Repeater using optical cables for radio paging system

A repeater which is installed as measures 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 and a plurality of slave devices which are allocated and installed in an underground shopping center or the like. The master device and the slaves devices both have current detectors for detecting current values of O/E transducers which transduce received lights to electric signals. Based on the detected current values, gain control signals are applied to variable gain amplifiers 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. The present invention greatly facilitates the installation of the repeater and additional installation of slave devices.

BACKGROUND OF THE INVENTION 
The present invention relates to a repeater which is installed to 
compensate 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 dialing 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 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 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 0 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 0 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 labor 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 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 (1+m sin pt) cos .omega.t, and the 
demodulated version of this output is 2.times.10 log A +20 log mdB! in dB 
terms. The first term of this equation, that is, 10 log AdB!, represents 
the optical level. 
FIG. 7 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 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 labor and much time. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a lightwave receiver from 
which an RF output signal of a prescribed level can be obtained without 
involving the necessity for manual adjustment of the amplification factor 
when the level of the received optical input varies. 
Another 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 installation of 
each slave device, permitting appreciable reduction of labor and time for 
the installation of the repeater itself. 
Still 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 labor and 
time for the installation of the repeater itself and ensuring the 
formation of radio zones of about the same area. 
To attain the above objects, the devices of the present invention have the 
constructions listed below. 
A repeater of the present invention with comprises: 
a master device which transduces an RF signal received via a down link from 
a radio base station into a light intensity-modulated signal, branches and 
outputs it from a plurality of output terminals and which transduces a 
plurality of light intensity-modulated signals of up links, received via a 
plurality of input terminals corresponding to a plurality of said output 
terminals, into RF signals by first opto-electric transducers, 
respectively, combines the RF signals into a composite signal to be sent 
out to the radio base station; 
a plurality of pairs of optical cables, each pair comprising one of 
down-link optical cables connected to a plurality of said output terminals 
of the master device, respectively, and one of up-link optical cables 
connected to the input terminals of the master device, respectively; and 
slave devices each of which is connected to one of a plurality of pairs of 
said optical cables, transduces a light intensity-modulated signal, 
received via the down link optical cable, into an RF signal by a second 
opto-electric transducer and sends it out to a portable station and each 
of which transduces an RF signal received from a portable station into a 
light intensity-modulated signal and provides it onto the up-link optical 
cable. 
The master device includes: a plurality of first current detectors for 
detecting current values of the first opto-electric transducers, 
respectively; a plurality of variable gain amplifiers for amplifying 
output signals from the first opto-electric transducers, respectively; and 
a first control circuit which compares the detected value from each of a 
plurality of said first current detectors with a preset reference value 
and applies a control signal to the corresponding one of the variable gain 
amplifiers to change its gain to reduce the difference to zero. 
The slave devices each include: a second current detector for detecting the 
current value of the second opto-electric transducer; a variable gain 
amplifier for amplifying the output signal from the second opto-electric 
transducer; and a second control circuit which compares the detected value 
from the second current detector with a preset reference value and applies 
a control signal to the variable gain amplifier to change its gain to 
reduce the difference to zero. 
With the repeater of the above construction, the levels of the RF signal 
outputs from the master device and the slave devices are held constant 
regardless of the lengths of the pairs of optical cables interconnecting 
the slave devices and the master device. 
A repeater of the present invention comprises: 
a master device which transduces an RF signal received via a down link from 
a radio base station into a light intensity-modulated signal, branches and 
outputs it from a plurality of output terminals and which transduces a 
plurality of light intensity modulated signals of up links, received via a 
plurality of input terminals corresponding to a plurality of said output 
terminals, into RF signals by first opto-electric transducers, 
respectively, combines the RF signal into a composite RF signal for 
sending the same to the radio base station; 
a plurality of pairs of optical cables, each pair comprising one of 
down-link optical cables connected to a plurality of said output terminals 
of the master device, respectively, and one of up-link optical cables 
connected to the input terminals of the master device, respectively; and 
slave devices each of which is connected to one of a plurality of pairs of 
said optical cables, transduces a light intensity-modulated signal, 
received via the down-link optical cable, into an RF signal by a second 
opto-electric transducer and sends it out to a portable station and each 
of which transduces an RF signal received from a portable station into a 
light intensity-modulated signal and provides it onto the up-link optical 
cable. 
The master device includes: a plurality of variable gain amplifiers for 
amplifying output signals from the first opto-electric transducers; a 
first modem for monitored data, for extracting control data added to 
monitored data included in the composite RF signal; and a first control 
circuit which follows the extracted control data to apply a control signal 
to the corresponding variable gain amplifier to change its gain. 
The slave devices each include: a second current detector for detecting the 
current value of the second opto-electric transducer; a variable gain 
amplifier for amplifying the output signal from the second opto-electric 
transducer; a second control circuit which compares the detected value 
from the second current detector with a preset reference value and applies 
a control signal to the variable gain amplifier to change its gain to 
reduce the difference to zero; and a second modem for monitored data which 
derives the control signal from the second control circuit, adds the 
monitored data with the control data composed of the number of the slave 
device concerned and a corrected value and superimposes the control data 
and the monitored data on the RF signal to be sent to the portable 
station. 
With the repeater of the above construction, the level of the RF signal 
outputs from the master device and the slave devices are held constant 
regardless of the lengths of the pair of optical cables interconnecting 
the slave device and the master device. 
A repeater of the present invention comprises: 
a master device which receives an RF signal from a radio base station in a 
radio paging system, superimposes the received RF signal on light of a 
fixed level and sends out a plurality of optical outputs from a branching 
circuit; 
a plurality of optical cables connected at one end to the branching circuit 
of the master device to transmit a plurality of said optical outputs, 
respectively, and 
a plurality of slave devices connected to a plurality of said optical 
cables, respectively, each slave device including a lightwave receiver 
which receives one of the optical outputs from the master device via the 
optical cable connected to the slave device and amplifies, by a variable 
gain amplifier, the RF signal extracted by an opto-electric transducer and 
outputs it at a prescribed level, and an antenna for sending out the RF 
signal. 
The lightwave receiver of each slave device includes: a current detector 
for detecting a current value of the opto-electric transducer; and a 
control circuit for applying to the variable gain amplifier a gain control 
signal based on the detected current value from the current detector so 
that the RF output signal has the prescribed level. 
With the repeater of the above construction, the RF signal output from each 
slave device has the prescribed level regardless of the length of the 
optical cable interconnecting the slave device and the master device. 
A lightwave receiver of the present invention, comprises: an opto-electric 
transducer which receives an optical output of a fixed level with an RF 
signal superimposed thereon via an optical cable of an unspecified length 
and transduces the received optical output into an RF signal, and a 
variable gain amplifier for amplifying the output from the opto-electric 
transducer to provide an RF output signal of a prescribed level. 
The lightwave receiver further comprises: 
a current detector for detecting the current value of the opto-electric 
transducer which linearly varies with a level fluctuation of the optical 
input into the opto-electric transducer; and 
a control circuit for applying to the variable gain amplifier a gain 
control signal based on the detected current value from the current 
detector so that the RF output signal has a prescribed level. 
With the above construction, a loss proportional to the length of the 
optical cable is compensated for so that the RF output signal has always 
the prescribed level regardless of the length of the optical cable. 
Moreover, the lengths of a plurality of pairs of the optical cables can be 
defined in the range of 1 m to 20 km.

PREFERRED EMBODIMENTS OF THE 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 photodide 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 diagram 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 salve 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 0/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 
base antennas of slave devices connected to the master device via optical 
cables of different lengths and the levels of output signals transmitted 
from the master device to the master device to the radio base station can 
be automatically made substantially the same. 
Accordingly, the variable range of each 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 labor 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.