JP2002217873A - Optical transmission system for radio access, and high- frequency optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmission system where a frequency conversion part or an electric-optical conversion part is shared by a plurality of radio base stations by optically frequency-converting an IF signal into an RF signal. SOLUTION: A plurality of modulation parts 110 modulate an inputted baseband signal into the IF signals of different frequencies. A multiplexing part 120 multiplexes a plurality of modulated IF signals. The electric-optic conversion part 130 converts the multiplexed IF signal into an optical signal by an intensity-modulating system. A local oscillation signal source 140 outputs a local oscillation signal which has previously been decided. An outer modulation part 150 modulates the intensity of the optical signal by the local oscillation signal. An optical branching part 160 branches the optical signal, whose intensify is modulated and outputs the signals to the radio base stations. An optic-electric conversion part 21k converts the inputted optical signal into an electrical signal and obtains the RF signal obtained by frequency-converting the IF signal. Only the desired radio frequency components in this RF signal extracted by a bandfilter part 22k in the RF signal is transmitted to a subscriber terminal from an antenna part 23k.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system for radio access and a high-frequency optical transmitter, and more particularly to a radio signal (microwave band or millimeter wave band) transmitted between a center station and a plurality of subscriber terminals. The present invention relates to an optical transmission system for connecting a center station and a wireless base station with an optical fiber in a wireless access that is connected by using a high-frequency wireless signal such as a high-frequency wireless signal, and a high-frequency optical transmitter used in the system.

[0002]

2. Description of the Related Art FIG. 1 shows a configuration example of a conventional optical transmission system used for wireless access connecting a center station and a subscriber terminal via a wireless base station for transmitting and receiving wireless signals.
It is shown in FIG. The conventional optical transmission system shown in FIG. 17 includes a center station 600 and a plurality of wireless base stations 701 to 70n (where n is 2).
The above integers (hereinafter the same in the present specification) are connected via a plurality of downstream optical fibers 801 to 80n, respectively. The center station 600 includes a plurality of electro-optical converters 611 to 61n corresponding to the plurality of wireless base stations 701 to 70n, respectively. Each radio base station 701-70
n is a photoelectric conversion unit 711-71n and a modulation unit 721-
72n, frequency conversion units 731 to 73n, local signal sources 741 to 74n, and antenna units 751 to 75n, respectively. Hereinafter, the operation of this conventional optical transmission system will be described.

In the center station 600, an input terminal 6k
(K = 1 to n, hereinafter the same in this specification) includes information transmitted to the subscriber terminal via the radio base station 70k,
It is input in the form of a baseband signal. Electro-optical converter 6
1k converts a baseband signal input from the input terminal 6k into an optical signal. The optical signal output from the electro-optical converter 61k is transmitted from the center station 600 to the wireless base station 70k via the downstream optical fiber 80k. In the wireless base station 70k, the optical signal transmitted from the center station 600 is input to the photoelectric conversion unit 71k. The photoelectric conversion unit 71k converts an input optical signal into an electrical signal. The modulator 72k converts the converted electric signal into an intermediate frequency signal (IF signal). The local oscillator signal source 74k outputs a local oscillator signal of a predetermined frequency. Frequency converter 7
3k inputs an IF signal and a local oscillation signal, and converts the IF signal into a radio frequency signal (RF signal) by the local oscillation signal. This RF signal is emitted to space via the antenna unit 75k.

FIG. 18 shows the concept of a service area covered by a radio base station when a center station 600 and a plurality of radio base stations 701 to 707 are connected. Note that areas 901 to 907 in FIG. 18 indicate service areas covered by the wireless base stations 701 to 707, respectively. Optical signals having different information are transmitted from the center station 600 to the respective wireless base stations 701 to 707 via the respective downstream optical fibers 801 to 807. The plurality of radio base stations 701 to 707 change the frequencies of the local oscillation signals output from the local oscillation signal sources 741 to 747 included in each of them to avoid interference between adjacent service areas, and convert the IF signals to different frequencies ( f
d1 to fd7) and converts the RF signals to wireless signals to the subscriber terminal. Note that the same radio frequency (fd1 = fd5 = fd6) is set for radio base stations covering non-adjacent service areas (the radio base stations 701, 705 and 706 correspond to the example in FIG. 18). No problem.

[0005]

However, FIG.
As shown in FIG. 18 and FIG. 18, when different information is transmitted to a large number of subscriber terminals by optically transmitting a base station between a center station 600 and a plurality of wireless base stations 701 to 70 n using baseband signals. Various issues arise. The first problem is that the center station 600 needs the electro-optical converters 611 to 61n by the number (= n) of the radio base stations 701 to 70n. The second problem is that a plurality of radio base stations 701 to
70n requires expensive frequency converters 731 to 73n for converting the frequency of the IF signal into an RF signal. A third problem is that when information to be transmitted to a large number of subscriber terminals is transmitted in a time-division multiplexed manner, the center station 6
That is, a multiplexing unit is required at 00. In this case, a demultiplexing unit is required for each of the radio base stations 701 to 70n, and a high-speed modulation process is required for each of the modulation units 721 to 72n. The fourth problem is that, in order to add a new subscriber terminal, one radio base station 70k
When transmitting different information to a large number of subscriber terminals by frequency division multiplexing by increasing the capacity (the amount of information that can be transmitted from the antenna unit 75k to the subscriber terminal), the radio base station 70
It is necessary to additionally install each component other than the antenna unit 75k in k, and a multiplexing unit for multiplexing each RF signal is required. In particular, when the position of the added subscriber terminal is at a position where a line-of-sight propagation path cannot be secured from the existing radio base station 70k, all the configurations shown in FIG. Need to be installed. The fifth problem is that in order to avoid interference between adjacent service areas, the frequency of the local oscillation signal output from the local oscillation signal source 74k of the corresponding radio base station 70k needs to be different. Individual components or individual adjustments (even for the same component) are required.

FIG. 1 shows another conventional optical transmission system having a simplified configuration of each of the radio base stations 701 to 70n.
It is shown in FIG. Another conventional optical transmission system shown in FIG.
The configuration is such that the center station 600 and a plurality of wireless base stations 701 to 70n are connected via a plurality of downlink optical fibers 801 to 80n, respectively. The center station 600 corresponds to each of the wireless base stations 701 to 70n, and
21 to 62n, frequency converters 631 to 63n, local signal sources 641 to 64n, and external modulators 651 to 65n
And light sources 661 to 66n. Each of the wireless base stations 701 to 70n includes a photoelectric conversion unit 711 to 71n.
And antenna units 751 to 75n.
Hereinafter, the operation of the conventional optical transmission system will be described.

[0007] In the center station 600, information to be transmitted to the radio base station 70k is input to the input terminal 6k in the form of a baseband signal. The modulator 62k modulates a baseband signal input from the input terminal 6k into an IF signal. The local oscillator signal source 64k outputs a local oscillator signal of a predetermined frequency. The frequency conversion unit 63k includes a modulation unit 62k
And the local oscillator signal output from the local oscillator signal source 64k, and converts the IF signal to R according to the local oscillator signal.
Frequency conversion to F signal. The light source 66k generates an optical signal having a predetermined wavelength. The external modulator 65k receives the RF signal converted by the frequency converter 63k and the optical signal output from the light source 66k, and modulates the intensity of the optical signal by the RF signal. The intensity-modulated optical signal is transmitted to the wireless base station 70k via the downstream optical fiber 80k.

[0008] The optical signal transmitted from the center station 600 is
The light propagates through the downstream optical fiber 80k and is input to the wireless base station 70k. In the wireless base station 70k, the photoelectric conversion unit 71k converts an input optical signal into an electrical signal,
Outputs F signal. The output RF signal is emitted as a radio signal from the antenna unit 75k to the space toward the subscriber terminal.

As described above, in the conventional optical transmission system,
Since the frequency conversion from the IF signal to the RF signal is performed in the center station 600, each of the radio base stations 701 to 70n
, Only the photoelectric conversion units 711 to 71n need to be installed. Therefore, in the above conventional optical transmission system,
There is an effect that each of the wireless base stations 701 to 70n can be provided in a reduced size.

However, in the configuration of another conventional optical transmission system shown in FIG.
n, external modulation units 651 to 65n and photoelectric conversion unit 711
To 71n are required to be high-frequency devices (active devices) operating in each radio frequency band. Such a high-frequency device is expensive. Therefore, like the other conventional optical transmission systems described above, the center station 60
In a configuration in which 0 manages a plurality of wireless base stations 701 to 70n, there is a problem that n expensive devices are required, and the entire system becomes very expensive.

[0011] Therefore, an object of the present invention is to adopt a configuration in which transmission signals destined for a plurality of radio base stations are collectively frequency-converted by a center station, and frequency conversion from IF signals to RF signals (and vice versa). The present invention provides an optical transmission system for radio access and a high-frequency optical transmitter that solves the above-mentioned problem by optically performing (frequency conversion) on an optical transmission path.

[0012]

A first aspect of the present invention is a radio access system for transmitting information between a center station and a subscriber terminal via a radio base station transmitting and receiving radio signals from an antenna unit. An optical transmission system for transmitting a radio signal bidirectionally by connecting a plurality of radio base stations and a center station each covering a different service area with a plurality of optical fibers, and The station is 1
One or more baseband signals, each being input as one or more modulated electric signals having a predetermined intermediate frequency, and an electro-optical converter that converts the electric signal into an optical signal by intensity modulation; A local oscillation signal source for outputting a local oscillation signal, an external modulation section for intensity-modulating the optical signal converted by the electro-optical conversion section with a local oscillation signal output from the local oscillation signal source, and an intensity modulation for the external modulation section At least an optical branching unit that splits the obtained optical signal and outputs the split signal to a plurality of optical fibers, respectively. The plurality of wireless base stations each convert the optical signal transmitted through the optical fiber into an electrical signal of a radio frequency band. At least a photoelectric conversion unit that converts the signal into a signal, and a band filter unit that extracts only an electric signal in a desired frequency band from the electric signal converted by the photoelectric conversion unit and supplies the extracted electric signal to the antenna unit.

As described above, according to the first aspect of the present invention, an electric-to-optical converter can be shared for a downstream system, and an electric frequency converter is not required. Can be shared. In addition, signal multiplexing can be performed more easily than before, and transmission capacity can be easily increased with the increase in subscriber terminals.

The second invention is used for wireless access for transmitting information between a center station and a subscriber terminal via a wireless base station for transmitting and receiving a wireless signal from an antenna unit, and covers different service areas. An optical transmission system in which a plurality of wireless base stations and a center station are connected by a plurality of optical fibers to transmit a wireless signal optically and bidirectionally, wherein the center station outputs a predetermined light. A light source, a local signal source that outputs a predetermined local signal, an external modulator that intensity-modulates light output from the light source with a local signal output from the local signal source, and an external modulator. An optical splitter that splits the intensity-modulated optical signal into a number of radio base stations, and one or more baseband signals, each of which has one or more modulated electrical signals having a predetermined intermediate frequency. Wireless to transmit At least a plurality of IF modulators, each of which is input to each of the base stations and which intensity-modulates the optical signal branched by the optical branching unit with the electric signal and outputs the modulated optical signal to a plurality of optical fibers, respectively. The plurality of wireless base stations each convert an optical signal transmitted via an optical fiber into an electric signal in a radio frequency band,
At least a photoelectric conversion unit for supplying to the antenna unit is provided.

As described above, according to the second aspect, an electrical frequency converter is not required for the downstream system, and an external modulator for performing optical frequency conversion can be shared. In addition, signal multiplexing can be performed more easily than before, and transmission capacity can be easily increased with the increase in subscriber terminals. In addition, since a band filter is not required, a wireless base station having the same configuration can be installed even when the service area to be covered is different.

The third invention is used for radio access for transmitting information between a center station and a subscriber terminal via a radio base station for transmitting and receiving radio signals from an antenna unit, and covers different service areas. An optical transmission system in which a plurality of wireless base stations and a center station are connected by a plurality of optical fibers to transmit a wireless signal bidirectionally, wherein the center station includes one or more baseband signals. Are input as one or more modulated electric signals each having a predetermined intermediate frequency, and output an electro-optical converter that converts the electric signal into an optical signal by intensity modulation, and outputs a predetermined local oscillation signal A local oscillation signal source, a first external modulation unit for intensity-modulating the optical signal converted by the electro-optical conversion unit with a local oscillation signal output from the local oscillation signal source, and intensity modulation by the first external modulation unit Optical signal A first optical branching unit for branching and outputting to a plurality of downstream optical fibers, and an optical signal transmitted from each of a plurality of radio base stations via the plurality of upstream optical fibers, to an electric signal of an intermediate frequency band. A plurality of first photoelectric conversion units for respectively converting the electric signals converted by the plurality of first photoelectric conversion units into a baseband signal, respectively. The radio base station wirelessly converts an optical signal transmitted via the downstream optical fiber into a second optical splitter that splits the optical signal into two, and one of the optical signals split by the second optical splitter. A second photoelectric conversion unit for converting the electric signal into a frequency band electric signal, a band filter unit for extracting only an electric signal of a desired frequency band from the electric signal converted by the second photoelectric conversion unit, and a band filter Extracted by part A circulator section for outputting the air signal to the antenna section and a radio signal received by the antenna section to the second external modulation section; and outputting the other optical signal branched by the second optical branch section from the circulator section. At least a second external modulator for intensity-modulating the received radio signal and outputting the modulated signal to an upstream optical fiber.

As described above, according to the third aspect, the electro-optical converter can be shared for both the upstream system and the downstream system, and the electrical frequency converter is not required. The external modulation unit to be performed can be shared. In addition, signal multiplexing can be performed more easily than before, and transmission capacity can be easily increased with the increase in subscriber terminals.

A fourth invention is used for wireless access for transmitting information between a center station and a subscriber terminal via a wireless base station for transmitting and receiving a wireless signal from an antenna unit, and covers different service areas. An optical transmission system in which a plurality of wireless base stations and a center station are connected by a plurality of optical fibers to transmit a wireless signal optically and bidirectionally, wherein the center station outputs a predetermined light. A light source, a local oscillator signal source that outputs a predetermined local oscillator signal, a first external modulator that intensity-modulates light output from the light source with a local oscillator signal output from the local oscillator signal source, A first optical branching unit for branching the optical signal intensity-modulated by one external modulation unit into a plurality of radio base stations, and one or more baseband signals;
One or more modulated electric signals each having a predetermined intermediate frequency are input to each of the radio base stations to be transmitted, and the optical signals branched by the first optical branching unit are respectively intensified by the electric signals. A plurality of IFs for modulating and outputting the modulated optical signal to a plurality of downstream optical fibers, respectively.
A modulator, a plurality of first optical-electrical converters for respectively converting optical signals transmitted from a plurality of radio base stations through a plurality of uplink optical fibers to electric signals in an intermediate frequency band, At least a plurality of demodulators for demodulating the electric signals respectively converted by the first opto-electrical converter into baseband signals, wherein the plurality of radio base stations are respectively transmitted via downlink optical fibers. A second optical splitter for splitting an optical signal into two, and a second optical-electrical converter for converting one of the optical signals split by the second optical splitter into an electric signal in a radio frequency band; A circulator section for outputting the electric signal converted by the second photoelectric conversion section to the antenna section and a radio signal received by the antenna section to the second external modulation section; The other optical signal And intensity-modulated by a radio signal output from the section comprises at least a second external modulator for outputting the upstream optical fiber.

As described above, according to the fourth aspect, an electrical frequency converter is not required for both the upstream system and the downstream system, and an external modulator for performing optical frequency conversion can be shared. it can. In addition, signal multiplexing can be performed more easily than before, and transmission capacity can be easily increased with the increase in subscriber terminals. In addition, since a band filter is not required, a wireless base station having the same configuration can be installed even when the service area to be covered is different.

A fifth invention is an invention according to the third and fourth inventions, wherein a downlink system for wirelessly transmitting from a wireless base station to a subscriber terminal and a wireless system from the subscriber terminal to the wireless base station are provided. It is characterized in that a radio frequency to be used is different between an upstream system and a transmitting system.

As described above, according to the fifth aspect, there is no possibility of radio signal interference between the upstream system and the downstream system.

The sixth invention is an invention according to the first to fourth inventions, wherein frequencies of radio signals used in the respective radio base stations are set to be different from each other. I do.

A seventh invention is an invention according to the second and fourth inventions, wherein the frequency of the radio signal used in each radio base station is determined by the radio base station between the radio base stations whose service areas are adjacent to each other. Are set differently.

As described above, according to the sixth and seventh aspects, there is no possibility that radio signals will interfere between adjacent service areas.

An eighth invention is the invention according to the first to fourth inventions, wherein the optical signal output from the external modulator (first or second external modulator) is composed of a carrier and one sideband. It is an optical single sideband signal comprising a wave component.

As described above, according to the eighth aspect, it is possible to avoid a decrease in the electric signal level after photoelectric conversion, which has conventionally occurred when the optical fiber has dispersibility.

The ninth invention is an invention according to the first to fourth inventions, wherein a Mach-Zehnder type external modulator is used for an external modulator (first or second external modulator). The bias point in the external modulator is set to a point at which the optical output becomes minimum or maximum, so that the intensity of the optical signal is modulated by a component twice the frequency of the local oscillation signal. .

As described above, according to the ninth aspect, it is possible to avoid a decrease in the electric signal level after photoelectric conversion, which has conventionally occurred when the optical fiber has dispersibility.

A tenth invention is according to the first and third inventions, wherein a semiconductor laser for converting an electric signal into an optical signal by using a direct modulation method is used for the electro-optical converter. It is characterized by.

As described above, according to the tenth aspect, the multiplexed electric signal in the intermediate frequency band is converted into an optical signal by a direct modulation method, thereby performing electro-optical conversion easily and at low cost. Can be.

An eleventh invention is an invention according to the tenth invention, wherein the wavelength of the optical signal output from the electro-optical converter and the zero dispersion wavelength are added to the optical fiber (downstream optical fiber). It is characterized in that substantially coincident optical fibers are used.

As described above, according to the eleventh aspect, distortion caused by dispersion can be avoided by making the optical signal wavelength substantially equal to the zero dispersion wavelength of the optical fiber, thereby realizing high quality transmission. Can be.

The twelfth invention is used for radio access for transmitting information between a center station and a subscriber terminal via a radio base station transmitting and receiving radio signals from an antenna unit, and covers different service areas. The first to n-th wireless base stations and the center station are connected by first to n-th upstream and downstream optical fibers provided corresponding to each wireless base station, and wireless signals are transmitted. An optical transmission system for bidirectional optical transmission, wherein a center station transmits one or more predetermined intermediate frequency signals of different wavelengths λd1 to λdn uniquely corresponding to the first to nth wireless base stations. First to n-th electro-optical converters for converting into first to n-th optical signals, respectively;
A wavelength multiplexing unit that multiplexes the converted first to n-th optical signals, a local oscillation signal source that outputs a local oscillation signal of a predetermined frequency, and a local oscillation signal that is output from the wavelength multiplexing unit. An optical modulator that intensity-modulates based on the signal, the intensity-modulated multiplexed optical signal is wavelength-separated into first to n-th modulated optical signals of wavelengths λd1 to λdn, and the k-th modulated optical signal is
A k-th wireless base station receives a k-th modulated optical signal of wavelength λdk transmitted through the k-th downstream optical fiber. A photoelectric conversion unit that converts the modulated optical signal into an electric signal in a radio frequency band and outputs the electric signal.

As described above, according to the twelfth aspect, the wavelength-multiplexed optical signals are collectively externally modulated to frequency-convert the signals in the intermediate frequency band to the radio frequency band. This eliminates the need for an electrical frequency converter that has been conventionally required, and allows a plurality of wireless base stations to share an optical modulator that performs optical frequency conversion. In addition, since signals in a radio frequency band to be transmitted to a plurality of radio base stations are separated in an optical wavelength region, separation can be easily performed even if the radio frequencies radiated from the plurality of radio base stations are the same. .

The thirteenth invention is used for radio access for transmitting information between a center station and a subscriber terminal via a radio base station transmitting and receiving radio signals from an antenna unit, and covers different service areas. The first to n-th wireless base stations and the center station are connected by first to n-th uplink and downlink optical fibers provided corresponding to the respective wireless base stations, and wireless signals are transmitted. An optical transmission system for bidirectional optical transmission, wherein a center station transmits one or more predetermined intermediate frequency signals of different wavelengths λd1 to λdn uniquely corresponding to the first to nth wireless base stations. The first to n-th electro-optical converters for converting to the first to n-th downstream optical signals, respectively, and the first to n-th of different wavelengths λu1 to λun which are different from any of the wavelengths λd1 to λdn. Output each upstream optical signal. The uplink light source of the first to n, and the downstream optical signals converted first to n, first through output
A wavelength multiplexing unit that multiplexes the nth upstream optical signal, a local oscillation signal source that outputs a local oscillation signal of a predetermined frequency, and a multiplexed optical signal output from the wavelength multiplexing unit based on the local oscillation signal. An optical modulator for intensity modulation, and a multiplexed optical signal with intensity modulation,
The wavelengths are separated into first to n-th modulated downstream optical signals of wavelengths λd1 to λdn and first to n-th modulated upstream optical signals of wavelengths λu1 to λun, and the k-th modulated downstream optical signal is converted to the k-th modulated downstream optical signal. Along with the modulated upstream optical signal, a wavelength demultiplexing unit for transmitting to the k-th downstream optical fiber, and first to first converting the optical signals transmitted through the first to n-th upstream optical fibers into electric signals. The k-th wireless base station receives the optical signal transmitted through the k-th downstream optical fiber, and outputs the k-th modulated downstream optical signal having the wavelength λdk and the wavelength a two-wavelength separation unit that separates the k-th modulated upstream optical signal into λuk, a photoelectric conversion unit that converts the k-th modulated downstream optical signal separated by the two-wavelength separation unit into an electric signal, and outputs the electric signal; 2
And an RF modulator for intensity-modulating the k-th modulated upstream optical signal separated by the wavelength separating unit with the input radio signal and transmitting the modulated signal to the k-th upstream optical fiber.

As described above, according to the thirteenth aspect, the first to n-th light sources for outputting light of different wavelengths for transmitting upstream signals are provided, and wavelength-multiplexed with downstream optical signals.
Thus, the optical modulation unit can be shared by a plurality of wireless base stations for optical modulation of not only the downstream optical signal but also the local signal of the upstream optical signal. Also, since the upstream optical signal modulated by the local oscillation signal is intensity-modulated by the radio signal received by the antenna unit, mixing of the local oscillation signal and the radio signal occurs in the optical domain. In addition, a frequency conversion function of converting a radio signal to an intermediate frequency can be provided.

The fourteenth invention is an invention according to the thirteenth invention, wherein the wavelengths λd1 to λd1 of the first to n-th downstream optical signals are provided.
λdn is set to belong to a predetermined first wavelength band, and the wavelengths λu1 to λun of the first to nth upstream optical signals are
The two-wavelength separation unit of the k-th wireless base station is set so as to belong to the predetermined second wavelength band, and converts the optical signal transmitted via the k-th downstream optical fiber into the first wavelength band. By separating the wavelength into a second wavelength band, the signal is separated into a k-th modulated downstream optical signal having a wavelength λdk and a k-th modulated upstream optical signal having a wavelength λuk.

As described above, according to the fourteenth aspect, the wavelengths of the optical signals output from the first to n-th electro-optical converters and the wavelengths of the optical signals output from the first to n-th light sources are determined. The wavelengths are set so as to belong to a set range of wavelength bands. This allows the two-wavelength demultiplexing unit of the radio base station to easily separate the modulated downstream optical signal and the modulated upstream optical signal.

The fifteenth invention is used for radio access for transmitting information between a center station and a subscriber terminal via a radio base station transmitting and receiving radio signals from an antenna unit, and covers different service areas. The first to n-th wireless base stations and the center station are connected by first to n-th upstream and downstream optical fibers provided corresponding to each wireless base station, and wireless signals are transmitted. An optical transmission system for bidirectional optical transmission, wherein a center station transmits one or more predetermined intermediate frequency signals to a predetermined first wavelength band uniquely corresponding to the first to n-th radio base stations. Belong to different wavelengths λd
First to n-th electro-optical converters for converting the first to n-th downstream optical signals of 1 to λdn, respectively;
And a first to n-th upstream light source for outputting first to n-th upstream optical signals of mutually different wavelengths λu1 to λun belonging to a predetermined second wavelength band, respectively. A first to n-th downstream optical signal, a wavelength multiplexing unit for multiplexing the output first to n-th upstream optical signal, and a local oscillator signal source for outputting a local oscillator signal of a predetermined frequency. An optical modulator that intensity-modulates a multiplexed optical signal output from the wavelength multiplexing unit based on a local oscillation signal, and converts the intensity-modulated multiplexed optical signal into first to n-th modulated downstream lights of wavelengths λd1 to λdn. The signal and the first to n-th modulated upstream optical signals of the wavelengths λu1 to λun are wavelength-separated, respectively, and the k-th modulated downstream optical signal is transmitted to the k-th downstream optical fiber together with the k-th modulated upstream optical signal. Transmitted through the wavelength separation unit for transmission and the first to n-th upstream optical fibers. And a first to n-th optical-to-electrical conversion units for converting optical signals into electrical signals, respectively. The k-th wireless base station receives the optical signal transmitted via the k-th downstream optical fiber. Then, a k-th modulated downstream optical signal of wavelength λdk and a k-th modulated upstream optical signal of wavelength λuk are separated,
The k-th modulated downstream optical signal in the first wavelength band representing the photoelectric conversion function is converted into an electric signal and output, and the k-th modulated upstream light in the second wavelength band representing the electro-optical conversion function is output. The signal is intensity-modulated by an input radio signal to perform k-th modulation.
And an electro-absorption type modulation unit for transmitting to the upstream optical fiber.

As described above, according to the fifteenth aspect, the wavelengths of the optical signals output from the first to n-th electro-optical converters and the wavelengths of the optical signals output from the first to n-th light sources are determined. The wavelength is appropriately set, and instead of the two-wavelength separation unit, the photoelectric conversion unit and the RF modulation unit in the seventeenth aspect, photoelectric conversion and electro-optical conversion are performed depending on the wavelength of the input optical signal. Is installed in the wireless base station. Thus, in addition to the effects obtained by the configuration of the seventeenth aspect, the configuration of the wireless base station can be greatly simplified.

A sixteenth invention is an invention according to the thirteenth to fifteenth inventions, wherein the first to n-th upstream light sources uniquely correspond to the first to n-th downstream optical signals, Wavelength λu1-λ
un is a predetermined amount fs for each of the wavelengths λd1 to λdn.
A different first to n-th upstream optical signal is output.

As described above, according to the sixteenth aspect of the present invention, the wavelength λd
It is possible to easily separate k and λuk together.

A seventeenth invention is a invention according to the twelfth to fifteenth inventions, wherein the first to n-th electro-optical converters of the center station output optical signals of wavelengths λd1 to λdn, respectively. The first to n-th light sources, and the first to n-th IF modulators for intensity-modulating the optical signals output from the first to n-th light sources with the respective intermediate frequency signals. Features.

As described above, according to the seventeenth aspect, the intermediate frequency signal to be transmitted to the radio base station is converted into an optical signal by external modulation. Therefore, in addition to the effects obtained by the configurations of the sixteenth to nineteenth aspects, even when an optical signal is transmitted through an optical fiber having dispersibility, an effect that chromatic dispersion distortion does not occur can be obtained.

An eighteenth invention is according to the twelfth to seventeenth inventions, wherein the frequencies of the radio signals used in the respective radio base stations are set differently. I do.

A nineteenth invention is an invention according to the twelfth to seventeenth inventions, wherein the frequency of the radio signal used in each radio base station is different between the radio base stations whose service areas are adjacent to each other. Are set differently.

As described above, according to the eighteenth and nineteenth aspects, there is no possibility that radio signals will interfere between adjacent service areas.

A twentieth aspect is the invention according to the twelfth to nineteenth aspects, wherein the optical signal output from the optical modulator is an optical single sideband signal comprising a carrier and a single sideband component. There is a feature.

A twenty-first invention is an invention according to the twelfth to nineteenth inventions, wherein a Mach-Zehnder type external modulator is used for an optical modulator, and a bias point in the external modulator is determined by an optical modulator. Set to the point where the output is minimum or maximum,
It is characterized in that the intensity of the optical signal is modulated with a component twice the frequency of the local oscillation signal.

As described above, according to the twentieth and twenty-first aspects, it is possible to avoid a decrease in the electric signal level after photoelectric conversion, which has conventionally occurred when the optical fiber has dispersibility. Becomes According to the twenty-fifth aspect, the oscillation frequency of the local oscillation signal may be half of the conventional frequency, and the operating frequencies of the local oscillation signal source and the external modulator may be half.

A twenty-second invention is directed to a high-frequency optical transmitter used for a center station for optically transmitting a radio signal by connecting a plurality of radio base stations each covering a different service area with a plurality of optical fibers. A three-branch unit that divides an input electric signal into a first electric signal and a second electric signal that are in phase with each other, and a third electric signal that is orthogonal to the first electric signal and the second electric signal. An electro-optical converter that converts the third electric signal into a light intensity modulated signal; a first delay controller that adjusts the propagation time of the first electric signal; and an electric wave that adjusts the propagation time of the second electric signal And a second delay control unit for converting the input local oscillation signal into first and second local oscillation signals having phases opposite to each other.
A bifurcating unit for branching, a third delay control unit for adjusting the propagation time of the first local oscillation signal, a fourth delay control unit for adjusting the propagation time of the second local oscillation signal, A first multiplexing unit that multiplexes the first electric signal output from the delay control unit and the first local signal output from the third delay control unit;
A second multiplexing unit that multiplexes the second electric signal output from the second delay control unit and the second local oscillation signal output from the fourth delay control unit; It has a second electrode, inputs signals multiplexed by the first and second multiplexing units to the first and second electrodes, respectively, and modulates a light intensity modulation signal output from the electro-optical conversion unit. And a differential type intensity modulator
The first and second electric signals input to the first and second electrodes of the differential type intensity modulator via the first and second multiplexing units are in phase with each other so that the first and second electric signals are in phase with each other. By adjusting the fourth delay control unit, the optical signal output from the electro-optical conversion unit is phase-modulated, and the same amount and the opposite amount are applied to the frequency deviation of the optical frequency modulation component of the optical signal. It is characterized by performing phase light modulation.

As described above, according to the twenty-second aspect, the electro-optical conversion unit uses a part of the electric signal input to the electro-optical conversion unit using the external modulation unit for performing frequency conversion. Phase-modulates the optical signal output from. As a result, the optical frequency modulation component (wavelength chirp) generated at the time of electro-optical conversion can be canceled without using additional optical components, and the chromatic dispersion distortion generated by the action of the optical frequency modulation and the chromatic dispersion of the optical fiber can be achieved. Therefore, high-performance optical transmission can be realized.

A twenty-third invention is a high-frequency optical transmitter used in a center station for optically transmitting a radio signal by connecting a plurality of radio base stations each covering a different service area with a plurality of optical fibers. Wherein the input electric signal is divided into first and second electric signals in phase with each other and a third electric signal having a phase difference of 90 ° with the first and second electric signals.
, A three-branch unit that divides the electric signal into three, an electro-optical converter that converts the third electric signal into a light intensity modulation signal, and a first delay controller that adjusts the propagation time of the first electric signal A second delay control unit that adjusts the propagation time of the second electric signal, and a first and second orthogonal signals that are mutually orthogonal.
A two-branch unit for branching into two local oscillation signals, a third delay control unit for adjusting the propagation time of the first local oscillation signal, and a fourth delay control for adjusting the propagation time of the second local oscillation signal A first multiplexing unit that multiplexes a first electrical signal output from the first delay control unit and a first local oscillation signal output from the third delay control unit; A second multiplexing unit that multiplexes the second electric signal output from the second delay control unit and the second local oscillation signal output from the fourth delay control unit; It has a second electrode, inputs signals multiplexed by the first and second multiplexing units to the first and second electrodes, respectively, and modulates a light intensity modulation signal output from the electro-optical conversion unit. And a first and second electrodes input to the first and second electrodes of the differential type intensity modulator via the first and second multiplexing units, respectively. Signal Retardation so that a 0 to each other, the first
And adjusting the second delay control unit to perform phase modulation on the optical signal output from the electro-optical conversion unit,
The same amount and opposite phase light modulation is performed on the frequency deviation of the optical frequency modulation component of the optical signal, and the first and second electrodes are applied to the first and second electrodes of the differential type intensity modulator. By adjusting the third and fourth delay controllers so that the first and second local signals input through the wave unit are orthogonal to each other, an optical signal having an optical carrier is provided. It is characterized by performing single sideband modulation.

As described above, according to the twenty-third aspect, in order to perform optical single sideband modulation, an optical signal that has been subjected to non-differential type external modulation (both optical sideband modulation) is transmitted over a long distance. Avoids the problem that the upper band wave and lower band wave of the light intensity modulation component cancel each other due to the effect of chromatic dispersion that occurs when photoelectric conversion is performed after the conversion, and the RF signal component obtained by frequency conversion of the electric signal is greatly reduced. can do.

A twenty-fourth aspect of the present invention is a high-frequency optical transmitter used in a center station for optically transmitting a radio signal by connecting a plurality of radio base stations each covering a different service area with a plurality of optical fibers. Wherein the input electric signal is converted into the first and second electric signals orthogonal to each other by 2
A bifurcating unit for branching, an electro-optical conversion unit for converting the first electric signal into a light intensity modulation signal, a delay control unit for adjusting a propagation time of the second electric signal, a phase modulation unit and an intensity modulation unit Are formed on the same substrate and the second output from the delay control unit is
An electric modulation signal is input to the phase modulation unit, an input local oscillation signal is input to the intensity modulation unit, and an integrated modulation unit that modulates a light intensity modulation signal output from the electro-optical conversion unit is provided.
The phase modulation unit performs phase modulation on the optical signal output from the electro-optical conversion unit, and performs optical modulation having an opposite phase to the frequency deviation amount (FM index) of the optical frequency modulation component of the optical signal. It is characterized by applying.

As described above, according to the twenty-fourth aspect, by integrating the phase modulation unit for canceling the optical frequency modulation component and the intensity modulation unit for performing frequency conversion, the optical frequency modulation component is Delay adjustment for canceling out can be easily performed. Further, since it is not necessary to combine the electric signal input to the phase modulation unit and the local oscillation signal input to the intensity modulation unit, the power of the electric signal input to the intensity modulation unit can be further reduced.

[0057]

DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) FIG. 1 is a block diagram showing a configuration of an optical transmission system for wireless access according to a first embodiment of the present invention. In FIG. 1, an optical transmission system according to the first embodiment includes a center station 100.
And a plurality of wireless base stations 201 to 20n are connected via a plurality of downlink optical fibers 301 to 30n, respectively. The center station 100 includes a plurality of modulation units 110
, A multiplexing unit 120, an electro-optical conversion unit 130, a local oscillation signal source 140, an external modulation unit 150, and an optical branching unit 160. Each of the wireless base stations 201 to 20n includes an opto-electric conversion unit 211 to 21n, a band-pass filter unit 221 to 22n,
Antenna units 231 to 23n are provided, respectively. Hereinafter, the operation of the optical transmission system according to the first embodiment will be described.

In the center station 100, different information to be transmitted to the subscriber terminal is input to each input terminal 1 in the form of a baseband signal. Multiple modulators 1
Reference numeral 10 modulates the baseband signal input from the input terminal 1 into IF signals having different predetermined frequencies. This frequency corresponds to each of the radio base stations 201 to 20n.
Are determined on the basis of the frequency of the radio signal transmitted from the terminal to the subscriber terminal. The multiplexing unit 120 multiplexes a plurality of IF signals output from the plurality of modulation units 110. The electro-optical converter 130 converts the IF signal multiplexed by the multiplexer 120 into an optical signal by an intensity modulation method. The local oscillation signal source 140 outputs a local oscillation signal having a predetermined frequency. The frequency of the local oscillation signal is determined based on the modulation frequency of modulation section 110 and the frequency of the radio signal. The external modulator 150 receives the optical signal converted by the electro-optical converter 130 and the local signal output from the local signal source 140, and modulates the intensity of the optical signal with the local signal. The optical branching unit 160 branches the optical signal intensity-modulated by the external modulation unit 150 by the number of radio base stations (= n), and transmits each of the radio base stations 201 to 2 via each of the downlink optical fibers 301 to 30n.
0n.

The optical signal output from the center station 100 is
The light propagates through each of the downstream optical fibers 301 to 30n and is input to each of the wireless base stations 201 to 20n. In the wireless base station 20k, the opto-electric conversion unit 21k converts an input optical signal into an electric signal. By this conversion, an RF signal whose IF signal has been frequency-converted can be obtained. This is because the electro-optical converter 130 and the external modulator 150 of the center station 100 perform double intensity modulation by the IF signal and the local oscillation signal, respectively. Then, the bandpass filter unit 22k receives the RF signal output from the photoelectric conversion unit 21k, and extracts only the RF signal of a desired radio frequency from the RF signal. The extracted RF signal is emitted to the space from the antenna unit 23k toward the subscriber terminal.

As described above, according to the present invention, the center station 100
Perform double intensity modulation at each of the wireless base stations 201 to 2
At 0n, an RF signal whose IF signal has been frequency-converted can be obtained only by converting the propagating optical signal into an electric signal. Therefore, the electro-optical converter 130 and the external modulator 150 for performing frequency conversion can be shared by the plurality of wireless base stations 201 to 20n. This allows
The number of components in each of the wireless base stations 201 to 20n can be significantly reduced as compared with the conventional case. In addition, each of the wireless base stations 201 to 20
n can be easily transported and installed. Also, the center station 100
Transmits information to be transmitted to a plurality of subscriber terminals to the multiplexing unit 120.
And performs analog optical transmission. For this reason, the center station 100 modulates the modulation units 1 by the number of information to be multiplexed.
All you have to do is prepare 10. Therefore, the optical transmission system of the present invention does not require a demultiplexing / multiplexing unit and a high-speed modulation unit, as compared with the case where information transmitted to a large number of subscriber terminals is transmitted in a time-division multiplexed manner as described in the prior art. .

Furthermore, in the present invention, since the intensity modulation is performed twice, the intensity modulation of the frequency-division multiplexed IF signal (electro-optical converter 130) can be applied only to a relatively low frequency signal. A semiconductor laser having better distortion characteristics than an external modulator is modulated by the intensity modulation of a local oscillation signal (external modulation unit 15).
0), it is possible to use an external modulator that operates up to a high frequency. In addition, since the frequency of the local oscillation signal is relatively high, it is possible to use an external modulator matched in a specific high frequency band for modulation for the intensity modulation of the local oscillation signal. Further, it is also possible to use a Mach-Zehnder type external modulator for intensity modulation of the local oscillation signal. When this Mach-Zehnder type external modulator is used, optical frequency conversion is performed at twice the frequency of the local oscillation signal by setting the bias point to the point where the optical output becomes maximum or minimum. It becomes possible. This method of setting the bias point cannot be used for intensity modulation of the frequency-division multiplexed IF signal because the second-order distortion component becomes large. However, since the local oscillation signal is only one carrier, it is used for the intensity modulation of the local oscillation signal. Can be used. Further, in this method, the frequency of the local oscillation signal used for intensity modulation may be １ ／ of that in normal frequency conversion, and therefore, a low-cost local oscillation signal source 140 can be used.

On the other hand, a single mode fiber (SMF) having a zero-dispersion wavelength at 1.3 μm is generally used as an optical fiber used in optical communication and the like. Usually, when an optical fiber is laid, not only the actually required number but also a spare optical fiber that is not used is laid at the same time in consideration of future use. Therefore, there is a possibility that the unused SMF already laid in this way is used as the downstream optical fiber 30k in the present invention. When it is necessary to use an optical amplifier to compensate for branch loss and transmission loss, a wavelength of 1.5 μm is used as a wavelength of a light source used in the electro-optical converter 130. At this time, when double sideband modulation (DSB modulation) is used as the modulation method performed by the external modulation unit 150 and a microwave or millimeter wave high frequency signal is optically transmitted by the SMF, the transmission distance is affected by the dispersion of the SMF. In some cases, the power of the high-frequency signal after optical transmission may be significantly reduced. In order to avoid the influence of the dispersion, an optical single sideband modulation (optical SSB modulation) including a carrier and a single sideband component may be used as a modulation method performed by the external modulation unit 150. Further, a Mach-Zehnder type external modulator may be used as the external modulator 150, and a point at which the optical output becomes minimum or maximum may be set as a bias point to perform external modulation.

It should be noted that a new downstream optical fiber 30 (n +
For example, when laying 1), the downstream optical fiber 30 (n +
If the zero-dispersion wavelength of 1) and the wavelength of the light output from the electro-optical conversion unit 130 can be selected, it is preferable that the two wavelengths are selected so as to match. In this case, even with the DSB modulation, there is no influence of the dispersion of the SMF.
When the frequency division multiplexed IF signal is converted into an optical signal by the direct modulation method in the electro-optical conversion unit 130, the dispersion according to the frequency after the frequency conversion is performed regardless of the external modulation method of the local oscillation signal. Distortion occurs. From the viewpoint of avoiding the occurrence of the dispersion distortion, the downstream optical fiber 30 (n +
It is desirable that the zero-dispersion wavelength of 1) and the wavelength of the light output from the electro-optical conversion unit 130 be selected so as to match.

Here, a case where the transmission capacity is increased by adding a subscriber terminal in the optical transmission system will be described. FIG. 2 is a block diagram showing a configuration when a subscriber terminal is added in the optical transmission system according to the first embodiment of the present invention. As described above, the center station 100
Is frequency division multiplexing information to be transmitted to a plurality of subscriber terminals. For this reason, as can be seen by comparing FIGS. 1 and 2, when a subscriber terminal is added, a corresponding additional modulation section 101 is added to the center station 100, and the output signal is added to the multiplexing section 120. Only need to be input. As a result, the IF signal output from the additional modulation unit 101 is frequency-division multiplexed and optically transmitted to all the wireless base stations 201 to 20n. After conversion into an RF signal by electric conversion, an RF signal of a desired radio frequency need only be extracted through the band-pass filter unit 22k. Therefore, according to the present invention, it can be understood that an increase in transmission capacity due to the addition of the subscriber terminal can be realized very easily as compared with the above-described conventional technology.
In addition, when the position of the added subscriber terminal is a position where a line-of-sight propagation path cannot be secured from the existing wireless base station, if there is room in the number of branches of the optical branching unit 160, the downstream optical fiber 30 (n + 1) And the new radio base station 20 (n + 1)
Need only be additionally installed, and the number of components that need to be additionally installed can be significantly reduced as compared with the above-described conventional technology.

As described above, according to the optical transmission system according to the first embodiment of the present invention, an IF signal in which a plurality of IF signals are frequency-division multiplexed is converted into an optical signal by intensity modulation. Is externally modulated with a local oscillation signal, so that a plurality of IF signals are collectively frequency-converted into RF signals as optical signals. Then, information to be transmitted to the subscriber terminal is optically transmitted from the center station 100 to the plurality of wireless base stations 201 to 20n in the form of an RF signal. This allows
The present invention has the following effects. First, since the center station 100 performs frequency division multiplexing on a plurality of IF signals for optical transmission, the electro-optical conversion unit 130 can be shared by a plurality of wireless base stations 201 to 20n. Secondly, since the frequency modulation is optically performed by the external modulation unit 150 in the center station 100, the electric frequency conversion unit, which has been an essential configuration in the related art, becomes unnecessary in each of the radio base stations 201 to 20n. , The external modulation unit 150
01 to 20n. Third, in the conventional configuration, a multiplexing / demultiplexing unit and a modulation unit having a high-speed modulation function, which are necessary when transmitting information to be transmitted to a plurality of subscriber terminals by time division multiplexing, are not required in the present invention. Becomes Fourth, since the capacity can be easily increased with the addition of system subscribers, an optical transmission system with excellent expandability can be provided.

(Second Embodiment) In the optical transmission system according to the first embodiment, a plurality of IF signals are multiplexed by frequency division multiplexing.
Double intensity modulation is performed on the F signal. For this reason, in each of the wireless base stations 201 to 20n, it is necessary to extract the RF signal component of the desired radio frequency from the transmitted RF signal using the bandpass filter units 221 to 22n, respectively. Therefore, in the second embodiment,
The bandpass filter unit 2 is added to the configuration of each of the wireless base stations 201 to 20n.
An optical transmission system that does not require 21 to 22n will be described.

FIG. 3 is a block diagram showing a configuration of an optical transmission system for wireless access according to the second embodiment of the present invention. 3, the optical transmission system according to the second embodiment includes a center station 100 and a plurality of wireless base stations 201 to 201.
20n is connected via a plurality of downstream optical fibers 301 to 30n. Center station 100
Are a plurality of modulation units 110 and a plurality of multiplexing units 121 to 12
n, a plurality of IF modulators 181 to 18n, and a light source 170
, A local oscillation signal source 140, an external modulation unit 150, and an optical branching unit 160. Each of the radio base stations 201 to 20n
Photoelectric conversion units 211 to 21n and antenna units 231 to 2
3n. Hereinafter, the operation of the optical transmission system according to the second embodiment will be described.

In the center station 100, different information to be transmitted to the subscriber terminal is input to each input terminal 1 in the form of a baseband signal. Multiple modulators 1
Reference numeral 10 modulates the baseband signal input from the input terminal 1 into IF signals having different predetermined frequencies. This frequency corresponds to each of the radio base stations 201 to 20n.
Are determined on the basis of the frequency of the radio signal transmitted from the terminal to the subscriber terminal. Each of the multiplexing units 121 to 12n
A plurality of IF signals output from the plurality of modulation units 110 are multiplexed for each of the radio base stations 201 to 20n. The light source 170 generates an optical signal having a predetermined wavelength. The local oscillation signal source 140 outputs a local oscillation signal having a predetermined frequency. The frequency of the local oscillation signal is determined based on the modulation frequency of modulation section 110 and the frequency of the radio signal. The external modulator 150 receives the optical signal output from the light source 170 and the local signal output from the local signal source 140, and modulates the intensity of the optical signal with the local signal. Optical branching section 160
Separates the optical signal intensity-modulated by the external modulator 150 by the number of radio base stations (= n), and splits the plurality of IF modulators 181.
To 18n. Each IF modulator 181-1
8n inputs the split optical signal and the multiplexed IF signal, respectively, and intensity-modulates the optical signal according to the IF signal. Here, the IF modulators 181 to 18n correspond to the radio base stations 201 to 20n, respectively, and transmit only the RF signal components used in the service areas respectively covered by the corresponding radio base stations 201 to 20n. As described above, the optical signal is intensity-modulated according to the IF signal. Each of these I
The optical signals intensity-modulated by the F modulators 181 to 18n are transmitted to the respective wireless base stations 201 to 20n via the respective downstream optical fibers 301 to 30n.

The optical signal output from the center station 100 is
The light propagates through each of the downstream optical fibers 301 to 30n and is input to each of the wireless base stations 201 to 20n. In the wireless base station 20k, the opto-electric conversion unit 21k converts an input optical signal into an electric signal. By this conversion, the IF signal is a frequency-converted RF signal and an RF signal of a desired radio frequency can be obtained. This is because the external modulation section 150 and the IF modulation section 18k of the center station 100 perform double intensity modulation by the local oscillation signal and the IF signal, respectively. The converted RF signal is emitted from the antenna unit 23k to the space toward the subscriber terminal.

As described above, according to the optical transmission system according to the second embodiment of the present invention, the radio base stations 201 to 20
The IF modulators 181 to 18n are installed in the center station 100 for every n, and only the RF signal components used in the service areas covered by the radio base stations 201 to 20n are transmitted to the radio base stations 201 to 20n, respectively. I do. Therefore, as described in the first embodiment, the radio base station 20k does not use the bandpass filter unit 22k for extracting the RF signal of the desired radio frequency, and uses different frequencies for each adjacent service area. Wireless transmission. Note that optical transmission is possible even at the same frequency due to the configuration. Also, the bandpass filter unit 22k is connected to the radio base station 20k.
Is not used, it is not necessary to select and use a radio base station capable of outputting a desired radio frequency for each service area, or a radio base station is set to output a desired radio frequency for each service area. There is no need to adjust (that is, in each service area, the same configuration of the radio base station can be used). Thereby, cost reduction of the optical transmission system can be realized.

(Third Embodiment) The first and second embodiments have a feature in transmitting information (in the down direction) from the center station 100 to the subscriber terminal. The optical transmission system provided with is described. Next, in the third embodiment, the RF signal in the optical signal state obtained by frequency-converting the IF signal is used also when information is transmitted from the subscriber terminal to the center station 100 (in the upstream direction). An optical transmission system having a simplified configuration required for uplink transmission will be described.

FIG. 4 is a block diagram showing the configuration of an optical transmission system for wireless access according to the third embodiment of the present invention. 4, the optical transmission system according to the third embodiment includes a center station 100 and a plurality of wireless base stations 201 to 201.
20n are connected via a plurality of downstream optical fibers 301 to 30n and a plurality of upstream optical fibers 351 to 35n, respectively. The center station 100 includes a plurality of modulation units 110, a plurality of demodulation units 102,
, An electro-optical converter 130, a local signal source 140, an external modulator 150, an optical splitter 160, and a plurality of opto-electric converters 191 to 19n. Each radio base station 201-2
0n is the optical branching units 261 to 26n and the photoelectric conversion unit 21
1 to 21n, external modulation units 251-25n, band-pass filter units 221-222n, and circulator units 241-2
4n and antenna units 231 to 23n. Hereinafter, the operation of the optical transmission system according to the third embodiment will be described.

First, from the center station 100 to each radio base station 2
Downward transmission to 01 to 20n will be described. In the transmission in the downstream direction, after a plurality of baseband signals are input to each input terminal 1 in the center station 100,
The processing from the transmission of the optical signal through each of the downstream optical fibers 301 to 30n to the output to each of the radio base stations 201 to 20n is the same as the processing described in the first embodiment. Omitted. In the radio base station 20k,
The optical splitter 26k splits the input optical signal into two,
One of the branched optical signals is output to the photoelectric conversion unit 21k, and the other is output to the external modulation unit 25k. The photoelectric conversion unit 21k
The optical signal branched and output from the optical branching unit 26k is converted into an electric signal. By this conversion, an RF signal whose IF signal has been frequency-converted can be obtained. Then, the bandpass filter unit 22k receives the RF signal output from the photoelectric conversion unit 21k, and extracts only the RF signal of a desired radio frequency from the RF signal. The extracted RF signal passes through the circulator 24k and is emitted from the antenna 23k to the space toward the subscriber terminal.

Next, uplink transmission from each of the radio base stations 201 to 20n to the center station 100 will be described. The RF signal transmitted from the subscriber terminal is transmitted to the antenna 23k
Received at. The received RF signal is output to the external modulator 25k through the circulator 24k. As described above, in the present invention, by providing the circulator 24k between the photoelectric conversion unit 21k and the antenna 23k, the antenna 23k is shared between the upstream and downstream directions. The external modulation unit 25k is configured to output the optical signal branched and output from the optical branching unit 26k and the R signal received by the antenna unit 23k.
An F signal is input, and the intensity of the optical signal is modulated according to the RF signal. The optical signal intensity-modulated by the external modulator 25k is output to the center station 100 via the upstream optical fiber 35k. The optical signals output from the wireless base stations 201 to 20n respectively propagate through the upstream optical fibers 351 to 35n and are input to the center station 100. Center station 100
, Each of the photoelectric conversion units 191 to 19n converts an input optical signal into an electric signal. By this conversion, an IF signal obtained by frequency-converting the RF signal can be obtained. Each demodulation unit 102 is connected to each photoelectric conversion unit 1
The IF signals converted at 91 to 19n are demodulated into baseband signals, respectively, and output from the output terminal 2.

Here, in the radio base station 20k, the optical signal branched and output to the external modulator 25k is transmitted to the center station 10k.
0, an external modulation unit 150 provided at
Are intensity-modulated by the local oscillation signal generated by. Therefore, by externally modulating the intensity of the optical signal branched and output by the external modulator 25k with the RF signal received by the antenna 23k, the local signal and the RF signal are double-modulated. Become. Therefore, the radio base station 20k
By converting the optical signal propagated from the external modulation unit 25k into an electric signal by the photoelectric conversion unit 19k of the center station 100, it is possible to obtain an IF signal whose RF signal is frequency-converted. In order to avoid signal interference between the uplink transmitted from the subscriber terminal and the downlink transmitted from the center station 100, the frequency of the RF signal used in each direction needs to be different. .

As described above, according to the present invention, the optical signal for the downlink, which is already intensity-modulated by the local oscillation signal, is split into two using the optical splitter 26k installed in the radio base station 20k. One of them is used as an upstream optical signal input to the external modulator 25k. Thereby, the external modulation unit 2
At 5k, the RF signal transmitted from the subscriber terminal is I
Since the frequency is optically converted into the F signal, the local oscillation signal source 1
40 and the modulating unit 110 can be installed in the center station 100. As a result, in the present invention, each radio base station 2
The size can be reduced from 01 to 20n.

Further, the processing performed in the optical transmission system according to the third embodiment will be specifically described with reference to FIG. FIG. 5A is a diagram illustrating an example of a spectrum of a downlink signal output from the photoelectric conversion unit 21k. In this example, an IF signal 1 (frequency: f _IF1 ) is converted into a down-stream RF signal 1 (frequency: f _LO ) by an LO signal (frequency: f _LO ).
f _RF1 ). FIG.
(B) is a diagram illustrating an example of a spectrum of an uplink signal received by the antenna unit 23k. This example shows a case where an uplink RF signal 2 (frequency: f _RF2 ) having a frequency difference Δf from a downlink RF signal 1 is received by the antenna unit 23k. The external modulator 25k modulates the intensity of the optical signal transmitted from the center station 100 with the RF signal 2 so that the signals having the spectra shown in FIGS. 5A and 5B can be mixed in an optical state. Is This mixed optical signal is converted to a photoelectric conversion unit 19k.
The signal spectrum when converted into an electric signal in FIG.
(C). That is, the IF signal 2 having a frequency of f _IF2 is obtained by mixing the LO signal and the RF signal 2 in the external modulation unit 25k. Therefore, by separating only the component of the frequency f _IF2 by the bandpass filter, the IF signal 2
Only the signal 2 can be extracted. At this time, the frequency difference between IF signal 1 and IF signal 2 is Δf. In this way, the frequency of the uplink RF signal 2 is detuned from the frequency of the downlink RF signal 1 by Δf, so that the uplink signal and the downlink signal do not adversely affect each other, and are low. Can be downconverted to frequency.

As described above, according to the optical transmission system according to the third embodiment of the present invention, in addition to the effects obtained with respect to the downlink system described in the first embodiment, each wireless system is also associated with the uplink system. This has the effect of reducing the size of the base stations 201 to 20n, and facilitates installation outdoors.

In the first to third embodiments, a plurality of modulators 110 are provided to modulate an input baseband signal into a plurality of IF signals having different frequencies. However, when it is only necessary to modulate the baseband signal into a single frequency IF signal, only one modulation unit 110 needs to be provided. In this case, the IF signal output from the modulator 110 may be directly input to the electro-optical converter 130 without providing the multiplexer 120.

(Fourth Embodiment) FIG. 6 shows a fourth embodiment of the present invention.
It is a block diagram which shows the structure of the optical transmission system for radio | wireless access which concerns on embodiment. In FIG. 6, the optical transmission system according to the fourth embodiment has a configuration in which a center station 100 and a plurality of radio base stations 201 to 20n are connected via a plurality of downlink optical fibers 301 to 30n, respectively. . The center station 100 includes a plurality of radio base stations 201 to 20
n, a plurality of modulation units 111 to 11n and a plurality of electro-optical conversion units 131 to 13n,
0, a local oscillation signal source 140, an optical modulator 450, and a wavelength separator 460. Each radio base station 201-20n
Are the photoelectric conversion units 211 to 21n and the antenna unit 231
To 23n. Hereinafter, an operation of the optical transmission system according to the fourth embodiment will be described.

In the center station 100, information to be transmitted to the radio base station 20k is input to the input terminal 1k in the form of a baseband signal. The modulator 11k modulates a baseband signal input from the input terminal 1k into an IF signal having a predetermined frequency. This frequency is determined based on the frequency of a radio signal transmitted from the radio base station 20k to the subscriber terminal. The electro-optical converter 13k converts the IF signal modulated by the modulator 11k into an optical signal by a direct modulation method. Here, each electro-optical converter 131
The wavelengths of the optical signals converted by .about.13n are assigned in advance so as to be different from each other. For example, the electro-optical converters 131 to 13n are set such that the wavelengths of the optical signals are at equal intervals. The wavelength multiplexing unit 430 includes:
13n are wavelength-multiplexed. Local oscillator signal source 140 outputs a local oscillator signal having a predetermined frequency f _LO . The frequency f _LO of the local oscillation signal is determined based on the modulation frequency of the modulators 111 to 11n and the frequency of the radio signal transmitted from each of the radio base stations 201 to 20n to the subscriber terminal. Light modulator 45
Numeral 0 inputs the optical signal wavelength-multiplexed by the wavelength multiplexing unit 430 and the local oscillation signal output from the local oscillation signal source 140, and collectively modulates the intensity of the optical signal wavelength-multiplexed by the local oscillation signal. By this intensity modulation, an optical signal in which the IF component has been optically converted into the RF component can be obtained. The wavelength separation unit 460 separates the optical signal intensity-modulated by the optical modulation unit 450 into a plurality of optical signals according to the wavelength, and separates the corresponding optical signal via each of the down-link optical fibers 301 to 30n into each radio base station. It transmits to the stations 201 to 20n, respectively.

The optical signal transmitted from the center station 100 is
Propagating through the respective downstream optical fibers 301 to 30n,
It is input to each of the radio base stations 201 to 20n. In the wireless base station 20k, the photoelectric conversion unit 21k converts an input optical signal into an electric signal and outputs an RF signal. By this conversion, an RF signal whose IF signal has been frequency-converted can be obtained. After only the desired frequency component is extracted from the output RF signal, the RF signal is emitted as a wireless signal from the antenna unit 23k to the subscriber terminal into space.

As described above, according to the optical transmission system according to the fourth embodiment of the present invention, IF signals having close or the same frequency are converted into light intensity modulated signals having different wavelengths. After each of the light intensity modulation signals is wavelength-multiplexed, the light intensity modulation signals are collectively intensity-modulated with a local oscillation signal by an external modulation method. Therefore, since the frequency conversion from the plurality of IF signals to the RF signals is performed optically collectively, the electrical frequency conversion unit is not required, and the optical modulation unit for performing the optical frequency conversion is provided by the plurality of wireless base stations. Since they can be shared, low-cost frequency conversion can be realized. In addition, since the optical signals that have been externally intensity-modulated collectively are separated according to the wavelength and then optically transmitted to each wireless base station, even when the frequencies of a plurality of RF signals are the same, they do not interfere with each other. Can be transmitted. In addition, since an optical signal can be easily separated in an optical wavelength region, a low-cost optical transmission system for wireless access can be provided.

In the fourth embodiment, the configuration in which the wavelength separation unit 460 is installed in the center station 100 has been described, but this is not always necessary. For example, the wavelength separation unit 46
0 may be installed separately as an independent relay station, or may be installed in each of the radio base stations 201 to 20n. However, in the latter case, it is necessary to newly provide a configuration in the center station 100 for branching the optical signal output from the optical modulation section 450 and outputting it to each of the radio base stations 201 to 20n. In the fourth embodiment, the electro-optical converter 1
Although the case where the IF signals input to 31 to 13n are one channel has been described, the same effects can be obtained even when the multi-channel IF signals are input. In this case, it is preferable that the IF signals of the respective channels are multiplexed by frequency division multiplexing and then input to the electro-optical converters 131 to 13n.

Further, in the fourth embodiment, since the intensity modulation is performed twice, the intensity modulation of the wavelength-multiplexed IF signal (the electro-optical converters 131 to 13n) is applied to the relatively low frequency signal. However, it is possible to use an external modulator that operates up to a high frequency for the intensity modulation of the local oscillation signal (optical modulation section 450). Become. In addition, since the frequency of the local oscillation signal is relatively high, it is possible to use an external modulator matched in a specific high frequency band for modulation for the intensity modulation of the local oscillation signal. Also, for the intensity modulation of the local oscillation signal,
It is also possible to use an external modulator of the Mach-Zehnder type. When this Mach-Zehnder type external modulator is used, optical frequency conversion is performed at twice the frequency of the local oscillation signal by setting the bias point to the point where the optical output becomes maximum or minimum. It becomes possible. This method of setting the bias point cannot be used for intensity modulation of a wavelength-multiplexed IF signal because the second-order distortion component becomes large, but since the local oscillation signal is only one carrier, it is not used for the intensity modulation of the local oscillation signal. Can be used. Further, in this method, the frequency of the local oscillation signal used for the intensity modulation may be half the frequency of the ordinary frequency conversion, and therefore, low-cost local oscillation signal source 140 and optical modulation section 450 are used. can do.

When it is necessary to use an optical amplifier to compensate for a branch loss or a transmission loss, the electro-optical converter 1
The wavelength of the light source used for 31 to 13n is 1.5
A μm wavelength is used. At this time, if the already laid optical fiber is the SMF and the DSB modulation is used as the modulation method performed by the optical modulator 450, and the high frequency signal in the microwave band or the millimeter wave band is optically transmitted by the SMF, the dispersion of the SMF is reduced. , The power of the high-frequency signal after optical transmission may greatly decrease depending on the transmission distance. In order to avoid the influence of this dispersion, optical SSB modulation including a carrier and a one-sideband component may be used as the modulation method performed by the optical modulator 450. Further, a Mach-Zehnder type external modulator may be used as the light modulator 450, and a point at which the light output becomes minimum or maximum may be set as a bias point to perform external modulation.

(Fifth Embodiment) In the optical transmission system according to the fourth embodiment, the electro-optical converters 131 to 13n
In the above, each IF signal is converted into an optical signal by a direct modulation method. Therefore, when the multi-channel IF signal is subjected to electro-optical conversion, wavelength chirp is generated. For this reason, when there is chromatic dispersion in the downstream optical fibers 301 to 30n, chromatic dispersion distortion occurs and transmission characteristics deteriorate. Therefore, in the fifth embodiment, an optical transmission system in which wavelength chirp does not occur will be described.

FIG. 7 is a block diagram showing a configuration of an optical transmission system for wireless access according to a fifth embodiment of the present invention. In FIG. 7, the optical transmission system according to the fifth embodiment includes a center station 100 and a plurality of wireless base stations 201 to 201.
20n are connected via a plurality of downstream optical fibers 301 to 30n. Center station 100
Are a plurality of modulators 111 to 11n corresponding to a plurality of wireless base stations 201 to 20n, respectively, and a plurality of light sources 171-1.
7n and a plurality of IF modulation sections 481 to 48n, a wavelength multiplexing section 430, a local oscillation signal source 140, an optical modulation section 450,
A wavelength separation unit 460. Each radio base station 201-2
0n indicates the photoelectric conversion units 211 to 21n and the antenna unit 2
31 to 23n. In the optical transmission system according to the fifth embodiment, the same components as those in the optical transmission system according to the fourth embodiment are denoted by the same reference numerals, and description thereof will be omitted. Hereinafter, operations of different components in the optical transmission system according to the fifth embodiment will be mainly described.

The light sources 171 to 17n output optical signals having different wavelengths. For example, the light sources 171 to 17n
Outputs optical signals such that the wavelengths are equally spaced. The IF modulator 48k inputs the IF signal modulated by the modulator 11k and the optical signal output from the light source 17k, respectively, and intensity-modulates the optical signal with the IF signal. These intensity-modulated optical signals are wavelength-multiplexed in the wavelength multiplexing unit 430, and then subjected to the above-described processing and optically transmitted to each of the radio base stations 201 to 20n.

As described above, in the optical transmission system according to the fifth embodiment shown in FIG. 7, the light sources 171 to 17n and the IF
Since the IF signal is converted into the optical signal by the external modulation method using the modulation units 481 to 48n, the wavelength chirp does not occur. Therefore, even when there is chromatic dispersion of the downstream optical fibers 301 to 30n, transmission can be performed without deteriorating the transmission characteristics. The configuration as in the fifth embodiment is particularly useful when the characteristics cannot be changed on the optical fiber side, for example, when a new system is constructed using the already laid optical fibers.

(Sixth Embodiment) The fourth and fifth embodiments have a feature in transmitting information (in the down direction) from the center station 100 to the subscriber terminal. The optical transmission system provided with is described. Next, in the sixth embodiment, the RF signal in the optical signal state obtained by frequency-converting the IF signal is also used when information is transmitted from the subscriber terminal to the center station 100 (in the upstream direction). An optical transmission system having a simplified configuration required for uplink transmission will be described.

FIG. 8 is a block diagram showing a configuration of an optical transmission system for wireless access according to a sixth embodiment of the present invention. 8, the optical transmission system according to the sixth embodiment includes a center station 100 and a plurality of wireless base stations 201 to 201.
20n is connected via a plurality of downstream optical fibers 301 to 30n and a plurality of upstream optical fibers 351 to 35n. The center station 100 includes a plurality of modulators 111 to 11n and a plurality of electro-optical converters 131 to 13 corresponding to the plurality of wireless base stations 201 to 20n, respectively.
n, a plurality of upstream light sources 471 to 47n, a plurality of photoelectric conversion units 421 to 42n, and a plurality of demodulation units 411 to 41n
, A wavelength multiplexing unit 431, a local signal source 140, an optical modulation unit 450, and a wavelength separation unit 460. Each of the radio base stations 201 to 20n includes two wavelength demultiplexers 271 to 27n,
RF modulation units 281 to 28n and photoelectric conversion units 211 to 2
1n, circulators 241 to 24n, and antennas 231 to 23n. The sixth
In the optical transmission system according to the fourth embodiment, the same components as those of the optical transmission system according to the fourth embodiment are denoted by the same reference numerals, and description thereof will be omitted. Hereinafter, operations of different components in the optical transmission system according to the sixth embodiment will be mainly described.

First, from the center station 100 to each radio base station 2
Downward transmission to 01 to 20n will be described. The light sources for uplink 471 to 47n are connected to the respective radio base stations 201 to 20n.
n to output optical signals to the center station 100. Here, each of the upstream light sources 471 to 47n is set so that the wavelength of the optical signal to be output is different from each other, and is also different from the wavelength previously assigned to each of the electro-optical converters 131 to 13n. The wavelength multiplexing unit 431 includes the electro-optical conversion units 131 to 13n.
And wavelength-multiplexed optical signals of different wavelengths respectively output from the upstream light sources 471 to 47n. The optical modulator 450 is configured to combine the optical signal wavelength-multiplexed by the wavelength multiplexer 431 with the local oscillation signal source 14.
A local signal output from 0 is input, and the optical signals wavelength-multiplexed by the local signal are intensity-modulated collectively. The wavelength separation unit 460 separates the optical signal intensity-modulated by the optical modulation unit 450 into a plurality of optical signals according to the wavelength, and separates the corresponding optical signal via each of the down-link optical fibers 301 to 30n into each radio base station. It transmits to the stations 201 to 20n, respectively. That is, the wavelength separation unit 460 converts the optical signal output from the electro-optical conversion unit 13k and the optical signal output from the upstream light source 47k via the downstream optical fiber 30k into the radio base station 2.
Transmit to 0k.

In the radio base station 20k, the two-wavelength separation unit 27k separates the wavelength of the optical signal transmitted from the center station 100, and transmits the optical signal output from the electro-optical conversion unit 13k to the photoelectric conversion unit 21k. The optical signal output from the light source 47k is output to the RF modulator 28k. Photoelectric conversion unit 2
1k converts the optical signal wavelength-separated by the two-wavelength separation unit 27k into an electric signal. By this conversion, an RF signal whose IF signal has been frequency-converted can be obtained. This RF signal passes through the circulator section 24k and passes through the antenna section 2
3k is released into the space toward the subscriber terminal.

Next, uplink transmission from each of the radio base stations 201 to 20n to the center station 100 will be described. The RF signal transmitted from the subscriber terminal is transmitted to the antenna 23k
Received at. The received RF signal is output to the RF modulator 28k through the circulator 24k. As described above, in the present invention, by providing the circulator 24k between the photoelectric conversion unit 21k and the antenna 23k, the antenna 23k is shared between the upstream and downstream directions. The RF modulator 28k inputs the optical signal wavelength-separated by the two-wavelength separator 27k and the RF signal received by the antenna 23k, and modulates the intensity of the optical signal by the RF signal. The optical signal intensity-modulated by the RF modulation unit 28k is
The signal is output to the center station 100 via the upstream optical fiber 35k.

The optical signal output from the radio base station 20k propagates through the upstream optical fiber 35k,
Is input to In the center station 100, the photoelectric conversion unit 42k converts an input optical signal into an electrical signal. By this conversion, an IF signal obtained by frequency-converting the RF signal can be obtained. Then, the demodulation unit 41k demodulates the IF signal converted by the photoelectric conversion unit 42k into a baseband signal and outputs the demodulated IF signal from the output terminal 4k.

Here, at the radio base station 20k, the RF
The optical signal of the upstream light source 47k output to the modulator 28k is intensity-modulated by the optical modulator 450 provided in the center station 100 by the local signal generated by the local signal source 140. Therefore, in the RF modulating unit 28k, the intensity of the wavelength-separated optical signal is modulated by the received RF signal, so that the local oscillation signal and the RF signal are double-modulated. Therefore, the radio base station 20k
The optical signal propagated from the RF modulation unit 28k is received and detected by the photoelectric conversion unit 42k of the center station 100, so that mixing between the two signals occurs, and the RF signal is
The frequency is converted to an F signal. As described above, in the sixth embodiment, in order to optically transmit the wireless signal received by the antenna unit 23k, the optical signal output from the upstream light source 47k is converted into the optical signal output from the electro-optical conversion unit 13k. After being intensity-modulated by the light modulation unit 450 together with the signal, the configuration is used for intensity modulation by the RF modulation unit 28k. Thereby, since the RF signal is frequency-converted into the IF signal in the photoelectric conversion unit 42k, there is an advantage that the number of devices for high-frequency signal processing can be reduced.

Next, with reference to FIG. 9, the wavelength separation operation performed by the wavelength separation unit 460 of the center station 100 will be described with a specific example. As the wavelength separation unit 460, for example, an n-output wavelength separator that separates n optical signals multiplexed at equal wavelength intervals can be used. For example, the n-output wavelength separator includes a magazine “O” published in November 1997.
plus E, an arrayed waveguide grating (AWG) introduced in "Wavelength multiplexing optical semiconductor component" (by Yoshikuni et al.) can be used. N with AWG configuration
The output wavelength separator has a periodic pass wavelength band when viewed from one output terminal side. Therefore, when such an n-output wavelength separator is used, as shown in FIG. 9A, the wavelength λdk of the optical signal output from the electro-optical converter 13k.
And the wavelength λu of the optical signal output from the upstream light source 47k.
k is previously adjusted so as to match the periodic passing wavelength band of the corresponding output terminal. As a result, optical signals of two different wavelengths λdk and λuk output from the electro-optical converter 13k and the upstream light source 47k can be taken out from the same output terminal together.

On the other hand, instead of the above-described n-output wavelength separator having a periodic passing wavelength band, a separator having a passing wavelength band having a constant width is used as the wavelength separating section 460 when viewed from one output terminal side. You can also. When such a separator is used, as shown in FIG. 9B, the wavelength λdk of the optical signal output from the electro-optical converter 13k and the wavelength λuk of the optical signal output from the upstream light source 47k are determined. ,
It is adjusted in advance so as to be close to each other within the transmission wavelength bandwidth of the output terminal. As a result, optical signals of two different wavelengths λdk and λuk output from the electro-optical converter 13k and the upstream light source 47k can be taken out from the same output terminal together.

Next, another configuration used for each of the radio base stations 201 to 20n will be described with reference to FIGS. FIG. 10 illustrates a two-wavelength separation unit 27k, a photoelectric conversion unit 21k, and a circulator unit 2 of the wireless base station 20k illustrated in FIG.
4k and an RF modulator 28k are connected to an electro-absorption modulator 29k.
FIG. 10 is a block diagram showing a configuration of a wireless base station 20k in which the configuration shown in FIG. Since the electro-absorption modulator 29k is a device having both the photoelectric conversion function and the electro-optical conversion function, FIG.
As described above, the configuration of the wireless base station 20k can be simplified. The electro-absorption modulator 29k is described in, for example, a document “IEEE Tran
“Full-Duplex Fiber-Optic RF Subcarrier Transm” published in s. Microwave Theory Tech. Vol. 47, No. 7
ission Using a Dual-Function Modulator / Demodulato
r "(Andreas Sto" hr et al.). The photoelectric conversion efficiency and the electro-optical conversion efficiency of the electro-absorption modulator 29k have wavelength dependence as shown by the dotted line in FIG. Therefore, the electro-optical converters 131 to 13n
The appropriate wavelength setting is performed so that the optical signals to be subjected to opto-electric conversion are output from the optical sources, and the optical signals to be subjected to electro-optical conversion are output from the upstream light sources 471 to 47n on the long wavelength side. By doing so (FIG. 11), the electro-absorption modulator 29k can be effectively used.

As described above, according to the optical transmission system according to the sixth embodiment of the present invention, in order to transmit a radio signal received by a radio base station to a center station, a plurality of unmodulated optical signals having different wavelengths are transmitted. Is wavelength-multiplexed into a downstream optical signal and externally modulated in advance with a local oscillation signal, whereby the radio signal can be frequency-converted into an IF signal as it is. Therefore, an electrical frequency converter is not required in the wireless base station, and an optical modulator for optically performing frequency conversion can be shared by a plurality of wireless base stations. In addition, there is no need to install a light source in the wireless base station, which facilitates maintenance. Further, by using an electro-absorption modulator for light reception and light modulation,
The configuration in the wireless base station can be simplified.

(Seventh Embodiment) As described above, in the center station 100 of the optical transmission system according to the present invention, an input IF signal is directly modulated in an electro-optical converter (for example, a semiconductor laser) by a direct modulation method. The optical signal is converted into an optical signal, and the optical signal is intensity-modulated again by the local modulation signal in the external modulator.

Here, cos (ωt) is used for the IF signal, sin (ω0t) is used for the optical signal, and the optical modulation degree at the time of direct modulation is m.
If the optical frequency modulation index by the IF signal is β1 and the optical phase modulation index at the time of external modulation is βLO, a double-modulated optical signal (electric field expression: E (t)) is expressed by the following equation (1). Given. E (t) = √ [[1 + J1 (βLO) cos (ωLOt)] [1 + mcos (ω (t−τ1))]] * sin [ω0t + β1sin (ωt)] = √ [1 + mcos (ω (t−τ1)) + J1 (βLO) cos (ωLOt) + mJ1 (βLO) cos (ωLOt) cos (ω (t−τ1))] * sin [ω0t + β1sin (ωt)] (1) From the above equation (1), the double-modulated light is obtained. The signal E (t) is
It can be seen that there is a light intensity modulation component obtained by frequency-modulating the modulation frequency (ω) by the electro-optical conversion unit by the modulation frequency (ωLO) by the external modulation unit.

Generally, an electro-optical converter such as a semiconductor laser has lower distortion characteristics than an external modulator, but has a relatively narrow frequency band. Conversely, the external modulator has wideband characteristics but poor distortion characteristics. Therefore, by adopting a configuration in which the electro-optic conversion unit and the external modulation unit are connected in cascade, the low distortion characteristics of the electro-optic conversion unit and the wide band characteristics of the external modulation unit can be utilized, and low-distortion transmission of high-frequency signals can be achieved. Can be realized.

However, when a configuration in which the electro-optical conversion unit and the external modulation unit are simply connected in cascade is used, the optical signal output from the direct modulation light source includes the optical frequency modulation component (wavelength chirp) and the wavelength. Waveform distortion occurs due to the effect of dispersibility. In particular, at the time of long-distance transmission, there is a problem that the transmission characteristics of the optical signal are greatly deteriorated. Therefore, in the seventh embodiment, a high-frequency optical transmitter applicable to the center station 100 that suppresses the optical frequency modulation component of the directly modulated optical signal and does not deteriorate the transmission characteristics even during long-distance transmission will be described. .

FIG. 12 is a block diagram showing a configuration of a high-frequency optical transmitter according to a seventh embodiment of the present invention. FIG.
In the high frequency optical transmitter according to the seventh embodiment,
A branching unit 510 and first to fourth delay control units 521 to 52
4, first and second multiplexing units 531 and 532, an electro-optical conversion unit 540, a differential light intensity modulation unit 550, a local oscillation signal source 560, and a two-branch unit 570. In the center station 100 according to the first and third embodiments, the high-frequency optical transmitter includes the electro-optical converter 130 and the local signal source 14.
0 and can be applied in place of the configuration of the external modulation unit 150. In the center station 100 of the second embodiment, the light source 17
0, local oscillation signal source 140, external modulator 150, optical splitter 1
60 and the corresponding IF modulator 18k can be applied instead. In the center station 100 according to the fourth to sixth embodiments, a signal obtained by multiplexing the IF signals output from the plurality of modulation units 111 to 11n is input to the IF input terminal 51, and the wavelength multiplexing unit 430 or 431 is The present invention can be applied by inputting an optical signal to be output to the differential light intensity modulator 550 as an optical signal output from the electro-optical converter 540, and inputting an output signal of the output terminal 54 to the wavelength separator 460. Hereinafter, the operation of the high-frequency optical transmitter according to the seventh embodiment of the present invention will be described.

The IF signal of the intermediate frequency input from the IF input terminal 51 is branched into first to third IF signals in a three-branch unit 510. The first and second IF signals are:
First and second delay control units 521 and 522, respectively
Then, the third IF signal is input to the electro-optical converter 540. Here, the phases of the first and second IF signals are set so as to be in phase with each other and differ from the third IF signal by 90 °. The third IF signal is converted into an optical signal by direct modulation in the electro-optical converter 540 and output. At this time, the directly modulated optical signal output from the electro-optical converter 540 is subjected to optical frequency modulation together with optical intensity modulation, and its electric field expression: ELD (t) converts the IF signal to cos (ωt).
, The optical signal sin (ω0t), the optical modulation degree m, and I
If the frequency modulation index by the F signal is β1, it is given by the following equation (2). ELD (t) = √ [1 + mcos (ωt)] sin [ω0t + β1sin (ωt)] (2)

On the other hand, after the first delay signal is adjusted to a predetermined value by the first delay control section 521, the first IF signal
The signal is output to one terminal of the first multiplexing unit 531. Similarly,
After the propagation delay amount is adjusted to a predetermined value by the second delay control unit 522, the second IF signal
Is output to one terminal. The local oscillation signal output from the local oscillation signal source 560 has a phase of 180
° It is branched into different first and second local signals. After the propagation delay amount is adjusted by the third delay control section 523 so that the propagation time of the first local oscillation signal becomes equal to that of the first IF signal, the other terminal of the first multiplexing section 531 Output to Further, after the propagation delay amount of the second local oscillation signal is adjusted by the fourth delay control unit 524 so that the propagation time is equal to that of the second IF signal, the second multiplexing unit 532
Is output to the other terminal. Then, the first multiplexing unit 531
Multiplexes the first IF signal and the first local oscillation signal, and the second multiplexing unit 532 multiplexes the second IF signal and the second local oscillation signal to obtain a difference. The light is output to the dynamic light intensity modulator 550.

The differential light intensity modulator 550 is a Mach-Zehnder type modulator having two optical waveguides, as shown in FIG. This is a configuration in which a voltage signal is applied to change the refractive index of each optical waveguide to give a difference in light propagation time, and then these are combined. At this time, the difference between the propagation times of the light passing through the two optical waveguides is converted into an optical phase, and π
/ 2, while applying a bias voltage to each electrode, and applying the first and second local oscillation signals to each electrode in opposite phases.

Here, the local oscillation signal is represented by cos (ωLOt),
Assuming that the optical phase modulation index of the first and second local signals in the differential light intensity modulator 550 is βLO, the optical signal output from the differential light intensity modulator 550 when no IF signal is input. (Electric field expression: EEMi (t)) is represented by the following equation (3). EEMi (t) = √ [[1 + J1 (2βLO) cos (ωLOt)] [1 + mcos (ω (t−τ1))]] * sin [ω0t + β1sin (ωt)] = √ [1 + mcos (ω (t−τ1)) + J1 (2βLO) cos (ωLOt) + mJ1 (2βLO) cos (ωLOt) cos (ω (t−τ1))] * sin [ω0t + β1sin (ωt)] (3) From the above equation (3), differential light intensity modulation The optical signal output from the unit 550 has a light intensity modulation component obtained by frequency-converting the modulation frequency (ω) of the electro-optical conversion unit 540 by the modulation frequency (ωLO) of the differential light intensity modulation unit 550. You can see that there is.

Next, a case is considered where only the IF signal is input to the differential light intensity modulator 550. In this case, the first and second IF signals are applied to the respective electrodes of the differential light intensity modulator 550 in the same phase. Here, the third IF signal is 3
The time from the output of the branching unit 510 to be converted to an optical signal by the electro-optical conversion unit 540 and then propagated to the differential type light intensity modulation unit 550 is τ1, and the first and second electric signals are converted to the three branching unit 510. From the output of the differential light intensity modulator 550 until the optical signal is modulated is τ2, and the optical phase modulation index by the first and second IF signals in the differential light intensity modulator 550 is Assuming that both are β2, the optical signal (electric field expression: EEMp (t)) output from the differential light intensity modulator 550 when the local oscillation signal is not input is expressed by the following equation (4).
It is represented by EEMp (t) = √ [1 + mcos (ω (t−τ1))] sin [ω0t + β1sin (ω (t−τ1)) + β2cos [ω (t−τ2) + π / 2]] (4)

Here, the first and second delay controllers 521
In 522 and 522, when the delay amount is adjusted so that τ2 = τ1, the above equation (4) becomes the following equation (5). EEMp (t) = √ [1 + mcos (ω (t−τ1))] sin [ω0t + (β1−β2) * sin (ω (t−τ1))] (5) As can be seen from the above equation (5), The optical frequency modulation index β1 generated by the direct modulation can be reduced to β1−β2 by the double modulation by the differential light intensity modulator 550. Also, by setting the phase modulation index β2 equal to β1, the frequency modulation component of the optical signal can be completely removed.

From the above, the differential light intensity modulating section 55
When both the local oscillation signal and the IF signal are input to 0, the optical signal output from the output terminal 54 (electric field expression: EEM (t))
Is given by the following equation (6) when β2 = β1. EEM (t) = √ [[1 + J1 (2βLO) cos (ωLOt)] [1 + mcos (ω (t−τ1))]] * sin (ω0t) = √ [1 + mcos (ω (t−τ1)) + J1 (2βLO) cos (ωLOt) + mJ1 (2βLO) cos (ωLOt) cos (ω (t−τ1))] sin (ω0t) (6) From the above equation (6), the light output from the differential light intensity modulator 550. From the signal, the optical frequency modulation component generated in the electro-optical converter 540 is removed, and
The modulation frequency (ω) by the differential light intensity modulator 5
It can be seen that there is a light intensity modulation component frequency-converted by the modulation frequency (ωLO) by 50.

FIG. 14 shows an example in which an optical frequency modulation component generated by direct modulation is actually suppressed by external modulation. FIG. 14A shows an optical spectrum accompanied by an optical frequency modulation component by direct modulation, and FIG. 14B shows an optical spectrum in which the optical frequency modulation component is canceled by external modulation. However, in FIG. 14, the local oscillation signal is not input to the differential light intensity modulator 550. FIG. 14 (a) and (b)
From this, it can be confirmed that the optical frequency modulation component can be suppressed by using the high-frequency optical transmitter according to the seventh embodiment.

In the above description, the two local oscillator input terminals 52
And 53 show examples in which the same local signal having a phase difference of 180 ° was input. At this time, the optical spectrum is as shown in FIG.
As shown in (a), it becomes an optical double sideband (DSB) signal having upper and lower sidebands. In general, an optical fiber has a wavelength dispersion property that the group velocity varies depending on the optical wavelength (optical frequency). For this reason, when such an optical DSB signal is transmitted, the group velocities of the upper band and the lower band do not match, and the upper band and the optical carrier, and the lower band and the optical carrier are subjected to square detection by the optical receiver. Each electric signal component obtained as a beat component has a phase difference, and particularly during long-distance transmission, there is a possibility that both electric signals have opposite phases and cancel each other.

As a method for avoiding this phenomenon, FIG.
The optical single sideband (SS) having only one sideband shown in FIG.
B) There is a modulation scheme or an optical double-sideband (DSB-SC) modulation scheme in which the optical carrier is suppressed as shown in FIG. In the configuration of the seventh embodiment described above, the phase difference is 90 °
The first and second local oscillation input terminals 52 and 53
, Optical SSB modulation can be easily realized. Also, a bias voltage is applied so that the propagation time difference of light passing through the two optical waveguides of the differential type light intensity modulator 550 becomes π in terms of an optical phase, and a local signal having a phase difference of 180 ° is applied. Is input to the first and second local oscillation input terminals 52 and 53, so that optical DSB-SC modulation can be easily realized. In the case of the optical DSB modulation and the optical DSB-SC modulation, of the two terminals of the differential light intensity modulation unit 550,
The same effect can be obtained even if the local oscillation signal is input to only one side.

As described above, according to the high-frequency optical transmitter according to the seventh embodiment of the present invention, the differential light intensity modulator 55
By optically modulating 0, an optical frequency modulation component generated at the time of direct modulation by an IF signal is canceled, and at the same time, an electric signal can be frequency-converted into a high-frequency signal by an optical intensity modulation operation by a local signal. . This allows the differential type light intensity modulation unit 550 to have two functions of optical frequency conversion and cancellation of frequency modulation. , Good transmission characteristics can be obtained.

(Eighth Embodiment) FIG. 16 is a block diagram showing a configuration of a high-frequency optical transmitter according to an eighth embodiment of the present invention. In FIG. 16, the high-frequency optical transmitter according to the eighth embodiment includes a two-branch unit 575 and an electro-optical converter 54.
0, a delay control unit 525, a local oscillation signal source 560, and a phase modulation unit 581 and an intensity modulation unit 582 included in the integrated modulation unit 580. Hereinafter, the operation of the high-frequency optical transmitter according to the eighth embodiment of the present invention will be described.

The IF signal input from the IF input terminal 51 is divided into two in a bifurcation section 575 into first and second IF signals having a phase difference of 90 ° from each other. The first I
The F signal is sent to the electro-optical converter 540, and the second IF signal is
This is input to the delay control unit 525. The first IF signal is converted into an optical signal by direct modulation in the electro-optical converter 540 and output to the phase modulator 581. The second IF signal is input to the optical waveguide of the phase modulation unit 581 via the delay control unit 525. The time τ3 during which the first IF signal is converted from the output of the two-branch unit 575 to an optical signal by the electro-optical conversion unit 540 and propagates to the phase modulation unit 581, and the second IF signal is The time τ4 from the output until the optical signal is modulated by the phase modulator 581 is
By making them equal, it is possible to reduce optical frequency modulation components generated at the time of direct modulation.

Generally, the amount of transmission delay is obtained by measuring the time difference between the input signal level and the received signal level. However, when the electric signal is phase-modulated by the phase modulation section 581, the electric signal component cannot be obtained even if the light signal is converted into the electric signal on the light receiving side, so that the transmission delay amount cannot be measured. Therefore, using an integrated modulator 580 in which the phase modulator 581 and the intensity modulator 582 are integrated,
The IF signal is input to the intensity modulator 582, and the transmission delay τ4 ′ when the IF signal is transmitted by intensity modulation is measured. Based on this result, first, the delay amount of the delay control unit 525 is roughly adjusted so that τ3 = τ4 ′. After that, the IF signal input to the intensity modulation section 582 is input again to the phase modulation section 581 to which the IF signal should be input, and the electro-optical conversion section 540
The optical spectrum after the direct modulation by the optical modulator and the phase modulation by the phase modulation unit 581 are measured by an optical heterodyne method or the like, and the delay amount of the delay control unit 525 is set so that the optical frequency modulation component is minimized. It may be adjusted precisely.

The advantage over the seventh embodiment is that the local oscillation signal is transmitted through the intensity modulation section 5 without passing through the multiplexing section or the like.
Since the voltage can be directly applied to the signal 82, there is an advantage that the loss for the local signal can be reduced.

As described above, according to the high-frequency optical transmitter according to the eighth embodiment of the present invention, the phase modulation section 581 and the intensity modulation section 582 are integrated in the integrated modulation section 580, so that the optical frequency modulation is performed. Delay adjustment for canceling out components can be easily performed. Also, an optical phase modulation operation for directly inputting to each of the phase modulation unit and the intensity modulation unit to suppress an optical frequency component generated at the time of direct modulation without multiplexing the IF signal and the local oscillation signal, By performing the light intensity modulation operation for conversion, the loss for each signal can be reduced, and light modulation can be performed more efficiently.

In the seventh and eighth embodiments,
The case where the high-frequency optical transmitter is applied to the configuration for transmitting information in the down direction from the center station 100 to the subscriber terminal has been described. Alternatively, a high-frequency optical transmitter can be similarly applied.

[Brief description of the drawings]

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

FIG. 2 is a block diagram showing a configuration when a subscriber terminal is added in the optical transmission system according to the first embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of an optical transmission system according to a second embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of an optical transmission system according to a third embodiment of the present invention.

FIG. 5 is a diagram showing an example of a signal spectrum processed in an optical transmission system according to a third embodiment of the present invention.

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

FIG. 7 is a block diagram showing a configuration of an optical transmission system according to a fifth embodiment of the present invention.

FIG. 8 is a block diagram showing a configuration of an optical transmission system according to a sixth embodiment of the present invention.

FIG. 9 is a diagram illustrating an example of a wavelength separation operation performed by a wavelength separation unit 460.

FIG. 10 is a block diagram showing another configuration example of the wireless base stations 201 to 20n of FIG. 8;

11 is a diagram showing an example of photoelectric conversion efficiency and electro-optical conversion efficiency obtained by the electro-absorption modulator 29k of FIG.

FIG. 12 is a block diagram showing a configuration of a high-frequency optical transmitter according to a seventh embodiment of the present invention.

FIG. 13 is a block diagram showing a specific configuration of a differential light intensity modulator 550 in FIG. 12;

FIG. 14 is a diagram showing an example of an optical spectrum measured using the high-frequency optical transmitter according to the seventh embodiment of the present invention.

FIG. 15 is a diagram illustrating a difference in an optical spectrum when a different external modulation scheme is used in the high-frequency optical transmitter according to the seventh embodiment of the present invention.

FIG. 16 is a block diagram showing a configuration of a high-frequency optical transmitter according to an eighth embodiment of the present invention.

FIG. 17 is a block diagram showing a configuration of a conventional optical transmission system.

FIG. 18 is a diagram showing the concept of service areas 901 to 907 covered by a plurality of radio base stations 701 to 707.

FIG. 19 is a block diagram showing a configuration of another conventional optical transmission system.

[Explanation of symbols]

1, 11-1n, 51-53, 61-6n ... input terminals 2, 41-4n, 54 ... output terminals 100, 600 ... center stations 101, 110-11n, 621-62n, 721-7
2n modulator 102, 411-41n demodulator 120-12n multiplexer 130-13n, 540, 611-61n electro-optical converter 140, 560, 641-64n, 741-74n local signal source 150 , 251 to 25n, 651 to 65n ... external modulators 160, 261 to 26n ... optical branching units 170 to 17n, 471 to 47n, 661 to 66n ... light sources 181 to 18n, 481 to 48n ... IF modulators 191 to 19n, 211 ~ 21n, 421-42n, 7
11 to 71n: photoelectric conversion units 201 to 20n, 701 to 70n: wireless base stations 221 to 22n: band filter units 231 to 23n, 751 to 75n: antenna units 241 to 24n: circulator units 271 to 27n: two-wavelength separation units 281 to 28n RF modulation section 29k Electroabsorption modulator 301 to 30n, 351 to 35n, 801 to 80n Optical fiber 430, 431 Wavelength multiplexing section 450 Optical modulation section 460 Wavelength separation section 510 Three branching section 521 to 525 delay control units 531 and 532 multiplexing unit 550 differential light intensity modulation units 570 and 575 two-branch units 580 integrated modulation units 581 phase modulation units 582 intensity modulation units 631 to 63n and 731 ... 73n ... frequency conversion parts 901-907 ... service area

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/06 10/142 10/152 10/28 H04J 14/02 F term (Reference) 2H079 AA02 AA12 AA13 BA01 CA04 EA05 EB04 5K002 AA01 BA02 CA14 DA02 DA08 FA01

Claims (24)

[Claims] 1. A plurality of radio stations each of which is used for radio access for transmitting information between a center station and a subscriber terminal via a radio base station which transmits and receives radio signals from an antenna unit and covers different service areas. An optical transmission system in which a base station and a center station are respectively connected by a plurality of optical fibers to optically transmit a radio signal bidirectionally, wherein the center station has one or more baseband signals. An electro-optical converter that is input as one or more modulated electric signals having a predetermined intermediate frequency and converts the electric signal into an optical signal by intensity modulation; and a local oscillation signal that outputs a predetermined local oscillation signal. A source, an external modulator that intensity-modulates the optical signal converted by the electro-optical converter with a local oscillation signal output from the local oscillation signal source, and branches the optical signal intensity-modulated by the external modulator. And a plurality of optical base stations, each of which outputs an optical signal to a corresponding one of the plurality of optical fibers, wherein each of the plurality of radio base stations converts an optical signal transmitted through the optical fiber into an electric signal in a radio frequency band. An optical transmission system, comprising: at least an electric conversion unit; and a band filter unit that extracts only an electric signal of a desired frequency band from the electric signal converted by the photoelectric conversion unit and supplies the extracted electric signal to the antenna unit. 2. A plurality of radio stations each of which is used for radio access for transmitting information between a center station and a subscriber terminal via a radio base station which transmits and receives radio signals from an antenna unit, and covers different service areas. An optical transmission system that connects a base station and a center station with a plurality of optical fibers, respectively, and optically transmits a wireless signal in two directions, wherein the center station outputs a predetermined light source, A local oscillation signal source that outputs a predetermined local oscillation signal, an external modulation unit that intensity-modulates light output from the light source with a local oscillation signal output from the local oscillation signal source, and the external modulation unit. An optical branching unit for branching the intensity-modulated optical signal into a plurality of radio base stations; and one or more modulated electric signals each having one or more baseband signals each having a predetermined intermediate frequency. As transmission Input for each of the radio base stations to be
At least a plurality of IF modulators each of which intensity-modulates the optical signal branched by the optical branching unit with the electric signal, and outputs the modulated optical signal to a plurality of optical fibers. Are optical transmission systems, each including at least a photoelectric conversion unit for converting an optical signal transmitted via the optical fiber into an electric signal in a radio frequency band and supplying the electric signal to the antenna unit. 3. A plurality of radio stations each of which is used for radio access for transmitting information between a center station and a subscriber terminal via a radio base station which transmits and receives radio signals from an antenna unit, and covers different service areas. An optical transmission system in which a base station and a center station are connected to each other by a plurality of optical fibers to optically transmit a radio signal bidirectionally, wherein the center station has one or more baseband signals, An electro-optical converter that is input as one or more modulated electric signals having a predetermined intermediate frequency and converts the electric signal into an optical signal by intensity modulation; and a local oscillation signal that outputs a predetermined local oscillation signal. A first external modulator for intensity-modulating the optical signal converted by the electro-optical converter with a local oscillation signal output from the local oscillation signal source; and an intensity modulation by the first external modulator. Was done A first optical branching unit that splits a signal and outputs each of the signals to a plurality of downstream optical fibers, and an optical signal transmitted from each of the plurality of wireless base stations through a plurality of upstream optical fibers to an intermediate frequency band. A plurality of first photoelectric conversion units for respectively converting the electric signals into a plurality of electric signals, and a plurality of demodulation units for demodulating the electric signals respectively converted by the plurality of first photoelectric conversion units into baseband signals. At least a plurality of the radio base stations each transmit an optical signal transmitted through the downlink optical fiber to 2
A second optical branching unit that branches, a second photoelectric conversion unit that converts one optical signal branched by the second optical branching unit into an electric signal in a radio frequency band, and the second light. From the electric signal converted by the electric conversion unit, a band filter unit that extracts only an electric signal in a desired frequency band, and the electric signal extracted by the band filter unit is received by the antenna unit to the antenna unit. A circulator section that outputs a radio signal to a second external modulation section; and the other optical signal branched by the second optical branch section is intensity-modulated by a radio signal output from the circulator section, An optical transmission system comprising at least the second external modulator for outputting to an upstream optical fiber. 4. A plurality of radio stations each of which is used for radio access for transmitting information between a center station and a subscriber terminal via a radio base station which transmits and receives radio signals from an antenna unit and covers different service areas. An optical transmission system that connects a base station and a center station with a plurality of optical fibers, respectively, and optically transmits a wireless signal in two directions, wherein the center station outputs a predetermined light source, A local oscillator signal source that outputs a predetermined local oscillator signal, a first external modulator that intensity-modulates light output from the light source with a local oscillator signal output from the local oscillator signal source, A first optical splitter for splitting an optical signal intensity-modulated by one external modulator into a number of the plurality of radio base stations; and one or more baseband signals each having a predetermined intermediate frequency One or more modulated As the air signal, each is input for each radio base station to be transmitted,
A plurality of IF modulators each of which intensity-modulates the optical signal split by the first optical splitter with the electric signal and outputs the modulated optical signal to a plurality of downlink optical fibers; A plurality of first photoelectric conversion units for respectively converting optical signals transmitted from the plurality of wireless base stations via an optical fiber into electric signals of an intermediate frequency band; and a plurality of the first photoelectric conversion units. A plurality of demodulators for demodulating the electric signals respectively converted by the units into baseband signals, wherein the plurality of wireless base stations each transmit an optical signal transmitted via the downlink optical fiber. , 2
A second optical branching unit that branches, a second photoelectric conversion unit that converts one optical signal branched by the second optical branching unit into an electric signal in a radio frequency band, and the second light. A circulator section that outputs the electric signal converted by the electric conversion section to the antenna section, a radio signal received by the antenna section to a second external modulation section, and a branch section that is branched by the second optical branch section. An optical transmission system, comprising: at least the second external modulation unit that intensity-modulates the other optical signal with a wireless signal output from the circulator unit and outputs the modulated signal to the upstream optical fiber. 5. A radio frequency used in a downlink system for wireless transmission from the radio base station to the subscriber terminal and for an uplink system in radio transmission from the subscriber terminal to the radio base station. The optical transmission system according to claim 3, wherein: 6. The optical transmission system according to claim 1, wherein a frequency of a radio signal used in each of said radio base stations is set to be different from each other. 7. The frequency of a radio signal used in each of the radio base stations is set such that a service area to be covered is different between adjacent radio base stations. 5. The optical transmission system according to 4. 8. The optical signal output from the external modulating section (first or second external modulating section) is an optical single sideband signal comprising a carrier and a single sideband component. The optical transmission system according to claim 1. 9. An external modulator of a Mach-Zehnder type is used for the external modulator (first or second external modulator),
The bias point in the external modulator is set to a point where the optical output is minimum or maximum, and the frequency of the local oscillation signal is set to 2
The optical transmission system according to any one of claims 1 to 4, wherein the intensity of the optical signal is modulated by a double component. 10. The optical transmission system according to claim 1, wherein a semiconductor laser that converts an electric signal into an optical signal using a direct modulation method is used for the electro-optical conversion unit. 11. The optical fiber (downstream optical fiber)
A wavelength of an optical signal output from the electro-optical converter,
The optical transmission system according to claim 10, wherein an optical fiber having substantially the same zero-dispersion wavelength is used. 12. A radio access system for transmitting information between a center station and a subscriber terminal via a radio base station for transmitting and receiving radio signals from an antenna unit, the first to the first to third radio stations covering different service areas. The n-th (n is an integer of 2 or more) radio base station and the center station are connected by first to n-th uplink and downlink optical fibers provided corresponding to each radio base station. And an optical transmission system that bidirectionally optically transmits a radio signal, wherein the center station uniquely associates one or more predetermined intermediate frequency signals with the first to n-th radio base stations. Mutually different wavelengths λd1
the first to n-th optical signals of λdn
To a n-th electro-optical conversion unit, a wavelength multiplexing unit for multiplexing the converted first to n-th optical signals, a local oscillation signal source for outputting a local oscillation signal of a predetermined frequency, and the wavelength multiplexing. An optical modulator for intensity-modulating the multiplexed optical signal output from the unit based on the local oscillation signal;
A wavelength separation unit that separates the wavelength into the first to n-th modulated optical signals, and sends the k-th (k = 1 to n) modulated optical signal to the k-th downstream optical fiber, The k-th wireless base station receives the k-th modulated optical signal having the wavelength λdk transmitted through the k-th downstream optical fiber, and converts the modulated optical signal into an electric signal in a radio frequency band. An optical transmission system including a photoelectric conversion unit that outputs a signal. 13. A radio access system for transmitting information between a center station and a subscriber terminal via a radio base station for transmitting / receiving a radio signal from / to an antenna unit. The n-th (n is an integer of 2 or more) radio base station and the center station are connected by first to n-th uplink and downlink optical fibers provided corresponding to each radio base station. And an optical transmission system that bidirectionally optically transmits a radio signal, wherein the center station uniquely associates one or more predetermined intermediate frequency signals with the first to n-th radio base stations. Mutually different wavelengths λd1
the first to n-th electro-optical converters respectively converting the first to n-th downstream optical signals of λ dn, and the first to n-th electro-optical converters of different wavelengths λ d1 to λ dn and different wavelengths A first to n-th upstream light source that respectively outputs an n-th upstream optical signal; the converted first to n-th downstream optical signal; and the first to n-th upstream optical signal to be output. A wavelength multiplexing unit that multiplexes the signals, a local oscillation signal source that outputs a local oscillation signal of a predetermined frequency, and a light that intensity-modulates the multiplexed optical signal output from the wavelength multiplexing unit based on the local oscillation signal. A modulating unit, and converting the intensity-modulated multiplexed optical signal to wavelengths
Of the first to nth modulated downstream optical signals and wavelengths λu1 to λun
Wavelength separation into the first to n-th modulated upstream optical signals,
A wavelength separation unit that sends the k-th (k = 1 to n) modulated downstream optical signal to the k-th downstream optical fiber together with the k-th modulated upstream optical signal; and the first to n-th upstream. A first to an n-th optical-electrical conversion unit for converting an optical signal transmitted via the optical fiber into an electrical signal, wherein the k-th wireless base station is the k-th downstream optical fiber A two-wavelength demultiplexer that receives an optical signal transmitted through the optical fiber and separates the signal into the k-th modulated downstream optical signal having a wavelength λdk and the k-th modulated upstream optical signal having a wavelength λuk; An opto-electric conversion unit that converts the k-th modulated downstream optical signal separated by the above into an electric signal and outputs the electric signal; and converts the k-th modulated upstream optical signal separated by the two-wavelength separation unit into an input radio signal The intensity is modulated by
Comprising an RF modulator for transmitting to the upstream optical fiber,
Optical transmission system. 14. A wavelength λ of the first to n-th downstream optical signals.
d1 to λdn are set to belong to a predetermined first wavelength band, and the wavelengths λu1 to λun of the first to nth upstream optical signals are:
The two-wavelength separation unit of the k-th wireless base station is set to belong to a predetermined second wavelength band.
By separating the optical signal transmitted through the downstream optical fiber into the first wavelength band and the second wavelength band, the k-th modulated downstream optical signal having the wavelength λdk and the wavelength λuk 14. The optical transmission system according to claim 13, wherein the optical signal is separated into the k-th modulated upstream optical signal. 15. A wireless access system for transmitting information between a center station and a subscriber terminal via a wireless base station for transmitting and receiving wireless signals from an antenna unit, wherein the first to the first services cover different service areas. The n-th (n is an integer of 2 or more) radio base station and the center station are connected by first to n-th uplink and downlink optical fibers provided corresponding to each radio base station. And an optical transmission system that bidirectionally optically transmits a radio signal, wherein the center station uniquely associates one or more predetermined intermediate frequency signals with the first to n-th radio base stations. A first to n-th electro-optical converters for respectively converting first to n-th downstream optical signals of different wavelengths λd1 to λdn belonging to a predetermined first wavelength band, and any of the wavelengths λd1 to λdn Different and predetermined second wavelength band Mutually different wavelength λu1~λu belonging
first to output the n-th to n-th upstream optical signals, respectively.
To a n-th upstream light source, a wavelength multiplexing unit that multiplexes the converted first to n-th downstream optical signals, and the output first to n-th upstream optical signals, and a predetermined frequency. A local oscillation signal source for outputting a local oscillation signal, an optical modulation section for intensity-modulating the multiplexed optical signal output from the wavelength multiplexing section based on the local oscillation signal, and an intensity-modulated multiplexed optical signal. , Wavelengths λd1 to λdn
Of the first to nth modulated downstream optical signals and wavelengths λu1 to λun
Wavelength separation into the first to n-th modulated upstream optical signals,
A wavelength separation unit that sends the k-th (k = 1 to n) modulated downstream optical signal to the k-th downstream optical fiber together with the k-th modulated upstream optical signal; and the first to n-th upstream. A first to an n-th optical-electrical conversion unit for converting an optical signal transmitted via the optical fiber into an electric signal, wherein the k-th wireless base station is the k-th downstream optical fiber. Receiving the optical signal transmitted through the first and second optical signals, and separating the optical signal into the k-th modulated downstream optical signal having the wavelength λdk and the k-th modulated upstream optical signal having the wavelength λuk, and performing the photoelectric conversion function.
The k-th modulated downstream optical signal in the second wavelength band representing the electro-optical conversion function is input while the k-th modulated downstream optical signal in the wavelength band is converted into an electric signal and output. An optical transmission system, comprising: an electro-absorption type modulation unit for intensity-modulating with a wireless signal transmitted to the k-th upstream optical fiber. 16. The first to n-th upstream light sources uniquely correspond to the first to n-th downstream optical signals and have wavelengths λu1 to λu1.
λun is a predetermined amount f for each of the wavelengths λd1 to λdn.
The optical transmission system according to claim 13, wherein the first to n-th different upstream optical signals are output. 17. The first to n-th electro-optical converters of the center station, wherein the first to n-th electro-optical converters output optical signals of wavelengths λd1 to λdn, respectively.
An n-th light source, and first to first light-modulating the optical signals output from the first to n-th light sources with the respective intermediate frequency signals.
16. The optical transmission system according to claim 12, comprising an n-th IF modulation unit. 18. The optical transmission system according to claim 12, wherein a frequency of a radio signal used in each of said radio base stations is set to be different from each other. 19. The frequency of a radio signal used in each of the radio base stations is set such that a service area to be covered is different between adjacent radio base stations. 18. The optical transmission system according to any one of 17. 20. The optical signal output from the optical modulator is an optical single sideband signal comprising a carrier and a single sideband component.
The optical transmission system according to any one of the above. 21. A Mach-Zehnder type external modulator is used for the optical modulator, and a bias point in the external modulator is set to a point where an optical output becomes minimum or maximum, and 20. The optical transmission system according to claim 12, wherein the intensity of the optical signal is modulated by a component twice as high as the frequency. 22. A high-frequency optical transmitter used in a center station for optically transmitting a radio signal by connecting a plurality of radio base stations each covering a different service area by a plurality of optical fibers, The input electric signal is divided into first and second electric signals in phase with each other and a third electric signal orthogonal to the first and second electric signals.
An electric signal, a three-branch unit that divides the electric signal into three, an electric-optical converter that converts the third electric signal into a light intensity modulation signal, and a first delay that adjusts a propagation time of the first electric signal. A control unit; a second delay control unit that adjusts the propagation time of the second electric signal; and bifurcating the input local oscillation signal into first and second local oscillation signals having opposite phases. A branching unit; a third delay control unit that adjusts the propagation time of the first local oscillation signal; a fourth delay control unit that adjusts the propagation time of the second local oscillation signal; A first multiplexing unit that multiplexes the first electric signal output from the delay control unit and the first local oscillation signal output from the third delay control unit; The second electric signal output from the delay control unit and the second local oscillation signal output from the fourth delay control unit A second multiplexing section for multiplexing; and a first and second electrode, and a signal multiplexed by the first and second multiplexing sections is input to the first and second electrodes, respectively. A differential intensity modulator that modulates the light intensity modulation signal output from the electro-optical converter. The first and second electrodes of the differential type intensity modulator The first and second electric signals input through the first and second multiplexing units are in phase with each other so that the first and second electric signals are in phase with each other.
By adjusting the fourth delay control unit, phase modulation is performed on the optical signal output from the electro-optical conversion unit, and the same amount is applied to the frequency deviation of the optical frequency modulation component of the optical signal. A high-frequency optical transmitter that performs opposite-phase optical modulation. 23. A high-frequency optical transmitter used as a center station for optically transmitting a radio signal by connecting a plurality of radio base stations each covering a different service area with a plurality of optical fibers, The input electric signal is first and second electric signals in phase with each other, and the first and second electric signals have a phase difference of 90 from each other.
° into a third electrical signal, a three-branch unit that branches into three, an electro-optical converter that converts the third electrical signal into a light intensity modulation signal, and a propagation time of the first electrical signal. A first delay control unit, a second delay control unit that adjusts the propagation time of the second electric signal, and an input local oscillation signal into first and second local oscillation signals that are orthogonal to each other. A two-branch unit that branches into two, a third delay control unit that adjusts the propagation time of the first local oscillation signal, a fourth delay control unit that adjusts the propagation time of the second local oscillation signal, A first multiplexing unit that multiplexes the first electric signal output from the first delay control unit and the first local oscillation signal output from the third delay control unit; The second electric signal output from the second delay control unit, and the second station output from the fourth delay control unit A second multiplexing unit that multiplexes the generated signal with the first and second electrodes; and a signal multiplexed by the first and second multiplexing units into the first and second electrodes. And a differential type intensity modulator for modulating the light intensity modulation signal output from the electro-optical conversion unit, respectively, and the first and second of the differential type intensity modulator. The first and second delay control units are controlled so that the phase difference between the first and second electric signals input to the electrodes via the first and second multiplexing units becomes 0 with each other. By adjusting,
Performing phase modulation on the optical signal output from the electro-optical conversion unit, and performing the same amount and opposite phase optical modulation on the frequency deviation of the optical frequency modulation component of the optical signal; The first and second local signals input to the first and second electrodes of the intensity modulator through the first and second multiplexing units are orthogonal to each other. 3
And a fourth delay control unit for performing optical single sideband modulation having an optical carrier on the optical signal. 24. A high-frequency optical transmitter used in a center station for optically transmitting a radio signal by connecting a plurality of radio base stations each covering a different service area with a plurality of optical fibers, A two-branch unit that bifurcates an input electric signal into first and second electric signals that are orthogonal to each other, an electro-optical converter that converts the first electric signal into a light intensity modulation signal, A delay control unit for adjusting the propagation time of the electric signal, a phase modulation unit and an intensity modulation unit are formed on the same substrate, and the second electric signal output from the delay control unit is transmitted to the phase modulation unit. An integrated modulator for inputting and inputting the local oscillation signal to the intensity modulator, and modulating the light intensity modulated signal output from the electro-optical converter. Output from the optical converter Performs phase modulation on an optical signal, and characterized by applying an optical modulation of the opposite phase to the frequency deviation of optical frequency modulation component to which the optical signal has (FM index), high frequency optical transmitter.