Abstract:
A mobile communication system includes using a mobile terminal, base stations and a base station control device. Each, each of the base stations cover respective areas for mobile communication service and is capable of relaying a conversation, made by the mobile terminal, by radio communication. The base station control device observes (or monitors) traffic (i.e., number of calls in progress) with respect to each of the base stations. In response to a conversation start event or a conversation end event that takes place in the mobile terminal which is located in an area of a base station whose traffic is large, the base station control device searches a surrounding base station whose traffic is relatively small. The base station control device then controls an area of the searched base station to enable radio communication with respect to the mobile terminal. For example, output of the searched base station is increased so that an area of the searched base station is broadened to cover a location of the mobile terminal. Thus, it is possible to avoid occurrence of call failure due to shortage of communication lines.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to mobile communication systems that are configured by mobile stations (or mobile terminals) and radio base stations, and particularly to traffic collection and control methods employed for radio base station control devices for controlling the radio base stations. This application is based on patent application No. Hei 9-152734 filed in Japan, the content of which is incorporated herein by reference. 
     2. Description of the Related Art 
     According to the conventional traffic control method of the mobile communication system, there is provided a threshold value which is subjected to “handoff” to or from a certain base station. For example, the conventional method controls call a quantity (or number of calls) of the base station, at which a number of calls concentrate, by changing its threshold value. An example of the above method is disclosed by the paper of Japanese Patent Application, Publication No. Hei 6-164477. 
     Now, the above control method will be described in conjunction with FIG.  9 . FIG. 9 shows an example of the mobile communication system that contains a mobile telephone exchange  4 , a first radio base station  5 , a second radio base station  6  and a mobile terminal  7 . Consider a case where a communication channel utilization rate α of the first radio base station  5 , which acts as a handoff source, is less than a prescribed threshold value, while a communication channel utilization rate β of the second radio base station  6 , which acts as a handoff destination, is greater than (or equal to) the prescribed threshold value. In such a case, a threshold value ΔAB is provided as a handoff condition and is used for making a decision with respect to a receiving level difference between a first receiving level, which is detected between the first radio base station  5  and the mobile terminal  7  located in proximity to the handoff source, and a second receiving level which is detected between the second radio base station  6  and the mobile terminal  7 . This threshold value ΔAB is corrected by a value X, which is calculated based on the communication channel utilization rates α and β, so the corrected one is used as a new threshold value “Δ+X”. Incidentally, a reference symbol A designates an area (or base station area) set for the first radio base station  5 , B designates an area (or base station area) set for the second radio base station  6 . In addition, ΔAB designates the threshold value for the receiving level difference calculated in connection with a direction from the first radio base station  5  to the second radio base station  6 , while ΔBA designates a threshold value for a receiving level difference calculated in connection with an inverse direction. 
     According to the aforementioned traffic control method which is conventionally known, controlling of traffic quantity of the base station area is performed by controlling the hand-over of the mobile terminal. So, it is not possible to reduce the traffic quantity of the base station based on where the mobile terminal is located unless the mobile terminal moves from the base station area corresponding to a hand-over source to the base station area corresponding to a hand-over destination. 
     In the aforementioned traffic control method, mobile terminals concentrate at the area of the base station where the handoff is performed. For this reason, if a number of the mobile terminals temporarily reaches a prescribed number of mobile terminals for which the base station allows communications, a mobile terminal which is over the prescribed number of mobile terminals cannot send or receive a call. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a mobile communication system with traffic collection and control for radio base stations, which is capable of reducing a number of call failure events due to shortage of communication lines of the radio base station and which is capable of effectively using the radio resource. 
     A mobile communication system of this invention is established using a mobile terminal, base stations and a base station control device. Herein, each of the base stations having respective areas for mobile communication services thereof is capable of relaying conversation, made by the mobile terminal, by radio communication. The base station control device observes (or monitors) traffic (i.e., number of calls in progress) with respect to each of the base stations. In response to a conversation start event or a conversation end event that takes place in the mobile terminal which is located in an area of a base station whose traffic is large, the base station control device searches a surrounding base station whose traffic is relatively small. For management of the statuses of the base stations, the base station control device provides a table storing a number of calls in progress as well as a list of surrounding base stations with respect to each of the base stations. 
     The base station control device controls an area of the searched base station to enable radio communication with respect to the mobile terminal. For example, output of the searched base station is increased so that an area of the searched base station is broadened to cover location of the mobile terminal. 
     Thus, it is possible to provide uniform call connection service. In addition, it is possible to avoid occurrence of call failure due to shortage of communication lines, so it is possible to effectively use radio resources. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects of the subject invention will become more fully apparent as the following description is read in light of the attached drawings wherein: 
     FIG. 1 is a block diagram showing an example of a mobile communication system in accordance with the invention; 
     FIG. 2 shows an example of content of a table, which is set in an area control unit shown in FIG. 1; 
     FIG. 3 is a block diagram showing an internal configuration of a base station; 
     FIG. 4 is a block diagram showing an internal configuration of a mobile terminal; 
     FIG. 5A is a sequence chart showing a sequence of operations with respect to start and end of conversation that take place in a base station control device shown in FIG. 1; 
     FIG. 5B shows an example of variations of numbers of calls in progress, which are observed by a traffic observation unit shown in FIG. 1; 
     FIG. 6 is a flowchart showing operations of an area control unit shown in FIG. 1; 
     FIG. 7 is a flowchart showing operations of a base station shown in FIG. 1; 
     FIG. 8 is a flowchart showing operations of a mobile terminal shown in FIG. 1; and 
     FIG. 9 is a block diagram showing an example of a mobile communication system, which is conventionally known. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a block diagram showing an example of a mobile communication system in accordance with the preferred embodiment of the invention. In FIG. 1, a base station control device  11  controls traffic of (radio) base stations  21 ,  22  and  23 . 
     In FIG. 1, mobile terminals  31  and  32  are located within areas  21   a,    22   a  and  23   a,  which are respectively set for the base stations  21 ,  22  and  23 . Conversations that take place in mobile terminals  31  and  32  are relayed using radio communication to the base station control device  11 , from which they are further relayed to an exchange station (not shown). 
     The base station control device  11  is configured using a conversation detection unit  111 , a traffic observation unit (or traffic monitoring unit)  112  and an area control unit  113 . Herein, the conversation detection unit  111  detects a conversation start event and a conversation end event of the mobile terminal ( 31 ,  32 ) at the base stations ( 21  to  23 ). So, the conversation detection unit  111  communicates the detected event(s) together with a base station ID (or identification number of the base station) to the traffic observation unit  112 . After detection of the conversation start event and conversation end event, the traffic observation unit  112  calculates traffic data with respect to each of the base stations  21  to  23 . The area control unit  113  broadens a conversation-enabled area of an adjacent base station, having a small number of calls, which is located adjacent to the base station whose number of calls is great. That is, the area control unit  113  controls the adjacent base station to increase its output in response to variations of the traffic data calculated for the base station whose number of calls is great, wherein the adjacent base station having a small number of calls is selected from among the base stations which are located adjacent to the base station whose number of calls is great. 
     At the start or end of the conversation of the base station, the conversation detection unit  111  detects a conversation start event or a conversation end event of the base station. So, the conversation detection unit  111  communicates a detection result together with an identification number of the base station to the traffic observation unit  112 . 
     Based on the identification numbers as well as the communication start events and communication end events which are communicated from the conversation detection unit  111 , the traffic observation unit  112  calculates a number of calls in progress that each of the base stations  21  to  23  presently makes, so the calculated number is communicated to the area control unit  113 . 
     When receiving the number of calls in progress, which is calculated for each of the base stations  21  to  23  and is communicated from the traffic observation unit  112 , the area control unit  113  communicates it to the mobile terminal ( 31 ,  32 ) over a control channel (not shown) of the corresponding base station. At an event that a certain base station increases a number of calls in progress, the area control unit  113  selects a base station whose traffic is small from among the base stations surrounding the certain base station. So, the area control unit  113  makes a request to increase radio output of the selected base station whose traffic is small. 
     Each of the base stations  21  to  23  has a function to control radio output in response to an instruction of the area control unit  113  of the base station control device  11 . So, the base station broadcasts traffic information, given from the base station control device  11 , to the mobile terminal over the control channel thereof. 
     In the case of the minimum output, the areas  21   a,    22   a  and  23   a  are set for the base stations  21 ,  22  and  23  respectively. Herein, each of the areas  21   a,    22   a  and  23   a  is capable of covering the full service area, which is determined in advance. In response to a request from the base station control device  11 , the base station  22  is solely capable of broadening the area thereof up to an area  22   b.    
     The mobile terminal ( 31 ,  32 ) receives the traffic information from each of the base stations  21  to  23  over its control channel. Based on the traffic information, the mobile terminal makes a call using a base station whose traffic is small and which is selected from among the base stations that do not obstruct the conversation. 
     FIG. 2 shows an example of a configuration of a table  113   a  provided in the area control unit  113  of the base station control device  11  of FIG.  1 . The table  113   a  of FIG. 2 stores multiple pieces of information with regard to each of the base stations, which are designated by base station names of “Base Station 21”, “Base Station 22”, “Base Station 23”, . . . In the table  113   a,  there are provided multiple items, i.e., “Number of Calls in Progress” and “Output Data” as well as “Data of Surrounding Base Stations”. As for the item of “Number of Calls in Progress”, numbers such as “6”, “3” and “4” are listed with regard to the base stations  21 ,  22  and  23  respectively. As for the item of “Output Data” which represents an output state of each base station, words such as “Minimum”, “Medium” and “Maximum” can be listed. As for the item of “Data of Surrounding Base Stations”, data and numerals designating the base stations, such as “21”, “22” and “23”, are listed. The content of the table  113   a  is changeable in response to the observation result of the traffic control unit  112  and the control result of the area control unit  113 . 
     FIG. 3 is a block diagram showing an internal configuration of the base station  21 , for example. Herein, the base station  21  is configured using an output control unit  211 , a maximum output data storage unit  212 , a minimum output data storage unit  213  and a transceiver unit  214 . 
     The output control unit  211  receives an area change request given from the area control unit  113  of the base station control device  11 . When the area change request represents a request to increase output, the output control unit  211  increases the output of the base station  21  if the output is less than a value of the maximum output data storage unit  212 . When the area change request represents a request to decrease output, the output control unit  211  decreases the output of the base station  21  if the output is greater than a value of the minimum output data storage unit  213 . 
     The transceiver unit  214  of the base station  21  receives data representing a communication channel and a control channel which is communicated thereto from the base station control device  11  so as to transmit them by radio. For convenience&#39; sake, detailed configurations of the other base stations  22  and  23  are not illustrated, however, they are similar to the aforementioned configuration of the base station  21 . So, operations of the base stations  22  and  23  are similar to the aforementioned operation of the base station  21 . 
     FIG. 4 is a block diagram showing an internal configuration of the mobile terminal  31 . Herein, the mobile terminal  31  is configured using a channel control unit  311  and a conversation quality threshold value storage unit  312 . 
     When the mobile terminal  31  starts a conversation, the channel control unit  311  receives all of control channels which are receivable. If the control channels do not contain the control channel whose conversation quality is better than a value of the conversation quality threshold value storage unit  312 , the channel control unit  311  selects the control channel whose conversation quality is the highest among the control channels. So, the channel control unit  311  uses the selected control channel to perform the conversation. 
     If there exists the control channel whose conversation quality is better than a value of the conversation quality threshold value storage unit  312 , the channel control unit  311  starts conversation by selecting a base station whose number of calls in progress is small. The aforementioned area control unit  113  of the base station control device  11  communicates information such a base station whose number of calls in progress is small to the mobile terminal  31  via a certain base station. Incidentally, an internal configuration of the other mobile terminal  32 , which is not illustrated, is similar to the aforementioned internal configuration of the mobile terminal  31 . So, operation of the mobile terminal  32  is similar to the aforementioned operation of the mobile terminal  31 . 
     FIG. 5A is a sequence chart showing operations of the base station control device  11  shown in FIG. 1 at detection of the start of conversation. FIG. 5B shows variations of observation results of the traffic observation unit  112  of the base station control device  11 . 
     FIG. 6 is a flowchart showing operations of the area control unit  113  of the base station control device  11 . FIG. 7 is a flowchart showing operations of the base station ( 21 - 23 ). FIG. 8 is a flowchart showing operations of the mobile terminal ( 31 ,  32 ). Using the aforementioned figures (i.e., FIG. 1 to FIG.  8 ), a description will be given with respect to the operation of the traffic control in accordance with the present example of the invention. 
     Incidentally, operations of the base station control device  11 , base stations  21 - 23  and mobile terminals  31 ,  32  are respectively actualized by execution of control programs stored in a control memory (not shown). As for the control memory, it is possible to employ the ROM (i.e., read-only memory), floppy disk and the like. 
     At first, the mobile terminal  32  receives a control channel from a surrounding base station thereof so as to start conversation using the base station  21 . Thus, the conversation detection unit  111  of the base station control device  11  detects the start of the conversation. The conversation detection unit  111  communicates with the traffic observation unit  112  to inform a conversation start event, representing the start of the conversation, and an identification number of the base station  21  at which the conversation is started. The traffic observation unit  112  manages a number of calls in progress within a traffic observation area (not shown) of the base station  21  corresponding to the identification number, which is communicated thereto from the conversation detection unit  111 . So, the traffic observation unit  112  adds “1” to a present number of the calls in progress within the traffic observation area (see FIG.  5 B). 
     When the mobile terminal  32  ends the conversation using the base station  21 , the conversation detection unit  111  detects an end of the conversation. Thus, the conversation detection unit  111  communicates with the traffic observation unit  112  to inform a conversation end event, representing the end of the conversation, and an identification number of the base station  21  at which the conversation is ended. So, the traffic observation unit  112  decreases “1” from the present number of calls in progress within the traffic observation area corresponding to the identification number of the base station  21 , which is communicated thereto from the conversation detection unit  111  (see FIG.  5 B). 
     When the conversation detection unit  111  detects a conversation start event or a conversation end event as described above, the traffic observation unit  112  communicates the number of calls in progress and the identification number of the base station  21  to the area control unit  113  (see FIG.  5 A). 
     The area control unit  113  makes a decision as to whether the number of calls in progress, which is assigned to the base station  21  and is communicated from the traffic observation unit  112 , is greater than a specific threshold value (e.g., “6”) or not in step S 1  shown in FIG.  6 . If the number of calls in progress is over the specific threshold value, a program control goes to step S 2  wherein the area control unit  113  searches surrounding base stations from the content of the table  113   a.  Among the surrounding base stations, the area control unit  113  extracts a base station (e.g., base station  22 ) whose traffic is small and whose amount of increasable output capacity is greater than a threshold value (e.g., “3”). So, the area control unit  113  issues an output increase request that requests the base station  22  to increase output. These operations are made in steps S 3 , S 4  and S 5  in FIG.  6 . Incidentally, if the base station  23  has a small traffic and an amount of increasable output capacity that is greater than the threshold value, the area control unit  113  issues an output increase request to the base station  23  as well. 
     When inputting the output increase request from the area control unit  113  (see step S 11  in FIG.  7 ), the base station  22  makes a decision with respect to radio output thereof. That is, if the radio output of the base station  22  is not the maximum output so that the area  22   b  is not established (see step S 12 ), the base station  22  increases the radio output thereof to enlarge the area thereof up to the area  22   b  (see step S 13 ). 
     On the other hand, when inputting an output decrease request from the area control unit  113  (see step S 11 ), the base station  22  makes a decision with respect to the radio output thereof. That is, if the radio output is not the minimum output so that the area  22   a  is not established (see step S 14 ), the base station  22  decreases the radio output thereof so as to downsize the area thereof to the area  22   a  (see step S 15 ). Incidentally, each of the base stations  21  to  23  broadcasts traffic information thereof over the control channel. 
     At completion of the process for obtaining search data (see step S 6 ), corresponding to the aforementioned search performed in step S 2 , the area control unit  113  proceeds to step S 7 . If the output of the base station  21 , to which a conversation start event is communicated, is not the minimum output so that the area of the base station  21  does not coincide with the area  21   a,  the area control unit  113  issues an output decrease request to the base station  21  (see step S 8 ). 
     When starting a new conversation, the mobile terminal  31  receives all of receivable control channels with respect to the base stations  21  to  23  (see step S 22  shown in FIG.  8 ), wherein the base station  21  has six channels while the base station  22  has three channels, for example. If there exists a channel whose conversation quality is good (see step S 22 ), the mobile terminal  31  selects a base station whose traffic is the smallest, i.e., base station  22  whose number of calls in progress is small (see step S 23 ). Using the selected base station  22 , the mobile terminal  31  performs conversation with a called party (see step S 24 ). 
     If there exists no channel whose conversation quality is good (see step S 22 ), the mobile terminal  31  selects a channel whose conversation quality is better within the existing channels (see step S 25 ). So, the mobile terminal  31  performs conversation with a called party via the base station over the selected channel. 
     The present example of the invention is described with respect to the case where the area of the base station  22  is broadened in response to an increase of the traffic of the base station  21 . The present example can be designed in such a way that an output decrease request is not made with respect to the base station  22  even in the case where the traffic quantity of the base station  21  is reduced in order to decrease the output of the base station  22  when the traffic of the base station  22  increases. 
     As described heretofore, the present example of the invention is designed as follows: 
     The traffic observation unit  112  normally observes the traffic quantity with respect to each of the base stations  21  to  23 . When a number of calls in progress of the base station  21 , to which a conversation start event is communicated, exceeds a prescribed threshold value, the system of the present example broadens the area of the base station  22  whose traffic quantity is small within adjacent base stations (i.e., surrounding base stations  22  and  23 ) of the base station  21 . Thus, it is possible to perform conversation at a same location by using multiple base stations. For this reason, it is possible to effectively use the radio resource (not shown) of the base stations  21  to  23  in the specific area at which traffic quantity changes in response to time bands. 
     In addition, each of the base stations  21  to  23  has its own threshold value for a number of calls. So, it is possible to increase the radio resource, on which conversations are enabled, by broadening the area of each of the surrounding base stations before call failure occurs due to the shortage of the radio resource. Therefore, it is possible to improve the communication service quality in the case where the multiple mobile terminals concentrate in the same area. 
     As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds are therefore intended to be embraced by the claims.