CN111064524B - Polarization-independent frequency offset estimation method and system - Google Patents
Polarization-independent frequency offset estimation method and system Download PDFInfo
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- CN111064524B CN111064524B CN201911267246.4A CN201911267246A CN111064524B CN 111064524 B CN111064524 B CN 111064524B CN 201911267246 A CN201911267246 A CN 201911267246A CN 111064524 B CN111064524 B CN 111064524B
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- 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
- H04B10/6164—Estimation or correction of the frequency offset between the received optical signal and the optical local oscillator
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- 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
- H04B10/6162—Compensation of polarization related effects, e.g., PMD, PDL
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/10—Frequency-modulated carrier systems, i.e. using frequency-shift keying
- H04L27/14—Demodulator circuits; Receiver circuits
- H04L27/142—Compensating direct current components occurring during the demodulation and which are caused by mistuning
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2657—Carrier synchronisation
- H04L27/2659—Coarse or integer frequency offset determination and synchronisation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2657—Carrier synchronisation
- H04L27/266—Fine or fractional frequency offset determination and synchronisation
Abstract
The invention discloses a polarization-independent frequency offset estimation method and system, and relates to the field of coherent reception passive optical networks. The method comprises the following steps: inserting pilot signals of which the X polarization components are orthogonal to the Y polarization components into a transmitting end; and the receiving end estimates the frequency offset according to the PSD offset of the pilot signal. The invention can effectively estimate the frequency offset under any polarization rotation under the condition of only receiving one polarization signal.
Description
Technical Field
The invention relates to the field of coherent reception passive optical networks, in particular to a polarization-independent frequency offset estimation method and system.
Background
The coherent receiving technology is used in a high-speed PON (Passive Optical Network), can effectively improve the sensitivity of a receiver (thereby increasing a splitting ratio and a transmission distance), and can effectively perform digital domain dispersion compensation due to the linear detection characteristic thereof.
However, the coherent transceiver used in the long-distance high-speed optical transmission system is too costly, and the application thereof in the PON is limited. In order to reduce the cost of the coherent PON, the coherent PON with single polarization receiving is provided, and the data of only one polarization can be received at the receiving end without a polarization controller, so that the number of photoelectric devices required by a coherent receiver is reduced by half.
In a coherent PON with single polarization reception, frequency offset estimation is required because carrier frequencies of a local oscillator laser at a receiving end and a laser at a transmitting end cannot be completely synchronized.
In the process of implementing the invention, the inventor finds that at least the following problems exist in the prior art:
conventional pilot-based frequency offset estimation algorithms face significant challenges. This is because the conventional digital pilot frequency offset estimation method aims at that signals of two polarizations are received, and in the case that only a signal of one polarization is received, power attenuation of the pilot frequency on the polarization due to polarization rotation may occur, so that frequency offset estimation cannot be accurately performed.
Disclosure of Invention
The present invention is directed to overcome the above-mentioned drawbacks of the prior art, and provides a method and a system for estimating frequency offset independent of polarization, which can effectively perform frequency offset estimation under any polarization rotation when only one polarized signal is received.
In a first aspect, a method for estimating a polarization-independent frequency offset is provided, which includes the following steps:
inserting pilot signals of which the X polarization components are orthogonal to the Y polarization components into a transmitting end;
and the receiving end estimates the frequency offset according to the PSD offset of the pilot signal.
According to the first aspect, in a first possible implementation manner of the first aspect, the X-polarization component of the pilot signal is: pX＝[s,-s,…s,-s]S is the digital modulation signal, and the Y polarization component is: pY＝[s,s,…s,s]。
According to the first aspect, in a second possible implementation manner of the first aspect, a receiving end estimates a frequency offset according to a shift of a PSD of the pilot signal, including the following steps:
calculating the PSD of the twice oversampled received pilot signal; moving the direct current component of the PSD to the center to obtain an intermediate variable Pf of the PSD; establishing a frequency vector, and calculating the maximum value Pf of the PfmaxAnd PfmaxNumber n ofmaxCalculating nmaxComponent f (n) of the corresponding frequency vectormax) (ii) a According to f (n)max) To estimate the frequency offset.
According to a second possible implementation form of the first aspect, in a third possible implementation form of the first aspect, the method is performed according to f (n)max) To estimate the frequency offset, comprising the steps of:
when in use
The estimated value of frequency offset is fsf(nmax)；
When in use
An estimate of the frequency offset is
When in use
An estimate of the frequency offset is
According to the second possible implementation manner of the first aspect, in a fourth possible implementation manner of the first aspect, the calculating the PSD of the twice oversampled received pilot signal includes the following steps:
calculating the square of the absolute value of N-point FFT of the received pilot signal, i.e. PSD, [ P (1), P (2), …, P (N) ], where N is the number of FFT points;
on the basis of the intermediate variable Pf of the PSD, [ Pf (1), Pf (2), …, Pf (N)]＝[P(N/2+1),P(N/2+2)…,P(N),P(1),P(2),…,P(N/2)]Frequency vector
In a second aspect, a polarization-independent frequency offset estimation system is provided, including a transmitting end and a receiving end, where the transmitting end is configured to: inserting a pilot signal with an X polarization component orthogonal to a Y polarization component; the receiving end is used for: estimating a frequency offset from the shift of the PSD of the pilot signal.
According to the second aspect, in a first possible implementation manner of the second aspect, the X-polarization component of the pilot signal is: pX＝[s,-s,…s,-s]S is the digital modulation signal, and the Y polarization component is: pY＝[s,s,…s,s]。
According to the second aspect, in a second possible implementation manner of the second aspect, the receiving end is specifically configured to:
calculating the PSD of the twice oversampled received pilot signal; moving the direct current component of the PSD to the center to obtain an intermediate variable Pf of the PSD; establishing a frequency vector, and calculating the maximum value Pf of the PfmaxAnd PfmaxNumber n ofmaxCalculating nmaxComponent f (n) of the corresponding frequency vectormax) (ii) a According to f (n)max) To estimate the frequency offset.
According to a second possible implementation form of the second aspect, in a third possible implementation form of the second aspect, the method is according to f (n)max) To estimate the frequency offset, comprising the steps of:
when in use
The estimated value of frequency offset is fsf(nmax)；
When in use
An estimate of the frequency offset is
When in use
An estimate of the frequency offset is
According to a second possible implementation manner of the second aspect, in a fourth possible implementation manner of the second aspect, the calculating the PSD of the twice oversampled received pilot signal includes the following steps:
calculating the square of the absolute value of N-point FFT of the received pilot signal, i.e. PSD, [ P (1), P (2), …, P (N) ], where N is the number of FFT points;
on the basis of the intermediate variable Pf of the PSD, [ Pf (1), Pf (2), …, Pf (N)]＝[P(N/2+1),P(N/2+2)…,P(N),P(1),P(2),…,P(N/2)]Frequency vector
Compared with the prior art, the invention has the following advantages:
the invention designs two polarized digital pilot signals at a transmitting end, wherein an X polarization component and a Y polarization component of the pilot signals are orthogonal; and the receiving end carries out corresponding digital signal processing and estimates frequency offset according to the PSD offset of the pilot signal. Under the condition that only one polarized signal is received, frequency offset estimation can be effectively carried out under any polarization rotation, the problem of frequency offset estimation in a PON system based on single-polarization coherent reception is solved, and the method can be used for a low-cost coherent passive optical network.
Drawings
FIG. 1 is a flow chart of a method for polarization independent frequency offset estimation in an embodiment of the present invention.
Fig. 2 is a schematic diagram of a frame structure of a signal based on digital pilot frequency offset estimation according to an embodiment of the present invention.
Fig. 3 is a diagram illustrating PSD of a frame signal according to an embodiment of the present invention.
Fig. 4 is a flowchart of frequency offset estimation performed by a receiving end in an embodiment of the present invention.
Detailed Description
Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the specific embodiments, it will be understood that they are not intended to limit the invention to the embodiments described. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. It should be noted that the method steps described herein may be implemented by any functional block or functional arrangement, and that any functional block or functional arrangement may be implemented as a physical entity or a logical entity, or a combination of both.
In order that those skilled in the art will better understand the present invention, the following detailed description of the invention is provided in conjunction with the accompanying drawings and the detailed description of the invention.
Note that: the example to be described next is only a specific example, and does not limit the embodiments of the present invention necessarily to the following specific steps, values, conditions, data, orders, and the like. Those skilled in the art can, upon reading this specification, utilize the concepts of the present invention to construct more embodiments than those specifically described herein.
In order to solve the problem that the traditional digital pilot frequency offset estimation method cannot accurately perform frequency offset estimation because the power of the pilot frequency is attenuated due to polarization rotation under the condition that only one polarization signal is received, an embodiment of the present invention provides a polarization-independent frequency offset estimation method, which is shown in fig. 1 and includes the following steps:
s1, inserting pilot signals with the X polarization components orthogonal to the Y polarization components into the transmitting end;
s2, the receiving end estimates the frequency offset according to the offset of the PSD (Power Spectrum Density) of the pilot signal.
As a preferred embodiment, the X-polarization component of the pilot signal is: pX＝[s,-s,…s,-s]S is the digital modulation signal, and the Y polarization component is: pY＝[s,s,…s,s]。
As a preferred embodiment, the receiving end estimates the frequency offset according to the shift of the PSD of the pilot signal, including the following steps:
calculating the PSD of the twice oversampled received pilot signal; moving the direct current component of the PSD to the center to obtain an intermediate variable Pf of the PSD; establishing a frequency vector, and calculating the maximum value Pf of the PfmaxAnd PfmaxNumber n ofmaxCalculating nmaxComponent f (n) of the corresponding frequency vectormax) (ii) a According to f (n)max) To estimate the frequency offset.
As a preferred embodiment, according to f (n)max) To estimate the frequency offset, comprising the steps of:
when in use
The estimated value of frequency offset is fsf(nmax)；
When in use
An estimate of the frequency offset is
When in use
Time, frequencyAn offset value is
As a preferred embodiment, calculating the PSD of the twice oversampled received pilot signal comprises the steps of:
calculating the square of the absolute value of N-point FFT of the received pilot signal, i.e. PSD, [ P (1), P (2), …, P (N) ], where N is the number of FFT points;
on the basis of the intermediate variable Pf of the PSD, [ Pf (1), Pf (2), …, Pf (N)]＝[P(N/2+1),P(N/2+2)…,P(N),P(1),P(2),…,P(N/2)]Frequency vector
The embodiment of the invention also provides a polarization-independent frequency offset estimation system, which comprises a sending end and a receiving end, wherein:
the sending end is used for: inserting a pilot signal with an X polarization component orthogonal to a Y polarization component;
the receiving end is used for: estimating a frequency offset from the shift of the PSD of the pilot signal.
As a preferred embodiment, the X-polarization component of the pilot signal is: pX＝[s,-s,…s,-s]S is the digital modulation signal, and the Y polarization component is: pY＝[s,s,…s,s]。
As a preferred embodiment, the receiving end is specifically configured to:
calculating the PSD of the twice oversampled received pilot signal; moving the direct current component of the PSD to the center to obtain an intermediate variable Pf of the PSD; establishing a frequency vector, and calculating the maximum value Pf of the PfmaxAnd PfmaxNumber n ofmaxCalculating nmaxComponent f (n) of the corresponding frequency vectormax) (ii) a According to f (n)max) To estimate the frequency offset.
As a preferred embodiment, according to f (n)max) To estimate the frequency offset, comprising the steps of:
when in use
The estimated value of frequency offset is fsf(nmax)；
When in use
An estimate of the frequency offset is
When in use
An estimate of the frequency offset is
As a preferred embodiment, calculating the PSD of the twice oversampled received pilot signal comprises the steps of:
calculating the square of the absolute value of N-point FFT of the received pilot signal, i.e. PSD, [ P (1), P (2), …, P (N) ], where N is the number of FFT points;
on the basis of the intermediate variable Pf of the PSD, [ Pf (1), Pf (2), …, Pf (N)]＝[P(N/2+1),P(N/2+2)…,P(N),P(1),P(2),…,P(N/2)]Frequency vector
The embodiment of the invention aims at a PON system based on single-polarization coherent reception, and comprises a digital signal frame structure designed at a sending end and a frequency offset estimation digital signal processing process at a receiving end. First, at the transmitting end, the frame structure of the digital signal is as shown in fig. 2, and each frame signal is composed of a pilot signal for frequency offset estimation and a data signal for carrying data information. Each pilot signal is composed of components on X-polarization and Y-polarization, where the X-polarization component of the pilot signal is: pX＝[s,-s,…s,-s]S is the digital modulation signal, and the Y polarization component of the pilot signal is: pY＝[s,s,…s,s]. s is flexibly selected according to the modulation format of the data signal. For example, if the modulation format of the data signal is 16QAM, s is a 16QAM modulated signal。
According to the reference, the polarization response of the fiber channel has also been modeled as a Jones matrix, expressed as:
wherein, [.]Denotes a conjugation operation, alpha and
representing the rotation angles in the horizontal and vertical directions for the two polarization states, respectively. Thus, without loss of generality, considering only the first two symbol periods, the received pilot signal after the fibre channel is passed can be expressed as:
wherein r isXAnd rYRepresenting X-polarized and Y-polarized received pilot signals, respectively.
Without loss of generality, assuming that only X-polarized pilot signals are received and twice oversampled, the received pilot signals are:
as can be seen from equation (4), the received pilot signal r has a period of 4TsIn which T issRepresents the over-sampled sampling period, and therefore its autocorrelation function can be calculated as:
according to the Winnozenkinje theorem, the received pilot signal is in the frequency range [ -1/2TS,1/2TS]PSD in the inner is an autocorrelation function R (k)The fourier transform of (a), and thus can be calculated as:
wherein (·) represents a unit impulse response, and F (m) represents a Fourier series of R (k). F (m) can be calculated as follows:
substituting equation (5) and equation (7) into equation (6) to obtain PSD of received pilot signal r (n) as:
equation (8) shows that the PSD of the pilot signal is at-1/4T in addition to the DC component (frequency of 0)SAnd +1/4TSThere is a component.
Since | a |2+|b|2The total power of all frequency components is 1/4TS, which is independent of any polarization rotation angle. Thus, at least one of the frequency components is sufficiently powerful to be used for frequency offset estimation, as shown in fig. 3. Let fs＝1/TsSince the frequency difference between two adjacent frequency components of the PSD is fs/4, so that the maximum frequency offset that the pilot can estimate is
According to the property of Fourier transform, the frequency offset causes the frequency component of PSD of received pilot frequency to shift, and the frequency offset estimation can be carried out by judging the position of the frequency component of PSD.
Specifically, the frequency offset estimation process at the receiving end is as shown in fig. 4:
first, calculating PSD of twice oversampled received pilot signal, specifically:
the square of the absolute value of the N-point Fast Fourier Transform (FFT), i.e., PSD, of the received pilot signal is calculated, and the result is denoted as PSD [ P (1), P (2), …, P (N) ], where N is the number of FFT points.
Secondly, moving the direct current component of P to the center to obtain an intermediate variable Pf of the PSD: pf ═ P (N/2+1), P (N/2+2) …, P (N), P (1), P (2), …, P (N/2) ];
establishing a frequency vector F:
calculating the maximum value Pf of PfmaxAnd recording PfmaxCorresponding serial number nmax(i.e., Pf in vector PfmaxIs the fourth component);
calculating nmaxComponent f (n) of the frequency vector corresponding to the indexmax)。
Thirdly, frequency offset value judgment is carried out, specifically, according to f (n)max) To estimate the frequency offset:
when in use
The estimated value of frequency offset is fsf(nmax)；
When in use
An estimate of the frequency offset is
When in use
An estimate of the frequency offset is
The embodiment of the invention designs digital pilot signals on two polarizations at a transmitting end, wherein an X polarization component and a Y polarization component of the pilot signals are orthogonal; and the receiving end carries out corresponding digital signal processing and estimates frequency offset according to the PSD offset of the pilot signal. Under the condition that only one polarized signal is received, frequency offset estimation can be effectively carried out under any polarization rotation, the problem of frequency offset estimation in a PON system based on single-polarization coherent reception is solved, and the method can be used for a low-cost coherent passive optical network.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (10)
1. A polarization-independent frequency offset estimation method is characterized by comprising the following steps:
inserting pilot signals of which the X polarization components are orthogonal to the Y polarization components into a transmitting end;
when the receiving end receives only one polarization signal, under the condition of arbitrary polarization rotation, the frequency offset is estimated according to the PSD offset of the pilot signal of the received polarization signal.
2. The method of claim 1, wherein:
the X-polarization component of the pilot signal is: pX＝[s，-s，…s，-s]S is the digital modulation signal, and the Y polarization component is: pY＝[s，s，…s，s]。
3. The method of claim 1, wherein: when a receiving end receives only one polarization signal, under any polarization rotation, the frequency offset is estimated according to the PSD offset of the pilot signal of the received polarization signal, and the method comprises the following steps:
calculating the PSD of the twice oversampled received pilot signal; moving the direct current component of the PSD to the center to obtain an intermediate variable Pf of the PSD; establishing a frequency vector, and calculating the maximum value Pf of the PfmaxAnd PfmaxNumber n ofmaxCalculating nmaxCorresponding frequencyComponent f (n) of the vectormax) (ii) a According to f (n)max) To estimate the frequency offset.
4. The method of claim 3, wherein:
according to f (n)max) To estimate the frequency offset, comprising the steps of:
when in use
The estimated value of frequency offset is fsf(nmax)，fs＝1/Ts，TsRepresents the sampling period of the oversampling;
when in use
An estimate of the frequency offset is
When in use
An estimate of the frequency offset is
5. The method of claim 3, wherein:
calculating the PSD of the twice oversampled received pilot signal, comprising the steps of:
calculating the square of the absolute value of N-point FFT of the received pilot signal, i.e. PSD, [ P (1), P (2), …, P (N) ], where N is the number of FFT points;
on the basis of the intermediate variable Pf of the PSD, [ Pf (1), Pf (2), …, Pf (N)]＝[P(N/2+1)，P(N/2+2)…，P(N)，P(1)，P(2)，…，P(N/2)]Frequency vector
6. A polarization-independent frequency offset estimation system comprises a sending end and a receiving end, and is characterized in that:
the sending end is used for: inserting a pilot signal with an X polarization component orthogonal to a Y polarization component;
the receiving end is used for: when only one polarization signal is received, the frequency offset is estimated from the shift of the PSD of the pilot signal of the received polarization signal at an arbitrary polarization rotation.
7. The system of claim 6, wherein:
the X-polarization component of the pilot signal is: pX＝[s，-s，…s，-s]S is the digital modulation signal, and the Y polarization component is: pY＝[s，s，…s，s]。
8. The system of claim 6, wherein: the receiving end is specifically configured to:
calculating the PSD of the twice oversampled received pilot signal; moving the direct current component of the PSD to the center to obtain an intermediate variable Pf of the PSD; establishing a frequency vector, and calculating the maximum value Pf of the PfmaxAnd PfmaxNumber n ofmaxCalculating nmaxComponent f (n) of the corresponding frequency vectormax) (ii) a According to f (n)max) To estimate the frequency offset.
9. The system of claim 8, wherein:
according to f (n)max) To estimate the frequency offset, comprising the steps of:
when in use
The estimated value of frequency offset is fsf(nmax)，fs＝1/Ts，TsRepresents the sampling period of the oversampling;
when in use
An estimate of the frequency offset is
When in use
An estimate of the frequency offset is
10. The system of claim 8, wherein:
calculating the PSD of the twice oversampled received pilot signal, comprising the steps of:
calculating the square of the absolute value of N-point FFT of the received pilot signal, i.e. PSD, [ P (1), P (2), …, P (N) ], where N is the number of FFT points;
on the basis of the intermediate variable Pf of the PSD, [ Pf (1), Pf (2), …, Pf (N)]＝[P(N/2+1)，P(N/2+2)…，P(N)，P(1)，P(2)，…，P(N/2)]Frequency vector
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