JP2001053688A - Communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a communication system which is superior in noise characteristics and distortion characteristics in a ROF(radio-on-fiber) system and relaxes the demand of a wide band to an optical transmitter. SOLUTION: In the communication system wherein a control station 1 and an antenna station 3 are connected by an optical fiber 3, the control station 1 after converting an IF signal and a local signal for frequency conversion into light signals by an optical transmitter 11 for IF signal and an optical transmitter 13 for local signal couples them by an optical coupler 14 and sends the resulting signal out to an optical fiber 3, and the antenna station 2 converts the light signal transmitted by the optical fiber 3 into an electric signal by a photoelectric converter 20. Filters 21-1 and 21-2 extracts, the IF signal and local signal for frequency conversion from the output of the photoelectric converter 20, and convert the IF signal by a frequency converter 22 into a desirable radio frequency signal, which is passed through a power amplifier 25 and then radiated from an antenna 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a communication system for transmitting a radio signal using an optical transmission line such as an optical fiber.

[0002]

2. Description of the Related Art A so-called Radio on Fi communication system in which laser light is modulated by a radio signal and transmitted through an optical fiber.
The ber (ROF) system is being put to practical use in mobile communication systems and the like. The ROF system mainly includes a control station and an antenna station having an antenna.

In the direction from the control station to the antenna station (downward direction), a radio signal generated in the control station is converted into an optical signal, and this optical signal is transmitted to the antenna station via an optical fiber. The antenna station converts the received optical signal into an electric signal to generate an original radio signal. On the other hand, in the direction from the antenna station to the control station (upward direction), the radio signal from the mobile station or the like received by the antenna at the antenna station is directly converted into an optical signal and transmitted to the control station via an optical fiber.

In such a ROF system, since the radio signal is not modulated and demodulated by the antenna station, the antenna station does not depend on the serviceability, and it is not necessary to replace the antenna station even if the service or the specification is changed. There is an advantage.

[0005] By the way, the wireless communication system is required to increase the communicable capacity. In response to this demand, research on wireless communication technology in the quasi-millimeter wave band and the millimeter wave band, which enables large-capacity transmission easily.・ Development is underway. Wireless communication in such a high frequency band is also an object of the ROF system.

[0006] As the radio frequency increases in the ROF system, it becomes difficult to employ direct modulation for modulating laser light with a radio frequency signal due to the problem of the modulation speed of the semiconductor laser. Therefore, the control station converts the intermediate frequency (IF) signal into an optical signal and transmits it through an optical fiber, and the antenna station up-converts the IF signal obtained by photoelectrically converting the received optical signal into a desired radio frequency signal. There is a way to do that.

At this time, a local signal for frequency conversion used for up-conversion or a subharmonic signal (subharmonics signal) which is a frequency which is an integer fraction of the fundamental frequency of the local signal for frequency conversion is transmitted along with an IF signal via an optical fiber. There is a way. As a result, the antenna station does not need an oscillator or a synthesizer for generating a local signal for frequency conversion.

In the case of transmitting the local signal for frequency conversion or its subharmonic signal as described above, conventionally, the local signal for frequency conversion or its subharmonic signal is superimposed on the IF signal, and the signal is provided at the subsequent stage of the semiconductor laser or the semiconductor laser. A method has been adopted in which the light is applied to an optical modulator for modulation. When these methods are used, it is necessary to increase the total degree of optical modulation in order to transmit the local signal for frequency conversion or its subharmonic signal without lowering the power of the IF signal. However, in both the direct modulation method and the external modulation method, there is a problem that the distortion characteristic deteriorates when the light modulation degree increases.

Further, the frequency of the local signal for frequency conversion and its subharmonic signal is often much higher than that of the radio frequency signal.
In the case of direct modulation, if a modulation signal exists at a high frequency of about several GHz, noise increases due to the effect of laser relaxation oscillation.

Further, the local signal for frequency conversion is
When the signal is directly converted into an optical signal by one optical transmitter together with the signal and transmitted, the frequency of the local signal for frequency conversion is significantly different from the IF signal, so that the semiconductor laser and the optical modulator constituting the optical transmitter are used. And an optical transmission system such as a driver for driving these components is required to have a wide band.

[0011]

As described above, in a conventional ROF system, when an IF signal and a local signal for frequency conversion or its subharmonic signal are transmitted from a control station to an antenna station, the frequency of the IF signal is Problems such as deterioration of noise characteristics and distortion characteristics due to superposition of the local signal for conversion and transmission by a common optical transmitter, and increase in system cost due to the requirement of a broadband optical transmission system There was a point.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and an object of the present invention is to provide a communication system which is excellent in a noise characteristic and a distortion characteristic in a ROF system and which can relax a requirement of a wide band for an optical transmitter. With the goal.

[0013]

In order to solve the above-mentioned problems, the present invention provides a communication system comprising a control station and an antenna station connected to the control station by a first optical transmission line. The basic feature is that the signal and the local signal for frequency conversion or its subharmonic signal are converted into an optical signal and then combined and transmitted to an antenna station via an optical transmission line.

According to the first communication system of the present invention, the control station converts the intermediate frequency signal into the first optical signal and transmits the first optical signal, and transmits the local signal for frequency conversion to the first optical transmitter. A second optical transmitter for converting the first optical signal into a second optical signal and transmitting the first optical signal and a second optical signal;
And a first optical coupler for transmitting to the optical transmission line.

On the other hand, the antenna station photoelectrically converts the optical signal transmitted through the first optical transmission line to obtain an intermediate frequency signal and a local signal for frequency conversion, and at least one photoelectric converter from the photoelectric converter. A first converting and outputting the output intermediate frequency signal into a desired radio frequency signal using the frequency conversion local signal output from the photoelectric converter;
And a first antenna for transmitting a radio frequency signal output from the first frequency converter.

In the first communication system, since the intermediate frequency signal and the local signal for frequency conversion modulate different lights, the distortion characteristics are not degraded due to an increase in the degree of modulation in the optical transmitter. Transmission becomes possible. Further, since each optical transmitter may be of a narrow band that operates only near the frequency of the intermediate frequency signal or the frequency conversion local signal, the cost can be reduced.

According to the second communication system of the present invention, the control station converts the intermediate frequency signal into the first optical signal and transmits the first optical signal, and the control station transmits the first optical signal and the fundamental frequency of the frequency conversion local signal. An optical transmitter for converting a sub-harmonic signal having a frequency of 1 / integer into a second optical signal for transmission, and combining the first optical signal and the second optical signal to form a first optical signal;
And an optical coupler for transmitting to the optical transmission line.

On the other hand, the antenna station photoelectrically converts the optical signal transmitted by the first optical transmission line to output an intermediate frequency signal and a sub-harmonic signal, and an output from the photoelectric converter. Multiplier that generates a local signal for frequency conversion by multiplying the sub-harmonic signal to be converted, and a desired radio frequency using the local signal for frequency conversion generated by the multiplier for the intermediate frequency signal output from the photoelectric converter. A first frequency converter that converts the signal into a signal and outputs the signal; and a first antenna that transmits a radio frequency signal output from the first frequency converter.

In the second communication system, the intermediate frequency signal and the sub-harmonics signal modulate different lights.
As in the first communication system, there is no deterioration in distortion characteristics due to an increase in the degree of modulation in the optical transmitter, and better transmission is possible.

Each of the optical transmitters may be of a narrow band operating only near the frequency of the intermediate frequency signal or the sub-harmonic signal. In addition, the sub-harmonic signal may be smaller than that of the local signal optical transmitter. Since the operating frequency of the second optical transmitter can be lowered, the cost can be further reduced.

The present invention can be similarly applied to a communication system including a control station and a plurality of antenna stations connected to the control station by a plurality of first optical transmission lines, like many ROF systems. .

According to the third communication system of the present invention in which a plurality of antenna stations are connected to one control station, the control station converts a plurality of intermediate frequency signals into a plurality of first optical signals. A plurality of first optical transmitters for converting the local signal for frequency conversion into a second optical signal for transmission, and a plurality of second optical signals for transmitting the second optical signal. A plurality of first optical signals and a plurality of second optical signals distributed by the optical splitter and respectively transmitted to a plurality of first optical transmission lines. And a first optical coupler.

On the other hand, each of the plurality of antenna stations performs at least one photoelectric converter for photoelectrically converting the optical signal transmitted by the first optical transmission line and outputting an intermediate frequency signal and a local signal for frequency conversion. A first frequency converter that converts an intermediate frequency signal output from the photoelectric converter into a desired radio frequency signal using a frequency conversion local signal output from the photoelectric converter and outputs the signal, A first antenna for transmitting a radio frequency signal output from the first frequency converter.

Further, according to the fourth communication system of the present invention in which a plurality of antenna stations are connected to one control station, the control station converts the plurality of intermediate frequency signals into a plurality of first optical signals. And a plurality of first optical transmitters for transmitting a sub-harmonic signal having a frequency which is an integer fraction of the fundamental frequency of the local signal for frequency conversion into a second optical signal. A second optical transmitter;
And a plurality of first optical transmission lines respectively combining the plurality of first optical signals and the plurality of second optical signals distributed by the optical distributor. And a plurality of first optical couplers for sending out the first optical coupler.

On the other hand, each of the plurality of antenna stations includes at least one photoelectric converter that photoelectrically converts the optical signal transmitted by the first optical transmission line and outputs an intermediate frequency signal and a sub-harmonic signal. A multiplier for multiplying the sub-harmonic signal output from the photoelectric converter to output a local signal for frequency conversion, and an intermediate frequency signal output from the photoelectric converter for the local signal for frequency conversion output from the multiplier. A first frequency converter that converts the signal into a desired radio frequency signal and outputs the signal, and a first antenna that transmits a radio frequency signal output from the first frequency converter.

When a plurality of antenna stations are connected to one control station via optical fibers as described above, different optical signals are transmitted to antenna stations using different intermediate frequency signals. If the configurations of the stations are the same, the frequency of the local signal for frequency conversion to be transmitted is the same.

Therefore, in the third or fourth communication system according to the present invention, the number of optical transmitters for transmitting the local signal for frequency conversion or its subharmonic signal is one, and the output light is distributed to a plurality. Then, the signal is transmitted to a plurality of different antenna stations depending on the optical transmission path. The local signal for frequency conversion is often very high in frequency, and the optical transmitter for modulating with the local signal for frequency conversion or the subharmonic signal is often expensive, but the third or fourth communication system In this case, since only one optical modulator for transmitting the local signal for frequency conversion or the sub-harmonic signal is required, the cost of the system can be reduced.

In the communication system according to the present invention, the second
The optical transmitter is preferably an external modulation type optical transmitter composed of a semiconductor laser and an optical modulator that modulates output light from the semiconductor laser with a local signal for frequency conversion or a subharmonic signal to obtain a second optical signal. Here, the second transmitter may further include means for directly modulating the semiconductor laser with a predetermined band signal. In this case, the optical modulator for external modulation has a function as a mixer.

That is, after directly modulating a semiconductor laser with a band signal, and then modulating the semiconductor laser with, for example, a local signal for frequency conversion, the output light includes a local signal for frequency conversion, a band signal, and a band signal. A signal that has been frequency-converted by the local signal for frequency conversion rides as an intensity modulation component. In this way, the antenna station can easily obtain a band signal that has been frequency-converted into a desired radio frequency signal without performing frequency conversion after photoelectrically converting the optical signal.

[0030] In the first or third communication system according to the present invention, the second optical transmitter includes a light source and a sub-harmonic signal having a frequency that is a fraction of an integer of a local signal for frequency conversion. A sub-harmonic signal is applied to the Mach-Zehnder optical modulator with a bias voltage and an amplitude at which harmonics are efficiently generated from the Mach-Zehnder optical modulator. The sub-harmonics signal may be converted to the frequency of the local signal for frequency conversion and the second optical signal may be transmitted.

When the radio frequency signal has a high frequency such as a millimeter wave or a quasi-millimeter wave, the local signal for frequency conversion and the subharmonic signal which can be easily multiplied and used are also high frequency. Modulators are expensive. On the other hand, the Mach-Zehnder type optical modulator has the characteristic that the light transmittance changes periodically with the applied voltage, and if this periodicity is used, the bias point and the amplitude of the applied voltage can be appropriately selected for comparison. It is possible to generate harmonics of the applied signal simply and easily, so the Mach-Zehnder type optical modulator multiplies the subharmonic signal oscillated by the oscillator at the frequency corresponding to the subharmonic of the local frequency for frequency conversion. By using millimeter waves,
A local signal for frequency conversion in a quasi-millimeter wave band can be easily obtained, and a multiplier or an optical modulator operating in a millimeter wave or quasi-millimeter wave band is not required.

In this case, even when the semiconductor laser is directly modulated by the band signal, the band signal appears at the frequency converted by the frequency conversion local signal obtained by multiplying the sub-harmonics, so that it operates without contradiction.

In the communication system according to the present invention, the antenna station may have means for transmitting the intermediate frequency signal output from the photoelectric converter by the first antenna or a second antenna different from the first antenna. Good.

For example, when a new broadband service is started in a millimeter wave or quasi-millimeter wave band, a signal of a narrow band service which has been performed in another lower frequency band is considered in consideration of the convenience of the service receiving side. May be frequency-converted into millimeter-wave and quasi-millimeter-wave bands and transmitted simultaneously with new services. This is so that the subscriber to the new service does not need to have a plurality of radios. On the other hand, some subscribers continue to subscribe only to the original narrowband service, so that it is necessary to provide the narrowband service at a low frequency for a certain period of time.

According to the present invention, assuming that the frequency of the intermediate frequency signal is the frequency of the narrow band service, the antenna station side receives the intermediate frequency signal received by the optical receiver together with the radio signal converted to the millimeter wave and the quasi-millimeter wave. Is radiated from the first antenna or the second antenna as it is, the same signal can be simultaneously provided in the low-frequency narrowband service and the millimeter-wave and quasi-millimeter-wave broadband service.

Further, at this time, by modulating the semiconductor laser with the band signal as described above, the optical signal from the first optical transmitter for the intermediate frequency signal is converted into a signal corresponding to the narrow band service and a signal for the local signal. It is also possible to use the band signal from the second optical transmitter as a signal provided only by the broadband service. By doing so, it is possible to cope with the transition period of the service without preparing a plurality of antenna stations corresponding to each service.

In the communication system according to the present invention, the antenna station uses a third antenna for receiving a radio frequency signal and converts the radio frequency signal received by the third antenna into a frequency using a local signal for frequency conversion. A second frequency converter that converts the intermediate frequency signal to generate an intermediate frequency signal, and converts the intermediate frequency signal generated by the second frequency converter into a third optical signal, between the control station and the antenna station. And a third optical transmitter for transmitting to the connected second optical transmission line.

The present invention basically relates to a transmission system in the down direction from the control station to the antenna station, and transmits an up signal from the antenna station to the control station received by the antenna station to the control station. A configuration is actually necessary. If a millimeter wave or a quasi-millimeter wave is converted into an optical signal as it is, an optical transmission / reception system becomes expensive. At this time, by using the local signal for frequency conversion for performing the down-conversion, the local signal for frequency conversion for the downlink signal optically transmitted from the control station or its subharmonic signal, the antenna station has a local oscillator There is no need at all.

In the communication system according to the present invention, the antenna station includes a fourth antenna for receiving the intermediate frequency signal, and an intermediate frequency signal received by the fourth antenna and generated by the second frequency converter. Means for combining the obtained intermediate frequency signal and supplying the combined signal to the third optical transmission line.

That is, in order to simultaneously transmit the uplink signal of the low-frequency narrowband service and the uplink signal of the high-frequency broadband service to the control station, the uplink signal of the narrowband service is converted into an optical signal without frequency conversion. Good. On the other hand, in the millimeter wave and quasi-millimeter wave bands, an upstream signal is down-converted and then converted into an optical signal. Since broadband communication is performed at such a high frequency, the uplink and downlink frequency intervals are expected to be wider than that of the narrowband service.

Therefore, when the local signal for frequency conversion used for frequency conversion of the downlink signal is used at the time of down-conversion, the intermediate frequency of the downlink signal differs from the intermediate frequency of the uplink signal by the interval between the uplink and downlink radio frequencies. Will be. Since the frequency of the uplink signal of the narrowband service is substantially the same as the frequency of the downlink signal, the intermediate frequency of the uplink signal of the narrowband service is slightly different from the intermediate frequency of the uplink signal of the downconverted broadband service, and does not overlap. Therefore, it is possible to combine these signals in the electric domain as described above and then convert them into an optical signal with one optical transmitter, which is very efficient.

[0042]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 is a diagram showing a configuration of a communication system according to a first embodiment of the present invention. This communication system is a ROF system including a control station 1, an antenna station 2, and an optical fiber 3 connected therebetween.

First, the control station 1 receives an IF signal input terminal 10,
An optical transmitter for IF signals (first optical transmitter) 11, a local signal generator 12, an optical transmitter for local signals (second optical transmitter) 13, and an optical coupler 14 are provided. The IF signal (intermediate frequency signal) input to the IF signal input terminal 10 is
The signal is converted into an optical signal by the signal optical transmitter 11.

The IF signal optical transmitter 11 is composed of a semiconductor laser 30, for example, as shown in FIG. That is, since the IF signal often has a relatively low frequency, for example, 5.8 GHz, the IF signal optical transmitter 1
For 1, a direct modulation method for modulating the semiconductor laser 30 with an IF signal can be applied. The direct modulation method of the semiconductor laser is lower in cost than an external modulation method in which the output light of the semiconductor laser is modulated by an optical modulator arranged at the subsequent stage of the semiconductor laser. However, when it is desired to avoid the problem of the wavelength chirp peculiar to the direct modulation method, or when the IF signal frequency is so high that the direct modulation is difficult, the external modulation method may be used.

On the other hand, the local signal generator 12 is a device for generating a local signal for frequency conversion composed of a sine wave signal, and the local signal for frequency conversion is converted into an optical signal by the local signal optical transmitter 13.

FIG. 3 shows an example of the configuration of the optical transmitter 13 for a local signal.
The optical modulator 32 is configured to modulate one output light with a local signal for frequency conversion from the local signal generator 12. This is called an external modulation method in contrast to the direct modulation method shown in FIG. 2, and the local signal for frequency conversion is often about several tens of GHz, which is much higher than that of the intermediate frequency signal. Therefore, an external modulation method in which the optical modulator 32 is provided separately from the semiconductor laser 31 is used.

That is, when the semiconductor laser is directly modulated by the local signal for frequency conversion having a high frequency, the relative intensity noise increases due to the relaxation oscillation of the semiconductor laser.
Although the noise characteristic of the entire output light deteriorates, in this embodiment, I
Since the F signal optical transmitter 11 and the local signal optical transmitter 13 are provided independently of each other, the local signal optical transmitter 1
3, an external modulation method can be used as shown in FIG. 3, and the problem of relaxation oscillation caused by directly modulating the semiconductor laser with the local signal for frequency conversion is solved.

The optical signals output from the IF signal optical transmitter 11 and the local signal optical transmitter 13 are combined by the optical coupler 14 and sent out to the optical fiber 3. The optical coupler 14 may have a function of simply mixing like an ordinary optical coupler, or may have a function of adding light of different wavelengths like an optical multiplexer. When an optical multiplexer is used for the optical coupler 14, it is necessary to match the output light wavelength of the IF signal optical transmitter 11 and the output light wavelength of the local signal optical transmitter 13 to the operating wavelength of the optical multiplexer.

From the optical coupler 14 of the control station 1 to the optical fiber 3
Is transmitted to the antenna station 2 via the optical fiber 3. The antenna station 2 includes a photoelectric converter 20, filters 21-1 and 21-2, a mixer 23 and a filter 24.
, A power amplifier 25 and the antenna 4.

In the antenna station 2, an optical signal transmitted from the control station 1 is received by the optical fiber 3, and is first converted into an electric signal by the photoelectric converter 20. The photoelectric converter 20 includes, for example, a photodiode and a preamplifier that amplifies the photocurrent output from the photodiode by current-to-voltage conversion.

In this embodiment, the optical signal received by the antenna station 2 includes an optical signal output from the IF signal optical transmitter 11 in the control station 1 and a local signal optical transmitter 13 output from the local signal optical transmitter 13. One optical signal, that is, an optical signal modulated by an IF signal and a frequency conversion local signal with different subcarriers is included, and these are converted into an electric signal by one photoelectric converter 20. Therefore, in the control station 1, it is necessary to sufficiently separate the output light wavelengths of the two optical transmitters 11 and 13 so that beat noise generated by interference between these two optical signals does not enter the reception band.

The output signal of the photoelectric converter 20 is split into two,
Input to filters 21-1 and 21-2. Filter 2
1-1 and 21-2 are typically band-pass filters. The filter 21-1 extracts only the IF signal, and the filter 21-2 extracts only the local signal for frequency conversion.

The IF signals and the local signals for frequency conversion output from the filters 21-1 and 21-2 are input to a frequency converter 22, where they are multiplied by a mixer 23. By removing unnecessary components from the output of the above, the IF signal having a frequency of 5.8 GHz is converted into a radio frequency signal having a frequency of 60 GHz, for example. The radio frequency signal output from the frequency converter 22
And transmitted to, for example, a mobile station (not shown).

As described above, in the present embodiment, the control station 1
The F signal and the local signal for frequency conversion are respectively subjected to optical modulation in different optical transmitters 11 and 13 to be converted into optical signals.
Good transmission becomes possible without causing cross modulation due to distortion.

In this embodiment, the IF signal optical transmitter 11 converts an IF signal having a relatively low frequency into an optical signal.
And a local signal optical transmitter 13 for converting a high-frequency local signal for frequency conversion into an optical signal is separately provided. As shown in FIG. Can be used,
The problem of relaxation oscillation caused by directly modulating the semiconductor laser with the local signal for frequency conversion can be solved to reduce noise.

(Second Embodiment) FIG. 4 shows a second embodiment of the present invention.
3 shows a configuration of the antenna station 2 in the communication system according to the embodiment. In the first embodiment shown in FIG.
In the antenna station 2, the optical signal transmitted by the optical fiber 3 is received by one photoelectric converter 20 and photoelectrically converted. However, in the present embodiment, the optical signal transmitted by the optical fiber 3 is an optical distributor. 26, and are divided into two at an appropriate ratio, and photoelectrically converted by the two photoelectric converters 20-1 and 20-2. Then, the photoelectric converters 20-1 and 20-
From the output signal of I.2 by filters 21-1 and 21-2.
The F signal and the local signal for frequency conversion are extracted.

Here, as described above, in many cases, IF
The signal is low-frequency, the local signal for frequency conversion is high-frequency, and the high-frequency photoelectric converter is expensive. Therefore, the IF converter 20-1 for receiving the IF signal is so slow and inexpensive that the IF signal can be photoelectrically converted. It is advantageous to use a suitable one. Further, the photoelectric converter 20-1 is constituted by a low-speed photodiode and a low-speed preamplifier, and the photoelectric converter 20-2 is operated before the high-speed photodiode and only near the frequency conversion local signal frequency. In this case, only the IF signal is output from the photoelectric converter 20-1, and only the local signal for frequency conversion is output from the photoelectric converter 20-2. 21-1 and 21-2 become unnecessary.

In this embodiment, the distribution ratio of the light distributor 26 does not need to be 1: 1.
1, 20-2, and the conversion efficiency and noise characteristics
The distribution ratio may be appropriately determined according to the required signal-to-noise ratio (CNR) of each of the F signal and the local signal for frequency conversion.

Further, in the present embodiment, an optical loss may occur at the time of the optical distribution by the optical distributor 26. Therefore, it is desirable to insert an optical amplifier for compensating the loss as necessary.

(Third Embodiment) FIG. 5 shows a third embodiment of the present invention.
1 shows a configuration of a communication system according to an exemplary embodiment. In the first embodiment shown in FIG. 1, the control station 1 converts the local signal for frequency conversion into an optical signal and transmits the optical signal. However, in the present embodiment, an integer of the fundamental frequency of the local signal for frequency conversion is used. The second embodiment is different from the first embodiment in that a subharmonic signal, which is a sine wave having a frequency of one-half, is converted into an optical signal and transmitted.

When the local signal for frequency conversion has a very high frequency, such as a millimeter wave or a quasi-millimeter wave,
The local signal for frequency conversion is generated by multiplying the subharmonic sine wave oscillator output. In general, the lower the frequency of a signal transmitted by an optical fiber, the lower the cost of an optical link.

Focusing on this point, in this embodiment, the sub-harmonic signal generated by the sub-harmonic signal generator 15 composed of a sub-harmonic sine wave oscillator is used instead of the frequency conversion local signal as shown in FIG. Is converted into an optical signal by the sub-harmonics signal optical transmitter 16. The optical signal output from the sub-harmonics signal optical transmitter 16 is converted by the optical coupler 14 into the IF signal optical transmitter 1.
After being combined with the optical signal from the antenna 1, the signal is transmitted to the antenna station 2 via the optical fiber 3.

On the other hand, in the antenna station 2, the optical signal transmitted from the control station 1 via the optical fiber 3 is converted into an electric signal by the photoelectric converter 20 as described above, and is branched into two. 1, 21-2. The filter 21-1 extracts only the IF signal,
At -2, only the sub-harmonics signal is extracted.

The sub-harmonics signal extracted by the filter 21-2 is multiplied by the multiplier 27, whereby a local signal for frequency conversion is generated. In this case, if the frequency of the subharmonics signal is 1 / N (N is an integer of 2 or more) of the fundamental frequency of the local signal for frequency conversion, the multiplication ratio of the multiplier 27 is set to N.

The IF signal output from the filter 21-1 and the local signal for frequency conversion output from the multiplier 27 are input to the frequency converter 22,
The IF signal is converted into a radio frequency signal, and this radio frequency signal is radiated from the antenna 4 and transmitted to a mobile station (not shown).

According to the present embodiment, the same effects as those of the first embodiment can be obtained. In addition, the operation of the sub-harmonics signal optical transmitter 16 is smaller than that of the local signal optical transmitter 13 in the first embodiment. Since the frequency is reduced, the cost can be reduced.

The configuration of the control station 1 in the present embodiment and the configuration of the antenna station 2 shown in FIG. 4 are combined, and the photoelectric converter 20-2 or the photoelectric converter 20-2 is provided on the antenna station 2 side in FIG. It is also possible to add a multiplier for multiplying the sub-harmonic signal output from the filter 21-2 connected to the output side of, and generate a local signal for frequency conversion.

(Fourth Embodiment) FIG. 6 shows a fourth embodiment of the present invention.
It is a figure showing the composition of the radio communications system concerning the embodiment of. In the present embodiment, a plurality of antenna stations 2 using a local signal for frequency conversion of the same frequency for one control station 1 are described.
This is an example in which an optical transmission function for i (i = 1, 2,..., n) is provided. In other words, the control station 1 is provided with n IF signal optical transmitters 11-i (i = 1, 2,..., N), and these IF signal optical transmitters 11-i (i = 1, 2). 2, ...,
n) converts an IF signal for the antenna station 2-i (i = 1, 2,..., n) input from the input terminal 10-i into an optical signal and outputs the optical signal.

On the other hand, the optical signal modulated by the local signal for frequency conversion generated by the local signal generator 12 in the local signal optical transmitter 13 is transmitted to the optical distributor 1.
7 and distributed to a plurality (n) systems. The optical signal output from each of the IF signal optical transmitters 11-i (i = 1, 2,..., N) and each one of the optical signals n-divided by the optical distributor 17 are combined into an optical coupler 14-. i (i = 1, 2,..., n) and transmitted to the optical fiber 3-i (i = 1, 2,..., n).

When the optical signal output from the local optical transmitter 13 is divided into n by the optical distributor 17, the optical power transmitted to each optical fiber 3-i (i = 1, 2,..., N) Decrease. If satisfactory transmission quality cannot be obtained due to such a decrease in optical power, an optical amplifier may be inserted in a stage preceding the optical distributor 17 to amplify an optical signal.

The antenna station 2-i (i = 1, 2,..., N)
Then, the optical signal transmitted by the optical fiber 3-i (i = 1, 2,..., N) is received. Antenna station 2-i
The configuration of (i = 1, 2,..., N) may be the same as the configuration of the antenna station 2 shown in FIG. 1 or FIG. 4, or may be configured as shown in FIG.

In FIG. 7, the optical fiber 3-i (i =
The optical signal transmitted by (1, 2,..., N) is first demultiplexed for each wavelength by the optical demultiplexer 28, and the IF signal optical transmitter 11-i (i = 1, 2, 2) of FIG. , N) and the optical signal output from the local signal light transmitter 13 are separated and extracted. In this case, the wavelength of the optical signal output from each of the IF signal optical transmitters 11-i (i = 1, 2,..., N) and the local signal light transmitter 13 is equal to the operating wavelength of the optical demultiplexer 28. The value must be compatible.

The IF signal optical transmitter 11-i (i = 1, 1)
If the wavelengths of the optical signals output from the local signal light transmitters 13 are multiplexed in a relatively close state, strict wavelength control is required. If the wavelengths of the two optical signals are sufficiently separated, for example, if rough wavelength multiplexing is performed, for example, the one having a 1.3 μm band and the other having a 1.5 μm band, wavelength control is not particularly necessary. In the latter case, light that does not match the zero-dispersion wavelength of the optical fiber suffers from the dispersion of the optical fiber. In a system in which the influence of dispersion is a problem, for example, an IF signal optical transmitter 1 for transmitting a slower optical signal
The influence of dispersion may be reduced, for example, by setting the output light wavelength of 1-i (i = 1, 2,..., N) to a wavelength that does not match the zero dispersion wavelength.

Among the optical signals output from the optical demultiplexer 28, the IF signal optical transmitters 11-i (i = 1, 2,...,
The optical signal from n) is input to the photoelectric converter 20-1, and the optical signal from the local signal optical transmitter 13 is input to the photoelectric converter 20-2. An IF signal re-converted into an electric signal is output from the photoelectric converter 20-1.
-2 outputs a local signal for frequency conversion reconverted into an electric signal.

The IF signal and the local signal for frequency conversion are input to the frequency converter 22, and the frequency converter 22 converts (up-converts) the IF signal into a radio frequency signal using the local signal for frequency conversion. Then, the signal is transmitted from the antenna 4-i (i = 1, 2,..., N) to, for example, a mobile station (not shown).

Since the local signal for frequency conversion is often of a high frequency, the optical transmitter for local signal 13 is more expensive than the optical transmitter for IF signal 11-i (i = 1, 2,..., N). Therefore, by using a configuration in which the optical signal output from the local optical transmitter 13 is distributed to a plurality of systems by the optical distributor 17 as in the present embodiment, the number of the local signal optical transmitters 13 is one. Any cost can be greatly reduced.

In general, an optical splitter has an intrinsic coupling loss, while an optical demultiplexer has no intrinsic coupling loss and only excess loss due to device imperfection. Therefore, in the configuration using the optical demultiplexer 28 as in the present embodiment, light loss is small and good photoelectric conversion characteristics are obtained, so that transmission quality is improved.

The local signal generator 12 and the local signal optical transmitter 13 of FIG. 6 are replaced by the sub-harmonic signal generator 15 and the sub-harmonic signal optical transmitter 16 in the same manner as in the third embodiment shown in FIG. It is also possible to adopt a configuration replaced with In that case, the antenna station 2-i (i
= 1, 2, ..., n), the subharmonics signal may be multiplied by an appropriate multiplication ratio and used for frequency conversion of the IF signal.

(Fifth Embodiment) FIG. 8 shows a fifth embodiment of the present invention.
It is a figure showing the composition of the radio communications system concerning the embodiment of. In the present embodiment, as in the fourth embodiment shown in FIG. 6, a plurality of antenna stations 2-i (i = 1, 2) using the same frequency conversion local signal of the same frequency for one control station 1.
This is an example in which an optical transmission function for (2,..., N) is provided.

This embodiment is different from the fourth embodiment in that the optical coupler 14-i (i
= 1, 2,..., N), and the IF signal optical transmitter 11
The optical signal from i (i = 1, 2,..., N) and the optical signal from the local signal optical transmitter 13 divided into n by the optical distributor 17 are separated into separate optical fibers 3-i1 and 3-i2. (I =
The antenna station 2-i (i = 1, 2, 3,..., N)
2,..., N).

On the other hand, the antenna station 2-i (i = 1, 2, 2)
.., N) are configured as shown in FIG. Optical fiber 3
-I-1 and 3-i-2 determine the antenna station 2-i (i =
The optical signals transmitted to (1, 2,..., N) are input to the photoelectric converters 20-1 and 20-2, respectively. The photoelectric converter 20-1 outputs an IF signal reconverted to an electric signal, and the photoelectric converter 20-2 outputs a local signal for frequency conversion reconverted to an electric signal.

The IF signal and the local signal for frequency conversion are input to the frequency converter 22, and the IF signal is frequency-converted (up-converted) into a radio frequency signal using the local signal for frequency conversion. i (i
= 1, 2,..., N) to, for example, a mobile station (not shown).

According to the present embodiment, except that twice as many optical fibers are required as in the fourth embodiment,
The same effects as in the fourth embodiment can be obtained. The local signal generator 12 and the local signal optical transmitter 1 shown in FIG.
3 is replaced with a sub-harmonic signal generator 15 and a sub-harmonic signal optical transmitter 16 as in the third embodiment, and the antenna station 2-i (i = 1, 2,...,
In n), the sub-harmonics signal may be multiplied by an appropriate multiplication ratio and used for frequency conversion of the IF signal.

(Sixth Embodiment) Next, a sixth embodiment of the present invention will be described. In the present embodiment, broadcast data can be distributed to a plurality of antenna stations by directly modulating a semiconductor laser, which is a light source of an optical transmitter for local signals, with a predetermined band signal, for example, broadcast data. is there. Since the configuration of the entire communication system is basically the same as that of the above-described embodiments, the description will focus on the local signal optical transmitter 13 and the antenna station 2.

FIG. 10 is a diagram showing the configuration of the local signal optical transmitter 13 in the present embodiment. The semiconductor laser 31 is used as a light source, and the output light from the semiconductor laser 31 is modulated by the external modulation optical modulator 32 by the local signal for frequency conversion from the local signal generator 12. At this time, the semiconductor laser 31 is directly modulated by the broadcast data 33. In this case, the optical modulator 3
2 has a function as a multiplier. Here, a band signal is used as the broadcast data 33. What is a band signal?
Unlike a baseband signal, a signal having a center frequency at a frequency other than DC.

When the output light from the semiconductor laser 31 directly modulated by the broadcast data 33 is externally modulated by the local signal for frequency conversion in the optical modulator 32, the output becomes the broadcast data 33 and the local signal for frequency conversion. Is multiplied by.

Next, the operation of the local signal optical transmitter 13 shown in FIG. 10 will be specifically described with reference to a frequency spectrum diagram shown in FIG. FIG. 11A shows broadcast data 3
3 shows the spectrum of the intensity modulation component of the output light from the semiconductor laser 31 which has been directly modulated. Center frequency f signal component of the broadcast data 33 in the vicinity of _B are present. Normally, the semiconductor laser 31 is modulated around a DC bias point, so that its output light has a DC, that is, a line spectrum at a frequency 0.

FIG. 11B shows an optical modulator 3 for external modulation.
2 shows a spectrum of a signal applied to the sample No. 2. Component of the frequency conversion local signal is present as a line spectrum in the frequency f _c. Since the optical modulator 32 also modulates around a certain DC bias, its output light has a line spectrum at a frequency 0.

The output light from the semiconductor laser 31 modulated as shown in FIG.
When the signal is modulated by the signal shown in FIG. 11B according to FIG. 2, light having a spectrum intensity modulation component as shown in FIG. 11C is output. That is, since the optical modulator 32 is a multiplier, the spectrum of the intensity modulation component of the output light is a convolution of FIG. 11A and FIG. 11B, and the frequency f as shown in FIG. _B, and in addition to the components of f _c, the sum frequency component of f _c + f _B, the difference frequency component of f _c -f _B appears.

The light thus modulated is transmitted from the local signal light transmitter 13 to the optical fiber 3 and transmitted to the antenna station 2. In the antenna station 2, the frequency f component or the frequency f _c + f _B components of _B shown in FIG. 11 (c) as a broadcast data as described below, the frequency f _c as the case
To use the components of the -f _B.

Next, the configuration of the antenna station 2 in this embodiment will be described. In the following examples, the optical signals output from the IF signal optical transmitter 12 and the local signal optical transmitter 13 as in the first embodiment and the fourth embodiment shown in FIGS. The case of coupling and transmission to the antenna station 2 by one optical fiber 3 will be described.

FIG. 12 is a diagram showing an I signal modulated by an IF signal.
3 shows the spectrum of the intensity modulation component of the output light from the F signal light transmitter 11. IF signal component is present in the center frequency f _D. The output light from the IF signal optical transmitter 11 and the local signal optical transmitter 13 shown in FIG.
The light having the intensity modulation component as shown in FIG.
And the signal is transmitted to the antenna station 2 by the optical fiber 3 and converted into an electric signal by one photoelectric converter 20 in the antenna station 2, the frequency spectrum becomes as shown in FIG. 13, and f _B , f _c , f _c + f _B, in addition to f _c -f _B having a frequency component of f _D.

FIGS. 14 to 19 show various configuration examples of the antenna station 2 in this embodiment. FIGS. 14 and 15 show an example in which two coupled optical signals transmitted to the antenna station 2 by the optical fiber 3 are collectively converted into an electric signal by one photoelectric converter 20 and processed.

FIG. 14 shows the broadcast data 33 having the frequency f
in the form of using the components of _c + f _B. After being trifurcated output signal suitable proportions from the photoelectric converter 20, is input to the filter 21-2,21-2,21-3, IF signal component of the frequency f _D by a filter 21-1, filter 21 frequency conversion local signal component of the frequency f _c by -2 and the frequency f _c + f _B by a filter 21-3
Are extracted respectively.

In the frequency converter 22, the filter 21-1
Is converted (up-converted) into a radio frequency signal by the local signal for frequency conversion output from the filter 21-2. The radio frequency signal component output from the frequency converter 22 is
At 9-1, it is combined with the broadcast data output from the filter 21-3 at an appropriate ratio and radiated from the antenna 4 via the power amplifier 25.

FIG. 15 shows the frequency f
_This is a mode when the component _B is used. Photoelectric converter 20
An output signal which is trifurcated in suitable proportions from, is input to the filter 21-1,21-2,21-4, IF signal component of the frequency f _D by a filter 21-1, the frequency by the filter 21-2 frequency conversion local signal component of f _c, the broadcast data component of the frequency f _B are respectively extracted by the filter 21-4.

The IF signal output from the filter 21-1 and the broadcast data output from the filter 21-4 are combined at an appropriate ratio by a combiner 29-2.
-2 is converted into a radio frequency signal by the frequency converter 22 using the local signal for frequency conversion outputted from -2. The radio frequency signal output from the frequency converter 22 is radiated from the antenna 4 via the power amplifier 25.

As shown in FIGS. 14 and 15, once the IF signal and the broadcast data are separated from each other, the combiners 29-1 and 29
The configuration of combining at −2 corresponds to the I received by the photoelectric converter 20.
It is mainly used when the power ratio between the F signal and the broadcast data is different from the power ratio when radiated from the antenna 4. In such a case, it is desirable to combine the couplers 29-1 and 29-2 with an appropriate coupling ratio so as to obtain a desired power ratio. If the ratio between the IF signal and the broadcast data received by the photoelectric converter 20 is appropriate, the IF 21 and the broadcast data are collectively extracted by the filter 21-1 using the same configuration as the antenna station 2 shown in FIG. do it.

FIG. 16 and FIG. 17 show that the two coupled optical signals transmitted to the antenna station 2 by the optical fiber 3 are divided into two by an optical distributor 26 at an appropriate ratio, and the photoelectric conversion for a low-speed signal is performed. This is an example in which a converter 20-1 and a photoelectric converter 20-2 for a high-frequency signal convert the signals into electric signals and process the electric signals.

In FIG. 16, one of the optical signals divided into two by the optical distributor 26 is converted into an electric signal by the photoelectric converter 20-1. The photoelectric converter 20-1 is a photoelectric converter that can photoelectrically convert a relatively low-speed signal of about the IF signal frequency.
The output signal of the photoelectric converter 20-1 is branched into two at an appropriate ratio, and then input to the filters 21-1, 21-4.
IF signal of a frequency f _D by a filter 21-1, the broadcast data of the frequency f _B are respectively extracted by the filter 21-4. The output signals of these filters 21-2 and 21-4 are combined at an appropriate ratio by a combiner 29-2.

The other of the two optical signals split by the optical splitter 26 is converted into an electric signal by the high-speed signal photoelectric converter 20-2. The output signal of the photoelectric converter 20-2 is applied to the filter 21.
Is input to -2, the frequency conversion local signal of a frequency f _c is extracted. The output signal of the coupler 29-2 is input to the frequency converter 22 and converted into a radio frequency signal using the local signal for frequency conversion output from the filter 21-2. Radiated.

Here, the IF signal output from the photoelectric converter 20-1 and the broadcast data are once separated and recombined by the combiner 29-2.
If the power ratio between the IF signal and the broadcast data output from 1 can be used as it is, the filter 21-1 may extract the IF signal and the broadcast data collectively by using the same configuration as the antenna station shown in FIG. Good.

If a photoelectric converter 20-1 having sufficiently low sensitivity to a high-frequency signal and not receiving a signal near the local frequency for frequency conversion at all is used, the filter 21-1 can be omitted.

[0104] Figure 17 is a modification using the broadcast data on the frequency f _c + f _B. Photoelectric converters 20-1, 20-
The configuration up to the point where the input optical signal is converted into an electrical signal in Step 2 is the same as the configuration in FIG. The output signal of the photoelectric converter 20-1 is a low-speed signal is input to the filter 21-1, IF signal component of the frequency f _D are extracted. The output signal of the photoelectric converter 20-2 is for high-speed signal is branched into two in suitable proportions, one of the frequency conversion local signal component of the input frequency f _c in the filter 21-2 is extracted, the other is is input to the filter 21-3 by the frequency f _c +
broadcast data component of f _B is extracted.

The IF signal output from the filter 21-1 is input to the frequency converter 22, and is converted into a radio frequency signal using the local signal for frequency conversion output from the filter 21-2. The output signal of the frequency converter 22 and the broadcast data output from the filter 21-3 are combined with the combiner 29-
1, and are radiated from the antenna 4 via the power amplifier 25.

FIG. 18 and FIG. 19 show that the optical signal transmitted to the antenna station 2 is wavelength-division multiplexed in a separable state. 20-2 shows an example of processing by converting into an electric signal.

[0107] Figure 18 is a modification using the component of the frequency f _B as broadcast data. The optical signal transmitted via the optical fiber 3 is separated by the optical demultiplexer 28 from the optical signal from the IF signal optical transmitter 11 and the local signal optical transmitter 1.
The optical signal is demultiplexed into the optical signal from the optical signal 3. The photoelectric converter 20-1 is
The optical signal from the IF signal optical transmitter 11 is converted into an electric signal and an IF signal is output. The photoelectric converter 20-2 converts the optical signal from the local signal optical transmitter 13 into an electric signal and outputs a frequency conversion local signal.

The output signal of the photoelectric converter 20-2 is branched into two at an appropriate ratio, and then input to filters 21-2 and 21-4. The filter 21-2 filters the local signal for frequency conversion of the frequency fc and the filter. broadcast data of frequency f _B are extracted respectively 21-4. After the IF signal output from the photoelectric converter 20-1 and the broadcast data output from the filter 21-4 are combined at an appropriate ratio by the combiner 29-2, the combined signal is input to the frequency converter 22 and the filter 21
-2 is converted into a radio frequency signal by the frequency conversion local signal output from the antenna 2 and radiated from the antenna 4 via the power amplifier 25.

FIG. 19 shows that the broadcast data has a frequency f _c +
a form to be used in the component of f _B. The process up to the point where the optical signal demultiplexed by the optical demultiplexer 28 is converted into an electric signal by the photoelectric converters 20-1 and 20-2 is the same as that in FIG.

The output signal of the photoelectric converter 20-2 is branched into two at an appropriate ratio, and then input to filters 21-2 and 21-3. The filter 21-2 filters the local signal for frequency conversion of the frequency fc and the filter. 21-3 by the frequency f _c
+ Broadcast data component of f _B are extracted, respectively. The IF signal reconverted into an electric signal by the photoelectric converter 20-1 is input to the frequency converter 22, and is converted into a radio frequency signal by the frequency conversion local signal output from the filter 21-2. This radio frequency signal is combined with the broadcast data output from the filter 21-3 at an appropriate ratio in the coupler 29-1 and passes through the power amplifier 25 to the antenna 4
Radiated from

The above description of the present embodiment is an example in which one optical fiber is connected from the control station 1 to the antenna station 2. For example, as shown in FIG. In the case where 3-i1 and 3-i2 are connected from the control station 1, for example, the optical demultiplexer 28 in the antenna station 2 is removed in the configuration of FIG. Optical fiber 3-i for transmitting an optical signal from
1 to the photoelectric converter 20-1 and the local signal optical transmitter 1
The optical fiber 3-i2 for transmitting the optical signal from the optical fiber 3 may be connected to the photoelectric converter 20-2.

The local signal generator 12 and the local signal optical transmitter 13 in FIG. 10 are replaced with a sub-harmonic signal generator 15 and a sub-harmonic signal optical transmitter 16 in the same manner as in the third embodiment, and a sub-harmonic signal having a frequency of 1 / N of the fundamental frequency of the local signal (f _c) may be transmitted as an optical signal. In that case, the frequency as a broadcast data f _c / N
Since the component of + f _B cannot be used, the component of the frequency f _B is used. Forms antenna station 2 which can be used at this time, for example 15, such as 16 and 18, is limited to a mode that uses the component of the frequency f _B. Also, in the antenna station 2, a multiplier for multiplying the sub-harmonic signal converted into an electric signal to generate a local signal for frequency conversion is inserted in front of the frequency converter 22 as in FIG. In the case where the optical loss is large and sufficient transmission quality cannot be ensured in the above embodiment, an optical amplifier may be appropriately inserted.

As described above, in the present embodiment, FIG.
As shown in (1), the output light of the semiconductor laser 31 directly modulated with the broadcast data 33 is used as the light source of the local signal optical transmitter 13 (or the sub-harmonics signal optical transmitter). By performing modulation with the local signal for frequency conversion, the distribution of broadcast data common to a plurality of antenna stations can be performed simultaneously with the distribution of the local signal for frequency conversion (or the subharmonic signal). Can be shared and cost can be reduced.

In this embodiment, for the purpose of distributing broadcast data to a plurality of antenna stations, the local signal optical transmitter 13 shown in FIG.
3, the semiconductor laser 31 is directly modulated. However, the signal for directly modulating the semiconductor laser 31 is not limited to the broadcast data 33, and any other band signal can be used.

(Seventh Embodiment) Next, a seventh embodiment of the present invention will be described. In the present embodiment, the oscillator output of the sub-harmonic frequency, which is a frequency that is a fraction of the frequency of the local signal for frequency conversion, is multiplied by using the periodic characteristic peculiar to the optical modulator to obtain the local signal for frequency conversion. It is an example. FIGS. 1 and 6 show the overall configuration of the communication system.
8 is the same as that in FIG.
Only the local signal optical transmitter 13 and the antenna station 2 will be described.

FIG. 20 shows the configuration of the local signal optical transmitter 13 in this embodiment. In this example, a semiconductor laser 31 is used as a light source, and the output light of the semiconductor laser 31 is modulated by a subharmonic signal from a subharmonic signal generator 15 in a Mach-Zehnder type optical modulator 34.

[0117] subharmonics signal generator 15 is constituted by a sine wave oscillator that oscillates at a frequency f _c / N (N is an integer of 2 or more) which corresponds to the sub-harmonics of the frequency conversion local signal frequency f _c, the output of the oscillator Is amplified or attenuated to have an appropriate amplitude, and then applied to the Mach-Zehnder optical modulator 34 with an appropriate bias voltage. Thus, the Mach-Zehnder type optical modulator 3
In 4, the input subharmonics signal is multiplied,
Harmonics, which are integer multiples of the frequency of the subharmonic signal,
That is, the local signal for frequency conversion is generated efficiently.

FIG. 21 shows a Mach-Zehnder type optical modulator 3
In addition to the graph showing the change in light transmittance with respect to the applied voltage of No. 4, the bias point for efficiently generating harmonics and the amplitude of the applied voltage are illustrated. As shown in the figure, since the light transmittance shows a sinusoidal periodic change with respect to the applied voltage, a harmonic of the applied voltage can be generated using this periodicity.

[0119] For example, the amplitude of the voltage = 0V as the bias point by multiplying the modulation by applying the following sinusoidal voltage V _[pi, 2 harmonic wave is generated. Furthermore, the amplitude of the voltage = V _{π /} 2 as the bias point beyond the V _[pi / 2, is multiplied modulation by applying 3V _[pi / 2 or less sine wave voltage, the third harmonic is generated. Similarly, when a sine wave voltage is applied to perform modulation, a fourth harmonic is generated. By the way, voltage = _Vπ /
When a sine wave voltage whose amplitude is less than _Vπ / 2 is applied to the Mach-Zehnder optical modulator 34 with the bias point 2 as the bias point, the sub-harmonic signal is output as it is, and FIG.
The optical signal generator 16 for the sub-harmonic signal is realized.

Therefore, by appropriately adjusting the bias point and the amplitude of the applied voltage in accordance with the required multiplication ratio and applying the same to the Mach-Zehnder type optical modulator 34, a desired signal required as a local signal for frequency conversion can be obtained. The harmonic component can be output from the Mach-Zehnder optical modulator 34.

In the local signal optical transmitter 13 having such a configuration, the bias point and the bias point are set so that the local signal frequency component for frequency conversion, which is a desired harmonic, is generated most efficiently from the Mach-Zehnder optical modulator. Even when the amplitude of the applied voltage is adjusted, unnecessary harmonics are generated in addition to the desired harmonics. For example, when N = 2, the output spectrum of the Mach-Zehnder optical modulator 34 is as shown in FIG.

When modulation is performed with the sine wave voltage shown in FIG.
In addition to the fourth harmonic, that is, the component of the frequency fc,
Even harmonics, such as the next, are generated in principle, albeit small. Further, due to imperfections of the Mach-Zehnder type optical modulator 34 as a device, the frequency f
The sub-harmonic signal component of c / 2 also remains at the output.
Further, if the bias point is not entirely suited to the apex of the sinusoidal light transmission characteristic of FIG. 21, the frequency 3f _c / 2, 5
f _c / 2 ... odd-order harmonics also occur. When these unnecessary waves become a problem, they may be removed by an antenna station.

The configuration of the antenna station in this embodiment is as follows.
Basically, it may be the same as the antenna station 2 shown in FIG. 1, FIG. 4, FIG. 7, or FIG. However, before the local signal for frequency conversion obtained by the photoelectric conversion by the photoelectric converters 20 and 20-2 in the antenna station 2 is input to the frequency converter 22, the local signal passes through a filter for removing unnecessary waves as described above. Need to be done.

Specifically, if the configuration of the antenna station 2 as shown in FIG. 1 or FIG. 4 is adopted, the filter 21-2 may have a characteristic capable of removing unnecessary waves. On the other hand, in the control station 1, it is necessary to appropriately select the multiplication ratio in the Mach-Zehnder type optical modulator 34 so that the unnecessary wave does not overlap the IF signal.

Further, the antenna station 2 shown in FIG. 7 and FIG.
In the configuration described above, it is necessary to newly insert a filter for removing the above-mentioned unnecessary wave between the photoelectric converter 20-2 and the frequency converter 22.

(Eighth Embodiment) FIG. 23 shows the configuration of a local signal optical transmitter 13 according to an eighth embodiment of the present invention. This embodiment is an embodiment in which the sixth embodiment described with reference to FIGS. 10 to 19 and the seventh embodiment described with reference to FIGS. 20 to 22 are combined.

The semiconductor laser 31 is directly modulated by the broadcast data 33. At this time, the spectrum of the intensity modulation component of the output light of the semiconductor laser 31 is the same as that of FIG. 11A, which is input to the Mach-Zehnder type optical modulator 34, and the sub-harmonics signal It is modulated by the sub-harmonic signal from the generator 15.

The subharmonic signal input as a drive signal to the Mach-Zehnder optical modulator 34 has a spectrum having a frequency component of f _c / 2, as shown in FIG. However, the function of the Mach-Zehnder type optical modulator 34 as a multiplier is not based on the sub-harmonic signal as the drive signal in FIG. 24 but by driving the light transmission characteristics of the Mach-Zehnder type optical modulator 34. It operates on the obtained waveform, that is, FIG. 22 which is the output spectrum of the Mach-Zehnder optical modulator 34 in the seventh embodiment. Therefore, the Mach-Zehnder optical modulator 34
The spectrum of the intensity modulation component of the output light from
FIG. 25 shows a convolution of the spectrum of FIG. 22A and the spectrum of FIG.

When FIG. 25 is compared with FIG. 11C, which is the spectrum of the intensity modulation component of the output light of the optical modulator 32 in FIG. 10, the unnecessary wave and the unnecessary waves generated on both sides thereof are convolved with the broadcast data. It can be seen that the component is newly present. As shown in FIGS. 1 and 6, the output lights of the IF signal optical transmitters 11 and 11i and the local signal optical transmitter 13 are coupled by the optical couplers 14 and 14-i, and then the optical fibers 3 and
In the transmission system for 3-i, it is necessary to select an appropriate multiplication ratio for the Mach-Zehnder optical modulator 34 so that such components do not overlap with the IF signal frequency.

In the antenna station 2, the frequency f
Filter 21-1 to extract the IF signal and _D, the filter 21-2 that extracts a local signal for frequency conversion of the frequency f _c,
Filter for extracting broadcast data of the frequency f _c + f _B 21-
3, the filter for extracting broadcast data of the frequency f _B 21-
All four need to have appropriate bands so as not to pass unnecessary waves other than signal components to be extracted.

As described above, according to the present embodiment, the local signal optical transmitter 13 modulates the Mach-Zehnder optical modulator 34 with the subharmonic signal adjusted to the appropriate bias point and voltage amplitude. Thus, it is possible to efficiently generate a harmonic of the subharmonic signal, that is, a local signal for frequency conversion. As a result, an optical modulator that is modulated by the local signal for frequency conversion, which is expensive at a high frequency of the local signal for frequency conversion, becomes unnecessary, and the cost can be reduced.

As shown in FIGS. 6 and 8, in a system in which a local signal for frequency conversion is distributed to a plurality of systems for a plurality of antenna stations 2-i, the local signal optical transmitter 13 according to the present embodiment is employed. Although there is an effect of cost reduction, especially in a system in which the control station 1 and the antenna station 2 are connected 1: 1 as shown in FIG. 1, the effect of cost reduction according to the present embodiment is further enhanced.

(Ninth Embodiment) Next, a ninth embodiment of the present invention will be described. Since the connection configuration of the entire communication system in this embodiment is the same as that in FIG. 1, FIG. 6, or FIG. 8, the configuration of the antenna station 2 which is a feature of this embodiment will be described here.

FIG. 26 shows the configuration of the antenna station 2 in this embodiment. The optical signal transmitted by the optical fiber 3 is converted by the photoelectric converter 20 into an electric signal. The output signal of the photoelectric converter 20 is branched into two at an appropriate ratio, and then input to the filters 21-1 and 21-2, where the IF signal is output by the filter 21-1 and the local signal for frequency conversion is output by the filter 21-2. Each is extracted. The output signal of the filter 21-1 is further branched into two at an appropriate ratio, and one of the two is supplied to the antenna 40 via the power amplifier 41. As a result, an intermediate frequency signal having a relatively low frequency, for example, a signal in the 5.8 GHz band is radiated from the antenna 40.

The other signal branched into two from the output of the filter 21-1 is output from the frequency converter 22 to the filter 21-1.
-2, a high frequency radio frequency signal such as a millimeter wave, a quasi-millimeter wave,
The signal is converted into a signal in the GHz band, and is radiated from the antenna 4 via the power amplifier 25.

As described above, according to the present embodiment, one type of antenna station can cope with a low frequency narrow band service to a high frequency broad band service, which is economical.

In this embodiment, a high-frequency radio signal and an intermediate frequency signal can be transmitted by adding a power amplifier 41 and an antenna 40 to the antenna station 1 shown in FIG. 4, 5, 7, 9, and 1
In the antenna station 2 shown in FIGS. 4 to 19, it is possible to add a similar configuration so that the intermediate frequency signal is branched and radiated from the antenna.

In the case where the antenna station 2 shown in FIGS. 14 to 19 has the same configuration, the intermediate frequency signal to be radiated from the antenna is branched according to the content of the service provided by the narrow band service. May be appropriately changed. If only the IF signal is provided as a narrow-band service, the IF signal is branched and radiated from the antenna immediately after the filter for extracting the IF signal. If both the IF signal and the broadcast data are provided as a narrowband service, they are radiated from the antenna after they are combined by the coupler 29-2 or at a point where both are combined as one signal. What should I do?

In FIG. 26, the high-frequency antenna 4 for radiating a high-frequency radio signal and the low-frequency antenna 40 for radiating an IF signal are separately installed. Antennas are usually
Such a configuration is appropriate because it does not correspond to two significantly different frequencies. However, if an antenna corresponding to both frequencies can be configured, a signal for high-frequency broadband service and a signal for low-frequency narrowband service may be combined and then radiated by one antenna.

(Tenth Embodiment) The main feature of the present invention relates to the configuration of the transmission system in the downlink direction from the control station 1 to the antenna station 2. However, since most of the radio communication is performed in two directions, Actually, an upstream transmission system from the antenna station 2 to the control station 1 is also required. In the present embodiment, an antenna station 2 in which a configuration for a transmission system in the uplink direction in addition to the downlink direction is added will be described.

FIG. 27 shows the configuration of the antenna station 2 in this embodiment. In this antenna station 2, an antenna 50 for the up direction, a low noise amplifier 51, and a frequency converter 52 are added to the antenna station 2 shown in FIG. The frequency converter 52 is a frequency converter 2 for the downlink direction.
As in the case of No. 2, the mixer 53 and the filter 54 are provided.

The upward radio frequency signal received by the antenna 50 is input to the frequency converter 52 via the low noise amplifier 51, where it is extracted by the filter 21-2 in the downstream transmission system. The local signal is down-converted into an intermediate frequency signal.
The intermediate frequency signal output from the frequency converter 52 is converted into an optical signal by the optical transmitter 55 and transmitted to the control station via the optical fiber 5.

According to the present embodiment, the optical transmitter 55 provided in the upstream transmission system and the optical receiver in the control station (not shown) need only be inexpensive enough to support the intermediate frequency signal. Further, the filter 5 in the frequency converter 52
4 may not be necessary. Since the image signal included in the output of the mixer 53 is on the higher frequency side than the radio frequency signal, if the optical transmitter 55 has a low speed enough to cope with the intermediate frequency signal, the signal to which the optical transmitter 55 responds is intermediate. This is because there is only a frequency signal.

The configuration of the antenna station 2 shown in FIG. 27 is obtained by modifying the antenna station 2 shown in FIG. 1 and adding an upstream transmission system. Can be dealt with in a similar manner. That is, in any of the antenna stations described above,
After splitting the local signal for frequency conversion obtained in the downstream transmission system, and using the local signal for frequency conversion, the frequency converter converts the upstream radio frequency signal into an intermediate frequency signal (down conversion). The optical transmitter may convert the optical signal into an optical signal and transmit the optical signal to the control station 1 through an optical fiber.

As shown in FIG. 5, when the control station 1 transmits a sub-harmonic signal having a frequency which is a fraction of the frequency conversion local signal in the downlink transmission system, the antenna station 1 Using the local signal for frequency conversion obtained by multiplying the sub-harmonics signal extracted in the above, the radio frequency signal in the upstream transmission system is frequency-converted (down-converted), and then converted to an optical signal and converted to an optical fiber 5 to the control station 1.

(Eleventh Embodiment) FIG. 28 shows the configuration of the antenna station 1 according to the eleventh embodiment of the present invention. This embodiment is directed to an antenna station capable of supporting both the low-frequency narrowband service and the high-frequency broadband service in the ninth embodiment described in FIG. 26, and the uplink station in the tenth embodiment described in FIG. This is one embodiment in which a direction transmission system is added.

The antenna station 2 of this embodiment combines the configurations shown in FIGS. 26 and 27 to radiate an intermediate frequency signal in the downstream transmission system from the antenna 40 via the power amplifier 41 and to transmit the intermediate transmission signal in the upstream transmission system. In the above, the radio frequency signal received by the antenna 50 is converted into an intermediate frequency signal by the frequency converter 52 through the low noise amplifier 51, converted into an optical signal by the optical transmitter 55, and transmitted to the control station via the optical fiber 5. Antenna 60, a low-noise amplifier 61, and a coupler 62 for receiving a signal of a low-frequency narrow-band service in an uplink transmission system.

That is, in the uplink transmission system, the low-frequency narrow-band service signal received by the antenna 60 is input to the coupler 62 via the low-noise amplifier 61. On the other hand, similarly to FIG. 27, the high-frequency broadband service signal received by the antenna 50 is input to the frequency converter 52 through the low-noise amplifier 51, and the frequency conversion extracted by the filter 21-2 in the downlink transmission system. Down-converted to an intermediate frequency signal using the local signal.

The intermediate frequency signal output from the frequency converter 52 and the low-frequency narrow-band service signal received by the antenna 60 and extracted through the low-noise amplifier 61 are combined by a combiner 62 to form an optical signal. The signal is converted into an optical signal by the transmitter 55 and transmitted to the control station via the optical fiber 5.

FIG. 29A shows the frequency spectrum of each signal in the air at this time. The spectrum frequency f _B of the signal (narrowband downlink signal) 71 of the narrowband service radiated from the antenna 40, also the spectrum of narrowband service signal (narrowband uplink signal) 72 that is received by the antenna 60 is frequency f _{In BUN} .

On the other hand, the spectrum of the broadband service signal (broadband downstream signal) 73 radiated from the antenna 4 is f
Only higher frequency f _c + f _B frequency f _c of the frequency conversion local signal from _B, also the spectrum of the signal of the broadband services (broadband uplink signal) 74 that is received by the antenna 50 f _BUN than f _c only high frequency f _c + F _BUB . Since the wideband service has abundant bandwidth, the frequency difference between the downstream signal 73 and the upstream signal 74 is large.

[0152] Here, the wideband uplink signal 74 is down-converted by the frequency conversion local signal of a frequency f _c in the frequency converter 52, although appearing in the frequency f _BUB as shown in FIG. 29 (b), which is narrow Band upstream signal 7
It does not overlap with the frequency f _BUN of frequency 2. Therefore, FIG.
As shown in FIG.
4 and the narrow-band upstream signal 72 are not overlapped even if they are combined by the combiner 62, so that they can be transmitted to the control station by one optical transmitter 55, which is economical.

[0153]

As described above, according to the present invention, in a communication system such as a ROF system for transmitting a radio signal having a high frequency such as a millimeter wave or a quasi-millimeter wave through an optical fiber, the radio signal is converted into an IF signal and a frequency. Deterioration of the distortion characteristics by converting the local signal for conversion or the sub-harmonic signal having a frequency that is a fraction of the integer of the local signal for frequency conversion into optical signals using different optical transmitters and transmitting the signals. In addition, the optical modulation degree can be increased, and further, each of the optical transmitters can have narrow band characteristics specialized in the frequency band of the drive signal, so that the cost can be reduced.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a communication system according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration example of an optical transmitter for IF signals according to the embodiment;

FIG. 3 is a block diagram showing a configuration example of a local signal optical transmitter according to the embodiment;

FIG. 4 is a block diagram illustrating a configuration of an antenna station according to a second embodiment of the present invention.

FIG. 5 is a block diagram showing a configuration of a communication system according to a third embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration of a communication system according to a fourth embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of an antenna station suitable for the embodiment.

FIG. 8 is a block diagram showing a configuration of a communication system according to a fifth embodiment of the present invention.

FIG. 9 is a block diagram showing the configuration of the antenna station according to the embodiment;

FIG. 10 is a block diagram showing a configuration of a local signal optical transmitter in a communication system according to a sixth embodiment of the present invention.

FIG. 11 is a frequency spectrum diagram for explaining the operation of the local signal optical transmitter in the embodiment.

FIG. 12 is a frequency spectrum diagram for explaining the operation of the antenna station in the embodiment.

FIG. 13 is a frequency spectrum chart for explaining the operation of the antenna station in the embodiment.

FIG. 14 is a block diagram showing a first configuration example of the antenna station according to the embodiment;

FIG. 15 is a block diagram showing a second configuration example of the antenna station according to the embodiment;

FIG. 16 is a block diagram showing a third configuration example of the antenna station according to the embodiment;

FIG. 17 is a block diagram showing a fourth configuration example of the antenna station according to the embodiment;

FIG. 18 is a block diagram showing a fifth configuration example of the antenna station in the embodiment.

FIG. 19 is a block diagram showing a sixth configuration example of the antenna station in the embodiment.

FIG. 20 is a block diagram showing a configuration of a local signal optical transmitter in a communication system according to a seventh embodiment of the present invention.

FIG. 21 is a diagram illustrating the operation of Mach.
A diagram showing a change in light transmittance with respect to an applied voltage of a Zener-type optical modulator and various bias and amplitude settings.

FIG. 22 is a diagram illustrating an example of a frequency spectrum of an output signal of a Mach-Zehnder optical modulator.

FIG. 23 is a block diagram showing a configuration of a local signal optical signal generator in a communication system according to an eighth embodiment of the present invention.

FIG. 24 is a diagram showing an example of a frequency spectrum of a drive signal of a Mach-Zehnder optical modulator.

FIG. 25 is a diagram illustrating an example of a frequency spectrum of an output signal of a Mach-Zehnder optical modulator.

FIG. 26 is a block diagram showing a configuration of an antenna station in the communication system according to the ninth embodiment of the present invention.

FIG. 27 is a block diagram showing a configuration of an antenna station in the communication system according to the tenth embodiment of the present invention.

FIG. 28 is a block diagram showing a configuration of an antenna station in the communication system according to the eleventh embodiment of the present invention.

FIG. 29 is a view showing frequency spectra of various signals for explaining the operation of the embodiment;

[Explanation of symbols]

1 ... control station 2, 2-1 to 2-n ... antenna station 3, 3-1 to 3-n, 3-11 to 3-n1, 3-12 to
3-n2: Optical fiber 5: Optical fiber 4, 4-1 to 4-n: Antenna 10: IF signal input terminal 11: Optical signal transmitter for IF signal 12: Local signal generator 13: Optical transmitter for local signal 14 ... Optical coupler 15 ... Sub-harmonic signal generator 16 ... Sub-harmonic signal optical transmitter 17 ... Optical distributor 20, 20-1, 20-2 ... Photoelectric converters 21-1, 21-2, 21-3 ... Filter 22 Frequency converter 23 Mixer 24 Filter 25 Power amplifier 26 Optical distributor 27 Multiplier 28 Optical demultiplexer 29-1, 29-2 Coupler 30, 31 Semiconductor laser 32 Light Modulator 33 Broadcast data 34 Mach-Zehnder type optical modulator 40 Antenna 41 Power amplifier 50 Antenna 51 Low noise amplifier 52 Frequency converter 53 Mixer 54 Fill 55: Optical transmitter 60: Antenna 61: Low noise amplifier 62: Coupler 71: Narrow band downstream signal 72: Narrow band upstream signal 73: Broadband upstream signal 74: Broadband upstream signal

Continuing on the front page (72) Inventor Hiroshi Harada 3-4 Hikarinooka, Yokosuka City, Kanagawa Prefecture Inside the Communications Research Laboratory, Yokosuka Radio Communication Center, Ministry of Posts and Telecommunications Toshiba R & D Center (72) Inventor Shigeru Oshima 1 Kosaka Toshiba-cho, Saiwai-ku, Kawasaki City, Kanagawa Prefecture F-term (reference) 2H079 AA02 AA13 BA01 CA04 EA05 HA11 5K002 AA01 AA03 BA04 BA13 CA14 DA02 FA01 5K067 AA23 AA41 BB02 DD57 EE10 EE16 EE23 EE34 EE37 EE41 EE61

Claims (10)

[Claims] 1. A communication system comprising a control station and an antenna station connected to the control station by a first optical transmission line, wherein the control station converts an intermediate frequency signal into a first optical signal and transmits the signal. Optical transmitter for converting a local signal for frequency conversion into a second optical signal and transmitting the second optical signal; the first optical signal and the second optical signal And an optical coupler for transmitting the optical signal to a first optical transmission line, wherein the antenna station photoelectrically converts an optical signal transmitted by the first optical transmission line to the intermediate signal. A frequency signal and at least one photoelectric converter that outputs the local signal for frequency conversion, and an intermediate frequency signal output from the photoelectric converter is converted into a desired signal using the local signal for frequency conversion output from the photoelectric converter. First to convert to radio frequency signal and output A communication system comprising: a frequency converter; and a first antenna for transmitting a radio frequency signal output from the first frequency converter. 2. A communication system comprising a control station and an antenna station connected to the control station by a first optical transmission line, wherein the control station converts an intermediate frequency signal into a first optical signal and transmits the signal. Transmitter for transmitting the signal, and an integer fraction of the fundamental frequency of the local signal for frequency conversion
An optical transmitter for converting a sub-harmonic signal having a frequency of 2 to a second optical signal and transmitting the second optical signal; and coupling the first optical signal and the second optical signal to a first optical transmission line. An optical coupler for transmitting the signal, and the antenna station photoelectrically converts an optical signal transmitted through the first optical transmission line and outputs the intermediate frequency signal and the sub-harmonic signal. A converter, a multiplier for multiplying the sub-harmonic signal output from the photoelectric converter to generate the local signal for frequency conversion, and an intermediate frequency signal output from the photoelectric converter generated by the multiplier. A first frequency converter that converts the output into a desired radio frequency signal using the local signal for frequency conversion and outputs the radio frequency signal output from the first frequency converter. Communication system, comprising a first antenna. 3. A communication system comprising a control station and a plurality of antenna stations respectively connected to the control station by a plurality of first optical transmission lines, wherein the control station converts a plurality of series of intermediate frequency signals into a plurality of intermediate frequency signals, respectively. A plurality of first optical transmitters for converting to a first optical signal and transmitting; a second optical transmitter for converting a frequency-converting local signal to a second optical signal and transmitting; An optical distributor for distributing the second optical signal into a plurality of optical signals; and a plurality of the plurality of first optical signals and the plurality of second optical signals distributed by the optical distributor being combined. A plurality of first optical couplers for transmitting the signals to the first optical transmission line, respectively, wherein each of the plurality of antenna stations photoelectrically converts the optical signal transmitted by the first optical transmission line. The intermediate frequency signal and the frequency conversion At least one photoelectric converter that outputs a cull signal, and converts an intermediate frequency signal output from the photoelectric converter into a desired radio frequency signal using a local signal for frequency conversion output from the photoelectric converter. A communication system, comprising: a first frequency converter that outputs a signal; and a first antenna that transmits a radio frequency signal output from the first frequency converter. 4. A communication system comprising a control station and a plurality of antenna stations respectively connected to the control station by a plurality of first optical transmission lines, wherein the control station transmits a plurality of series of intermediate frequency signals to a plurality of intermediate frequency signals, respectively. A plurality of first optical transmitters for converting to a first optical signal and transmitting the first optical signal;
A second optical transmitter for converting a sub-harmonic signal having a frequency of the second optical signal into a second optical signal and transmitting the second optical signal; an optical distributor for distributing the second optical signal to a plurality of optical signals; A first optical signal and a plurality of first optical couplers each of which couples the plurality of second optical signals distributed by the optical distributor and sends the combined signal to a plurality of first optical transmission lines, Each of the plurality of antenna stations, at least one photoelectric converter that photoelectrically converts an optical signal transmitted through the first optical transmission line and outputs the intermediate frequency signal and the sub-harmonic signal; A multiplier for multiplying the sub-harmonic signal output from the photoelectric converter and outputting the local signal for frequency conversion, and an intermediate frequency signal output from the photoelectric converter for the frequency conversion output from the multiplier. B A first frequency converter for converting a desired frequency signal into a desired radio frequency signal using a cull signal and outputting the signal; and a first antenna for transmitting a radio frequency signal output from the first frequency converter. A communication system characterized by the above-mentioned. 5. The semiconductor device according to claim 1, wherein the second optical transmitter includes: a semiconductor laser;
5. The optical modulator according to claim 1, further comprising: an optical modulator that modulates output light of the semiconductor laser with the local signal for frequency conversion or the subharmonic signal to obtain the second optical signal. The communication system according to any one of the preceding claims. 6. The communication system according to claim 5, further comprising means for directly modulating said semiconductor laser with a predetermined band signal. 7. The second optical transmitter comprises: a light source; and a light from the light source, which is a fraction of the local signal for frequency conversion.
Having a Mach-Zehnder type optical modulator that modulates with a subharmonic signal having a frequency of the Mach-Zehnder type optical modulator with a bias voltage and an amplitude at which harmonics are efficiently generated from the Mach-Zehnder type optical modulator. 4. The communication system according to claim 1, wherein the sub-harmonics signal is applied to a transmitter, the sub-harmonics signal is converted into a frequency of the frequency conversion local signal, and the second optical signal is transmitted. . 8. The antenna station has means for transmitting an intermediate frequency signal output from the photoelectric converter by the first antenna or a second antenna different from the first antenna. The communication system according to any one of claims 1 to 7, wherein 9. The antenna station according to claim 1, further comprising: a third antenna for receiving a radio frequency signal; and frequency converting the radio frequency signal received by the third antenna using the local signal for frequency conversion. Second to generate the intermediate frequency signal
And a second optical transmission line connected between the control station and the antenna station by converting an intermediate frequency signal generated by the second frequency converter into a third optical signal. The communication system according to any one of claims 1 to 8, further comprising a third optical transmitter for transmitting the data to a third optical transmitter. 10. An antenna station comprising: a fourth antenna for receiving an intermediate frequency signal; an intermediate frequency signal received by the fourth antenna; and an intermediate frequency signal generated by the second frequency converter. 10. The communication system according to claim 9, further comprising: means for combining a signal and supplying the combined signal to the third optical transmission line.