CN101882968B - Time synchronization method of access system of optical orthogonal frequency division multiplexing-based passive optical network (OOFDM-PON) - Google Patents

Abstract

The invention discloses a time synchronization method in a system of an optical orthogonal frequency division multiplexing-based passive optical network (OOFDM-PON). The scheme comprises an optical line terminal (OLT) and an optical network unit (ONU), wherein the OLT comprises a laser, a modulator, an OFDM signal generating module and an optical multiplexer. The ONU comprises an optical demultiplexer, a time delay compensator, an optical detector and an OFDM receiving module. At the transmitting end of the OLT, optical signals for modulating optical OFDM signals and timing information are divided into two branches. The optical signals are divided into two branches after transmitted by optical fibers, wherein a timing signal is extracted from the branch for carrying the timing information by the optical detector after time-delay compensation, and the OFDM signals are accurately received at the receiving end of the ONU. The invention has the characteristics of simple design principle, easy realization, low cost and the like.

Description

The method for synchronizing time of the EPON connecting system based on OOFDM

Technical field

The invention belongs to the optical communication technique field, specifically, relate to EPON (OOFDM-PON) access and the Time synchronization technique of handing over frequency division multiplexing based on light positive.

Background technology

Simultaneous techniques is one of key technology of OFDM (OFDM) communication system, is also the key that can light positive friendship frequency division multiplexing (OOFDM) and EPON (OOFDM-PON) connecting system based on light positive friendship frequency division multiplexing practical.The OOFDM-PON access is divided into relevant and incoherent two kinds of systems, the OOFDM-PON connecting system better performances that wherein is concerned with, but realization is more difficult, cost is higher; Incoherent OOFDM-PON access technology adopts direct modulate intensity to detect, and realizes that easy cost is lower, and therefore, incoherent OOFDM-PON connecting system will be that the following OOFDM of employing realizes one of preferred version in access.Synchronous method in the OOFDM-PON connecting system directly has influence on the flexibility of design cost, transmission rate, the error rate and the system of OOFDM-PON system, and synchronous method is one of practical decisive factor of OOFDM-PON system efficiently.Therefore, in solution OOFDM-PON system, stationary problem is particularly important for advancing this technical applicationization.

The theory of OOFDM-PON system has been suggested when many, but the Study on Synchronization Techniques in this system is less, studies at present in the document of OOFDM-PON the stationary problem in this system of concentrated discussion not.There is the part document directly just to quote some synchronous method commonly used in the wireless OFDM technology, as M.Schmidl and D.Cox, " Robust frequency and timing synchronization for OFDM; " IEEE Trans.Commun., vol.45, pp.1613-1621, Dec.1997; H.Minn, M.Zeng and V.Bhargava, " On timing offset estimation for OFDM systems, " IEEE Commun.Lett., vol.4, pp.242-244, July 2000; B.Park, H.Cheon, C.Kang, " A Novel Timing EstimationMethod for OFDM Systems; " IEEE Lett.Commun., 2003,7 (5): the 239-241 scheduling algorithm, but in these wireless channels, classical algorithm substantially all needs pilot signal, reduced on the one hand the validity of system, and also need to carry out complicated data at receiving terminal and process, in high speed transmission system, can take a large amount of data processing resources, cost will be very high.In high speed OOFDM-PON system, realize that at receiving terminal it is obviously irrational that data are at a high speed processed, because need to take a large amount of resources, and cost is also higher.

Therefore, overcome the deficiency of simultaneous techniques in above-mentioned OOFDM-PON connecting system, the invention provides a kind of time synchronized new method of the connecting system of the OOFDM-PON for the monomode fiber channel, it is extremely important that the method is moved towards through engineering approaches to the OOFDM-PON system, below the present invention is described in detail.

Summary of the invention

The object of the invention is to overcome the deficiency of existing simultaneous techniques in above-mentioned OOFDM-PON system, provide a kind of with low cost, easily realize the synchronous method of the OOFDM-PON system that efficiency is higher.The method has improved the availability of frequency spectrum of OOFDM-PON connecting system, has reduced the data processing amount of this system receiving terminal, has effectively improved resource utilization ratio.

In order to describe easily content of the present invention, some technical terms are described:

OFDM: OFDM;

OOFDM: light positive is handed over frequency division multiplexing;

PON: EPON;

OOFDM-PON: the EPON of handing over frequency division multiplexing based on light positive;

OLT: optical line terminal;

ONU: Optical Network Terminal;

ADC: analog-to-digital conversion;

FFT: Fourier transform.

For achieving the above object, method for synchronizing time of the present invention is characterised in that, the mode of having abandoned dependence data processing in the past obtains synchronously, but use based on the pulse modulation timing signal, direct detection timing signal and the mode that adopts the wavelength division multiplexing realization to process light OFDM and timing signal realize synchronously, and do not take any dedicated bandwidth that has.The method basic principle of the OOFDM-PON system synchronization that the present invention proposes is: in the OOFDM-PON system based on the monomode fiber channel, produce the light signal of two branches at the transmitting terminal of system, the optical signal modulation light ofdm signal of one of them branch, be used for passing user's data, be designated as A branch; The optical signal modulation synchronizing information of another branch, be used for transmitting the synchronous timing information in the OOFM-PON system, is designated as B branch.Finally, by two branched optical cable information couplings having modulated respectively user data and synchronous timing information, in fiber channel, the optical information of these two branches adopts the light carrier of different frequency to transmit in same optical fiber in the method.At transmitting terminal, in A branch, each carries the zero hour of the light OFDM data of Cyclic Prefix, in B branch, send the pulse signal of a sign synchronization timing, the length that the synchronization timing pulse duration is Cyclic Prefix, after the light signal of these two branches transmits in optical fiber, suppose the impact of not considering group velocity, the propagation delay time of the light signal of two branches in optical fiber is identical.At receiving terminal, by optical demultiplexer the light signal of these two branches separately, wherein carried the light signal of timing information, completed the reception of optical information by photodetector, the timing signal on electric territory still occurs in the zero hour of a light OFDM data-signal.Therefore, can carry out with the synchronization timing pulse signal of a branch determining of time synchronized.But in the real system of OOFDM-PON, impact due to group velocity, there will be delay inequality after two A of branch and B transmit in fiber channel, the commutator pulse that causes sending in receiving terminal B branch can accurately not appear at the original position of the light OFDM data of A branch.Therefore, the delay inequality of calculating and compensate two branches is to realize one of key factor that the method is applied in the OOFDM-PON system, below will introduce the computational process of two branch's transmission time delay differences in this system.

In the OOFDM-PON system, the topmost characteristics that are different from wireless channel due to fiber channel are that fiber channel is static channel, and wireless channel is time varying channel.In the OOFDM-PON system, the synchronous method key that the present invention proposes is that receiving terminal determines the initial time of light OFDM data according to the commutator pulse received, select a certain threshold level, the commutator pulse received rises to the moment of the moment of this threshold value as an OFDM symbol arrival.Therefore, the key of this problem is to determine this threshold value.

The transmission rate of light carry information is generally used the light group velocity

mean, group velocity is the function of frequency.The β representative, along the propagation constant of shaft axis of optic fibre, is the slowly varying function of ω, near frequency of heart ω 0, it is launched into to Taylor series therein, obtains

$\mathrm{\β}\left(\mathrm{\ω}\right)={\mathrm{\β}}_0+{\mathrm{\β}}_1\left(\mathrm{\ω}-{\mathrm{\ω}}_0\right)+\frac{1}{2}{\mathrm{\β}}_2{\left({\mathrm{\ω}-\mathrm{\ω}}_0\right)}^2+\frac{1}{6}{\mathrm{\β}}_3{\left({\mathrm{\ω}-\mathrm{\ω}}_0\right)}^3+...+\frac{1}{m!}{\mathrm{\β}}_m{\left({\mathrm{\ω}-\mathrm{\ω}}_0\right)}^m---\left(1\right)$

Wherein

${\mathrm{\β}}_m={\left(\frac{{\∂}^m\mathrm{\β}}{{\∂\mathrm{\ω}}^m}\right)}_{\mathrm{\ω}={\mathrm{\ω}}_0},m=\mathrm{0,1,2,3}...$

Solve the time delay τ after light unit of transfer length _g,

${\mathrm{\τ}}_g=\frac{1}{v_g}=\frac{\∂\mathrm{\β}}{\∂\mathrm{\ω}}---\left(2\right)$

Therefore, two bundle angular frequencies are respectively ω ₁and ω ₂light delay inequality after unit of transfer's length in optical fiber be

$\mathrm{\Δ\τ}={\∫}_{{\mathrm{\ω}}_1}^{{\mathrm{\ω}}_2}{\mathrm{d\τ}}_g={\∫}_{{\mathrm{\ω}}_1}^{{\mathrm{\ω}}_2}\left(\frac{{\∂\mathrm{\τ}}_g}{\∂\mathrm{\ω}}\right)\mathrm{d\ω}={\∫}_{{\mathrm{\ω}}_1}^{{\mathrm{\ω}}_2}\frac{{\∂}^2\mathrm{\β}}{{\∂\mathrm{\ω}}^2}\mathrm{d\ω}---\left(3\right)$

Ignore the above dispersion of quadravalence, (3) formula can be written as

$\mathrm{\Δ\τ}={\∫}_{{\mathrm{\ω}}_1}^{{\mathrm{\ω}}_2}({\mathrm{\β}}_2+{\mathrm{\β}}_3\left({\mathrm{\ω}-\mathrm{\ω}}_0\right))\mathrm{d\ω}---\left(4\right)$

Angular frequency is respectively ω ₁and ω ₂the light optical fiber that is L through length after, the delay inequality of two-beam is

Δt＝LΔτ (5)

By (5) formula, can obtain the delay inequality of the light arrival receiving terminal of two branches.

Suppose that the rectangular pulse of one fixed width is as commutator pulse, at receiving terminal, set a threshold value, when the rectangular pulse ascent stage arrives threshold value, decision circuit sends a synchronizing signal, means the beginning of light OFDM data.

In the method, threshold value is definite as follows:

Suppose that it is plane wave that fiber channel is propagated each frequency component of light field, can be write as

$\tilde{E}(r,\mathrm{\ω})=\hat{e}F(x,y)\tilde{G}(0,\mathrm{\ω})e^{\mathrm{j\βz}}---\left(6\right)$

In formula,

for the unit polarization vector;

for initial amplitude; β is propagation constant; F (x, y) is the mould field distribution, usually with frequency and non-linear relevant, but spectrum width Δ ω is far smaller than to light carrier frequency ω ₀light pulse and small nonlinearity under, its dependence can be ignored.The light field of the different spectral components in Δ ω scope is transmitted by following relation in optical fiber

$\tilde{G}(z,\mathrm{\ω})=\tilde{G}(0,\mathrm{\ω})e^{\mathrm{j\βz}}---\left(7\right)$

To after (7) inverse Fourier transform being

$G(z,t)=\frac{1}{2\mathrm{\π}}{\∫}_{-\∞}^{\∞}\tilde{G}(z,\mathrm{\ω})e^{-\mathrm{j\ωt}}\mathrm{d\ω}---\left(8\right)$

To after (8) conversion, obtain

$G(z,t)=A(z,t)e^{j({\mathrm{\β}}_{0}z-{\mathrm{\ω}}_{0}t)}---\left(9\right)$

Wherein becoming slowly amplitude can be expressed as

$A(z,t)=\frac{1}{2\mathrm{\π}}{\∫}_{-\∞}^{\∞}\tilde{A}(0,\mathrm{\Δ\ω})e^{j({\mathrm{\β}}_1\mathrm{z\Δ}\mathrm{\ω}+\frac{1}{2}{\mathrm{\β}}_{2}z{\mathrm{\Δ\ω}}^2+\frac{1}{6}{\mathrm{\β}}_3{\mathrm{\Δ\ω}}^3-\mathrm{\Δ\ω})}\mathrm{d\Δ\ω}---\left(10\right)$

In formula

be A (0, Fourier transform t).In order to analyze the variation of slow envelope, to the z differentiate, in the situation that luminous power is less, neglect nonlinear impact, only consider the decay of optical fiber, obtain

$\frac{\&PartialD;A}{\&PartialD;z}+{\mathrm{\&beta;}}_1\frac{\&PartialD;A}{\&PartialD;t}+\frac{\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HSA00000157403600011.png"\; state="\{\{state\}\}">j</figure-callout>}}{2}{\mathrm{\&beta;}}_2\frac{{\&PartialD;}^{2}A}{{\&PartialD;t}^2}-\frac{1}{6}{\mathrm{\&beta;}}_3\frac{{\&PartialD;}^{3}A}{{\&PartialD;t}^3}=-\frac{\mathrm{\&alpha;}}{2}A---\left(11\right)$

α means attenuation coefficient.The impact of transmitting in optical fiber in order to be beneficial to the analysis pulse, investigate the differentiation of light pulse in the new coordinate system moved with group velocity, define new coordinate and be

$T=t-\frac{z}{v_g}=t-{\mathrm{\&beta;}}_{1}z---\left(12\right)$

When | β ₂|>0.1ps ²in the time of/km, usually ignore β ₃impact, so obtain

$\frac{\&PartialD;A}{\&PartialD;z}=-\frac{\mathrm{\&alpha;}}{2}A-\frac{\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HSA00000157403600011.png"\; state="\{\{state\}\}">j</figure-callout>}}{2}{\mathrm{\&beta;}}_2\frac{{\&PartialD;}^{2}A}{{\&PartialD;T}^2}---\left(13\right)$

Character according to Fourier transform, obtain fourier transform be

have

$\frac{\&PartialD;\tilde{A}(z,\mathrm{\&omega;})}{\&PartialD;z}=-\frac{\mathrm{\&alpha;}}{2}\tilde{A}-\frac{\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HSA00000157403600011.png"\; state="\{\{state\}\}">j</figure-callout>}}{2}{\mathrm{\&beta;}}_2{\left(\mathrm{j\&omega;}\right)}^2\tilde{A}(z,\mathrm{\&omega;})---\left(14\right)$

Separate (14), obtain

$\tilde{A}(z,\mathrm{\&omega;})=\tilde{A}(0,\mathrm{\&omega;})e^{(\frac{\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HSA00000157403600011.png"\; state="\{\{state\}\}">j</figure-callout>}}{2}{\mathrm{\&beta;}}_2{\mathrm{\&omega;}}^2-\frac{\mathrm{\&alpha;}}{2})z}---\left(15\right)$

that (0, Fourier transform T), will for initial value A

carry out Fourier inversion, can obtain A (z, T)

$A(z,T)=\frac{1}{2\mathrm{\&pi;}}{\&Integral;}_{-\&infin;}^{\&infin;}{\tilde{A}(0,\mathrm{\&omega;})e}^{(\frac{\mathrm{<figure-callout\; id="2"\; label="j"\; filenames="HSA00000157403600011.png"\; state="\{\{state\}\}">j</figure-callout>}}{2}{\mathrm{\&beta;}}_2{\mathrm{\&omega;}}^2-\frac{\mathrm{\&alpha;}}{2})z}{e}^{\mathrm{j\&omega;T}}\mathrm{d\&omega;}---\left(16\right)$

What suppose transmission is rectangular pulse, and A (0, T)=1,0≤T≤T ₀, its Fourier transform is

$\tilde{A}(0,\mathrm{\&omega;})=T_0{e}^{-\mathrm{j\&omega;}{T}_0/2}\frac{\sin \mathrm{\&omega;}{T}_0/2}{{\mathrm{\&omega;T}}_0/2}---\left(17\right)$

By (17) substitution (16) formula, the light pulse A (z, T) that obtains receiving terminal is

$A(z,T)=\frac{e^{\frac{-\mathrm{\&alpha;}}{2}z}}{\mathrm{\&pi;}}{\&Integral;}_{-\&infin;}^{\&infin;}\frac{\sin ({\mathrm{\&omega;T}}_0/2)}{\mathrm{\&omega;}}{\mathrm{<figure-callout\; id="2"\; label="e\; j"\; filenames="HSA00000157403600011.png"\; state="\{\{state\}\}">e</figure-callout>}}^{\frac{j}{}}\mathrm{d\&omega;}---\left(18\right)$

Therefore, can obtain the receiving terminal decision threshold by formula (18).

The accompanying drawing explanation

The schematic diagram of method for synchronizing time in Fig. 1 OOFDM-PON system.

The schematic diagram of the transmitting terminal in Fig. 2 system and receiving terminal light signal.

The synchronous processing block diagram of receiving terminal in Fig. 3 OOFDM-PON system.

Fig. 4 commutator pulse transmits the delay inequality calculation flow chart after rear and ofdm signal transmission.

The definite flow chart of receiving terminal threshold value after the transmission of Fig. 5 B branch.

Embodiment

Below in conjunction with embodiment, the method that the present invention is applied to time synchronized in the OOFDM-PON connecting system is described in further detail.

Fig. 1 is the schematic diagram of method for synchronizing time in the OOFDM-PON system.The OOFDM-PON system mainly is comprised of optical line terminal (OLT) and a plurality of Optical Network Terminal (ONU).OLT has comprised laser, ofdm signal generation module, modulator and optical multiplexer.ONU has comprised optical demultiplexer, delay inequality compensation, photo-detector and OFDM receiver module.At the OLT end, by two laser constitutions, wherein No. 1 laser, for light modulated OFDM data-signal, is expressed as A branch, and wherein ofdm signal is produced by the ofdm signal generation module; Another No. 2 lasers, for modulating timing pulse signal, are expressed as B branch, and these two branch informations are multiplexed into the channel based on monomode fiber by optical multiplexer.At receiving terminal, by splitter, the information of transmission is divided into to a plurality of branches, each branch enters into different ONU.As in any one ONU, the signal of input is divided into two branches by optical demultiplexer, and a branch of wherein having carried light OFDM data-signal enters into serial/parallel modular converter after by ADC; The signal that has carried another branch of timing information enters into photo-detector by the delay compensation module and extracts timing information, be converted to the signal of telecommunication, this signal is directly inputted to the FFT module, for the Timing Processing of light OFDM data-signal, finally by data reception module, completed the correct reception of light OFDM.

Fig. 2 is transmitting terminal in system and the schematic diagram of receiving terminal light signal.First commutator pulse is carried out to delay compensation at the ONU receiving terminal, the delay inequality that the optical signals that makes two branches causes in the group velocity impact is cancelled out each other.At transmitting terminal, A branch light source has carried after the waveform signal of light ofdm signal and the modulation of B branch the waveform that has carried timing information and has illustrated respectively as Fig. 2 (a) with (b) after modulation.At receiving terminal, due to the light ofdm signal waveform after the impact transmission of fiber channel, as shown in Fig. 2 (c), commutator pulse is illustrated as shown in Fig. 2 (d) through the waveform after delay compensation.Carried the B branch of timing information after fibre channel transmission, at receiving terminal, will extract exactly timing information, when timing signal extracts, set a threshold value, reached threshold value p in light pulse ₀the time, show the beginning of light ofdm signal, to realize the timing in OOFDM-PON system accurately.

Fig. 3 is the synchronous processing block diagram of receiving terminal in the OOFDM-PON system.Mainly by optical demultiplexer, ADC, OFDM demodulation, delay compensation and light pulse, detect and form.At the receiving terminal of OOFDM-PON, by the light signal after fibre channel transmission, through optical demultiplexer, be divided into A branch and B branch.After processing by ADC, the A branch of having carried the light ofdm signal enters into the OFDM demodulation module.For ease of the compensation that realizes that Time Delay of Systems is poor, optical signal transmission time delay for transmit timing information is not less than the time delay for the light signal of transmission light ofdm signal, the B branch of having carried timing information is converted to the signal of telecommunication by the light pulse detector by light signal after by delay compensation, be input to again in the OFDM demodulation module, realize the accurate reception of ofdm signal.

Fig. 4 is after the commutator pulse transmission and the delay inequality calculation flow chart after the ofdm signal transmission.Given Optical Fiber Transmission length L and centre frequency ω ₀, work as ω ₁≤ ω≤ω ₂the time, calculating B branch is the delay volume differed with A branch after L by Optical Fiber Transmission, determines thus the delay compensation of B branch, realizes that timing signal exactly regularly, recovers ofdm signal at receiving terminal.

Fig. 5 is the definite flow chart of receiving terminal threshold value after the transmission of B branch.Given fiber transmission attenuation α and centre frequency ω ₀, determine the Optical Fiber Transmission constant beta ₁, β ₂and β ₃, calculate slow transforming amplitudes A (z, T), as 0≤T≤T ₀the time, calculate the Fourier transform amount of slow amplitude by

calculate the amplitude of light pulse.Therefore, can judge the threshold quantity of receiving terminal, finally realize the accurate judgement of timing information.

Beneficial effect of the present invention

The present invention can realize that, to the time synchronized in the OOFDM-PON connecting system, design principle of the present invention is simple, easily realizes, with low cost.In design process, the model of this invention is simulated to required time shorter, the preferred parameter simple and fast.Adopting the present invention to carry out the synchronous of OOFDM-PON system easily realizes, do not take the effective bandwidth of system, particularly the present invention does not rely at receiving terminal and carries out a large amount of data processing, only carrying out simple and reliable processing on analog signal can realize synchronously, obtain performance preferably, the present invention has very important significance to the practical tool of OOFDM-PON connecting system.

Claims (4)

1. the method for synchronizing time in EPON (OOFDM-PON) connecting system of handing over frequency division multiplexing based on light positive, the OOFDM-PON connecting system is comprised of optical line terminal (OLT) and Optical Network Terminal (ONU), OLT is comprised of laser, OFDM (OFDM) signal generator module, modulator and optical multiplexer, and ONU is comprised of optical demultiplexer, delay inequality compensation, photo-detector and OFDM receiver module; The performing step of the method: at first at the OLT transmitting terminal, produce the light signal of two branches, two branches have adopted respectively the carrier wave of different frequency, and the optical signal modulation light ofdm signal of one of them branch is used for passing user's number pick; The optical signal modulation synchronizing information of another branch, be used for transmitting the timing pulse signal in the OOFM-PON system; Then two branched optical cable information couplings will having modulated respectively user data and synchronous timing information transmit in fiber channel; At the ONU receiving terminal, by optical demultiplexer the light signal of these two branches separately, wherein carried the light signal of synchronous timing information, the reception that completes optical information by photo-detector realizes that the OOFDM-PON system time is synchronous; Due to the impact of fiber channel, produced delay inequality after light ofdm signal and the transmission of timing pulse signal two branches, in the present invention, adopt fibre delay line or electric territory time-delay method to be compensated the delay inequality of two branch signals.

2. the method for synchronizing time in a kind of EPON OOFDM-PON connecting system of handing over frequency division multiplexing based on light positive as claimed in claim 1, it is characterized in that, for ease of the compensation that realizes that Time Delay of Systems is poor, for the optical signal transmission time delay of transmit timing information, be not less than the time delay for the light signal of transmission light ofdm signal.

3. the method for synchronizing time in a kind of EPON OOFDM-PON connecting system of handing over frequency division multiplexing based on light positive as claimed in claim 1, it is characterized in that, in the OOFDM-PON system, the ONU receiving terminal is detected and is formed by demodulation multiplexer, analog-to-digital conversion (ADC), OFDM demodulation, delay compensation and light pulse.

4. the method for synchronizing time in a kind of EPON OOFDM-PON connecting system based on light positive friendship frequency division multiplexing as claimed in claim 1, is characterized in that given Optical Fiber Transmission length L and centre frequency ω ₀, as the angular frequency ω that satisfies condition ₁≤ ω≤ω ₂the time, calculate the delay inequality after the light signal that has carried OFDM information and carried timing information passes through Optical Fiber Transmission:

The transmission rate of light carry information is generally used the light group velocity

mean, group velocity is the function of frequency; Solve the time delay τ after light unit of transfer length _g,

${\mathrm{\&tau;}}_g=\frac{1}{v_g}=\frac{\&PartialD;\mathrm{\&beta;}}{\&PartialD;\mathrm{\&omega;}}$

The β representative, along the propagation constant of shaft axis of optic fibre, is the slowly varying function of ω; Therefore, two bundle angular frequencies are respectively ω ₁and ω ₂light delay inequality after unit of transfer's length in optical fiber be,

$\mathrm{\&Delta;\&tau;}={\&Integral;}_{{\mathrm{\&omega;}}_1}^{{\mathrm{\&omega;}}_2}({\mathrm{\&beta;}}_2+{\mathrm{\&beta;}}_3(\mathrm{\&omega;}-{\mathrm{\&omega;}}_0))\mathrm{d\&omega;}$

Angular frequency is respectively ω ₁and ω ₂the light optical fiber that is L through length after, the delay inequality of two-beam is,

△t=L△τ

Determine thus the delay compensation amount, realize that timing signal exactly regularly.

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