Repeater for radio paging system

Abstract: A repeater for installation as a measure for silent zones, which automatically compensates for a variation in a light loss caused by a difference in length among optical cables interconnecting a master device (3) and a plurality of slave devices (4) which are allocated and installed in an underground shopping center or the like. The master device and the slaves devices each have current detectors (35-1 to 35-n; 44) for detecting current values of O/E transducers which transduce received light to electric signals. Based on the detected current values, gain control signals are applied to variable gain amplifiers (36-1 to 36-n) to change their amplification factors so that the levels of output RF signals for transmission to a radio base station and a portable station have a prescribed value, thereby compensating for the variation in the light loss of the optical cables (5). This repeater greatly facilitates its installation and additional installation of slave devices.

Description: The present invention relates to a repeater for installation as measures for silent zones in portable, car and similar telephone systems and, more particularly, to a repeater using an optical cable for optical analog transmission and a lightwave receiver for use in the repeater. For example, a cellular telephone system comprises a switching center connected to a telephone network, a plurality of distributed radio base stations connected to the switching center via inter-station links and a number of portable cellular telephones. Upon dialling a call number of a portable telephone set from a telephone set, the radio base station of the service area concerned sends radio waves to call up the portable cellular telephone, permitting two-way communication. With a view to further upgrading services of such a cellular telephone system, repeaters have been introduced in silent zones where no radio waves can be received, such as underground shopping centers and tunnels, to repeat and amplify radio waves from radio base stations, enabling communication by portable cellular telephones (mobile stations) even in the silent zones. Fig. 4 of the accompanying drawings is a block diagram of a repeater 2 to which the present invention is intended to apply. In Fig. 4, reference numeral 1 denotes a radio base station, 31 a radio frequency stage, 32 an electro-optic transducer (E/O) for transducing an electric signal to a lightwave, 33 a branching circuit, 5 optical cables, 4 a plurality of slave devices, 34 opto-electric transducers (O/E) each for transducing a lightwave to an electric signal, 40 a signal combiner, 3 a master device and 6-1 to 6-n radio zones. As shown in Fig. 4, the repeater 2 is composed of the master device 3, the plurality of slave devices 4-1 to 4-n and optical cables 5 interconnecting them. The radio frequency stage 31 receives radio waves from the radio base station 1 and outputs an amplified RF signal. The RF signal from the radio frequency stage 31 is transduced, by the E/O transducer 32 using a light-emitting diode such as a semiconductor laser diode, to an intensity-modulated lightwave, which is branched by the branching circuit 33 to the plurality of slave devices 4. The plurality of slave devices 4 are dispersed over the range of 1 m to 20 km, for instance, so that the respective zones 6-1 through 6-n cover the entire area of a silent zone such as an underground shopping center or tunnel. The slave devices 4 and the master device 3 are interconnected using low-loss (0.3 dB/km to 0.5 dB/km) optical cables 5 for optical analog transmission. The slave devices 4 each has an O/E transducer for directly detecting the intensity modulated lightwave and a radio frequency stage for amplifying and transmitting the detected RF signal from an antenna. Transmitted waves from mobile stations (portable cellular telephone sets) in the radio zones 6 of the respective slave devices 4 are received and amplified in the radio frequency stages via the antennas of the slave devices 4. As in the master device 3, the received waves are transduced by E/O transducers to intensity modulated lightwaves, which are sent via the cables 5 to the master device 3 together with monitor data; in the master device 3, the lightwaves are transduced by the opto-electric (O/E) transducers 34 to electric signals, then combined by the combiner 40 and input into the radio frequency stage 31. Fig. 5 of the accompanying drawings is a block diagram showing a conventional two-way communication circuit of the master device 3 and one of the slave devices 4 in Fig. 5. The construction of the master device 3 is the same as depicted in Fig. 4, but the illustrated construction of the slave device 4 is a specific operative example. In the slave device 4, reference numeral 41 denotes a radio frequency stage, 42 an O/E transducer, 43 an E/O transducer and 46 a variable gain amplifier. An antenna connected to a branching circuit is a duplex antenna for transmission to and reception from mobile stations which move in the zone. The variable gain amplifier 46 is to provide a prescribed RF output for transmission by adjusting a circuit gain difference resulting from the difference in length between the optical cables 5 at the time of installation of the slave device 4. In a long and narrow area such as a tunnel, the slave devices 4 of the conventional system are allocated and arranged for a required distance from the master device 3 in such a manner that the zones 6 link one after another just like a chain, whereas in an area such as an underground shopping center, they are arranged in a big matrix form. In the two cases, the optical cables interconnecting the master device 3 and the slave devices 4 differ from one another in length with the slave devices 4; the cable to the nearest slave device is one to several meters but the cable to the remotest slave device may sometimes become as long as 20 km. As referred to previously, the optical cable is low-loss but has a loss of 0.3 to 0.5 dB/km. Hence, in the case of installing such a repeater 2, the loss of light in the optical cable varies or scatters from O to 10 dB according to the position of the respective slave device 4, and in terms of an electric signal, the loss varies in the range of O to 20 dB, twice as large as that of the light loss. Thus, the level of the output RF signal from the antenna of each slave device 4 varies with the length of the optical cable between it and the master device 3. In the CATV or similar system of the type generating a reconstructed image in an image receiver connected directly to the antenna terminal, a little variation in the antenna output level does not matter. In the portable cellular telephone system which sends out radio waves from the antenna of each slave device 4, however, it presents problems as the distance of travel of radio waves and the area of the radio zone inevitably vary with slave devices 4. To avoid this, it is customary in the prior art to manually adjust the gain of the variable gain amplifier 46 at the time of installation of each slave device to compensate for differences among losses by the optical cables of different lengths so that the radio zones of the respective slave devices 4 have about the same area and that radio waves from the antennas are set out at substantially the same level. Hence, the installation of the conventional repeater requires much labour and much time. Next, a description will be given of the variation in the output RF signal level which is attributable to different lengths of the optical cables. Fig. 6 of the accompanying drawings is a graph showing an example of the light intensity modulation characteristic of the E/O transducer 32 formed by a light emitting diode, that is, the relationship of the modulated optical output to the input RF signal, or the relationship between the current of the light emitting diode and the optical level thereof. It is seen from Fig. 6 that, when driven with a high-frequency signal (the input RF signal) superimposed on its bias current, the light emitting diode generates an analog intensity modulated optical output. Letting the modulation factor for the input RF signal be represented by m , the optical output is expressed by A (l+m sin pt) cos t, and the demodulated version of this output is 2x10 log A + 20 log m[dB] in dB terms. The first term of this equation, that is, 10 log A[dB], represents the optical level. Fig. 7 of the accompanying drawings is a graph explanatory of the direct detecting operation (i.e. the modulating operation) of a photodiode used as the O/E transducer 42, 34-1 or 34-2 in Fig. 5. Upon application of an optical input of a level A to the photodiode from the optical cable 5, a current B flows in the photodiode. Hence, when the optical input A is intensity-modulated by an RF signal a , an RF signal b is superimposed on the photodiode current B in the optical output from the photodiode 46. That is, when the optical input level is A and the modulation factor for the RF signal a is m , the detected output RF current is b . When the optical cable 5 is long and hence is high-loss and the optical input level is A', however, since the modulation factor m is fixed, the photodiode current decreases from B to B' and the output RF current also decreases from b to b ' as shown in Fig. 7. It is necessary, therefore, to compensate for, by the variable gain amplifier 46, the decrease in the output RF signal from b to b ', that is, an increase in the loss of the electric signal corresponding to a value twice the loss of the optical cable which causes the increase in the optical input level from A to A'. While in the above a defect of a down-link from the master device 4 to the slave device 4 has been described, an up-link has the same defect. As described above, when the conventional repeater using the optical analog transmission scheme is installed or the slave devices 4 are newly provided, the variable gain amplifier 46 must be adjusted manually. This involves the transportation of a measuring instrument to the field for measurement and adjustment of the variable gain amplifier 46, and hence requires much labour and much time. An object of the present invention is to provide a repeater for a radio paging system which automatically compensates for variations in a loss by the optical fiber according to the position of installacion of each slave device, permitting appreciable reduction of labour and time for the installation of the repeater itself. Another object of the present invention is to provide a repeater for a two-way communication system such as a portable telephone system which automatically compensates for variations in the received optical input level by different lengths of optical cables between the master device and the slave devices, thereby permitting appreciable reduction of labour and time for the installation of the repeater itself and ensuring the formation of radio zones of about the same area. European Patent Application having publication number 0,538,635 discloses a microcellular mobile radio communication system having a central base station and a plurality of radio base stations interconnected by a shared optical fibre transmission line. The radio signals in different frequency bands assigned to each radio base station can be simultaneously transmitted through the shared optical fibre transmission line between the radio and central base station. According to a first aspect of the present invention there is provided a repeater for repeating RF electric waves between a radio base station and portable stations in a radio paging system, comprising: a master device for receiving one of said RF electric waves from said radio base station, superimposing said received one of said RF electric waves on a light wave of a fixed level, branching said light wave by a branching circuit to a plurality of branched outputs for sending out to a plurality of optical outputs from a branching circuit; a plurality of optical cables connected at one end to said branched outputs of said branching circuit of said master device to transmit a plurality of said optical outputs; and a plurality of slave devices connected to other ends of a plurality of said optical cables, each slave device comprising a lightwave receiver for receiving one of said optical outputs from said master device via the optical cable connected to said slave device to provide said RF electric waves transmitted by an antenna; characterised in that said lightwave receiver of each slave device comprising: a first variable gain amplifier for amplifying an RF electric signal extracted by an opto-electric transducer in said lightwave receiver and outputting said extracted RF electric signal at a prescribed level, a current detector for detecting the direct-current value of said opto-electric transducer; and a control circuit for applying to said variable gain amplifier a gain control signal based on the detected value from said current detector so that RF electric signal has said prescribed level; said RF electric waves transmitted from said antenna of said slave device having said prescribed level regardless of the lengths of said optical cables interconnecting respective ones of said slave devices and said master device. Preferably, the master device further comprises: a plurality of first input terminals provided one to one correspondence to a plurality of said first branched outputs and each connected to one end of each up-link optical cable to receive up-link optical signals respectively transmitted from corresponding ones of said slave devices, a plurality of first opto-electric transducers connected respectively to said first input terminals for transducing respectively said up-link optical signals into up-link electric signals, a plurality of first variable-gain amplifiers for amplifying respectively said up-link electric signals, a plurality of first current detectors for detecting respectively direct-current values of said first opto-electric transducers, a first control circuit for comparing, respectively, first detected values from a plurality of said first current detectors with first preset reference values and for applying each of first control signals to corresponding one of said first variable gain amplifier to change an amplification factor thereof to reduce a corresponding first difference between a corresponding one of the first detected values and a corresponding one of the first preset reference values to zero, a signal combiner for combining said up-link electric signals amplified into a composite RF electric signal, and means for providing one of said RF electric waves from said composite RF electric signal to transmit said one of said RF electric waves to said radio base station; the levels of said RF electric waves transmitted from said master device to said radio base station being constant regardless of the lengths of a plurality of pairs of said optical cables respectively interconnecting said master device and said slave devices. Preferably, the lightwave receiver of each slave device further comprises a second variable gain amplifier for amplifying an up-link RF electric signal received by said antenna of the same slave device, an electro-optic transducer for transducing said up-link RF electric signal into an up-link light intensity-modulated signal, a second output terminal connected to the other end of said up-link optical cable to send out said up-link light intensity-modulated signal to said master device, a modem for monitored data responsive to said second control signal from second control circuit for deriving, from said second control signal, said monitored data composed of an identification number of said slave device concerned and said compensation value, and means for superimposing said monitored data on said RF electric signal for transmission to said light -intensity modulated signal transmitted on said corresponding one of said up-link optical cables; the levels of said branched outputs of said light intensity-modulated signals for transmission from said master device to said slave devices being constant regardless of the lengths of a plurality of pairs of said optical cables respectively interconnecting said master device and said slave devices. Preferably, the master device further comprises a plurality of first input terminals providing one to one correspondence to a plurality of said first output terminals and each connected to one end of said up-link optical cable to receive up-link optical signals respectively transmitted from corresponding ones of said slave devices, a plurality of first opto-electric transducers connected respectively to said first input terminals for transducing respectively said up-link optical signals into up-link electric signals, a plurality of first variable-gain amplifiers for amplifying respectively said up-link electric signals, a signal combiner for combining said up-link electric signals amplified into a composite RF electric signal, a modem for monitored data, for extracting control data added to said monitored data included in said composite RF electric signal, and a first control circuit responsive to said extracted control data to apply one of first control signals to a corresponding one of said first variable gain amplifiers to change an amplification factor thereof; the levels of said RF electric waves transmitted from said master device to said radio base station being constant regardless of the lengths of a plurality of pairs of said optical cables respectively interconnecting said master device and said slave devices. Moreover, the lengths of a plurality of pairs of the optical cables are preferably defined in the range of 1 m to 20 km. Embodiments of the present invention will now be described in detail below with reference to accompanying drawings, in which: Fig. 1 is a block diagram illustrating a first embodiment of the present invention; Fig. 2 is a block diagram illustrating the slave device side of a second embodiment of the present invention; Fig. 3 is a block diagram illustrating the master device side of the second embodiment of the present invention; Fig. 4 is an example of a system to which the present invention is applied; Fig. 5 is a block diagram showing a prior art example; Fig. 6 is a graph showing an example of an electro-optic transducing (direct modulation) characteristic; and Fig. 7 is a graph showing an example of an opto-electric transducing (direct detection) characteristic, explanatory of the operation of the present invention. Fig. 1 illustrates in block form a first embodiment of the present invention, in which the parts corresponding to those in Fig. 4 are denoted by the same reference numerals. This embodiment differs from the prior art example in that the optical input detecting stage of either of the master device 3 and the slave device 4 includes means for detecting the level of the optical input from the optical cable 5 and means for effecting control on the basis of the detected level to hold the detected RF signal at about the same level. In the master device 3 there are provided current detectors (I-DET) 35-1 to 35-n for detecting current values of O/E transducers 34-1 to 34-n which detect lightwaves sent via up-link optical cables 5 from the respective slave devices 4. Disposed between the O/E transducers 34-1 to 34-n and the signal combiner 40 are variable gain amplifiers 36-1 to 36-n. The current values detected by the current detectors 35-1 to 35-n are input into a control circuit 37. The control circuit 37 converts the respective detected current value by an A/D converter to a digital value, then estimates the loss of the optical cable concerned from a DC current value, which is a reference value (a value when the length of the optical cable is 0 m), and applies a gain control signal to the concerned one of the variable gain amplifiers 36-1 to 36-n to change its amplification factor so that its output level takes a predetermined value. The DC current value and the gain correcting value are prestored in the control circuit 37 after being calibrated. In each of the slave devices 4, the current value of an O/E transducer 42 formed by a photodiode or the like, which detects a lightwave sent via the down-link optical cable 5 from the master device 3, is detected by a current detector (I-DET) 44, and a control circuit 45 uses the detected value to estimate the loss of the optical cable from the same DC current value as mentioned above and controls the gain of the variable gain amplifier 46 as in the master device 3. The variable gain amplifiers 36-1 to 36-n and 46 may each be a variable attenuator or a combination of an amplifier and a variable attenuator. When a photodiode is used as each of the O/E transducer 34 and 42, the current flowing in the photodiode linearly varies with a variation in the optical input level as depicted in Fig. 7. That is, the DC current value of the O/E transducer 42 or 34 corresponding to the level of the input light which is received via the optical cable 5 from the master device 3 or slave device 4 of the repeater 2 is detected by the current detector 44 or 35. The control circuit 37 (45 in the slave device 4) converts the detected current value, then estimates the loss of the optical cable from the above-mentioned reference DC current value and a gain control signal to the variable gain amplifier 36 (46 in the slave device 4) to change its amplification factor so that its output RF signal voltage takes a predetermined value. A DC current value vs. optical cable loss (the gain of the variable gain amplifier, in practice) is prestored in a memory of the control circuit, and hence the loss of the optical cable can easily be estimated. Figs. 2 and 3 are block diagrams illustrating a second embodiment of the present invention, Fig. 2 showing the slave device side and Fig. 3 the master device side. The parts corresponding to those in Fig. 1 are identified by the same reference numerals. In this embodiment, since the up- and down-link optical cables 5 interconnecting the master device 3 and the slave device 4 have about the same length, control data for compensating for the loss of the down-link optical cable detected in the slave device 4 is added to monitored data which is sent from the slave device 4 to the master device 3, the control data is used to control the gain of the variable gain amplifier in the master device 3. In the slave device 4 of Fig. 2, reference numeral 47 denotes a modem for monitored data, which modulates gain control data from the control circuit 45 together with other monitored data and inputs the modulated data into an amplifier 48, from which it is provided to the E/O transducer 43 together with the send signal (i.e. the up-link signal) to the master device 3. In the master device 3 of Fig. 3, reference numeral 38 denotes a modem for monitored data, which demodulates monitored data and control data from the output of the signal combiner 40 and inputs into the control circuit 39 the control data composed of the number of the slave device and cable loss data. The control circuit 39 controls the gain of the variable gain amplifier 36 corresponding to the slave device 4 concerned. In this instance, the control data from the respective slave devices are sequentially processed by instructions (polling) from the master device 3, hence the control signals will not collide. Incidentally, the reason for which the gain of the amplifier 48 for the up-link signal in each slave device 4 is that the operation of a laser diode, which is used as the E/O transducer 43, is set in the optimal condition. That is, an increase in the input level of the E/O transducer 43 improves the CN ratio but degrades the inter modulation, whereas a decrease in the input level improves the latter but degrades the former; therefore, the input level of the E/O transducer 43 is always set at the optimum value. As described above, the embodiment of Fig. 1 differs from the prior art example in the provision of the current detector 35 for detecting the current value of the O/E transducer 34 and the control circuit 37 for controlling the gain of the variable gain amplifier 36 in accordance with the detected current value. When a photodiode is used as the O/E transducer 34, the current flowing therein linearly varies with the optical input level. That is, the current detector 35 is used to detect the DC current value of the O/E transducer 34 which corresponds to the level of the optical input which is received via the optical cable 5 from the master device 3 of the repeater 2. The control circuit 37, for example, converts the detected current value by an A/D converter to a digital value, then estimates the loss of the optical cable from a DC current value which is a reference value (when the length of the optical cable is 0 m) and applies a gain control signal to the variable gain amplifier 36 to change its amplification factor so that the output RF signal voltage at its output terminal takes a predetermined value. As described above in detail, according to the present invention, the levels of RF signals which are sent out from the output signal from the master device to the radio base antennas of slave devices connected to the master device via optical cables of different lengths and the level of station can be automatically made substantially the same. Accordingly, the variable range of the gain of the variable gain amplifier can set in accordance with the distance from the master device to the remotest slave device, taking into account the area and planar landform of the silent zone where the repeater is installed. Thus, the present invention permits appreciable reduction of labour and time for the installation of the repeater or additional installation of the slave device, and hence is of great utility when employed in practice.

Claims: A repeater for repeating RF electric waves between a radio base station and portable stations in a radio paging system, comprising: a master device (3) for receiving (31) one of said RF electric waves from said radio base station, superimposing (32) said received one of said RF electric waves on a light wave of a fixed level, branching said light wave by a branching circuit (33) to a plurality of branched outputs for sending out to a plurality of optical outputs from a branching circuit; a plurality of optical cables (5) connected at one end to said branched outputs of said branching circuit (33) of said master device (3) to transmit a plurality of said optical outputs; and a plurality of slave devices (4) connected to other ends of a plurality of said optical cables (5), each slave device comprising a lightwave receiver for receiving one of said optical outputs from said master device via the optical cable connected to said slave device to provide said RF electric waves transmitted by an antenna; characterised in that said lightwave receiver of each slave device comprising: a first variable gain amplifier (46) for amplifying an RF electric signal extracted by an opto-electric transducer (42) in said lightwave receiver and outputting said extracted RF electric signal at a prescribed level, a current decector (44) for detecting the direct-current value of said opto-electric transducer (42); and a control circuit (45) for applying to said variable gain amplifier (46) a gain control signal based on the detected value from said current detector (44) so that RF electric signal has said prescribed level; said RF electric waves transmitted from said antenna of said slave device (4) having said prescribed level regardless of the lengths of said optical cables (5) interconnecting respective ones of said slave devices (4) and said master device (3). A repeater for repeating RF electric waves between a radio base station and portable stations in a radio paging system according to claim 1 in which the master device (3) further comprises: a plurality of first input terminals provided one to one correspondence to a plurality of said first branched outputs and each connected to one end of each up-link optical cable (5) to receive up-link optical signals respectively transmitted from corresponding ones of said slave devices (4), a plurality of first opto-electric transducers (34-1∼34-n)connected respectively to said first input terminals for transducing respectively said up-link optical signals into up-link electric signals, a plurality of first variable-gain amplifiers (36-1∼36-n) for amplifying respectively said up-link electric signals, a plurality of first current detectors (35-1∼35-n)for detecting respectively direct-current values of said first opto-electric transducers (34-1∼34-n), a first control circuit (37) for comparing, respectively, first detected values from a plurality of said first current detectors (35-1∼35-n)with first preset reference values and for applying each of first control signals to corresponding one of said first variable gain amplifier (36-1∼36-n)to change an amplification factor thereof to reduce a corresponding first difference between a corresponding one of the first detected values and a corresponding one of the first preset reference values to zero, a signal combiner (40) for combining said up-link electric signals amplified into a composite RF electric signal, and means for providing one of said RF electric waves from said composite RF electric signal to transmit said one of said RF electric waves to said radio base station; the levels of said RF electric waves transmitted from said master device to said radio base station being constant regardless of the lengths of a plurality of pairs of said optical cables respectively interconnecting said master device and said slave devices. A repeater for repeating RF electric waves between a radio base station and portable stations in a radio paging system according to claim 1, in which the lightwave receiver of each slave device further comprises a second variable gain amplifier (48) for amplifying an up-link RF electric signal received by said antenna of the same slave device, an electro-optic transducer (43)for transducing said up-link RF electric signal into an up-link light intensity-modulated signal, a second output terminal connected to the other end of said up-link optical cable to send out said up-link light intensity-modulated signal to said master device, a modem for monitored data (47) responsive to said second control signal from second control circuit (45) for deriving, from said second control signal, said monitored data composed of an identification number of said slave device concerned and said compensation value, and means for superimposing said monitored data on said RF electric signal for transmission to said light-intensity modulated signal transmitted on said corresponding one of said up-link optical cables (5) ; the levels of said branched outputs of said light intensity-modulated signals for transmission from said master device to said slave devices being constant regardless of the lengths of a plurality of pairs of said optical cables respectively interconnecting said master device and said slave devices. A repeater for repeating RF electric waves between a radio base station and portable stations in a radio paging system according to claim 1, in which the master device (3) further comprises: a plurality of first input terminals providing one to one correspondence to a plurality of said first output terminals and each connected to one end of said up-link optical cable (5) to receive up-link optical signals respectively transmitted from corresponding ones of said slave devices (4), a plurality of first opto-electric transducers (34-1∼34-n)connected respectively to said first input terminals for transducing respectively said up-link optical signals into up-link electric signals, a plurality of first variable-gain amplifiers (36-1∼36-n)for amplifying respectively said up-link electric signals, a signal combiner (40) for combining said up-link electric signals amplified into a composite RF electric signal, a modem for monitored data (38), for extracting control data added to said monitored data included in said composite RF electric signal, and a first control circuit (39) responsive to said extracted control data to apply one of first control signals to a corresponding one of said first variable gain amplifiers (36-1∼36-n)to change an amplification factor thereof; the levels of said RF electric waves transmitted from said master device to said radio base station being constant regardless of the lengths of a plurality of pairs of said optical cables respectively interconnecting said master device and said slave devices. A repeater according to any preceding claim, in which the lengths of said plurality of optical cables are in the range of 1 m to 20 km.

Inventors: AKIYAMA TADASHI, NORICHIKA MICHIO, OKUBO YOICHI, SUGANUMA JUN, SUZUKI HIROSHI, YOKOTE TAKASHI, AKIYAMA, TADASHI, NORICHIKA, MICHIO, OKUBO, YOICHI, SUGANUMA, JUN, SUZUKI, HIROSHI, YOKOTE, TAKASHI, AKIYAMA, TADASHI,
Applicants: HITACHI INT ELECTRIC INC, NIPPON TELEGRAPH & TELEPHONE, HITACHI KOKUSAI ELECTRIC INC., NTT MOBILE COMMUNICATIONS NETWORK INC.
Application year: 1995
Main classification: H04Q7
IPC codes: H04B10, H04W88