Abstract:
A mobile station, that includes a receiving means for receiving first PDCP PDU from a source base station and a receiving means for receiving second PDCP-PDU from a target base station, in which the second PDCP-PDU being created using sequence number and PDCP SDU that are transferred from the source base station to the target base station and a storage means for storing PDCP SDU conforming to the first PDCP PDU and the PDCP SDU conforming to the second PDCP PDU; and a reordering means for performing in order delivery of the stored PDCP SDUs based on the sequence numbers.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a Divisional of U.S. application Ser. No. 12/603,125, filed Oct. 21, 2009, now pending, which is a continuation of International Application No. PCT/JP2007/059037, which was filed on Apr. 26, 2007, the contents of each are herein wholly incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a base station, mobile station, communication system, transmission method and reordering method. 
         [0003]    Currently, for mobile communication systems such as mobile telephones, third-generation type service using CDMA has begun, however, research of a next generation mobile communication system (LTE: Long Term Evolution) in which even faster communication is possible is being advanced by the 3GPP (3rd Generation Partnership Project) (refer to 3GPP, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTAN),” TR25.913 V7.3.0, Release 7, March 2006). Some of the large challenges in this research include increasing the transmission rate and reducing transmission delay. 
         [0004]    In the LTE communication system, in order to increase the transmission rate and reduce transmission delay, the handover method used is designed to be at a higher level than that of a conventional system. In a mobile communication system, when a mobile station moves during communication, the base station that the mobile station is communicating with is switched (handover) according to the reception status. Therefore, in order to perform communication at increased transmission speed and with low transmission delay, improving the level of the handover is essential. In a LTE communication system, a packet exchange system is basic, so handover is a hard handover. In a hard handover, after the line connection with the base station that the mobile station communicates with before moving is cut, a line is connected between the mobile station and a target base station. In a hard handover, by obtaining system information about the target base station immediately before performing the handover, it is possible to perform the handover in a short time; however, transmission of user data becomes interrupted during the handover. 
         [0005]    Therefore, in order to reduce transmission delay it is important that the state of interrupted transmission be shortened and that the loss of packets while transmission is interrupted be prevented. In the case that packets do become lost while transmission is interrupted, the lost packets are recovered in end-to-end retransmission of the packets, so transmission delay becomes large. 
         [0006]    Therefore, in a handover in a LTE communication system, a method is specified in which among data that includes control information and packets for the mobile station, the transmission of packets that have not yet been transmitted to the mobile station from the source base station is taken over by forwarding those packets to the target base station from the source base station (refer to 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN),” TS36.300, Release 8, V8.0.0, April 2007). 
         [0007]    •Takeover During a Handover 
         [0008]      FIG. 21  is a drawing explaining takeover during a handover. In (A) of  FIG. 21 , two base stations  1   a  and  1   b  are connected to a host station (for example a MME/SAE gateway)  2 . A mobile station  4  exists within the cell  3   a  of the base station  1   a , and is currently communicating with the base station  1   a . In this state, as shown in (B) of  FIG. 21 , when the mobile station  4  moves in the direction toward the base station  1   b  and enters into the cell  3   b , a handover is executed and the base station with which the mobile station  4  communicates is switched from the base station  1   a  to the base station  1   b . Here, a base station that is in communication with a mobile station before the mobile station moves is called the source base station, and the base station that communicates with the mobile station after the mobile station moves is called the target base station. 
         [0009]    The source base station  1   a  stores packets that were received from the host station  2  in an internal buffer, and sequentially sends the packets that are stored in that buffer to the mobile station  4 . Therefore, when a handover occurs, the packets that are not sent to the mobile station are stored and exist in the buffer. In (B) of  FIG. 21 , it is necessary that before a handover, the packets n−2 to n be received and stored in the buffer without being sent to the mobile station, and that after the handover it is necessary that these packets be sent to the mobile station  4  from the target base station. Therefore, when executing the handover sequence, the source base station  1   a  transfers (forwards) the packets n−2 to n to the target base station  1   b . By using this forwarding method, the target base station sends those packets to the mobile station  4  immediately after the handover, so no interruption in the packets occurs. Therefore, it becomes possible to execute high-speed handover without performing end-to-end retransmission of the packets. In the explanation above, n−2 to n are numbers (sequence numbers) that indicate the order of the packets. 
         [0010]    •Handover 
         [0011]      FIG. 22  is a drawing explaining a handover in a LTE communication system, and  FIG. 23  is a drawing explaining the handover procedure that is currently presumed for a LTE communication system. 
         [0012]    Using a Measurement Report (report of the reception status of the base station  1  and other surrounding base stations), the mobile station  4  notifies the source base station  1  that a handover HO (Handover) is necessary (1. Measurement Control). 
         [0013]    The source base station  1  decides a target base station  1   b  according to the contents of the Measurement Report (2. HO Decision), and sends a handover request (3. Handover Request) to that target base station  1   b . At that time, the source base station  1   a  also sends information about the mobile station (mobile station ID, QoS (Quality of Service) information, etc.). The target base station  1   b  performs call-receiving control based on that information (4. Call-Receiving Control). 
         [0014]    After allowing the acceptance of the mobile station, the target base station  1   b  returns a handover response to the source base station (5. HO Response). After that, the source base station  1   a  sends a handover instruction to the mobile station  4  (6. HO Instruction), then immediately afterwards starts taking over the data (packets) (Packet transfer: Forwarding). 
         [0015]    The mobile station  4  receives the handover instruction, and then obtains synchronization using L1/L2 signaling (7. Synchronization), and after synchronization has been obtained, sends a handover complete report to the target base station  1   b  (8. HO Complete). 
         [0016]    After this, the target base station  1   b  sends a handover complete report to the host station  2  (9. HO Complete). After receiving the handover complete report, the host station  2  changes the packet transmission path from the source base station  1   a  to the target base station  1   b  (10. Path Change), and returns a HO complete response to the target base station  1   b  (11. HO Complete Response). The target base station  1   b  uses a HO complete response to notify the source base station  1   a  that the handover HO is complete (12. Resource Release). After that, the path between the source base station  1   a  and the host station  2  is eliminated (13. Resource Release). 
         [0017]    •Packet Order Alignment Control 
         [0018]    When packets are forwarded (forwarded) during execution of the handover sequence described above, there is a possibility that packets that are transferred by the target base station  1   b  will be jumped over by the packets that flow from the host station  2  resulting in the sequence numbers becoming mixed up. When packets are transferred from the target base station  1   b  to the mobile station  4  as are with the sequence numbers mixed up, the mobile station  4  is unable to receive the packets in the correct order, so the quality of communication is degraded, and as a result high-quality communication cannot be achieved just before and after a handover. 
         [0019]    Therefore, in the LTE communication system, by using a method as described below, the base station and mobile station are capable of maintaining packet order.  FIG. 24  is a drawing explaining packet order alignment wherein the target base station  1   b  maintains the packet order by sending packets that are transferred from the source base station  1   a  with higher priority than packets that are received from the host station. 
         [0020]    In  FIG. 24 , before a handover, packets n−5 to n are stored in the source base station  1   a , and of these packets, packets n−5 to n−2 are sent to the mobile station  4 , however, packets n−1 and n are not sent to the mobile station  4 . In addition, of the packets that are sent to the mobile station  4 , packets n−5 and n−3 are not rightly received by the mobile station  4  (NACK), and packets n−4 and n−2 are rightly received (ACK). Therefore, the mobile station  4  save the packets n−4 and n−2 in a buffer BF 1 , and does not save the packets n−5 and n−3. 
         [0021]    When a handover occurs in this state, the source base station  1   a  transfers (forwards) the packets n−5 and n−3, which were not rightly received by the mobile station  4 , and the packets n−1 to n, which have not yet been sent to the mobile station  4 , to the target base station  1   b , and the target base station  1   b  stores those packets in a buffer BF. Also, after the handover, the host station  2  sends two packets m to m+1 that are intended for the mobile station  4  to the target base station  1   b , and the target base station  1   b  stores those packets in the buffer BF. 
         [0022]    After communication with the mobile station  4  becomes possible, the target base station  1   b  first sends the packets n−5, n−3 and n−1 to n that were forwarded from the source base station  1   b  to the mobile station  4 . Next, the target base station  1   b  sends the packets m to m+1 that were received from the host station  2 . As shown in  FIG. 25 , the mobile station  4  rearranges the sequence order of the packets n−4 and n−2 that were received before the handover, and the packets n−5, n−3, n−1 and m to M+1 that were received after the handover, and gives those packets in order to an upper layer. 
         [0023]    In the explanation above, the case was explained in which all of the packets n−5, n−3 and n−1 to n were transferred (forwarded) to the target base station  1   b , however, in some cases, only the packets n−5 and n−3 will be transferred, and transferring of packets n−1 to n will be delayed.  FIG. 26  is a drawing explaining packet order arrangement in that case. The target base station  1   b  stores the transferred packets n−5 and n−3 together with the packets m to m+1 that come from the host station  2  in a buffer BF, however, sends the packets n−5 and n−3 that were forwarded from the source base station  1   a  to the mobile station  4  first. After that, in the case that packets n−1 and n are forwarded from the source base station  1   a  later, the target base station  1   b  monitors whether a set time (waiting time) has elapsed, and when the packets n−1 and n still have not been forwarded from the source base station  1   a  even after the waiting time has elapsed, the target base station  1   b  determines that forwarding has been completed and sends the packets m and m+1 that were received from the host station to the mobile station to the mobile station. Even though the packets n−1 and n may be received from the source base station  1   a  after forwarding has been completed, the target base station  1   b  discards those packets. 
         [0024]    The mobile station  4  executes a process for rearranging the order of the sequence numbers of the received packets (reordering). As shown in  FIG. 27 , the mobile station  4  rearranges the sequence numbers of the packets n−4 and n−2 that were received before the handover, and the packets n−5, n−3, m and m+1 that were received after the handover, and gives those packets in order to an upper layer. 
         [0025]    •Protocol Configuration 
         [0026]    As described above, in a handover in a LTE communication system, packet transfer (forwarding) and packet reordering are necessary techniques. The relationship between these functions will be explained in more detail here. 
         [0027]      FIG. 28  is a drawing explaining the protocol configuration between a mobile station and a base station. Between a mobile station and a base station there is at least a PDCP (Packet Data Convergence Layer) layer, RLC (Radio Link Control) layer and a lower layer (MAC layer/physical layer MAC/PHY). A packet routing function or the like is provided in MME/S-GW. 
         [0028]    The main features of each protocol are as described below. 
         [0029]    (1) PDCP: In the PDCP layer, the transmitting side compresses the upper protocol header, as well as attaches a sequence number and performs transmission. The receiving side checks the sequence number, and by doing so, performs a discarding process for redundant reception. Retransmission is not performed in the PDCP layer. 
         [0030]    (2) RLC: The RLC layer is a layer having a retransmission function, and in the RLC layer, a sequence number is newly attached to the data that is different from the sequence number that is attached to the data from the PDCP layer, and the data is then transmitted. For example, when data having a sequence number n is received from the PDCP layer, that data is divided into a plurality of data, and to each division of data, sequence numbers I( 1 ), I( 2 ), I( 3 ), . . . are attached in the RLC layer, after which the data is transmitted. The receiving side notifies the sending side by using those sequence numbers I(•) to send a transmission confirmation (Ack/Nack signal) indicating whether the data were received rightly or improperly. When an Ack signal is returned, the sending side deletes the data that is saved, however, when a Nack signal is returned, the sending side retransmits the saved data. 
         [0031]    (3) Lower Layer
       MAC: The MAC layer is a layer that multiplexes/demultiplexes the data of the RLC layer. In other words, the sending side multiplexes the data of the RLC layer and transmits the data, and the receiving side demultiplexes the received data of the MAC layer to RLC layer data.   PHY: The PHY layer is a layer for transmitting and receiving data by radio signals between the user terminal  4  and base station  1 , and converts MAC layer data to radio data, or converts radio data to MAC layer data.       
 
         [0034]    Data destined for a mobile station first flows from the upper layer (for example IP layer) to the PDCP layer to become a PDCP SDU (Service Data Unit), then header information (PDCP layer sequence number, etc.) is attached to become a PDCP PDU (Protocol Data Unit). 
         [0035]    The PDCP PDU is sent to the RLC to become a RLC SDU, where header information (RLC layer sequence number, etc.) is further attached to become a RLC PDU. The RLC PDU passes through the lower layer processing after which it arrives at the RLC layer of the mobile station. In this RLC layer, the header is deleted and the RLC SDU is reassembled, then in the PDCP layer, the PDCP PDU header is deleted to become a PDCP SDU, then the data is sent to the upper layer. 
         [0036]    In this kind of protocol configuration, in a LTE communication system, the forwarding of packets is performed in PDCP SDU units, and reordering is performed in PDCP PDU units. When forwarding is performed in PDCP SDU units, header information such as a sequence number is not attached to the PDCP SDU unit packet, so that sequence number is not forwarded. 
         [0037]    Therefore, in a case where the forwarding is performed in PDCP SDU units, it is necessary to forward PDCP SDU data and harder information including the sequence number separately. 
         [0038]    The RLC SDU data and PDCP PDU data are essentially the same data, so in the specification for the present invention, unless specifically specified, these will simply be called packets, and when a packet number is provided, that number is the sequence number of the PDCP PDU data. 
         [0039]    •Operation of the Source Base Station 
         [0040]      FIG. 29  is a flowchart of the operation of a source base station during a handover. 
         [0041]    When the source base station  1   a  receives the reception field strength from the user terminal  4  by way of a Measurement Report (step  101 ), the source base station  1   a  determines whether or not a handover HO is necessary (step  102 ), and when a handover is not necessary, returns to the beginning. 
         [0042]    However, when it is determined that a handover is necessary, the source base station  1   a  decides a target base station  1   b  according to the contents of the Measurement Report, and sends a handover request to that target base station  1   b  (step  103 ). 
         [0043]    After that, the source base station  1   a  receives a handover response that is sent from the target base station  1   b  (step  104 ), and forwards the remaining packets to a target base station (step  107 ). 
         [0044]    After receiving a resource release message that is sent from the target base station  1   b  (step  108 ), the source base station  1   a  executes the resource release (step  109 ). 
         [0045]    •Operation of the Target Base Station 
         [0046]      FIG. 30  is a flowchart of the operation of a target base station during a handover. 
         [0047]    After receiving a HO request from the source base station  1   a  (including the mobile station ID, QoS information, etc.) (step  121 ), the target base station  1   b  performs call receiving control based on that information, and determines whether to allow the acceptance of the mobile station (step  122 ), and when the mobile station is not allowed, performs post processing (step  130 ) and ends handover control. 
         [0048]    On the other hand, when acceptance of the mobile station is allowed, the target base station  1   b  returns a HO response to the source base station  1   a  (step  123 ). The target base station  1   b  then stores the packets that are forwarded from the source base station  1   a  in a buffer (step  124 ), and receives a HO complete report from the mobile station  4  (step  125 ). After receiving the HO complete report, the target base station  1   b  sends a HO complete report to the host station  2  (step  126 ). The host station  2  receives the handover complete report, then changes the packet transmission path from the source base station  1   a  to the target base station  1   b , and returns a HO complete response to the target base station  1   b . After receiving the HO complete response from the host station  2  (step  127 ), the target base station  1   b  starts sending the packets that were forwarded from the source base station  1   a  to the mobile station preferentially, and after those packets have been sent, sends the packets that were received from the host station  2  to the mobile station (scheduling: step  128 ). In addition, at the same time as step  128 , the target base station  1   b  sends a resource release to the source base station  1   a  (step  129 ), then performs post processing (step  130 ) and ends handover control. In the scheduling process of step  128 , when forwarding of packets from the source base station  1   a  is delayed, the target base station  1   b  monitors whether a set time (Waiting Time) has elapsed, and when no packets have been transferred even though the Waiting Time has elapsed, the target base station  1   b  determines that forwarding has finished and sends all the packets that were received from the host station  2 , and even though packets may be received from the source base station  1   a  after forwarding has finished, the target base station  1   b  discards those packets. 
         [0049]    •Operation of a Mobile Station 
         [0050]      FIG. 31  is a flowchart of the operation of a mobile station during a handover. 
         [0051]    The measurement unit of the mobile station  4  sends a notification of the reception field strength or the like to the source base station by way of a Measurement Report (step  151 ). After that, the mobile station  4  waits for a HO instruction from the source base station  1   a , and after receiving a HO instruction (step  152 ), obtains synchronization with the target base station  1   b  using L1/L2 signaling (step  153 ), and after synchronization has been obtained, sends a handover complete report to the target base station  1   b  (step  154 ), then, in the case where packets are received from the target base station  1   b , the mobile station  4  executes a reordering process (steps  155  to  160 ). 
         [0052]    In other words, when the control unit of the mobile station receives lower layer packets from the target base station  1   b , the control unit creates RLC SDU data and delivers that RLC SDU (PDCP PDU) data to a reordering unit (step  155 ). The reordering unit checks whether there are any missing sequence numbers (step  156 ), and when there are no missing sequence numbers and the sequence numbers are continuous, delivers PDCP SDU data obtained by removing a header from the RLC SDU (PDCP PDU) data to the upper layer (step  160 ). However, when there is a missing sequence number, the control unit instructs the reordering unit to save the RLC SDU (PDCP PDU) data. By doing so, the reordering unit saves the PDCP PDU data (step  157 ), and checks whether RLC SDU (PDCP PDU) data having a continuous sequence number has been received (step  158 ). When RLC SDU (PDCP PDU) data having a continuous sequence number has been received, the reordering unit delivers PDCP SDU data obtained by removing a header from the RLC SDU (PDCP PDU) data to the upper layer, a well as delivers PDCP PDU data obtained by removing a header from the saved PDCP PDU data to the upper layer (step  160 ). 
         [0053]    However, in step  158 , when RLC SDU (PDCP PDU) data having a continuous sequence number is not received, the reordering unit monitors whether a preset amount of time has elapsed (step  159 ), and when that set amount of time has not elapsed, repeats the process from step  157 ; however, when the set time has elapsed, the reordering unit delivers PDCP SDU data obtained by removing a header from the saved PDCP PDU data to the upper layer even though the sequence numbers may not be continuous (step  160 ). 
         [0054]    •Problems 
         [0055]    In a LTE communication system, the following problems exist when executing packet forwarding during a handover. That is, as was described above, when a handover is executed in a LTE communication system, a process for taking over the packet for the mobile station that remains at the source base station  1   a  is executed, and packets are forwarded to a target base station during that taking over process. However, in the handover control described above, if the value of the waiting time by the target base station  1   b  is small, the target base station  1   b  begins sending the packets received from the host station regardless of whether or not all of the packets have been forwarded, so a problem occurs in that the packets that have not yet been forwarded are discarded. On the other hand, if the value of the waiting time is large, there is a problem in that the target base station  1   b  cannot send the packets received from the host station until the waiting time has elapsed even though all of the packets may have already been forwarded, so a transmission delay occurs. In other words, in conventional handover control there is an increase in communication delays and throughput is degraded, making it impossible to maintain high-quality communication immediately before and after a handover. 
         [0056]    As first related art is a method of notifying the target base station of the last packet to be forwarded from the source base station (refer to Samsung, “Method to Release Resources at Source ENB During Handover,” R3-061032, RAN3#53, September 2006). When forwarding is delayed, the target base station that has been notified of the last packet can transmit the packets received from the host station by starting the sequence numbers of those packets from the sequence number of the last packet+1. In addition, by comparing the sequence numbers of the packets that are forwarded with the sequence number of the last packet, it is possible to forcibly end the waiting time and to detect the end of forwarding with optimum timing. However, in the case that the last packet is deleted during forwarding, the target base station will be unable to accurately detect the last packet. 
         [0057]    Moreover, as second related art is a mobile communication system for making high-speed packet data transfer possible with no data loss during a handover between base stations during high-speed packet communication (refer to Japanese patent publication No. JP2004-282652A). When a handover occurs in this mobile communication system, the source base station of the handover transfers (forwards) packet data to the target base station of the handover. However, there is an increase in communication delay due to reordering by the mobile station, and there is no improvement in the degraded throughput. 
         [0058]    Furthermore, as third related art is a method in which the target base station jumps the packets sent from the host station without waiting for forwarded packets to arrive (refer to Japanese patent application No. 2006-086537A). In this method, by distinguishing the packets that are received from the source base station from the packets that are received from the host station, transmission is possible in which the packets jump. However, it is necessary for the mobile station to have two order control functions, and the control thereof becomes complicated. 
       SUMMARY OF THE INVENTION 
       [0059]    Taking into consideration the aforementioned problems, it is the object of the present invention to quickly transmit packets that are sent from a host station (for example, a device that is different from a source base station, and that sends data (packets) to a target base station) to a target base station to a mobile station. 
         [0060]    Another object of the present invention is to properly reorder forwarded packets even when there is only one order-control function and even when packets that are forwarded from a source base station are mixed with packets that are sent from the host station by excluding the packets that are sent from the host station as the object of order control. 
         [0061]    Providing items that are described in the embodiments that are not described in the related art can also be considered as another object of the present invention. Preferably, such items are necessary for obtaining advantages that cannot be obtained with the related art. 
         [0062]    •Reordering Method 
         [0063]    The reordering method of the present invention is a reordering method of sending data (for example, packets), to which information (for example, numbers) that indicates the order of the packets is added, from a target base station to a mobile station, and rearranging the data (packets) at the mobile station according to this order information. 
         [0064]    In the reordering method of the present invention, the target base station correlates discriminating information which makes it possible to discriminate data that is forwarded from a source base station from data that is not obtained via the source base station, with data that is sent to the mobile station, and transmits the discrimination information with the data from the target base station to the mobile station. Preferably, in performing the reordering process, the mobile station discriminates the data forwarded from the base station from the data that is not obtained via the source base station based on the discrimination information. Moreover, preferably, the data that is not obtained via the source base station is data that is received from a gateway device. Furthermore, preferably, the data that is sent to the mobile station is in the form of packets. 
         [0065]    The reordering method of the present invention, comprises: a step of forwarding packets, which has not yet been sent to the mobile station, or data, for which a confirmation response of right reception has not been received from the mobile station, to a target base station from a source base station; sending that data to the mobile station from the target base station, as well as sending data that is received from a host station to the mobile station; a step of, in the case of sending data received from the host station with the order jumped, giving the data information indicating that the data is jump data, then sending the data from the target base station to the mobile station; and a step of deciding the jump data is not the object of reordering and performing the reordering process on data that is the object of reordering. 
         [0066]    Another reordering method of the present invention comprises: a step of forwarding data that has not yet been sent to the mobile station or data for which a confirmation response of proper reception has not been received from the mobile station to a target base station from a source base station; a step of sending data that is forwarded from the source base station before a first set period of time ends and data received from a host station from the target base station to the mobile station; a step of discarding data that is forwarded from the source base station after the first time period has ended; a step of adding information to data to specify the data is the last data that is object of reordering from among data that was received before the first time period ends or adding the information to other data that is sent immediately after that data and sending the data from the target base station to the mobile station and a step of performing reordering process until a second time period ends, and when the data that contains the aforementioned information is received, ending the reordering process even though the second time period has not yet ended. 
         [0067]    The reordering method described above, also preferably comprises: a step of giving packets information indicating that the packets are jump packets when sending packets to the mobile station that were received from a gateway with the order jumped and sending the packets to the mobile station from the target base station; and a step of deciding those jump packets are not objects of reordering, and performing the reordering process on packets that are the objects of reordering at the mobile station. 
         [0068]    •Communication System 
         [0069]    The present invention is a communication system in which data (for example, packets), to which information that indicates the order of the data is added, is sent from a target base station to a mobile station, and the data is rearranged in number order at the mobile station. 
         [0070]    The communication system of the present invention comprises a mobile station, a source base station that communicates with the mobile station before a handover, and a target base station that communicates with the mobile station after a handover; where (1) the source base station comprises: a buffer that saves data that was received from a host station; a data transmission unit that transmits the data that is saved in the buffer to the mobile station; and a control unit that forwards data that was not sent to the mobile station before execution of a handover sequence, or data for which a confirmation response of right reception has not been received from the mobile station to the target base station; (2) the target base station comprises: a buffer that saves data that is received from the source base station during execution of a handover sequence and data that is received from the host station; a control unit that performs control so that data that is received from the source base station is sent preferentially to the mobile station, and when sending data to the mobile station that is received from the host station with the order jumped, adds information to the data to indicate that the data is jump data; and a transmission unit that transmits data to the mobile station; and (3) the mobile station comprises: a buffer that saves data that is received from a base station; and a reordering control unit that decides the jump data is not an object of reordering, and performs reordering on data that is the object of reordering. 
         [0071]    Another communication system of the present invention comprises: a mobile station, a source base station that communicates with the mobile station before a handover, and a target base station that communicates with the mobile station after a handover; where (1) the source base station comprises: a buffer that saves data that is received from a host station; a data transmission unit that transmits the data that is stored in the buffer to the mobile station; and a control unit that forwards data that is not sent to mobile station before execution of a handover sequence, or data for which a confirmation response of right reception has not been received from the mobile station, to the target base station; (2) the target base station comprises: a buffer that saves data that is received from the source base station during execution of a handover sequence and data that is received from the host station; a control unit that performs control to send data that is forwarded from the source base station before a first set period of time ends to the mobile station preferentially as the object of reordering, as well as adds information to data to indicated that the data is last data received as an object of reordering before the first set period of time ends, or adds the information to other data that is transmitted immediately after that data and sends the data from the target base station to the mobile station, and discards data that is forwarded from the source base station after the first set period of time has ended; and a transmission unit that transmits data to the mobile station; and (3) the mobile station comprises: a buffer that saves data that is received from the base station; and a reordering control unit that performs reordering until a second set period of time ends, and when data that contains information identifying that data is the last data that is an object of reordering, is received or when data that contains information indicating that the last data that is an object of reordering has already been received is, is received, ends the reordering process even though the second set period of time has not yet ended. 
         [0072]    •Base Station/Mobile Station 
         [0073]    Another form of the present invention is a base station and a mobile station that form a first or second communication system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0074]      FIG. 1  is a drawing explaining a first embodiment of the invention. 
           [0075]      FIG. 2  shows an example of the format of a PDCP PDU packet. 
           [0076]      FIG. 3  is a drawing explaining packet processing in the PDCP layer and RLC layer before a handover sequence. 
           [0077]      FIG. 4  is a drawing explaining packet processing in the PDCP layer and RLC layer during a handover sequence. 
           [0078]      FIG. 5  is a drawing showing the construction of a base station. 
           [0079]      FIG. 6  is a drawing showing the construction of a mobile station. 
           [0080]      FIG. 7  is a flowchart of the operation of a target base station in a first embodiment of the invention. 
           [0081]      FIG. 8  is a flowchart of the operation of a source base station in a first embodiment of the invention. 
           [0082]      FIG. 9  is a flowchart of the operation of a mobile station in a first embodiment of the invention. 
           [0083]      FIG. 10  is a flowchart of the reordering process by a mobile station. 
           [0084]      FIG. 11  is a drawing explaining a second embodiment of the invention. 
           [0085]      FIG. 12  shows an example of the format of a PDCP PDU packet. 
           [0086]      FIG. 13  is a drawing explaining packet processing in the PDCP layer and RLC layer during a handover (1/2). 
           [0087]      FIG. 14  is a drawing explaining packet processing in the PDCP layer and RLC layer during a handover (2/2). 
           [0088]      FIG. 15  is a flowchart of the operation of a target base station in a second embodiment of the invention. 
           [0089]      FIG. 16  is a flowchart of the operation of a mobile station in a second embodiment of the invention. 
           [0090]      FIG. 17  shows an example of attaching PDCP PDU sequence numbers SN to PDCP SDU data and performing notification via the data plane (U-plane). 
           [0091]      FIG. 18  shows an example of attaching PDCP PDU sequence numbers SN to PDCP SDU data and performing notification via the U-plane, as well as performing notification of sequence numbers via the C-plane. 
           [0092]      FIG. 19  is a drawing showing the procedure of the handover sequence in  FIG. 18 . 
           [0093]      FIG. 20  shows an example when there is absolutely no PDCP PDU data to forward. 
           [0094]      FIG. 21  is a drawing explaining the takeover of communication during a handover. 
           [0095]      FIG. 22  is a drawing explaining a handover in a LTE communication system. 
           [0096]      FIG. 23  is a drawing explaining the handover procedure that is currently presumed for a LTE communication system. 
           [0097]      FIG. 24  is a first drawing explaining the reordering process by a mobile station. 
           [0098]      FIG. 25  is a second drawing explaining the reordering process by a mobile station. 
           [0099]      FIG. 26  is a third drawing explaining the reordering process by a mobile station. 
           [0100]      FIG. 27  is a fourth drawing explaining the reordering process by a mobile station. 
           [0101]      FIG. 28  is a drawing explaining the protocol configuration between a mobile station and a network. 
           [0102]      FIG. 29  is a flowchart of the operation of a source base station during a handover. 
           [0103]      FIG. 30  is a flowchart of the operation of a target base station during a handover. 
           [0104]      FIG. 31  is a flowchart of a mobile station during a handover. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (A) Theory of the Present Invention 
       [0105]    With the present invention, the problems described above are solved by making it possible for a base station and a mobile station to execute the two following procedures. 
         [0106]    Procedure 1: When there is a delay in data forwarding from the source base station after a handover, the target base station sends data that has already been received from the host station without waiting to receive the delayed data (jump transmission) and the mobile station is made to be able to recognize that the data. In other words, discrimination information is included in, attached to or correlated with the data in order to recognize that the data is jump data, and transmitted with the data or transmitted using a control channel. 
         [0107]    Procedure 2: When it is detected that jump transmission has occurred, the mobile station decides the jump data is not the object of reordering and holds it in a buffer, and the mobile station waits for the data being forwarded from the source base station to arrive. In other words, the mobile station discriminate data that is forwarded from the source station from data that is transmitted without going through the source base station using the discrimination information and performs reordering of the data that is forwarded from the source station. 
         [0108]    In the conventional method, when data forwarding from the source base station to the target base station is delayed, the mobile station must wait transmission of the data that is forwarded from the host station to the target base station. Therefore, by waiting until a specified amount of time (Waiting Time) has elapsed for that data from the source base station to arrive, a problem occurs in that delay in communication increases and the throughput is degraded. However, as described above, by performing jump transmission of data, it is possible to quickly send data that was received from the host station to the mobile station even when data transfer (forwarding) from the source base station is delayed, and in doing so, communication delay is reduced. Therefore, when compared with the conventional method, the present invention is able to maintain high-quality communication immediately before and after a handover. 
       (B) First Embodiment 
       [0109]      FIG. 1  is a drawing explaining a first embodiment of the invention, and in this embodiment, the case of adding order information to each packet is explained, however, it is also possible to use data having a specified size. 
         [0110]    Here it is presumed that before a handover, packets n−5 to n are stored in the source base station  11   a , and of these packets, packets n−5 to n−2 are transmitted to the mobile station  14 , however, packets n−1 and n are not transmitted to the mobile station  14 . For example, the packets n−1 and n arrived at the source base station  11   a  after the radio communication line between the source base station  11   a  and mobile station  14  was cut, so these packets n−1 and n could not be transmitted to the mobile station  14 . Also, of the packets that were transmitted to the mobile station  14 , it is presumed that the mobile station  14  could not rightly receive the packets n−5 and n−3 (NACK), however was able to rightly receive the packets n−4 and n−2 (ACK). Therefore, the mobile station  14  saves packets n−4 and n−2, but does not save packets n−5 and n−3. 
         [0111]    When a handover occurs in this state, the source base station  11   a  transfers (forwards) the packets n−5 and n−3 that could not be rightly received by the mobile station  14  and the unsent packets n−1 and n to the target base station  11   b . The forwarding of the packets will be explained below, however, the invention is not limited to this example. 
         [0112]    In addition, after the handover, the host station  12  transmits two packets m to m+1 that are destined for the mobile station  14  to the target base station  11   b . It is presumed that the transfer (forwarding) of packets n−5 to n is delayed. 
         [0113]    When that the target base station  11   b  receives packets m and m+1 from the host station  12  before packets n−5, n−3, and n−1 to n are forwarded from the source base station  11   b , the target base station  11   b  adds jump ID code F to the packets m and m+1 that are received from the host station  12  and sends those packets to the mobile station  14  first (jump transmission). 
         [0114]    The mobile station  14  saves the packets that were received from the base station and to which jump ID code F is added in a buffer BF 2 , and excludes those packets as objects of the reordering process. In (A) of  FIG. 1 , the state is shown in which the mobile station  14  saves packets m and m+1 in the buffer BF 2 , and saves the packets n−4 and n−2 that were received before the handover in buffer BF 1 . 
         [0115]    Next, the target base station  11   b  transmits the packets n−5, n−3 and n−1 to n that were forwarded from the source base station  11   a  to the mobile station  14 . The mobile station  14  saves the packets n−5, n−3 and n−1 to n that were received from the target base station  11   b  in the buffer BF 1 , then executes the reordering process for these packets that were received after the handover and the packets n−4 and n−2 that were received before the handover (see (B) of  FIG. 1 ), and delivers the packets to the upper layer in the order of continuous sequence numbers. 
         [0116]    In a case where the mobile station  14  do not receive packets having continuous sequence number even when a prescribed time has been elapsed, the mobile station ends reordering process, and rearranges the order of the packets that have been already received and delivers them to the upper layer. 
         [0117]    The mobile station  14  then delivers the packets to which the jump ID code F has been added in order to the upper layer. During forwarding, a source base station can transfer an entire packet to a target base station, or can transfer data of just part of the packet (user data portion). Preferably, order information is added to the forwarded data. 
         [0118]    In addition, in  FIG. 1 , sequence numbers (order information) m, m+1 are assigned to the packet for which jump transmission is performed, however, the target base station  11   b  can assign arbitrary sequence numbers, can add numbers that overlap the sequence numbers that are added to the forwarded packets, or can add numbers that do not overlap. 
         [0119]    •Jump ID Code 
         [0120]    As an example of jump ID code F that is added to packets, a 3-bit ‘type’ field that is included in the PDCP PDU header is used. In other words, in that type field, a new type number is defined as jump ID code F, and that type number is added to a packet for which jump transmission is performed.  FIG. 2  is an example of the PDCP PDU format, where (A) is an example of format when there is no header, (B) is an example of format when the PDCP PDU sequence number is not added, and (C) and (D) are examples of format when the PDCP PDU sequence number is added. In the format of (C) and (D), a type field and PID field are defined in the header HD, where the type field indicates the PDCP PDU type. The PID field is a field that indicates the header compression type that is used for the data included in the data portion. In the type field, ‘type=000’ and ‘type=001’ are already regulated, however, the types ‘type=010 to 111’ are not regulated and are unused. Therefore, ‘type=010’ is used as the type number (jump ID code) for discriminating the PDCP PDU for which jump transmission is performed. 
         [0121]    •PDCP Layer and RLC Layer Processing Before and After Handover Control 
         [0122]      FIG. 3  is a drawing explaining the packet processing of the PDCP layer and RLC layer before a handover. The source base station  11   b  stores packets n−5 to n−2 of the PDCP layer in a buffer ((A) of  FIG. 3 ), and in the RLC layer the packets are divided into a plurality of data as shown in (B), then RLC layer sequence numbers I, I+1, I+2, . . . , I+6 are added to the divided data, and the data (RLC PDU data) are sent to the mobile station  14 . Before a handover, the mobile station performs order control for the PDCP layer using RLC SDU(PDCP PDU) and PLC PDU in the RLC layer. In (C), (D) of  FIG. 3 , the mobile station  14  did not rightly receive the divided data I, I+4 and I+5, or in other words, packets n−5 and n−3, however, the mobile station  14  did rightly receive packets n−4 and n−2. Before a handover, the mobile station  14  performs order control for the PDCP layer using RLC SDU (PDCP PDU) and PLC PDU in the RLC layer, however, after a handover, the RLC layer is regulated to be initialized. Therefore, processing for order control moves to the PDCP layer, and the mobile station  14  executes the handover sequence in the PDCP layer. 
         [0123]    During reordering, the mobile station  14  starts a timer in order to determine when reordering ends. 
         [0124]    The arrival of the packets m and m+1 that are received from the host station  2  is early, so jump ID code is attached to those packets m, m+1 and they are sent to the mobile station  14  first (jump transmission). The mobile station  14  saves the packets that are received from the base station and to which jump ID code is attached in a buffer B 2  (see  FIG. 1 ), and removes them as objects of the reordering process. After that, the mobile station  14  executes the reordering process for the packets n−5, n−3, and n−1 to n that were received from the target base station  11   b  and the packets n−4 and n−2 that were received before the handover, then delivers the data to the upper layer. 
         [0125]    When the specified time TM of the timer has elapsed, the mobile station  14  ends the reordering process and sends the packets that have been received to the upper layer even though there may be missing packets. 
         [0126]    •Window Control 
         [0127]      FIG. 4  is a drawing explaining the packet processing of the PDCP layer and RLC layer after a handover, and shows in detail window control. 
         [0128]    In consideration of the fact that more than the allowable amount of data arrives, the mobile station  14  internally generates a buffer size window, and during the reordering process, performs the following window control. In the example shown in  FIG. 4 , the left end of the window WD is initially the number n−5 of the expected data, and the right end of the window is the number decided according to the amount of allowable data (window buffer size), and is taken to be n−2. The mobile station  14  does not apply the window process to the packets to which jump ID code F has been added. 
         [0129]    In the initial state (window state (0)), when the mobile station  14  receives the expected packet (n−5) and executes the reordering process, the window state becomes as shown in (1). Therefore, the mobile station  14  delivers the expected packet (n−5) and the packet (n−4) that continues after the packets (n−5) to the upper layer. Thereafter, the window state becomes as shown in (2), and in this state, when the mobile station  14  receives the expected packet (n−3) and performs the reordering process, the window state becomes as shown in (3). Therefore, the mobile station  14  delivers the expected packet (n−3) and the packet (n−2) that continues after the packet (n−3) to the upper layer. The window state then becomes as shown in (4), and in this state, the mobile station  14  waits to receive the packets (n−1) and n, and when the packets are received within the set period of time T M , the mobile station  14  then delivers the packets to the upper layer. However, when the mobile station  14  does not receive the packets (n−1) and n within the set period of time T M , the mobile station  14  delivers the packets starting from packet m that are saved in the buffer BF 2  to the upper layer, and moves to normal control as before handover. As was explained above, by removing the jump packets from the reordering process and performing window control as described above, it is possible to execute the reordering processing on just packets n−5 to n. 
         [0130]    In this window control, when a packet is received that has a sequence number that is smaller than the sequence number on the left end of the window, the mobile station  14  deletes that packet. Also, when a packet is received that has a sequence number that is larger than the sequence number on the right side of the window, the mobile station  14  takes the sequence number of that packet to be the sequence number on the right side of the window and changes the sequence number on the left side of the window according to the window buffer size, while at the same time, delivers the packets that have moved out of the window range and that have been received to the upper layer. 
         [0131]    If this window processing is applied to the jump packets as well, at the instant that packet m is received, the window state will be such that the left side of the window is n−2 and the right side of the window is m. In this case, the mobile station  14  gives up on receiving the packets n−5 and n−3 that have not yet been received, and delivers the already received packets n−4 and n−2 to the upper layer. In addition, the mobile station decides the sequence number of the right side of the window to m, and decides the sequence number of the left side of the window to the number n−1 that is determined according to the window size, then after that waits for packet n−1, packet n, packet n+1, . . . , packet m−1 to arrive. However, since packet n+1 to packet m−1 are packets that do not actually exist, the mobile station uselessly waits to receive those packets, and trouble occurs in packet processing. 
         [0132]    •Construction of a Base Station 
         [0133]      FIG. 5  is a drawing that shows the construction of a base station, and shows the buffer unit, scheduler unit, transmission/reception unit and control unit. 
         [0134]    The buffer unit  21  is memory for storing packets that come from the host station, and packets that are forwarded from an adjacent base station (source base station). In the  FIG. 5 , two buffers  21   a ,  21   b  are physically provided, however, construction is also possible in which just one memory is physically provided, and that one memory is used by dividing it using software. 
         [0135]    The scheduler unit  22  selects a mobile station from among a plurality of communicating mobile stations with which to perform radio transmission, fetches packets for that mobile station that are stored in the buffer unit and sends them to the transmission/reception unit  23 . The transmission/reception unit  23  encodes and modulates the packets that are input from the scheduler unit  22 , and transmits the actual data using radio communication. In addition, the transmission/reception unit  23  receives and demodulates control signals and various data that are sent from the mobile station. 
         [0136]    The control unit  24  comprises a buffer management unit  24   a , HO control unit  24   b  and measurement control unit  24   c . The buffer management unit  24   a  manages the various packets that are stored in the buffer  21 . When data is taken over in a handover, the control unit  24  forwards at least the packets that are stored in the buffer unit  21   b  for which a confirmation (ACK) indicating that the packets were rightly received has not been obtained from the mobile station to the target base station  11   b.    
         [0137]    On the other hand, when the arrival of packets that are being forwarded from the source base station  11   a  are delayed due to the take over of data, and jump transmission is performed for the packets that are received from the host station  12 , ‘type=010’ is entered into the type field of the headers of the packets that will jump. 
         [0138]    The HO control unit  24   b  executes handover control as explained in  FIG. 23 , and the measurement control unit  24   c  gathers various measurement data that is sent from the mobile station, such as the radio communication quality CQI (Channel Quality Information) of the mobile station and the like. 
         [0139]    •Construction of a Mobile Station 
         [0140]      FIG. 6  is a drawing showing the construction of a mobile station, and shows the transmission/reception unit  31 , buffer unit  32 , reordering unit  33  and control unit  34 . The transmission/reception unit  31  transmits packets and control information to or receives packets and control information from a base station. When RLC PDU data could not be created from a lower-layer packet that was received, the buffer unit  32  holds that lower-layer packet until that RLC PDU data can be created, and after the RLC PDU data is created, the buffer unit  32  removes the header and delivers that data to the reordering unit  33  as RLC SDU (PDCP PDU) data. The reordering unit  33  has a function for rearranging the PDCP PDU data in order of sequence numbers and delivers the data to the upper layer. When a missing PDCP PDU sequence number is detected, the reordering unit  33  saves the PDCP PDU data following that PDCP PDU data in internal memory until the PDCP PDU data with the continuing sequence number is received. However, when that PDC PDU data has still not arrived after a set period of time has elapsed, the reordering unit  33  stops the reordering process and delivers all of the stored PDCP PDU data to the upper layer. Moreover, the reordering unit  33  performs window control so that the amount of data being processed does not exceed the allowable amount. 
         [0141]    The control unit  34  comprises a measurement unit  34   a , reordering management unit  34   b  and retransmission management unit  34   c . The measurement unit  34   a  measures various kinds of measurement information that are sent to a base station. For example, the measurement unit  34   a  measures the radio communication quality (Channel Quality Information) of the mobile station. The reordering management unit  34   b  controls the reordering unit  33 , and when there is a missing sequence number in the PDCP PDU data that is held by the reordering unit  33 , the reordering management unit  34   b  instructs the reordering unit  33  to wait for that PDCP PDU data having the continuing sequence number to arrive. Moreover, when a set amount of time for waiting for that packet to arrive has elapsed, the reordering management unit  34   b  instructs the reordering unit  33  to stop the reordering process, as well as instructs the reordering unit  33  to delete the headers of all of the saved PDCP PDU data and to delivers that data to the upper layer as PDCP SDU data, then sets the reordering unit  33  to a state in which it is able to receive new PDCP PDU data. In addition, the reordering management unit  34   b  finds the maximum sequence number from among sequence numbers that have been received up to that time as the sequence number on the right end of the window, and determines and sets the sequence number of the left end of the window by taking into considering the window size. Here, in the case where there is a packet that has already been received that has a sequence number that is smaller than the sequence number on the left end of the window, that packet is immediately delivered to the upper layer. When there is retransmission control, the retransmission management unit  34   c  sends a retransmission request signal to a base station via the transmission/reception unit by a route indicated by the dashed line. 
         [0142]    •Operation of the Target Base Station 
         [0143]      FIG. 7  is a flowchart showing the operation of the target base station in a first embodiment of the invention. 
         [0144]    When the handover control unit  24   b  of the target base station  11   b  receives a HO request from the source base station  11   a  (including the mobile station ID, QoS information, and the like) (step S 201 ), the handover control unit  24   b  determines whether to allow accepting the mobile station (step  202 ). When accepting the mobile station is not allowed, the handover control unit  24   b  performs post processing (step  213 ) and ends handover control. 
         [0145]    On the other hand, in a case where the handover control unit  24   b  allows the acceptance of the mobile station, the handover control unit  24   b  return a HO request response message to the source base station  11   a  (step  203 ). Then the target base station waits the packets that are forwarded from the source base station  11   a . When the packets arrivers from the source base station, the buffer  21  stores them (step  204 ). 
         [0146]    When the handover control unit  24   b  receives a HO complete report from the mobile station  14  (step  205 ), the handover control unit  24   b  then sends a HO complete report to the host station  12  (step  206 ). After receiving the HO complete report, the host station  12  changes the transmission path for packets from the source base station  11   a  to the target base station  11   b , and returns a HO complete response to the target base station  11   b . After the handover control unit  24   b  of the target base station  11   b  receives the HO complete response from the host station  12  (step  207 ), the handover control unit  24   b  sends an instruction to the scheduler unit  22  to start transmitting packets (step  208 ). 
         [0147]    From this, the scheduler unit  22  checks whether jump transmission of packets is necessary (step  209 ), and when jump transmission is not necessary, the scheduler unit  22  sends the packets that were forwarded from the source base station  11   a  to the mobile station  14  first (step  210 ). However, when forwarding of packets from the source base station  11   a  is delayed, and it is necessary to perform jump transmission of packets, the scheduler unit  22  executes jump transmission of the packets that were received from the host station  12 . 
         [0148]    In order to execute jump transmission, the scheduler unit  22  sets the type number in the type fields of the packets for which jump transmission will be performed to 010 (type=010) in order that the mobile station  14  will be able to recognize that the packets are jump packets (step  211 ), then after that, sends the packets to the mobile station  14  (step  210 ). 
         [0149]    At the same time as this, the handover control unit  24   b  sends a resource release to the source base station  11   a  (step  212 ), then performs post processing (step  213 ) and ends handover control. 
         [0150]    •Operation of the Source Base Station 
         [0151]      FIG. 8  is a flowchart showing the operation of a source base station in a first embodiment of the invention. 
         [0152]    In  FIG. 8 , when the measurement control unit  24   c  of the source base station  11   a  receives a Measurement Report which indicates reception state information from the mobile station  14  (step  251 ), the handover control unit  24   a  determines whether or not a handover (HO) is necessary based on that reception state information (step  252 ), and when a handover is not necessary, returns to the start. 
         [0153]    However, when the handover control unit  24   b  determines that a handover HO is necessary, the handover control unit  24   b  decides a target base station  11   b  according to the contents of the Measurement Report and sends a handover request to that target base station  11   b  (step  253 ). 
         [0154]    After that, when a HO response message that is sent from the target base station  11   b  is received (step  254 ), the HO control unit  24   b  sends a HO instruction message to the mobile station  14  (step  255 ), and instructs the buffer management unit  24   a  to forward packets that are saved in the buffer  21   b  to the target base station  11   b . Thereby, using the route indicated by the dotted line, the buffer management unit  24   a  forwards packets that are saved in the buffer  21   b  and that were not sent to the mobile station  14 , or packets that were not received rightly (NACK packets) by the mobile station, to the target base station  11   b  (step  256 ). After that, when a resource release message is received from the target base station  11   b  (step  257 ), the HO control unit  24   a  performs a resource release (step  258 ). 
         [0155]    •Operation of the Mobile Station 
         [0156]      FIG. 9  is a flowchart showing the operation of the mobile station. The measurement unit  34   a  of the mobile station  14  notifies the source base station  11   a  of the reception status using a Measurement Report (step  271 ). The control unit  34  then waits for a HO instruction message to be sent from the source base station  11   a , and when a HO instruction message is received (step  272 ), the control unit  24  establishes synchronization between the mobile station and the target base station  11   b  using L1/L2 signaling (step  273 ), and after synchronization is established, sends a handover complete report to the target base station  11   b  (step  274 ). After that, the control unit  34  checks whether a received packet is a jump packet, or in other words whether the type number of a packet is type=010 (step  275 ), and when a packet is a jump packet, the control unit  34  removes that packet as an object of reordering and saves the packet in a buffer  32 , then after a set period of time has elapsed, removes the header and delivers the packet to the upper layer as a PDCP SDU packet (step  276 ). On the other hand, when a packet is not a jump packet, the control unit  24  executes reordering, and rearranges the order according to the sequence number, then removes to header from the reordered packets and delivers them to the upper layer as a PDCP SDU packets (step  277 ). 
         [0157]      FIG. 10  is a flowchart showing the reordering process by the mobile station. 
         [0158]    When the transmission/reception unit  31  of the mobile station  31  receives a lower layer packet from the target base station  11   b  (step  301 ), the reordering management unit  34   b  checks whether it is possible to create RLC PDU data (step  302 ), and when it is not possible to create RLC PDU data, checks whether a set amount of time has elapsed (step  303 ), and when the set amount of time has not elapsed, saves the lower layer packet in the buffer  32  (step  304 ), then performs processing again from step  301 . When it is not possible to create RLC PDU data even though a set amount of time has elapsed since the lower layer packet was received, that lower layer packet is deleted from the buffer (step  305 ). 
         [0159]    On the other hand, in step  302 , when it is possible to create RLC PDU data using the received lower layer packet, that RLC PDU data is delivered to the reordering unit  33  as RLC SDU (PDCP PDU) data (step  306 ). After receiving the RLC SDU (PDCP PDU) data, the reordering unit  33  checks whether there is a missing sequence number (step  307 ), and when there is no missing sequence number and the sequence numbers are continuous, the reordering unit  33  removes the header of that RLC SDU (PDCP PDU) data and delivers it to the upper layer as PDCP SDU data (step  311 ). However, when there is a missing sequence number, the reordering management unit  34   b  instructs the reordering unit  33  to save the PDCP PDU data (step  308 ). Thereby, the reordering unit  33  saves the RLC SDU (PDCP PDU) data, and checks whether RLC SDU (PDCP PDU) data with a continuing sequence number is received (step  309 ). After receiving RLC SDU (PDCP PDU) data with a continuing sequence number, the reordering unit  33  removes the header of the RLC SDU (PDCP PDU) data and delivers it to the upper layer as PDCP SDU data, and then delivers similarly the saved PDCP PDU as PDCP SDU to the upper layer (step  311 ). 
         [0160]    Moreover, in step  309 , when RLC SDU (PDCP PDU) data having a continuing sequence number is not received, the reordering unit  33  monitors a preset period of time T M  (step  310 ), and when the set period of time has not yet elapsed, repeats processing from step  308 , however, when the set period of time T M  has elapsed, the reordering unit  33  removes the header of the saved PDCP PDU data and delivers it to the upper layer even though the sequence numbers are not continuous (step  311 ). 
         [0161]    With the first embodiment of the invention as described above, packets that are transmitted from the host station  12  to the target base station  11   b  can be transmitted to the mobile station  14  as jump packets without waiting, so it is possible to eliminate delay time of the data, and to improve the throughput of the overall system. Moreover, the mobile station  14  removes jump packets as objects of reordering control (sequence order control), and performs sequence order control on packets other than jump packets and delivers those packets to the upper layer in order of sequence numbers. As a result, the mobile station is able to perform sequence order control of packets even when there is only one order control function. 
       (C) Second Embodiment 
       [0162]    In the first embodiment, the mobile station  14  performed the reordering process during a set period of time TM, and ended the reordering process when that set period of time TM elapsed (see step  310  in  FIG. 10 ). In that case, when the set period of time TM has not yet elapsed even though all of the packets that have been forwarded from the source base station  11   a  to the target base station  11   b  are received, the mobile station  14  continues the reordering process. Therefore, in a second embodiment of the invention, in order that the mobile station  14  is able to recognize the last packet that is an object of the reordering process, the target base station  11   b  adds an identifier to a specified packet (last packet) and transmits that packet to the mobile station  14 , and after receiving that last packet, the mobile station  14  immediately ends the reordering process even though the set time TM has not elapsed. 
         [0163]      FIG. 11  is a drawing explaining a second embodiment of the invention, where it presumed that packets n−5 to n are stored at the source base station  11   a  before a handover, and of these packets, packets n−5 to n−2 were sent to the mobile station  14 , however, packets n−1 and n were not sent to the mobile station  14 . For example, packets n−1 and n arrived after the radio communication line between the source base station  11   a  and the mobile station had been cut, so these packets n−1, n were not sent to the mobile station  14 . In addition, it is presumed that of the packets that were sent to the mobile station  14 , packets n−5 and n−3 were not received rightly by the mobile station  14  (NACK), and that packets n−4 and n−2 were received rightly (ACK). Therefore, the mobile station  14  saves packets n−4 and n−2, and does not save packets n−5 and n−3. 
         [0164]    When a handover occurs in this state, the source base station  11   a  forwards the packets n−5 and n−3 that were not rightly received by the mobile station  14 , and the unsent packets n−1 and n to the target base station  11   b . Also, the host station  12  sends two packets m to m+1 to the target base station  11   b  after the handover. It is presumed that the forwarding of the packets n−5, n−3 and n−1 to n is delayed. 
         [0165]    When the target base station  11   b  receives the packets m and m+1 from the host station  12  before the packets n−5, n−3 and n−1 to n are forwarded from the source base station  11   a , the target base station  11   b  adds jump ID code F to the packets m and m+1 that were received from the host station and sends them to the mobile station  14  first (jump transmission). The mobile station  14  saves the packets received from the base station with the jump ID code added in the buffer BF 2 , and removes them as objects of the reordering process. In (A) of  FIG. 11 , the state is shown in which the mobile station  14  saves packets m and m+1 in the buffer BF 2 , and saves the packets n−4 and n−2 that were received before the handover in the buffer BF 1 . 
         [0166]    The target base station  11   b  sends the packets that are forwarded until a preset time T M  (called the Waiting Time) has elapsed to the mobile station  14 , and when the Waiting Time has elapsed the target base station  11   b  discards any packets that may be forwarded. Therefore, the target base station  11   b  receives the packets n−5, n−3 and n−1 that were forwarded from the source base station  11   a  before the Waiting Time elapsed and saves them in the buffer BF, then sends the packets one by one to the mobile station  14 . In addition, the target base station  11   b  receives packet n from the source base station  11   a , however, the Waiting Time was complete before the packet n could be sent to the mobile station  14 . In this case, the target base station  11   b  adds an identifier (L) to packet n to indicate that it is the last packet forwarded from the source base station, and sends packet n to the mobile station  14 . 
         [0167]    The mobile station  14  saves the packets n−5, n−3, n−1 and n that were received from the target base station  11   b  in the buffer BF 1 , and as shown in (B) of  FIG. 11 , executes the reordering process to rearrange these packets and the packets n−4 and n−2 that were received before handover. Moreover, after detecting packet n with the identifier (L) added, the mobile station  14  determines that forwarding has ended, so delivers the packets that have been reordered to the upper layer and ends the reordering process even though the set time period T M  for reordering has not ended. 
         [0168]    The case in which the Waiting Time T w  endsbefore packet n is sent is described above, however, the Waiting Time T w  may end after the packet n has been sent to the mobile station. In that case, the target base station  11   b  adds an identifier (L) to a packet m+2 that will be received from the host station  12  indicating that it is the last packet, and sends the packet m+2 to the mobile station  14 . After receiving the last packet to which that identifier (L) has been added, the mobile station  14  determines that forwarding has ended, and immediately ends reordering even though the set period of time T M  for reordering has not ended. 
         [0169]    Moreover, packets n−5 and n−3 are received by forwarding, however, the Waiting Time T w  may end in the stage before packets n−1 and n are received. In that case, the target base station  11   b  may add an identifier (L) to packet n−3 indicating that it is the last packet, and sends that packet to the mobile station  14 . The mobile station  14  detects the Last packet to which that identifier (L) is added and ends the reordering process. 
         [0170]    •Last Packet ID Code L 
         [0171]    To add an identifier (last packet ID code) L to a packet identifying that the packet is the last packet, a 3-bit ‘type’ field that is included in the PDCP PDU header is used. In other words, a new type number is defined in that ‘type’ field as the last packet ID code L, and that type number is assigned to the first packet to be sent after the Waiting Time T w  ends.  FIG. 12  shows an example of the PDCP PDU format, where (A) is an example of format without a header, (B) is an example of format in which the PDCP PDU sequence number is not added, and (C) and (D) are examples of format in which the PDCP PDU sequence number is added. In the format shown in (C) and (D), a type field and a PID field are defined in the header HD, where the type field indicates the type of PDCP PDU. For the type field, ‘type=000’ and ‘type=001’ are already regulated, however, the type numbers ‘type=010 to 111’ are not regulated and are unused. Therefore, ‘type=011’ is used as the type number for recognizing the PDCP PDU packet (last packet) that is to be sent first after the Waiting Time T w  has ended. 
         [0172]    •PDCP Layer and RLC Layer Processing 
         [0173]      FIG. 13  and  FIG. 14  are drawings explaining packet processing for the PDCP layer and RLC layer after a handover. 
         [0174]      FIG. 13  shows the case in which the last packet ID code L is added to the packet n that is sent to the mobile station  14 . In the first embodiment, during reordering, the mobile station  14  starts a timer in order to determine the end of reordering. Here, when the set time T M  is set to be a large value, the reordering process continues regardless of whether or not packet n is the last packet forwarded, and the packets cannot be deliverd to the upper layer until the set time T M  ends. However, in this second embodiment, after receiving packet n (Last packet) to which the last packet ID code L has been added, the mobile station  14  immediately ends reordering and delivers all of the PDCP PDU packets to the upper layer. 
         [0175]    When more than the allowable amount of data arrives at the mobile station  14 , the mobile station  14  performs window control during reordering. The left side of the window is n−5, which is the number expected for arrival, and the right side of the window is the value of the upper limit of the allowable amount of data (in the figure, this value is n). However, window processing is not applied to ‘type=010’ packets (jump packets) as in the first embodiment. By performing processing in this way, it is possible to execute reordering from packet n−5 to packet n. 
         [0176]      FIG. 14  shows the case in which last packet ID code L is added to packet m+2. At the instant that packet m+2 (Last packet) is received, the mobile station  14  immediately ends reordering even though the set time T M  may not have ended yet, and delivers all of the received PDCP PDU packets to the upper layer. Also, in the case of window control, the mobile station  14  executes window control in the same way as shown in  FIG. 13 . 
         [0177]    •Operation of the Target Base Station 
         [0178]      FIG. 15  is a flowchart showing the operation of the target base station in the second embodiment of the invention, where the same reference numbers are given to steps that are the same as the steps of the first embodiment shown in  FIG. 7 . The base stations and mobile station of this second embodiment have the construction shown in  FIG. 5  and  FIG. 6 . 
         [0179]    When the handover control unit  24   b  of the target base station  11   b  receives a HO request from the source base station  11   a  (includes the mobile station ID, QoS information, etc.), the handover control unit  24   b  performs call-receiving control based on that information and determines whether to allow acceptance of the mobile station. When acceptance is not allowed, the handover control unit  24   b  performs post processing, and ends handover control (steps  201  to  202 , and  213 ). 
         [0180]    On the other hand, when the handover control unit  24   b  allows the acceptance of the mobile station  14 , the handover control unit  24   b  returns a HO request response message to the source base station  11   a , and starts measuring the elapsed time. After that, the target base station  11   b  waits for packets forwarded from the source base station  11   a , and stores the packets in the buffer unit  21  (steps  203  and  204 ). 
         [0181]    In this state, after receiving a HO complete report from the mobile station  14 , the handover control unit  24   b  sends a HO complete report to the host station  12  (steps  205  and  206 ). After the host station  12  receives the handover complete report, the host station  12  then changes the transmission path for the packets from the source base station  11   a  to the target base station  11   b , and returns a HO complete response to the target base station  11   b . The handover control unit  24   b  of the target base station  11   b  receives the HO complete response from the host station  12 , and then instructs the scheduler unit  22  to start transmitting packets to the mobile station (steps  207  and  208 ). 
         [0182]    Next, the scheduler unit  22  monitors whether or not the elapsed time has exceeded a set period of time T w , that is, whether or not the Waiting Time has ended (step  501 ), and when the Waiting Time has not ended, checks whether it is necessary to perform jump transmission of packets (step  209 ), and when it is not necessary, starts sending packets that were forwarded from the source base station  11   a  (step  210 ) preferentially to the mobile unit  14  via transmission/reception unit  23 . However, when forwarding is delayed and it is necessary to perform jump transmission of packets, the scheduler unit  22  executes jump transmission of packets that are received from the host station  12 . In order to execute jump transmission, the scheduler unit  22  sets the type number of the type field in the header of the packets for which jump transmission will be performed to 010 (type=010), thereby the mobile station  14  will be able to identify that the packets are jump packets (step  211 ). After that, the transmission/reception unit  23  sends the jump packets to the mobile station. 
         [0183]    At the same time as this, the handover control unit  24   b  sends a resource release to the source base station  11   a  (step  212 ), then returns to step  501  to check whether the Waiting Time has ended, and when the Waiting Time has not ended, repeats the process from step  209 . 
         [0184]    On the other hand, in step  501 , when the Waiting Time T w  hasended, the scheduler unit  22  sets the type number of the packet that will be transmitted immediately after the Waiting Time has ended to ‘011’. In other words, the scheduler unit  22  adds a last packet ID code L to the packet that will be transmitted immediately after the Waiting Time ends, and transmits that packet (Last packet) to the mobile station  14  (steps  502  and  503 ), then after that, performs post processing and ends handover control (steps  212  and  213 ). 
         [0185]    In step  502 , in order to be able to identify whether the last packet to which the last packet ID code L has been added is a packet that has been forwarded from the source base station  11   a , or is a packet that was received from the host station  12 , the scheduler unit  22  sets the type number to ‘011’ in the case of the former, and sets the type number to ‘100’ in the case of the latter. By doing this, it becomes easy for the mobile station  14  to perform the reordering process as will be described later. 
         [0186]    •Operation of the Mobile Station 
         [0187]      FIG. 16  is a flowchart showing the operation of the mobile station, where the same reference numbers are used for steps that are the same those in the operation flowchart shown in  FIG. 98 . The measurement unit  34   a  of the mobile station  14  uses a Measurement Report to notify the source base station  11   a  of the reception status. The control unit  34  then waits for a HO instruction message to be sent from the source base station  11   a , and after receiving the HO instruction message, establishes synchronization between the mobile station  14  and the target base station  11   b  using L1/L2 signaling, and after synchronization has been established, sends a handover complete report to the target base station  11   b  (steps  271  to  274 ). 
         [0188]    After that, the control unit  34  checks whether a received packet is a jump packet, or in other words, a packet having a type number type=010 (sep  600 ), and when the packet is a jump packet, removes that packet from being an object of reordering and saves it in the buffer  32  (step  601 ). 
         [0189]    On the other hand, when the packet is not a jump packet, the control unit  34  checks whether the received packet is the Last packet, or in other words, a packet having a type number type=011 (sep  602 ). In the case where the packet is not the Last packet, the control unit  34  starts the reordering process, and arranges the PDCP PDU packets according to sequence numbers, then creates PDCP SDU data and delivers that data to the upper layer (step  603 ). 
         [0190]    The control unit  34  then checks whether the preset time period T M  has ended (step  604 ), and when the set time period T M  has not yet ended, the control unit  34  repeats processing from step  600 , and when the set time period T M  has ended, the control unit  34  ends the reordering process, then deletes the header from RLC SEU (PDCP PDU) packets that have not yet been delivered to the upper layer, creates PDCP SDU data and delivers that data to the upper layer (step  605 ). Next, the control unit  34  delivers packets that are not the object of reordering to the upper layer and ends processing (step  606 ). 
         [0191]    However, in step  602 , when the received packet is the Last packet, the control unit  34  immediately stops the reordering process (step  607 ). At that time, in the case where the Last packet is a forwarded packet, the control unit  34  creates a PDCP SDU packet from that Last packet and delivers that packet to the upper layer, then stops reordering. When the Last packet is a packet received from the host station  12 , the control unit  34  immediately stops reordering, and connects the Last packet to the last packet that is not an object of reordering and stored in the buffer. After that, the control unit  34  delivers the packets that were not the object of reordering to the upper layer and ends processing (step  606 ). 
         [0192]    As described above, with this second embodiment of the invention, the mobile station detects the end of the forwarded packets by referencing the Last packet ID code, making it possible to quickly stop the reordering process. Therefore, it is possible to eliminate the data delay time, and improve the throughput of the overall system. 
       (D) Third Embodiment 
       [0193]    The order information (sequence numbers n, m) that has been presented up until now has been convenient numbers for simplifying the explanation, however, actually, it is information that is added at the base station. 
         [0194]    As was explained above, in a LTE communication system, forwarding is performed in PDCP SDU data units. Therefore, when forwarding occurred, it was not possible to send the Sequence Number field from the source base station  11   a  to the target base station  11   b , and it was necessary to notify the target base station  11   b  of the sequence numbers using some method. 
         [0195]    In this embodiment, the source base station  11   a  notifies the target base station  11   b  of the sequence numbers together with the PDCP SDU data, and based on those sequence numbers, the target base station  11   b  adds a header including the sequence numbers to the forwarded PDCP SDU data (packets) thereby PDCP PDU(RLC SDU) packet is created. 
         [0196]      FIG. 17  is an example of sending the PDCP PDU sequence number together with the PDCP SDU data (packets) via the data plane (U-plane). In  FIG. 17 , only the PDCP SDU data and the sequence numbers that accompany that data are shown, however, in order that the PDCP SDU data is recognized to be a bunch of packets, control information (header information) must be added. Each time the source base station  11   a  forwards PDCP SDU data, the source base station  11   a  notifies the target base station  11   b  of the sequence number that accompanies that PDCP SDU data using the format shown in  FIG. 17 , and based on that sequence number, the target base station  11   b  adds the sequence number to the forwarded PDCP SDU data (packet). 
         [0197]    In other words, in the condition shown in  FIG. 17 , the source base station  11   a  first forwards sequence number n−5 and the first PDCP SDU data via the data plane U-plane. Next, the source base station  11   a  forwards sequence number n−3 and the next PDCP SDU data via the data plane U-plane. After that, when forwarding following PDCP SDU data that were received from the host station  12 , the source base station  11   a  similarly forwards the PDCP SDU data and the sequence numbers n−1 and n thereof to the target base station  11   b  via the U-plane. At the target base station  11   b , by receiving the sequence numbers n−5, n−3, n−1 and n that were notified via the U-plane, it is possible for the target base station  11   b  to recognize the numbers as the sequence numbers for the forwarded data. 
         [0198]      FIG. 18  is an example of the source base station  11   a  notifying the target base station  11   b  via the U-plane of the PDCP PDU sequence number together with the appropriate PDCP SDU data, and notifying the target base station  11   b  via the C-plane of the sequence number of PDCP SDU data, for which right reception by the mobile station could not be confirmed, as the Next SN. Notification of the sequence number (Next SN) via this C-plane is shown as taking over of the SN (SN takeover) in the handover sequence shown in  FIG. 19 . It may possible to perform SN taking over at the same time that the source base station  11   a  sends a HO request to the target base station  11   b.    
         [0199]    In the state shown in  FIG. 18 , the source base station  11   a  first forwards the sequence number n−5 and the first PDCP SDU data via the U-plane. Also, at the same time, the source base station  11   a  notifies the target base station via the C-plane of the just the sequence number n−5 of the PDCP SDU data, for which proper reception by the mobile station could not be confirmed, as the Next SN (SN takeover). Next, the source base station  11   a  forwards the sequence number n−3 and the next PDCP SDU data via the U-plane. After that, when forwarding the PDCP SDU data that was received from the host station  12 , the source base station  11   a  similarly forwards that PDCP SDU data and the sequence number n−1, n thereof to the target base station  11   b  via the U-plane. The target base station  11   b  receives the sequence number n−5 that was notified via the C-plane, and the sequence numbers n−5, n−3, n−1 and n that were notified via the U-plane, however, since the sequence numbers that were notified via the U-plane are larger than the sequence number notified via the C-plane, the target base station  11   b  adds the sequence numbers that were notified via the U-plane to the following PDCP SDU data received via the U-plane, and transmits the data to the mobile station. In other words, the target base station  11   b  ignores the sequence number n−5 that was notified via the C-plane. 
         [0200]      FIG. 20  is an example of the case in which there is no PDCP SDU data to forward. 
         [0201]    The source base station  11   a  notifies the target base station  11   b  via the C-plane of the next sequence number n+1, as the Next SN. When the Waiting Time ends without the target base station  11   b  receiving PDCP SDU data from the source base station  11   a , the target base station  11   b  adds the sequence number to the following PDCP SDU data based on the sequence number n+1 that was notified via the C-plane, and transmits the data to the mobile station. In other words, the source base station  11   b  sets the sequence number of the packet m that was received from the host station to n+1 and transmits that data to the mobile station. 
       ADVANTAGE OF THE PRESENT INVENTION 
       [0202]    With the present invention described above, it is possible to quickly send packets that were sent from host station to the target base station to the mobile station as jump packets, thus making it possible to eliminate the delay time of data, and to improve the throughput of the overall system. Moreover, the mobile station removes jump packets from being the object of reordering control (sequence order control), and performs order sequence control only on packets other than jump packets, then delivers the packets to a higher apparatus (upper layer) in order of sequence numbers. As a result, the mobile station is able to properly perform sequence order control of packets that are the object of reordering even when there is only one sequence order function. 
         [0203]    Furthermore, with the present invention, the mobile station detects the end of the forwarded packets by referencing last packet ID code, and so is able to quickly stop reordering. Therefore, it is possible to eliminate the delay time of data, and to improve the throughput of the overall system.