Patent Publication Number: US-2012026983-A1

Title: Mobile wireless communication system and method of carrying out handover in the system

Description:
REFERENCE TO RELATED APPLICATION 
     This is a continuation application of U.S. patent application Ser. No. 12/300,147 filed Nov. 10, 2008 and claims the benefit of its priority. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to handover technique in a mobile wireless communication network. In particular, the invention relates to a mobile wireless communication system, a method of carrying out handover in the system, a wireless station used in the system, a mobile communication terminal device used in the system, a program for causing the wireless station to carry out the method, and a program for causing the mobile communication terminal device to carry out the method. 
     2. Description of the Related Art 
     Many attempts have been made with respect to handover between adjacent wireless stations. 
     For instance, one of examples is disclosed in Japanese Patent Application Publication No. 2005-509327. 
     It is supposed that a mobile communication terminal device is making wireless communication with a first base station, and is moving in a direction from the first base station towards a second base station disposed adjacent to the first base station. If the mobile communication terminal device detects reduction in a strength of an electric field for receiving signals from the first base station, increase in power for transmitting signals, or increase in a signal-interference ratio (SIR) in the second base station, the mobile communication terminal device treats them as triggers for starting carrying out handover to the second base station from the first base station. 
     DISCLOSURE OF THE INVENTION 
     Recently, there has been suggested a mobile communication terminal device including a plurality of wireless interfaces different from one another with respect to a communication system. 
     For instance, if a mobile communication terminal device including a WLAN wireless interface and a W-CDMA wireless interface, during making wireless communication in accordance with WLAN, could not maintain making the communication, the mobile communication terminal device switches a communication system from WLAN to W-CDMA to thereby restart making wireless communication. 
     However, since it takes time to restart the communication by switching a communication system, serviceability to a user is much deteriorated in real-time service such as voice communication or visual telephone communication. 
     In comparison with the conventional cellular system, a bandwidth is not ensured for each user in WLAN system in which packet communication is basically carried out. Accordingly, even if wireless quality were high, communication might be much delayed, voices might be interrupted, or voices might be delayed, resulting in much deterioration in serviceability to users. 
     Accordingly, if only a strength of an electric field for receiving wireless signals is used as a factor in accordance with which whether handover should be carried out is determined, only signal-receiving quality would be considered. This means that a strength of an electric field for receiving wireless signals is not sufficient for measuring communication quality in wireless communication. 
     For instance, if a strength of an electric field for receiving wireless signals were high, but the communication in the area were very crowded, it would be substantially impossible to provide sufficient real-time service (voice communication and visual communication) to users. 
     Furthermore, even if a strength of an electric field for receiving wireless signals were sufficiently high, wireless signals sometimes could not reach an access point. Thus, if only a strength of an electric field for receiving wireless signals is used as a timing factor in accordance with which whether handover should be carried out is determined, communication might be instantaneously interrupted. 
     In addition, even if wireless link between an access point and a mobile communication terminal device were sufficiently qualified, a network between the access point and a mobile communication terminal device with which the first mentioned mobile communication terminal device is going to make communication might be crowded, resulting in that a user cannot receive sufficient real-time service. 
     Furthermore, power for transmitting wireless signals is weak in a wireless communication system in which an administrator is not necessary to have a license to make wireless communication, such as WLAN, an area in which wireless signals can reach is not so wide. Thus, it is difficult to maintain communication quality high around an area in which wireless signals can reach. 
     Moreover, since internal interferences occur in a mobile communication terminal device including a plurality of wireless interfaces different from one another with respect to a communication system, it sometimes takes much time to retrieve a wireless station to which handover is directed. 
     SUMMARY OF THE INVENTION 
     In view of the above-mentioned problems in the conventional mobile wireless communication system, it is an object of the present invention to provide a mobile wireless communication system, a method of carrying out handover in the system, a wireless station used in the system, a mobile communication terminal device used in the system, a program for causing the wireless station to carry out the method, and a program for causing the mobile communication terminal device to carry out the method, all of which are capable of carrying out handover without interruption or with no interruption in communication. 
     Solution to the Problems 
     In order to achieve the above-mentioned object, the present invention provides a mobile wireless communication system including a network, a retrieval server connected in wireless communication to the network, a plurality of wireless stations connected in wireless communication to the network, and a mobile communication terminal device making communication, keeping a first wireless link comprising a pair of uplink and downlink with one of the wireless stations, wherein when a strength of an electric field for receiving wireless in the downlink is equal to or smaller than a predetermined strength, the mobile communication terminal device transmits a retrieval request to the retrieval server through a wireless station through which the mobile communication terminal device is making wireless communication, to cause the retrieval server to start selecting a new wireless station, changes at least one of profiles (UTP) for transmitting wireless in the uplink, and notifies the wireless station of the changed profile(s), the wireless station changes a profile or profiles (URP) for receiving wireless in uplink, corresponding to the changed profile(s) (UTP) for transmitting wireless in the uplink, received from the mobile wireless terminal device, changes at least one of profiles (DTP) for transmitting wireless in the downlink, and notifies the mobile wireless terminal device of the changed profile(s) (DTP) for transmitting wireless in the downlink, the mobile communication terminal device, until the mobile communication terminal device establishes a second wireless link with the new wireless station notified from the retrieval server, changes a profile or profiles (DRP) for receiving wireless in downlink, corresponding to the changed profile(s) for transmitting wireless in the downlink, received from the wireless station, to maintain the first wireless link with the wireless station. 
     The wireless station may be comprised of an access point having a WLAN (Wireless Local Area Network) function, for instance. 
     The mobile communication terminal device changes at least one of profiles (UTP) for transmitting wireless in the uplink, based on a strength of an electric field for receiving wireless, an electric power for transmitting wireless, and an error rate, for instance. 
     At least one of profiles (UTP) for transmitting wireless in the uplink is modulation, for instance. 
     At least one of profiles (UTP) for transmitting wireless in the uplink is a coding rate, for instance. 
     In the case that the first wireless link is established in accordance with OFDMA (Orthogonal Frequency Division Multiple Access), at least one of profiles (UTP) for transmitting wireless in the uplink is a number of sub-channels in the OFDMA, for instance. 
     It is preferable that the wireless station, on receipt of notification that a profile or profiles for transmitting wireless in the uplink has(have) been changed, from the mobile communication terminal device, changes at least one of profiles (DTP) for transmitting wireless in the downlink, based on a strength of an electric field for receiving wireless, an electric power for transmitting wireless, and an error rate. 
     At least one of profiles (DTP) for transmitting wireless in the downlink may be modulation, for instance. 
     At least one of profiles (DTP) for transmitting wireless in the downlink may be a coding rate, for instance. 
     At least one of profiles (DTP) for transmitting wireless in the downlink may be a number of sub-channels in the OFDMA, for instance. 
     The mobile communication terminal device may be designed to transmit the retrieval request to the retrieval server through the wireless station when a number of packet collisions exceeds a predetermined threshold. 
     The mobile communication terminal device may be designed to transmit the retrieval request to the retrieval server through the wireless station when propagation delay of packets exceeds a predetermined threshold. 
     The mobile communication terminal device may be designed to include a plurality of wireless interfaces different from each other with respect to a communication system, in which case, the mobile communication terminal device may be designed to simultaneously establish a plurality of wireless links with a plurality of wireless stations including the wireless station through which the mobile communication terminal device is making wireless communication, through the plurality of wireless interfaces, detect internal interference among the plurality of wireless interfaces, and cancel the internal interference by changing profiles for operating one of the plurality of wireless interfaces. 
     The mobile communication terminal device may be comprised of a mobile phone, for instance. 
     The present invention further provides a method of carrying out handover in a mobile wireless communication system including: a network, a retrieval server connected in wireless communication to the network, a plurality of wireless stations connected in wireless communication to the network, and a mobile communication terminal device making communication, keeping a first wireless link comprising a pair of uplink and downlink with one of the wireless stations, the method including a first step in which, when a strength of an electric field for receiving wireless in the downlink is equal to or smaller than a predetermined strength, the mobile communication terminal device transmits a retrieval request to the retrieval server through a wireless station through which the mobile communication terminal device is making wireless communication, to cause the retrieval server to start selecting a new wireless station, changes at least one of profiles (UTP) for transmitting wireless in the uplink, and notifies the wireless station of the changed profile(s), a second step in which the wireless station changes a profile or profiles (URP) for receiving wireless in uplink, corresponding to the changed profile(s) (UTP) for transmitting wireless in the uplink, received from the mobile wireless terminal device, changes at least one of profiles (DTP) for transmitting wireless in the downlink, and notifies the mobile wireless terminal device of the changed profile(s) (DTP) for transmitting wireless in the downlink, a third step in which the mobile communication terminal device, until the mobile communication terminal device establishes a second wireless link with the new wireless station notified from the retrieval server, changes a profile or profiles (DRP) for receiving wireless in downlink, corresponding to the changed profile(s) (DTP) for transmitting wireless in the downlink, received from the wireless station, to maintain the first wireless link with the wireless station, and a fourth step in which the mobile communication terminal device establishes a second wireless link with the new wireless station notified from the retrieval server. 
     The wireless station is comprised of an access point having a WLAN (Wireless Local Area Network) function, for instance. 
     It is preferable that at least one of profiles (UTP) for transmitting wireless in the uplink is changed in the first step, based on a strength of an electric field for receiving wireless, an electric power for transmitting wireless, and an error rate. 
     For instance, modulation may be selected as at least one of profiles (UTP) for transmitting wireless in the uplink. 
     For instance, a coding rate may be selected as at least one of profiles (UTP) for transmitting wireless in the uplink. 
     In the case that the first wireless link is established in accordance with OFDMA (Orthogonal Frequency Division Multiple Access), at least one of profiles (UTP) for transmitting wireless in the uplink may be a number of sub-channels in the OFDMA, for instance. 
     It is preferable that the wireless station, on receipt of notification that a profile or profiles for transmitting wireless in the uplink has(have) been changed, from the mobile communication terminal device, changes in the second step at least one of profiles (DTP) for transmitting wireless in the downlink, based on a strength of an electric field for receiving wireless, an electric power for transmitting wireless, and an error rate. 
     For instance, modulation may be selected as at least one of profiles (DTP) for transmitting wireless in the downlink. 
     For instance, a coding rate may be selected as at least one of profiles (DTP) for transmitting wireless in the downlink. 
     For instance, a number of sub-channels in the OFDMA may be selected as at least one of profiles (DTP) for transmitting wireless in the downlink. 
     It is preferable that the mobile communication terminal device transmits the retrieval request to the retrieval server through the wireless station in the first step when a number of packet collisions exceeds a predetermined threshold. 
     It is preferable that the mobile communication terminal device transmits the retrieval request to the retrieval server through the wireless station in the first step when propagation delay of packets exceeds a predetermined threshold. 
     The mobile communication terminal device may be designed to include a plurality of wireless interfaces different from each other with respect to a communication system, in which case, it is preferable that the method further includes a step in which the mobile communication terminal device simultaneously establishes a plurality of wireless links with a plurality of wireless stations including the wireless station through which the mobile communication terminal device is making wireless communication, through the plurality of wireless interfaces, detects internal interference among the plurality of wireless interfaces, and cancels the internal interference by changing profiles for operating one of the plurality of wireless interfaces. 
     The present invention further provides a wireless station to be used in a mobile wireless communication system including: a network, a retrieval server connected in wireless communication to the network, a plurality of wireless stations connected in wireless communication to the network, and a mobile communication terminal device making communication, keeping a first wireless link comprising a pair of uplink and downlink with one of the wireless stations, wherein, in the case that, when a strength of an electric field for receiving wireless in the downlink is equal to or smaller than a predetermined strength, the mobile communication terminal device transmits a retrieval request to the retrieval server through a wireless station through which the mobile communication terminal device is making wireless communication, to cause the retrieval server to start selecting a new wireless station, changes at least one of profiles (UTP) for transmitting wireless in the uplink, and notifies the wireless station of the changed profile(s), the wireless station changes a profile or profiles (URP) for receiving wireless in uplink, corresponding to the changed profile(s) (UTP) for transmitting wireless in the uplink, received from the mobile wireless terminal device, changes at least one of profiles (DTP) for transmitting wireless in the downlink, and notifies the mobile wireless terminal device of the changed profile(s) (DTP) for transmitting wireless in the downlink. 
     The wireless station is comprised of an access point having a WLAN (Wireless Local Area Network) function, for instance. 
     It is preferable that the mobile communication terminal device, on receipt of notification that a profile or profiles for transmitting wireless in the uplink has(have) been changed, from the mobile communication terminal device, changes at least one of profiles (UTP) for transmitting wireless in the uplink, based on a strength of an electric field for receiving wireless, an electric power for transmitting wireless, and an error rate. 
     For instance, modulation may be selected as at least one of profiles (UTP) for transmitting wireless in the uplink. 
     For instance, a coding rate may be selected as at least one of profiles (UTP) for transmitting wireless in the uplink. 
     For instance, a number of sub-channels in the OFDMA may be selected as at least one of profiles (UTP) for transmitting wireless in the uplink. 
     The present invention further provides a mobile communication terminal device in a mobile wireless communication system including: a network, a retrieval server connected in wireless communication to the network, a plurality of wireless stations connected in wireless communication to the network, and the mobile communication terminal device, the mobile communication terminal device making communication, keeping a first wireless link comprising a pair of uplink and downlink with one of the wireless stations, wherein when a strength of an electric field for receiving wireless in the downlink is equal to or smaller than a predetermined strength, the mobile communication terminal device transmits a retrieval request to the retrieval server through a wireless station through which the mobile communication terminal device is making wireless communication, to cause the retrieval server to start selecting a new wireless station, changes at least one of profiles (UTP) for transmitting wireless in the uplink, and notifies the wireless station of the changed profile(s), when the wireless station changes a profile or profiles (URP) for receiving wireless in uplink, corresponding to the changed profile(s) (UTP) for transmitting wireless in the uplink, received from the mobile wireless terminal device, changes at least one of profiles (DTP) for transmitting wireless in the downlink, and notifies the mobile wireless terminal device of the changed profile(s) (DTP) for transmitting wireless in the downlink, the mobile communication terminal device, until the mobile communication terminal device establishes a second wireless link with the new wireless station notified from the retrieval server, changes a profile or profiles (DRP) for receiving wireless in downlink, corresponding to the changed profile(s) for transmitting wireless in the downlink, received from the wireless station, to maintain the first wireless link with the wireless station. 
     It is preferable that the mobile communication terminal device changes at least one of profiles (UTP) for transmitting wireless in the uplink, based on a strength of an electric field for receiving wireless, an electric power for transmitting wireless, and an error rate. 
     There may be selected modulation as at least one of profiles (UTP) for transmitting wireless in the uplink. 
     There may be selected a coding rate as at least one of profiles (UTP) for transmitting wireless in the uplink for instance. 
     It is preferable that the mobile communication terminal device transmits the retrieval request to the retrieval server through the wireless station when a number of packet collisions exceeds a predetermined threshold. 
     It is preferable that the mobile communication terminal device transmits the retrieval request to the retrieval server through the wireless station when propagation delay of packets exceeds a predetermined threshold. 
     The mobile communication terminal device may be designed to include a plurality of wireless interfaces different from each other with respect to a communication system, in which case, it is preferable that the mobile communication terminal device simultaneously establishes a plurality of wireless links with a plurality of wireless stations including the wireless station through which the mobile communication terminal device is making wireless communication, through the plurality of wireless interfaces, detects internal interference among the plurality of wireless interfaces, and cancels the internal interference by changing profiles for operating one of the plurality of wireless interfaces. 
     The mobile communication terminal device may be comprised of a mobile phone, for instance. 
     The present invention further provides a program for causing a computer mounted in a wireless station to carry out a method of carrying out handover in a mobile wireless communication system including: a network, a retrieval server connected in wireless communication to the network, a plurality of wireless stations connected in wireless communication to the network, and a mobile communication terminal device making communication, keeping a first wireless link comprising a pair of uplink and downlink with one of the wireless stations, wherein steps executed by the computer in accordance with the program includes a step in which in the case that, when a strength of an electric field for receiving wireless in the downlink is equal to or smaller than a predetermined strength, the mobile communication terminal device transmits a retrieval request to the retrieval server through a wireless station through which the mobile communication terminal device is making wireless communication, to cause the retrieval server to start selecting a new wireless station, changes at least one of profiles (UTP) for transmitting wireless in the uplink, and notifies the wireless station of the changed profile(s), the wireless station changes a profile or profiles (URP) for receiving wireless in uplink, corresponding to the changed profile(s) (UTP) for transmitting wireless in the uplink, received from the mobile wireless terminal device, changes at least one of profiles (DTP) for transmitting wireless in the downlink, and notifies the mobile wireless terminal device of the changed profile(s) (DTP) for transmitting wireless in the downlink. 
     The present invention further provides a program for causing a computer mounted in a mobile communication terminal device to carry out a method of carrying out handover in a mobile wireless communication system including: a network, a retrieval server connected in wireless communication to the network, a plurality of wireless stations connected in wireless communication to the network, and a mobile communication terminal device making communication, keeping a first wireless link comprising a pair of uplink and downlink with one of the wireless stations, wherein steps executed by the computer in accordance with the program includes a first step in which in the case that, when a strength of an electric field for receiving wireless in the downlink is equal to or smaller than a predetermined strength, the mobile communication terminal device transmits a retrieval request to the retrieval server through a wireless station through which the mobile communication terminal device is making wireless communication, to cause the retrieval server to start selecting a new wireless station, changes at least one of profiles (UTP) for transmitting wireless in the uplink, and notifies the wireless station of the changed profile(s), a second step in which when the wireless station changes a profile or profiles (URP) for receiving wireless in uplink, corresponding to the changed profile(s) (UTP) for transmitting wireless in the uplink, received from the mobile wireless terminal device, changes at least one of profiles (DTP) for transmitting wireless in the downlink, and notifies the mobile wireless terminal device of the changed profile(s) (DTP) for transmitting wireless in the downlink, the mobile communication terminal device changes a profile or profiles (DRP) for receiving wireless in downlink, corresponding to the changed profile(s) (DTP) for transmitting wireless in the downlink, received from the wireless station, to maintain the first wireless link with the wireless station, and a third step in which the mobile communication terminal device establishes a second wireless link with the new wireless station notified from the retrieval server. 
     The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptional view of the mobile wireless communication system in accordance with the first exemplary embodiment of the present invention. 
         FIG. 2  is a block diagram illustrating a structure of the mobile communication terminal device used in the mobile wireless communication system in accordance with the first exemplary embodiment of the present invention. 
         FIG. 3  is a block diagram showing an example of a control circuit mounted in the mobile communication terminal device used in the mobile wireless communication system in accordance with the first exemplary embodiment of the present invention. 
         FIG. 4  is a block diagram illustrating a structure of the first wireless station used in the mobile wireless communication system in accordance with the first exemplary embodiment of the present invention. 
         FIG. 5  is a flow-chart showing an operation of the control circuit mounted in the mobile communication terminal device used in the mobile wireless communication system in accordance with the first exemplary embodiment of the present invention. 
         FIG. 6  is a flow-chart showing an operation of a control circuit equipped in the first wireless station used in the mobile wireless communication system in accordance with the first exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
       FIG. 1  is a conceptional view of the mobile wireless communication system in accordance with the first exemplary embodiment of the present invention. 
     The mobile wireless communication system in accordance with the first exemplary embodiment is comprised of a network  100 , a retrieval server  102  capable of making wireless communication through the network  100 , a plurality of wireless stations each capable of making wireless communication through the network  100  (only a first wireless station  103  and a second wireless station  104  are illustrated in  FIG. 1 ), and a mobile communication terminal device  105  making wireless communication, keeping a first wireless link comprising a pair of uplink and downlink with one of the wireless stations. 
     The network  100  is comprised of Internet, for instance. 
     The retrieval server  102  includes a database indicating a location of each of the wireless stations. On receipt of a retrieval request from the mobile communication terminal device  105 , the retrieval server  102  retrieves the database to identify a wireless station located in the vicinity of the mobile communication terminal device  105  in accordance with both a location of the mobile communication terminal device  105  and a direction in which the mobile communication terminal device  105  moves. 
     Each of the first wireless station  103  and the second wireless station  104  is comprised of a WLAN access point or a mobile communication network base station. In the first exemplary embodiment, the first wireless station  103  is comprised of a WLAN access point, and the second wireless station  104  is comprised of a WLAN access point or a mobile communication network base station. 
     The mobile communication terminal device  105  is comprised of a mobile phone, for instance. 
       FIG. 1  illustrates that the mobile communication terminal device  105  establishes wireless link with the first wireless station  103 , is making wireless communication with a communication terminal device  106  through the network  100 , and starts moving in a direction A during the communication, approaching the second wireless station  104 . 
       FIG. 2  is a block diagram of the mobile communication terminal device  105  in the first exemplary embodiment. 
     As illustrated in  FIG. 2 , the mobile communication terminal device  105  includes an OFDMA (Orthogonal Frequency Division Multiple Access) wireless interface  201 , a W-CDMA (Wideband Code Division Multiple Access) wireless interface  202 , a decoder  203 , a circuit  204  for disassembling packets, a packet  205  for assembling packets, an encoder  206 , a measuring circuit  207 , a codec  208 , a control circuit  209 , a circuit  210  for measuring an error rate, a circuit  211  for detecting packet collision, a circuit  212  for measuring reciprocating propagation delay, and an antenna  213 . 
     The OFDMA wireless interface  201  establishes a plurality of sub-channels with a wireless station comprised of a WLAN access point. 
     The W-CDMA wireless interface  202  establishes a spread spectrum channel with a wireless station comprised of a mobile communication network base station. 
     The mobile communication terminal device  105  and the communication terminal device  106  transmit signals to and receive signals from each other through the codec  208  and the W-CDMA wireless interface  202 . 
     Signals having been received at the OFDMA wireless interface  201  through the antenna  213  are decoded in the decode  203 , and then, converted into original digital signals in the packet - disassembling circuit  204 . 
     User data to be transmitted are assembled into packets in the packet-assembling circuit  205 , encoded in the encoder  206 , and then, transmitted through the OFDMA wireless interface  201  and the antenna  213 . 
       FIG. 3  is a block diagram of an exemplary structure of the control circuit  209 . 
     The control circuit  209  is comprised of a central processing unit (CPU)  2091 , a first memory  2092 , a second memory  2093 , an input interface  2094  through which a command and/or data is input into the central processing unit  2091 , an output interface  2095  through which a result of steps having been executed by the central processing unit  2091  is output, and buses  2096  through which the central processing unit  2091  is electrically connected with the other parts. 
     Each of the first and second memories  2092  and  2093  is comprised of a semiconductor memory such as a read only memory (ROM), a random access memory (RAM) or an IC memory card, or a storage device such as a flexible disc, a hard disc or an optic magnetic disc. In the present exemplary embodiment, the first memory  2092  comprises a ROM, and the second memory  2093  comprises a RAM. 
     The first memory  2092  stores therein fixed data such as a program to be executed by the central processing unit  2091 . The second memory  2093  stores therein various data and parameters, and provides an area for the central processing unit  2091  to operate therein. That is, the second memory  2093  stores data therein temporarily necessary for the central processing unit  2091  to execute the program. 
     The central processing unit  2091  reads the program out of the first memory  2092 , and executes the program. In other words, the central processing unit  2091  operates in accordance with the program stored in the first memory  2092 . 
     The measuring circuit  207  is comprised of a circuit  221  for measuring a strength of an electric field for receiving signals, and a circuit  222  for measuring a power for transmitting signals. The measuring circuit  207  is connected to the OFDMA wireless interface  201  and the W-CDMA wireless interface  202 . The circuit  221  measures a strength of an electric field for receiving signals, and transmits a signal indicative of the measured strength, to the control circuit  209 . The circuit  222  measures a power for transmitting signals in the mobile communication terminal device  105 , and transmits a signal indicative of the measured power to the control circuit  209 . 
     The error-rate measuring circuit  210  is connected to an output of the decoder  203 . The error-rate measuring circuit  210  measures an error rate in signals having been received in the OFDMA wireless interface  201 , and transmits the measures error rate to the control circuit  209 . 
     The control circuit  209  judges quality of a wireless link established between the mobile communication terminal device  105  and the first wireless station  103 , based on a strength of an electric field for receiving signals, received from the circuit  221 , a power for transmitting signals, received from the circuit  222 , and an error rate received from the error-rate measuring circuit  210 . 
     For instance, if signals transmitted from the first wireless station (access point)  103  can readily reach the mobile communication terminal device  105 , and signals transmitted from the mobile communication terminal device  105  can readily reach the first wireless station (access point)  103 , the control circuit  209  judges that a wireless link established between the mobile communication terminal device  105  and the first wireless station  103  has high quality. 
     The control circuit  209  varies profiles in the mobile communication terminal device  105  for transmitting signals, in accordance with the judgment results, as mentioned later. 
     When a strength of an electric field for receiving signals is equal to or smaller than a predetermined strength, the control circuit  209  transmits a retrieval request packet to the retrieval server  102  through the packet-assembling circuit  205 , the encoder  206 , the OFDMA wireless interface  201 , and the antenna  213 . 
     The control circuit  209  simultaneously determines modulation, a number of sub-channels, and a coding rate in uplink in accordance with the above-mentioned three measurements for reestablishing the OFDMA wireless interface  201  and the encoder  206 , and transmits the thus determined operational profiles in uplink to the first wireless station  103  as control packets (a request for changing profiles) through the packet-assembling circuit  205 . 
     On receipt of the control packets from the mobile communication terminal device  105 , the first wireless station  103  determines modulation, a number of sub-channels, and a coding rate in downlink in accordance with the above-mentioned three measurements having been measured by the first wireless station  103 , and changes current modulation, a current number of sub-channels, and a current coding rate into the thus determined modulation, number and rate, and further, notifies the mobile communication terminal device  105  of the thus determined operational profiles in downlink. 
     Signals indicative of the operational profiles in downlink transmitted to the mobile communication terminal device  105  from the first wireless station  103  are converted in the packet-disassembling circuit  204  into control data used for controlling receiving signals, and then, transmitted to the control circuit  209 . The control circuit  209  changes demodulation in the OFDMA wireless interface  201 , and a number of sub-channels in accordance with the received control data. 
     The circuit  211  detects collision of packets. The packet-collision detecting circuit  211  is connected to both the packet-assembling circuit  205  and the packet-disassembling circuit  204 . The packet-collision detecting circuit  211  counts a number by which packets having been transmitted from the mobile communication terminal device  105  collide with other packets on a transmission path, to thereby judge communication quality except the above-mentioned wireless link. 
     When communication traffic increases in a common area, the mobile communication terminal device  105  attempts to transmit data such that the data does not interfere with data transmitted from other mobile communication terminal devices. However, the greater a number of collision is, the lower estimate to communication quality in the area is. 
     The circuit  212  is connected to both the packet-assembling circuit  205  and the packet-disassembling circuit  204 , and measures a reciprocating propagation time in which packets transmitted from the mobile communication terminal device  105  are received by the communication terminal device  106 , and reply packets transmitted from the communication terminal device  106  reach the mobile communication terminal device  105 . 
     The measured reciprocating propagation time is transmitted to the control circuit  209 . The control circuit  209  judges quality of communication made between the mobile communication terminal device  105  and the communication terminal device  106 , in accordance with how much degree the measured reciprocating propagation time delays in comparison with standard reciprocating propagation time. 
     Specifically, since a lot of routers exist between the first wireless station  103  and the communication terminal device  106 , a period of time necessary for signals to arrive at the communication terminal device  106  from the mobile communication terminal device  105  varies in dependence on communication traffic passing through the routers. The control circuit  109  judges that the shorter the propagation delay time is, the higher communication quality on network between the mobile communication terminal device  105  and the communication terminal device  106  is, and vice versa. 
     The control circuit  209  further compares a number of packet collisions indicated in the signal transmitted from the circuit  211  to a predetermined number. If a number of packet collisions is equal to or greater than the predetermined number, the control circuit  209  transmits a retrieval request to the retrieval server  102 . 
     The control circuit  209  further compares the reciprocating propagation time indicated in the signal transmitted from the circuit  212  to a predetermined time. If the reciprocating propagation time is equal to or greater than the predetermined time, the control circuit  209  transmits a retrieval request to the retrieval server  102 . 
     The measuring circuit  207  monitors whether internal interference, which occurs between the OFDMA wireless interface  201  and the W-CDMA wireless interface  202  when both of them operate, occurs. If the measuring circuit  207  detects the internal interference, the measuring circuit  207  transmits a signal  207   a  indicative of the detection of the internal interference to the control circuit  209 . 
     On receipt of the signal  207   a  indicative of the detection of the internal interference, the control circuit  209  varies wireless profiles of the W-CDMA wireless interface  202  to thereby stop the internal interference. It is possible to stop the internal interference by varying wireless profiles of the OFDMA wireless interface  201 , if handover is not being carried out. 
       FIG. 4  is a block diagram showing a structure of the first wireless station  103 . 
     As illustrated in  FIG. 4 , the first wireless station  103  is comprised of an OFDMA wireless interface  301 , a decoder  302 , a network interface  303 , an encoder  304 , a circuit  305  for measuring an error rate, a measuring circuit  306 , a control circuit  307 , and an antenna  308 . 
     Signals having been transmitted from the mobile communication terminal device  105  are received by the OFDMA wireless interface  301 , input into the decoder  302 , and decoded in the decoder  302 . 
     The decoded signals are transmitted to the communication network  100  through the network interface  303 , and then, transmitted to the communication terminal device  106  through the communication network  100 . 
     Signals transmitted from the communication terminal device  106  are input into the network interface  303  through the communication network  100 , and then, encoded in the encoder  304 . Then, the signals are transmitted to the mobile communication terminal device  105  through the OFDMA wireless interface  301 . 
     The error-rate measuring circuit  305  is connected to an output of the decoder  302 . The error-rate measuring circuit  305  measures an error rate of signals having been transmitted from the mobile communication terminal device  105 , and notifies the control circuit  307  of the measurement. 
     The measuring circuit  306  is comprised of a circuit  311  for measuring a strength of an electric field for receiving signals, and a circuit  312  for measuring a power for transmitting signals. The measuring circuit  306  is connected to the OFDMA wireless interface  301 . The circuit  311  measures a strength of an electric field for receiving signals, and transmits a signal indicative of the measured strength, to the control circuit  307 . The circuit  312  measures a power for transmitting signals in the first wireless station  103 , and transmits a signal indicative of the measured power to the control circuit  307 . 
     The control circuit  307  has the same structure as that of the control circuit  209  of the mobile communication terminal device  105  illustrated in  FIG. 3 . Specifically, a central processing unit in the control circuit  307  reads a program out of a first memory, and executes the program. That is, the central processing unit in the control circuit  307  operates in accordance with the program stored in the first memory. 
     On receipt of the retrieval request from the mobile communication terminal device  105 , the control circuit  307  transmits a retrieval request packet to the retrieval server  102  through the network interface  303 , and receives a reply thereof as data indicative of an access point (AP). 
     On receipt of a request to change a profile or profiles from the mobile communication terminal device  105 , the control circuit  307  determines modulation, a number of sub-channels, and a coding rate in accordance with a strength of an electric field for receiving signals, a power for transmitting signals, and an error rate in received signals in the first wireless station  103 , applies the determined modulation, number and rate to associated devices thereof, and notifies the mobile communication terminal device  105  of the determined modulation, number and rate. 
       FIG. 5  is a flow-chart showing an operation of the control circuit  209  of the mobile communication terminal device  105  in the mobile wireless communication system in accordance with the present exemplary embodiment. 
     The operation of the control circuit  209  of the mobile communication terminal device  105  in the present exemplary embodiment is explained hereinbelow with reference to  FIG. 5 . 
     It is supposed that the mobile communication terminal device  105  presently uses the OFDMA wireless interface  201 , and has wireless link with the first wireless station  103  comprised of a WLAN access point. 
     First, the control circuit  209  judges whether a strength of an electric field in the signals having been transmitted from the first wireless station  103  is equal to or smaller than a predetermined strength (step  401 ). 
     If a strength of an electric field in the signals having been transmitted from the first wireless station  103  is equal to or smaller than a predetermined strength (YES in step  401 ), the control circuit  209  transmits a retrieval request packet to the retrieval server  102  (step  402 ). 
     Then, the control circuit  209  receives both a strength of an electric field for receiving signals in downlink and a power for transmitting signals in uplink from the measuring circuit  207 , and further receives an error rate in downlink from the error-rate measuring circuit  210  to thereby determine profiles (UTP) (modulation, a number of sub-channels for transmitting signals, and a coding rate in uplink) for transmitting signals in uplink, which indicate a plurality of operational profiles in uplink in the mobile communication terminal device  105  (step  403 ). 
     Then, the control circuit  209 , as a result of the uplink signal-transmission profiles (UTP) being changed, transmits how the uplink signal-transmission profiles (UTP) was changed by the mobile communication terminal device  105  to the first wireless station  103  as a message for requesting changing profiles (URP) for receiving signals in uplink in the first wireless station  103  (step  404 ). 
     Then, the control circuit  209  receives a message for requesting changing downlink signal-receipt profiles (DRP) from the first wireless station  103  (step  405 ). 
     Then, the control circuit  209  changes modulation, demodulation, a number of sub-channels for transmitting signals, a number of sub-channels for receiving signals, and a coding rate in accordance with both the uplink signal-transmission profiles (UTP) having been determined before, and the downlink signal-receipt profiles (DRP) having been received from the first wireless station  103  (step  406 ). 
     It is possible for signals to reach as far as possible by changing the modulation. 
     Specifically, by changing 64 QAM into modulation having a low modulation rate, such as 16 QAM or 4 PSK, a signal can reach remoter in modulation having a low modulation rate than in modulation having a high modulation rate, even if the signal is transmitted at the same signal-transmission power. 
     Furthermore, it is possible to enhance ability of correcting an error for readily reproducing data by changing a coding rate such that redundant data increases. 
     In addition, it is possible to increase a maximum distance by which a signal can reach, by reducing a number of sub-channels to thereby raise an energy density per a sub-channel. 
     Then, the control circuit  209  receives a list of candidates of wireless stations from the retrieval server  102 , and selects the most optimal wireless station among them (step  407 ). 
     Thereafter, the control circuit  209  judges whether the selected wireless station is a base station for a mobile communication network (step  408 ). Specifically, the control circuit  209  judges whether the selected wireless station is a base station for a mobile communication network or a wireless access point. 
     If the selected wireless station is a wireless access point (NO in step  408 ), the control circuit  209  establishes wireless link with the newly selected wireless station through the OFDMA wireless interface  201  which the mobile communication terminal device  105  presently uses, and notifies the newly selected wireless station of an address of the communication terminal device  106  (step  412 ). 
     Then, the control circuit  209  releases the previously used wireless link (step  413 ). 
     Thereafter, the control circuit  209  repeatedly carries out step  401  and the subsequent steps. 
     If the selected wireless station is a base station for a mobile communication network (YES in step  408 ), the control circuit  209  establishes wireless link with the newly selected wireless station through the W-CDMA wireless interface  202 , and notifies the newly selected wireless station of an address of the communication terminal device  106  (step  409 ). 
     Then, the control circuit  209  judges whether there exists internal interference between the OFDMA wireless interface  201  and the W-CDMA wireless interface  202 , in accordance with the signal  207   a  received from the measuring circuit  207  (step  410 ). 
     If there exists no internal interference (NO in step  410 ), the control circuit  209  releases the previously used wireless link (step  413 ). 
     If there exists internal interference (YES in step  410 ), the control circuit  209  prevents internal interference by changing wireless profiles of the W-CDMA wireless interface  202  (step  411 ). 
     Then, the control circuit  209  releases the previously used wireless link (step  413 ). 
     Thereafter, the control circuit  209  repeatedly carries out step  401  and the subsequent steps. 
     If a strength of an electric field in the signals having been transmitted from the first wireless station  103  is greater than the predetermined strength (NO in step  401 ), the control circuit  209  judges whether a number of packet collisions indicated in a signal transmitted from the packet-collision detecting circuit  211  is greater than a predetermined number (step  421 ). 
     If a number of packet collisions is greater than the predetermined number (YES in step  421 ), the control circuit  209  transmits a retrieval request to the retrieval server  102  (step  423 ). 
     Thereafter, the control circuit  209  carries out step  407  and the subsequent steps to thereby newly select a wireless station, and establishes wireless link with the newly selected wireless station (steps  407 - 413 ). 
     If a number of packet collisions is equal to or smaller than the predetermined number (NO in step  421 ), the control circuit  209  judges whether t 
     If the reciprocating propagation delay of packets is greater than the predetermined delay (YES in step  422 ), the control circuit  209  transmits a retrieval request to the retrieval server  102  (step  423 ). 
     If the reciprocating propagation delay of packets is equal to or smaller than the predetermined delay (NO in step  422 ), the control circuit  209  carries out again step  410  and the subsequent steps. 
       FIG. 6  is a flow-chart showing an operation of the control circuit  307  equipped in the first wireless station  103 . 
     The operation of the control circuit  307  equipped in the first wireless station  103  is explained hereinbelow with reference to  FIG. 6 . 
     The control circuit  307  equipped in the first wireless station  103  monitors whether the message (step  404  in  FIG. 5 ) requesting changing a profile (URP) for receiving signals in uplink is received from the mobile communication terminal device  105  (step  501 ). 
     If the message for requesting changing a profile (URP) for receiving signals in uplink is received from the mobile communication terminal device  105  (YES in step  501 ), the control circuit  307  receives both a strength of an electric field for receiving signals in uplink and a power for transmitting signals in downlink from the circuits  311  and  312  of the measuring circuit  306 , respectively, and further receives an error rate in uplink from the error-rate measuring circuit  305 . Then, the control circuit  307  determines profiles (DTP) (modulation, a number of sub-channels for transmitting signals, and a coding rate) for transmitting signals in downlink in accordance with the above-mentioned strength, power and error rate  (step  502 ) . 
     Then, the control circuit  307  transmits the profiles (DTP) for transmitting signals in downlink to the mobile communication terminal device  105  as a request for changing profiles (DRP) for receiving signals in downlink (step  503 ). 
     Then, the control circuit  307  changes modulation, demodulation, a number of sub-channels for transmitting signals, a number of sub-channels for receiving signals, and a coding rate in uplink in accordance with both the profiles (URP) for receiving signals in uplink received from the mobile communication terminal device  105 , and the profiles (DTP) for transmitting signals in downlink (step  504 ). 
     Thereafter, the control circuit  307  repeatedly carries out step  501  and the subsequent steps. 
     If the message for requesting changing a profile (URP) for receiving signals in uplink is not received from the mobile communication terminal device  105  (NO in step  501 ), the control circuit  307  judges whether the retrieval request is received from the mobile communication terminal device  105  (step  511 ). 
     If the retrieval request is received from the mobile communication terminal device  105  (YES in step  511 ), the control circuit  307  transmits the received retrieval request to the retrieval server  102  (step  512 ). 
     Thereafter, the control circuit  307  repeatedly carries out step  501  and the subsequent steps. 
     Supposing that a certain wireless station has received a list of wireless station candidates from the retrieval server  102 , the wireless station is the second wireless station  104  which is comprised of a wireless access point or a base station for a mobile wireless network and to which handover is directed. This means that the first wireless station  103  does not receive a retrieval request from the mobile communication terminal device  105  (NO in step  511 ). 
     Thus, the later-mentioned steps  521  to  524  (surrounded with a broken line) are carried out by the second wireless station  104 . 
     The second wireless station  104  judges whether a list of wireless station candidates is received from the retrieval server  102  (step  521 ). 
     If a list of wireless station candidates is received from the retrieval server  102  (YES in step  521 ), the second wireless station  104  establishes wireless link with the mobile communication terminal device  105 , and transmits the list to the mobile communication terminal device  105 . 
     The second wireless station  104  receives an address of the communication terminal device  106  from the mobile communication terminal device  105  (step  523 ). 
     Then, the second wireless station  104  establishes wireless link with the communication terminal device  106  through the network  100  in accordance with the received address (step  524 ). 
     Thereafter, the control circuit  307  repeatedly carries out step  501  and the subsequent steps. 
     As explained above, the mobile communication terminal device  105  in the mobile wireless communication system in accordance with the present exemplary embodiment, if a strength of an electric field therein for receiving signals is equal to or smaller than a predetermined strength, changes profiles for transmitting signals in accordance with a plurality of operational parameters in order to maintain wireless link with the first wireless station  103  as long as possible, and notifies the first wireless station  103  of the changed profiles. 
     Before changing the profiles, the mobile communication terminal device  105  transmits a retrieval request to the retrieval server  102  through the first wireless station  103  to request the retrieval server  102  to find another wireless station. 
     The retrieval server  102  retrieves a database to select a wireless station as the second wireless station  104  in accordance with a current location of the mobile communication terminal device  105 , and notifies the mobile communication terminal device  105  of an identification number of the selected wireless station through the first wireless station  103 . 
     On receipt of the notification, the mobile communication terminal device  105  starts carrying out handover, establishes wireless link with the second wireless station  104 , and notifies the second wireless station  104  of an address of the communication terminal device  106 . 
     Receiving the notification, the second wireless station  104  establishes wireless link with the communication terminal device  106  through the network  100 . Thus, the handover is achieved to the second wireless station  104  from the first wireless station  103 . 
     The mobile communication terminal device  105  in the mobile wireless communication system in accordance with the present exemplary embodiment maintains wireless link through which the mobile communication terminal device  105  is presently making wireless communication with the first wireless station  103 , until the mobile communication terminal device  105  newly establishes wireless link with the second wireless station  104  to which handover is directed. Thus, continuity in communication is ensured as long as possible after a strength of an electric field for receiving signals in downlink has fallen below a predetermined strength, resulting in stable quality in wireless link. That is, the handover is carried out without interruption in communication. 
     INDUSTRIAL APPLICABILITY 
     The present invention presents handover technique suitable to a system for combining mobile communication and IP (internet protocol) with each other. 
     Furthermore, the present invention uses both an OFDMA wireless interface and a W-CDMA wireless interface to make it possible to carry out handover over systems different from each other. For instance, the present invention can be applied to handover to be carried out between WLAN and W-CDMA, and further between WiMAX, Bluetooth, iBurst or wireless USB and a conventional cellular system. 
     The exemplary advantages obtained by the above-mentioned exemplary embodiments are described hereinbelow. 
     In the mobile wireless communication system in accordance with the present invention, the mobile communication terminal device maintains wireless link with a wireless station through which the mobile communication terminal device is presently making wireless communication, by changing a profile or profiles for transmitting or receiving wireless signals, until the mobile communication terminal device newly establishes wireless link with a wireless station to which handover is directed. Thus, it is possible to ensure continuity in communication as long as possible after a strength of an electric field for receiving wireless signals in downlink has been equal to or smaller than a predetermined threshold, and hence, keep quality of the wireless link stable. That is, it is possible to guarantee carrying out handover without interruption. 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.