Abstract:
A system for controlling handover is disclosed that includes a mobile terminal that enables communications with multiple wireless systems having wireless access stations, and a handover-controlling apparatus that executes control of handover for the mobile terminal, which travels among multiple types of wireless systems. The mobile terminal includes a destination wireless system determining unit that receives downlink signals from the corresponding wireless access stations so as to determine which wireless system accommodates a destination wireless access station based on reception quality of downlink signals. The mobile terminal also includes a destination wireless system reporting unit that sends information indicating the determined wireless system to the handover-controlling apparatus. The handover-controlling apparatus includes a handover-controlling unit that controls handover operations for the mobile terminal based on the information indicating the determined wireless system, which is sent from the destination wireless system reporting unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. patent application Ser. No. 10/778,106, filed Feb. 17, 2004, and claims priority to Japanese Patent Application No. 2003-047761, filed Feb. 25, 2003. The entire contents of these applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a system for and its method of controlling handover, a mobile terminal, and an apparatus for controlling handover, and particularly relates to a system for and its method of controlling handover between homogeneous or heterogeneous wireless systems at the network side so as to enable a reduction in the throughput of the mobile terminal. 
     Furthermore, the present invention relates to the mobile terminal and the apparatus for controlling handover which are used in such a system for controlling handover. 
     2. Description of the Related Art 
     In recent years, the Wireless Local Area Network (WLAN) has been spreading rapidly and also has been watched with keen interest as means for connecting to the Internet. 
     In the WLAN, when changing on the move from one connecting access point to another, the IP address of the mobile terminal changes so that a problem of intermittent data flow arises in a case such as streaming. To this end, as a method of solving this problem, the “Mobile IP” has been proposed. In the Mobile IP, having a home address and an agent address enables the mobile terminal to retain connectivity even when the connecting access point changes from one to another. 
     Furthermore, although the transmission data rate of the WLAN is higher than that of a mobile telephone network, in order for the WLAN to achieve the same level of ease-of-use as the mobile telephone network, the problem of handover needs to be solved so as to be able to use the WLAN consistently without a break in data communications even when the mobile terminal is on the move. In order to respond to this problem, the Hierarchical Mobile IP (HMIP) which is based on the Mobile IP has been proposed. This HMIP enables the enhancement in the speed of handover as registration at the home agent is not performed every time a location is registered. More specifically, a server called the Mobility Anchor Point (MAP) having a function similar to that of the home agent is provided within a foreign network, and the location information is registered within the MAP so as to implement the enhancement in the handover speed. 
     Non-Patent Document 1 
     Hesham Soliman, Claude Castelluccia, Karim El-Malki, Ludovic Bellier, “Hierarchical MIPv6 mobility management”, Internet draft, draft-ietf-mobileip-hmipv6-03.txt., 2001 
     However, as seen in the HMIP described above, in the handover using the WLAN, the terminal itself needs to designate a route in the network. An IP (Internet Protocol)-centric network (the so-called IP network) is layered so that the own layer number and the IP address are provided as broadcast information to the terminal. At the terminal side, its own optimal route (which does not produce redundant routes) is determined using the broadcast information. 
     Now it is expected that, in the next generation mobile telephone system which follows the current mobile telephone system, a wide variety of wireless systems including the mobile telephone and the WLAN systems will be interconnected via the IP network. For example, in the case of accommodating such a wide variety of wireless systems in a common IP network, the development of handover technology to seamlessly connect the wide variety of wireless systems in response to the location and the taste of the user, the communications content, etc., will become quite important. 
     Furthermore, at the terminal side, although study on a software-defined radio that enables the handling of a variety of wireless methods and protocols is underway, it is expected that the amount of processing at the terminal side will increase in order to achieve such greater functionality. Therefore, if the HMIP of the Mobile IP as described above is implemented as it is in the software-defined radio, a problem arises such that there will be a further increase in the amount of processing at the terminal so as to make minituarization difficult. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a system for and its method of controlling handover, a mobile terminal, and an apparatus for controlling handover that substantially obviate one or more problems caused by the limitations and disadvantages of the related art. 
     In light of the problems as described above, it is a more particular object of the present invention to provide a system for and its method of controlling handover between homogeneous or heterogeneous wireless systems at the network side so as to enable the seamless implementation of handover between homogeneous or heterogeneous wireless systems while seeking a reduction in the amount of processing at the mobile terminal for controlling handover in the case of accommodating multiple types of wireless networks in the common network. 
     Furthermore, the present invention relates to the mobile terminal and the apparatus for controlling handover which are used in such a system for controlling handover. 
     According to the invention, a system for controlling handover includes a mobile terminal which enables communication with a plurality of wireless systems having wireless access stations, and a handover-controlling apparatus which executes control of handover for the mobile terminal which travels among a plurality of types of the wireless systems, wherein the mobile terminal includes a destination wireless system determining unit which receives downlink signals from the respective wireless access stations so as to determine a wireless system which accommodates a destination wireless access station based on reception quality of the downlink signals, and a destination wireless system reporting unit which sends information indicating said determined wireless system to said handover-controlling apparatus, and said handover-controlling apparatus includes a handover-controlling unit which controls handover operations for said mobile terminal based on the information indicating the wireless system which is sent from said destination wireless system reporting unit. 
     The system for controlling handover as described above enables the reduction of handover control processing at the mobile terminal side as the apparatus for controlling handover at the network side selects the handover method which fulfills the request of the mobile terminal based on the wireless system type information which is provided from the mobile terminal. This enables the simplification of said apparatus even in such a case as a software-defined radio in which the mobile terminal is enabled to conduct communications with the wireless systems. 
     According to another aspect of the invention, an apparatus for controlling handover which executes control of handover for a mobile terminal that travels among a plurality of types of wireless systems includes a handover-controlling unit which controls handover operations for said mobile terminal based on the information indicating a wireless system which is sent from said mobile terminal. 
     The apparatus for controlling handover as described above enables the reduction of handover control processing at the mobile terminal side as said apparatus at the network side selects the handover method which fulfills the request of the mobile terminal based on the wireless system type information which is provided from the mobile terminal. 
     According to another aspect of the invention, a mobile terminal enables communication with a plurality of wireless systems having wireless access stations and the handover to a plurality of types of the wireless systems includes a destination wireless system determining unit which receives downlink signals from respective wireless access stations so as to determine the wireless system which accommodates the destination wireless access station based on the reception quality of the received downlink signals, and a destination wireless system reporting unit which sends to said apparatus at the network side the information indicating said determined wireless system. 
     The mobile terminal as described above enables the reduction of handover control processing at said terminal side as the apparatus for controlling handover at the network side selects the handover method which fulfills the request of said terminal based on the wireless system type information which is provided from said terminal. This enables the simplification of the apparatus even in such a case as a software-defined radio in which said terminal is enabled to conduct communications with the wireless systems. 
     According to another aspect of the invention, a method of controlling handover in the system for controlling handover which includes the mobile terminal that enables the communication with the plurality of wireless systems having wireless access stations and the apparatus for controlling handover that executes control of handover for said mobile terminal that travels among the plurality of types of wireless systems, said method including the steps of receiving, at said mobile terminal, the downlink signals from the respective wireless access stations so as to determine the wireless system which accommodates the destination wireless access station based on the reception quality of the received downlink signals and sending, from said mobile terminal, information indicating said determined wireless system to said apparatus for controlling handover, and controlling, by said apparatus, handover operations for said mobile terminal based on the information indicating said reported wireless system. 
     The method of controlling handover as described above enables the reduction of handover control processing at the mobile terminal side as the apparatus for controlling handover at the network side selects the handover method which fulfills the request of the mobile terminal based on the wireless system type information which is provided from the mobile terminal. This enables the simplification of the apparatus even in such a case as a software-defined radio in which the mobile terminal is enabled to conduct communications with the wireless systems. 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system for controlling handover which is concerned with a working form of the invention; 
         FIG. 2  is a block diagram of an IP network as illustrated in  FIG. 1 ; 
         FIG. 3  is a block diagram of a mobile terminal as illustrated in  FIG. 1 ; 
         FIG. 4  is a schematic diagram which illustrates a state of communications for describing handover operations of a system for controlling handover according to the working form of the invention; 
         FIG. 5  is the first part of a sequence diagram which illustrates handover operations of a system for controlling handover according to the working form of the invention; 
         FIG. 6  is the second part of the sequence diagram which illustrates handover operations of a system for controlling handover according to the working form of the invention; and 
         FIG. 7  is a table which illustrates an exemplary structure of an AP-AR management database. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention are described with reference to the accompanying drawings. 
       FIG. 1  is a block diagram of a system for controlling handover which is concerned with a working form of the invention. 
     In  FIG. 1 , this handover control system  100  comprises a WCDMA (Wideband Code Division Multiple Access)/WLAN multi-functional wireless communications terminal (hereinafter abbreviated as the mobile terminal)  10  which handles both the WCDMA method based on the IMT-2000 standards specifications and the WLAN method, a WCDMA access point  20  (for example, a wireless base station), a WLAN access point  30 , and an IP network  40 , the mobile terminal  10  being enabled for connection to the Internet  50  via the IP network  40 . 
       FIG. 2  is a block diagram of the IP network  40  which is illustrated in  FIG. 1 . 
     In  FIG. 2 , this IP network  40  comprises access routers  41  through  45  which connect to the WLAN access point  30  and the WCDMA access point  40 , a handover method selector  46 , a handover method effector  47 , and a database  48  which manages the corresponding relationships between the access routers and the access points (or an AP-AR management database). 
     The handover method selector  46  refers to the AP-AR management database  48  based on the information provided from the mobile terminal  10  so as to select an optimal handover method and notifies the handover method effector  47  of the selection. The handover method effector  47  performs the handover processing according to the handover method as provided from the handover method selector  46 . The specific processing procedures are described below. 
       FIG. 3  is a block diagram of a mobile terminal  10  as illustrated in  FIG. 1 . 
     In  FIG. 3 , this mobile terminal  10  comprises an antenna  11  which receives a WCDMA wireless or a WLAN wireless signal, a multi-band RF/IF unit  12  which switches to a filter that is relevant to the corresponding wireless methods of said received signal so as to convert the wireless signal to an IF signal, an ADC (Analog-to-Digital Converter) and DAC (Digital-to-Analog Converter)  13  which performs conversion processing between an analog IF signal and a digital IF signal, a signal processor  14  which performs signal processing that is relevant to the method (for example, despreading processing, modulation/demodulation processing), an external interface unit  16  which is provided with an interface function for outputting to an external Ethernet (a registered trademark), etc., the output received from the signal processor  14 , a Central Processing Unit (CPU)  15  which is in charge of controlling the overall system and determines a destination wireless system based on the received input signal output from the multi-band RF/IF unit  12 , and the network interface cards (NICs)  17  and  18  which accommodate, for example, the communications protocols for performing wireless connections according to said corresponding wireless methods. 
     The mobile terminal  10  as described above comprises, for example, a software-defined radio which enables the operation, by downloading to the multi-band RF/IF unit  12 , the ADC and DAC  13 , and the signal processor  14 , the software of the communications method accommodated in NIC  17  or NIC  18 , as an apparatus at the terminal side for the corresponding methods. Besides, the software of the corresponding methods may be accommodated in one NIC, or one NIC may be provided per wireless method. In the present embodiment, a NIC is provided per wireless method so that a WLAN NIC  17  and a WCDMA NIC  18  are implemented in the mobile terminal  10 . 
     Besides, the example described above is an example in which the NIC and the antenna  11  are separately configured, but there may be a case in which a NIC includes an antenna. In such a case, such antenna serves the function of the antenna  11  as described above. 
     Referring to  FIG. 4  through  FIG. 6 , the handover operations of the system for controlling handover which is configured as described above are described. 
       FIG. 4  is a diagram which illustrates a state of communications so as to describe the handover operations of a system for controlling handover according to a working form of the invention. 
       FIG. 5  and  FIG. 6  are sequence diagrams which illustrate the handover operations of a system for controlling handover according to a working form of the invention. 
     In  FIG. 4 , the example is intended to represent a case in which the mobile terminal  10  is located in the vicinity of the boundary between a wireless zone that comprises a WLAN access point AP-LAN  30  and a wireless zone that comprises a WCDMA access point AP-WCDMA  20  and in which the mobile terminal  10  is seeking a handover from a WLAN wireless system to a WCDMA wireless system. 
     Moreover, taking an access router that is proximate to the WLAN access point AP-LAN  30  as AR-A  101  and an access router that is proximate to the WCDMA access point AP-WCDMA  20  as AR-B  102 , the access routers AR-A  101  and AR-B  102  are connected to a router R-A  103 , the handover method selector  46 , and the handover method effector  47  so as to form the respective communications routes. Furthermore, a transmitting mobile terminal  70  which transmits a packet to the mobile terminal  10  conducts wireless communications with an access point AP-TX  60  which is currently communicating so that a packet which is received at the AP-TX  60  is transferred via a proximate router R-B  104 . 
     Now referring to  FIG. 5  and  FIG. 6 , the handover operations of a system for controlling handover according to a working form of the invention are described in detail. 
     A user packet which is transmitted at the transmitting mobile terminal  70  is received at the wireless system access point AP-TX  60  and then is sequentially transferred in the order of R-B  104 , R-A  103 , AR-A  101 , and AP-LAN  30  so as to be converted to a wireless signal at the AP-LAN  30  and sent to the mobile terminal  10  (S 1 ). The wireless signal which transmits said user packet is input to the multi-band RF/IF unit  12  of the mobile terminal  10 , which measures the received input value. The measured value is converted to a predetermined signal format at the W-LAN NIC  17  so as to be sent to the CPU  15 . Furthermore, as the mobile terminal  10  is then in the vicinity of the boundary of the wireless zone which comprises the WCDMA access point AP-WCDMA  20 , the wireless signal which is being transmitted from said AP-WCDMA  20  is also received. Therefore, as described above, the measurement result of the received input of the wireless signal which is transmitted from AP-WCDMA  20  is also sent to the CPU  15  (S 3 ). 
     The CPU  15  of the mobile terminal  10  compares the received input value from AP-LAN  30  and that from AP-WCDMA  20  so as to determine the wireless system with the higher received input value as the destination wireless system. Here it is assumed that the WCDMA wireless system is determined as the destination wireless system. Also, then, the CPU  15  detects a change in the respective received inputs at AP-LAN  30  and at AP-WCDMA  20  so as to estimate from said detected amount of change the travelling speed of the mobile terminal  10 . For example, the Doppler velocity is calculated based on the distribution of the received power so as to assume the Doppler velocity as the travelling speed of the mobile terminal (S 4 ) 
     The CPU  15 , as described above, determines the WCDMA wireless system as the destination wireless system and, when the projection of the travelling speed is completed, a startup instruction which triggers the startup of WCDMA wireless communications is output to NIC  17  (S 5 ) Upon receiving this startup instruction, the WCDMA NIC  18  outputs and sets up at the signal processor  14 , the ADC/DAC  13 , and the multi-band RF/IF unit  12 , the software in which various parameters (such as modulation parameters) for receiving the WCDMA wireless signal are stated so as to enable the reception of the broadcast information which is periodically broadcast from the AP-WCDMA  20 . This broadcast information is received at the CPU  15  via the multi-band RF/IF unit  12 , the ADC/DAC  13  and the signal processor  14  (S 6 ), said CPU  15  determining the destination access point (hereinafter abbreviated as the destination AP) according to the address information of the access router which is included in this broadcast information (in this case, determined as AR-B  102 ) (S 7 ). 
     The CPU  15  outputs to the signal processor  14  the information of the destination AP which is determined in said manner (1), the travelling speed information of the mobile terminal  10  which is previously predicted (2) and the information on the wireless communications system previously determined (to be called the wireless system type information (3)). A set of information items comprising (1), (2) and (3) as described above which is input to the signal processor  14 , after undergoing modulation processing, etc., is converted to an analog signal at the DAC portion of the ADC/DAC  13  and then is converted to a wireless signal at the multi-band RF/IF unit  12  so as to be transmitted to the AP-WCDMA  20 . Subsequently, the information set is sent to the handover method selector  46  via the AR-B  102  within the IP network  40  (S 8 ). 
     The handover method selector  46 , upon receiving the set of information items (1), (2) and (3), or the destination AP information, the speed information of the mobile terminal  10 , and the wireless system type information, accesses the AP-AR management database  48  so as to determine whether the access router which is in the proximity of the destination access point differs from the access router which is in the proximity of the access point prior to travelling (in this case, referred to as the determination on the compatibility of the access routers) (S 9 ). 
     More specifically, the compatibility of the access routers is determined as described below.  FIG. 7  is a table which illustrates an exemplary structure of the AP-AR management database. This AP-AR management database is a database for registering and/or inquiring about the corresponding relationships between the access points and the access routers per user and may be a database server, for example. 
     In  FIG. 7 , this AP-AR management database  48  manages per user the Access Point address (the AP address), the Access Router address (the AR address), the wireless QoS parameter which corresponds to the required QoS class, the wireless system type information and the wireless quality condition information. 
     Hereupon, it is assumed that a user A of the mobile terminal  10  is currently conducting wireless communications with the AP-LAN  30  so as to transfer a user packet via AR-A  101 . Therefore, in the AP-AR management database  48 , the AP-LAN  30  address as the AP address for the user A and the AR-A  101  address as the AR address are registered. 
     The handover method selector  46 , upon receiving the wireless system type information and the notification of AP-WCDMA  20  as the destination AP, refers to the AP-AR management database  48  so as to retrieve the AP address which corresponds to the AP address of the AP-WCDMA  20 , the destination access point. In this example, the fact that the AR address which corresponds to the AP address of the AP-WCDMA  20  is AR-B  102  may be determined from the same database. In this connection, this AR-B  102  is used for transferring the user packet from the mobile terminal of a certain user x. 
     The handover method selector  46  upon identifying the destination access router as described above determines whether this destination access router and the access router prior to travelling are compatible. 
     As described below, there are, for example, two methods of determining this compatibility of access routers. 
     Method 1 
     Method 1 is a method which corresponds to a handover between heterogeneous wireless systems in which the compatibility of the access routers is determined based on the wireless system type information. For example, in the example as described above, the wireless system type information of AR-A  101  and the wireless system type information of AR-B  102  are compared so as to determine whether there is compatibility of the access points in the case that the types are different. 
     Method 2 
     Method 2 is a method which corresponds to a handover between homogeneous wireless systems in which a prefix is set up per access router so as to determine whether there is compatibility of the access routers in the case that the prefixes are different. 
     The handover method selector  46 , upon completing the determination of the compatibility of the access routers as described above, transmits to the mobile terminal  10  a request for setting up a WCDMA wireless link (S 10 ). Thereafter, a predetermined process to set up a wireless link between the AP-WCDMA  20  and the mobile terminal  10  is executed (S 11 ) so that a wireless link setup completion notification is sent from the mobile terminal  10  to the handover method selector  46  upon completing this setup process. 
     The handover method selector  46 , upon receiving the wireless link set-up completion notification (S 12 ), determines a handover method based on the wireless characteristics of the destination wireless system (S 13 ). More specifically, a network method is selected (or a handover algorithm is determined) based on the wireless system type information sent from the mobile terminal  10 , and then a determination is made as to whether a high-speed handover is to be performed based on the information of the travelling speed of the mobile terminal. 
     Selection of Handover Method 
     The handover method selector  46  selects a handover algorithm based on the destination wireless system according to the criteria as described below. 
     Selection Criteria of Handover Method 
     (1) In the case of WCDMA: the soft-handover method 
     (2) In the case of HSDPA (High Speed Downlink Packet Access), an enhanced WCDMA method: the IP-packet-buffering method 
     (3) In the case of WLAN: the IP-packet-buffering method 
     (4) In the case of PDC (Personal Digital Cellular): the hard-handover method 
     Below, the respective handover methods as listed above according to an embodiment of the invention are described. 
     The soft-handover method of (1) refers to a method in which the same packet is sent at the time of handover to the router of the destination and the router prior to travelling. 
     The IP-packet-buffering method of (2) and (3) refers to a method in which an IP packet is buffered temporarily at the time of handover, the buffering being completed at the time the mobile terminal changes over to the destination router. 
     The hard-handover method of (4) refers to a method in which at the time of handover a routing cache table is newly created while the routing cache table of the router prior to travelling is deleted. 
     In the case of the present embodiment, since WCDMA is selected as the destination wireless system based on the wireless system type information, the soft-handover method (1) is selected. 
     Then, the handover method selector  46  selects whether a high-speed handover is to be performed based on the criteria as described below. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Selection Criteria of High-speed Handover 
               
             
          
           
               
                   
                   
                 High-speed handover 
               
               
                   
                 Speed of terminal 10 
                 selection 
               
               
                   
                   
               
               
                   
                 High speed 
                 Required 
               
               
                   
                 Medium speed 
                 Not required 
               
               
                   
                 Low speed 
                 Not required 
               
               
                   
                   
               
             
          
         
       
     
     Herein, the high-speed handover method refers to a method in which the signaling procedure (referring to a signal procedure to set up a communications channel with the other party before starting communications and to carry out the connection with the other party) of the wireless link is simplified, or to multicasting to the candidate destination routers at the network side, in order to enable, at the time the mobile terminal  10  is travelling at high speed, the accomplishment of the handover for the mobile terminal. 
     Based on the travelling speed information from the mobile terminal  10 , the handover method selector  46  selects the high-speed handover method in the case where the mobile terminal  10  is determined to be travelling at high speed while the high-speed handover method is not selected in the cases where the mobile terminal  10  is determined to be travelling at either low speed or medium speed. 
     As described above, the handover method selector  46  selects a handover method according to the type of the wireless system and determines whether the high-speed handover method is required so that the result (for example, a combination of the handover method and a determination as to whether the high-speed handover method is required) is provided to the handover method effector  47  (S 14 ). The handover effector  47  performs processing for the mobile terminal  10  to make the handover based on the provided result (S 15 ). More specifically, the handover method effector  47  transfers the user packet which is transmitted from the transmitting mobile terminal  70  to AP-WCDMA  20  and an access point which neighbors said AP-WCDMA (tentatively referred to herein as the AP-WCDMA  2 ) via R-A  103  and AR-B  102  (S 16 ). In the case where the mobile terminal  10  is located in the vicinity of the boundary of the wireless zones comprising the respective access points, the mobile terminal  10  receives and modulates the user packets which are transmitted from the respective access points. Furthermore, the mobile terminal  10 , in the case where the terminal is not located in the vicinity of the boundary, receives and modulates the user packet which is received from either access point. Besides, in the case that the handover method selector  46  determines that the high-speed handover method is required, a further transfer of the user packet is performed, omitting the signaling procedure as described above. 
     As described above, according to the present working form, the handover method selector  46  at the network side selects a handover method which fulfills the requirement of the mobile terminal  10  based on the destination AP information, the wireless system type information and the travelling speed information which are provided by the mobile terminal  10  so that the selection of the handover method at the side of the mobile terminal  10  is not required. In other words, the identification of the handover method at the mobile terminal  10  is not required so as to enable a reduction in the amount of handover control processing at the mobile terminal. Therefore, even in the case of the mobile terminal  10  in which the handover between homogeneous or heterogeneous wireless systems is implemented by a software-defined radio, the configuration of the apparatus at the mobile side does not become complex, enabling the simplification of the apparatus. 
     Although the embodiment describes an example in which the handover method selector  46  and the handover method effector  47  are configured separately, they do not necessarily have to be set up separately and may be configured in one server unit which includes both functions. 
     Moreover, although the embodiment describes a case in which the handover method selector  46  selects the handover method based on the wireless system type information which is received from the mobile terminal  10 , the present invention is not limited to that case. For example, it may take a form in which the mobile terminal  10  monitors the wireless link quality conditions prior to and after travelling, and the monitoring results are provided to the handover method selector  46  at the network side so that the selector selects the handover method based on the wireless link quality conditions. In this case, the mobile terminal  10  calculates the BER (Bit Error Rate) or the S/N (Signal-to-Noise ratio) using the received inputs from the access point prior to travelling (the WLAN system in the case of the present embodiment) and the access point after travelling (the WCDMA system in the case of the present embodiment) so as to provide the calculation results as the wireless link quality conditions to the handover method selector  46  at the network side. The handover method selector  46  performs the selection of the handover method according to the criteria as described below: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Handover method 
               
               
                   
                   
                 before/after 
               
               
                   
                 Wireless link quality conditions 
                 travelling 
               
               
                   
                   
               
             
             
               
                   
                 Good/good 
                 Hard-handover 
               
               
                   
                   
                 method 
               
               
                   
                 Good/bad 
                 Soft-handover 
               
               
                   
                   
                 method 
               
               
                   
                 Bad/good 
                 IP-packet- 
               
               
                   
                   
                 buffering 
               
               
                   
                   
                 method 
               
               
                   
                 Bad/bad 
                 Soft-handover 
               
               
                   
                   
                 method 
               
               
                   
                   
               
             
          
         
       
     
     For example, in the case such that the BER is good before travelling and bad after travelling, the soft-handover method in which the packets are sent from two or more access points is selected as the handover method to obtain a predetermined reception quality. This enables the execution of handover without degrading the reception quality at the mobile terminal  10 . 
     Moreover, although the AP-AR management database  48  in the embodiment described above assumes a database server which is connected externally to the handover method selector  46 , the configuration is not limited to this form, but may take, for example, a form such that the functions of the AP-AR management database  48  are provided within the handover method selector  46 . 
     Furthermore, although the example as described thus far indicates a case in which communications with multiple wireless systems is enabled by having the mobile terminal  10  either to switch NICs which store the respective wireless system communications protocols or to implement multiple NICs, the present invention is not limited to this form. For example, it may take a form in which the software which is stored in the NICs is downloaded from a server, etc., that is provided externally. In such a case, as the software in which a wireless protocol most suitable for the location as stated is automatically downloaded to the mobile terminal  10 , the user of the mobile terminal  10  does not have to keep possession of multiple NICs, thus improving the ease-of-use for the user. Also, although the embodiment describes a case in which the mobile terminal  10  is a multi-functional WCDMA/WLAN terminal which handles both WCDMA and WLAN communications methods, it is possible as a matter of course to apply to the present invention a multi-functional terminal which handles more than two types of wireless systems.