CN115296729A - OFDM-Simplex code encoding and decoding method and system and optical time domain reflectometer - Google Patents
OFDM-Simplex code encoding and decoding method and system and optical time domain reflectometer Download PDFInfo
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- 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
- H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
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- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
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- H04L27/2626—Arrangements specific to the transmitter only
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- H04L27/2634—Inverse fast Fourier transform [IFFT] or inverse discrete Fourier transform [IDFT] modulators in combination with other circuits for modulation
Abstract
The invention provides a coding and decoding method and a system of an OFDM-Simplex code and an optical time domain reflectometer, wherein the method comprises the following steps: generating an L-order Simplex coding matrix according to the Hadamard matrix, and converting the L-order Simplex coding matrix into an L-order pulse matrix to be transmitted; expanding the column length of the L-order pulse matrix to be transmitted, and carrying out Hermitian symmetric transformation and IFFT operation to obtain an OFDM modulation matrix; performing parallel/serial conversion on the OFDM modulation matrix to obtain an OFDM-Simplex coding signal; performing serial/parallel conversion on the OFDM-Simplex coding signal to obtain a parallel signal matrix; performing FFT operation and modular operation on the parallel signal matrix to obtain an OFDM decoding matrix; and carrying out Simplex decoding on the OFDM decoding matrix to obtain an OTDR curve. Compared with the traditional single-pulse OTDR, the OTDR curve after being coded by the invention has higher signal-to-noise ratio, and only needs to transmit coded probe light to the optical fiber once and receive the coded probe light once again, thereby greatly improving the time efficiency of the system.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to an OFDM-Simplex code encoding and decoding method and system and an optical time domain reflectometer.
Background
Currently, optical Time Domain Reflectometry (OTDR) is a distributed fiber sensing technique. The basic principle is to emit a probing laser pulse (typically 10-500 ns pulse width) at one end of the fiber. When the probe pulse propagates along the fiber, rayleigh scattering (Rayleigh scattering) occurs at various positions of the fiber due to elastic collision of photons with molecules in the fiber, and a part of the laser light is scattered back in the direction opposite to the propagation direction. The rayleigh scattered light at each position of the optical fiber has different arrival times at the emission end of the optical fiber according to the distance. Furthermore, fresnel reflection (Fresnel reflection) occurs due to a sudden change in refractive index at a position such as a fiber break or a splice, and an optical signal with high intensity is returned to the incident end. Therefore, the distribution of the collected rayleigh scattered light signal and the fresnel reflected light signal in time corresponds to the distribution of the scattered light in space, and a set of OTDR curves is formed, and the curves can reflect the integrity and fault status of the optical fiber link.
The signal-to-noise ratio (SNR) is the ratio of the power of the back-scattered signal of the OTDR to the power of the noise, and is one of the key indicators of the OTDR system, and the OTDR with high SNR can usually better distinguish each event point, and the measurable maximum length is also longer. Rayleigh scattered signals are weak, so in order to improve the signal-to-noise ratio of the received signals, it is often necessary to use wider detection pulses. But wider pulses also occupy a larger range in space, thus sacrificing spatial resolution. In order to improve the signal-to-noise ratio without sacrificing the spatial resolution, an accumulation averaging method can be adopted, since the noise in the received signal is generated by the dark current of the receiver, the noise has non-correlation, and the ratio of the noise in the signal can be reduced and the ratio of the effective signal in the signal can be improved by superposing the signal of the receiving end for many times and averaging the signals. However, with the conventional cumulative averaging method, as the averaging times increase, the time cost of the system will gradually increase, and the time required for the system to measure an OTDR curve will increase more and more. And the increment of the OTDR signal-to-noise ratio brought by the accumulative average method is very limited, the averaging times and the signal-to-noise ratio are in an exponential relation, the amplitude of the increment of the signal-to-noise ratio is gradually reduced and tends to be stable, and the method has limitation in practical engineering application.
The SNR can also be improved by adopting a pulse coding technology, and a plurality of groups of sequences with good autocorrelation characteristics are used for coding the detection pulse. The total energy of the coded detection pulse is increased, so that the signal-to-noise ratio of the system can be improved, and the spatial resolution of the system cannot be changed after the received signal is decoded. Simplex coding is a pulse coding technique with high coding gain, and the coding needs to use an L × L S matrix as the coding basis. The matrix is converted from an (L + 1) -order Hadamard matrix, the first row and the first column of the Hadamard matrix are removed, an element '1' in the Hadamard matrix is changed into '1', and an element '0' is changed into '1', so that an S matrix can be obtained. Each row in the S matrix is a set of Simplex codes. And finally, carrying out decoding operations such as inverse Hadamard transformation, time-shift addition and the like on the received group results respectively to obtain a decoded OTDR curve.
Based on the Simplex coded OTDR, the longer the code length, the higher the signal-to-noise ratio of the system. But as the code length increases, the time cost of the system also increases. If the code length is increased to 255 bits, 255 groups of detection light pulses need to be transmitted into the optical fiber respectively and then received 255 times respectively, and therefore, the time required for system measurement is long.
Disclosure of Invention
The purpose of the invention is: the method and the system for encoding and decoding the OFDM-Simplex code and the optical time domain reflectometer are provided, and the detection pulse light is encoded by using a Simplex encoding technology on the basis of the traditional OTDR (optical time Domain reflectometer) accumulation average method, so that the signal-to-noise ratio of the system is further improved; on the basis of Simplex coding, an OFDM-Simplex coding sequence is formed by using an orthogonal frequency division multiplexing technology, and the signal-to-noise ratio of a system can be improved and the time efficiency of the system can be greatly improved on the premise of not sacrificing the spatial resolution of an OTDR system only by single transmission and single measurement.
In order to achieve the above object, a first aspect of the present invention provides a method for coding and decoding OFDM-Simplex codes, the method comprising: generating an L-order Simplex coding matrix according to the Hadamard matrix, and converting the L-order Simplex coding matrix into an L-order pulse matrix to be transmitted; expanding the column length of the L-order pulse matrix to be transmitted, and carrying out Hermitian symmetric transformation and IFFT operation to obtain an OFDM modulation matrix; performing parallel/serial conversion on the OFDM modulation matrix to obtain an OFDM-Simplex coding signal; performing serial/parallel conversion on the OFDM-Simplex coding signal to obtain a parallel signal matrix; performing FFT operation and modular operation on the parallel signal matrix to obtain an OFDM decoding matrix; and performing Simplex decoding on the OFDM decoding matrix to obtain an OTDR curve.
Preferably, the generating an L-order Simplex coding matrix according to the Hadamard matrix and transforming the L-order Simplex coding matrix into an L-order to-be-transmitted pulse matrix includes the following steps: transforming the Hadamard matrix by the following formula (2) to obtain an L +1 order Hadamard matrix;
wherein, formula (1) is Hadamard matrix H L The basic constitution of (1);
removing the first row and the first column of the L + 1-order Hadamard matrix, changing the element '1' into '0' and changing the element '1' into '1', and obtaining the L-order Simplex coding matrix; the L-order Simplex coding matrix has the following form:
the L-order Simplex coding matrix S L Constructing a pulse sequence to obtain the L-order pulse matrix to be transmitted:
wherein eta is 1 (t)、η 2 (t)…η L (t) consists of a pulse P (t) of width τ.
Preferably, the expanding the column length of the L-order pulse matrix to be transmitted, and performing Hermitian symmetric transformation and IFFT operation to obtain an OFDM modulation matrix includes the following steps: expanding the column length of the L-order pulse matrix to be transmitted into the number of points of the IFFT operation; performing Hermitian symmetric transformation on the expanded L-order pulse matrix to be transmitted to obtain a matrix with a Hermitian symmetric structure
The matrix
Has the following form:
wherein eta * A conjugate vector representing η;
combining the matrix
The following calculation is performed to obtain the coordinate position, and the remaining positions are complemented by 0 to obtain a matrix
The calculation formula is as follows:
C co ＝L IFFT -C ca +2 (7)
wherein, C ca And C co Respectively representing an original signal coordinate set and a conjugate signal coordinate set; c cL The expression interval is [1,L]Coordinate vectors with an interval of 1; l is a radical of an alcohol ca Represents the number of subcarriers, numerically equal to the code length L; l is IFFT The number of IFFT points is represented;
representing a floor function for element x;
the matrix
The form of (A) is as follows:
wherein z (t) represents a zero vector; eta (t) is represented by C ca Composition is carried out; eta (t) * From C co Composition is carried out;
for the matrix
Performing IFFT operation on each column to obtain the OFDM modulation matrix.
Preferably, the performing FFT operation and modulo operation on the parallel signal matrix to obtain an OFDM decoding matrix includes the following steps: performing FFT operation on each column of the parallel signal matrix to obtain a matrix R (t), wherein the matrix R (t) has the following form:
wherein ca represents the original signal initial coordinate, co represents the conjugate signal initial coordinate,
the original signal vector is used as the vector of the original signal,
is a conjugate signal vector;
selecting an original signal vector for the matrix R (t) to obtain a matrix R c (t)：
For the matrix R c Performing modular operation on elements in the (t) to obtain the OFDM decoding matrix r (t):
where e represents the noise carried each time a signal is received, and ψ (t) represents the OTDR curve function obtained after different delays are injected into the fiber.
Preferably, the obtaining the OTDR curve by performing Simplex decoding on the OFDM decoding matrix includes the following steps: calculating the L-order Simplex coding matrix S L Inverse matrix of
Applying the inverse matrix
Multiplying the OFDM decoding matrix r (t), performing inverse Hadamard transformation, performing time shift and accumulation by taking pulse width tau as a unit, and finally averaging to obtain the OTDR curve:
the invention provides a coding and decoding system of OFDM-Simplex codes, which comprises a first matrix module, a first operation module, a coding module, a second matrix module, a second operation module and a decoding module, wherein the first matrix module is used for coding OFDM-Simplex codes; wherein the content of the first and second substances,
the first matrix module is used for generating an L-order Simplex coding matrix according to the Hadamard matrix and converting the L-order Simplex coding matrix into an L-order pulse matrix to be transmitted;
the first operation module is used for expanding the column length of the L-order pulse matrix to be transmitted and carrying out Hermitian symmetric transformation and IFFT operation to obtain an OFDM modulation matrix;
the encoding module is used for carrying out parallel/serial conversion on the OFDM modulation matrix to obtain an OFDM-Simplex encoding signal;
the second matrix module is used for performing serial/parallel conversion on the OFDM-Simplex coded signal to obtain a parallel signal matrix;
the second operation module is used for performing FFT operation and modular operation on the parallel signal matrix to obtain an OFDM decoding matrix;
the decoding module is configured to perform Simplex decoding on the OFDM decoding matrix to obtain an OTDR curve.
A third aspect of the present invention provides an optical time domain reflectometer, comprising: the laser is used for inputting continuous laser to the modulation module; the sequence generating module is used for acquiring the OFDM-Simplex coding sequence according to the coding and decoding method of the OFDM-Simplex codes; the modulation module is used for carrying out orthogonal frequency division multiplexing on the continuous laser according to the OFDM-Simplex coding sequence and modulating the continuous laser into probe light carrying OFDM-Simplex coding information; the optical processing module is used for performing backward Rayleigh scattering on the detection light; a photodetector for converting the optical signal after the backward Rayleigh scattering into an electrical signal; the electric signal processing module is used for carrying out analog-to-digital conversion and average processing on the electric signal to obtain a signal to be decoded; and the decoding module is used for decoding the signal to be decoded according to the encoding and decoding method of the OFDM-Simplex code to obtain an OTDR curve.
Preferably, the modulation module comprises an arbitrary waveform generator and an electro-optical modulator; the random waveform generator is used for generating a modulation signal according to the OFDM-Simplex coding sequence; and the electro-optical modulator is used for carrying out orthogonal frequency division multiplexing on the input continuous light through the modulation signal and modulating the continuous light into probe light carrying OFDM-Simplex coding information.
Preferably, the optical processing module comprises an optical amplifier, an optical circulator and a single-mode optical fiber; the optical amplifier is used for performing power amplification processing on the detection light received from the modulation module; and the optical circulator is used for receiving the amplified detection light from the optical amplifier and outputting the detection light to the single-mode optical fiber to generate backward Rayleigh scattering detection light.
Preferably, the light processing module further comprises an attenuator; the attenuator is used for receiving the detection light which is subjected to backward Rayleigh scattering from the optical circulator, performing power attenuation processing, and outputting the detection light to the photoelectric detector.
Compared with the prior art, the encoding and decoding method and system of the OFDM-Simplex code and the optical time domain reflectometer provided by the invention have the following advantages:
(1) The signal-to-noise ratio is improved. Compared with the traditional single-pulse OTDR, the signal-to-noise ratio of the coded OTDR curve under the same pulse width and the same average times is higher than that of the traditional single-pulse OTDR.
(2) The time efficiency is improved. Compared with the traditional OTDR encoding technology, the encoding OTDR based on the orthogonal frequency division multiplexing technology does not need to transmit the encoding probe light into the optical fiber for multiple times and receive the scattering signal for multiple times. The coded detection light is transmitted into the optical fiber once and received once again, so that the time efficiency of the system is greatly improved.
Drawings
In order to more clearly illustrate the technical solution of the present invention, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a flowchart of a method for encoding and decoding OFDM-Simplex codes according to an embodiment of the present invention;
fig. 2 is a system block diagram of a coding/decoding system for OFDM-Simplex codes according to another embodiment of the present invention;
FIG. 3 is a block diagram of an optical time domain reflectometer according to another embodiment of the present invention;
FIG. 4 is a block diagram of a modulation module in an optical time domain reflectometer according to another embodiment of the present invention;
FIG. 5 is a block diagram of an optical processing module in an optical time domain reflector according to another embodiment of the present invention;
fig. 6 is an OTDR curve and a single pulse OTDR curve with different code lengths according to another embodiment of the present invention: (a) code length 31; (b) code length 15; (c) a code length of 7; (d) a single pulse.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
It should be understood that the step numbers used herein are for convenience of description only and are not intended as limitations on the order in which the steps are performed.
It is to be understood that the terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in the specification of the present invention and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.
The terms "comprises" and "comprising" indicate the presence of the described features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
The term "and/or" refers to any and all possible combinations of one or more of the associated listed items and includes such combinations.
In one embodiment, as shown in fig. 1, there is provided a method for encoding and decoding OFDM-Simplex codes, comprising the steps of:
s10, generating an L-order Simplex coding matrix according to the Hadamard matrix, and converting the L-order Simplex coding matrix into an L-order pulse matrix to be transmitted;
the method comprises the following steps of generating an L-order Simplex coding matrix according to an Hadamard matrix, wherein the L + 1-order Hadamard matrix needs to be generated firstly, and the Hadamard matrix basically comprises the following components:
the Hadamard matrix of order L +1 can be obtained by the following formula:
then, the first row and the first column of the matrix are removed, and the element "1" in the matrix is changed to "0" and the element "1" in the matrix is changed to "1", so that an L-order Simplex matrix can be obtained, and the form of the L-order Simplex matrix is shown as the following formula:
will matrix S L Constructing a pulse sequence, and then having the following L-order pulse matrix to be transmitted:
wherein eta is 1 (t)、η 2 (t)…η L (t) consists of a pulse P (t) of width τ.
S20, expanding the column length of the L-order pulse matrix to be transmitted, and carrying out Hermitian symmetric transformation and IFFT operation to obtain an OFDM modulation matrix;
the L-order pulse matrix to be transmitted is subjected to Hermitian symmetry to eliminate complex numbers generated in the subsequent fast inverse Fourier transform (IFFT) operation, and according to the property of discrete Fourier transform, signals with the Hermitian symmetric structure do not contain complex numbers after IFFT. When the IFFT is performed, the number of IFFT points is required to be a power of 2, where the number of IFFT points is λ (λ > L), that is, λ signals on a matrix column are subjected to IFFT. Because each column element of the signal matrix after Hermitian symmetry is smaller than the number lambda of IFFT points, the length of the original matrix column needs to be expanded to lambda, and the coordinate of the Hermitian symmetric structure needs to be calculated.
Therefore, to obtain an OFDM modulation matrix, the column length of the L-order pulse matrix to be transmitted needs to be expanded to the number of points of IFFT operation; and then the Hermitian symmetric transformation is carried out on the expanded L-order pulse matrix to be transmitted to obtain a matrix with a Hermitian symmetric structure
The matrix
Has the following form:
wherein eta is * A conjugate vector representing η;
then the matrix is formed
The following calculation is performed to obtain the coordinate position, and the remaining positions are complemented by 0 to obtain a matrix
The calculation formula is as follows:
C co ＝L IFFT -C ca +2 (7)
wherein, C ca And C co Respectively representing an original signal coordinate set and a conjugate signal coordinate set; c cL The expression interval is [1,L]Coordinate vectors with an interval of 1; l is ca Represents the number of subcarriers, numerically equal to the code length L; l is IFFT The number of IFFT points is represented;
representing a floor function for element x;
matrix array
The form of (A) is as follows:
wherein z (t) represents a zero vector; eta (t) is represented by C ca Composition is carried out; eta (t) * From C co Composition is carried out;
for matrix
Every column of the OFDM modulation matrix is subjected to IFFT operation to obtain the OFDM modulation matrix.
S30, performing parallel/serial conversion on the OFDM modulation matrix to obtain an OFDM-Simplex coding signal;
wherein the matrix is finally divided
And performing parallel/serial conversion to obtain the OFDM-Simplex coding sequence to be transmitted. Since the encoded signal contains negative numbers, it is necessary to base the signal onThe direct current bias is added to make the optical signal have single polarization, so that the optical signal can be transmitted in the optical fiber.
S40, performing serial/parallel conversion on the OFDM-Simplex coded signal to obtain a parallel signal matrix;
after receiving OFDM-Simplex coding signals, serial/parallel conversion is carried out on the signals, so that the signals become a parallel signal matrix of NxLambda.
S50, performing FFT operation and modulus operation on the parallel signal matrix to obtain an OFDM decoding matrix;
firstly, fast Fourier Transform (FFT) is carried out on each column of a parallel signal matrix to obtain a matrix R (t), wherein the matrix R (t) has the following form:
wherein ca represents the original signal initial coordinate, co represents the conjugate signal initial coordinate, which can be known from the formulas (6) and (7),
is the original signal vector and is the original signal vector,
for the conjugate signal vector, only the original signal vector needs to be selected to obtain the matrix R c (t)：
The matrix elements obtained by FFT also include complex numbers, and the OFDM decoding matrix r (t) can be obtained by performing modulo on the elements in the matrix:
where e represents the noise carried each time a signal is received, and ψ (t) represents the OTDR curve function obtained after different delays are injected into the fiber.
S60, simplex decoding is carried out on the OFDM decoding matrix to obtain an OTDR curve;
in which, to perform Simplex decoding, a matrix S is first calculated L Inverse matrix of
To recover the original OTDR curve, the matrix is applied
Multiplying by a matrix r (t), and performing inverse Hadamard transform:
and (3) performing time shifting and accumulation on all the results of the formula (13) by taking the pulse width as a unit, and finally averaging to obtain a final original OTDR curve:
in the embodiment of the application, based on the problems that the signal-to-noise ratio improvement is limited and the time cost of a system is increased along with the increase of the code length caused by the traditional single-pulse OTDR, an OFDM-Simplex code encoding and decoding method is designed, and the L-order Simplex encoding matrix is generated according to a Hadamard matrix and is converted into the L-order pulse matrix to be transmitted; expanding the column length of the L-order pulse matrix to be transmitted, and carrying out Hermitian symmetric transformation and IFFT operation to obtain an OFDM modulation matrix; performing parallel/serial conversion on the OFDM modulation matrix to obtain an OFDM-Simplex coding signal; performing serial/parallel conversion on the OFDM-Simplex coded signal to obtain a parallel signal matrix; performing FFT operation and modular operation on the parallel signal matrix to obtain an OFDM decoding matrix; and carrying out Simplex decoding on the OFDM decoding matrix to obtain the OTDR curve.
The OFDM-Simplex code adopts a mode of combining Simplex coding with OFDM technology, based on the principle that orthogonal subcarriers in the OFDM technology cannot interfere with each other, simplex coding sequences needing to be transmitted for multiple times are respectively modulated onto a plurality of subcarriers, and are simultaneously transmitted into an optical fiber after being superposed.
It should be noted that, although the steps in the above-described flowcharts are shown in sequence as indicated by arrows, the steps are not necessarily executed in sequence as indicated by the arrows. The steps are not limited to being performed in the exact order illustrated and, unless explicitly stated herein, may be performed in other orders.
In one embodiment, as shown in fig. 2, there is provided a coding and decoding system of OFDM-Simplex codes, the system comprising:
the first matrix module 1 is used for generating an L-order Simplex coding matrix according to a Hadamard matrix and converting the L-order Simplex coding matrix into an L-order pulse matrix to be transmitted;
the first operation module 2 is used for expanding the column length of the L-order pulse matrix to be transmitted and carrying out Hermitian symmetric transformation and IFFT operation to obtain an OFDM modulation matrix;
the encoding module 3 is used for performing parallel/serial conversion on the OFDM modulation matrix to obtain an OFDM-Simplex encoding signal;
a second matrix module 4, configured to perform serial/parallel conversion on the OFDM-Simplex encoded signal to obtain a parallel signal matrix;
the second operation module 5 is configured to perform FFT operation and modulo operation on the parallel signal matrix to obtain an OFDM decoding matrix;
and the decoding module 6 is used for performing Simplex decoding on the OFDM decoding matrix to obtain an OTDR curve.
The modules in the coding and decoding system of the OFDM-Simplex code can be implemented in whole or in part by software, hardware and a combination thereof. The modules can be embedded in a hardware form or independent from a processor in the computer device, and can also be stored in a memory in the computer device in a software form, so that the processor can call and execute operations corresponding to the modules.
In an embodiment, based on the above OFDM-Simplex code encoding/decoding method, as shown in fig. 3, the present invention provides an optical time domain reflectometer, including:
a laser 11 for inputting continuous laser to the modulation module;
in order to ensure sufficient frequency stability of the laser and avoid interference with the measurement result, the laser used herein emits laser light with a wavelength of 1550nm.
A sequence generating module 12, configured to obtain an OFDM-Simplex coding sequence by using the encoding and decoding method for the OFDM-Simplex code;
the OFDM-Simplex coding sequence is formed by modulating each Simplex coding sequence to be transmitted to orthogonal parallel subcarriers by using an orthogonal frequency division multiplexing technology (OFDM technology) on the basis of Simplex coding, and finally combining the subcarriers; the method can be used for the coding and decoding method of the OFDM-Simplex code to obtain the OFDM-Simplex coding sequence.
The modulation module 13 is configured to perform orthogonal frequency division multiplexing on the continuous laser according to the OFDM-Simplex coding sequence, and modulate the continuous laser into probe light carrying OFDM-Simplex coding information;
as shown in fig. 4, the modulation module 13 includes an arbitrary waveform generator 131 and an electro-optical modulator 132.
Specifically, the arbitrary waveform generator 131 is configured to generate a modulation signal according to the OFDM-Simplex code sequence. Meanwhile, the sampling rate of the arbitrary waveform generator is as high as possible, so that the orthogonality of each subcarrier is guaranteed not to be damaged, interference on decoding caused by subcarrier distortion due to too low sampling rate is avoided, and the sampling rate used by the arbitrary waveform generator is 2.5GHz.
And the electro-optical modulator 132 is configured to perform orthogonal frequency division multiplexing on the input continuous light through the modulation signal, and modulate the input continuous light into probe light carrying OFDM-Simplex coding information.
The electro-optical intensity modulator can be a conventional lithium niobate waveguide electro-optical Mach-Zehnder intensity modulator. The modulator needs to be dc biased to the lowest output power when operating, i.e. the output power of the modulator is lowest when no modulation signal is applied. The modulator is driven with a bipolar signal (having both a positive and a negative level signal) so that the modulator converts the input continuous light into a switching between two phases that differ by pi, according to the input code sequence. In theory, the electro-optic intensity modulator could be replaced by other types of modulators as long as "switching the input continuous light between two phases with a phase difference of π according to the input code sequence" is achieved. However, the electro-optical intensity modulator described herein is widely adopted and has better performance.
A light processing module 14, configured to perform backward rayleigh scattering on the probe light;
as shown in fig. 5, the optical processing module 14 includes an optical amplifier 141, an optical circulator 142, a single-mode fiber 143, and an attenuator 144.
An optical amplifier 141 for performing power amplification processing on the probe light received from the modulation module 13; the optical circulator 142 has three ports, wherein, the port 1 is connected to the optical amplifier 141 and is configured to receive the probe optical signal subjected to the power amplification processing by the optical amplifier 141; the No. 2 port is connected with the single mode fiber 143, and the detection light is output to the single mode fiber 143 connected to the No. 2 port of the optical circulator 142, and then backward Rayleigh scattering detection light is generated and output along the fiber; the attenuator 144 is connected to port No. 3, and is configured to receive the backward rayleigh scattering probe light generated on the single-mode optical fiber 143 from port No. 3 of the optical circulator 142, perform power attenuation processing, and output the power attenuation processing to the photodetector.
A photodetector 15 for converting the optical signal after the backward rayleigh scattering into an electrical signal;
when the photodetector 15 converts an optical signal into an electrical signal, the sampling rate of the photodetector needs to satisfy the nyquist sampling law, that is, the sampling rate is twice the sampling rate of an arbitrary waveform generator, so that the situation that the subcarrier orthogonality is destroyed to cause the decoding incapability is prevented. The photodetector sampling rate used here is 9GHz.
The electric signal processing module 16 is configured to perform analog-to-digital conversion and average processing on the electric signal to obtain a signal to be decoded;
the electrical signal processing module 16 is an analog-to-digital converter, and is configured to convert an electrical signal received from the photodetector 15 into a digital signal, and perform multiple acquisition and averaging on the digital signal to obtain a signal to be decoded. The signal processing after this module works in the digital domain. The average number of times is required to be as high as possible so that each orthogonal subcarrier is not affected by noise, and the average number of times is 2048 times.
And a decoding module 17, configured to decode the signal to be decoded according to the encoding and decoding method of the OFDM-Simplex code, to obtain an OTDR curve.
The working process of the optical time domain reflectometer provided by the invention is as follows: a laser in the optical time domain reflectometer emits 1550nm continuous laser, a coded detection sequence is input into an arbitrary waveform generator, the laser passes through a modulation module consisting of the arbitrary waveform generator and an electro-optical modulator, the continuous light is modulated into detection light carrying OFDM-Simplex coded information, the detection light is amplified to required power by an optical amplifier, and then the detection light is injected into a port No. 1 of an optical circulator; the detection light is output to a single mode fiber connected to the No. 2 port of the optical circulator; the back scattering light generated in the optical fiber by the detection light is transmitted backwards along the optical fiber, enters the attenuator from the 3 ports of the optical fiber circulator to be attenuated to required power, and then reaches the photoelectric detector to convert the optical signal into an electric signal; and finally, finishing multiple acquisition and averaging of the signals by the analog-to-digital converter, and finally outputting the result to a decoding module for decoding the signals to obtain a decoded OTDR curve.
In one embodiment, as shown in fig. 6, the results obtained by using 31, 15, and 7-bit OFDM-Simplex codes and 50ns single pulses as OTDR probe light respectively show that the signal-to-noise ratio of the coded OTDR curve is greatly improved compared with the single pulse OTDR curve, and the signal-to-noise ratio is further improved with the increase of the code length.
In summary, the invention discloses a coding and decoding method, a system and an optical time domain reflectometer of an OFDM-Simplex code, compared with the traditional single pulse OTDR, the signal to noise ratio of the coded OTDR curve is higher under the same pulse width and the same average number of times, and the coded OTDR based on the orthogonal frequency division multiplexing technology does not need to transmit coded probe light into an optical fiber for multiple times and receive scattered signals for multiple times; the coded detection light is transmitted into the optical fiber once and received once again, so that the time efficiency of the system is greatly improved.
The embodiments in this specification are described in a progressive manner, and all the same or similar parts of the embodiments are directly referred to each other, and each embodiment is described with emphasis on differences from other embodiments. In particular, for the system embodiment, since it is substantially similar to the method embodiment, the description is simple, and for the relevant points, reference may be made to the partial description of the method embodiment. It should be noted that, the technical features of the embodiments may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments are not described, but should be considered as the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express some preferred embodiments of the present application, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for those skilled in the art, various modifications and substitutions can be made without departing from the technical principle of the present invention, and these should be construed as the protection scope of the present application. Therefore, the protection scope of the present patent shall be subject to the protection scope of the claims.
Claims (10)
1. A method for encoding and decoding OFDM-Simplex codes, the method comprising:
generating an L-order Simplex coding matrix according to the Hadamard matrix, and converting the L-order Simplex coding matrix into an L-order pulse matrix to be transmitted;
expanding the column length of the L-order pulse matrix to be transmitted, and carrying out Hermitian symmetric transformation and IFFT operation to obtain an OFDM modulation matrix;
performing parallel/serial conversion on the OFDM modulation matrix to obtain an OFDM-Simplex coding signal;
performing serial/parallel conversion on the OFDM-Simplex coding signal to obtain a parallel signal matrix;
performing FFT operation and modular operation on the parallel signal matrix to obtain an OFDM decoding matrix;
and carrying out Simplex decoding on the OFDM decoding matrix to obtain an OTDR curve.
2. The method of claim 1, wherein the step of generating an L-order Simplex coding matrix according to a Hadamard matrix and transforming the L-order Simplex coding matrix into an L-order pulse matrix to be transmitted comprises the steps of:
transforming the Hadamard matrix by the following formula (2) to obtain an L +1 order Hadamard matrix;
wherein, formula (1) is Hadamard matrix H L The basic constitution of (1);
removing the first row and the first column of the L + 1-order Hadamard matrix, changing the element '1' into '0' and changing the element '1' into '1', and obtaining the L-order Simplex coding matrix; the L-order Simplex coding matrix has the following form:
the L-order Simplex coding matrix S L Constructing a pulse sequence to obtain the L-order pulse matrix to be transmitted:
wherein eta is 1 (t)、η 2 (t)…η L (t) consists of a pulse P (t) of width τ.
3. The encoding and decoding method of the OFDM-Simplex code according to claim 2, wherein the expanding the column length of the L-order pulse matrix to be transmitted, and performing Hermitian symmetric transformation and IFFT operation to obtain the OFDM modulation matrix, comprises the following steps:
expanding the column length of the L-order pulse matrix to be transmitted into the number of points of the IFFT operation;
carrying out Hermitian symmetric transformation on the expanded L-order pulse matrix to be transmitted to obtain a matrix with a Hermitian symmetric structure
The matrix is formed by
Has the following form:
wherein eta * A conjugate vector representing η;
combining the matrix
The following calculation is performed to obtain the coordinate position, and the remaining positions are complemented by 0 to obtain a matrix
The calculation formula is as follows:
C co ＝L IFFT -C ca +2 (7)
wherein, C ca And C co Respectively representing an original signal coordinate set and a conjugate signal coordinate set; c cL The expression interval is [1,L]Coordinate vectors at an interval of 1; l is a radical of an alcohol ca Represents the number of subcarriers, numerically equal to the code length L; l is IFFT The number of IFFT points is represented;
representing a floor function for element x;
the matrix
Is of the form:
wherein z (t) represents a zero vector; eta (t) is represented by C ca Composition is carried out; eta (t) * From C co Composition is carried out;
for the matrix
Performing IFFT operation on each column to obtain the OFDM modulation matrix.
4. The method as claimed in claim 3, wherein the performing FFT operation and modulo operation on the parallel signal matrix to obtain the OFDM decoding matrix comprises the following steps:
performing FFT operation on each column of the parallel signal matrix to obtain a matrix R (t), wherein the matrix R (t) has the following form:
wherein ca represents the original signal initial coordinate, co represents the conjugate signal initial coordinate,
is the original signal vector and is the original signal vector,
is a conjugate signal vector;
selecting an original signal vector for the matrix R (t) to obtain a matrix R c (t)：
For the matrix R c Performing modular operation on elements in the (t) to obtain the OFDM decoding matrix r (t):
where e represents the noise carried each time a signal is received, and ψ (t) represents the OTDR curve function obtained after different delays are injected into the fiber.
5. The method of claim 4, wherein the step of Simplex decoding the OFDM decoding matrix to obtain an OTDR curve comprises the steps of:
calculating the L-order Simplex coding matrixS L Inverse matrix of (2)
Applying the inverse matrix
Multiplying the OFDM decoding matrix r (t), performing inverse Hadamard transformation, performing time shift and accumulation by taking pulse width tau as a unit, and finally averaging to obtain the OTDR curve:
6. an OFDM-Simplex code coding and decoding system is characterized by comprising a first matrix module, a first operation module, a coding module, a second matrix module, a second operation module and a decoding module; wherein the content of the first and second substances,
the first matrix module is used for generating an L-order Simplex coding matrix according to the Hadamard matrix and converting the L-order Simplex coding matrix into an L-order pulse matrix to be transmitted;
the first operation module is used for expanding the column length of the L-order pulse matrix to be transmitted and carrying out Hermitian symmetric transformation and IFFT operation to obtain an OFDM modulation matrix;
the encoding module is used for carrying out parallel/serial conversion on the OFDM modulation matrix to obtain an OFDM-Simplex encoding signal;
the second matrix module is used for carrying out serial/parallel conversion on the OFDM-Simplex coded signal to obtain a parallel signal matrix;
the second operation module is used for performing FFT operation and modular operation on the parallel signal matrix to obtain an OFDM decoding matrix;
the decoding module is configured to perform Simplex decoding on the OFDM decoding matrix to obtain an OTDR curve.
7. An optical time domain reflectometer, comprising:
the laser is used for inputting continuous laser to the modulation module;
a sequence generating module, for obtaining the OFDM-Simplex coding sequence according to the coding and decoding method of claims 1-5;
the modulation module is used for carrying out orthogonal frequency division multiplexing on the continuous laser according to the OFDM-Simplex coding sequence and modulating the continuous laser into probe light carrying OFDM-Simplex coding information;
the optical processing module is used for performing backward Rayleigh scattering on the detection light;
a photodetector for converting the optical signal after the backward Rayleigh scattering into an electrical signal;
the electric signal processing module is used for carrying out analog-to-digital conversion and average processing on the electric signal to obtain a signal to be decoded;
a decoding module, configured to decode the signal to be decoded according to the coding and decoding method in claims 1 to 5, so as to obtain an OTDR curve.
8. An optical time domain reflectometer as in claim 7, wherein: the modulation module comprises an arbitrary waveform generator and an electro-optical modulator; wherein the content of the first and second substances,
the arbitrary waveform generator is used for generating a modulation signal according to the OFDM-Simplex coding sequence;
and the electro-optical modulator is used for carrying out orthogonal frequency division multiplexing on the input continuous light through the modulation signal and modulating the continuous light into probe light carrying OFDM-Simplex coding information.
9. An optical time domain reflectometer as in claim 7, wherein: the optical processing module comprises an optical amplifier, an optical circulator and a single-mode optical fiber; wherein the content of the first and second substances,
the optical amplifier is used for carrying out power amplification processing on the detection light received from the modulation module;
and the optical circulator is used for receiving the amplified detection light from the optical amplifier and outputting the detection light to the single-mode optical fiber to generate backward Rayleigh scattering detection light.
10. An optical time domain reflectometer as in claim 7, wherein: the optical processing module further comprises an attenuator; wherein the content of the first and second substances,
and the attenuator is used for receiving the detection light subjected to backward Rayleigh scattering from the optical circulator, performing power attenuation processing and outputting the detection light to the photoelectric detector.
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