Optical communication system

An optical communication system for optically transmitting transmission data from a transmitting station to a transmitting device includes an adder for adding an intermediate frequency subcarrier signal modulated with data to be transmitted to a pilot carrier signal as a sinusoidal wave, and an electro-optical converter for electro-optically converting the above sum signal to an optical signal by directly modulating a semiconductor laser element having a resonant frequency fr in accordance with the sum signal and transmitting the signal to an optical fiber for a down link. The frequency fIF of the intermediate frequency subcarrier signal and the frequency fLO of the pilot carrier signal satisfyfLO−fIF≧1[GHz],2×fIF<fLO<(⅔)×fr,fIF<1[GHz], and2 [GHz]<fLO.

BACKGROUND OF THE INVENTION

The present invention relates to an optical communication system for transmitting a high-frequency analog signal such as a radio signal via an optical fiber.

This application is based on Japanese Patent Application No. 10-163561, filed Jun. 11, 1998 and Japanese Patent Application No. 10-309981, filed Oct. 30, 1998, the contents of which are incorporated herein by reference.

Along with recent development of mobile communication, expansion of radio communication service areas is required. To effectively utilize radio wave frequency resources and reduce cost of base station equipment, a scheme in which individual radio zones (cells) are made small, and instead, a number of radio zones are arranged at a high density has received a great deal of attention. This is called a picocell radio zone. To realize the picocell radio zone, a radio communication base station arrangement in which transmitting/receiving devices and transmitting/receiving stations are connected through optical fibers has been examined.

More specifically, a radio base station has transmitting/receiving stations and transmitting/receiving devices. A plurality of transmitting/receiving devices are prepared for one transmitting/receiving station. The output power from each transmitting/receiving device is made small for the picocell radio zones. The transmitting/receiving devices and the transmitting/receiving station are connected through optical fibers. The transmitting/receiving devices transmit signals received from a common transmitting/receiving station to subscribers and transmit signals received from subscribers to a common transmitting/receiving station. The output from each transmitting/receiving device is made small to reduce cost.

A transmitting/receiving device is mainly formed from an antenna section and placed in each cell. A transmitting/receiving station has a modem and a controller corresponding to the plurality of transmitting/receiving devices in the cells. Therefore, the transmitting/receiving station is also called a central control terminal station. An analog radio signal is optically transmitted through an optical fiber between the transmitting/receiving device and transmitting/receiving station. With this arrangement, each transmitting/receiving device can be made simple, compact, and low-cost, and one radio communication base station can provide a number of cells.

In this arrangement, the basic arrangement of a transmitting/receiving device includes only an antenna, and opto-electric and electro-optic conversion devices and does not depend on the data rate or modulation scheme of a radio signal. Therefore, even when the radio transmission scheme is changed, replacement of the transmitting/receiving device or change in constituent elements of the transmitting/receiving devices is unnecessary.

For the above optical analog transmission, an electro-optical converter (E/O converter) is required to convert an electrical signal into an optical signal. At the E/O converter, light intensity of a semiconductor laser element is modulated with a radio frequency signal. As the modulation scheme, a scheme of directly modulating a semiconductor laser element or a scheme using an external optical modulator is employed.

Advantages and disadvantages of these two schemes will be compared. In terms of modulation distortion characteristics, device scale, and device cost, the scheme of directly modulating a laser element is more advantageous.

However, the trend of technology obviously indicates carrier frequency shift to a higher frequency band, e.g., shift to the 2- to 5-[GHz] band as the capacity of a radio frequency signal increases. However, in a distributed feedback laser element (DFB-LD) as a representative laser element, the modulation frequency range with a relatively small modulation distortion is as low as 2 to 3 [GHz]. Therefore, direct modulation of a laser element using a radio frequency signal is becoming difficult.

As disclosed in, e.g., Japanese Patent Publication (KOKAI) No. 6-164427, a scheme (subcarrier transmission) of superposing an intermediate frequency subcarrier signal fIFmodulated by a data signal on a pilot carrier signal fLOas a sinusoidal wave and optically transmitting the superposed analog signal from a transmitting/receiving station to a transmitting/receiving device has been proposed.

In the transmission scheme proposed in this prior art, the intermediate frequency subcarrier signal fIFis frequency-converted (up-converted) by a multiplied signal obtained by multiplying the received pilot carrier signal fLOon the transmitting/receiving device side, thereby obtaining a radio frequency signal. The laser element is used in a low frequency band with excellent modulation distortion characteristics, and the pilot carrier signal fLOis superposed on a frequency close to the intermediate frequency subcarrier signal fIF.

According to an embodiment described in the above prior art, a pilot carrier signal fLOhaving a frequency of 300 [MHz] is superposed near an intermediate frequency subcarrier signal fIFin the 200-[MHz] band, as shown in FIG.1. In this scheme, on the transmitting device side, to ensure the noise characteristics of the radio frequency signal and increase the frequency stability, the CNRs (Carrier-to-Noise Ratios) of the received intermediate frequency subcarrier signal fIFand pilot carrier signal fLOmust be high. That is, the noise level must be low.

However, in the frequency band near the pilot carrier signal fLO, the RIN (Relative Intensity Noise) increases. Therefore, when the pilot carrier signal fLOis arranged near the frequency band of the intermediate frequency subcarrier signal fIF, as in the prior art, the CNR decreases.

FIG. 2shows the result of an experiment conducted by the present inventors. When the intermediate frequency subcarrier signal fIFis set at 500 [MHz] and the pilot carrier signal fLOis set at 550 [MHz], the RIN characteristics largely degrade in accordance with the optical modulation index of the pilot carrier signal fLOand, more particularly, at an optical modulation index of 15 [%] or more, as shown in FIG.2. Therefore, the communication quality of a radio frequency signal greatly degrades.

Especially, when the optical modulation index of the pilot carrier signal fLOincreases, degradation in RIN becomes conspicuous. Hence, a radio frequency signal generated by frequency-converting the intermediate frequency subcarrier signal fIFusing the pilot carrier signal fLOcontains a number of noise components and therefore has poor transmission characteristics. When a radio frequency signal containing a number of noise components is transmitted, the noise components adversely affect other radio frequency signals to impede radio communication. Solutions to this problem are required.

To cope with a shortage in channels due to the recent increase in number of subscribers or an increase in transmission rate, extensive studies have been made for radio communication using a frequency band higher than the conventional frequency band, e.g., millimeter waves or submillimeter waves. For this system as well, an arrangement for connecting transmitting/receiving devices and transmitting/receiving stations through optical fibers has been examined.

As a connection form using optical fibers, a PON (Passive Optical Network) is used. In the PON, as shown inFIGS. 3 and 4, a transmitting/receiving station1and a plurality of transmitting/receiving devices2are connected through optical fibers4in which a passive optical divider3is inserted. An optical signal transmitted from the transmitting/receiving station1to the optical fiber4is divided by the optical divider3inserted into the optical fiber4, and distributed to the transmitting/receiving devices2.

In the PON, a passive optical divider3is inserted midway along optical fibers4to accommodate the plurality of transmitting/receiving devices2. Hence, the optical transmission/reception device of the transmitting/receiving station1and optical fibers4can be shared, and accordingly, the equipment can be made compact.

In the PON, an optical signal transmitted from the transmitting/receiving station1is divided, so the same signal reaches the plurality of transmitting/receiving devices2. There is no problem when radio signals transmitted from the plurality of transmitting/receiving devices are completely equal. However, different transmitting/receiving devices2normally transmit different radio signals.

Conventionally, as shown in the spectrum arrangement inFIG. 5, an optical signal to be transmitted from the transmitting/receiving station to the transmitting/receiving device is frequency-multiplexed while changing the frequency of the intermediate frequency subcarrier signal fIFcorresponding with each transmitting/receiving devices and sent (subcarrier multiplex transmission scheme). In this case, each transmitting/receiving device receives the optical signal, extracts a component to be transmitted from the self station, converts the component into a radio signal frequency, and transmits the signal from the antenna.

In the example shown inFIG. 5, the frequencies of the intermediate frequency signal are assigned at an appropriate interval and frequency-multiplexed: for example, a signal fIF1to a transmitting/receiving device2-1is assigned near 100 [MHz], a signal fIF2to a transmitting/receiving device2-2is assigned near 200 [MHz], and a signal fIF3to a transmitting/receiving device2-3is assigned near 300 [MHz]. Therefore, if radio signals sent from the transmitting/receiving devices2-1, . . . ,2-3are in the 2 [GHz] band, the transmitting/receiving device2-1must up-convert the signal fIF1by 1.9 [GHz], the transmitting/receiving device2-2must up-convert the signal fIF2by 1.8 [GHz], and the transmitting/receiving device2-3must up-convert the signal fIF3by 1.7 [GHz].

For a conventional radio system using optical subcarrier transmission, a method has been proposed in which not only the intermediate frequency subcarrier signal fIFbut also the pilot carrier signal fLOas a signal for maintaining the frequency stability of the radio wave transmitted from the transmitting/receiving devices is transmitted, and each transmitting/receiving device frequency-converts (up-converts) the intermediate frequency subcarrier signal fIFusing the pilot carrier signal fLO, as shown in FIG.1.

As a consequence, when the frequencies of intermediate frequency subcarrier signals for the individual transmitting/receiving devices are different, as shown inFIG. 5, pilot carrier signals fLOfor frequency conversion must be prepared for the respective intermediate frequency subcarrier signals fIF. Pilot carrier signals fLOcorresponding to the number of intermediate frequency subcarrier signals multiplexed must be sent. These signals are multiplexed and sent in optical transmission.

As a result, the total number of signals including the pilot carrier signal fLOincreases. Since the optical modulation index of the intermediate frequency subcarrier signals in optical transmission is shared by the pilot carrier signals fLO, the optical modulation index decreases to degrade the transmission quality.

In the radio system, when a plurality of radio base stations (transmitting/receiving devices) provide the same service, frequencies slightly different from each other in the same frequency band are sometimes used to prevent interference between signals from adjacent base stations.

For example, frequencies are separated at an interval of 100 [kHz] in the 2 [GHz] band. When such transmitting/receiving devices are accommodated through one fiber, a system in which subcarrier signals with different frequencies are multiplexed in the radio frequency band while only one pilot carrier signal is transmitted can be constructed.

In this case, however, each transmitting/receiving device2that has received the optical signal from the transmitting/receiving station1must select a signal to be used in the self station from signals arranged at an interval as small as 100 [kHz]. For this purpose, a very steep filter with high frequency stability is required, resulting in an increase in cost. In a radio system using a radio scheme other than frequency multiplexing, e.g., CDMA, signals transmitted from transmitting/receiving devices are in the same frequency band. Therefore, the method of transmitting only one pilot carrier signal fLOusing a steep filter cannot be used.

To simplify the arrangement, an intermediate frequency signal to be used in the self station must be separated from intermediate frequency signals, which are multiplexed as subcarriers, using a simple filter, as described above. For this purpose, subcarriers are preferably multiplexed at a large frequency interval.

However, to do this, a plurality of pilot carrier signals fLOcorresponding to the number of the intermediate signals fIFare necessary. To stabilize transmission quality, the optical modulation index should not be decreased. Sending more signals including a plurality of pilot carrier signals fLOmeans increasing the optical modulation index for the total signals. The amount of RIN corresponds to the optical modulation index. There is the effect of interference modulation as one of the others noise decreasing transmission quality. The effect of the interference modulation also corresponds to the number of signals and the optical modulation index. The plurality of pilot carriers inevitable degrades the data transmission quality.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an optical communication system which can generate a radio frequency signal excellent in noise characteristics without decreasing the CNR of an intermediate frequency subcarrier signal fIFwhen a laser beam is directly modulated using a signal obtained by synthesizing the intermediate frequency subcarrier signal fIFand a pilot carrier signal fLOto optically transmit the analog intermediate frequency subcarrier signal fIFand pilot carrier signal fLOwith a large optical modulation index.

It is a second object of the present invention to provide an inexpensive and simple optical communication system which can reduce the number of pilot carrier signals fLOto be sent from a transmitting/receiving station to transmitting/receiving devices, that are necessary for frequency conversion, multiplex subcarriers at a large frequency interval, and separate the intermediate frequency subcarrier signals using a simple filter.

An optical communication system according to the present invention, which multiplexes a subcarrier signal and a pilot carrier signal and optically transmits the multiplexed signal from a transmitting/receiving station to a transmitting/receiving device has the following arrangement.

The frequency band of a pilot carrier signal fLOand that of a subcarrier signal fIFare arranged such that fLO−fIF≧1[GHz] and 2×fIF<fLO<(⅔)×fr (resonant frequency of a laser) are satisfied.

With this arrangement, the RIN characteristics of the subcarrier signal fIFcan be prevented from degrading due to multiplex of the pilot carrier signal fLO. Since satisfactory CNR characteristics can be provided on the transmitting/receiving device side, the communication quality of the transmitted optical signal is improved. When the above conditions are satisfied, degradation in RIN characteristics can be suppressed even when a large optical modulation index is set for the pilot carrier signal. Since the optical modulation index of the pilot carrier signal can be made large, the pilot carrier signal fLOwith excellent CNR characteristics can be provided on the transmitting/receiving device side. Since the pilot carrier signal fLOis used by a multiplier as a local oscillation signal for frequency conversion, an additive noise amount in the output from the multiplier decreases, so a radio frequency signal with few noise components can be obtained.

When the pilot carrier signal fLOis excellent in CNR characteristics, the Q value of the filter for extracting the pilot carrier signal fLOcan be made small, so the frequency band of the pilot carrier signal fLOto be transmitted becomes wide. That is, since the frequency range of the radio frequency signal to be processed on the transmitting/receiving device side is widened, a transmitting/receiving station with a large application range can be provided.

On the transmitting/receiving device side, the pilot carrier signal fLOtransmitted from the transmitting/receiving station side can be extracted with a high CNR. Therefore, the frequency of the radio frequency signal can be up- or down-converted while suppressing the additive noise amount. Since the degradation in CNR characteristics of the subcarrier signal and pilot carrier signal is small, the optical transmission distance between the transmitting/receiving station and transmitting/receiving device can be increased. More specifically, the setting range of a transmitting/receiving device connected to one transmitting/receiving station can increase, the number of transmitting/receiving devices which can be connected can be increased, and the radio communication service area can be efficiency expanded.

Except the RIN value, a modulation distortion also degrades the CNR characteristics. The frequency band with good laser modulation distortion characteristics is lower than 1 [GHz]. Hence, when fIF<1[GHz] and 2 [GHz]<fLOare satisfied, a transmission system in which degradation in CNR due to not only the RIN value but also modulation distortion is suppressed can be provided.

According to the present invention, when a plurality of transmitting/receiving devices are connected to a transmitting/receiving station through a PON, the frequency stability between the transmitting/receiving devices can be maintained using a simpler optical transmission system. More specifically, the frequency of a radio wave is set such that when data signals subcarrier-multiplexed are to be distributed from a transmitting/receiving station to a plurality of transmitting/receiving devices, the data signals to be used by the transmitting/receiving devices are subcarrier-multiplexed at a sufficiently large frequency interval so that the data signals can be separated by a simple filter after reception of an optical signal, and only two pilot carrier signals suffice to synchronize the frequencies of radio waves radiated from the transmitting/receiving devices (independently of the number of transmitting/receiving devices). As a consequence, an optical communication system in which while establishing frequency synchronization between the transmitting/receiving devices, satisfactory transmission can be performed without sacrificing the optical modulation index of the data signal in optical subcarrier transmission due to transmission of the pilot carrier signal, and the process of extracting necessary signals after reception of an optical signal is easy and inexpensive can be provided.

Additional objects and advantages of the present invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention.

The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of an optical communication system according to the present invention will now be described with reference to the accompanying drawings.

First Embodiment

FIG. 6shows the arrangement of a transmitting station as part of the first embodiment of the present invention. A transmitting station10comprises a modulator12, an oscillator14, an adder16, and an electro-optical converter (E/O) converter18.

The modulator12modulates an intermediate frequency signal, which is output from an oscillator (not shown), with a data signal to be transmitted and supplies an intermediate frequency subcarrier signal fIFas the modulation result to the first input terminal of the adder16. The adder16adds the intermediate frequency subcarrier signal fIFto a pilot carrier signal fLOoutput from the oscillator14.

The E/O converter18converts the sum signal of the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOinto an optical signal. The E/O converter18comprises a driver amplifier20, a current source22, a semiconductor laser element24, an inductor26, and a resistor28. The inductor26applies a bias corresponding to the output from the current source22to an output signal from the driver amplifier20. The resistor28is an input resistor for supplying the biased output from the driver amplifier20to the semiconductor laser element24as a direct modulation signal. The semiconductor laser element24emits a laser beam modulated in correspondence with the output from the adder16and sends the laser beam to an optical fiber30as a transmission line. The semiconductor laser element24used is a DFB laser diode for analog transmission.

The laser beam output from the semiconductor laser element24is transmitted to a transmitting device (not shown) through the optical fiber30. The resonant frequency of the semiconductor laser element24is represented by fr.

In this arrangement, a condition required for the frequency arrangement relationship between the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOis fLO−fIF≧1[GHz].FIG. 7shows the frequency arrangement of the intermediate frequency subcarrier signal fIFand the pilot carrier signal fLO.

Normally, the lower the frequency band becomes, the more excellent the modulation distortion characteristics and RIN characteristics of the semiconductor laser element24become. Therefore, the intermediate frequency subcarrier signal fIFmodulated with the data to be transmitted is arranged on the lower side of the pilot carrier signal fLO. The pilot carrier signal fLOis a sinusoidal wave and can stand the distortion.

FIG. 8shows the RIN characteristics of the intermediate frequency subcarrier signal fIFwith respect to the frequency of the pilot carrier signal fLOin the semiconductor laser element24in the electro-optical converter18. The intermediate frequency subcarrier signal fIFhas a frequency of 1 [GHz]. The optical modulation index of the pilot carrier signal fLOis 40 [%]. The frequency of the intermediate frequency subcarrier signal fIFis changed to 1.2 [GHz], 2 [GHz], 3 [GHz], 3.5 [GHz], and 4 [GHz]. The RIN obtained when the pilot carrier signal fLOis not superposed is −152 [dB/Hz]. The RIN value is influenced by the spectral component of the pilot carrier signal fLO. As the frequency becomes close to the pilot carrier signal fLO, the degradation becomes large.

When the frequency is separated from the pilot carrier signal fLO, the RIN value is improved. As shown inFIG. 8, when fLO−fIF≧1 [GHz], the RIN value is asymptotic to the value “−152” obtained when the pilot carrier signal fLOis not superposed, and stabilizes.

FIG. 9shows the RIN value to the pilot carrier signal fLOwhen the intermediate frequency subcarrier signal fIFhas a frequency of 120 [MHz]. The optical modulation index of the pilot carrier signal fLOis 40 [%], as in FIG.8. The RIN value obtained when the pilot carrier signal fLOis not superposed is −164.0 [dB/Hz].

As is apparent fromFIG. 9, when the pilot carrier signal fLOis close to the intermediate frequency subcarrier signal fIF, and the difference between the two signals is 1 [GHz] or less (i.e., when fLO−fIF<1 [GHz]), the RIN value is −160 [dB/Hz], and the degradation is great. For a pilot carrier signal fLO=2 [GHz] satisfying fLO−fIF≧1 [GHz], the RIN value becomes −162 [dB/Hz], and the degradation is apparently suppressed.

As is apparent from the above description, when fLO−fIF≧1 [GHz], the RIN characteristics can be improved.

Normally, the semiconductor laser element24has non-linear E/O conversion characteristics. When a laser element is modulated directly by a sum signal of fLOand fIF, intermodulation components are appeared at frequency bands fLO−fIFand fLO+fIF. If the intermodulation components overlap fIFand a resonant frequency fr, the noise characteristic for fIFis distorted. It is because the characteristics of laser elements becomes unstable by modulating the resonant frequency fr and RIN increased through the signal frequency band.

Therefore, the pilot carrier signal fLOmust be arranged such that the frequency fLO+fIFof the higher-band side distortion becomes lower than the resonant frequency fr and the frequency fLO−fIFof the lower-band side distortion becomes higher than the signal frequency fIF. Since 2×fIF<fLO, and fLO+fIF<fr, fLO<(⅔)×fr.FIG. 7shows the frequency arrangement of the intermediate frequency subcarrier signal fIFand pilot carrier signal fLO, which satisfies these conditions.

As the semiconductor laser element24, i.e., the semiconductor laser diode, a distributed feedback semiconductor laser (DFB-LD) or Fabri-Pérot semiconductor laser element (FP-LD) is used. Especially, a distributed feedback semiconductor laser element has a small modulation distortion that suppresses the dynamic range of a multi-channel signal, and is suitable for analog transmission. However, even in the distributed feedback semiconductor laser element, the frequency with a small modulation distortion and noise amount is normally 1 [GHz] or less.

When the intermediate frequency subcarrier signal fIFas an intermediate frequency subcarrier signal is arranged within the range of fIF>1 [GHz], the dynamic range is suppressed because of degradation in modulation distortion characteristics and an increase in noise. Therefore, the intermediate frequency subcarrier signal fIFis preferably arranged within the range of fIF<1 [GHz].

As is apparent from the RIN value when fIF=1 [GHz], which is shown inFIG. 8as the graph showing the pilot carrier signal fLOfrequency vs. RIN characteristics, and the RIN value when fIF=120 [MHz], which is shown inFIG. 9as the graph showing the pilot carrier signal fLOfrequency vs. RIN characteristics in the relatively high frequency band, the RIN value on the frequency band of the intermediate frequency subcarrier signal fIF, 120 MHz, is smaller by about 10 dB/Hz than the RIN value on the frequency band of fIF, 1 GHz.

As described above, when the frequency arrangement of the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOsatisfies fIF<1 [GHz] and fLO>2 [GHz], as shown inFIG. 7, satisfactory transmission characteristics can be maintained without any influence of the RIN characteristics and modulation distortion.

As described above, as a characteristic feature of the first embodiment, the intermediate frequency subcarrier signal fIFmodulated with data to be transmitted is added to the pilot carrier signal fLOas a sinusoidal wave. The sum signal is electro-optically converted by directly modulating the semiconductor laser element24having the resonant frequency fr and transmitted to the down link optical fiber. The frequency fIFof the intermediate frequency subcarrier signal and the frequency fLOof the pilot carrier signal satisfy fLO−fIF≧1 [GHz] and 2×fIF<fLO<(⅔)×fr.

Normally, the lower the frequency band becomes, the more excellent the modulation distortion characteristics and RIN characteristics of the semiconductor laser element become. Therefore, when the intermediate frequency subcarrier signal fIFis arranged on the lower side of the pilot carrier signal fLO, the pilot carrier signal fLOas a sinusoidal wave can stand the distortion.

In the RIN characteristics in the fIFband with respect to the pilot carrier signal fLOin the semiconductor laser element, which are shown inFIG. 8, the intermediate frequency subcarrier signal fIFhas a frequency of 1 [GHz]. The optical modulation index of the pilot carrier signal fLOis 40 [%]. The frequency of the intermediate frequency subcarrier signal fIFis changed to 1.2 [GHz], 2 [GHz], 3 [GHz], 3.5 [GHz], and 4 [GHz]. The RIN obtained when the pilot carrier signal fLOis not superposed is −152 [dB/Hz]. The RIN value is influenced by the spectral component of the pilot carrier signal fLO. As the frequency becomes close to the pilot carrier signal fLO, the degradation becomes large.

When the frequency is separated from the pilot carrier signal fLO, the RIN value decreases. As shown inFIG. 8, when fLO−fIF≧1 [GHz], the RIN value is asymptotic to the value “−152” obtained when the pilot carrier signal fLOis not superposed, and stabilizes.

As shown inFIG. 9, in the RIN characteristics with respect to the pilot carrier signal fLOwhen the intermediate frequency subcarrier signal fIFhas a frequency of 120 [MHz], the optical modulation index of the pilot carrier signal fLOis 40 [%], as in FIG.8. The RIN value obtained when the pilot carrier signal fLOis not superposed is −164.0 [dB/Hz].

As is apparent fromFIG. 9, when the pilot carrier signal fLOis close to the intermediate frequency subcarrier signal fIF, and the difference between the two signals is 1 [GHz] or less (i.e., when fLO−fIF<1 [GHz]), the RIN value is equal to or larger than −160 [dB/Hz], and the degradation is large. However, when the pilot carrier signal fLOmaintains the relation to the fIFsuch that fLO−fIF≧1 [GHz], the degradation is suppressed.

Hence, when the arrangement satisfies fLO−fIF≧1 [GHz], the RIN characteristics can be improved.

Normally, the semiconductor laser element has non-linear E/O conversion characteristics. When the laser element is directly modulated, a frequency corresponding to fLO±fIFhas an intermodulation distortion between the signals fLOand fIF, resulting in an increase in noise. Therefore, it is important to arrange the pilot carrier signal fLOwith respect to the intermediate frequency subcarrier signal fIFin consideration of fLO±fIF. When the frequency fLO−fIFof the lower-band side distortion overlaps the frequency band of the intermediate frequency subcarrier signal fIF, the RIN characteristics in the peripheral band degrade, as described above.

To prevent this, fLO>2×fIFis set to satisfy fLO−fIF>fIF, thereby avoiding the influence of degradation in RIN characteristics. In addition, the semiconductor laser element has the resonant frequency fr which is a specific frequency for each laser element.

Modulation efficiency at the resonant frequency fr band becomes much large comparing with the lower frequency band than fr. The resonant frequency is explained under. If a laser diode is suddenly turned-on from zero bias, a turn-on delay and an exponential rise in the optical output will be observed. The optical output initially overshoots and goes through a few cycles of damped oscillation before reaching equilibrium. The oscillation frequency of this behavior is called “resonant frequency fr”. This behavior is caused by the inverse relationship between carrier density and photon density in the semiconductor. If the resonant frequency fr is modulated, the characteristics of the laser element become unstable, and the RIN increases throughout the frequency band. Therefore, the pilot carrier signal fLOmust be arranged such that the frequency fLO+fIFof the higher-band side distortion becomes lower than the resonant frequency fr. Since 2×fIF<fLO, and fLO+fIF<fr, fLO<(⅔)×fr.

As another characteristic feature of the first embodiment, the frequency band fIFof the intermediate frequency subcarrier signal satisfies is lower than 1 [GHz], and the frequency band fLOof the pilot carrier signal fLOis higher than 2 [GHz].

As the semiconductor laser element, i.e., the semiconductor laser diode, a distributed feedback semiconductor laser (DFIB-LD) or Fabri-Pérot semiconductor laser element (FP-LD) is used. Especially, a DFB-LD has a small modulation distortion that suppresses the dynamic range of a multi-channel signal, and is suitable for analog transmission. However, even in the DFB-LD, the frequency band with a small modulation distortion and noise amount is normally 1 [GHz] or less.

When the frequency fIFof the intermediate frequency subcarrier signal fIFas an intermediate frequency subcarrier signal is arranged within the range of fIF>1 [GHz], the dynamic range is suppressed because of degradation in modulation distortion characteristics and an increase in noise. Therefore, the intermediate frequency subcarrier signal fIFis preferably arranged within the range of fIF<1 [GHz].

As is apparent from the RIN value when fIF=1 [GHz], which is shown inFIG. 8as the graph showing the pilot carrier signal fLOfrequency vs. RIN characteristics in the relatively low frequency band, and the RIN value when fIF=120 [MHz], which is shown inFIG. 9as the graph showing the pilot carrier signal fLOfrequency vs. RIN characteristics in the relatively high frequency band, the RIN value on the frequency band of the intermediate frequency subcarrier signal fIF, 120 [MHz], is smaller by about 10 dB/Hz than the RIN value on the frequency band of fIF, 1 [GHz].

As described above, when the frequency arrangement of the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOsatisfies fIF<1 [GHz] and fLO>2 [GHz], satisfactory transmission characteristics can be maintained without any influence of the RIN characteristics and modulation distortion.

Other embodiments of the optical transmission apparatus according to the present invention will be described. The same portions as those of the first embodiment will be indicated in the same reference numerals and their detailed description will be omitted.

Second Embodiment

FIG. 10shows an optical communication system according to the second embodiment of the present invention. The second embodiment is associated with an entire optical communication system including the transmitting station of the first embodiment of the present invention, and a radio communication base station device including a transmitting device.

As shown inFIG. 10, a transmitting station10is connected to transmitting devices32-1,32-2, . . . through an optical fiber30.

Each transmitting device32is connected to the optical fiber30through an optical divider34. The transmitting devices32are set at separate locations. A range where radio waves reach is a radio zone (cell or service area), and each transmitting device32can transmit/receive radio waves to/from communication terminals in the cell.

The transmitting station10is the same as in the first embodiment shown in FIG.6.

The optical fiber30is an optical transmission line connecting an E/O converter18in the transmitting station10to an opto-electrical converter (O/E converter)34in each transmitting device32.

The O/E converter34receives an optical signal transmitted through the optical fiber30and converts the optical signal into an electrical signal. The divider36receives the electrical signal output from the optical divider34and supplies it to the bandpass filters38and40.

The bandpass filter38extracts an intermediate frequency subcarrier signal fIFand contains the frequency fIFin the passband. The bandpass filter40extracts a pilot carrier signal fLOand contains the frequency fLOin the passband.

The multiplier42multiplies output signals from the two bandpass filters38and40and outputs the multiplied signal. The bandpass filter44extracts a predetermined radio frequency signal from the output from the multiplier42. The power amplifier46power-amplifies the radio frequency signal output from the bandpass filter44. The antenna48radiates the amplified signal into air as a radio wave.

In the system with the above arrangement, a radio data signal obtained by adding the pilot carrier signal fLOfrom a local oscillator14to the intermediate frequency subcarrier signal fIFfrom a radio signal modulator12by an adder16in the transmitting station10is input to an E/O converter18.

The E/O converter18directly modulates a laser beam with the radio data signal to obtain an optical signal. This optical signal is transmitted to the transmitting devices32-1,32-2, . . . through the optical fiber30.

On the side of each of the transmitting devices32-1,32-2, . . . , the optical signal transmitted through the optical fiber30is received by the O/E converter34, converted into an electrical signal and separated into two paths by the divider36. One is supplied to the bandpass filter38having the passband for the intermediate frequency subcarrier signal fIF, and the other is supplied to the bandpass filter40having the passband for the pilot carrier signal fLO, thereby reproducing the original intermediate frequency subcarrier signal fIFand pilot carrier signal fLO.

The reproduced intermediate frequency subcarrier signal fIFand pilot carrier signal fLOare input to the multiplier42and multiplied.

The output from the multiplier42is passed through the bandpass filter44to extract a predetermined radio frequency signal. The extracted radio frequency signal is amplified through the power amplifier46, radiated from the transmission antenna48into air as a radio wave, and transmitted to a terminal side in the cell.

According to the second embodiment, the frequency arrangement of the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOis set to satisfy fLO−fIF≧1 [GHz] and 2×fIF<fLO<(⅔)×fr, or fIF<1 [GHz] and fLO>2 [GHz], as in the first embodiment.

In the above-described manner, the first embodiment can be applied to the base station device for radio communication. The intermediate frequency subcarrier signal fIFis a single-channel or a frequency-division multiplexed signal. In case the intermediate subcarrier signal fIFis a frequency-division multiplexed signal, each channel frequency of fIFmay be changed for each unit of transmitting device, or the same frequency of fIFmay be used.

The second embodiment of the present invention, in which the first embodiment is applied to the base station device for radio communication, has been described above. Next, the third embodiment in which transmitting devices with the same specifications are used between adjacent cells against the different frequency band of an intermediate frequency signal, a pilot carrier signal and a radio frequency signal, thereby reducing cost of the system without exchanging the hardware for each transmitting device.

Third Embodiment

FIG. 11shows the third embodiment of the present invention. The transmitting station10has the same arrangement as in the first and second embodiments. In the transmitting device32as well, the same reference numerals as in the transmitting device32of the second embodiment denote the same parts in the third embodiment.

The transmitting device32of the third embodiment comprises the O/E converter34, the divider36, the multiplier42, the bandpass filter44, the power amplifier46, the antenna48, a low-pass filter52, and a high-pass filter54.

In this system as well, in the transmitting station10, a radio data signal obtained by adding the pilot carrier signal fLOfrom the local oscillator14to the intermediate frequency subcarrier signal fIFfrom the radio signal modulator12by the adder16is input to the E/O converter18, a semiconductor laser element in the E/O converter18is directly modulated by the radio data signal to obtain an optical signal, and this optical signal is transmitted to the transmitting device32through the optical fiber30, as in the first and second embodiments.

Of the constituent elements of the transmitting device32, the O/E converter34receives the optical signal transmitted from the transmitting station10through the optical fiber30and converts the optical signal into an electrical signal. The divider36supplies the electrical signal output from the O/E converter34to the low-pass filter52and the high-pass filter54.

The low-pass filter52has the passband of the intermediate frequency subcarrier signal fIF, and the high-pass filter54has the passband of the pilot carrier signal fLO.

The multiplier42multiples output signals from the two filters52and54and outputs the multiplied signal. The bandpass filter44extracts a predetermined radio frequency signal from the output from the multiplier42. The power amplifier46power-amplifies the radio frequency signal output from the bandpass filter44and outputs the amplified signal. The antenna48radiates the amplified signal into air as a radio wave.

In the system with the above arrangement, on the transmitting device side, the optical signal transmitted through the optical fiber30is received by the O/E converter34, converted into an electrical signal, and supplied through the divider36to the low-pass filter52having the passband for the intermediate frequency subcarrier signal fIFand the high-pass filter54having the passband for the pilot carrier signal fLO, thereby reproducing the original intermediate frequency subcarrier signal fIFand pilot carrier signal fLO.

The reproduced intermediate frequency subcarrier signal fIFand pilot carrier signal fLOare input to the multiplier42and multiplied.

The output from the multiplier42is passed through the bandpass filter44to extract a predetermined radio frequency signal. The extracted radio frequency signal is amplified through the power amplifier46, radiated into air as a radio wave through the antenna48, and transmitted to a terminal side in the cell.

As described above, in the system according to the third embodiment, on the transmitting device32side, two outputs from the divider36are input to the low-pass filter52and high-pass filter54, respectively, to extract the intermediate frequency subcarrier signal fIFand pilot carrier signal fLO. That is, not the bandpass filters38and40but the low-pass filter52and high-pass filter54are used to extract the intermediate frequency subcarrier signal fIFand pilot carrier signal fLO.

According to this arrangement, if the passband of the bandpass filter44connected to the output side to extract a radio frequency signal has a margin, the flexibility for frequency selection increases so that the carrier of the radio frequency signal can be changed without exchanging the hardware of the transmitting device since the low-pass filter52and high-pass filter54have wider frequency passbands than the passband of the bandpass filter.

To extract the original pilot carrier signal fLOand intermediate frequency subcarrier signal fIFfrom the sum signal of the pilot carrier signal fLOand intermediate frequency subcarrier signal fIFusing two bandpass filters, bandpass filters having large Q values depending on the CNR value of the signal to be processed are normally used. More specifically, to prevent any decrease in CNR value of a received signal obtained by receiving an optical signal from the transmitting station10through the optical fiber30and photoelectrically converting the signal by the O/E converter34, filters having large Q values must be used as the bandpass filters38and40, and the range of selection of the frequency of a radio frequency signal becomes narrow.

However, according to the system of the third embodiment, even when the pilot carrier signal fLOis superposed on the transmitting station10side, an increase in RIN value of the intermediate frequency subcarrier signal fIFband can be suppressed. Hence, the optical modulation index of the pilot carrier signal fLOcan be made large without increasing the RIN value of the intermediate frequency subcarrier signal fIFband, and the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOcan be separated using the low-pass filter52and high-pass filter54. Therefore, flexibility of the radio frequency signal can be increased, so a radio communication station with a wide application range can be provided.

A fourth embodiment in which the frequency of a radio frequency signal can be highly increased even when the frequencies of intermediate frequency subcarrier signal fIFand pilot carrier signal fLOare not high.

Fourth Embodiment

FIG. 12shows the fourth embodiment of the present invention. The arrangement of a transmitting station10is the same as in the first and second embodiments. In a transmitting device32as well, the same reference numerals as in the first and second embodiments denote the same parts in the fourth embodiment.

The transmitting device32of the fourth embodiment is different from the transmitting device32of the third embodiment in that a multiplier56and a bandpass filter58are connected between the high-pass filter54and the multiplier42. The multiplier56is multiples a filtered output from the high-pass filter54by n and outputs the signal. The bandpass filter58extracts a predetermined frequency band component from the output multiplied by n. Note that n is a positive integer.

In the system having the above arrangement, on the transmitting device32side, an optical signal transmitted through the optical fiber30is received by the O/E converter34, converted into an electrical signal. The electrical signal is supplied through the divider36to the low-pass filter52having the passband for an intermediate frequency subcarrier signal fIFand the high-pass filter54having the passband for a pilot carrier signal fLO, thereby reproducing the original intermediate frequency subcarrier signal fIFand pilot carrier signal fLO.

Of the reproduced intermediate frequency subcarrier signal fIFand pilot carrier signal fLO, the pilot carrier signal fLOis multiplied by n by the multiplier56and then passed through the bandpass filter58to obtain a pilot carrier signal n×fLOmultiplied by a desired value. This signal is input to the multiplier42and used for frequency conversion.

The multiplier42multiplies the intermediate frequency subcarrier signal fIFfrom the low-pass filter52by the multiplied pilot carrier signal n×fLO. The obtained signal output is passed through a bandpass filter44to extract a predetermined radio frequency signal. The radio frequency signal output from the bandpass filter44is power-amplified by the power amplifier46and radiated from the antenna48into air as a radio wave.

In the fourth embodiment, the multiplier56and bandpass filter58are added to the arrangement of the third embodiment. The filtered output from the high-pass filter54is multiplied by n, and a predetermined frequency band component is extracted, by the bandpass filter58, from the output multiplied by n, thereby obtaining the pilot carrier signal n×fLOmultiplied by a desired value, which is to be used for frequency conversion. This point is different from the third embodiment.

As a laser light source used in the electro-optical converter18of the transmitting station10, a distributed feedback semiconductor laser element (DFB-LD) or a Fabri-Pérot semiconductor laser element (FP-LD) is used.

Normally, a modulation band fc of a DFB-LD is 3 [GHz], and that of an FP-LD is 1 to 2 [GHz]. If the frequency band becomes higher, the modulation efficiency degrades. Therefore, except for a special processed laser element which can be modulated by a high frequency signal, the frequency band of the pilot carrier signal fLOthat can be superposed is limited to about 3 to 5 [GHz].

Since the frequency of the intermediate frequency subcarrier signal fIFused in this system is lower than 1 [GHz], a frequency fMWof a radio frequency signal corresponding to fIF+fLO(output from the bandpass filter44) is limited to 4 to 6 [GHz].

However, in the arrangement having the multiplier56, as shown inFIG. 12, the pilot carrier signal fLOto be used for frequency conversion is multiplied by a desired value. Since the frequency fMWof the radio frequency signal can be set as fMW=fIF+n×fLO, this system can generate a radio frequency signal in a higher frequency band without being limited to the modulation band of the laser element in the electro-optical converter18.

However, noise is inevitably added upon multiplying the pilot carrier signal fLOby n, and this normally degrades the quality of the radio frequency signal. According to the present invention, however, the optical modulation index of the pilot carrier signal fLOcan be made large without increasing the RIN value of the intermediate frequency subcarrier signal fIFband, so both the pilot carrier signal and subcarrier signal can maintain satisfactory CNR characteristics.

Therefore, the system of the fourth embodiment can prevent any large degradation in quality of the radio frequency signal even when the multiplier56is used. In place of the low-pass filter52and high-pass filter54, bandpass filters38and40may be used, as in the first and second embodiments.

According to the fourth embodiment, the transmitting device comprises an opto-electrical converter for receiving an optical signal transmitted through an optical fiber, converting the optical signal into an electrical signal, and outputting the electrical signal, a filter for extracting the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOfrom the converted and output electrical signal, a frequency multiplier for multiplying the extracted pilot carrier signal fLO, a frequency converter for frequency-converting the extracted intermediate frequency subcarrier signal fIFusing the multiplied pilot carrier signal fLOto obtain a radio frequency signal, and an antenna for transmitting the obtained radio frequency signal.

On the transmitting station side, the pilot carrier signal fLOas a sinusoidal wave used to up-converting the intermediate frequency subcarrier signal fIFinto a radio frequency signal (frequency F0) is added to the intermediate frequency subcarrier signal fIFas a signal in the intermediate frequency band, which is modulated with data to be transmitted. The sum signal is converted into an optical signal and sent to the optical fiber. This conversion to an optical signal is performed by controlling a current from a semiconductor laser element in accordance with the sum signal.

As the semiconductor laser element, a distributed feedback semiconductor laser element (DFB-LD) or a Fabri-Pérot semiconductor laser element (FP-LD) is used. Normally, the modulation band fc of a DFB-LD is 3 [GHz], and that of a FP-LD is 1 to 2 [GHz]. If the frequency band becomes higher, the modulation efficiency degrades. Therefore, except for a special processed laser element which can be modulated by a high frequency signal, the frequency band of the pilot carrier signal fLOthat can be superposed is limited to about 3 to 5 [GHz].

Since the frequency of the intermediate frequency subcarrier signal fIFused in this embodiment is lower than 1 [GHz], the frequency F0of a radio frequency signal corresponding to fIF+fLOis originally limited to 4 to 6 [GHz].

However, when a multiplier is used, the pilot carrier signal fLOto be used for frequency conversion is multiplied. Since the frequency F0of the radio frequency signal can be set as F0=fIF+n×fLO(n is a positive integer), a radio frequency signal in a higher frequency band can be generated without being limited to the modulation band of the laser element in the electro-optical converter18.

The above embodiments have been described mainly in association with transmission. An actual system need allow transmission and reception. The fifth embodiment for such a system will be described next.

Fifth Embodiment

FIGS. 13 and 14show the fifth embodiment of the present invention. The main arrangement is the same as in the second embodiment, and the same reference numerals as in the above embodiments denote the same parts in the fifth embodiment. In the fifth embodiment, the present invention is applied to a base station device for radio communication, as in the second embodiment.

In the embodiment shown inFIG. 13, to allow bi-directional communication, the terminal station is constructed as a transmitting/receiving station10A including not only the transmission function but also the reception function. The transmitting device connected to the transmitting/receiving station10A is also constructed as a transmitting/receiving device32A having not only the transmission function but also the reception function.

The transmitting/receiving station10A and transmitting/receiving device32A are connected through optical fibers30aand30b. AlthoughFIG. 13illustrates only one transmitting/receiving device32A, a plurality of transmitting/receiving devices32A may be arranged in correspondence with one transmitting/receiving station10A. When a plurality of transmitting/receiving devices32A are present, the transmitting/receiving device32A are set at separate locations. A range where radio waves reach is a radio zone (cell or service area), and each transmitting/receiving device32A can transmit/receive radio waves to/from communication terminals in the cell.

Of the optical fibers30aand30b, the former is used for a down link (for a transmission line), and the latter is used for an up link (for a reception line).

The transmitting/receiving station10A has the radio signal modulator12, the local oscillator14, the adder16, and the E/O converter18for the down link (for transmission), and an O/E converter62and a demodulator64for the up link (for reception).

The transmitting/receiving device32A has the O/E converter34, the divider36, the bandpass filters38,40, and44, the multiplier42, and the power amplifier46for the down link (for transmission), a circulator (or duplexer)66, and a transmission/reception antenna68. The transmitting/receiving device32A further includes a low-noise amplifier70, a bandpass filter72, a multiplier74, a bandpass filter76, and an E/O converter78for the up link (for reception).

Of these elements, the E/O converter18incorporates a semiconductor laser element as a light source and has a function of outputting an optical signal modulated with a radio data signal output from the adder16by controlling the current of the semiconductor laser element in accordance with the radio data signal. The E/O converter18is connected to the optical fiber30a. The optical signal output from the E/O converter18is output to the optical fiber30a.

The O/E converter34as a constituent element of the transmitting/receiving device32A converts the optical signal transmitted through the optical fiber30ainto an electrical signal. The divider36supplies the received electrical signal output from the O/E converter34to the bandpass filters38and40.

The bandpass filter38extracts an intermediate frequency subcarrier signal fIFand contains the frequency fIFin the passband. The bandpass filter40extracts a pilot carrier signal fLOand contains the frequency fLOin the passband.

The multiplier42multiplies output signals from the two bandpass filters38and40and outputs the multiplied signal. The bandpass filter44extracts a predetermined radio frequency signal from the output from the multiplier42. The power amplifier46power-amplifies the radio frequency signal output from the bandpass filter44and outputs the amplified signal. The antenna68receives the amplified signal through the circulator (or duplexer)66and radiates the signal into air as a radio wave. The antenna68also receives a radio wave arriving from air and supplies the signal to the low-noise amplifier70through the circulator66.

The circulator66is a device for switching between the path for guiding the radio frequency signal to be transmitted to the antenna68and the path for guiding a received radio frequency signal received by the antenna68to the low-noise amplifier70.

The low-noise amplifier70has performance for amplifying the received radio frequency signal with low noise. The bandpass filter72passes an output from the low-noise amplifier70through a predetermined passband to extract a predetermined passband component. The multiplier74multiplies the output from the bandpass filter72by the pilot carrier signal fLOoutput from the bandpass filter40.

The bandpass filter76passes the output from the multiplier74in a predetermined passband to extract a predetermined passband component as a radio data signal. The E/O converter78converts the radio data signal obtained through the bandpass filter76into an optical signal and outputs the optical signal. The E/O converter78incorporates a semiconductor laser element as a light source and has a function of outputting an optical signal modulated with the radio data signal by controlling the current of the semiconductor laser element in accordance with the radio data signal. The optical signal output from the E/O converter78is output to the optical fiber30b.

The O/E converter62of the transmitting/receiving station10A is connected to the optical fiber30bto convert the optical signal transmitted from the transmitting/receiving device32A through the optical fiber30binto an electrical signal and outputs the signal. The demodulator64receives the electrical signal converted by the O/E converter62and demodulates the signal into the original radio data signal.

In the fifth embodiment having the above arrangement, in the transmitting/receiving station10A, a radio data signal obtained by adding the pilot carrier signal fLOfrom the local oscillator14to the intermediate frequency subcarrier signal fIFfrom the radio signal modulator12by the adder16is input to the E/O converter18.

In the E/O converter18, the laser element is directly modulated with the radio data signal to obtain an optical signal. This optical signal is transmitted to the transmitting/receiving device32A through the optical fiber30a.

On the transmitting/receiving device32A side, the optical signal transmitted through the optical fiber30ais received by the O/E converter34, converted into an electrical signal. The electrical signal is supplied to the bandpass filter38having the passband for the intermediate frequency subcarrier signal fIFand the bandpass filter40having the passband for the pilot carrier signal fLO, thereby reproducing the original intermediate frequency subcarrier signal fIFand pilot carrier signal fLO.

The reproduced intermediate frequency subcarrier signal fIFand pilot carrier signal fLOare input to the multiplier42and multiplied.

The output from the multiplier42is passed through the bandpass filter44to extract a predetermined radio frequency signal. The extracted radio frequency signal is amplified through the power amplifier46, radiated from the antenna68into air through the circulator66as a radio wave, and transmitted to a terminal side in the cell.

On the other hand, a radio wave transmitted from a terminal side in the cell is received by the antenna68, input to the low-noise amplifier70through the circulator66, and amplified. A predetermined band component is extracted by the bandpass filter72. The extracted component signal is multiplied by the pilot carrier signal fLOfrom the bandpass filter40by the multiplier74and down-converted. A predetermined band component is extracted by the bandpass filter76, converted into an optical signal by the E/O converter78, sent to the optical fiber30bas an up link optical signal, and sent to the transmitting/receiving station10A.

As the characteristic feature of this embodiment, the pilot carrier signal fLOis extracted in the transmitting/receiving device32A using the bandpass filter40, the extracted pilot carrier signal fLOis input to the multiplier42on the transmission system side and the multiplier74on the reception system side. In the transmission system, the extracted pilot carrier signal fLOis multiplied by the intermediate frequency subcarrier signal fIFby the multiplier42to up-convert the frequency of the intermediate frequency subcarrier signal fIF. In the reception system, the radio frequency signal is multiplied by the pilot carrier signal fLOby the multiplier74to down-convert the frequency of the radio frequency signal.

That is, the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOare extracted from two outputs from the divider36in the transmitting/receiving device32A using the bandpass filters38and40. The extracted pilot carrier signal fLOis separated from the transmission system and also input to the multiplier74of the reception system.

The multiplier42up-converts the frequency of the intermediate frequency subcarrier signal fIFtransmitted from the transmitting/receiving station10A using the pilot carrier signal fLOto obtain a radio frequency signal, and this radio frequency signal is transmitted by radio through the power amplifier46and antenna68, as described above.

The multiplier74multiplies a radio frequency signal by the pilot carrier signal fLOto down-convert the frequency of the radio frequency signal.

More specifically, the radio frequency signal transmitted by radio is received by the antenna68and input to the low-noise amplifier70through the circulator or duplexer66, and a desired band is extracted by the bandpass filter72. The frequency of the extracted band component of the radio frequency signal is down-converted by the multiplier74using the pilot carrier signal fLO. The image frequency and the like are removed by the bandpass filter76to extract a desired band, thereby obtaining an up link intermediate frequency signal. The up link intermediate frequency signal is converted into an optical signal by the E/O converter78and transmitted to the transmitting/receiving station10A through the optical fiber30b.

In the transmitting/receiving station10A, the optical signal transmitted from the transmitting/receiving device32A side is received by the O/E converter62and input to the demodulator64to extract data.

As described above, in the fifth embodiment, in the transmitting/receiving device32A, the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOare extracted from the sum signal of the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOfor radio transmission, which are transmitted from the transmitting/receiving station10A, using the bandpass filters. The extracted pilot carrier signal fLOis used for frequency up conversion in the transmission system and for frequency down conversion in the reception system.

Hence, the transmitting/receiving device32A can down-converts the frequency of an up link signal from the transmitting/receiving device32A to the transmitting/receiving station10A without requiring a component such as a local oscillator. Therefore, the constituent elements of the reception system can be simplified. In addition, a received signal in a radio frequency band, which is received by the antenna is down-converted, input to the E/O converter78, converted into an optical signal, and sent to the transmitting/receiving station10A. Therefore, the frequency band required for the E/O converter78of the reception system can be made low. Since the frequency band of the signal to be processed is low, specifications of the laser element, driver amplifier, and the like incorporated in the E/O converter78can be lenient, and inexpensive elements can be used.

With the above arrangement, the transmitting/receiving device32A can be made compact and simple, so an inexpensive transmitting/receiving device32A can be provided.

FIG. 14shows another arrangement of the transmitting/receiving device32A. In the transmitting/receiving device32A shown inFIG. 14, the multiplied pilot carrier signal fLOis used for frequency conversion.

In the example, as in the fourth embodiment shown inFIG. 12, the intermediate frequency subcarrier signal fIFis extracted from the output from the divider36by the low-pass filter52, the pilot carrier signal fLOis extracted by the high-pass filter54, the extracted pilot carrier signal fLOis multiplied by n by the multiplier56and passed through the multiplier56, and this pilot carrier signal n×fLOmultiplied by a desired value is used for frequency up conversion in the transmission system and frequency down conversion in the reception system.

Except that the pilot carrier signal n×fLOmultiplied by a desired value is input to the multiplier42and used for up conversion or input to the multiplier74and used for down conversion, the arrangement is the same as inFIG. 13, and a detailed description thereof will be omitted.

In this example, since the multiplier56and bandpass filter58are added, the circuit scale becomes larger than that of the example shown in FIG.13. Even with this arrangement, the frequency band required for the E/O converter78in the reception system can be made low, as in the example shown in FIG.13. Specifications of the laser element, driver amplifier, and the like incorporated in the E/O converter78can be lenient because the frequency band of a signal to be processed becomes low. Hence, inexpensive elements can be used. In addition, since the multiplier is added, the pilot carrier signal fLOto be used for frequency conversion is multiplied by a desired value. The frequency of a radio frequency signal can be made higher by n, so a radio frequency signal in a higher frequency band can be generated without being limited by the modulation band in the E/O converter18in the transmitting/receiving station10A.

According to the fifth embodiment, even when the optical modulation index of the pilot carrier signal fLOis increased, the CNR of the intermediate frequency subcarrier signal fIFdoes not decrease. Therefore, on the transmitting/receiving device32A side, the pilot carrier signal fLOwith a satisfactory CNR can be obtained. In the transmitting/receiving device32A, the radio frequency signal received by the antenna68is sometimes weak, and the signal for frequency conversion by the multiplier74is required to have a high CNR. As the signal for frequency conversion, the pilot carrier signal fLOfrom the transmitting/receiving station10A can be provided. Additionally, when the pilot carrier signal fLOis multiplied as a signal for frequency conversion, the noise characteristics are not largely degraded in frequency conversion because the CNR of the received pilot carrier signal fLOis large.

In place of the low-pass filter52and high-pass filter54, the bandpass filters38and40may be used, as in the first and second embodiments.

Sixth Embodiment

An embodiment in which when modulated and multiplexed subcarrier signals are to be transmitted from a transmitting/receiving station to a plurality of transmitting/receiving devices, the number of pilot carrier signals for maintaining the frequency stability of radio waves to be radiated from the transmitting/receiving devices is decreased, and a data signal to be radiated can be separated from the received optical signal by a simple filter in each transmitting/receiving device will be described. In this case, only two pilot carrier signals are used regardless of the number of the subcarrier signals, and subcarrier signals are multiplexed at a sufficiently large frequency interval such that the subcarrier signals can be separated by a simple filter. More specifically, the frequency interval between the two pilot carrier signals and that between the subcarrier signals are made equal.

For three or more systems of subcarrier signals to be transmitted as radio signals, only two pilot carrier signals fLOare prepared. Each of the three or more systems of subcarrier signals is converted into a radio signal having a desired carrier frequency. Since transmission and reception systems have the same arrangement, the down link signal processing system will be described in the sixth embodiment for the descriptive convenience.

The first and second pilot carrier generators14-1and14-2are circuits for generating the pilot carrier signals fLO1and fLO2having different frequencies. The adder16synthesizes the two pilot carrier signals fLO1and fLO2with the outputs fIF1, fIF2, fIFpfrom the frequency converters88-1,88-2, . . . ,88-p. The laser driver84drives the laser element86in accordance with the signal synthesized by the adder16. The laser element86is caused by the laser driver84to output a laser beam optically modulated in accordance with the synthesized signal from the adder16and send the signal to the optical fiber30.

Each of the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p comprises the O/E converter34, bandpass filter38for separating the intermediate frequency subcarrier signal fIF1, bandpass filters40-1and40-2for separating the pilot carrier signals fLO1and fLO2, a frequency converter82formed from a multiplier and a power amplifier, and the antenna48.

The O/E converter34converts an optical signal sent through the optical fiber30into an electrical signal. The bandpass filter38separates the intermediate frequency subcarrier signal from the electrical signal. The bandpass filters40-1and40-2separate the first and second pilot carrier signals from the electrical signal from the O/E converter34.

The frequency converter82frequency-converts the separated first and second pilot carrier signals and intermediate frequency subcarrier signal and sends a data signal to the antenna48.

The optical fiber30is an optical transmission line connecting the transmitting/receiving station10B to the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p and has the optical divider34inserted in the midway. The optical divider34divides the optical signal from the laser element86and distributes the optical signals to all the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p connected.

In this system, the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFpoptically transmitted from the transmitting/receiving station10B to the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p are subcarrier-multiplexed at a large frequency interval, as shown inFIG. 16, such that the signals can be separated by a simple filter.

In addition to the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFp, the pilot carrier signals fLO1and fLO2are transmitted. In this system, the number of pilot carrier signals fLO1and fLO2is always two independently of the number of intermediate frequency subcarrier signals. The frequencies of the two pilot carrier signals fLO1and fLO2are set such that a frequency to be sent from the antenna is obtained when integral multiples of the frequencies fLO1and fLO2of the pilot carrier signals are appropriately added/subtracted to/from the frequencies of the intermediate frequency subcarrier signals fIF1, fIF2, fIFp.

In the system having the above arrangement, in the transmitting/receiving station10B, data to be transmitted to the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p are supplied to the modulators12-1,12-2, . . . ,12-p corresponding to the transmitting/receiving devices and modulated into an appropriate form by QPSK, QAM, or the like.

At this time, the frequencies of the plurality of intermediate frequency subcarrier signals are sufficiently separated from each other, as shown in FIG.16. For example, the bandwidth of each intermediate frequency subcarrier signal is about 20 [MHz], an interval of about 100 [MHz] is set.

The first and second pilot carrier generators14-1and14-2generate the pilot carrier signals fLO1and fLO2having different frequencies, respectively. These pilot carrier signals fLO1and fLO2are input to the adder16.

The adder16multiplexes the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFpoutput from the frequency converters88-1,88-2, . . . ,88-p with the first and second pilot carrier signals fLO1and fLO2generated by the first and second pilot carrier generators14-1and14-2, respectively. The multiplexed signal is input to the laser driver84and converted into an optical signal by the laser element86.

The optical signal converted by the laser element86and output is input to the optical fiber30. The optical divider34is inserted midway along the optical fiber30, so the optical signal from the laser element86is divided and distributed to all the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p connected.

The optical signal transmitted to a transmitting/receiving device, e.g., the transmitting/receiving device32B-1is converted into an electrical signal by the O/E converter34. The intermediate frequency subcarrier signal fIF1that is sent to the self station is separated from the obtained electrical signal by the bandpass filter38. The bandpass filter38can be formed from a simple filter having a relatively small Q value. The bandpass filters40-1and40-2further extract the first pilot carrier signal fLO1and second pilot carrier signal fLO2from the signal obtained by the O/E converter34, respectively. The bandpass filters40-1and40-2can also be constructed by simple filters having relatively small Q values.

The intermediate frequency subcarrier signal fIF1separated by the bandpass filter38and the two pilot carrier signals fLO1and fLO2separated by the bandpass filters40-1and40-2are input to the frequency converter82. The frequency converter82appropriately converts the intermediate frequency subcarrier signal fIF1into a target radio frequency by appropriately multiplying, adding, and subtracting the frequencies of these three signals.

The frequency converter82mainly comprises a mixer, a multiplier, a filter, a switch, and the like. Details of the arrangement will be described later.

The intermediate frequency subcarrier signal fIF1converted into a desired radio frequency by the frequency converter82is radiated from the antenna48of the self station into air and sent to a terminal in the service area of the self station.

According to the sixth embodiment, when data signals are to be optically transmitted from the transmitting/receiving station10B to the plurality of transmitting/receiving devices32B-1,32B-2, . . . ,32B-p as subcarriers, only two pilot carrier signals fLO1and fLO2are used while setting a large frequency interval between the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFp, and the frequencies are set in advance such that a frequency to be sent from the antenna is obtained when integral multiples of the frequencies of the pilot carrier signals fLO1and fLO2are appropriately added/subtracted to/from the frequencies of the intermediate frequency subcarrier signals fIF1, fIF2, fIFp.

Since the frequency interval between the intermediate frequency subcarrier signals is set to be sufficiently large, each transmitting/receiving device can extract the intermediate frequency subcarrier signal addressed to the self station from the subcarrier-multiplexed data signal using a simple filter.

The frequencies of the pilot carrier signals fLO1and fLO2are set to satisfy the above relationship. Therefore, the advantage in use of the pilot carrier signals fLO1and fLO2can be maintained: the signal to be sent from the antenna has high frequency stability although only two pilot carrier signals fLO1and fLO2are used. Additionally, since the number of pilot carrier signals fLO1and fLO2is as small as two, high-quality transmission can be performed without largely decreasing the optical modulation index of the intermediate frequency subcarrier signal in optical transmission.

In the present invention, the intermediate frequency subcarrier signals to be optically transmitted from the transmitting/receiving station to the plurality of transmitting/receiving devices are subcarrier-multiplexed at a large frequency interval such that the signals can be separated by a simple filter.

In addition to the intermediate frequency subcarrier signals fIF1, fIF2, . . . fIFp, the pilot carrier signals fLO1and fLO2are transmitted from the transmitting/receiving station10B to the plurality of transmitting/receiving devices32B-1,32B-2, . . . ,32B-p. Unlike the prior art, the number of pilot carrier signals fLO1and fLO2is only two independently of the number of intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFp. The frequencies of the two pilot carrier signals fLO1and fLO2and the frequencies of the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFpare set such that the frequency of each intermediate frequency subcarrier signal can be up-converted into a frequency to be sent from the antenna when integral multiples of the frequencies of the two pilot carrier signals fLO1and fLO2are appropriately added/subtracted to/from the frequencies of the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFp. Note that “integers” for “integral multiples” include “0” and all positive and negative integers.

In the frequency converter82, the pilot carrier signals fLO1and fLO2are multiplied by a multiplier or mixer. Addition/subtraction of signals or the frequencies of multiplied pilot carrier signals is also performed using the mixer.

With this arrangement, even when three or more transmitting/receiving devices are accommodated in a PON, only two pilot carrier signals fLO1and fLO2are necessary to be sent to all the transmitting/receiving devices. In addition, the frequency interval between the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFpcan be set to be sufficiently large such that each signal can be extracted using a simple filter.

As a consequence, the optical modulation index of the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFpis not sacrificed by a number of pilot carrier signals, and satisfactory transmission can be performed. In addition, the process of extracting a necessary signal after reception of the optical signal becomes simple and inexpensive.

Next, a specific example of the frequency arrangement of the two pilot carrier signals fLO1and fLO2will be described as a seventh embodiment.

Seventh Embodiment

FIG. 17is a graph showing a specific frequency arrangement of two pilot carrier signals fLO1and fLO2. As shown inFIG. 17, modulated data signals to be transmitted to transmitting/receiving devices32B-1,32B-2, . . . ,32B-p are subcarrier-multiplexed with center frequencies fIF1, fIF2, . . . , fIFp. The frequency difference between adjacent intermediate frequency subcarrier signals is ΔF or an integral multiple of ΔF.

The frequency difference between the pilot carrier signals fLO1and fLO2is also ΔF. In the seventh embodiment, as shown inFIG. 17, the pilot carrier signals fLO1and fLO2are arranged in a frequency region higher than that range where the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFpare arranged.

The operation of the present invention will be described using specific numerical data.

Assume that four intermediate frequency subcarrier signals fIF1, fIF2, fIF3, fIF4having frequencies of 100 [MHz], 200 [MHz], 400 [MHz], and 500 [MHz], respectively, are subcarrier-multiplexed, and the first and second pilot carrier signals fLO1and fLO2have frequencies of 2 [GHz] and 1.9 [GHz], respectively. A radio frequency F0to be sent from an antenna48of the transmitting/receiving device32B is 22 [GHz].

In the transmitting/receiving device32B-1which uses the intermediate frequency subcarrier signal fIF1, the bandpass filter38extracts the intermediate frequency subcarrier signal fIF1from the subcarrier-multiplexed optical signal, and the bandpass filters40-1and40-2extract the first and second pilot carrier signals fLO1and fLO2, respectively.

To up-convert the intermediate frequency subcarrier signal fIF1(=100 [MHz]) into the radio frequency F0(=22 [GHz]) using these signals, the second pilot carrier signal fLO2(=1.9 [GHz]) is added to a frequency (=20 [GHz]) obtained by multiplying the first pilot carrier signal fLO1by 10, and the frequency fIF1(=100 [MHz]) of the intermediate frequency subcarrier signal is added to the resultant frequency. That is,fLO1⁢(=2⁢[GHz])×10+fLO2⁢(=1.9⁢[GHz])+fIF1⁢(=100⁢[MHz])=20+1.9+0.1=22⁢[GHz]

As a result, the intermediate frequency subcarrier signal fIF1having a frequency of 100 [MHz] can be up-converted into the radio frequency F0of 22 [GHz] using the first and second pilot carrier signals fLO1and fLO2.

To up-convert the intermediate frequency subcarrier signal fIF2(=200 [MHz]) into the radio frequency F0, a frequency (=3.8 [GHz]) obtained by multiplying the second pilot carrier signal fLO2by 2 is added to a frequency (=18 [GHz]) obtained by multiplying the first pilot carrier signal fLO1by 9, and the intermediate frequency subcarrier signal fIF2(=200 [MHz]) is added to the resultant frequency. That isfLO1⁢(=2⁢[GHz])×9+fLO2⁢(=1.9⁢[GHz])×2+fIF2⁡(=200⁢[MHz])=18+3.8+0.2=22⁢[GHz]

As a result, the intermediate frequency subcarrier signal fIF2having a frequency of 200 [MHz] can be up-converted into the radio frequency F0of 22 [GHz] using the first and second pilot carrier signals fLO1and fLO2.

To up-convert the intermediate frequency subcarrier signal fIF3(=400 [MHz]) into the radio frequency F0, a frequency (=7.6 [GHz]) obtained by multiplying the second pilot carrier signal fLO2by 4 is added to a frequency (=14 [GHz]) obtained by multiplying the first pilot carrier signal fLO1by 7, and the intermediate frequency subcarrier signal fIF3(=400 [MHz]) is added to the resultant frequency. That isfLO1⁢(=2⁢[GHz])×7+fLO2⁢(=1.9⁢[GHz])×4+fIF3⁢(=400⁢[MHz])=14+7.6+0.4=22⁢[GHz]

As a result, the intermediate frequency subcarrier signal fIF3having a frequency of 400 [MHz] can be up-converted into the radio frequency F0of 22 [GHz] using the first and second pilot carrier signals fLO1and fLO2.

To up-convert the intermediate frequency subcarrier signal fIF4(=500 [MHz]) into the radio frequency F0, a frequency (=9.5 [GHz]) obtained by multiplying the second pilot carrier signal fLO2by 5 is added to a frequency (=12 [GHz]) obtained by multiplying the first pilot carrier signal fLO1by 6, and the frequency fIF4(=500 [MHz]) of the intermediate frequency subcarrier signal is added to the resultant frequency. That isfLO1⁢(=2⁢[GHz])×6+fLO2⁢(=1.9⁢[GHz])×5+fIF4⁢(=500⁢[MHz])=12+9.5+0.5=22⁢[GHz]

As a result, the intermediate frequency subcarrier signal fIF4having a frequency of 500 [MHz] can be up-converted into the radio frequency F0of 22 [GHz] using the first and second pilot carrier signals fLO1and fLO2.

As described above, using only the two pilot carrier signals fLO1, and fLO2, the intermediate frequency subcarrier signals can be up-converted into the radio frequency F0in all transmitting/receiving devices.

Arrangements of the frequency converter82will be described next with reference toFIGS. 18,19, and22.

[First Arrangement of Frequency Converter]

FIG. 18shows a first arrangement of the frequency converter82in the transmitting/receiving device.

The multiplier 92 multiplies the first pilot carrier signal fLO1by |n| and supplies the signal to the mixer96. The mixer96also receives the intermediate frequency subcarrier signal fIFand mixes this signal with the signal from the multiplier92, which is multiplied by |n|. The bandpass filter100extracts a desired frequency component from the signal from the mixer96.

The multiplier94multiplies the second pilot carrier signal fLO2by |m| and supplies the signal to the mixer98. The mixer98also receives the signal from the bandpass filter100and mixes this signal with the second pilot carrier signal fLO2from the multiplier94, which is multiplied by |m|. The bandpass filter102extracts a desired frequency component from the signal from the mixer98.

In the frequency converter82having the arrangement shown inFIG. 18, the first pilot carrier signal fLO1is multiplied by a necessary multiplying factor (|n|) by the multiplier92. In the example of the above-described intermediate frequency subcarrier signal fIF1, the first pilot carrier signal fLO1is multiplied by 10 and the second pilot carrier signal fLO2is multiplied by a necessary multiplying factor (|m|) by the multiplier94. In the above example, the second pilot carrier signal fLO2is multiplied by 1, i.e., passes through the multiplier94without any multiplication. The intermediate frequency subcarrier signal fIF1(100 [MHz]) is mixed with the first pilot carrier signal fLO1(20 [GHz]) multiplied by |n| by the mixer96. Of the sum frequency component (20.1 [GHz]) and difference frequency component (19.9 [GHz]) output from the mixer96, the sum frequency component (20.1 [GHz]) is selected by the filter100and outputted.

The output from the filter100is mixed with the second pilot carrier signal fLO2(1.9 [GHz]) multiplied by |m| by the mixer98. Of the sum frequency component (22 [GHz]) and difference frequency component (18.2 [GHz]) output from the mixer98, the sum frequency component (22 [GHz]) is selected by the bandpass filter102and outputted.

In this way, the frequency converter82can obtain the intermediate frequency subcarrier signal up-converted to the target frequency.

[Second Arrangement of Frequency Converter]

FIG. 19shows another arrangement of the frequency converter82in the transmitting/receiving device.

The multiplier92multiplies the first pilot carrier signal fLO1by |n| and supplies the signal to the mixer96. The multiplier94multiplies the second pilot carrier signal fLO2by |m| and supplies the signal to the mixer96. The mixer96mixes the multiplied outputs from the multipliers92and94.

The bandpass filter100extracts a desired frequency component from the signal from the mixer96and outputs the frequency component to the mixer98. The mixer98also receives the intermediate frequency subcarrier signal fIF, mixes this signal with the signal passed through the bandpass filter100, and outputs the mixed signal to the filter102. The bandpass filter102extracts a desired frequency component from the signal from the mixer98.

In the frequency converter82having the arrangement shown inFIG. 19, the first pilot carrier signal FLO1is multiplied by a necessary multiplying factor (|n|) by the multiplier92. In the example of the above-described intermediate frequency subcarrier signal fIF1, the first pilot carrier signal fLO1is multiplied by 10 and the second pilot carrier signal fLO2is multiplied by a necessary multiplying factor (|m|) by the multiplier94. In the above example, the second pilot carrier signal fLO2is multiplied by 1, i.e., passes through the multiplier94without any multiplication. The first pilot carrier signal fLO1(20 [GHz]) multiplied by |n| and second pilot carrier signal fLO2(1.9 [GHz]) multiplied by |m| are mixed by the mixer96, and a sum frequency component (21.9 [GHz]) and difference frequency component (18.1 [GHz]) are output.

Of these frequency components, the sum frequency component (21.9 [GHz]) is selected by the bandpass filter100and outputted. The intermediate frequency subcarrier signal fIF1(100 [MHz]) and output from the bandpass filter100are mixed by the mixer98. Of the sum frequency component (22 [GHz]) and difference frequency component (21.8 [GHz]) output from the mixer98, the sum frequency component (22 [GHz]) is selected by the filter102and outputted.

In this manner, the frequency converter82can obtain the intermediate frequency subcarrier signal up-converted to the target frequency (22 [GHz]).

As the multipliers92and94used for the frequency converter82, conventional frequency multipliers with fixed multiplying factors are used if the multiplying factors |n| and |m| can be fixed. However, the multiplying factors may change sometimes depending on the system arrangement. That is, the frequency of an intermediate frequency subcarrier signal sent to the self station may change. In such a case, the multipliers92and94are constructed as shown in FIG.20. In this example, the multiplying factor can be changed from 1 (without any multiplication) to k.

An input is supplied to a divider114through a switch110-1. The switch110-1is a path change-over switch for selectively supplying the input to the divider114side or a selector112side. When the path is switched to the selector112side, the input signal is output without any multiplication.

The divider114distributes the input signal to k paths (outputs). The first and second outputs are input to a mixer116-1. The mixer116-1outputs the sum frequency component and difference frequency component of the two input signals. From the output from the mixer116-1, the sum frequency component is extracted by a bandpass filter118-1, and a signal multiplied by 2 is output.

The output from the bandpass filter118-1is supplied to a switch110-2. The switch110-2is a path change-over switch for supplying the signal to a mixer116-2of the next stage or the selector112side. The mixer116-2mixes the output from the bandpass filter118-1with the output from the divider114and outputs the mixed signal. The mixer116-2mixes the output from the mixer116-1for multiplication by 2 with the output from the divider114, i.e., the original frequency signal. Hence, frequency up conversion of multiplication by 3 is performed. A bandpass filter118-2extracts the sum frequency component from the output from the mixer116-2.

In a similar manner, an output from a bandpass filter118-i is supplied to a switch110-(i+1). The switch110-(i+1) is a path change-over switch for supplying this signal to a mixer116-(i+1) or the selector112side. The mixer116-(i+1) mixes the output from the bandpass filter118-i with the output from the divider114and outputs the mixed signal. The output from the mixer116-(i+1) is supplied to a bandpass filter118-(i+1) for extracting the sum frequency component, and frequency up conversion of (i+1) multiplication is performed.

The selector112selects one of the signal (original signal) from the switch110-1and signals frequency-up-converted by the respective stages and outputs the selected signal.

In the example shown inFIG. 20, multiplication by 1 (without any multiplication) to k can be performed. The input pilot carrier signals fLO1and fLO2are input to the switch110-1. When the signals are to be multiplied by 1, i.e., output without any multiplication, the switch110-1is switched to the selector112side, and the selector112is switched to the switch110-1side.

In this way, a carrier signal multiplied by 1 is output. For multiplication by 2 or more, the switch110-1is switched and connected to the divider114side.

The divider114divides the input signal into k components. Two components are input to the two terminals of the mixer116-1. The sum frequency component generated by the mixer116-1is selected by the bandpass filter118-1and outputted.

The output from the bandpass filter118-1is connected to the switch110-2. When the switch110-2is connected to the selector112side, a signal multiplied by 2 is output. When the switch110-2is connected to the mixer116-2on the output side, the signal is multiplied by 3 or more. Similarly, the switches110-2,110-3, . . . and selector112are controlled such that the mixers and filters are alternately connected, and the signal multiplied by a necessary multiplying factor is connected to the output terminal.

With this arrangement, a multiplier with a variable multiplying factor can be constructed.

Another arrangement may be employed for the multiplier with a variable multiplying factor.FIG. 21shows a multiplier92or94having a nonlinear element such as a diode119to which an input signal is supplied, a filter bank formed of filters118-1,118-2, . . . ,118-k to which an output signal of the diode119is supplied via a divider114, and a selector112selecting one of output signals from the filters118-1,118-2, . . . ,118-k.

Each of filters118-1,118-2, . . . ,118-k of the filter bank has pass characteristics corresponding to its harmonics. A filter corresponding to a desired multiplying factor is selected from the filter bank, thereby constructing a multiplier with a variable multiplying factor. Each output from the filter bank118is supplied to a selector112. The selector112is controlled to select any path from the filter bank118and the signal multiplied by a necessary multiplying factor is connected to the output terminal.

[Third Arrangement of Frequency Converter]

Still another arrangement of the frequency converter82with a variable multiplying factor will be described with reference to FIG.22.

The first pilot carrier signal fLO1is distributed into (n+m) components by a divider128-1. The second pilot carrier signal fLO2is distributed into (n+m) by a divider128-2.

Switches126-1,126-2, . . . ,126-(n+m) select outputs from the divider128-1or128-2. Outputs from the switches126-1and126-2are supplied to a mixer122-1. An output from the mixer122-1is supplied to a mixer122-2through a bandpass filter124-1.

In a similar manner, a mixer122-i mixes an output from a bandpass filter124-(i−1) with an output from a switch126-(i+1). A mixer122-(n+m) at the final stage mixes an output from a bandpass filter124-(n+m−1) with the intermediate frequency subcarrier signal fIF. An output from the mixer122-(m+n) is output through a bandpass filter124-(n+m). Even when the values n and m change, the value (n+m) does not change.

In this arrangement, the two pilot carrier signals fLO1and fLO2are distributed, by the dividers128-1and128-2, into plural signals whose number equals a maximum multiplying factor. The mixer122-1has two terminals to which one of the signals fLO1, fLO2, and fIFis directly input, and each of the mixers122-2to122-(n+m−1) has one terminal to which one of the signals fLO1, fLO2, and fIFis directly input. Hence, a total of n+m input terminals are present.

The switches126-1,126-2, . . . are controlled such that the first pilot carrier signal fLO1is input to n terminals and the second pilot carrier signal fLO2is input to m terminals. The filters124-1,124-2, connected to the output sides of the mixers122-1,122-2, . . . select sum frequency components from the difference frequency components and sum frequency components between the mixed signals output from the122-1,122-2, . . . and the directly input signals fLO1, fLO2, and fIFand output the sum frequency components.

In this arrangement, the filter124-(n+m−1) outputs a sum carrier of the first pilot carrier signal fLO1multiplied by n and the second pilot carrier signal fLO2multiplied by m. This signal is mixed with the intermediate frequency subcarrier signal fIFby the mixer 122-(n+m). As a result, the sum frequency component and difference frequency component are output from the mixer122-(n+m), and the sum frequency component is selected by the bandpass filter124-(m+n) and output, so a multiplied signal fLO1×n+fLO2×m+fIFis obtained.

With this arrangement, a frequency converter whose multiplying factor can be changed to a desired value can be constructed.

[Fourth Arrangement of Frequency Converter]

Still another arrangement of the frequency converter82will be described. In the above examples, n and m are “0” or positive integers.

However, in the present invention, n and m can be negative values. For example, assume that a relation
10×fLO1+fIfi=F0
holds between the target frequency F0, pilot carrier signals fLO1and fLO2(=fLO1−Δf), and intermediate frequency subcarrier signal fIF1.

For an intermediate frequency subcarrier signal fIFi+1 (=fIFi+ΔF),
9×fLO1+fLO2+fIfi=F0
That is, the target frequency F0can be synthesized using positive values for both n and m, i.e., n=9 and m=1.

On the other hand, for an intermediate frequency subcarrier signal having a frequency of fIFi−1(=fIFi−ΔF),
40×fLO1−fLO2+fIfi−1=F0
That is, one of n and m is set to be negative: n=40 and m=−1.

In the arrangement of the frequency converter82shown in, e.g.,FIG. 18, frequency synthesis using a negative value, i.e., subtraction can be executed by selecting, by the bandpass filter102, the difference frequency component from the sum frequency component and difference frequency component generated by the mixer98and outputting the difference frequency component.

When negative values can be used as n and m, the upper and lower limits of the intermediate frequency subcarrier signal, i.e., limitation on the number of channels of subcarrier multiplex is moderated, and a flexible system can be constructed.

Various arrangements of the frequency converter have been described above. A pilot carrier signal separator will be described next.

[Arrangement of Pilot Carrier Signal Separator]

The pilot carrier signal separators (bandpass filters40-1and40-2inFIG. 15) in each of the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p can be realized by filters having small Q values, as described above. However, when PLLs (Phase-Locked Loops) are used together with the filters, the pilot carrier signals fLO1and fLO2with higher quality can be separated.

An optical signal sent from the transmitting/receiving station10B contains not only the necessary intermediate frequency subcarrier signals fIFand pilot carrier signals fLO1and fLO2but also various noise components.

There are noise called relative intensify noise originally contained in the optical signal, thermal noise generated by the optical receiver, and shot noise generated when a photocurrent flows to the photodiode. These noise components are generally white noise.

When the pilot carrier signals fLO1and fLO2are separated from the optical signal containing such white noise using only filters with small Q values as the bandpass filters40-1and40-2, a number of noise components are also extracted.

The demand for the noise amount changes depending on the system. Some systems can directly use components extracted by filters. However, in a system with a strict demand for noise, a PLL is connected to the output side of a filter having a small Q value. With this arrangement, the carrier-to-noise ratios of the pilot carrier signals fLO1and fLO2can be made high.

As mentioned in the description of prior arts, in some systems, the center frequency of a radio signal radiated from the antenna48of the self station slightly changes in units of the plurality of transmitting/receiving devices32B-1,32B-2, . . . ,32B-p.

For example, when radio waves in the same 2 [GHz] band are radiated, the center frequency of the radio signal radiated from one transmitting/receiving device is 2.000000 [GHz], and the center frequency of the radio signal radiated from another transmitting/receiving device is 2.000100 [GHz]. That is, the center frequency changes at an interval of, e.g., 100 [kHz].

In this case as well, in the present invention, the frequency interval between subcarrier signals to be optically transmitted is set to be much larger than the frequency interval (e.g., 100 [kHz]) of the radio range. As a result, processing such as data separation in the transmitting/receiving device is facilitated.

When the present invention is applied to a system in which the frequencies of radio signals radiated from the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p are slightly different from each other, a frequency difference corresponding to the frequency difference (e.g., 100 [kHz]) between the radio waves of the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p is applied to the frequencies of subcarrier signals to be optically transmitted in advance as an offset.

Since the frequency interval between the subcarrier signals to be optically transmitted is very large, this offset does not affect the operation of the present invention at all.

An example will be described below.

Assume that a signal radiated from the self station antenna48of the transmitting/receiving device32B-1has a center frequency F01, and a signal radiated from the self-station antenna48of the transmitting/receiving device32B-2has a center frequency F02(=F01+ΔFR).

The center frequency of the first and second pilot carrier signals fLO1and fLO2to be optically transmitted is fLO2(=fLO1+ΔF). The intermediate frequency subcarrier signal fIF1is subcarrier-transmitted with a center frequency F01that is given by
F01=7×fLO1+3×fLO2+fIF1.

The intermediate frequency subcarrier signal fIF2is subcarrier-transmitted with a center frequency F02that is given by
F02=8×fLO1+2×fLO2+fIF2.

At this time, the intermediate frequency subcarrier signals fIF1and fIF2are determined such that the difference between the center frequencies fIF1and fIF2of the two intermediate frequency subcarrier signals is represented by
ΔF+ΔFR.
Since
ΔF>>ΔFR
then
ΔF+ΔFR=ΔF.

In this case, ΔFRis the above-described offset amount of the subcarrier frequency.

In the embodiments of the present invention, such a small offset has not been specified so far and will not particularly be described in examples later. However, note that the method described in the seventh embodiment may be employed in practicing the present invention.

Eighth Embodiment

FIG. 23is a graph showing the frequency arrangement in optical transmission of the eighth embodiment. Intermediate frequency subcarrier signals are subcarrier-multiplexed to center frequencies fIF1, fIF2, . . . , fIFp. The center frequency interval between adjacent intermediate frequency subcarrier signals is an integral multiple (≧1) of ΔF. A first pilot carrier signal fLO1is set in a frequency band higher than that of the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFp, as in the case described in the seventh embodiment. On the other hand, a second pilot carrier signal fLO2is set in a frequency band lower than that of the intermediate frequency subcarrier signals fIF1, fIF2, . . . , fIFpand has the frequency ΔF in this embodiment.

Assume that for the intermediate frequency subcarrier signal fIF1, the frequency F0of a signal to be radiated from the antenna is F0=n×fLO1+fIF1. The intermediate frequency subcarrier signal fIF2(=fIF1+ΔF) is synthesized such that the frequency F0becomes F0=n×fLO1−fLO2+fIF2. That is, the intermediate frequency subcarrier signal fIF1is synthesized while setting a multiplying factor m of the second pilot carrier signal fLO2at 0, and the intermediate frequency subcarrier signal fIF2is synthesized while setting the multiplying factor m at −1. The intermediate frequency subcarrier signals fIF3, fIF4, . . . , fIFpare also synthesized in a similar manner.

If a negative multiplying factor m is not preferable, for the intermediate frequency subcarrier signal fIFphaving the highest frequency, F0=n×fLO1+fIFpis set. For an intermediate frequency subcarrier signal fIFp-1(=fIFp−ΔF), F0=n×fLO1+fLO2+fIFp-1is set. Frequencies are sequentially synthesized in a similar manner.

For an intermediate frequency subcarrier signal fIFi, F0=n×fLO1+fIFiis set. For an intermediate frequency subcarrier signal having a frequency lower than fIFi, an integral multiple of fLO2is appropriately added. For an intermediate frequency subcarrier signal having a frequency higher than fIFi, an integral multiple of fLO2is appropriately subtracted.

The arrangement of the system to which the eighth embodiment is applied is the same as in FIG.15. The arrangement of a frequency converter82in each of the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p is the same as inFIG. 18or19.

Although the arrangement is the same as in the eighth embodiment, the operation is slightly different. The operation of the arrangement shown inFIG. 18will be described, assuming that the intermediate frequency subcarrier signal used by the station is fIF3(=fIF1+2×ΔF), and the target center frequency of frequency conversion is F0=n×fLO1+fIF1.

Referring toFIG. 18, the intermediate frequency subcarrier signal fIF3is input to the mixer96. On the other hand, the first pilot carrier signal fLO1is input and multiplied by n (>0) by the multiplier92. Unlike the arrangement inFIG. 18, the value n is constant independently of the center frequency in subcarrier transmission of the intermediate frequency subcarrier signal.

The intermediate frequency subcarrier signal fIF3and the first pilot carrier signal fLO1multiplied by n are mixed by the mixer96, so a sum frequency component (n×fLO1+fIF3) and difference frequency component (n×fLO1−fIF3) are obtained. Of the sum frequency component (n×fLO1+fIF3) and difference frequency component (n×fLO1−fIF3) output from the mixer96, the sum frequency component is selected by the bandpass filter100and outputted.

The second pilot carrier signal fLO2is multiplied by |m| by the multiplier94. In this example, since the multiplying factor m is −2, the second pilot carrier signal fLO2is multiplied by 2 by the multiplier94.

The output from the bandpass filter100and the output from the multiplier94are mixed by the mixer98. Consequently, the sum frequency component (n×fLO1+fIF3+|m|×fLO2) and difference frequency component (n×fLO1+fIF3−|m|×fLO2) between the two signals are output from the mixer98.

Since the multiplying factor m is a negative value, the bandpass filter102selects the difference frequency component and outputs this component. As a result, the target frequency F0is output from the bandpass filter102.

The frequency converter82may have the arrangement shown in FIG.19.

In the frequency converter82having this arrangement, the first pilot carrier signal fLO1is multiplied by n (>0) by the multiplier92. In this case as well, the value n is constant independently of the subcarrier center frequency of the intermediate frequency subcarrier signal, unlike the arrangement in FIG.18.

The second pilot carrier signal fLO2is multiplied by |m| by the multiplier94. For the intermediate frequency subcarrier signal fIF3, the multiplying factor m is −2, the second pilot carrier signal fLO2is multiplied by 2 by the multiplier94.

The first pilot carrier signal fLO1multiplied by n and second pilot carrier signal fLO2multiplied by |m| are supplied to the mixer96and mixed. As a consequence, the sum frequency component (n×FLO1+|m|×fLO2) and difference frequency component (n×fLO1−51m|×fLO2) are output from the mixer96.

As described above, since the value m is negative, of the sum frequency component (n×fLO1+|m|×fLO2) and difference frequency component (n×fLO1−|m|×fLO2) output from the mixer96, the difference frequency component is selected by the filter100. The output from the filter100is input to the mixer98.

The mixer98mixes the intermediate frequency subcarrier signal fIF3with the output from the filter100. As a result, the sum frequency component (n×fLO1−|m|×fLO2+fIF3) and difference frequency component (n×fLO1−|m|×fLO2−fIF3) between the two signals is output from the mixer98. This output signal is supplied to the filter102.

Of the sum frequency component (n×fLO1−|m|×fLO2+fIF3) and difference frequency component (n×fLO1−|m|×fLO2−fIF3) output from the mixer98, the difference frequency component is selected by the filter102. Consequently, the target frequency F0is output from the filter102.

In the above examples, the value n is constant independently of the subcarrier frequency in optical transmission. If the value n can change depending on the transmitting/receiving device, the absolute values of n and m can be decreased in accordance with a condition. This condition is that the first pilot carrier signal fLO1is an integral multiple of the second pilot carrier signal fLO2(=ΔF). An example will be described.

For example, an intermediate frequency subcarrier signal having a center frequency fIF4(=fIF1+4×ΔF) when fLO1=3×fLO2will be considered.

Assume that the center frequency of a radio signal radiated from the antenna48is F0(=n×fLO1+fIF1) In this case, as in the above examples, the first pilot carrier signal fLO1is multiplied by n, a difference between the resultant signal and the second pilot carrier signal fLO2multiplied by 4 is calculated, and the intermediate frequency subcarrier signal fIF4is added to the difference. That is, n×fLO1−4×fLO2+fIF4is calculated. With this frequency conversion, the target frequency F0can be obtained.

Alternatively, since fLO1=3×fLO2, the target frequency F0can be obtained by frequency conversion (n−1)×fLO1−fLO2+fIF4. At this time, the first pilot carrier signal fLO1is multiplied by (n−1), and the second pilot carrier signal fLO2is multiplied by (|m|−3).

<Arrangement Using Transmitting/receiving Device with A Plurality of Antennas>

The above description has been made in association with a system using a transmitting/receiving device having one antenna. Each of the transmitting/receiving devices32B-1,32B-2, . . . ,32B-p extracts one of a plurality of intermediate frequency subcarrier signals frequency-multiplexed and transmitted from the transmitting/receiving station10B.

However, an arrangement using a transmitting/receiving device with a plurality of antennas may also be used. In this case, a plurality of intermediate frequency subcarrier signals need be used by one transmitting/receiving device.

FIG. 24shows the arrangement of such a transmitting/receiving device32C.

The transmitting/receiving device32C comprises the O/E converter34, intermediate frequency subcarrier signal separation bandpass filters38-1,38-2, . . . ,38-N, the pilot carrier signal separation bandpass filters40-1and40-2, the frequency converter82formed from a multiplier and a power amplifier, and antennas48-1,48-2, . . . ,48-N. The bandpass filters38-1, . . . ,38-N are arranged in correspondence with the antennas48-1, . . . ,48-N, respectively. The bandpass filters38-1, . . . ,38-N extract intermediate frequency subcarrier signals for corresponding antennas48-1, . . . ,48-N from an electrical signal from the O/E converter34and are formed from simple filters having small Q value.

In this arrangement, an optical signal transmitted from the transmitting/receiving station10B as shown inFIG. 15is photoelectrically converted into an electrical signal. From the electrical signal obtained by photoelectrically converting a received optical signal by the O/E converter34, the first pilot carrier signal separator40-1extracts the first pilot carrier signal fLO1, and the second pilot carrier signal separator40-2extracts the second pilot carrier signal fLO2.

When the transmitting/receiving device32C uses N (N≧2) intermediate frequency subcarrier signals, the intermediate frequency subcarrier signals are separated by the bandpass filters38-1,38-2, . . . ,38-N formed from simple filters.

The separated intermediate frequency subcarrier signals are converted into frequencies to be radiated from the antennas48-1,48-2, . . . ,48-N by the frequency converter82. The signals are sent to the antennas48-1,48-2, . . .48-N and radiated into air.

For the frequency converter82, the arrangement shown inFIG. 18,19, or22may be prepared in number corresponding to the number of intermediate frequency subcarrier signals. Alternatively, portions where the frequencies of the pilot carrier signals fLO1and fLO2are multiplied by a desired value and added, which are common to the intermediate frequency subcarrier signals, may be shared by the intermediate frequency subcarrier signals.

In this embodiment, if the value N is large, the transmitting/receiving stations10B and transmitting/receiving devices32C may be connected in a one-to-one correspondence instead of connecting one transmitting/receiving station to a plurality of transmitting/receiving devices as in the above examples.

The above description has been made about the transmission system (down link signal processing system). A reception system is also necessary, and the reception system (up link signal processing system) of this system will be described next as a ninth embodiment.

Ninth Embodiment

The ninth embodiment is associated with a reception system (up link signal processing system). When three or more intermediate frequency subcarrier signals, i.e., intermediate frequency subcarrier signals of three or more systems are to be received by different antennas, only two pilot carrier signals fLO1and fLO2are used for frequency conversion of the intermediate frequency subcarrier signals, an intermediate frequency subcarrier signal of a system is converted into a signal with a desired frequency using the pilot carrier signals and the intermediate frequency subcarrier signal of that system and optically transmitted to a transmitting/receiving station.

FIG. 25is a block diagram showing an embodiment of the reception system, i.e., up link signal processing system of such a system. As shown inFIG. 25, as the system arrangement of the reception system, a receiving station10D incorporates a data separation demodulator140and an O/E converter138.

Each of a plurality of receiving devices32D-1,32D-2, . . . ,32D-p has an E/O converter136, a frequency converter134, and an antenna132. The frequency converter134receives the first and second pilot carrier signals fLO1and fLO2.

The plurality of receiving devices32D-1,32D-2, . . . ,32D-p and receiving station10D are connected through optical fibers30. Optical signals received by the receiving devices32D-1,32D-2, . . . ,32D-p are coupled by an optical divider34inserted midway along the optical fibers30, and are guided to the receiving station10D.

The frequency converter134frequency-converts a received radio signal into an intermediate frequency subcarrier signal fIFusing the first and second pilot carrier signals fLO1and fLO2and the carrier component of the radio signal. The first and second pilot carrier signals fLO1and fLO2are transmitted from a device with exception of the receiving device32D-1. It is most reasonable to use the pilot carrier signals fLO1and fLO2from the terminal station10D, which are separated in the transmission system (down link signal processing system).

The E/O converter136converts the intermediate frequency subcarrier signal fIFfrequency-converted by the frequency converter134into an optical signal and optically subcarrier-transmits the optical signal to the optical fiber30.

The O/E converter138in the receiving station10D converts the optical signal optically subcarrier-transmitted through the optical fiber30into an electrical signal. The data separation demodulator140separates the electrical signal converted by the O/E converter138into intermediate frequency subcarrier signals in units of channels and demodulates the signals.

In the system having this arrangement, the radio signals having center frequencies F0, which are received by the antennas132of the receiving devices32D-1, . . . ,32D-p, are frequency-converted into intermediate frequency subcarrier signals fIF1, . . . , fIFpby the frequency converters134. The first and second pilot carrier signals fLO1and fLO2are input to each frequency converter134. Signals obtained by appropriately multiplying the two pilot carrier signals fLO1and fLO2are appropriately added/subtracted to/from the frequencies of the radio signals having the center frequencies F0to obtain the intermediate frequency subcarrier signals fIF1, . . . , fIFp. The signals converted into the intermediate frequency subcarrier signals fIF1, . . . , fIFpare converted into optical signals by the E/O converters136and optically subcarrier-transmitted to the receiving station10D through the optical fibers30.

FIG. 26is a graph showing the frequency arrangement of the intermediate frequency subcarrier signals fIF1, . . . , fIFp.The frequency converters134of the receiving devices32D-1, . . . ,32D-p use different multiplying factors for the two pilot carrier signals fLO1and fLO2and different signs for addition/subtraction.

Optical signals output from the plurality of receiving devices32D-1, . . . ,32D-p are coupled by the optical divider34and converted into an electrical signal by the O/E converter138in the receiving station10D. For the spectrum of the electrical signal received and converted by the O/E converter138, the center frequencies have a difference such that the signal can be separated in units of devices, as shown in FIG.26. At the demodulator140, the received signal is separated to each subcarrier signal corresponding with each intermediate frequency band and demodulated to the data.

With this arrangement, each different subcarrier frequency which is converted data signals in the same radio frequency band into at the receiving devices becomes stable.

Like the down link signal (transmission signal) processing system shown inFIG. 24, the up link signal (reception signal) processing system may also employ an arrangement using one transmitting/receiving device having a plurality of antennas. In this arrangement, intermediate frequency subcarrier signals received by the plurality of antennas are converted into different frequencies and optically transmitted to the transmitting/receiving station.

FIG. 27shows the arrangement of a receiving device32E. Referring toFIG. 27, antennas150-1, . . . ,150-N are antennas of different systems. Outputs from the antennas150-1, . . . ,150-N are transmitted to the optical fiber30through a frequency converter152, a mixer154, and an E/O converter156. The frequency converter152converts the radio signals received by the plurality of antennas150-1, . . . ,150-N into different subcarrier frequencies using the first and second pilot carrier signals fLO1and fLO2.

The mixer154mixes the subcarrier signals of different antenna systems, which are output from the frequency converter152. The E/O converter156modulates the mixed subcarrier signal into an optical signal and outputs the optical signal to the optical fiber30.

In the system having the above arrangement, radio signals are received by the plurality of antennas150-1, . . . ,150-N of the receiving device32E. The received radio signals are converted into different subcarrier frequencies by the frequency converter152using the first and second pilot carrier signals fLO1and fLO2.

The pilot carrier signals fLO1and fLO2are transmitted from an external device of the receiving device32E. The frequency converter152appropriately multiplies and adds/subtracts the pilot carrier signals fLO1and fLO2to frequency-convert the radio signals received by the antennas.

The multiplying factors and, in some cases, the signs for addition/subtraction of the pilot carrier signals fLO1and fLO2change in units of signals received by the antennas.

The frequency converter152may have independent multiplication and addition/subtraction means in correspondence with the antennas, or a common portion may be shared.

The signals frequency-converted into different subcarrier frequencies by the frequency converter152are added by the adder154, converted into an optical signal by the E/O converter156, and transmitted to the receiving station. As inFIG. 25, signals from the plurality of receiving devices may be mixed and transmitted to the receiving station. Alternatively, the receiving devices and receiving stations may be connected in a one-to-one correspondence.

In the above-described examples, the transmitting/receiving device has only the down link signal processing system or only the up link signal processing system. However, a transmitting/receiving device incorporating both a down link signal processing system and up link signal processing system is also necessary. An example, will be described below.

Tenth Embodiment

An example of a transmitting/receiving device incorporating a down link signal processing system and up link signal processing system will be described. The communication system to be described below has the characteristic features of both the above-described arrangement applied to a down link signal and that applied to an up link signal. The transmitting/receiving device also uses two pilot carrier signals fLO1and fLO2contained in an optical signal sent from a transmitting/receiving station for frequency conversion of an up link signal received by the antenna.

FIGS. 28 and 29show arrangements of the transmitting/receiving device. The transmitting/receiving device shown inFIG. 28has both the transmitting/receiving device structures shown inFIGS. 15 and 25. In addition, the pilot carrier signals fLO1and fLO2obtained by the system for processing a down link signal are divided and input to the system for processing an up link signal.

More specifically, the down link signal processing system of a transmitting/receiving device32F comprises the O/E converter34, bandpass filter38for extracting an intermediate frequency subcarrier signal, bandpass filters40-1and40-2for extracting pilot carrier signals, frequency converter82, and antenna48. The up link signal processing system comprises an antenna132, a frequency converter134, and an E/O converter136.

The bandpass filters40-1and40-2extract the first and second pilot carrier signals fLO1and fLO2. The pilot carrier signals fLO1and fLO2are used by the frequency converter82in the down link signal processing system and also supplied to the frequency converter134in the up link signal processing system. The frequency converter134frequency-converts a received radio signal into an intermediate frequency subcarrier signal fIFusing the first and second pilot carrier signals fLO1and fLO2.

As described above, the first and second pilot carrier signals fLO1and fLO2extracted by the bandpass filters40-1and40-2in the down link signal processing system are used not only by the frequency converter82in the down link signal processing system but also by the frequency converter134in the up link signal processing system to frequency-convert an up link signal.

In the arrangement shown inFIG. 28, the transmitted pilot carrier signals fLO1and fLO2are separated and independently multiplied and added/subtracted in the frequency converters82and134. After this, frequency conversion is performed to obtain a predetermined frequency. A portion capable of partially sharing the multiplication and addition/subtraction functions may be shared. An example is shown in FIG.29.

InFIG. 29, a frequency converter160which integrates the frequency converters82and134of the arrangement shown inFIG. 28is used. In this frequency converter160, the multiplication and addition/subtraction functions are partially shared by up and down link signals.

In the frequency converter160, a multiplier output and, as needed, a mixer output of the frequency converter for a down link signal inFIG. 18or19are divided and used for frequency conversion of an up link signal.

In addition, as shown inFIG. 30, outputs from the bandpass filters40-1and40-2are input to a local carrier generation unit166. The local carrier generation unit166generates local carrier signals to be used to frequency-convert up link and down link signals.

A frequency converter168for an up link signal and a frequency converter170for a down link signal only add/subtract the local carrier signals generated by the local carrier generation unit166to/from the frequencies of signals before frequency conversion. Hence, each of the frequency converters168and170has a simple arrangement mainly comprising a mixer and a filter.

In the above example, the antennas for up link and down link signals are separated. However, when one antenna can be used for both up link and down link signals, a circulator is connected to one antenna such that the up link system and down link system can share the antenna through the circulator.

In this form, in a transmitting/receiving station10I, local carrier signals can be used to generate subcarrier signals for a down link signal and also to frequency-convert subcarrier signals for an up link signal before modulation.FIG. 31is a block diagram showing an example of the transmitting/receiving station10I with such an arrangement.

The transmitting/receiving station10I shown inFIG. 31has, as a down link system (transmission system), the E/O converter18formed from the laser element86and the laser driver84, adder16, frequency converters88-1,88-2, . . . ,88-p each comprising a local carrier generator172and a mixer174, modulators12-1,12-2, . . . ,12-p, and first and second pilot carrier generators14-1and14-2.

Local carrier generators172-1, . . . ,172-p generate different local carrier signals. The local carrier generators172-1, . . . ,172-p are arranged, respectively, corresponding to frequency converters88-1,88-2, . . . ,88-p and output local carrier signals to corresponding mixers174-1,174-2, . . . ,174-p, respectively. Each of the mixers174-1,174-2, . . . ,174-p converts the modulated input signal into the intermediate frequency subcarrier signal fIFhaving a desired center frequency using the frequency of the input signal and local carrier signal and outputs the converted signal.

The first and second pilot carrier generators14-1and14-2generate the first and second pilot carrier signals fLO1and fLO2having different frequencies, respectively. The adder16synthesizes the two pilot carrier signals fLO1and fLO2and outputs from the frequency converters88-1,88-2, . . . ,88-p. The laser driver84drives the laser element86in accordance with the signal synthesized by the adder16. The laser element86outputs a laser beam optically modulated in accordance with the synthesized signal from the adder16and sends the laser beam to the optical fiber30.

The O/E converter138is connected to an optical divider (not shown) and receives an optical signal transmitted from the transmitting/receiving device side through the optical fiber30and converts the optical signal into an electrical signal. Each of the intermediate frequency subcarrier signal separators178-1, . . . ,178-p separates and extracts an intermediate frequency subcarrier signal for a specific one of the plurality of transmitting/receiving devices from the electrical signal from the O/E converter138and is formed from a simple filter or the like.

The mixers180-1, . . . ,180-p are arranged, respectively, corresponding to intermediate frequency subcarrier signal separators178-1, . . . ,178-p. Each of the mixers180-1, . . . ,180-p receives a local carrier signal from a corresponding one of the local carrier generators172-1, . . . ,172-p and frequency-converts an intermediate frequency subcarrier signal using the intermediate frequency subcarrier signal obtained from a corresponding one of the intermediate frequency subcarrier signal separators178-1, . . . ,178-p.

In this arrangement, data signals from transmitting/receiving devices are modulated by, e.g., QSPK by the corresponding modulators12-1, . . . ,12-p.

The signals modulated by the modulators12-1, . . . ,12-p are frequency-converted into intermediate frequency subcarrier signals for optical transmission by the corresponding frequency converters88-1, . . . ,88-p, respectively. More specifically, each of the frequency converters88-1, . . . ,88-p adds/subtracts the frequency of the local carrier signal generated by a corresponding one of the local carrier generators172-1, . . . ,172-p to/from the input band signal, thereby performing frequency conversion.

The first and second pilot carrier generators14-1and14-2generate the pilot carrier signals fLO1and fLO2having different frequencies, respectively. The adder16synthesizes the two pilot carrier signals fLO1and fLO2and outputs from the frequency converters88-1,88-2, . . . ,88-p, and outputs the synthesized signal to the laser driver84. The laser driver84drives the laser element86in accordance with the signal synthesized by the adder16to generate a laser beam optically modulated in accordance with the synthesized signal from the adder16, and sends the laser beam to the optical fiber30.

In the up link signal processing system, an optical signal transmitted through the optical fiber30is converted into an electrical signal by the O/E converter138and supplied to the intermediate frequency subcarrier signal separators178-1, . . . ,178-p.

The intermediate frequency subcarrier signal separators178-1, . . . ,178-p separate intermediate frequency subcarrier signals of predetermined channels from the electrical signal. Each of the mixers180-1, . . . ,180-p converts the intermediate frequency subcarrier signal supplied from a corresponding one of the intermediate frequency subcarrier signal separators178-1, . . . ,178-p into a signal having a predetermined center frequency using the local carrier signal.

More specifically, each of the mixers180-1, . . . ,180-p receives a local carrier signal from a corresponding one of the local carrier generators172-1, . . . ,172-p. The intermediate frequency subcarrier signal is converted into a specific center frequency using the local carrier signal and the carrier component of the intermediate frequency subcarrier signal obtained from a corresponding one of the intermediate frequency subcarrier signal separators178-1, . . . ,178-p. The specific center frequency is the same as the output frequency from the modulators12-1,12-2, . . . ,12-p.

In this embodiment, in the arrangement having the down link signal processing system and up link signal processing system in the transmitting/receiving station10I, local carrier signals are shared by the down link signal processing system and up link signal processing system.

More specifically, a local carrier signal is input from the local carrier generator172-1to the mixer174-1in the frequency converter88-1, another local carrier signal is input from the local carrier generator172-2to the mixer174-2in the frequency converter88-2, and still another local carrier signal is input from the local carrier generator172-p to the mixer174-p in the frequency converter88-p. In this manner, local carrier signals are supplied from local carrier generators of the corresponding systems and also supplied to the mixers180-1, . . . ,180-p in the up link signal processing system. More specifically, a local carrier signal is input from the local carrier generator172-1to the mixer180-1, another local carrier signal is input from the local carrier generator172-2to the mixer180-2, and still another local carrier signal is input from the local carrier generator172-p to the mixer180-p.

In the present invention, up link subcarrier signals transmitted from the transmitting/receiving device having the arrangement as shown inFIG. 28,29, or30are multiplexed at a sufficiently large frequency interval, as shown inFIG. 5, like the down link subcarrier signals. The frequency interval is the same as that between the down link subcarrier signals.

In the transmitting/receiving station10I, the output from the local carrier generator172-1is divided into two paths. One path is connected to the mixer174-1in the frequency converter88-1, and the other path is connected to the mixer180-1. The up link intermediate frequency subcarrier signals fIF1, . . . , fIFpobtained by converting an optical signal into an electrical signal and separating the signal by the intermediate frequency subcarrier signal separators178-1, . . . ,178-p are converted by the mixers180-1, . . . ,180-p, into frequencies suitable for demodulation by the demodulators182-1, . . . ,182-p, respectively.

Since the local carriers from the local carrier generators172-1, . . . ,172-p are used by the mixers180-1, . . . ,180-p, the frequencies of signals to be input to the demodulators182-1, . . . ,182-p can be easily and accurately controlled.

With this arrangement, the subcarrier frequencies between the transmitting/receiving devices when up link signals are subcarrier-multiplexed can be easily stabilized, and the frequencies of demodulator inputs of up link signals in the transmitting/receiving station are stabilized, so frequency control for demodulation is facilitated. Additionally, since the local carrier generator can be shared in the transmitting/receiving station, the equipment can be reduced. Furthermore, since local carriers are generated in the transmitting/receiving device and used for frequency conversion of both of the up link and down link signals, the frequency conversion unit of the transmitting/receiving device can be simplified.

The conversion frequency interval for frequency conversion of up link and down link signals in the transmitting/receiving devices may be different between the up link and down link. When down link signals are subcarrier-multiplexed as shown inFIG. 5, the up link signals need not be arranged in the same order as that of the down link signals, i.e., in the order of transmitting/receiving devices32B-1,32B-2, . . . ,32B-p in ascending order of frequencies. The order may be changed by appropriately changing the conversion frequency interval of the up link signals.

When the radio signal frequency radiated from the antenna largely changes between the up link and down link, the conversion frequency interval of up link signals is intentionally made different from that of down link signals in the transmitting/receiving devices such that the frequencies are in almost the same frequency band in optical subcarrier transmission. With this arrangement, the cost can be reduced because the subcarrier frequency band which allows inexpensive optical subcarrier transmission is limited.

In the above examples, the transmitting/receiving device has antennas independently prepared for the up link signal system and down link signal system. However, as in the arrangement for the up link signal system or down link signal system (FIG. 24or27), a plurality of antennas for the up link signal system or down link signal system may be arranged.

The above embodiments have been described about only an optical fiber network called a PON. However the present invention can be applied to another form such as a cable coaxial transmission or HFIFC (Hybrid fiber Coax) in which a signal is transmitted through an optical fiber and then divided through a coaxial cable.

The present invention is not limited to the above-described embodiments, and various changes and modifications can be made. For example, the multiplying factor of the frequency multiplier for multiplying a pilot carrier signal is not limited to an integral multiple and may be a decimal multiple.

As has been described above, according to the present invention, even when a large optical modulation index is set for the pilot carrier signal fLO, the CNR of the intermediate frequency subcarrier signal fIFdoes not decrease, and the pilot carrier signal fLOwith a high CNR can be obtained on the transmitting device side. Since a radio frequency signal received by the antenna of the receiving device may be weak, a signal for frequency conversion by the multiplier is required to have a high CNR. As the signal for frequency conversion, the pilot carrier signal fLOcan be provided from the transmitting station. When the pilot carrier signal fLOis multiplied as the signal for frequency conversion, the noise characteristics are not largely degraded in frequency conversion because the CNR of the received pilot carrier signal fLOis high.

By adding the pilot carrier signal fLO, an increase in RIN value of the intermediate frequency subcarrier signal fIFband can be suppressed. Hence, degradation in CNR of the intermediate frequency subcarrier signal fIFband can be reduced, and the optical modulation index of the pilot carrier signal fLOto be transmitted to the transmitting device side can be made large without increasing the RIN of the intermediate frequency subcarrier signal fIFband. Since the CNR of the pilot carrier signal fLOreceived on the transmitting device side can be made high, a high-quality radio frequency signal can be obtained while suppressing additive noise in frequency conversion. Since any degradation in CNR characteristics of the intermediate frequency subcarrier signal fIFand pilot carrier signal fLOcan be suppressed, the optical fiber transmission distance can be increased. For example, when optical analog transmission of the present invention is applied to a radio communication base station, the communication service area covered by one transmitting station can be expanded.

According to the present invention, when one transmitting/receiving station accommodates a plurality of transmitting/receiving devices through a PON, the frequency stability between the transmitting/receiving devices can be maintained using a simpler optical transmission system. More specifically, when data signals subcarrier-multiplexed are to be distributed from a transmitting/receiving station to a plurality of transmitting/receiving devices, the intermediate frequency subcarrier signals to be used by the transmitting/receiving devices are subcarrier-multiplexed at a sufficiently large frequency interval such that the intermediate frequency subcarrier signals can be separated by a simple filter after reception of an optical signal. In addition, the radio frequency is set such that only two pilot carrier signals suffice to synchronize the frequencies of radio waves radiated from the transmitting/receiving devices (independently of the number of transmitting/receiving devices).

As a consequence, a communication system in which while establishing frequency synchronization between the transmitting/receiving devices, satisfactory transmission can be performed without sacrificing the optical modulation index of the intermediate frequency subcarrier signal in optical subcarrier transmission due to transmission of the pilot carrier signal, and the process of extracting necessary signals after reception of an optical signal is easy and inexpensive can be provided.