CN106850497A - A kind of method of coherent-light OFDM communication system cascade compensation - Google Patents

A kind of method of coherent-light OFDM communication system cascade compensation Download PDF

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CN106850497A
CN106850497A CN201710032907.XA CN201710032907A CN106850497A CN 106850497 A CN106850497 A CN 106850497A CN 201710032907 A CN201710032907 A CN 201710032907A CN 106850497 A CN106850497 A CN 106850497A
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transmitting terminal
coherent
communication system
phase
ofdm communication
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CN106850497B (en
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李超
徐正元
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University of Science and Technology of China USTC
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/38—Demodulator circuits; Receiver circuits
    • H04L27/3845—Demodulator circuits; Receiver circuits using non - coherent demodulation, i.e. not using a phase synchronous carrier
    • H04L27/3854—Demodulator circuits; Receiver circuits using non - coherent demodulation, i.e. not using a phase synchronous carrier using a non - coherent carrier, including systems with baseband correction for phase or frequency offset
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/60—Receivers
    • H04B10/61—Coherent receivers
    • H04B10/616—Details of the electronic signal processing in coherent optical receivers
    • H04B10/6165—Estimation of the phase of the received optical signal, phase error estimation or phase error correction
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
    • H04L27/2695—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation with channel estimation, e.g. determination of delay spread, derivative or peak tracking

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Optical Communication System (AREA)

Abstract

The invention discloses a kind of method of coherent-light OFDM communication system cascade compensation, the transmitting terminal in coherent-light OFDM communication system first designs pilot sub-carrier allocative decision；The laser phase noise of the transmitting terminal is compensated using phase conjugation pilot aided method in the receiving terminal of coherent-light OFDM communication system；The IQ amplitudes and the unbalance in phase factor of the transmitting terminal are further estimated using frequency domain second order algorithm for estimating in the receiving terminal；The IQ for further eliminating the transmitting terminal mismatches the system performance degradation for causing, and the signal to noise ratio of signal is received to be lifted.The method can solve the problem that the influence that laser phase noise is demodulated to signal in relevant optical OFDM system, estimates and compensate for transmitting terminal IQ unbalance factors, improve Transmission system performance.

Description

A kind of method of coherent-light OFDM communication system cascade compensation
Technical field
The present invention relates to optical communication system technical field, more particularly to a kind of compensation of coherent-light OFDM communication system cascade Method.
Background technology
At present, be concerned with first-selection of the detection as high speed long distance optic transmission system, and single carrier 100G coherent receptions have been realized Commercialization, coherent light ofdm communication technology is the strong alternative of following 400G, 1T optical transmission system.OFDM technology is to optical fiber color Degree dispersion (CD) and polarization mode dispersion (PMD) with tolerance very high, and be easy to higher order modulation formats (such as m-QAM and M-PSK) it is combined, obtains spectral efficient.But relevant detection is especially sensitive to the phase noise caused by laser linewidth, Especially ofdm signal has a long symbol cycle due to it, and phase noise will have a strong impact on and deteriorate systematic function, therefore Relevant detection receiving terminal needs strict numeric field tracking, estimates and compensate.
Simultaneously as the characteristics of the intrinsic signal modulation of coherent light communication and demodulation, I roads and Q roads are strict with theory It is mutually orthogonal between signal, and the amplitude of two-way is equal.But in practical application, the mismatch of I/Q modulator bias point, I/Q Two-way cable is mismatched, nonideal 90 ° of optical mixer units and photodiode responsiveness be not equal, can all cause reception signal Amplitude and phase mismatch.The phase and amplitude of modulated signal is mismatched and is referred to as IQ imbalances, and IQ is uneven again comprising hair Penetrate end IQ uneven uneven with receiving terminal IQ, either the IQ of transmitting terminal or receiving terminal is uneven can all have a strong impact on system Performance.
The content of the invention
It is an object of the invention to provide a kind of method of coherent-light OFDM communication system cascade compensation, this method solve The influence that laser phase noise demodulate to signal in relevant optical OFDM system, estimate with compensate for transmitting terminal IQ imbalances because Son, improves system transfers performance.
A kind of method of coherent-light OFDM communication system cascade compensation, methods described includes：
Step 1, the transmitting terminal design pilot sub-carrier allocative decision in coherent-light OFDM communication system；
Step 2, the transmitting is compensated using phase conjugation pilot aided method in the receiving terminal of coherent-light OFDM communication system The laser phase noise at end；
Step 3, the receiving terminal further using frequency domain second order algorithm for estimating estimate the transmitting terminal IQ amplitudes and The unbalance in phase factor；
Step 4, the IQ for further eliminating the transmitting terminal mismatch the system performance degradation that causes, to lift reception signal Signal to noise ratio.
In the step 1：
The pilot sub-carrier allocative decision of the design includes：Localized sub-carrier distributes and interleaves subcarrier distribution.
In the step 2, the laser phase that the use phase conjugation pilot aided method compensates the transmitting terminal is made an uproar The process of sound is specially：
Each pilot sub-carrier is multiplied by a phase massφmIt is common phase error that laser linewidth causes；
Using the conjugated pilot sequence of transmitting terminal design, the estimate expression formula for obtaining common phase error is：
Wherein, arg () represents the phase for taking and tiring out and multiply rear pilot frequency sequence,It is the public phase estimated using pilot frequency sequence Position error amount；
The channel distortion experienced of coherent-light OFDM communication system is multiplied by againSo as to remove the shadow of laser phase noise Ring.
In the step 3, the IQ amplitudes and unbalance in phase factor expression of estimated transmitting terminal are：
Wherein, ε and Δ φ represent the IQ amplitudes and the unbalance in phase factor of transmitting terminal respectively；
C is known constant, NPFor the number of pilot sub-carrier, I and Q represents the two-way letter of coherent optical communication system transmitting terminal Number, rIAnd rQRepresent receiving terminal I roads and Q roads signal.
In the step 4, the process that the IQ for eliminating transmitting terminal mismatches the system performance degradation for causing is specially：
Assuming that the I roads and Q roads of coherent optical communication system transmitting end signal are respectively SIAnd SQ, and there is transmitting terminal I/Q two-way Amplitude and unbalance in phase ε and Δ φ, then the signal expression for obtaining receiving terminal is：
Finally giving the signal expression after compensation is
Wherein,
As seen from the above technical solution provided by the invention, the above method solves laser in relevant optical OFDM system The influence that device phase noise is demodulated to signal, estimates and compensate for transmitting terminal IQ unbalance factors, improves system transfers Energy.
Brief description of the drawings
Technical scheme in order to illustrate more clearly the embodiments of the present invention, below will be to that will use needed for embodiment description Accompanying drawing be briefly described, it should be apparent that, drawings in the following description are only some embodiments of the present invention, for this For the those of ordinary skill in field, on the premise of not paying creative work, other can also be obtained according to these accompanying drawings Accompanying drawing.
Fig. 1 is provided the method flow schematic diagram of coherent-light OFDM communication system cascade compensation by the embodiment of the present invention；
The scheme schematic diagram of the pilot sub-carrier design that Fig. 2 is provided by the embodiment of the present invention.
Specific embodiment
With reference to the accompanying drawing in the embodiment of the present invention, the technical scheme in the embodiment of the present invention is carried out clear, complete Ground description, it is clear that described embodiment is only a part of embodiment of the invention, rather than whole embodiments.Based on this Inventive embodiment, the every other implementation that those of ordinary skill in the art are obtained under the premise of creative work is not made Example, belongs to protection scope of the present invention.
The uneven docking collections of letters number of uneven to transmitting terminal IQ in coherent optical communication system first and receiving terminal IQ are produced Influence is illustrated：
Assuming that there is transmitting terminal phase mismatches factor θtFactor ε is mismatched with amplitudet, through light I/Q modulator after, output Electric-field intensity expression formula is：
Wherein,It is the electric-field intensity for being input into laser, ωSWithTransmitting terminal is represented respectively The initial angular frequency and phase of laser.In the ideal case, θt=0, εt=1, now export electric-field intensity and be changed into Eout(t)= Ein(t)·(IS+jQS)。
Coherent receiver based on three-dB coupler is made up of 4 three-dB couplers and 90 ° of phase shifters, for simplification Without loss of generality, it is assumed that coupler C1 has amplitude and mismatches factor εr(being the coefficient of coup of coupler), remaining 3 coupling Clutch is perfect condition, while there is phase mismatches factor θr.The ideally coefficient of coup ε of three-dB couplerr=1/2, Transmission function now can be expressed as：
It is expressed as by the electric field transmission function relational expression of coupler C1, C2, C3 and C4：
More than cascade 4 formulas, can obtain input signal electric field and local oscillation signal electric field with output E1(t)、E2(t)、E3 (t) and E4T the relation of () is：
By after balanced reciver, the expression formula that can obtain output light electric current is：
Wherein,It is the responsiveness of photodiode, in relevant detection ideallyThe pole of photoelectricity two is not considered The unmatched situation of responsiveness of pipe, the electric field expression formula solved in wushu (9) is obtained after substituting into formula (10) and (11)：
Obtaining expression formula after further simplifying is：
()*Conjugation is represented, using X (t)+X*T the property of ()=2Re { X (t) }, can obtain following simplified style
Assuming that not considering the influence of noise, EST () is the photoelectricity field intensity by the carrying signal after Optical Fiber Transmission, it Shown in expression formula such as formula (3), the electric-field intensity expression formula of local oscillator isωLOWithIt is respectively the initial angular frequency and phase of local oscillator.
By ES(t) and ELOT the expression formula of () substitutes into formula (16) and (17) and can obtain
Wherein,It is identical with local oscillator laser frequency in signal laser and made an uproar without phase θ=0 under conditions of sound, formula (18) and (19) are rewritten as：
Above formula is has transmitting terminal I roads and Q roads signal and the relational expression of receiving end signal in the case of IQ imbalances, uses square The form of battle array can be expressed as：
In the ideal case, θt=0, θr=0, εt=1,Substitution formula (22) can be obtained
Now, I roads and Q roads are completely orthogonal and amplitude is equal.
Start with from ofdm signal, when transmitting terminal has IQ imbalances, can obtain following derivation：
In transmitting terminal, OFDM baseband signals are x (t), and the expression formula that m-th symbol of time domain can be obtained after being converted through IDFT is
Wherein, N is the subcarrier number of ofdm signal, dt[k] represents k-th subcarrier, TSIt is OFDM symbol cycle, fk It is k-th frequency of subcarrier, it is ensured that the orthogonality requirement f of each subcarrierk=(k-1)/TS。
In receiving terminal, by transforming to frequency domain after DFT, then the expression formula of k-th subcarrier is
W [m] represents m-th OFDM symbol for receiving, and in the case where IQ imbalances and interchannel noise is not considered, there is w T ()=z (t), z (t)=x (t), z (t) are the OFDM baseband signals for receiving, while receiving terminal d can be obtainedr[k] is equal to transmitting terminal Value dt[k]。
Assuming that the amplitude and phase that there is transmitting terminal mismatch the factor, represented with variable ε and δ respectively, the time domain of receiving terminal Baseband signal is subject to the unbalanced influences of transmitting terminal IQ, and its expression formula is rewritten as：
W (t)=μ z (t)+λ z*(t) (26)
Wherein, introduce due to transmitting terminal IQ uneven caused distortion parameter μ and λ, their amplitudes and phase with transmitting terminal Mismatch amount ε is relevant with δ for position, μ=cos (δ)+j ε sin (δ), λ=ε cos (δ)-j sin (δ).
When transmitting terminal I roads and Q roads are in poised state, ε=0 in formula (26), δ=0 can obtain μ=1, λ=0.
Therefore, in the case where there is transmitting terminal IQ non-equilibrium states, the expression of k-th subcarrier of the frequency domain ofdm signal for receiving Formula is changed into
When noise is not considered, z (t)=x (t), thenAfter being substituted into formula (27) Obtain
Wherein,It isMirror image.
When there is transmitting terminal IQ and be uneven, OFDM subcarriers produce a mirror by symmetrical centre of transmitting terminal light carrier Picture；Similarly, when there is receiving terminal IQ and be uneven, the OFDM subcarriers for receiving can be produced centered on local oscillator frequencies A raw mirror image.Therefore due to symmetric position subcarrier mirror image interference, the planisphere of modulated signal can be distorted, cause Inter-carrier interference, has a strong impact on and reduces the transmission performance of signal.
The embodiment of the present invention is described in further detail in conjunction with accompanying drawing below, as shown in Figure 1 for the present invention is implemented Example provides the method flow schematic diagram of coherent-light OFDM communication system cascade compensation, and methods described includes：
Step 1, the transmitting terminal design pilot sub-carrier allocative decision in coherent-light OFDM communication system；
In the step 1, the pilot sub-carrier allocative decision of the design includes：Localized sub-carrier distributes and interleaves son Carrier wave is distributed.
The scheme schematic diagram of the pilot sub-carrier design that the embodiment of the present invention is provided is illustrated in figure 2, a in figure is represented Localized sub-carrier allocative decision, b represents and interleaves subcarrier distribution scheme, specifically：
Assuming that the number of pilot sub-carrier is 2NP, the both sides centered on DC component DC are symmetrically distributed in, and meet such as Lower relation：
It is assumed that 0 position is labeled as at DC component DC, DC left sides NPIndividual pilot tone is located at negative semiaxis, DC the right NPIndividual pilot bit In positive axis.Adjacent pilot frequencies sub-carrier phase difference is for accordingly pilot tone meets phase conjugation on pi/2, and positive and negative semiaxis on two-semiaxle Relation.
Step 2, the transmitting is compensated using phase conjugation pilot aided method in the receiving terminal of coherent-light OFDM communication system The laser phase noise at end；
In the step 2, the mistake of the laser phase noise of the transmitting terminal is compensated using phase conjugation pilot aided method Journey is specially：
Each pilot sub-carrier is multiplied by a phase massφmIt is common phase error that laser linewidth causes；
Using the conjugated pilot sequence of transmitting terminal design, the estimate expression formula for obtaining common phase error is：
Wherein, arg () represents the phase for taking and tiring out and multiply rear pilot frequency sequence,It is the public phase estimated using pilot frequency sequence Position error amount；
The channel distortion experienced of coherent-light OFDM communication system is multiplied by againSo as to remove the shadow of laser phase noise Ring.
Step 3, the receiving terminal further using frequency domain second order algorithm for estimating estimate the transmitting terminal IQ amplitudes and The unbalance in phase factor；
Specifically, the IQ amplitudes and the unbalance in phase factor of the transmitting terminal are estimated using frequency domain second order algorithm for estimating Process is：
Set the pilot subcarrier signals expression formula of the transmitting terminal as：
P [k]=ck·ejkπ/2, | k |=0,1,2..., NP. (31)
P [k] represents k-th expression formula of subcarrier, ckIt is k-th amplitude of intensity pilot tone；
It is by pilot subcarrier signals Frequency-variable Modulation, the signal expression for obtaining the transmitting terminal：
PI(k)=cos (2 π f0T+k pi/2s), PQ(k)=sin (2 π f0t+kπ/2)；
Assuming that existing, transmitting terminal IQ is uneven, receiving terminal I/Q two-way is perfect condition, not within limit of consideration.According to The expression formula of base electrical signal of the formula (22) by being received after fiber channel is
Wherein, ε and Δ φ are respectively the amplitude and the unbalance in phase factor of transmitting terminal, HkIt is channel response transmission function, f0 The frequency of modulated optical carrier is represented, ω [k] is channel Gaussian noise.
Then the value of ε and Δ φ is estimated using intensity pilot signal, process is implemented as described below：
OrderSubstitution formula (33), to rI[k] makees such as down conversion, can obtain
After channel equalization, transmitting terminal IQ unbalance factors are extracted using frequency domain second moment algorithm for estimating.It is adjacent to lead The phase difference of frequency is pi/2, can obtain following desired relationship formula
The c during pilot designkWith ck+1Range value it is consistent, using the pilot signal for receiving, what is be calculated Average value substitutes into formula (35) and (36) and can obtain：
WhereinIQ amplitudes can be solved during the value of A and B is substituted into formula (37) It is with the unbalance in phase factor
C is known constant, NPFor the number of pilot sub-carrier, I and Q represents the two of coherent-light OFDM communication system transmitting terminal Road signal.
Step 4, the IQ for further eliminating the transmitting terminal mismatch the system performance degradation that causes, to lift reception signal Signal to noise ratio.
In the step 4, the process that the IQ for eliminating transmitting terminal mismatches the system performance degradation for causing is specially：
Assuming that the I roads and Q roads of coherent-light OFDM communication system transmitting end signal are respectively SIAnd SQ, and there is transmitting terminal I/Q Two-way amplitude and unbalance in phase ε and Δ φ, then the signal expression for obtaining receiving terminal is：
Finally giving the signal expression after compensation is
Wherein,
In sum, laser phase is made an uproar during the method that the embodiment of the present invention is provided can solve the problem that relevant optical OFDM system The influence that sound is demodulated to signal, estimates and compensate for transmitting terminal IQ unbalance factors, Transmission system performance is improved, with low Computation complexity, easily can realize in hardware platform.
The above, the only present invention preferably specific embodiment, but protection scope of the present invention is not limited thereto, Any one skilled in the art in the technical scope of present disclosure, the change or replacement that can be readily occurred in, Should all be included within the scope of the present invention.Therefore, protection scope of the present invention should be with the protection model of claims Enclose and be defined.

Claims (5)

1. a kind of method that coherent-light OFDM communication system cascade is compensated, it is characterised in that methods described includes：
Step 1, the transmitting terminal design pilot sub-carrier allocative decision in coherent-light OFDM communication system；
Step 2, the transmitting terminal is compensated using phase conjugation pilot aided method in the receiving terminal of coherent-light OFDM communication system Laser phase noise；
Step 3, the IQ amplitudes and phase of further estimating the transmitting terminal using frequency domain second order algorithm for estimating in the receiving terminal Unbalance factor；
Step 4, the IQ for further eliminating the transmitting terminal mismatch the system performance degradation that causes, and the letter of signal is received to be lifted Make an uproar ratio.
2. the method that coherent-light OFDM communication system cascade is compensated according to claim 1, it is characterised in that in the step In rapid 1：
The pilot sub-carrier allocative decision of the design includes：Localized sub-carrier distributes and interleaves subcarrier distribution.
3. the method that coherent-light OFDM communication system cascade is compensated according to claim 1, it is characterised in that in the step In rapid 2, the process that the use phase conjugation pilot aided method compensates the laser phase noise of the transmitting terminal is specially：
Each pilot sub-carrier is multiplied by a phase massφmIt is common phase error that laser linewidth causes；
Using the conjugated pilot sequence of transmitting terminal design, the estimate expression formula for obtaining common phase error is：
φ m ^ = arg ( Σ k = 1 N P p m , k · p m , - k ) 2 N P
Wherein, arg () represents the phase for taking and tiring out and multiply rear pilot frequency sequence,It is that the common phase estimated using pilot frequency sequence is missed Difference；
The channel distortion experienced of coherent-light OFDM communication system is multiplied by againSo as to remove the influence of laser phase noise.
4. the method that coherent-light OFDM communication system cascade is compensated according to claim 1, it is characterised in that in the step In rapid 3, the IQ amplitudes and unbalance in phase factor expression of estimated transmitting terminal are：
ϵ = A + B - c 2 c Δ φ = ± a r c t a n A - c 2 B
Wherein, ε and Δ φ represent the IQ amplitudes and the unbalance in phase factor of transmitting terminal respectively；
A = Σ k = 1 2 N P r I 2 [ k ] / N P ;
B = Σ k = 1 2 N P r Q 2 [ k ] / N P ;
C is known constant, NPFor the number of pilot sub-carrier, I and Q represents the two-way letter of coherent-light OFDM communication system transmitting terminal Number.
5. the method that coherent-light OFDM communication system cascade is compensated according to claim 4, it is characterised in that in the step In rapid 4, the process that the IQ for eliminating transmitting terminal mismatches the system performance degradation for causing is specially：
Assuming that the I roads and Q roads of coherent-light OFDM communication system transmitting end signal are respectively SIAnd SQ, and there is transmitting terminal I/Q two-way Amplitude and unbalance in phase ε and Δ φ, then the signal expression for obtaining receiving terminal is：
r I r Q = 1 - ϵ s i n Δ φ 0 ϵ cos Δ φ S I S Q - - - ( 39 )
Finally giving the signal expression after compensation is
r I _ c o m . r Q _ c o m . = f [ ϵ , Δ φ ] r I r Q - - - ( 40 )
Wherein,
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CN108718217A (en) * 2018-03-16 2018-10-30 华北电力大学扬中智能电气研究中心 A kind of compensation method and device based on coherent optical communication system
CN109728856A (en) * 2017-10-27 2019-05-07 富士通株式会社 The unbalanced estimation device of optical sender IQ, compensation device and electronic equipment
CN112769497A (en) * 2021-02-23 2021-05-07 苏州大学 Method for carrying out nonlinear compensation on high-capacity high-order QAM coherent light

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Publication number Priority date Publication date Assignee Title
CN109728856A (en) * 2017-10-27 2019-05-07 富士通株式会社 The unbalanced estimation device of optical sender IQ, compensation device and electronic equipment
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CN112769497A (en) * 2021-02-23 2021-05-07 苏州大学 Method for carrying out nonlinear compensation on high-capacity high-order QAM coherent light
CN112769497B (en) * 2021-02-23 2022-02-22 苏州大学 Method for carrying out nonlinear compensation on high-capacity high-order QAM coherent light

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