JP2001285204A - Optical transmitter and optical receiver, and optical communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical communication system that employs an optical fiber as a transmission line of a portable telephone system and can suppress intermodulation distortion in the case that frequency multiplex processing is applied to a portable telephone signal adopting the PDC system and the CDMA system and the processed signal is transmitted through the optical fiber. SOLUTION: The optical communication system of this invention consists of an optical transmitter 1 that optically modulates 1st and 2nd high frequency signals 51, 52 having different frequencies at different optical modulation degrees and transmits the modulated signals to an optical transmission line and of an optical receiver 2 that optically demodulates the optical signal received from the optical transmission line 3, separates the 1st and 2nd high frequencies signals from the demodulated signal, and corrects a level deviation given between both the signals for optical modulation with different optical modulation degrees by individually adjusting the levels of both the signals.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitting apparatus, an optical receiving apparatus, and an optical communication system when an optical fiber is used as a transmission line of a portable telephone system.

[0002]

2. Description of the Related Art Conventionally, a portable telephone system starts with an analog system and uses a digital system as the number of subscribers increases. The digital system in Japan is a PDC (Personal Digital Cellular) system using time division multiple access (TDMA).
In this system, the frequency band is 800 MHz band and 1.5 GHz
This is the z band, and the carrier frequency interval is 25 kHz.

[0003] As a next-generation mobile phone system,
CDMA (Code D based on spread spectrum system)
ivision Multiple Access) system has been proposed. Using this method will dramatically increase subscriber capacity,
There are many advantages such as the ability to perform an exchange without instantaneous interruption at the time of tracking exchange required when a subscriber moves. In this method, the frequency band is 2 GHz band, and the used band is 4 MHz to 20 MHz.
z.

[0004]

Since the above-mentioned PDC system and CDMA system are not compatible with each other, it is necessary to construct each system individually. However, for the purpose of cost reduction, it is conceivable to use a common optical fiber for the transmission line, multiplex the signals of both systems, and transmit them collectively.

However, if both signals are simply multiplexed, in addition to the intermodulation distortion generated in the PDC band, the CDM
Since intermodulation distortion due to the A band occurs, the transmission signal is adversely affected.

FIG. 7A is a spectrum diagram showing the relationship between the second-order intermodulation distortion and the third-order intermodulation distortion with respect to the first to fourth carrier signals f1 to f4 by the PDC mobile phone signal. Note that “Δ = f2-
f1 ”.

As is apparent from FIG. 1, the carrier signal f
1, 2nd-order distortions “2f1” and “2f” caused by f2
"2" and "f1 + f2" occur outside the band, and thus have little effect. However, third-order distortion “2f1-f2”, “2f2-
Since “f1” is generated in the band, it adversely affects the carrier signal. For example, if the carrier signal f1 is 800 MHz and the carrier signal f2 is 801 MHz, the third-order distortion “2”
f1-f2 "is 799 MHz, and the third-order distortion is" 2f2-f "
"1" becomes 802 MHz. The same applies to the second-order distortion and the third-order distortion caused by the carrier signals f3 and f4.

In addition to this, as shown in FIG.
Third-order distortion due to the PDC mobile phone signal and the CDMA mobile phone signal occurs within the range shown by the broken line in the figure. That is, the mobile phone signal of the CDMA system has a frequency f
Assuming that the signal has a band of 20 MHz in a to fb,
“F1 + fa−fb” to “f2 + fb−f” in the PDC band
a ”and“ fa + f1-f2 ”to“ fb +
A third-order distortion occurs in “f2−f1”.

FIG. 8 is an input / output characteristic diagram of an amplifier circuit or an optical modulation circuit. As is apparent from FIG. 8, the third-order distortion level increases in proportion to the third power of the carrier level. Therefore, if the carrier level rises by 3 dB, the third-order distortion level rises by 9 dB, which adversely affects the carrier signal.

SUMMARY OF THE INVENTION The present invention has been made to solve such a conventional problem, and it is intended to reduce the influence of intermodulation distortion when frequency-multiplexing PDC and CDMA mobile phone signals and transmitting them over an optical fiber. It is an object of the present invention to provide an optical transmitting device, an optical receiving device, and an optical communication system that are less susceptible to interference.

[0011]

An optical transmitting apparatus according to the present invention comprises:
First and second level adjusting means for individually adjusting the levels of the first and second high-frequency signals having different frequencies; and first and second high-frequency signals adjusted by the first and second level adjusting means. A synthesizing unit for synthesizing, and an optical modulation unit for optically modulating the first and second high-frequency signals synthesized by the synthesizing unit and sending the modulated signal to an optical transmission line, wherein the first and second level adjusting units are first and second level adjusting units. A configuration is provided in which the level of the second high-frequency signal is deviated so that the optical modulation means varies the optical modulation degrees of the first and second high-frequency signals.

With this configuration, when the first and second high-frequency signals having different frequencies are frequency-multiplexed and transmitted through the optical transmission line, the level of intermodulation distortion is suppressed by making the optical modulation degrees of the two signals different. And both signals can be transmitted collectively.

Further, in the optical transmission apparatus of the present invention, the first
Of the high-frequency signal of P is lower in frequency than the second high-frequency signal.
It is a DC type mobile phone signal, and the second high frequency signal is C
The signal is a DMA type mobile phone signal, and the first and second level adjusting means adjust the level of the first high-frequency signal relatively smaller than the level of the second high-frequency signal so that the light in the light modulating means can be adjusted. It has a configuration to make the modulation degree relatively small.

With this configuration, when the PDC mobile phone signal and the CDMA mobile phone signal are frequency-multiplexed and transmitted through an optical transmission line, the optical modulation degree of the PDC mobile phone signal is made relatively small. Thus, the level of intermodulation distortion can be suppressed, and both signals can be transmitted collectively.

Further, in the optical transmission apparatus of the present invention, the first
After the first and second level adjustment means, the first and second level adjustment means are provided.
And a configuration in which the first and second high-frequency signals are individually amplified and then combined by the combining unit.

With this configuration, the first and second high-frequency signals are individually amplified to a required level by the first and second amplifying means before being synthesized, and then synthesized by the respective amplifying means if they are synthesized. Since the number of signals can be reduced, the third-order distortion level can be suppressed, and the distortion can be reduced even when it is desired to increase the degree of optical modulation.

Further, the optical receiving apparatus of the present invention receives an optical signal obtained by optically modulating first and second high-frequency signals having different frequencies with mutually different optical modulation degrees via an optical transmission line and converts the signal into an electric signal. Optical demodulation means, separation means for separating the first and second high-frequency signals from the electric signal, and first and second level adjustment means for individually adjusting each level of the first and second high-frequency signals; Wherein the first and second level adjusting means perform first light modulation with different light modulation degrees.
And a configuration for correcting a level deviation given between the second high-frequency signals.

With this configuration, when the first and second high-frequency signals having different frequencies are frequency-multiplexed and optically transmitted, the signals are transmitted with different optical modulation degrees for the purpose of suppressing the level of intermodulation distortion. For this purpose, the level deviation given between the two signals can be corrected and taken out by the level adjusting means.

Further, in the optical receiver according to the present invention, the first
The first amplifier is provided after the second level adjuster, and the second amplifier is provided after the second level adjuster.

With this configuration, the level deviation between the first and second high-frequency signals can be corrected by the amplifying means together with the level adjusting means, and the final-stage amplifier is easily distorted.
Since the number of transmission signals is separated and reduced, the generated distortion level is small.

In the optical receiving apparatus according to the present invention, an amplifying means is provided between the optical demodulating means and the separating means, and the electric signal demodulated by the optical demodulating means is amplified by the amplifying means at a certain degree of amplification or more. It is.

With this configuration, if two signals are separated immediately after optical demodulation, the level is attenuated due to the separation, so that CN
Although R deteriorates, the CNR can be prevented from deteriorating by amplifying by the amplifying unit and then separating by the separating unit.

In the optical communication system of the present invention, an optical transmitter for optically modulating first and second high-frequency signals having different frequencies with different optical modulation degrees and transmitting the modulated signals to an optical transmission line, and an optical transmission device receiving the signals from the optical transmission line. After the optical signal is optically demodulated, the first and second high-frequency signals are separated and the levels of the two signals are individually adjusted to correct the level deviation given between the two signals in order to perform optical modulation with different optical modulation degrees. And an optical receiver.

With this configuration, when two high-frequency signals, for example, a mobile phone signal of the PDC system and the CDMA system are frequency-multiplexed and transmitted on the optical transmission line, the degree of light modulation of the two signals is made different so that the intermodulation distortion is reduced. Can be suppressed, and collective transmission becomes possible.

[0025]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of an optical communication system according to the present invention, and shows a configuration in which an optical transmitting device 1 and an optical receiving device 2 are connected via an optical fiber 3.

The optical transmitter 1 combines the level adjusting circuits 11 and 12 having variable attenuators for individually adjusting the levels of the high frequency signals S 1 and S 2, and the high frequency signals S 1 and S 2 adjusted by the level adjusting circuits 11 and 12. A circuit 13;
There is provided an optical modulation circuit 14 for optically modulating the high-frequency signals S1 and S2 synthesized by the synthesizing circuit 13 with, for example, a semiconductor laser and transmitting the modulated light to the optical fiber 3. The high frequency signal S1 is a PDC mobile phone signal, and the high frequency signal S2 is a CDMA mobile phone signal.

On the other hand, the optical receiver 2 includes an optical demodulation circuit 21 for converting an optical signal received from the optical fiber 3 into an electric signal,
The electric signal converted by the optical demodulation circuit 21 is converted into a high-frequency signal S1,
Separation circuit 22 having a filter configuration for separation into S2, and level adjustment circuit 2 having a variable attenuator configuration for individually adjusting the levels of high-frequency signals S1 and S2 separated by separation circuit 22
3 and 24.

Next, the operation of the optical communication system configured as described above will be described. First, the optical transmitter 1 is a PDC
A high-frequency signal S1 which is a mobile phone signal of the CDMA system is attenuated by the level adjustment circuit 11, and a high-frequency signal S2 which is a mobile phone signal of the CDMA system is attenuated by the level adjustment circuit 12, and a level deviation is given to both signals, and then combined. Frequency multiplexing is performed by the circuit 13. Then, the multiplexed high-frequency signals S1 and S2 are converted into optical signals by the optical modulation circuit 14 and output to the optical fiber 3.

The optical receiver 2 converts the optical signal transmitted through the optical fiber 3 into an electric signal by an optical demodulation circuit 21 and separates it into high-frequency signals S1 and S2 by a separation circuit 22 having a filter configuration. Then, the separated high-frequency signals S1 and S2 are individually adjusted by the level adjustment circuits 23 and 24, respectively.
In steps 1 and 12, the level deviation given to both signals is restored.

Here, the level adjustment in the level adjustment circuits 11 and 12 is performed so that the attenuation of the high-frequency signal S1 is relatively larger than the attenuation of the high-frequency signal S2. When a deviation is given to the levels of the high-frequency signals S1 and S2 and modulated by the optical modulation circuit 14, the light modulation m1 of the high-frequency signal S1 is small and the light modulation m2 of the high-frequency signal S2 is large.

Here, the degree of light modulation m will be described with reference to the input / output characteristic diagram of the light modulation circuit 14 shown in FIG. 2, where m = If / Ib = B / A. If is the input signal level, Ib is the bias current,
B is an optical signal output level, and A is an optical signal bias level. In the drawing, the solid line waveform indicates the high-frequency signal S1, and the broken line waveform indicates the high-frequency signal S2.

Accordingly, the light modulation m1 of the high-frequency signal S1 and the light modulation m2 of the high-frequency signal S2 are as follows: m1 = B1 / A m2 = B2 / A As a result, since B1 <B2, m1 <m
2.

The CNR (carrier-to-noise power ratio) depends on the modulation factor, and the difference between the optical modulation factors m1 and m2 becomes the CNR difference almost as it is. Moreover, the signal of the PDC system is a CDM.
Since the specification of the CNR is a few dB lower than that of the signal of the A system, the optical modulation degree m1 of the high-frequency signal S1 which is a PDC signal is reduced, and the optical modulation degree m2 of the high-frequency signal S2 which is a CDMA signal is increased.

FIG. 3 is a spectrum diagram corresponding to FIG. 7B, and shows the characteristics when the optical modulation factor m1 of the high-frequency signal S1 is reduced by 3 dB. At this time, the third-order distortion level generated in the PDC band is reduced by 3 dB, and the third-order distortion level generated in the CDMA band is reduced by 6 dB. in this way,
It is also effective in suppressing the distortion generated in the PDC band, but the CDM
This is particularly effective for suppressing the distortion in the A band. Thus, the intermodulation distortion can be suppressed.

FIG. 4 is a block diagram showing another embodiment of the optical transmission device 1. In the optical transmission device 1 according to the present embodiment, the amplification circuit 15 is provided between the level adjustment circuit 11 and the synthesis circuit 13, and the level adjustment circuit 12 and the synthesis circuit 1
It has a configuration in which an amplifier circuit 16 is provided between the three.

When the high frequency signals S1 and S2 are combined and then amplified by an amplifier circuit, distortion in the amplifier circuit may increase. In particular, when the degree of light modulation is large, the level to be input to the light modulation circuit 14 becomes large, so that possibility is high.

Therefore, as in the present embodiment, both signals S
If the amplifiers 15 and 16 individually amplify to the required level before combining S1 and S2, and then combine them, the number of signals to be amplified by each amplifier circuit can be reduced, so that the third-order distortion level can be suppressed. Thus, even when it is desired to increase the degree of optical modulation, an optical transmission device with small distortion can be realized.

FIG. 5 is a block diagram showing another embodiment of the optical receiver 2. The optical receiver 2 according to the present embodiment has a configuration in which an amplifier circuit 25 is newly provided after the level adjustment circuit 23 and an amplification circuit 26 is newly provided after the level adjustment circuit 24.

With this configuration, the level deviation between the high frequency signals S1 and S2 can be reduced by the level adjustment circuit 23,
It can be restored by the amplifier circuits 25 and 26 together with 24. Although the final stage amplifier is easily distorted, the level of distortion generated is small because the number of transmission signals is reduced by being separated. Further, by providing the amplifier circuits 25 and 26 after the level adjustment circuits 23 and 24, distortion can be suppressed as compared with the case where the amplifier circuits 25 and 26 are provided before.

FIG. 6 is a block diagram showing still another embodiment of the optical receiver 2. In FIG. The optical receiver 2 according to the present embodiment has an optical demodulation circuit 21 in the configuration shown in FIG.
In addition, an amplifier circuit 27 having an amplification degree of at least 10 dB is newly provided between the separation circuits 22.

In general, when the level is attenuated after optical demodulation, the CNR deteriorates accordingly. Therefore, usually, the signal is amplified by the amplifier circuit immediately after the optical demodulation. However, in the case of the present embodiment, if the two signals are separated immediately after the optical demodulation, the CNR is deteriorated because the level is attenuated by the separation. Therefore, if the signal is amplified at least by 10 dB or more by the amplifier circuit 27 and then separated, the influence on the CNR can be neglected.

Further, since both signals are collectively amplified, the number of components can be reduced and the power consumption can be reduced, and the device can be reduced in size and cost. In general, since the level is low after demodulation, the distortion level does not increase even when the collective transmission is performed. Therefore, such a configuration is sufficiently possible.

[0044]

As described above, the present invention provides the first and second high-frequency signals (for example, PDC system and CDMA).
System), the level of intermodulation distortion can be suppressed by changing the optical modulation degree of both signals when transmitting them over an optical fiber after frequency multiplexing, so that both signals can be transmitted collectively. In addition, it is possible to provide an optical communication system capable of improving transmission efficiency and reducing laying cost.

[Brief description of the drawings]

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

FIG. 2 is an input / output characteristic diagram of an optical modulation circuit.

FIG. 3 is a spectrum diagram showing generation of intermodulation distortion caused by a PDC mobile phone signal and a CDMA mobile phone signal.

FIG. 4 is a block diagram showing another embodiment of the optical transmission device.

FIG. 5 is a block diagram showing another embodiment of the optical receiver.

FIG. 6 is a block diagram showing still another embodiment of the optical receiver.

7A is a spectrum diagram showing the occurrence of intermodulation distortion in a PDC mobile phone signal. FIG. 7B is a spectrum diagram showing the occurrence of intermodulation distortion caused by a PDC mobile phone signal and a CDMA mobile phone signal.

FIG. 8 is an input / output characteristic diagram of an amplifier circuit or an optical modulation circuit.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 optical transmission device 2 optical reception device 3 optical fiber 11, 12 level adjustment circuit 13 combining circuit 14 optical modulation circuit 15, 16 amplification circuit 21 optical demodulation circuit 22 separation circuit 23, 24 level adjustment circuit 25, 26, 27 amplification circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/02 10/18 F term (Reference) 5K002 AA02 CA01 CA08 CA16 DA21 FA01 5K067 AA03 BB04 DD57 EE10 EE16 EE37 GG08

Claims (7)

[Claims] A first and a second level adjuster for individually adjusting the levels of first and second high-frequency signals having different frequencies; and the first and the second level adjusters adjusted by the first and the second level adjusters. A synthesizing unit for synthesizing the first and second high-frequency signals, and an optical modulation unit for optically modulating the first and second high-frequency signals synthesized by the synthesizing unit and sending the modulated light to an optical transmission line. The second level adjusting means varies the light modulation degrees of the first and second high-frequency signals with the light modulation means by giving a deviation to the levels of the first and second high-frequency signals. Optical transmitter. 2. The first high-frequency signal is a PDC mobile phone signal having a lower frequency than the second high-frequency signal. The second high-frequency signal is a CDMA mobile phone signal. The first and second level adjusting means adjust the level of the first high-frequency signal relatively small with respect to the level of the second high-frequency signal, so that the degree of light modulation in the light modulating means is relatively small. The optical transmission device according to claim 1, wherein the optical transmission device has a configuration that performs the following. 3. A first and a second amplifying means are provided at a stage subsequent to the first and the second level adjusting means, and the first and the second high frequency signals are individually amplified and then combined by the combining means. The optical transmission device according to claim 1, wherein: 4. An optical demodulator for receiving, via an optical transmission line, an optical signal obtained by optically modulating first and second high-frequency signals having different frequencies with optical modulations different from each other, and converting the optical signal into an electric signal; Separating means for separating the first and second high-frequency signals from each other; and first and second level adjusting means for individually adjusting each level of the first and second high-frequency signals, And an optical receiving device, wherein the second level adjusting means corrects a level deviation given between the first and second high-frequency signals in order to perform optical modulation at different optical modulation degrees. 5. The optical receiving apparatus according to claim 4, wherein a first amplifying means is provided after said first level adjusting means, and a second amplifying means is provided after said second level adjusting means. . 6. An amplification means is provided between said optical demodulation means and said separation means, and the electric signal demodulated by said optical demodulation means is amplified by said amplification means at a certain degree of amplification or higher. The optical receiving device as described in the above. 7. An optical transmission device for optically modulating first and second high-frequency signals having different frequencies with different optical modulation degrees and transmitting the modulated optical signals to an optical transmission line, and after optically demodulating an optical signal received from the optical transmission line. An optical receiving device that separates the first and second high-frequency signals and individually adjusts the levels of both signals to correct the level deviation given between the two signals in order to perform optical modulation at the different optical modulation degrees; An optical communication system comprising: