GB2298996A

GB2298996A - Communication system

Info

Publication number: GB2298996A
Application number: GB9524229A
Authority: GB
Inventors: Hideo Ashida; Masashi Nakayama; Junichi Hasegawa; Kazuo Ogawa
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1995-03-17
Filing date: 1995-11-27
Publication date: 1996-09-18
Also published as: GB9524229D0; JPH08256099A

Abstract

A mobile communication system applied to communication using terminals such as portable telephones, mobile telephones and car phones, and a radio station and a toll centre. The mobile communication system is provided with a radio station (1) for establishing communication with the mobile terminal (3) through a first radio communication link (4), and a toll centre (2) which transmits and receives communication signals to and from the radio station (1) and which has an exchange function. The radio station (1) and the toll centre (2) are connected via a second radio communication link (5) which uses a frequency band different from that of the first radio communication link (4). The size of the radio station can be reduced, thereby facilitating the installation of the radio station and decreasing the installation costs. Also, additional lines can easily be provided when the number of radio stations increases. Various embodiments involving modifications of the frequency interfaces or conversion units used in the radio station and toll centre are described.

Description

COMMUNICATION SYSTEM The present invention relates to a mobile communication system which uses terminals such as portable telephones and car phones. The invention further relates to radio stations and a toll centre used in such a mobile communication system.

Fig. 20 of the accompanying drawings is a schematic diagram showing the outline of a general mobile communication system. In Fig. 20, reference numeral 101 denotes an exchange office installed in a building. The exchange office 101 is connected via optical fibres 103 to a plurality of base stations BSS, there being seven in the illustration, labelled 102-1 through to 102-7. The exchange office 101 controls the connection between each of the base stations 102-1 through to 102-7 and a public network (not shown in Fig. 20).

A service area covered by the exchange office 101 is divided into a plurality of areas labelled 104-1 through to 104-7. The base stations 102-1 through to 102-7 are respectively disposed in the areas 104-1 through to 104-7. The base stations 102-1 through to 102-7 can establish radio communication with mobile terminal equipment (terminal) 105 in the area concerned by means of an attached antenna.

Fig. 21 of the accompanying drawings is a block diagram showing the previously mentioned exchange office 101 and base stations 102-1 through to 102-7.

As shown in Fig. 21, each of the base stations 102-1 through to 102-7 is provided with an antenna 106, a transmission/reception unit 102a (transceiver), a modulation/demodulation unit 102b (modem), and a multiplexer 102c suitable for the number of transmission/reception channels. The exchange office 101 is provided with an exchange 101a for establishing connection with the public network.

In the mobile communication system having the above-described configuration, it is possible to establish communication between a mobile terminal 105, which is located in the service area covered by the exchange office 101, and another terminal via the base station 102 in the corresponding area where the mobile terminal 105 is currently situated, via the exchange office 101, and via the public network.

In other words, when the signal sent from a mobile terminal 105 is received by the transmission/reception unit 102a of any of the base stations 102-1 to 102-7 in the area in which the terminal 105 is located, the received signal is demodulated by the modulation/demodulation unit 102b (modem) and is then multiplexed by the multiplexer 102c. Then, the signal is transmitted to the exchange office 101 via the optical fibres 103.

The signal received from the particular one of the base stations 102-1 to 102-7 is transmitted to the public network through the exchange 101a of the exchange office 101, by which the signal sent from the mobile terminal 105 is transmitted to another terminal at the receiving end.

Further, when the mobile terminal 105 receives a signal from another terminal through the public network, the exchange 101a, and the optical fibres 103, the signal is modulated by the modulation/demodulation unit 102b, and the thus modulated signal is radio transmitted to the mobile terminal 105 by the transmission/reception unit 102a. Thus, the mobile terminal 105 can receive the signal from another terminal.

As mentioned above, although the exchange office 101 is typically installed in a building, the base stations 102-1 to 102-7 need to be set up close to the antennas. For this reason, the base stations have to be installed at an elevated position such as on the rooftop of a building.

Moreover, the base stations 102-1 to 102-7 require one modulation/demodulation unit 102b for each of the transmission/reception channels, thereby rendering equipment at the base stations bulky. For this reason, it is necessary to build sheds for accommodating the base stations.

However, it is not always convenient or possible to construct a shed on the rooftop of a building. It is for example necessary that the building has sufficient strength to allow the construction of such a shed. For these reasons, it is difficult to install all of the base stations on buildings.

Accordingly, it is not easy to install new base stations in response to expansion of the service area, and the installation costs of the base stations are considerable. Since large-scale equipment is installed at each of the base stations, the base 'stations require maintenance.

A A mobile communication system, shown in Fig. 22, which employs optical SCM technology has also been proposed.

The mobile communication system shown in Fig. 22 is also composed of a mobile terminal 105, a radio station 112 disposed in each area and a toll centre 111 which is connected to the radio stations 112 via optical fibres 103.

SCM technology is used for converting a radio signal into an optical signal and connecting the toll centre 111 with the radio stations 112 through the optical fibres. This simplifies the configuration of the radio stations 112.

In other words, the radio station 112 is provided with a transmission/reception unit 112a for sending and receiving radio signals, and a photo-electric converter 112b for converting a radio signal into an optical signal, and vice versa. The toll centre 111 is provided with a photoelectric converter llla for converting a radio signal into an optical signal and vice versa, a modulation/demodulation unit 111b having the same function as that of the previously mentioned modulation/demodulation unit shown in Fig. 21, and an exchange 111c (see reference numeral 101a).

In the mobile communication system shown in Fig.

22, by virtue of its configuration, communication between the mobile terminal 105 and another terminal is established via the base station 112 in the corresponding area, via the toll centre 111, and via the public network in the same manner as in the previously mentioned mobile communication system shown in Fig. 21.

In the mobile communication system shown in Fig.

22, the modulation/demodulation unit 111b is disposed not in the radio station 112 but in the toll centre 111, and the radio station 112 is composed of the transmission/reception unit 112a and the photo-electric converter 112b. Accordingly, the equipment at the radio station 112 can be made more compact and easier to install. This results in a less expensive installation of the radio station.

However, in the mobile communication systems shown in Figs. 21 and 22, the optical fibres 103 are used to connect the toll centre (exchange office) 101 and 111 to the radio stations 102-1 to 102-7 and 112. For this reason, it is necessary to lay more and more optical fibres 103 as the number of radio stations increases.

This increases the cost and time necessary for laying the optical fibres 103, and hence expansion of the system resulting from expansion of the service area requires a huge amount of money and time.

One aspect of the present invention provides a mobile communication system which comprises a radio station which communicates with a radio terminal through a first radio communication line, and a toll centre which sends and receives communication signals to and from the radio station and has an exchanging function, wherein the radio station and the toll centre are connected together through a second radio communication line which uses a frequency band different from that used for the first radio communication line.

Another aspect of the invention provides a radio station for use in a mobile communication system to communicate with a toll centre having an exchanging function while communicating with the radio terminal, the radio station comprising a transmission/reception unit for sending and receiving communication signals to and from the radio terminal through the first radio communication line, and a frequency conversion unit connected to the transmission/reception unit and providing frequency conversion between an interface frequency between the radio terminal and the frequency conversion unit and an interface frequency between the radio station and the toll centre, thereby making it possible to send and receive communication signals to and from the toll centre via a second radio communication line using a frequency band different from that used for the first radio communication line.

According to a further aspect of the invention, there is provided a toll centre used in a mobile communication system to communicate with a radio station which communicates with a radio terminal through the first radio communication line, the toll centre comprising a modulation/demodulation unit and an exchange connected to the modulation/demodulation unit.

The toll centre is further provided with a transmission/reception unit which performs communication via the second radio communication link using a frequency band different from that used for the first radio communication link, and a frequency conversion unit connected between the transmission/reception unit and the modulation/demodulation unit to provide frequency conversion between the interface frequency between the radio station and the toll centre and the interface frequency between the frequency conversion unit and the modulation/demodulation unit.

As described above, the radio station and the toll centre can be connected with each other via the second radio communication line using a frequency band different from that used for the first radio communication line. Accordingly, in preferred embodiments of the present invention, the radio station no longer needs to be provided with a modulation/demodulation unit. This makes it possible to construct a compact radio station. Also, an optical fibre link need not exist between the radio station and the toll centre. Therefore, the installation of the radio station can be less difficult and installation costs can be decreased. Moreover, even when communication lines need to be added due to an increase in the number of radio stations, the addition of the communication lines can be performed easily.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a block diagram showing one aspect of the present invention; Fig. 2 is a block diagram showing a mobile communication system according to a first embodiment of the present invention; Fig. 3 is a block diagram showing a mobile communication system according to a second embodiment of the present invention; Fig. 4 is a block diagram showing a mobile communication system according to a third embodiment of the present invention; Fig. 5 is a block diagram showing a mobile communication system according to a fourth embodiment of the present invention; Fig. 6 is a block diagram showing a mobile communication system according to a fifth embodiment of the present invention;; Fig. 7 is a block diagram showing a mobile communication system according to a sixth embodiment of the present invention; Fig. 8 is a block diagram showing a mobile communication system according to a seventh embodiment of the present invention; Figs. 9 to 12 are illustrations for explaining variations in signal level in the mobile communication system according to the seventh embodiment of the present invention; Fig. 13 is a block diagram showing a mobile communication system according to an eighth embodiment of the present invention; Fig. 14 is a block diagram showing a mobile communication system according to a ninth embodiment of the present invention; Figs. 15 and 16 are illustrations for explaining variations in signal level in the mobile communication system according to the ninth of the present invention;; Fig. 17 is a block diagram showing a mobile communication system according to a tenth embodiment of the present invention; Fig. 18 is a block diagram showing a mobile communication system according to an eleventh embodiment of the present invention; Fig. 19 is a block diagram showing a mobile communication system according to a twelfth embodiment of the present invention.

An aspect of the present invention will be described with reference to the accompanying drawings.

Fig. 1 is a block diagram showing the aspect of the present invention. In Fig. 1, reference numeral 1 denotes a radio station; 2, a toll centre with an exchange function; 3, a radio terminal; 4, a first radio communication link; and 5, a second radio communication link.

The radio station 1 communicates with the radio terminal 3 via the first radio communication link 4.

The toll centre 2 sends and receives communication signals to and from the radio station 1 while providing an exchanging function.

The radio station 1 and the toll centre 2 are connected together via a second radio communication link 5 which uses a frequency band different from that used for the first radio communication link 4.

In other words, the radio station 1 communicates with the toll centre 2 having an exchanging function while communicating with the radio terminal 3. In detail, the radio station 1 is provided with a transmission/reception unit la and a frequency conversion unit lb.

The transmission/reception unit la sends and receives communication signals to and from the radio terminal 3 via the first radio communication link 4.

The frequency conversion unit lb is connected to the transmission/reception unit la, and provides frequency conversion between an interface frequency between the transmission/reception unit la and the radio terminal 3 and an interface frequency between the radio station 1 and the toll centre 2. The frequency conversion unit lb sends and receives communication signals to and from the toll centre 2 via the second radio communication line 5 which uses a frequency band different from that used for the first radio communication line 4.

The toll centre 2 communicates with the radio station 1, and is provided with a transmission/reception unit 2a and a frequency conversion unit 2b as well as with a modulation/demodulation unit 2c and an exchange 2d connected to the modulation/demodulation unit 2c.

The transmission/reception unit 2a communicates with the radio station 1 via the second radio communication line 5 which uses a frequency band different from that used for from the first radio communication line 4. The frequency conversion unit 2b is connected between the transmission/reception unit 2a and the modulation/demodulation unit 2c, and provides frequency conversion between an interface frequency between the radio station 1 and the toll centre 2 and an interface frequency between the modulation/demodulation unit 2c and the frequency conversion unit 2b.

The frequency band used for the first radio communication line 4 can be a microwave band, and the frequency band used for the second radio communication line 5 can be a submillimeter wave band.

The radio station 1 communicates with the radio terminal 3 via the first radio communication line 4, whereas the toll centre 2 with an exchanging function sends and receives communication signals to and from the radio station 1 via the second radio communication line S in the frequency band (for example, a submillimeter wave band) which is different from the frequency band (for example, a microwave band) used for the first radio communication line 4.

In other words, the transmission/reception unit la of the radio station 1 sends and receives communication signals to and from the radio terminal 3 through the first radio communication line 4, and the frequency conversion unit lb provides frequency conversion between the interface frequency between the radio station and the radio terminal 3 and the interface frequency between the radio station 1 and the toll centre 2. In this way, communication signals are transferred between the radio station 1 and the toll centre 2 via the second radio communication line 5 which uses a frequency band different from that used for the first radio communication line 4. Hence, the radio station 1 communicates with the toll centre 2 with an exchanging function while communicating with the radio terminal 3.

Moreover, the transmission/reception unit 2a of the toll centre 2 communicates with the radio station 1 through the second radio communication line 5 which uses a frequency band different from that used for the first radio communication line 4. The frequency converter 2b provides frequency conversion between the interface frequency between the radio station 1 and the toll centre 2 and the interface frequency between the frequency conversion unit 2b and the modulation/demodulation unit 2c. Therefore, the toll centre 2 communicates with the radio station 1 which communicates with the radio terminal 3 through the first radio communication line 4.

Thus, the radio station 1 is connected to the toll centre 2 through the second radio communication line 5 which uses a frequency band different from that used for the first radio communication line 4, and hence it is unnecessary to provide the radio station 1 with a modulation/demodulation unit, which makes it possible to reduce the size of the radio station 1. Further, it becomes unnecessary to lay optical fibres between the radio station 1 and the toll centre 2, thereby resulting in easy installation of the radio station 1 and reduced installation costs. In addition, it becomes possible to easily provide communication lines when the number of radio station increases.

A first embodiment of the present invention will now be described.

Fig. 2 is a block diagram showing a mobile communication system according to the first embodiment of the present invention. In the mobile communication system shown in Fig. 2, reference numeral 11-1 denotes a radio station; 12-1, a toll centre with an exchanging function; 13, a mobile terminal; 14, a first radio communication line; and 15, a second radio communication line.

In the mobile communication system shown in Fig.

2, a service area covered by the toll centre 12-1 is divided into a plurality of areas, and the radio station 11-1 is disposed in each of the areas in the same manner as in the general mobile communication system shown in Fig. 20. When the mobile terminal 13 is located in the area covered by the radio station 11-1, the mobile terminal 13 can communicate with another terminal via the toll centre 12-1 and a public network (not shown).

The radio station 11-1 communicates with the mobile terminal (a radio terminal) 13 through the first radio communication line 14 using a microwave band such as 800 MHz band or 1.5 GHz band. On the other hand, the radio station 11-1 communicates with the toll centre 12-1 through the second radio communication line 15 using a frequency band (submillimeter wave band such as 18 GHz band) different from the frequency band used for the first radio communication line 14. The radio station 11-1 is provided with a transmission/reception unit 16-1, a frequency conversion unit 17-1, and antennas 21 and 22.

The toll centre 12-1 is connected to the radio station 11-1 through the second radio communication line 15 which uses the frequency band (for example, a submillimeter wave band such as 18 GHz band) different from the frequency band used for the first radio communication line 14. The toll centre 12-1 sends and receives communication signals to and from the radio station 11-1, and is composed of a frequency conversion unit 18-1, a modulation/demodulation unit 19, an exchange 20, and an antenna 23.

The transmission/reception unit 16-1 of the radio station 11-1 is composed of a duplexer (DUP) 16a, a power amplifier (PA) 16b, and a low-noise amplifier (LNA) 16c. The duplexer (DUP) 16a outputs a radio signal, which is received from the mobile terminal 13 via the antenna 21, to a low-noise amplifier 16c, and outputs a radio signal, which is sent from the second radio communication line 15, to the antenna 21. The power amplifier (PA) 16b amplifies the radio signal sent from the second radio communication line 15 to a desired transmission level. The low-noise amplifier (LNA) 16c amplifies a radio signal received from the duplexer 16a with low noise.

Accordingly, the radio station 11-1 sends and receives communication signals to and from the mobile terminal 13 by means of the previously mentioned transmission/reception unit 16-1 and the antenna 21 via the first radio communication line 14.

The frequency conversion unit 17-1 performs frequency conversion between a signal in the 800 MHz band or 1.5 GHz band used for the first radio communication line 14 and a signal in the 18 GHz band used for the second radio communication line 15. The frequency conversion unit 17-1 is provided with a band-pass filter (BPF) 17a, a mixer (MIX) 17b, a local oscillator (to3) 17c, an amplifier (AMP) 17d, a mixer (MIX) 17e, a local oscillator (LO4) 17f, a band-pass filter (BPF) 17g, an amplifier (AMP) 17h, and a duplexer (DUP) 17i.

In other words, a radio signal in the 800 MHz band or 1.5 GHz band received from the low-noise amplifier 16c is mixed with a signal The from the local oscillator 17f by the mixer 17e, and the thus mixed signal is output. The output signal is then passed through the band-pass filter 17g, and the amplifier 17h, and the duplexer 17i, and the signal is transmitted to the toll centre 12-1 through the antenna 22 after being converted into a signal in the 18 GHz band.

A radio signal sent from the second radio communication line 15 is amplified by the amplifier 17d, and the thus amplified signal and a signal fL3 from the local oscillator 17c are mixed together by the mixer 17b. The thus mixed signal is then output to the power amplifier 16b of the transmission/reception unit 16-1 via the band-pass filter 17a. In other words, the frequency conversion unit 17-1 converts the signal in the 18 GHz band into a signal fOT in the 800 MHz band or 1.5 GHz band, and the thus frequency-converted signal is output to the power amplifier 16b.

Accordingly, the frequency conversion unit 17-1 and the antenna 22 provides frequency conversion between the interface frequency between the radio station and the mobile terminal 13 and the interface frequency between the radio station 11-1 and the toll centre 12-1, and transmission of communication signals to and from the toll centre 12-1 through the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14.

The frequency conversion unit 18-1 of the toll centre 12-1 converts a modulator output signal fIT output from the modulation/demodulation unit 19 into a transmission signal in the 18 GHz band, and also converts a signal in the 18 GHz band which is received from the second radio communication line 15 via the antenna 23 into a demodulator input signal fIR to be input to the modulation/demodulation unit 19. The frequency conversion unit 18-1 is provided with a duplexer (DUP) 18a, an amplifier (AMP) 18b, a band-pass filter (BPF) 18c, a mixer (MIX) 18d, a local oscillator (LO1) 18e, an amplifier (AMP) 18f, a mixer (MIX) 18g, a local oscillator (LO2) 18h, and a band-pass filter (BPF) 18i.

In other words, the modulator output signal fIT output from the modulation/demodulation unit 19 is mixed with a signal fLl from the local oscillator 18e by the mixer 18d, and the thus mixed signal is output.

The output signal is passed through the band-pass filter 18c, the amplifier 18b, and the duplexer 18a, and it is sent to the radio station 11-1 through the antenna 23 after being converted into a signal in the 18 GHz band.

A signal received from the second radio communication line 15 is passed through the antenna 23, the duplexer 18a, and the amplifier 18f, and the signal is then mixed with a signal fL2 from the local oscillator 18h by the mixer 18g. As a result, the signal in the 18 GHz band is converted into a demodulator input signal fIR, and the thus converted signal is output to the modulation/demodulation unit 19.

The modulation/demodulation unit 19 modulates signals from the exchange 20 to obtain the modulator output signal fIT, which is output to the frequency conversion unit 18-1. On the other hand, the demodulator input signal fIR input from the frequency conversion unit 18-1 is demodulated by the modulation/demodulation unit 19 to obtain the demodulator output signals, which are output to the exchange 20. The modulation/demodulation unit 19 is provided with a compositing unit l9a, modulators (MOD1 to MODn) l9a-1 to 19a-n for each channel, a distributing unit l9b, and demodulators (DEM1 to DEMn) 19be1 to 19b-n for the respective channels.

The exchange 20 controls the connection between the toll centre and the public network.

Even in the mobile communication system according to the first embodiment of the present invention, by virtue of the previously mentioned configuration, if the mobile terminal 13 is currently situated in the area of the radio station 11-1 within the service area covered by the toll centre 12-1, the mobile terminal 13 can communicate with another terminal via the radio station 11-1, the toll centre 12-1, and the public network.

In other words, when a signal is sent from the mobile terminal 13, this signal is received by the transmission/reception unit 16-1 of the radio station 11-1 via the first radio communication line 14 using a frequency in the 800 MHz band or 1.5 GHz band.

The duplexer 16a of the transmission/reception unit 161 outputs the radio signal received through the antenna 21 to the low-noise amplifier 16c. The signal is then amplified with low noise by the low-noise amplifier 16c, and the thus amplified signal is then output to the frequency conversion unit 17-1 (see the signal foR) The mixer 17e of the frequency conversion unit 17-1 mixes the radio signal in the 800 MHz band or 1.5 GHz band from the low-noise amplifier 16c with the signal fL4 from the local oscillator 17f, and the thus mixed signal is output.

The signal output from the mixer 17e, which has been converted into a signal in the 18 GHz band, is passed through the band-pass filter 17g, the amplifier 17h, and the duplexer 17i, and is sent to the toll centre 12-1 through the antenna 22.

The toll centre 12-1 receives a signal from the radio station 11-1 via the second radio communication line 15 and the antenna 23, and the thus received signal is then input to the duplexer 18a of the frequency conversion unit 18-1. The signal from the duplexer 18a is input to the amplifier 18f, and the amplifier 18f amplifies the input signal.

The mixer 18g mixes the signal amplified by the amplifier 18f with the signal fL2 from the oscillator 18h, and the thus mixed signal is then output to the modulation/demodulation unit 19. As a result, the signal in the 18 GHz band is converted into the demodulator input signal fIR, and the thus converted signal is output to the modulation/demodulation unit 19. The distributing unit 19b and the demodulators 19b-1 to 19b-n of the modulation/demodulation unit 19 demodulate the demodulator input signal fIR from the frequency conversion unit 18-1, and the demodulated signal is then output to the exchange 20. Thereafter, the exchange 20 connects the toll centre 12-1 with the public network, whereby the signal is sent to another terminal.

When the mobile terminal 13 receives a signal from another terminal, the signal from the exchange 20 is input to the modulation/demodulation unit 19. The signal from the exchange 20 is modulated by the modulators 19a-1 to 19a-n and the compositing unit 19a of the modulation/demodulation unit 19, and the thus modulated signal is output to the frequency conversion unit 18-1 as the modulator output signal fIT.

The modulator output signal fIT output from the modulation/demodulation unit 19 is mixed with the signal fLl from the local oscillator 18e by the mixer 18d, and the thus mixed signal is output. This output signal, which has been converted into a signal in the 18 GHz band, is passed through the band-pass filter 18c, the amplifier 18b, and the duplexer 18a, and is sent to the radio station 11-1 via the antenna 23.

The radio station 11-1 receives a signal from the second radio communication line 15 using the antenna 22. The thus received signal is input from the duplexer 17i of the frequency conversion unit 17-1 to the amplifier 17d, and the signal is then amplified by the amplifier 17d.

The mixer 17b mixes the signal amplified by the amplifier 17d with the signal fL3 from the local oscillator 17c, whereby the signal foT, which has been converted from the signal in the 18 GHz band to the signal in the 800 MHz band or 1.5 GHz band, is output to the power amplifier 16b of the transmission/reception unit 16-1.

In the power amplifier 16b, the radio signal, which is received from the frequency conversion unit 17-1, is amplified to a desired transmission level before being sent to the mobile terminal 13.

Thereafter, the amplified radio signal is passed through the duplexer 16a and is transmitted to the mobile terminal 13 via the antenna 21 and the first radio communication line 14.

In this way, according to the mobile communication system of the first embodiment, the radio station 11-1 and the toll centre 12-1 are connected together via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14. Hence, the radio station 11-1 and the toll centre 12-1 can be connected together by radio signals, and it becomes unnecessary to provide the radio station 11-1 with a modulation/demodulation unit. This makes it possible to reduce the size of the radio station 11-1. Further, it becomes unnecessary to lay optical fibres between the radio station 11-1 and the toll centre 12-1. This results in the easy installation of the radio station and reduced installation costs. Moreover, when the number of radio station increases, it is possible to easily provide communication lines.

Furthermore, the local oscillators 17c, 17f, 18e, and 18h of the frequency conversion units 17-1 and 18-1 in the radio station 11-1 and the toll centre 12-1 are separated into a transmission system and a reception system. For this reason, each of the local oscillators 17c, 17f, 18e and 18h can optionally select a frequency, and therefore a degree of freedom of the frequency selection can be increased.

A second embodiment of the present invention is now described.

Fig. 3 is a block diagram showing a mobile communication system according to a second embodiment of the present invention. The mobile communication system shown in Fig. 3 is applied to the case where the frequency difference fIT - fIR between a transmission signal fIT and a reception signal fIR, at the interface between a frequency conversion unit 18-2 and a modulation/demodulation unit 19 in a toll centre 12-2, is the same as the frequency difference fOT foR between a transmission signal foT and a reception signal foR at the interface between a mobile terminal 13 and a frequency conversion unit 17-2.

The mobile communication system according to the second embodiment is similar to that of the first embodiment, except that the frequency conversion units 17-2 and 18-2 are provided with local oscillators 17c and 18e which are shared between the transmission system and the reception system.

In other words, the mixer 17b of the frequency conversion unit 17-2 mixes a signal from the amplifier 17d with a signal fL2 from the local oscillator 17c, whereas the mixer 17e mixes a signal fOR from the low-noise amplifier 16c with a signal fL2 from the local oscillator 17c.

The mixer 18d of the frequency conversion unit 18-2 mixes a modulator output signal fIT from the modulation/demodulation unit 19 with a signal Th1 from the local oscillator 18e, whereas the mixer 18e mixes a signal from the amplifier 18f with a signal fL1 from the local oscillator 18e.

Even in the mobile communication system according to the second embodiment, by virtue of the above configuration, if the mobile terminal 13 is currently situated in the area of the radio station 11-2 within the service area covered by the toll centre 12-2, the mobile terminal 13 can communicate with another terminal via, the radio station 11-2, the toll centre 12-2 and the public network.

In other words, the transmission/reception unit 16-1 of the radio station 11-2 sends and receives communication signals to and from the mobile terminal 13 via the first radio communication line 14. The frequency conversion unit 17-2 performs frequency conversion between an interface frequency (microwave band such as 800 MHz band or 1.5 GHz band) between the frequency conversion unit 17-2 and the mobile terminal 13 and an interface frequency (submillimeter wave band such as 18 GHz band) between the radio station 112 and the toll centre 12-2. The frequency conversion unit 17-2 sends and receives communication signals to and from the toll centre 12-2 via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14.As a result, the radio station. 11-2 communicates with the toll centre 12-2 while communicating with the mobile terminal 13.

In addition, the toll centre 12-2 carries out communication via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14. The frequency conversion unit 18-2 provides frequency conversion between the interface frequency between the radio station 11-2 and the toll centre 12-2 and the interface frequency between the modulation/demodulation unit 19 and the frequency conversion unit. As a result, the toll centre 12-2 communicates with the radio station 11-2 which in turn communicates with the mobile terminal 13 through the first radio communication line 14.

In this way, even in the mobile communication system according to the second embodiment, the radio station 11-2 is connected to the toll centre 12-2 via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14. Hence, it becomes unnecessary to provide the radio station 11-2 with a modulation/demodulation unit, and similar results as in the first embodiment can be obtained.

Also, since the difference between transmission and reception frequencies, i.e. fIT fIR, at the interface between the frequency conversion unit 18-2 and the modulation/demodulation unit 19 in the toll centre 12-2 is set to be equal to the difference between transmission and reception frequencies, i.e.

fOT fOR' at the interface between the mobile terminal 13 and the frequency conversion unit 17-2, it becomes possible to reduce the number of local oscillators by sharing the local oscillators 17c and 18e between the transmission system and the reception system.

Therefore, it is possible to reduce the size of the radio station and the toll centre.

A third embodiment of the invention is now described.

Fig. 4 is a block diagram showing a mobile communication system according to a third embodiment of the present invention. The mobile communication system, shown in Fig. 4, is applied to the case where the frequency fIT of a transmission signal at the interface between a frequency conversion unit 18-3 and a modulation/demodulation unit 19 in a toll centre 12-3 is the same as the frequency fOT of a transmission signal at the interface between a mobile terminal 13 and a frequency conversion unit 17-3, and where the frequency fIR of a reception signal at the interface between the frequency conversion unit 18-3 and the modulation/demodulation unit 19 in the toll centre 12-3 is the same as the frequency fOR of a reception signal at the interface between the mobile terminal 13 and a frequency conversion unit 17-3.

The mobile communication system according to the third embodiment is similar to that of the first embodiment, except that a local oscillation signal from a local oscillator (LO2) 17m of the frequency conversion unit 17-3 is shared between the frequency conversion units 17-3 and 18-3 when the frequency of a reception signal is converted, and a local oscillation signal from a local oscillator (LO1) 18e of the frequency conversion unit 18-3 is shared between the frequency conversion units 17-3 and 18-3 when the frequency of a transmission signal is converted.

In other words, the frequency conversion unit 17-3 of the radio station 11-3 is provided with a band-pass filter (BPF) 17a, a mixer (MIX) 17b, an amplifier (AMP) 17d, a mixer (MIX) 17e, a band-pass filter (BPF) 179, an amplifier (AMP) 17h, and a duplexer (DUP) 17i, which are the same as those used in the first embodiment, and is also provided with a duplexer (DUP) 17j, filter (FIL)17k, the local oscillator 17m, and a hybrid circuit (HYB) 17n.

In the frequency conversion unit 17-3 of the radio station 11-3, the mixer 17e mixes a signal fOR from the transmission/reception unit 16-1 with a signal fL2 from the local oscillator 17m, whereas the mixer 17b mixes a signal from the amplifier 17d with a signal fL1 which is generated by the local oscillator 18e of the toll centre 12-3 and is input to the mixer 17b via a hybrid circuit 18j, a duplexer 18a, a second radio communication line 15, the duplexers 17i and 17j, and the filter 17k.

Similarly, the frequency conversion unit 18-3 of the toll centre 12-3 is provided with the duplexer (DUP) 18a, an amplifier (AMP) 18b, a band-pass filter (BPF) 18c, a mixer (MIX) 18d, a local oscillator (LO1) 18e, an amplifier (AMP) 18f, a mixer (MIX) 18g, and a band-pass filter (BPF) 18i, which are the same as those used in the first embodiment (see reference numeral 18-1), and is also provided with the hybrid circuit (HYB) 18j, a duplexer (DUP) 18k, and a filter (FIL) 18m.

The mixer 18d mixes a modulator output signal fIT from the modulation/demodulation unit 19 with a signal fL1 from the local oscillator 18e, whereas the mixer 18g mixes a signal from the amplifier 18f with a signal fL2 which is generated by the local oscillator 17m of the radio station 11-3 and input to the mixer 18g via the hybrid circuit 17n, the duplexer 17i, a second radio communication line 15, the duplexers 18a and 18k, and the filter 18m.

With such a configuration, even in this mobile communication system according to the present embodiment, if fIT fOT and fIR = foR, when the mobile terminal 13 is currently situated in the area of the radio station 11-3 within the service area covered by the toll centre 12-3 in the same manner as in the previous first and second embodiments, the mobile terminal 13 can communicate with another terminal via the radio station 11-3, the toll centre 12-3, and the public network.

When the frequency of the signal received from the mobile terminal 13 is converted, the frequency conversion unit 17-3 uses a local oscillation signal fL2 supplied from the local oscillator (LO2) 17m. The local oscillation signal fL2 supplied from the local oscillator (LO2) 17m is also used in frequency conversion of the frequency conversion unit 18-3 by extracting the oscillation signal using the filter 18m.

In other words, the local oscillation signal supplied from the local oscillator 17m is input to the filter 18m via the hybrid circuit 17n, the duplexer 17i, the antennas 22 and 23, and the duplexers 18a and 18k. The local oscillation signal fL2 can be extracted by subjecting the received local oscillation signal to filtering in the filter 18m.

When the frequency of a signal to be sent to the mobile terminal 13 is converted, the frequency conversion unit 18-3 uses the local oscillation signal from the local oscillator (L01) 18e. The local oscillation signal fLl used in frequency conversion of the frequency conversion unit 17-3 is extracted from the local oscillation signal supplied from the local oscillator (LO1) 18e using the filter 17k.

In other words, the local oscillation signal from the local oscillator 18e is input to the filter 17k via the hybrid circuit 18j, the duplexer 18a, the antennas 22 and 23, and the duplexers 17i and 17j. The local oscillation signal fLl can be extracted by subjecting the received local oscillation signal to filtering using the filter 17k.

As a result, it is possible for the reception system to share the local oscillator 17m between the frequency conversion units 17-3 and 18-3, whilst it is possible for the transmission system to share the local oscillator 18e between the frequency conversion units 18-3 and 17-3.

In this way, even in the mobile communication system according to the third embodiment of the present invention, the radio station 11-3 and the toll centre 12-3 are connected together via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14. Hence, it becomes unnecessary to provide the radio station 11-3 with a modulation/demodulation unit, and similar results as in the first embodiment can be obtained.

Further, if fIT fOT and fIR = foR, the frequency conversion units 17-3 and 18-3 are respectively provided with local oscillators 17m and 18e, and these local oscillators can be shared in the transmission system and the reception system, respectively. It is possible to eliminate the influence of frequency deviations and phase noise of the local oscillators, which enables the suppression of signal degradation.

In addition, the reduction of the local oscillators makes it possible to reduce the size of the radio station and the toll centre to a much greater extent.

A fourth embodiment of the present invention is now described.

Fig. 5 is a block diagram showing a mobile communication system according to a fourth embodiment of the present invention. The mobile communication system shown in Fig. 5 is applied to the case where the frequency difference fIT fIR between a transmission signal fIT and a reception signal fIR, at the interface between a frequency conversion unit 18-4 and a modulation/demodulation unit 19 in a toll centre 12-4, matches with the frequency difference fOT foR between a transmission signal fOT and a reception signal fOR, at the interface between a mobile terminal 13 and a frequency conversion unit 17-4, and where additionally, fIT = fOT and fIR fOR The mobile communication system according to the fourth embodiment is similar to that of the first embodiment, except in the following respects.

Specifically, a frequency conversion unit 17-4 is provided with a duplexer (DUP) 17j, a filter (FIL) 17k, and a hybrid circuit (HYB) 17p, and a frequency conversion unit 18-4 is provided with a local oscillator (LO1) 18e, and hybrid circuits (HYB) 18j and 18n. Further, a signal fL1 from the local oscillator 18e is shared between the radio station 11-4 and the toll centre 12-4.

In other words, a mixer 18d of the frequency conversion unit 18-4 of the toll centre 12-4 mixes a modulated signal fIT from the modulation/demodulation unit 19 with a signal fLl from the local oscillator 18e, whereas a mixer 18g mixes a signal from an amplifier 18f with a signal fL1 input from the local oscillator 18e via the hybrid circuit 18n.

The hybrid circuit 17p receives the signal fL1 generated by the local oscillator 18e of the frequency conversion unit 18-4 via the hybrid circuits 18n and 18j, the duplexers 18a, 17i, and 17j, and the filter 17k, and outputs the signal fLl Moreover, a mixer 17e of the frequency conversion unit 17-4 in the radio station 11-4 mixes a signal fOR from a transmission/reception unit 16-1 with the signal fL1 from the hybrid circuit 17p, whereas a mixer 17b mixes a signal from the amplifier 17d with the signal fL1 from the hybrid circuit 17p.

Even in the mobile communication system according to the fourth embodiment, by virtue of the above configuration, when the mobile terminal 13 is currently situated in the area of the radio station 11-4 within the service area covered by the toll centre 12-4 in the same manner as in the previous first to third embodiments, the mobile terminal 13 can communicate with another terminal via the radio station 11-4, the toll centre 12-4, and the public network.

Also, in the mixer 18d of the frequency conversion unit 18-4, the modulated signal fIT from the modulation/demodulation unit 19 is mixed with the signal fL1 from the oscillator 18e, whereas in the mixer 18g, the signal from an amplifier 18f is mixed with the signal fL1 input from the local oscillator 18e via the hybrid circuit 18n.

Moreover, the hybrid circuit 17p receives the signal fL1 generated by the local oscillator 18e of the frequency conversion unit 18-4 via the hybrid circuits 18n and 18j, the duplexers 18a, 17i, and 17j, and the filter 17k, and outputs the signal fL1 to the mixers 17b and 17e.

Also, in the mixer 17e of the frequency conversion unit 17-4, the signal foR from a transmission/reception unit 16-1 is mixed with the signal fL1 from the hybrid circuit 17p, whereas in the mixer 17b, the signal from the amplifier 17d is mixed with the signal fL1 from the hybrid circuit 17p.

As a result, when the frequency conversion units 17-4 and 18-4 carry out frequency conversion, the transmission system and the reception system can share the signal fL1 from the local oscillator 18e.

Accordingly, even in the mobile communication system according to the fourth embodiment, the radio station 11-4 and the toll centre 12-4 are connected together via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14, as a result of which it becomes unnecessary to provide the radio station 11-4 with a modulation/demodulation unit. It becomes possible to obtain similar results as in the first embodiment.

When fIT fIR fOT fOR and fIT fOT and fIR = fOR, he signal fL1 generated by the local oscillator 18e of the frequency conversion unit 18-4 can be shared between the mixers 17b and 17e of the frequency conversion unit 17-4 and the mixers 18d and 18g of the frequency conversion unit 184. Therefore, the influence of frequency deviations and phase noise of the local oscillators can be eliminated, and the degradation of the signals can be suppressed.

Moreover, the reduction of the local oscillators makes it possible to reduce the size of the radio station and the toll centre to a much smaller extent.

A fifth embodiment of the present invention is now described.

Fig. 6 is a block diagram showing a mobile communication system according to a fifth embodiment of the present invention. The mobile communication system shown in Fig. 6 is similar to that of the first embodiment, except in the following respects.

Specifically, a frequency conversion unit 17-5 is provided with a duplexer (DUP) 17j, a filter (FIL) 17k, a local oscillator 17m, a hybrid circuit (HYB) 17n, and a PLL circuit (PLL) 17q, and a frequency conversion unit 185 is provided with a local oscillator (LO1) 18e, a hybrid circuit (HYB) 18j, a duplexer (DUP) 18k, a filter (FIL) 18m, and a PLL circuit (PLL) 18p. The local oscillation signals in a station serving as a receiving end of the second radio communication line 15 are respectively synchronized with the local oscillation signals in a station serving as a sending end of the second radio communication line 15.

In other words, when a signal is sent from a radio station 11-5 to a toll centre 12-5, a mixer 17e of the frequency conversion unit 17-5 sends a signal by converting the frequency of a signal fL2 received from the local oscillator 17m. On the other hand, when the toll centre 125 converts the frequency of the signal received from the radio station 11-5, the local oscillation signal fL2 is input to the mixer 18g of the frequency conversion unit 18-5 via the hybrid circuit 17n, the duplexers 17i, 18a, 18k, the filter 18m, and the PLL circuit 18p while being synchronized with the local oscillation signal input to the mixer 17e.

Similarly, when a signal is sent from the toll centre 12-5 to the radio station 11-5, a mixer 18d of the frequency conversion unit 18-5 converts the frequency of a signal fL1 received from the local oscillator 18e and sends the thus frequency-converted signal. On the other hand, when the radio station 11-5 converts the frequency of the signal received from the toll centre 12-5, the mixer 17b of the frequency conversion unit 17-5 receives the local oscillation signal fL1 from the local oscillator 18e via the hybrid circuit 18j, the duplexers 18a, 17i, 17j, the filter 17k, and the PLL circuit 17q while the local oscillation signal is synchronized with the local oscillation signal input to the mixer 18d.

Even in the mobile communication system according to the fifth embodiment, by virtue of the above configuration, when the mobile terminal 13 is currently situated in the area of the radio station 11-5 within the service area covered by the toll centre 12-5 in the same manner as in the previous first to fourth embodiments, the mobile terminal 13 can communicate with another terminal via the radio station 11-5, the toll centre 12-5, and the public network.

When the frequency of a signal received from the mobile terminal 13 is converted, the frequency conversion unit 17-5 converts the frequency of the reception signal using the local oscillation signal fL2 from the local oscillator 17m. The frequency conversion unit 18-5 generates a local oscillation signal fL4 using the signal fL2 from the local oscillator 17m, and converts the frequency of the signal from the mobile terminal 13 using the thus generated local oscillation signal fL4 Specifically, the signal fL2 from the local oscillator 17m is input to the filter 18m via the hybrid circuit 17n, the duplexer 17i, the antennas 22 and 23, and the duplexers 18a and 18k. The filter 18m and the PLL circuit 18p produce the local oscillation signal having a frequency fL4 by the use of the signal fL2 from the local oscillator 17m.As a result, the frequency conversion unit 18-5 can convert the frequency of the signal received from the mobile terminal 13 using the local oscillation signal fL4 Similarly, when the frequency of a radio signal to be sent to the mobile terminal 13 is converted, the frequency conversion unit 18-5 converts the frequency of the radio signal using the local oscillation signal fLl from the local oscillator 18e. On the other hand, the frequency conversion unit 17-5 produces the local oscillation signal fL3 using the signal fLl from the local oscillator 18e, and converts the frequency of the signal to be sent to the mobile terminal 13 using the thus produced local oscillation signal fL3 In other words, the signal fL1 from the local oscillator 18e is input to the filter 17k via the hybrid circuit 18j, the duplexer 18a, the antennas 23 and 22, and the duplexers 17i and 17j. The filter 17k and the PLL circuit 17p produce a local oscillation signal having a frequency fL3 using the signal fLl from the local oscillator 18e. As a result, the frequency conversion unit 17-5 can convert the frequency of a signal to be sent to the mobile terminal 13 using the local oscillation signal fL3.

Hence, even in the mobile communication system according to the fifth embodiment of the present invention, the radio station 11-5 and the toll centre 12-5 are connected together via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14, as a result of which it becomes unnecessary to provide the radio station 11-5 with a modulation/demodulation unit. Similar results in the first embodiment can be obtained.

Further, the local oscillators 17m and 18e of the frequency conversion units 17-5 and 18-5 are arranged in such a way that a local oscillation signal of one station (the receiving end in communication between stations) is synchronized with a local oscillation signal of the other station (the sending end in communication between the stations) during both transmission and reception. By virtue of such a configuration, the local oscillation signals used in the radio station 11-5 and the toll centre 12-5 are synchronized with each other, and hence the influence of frequency deviations and phase noise can be eliminated.

A sixth embodiment of the present invention is now described.

Fig. 7 is a block diagram showing a mobile communication system according to a sixth embodiment of the present invention. The mobile communication system, shown in Fig. 7, is applied to the case where the frequency difference fIT - fIR between a transmission signal fIT and a reception signal fIRt at the interface between the frequency conversion unit 18-6 of a the modulation/demodulation unit 19 in a toll centre 12-6, matches with the frequency difference fOT foR between a transmission signal foT and a reception signal foR at the interface between a mobile terminal 13 and a frequency conversion unit 17-6.

The mobile communication system according to the sixth embodiment is similar to that of the first embodiment, except for the following points. The frequency conversion unit 17-6 is provided with a duplexer (DUP) 17j, a filter (FIL) 17kj and a PLL circuit 17q, whereas the frequency conversion unit 18-6 is provided with a local oscillator (LO1) 18e, and hybrid circuits (HYB) 18j and 18n. The local oscillators of the frequency conversion units 17-6 and 18-6 are shared between the radio station 11-6 and the toll centre 12-6 during transmission and reception.

The local oscillation signal of the radio station 11-6 is synchronized with the local oscillation signal of the toll centre 12-6.

In other words, a mixer 18d of the frequency conversion unit 18-6 in the toll centre 12-6 mixes a modulator output signal fIT from the modulation/demodulation unit 19 with a signal fL1 from the local oscillator 18e, whereas a mixer 18g mixes a signal from an amplifier 18f with a signal fL1 input from the local oscillator 18e via the hybrid circuit 18n.

A mixer 17e of the frequency conversion unit 17-6 in the radio station 11-6 mixes a signal fOR from a transmission/reception unit 16-1 with a signal fL2 from the PLL circuit 17q, whereas a mixer 17b mixes a signal from the amplifier 17d with the signal fL2 from the PLL circuit 17q.

The PLL circuit 17q receives the signal f generated by the local oscillator 18e of the frequency conversion unit 18-6 via the hybrid circuits 18n and 18j, the duplexers 18a, 17i, 17j, and the filter 17k.

Thereby, the local oscillation signal fL2 shared between transmission and reception in the frequency conversion unit 17-6 can be output to the mixers 17b and 17e while being synchronized with the signal fL used by the frequency conversion unit 18-6.

Even in the mobile communication system according to the sixth embodiment, by virtue of the above configuration, when the mobile terminal 13 is currently situated in the area of the radio station 11-6 within the area covered by the toll centre 12-6 in the same manner as in the first to fifth embodiments, the mobile terminal 13 can communicate with another terminal via the radio station 11-6, the toll centre 12-6, and the public network.

Moreover, the mixer 18d of the frequency conversion unit 18-6 in the toll centre 12-6 mixes the modulator output signal fIT from the modulation/demodulation unit 19 with the signal fL1 from the local oscillator 18e, whereas the mixer 18g mixes the signal from the amplifier 18f with the signal fL1 input from the local oscillator 18e via the hybrid circuit 18n.

The mixer 17e of the frequency conversion unit 17-6 in the radio station 11-6 mixes the signal fOR from the transmission/reception unit 16-1 with the signal fL2 from the PLL circuit 17q, whereas the mixer 17b mixes the signal from the amplifier 17d with the signal fL2 from the PLL circuit 17q.

The PLL circuit 17q receives the signal fLl generated by the local oscillator 18e of the frequency conversion unit 18-6 via the hybrid circuits 18n and 18j, the duplexers 18a, 17i, 17j, and the filter 17k.

Hence, even in the mobile communication system according to the sixth embodiment of the present invention, the radio station 11-6 and the toll centre 12-6 are connected together via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14, as a result of which it becomes unnecessary to provide the radio station 11-6 with a modulation/demodulation unit. Similar results as in the first embodiment can be obtained.

Further, the local oscillators of the frequency conversion units 17-6 and 18-6 can be arranged so as to be shared between the radio station 11-6 and the toll centre 12-6, and the local oscillation signal of the radio station 11-6 is synchronized with the local oscillation signal of the toll centre 12-6. Therefore, the size of the radio station can be reduced further, and the influence of frequency deviations and phase noise can be eliminated.

Although the local oscillation signal of the radio station 11-6 is synchronized with the local oscillation signal of the toll centre 12-6 in the above embodiment, the present invention is not limited thereto. It is also possible to synchronize the local oscillation signal of the toll centre 12-6 with the local oscillation signal of the radio station 11-6.

A seventh embodiment of the present invention is now described.

Fig. 8 is a block diagram showing a mobile communication system according to a seventh embodiment of the present invention. The mobile communication system of the seventh embodiment shown in Fig. 8 is similar to that of the first embodiment, except in the following respects. The transmission system of a radio station 11-7 is provided with an automatic gain controller (AGC) 17r. The automatic gain controller (AGC) 17r maintains the level of a signal, which is to be transmitted to the mobile terminal 13, constant irrespective of the attenuation during the transmission from the toll centre 12-7 to the radio station 11-7.

Even in the mobile communication system according to the seventh embodiment, by virtue of the above configuration, when the mobile terminal 13 is currently situated in the area of the radio station 11-7 within the service area covered by the toll centre 12-7 as in the previous first through sixth embodiments, the mobile terminal 13 can communicate with another terminal via the radio station 11-7, the toll centre 12-7, and the public network.

If the automatic gain controller 17r provides no control when the toll centre 12-7 sends a signal to the radio station 11-7, the level of the signal sent to the mobile terminal 13 varies as shown in Figs. 9 and 10.

Contrary to this, if the automatic gain controller 17r provides control, the level of the signal becomes stable as shown in Figs. 11 and 12.

In detail, since the radio station 11-7 and the toll centre 12-7 are connected together by the second radio communication line 15 using a submillimeter wave band of 18 GHz, a signal sent from the toll centre 12-7 attenuates while being transmitted through the second radio communication line 15 if the automatic gain controller 17r provides no automatic gain control.

When the level of the signal sent from the toll centre 12-7 is S9, as shown in Fig. 10, the attenuation of the signal through the second radio communication line 15 is S1 if the weather is fine. However, the attenuation of the signal through the second radio communication line 15 increases to S2 during rainfall. In this way, the level of a signal arrived at the radio station 11-7 varies in a considerably wide range.

The level of the signal received by the radio station 11-7 is amplified while it is passed from the frequency conversion unit 17-7 to the transmission/reception unit 16-1 (see signal levels S3, S5, and 57 in Fig. 10 for fine weather, and see signal levels S4, S6, and S8 in Fig. 10 for rainfall).

As a result, the level of the signal sent to the mobile terminal 13 becomes S10-1 in Fig. 10 during fine weather, but the level becomes S10-2 in Fig. 10 during rainfall. Thus, the level of the signal sent to the mobile terminal 13 varies depending on the weather.

Fig. 12 shows changes in the level of a signal sent from the radio station 12-7 when the automatic gain controller 17r performs automatic gain control. As shown toll centre 12-7 is S19, the attenuation of the signal through the second radio communication line 15 is Sil during fine weather but increases to S12 during rainfall.

The level of the signal received by the radio station 11-7 is amplified while it passes from the frequency conversion unit 17-7 to the transmission/reception unit 16-1 (see signal levels S13, S15, and S17 in Fig. 12 for fine weather, but see signal levels S14, S16, and S17 in Fig. 12 for rainfall).

In other words, the signal level during rainfall is controlled so as to be the same signal level as that during fine weather by means of automatic gain control of the automatic gain controller 17r, whereby it becomes possible for the transmission/reception unit 16-1 to send a signal S18 having a constant level irrespective of factors which cause signal attenuation, such as the weather.

Accordingly, even in the mobile communication system according to the seventh embodiment of the present invention, the radio station 11-7 and the toll centre 12-7 are connected together via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14, as a result of which it becomes unnecessary to provide the radio station 11-7 with a modulation/demodulation unit. Hence, similar results as in the first embodiment can be obtained.

Moreover, the automatic gain controller 17r controls the transmission level of the radio station 11-7 so as to be constant, whereby the quality of the communication line between the radio station and the mobile terminal 13 can be improved.

In the seventh embodiment, although the automatic gain controller 17r is provided at the sending side of the radio station 11-1 in the first embodiment, the present invention is not limited to such a configuration. The above-described structure can be applied to any one of the above second to sixth embodiments. Even if the communication system is configured in this way, the advantageous results inherent to each embodiment will be ensured. In addition, the quality of the communication line between the radio station and the mobile terminal 13 can be improved.

An eighth embodiment of the present invention is now described.

Fig. 13 is a block diagram showing a mobile communication system according to an eighth embodiment of the present invention. The mobile communication system of the eighth embodiment shown in Fig. 13 is similar to that of the seventh embodiment, except in the following respects. Specifically, the transmission system of a frequency conversion unit 17-8 of a radio station 11-8 is not provided with an automatic gain controller, but the reception system of a toll centre 12-8 is provided with an automatic gain controller 18q which is the same as the automatic gain controller (see reference numeral 17r) in the seventh embodiment.

In other words, the mobile communication system according to the eighth embodiment is different from the mobile communication system according to the first embodiment in that the automatic gain controller 18q is disposed at the receiving side of the toll centre 12-8.

Even in the mobile communication system according to the eight embodiment of the present invention, by virtue of the above configuration, when the mobile terminal 13 is currently situated in the area of the radio station 11-8 within the service area covered by the toll centre 12-8 in the same manner as in the first to seventh embodiments, the mobile terminal 13 can communicate with another terminal via the radio station 11-8, the toll centre 12-8, and the public network.

The radio station 11-8 and the toll centre 12-8 are connected together by the second radio communication line 15 having a submillimeter wave band, and hence the signal level varies by attenuations due to rainfall or the like during the course of transmission, in the same manner as in the seventh embodiment. Therefore, when the toll centre 12-8 receives a signal from the radio station 11-8, the level of the signal arrived at toll centre 12-8 varies in a wide range. However, the level of a signal input to the modulation/demodulation unit 19 can be made constant by using the automatic gain controller 18q.

Hence, even in the mobile communication system according to the eighth embodiment of the present invention, the radio station 11-8 and the toll centre 12-8 are connected together by means of the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14, as a result of which it becomes unnecessary to provide the radio station 11-8 with a modulation/demodulation unit. Therefore, similar results as in the first embodiment can be obtained.

Further, the automatic gain controller 18q controls the level of the signal input to the modulation/demodulation unit 19 so as to be constant, and hence it is possible to use a conventional modulation/demodulation unit as the modulation/demodulation unit 19 disposed in the toll centre 12-8. This minimizes an increase in costs required to implement the system.

In the above embodiment, the local oscillators 17c, 17f, 18e, and 18h are provided in the transmission and reception systems of the radio station 11-8 and the toll centre 12-8, respectively. However, the present invention is not limited to such a configuration. For example, this configuration can be applied to any of the second to seventh embodiments. Even if the communication system is configured in this way, the advantageous results inherent to each embodiment will be ensured. Furthermore, a cost increase due to the implementation of the system can be minimized.

A ninth embodiment of the present invention is now described.

Fig. 14 is a block diagram showing a mobile communication system according to a ninth embodiment of the present invention. The mobile communication system of the ninth embodiment shown in Fig. 14 is similar to that of the seventh embodiment of the present invention, except in the following respects.

Specifically, the transmission system of a radio station 11-9 is provided with a signal branching circuit (HYB) 17s, a detector (DET) 17t, a level comparator (COMP) 17u, and changeover switches 17v and 17w.

The signal branching circuit 17s branches a signal sent from the toll centre 12-9 into two signals, and the detector 17t detects the level of the signal from the signal branching circuit 17s. The level comparator 17u outputs a control signal to the changeover switches 17v and 17w based on the level of the signal from the detector 17t, thereby selecting a route including the automatic gain controller 17r or a bypass route without the automatic gain controller 17r.

Specifically, if the level of the signal detected by the detector 17t is in, for example, a range T1 shown in Fig. 16, a bypass route 41 shown in Fig. 15 (the signal branching circuit 17s - > the changeover switch 17v - > the changeover switch 17w) is selected, because the signal level is sufficient for the mobile terminal 13 to receive the signal transmitted from the radio station 11-9 even if the output level of the signal decreases.

When the level of the signal detected by the detector 17t is in, for example, a range T2 shown in Fig. 16, a route 42 shown in Fig. 15 (the signal branching circuit 17s - > the changeover switch 17v - > the automatic gain controller 17r - > the changeover switch 17w) which includes the automatic gain controller 17r disposed in a signal path, because a drop in the transmission level of the radio station 11-9 makes it difficult for the mobile terminal 13 to receive the signal.

In the mobile communication system according to the ninth embodiment, a relative distance between the radio station 11-9 and the mobile terminal 13 varies as the mobile terminal 13 moves. For this reason, it is necessary for the mobile terminal 13 itself to have a wide dynamic range with respect to the receiving level.

Further, as regards attenuation of the level of a radio signal for use in communication between stations due to rainfall, there is very small temporal probability of the occurrence of the maximum attenuation estimated in designing the communication line.

Accordingly, if the range of attenuation of a radio signal for use in communication between stations estimated during system design is narrower than a receiving dynamic range of the mobile terminal 13, the automatic gain controller 17r does not provide control, whereby the generating of noise from the radio station 11-9 can be suppressed.

Even in the mobile communication system according to the ninth embodiment, by virtue of the above configuration, when the mobile terminal 13 is in the area of the radio station 11-9 within the service area covered by the toll centre 12-9 in the same manner as in the previous first to eighth embodiments, the mobile terminal 13 can communicate with another terminal via the radio station 119, the toll centre 12-9, and the public network.

The radio station 11-9 and the toll centre 12-9 are connected together via the second radio communication line 15 using a submillimeter wave band of 18 GHz. Hence, if the level of the signal sent from the toll centre 12-9 is T15 as shown in fig. 16, the attenuation of the signal through the second radio communication line 15 during fine weather will be T3, whereas the attenuation of the signal through the second radio communication line 15 during rainfall will be T4.

The signal received by the radio station 11-9 is input to the mixer 17b, and the level of the signal received during fine weather varies as designated by T5, but the signal level during rainfall varies as designated by T6.

The detector 17t detects the level of the signal received from the mixer 17b. The level comparator 17u selects the bypass route 41 as a signal path when the level of the detected signal is in the range T1 shown in Fig. 16. On the other hand, when the level of the detected signal is in the range T2 shown in Fig. 16, the level comparators 17u controls the changeover switches 17v and 17w so as to select the route 42 including the automatic gain controller 17r disposed in the signal path.

With this operation, during fine weather, the level of the signal is amplified as indicated by symbols T7 and T9, whereas during rainfall, the level of the signal is amplified as indicated by the symbols T8 and T10. Thereafter, the amplified signal is sent to the mobile terminal 13.

Therefore, the level of the signal sent from the radio station 11-9 to the mobile terminal 13 can be sent between the maximum signal output level T13 and the minimum signal output level T14, irrespective of a factor which cause the attenuation of the signal such as weather. Further, the maximum noise output level can be decreased to T11 so that the absolute level of a noise output can be reduced.

Even in the mobile communication system according to the ninth embodiment of the present invention, the radio station 11-9 and the toll centre 12-9 are connected together by the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14, as a result of which it becomes unnecessary to provide the radio station 11-9 with a modulation/demodulation unit.

Thus, similar results as in the first embodiment can be obtained.

With the automatic gain controller 17r, the signal branching circuit (HYB) 17s, the detector (DET) 17t, the level comparator (COMP) 17u, and the changeover switches 17v and 17w, even if the output level of the signal sent from the radio station 11-9 drops, the automatic gain controller 17r does not provide control so long as the mobile terminal 13 can receive the signal. Contrary to this, if the level of the signal which is transmitted from the toll centre 12-9 and received by the radio station 11-9 drops, thereby making difficult for the mobile terminal 13 to receive the signal, the automatic gain controller 17r provides control. This suppresses the noise output level, and ensures the quality of lines during rainfall.

In the ninth embodiment, the transmission systems and the reception systems of the radio station 11-9 and the toll centre 12-9 are respectively provided with the local oscillators 17c, 17f, 18e, and 18h. However, the present invention is not limited to such a configuration. For example, this configuration can be applied to any of the second to eighth embodiments.

Even if the communication system is configured in this way, the advantageous results inherent to each embodiment will be ensured. Furthermore, it is possible to ensure the quality of lines during rainfall while the noise output level is suppressed.

A tenth embodiment of the present invention is now described.

Fig. 17 is a block diagram showing a mobile communication system according to a tenth embodiment of the present invention. The mobile communication system of the tenth embodiment shown in Fig. 17 is similar to that of the first embodiment, except in the following respects. Specifically, a frequency conversion unit 17-10 of a radio station 11-10 is provided with a duplexer (DUP) 17j, a filter (FIL) 17k, a hybrid circuit (HYB) 17n, PLL circuits (PLL2, PLL3) 17q-1 and 17q-2, and a reference oscillator (REF OSC) 17x. On the other hand, a frequency conversion unit 18-10 of a toll centre 12-10 is provided with a hybrid circuit (HYB) 18j, a duplexer (DUP) 18k, a filter (FIL) 18m, PLL circuits (PLL1, PLL4) 18r-1 and 18r-2, and a reference oscillator (REF OSC) 18s.

In the mobile communication system according to the tenth embodiment, a signal, within a communication band assigned to communication between stations, which is in synchronism with a reference signal of a local oscillation signal of one station (for example, the toll centre 12-10), is used as a reference signal for a local oscillation signal of another station (for example, the radio station 11-10), whereby the local oscillation signals of both stations can be synchronized with each other.

In detail, the toll centre 12-10 sends to the radio station 11-10 a signal having a frequency fRF and a signal having a frequency fL5 which is used as a reference signal of the local oscillation signal of the radio station 11-10, via the second radio communication line 15 using the 18 GHz band. The frequency fL5 of the signal used as the reference signal is selected to be within the communication band (18 GHz) assigned to the communication between the toll centre 12-10 and the radio station 11-10.

Similarly, the radio station 11-10 sends to the toll centre 12-10 the signal having the frequency fRF and a signal having a frequency fL6 which is used as a reference signal of the local oscillation signal of the toll centre 12-10, via the second radio communication line 15 using the 18 GHz band. The frequency fL6 of the signal used as the reference signal is selected to be within the communication band (18 GHz) assigned to the communication between the toll centre 12-10 and the radio station 11-10.

Thereby, the oscillation signals of both the radio station 11-10 and the toll centre 12-10 can be synchronized with each other.

Even in the mobile communication system according to the tenth embodiment, by virtue of the above configuration, when the mobile terminal 13 is in the area of the radio station 11-10 within the service area covered by the toll centre 12-10 in the same manner as in the previous first to ninth embodiments, the mobile terminal 13 can communicate with another terminal via the radio station 11-10, the toll centre 12-10, and the public network.

A signal, within a communication band assigned to communication between stations, which is in synchronous with a reference signal of a local oscillation signal of either the radio station 11-10 or the toll centre 12-10 (for example, the toll centre 12-10), i.e., the signal within the band of the second radio communication line 15, is used as a reference signal for a local oscillation signal of another station (for example, the radio station 11-10) whereby the local oscillation signals of both the stations can be synchronized with each other.

As explained further above, in the mobile communication system according to the sixth embodiment, when the interface frequency fIT between a modulation/demodulation unit 19 and a frequency conversion unit 18-6 in the toll centre 12-6 is in, for example, 800 MHz band or 1.5 GHz band, the signal transmission frequency (18 GHz band) and the local oscillation signal frequency fL1, both being used in communication between stations, are spaced apart from each other by a frequency of 800 MHz or 1.5 GHz. In such a case, a signal input to the radio station 11-6 as a reference signal of a local oscillation signal may go out of the frequency band assigned to transmission between the stations.In contrast, in the mobile communication system according to the tenth embodiment, frequencies within the band assigned to the transmission between stations can be used as the reference signal of the local oscillation signal of the radio station 11-10.

Accordingly, even in the mobile communication system according to the tenth embodiment of the present invention, the radio station 11-10 and the toll centre 12-10 are connected together via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14, as a result of which it becomes unnecessary to provide the radio station 11-10 with a modulation/demodulation unit. Similar results as in the first embodiment can be obtained.

The frequency conversion unit 17-10 of a radio station 11-10 is provided with the duplexer 17j, the filter 17k, the hybrid circuit 17n, the PLL circuits (PLL2, PLL3) 17q-1 and 17q-2, and the reference oscillator (REF OSC) 17x. On the other hand, the frequency conversion unit 18-10 is provided with the hybrid circuit 18j, the duplexer 18k, the filter 18m, the PLL circuits (PLL1, PLL4) 18r-1 and 18r-2, and the reference oscillator (REF OSC) 18s. By virtue of these components, a signal, within a communication band assigned to communication between stations, which is in synchronism with a local oscillation signal of one station is used as a reference signal for a local oscillation signal of another station, whereby the local oscillation signals of both stations can be synchronized with each other, and the influence of frequency deviations and phase noise can be eliminated.

In the tenth embodiment, a signal, within a communication band assigned to communication between stations, which is in synchronism with a local oscillation signal of one station is used as a reference signal for a local oscillation signal of another station, whereby the local oscillation signals of both stations can be synchronized with each other.

However, the present invention is not thus limited.

For example, this configuration can be applied to any of the second to ninth embodiments. Even if the communication system is configured in this way, the advantageous results inherent to each embodiment will be ensured. Furthermore, it is possible to eliminate the influence of frequency deviations and phase noise.

An eleventh embodiment of the present invention is now described.

Fig. 18 is a block diagram showing a mobile communication system according to an eleventh embodiment of the present invention. The mobile communication system of the eleventh embodiment shown in Fig. 18 is similar to that of the first embodiment except that it adopts a receiving space diversity (SD) system.

In other words, in the mobile communication system according to the eleventh embodiment, a radio station 11-11 is provided with two antennas 21a and 21b which are spaced apart from each other to send and receive signals to and from the mobile terminal 13.

Reference numeral 16-2 denotes a transmission/reception unit, and this transmission/reception unit 16-2 is provided with a band-pass filter (BPF) 16d and a low-noise amplifier (LNA) 16e as well as with a duplexer (DUP) 16a, a power amplifier (PA) 16b, and a low-noise amplifier (LNA) 16c which have the same functions as those used in the first embodiment.

Specifically, the duplexer 16a is connected to the antenna 21a so as to receive a signal from a main system, whereas the band-pass filter 16d is connected to the antenna 21a to receive a signal from a sub-system.

Differing from the frequency conversion unit of the radio station in the first embodiment, a frequency conversion unit 17-11 is provided with a hybrid circuit (HYB) 17y, a local oscillator (LO5) 17z-1, and a mixer (MIX) 17z-2 for a reception system. The remaining portions of the frequency conversion unit 17-11 have the same functions as those in used in the first embodiment.

Differing from the frequency conversion unit of the toll centre in the first embodiment, a frequency conversion unit 18-11 of the toll centre 12-11 is provided with a local oscillator (LO6) 18t, a mixer (MIX) 18u, a band-pass filter 18v, and a hybrid circuit (HYB) 18w. The remaining portions of the frequency conversion unit 18-11 have the same functions as those in used in the first embodiment.

Even in the mobile communication system according to the eleventh embodiment, by virtue of the above configuration, when the mobile terminal 13 is in the area of the radio station 11-11 within the service area covered by the toll centre 12-11 in the same manner as in the previous first to tenth embodiments, the mobile terminal 13 can communicate with another terminal via the radio station 11-11, the toll centre 12-11, and the public network.

The signal sent from the mobile terminal 13 is received by the antennas 21a and 21b. The signal received by the antenna 21a is output to the frequency conversion unit 17-11 via the duplexer 16a and the low-noise amplifier 16c, whereas the signal received by the antenna 21b is output to the frequency conversion unit 17-11 via the band-pass filter 16d and the low-noise amplifier 16e.

A signal fOR for the sub-system delivered from the low-noise amplifier 16e is mixed with the signal fL5 delivered from the local oscillator 17z-1, and the thus mixed signal is output to the hybrid circuit 17y (see signal for) The hybrid circuit 17y of the frequency conversion unit 17-11 combines the signal fOR for the main system delivered from the transmission/reception unit 16-1 with a signal for for the sub-system, and converts the frequency of the signal.

When the toll centre 12-11 receives a signal from the radio station 11-11, the frequency of that signal is converted while passing through the duplexer 18a, the amplifier 18f, the mixer 18g, and the local oscillator 18h.

The frequency-converted signal fIr is output to the modulation/demodulation unit 19 as a signal fIR via the hybrid circuit 18w and the band-pass filter 18i.

Also, the frequency-converted signal fIr is mixed with a signal fL6 delivered from the local oscillator 18t by the mixer 18u, and the thus mixed signal is output to the modulation/demodulation unit 19 as the signal fIR via the band-pass filter 18v.

Accordingly, even in the mobile communication system according to the eleventh embodiment of the present invention, the radio station 11-11 and the toll centre 12-11 are connected together via the second radio communication line 15 which uses a frequency band different from that used for the first radio communication line 14, as a result of which it becomes unnecessary to provide the radio station 11-11 with a modulation/demodulation unit. Similar results as are obtained in the first embodiment can be obtained.

Also, the receiving SD (Space Diversity) system is employed to combine the signal foR of the main system and the signal for of the sub-system delivered from the transmission/reception unit 16-1 before frequency conversion. It is therefore possible to prevent the deterioration of the quality of the lines due to fading occurring in the first radio communication line 14 between the radio station 11-11 and the mobile terminal 13.

Although the eleventh embodiment is similar to the first embodiment except for the adoption of the receiving SD (Space Diversity) system, it is not limited thereto. For example, this configuration can be applied to any of the second to tenth embodiments.

Even if the communication system is configured in this way, the advantageous results inherent to each embodiment will be ensured. Furthermore, it is possible to prevent the deterioration of the quality of lines due to fading occurring in the first radio communication line 14 between the radio station and the mobile terminal 13.

A twelfth embodiment of the present invention is now described.

Fig. 19 is a block diagram showing a mobile communication system according to a twelfth embodiment of the present invention. The mobile communication system shown in Fig. 19 is similar to those of each of the previous embodiments, except in the following respects. Specifically, the transmission and reception systems of the radio station 11 and the toll centre 12 are respectively provided with three systems presently used and one spare system. However, the mobile communication system in the twelfth embodiment is similar to that of each of the previous embodiments in that the radio station 11 and mobile terminals 13A to 13C are connected together via the first radio communication line 14, and that the toll centre 12 and the radio station 11 are connected to each other via the second radio communication line 15.

The radio station 11 is provided with three antennas 21A to 21C for establishing communication between the radio station and the mobile terminals.

These antennas 21A to 21C divide the service area covered by the radio station 11 into three sectors.

For instance, the antenna 21A is used to send and receive signals to and from the mobile terminal 13A existing in the corresponding sector, the antenna 21B is used to send and receive signals to and from the mobile terminal 13B existing in the corresponding sector, and the antenna 21C is used to send and receive signals to and from the mobile terminal 13C existing in the corresponding sector.

The radio station 11 is composed of four transmission/reception units 16A to 16D, four frequency conversion units 17A to 17D, an antenna switch 31, and a branching line 32.

Each of the four transmission/reception units 16A to 16D can be formed by the transmission/reception unit 16-1 used in the first embodiment, while each of the four frequency conversion units 17A to 17D can be formed by the frequency conversion unit 17-1 used in the first embodiment.

The toll centre 12 is composed of four frequency conversion units 18A to 18D, three modulation/demodulation units 19A to 19C, a branching line 33, an IF switch 34, and the exchange 20 which is the same as the exchange used in each of the previous embodiments.

Like the radio station 11, each of the four frequency conversion units 18A to 18D can be formed by the frequency conversion unit 18-1 used in the first embodiment, while each of the three modulation/demodulation units 19A to 19C can be made of the modulation/demodulation unit 19 used in the first embodiment.

When a malfunction occurs in any one of the transmission/reception units 16A to 16D and the frequency conversion units 17A to 17D of the radio station 11, and the frequency conversion units 18A to 18D of the toll centre 12, the antenna switch 31 and the IF switch 34 switch a communication system to use a spare system instead of a faulty one.

The second radio communication line 15 uses a frequency band of 18 GHz, for example. In this frequency band, intervals are provided among the three systems presently used and one spare system.

Even in the mobile communication system according to the twelfth embodiment of the present invention, by virtue of the above configuration, when the mobile terminal 13 is in any of the sectors of the service area covered by the radio station 11-11, the mobile terminal 13 can communicate with another terminal via any one of the presently used three systems corresponding to the sector where the mobile terminal is currently situated.

If system trouble arises in any of the transmission/reception units 16A to 16D and the frequency conversion units 17A to 17D of the radio station 11, and the frequency conversion units 18A to 18D of the toll centre 12, it is possible to avoid an interruption of the line at the time of the system failure by switching the antenna switch 31 and the IF switch 34 to use the spare system instead of the faulty system.

In this way, in the mobile communication system according to the twelfth embodiment of the present invention, the communication system is made of a current system and a spare system. If system trouble arises in the current system, it is possible to avoid the interruption of the line by switching the system from the faulty system to the spare system, thereby resulting in improved reliability of the system.

Although each of the transmission/reception units 16A to 16D, the frequency conversion units 17A to 17D and 18A to 18D, and the modulation/demodulation units l9A to l9C is made by the use of the components employed in the first embodiment, the present invention is not limited thereto. For example, this configuration can be applied to any of the second to eleventh embodiments. Even if the communication system is configured in this way, the advantageous results inherent to each embodiment will be ensured.

Furthermore, the communication system is made of the current system and the spare system. If system trouble arises in the current system, it is possible to avoid the interruption of the line by switching the system from the faulty system to the spare system, thereby resulting in improved reliability of the system.

Further possible variations and modifications are now described.

As previously mentioned, a plurality of radio stations according to each of the previous embodiments (see reference numerals 11-1 to 11-11, and 11) can be disposed in each segment of the service area covered by the toll centre (see reference numerals 12-1 to 12-11, and 12) in the same manner as the common mobile communication system shown in Fig. 20. In such a case, each of different frequencies within the 18 GHz band is selected in communication between the toll centre and the radio station disposed in each area.

The toll centre according to the previous first to eleventh embodiments (see reference numerals 12-1 to 12-11) can be provided with the antenna 23, the frequency conversion units 18-1 to 18-11, and the modulation/demodulation unit 19, which are the same as those used in each of the previous embodiments, for a predetermined number of radio stations disposed in each area. Moreover, the toll centre according to the twelfth embodiment (see reference numeral 12) can also be provided with the antenna 23, the branching line 33, the frequency conversion units 18A to 18D, the IF switch 34, and the modulation/demodulation units 19A to l9C, which are the similar to those used in the twelfth embodiment, for a predetermined number of radio stations disposed in each area.

Claims

1. A mobile communication system comprising: a radio station, a radio terminal and a toll centre, the radio station and the radio terminal having means for communication with each other via a first radio communication link having a first operating frequency band and the radio station and the toll centre having means for communicating with each other via a second radio communication link having a second operating frequency band different from the first operating frequency band, the toll centre having means for transmitting and receiving communication signals to and from the radio station and having an exchange function.

2. A mobile communication system according to claim 1, wherein the first operating frequency band is a microwave band and the second operating frequency band is a submillimeter wave band.

3. A mobile communication system according to claim 1 or 2, the radio station comprising a transmission/reception unit for sending and receiving communication signals to and from the radio terminal.

4. A mobile communication system according to claim 3, having a frequency conversion unit connected to the transmission/reception unit and providing frequency conversion between an interface frequency between the radio terminal and the frequency conversion unit and an interface frequency between the radio station and the toll centre.

5. A mobile communication system according to any one of the preceding claims, the toll centre comprising a modulation/demodulation unit; an exchange which is connected to the modulation/demodulation unit; a transmission/reception unit for the said second radio communication link; and a frequency conversion unit connected between the transmission/reception unit and the modulation/demodulation unit to provide frequency conversion between an interface frequency between the radio station and the toll centre and an interface frequency between the frequency conversion unit and the modulation/demodulation unit.

6. A radio station for communication with a toll centre with an exchange function while communicating with a radio terminal, the radio station comprising: a transmission/reception unit for transmitting and receiving communication signals to and from a radio terminal via a first radio communication link; and a frequency conversion unit connected to the transmission/reception unit and providing frequency conversion between an interface frequency between a radio terminal and the frequency conversion unit and an interface frequency between the radio station and the toll centre, thereby making it possible to transmit and receive communication signals to and from the toll centre via a second radio communication link having an operating frequency band different from that of the first radio communication link.

7. A radio station according to claim 6, wherein the frequency band of the first radio communication link is a microwave band, and the frequency band of the second radio communication link is a submillimeter wave band.

8. A toll centre for communication with a radio station which in turn communicates with a radio terminal via a first radio communication link over a first operating frequency band, the toll centre comprising: a modulation/demodulation unit; an exchange which is connected to the modulation/demodulation unit; a transmission/reception unit for communication between the toll centre and the radio station via a second radio communication link over a second operating frequency band different from the first operating frequency band; and a frequency conversion unit connected between the transmission/reception unit and the modulation/demodulation unit to provide frequency conversion between an interface frequency between the radio station and the toll centre and an interface frequency between the frequency conversion unit and the modulation/demodulation unit.

9. A toll centre according to claim 8, characterized in that the first operating frequency band is a microwave band, and the second operating frequency band is a submillimeter wave band.

10. A mobile communication system substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

11. A radio station as hereinbefore described with reference to Figure 1 of the accompanying drawings.

12. A toll centre as hereinbefore described with reference to Figure 1 of the accompanying drawings.

13. A mobile communication system substantially as hereinbefore described with reference to any one of the first to twelfth embodiments of Figures 2, 3, 4, 5, 6, 7, 8-12, 13, 14-16, 17, 18 and 19 respectively of the accompanying drawings.