm message from the call processing block  522  in step  544 , the relay station  515  releases the timer in operation in step  546 . 
     Through the foregoing procedure, the BS switching from the serving BS  510  to the target BS  520  is performed, completing handover. 
     Thereafter, upon receiving CQICH_Alloc_IE( ) from the target BS  520  in step  548 , the MS  500  sends a CQICH report to the target BS  520  in step  550 . In step  552 , the modem  524  sends a Handover Complete Indication message to the call processing block  522 . In step  554 , the call processing block  522  sends an Acknowledgement (ACK) message for the CQICH report to the relay station  515 . 
       FIG. 6  illustrates a call processing flow for FBSS handover according to an exemplary embodiment of the present invention. Here, the FBSS handover occurs between an MS  600 , a serving BS  610  that includes a modem  612  and a call processing block  614 , a relay station  615 , and a target BS  620  that includes a call processing block  622  and a modem  624 . 
     In this exemplary case, the serving BS  610  fails to detect target BS indicator transmitted by the MS  600  because the channel state for the radio link in the serving BS  610  is poor. That is, in the state where the serving BS  610  has never recognized the completed execution of FBSS handover, the target BS  620  receives data from the MS  600 . Thereafter, though the serving BS  610  continuously transmits DL data to the MS  600 , the data is missed because the MS  600  has actually moved to the target BS  620 . Therefore, the target BS  620  sends a transmission stop indication for the DL data to the serving BS  610  via the relay station  615 , and then restarts the DL data transmission. 
     Referring to  FIG. 6 , in step  626 , the MS  600  sends a CQICH including an indicator of the target BS  620  to serving BS  610  for an ASR period. Here, as the modem  612  has failed to detect the target BS indicator or has missed the CQICH, the serving BS  610  continuously maintains the state before the occurrence of the handover. 
     Thereafter, if the ASR period expires and the MS  600  moves to the target BS  620  and acquires synchronization therewith, the MS  600  sends CQICH_Allocation_Request_Header for requesting allocation of a CQI channel, to the target BS  620  in step  628 . The modem  624  sends CQICH_Alloc_IE( ) for CQI channel allocation to the MS  600  in step  630 , and sends a CQI Channel Status Report Response message to the call processing block  622  in step  632 . 
     In step  634 , the call processing block  622  sends a DL Data Transmission Stop Request message to the relay station  615 . The DL Data Transmission Stop Request message includes a Media Access Control (MAC) Address value, an address of the MS  600 , to indicate whether the handover is BS switching handover for the MS  600 . In step  636 , the relay station  615  sends a DL Data Transmission Stop Request message to the call processing block  614 . The DL Data Transmission Stop Request message includes a Basic Connection Identifier (CID) of MS and BS_ID information, based on which the serving BS  610  will stop the data transmission in the corresponding sub-cell. In step  638 , the call processing block  614  sends a DL Data Transmission Stop command message with the Basic CID of MS and the BS_ID to the modem  612 . 
     Thereafter, the modem  612  stops the transmission of DL data, and sends a DL Data Transmission Stop message to the call processing block  614  in step  640 . The DL Data Transmission Stop message includes the DL data packet number information that the serving BS  610  has last transmitted. 
     In step  642 , the call processing block  614  sends a Data Transmission Stop Confirm message to the relay station  615  to indicate the stop of the data transmission by the serving BS  610 . The Data Transmission Stop Confirm message includes the last transmitted UL/DL data packet number. 
     In step  644 , the relay station  615  sends a DL Data Transmission Start Indication message to the call processing block  622  to instruct the transmission restart of the DL data. The DL Data Transmission Start Indication message includes the DL packet number that the serving BS  610  has last transmitted. 
     In step  646 , if the call processing block  622  sends a DL Data Transmission Restart Command message with the last packet number to the modem  624 , the modem  624  restarts the transmission of the DL data. Also, if the call processing block  622  sends the DL Data Transmission Restart Command message to the modem  624 , then in step  648  the call processing block  622  sends a DL Data Transmission Restart Confirm message to the relay station  615 . Thereafter, upon receipt of CQICH_Alloc_IE( ) from the target BS  620 , the MS  600  sends a CQICH report to the modem  624  in step  650 . In step  652 , the modem  624  sends a Handover Complete Indication message to the call processing block  622 . In step  654 , the call processing block  622  sends an ACK message for the CQICH report to the relay station  615 . 
       FIG. 7  illustrates a configuration of a communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 7 , the communication system includes an MS  700  intending to perform FBSS handover, a serving BS  710  that includes a modem  712  and a call processing block  714 , a relay station  720 , and a target BS  730  that includes a modem  732  and a call processing block  734 . 
     The MS  700  sends an indicator of the target BS  730 , to which it will perform FBSS handover, to the serving BS  710  over a CQICH. Upon normally receiving information on the indicator received from the MS  700  in the second ASR frame of the ASR period, the serving BS  710  continuously performs the handover to the target BS  730  according to the procedures of  FIGS. 3 and 5  based on an exemplary embodiment. A detailed description of the procedures has been made above. Upon failure to normally receive the information on the indicator received from the MS  700  in the second ASR frame, the serving BS  710  cancels the handover to the target BS  730  according to the procedures of  FIGS. 4 and 6  based on an exemplary embodiment. 
     As is apparent from the foregoing description, the present invention uses the optimized value of the time information for BS switching, thereby minimizing the DL transmission delay time for the traffic data, which may occur during handover. In addition, exemplary embodiments of the present invention can prevent the possible deviation for the BS switching period between the MS and the system, and remarkably reduce the number of various abnormal cases possibly caused by the deviation, thereby facilitating maximized stability of the system that performs handover. 
     While the invention has been shown and described with reference to certain exemplary embodiments 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 and their equivalents.