Patent Publication Number: US-2011075634-A1

Title: Handover method, radio base station, and mobile terminal

Description:
TECHNICAL FIELD 
     The present invention relates to a handover of a terminal in mobile communications including MIMO (Multi Input Multi Output) communications. 
     BACKGROUND ART 
     MIMO communications are attracting attention as a communication technology for increasing the capacity of mobile communications. Transmitting/receiving devices for MIMO communications are provided with an array antenna which includes a plurality of antennas. MIMO communications transmit different pieces of information data from the respective antennas of the array antenna at the same frequency in parallel by utilizing differences in the transmission conditions between the individual antennas and the other party of the communication. The radio signals are thereby spatially multiplexed for increased transmission quantity. 
       FIG. 6  shows a configuration example of an MIMO transceiver  1  with a four-system array antenna. A transmission unit  2  of the MIMO transceiver  1  includes: an S/P conversion unit  2   a  which performs serial/parallel conversion of transmission signals; and modulation units  2   b  and RF (Radio Frequency) transmission units  2   c  which are arranged for the respective antennas of an array antenna  4 . A reception unit  3  includes: RF reception units  3   a  and demodulation units  3   c  which are arranged for the respective antennas of the array antenna  4 ; a signal separation circuit  3   b  for separating the reception signals resulting from the RF reception units  3   a  and supplying the resultant to the demodulation units  3   c ; and a P/S conversion unit  3   d  which performs parallel/serial conversion of the demodulated signals. The signal separation circuit  3   b  is a circuit specific to the reception unit  3  for MIMO communications. 
     MIMO communications are intended to transmit a large amount of data seamlessly, whereas the seamless transmission can be hindered at the time of a handover in a terminal, i.e., when the terminal switches a base station to another base station for movement or other reasons. Originally, communication environment where a handover is needed is often unstable because of radio wave interference between a plurality of base stations. During a MIMO handover, the signal separation circuit ( FIG. 6 :  3   b ) can thus fail to properly separate the reception signals, causing transmission losses such as retransmission and a greater instantaneous interruption time. 
     Examples of the technologies related to a MIMO communication handover include one described in PTL 1 below. The literature describes the technology for improving the throughput in handover areas to expand the cell coverage. 
     CITATION LIST 
     Patent Literature 
     
         
         {PTL 1} JP-A-2006-157848 
       
    
     Non-Patent Literature 
     
         
         {NPL 1} 3GPP, “TS36.300 V8.3.0,” [online], December 2007, [Searched on 20 May 2008], the Internet &lt;http://www.3gpp.org/ftp/Specs/html-info/36300.htm&gt;, pp. 13-14, FIG. 4 
         {NPL 2} 3GPP, “TS 36.300 V8.3.0,” [online], December 2007, [Searched on 20 May 2008], the Internet &lt;http://www.3gpp.org/ftp/Specs/html-info/36300.htm&gt;, pp. 40-42, FIG. 10.1.2.1 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     According to the technology of the foregoing PTL 1, the MIMO communication throughput at handover time improves. As described in the literature, such an improvement, however, is predicated on complicated analyses of the reception signals. As described with reference to  FIG. 6 , the MIMO transceiver ( 1 ) needs the modulation/demodulation circuits ( 2   b ,  3   c ), the RF signal processing circuits ( 2   c ,  3   a ), and the like for the respective antennas. This tends to increase the circuit scale and the power consumption. The application of the configuration of the foregoing literature  1  to such a MIMO transceiver can thus cause the problem of a further increase in the circuit scale and power consumption of each node. 
     It is an object of the present invention to provide technology for preventing each node from increasing in circuit scale and power consumption in order to facilitate a handover of the mobile terminal in MIMO communications. 
     Solution to Problem 
     A handover method according to the present invention is a method including: determining whether or not to perform a handover to switch a destination of connection of a mobile terminal from a first radio base station to a second radio base station, the mobile terminal holding data communications with the first radio base station by MIMO (Multi Input Multi Output) communications using a plurality of transmission channels; suspending, if it is determined to perform the handover, the data communications between the mobile terminal and the first radio base station and starting communications pertaining to the handover between the mobile terminal and each of the first and second radio base stations by ordinary communications using a single transmission channel; and switching communication mode between the mobile terminal and the second radio base station from the ordinary communications to MIMO communications after completion of the handover. 
     A radio base station according to the present invention includes: a transmission and reception unit that radioly performs MIMO (Multi Input Multi Output) communications using a plurality of transmission channels and ordinary communications using a single transmission channel; and a control unit that determines whether or not to perform a handover to switch a destination of connection of a mobile terminal to another station, the mobile terminal holding data communications with the own station by MIMO communications, suspends, if it is determined to perform the handover, the data communications between the mobile terminal and the own station and starts communications pertaining to the handover between the mobile terminal and the own station by ordinary communications using a single transmission channel, and switches communication mode between a mobile terminal and the own station from ordinary communications to MIMO communications after completion of another handover, a destination of connection of the mobile terminal being switched from another station to the own station by the ordinary communications for the another handover. 
     A mobile terminal according to the present invention is a mobile terminal including a transmission and reception unit that radioly performs MIMO (Multi Input Multi Output) communications using a plurality of transmission channels and ordinary communications using a single transmission channel. When performing a handover to switch a destination of connection to a second radio base station while holding data communications with a first radio base station by MIMO communications, the mobile terminal starts communications pertaining to the handover between the own terminal and each of the first and second radio base stations by ordinary communications, and switches communication mode between the own terminal and the second radio base station from the ordinary communications to MIMO communications after completion of the handover. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to the present embodiment, it is possible to prevent each node from increasing in circuit scale and power consumption in order to facilitate a handover of the mobile terminal during MIMO communications. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  A block diagram of a system according to an embodiment of the present invention. 
         FIG. 2  A block diagram showing the configuration of each base station and mobile terminal according to the embodiment of the present invention. 
         FIG. 3  An explanatory diagram about antenna switching according to the embodiment of the present invention. 
         FIG. 4  A sequence diagram showing the operation of the embodiment of the present invention. 
         FIG. 5  A sequence diagram showing the operation of another embodiment of the present invention. 
         FIG. 6  A block diagram showing the configuration of a transceiver for MIMO communications. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows the system configuration according to an embodiment of the present invention. The system  100  of the present embodiment is based on the configuration of an LTE (Long Term Evolution)/SAE (System Architecture Evolution) network which has been discussed in 3GPP (3rd Generation Partnership Project). An example of the LTE/SAE network configuration is described in NPL 1 ( FIG. 4 ). 
     The system  100  includes: a source base station  20  and a target base station  30  which are “eNBs” (E-UTRAN NodeBs) constituting “E-UTRAN” (Evolved Universal Terrestrial Radio Access Network) described in NPL 1; a core network (Evolved CN)  40  which includes “MME/S-GW” (Mobility Management Entity/Serving Gateway) described in the same literature; and a mobile terminal  10  which attempts a handover from the source base station  20  to the target base station  30 . The source base station  20  is a node corresponding to the first radio base station according to the present invention. The target base station  30  is one corresponding to the second radio base station. 
     The foregoing “eNBs,” or the source base station  20  and the target base station  30 , are nodes in which part of 3G network RNC (Radio Network Controller) functions and “NodeB” (radio base station) functions are integrated. The core network  40  is connected to an external network such as the Internet. 
     The source base station  20  and the target base station  30  are mutually connected by an interface  101  called “X 2 .” The base stations  20  and  30  and the core network  40  are connected by interfaces  102  called “S 1 .” The interfaces such as “X 2 ” and “S 1 ” are logical ones. For physical connections, “S 1 ” ( 102 ) may also be used for “X 2 ” ( 101 ). 
     If, in the system  100 , the mobile terminal  10  is currently connected with the source base station  20 , downstream data from the external network to the mobile terminal  10  is delivered to the mobile terminal  10  from the core network  40  through the source base station  20 . At the time of a handover of the mobile terminal  10 , i.e., when the mobile terminal  10  switches the connection from the source base station  20  to the target base station  30 , the following procedure is performed. 
     Initially, the source base station  20  once suspends the data communications with the mobile terminal  10 , and transfers pieces of downstream data from the core network  40  to the target base station  30  through the interface  102  (X 2 ). The pieces of downstream data are data that have not been transmitted to the mobile terminal  10  yet or data that receipt acknowledgement has not been received from the mobile terminal  10  yet. The target base station  30  transfers the data that is received from the source base station  20  to the mobile terminal  10  which is handed over from the source base station  20 . Consequently, the mobile terminal  10  can continue the once-suspended data communications with the target base station  30 . 
       FIG. 2  shows the configuration of the base stations  20  and  30 . The shown configuration is in charge of radio communications between each base station and the mobile terminal  10 . Similar configuration is applied to the mobile terminal  10 . A transmission and reception unit  51  is carries out signal processing for radio communications including ordinary communications through a single transmission channel and MIMO communications mentioned above. The transmission and reception unit  51  has the same basic configuration as that of the MIMO transceiver  1  of  FIG. 6 . Specifically, a transmission unit  53 , a reception unit  54 , and an array antenna  55  including antennas  55 - 1 , . . . ,  55 - n  of the transmission and reception unit  51  correspond to the transmission unit  2 , the reception unit  3 , and the array antenna  4  of  FIG. 5 . 
     A control unit  52  controls the mode of the radio communications to be performed by the transmission and reception unit  51 . For MIMO communications, the control unit  52  instructs the transmission and reception unit  51  to use all or the plurality of systems of the array antenna  55  for communication. For ordinary communications through a single transmission channel, the control unit  52  instructs the transmission and reception unit  51  to use any one of the systems of antennas of the array antenna  55  for communication. 
     If the control unit  52  belongs to a base station  20  or  30 , the control unit  52  determines whether the mobile terminal  10  in connection with the own station by MIMO communications needs to be handed over to another station. If a handover is needed, the control unit  52  switches the communication mode with the mobile terminal  10  from MIMO communications to ordinary communications, and executes a series of handover processing by ordinary communications. 
     For the sake of switching of the communication mode, information on the correspondence between communication modes and antennas such as shown in  FIG. 3  is set in the base stations  20  and  30  and the mobile terminal  10 . The shown setting shows that all the systems ( 55 - 1 , . . . ,  55 - n ) of the array antenna  55  are used for MIMO communications, and only one of the systems of antennas ( 55 - 1 ) of the array antenna  55  is used for ordinary communications. It should be appreciated that the shown setting is just an example, and which antennas to use may be arbitrarily set depending on the numbers of antennas needed for MIMO communications and ordinary communications, respectively. The antennas to use and the number of antennas for MIMO communications may be changed depending on the quality needed of the current communications. 
     Referring to the sequence shown in  FIG. 4 , the operation of the present embodiment will be described. As will be described, the sequence described in NPL 2 (FIG. 10.1.2.1) is applied to part of the shown sequence. 
     Suppose that the mobile terminal  10  and the source base station  20  are currently holding MIMO data communications with each other. In order to determine whether the mobile terminal  10  needs to be handed over, the source base station  20  performs the processing of checking reception status of radio waves of the mobile terminal  10  (steps S 1  to S 4 ). Such processing is the same as the one described in NPL 2. More specifically, the source base station  20  transmits “Measurement Control” to the mobile terminal  10  (S 1 ). “Packet Data” is exchanged between the mobile terminal  10  and the source base station  20 , and between the source base station  20  and the core network  40  (S 2 ). The source base station  20  then transmits “UL (uplink) allocation” to the mobile terminal  10  (S 3 ), and the mobile terminal  10  returns “Measurement Reports” to the source base station  20  (S 4 ). 
     Suppose here that the source base station  20  determines that the mobile terminal  10  needs to be handed over (step S 5 ). Whether or not a handover is needed can be determined, for example, based on the reception status of radio waves of the mobile terminal  10  estimated from the terminal&#39;s SIR (Signal to interference Power ratio) which is notified by the foregoing “Measurement Reports.” Specifically, the source base station  20  can be determine that a handover is needed if more than half of all the antennas fall below a reference value in SIR. 
     If the source base station  20  determines to perform a handover, the source base station  20  stops the data communications with the mobile terminal  10 , and instructs the mobile terminal  10  to switch the communication mode from MIMO to ordinary communications (step S 6 : “MIMO Stop”). Consequently, the mobile terminal  10  performs subsequent radio communications with the source base station  20  and the target base station  30  by ordinary communications using a single system of antennas. 
     The source base station  20  transmits a handover request to the target base station  30  which is the destination of the handover (step S 7 ). While the handover request may be in the same format as with “4 Handover Request” described in NPL 2, the source base station  20  adds to the format the information that instructs the target base station  30  to perform ordinary communications with the mobile terminal  10 . Recognizing the handover request from the source base station  20 , the target base station  30  communicates with the mobile terminal  10  by ordinary communications. 
     Subsequently, the entire system ( 10 ,  20 ,  30 ,  40 ) performs a series of handover processing (step S 8 ). The handover processing is the same as the procedure from “5. Admission Control” to “18. Release Resources” described in NPL 2. In the meantime, the mobile terminal  10  to be handed over from the source base station  20  to the target base station  30  communicates with each base station ( 20 ,  30 ) by ordinary communications. 
     When the resources of the source base station  20  are released to complete the handover (NPL 2, “18. Release Resources”), the target base station  30  instructs the mobile terminal  10  to switch the communication mode with the own station from ordinary communications to MIMO (step S 9 : “MIMO start”). The mobile terminal  10  performs subsequent communications with the target base station  30  by MIMO. Consequently, the data communications suspended at the time of the handover are resumed between the mobile terminal  10  and the target base station  30  by MIMO (step S 10 ). 
     As described above, according to the present embodiment, when a handover is needed during MIMO communications, the MIMO are once switched to ordinary communications and handover processing is carried out. In general, situations where a handover of the mobile terminal ( 10 ) is needed are where the reception quality of the terminal drops. In such situations, a MIMO-based handover, if attempted, is likely to cause transmission losses such as disconnection and a packet loss. 
     Therefore, the handover processing is performed by ordinary communications instead of MIMO so as to reduce transmission losses that can occur during a handover. Since ordinary communications use a single system of antennas, part of the circuit configuration for MIMO communications can be utilized. 
     Consequently, according to the present embodiment, it is possible to prevent each node from increasing in circuit scale and power consumption in order to facilitate a handover of the mobile terminal  10  during MIMO communications. This can improve the performance of the entire system. 
     Other Embodiments 
     In the foregoing embodiment, the communication mode is switched from ordinary communications to MIMO when the data communications are resumed after the completion of the handover. The switching timing is not limited to that of the foregoing embodiment, however. For example, the data communications may be resumed by ordinary communications, and then switched to MIMO if the communication stability is confirmed.  FIG. 5  schematically shows the procedure of such an embodiment. 
     In  FIG. 5 , when the foregoing series of handover processing (step S 8 ) is completed, the mobile terminal  10 , the target base station  30 , and the core network  40  resume data communications (step S 20 ). Here, the mobile terminal  10  and the target base station  30  resume the data communication not by MIMO but by ordinary communications continuous from the handover processing. 
     After the data communications are resumed, it is examined whether the communications are normally performed between the mobile terminal  10  and the target base station  30  (step S 21 ). An arbitrary method of examination may be used as long as the communication status between the mobile terminal  10  and the target base station  30  can be checked. For example, the target base station  30  may estimate the reception of the mobile terminal  10  based on an error rate that is derived from a pilot signal in the control channel, and determine whether the current communications are normal or not from the result of estimation. If the target base station  30  determines that the communications between the own station and the mobile terminal  10  are normal (step S 22 ), the target base station  30  instructs the mobile terminal  10  to switch the communication mode from ordinary communications to MIMO (step S 23 : “MIMO start”). 
     According to the present embodiment, when the communication environment is unstable immediately after a handover, the switching to MIMO communications is suspended until the communication environment is stabilized into a normal state. The mobile terminal  10  can thus resume MIMO communications in appropriate communication environment. 
     It should be noted that the present invention is not limited to the foregoing embodiments. The present invention may be carried out with appropriate modifications within the scope of claims of the present application. For example, the present invention may be practiced as a computer program that corresponds to the operation of the radio base station ( 20 ,  30 ) or the mobile terminal ( 10 ), or a recording medium that contains the program. The foregoing embodiments have dealt with the cases where the present invention is applied to a 3GPP LTE/SAE network. However, the present invention may be applied to a conventional 3G network and networks of other standards as long as the networks are capable of radio communications including MIMO communications. 
     This application is based on and claims priority from prior Japanese Patent Application No. 2008-146878, filed 4 Jun. 2008, the entire contents of which are incorporated herein. 
     REFERENCE SIGNS LIST 
     
         
           100 : system 
           101 : interface (X 2 ) 
           102 : interface (S 1 ) 
           10 : mobile terminal 
           20 : source base station 
           30 : target base station 
           40 : core network 
           51 : transmission and reception unit, 
           52 : control unit, 
           53 : transmission unit, 
           54 : reception unit, 
           55 : array antenna, and 
           55 - 1  to  55 - n : antenna