JPH11275009A - Optical transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transmitter for transmitting subcarrier light with high reliability and high stability at a lower cost and with less deterioration to distortion in a signal due. SOLUTION: This transmitter is provided with an adder 9, that adds a dummy carrier to an electric signal to be sent, a pre-distortion circuit 2 that adds distortion of a prescribed amount to the electric signal to which the dummy carrier is added, a semiconductor laser 1 that provides an output of the light modulated by the output signal of the circuit 2, a photodetector 5 that applies photoelectric conversion to a part of the modulated light, an error signal detection section 6 that detects the level of an error signal generated from the distortion of the dummy carrier included in an output signal of the photodetector, and a digital controller that controls the distortion amount applied from the pre- distortion circuit 2, so as to decrease the error signal detected by the error signal detection section 6 by allowing, e.g. a processor to execute a predetermined procedure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical transmitter for performing subcarrier optical transmission.

[0002]

2. Description of the Related Art Although portable communication terminals have become widespread, communication costs are still higher than those of wired telephones, and there is a need for further reduction in the price of cellular telephone systems.

As one method of reducing the price of a wireless base station which is a component of a mobile phone system, as shown in FIG. 13, a base station attached to an antenna 49 for mutually communicating with a mobile terminal 31 is used. 32 equipment is basically only an antenna 49, an optical-electrical converter 29 and an electrical-optical converter 30, and the signal received by the antenna 49 is sent to the central control station 25 via the optical fiber 4-1 as it is, In addition, antenna 4
A scheme has been proposed in which the central control station 25 sends a signal to be transmitted at 9 to the base station 32 via the optical fiber 4-2. At this time, a circuit such as an amplifier or a filter is appropriately inserted between the antenna and the photoelectric converter as needed.

[0004] In this case, since the centralized control station performs complicated processing such as modulation and demodulation and power control, the equipment of the base station can be simplified and the cost can be reduced. In addition, since the processing of a plurality of base stations is collectively performed by the central control station, the number of facilities can be reduced due to the statistical multiplexing effect. Further, processing across base stations, such as handover processing, is facilitated.

[0005] A method of modulating and transmitting light with a signal obtained by modulating a frequency carrier such as a radio signal is called a subcarrier optical transmission system. In the subcarrier optical transmission system, a device that converts an electric signal into an optical signal (a semiconductor laser in the case of a direct modulation system, an external modulator in the case of an external modulation system) has a non-linear characteristic of a current-light conversion function or a voltage-light conversion function. As a result, intermodulation distortion, harmonic distortion and the like are generated, and the signal quality is degraded.

In order to avoid such deterioration, usually a laser having a very good linearity is directly modulated and used, but such a laser is very expensive. In a system where the wavelength spread of light due to the direct modulation of the semiconductor laser is unacceptable, an external modulator needs to be used, but currently available external modulators have extremely poor linearity.

[0007] When modulation is performed with a device having poor linearity, distortion compensation is required. One of them is a method called pre-distortion. This is shown in FIG.
As shown in FIG. 4, a signal having the opposite characteristic to the distortion of the modulating element 33 is previously applied to the signal by applying a reverse distortion to the signal and applied to the modulating element 33 to obtain a signal without the original distortion. is there.

A frequently used configuration of the pre-distortion circuit is, for example, as shown in FIG. The non-linearity which is a problem in the modulation element is usually a second-order distortion or a third-order distortion. These can approximately compensate for distortion by adding or subtracting the squared or cubed signal to the original signal.

[0009] However, the degree of distortion of the modulation element changes due to temperature change and device deterioration. In order to follow such a change, "adaptive predistortion (adaptive predistortion)
n) "(M. Bertel)
smier and others "Linearization of
Broadband Optical Transm
issue Systems by Adaptiv
e Predistortion ", Frequen
z, Vol. 38, no. 9 p. 206 (198
4).

FIG. 16 shows an example of an optical transmitter to which the adaptive predistortion technique is applied. In this circuit, dummy carriers f _T1 and f _T2 for distortion measurement generated by the test signal generator 39 are added to the original signal V _O. On the other hand, the test signal generator 39 intentionally generates the intermodulation components f _T2 −2 · f _T1 (or f _T1 ) and f _T2 −f _T1 (or 2 · f _T1 ) of the two dummy carriers.
The light modulation element (in this case, the semiconductor laser 4
The distortion generated in 3) is synchronously detected, and negative feedback is applied to the distortion amount of the predistortion circuit. In this method, the loop gain of the negative feedback loop, which is the degree of correcting the distortion, changes according to the phase input to the mixer 46 for synchronous detection, and as a result, the degree of the distortion correction is the same as that of the signal input to the mixer 46. Depends on phase fluctuation.

The degree of distortion correction is determined by a secondary distortion generator 36-
It is greatly affected by the phase relationship between the squared and cubed signals and the original signal by the first and third order distortion generators 36-2, respectively. However, this phase relationship changes depending on the temperature characteristics of the line. Further, a variable attenuator (or a variable gain amplifier) (40-
In general, when the transmission coefficient is changed, the transmission phase also changes accordingly.

Therefore, in an automatic negative feedback system using an analog circuit as shown in FIG. 16, a phase shift occurs in which the transmission coefficient is changed in order to adjust the amount of predistortion, and the amount of distortion differs from the amount determined by the negative feedback loop. The state of occurrence occurs repeatedly, and as a result, the amount of distortion fluctuates so as to always vibrate, and sometimes causes oscillation. Such a circuit configuration is very unstable and is not suitable for practical use.

[0013]

As described above, the conventional adaptive predistortion has an unstable negative feedback loop and is not suitable for practical use.

The present invention has been made in view of the above circumstances, and is an optical transmitter including an adaptive pre-distortion device, which is lower in cost, has less signal degradation due to distortion, and has high reliability and high stability. An object of the present invention is to provide a transmitter for carrier optical transmission.

[0015]

Means for Solving the Problems The present invention (claim 1) provides:
An optical transmitter for subcarrier optical transmission, an adding unit that adds one or more dummy carriers to an electric signal to be transmitted, and a predetermined amount of the electric signal to which the one or more dummy carriers are added. Predistortion means for applying distortion, modulation means for modulating light by an output signal of the predistortion means, photoelectric conversion means for photoelectrically converting a part of the modulated light output from the modulation means, and photoelectric conversion means Detecting means for detecting the magnitude of an error signal generated by distortion of the one or more dummy carriers included in the output signal of the above, and detecting by the detecting means by executing a predetermined procedure. Digital control means for controlling the amount of distortion applied by the pre-distortion means so as to reduce the error signal to be generated. It is characterized in.

The modulating means is, for example, a semiconductor laser in the case of the direct modulation system, a laser in the case of the external modulation system, and an external modulator.

In the present invention, the amount of distortion of the pre-distortion means is adjusted based on an error signal generated by distortion of the dummy carrier, for example, a cross-modulation distortion component or a harmonic distortion component. At this time, control is performed using a digital processor or the like based on the procedure stored therein.
As a result, it is possible to avoid the control instability caused by the phase fluctuation, which is observed when using the analog circuit of the related art. When digital processing is performed, even if a change in the amount of distortion different from the expected is measured by changing the amount of attenuation of the pre-distortion means (for example, of the variable attenuator) by a small amount, the amount of distortion may be smaller than the threshold. If the value is a value), by ending the control, it is possible to avoid a state in which the distortion amount constantly oscillates or oscillates (unstable operation such as control oscillation due to imperfect device or the like). .

According to a second aspect of the present invention, in the optical transmitter according to the first aspect, the error signal detecting unit separates an error signal and another signal from an output signal of the optical-electrical conversion unit. And an asynchronous detection means for detecting the magnitude of the error signal by asynchronously detecting the error signal separated and output by the filter means.

In the analog feedback circuit described in the prior art, since the error signal is synchronously detected, it is sensitive to a phase shift at the time of synchronous detection. The detection amount of the error signal differs depending on the phase shift, and as a result, the loop gain fluctuates, and the distortion compensation amount fluctuates.

In the present invention, the magnitude of the error signal is reliably detected by asynchronously detecting the error signal using a diode or the like, so that an appropriate distortion compensation amount can be obtained.

The present invention (claim 3) is based on claim 1 or 2
3. The optical transmitter according to claim 1, further comprising: a unit for determining a magnitude of the electric signal to be transmitted or an output signal of the predistortion unit, wherein the digital control unit determines the magnitude of the determined output signal and the predistortion. Estimate the magnitude of the error signal expected from the state of the distortion amount of the distortion means, based on the difference between the estimated value and the measured value of the error signal, the control amount of the distortion amount of the pre-distortion means It is characterized in that it is determined.

It is possible to simply provide feedback based on only the value of the error signal generated by the distortion of the dummy carrier so that the error signal becomes smaller.
This method has the advantage that the configuration is simple. However, the amount of the distortion signal generated by the nonlinear distortion is not only the size of the original signal, that is, the size of the dummy carrier, but also
It depends on the magnitude of all the signals input to the mechanism where distortion occurs. Therefore, even if the size of the dummy carrier is kept constant, the size of the error signal changes when the size of another signal changes. In a system that simply controls the error signal to be smaller, even if the error signal becomes smaller because the total magnitude of the signal input to the distortion generating mechanism becomes smaller, it is as if the control was successful. Recognize. Conversely, if the total magnitude of the signal exceeds the system allowable value, the error signal becomes extremely large, and control may be started even in a range where control is not originally required.

In the present invention, since the control is performed by digital processing using a digital processor or the like, the amount of distortion of the pre-distortion means and the amount of distortion of the block for modulating light are always stored at what time. It is possible. If the magnitude of the added signal is known when the signal is added, the magnitude of the error signal generated by the distortion of the dummy carrier can be predicted. By comparing this predicted value with the actually measured magnitude of the error signal, it is possible to know how much the mechanism that generates the distortion deviates from the stored state. By adjusting the amount of distortion of the pre-distortion unit based on the amount of deviation, appropriate distortion correction can be performed.

The present invention (Claim 4) provides Claims 1 to 3
In the optical transmitter according to any one of the above, the pre-distortion means, a distortion generating means for outputting a distortion signal, and when synthesizing the distortion signal and the signal to be transmitted, the distortion signal and the signal Means for changing a relative delay amount between the transmitted signal and the signal to be transmitted.

As described in the prior art, when the distortion signal generated in the predistortion circuit is combined with the original signal, strict precision is required for the relative phase relationship. However, many variable attenuators and variable gain amplifiers for changing the amount of distortion often change the transmission phase as the amount of transmission changes. To compensate for this, the present invention includes a variable phase shifter in the pre-distortion means.

If it is attempted to simultaneously control the phase and the transmission amount using a variable attenuator whose phase changes and a phase shifter whose transmission amount changes under analog control as described in the prior art. In addition, the control circuit becomes very complicated and unstable.

In digital control in which control is performed according to stored procedures as in the present invention, stable and more precise control becomes possible by grasping the characteristics of each device.

According to the present invention (claim 5), in the optical transmitter according to any one of claims 1 to 4, when the adding means adds a plurality of dummy carriers to the electric signal to be transmitted, The frequency interval between the plurality of dummy carriers is set to one of the average values of the frequencies of the plurality of dummy carriers.
/ 1000 or more.

According to the present invention, an error signal generated by distortion of the dummy carrier is cut out by a filter and detected. The specific band (transmission bandwidth / transmission center frequency) of the filter is 1/100 in a normal relatively low-cost filter.
About 0 is the minimum. On the other hand, a carrier of a frequency-multiplexed mobile communication radio signal is, for example, 25 kHz in a 900 MHz band in a system called PDC of a digital mobile phone.
z interval. This is 1/3600 as a fractional band.
Very small, 0.

According to the present invention, the interval between the dummy carriers is made larger than the carrier interval of the normal mobile radio so that the third-order intermodulation distortion (IMD3) of the dummy carrier can be easily cut out by the filter and asynchronous detection can be performed. . By setting the interval between the dummy carriers to be equal to or more than the specific bandwidth of a filter that can be obtained at a low price, the cost of the system can be reduced.

Preferably, the adding means, the pre-distortion means, the modulating means, the photoelectric conversion means and the detecting means are collectively constituted as a first device, and the digital control means is provided as a first device. The second device may be configured as a second device different from the first device, and exchange of signals between the first device and the second device may be performed using a predetermined communication line.

Preferably, the first device may be installed in a base station in a wireless communication system, and the second device may be installed in a central control station in a wireless communication system.

[0033]

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

The drawings and description show only parts relevant to the present invention, and do not show parts and the like which may be actually used but are not relevant to the description of the embodiments of the present invention. . In implementation, such components and the like are inserted as appropriate.

The present invention is applicable to a direct modulation type optical transmitter and an external modulation type optical transmitter.

FIG. 1 shows a configuration example of a direct modulation type optical transmitter according to an embodiment of the present invention.

As shown in FIG. 1, the optical transmitter comprises a semiconductor laser 1, a predistortion circuit 2, an optical splitter 3, a photodetector 5, an error signal detector 6, a digital controller 7, a dummy carrier generator 8, , And an adder 9.

In this optical transmitter, the semiconductor laser 1 is directly modulated by a transmitted signal to obtain subcarrier modulated light. The electric signal to be transmitted is, for example, a radio signal received by a base station antenna (not shown) for mobile communication.

The signal to be transmitted is added by the adder 9 to the dummy carrier generated by the dummy carrier generator 8. These signals are pre-distorted by the pre-distortion circuit 2 and applied to the semiconductor laser 1. A part of the modulated light is extracted by the optical splitter 3, and the rest is transmitted to the destination by the optical fiber 4.

The light split by the optical splitter 3 is detected by the photodetector 5 (photo-electric conversion). This light-
The error signal detector 6 detects the magnitude of the error signal generated by the distortion of the dummy carrier from the electrical converted signal. The magnitude of the detected error signal is input to the digital controller 7. The digital control device 7 has a processor and controls the amount of distortion generated by the predistortion circuit 2 based on the detected error signal.

In the above description, an error signal is obtained from the light split by the optical splitter 3. However, a photodiode for power monitoring is mounted in a module package in which the semiconductor laser 1 is sealed. In this case, the optical splitter may be omitted, and photoelectric conversion may be performed using the power monitoring photodiode in order to detect an error signal.

Next, FIG. 2 shows a configuration example of an external modulation type optical transmitter according to an embodiment of the present invention.

As shown in FIG. 2, the present optical transmitter comprises a laser 10, an external modulator 11, a predistortion circuit 12, an optical splitter 3, a photodetector 5, an error signal detector 6, a digital controller 7, a dummy It is configured using a carrier generator 8 and an adder 9.

In this optical transmitter, a subcarrier modulated light is obtained by externally modulating the unmodulated light output from the laser 10 with a transmitted signal using the external modulator 11. As described above, the electric signal to be transmitted is, for example, a radio signal received by a base station antenna (not shown) for mobile communication.

The unmodulated light output from the laser 10 is input to the external modulator 11. The signal to be transmitted is mixed with the dummy carrier by the adder 9, subjected to pre-distortion via the pre-distortion circuit 12, and then applied to the external modulator 11. The external modulator 11 modulates unmodulated light with the input signal. Part of the modulated light is split by the optical splitter 3, the light is converted into light by electricity by the photodetector 5, and the rest is transmitted to the destination by the optical fiber 4.

The magnitude of the error signal generated due to the distortion of the dummy carrier among the signals optical-to-electrically converted by the error signal detector 6 is determined by the error signal detector 6.
Is detected by The magnitude of the detected error signal is input to the digital controller 7. Digital control device 7
Has a processor and controls the amount of distortion generated by the predistortion circuit based on the detected error signal.

The error signal detector 6 shown in FIGS. 1 and 2 does not measure only the absolute magnitude of the error signal, but a ratio to the magnitude of the entire signal or a specific portion (dummy carrier) in the signal. Is measured.

The operation of the present embodiment and details of each component will be described below. The following description basically applies to both the direct modulation type and the external modulation type optical transmitters.

First, an operation example of the present embodiment will be described with reference to FIG. FIG. 3 is a diagram showing electrical frequencies.

FIG. 3A shows an example of an electrical frequency of a signal to be transmitted. Here, a frequency-multiplexed mobile communication signal is taken as an example. In FIG. 3A, a plurality of signals are frequency-multiplexed in a band from f1 to f2.

When a dummy signal is added to the signal shown in FIG. 3A by the adder 9, as shown in FIG. 3B, the dummy carrier, fd1, fd2 is added. In this example, two dummy carriers are used.

The frequency of the dummy carrier may be higher than the signal to be transmitted as in the example of FIG. 3B, or may be lower than the signal to be transmitted. Further, since the magnitude of the nonlinearity of a semiconductor laser or an external modulator generally varies depending on the frequency, it is preferable that the frequency of the dummy carrier does not significantly deviate from the signal to be transmitted. However, it is better not to be too close to the signal band.
Further, it is better that the interval between the dummy carriers is slightly widened. These reasons and desirable frequency intervals will be described later.

When the signal illustrated in FIG. 3B is distorted by the pre-distortion circuit 2 (in the case of the example of FIG. 1) or the pre-distortion circuit 12 (in the case of the example of FIG. 2), the signal shown in FIG. The components generated by the distortion are added as shown in FIG. FIG. 3C shows an intermodulation component due to third-order distortion. The order and frequency of the generated distortion differ depending on the mechanism of the distortion generation in the predistortion circuit.
The amount of distortion generated by the distortion generating mechanism in the pre-distortion circuit is determined by the following optical modulator (that is, the semiconductor laser 1 in the example of FIG. 1; the external modulator 1 in the example of FIG. 2).
Control is performed so as to cancel the distortion of 1).

When the light is modulated using the signal illustrated in FIG. 3C, the distortion generated in the predistortion circuit cancels the distortion of the semiconductor laser 1 or the external modulator 11. When this modulated light is received by the photodiode, a spectrum close to the spectrum of FIG. 3B is obtained as shown in FIG. 3D, but at this time, a slightly distorted component remains. However, these distortion components are smaller than those without the pre-distortion circuit.

Among these distortion components, a component fim caused by the intermodulation of the dummy carrier is detected as an error signal (by the error signal detection unit 6), and the distortion of the pre-distortion circuit 2 is reduced so as to be less than a certain specified value. (By the digital controller 7).

The above is an example of the basic operation of the present embodiment.

In the above example, an intermodulation wave due to the third-order distortion of two dummy carriers was detected as an error signal. In this example, the third-order distortion is controlled. When the second-order distortion is to be controlled, the second-order distortion component between the dummy carriers may be detected. Similarly, an error signal may be selected according to the order of distortion to be controlled.

In this example, two dummy carriers are used. However, only one wave may be used and its harmonic may be used as an error signal. In this case, the second harmonic may be used for the error signal of the second distortion, and the third harmonic may be used for the error signal of the third distortion.

It is preferable that the frequency of the dummy carrier is slightly away from the frequency band of the signal to be transmitted, as shown in FIG. The reason is that if it is too close, the "distortion component generated by distorting the signal to be transmitted" overlaps the frequency of the error signal, making it difficult to detect only the "distortion component generated by distorting the dummy carrier" Because.

As a mechanism for detecting an error signal from light received by the photodetector 5, for example, as shown in FIG. 5, an error signal is directly cut out by the filter 13 and asynchronously detected using the diode detector 14 or the like. Is preferred. Although the synchronous detection may be performed as in the conventional example, the asynchronous detection is simpler and does not require a complicated mechanism such as phase shift, so that the cost is reduced. Furthermore, the detection accuracy is good because there is no influence of the DC offset due to the secondary distortion of the mixer used for the synchronous detection.

The magnitude of the error signal is detected by power (square), amplitude (first power), or an intermediate value (x power; 1 <x <2). , Its multiplier and coefficient should be constant. At this time, the digital control device 7 needs to know what multiplier and coefficient are used for detection.

By the way, the intermodulation distortion component caused by the third-order distortion of the two dummy carriers appears at a distance Δf from the dummy carrier when the frequency interval between the dummy carriers is Δf as shown in FIG. In order to cut this out by a filter so that other signals do not leak, it is necessary to use a filter having a bandwidth equal to or smaller than Δf. However, there is a lower limit to the fractional bandwidth of the filter that can be realized at relatively low cost, and is usually about 1/1000. Therefore, the dummy carrier interval Δf is set to be equal to or more than 1/1000 of the frequency fd1 or fd2 or the average value thereof. For example, if the frequency of the dummy carrier is 900
If it is MHz, the frequency interval of the dummy carrier is 9
00 kHz or more. By doing this,
An error signal can be cut out by a low-cost filter.

When it is desired to use a filter having a larger fractional band or to configure a lower frequency in order to make the configuration of parts easier, as shown in FIG. 6, the frequency is reduced by the down converter 51 before the diode detection. Convert it. Also in this case, the filter 5
The error signal is cut out at 3 and its magnitude is detected by the diode detector 54. Before cutting out with the filter 53,
The frequency is reduced by the down converter 55. At this time, in order to avoid cross-modulation distortion and DC offset due to the nonlinearity of the mixer included in the down converter, signals other than those around the error signal can be generated before down-conversion using the filter 50 having a relatively gentle ratio band. Remove as long as possible.

The downconverter requires a local carrier for lowering the frequency, but preparing an oscillator or a synthesizer for this purpose leads to an increase in cost.
Therefore, a dummy carrier generated from the dummy carrier generator 8 may be branched and used as the local carrier. As a result, the frequency stability of the down-converted error signal is good, and the detection accuracy is high.

This will be described with reference to a frequency spectrum as shown in FIG. That is, as shown in FIG.
The error signal fim is cut out by the filter 50. Since the filter 50 has a large fractional band, a component of the dummy carrier fd2 leaks in addition to the error signal fim as shown in FIG. 7B. When this is down-converted using the local carrier of the frequency fd1, the result is as shown in FIG. 7C. When this is cut out by the filter 53, only the error signal component can be extracted as shown in FIG.

The procedure of control by the digital control device 7 will be described below.

The digital controller 7 has a processor (DSP, microcomputer, etc.) inside, and controls the distortion of the pre-distortion circuit by a stored algorithm. The processor may be an IC in which logic is semi-fixed, such as an FPGA.

In the digital controller 7, the optical transmitter according to the present embodiment may be continuously controlled so that the error signal is minimized. However, the time-dependent change of the function that determines the amount of the strain component of the mechanism in which the strain occurs is relatively slow, and the time constant is shorter, which is about several hours. In a communication system, there is generally an allowable distortion value, and it is sufficient that the distortion value is not exceeded. Therefore, it is not always necessary to control so that the distortion always approaches zero.

Since the distortion control according to the present embodiment is software control by the processor, when the control is unnecessary, only the monitoring of the distortion amount can be performed. As a result, the control load is reduced.

As an example of the control method, for example, in the present optical transmitter, the amount of distortion estimated from data such as the measured error signal and the power of the signal to be transmitted increases, and the allowable limit value of the system is increased. , And when it exceeds the first threshold value, the control is started. The first threshold value is a value smaller than the maximum distortion amount allowable in system design. This is because, when starting the control, the amount of distortion is changed a small amount to find a direction in which the distortion is reduced, so that when the distortion is changed a small amount, the allowable value in the system design is not exceeded. .

When the control is started, first, the amount of distortion of the predistortion circuit is slightly changed as described above.
At this time, try changing the amount of distortion in both the direction in which the amount of distortion of the pre-distortion circuit increases and the direction in which the amount of distortion of the pre-distortion circuit decreases. Find a direction to be less. When a direction in which the distortion becomes smaller is found, the amount of distortion of the pre-distortion circuit is gradually changed in that direction. When the distortion amount becomes sufficiently small and becomes smaller than the second threshold value, the control ends. Of course, the second threshold is smaller than the first threshold.

Next, the pre-distortion circuit will be described.

FIG. 8 shows a configuration example of the pre-distortion circuit 2.

FIG. 8A shows a configuration example in the case where the order of distortion to be compensated is one. When it is desired to compensate for the order of a plurality of types of distortion, a configuration is generally used in which the number of types of distortion branches as shown in FIG. However, one distortion generating section generates a plurality of orders of distortion, and the ratio of the coefficients is equal to the ratio of the distortion coefficients generated by an optical modulator such as a semiconductor laser or an external modulator. If so, it is possible to reduce the number of branches.

If the distortion to be compensated is the third-order distortion, the distortion generator is a non-linear circuit whose input and output have a cubic characteristic. If it is desired to compensate for the second-order distortion, a non-linear circuit having a square characteristic is used. The element for controlling the magnitude is a variable attenuator in the example of FIG. 8, but a variable gain amplifier can also be used, and the element can be appropriately selected depending on the magnitude of the generated distortion signal and the magnitude of the distortion to be compensated. use.

Variable attenuator (or variable gain amplifier) 1
6 is controlled by an electric signal from a digital control device 7. The distortion signal whose magnitude is controlled is added by the adder 17 to the signal to be transmitted. In addition, it is preferable to adjust the delay of one of the paths so that the original signal and the distortion signal are added in a correct phase relationship.

The variable attenuator or the variable gain amplifier may be an ideal device that purely controls only the transmittance, but in many cases, the transmission phase changes with the change in the transmittance. As a result, when the signal is added to the original signal by the adder, the signal is not added with a correct phase relationship, and distortion compensation may not be performed correctly. Even in the variable attenuator, there is a point that the transmission phase does not change even if the transmittance changes depending on the frequency used, so that a variable attenuator that becomes such at the frequency of the signal to be transmitted may be used.

When such a variable attenuator is not available, or when it is desired to perform more accurate distortion compensation, for example, a variable phase shifter 19 is inserted in series as shown in FIG. It is preferable to compensate for the phase that has changed.

In this case, distortion control by the digital control device 7 can be roughly divided into two methods.

The first is simple hill-climbing control. The control is started when it is detected that the amount of distortion estimated from data such as the detected error signal exceeds a first threshold value determined from an allowable value of the system.

First, either a variable attenuator or a variable phase shifter is selected as a control target. Here, a variable attenuator is temporarily selected.

By making small changes in both the direction of increasing and decreasing the attenuation of the variable attenuator, find the direction in which the amount of distortion decreases. The attenuation is changed little by little in that direction. At this stage, when the distortion becomes sufficiently small and becomes smaller than the second threshold value, the control is terminated.

On the other hand, no matter which direction the attenuation is changed due to the phase change accompanying the change of the attenuation, the attenuation does not decrease any more, and the distortion does not reach the second threshold value only by controlling the variable attenuator. In this case, the amount of attenuation of the variable attenuator is fixed near the minimum value of the amount of distortion, and control is shifted to the variable phase shifter. Similarly, the variable phase shifter also detects a direction in which the amount of distortion decreases by changing the phase by a small amount in a direction in which the phase advances and in a direction in which the phase lags. After detection, the phase is changed little by little in that direction. When the distortion amount becomes smaller than the second threshold value, the control ends.

In many cases, the signal transmittance of the variable phase shifter slightly changes with the phase change. If the amount of distortion does not reach the second threshold due to such a change in transmittance, the process returns to the control of the variable attenuator. Similarly, until the amount of distortion reaches the second threshold, the variable attenuator → the variable phase shifter →
The variable attenuator control is repeated.

The second method is as follows. The amount of attenuation of the variable attenuator and the accompanying phase change, and the amount of phase shift of the variable phase shifter and the resulting change in transmittance with respect to the frequency to be used are measured in advance and stored in the digital controller 7. From these data, the optimal attenuation of the variable attenuator and the optimal phase shift of the variable phase shifter are calculated and changed to these values at once. In this case, there is a possibility that the control cannot be performed as calculated, and in that case, the deviation is corrected by using the first method, and the digital control device 7 is controlled by using the control data at that time. The stored data is corrected.

In the various control procedures according to the present embodiment, control is terminated when the amount of distortion becomes smaller than the second threshold value. Therefore, unlike feedback using an analog circuit as in a conventional example, device control is not performed. Integrity and track temperature characteristics,
Further, the control does not vibrate or oscillate due to a control error due to aging or the like.

Incidentally, the magnitude of the error signal depends not only on the magnitude of the dummy carrier but also on the magnitude of the amplitude of the entire signal including the signal to be transmitted. The dependence is not linear, but non-linearly dependent on the amplitude of the entire signal. If the amplitude of the entire signal is always constant, only the magnitude of the error signal needs to be detected. However, for example, in the case of a mobile communication signal, the amplitude of the entire signal is constantly changing,
As a result, the magnitude of the error signal is constantly changing.

In order to correct the coefficient of the distortion generating mechanism without being influenced by the fluctuation of the error signal resulting from the fluctuation of the amplitude of the entire signal, for example, the following configuration may be added. That is, as shown in FIG. 10, a signal to be transmitted to which the dummy carrier has been added is partially branched by the branching unit 20 for the optical transmitter of FIG.
And input to the digital controller 7 (the same applies to the optical transmitter of FIG. 2). The device for detecting the magnitude of the signal may be an amplitude detector, and the digital controller 7 only needs to know the multiplier and coefficient of the detector whether the detected magnitude is power or amplitude. In addition, as described above, the magnitude of the error signal is input from the error signal detector 6 to the digital controller 7. The digital controller 7 estimates a coefficient of a distortion generating mechanism in a transmitter including an optical modulator such as a pre-distortion circuit, a semiconductor laser, or an external modulator from the magnitude of the entire input signal and the magnitude of the error signal. I do.

The magnitude of the error signal is not represented by the absolute amount of the power or amplitude of the error signal but by the ratio of the error signal to other signals. To this end, the error signal detection unit 6
For example, as shown in FIG. 11, a filter 13 corresponding to the frequency of the error signal, a diode 14 for detecting the power (or amplitude) of the transmitted signal, and a filter 23 and a diode 24 for detecting the magnitude of the entire signal Constitute. When detecting the magnitude of the entire signal, the filter 23 may be omitted if the signal that has been subjected to the photoelectric conversion by the photodetector 5 has little noise.
Alternatively, only the size of the dummy carrier, not the entire signal, may be cut out by the filter 23 and detected by the diode 24. Further, it is necessary to know at what ratio the dummy carrier generated by the dummy carrier generator 8 is added to the signal to be transmitted by the adder 9. For this purpose, the output power of the dummy carrier generator 8 and the addition ratio of the adder 9 need only be known. Alternatively, only the portion of the signal to be transmitted by the filter 23 may be cut out.

The error signal detecting section 6 detects the frequency of the error signal from the semiconductor laser 1 (or the external modulator 11).
If the transmission characteristics up to this point are known and there is no fluctuation or if it can be detected, then only the error signal may be detected as shown in FIG.

Although FIG. 10 shows an example in which the magnitude of the signal between the adder 9 and the pre-distortion circuit 2 is measured, the pre-distortion circuit 2 and the semiconductor laser 1 (or the external modulator 11) are measured. May be measured. The error signal detector 6
In order to measure the content ratio of the error signal, if the magnitude of the entire signal is measured, it may be used.

The digital controller 7 estimates the coefficient of the distortion generating mechanism in the optical transmitter based on the data such as the magnitude of the error signal, the content ratio, and the magnitude of the original signal. Control is performed to reduce the coefficient. In this manner, control can be performed without being affected by a change in the strength of a signal to be transmitted.

Although the digital controller 7 has been described as being provided inside each optical transmitter, it is not always necessary to arrange the digital controller 7 in each optical transmitter. In the system assumed in the present invention, the change in distortion is relatively slow, so that the control time constant does not matter much. Therefore, the digital controller 7 need only be somewhere in the system.

For example, when the present invention is applied to an optical transmitter disposed at a remote base station of a wireless system, a digital controller 7 may be provided in the optical transmitter of the remote base station, The digital controller 7 may be arranged at a centralized control station, which is a destination of the optical signal from the optical transmitter of the station. In the latter case, optical fibers 4-1 and 4- for exchanging wireless data signals as shown in FIG.
2, control lines 27-1 for exchanging control signals,
27-2 is provided to exchange signals for distortion control. Alternatively, as shown in FIG. 12B, a control signal is also multiplexed on the radio data signal and the fibers 4-1 and 4-
You may exchange in two. In such a case, the digital controller 7 in the central control station controls the optical transmitters of a plurality of remote base stations instead of controlling only the optical transmitters of one remote base station. It leads to reduction.

The present invention is not limited to the above-described embodiment, but can be implemented with various modifications within the technical scope.

[0096]

According to the present invention, the control of adaptive predistortion is performed according to a stored procedure using a digital processor or the like, thereby achieving higher stability and higher accuracy than the conventional analog circuit. And fine-grained control becomes possible. As a result, a subcarrier optical transmission system having high reliability and stability can be configured with low-cost devices.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration example of a direct modulation type optical transmitter according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration example of an external modulation type optical transmitter according to an embodiment of the present invention.

FIG. 3 is an exemplary view for explaining a basic operation example of the embodiment.

FIG. 4 is a diagram for explaining frequencies of dummy carriers;

FIG. 5 is a diagram illustrating a configuration example of an error signal detection unit;

FIG. 6 is a diagram showing another configuration example of the error signal detection unit.

FIG. 7 is a diagram for explaining an example of detecting an error signal by down-conversion.

FIG. 8 is a diagram illustrating a configuration example of a predistortion circuit;

FIG. 9 is a diagram illustrating another configuration example of the predistortion circuit;

FIG. 10 is a diagram showing another configuration example of the optical transmitter according to the embodiment of the present invention;

FIG. 11 is a diagram illustrating still another example of the configuration of the error signal detection unit;

FIG. 12 is a diagram for explaining a case where a control device is arranged outside the optical transmitter;

FIG. 13 is a diagram for explaining a conventional mobile phone system.

FIG. 14 is a diagram for explaining a conventional pre-distortion technique;

FIG. 15 is a diagram showing an example of a conventional pre-distortion circuit.

FIG. 16 is a diagram showing a conventional optical transmitter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser 2 ... Predistortion circuit 3 ... Optical splitter 4,4-1,4-2 ... Optical fiber 5 ... Photodetector 6 ... Error signal detector 7 ... Digital controller 8 ... Dummy carrier generator 9 ... Adder Reference Signs List 10 laser 11 external modulator 12 predistortion circuit 13 filter 14 diode detector 15, 15-1 to 15-m distortion generator 16, 16-1 to 16-m variable attenuator 17 Adder 18 Adder 19 Variable phase shifter 20 Splitter 21 Power detector 22 Splitter 23 Filter 24 Diode detector 25 Central control station 26 Remote base station 27, 27-1, 27 -2: control line 28: circulator 29: optical-electrical converter 30: electrical-optical converter 31: portable terminal 32: base station 33: modulating element 34: pridy Torsion circuit 35 ... Phase shifter 36 ... Distortion generator 37 ... Attenuator 38 ... Adder 39 ... Test signal generator 40 ... Variable attenuator 41 ... Adder 42 ... Adder 43 ... Semiconductor laser 44 ... Optical splitter 45 ... Photodetector 46 ... Mixer 47 ... Filter 48 ... Integrator 49 ... Antenna 50 ... Filter 51 ... Downconverter 53 ... Filter 54 ... Diode detector 55 ... Local carrier

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04B 10/14 10/04 10/06

Claims (7)

[Claims] 1. An optical transmitter for subcarrier optical transmission, comprising: an adder for adding one or more dummy carriers to an electric signal to be transmitted; and an electric transmitter to which the one or more dummy carriers are added. Pre-distortion means for applying a predetermined amount of distortion to a signal, modulation means for modulating light by an output signal of the pre-distortion means, and photoelectric conversion means for photoelectrically converting a part of modulated light output from the modulation means, In the output signal of the photoelectric conversion means, detection means for detecting the magnitude of an error signal generated by the distortion of the one or more dummy carriers, by executing a predetermined procedure, Digital control for controlling the amount of distortion applied by the pre-distortion means so as to reduce the error signal detected by the detection means. Optical transmitter, characterized in that a control means. 2. The filter according to claim 1, wherein said error signal detecting means separates an error signal and another signal from an output signal of said optical-electrical converting means and outputs an error signal. The optical transmitter according to claim 1, further comprising: asynchronous detection means for detecting a magnitude of the error signal by asynchronously detecting the error signal. 3. The apparatus according to claim 2, further comprising: means for determining the magnitude of the electric signal to be transmitted or the output signal of the predistortion means, wherein the digital control means determines the magnitude of the determined output signal and the magnitude of the predistortion means. Estimating the magnitude of the error signal expected from the state of the distortion amount, and determining the control amount of the distortion amount of the pre-distortion means based on the difference between the estimated value and the actually measured value of the error signal. The optical transmitter according to claim 1, wherein: 4. The pre-distortion means includes: a distortion generating means for outputting a distortion signal; and a synthesizing means for synthesizing the distortion signal and the signal to be transmitted. The optical transmitter according to any one of claims 1 to 3, further comprising means for changing a relative delay amount between the optical transmitters. 5. When a plurality of dummy carriers are added to an electric signal to be transmitted by said adding means, a frequency interval between said plurality of dummy carriers is set to be not less than 1/1000 of an average value of frequencies of said plurality of dummy carriers. The optical transmitter according to any one of claims 1 to 4, wherein: 6. The first device, wherein the adding device, the pre-distortion device, the modulating device, the photoelectric conversion device, and the detecting device are collectively configured as a first device, and the digital control device is different from the first device. 2. The communication device according to claim 1, wherein the communication device is configured as a second device, and exchanges signals between the first device and the second device using a predetermined communication line. Optical transmitter. 7. The optical transmission system according to claim 6, wherein said first device is installed at a base station in a wireless communication system, and said second device is installed at a central control station in a wireless communication system. vessel.