Channel changing system

Abstract: The present invention is a channel changing system in a mobile unit communication system comprising a plurality of radio base stations (3) making communication with moveable stations (7) through radio channels and a center station for accommodating such base stations (3) and connecting these base stations (3) with the public telephone network (6). The center station is configurated comprising a private branch exchanger (1) for accommodating the public telephone network (6) and the lines (4) extending from a plurality of base stations (3) and a controller (2) for controlling changing of communication channels by supervising the signal receiving condition between a moveable station (7) and a plurality of base stations (3). The controller (2) is configurated so that it issues a channel changing command by detecting, based on the signal receiving condition, that a moveable station (7) has transferred to the radio zone of an adjacent base station (3) from the radio zone of a certain base station (3), while the private branch exchanger (1) conducts line changing depending on such command through the extension transfer to the line (4) of adjacent base station (3) from the line (4) to the relevant base station (3).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a channel changing system. 2. Description of the Related Arts As illustrated in Fig. 1, a mobile unit communication system has a configuration wherein radio base stations BS1 ∼ BSn are arranged in a plurality of radio zones #1 ∼ #n, causing movable stations MS scattered therein to be capable of radio communication with such radio base stations BS1 ∼ BSn and with regular telephone subscribers through one or more center stations 100, accommodating radio base stations BS1 ∼ BSn. Such a system is known from WO-A-8 606 915. In such a mobile unit communication system, if a movable station moves under the busy condition to the adjacent radio zone from a certain radio zone, a channel changing process is required for continuation of communication. Fig. 2 illustrates a diagram for explaining the busy channel changing system through frequency changing. That is, if a movable station MS having existed in the radio zone #(n-1) moves, for example, under the busy condition, to an adjacent radio zone #n, the radio zone #(n-1) is requested to change the radio frequency fa assigned to the radio base station BS(n-l)to the radio frequency fb assigned for the use to the radio base station BSn in the radio zone #n after the movement. Because the radio frequency to be used is changed to different frequencies respectively for each radio zone, this frequency changing system provides less frequency interference between respective radio zones and can therefore be said to be a system which enables easy management. Accordingly, the system described here is suitable, for example, fora large capacity and wide range system such as an automobile telephone system linking a plurality of zones. The mobile unit communication system described above makes a communication channel change, if the mobile unit transfers to an adjacent radio zone, at the region wherein the receiving electric field between the movable stations in the boundary of radio zones is lowered. Therefore a protection circuit or control circuit is complicated and becomes expensive. Accordingly, such channel changing is controlled, for example, by a particular exchange control apparatus such as an automobile telephone exchanger. Such system is suitable for a large capacity and wide range system but not for a regional small scale mobile unit communication system to which the applicable region is restricted, because an expensive exchange control apparatus is required. GB 1 564 053 shows a cellular mobile telecommunications system including a switching office for switching a connection from one cell to another without interruption. US-A-3 906 166 shows a radio telephone system with a central control unit for switching land lines corresponding to base stations, according to the signal strengths received. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide a communication channel changing system of a mobile unit communication which is capable of easily and economically realizing the channel changing even under the busy condition. According to the present invention, there is provided a channel changing system, as set out in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a structural diagram of an ordinary mobile unit communication system; Fig. 2 is a diagram for explaining frequency change of communication channel due to zone transfer in the mobile unit communication system; Fig. 3 is a block diagram of an embodiment of the present invention; Fig. 4 is a block diagram of a private branch exchanger; Fig. 5 is a block diagram of connection controller in an embodiment of the present invention; Fig. 6 is a block diagram of radio base station in an embodiment of the present invention; Fig. 7 is a block diagram of a movable station in an embodiment of the present invention. PREFERRED EMBODIMENT OF THE PRESENT INVENTION Fig. 3 illustrates a mobile unit communication system utilizing a channel changing system in mobile unit communication as an embodiment of the present invention. In the same figure, 3 1 ∼ 3 n designate radio base stations. The base stations 3 1 ∼ 3 n respectively have their own radio zones #1 ∼#n, forming a communication service area. For example, m radio frequencies (radio channels) from f 1 ∼ f m are used for the movable station 7 and radio base stations 3 1 ∼ 3 n in this communication service area and the movable station 7 is capable of making radio communication with the radio base station using only one frequency among the radio frequencies f 1 ∼ f m . The sending and receiving frequency can be selected by changing, with a frequency synthesizer, the local oscillation frequency signal for converting the radio frequency (800 MHz, for example) and the intermediate frequency(455 kHz, for example). Each radio base station 3 1 ∼ 3 n makes communication with a movable station using a channel among the radio frequencies f 1 ∼ f m . However, while the movable station 7 uses the radio frequency f 1 for communication with the base station 3 1 , the other radio base stations 3 2 ∼ 3 n in the same service area cannot use the radio frequency f 1 for communication with the other movable station. In addition to the radio communication with a movable station using a certain radio frequency, the radio base stations 3 1 ∼ 3 n also monitor radio communication using the other radio frequencies and thereby send a call termination level signal to a connection controller 2 by supervising the termination level of all radio frequencies. The private branch exchanger 1 and connection controller 2 configure a center station. The radio base station lines 4 1 ∼ 4 n extending from the radio base stations 3 1 ∼ 3 n are accommodated within the private branch exchanger 1 via the exchanger lines 5 1 ∼ 5 n . The private branch exchanger 1 is then extended to a public telephone network 5. Fig. 4 illustrates a structure of the private branch exchanger. A switching circuit 11 is connected to the public telephone network 6 via a main wire interface 12 and also connected to the exchanger lines 5 1 ∼ 5 n via an extension interface 13. A central processor 14 processes various control signals for the main wire and extension via respective interfaces and issues an instruction to the switching circuit for connection with the main wire or extension wire. Fig. 5 illustrates detail block diagram of the connection controller 2. In the same figure, 201 designates a central processor, while 202 and 203 designate dial oulse generators, 204 and 205 designate level detectors, 206 and 207 designate multiplex circuits, 208 and 209 designate separation circuits, 210 ∼ 215 designate switches. The connection controller 2 provides the circuits for similar functions as is illustrated for the channels used by respective base stations among the stations 3 1 ∼ 3 n . In Fig. 5, only the circuit of one channel is shown for the radio base stations 3 1 and 3 2 for simplification of drawings. Fig. 6 illustrates a block diagram of the radio base station 3. Fig. 7 illustrates a block diagram of a movable station 7. In the same figures, 601 is separation circuit, 602 is multiplex circuit, 603 and 707 are modulators, 605 and 706 are demodulators, 606, 608, 703 and 705 are mixers, 607 and 704 are frequency synthesizers, 609 and 702 are duplexers, 610 and 701 are antennae, 604 and 708 are control circuits. The operation of the system are explained hereinafter. It is assumed that a movable station 7 is located within the radio zone #1 of the radio base station 3 1 , making communication with the base station 3 1 using the radio frequency f 1 and further making communication with an ordinary public telephone subscriber by extending the connection to the public telephone network 6 from such base station 3 1 via the line 4 1 , connection controller 2, line 5 1 , base station 3 1 and private branch exchanger 1. In this case, the sending signal of frequency f 1 from the movable station 7 is also monitored by the other radio base stations 3 2 ∼ 3 n and the termination level at these base stations are output from the demodulator 605 and supervised by the control circuit 604. The termination level signals from the base stations 3 1 ∼ 3 n are frequency-multiplexed into the voice signals in the multiplex circuit 602 and are then sent to the connection controller 2 of the center station. Therefore, if the movable station 7 moves, for example, to the radio zone #2 from the radio zone #1, the receiving level of radio channel f 1 of the base station 3 1 in the radio zone #1 is lowered, while the receiving level of radio channel f 1 of the base station 3 2 in the radio zone #2 rises. Now that, the connection controller 2 detects that the movable station 7 has transferred to the radio zone #2 from the zone #1. Sending and receiving of control signal between base stations can be made by multiplexing the frequency to the voice signal. Upon detecting movement of movable station 7 of radio zone, the connection processor 2 issues a line changing command to the base stations 3 1 , 3 2 and the private branch exchanger 1 in order to change the communication line for the movable station 7 to the base station 3 2 from the base station 3 1 . For issuance of the line changing command, a dial pulse signal corresponding to the exchanger accommodation terminal location (exchanger line 5 1 ) accommodating the base station line 4 1 and a dial pulse signal corresponding to the exchanger accommodation terminal location (exchanger line 5 2 ) accommodating the base station line 4 2 are sent to the private branch exchanger 1. Thereby, the private branch exchanger 1 changes the connection line to the movable station 7 to the exchanger line 5 2 from the exchanger line 5 1 through extension transfer. Simultaneously, the radio base station 3 1 once stops transmission of radio frequency f 1 used for the movable station 7 to suspend communication therewith and on the other hand, the radio base station 3 2 starts communication with the movable station 7 using the radio frequency f 1 which has been used for communicaiton with the base station 3 1 before the transfer of radio zone. Thereby, communication with the movable station 7 through the base station line 4 1 of the base station 3 1 is changed to that through the base station line 4 2 of the base station 3 2 . Channel changing during communication can thus be attained. Detailed opeartions of connection controller 2 are explained with reference to Fig. 5. The termination level signal of frequency f 1 of the base station 3 1 can be detected by the level detector 204 through the base station line 4 1 , but when reduction of termination level due to transfer of radio zone is detected by the level detector and rise of termination level of frequency f 2 in the base station 3 2 is also detected by the level detector 205, it is notified to the central processor 201. The central processor 201 issues a command to suspend communication with the movable station 7 by the frequency f 1 to the base station 3 1 and also a command to start communication with the movable station 7 using the frequency f 1 to the base station 3 2 . Moreover, the central processor 201 also generates, by the dial pulse generator 202, the dial pulse signal indicating origination address (namely, the dial pulse signal corresponding to the base station line 4 1 ) and the dial pulse signal indicating transfer destination (namely, the dial pulse signal corresponding to the base station ine 4 2 ) and then sends these dial pulse signals together with the extension transfer command to the private branch exchanger through the switch 210. The switches 212 and 213 are turned off, while the switches 214 and 215, are turned on. When the private branch exchanger 1 makes extension transfer to the exchanger line 5 2 from the line 5 1 , the communication signal from the publich telephone network 6 is thereafter sent and received by the base station line 4 2 and thereby communication with the movable station 7 is established via the base station 3 2 . As described, in the system of the present invention, the movable station is capable of continuously using the preset radio frequency until the end of conversation even after the communication is once started and it moves to the other radio zone. In addition, since changing of busy channel is carried out through the changing of wire system in the side offixed station side, it is no longer necessary for the movable station to provide the channel changing mechanism during the conversation due to the transfer of radio zone by the movable station and structure of movable station can be simplified. Moreover, the fixed station can also be formed easily and economically by direct use of the extension transfer function without any design change of the existing private branch exchanger. The embodiment of the present invention also allows various modifications and changes. For example, in the above explanation, the radio frequency used for communication between the movable station and base stations can also be used even after the movable station has transferred to the other radio zone. Of course, for instance, it is also allowed to change the radio frequency used in the adjacent radio zones as is carried out in the ordinary automobile telephone system. Moreover, the connection controller is not limited only a single unit and a plurality of connection controllers may be accommodated within the private branch exchanger. In addition, it is further possible to provide a plurality of private branch exchangers arranged for each region in such a case that the service area is comprised of separated areas.

Claims: A channel changing system, which includes center station means for connecting a public telephone network (6) to a plurality of radio base stations (3) which communicates with movable stations (7) through radio channels, wherein said center station comprises: a private branch exchanger (1) electrically. connected to the public telephone network (6) and to a control means (2) which is connected to lines (4) extending from said plurality of radio base stations (3), wherein said control means (2) is operable to detect that the movable station (7) has transferred from the radio zone of a first one of said radio base stations (3) to the radio zone of an adjacent one of said radio base stations (3), by monitoring the signal 'receiving condition between the movable station (7) and said plurality of base stations (3), wherein said private branch exchanger (1) is connected to the control means via exchanger lines (5) which correspond to the plurality of radio base stations (3) respectively, and wherein said control means is operable to control the private branch exchanger (1) to transfer the extension from the exchanger line (5) corresponding to said first radio base station (3) to the exchanger line (5) corresponding to said adjacent radio base station (3). A channel changing system according to claim 1, wherein the instruction for extension transfer is issued with the dial pulse signal indicating the originating address and the dial pulse signal indicating the transfer destination. A channel changing system according to claim 1 or 2, wherein said plurality of radio base stations (3) comprise a multiplex circuit (602) for multiplexing the signal indicating the signal receiving condition into a communication signal and said controller (2) comprises a demultiplexing circuit (208,209) for extracting the signal indicating the signal receiving condition from a communication signal.

Inventors: SASAKI SUSUMU, SASAKI, SUSUMU
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: H04Q7
IPC codes: H04W36, H04W84