KR102061944B1 - Receiver and method for digital signal processing based compensation of dispersion included composite second order distortion in anlog optical system - Google Patents

Abstract

Disclosed are a distributed induced secondary distortion compensation receiving apparatus and an analog signal processing method of an analog optical transmission system. A CSO distortion compensation method performed by an analog optical transmission system, the method comprising: receiving a signal transmitted from an optical transmission apparatus through an optical fiber, converting a signal received through the optical fiber into a digital signal, and a digital signal processing technique Compensating for the CSO distortion included in the converted digital signal based on the dispersion-derived composite second order (CSO) distortion predicted using the method, and the compensating for the CSO distortion comprises: Subject to predicting a component corresponding to the CSO distortion induced by variance using a numerical differential operation, and removing the component corresponding to the predicted CSO distortion from the transformed digital signal. Can be.

Description

Dispersion-induced secondary distortion compensation receiver and digital signal processing method of analog optical transmission system {RECEIVER AND METHOD FOR DIGITAL SIGNAL PROCESSING BASED COMPENSATION OF DISPERSION INCLUDED COMPOSITE SECOND ORDER DISTORTION IN ANLOG OPTICAL SYSTEM}

The present invention relates to a technique for compensating for secondary nonlinear distortion induced by dispersion (i.e., CSO distortion) in an analog optical transmission system.

Since the fourth generation of mobile communication, radio access networks (RANs) have evolved into a Cloud RAN (CRAN) structure. In the CRAN structure, a base band unit (BBU) for processing a baseband signal is located at a central base station, and a remote radio head (RRH) for converting and transmitting / receiving a base signal into a radio signal is located in a cell. The mobile fronthaul network, which is the connection between the BBU and the RRH, is typically implemented as a digital optical transmission link utilizing the common public radio interface (CPRI) protocol. However, since the digital optical transmission link-based mobile fronthaul network digitally samples and transmits a radio signal, an excessively high transmission capacity is required compared to the radio signal bandwidth.

In order to solve the problems caused by digital sampling, a mobile fronthaul network using an analog optical transmission link has been proposed. However, in a mobile optical fronthaul network based on an analog optical transmission link, narrowband radio signals of various channels are transmitted only by frequency conversion. In addition, unlike a CPRI-based digital optical transmission link, a mobile fronthaul network using an analog optical transmission link does not perform format conversion, and thus does not digitally sample a radio signal and thus has high frequency efficiency.

In addition, since there is no additional time delay due to format conversion, the time delay in the analog optical transmission link-based mobile fronthaul network is relatively shorter in the link than the mobile fronthaul network utilizing the digital optical transmission link. The analog optical transmission link-based mobile fronthaul network is implemented as an optical transmitter and photodetector with a small bandwidth, which is economical. Since the analog optical transmission link is mainly used in an access network, the economic efficiency of the system is important, and thus, mainly implemented as an intensity modulation-direct receiver optical link.

However, in the case of the intensity modulation-direct reception-based analog optical transmission link, performance is greatly degraded by composite second-order (CSO) distortion induced by dispersion.

While CSO distortion can be compensated for using analog signal processing, compensation techniques using analog signal processing require precise tuning and complex calibration procedures for analog devices such as attenuators and delay lines. In addition, since the characteristics of the optical transmitter and the analog elements are different for each analog optical transmission link, precise tuning and complicated calibration procedures must be performed for each analog optical transmission link. In addition, there is a difficulty in applying to an analog optical transmission link using a bandwidth of several GHz due to the characteristics of the signal processing technology used for analog signal processing.

Accordingly, there is a need for a technique that can be applied to analog optical transmission links over a few GHz bandwidth without requiring precise tuning and complex calibration procedures, and to eliminate secondary nonlinear (CSO) distortion induced by dispersion.

US Patent No. 5,418,637 describes a technique for compensating for CSO distortion using analog signal processing techniques.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for compensating CSO distortion induced by dispersion based on digital signal processing (DSP) techniques in an analog optical transmission system consisting of an optical transmission device, an optical fiber, and an optical reception device.

A CSO distortion compensation method performed by an analog optical transmission system, the method comprising: receiving a signal transmitted from an optical transmission apparatus through an optical fiber, converting a signal received through the optical fiber into a digital signal, and a digital signal processing technique Compensating for the CSO distortion included in the converted digital signal based on the dispersion-derived composite second order (CSO) distortion predicted using the method, and the compensating for the CSO distortion comprises: Subject to predicting a component corresponding to the CSO distortion induced by variance using a numerical differential operation, and removing the component corresponding to the predicted CSO distortion from the transformed digital signal. Can be.

According to one aspect, the predicting step, after converting the sampling rate of the digital signal processing (DSP), it is possible to predict the component corresponding to the CSO distortion.

According to another aspect, the compensating the CSO distortion may compensate for the CSO distortion based on any one of Newton's difference quotient, symmetric difference quotient, finite difference coefficient method, and differential quadrature method for numerical differential calculation.

According to another aspect, the method may further include converting an optical signal received through the optical fiber into an electrical signal. Then, in the converting into the digital signal, the converted electrical signal may be converted into a digital signal.

A CSO distortion compensation method performed by an analog optical transmission system, the method comprising: converting an electrical signal to be transmitted into an optical signal, and transmitting the converted optical signal to an optical receiving apparatus through an optical fiber, and receiving the optical signal The optical signal received at the device may be compensated for the CSO distortion included in the received optical signal based on the predicted CSO distortion based on digital signal processing. In this case, the predicted CSO distortion may represent a component corresponding to the CSO distortion induced by variance using a numerical differential operation.

According to one aspect, the output optical frequency of the analog optical transmission system may be proportional to the light intensity of the converted optical signal.

According to another aspect, the converting into the optical signal may directly modulate a binary electrical signal into the optical signal based on a directly modulated laser (DML).

The analog optical transmission system includes a direct detector for receiving a signal transmitted from an optical transmission device through an optical fiber, an AD converter for converting a signal received through the optical fiber into a digital signal, and a dispersion predicted using a digital signal processing technique. And a distortion compensator configured to compensate for the CSO distortion included in the converted digital signal based on the derived CSO distortion. According to one aspect, the distortion compensator is configured to target the converted digital signal. A distortion predictor for predicting a component corresponding to the CSO distortion induced by variance using a numerical differential operation, and a distortion remover for removing a component corresponding to the predicted CSO distortion from the converted digital signal; Can be.

According to one aspect, the distortion predicting unit, after converting the sampling rate of the digital signal processing (DSP), may predict the component corresponding to the CSO distortion.

According to another aspect, the distortion compensator may compensate for the CSO distortion based on any one of Newton's difference quotient, symmetric difference quotient, finite difference coefficient method, and differential quadrature for numerical differential calculation.

According to another aspect, the direct detector may convert an optical signal received through the optical fiber into an electrical signal, and the AD converter may convert the converted electrical signal into a digital signal.

The analog optical transmission system includes an optical modulator for converting an electrical signal to be transmitted into an optical signal, and a transmitter for transmitting the converted optical signal to an optical receiver through an optical fiber, and the optical signal received by the optical receiver The CSO distortion included in the received optical signal may be compensated based on the CSO distortion predicted based on the digital signal processing. In this case, the predicted CSO distortion may represent a component corresponding to the CSO distortion induced by variance using a numerical differential operation.

According to one aspect, the output optical frequency of the analog optical transmission system may be proportional to the light intensity of the converted optical signal.

According to another aspect, the optical modulator may directly modulate a binary electrical signal into the optical signal based on a directly modulated laser (DML).

According to embodiments of the present invention, the CSO distortion induced by variance is compensated for by using a numerical differential operation, which is a digital signal processing technique, so that the CSO distortion included in the received signal can be compensated without precise tuning and complicated correction procedures. .

1 is a block diagram showing the configuration of an optical transmitting apparatus and an optical receiving apparatus according to an embodiment of the present invention.
2 is a flowchart illustrating a CSO distortion compensation method according to an embodiment of the present invention.
3 is a block diagram illustrating a detailed configuration of a distortion compensator according to an exemplary embodiment of the present invention.
4 is a flowchart illustrating a method of predicting and removing CSO distortion according to an embodiment of the present invention.
5 is a graph illustrating CSO distortion according to a transmission rate according to an embodiment of the present invention.
FIG. 6 is a block diagram illustrating an internal configuration of a CSO distortion compensator using a symmetric difference quotient according to an embodiment of the present invention.
7 is a flowchart illustrating a method of transmitting an optical signal through an optical fiber in an optical transmission apparatus according to an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

The present invention relates to a technique for compensating for secondary nonlinear distortion induced by dispersion (i.e., CSO distortion) using digital signal processing (DSP). In particular, the present invention theoretically calculates (i.e. predicts) the amount of CSO distortion induced by variance based on numerical differential operations and removes the CSO distortion included in the received signal based on the calculated CSO distortion. It's about technology.

In the present embodiments, the "analog optical transmission system" includes an optical transmission apparatus, an optical fiber, and an optical reception apparatus, and an "optical transmission apparatus" corresponding to a transmitting side for transmitting an optical signal may be represented by an analog optical transmission system. In addition, the "light receiving apparatus" corresponding to the receiving side for receiving the optical signal may be represented by an analog light transmission system.

1 is a block diagram showing the configuration of an optical transmitting apparatus and an optical receiving apparatus according to an embodiment of the present invention.

According to FIG. 1, the analog light transmission system 100 may include an optical transmission device 110, an optical reception device 120, and an optical fiber, and the optical transmission device 110 and the optical reception device 120 may be connected by an optical fiber. have. That is, the optical transmitter 110 and the optical receiver 120 may exchange data (ie, a signal) through the optical fiber.

The optical transmitter 110 may include an optical modulator 111, and the optical modulator 111 may be embodied in a form including a transmitter (not shown) for transmitting a modulated optical signal. The modulator 111 and the transmitter (not shown) may be implemented in separate configurations.

The optical receiver 120 may include a direct detector 121, an analog-digital converter (AD converter) 122, and a distortion compensator 123. Here, the operation of compensating the CSO distortion in the optical receiving apparatus 120 and transmitting the directly modulated optical signal in the optical transmitting apparatus 110 will be described in detail with reference to FIGS. 2 to 7.

2 is a flowchart illustrating a CSO distortion compensation method according to an embodiment of the present invention.

Each step (ie, steps 210 to 230) of FIG. 2 is performed by the direct detector 121, the AD converter 122, and the distortion compensator 123, which are components of the optical receiver 120 of FIG. 1. Can be performed.

In operation 210, the direct detector 121 may receive an optical signal transmitted from the optical transmitter 110 through an optical fiber. In this case, the direct detector 121 may convert the received optical signal into an electrical signal.

In step 220, the AD converter 122 may convert an analog signal corresponding to the converted electrical signal into a digital signal. For digital signal processing, a process of converting a received signal into a digital signal may be performed.

In operation 230, the distortion compensator 123 may predict the second order nonlinear distortion (that is, the CSO distortion) by using the digital signal processing technique on the converted digital signal, and the predicted second order nonlinear distortion (that is, the CSO). The distortion may be compensated by removing the second order nonlinear distortion (ie, CSO distortion) included in the received signal based on the distortion.

3 is a block diagram illustrating a detailed configuration of a distortion compensator according to an embodiment of the present invention, and FIG. 4 is a flowchart illustrating a method of predicting and removing CSO distortion according to an embodiment of the present invention.

Referring to FIG. 3, the distortion compensator 123 may include a distortion predictor 310 and a distortion remover 320, and each of steps 410 to 420 of FIG. 4 may be a distortion predictor 310. ) And the distortion canceller 320.

First, the digital signal converted by the AD converter 122 may be input to the distortion compensator 123 for CSO distortion compensation based on digital signal processing.

In operation 410, the distortion predicting unit 310 may predict a component corresponding to the CSO distortion induced by the variance by using a numerical differential operation on the digital signal.

For example, the distortion predicting unit 310 is a second order nonlinear component (ie, a component corresponding to CSO distortion) based on any one of Newton's difference quotient, symmetric difference quotient, finite difference coefficient method, and differential quadrature for numerical differential calculation. Calculate (ie, predict) the amount of.

In operation 420, the distortion removing unit 320 may include the CSO included in the received signal (ie, the received signal converted into the digital signal) based on the calculated amount of the second nonlinear component (ie, the component corresponding to the CSO distortion). By removing the distortion, the CSO distortion can be compensated.

Hereinafter, the distortion compensator 123 calculates (ie, predicts) the CSO distortion from the received signal based on a numerical differential operation and then removes the CSO distortion included in the received signal based on the calculated CSO distortion. Will be described with reference to equations.

The signal transmitted from the optical transmitting device 110 may be received by the optical receiving device 120 through an optical fiber, and at this time, the received signal received by the optical receiving device 120 may be expressed by Equation 1 below. have.

[Equation 1]

은 신호의 크기 성분인 직접 수신기(예컨대, 도 1의 직접 검출부)의 효율을 나타내고,

는 링크 손실, η는 광 송신 장치의 변조효율, I_TX(t)는 광 송신 장치에 인가된 아날로그 신호, D는 분산 파라미터, L은 전송거리, β_FM는 광 송신 장치의 FM 효율, c는 빛의 속도를 나타낼 수 있다. 수학식 1에서, 첫 번째 항에 해당하는 I_TX(t)는 전송하고자 하는 신호에 해당하고, 두 번째 항에 해당하는

이 분산에 의해 유도된 CSO 왜곡에 해당할 수 있다.In Equation 1,

Denotes the efficiency of a direct receiver (eg, the direct detector of FIG. 1), which is a magnitude component of the signal,

Is the link loss, η is the modulation efficiency of the optical transmitter, I _TX (t) is the analog signal applied to the optical transmitter, D is the dispersion parameter, L is the transmission distance, β _FM is the FM efficiency of the optical transmitter, c is It can indicate the speed of light. In Equation 1, I _TX (t) corresponding to the first term corresponds to a signal to be transmitted and corresponds to the second term.

This may correspond to the CSO distortion induced by this dispersion.

Referring to FIG. 5, when 0 km is transmitted (510), since the term corresponding to the CSO distortion in Equation 1 is 0 (that is, L = 0), it can be confirmed that the CSO distortion is not observed. Referring to 520, when the optical signal is transmitted at 20 km, it can be seen that nonlinear distortion due to color dispersion is observed. Then, since the modulated signal component in the received signal is much larger than the variance induced CSO distortion component, the modulated signal can be approximated from the received signal. Herein, the modulated signal from the received signal may be expressed by Equation 2 below.

[Equation 2]

For the digital signal processing (DSP), the modulated signal of equation (2) must represent an undefined derivative in the digital domain, and to express the modulated signal as a derivative, a numerical differential operation can be used in the DSP. In other words, the distortion compensator 123 may use a positional differential operation to represent the modulated signal as a derivative, and may predict the CSO distortion corresponding to the received signal through the numerical differential operation. In this case, a symmetric difference quotient, a finite difference coefficient method, and the like may be used. When using the symmetric difference quotient, Equation 2 may be expressed as Equation 3 below. That is, the modulated signal of Equation 2 may be expressed as a derivative as in Equation 3 below.

[Equation 3]

이 대칭 계차몫을 이용하여 CSO 왜곡을 예측한 부분(즉, 성분)에 해당할 수 있다.In Equation 3, T _s may correspond to a sampling interval (ie, a sampling rate) of the DSP. Second term of equation (3)

It may correspond to a portion (ie, a component) for predicting the CSO distortion using this symmetric series quotient.

FIG. 6 is a block diagram illustrating an internal configuration of a CSO distortion compensator using a symmetric difference quotient according to an embodiment of the present invention.

Referring to FIG. 6, the distortion compensator 600 may include a distortion predictor 610 and a distortion remover 620. Since the distortion prediction unit 610 includes the CSO distortion (that is, the second order nonlinear component) predicted based on Equation 3 above, the distortion removing unit 620 receives the received signal and the predicted CSO distortion. By adding a negative value of, the CSO distortion included in the received signal can be removed. At this time, in order to increase the accuracy of the CSO distortion prediction, the sampling rate may be changed (eg, the sampling rate is increased) or several adjacent samples may be used. In other words, a more precise numerical differential method such as a finite difference coefficient method may be used.

7 is a flowchart illustrating a method of transmitting an optical signal through an optical fiber in an optical transmission apparatus according to an embodiment of the present invention.

Each step (ie, steps 710 to 720) in FIG. 7 may be performed by the optical receiving device 110 of FIG. 1.

In operation 710, the optical modulator 111 may convert an electrical signal to be transmitted into an optical signal.

For example, the optical modulator 111 may directly modulate a binary electrical signal into the optical signal based on a directly modulated laser (DML). For example, a precoded binary electrical signal can be directly modulated with the laser, and the modulated optical signal can be transmitted to the optical receiving device 120 via the optical fiber. In this case, the optical signal modulated through the single mode optical fiber (SSMF) as well as the optical fiber may be transmitted to the optical receiving device 120. For example, the light modulator 111 may directly modulate a binary electrical signal using the laser diode LD and transmit a signal. In addition, the optical modulation unit 111 may use a distributed feedback laser (DFB laser) for directly modulating the precoded binary electrical signal, and a vertical cavity surface light emitting laser (vertical cavity) for directly modulating the precoded binary electrical signal. A surface emitting laser (VCSEL) may be used, or direct modulation may be performed using a distributed bragg reflection laser (DBR laser) that directly modulates a precoded binary electrical signal. As such, when directly modulated with a directly modulated laser (DML), a separate modulator is not required, so it is efficient in terms of cost and high optical power can be obtained without a separate insertion loss.

In operation 720, the transmitter may transmit a directly modulated optical signal to the optical receiver 120 through the optical fiber. In this case, the output optical frequency of the optical transmission device 110 may be proportional to the optical intensity of the converted optical signal (that is, the optical intensity of the directly modulated signal).

Here, the transmitter may be implemented in a form included in the light modulator 111, the transmitter and the light modulator 111 may be implemented separately. Then, the optical signal received by the optical receiver 120 may compensate for the CSO distortion by removing the CSO distortion included in the received optical signal based on the predicted CSO distortion based on the digital signal processing. have.

In this case, the predicted CSO distortion may represent a component corresponding to the CSO distortion induced by variance using a numerical differential operation, and based on Newton's difference quotient, symmetric difference quotient, finite difference coefficient method, and differential quadrature method. After the CSO distortion (i.e., amount of secondary nonlinear components) is predicted (i.e., calculated), the CSO distortion included in the received signal may be removed based on the predicted CSO distortion. Here, since the operation of compensating for the CSO distortion based on the numerical derivative operation has been described in detail with reference to FIGS. 3, 4, and 6, redundant descriptions thereof will be omitted.

Although the embodiments have been described by the limited embodiments and the drawings as described above, various modifications and variations are possible to those skilled in the art from the above description. For example, the described techniques may be performed in a different order than the described method, and / or components of the described systems, structures, devices, circuits, etc. may be combined or combined in a different form than the described method, or other components. Or even if replaced or substituted by equivalents, an appropriate result can be achieved.

Therefore, other implementations, other embodiments, and equivalents to the claims are within the scope of the claims that follow.

Claims (14)

In the CSO distortion compensation method performed by the analog optical transmission system,
Receiving, by the direct detection unit, the optical signal transmitted from the optical transmission device through the optical fiber;
Converting, by an AD converter, a signal received through the optical fiber into a digital signal; And
Compensating for the CSO distortion included in the converted digital signal based on the dispersion-derived composite second order (CSO) distortion predicted by the digital signal processing technique using a distortion compensator
Including,
Compensating the CSO distortion,
Predicting a component corresponding to the CSO distortion induced by variance using a numerical differential operation on the converted digital signal; And
Removing a component corresponding to the predicted CSO distortion from the converted digital signal
Including,
The signal received through the optical fiber is expressed as the following equation,

Where

Denotes the efficiency of the direct detection unit,

Denotes a link loss, η denotes a modulation efficiency of the optical transmission device, I _TX (t) denotes a signal transmitted from the optical transmission device, D denotes a dispersion parameter, and L denotes a transmission distance Β _FM denotes the FM efficiency of the optical transmission device, and c denotes the speed of light.
The converted digital signal is approximated as in the following equation,

The predicting step,
By using the numerical differential operation, the converted digital signal is expressed as a derivative such as

Second term from the derivative

Is predicted as the component corresponding to the CSO distortion,
Removing the component corresponding to the predicted CSO distortion,
And adding a negative value of a component corresponding to the predicted CSO distortion to the converted digital signal. The method of claim 1,
The predicting step,
Predicting the component corresponding to the CSO distortion after converting the sampling rate of the digital signal processing (DSP)
CSO distortion compensation method characterized in that. The method of claim 1,
Compensating the CSO distortion,
Compensating for the CSO distortion based on any one of Newton's series quotient, symmetric series quotient, finite difference coefficient method, and differential quadrature method for the numerical differential operation
CSO distortion compensation method characterized in that. The method of claim 1,
Converting the optical signal received through the optical fiber into an electrical signal
More,
Converting to the digital signal,
Converting the converted electrical signal into a digital signal
CSO distortion compensation method characterized in that. delete delete delete A direct detector for receiving a signal transmitted from the optical transmission device through an optical fiber;
An AD converter converting a signal received through the optical fiber into a digital signal; And
Distortion compensator for compensating CSO distortions included in the converted digital signal based on dispersion-derived composite second order (CSO) distortion predicted using a digital signal processing technique
Including,
The distortion compensator,
A distortion prediction unit for predicting a component corresponding to the CSO distortion induced by variance using a numerical differential operation on the converted digital signal; And
A distortion remover that removes a component corresponding to the predicted CSO distortion from the converted digital signal.
Including,
The signal received through the optical fiber is expressed as the following equation,

Where

Denotes the efficiency of the direct detection unit,

Denotes a link loss, η denotes a modulation efficiency of the optical transmission device, I _TX (t) denotes a signal transmitted from the optical transmission device, D denotes a dispersion parameter, and L denotes a transmission distance Β _FM denotes the FM efficiency of the optical transmission device, and c denotes the speed of light.
The converted digital signal is approximated as in the following equation,

The distortion prediction unit,
By using the numerical differential operation, the converted digital signal is expressed as a derivative such as

Second term from the derivative

Is predicted as the component corresponding to the CSO distortion,
The distortion removing unit,
And a negative value of a component corresponding to the predicted CSO distortion to the converted digital signal. The method of claim 8,
The distortion prediction unit,
Predicting the component corresponding to the CSO distortion after converting the sampling rate of the digital signal processing (DSP)
Analog optical transmission system, characterized in that. The method of claim 8,
The distortion compensator,
Compensating for the CSO distortion based on any one of Newton's series quotient, symmetric series quotient, finite difference coefficient method, and differential quadrature method for numerical differential operation
Analog optical transmission system, characterized in that. The method of claim 8,
The direct detection unit,
Converts the optical signal received through the optical fiber into an electrical signal,
The AD conversion unit,
Converting the converted electrical signal into a digital signal
Analog optical transmission system, characterized in that. delete delete delete

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