Mobile communication system

Abstract: A mobile communication system wherein a plurality of signposts (5) enable communications among mobile stations and a switching system in a predetermined communicating area, and are connected through a transmission network connected to a fixed network; each of the signposts (s) has dual zone communications ability including a communicating area (bz) which overlaps the communicating areas or adjacent signposts, and a zone switching signal area (cz) which does not overlap the zone switching signal areas of adjacent signposts. The signpost and mobile station, jointly detect on-going signal levels of the mobile station, and communicating area switching operation are carried out as needed. In the mobile communication system of the invention, a transmitting and receiving equipment can be a single transceiver which switches over the frequencies to communicating channel frequencies (ft) and to zone switching controlling frequencies (ftc).

Description: BACKGROUND OF THE INVENTION This invention relates to a zone type mobile communication system, and more particularly to a mobile communication system wherein a plurality of signposts for relaying communications among mobile stations and a switching system in a predetermined communicating zone are disposed in a road traffic network. The communication system of the present invention is capable of interconnection with a fixed communication network. DESCRIPTION OF THE PRIOR ART Heretofore, a zone type mobile communication system included a zone switching operation that provided communications for the ordinary automobile telephone system by utilizing a plurality of mobile radio base stations to detect the received levels of radio waves transmitted by the mobile stations. A mobile control station, according to the received levels transmitted by the mobile radio base stations, selects the mobile radio base station of which the received level is highest. This will be described in more detail. A given mobile radio base station monitors the received level of radio waves transmitted by a predetermined mobile station, and when the received signal level becomes lower than a predetermined value, assumes that the mobile station of interest has entered another zone; the base station then transmits a level deterioration signal to the mobile control station. Upon reception of the level deterioration signal, the mobile control station commands the remaining mobile radio base stations to monitor the signal levels of the mobile station of interest. The mobile control station compares the signal level of the mobile station as reported by each of the mobile radio base stations and selects the base station reporting the highest received signal level from the mobile station of interest. The mobile control station then assigns a new channel between the mobile base station so selected and the automobile switching center, and instructs the previous mobile radio base station to disconnect its channel. From US-A-4 144 409 a prior art communication system is known, that uses communicating channels with different radio frequencies for adjacent communicating zones and identical radio frequencies for distant communicating zones. The switching of the radio frequencies is transmitted through the communicating channels. The radio communication system disclosed in EP-A-0240051 uses signposts located along a road. These signposts communicate with the mobile stations. Beacon transmitters, which work stand alone, continuously transmit a zone switching signal. PROBLEMS TO BE SOLVED BY THE INVENTION As can be seen from the above, in many prior art systems, zone switching is carried out by the mobile radio base station and the mobile control station, which are network facilities. Applying this conventional system architecture to a mobile communication system utilizing numerous signposts therefore causes following disadvantages: because of the use of the number of signposts, control operations such as detecting the position of a mobile station and switching the communicating zones become complicated and take a relatively long time to accomplish. In view of the foregoing, an object of this invention is to provide a mobile communication system wherein zone switching operations are achieved quickly by a signpost and a mobile station. MEANS FOR SOLVING THE PROBLEM The foregoing object of the invention is achieved by the features of claim 1. SUMMARY OF THE INVENTION In the mobile communication system of the invention, each signpost has dual zone capabilities overlapping the communicating area of adjacent signposts. A zone switching signal area is also included which does not overlap the zone switching signal areas of adjacent signposts. Therefore, a mobile station in the zone switching signal area of a signpost, upon reception of the zone switching signal, detects the zone in which it is presently located from the signal levels thereof. By switching the communicating channel of the previous signpost over to that of the signpost in the zone thus detected, the mobile station receives signals with a corresponding higher signal level. This will be described in more detail. The antenna of each signpost, as shown in FIG. 1, includes two types or directive antennas: a communicating antenna, and a zone switching antenna for transmitting a zone switching signal radio wave. (Physically stated, one antenna can radiate two different radio waves when excited utilizing differing technique.) The directivity of the communicating antenna is such that it provides a propagation path coincident with a road, and overlaps the path of adjacent signposts. On the other hand, the zone switching antenna is highly directive and is meant only to be able to communicate along the transmission path of interest of a particular signpost (cf. FIG. 2). As was described above, two areas, a communicating area and a zone switching area, are provided. Therefore, when a mobile station is at a position where the plural communicating areas overlap, the mobile station receives signal by using the communicating channel of the preceding (old) signpost. When the mobile station enters a zone switching signal area of another (new) signpost, it receives the zone switching signal radio waves therefrom, as a result of which it can detect its present position from the signal levels thereof. For the purpose of detecting the position of a mobile station, it is preferable that the zone switching antenna is highly directional; however, the directivity is so designed that reliability does not suffer. Signal transmitting and receiving frequencies to be switched over to communicating channel frequencies and to zone switching controlling frequencies at a high speed, are therefore provided at the mobile station in one unit. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing an arrangement of signpost antennas. FIG. 2 is an explanatory diagram showing the directivities in a vertical plane of a communicating antenna and a zone switching antenna which together form a signpost antenna. FIG. 3 is an explanatory diagram showing propagation areas covered by the communicating antenna and the zone switching antenna. FIG. 4 is a block diagram showing one example of the arrangement of a signpost according to a FDMA system. FIG. 5 is a block diagram showing one example of the arrangement of a signpost according to a TDMA system. FIG. 6 is a block diagram showing one example of the constitution of a mobile station. FIG. 7 is an explanatory diagram showing a communicating channel switching procedure. FIG. 8 is an explanatory diagram showing a procedure of a call terminating. FIG. 9 is an explanatory diagram showing the relation between two high and low threshold values and a controlling radio wave. FIG. 10 is an explanatory diagram showing a procedure of a call originating. FIG. 11 is an block diagram showing another example of a mobile station. FIG. 12 is an explanatory diagram showing a frame format in the case of the TDMA communication system, for a description of a receiver at a mobile station; and FIG. 13 is an explanatory diagram showing a frame arrangement in the case of the FDMA communication system, for a description of a receiver at a mobile station, wherein 1 -- Mobile station; bZ -- Communicating region; cZ -- Zone switching signal region; S -- Signpost DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT One preferred embodiment of this invention will be described with reference to the accompanying drawings. FIG. 1 is an explanatory diagram showing an arrangement of signpost antennas a1, a2, ... in a mobile communication system according to the present invention. The signpost antennas a1, a2, ... an include communicating antennas b1, b2, ... and bn, and zone switching antennas c1, c2, ... and cn for transmitting zone switching signals. FIG. 2 shows the directivities in a vertical plane of the communicating antennas and the zone switching antennas. Each directivity of the communicating antennas b1, b2, ... and bn is appropriately long along the road. Each directivity of the zone switching antennas c1, c2, ... and cn is suppressed both in horizontal direction and in upward direction, but extends in a downward direction, or towards the road of interest. FIG. 3 shows propagation areas covered by the communicating antennas and the zone switching antennas. The communicating antennas b1, b2 and b3 include communication areas bZ1, bZ2 and bZ3, respectively; disposed in such a manner that the communication areas overlap; that is, communication area bZ1 overlaps the communication area bZ2, and the communication area bZ2 overlaps the communication region bZ3. It is assumed that, at the present, signpost S1 relays communications with a mobile station 1, traveling in the direction of the arrow A, by using a transmitting frequency ft1 and a receiving frequency fr1 assigned to the signpost S1. The zone switching antennas c1, c2 and c3 have zone switching signal areas cZ1, cZ2 and cZ3, respectively, which are separated from one another (not overlapping), and they transmit zone switching signals towards the road by using the frequency ftc which is assigned to all of the signposts. On the other hand, various control signals, for instance, for paging for mobile stations 1, and signals representing the numbers of repeaters transmitting those particular control signals are carried by the frequency ftc. In addition, empty channels not used by the signpost may carry data. FIG. 4 is a block diagram showing one example of the arrangement of a signpost (S). The signpost comprises: communicating transceivers 2; a controlling transceiver 3 for controlling a zone switching operation, terminating or originating a call (described later); a circuit switching unit 4 for selectively connecting the communicating transceivers to a network; a communicating antenna b; a zone switching antenna c; a memory unit 5 for storage, utilized for instance by a wireless channel and mobile station ID control table; and a control unit 6. The communicating transceivers 2 include as many channels as are required, and are connected to predetermined channel circuits and networks by means of the circuit switching unit 4. Assignment of a frequency; i.e., assignment of a wireless channel to the mobile station 1, or connection to a network are carried out as functions of the signpost S itself, under the control of the control unit 6 according to a "wire and wireless channel assignment control table" stored in the memory unit 5. The controlling transceiver 3 is connected to the zone switching antenna c, so that various data signals necessary for switching zones and control signals for the operation of call terminating or originating are transmitted between the controlling transceiver 3 and the mobile station 1. FIG. 5 is a block diagram showing the arrangement of a signpost according to a TDMA system. The signpost of FIG. 5 is different from that of FIG. 4 only in that instead of the wireless channels, time slots are employed. In both a FDMA signpost and the TDMA signpost according to the invention, the communicating channel frequency is not the same as the controlling radio wave frequency. For the purpose of eliminating signal interference, different communication channel frequencies are assigned to different signposts; however, one and the same zone switching radio wave frequency is assigned to all of the signposts, because the zone switching areas do not overlap, as was hereinabove described. FIG. 6 is a block diagram showing one example of a constitution of the mobile station 1. The mobile station 1, as shown in FIG. 6, comprises: a communicating transceiver 11 for converting an input signal transmitted through the communicating antenna b into an audio signal, and for converting an audio signal into a modulated radio signal; a controlling transceiver 12 having a low threshold value for receiving a paging signal from a signpost at all times, and a zone switching threshold value (cf. FIG. 9) to monitor the controlling radio waves; a control unit 13 for switching the two kinds of threshold values and for switching the frequencies of the communicating transceiver 11; and an antenna 14. The controlling transceiver 12 monitors the controlling radio waves at all times, and according to the control signals extracted from the controlling radio waves, detects the levels of received signals for switching zones, identifies a signpost ID, and transmits control signals for the operation of a call terminating or originating. Signal transmission between the mobile station 1, the signpost S, and an exchange system will now be described with particular reference to an FDMA system. FIG. 7 shows a communicating channel switching procedure at the point "1" in the communicating region 1 (FIG. 3), the mobile station 1 is transmitting to the communicating antenna of the sign post S1 via frequencies ft1 and fr1. Concurrently, the mobile station 1 receives the zone switching radio wave ftc via the controlling transceiver, recognizing from the signpost number included in the zone switching radio wave ftc that the mobile station is in the zone 1 (hereinafter, the zone in which the mobile station is located will be referred to as an "in-zone", when applicable). When the mobile station 1 reaches the point "2" from the point "1", within the controlling transceiver of the mobile station the received signal level becomes lower than the predetermined threshold value, as a result of which it is determined that the mobile station is now out of the zone switching signal area cZ1. However, the in-zone 1 information has been stored by the mobile station, and since the mobile station is still in the communicating area bZ1, communication with the signpost S1 is maintained. when the mobile station 1 reaches the point "3"; that is, it travels into the zone switching signal area cZ2, the signpost number "2" is transmitted with the zone switching radio wave, and therefore it determines that the mobile station 1 has entered the next zone; that is, its new in-zone number is "2". In order to maintain the communicating frequency of the communication area bZ2, the mobile station 1 queries the signpost S2 for a new frequency by using the zone switching signal radio wave up-link (the term "up" as used herein is intended to represent hailing from a mobile station), and transmits a signal representing the previous in-zone number "1" to the signpost S2. The signpost S2 transmits the communicating frequency assigned thereto to the mobile station by using the zone switching radio wave down-link ftc (the term "down" as used herein is intended to represent a hailing for a mobile station), and transmits the previous in-zone number "1" and the mobile station ID to the exchange system, requesting an exchange of the communication circuits. After achieved the link between the exchange system and the signposts, the communicating frequencies are then switched over to the frequencies ft2i and fr2i of the signpost S2. Thereafter, the preceding signpost S1 is informed that communication with it has been ended; the communication channels with the frequencies ft1i and fr1i are reset, and the wireless circuit to the signpost S1 from the exchange system is opened. Upon switching of the communication, the in-zone number of the mobile station 1 is changed from "1" to "2". Thus, the switching period of communications is minimized. In cases where the signpost S2 transmits informations about empty channels by using the down-link ftc, the mobile station may select one of the empty channels and report it to the signpost S2 so that the signpost S2 can enable communications by using the empty channel thus reported. The signpost then instructs the mobile station to switch the communication channel. As is apparent from the above description, the zone switching operation is readily achieved centering around the mobile station. In addition, the mobile station can detect its present position by detecting the current in-zone number, while the signpost detects the position of the mobile station from the ID of the mobile station 1. Where the communication channels are of the TDMA type, the zone switching operation is achieved substantially similar to the case where the communicating channels are of the FDMA system, but employs times slots instead of frequency domains. FIG. 8 shows a procedure of a call terminating. FIG. 9 is an explanatory diagram showing the relation between the controlling radio wave and the two high and low threshold values. The procedure of a call terminating will be described with reference to FIGS. 8 and 9. The exchange system issues a calling request, and the signpost uses the zone switching antenna to transmit a paging signal to all mobile stations 1. In response to the paging, each of the mobile stations 1 utilizes the controlling transceiver 12 to detect the controlling radio wave according to a low threshold value, thus receiving the controlling radio wave continuously. That is, all of the mobile stations 1 in the communicating area receives the general calling. Upon reception of the paging signal, the concerned mobile station 1 answers by using the up-link frc. In this operation, the ID of the mobile station is also transmitted (in addition, the in-zone number may be transmitted so that the answer to the paging is more succinct). In response to the answer from the mobile station, the signpost uses the down-link ftc to specify the communicating channel assigned thereto, and requests the exchange system to connect the network. The exchange system sends a network connection completion signal to the signpost, the signpost then instructs the mobile station 1 to switch the communicating channel over to that specified, and the mobile station enables communications by using the communicating channel specified. A procedure of a call terminating will be described with reference to FIG. 10. A originating call from the mobile station 1 is carried out by using the up-link frc. In this operation, the ID of the mobile station 1 is also transmitted (in addition to the ID, the in-zone number may be transmitted so that the calling be more succinct). In response to the call, the signpost uses the down-link ftc to specify the communicating channel assigned thereto. According to the instruction from the signpost, the mobile station 1 switches the communicating channel over to that specified for connection to the exchange system. If, in the signpost, similarly as in the mobile station, the signal reception threshold value of the zone switching antenna is low, then the signpost continuously receives the originating call from the mobile station 1. FIG. 11 is a block diagram showing another example of the mobile station. The mobile station of FIG. 11 differs from the mobile station 1 shown in FIG. 6 in that the communicating transceiver 11 and the controlling transceiver 12 are formed into one unit 11′. where a TDMA communication system is employed, the frame format is as shown in FIG. 12. That is, signals are received by using the communicating frequencies fri and fti during the communicating time slow interval Tj assigned to the mobile station 1; however, it is unnecessary to do so during the period of time other than the communicating time slot interval Tj. A time slot therefore utilized to transmit and receive the zone switching control radio waves ftc and frc; the signal level for switching zones is detected, the in-zone number is recognized, and the operation of a call terminating or originating is controlled. Thus, the need for a controlling transceiver is eliminated. On the other hand, where a FDMA communication system is employed, the frame format is as shown in FIG. 13. Blank intervals (Blank) are provided between frames in the communicating channel of the signpost, and in synchronization with the blank intervals, the zone switching signal is transmitted with the zone switching control channel ftc. In compliance with the frame format described above the mobile station receives the communicating channel ft1 and the zone switching control channel ftc alternately. Thus, similarly as in the case of the TDMA communication system, the signal level for switching zones is detected, the in-zone number is recognized, and the operation of a call terminating or originating is controlled, without a controlling transceiver. In a FDMA communication system, the controlling transceiver may therefore be eliminated with the frame format unchanged. That is, the frequency of the transceiver of the mobile station 1 is switched over to the communicating frequency for signal transmission and reception, and it is regularly switched over to the zone switching control frequency, thereby eliminating the need for a controlling transceiver. EFFECTS OF THE INVENTION As was described above, each signpost has a dual zone arrangement comprising a communicating area overlapping the communicating areas of adjacent signposts, and a zone switching signal area which does not overlap those of the adjacent signposts. The mobile station in the zone switching signal region of a given signpost therefore receives the zone switching signal, so that it can detect the zone from its signal level in which it is located. The mobile station switches the communicating area of the preceding signpost over to that of the signpost in the zone thus detected, so that it can receive signals in the communicating area higher in signal level. Thus, the switching of zones can be quickly achieved only by the signpost and the mobile station.

Claims: Method of communication between a mobile station (1) and a plurality of signposts (S1,...Sn) with (a) signposts (S1,...Sn) covering first transmission zones (bZ1,...bZn) and second transmission zones (cZ1,...cZn), the first transmission zones (bZ1,...bZn) representing communicating areas, the second transmission zones (cZ1,...cZn) representing zone switching signal areas; (b) the first transmission zones (bZ1,...bZn) of adjacent signposts overlap each other; (c) the second transmission zones (cZ1,...cZn) do not overlap each other; characterised in that the method includes the following steps: (d) each single signpost (S1,...Sn) covers a first transmission zone (bZ1,...bZn) and a second transmission zone (cZ1,...cZn); (e) the signposts (S1,...Sn) communicate with the mobile station (1) in a communicating area by using a particular frequency (ft1i, fr1i,...) assigned to the respective signpost; and (f) the signposts (S1,...Sn) perform switching control in its zone switching signal areas by using a frequency (ftc) which is common to all signposts. Method of communication as set forth in claim 1 characterised in that (a) a signpost within a zone switching signal area transmits a zone switching controlling signal including the zone number assigned to the respective signpost; (b) the signpost establishes a connection to a mobile station (1) and assigns a communicating channel to the mobile station. Method of communication as set forth in claim 1 or 2 characterised in that (a) a FDMA communication method is used with the frames of the communicating channels including blanked intervals. Method of communication as set forth in claim 1 or 2 characterised in that (a) a TDMA communication method is used. Mobile station carrying out the method of any of the claims 1 to 4, the mobile station comprising: a) means for communicating (11,..,14) with signposts in overlapping communicating zones by using a particular frequency (ft1i, fr1i,...) assigned to the respective signpost only; b) means for transmitting and receiving (11,...,14) zone switching controlling signals with signposts in non overlapping switching zones by using a frequency (ftc) which is common to all signposts, characterised in that there are further provided: (c) means for detecting (11,12,13,14;11′,13′,14) a new zone number from a zone switching controlling signal, when the mobile station travels from a zone switching signal area into an adjacent zone switching signal area; (d) means for querying (11,12,13,14;11′,13′,14) the signpost having the new zone number to establish a connection thereto and to assign a new communicating channel; (e) transceiver (11;11′)for enabling communication with the previous signpost while a new zone number is detected and the zone switching operation is performed. Mobile station as set forth in claim 5 characterised in that there is provided: (a) a receiver (11,13;11′,13′) comprising at least two signal reception threshold values for a controlling signal radio wave. Mobile station as set forth in claim 5 or 6 characterised in that there is provided: (a) means for switching (13;13′) the signal transmitting and receiving frequencies of the tranceivers alternately between communicating frequencies and zone switching controlling frequencies at high speed, the transceivers being formed into one unit (11′). Mobile station as set forth in any of claims 5 to 7 carrying out the method of claim 4 characterised in that there are provided: (a) means for operating (11,12,13;11′,13′) according to a TDMA communication system; and (b) means for switching (13,13′) the signal transmitting and receiving frequencies of a transceiver from communicating frequencies (fri,fti) over to zone switching controlling frequencies (frc,ftc) during time periods other than the periods in which a communicating time slot is assigned to the mobile station. Mobile station as set forth in any of claim 5 to 7 carrying out the method of claim 3 characterised in that there is provided: (a) means for operating (11,12,13;11′,13′) according to an FDMA system with blanked intervals within frames of a communicating channel of a signpost; and (b) means for switching (13;13′) frequencies of the transceiver from communicating frequencies to zone switching controlling frequencies during the blanked interval of each frame of the communicating channel. Mobile station as set forth in any of claims 6, 7 or 9 carrying out the method of claim 3 characterised in that there are provided: (a) means for operating (11,12,13; 11′,13′) according to an FDMA communication system; (b) means for switching (13;13′) predetermined frequencies of a transceiver regularly over to a zone switching controlling frequency independently of the contents and the frame arrangement of a communicating channel; (c) means for error correction and waveform estimation (11,12,13;11′,13′) for suppressing the deterioration of communication quality.

Inventors: HATTORI SEIJI C O THE TOKYO EL, KASUKAWA AKIHIDE C O THE TOKYO, KOBAYASHI YOSHINOBU C O OSAKA, SHIBANO YOSHIZO C O OSAKA WORK, SUZUKI SHINJI C O OSAKA WORKS, HATTORI, SEIJI C/O THE TOKYO ELECTRIC, KASUKAWA, AKIHIDE C/O THE TOKYO ELECTRIC, KOBAYASHI, YOSHINOBU C/O OSAKA WORKS OF, SHIBANO, YOSHIZO C/O OSAKA WORKS OF, SUZUKI, SHINJI C/O OSAKA WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, TOKYO ELECTRIC POWER CO, SUMITOMO ELECTRIC INDUSTRIES LIMITED, THE TOKYO ELECTRIC POWER CO., INC.
Application year: 1989
Main classification: H04Q7
IPC codes: H04W36