CN111970222A - Uplink beat frequency noise suppression method based on chip and mode coherence - Google Patents
Uplink beat frequency noise suppression method based on chip and mode coherence Download PDFInfo
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- CN111970222A CN111970222A CN202011128495.8A CN202011128495A CN111970222A CN 111970222 A CN111970222 A CN 111970222A CN 202011128495 A CN202011128495 A CN 202011128495A CN 111970222 A CN111970222 A CN 111970222A
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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
- H04L27/345—Modifications of the signal space to allow the transmission of additional information
- H04L27/3461—Modifications of the signal space to allow the transmission of additional information in order to transmit a subchannel
- H04L27/3483—Modifications of the signal space to allow the transmission of additional information in order to transmit a subchannel using a modulation of the constellation points
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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
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/69—Electrical arrangements in the receiver
- H04B10/697—Arrangements for reducing noise and distortion
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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
- H04L27/2668—Details of algorithms
- H04L27/2681—Details of algorithms characterised by constraints
- H04L27/2688—Resistance to perturbation, e.g. noise, interference or fading
Abstract
The invention discloses an uplink beat frequency noise suppression method based on chip and mode coherence, belonging to the technical field of optical access networks, and the invention relates to an uplink transmission method for suppressing beat frequency noise caused by frequency difference generated by different lasers, wherein each ONU is allocated with an appointed chip, and the chips are mutually orthogonal; the uplink data is firstly subjected to constellation mapping, then each uplink data is subjected to spread spectrum operation by using a chip respectively, and then the uplink data is converted into an optical signal by an optical transceiver and then transmitted back to the OLT. Due to the fact that the chips have orthogonality, uplink signal mutual interference of the ONUs is greatly reduced; in addition, the coding gain of the chip can also inhibit the uplink beat noise; meanwhile, multimode optical fiber is used as a transmission medium, each mode can independently transmit signals from a user to a terminal, and an optical orthogonal mode division multiplexing method is provided, so that crosstalk between the modes is eliminated, and the information transmission quality of the system is greatly improved.
Description
Technical Field
The invention belongs to the technical field of optical access networks, and particularly relates to an uplink beat frequency noise suppression method based on chip and mode coherence.
Background
In recent years, the proliferation of high bandwidth services and applications (e.g., High Definition Television (HDTV) and 3D video telephony, online gaming, and cloud computing) has led to an exponential increase in the bandwidth demand on optical access networks. In the past decade, researchers have made various efforts on single mode optical fiber to increase the capacity of optical fiber communication, but the inherent non-linear characteristics limit the increase of transmission capacity, which is approaching the shannon limit. In the face of the great demand of the increasingly developing communication networks, a new communication technology is urgently needed to improve the system capacity.
Multimode fibers have a larger field area and therefore a higher non-linearity tolerance than single mode fibers. The mode multiplexing technology is a novel optical communication technology based on an optical fiber waveguide transmission mode, adopts a higher-order mode as a carrier in addition to a basic transmission mode, performs mode division multiplexing, and realizes higher capacity and higher transmission rate. In a mode multiplexing transmission system, because a multimode fiber (generally, the number of transmission modes is 2-10) has a larger mode field area, the influence of intermodal dispersion, intermodal coupling and nonlinear damage in mode transmission is slightly smaller than that of the traditional multimode fiber, and therefore the mode multiplexing transmission system has great research value. However, since the multimode optical fiber inevitably has defects of refractive index distribution caused by materials, processes and the like in the manufacturing process, and is influenced by microbending, optical fiber span mismatch and the like caused by external force in the laying engineering, mutual coupling crosstalk occurs in the originally orthogonal transmission mode in transmission, and the coupling is random, which causes the ambiguity of mode signals at the receiving end, and the transmission performance is limited. The invention adopts the orthogonal mode division multiplexing method to effectively improve the limitation.
Code Division Multiplexing (ECDM) is a multiplexing method that distinguishes the original signals by different codes. The ECDM PON allows all users to use a single wavelength, can support all users using the same light source and detector, and furthermore, each user is allocated a separate chip, which is orthogonal to each other, so that the mutual interference between users is greatly reduced. However, for uplink transmission, the beat noise generated by the laser seriously affects the transmission performance, and the beat noise is greatly suppressed because code division multiplexing has high code gain.
Disclosure of Invention
The purpose of the invention is as follows: the invention aims to provide an uplink beat noise suppression method based on chip and mode coherence, which is used for suppressing beat noise caused by frequency difference generated by different laser sources by adopting an orthogonal mode and chip coherence method based on a beat noise generation mechanism.
The technical scheme is as follows: in order to achieve the purpose, the invention adopts the following technical scheme:
an uplink beat frequency noise suppression method based on chip and mode coherence comprises the following steps:
1) adopts the principle of code division multiplexing to carry out the treatment on the QAM modulated pseudo-random sequence and the orthogonal spread spectrum codeW L Performing direct spread spectrum;
2) the electrical signal is transmitted to the optical transceiver module through the interface, the electrical signal is converted into an optical signal, the optical signal sent by each user is converted into a path of optical signal through the power combination module, and the optical signal is modulated into an orthogonal mode through the optical signal modulation module;
3) each orthogonal mode is coupled into the multimode fiber after mode multiplexing of the orthogonal mode multiplexing module, and is transmitted into the orthogonal mode demultiplexing module after passing through the erbium-doped fiber amplifier;
4) each mode demultiplexed by the orthogonal mode demultiplexing module is converted into basic mode light from an orthogonal mode through the optical signal demodulation module, and the basic mode light signal is transmitted into the optical line terminal;
5) in the optical line terminal, an optical signal is converted into an electrical signal through an optical transceiver module, and the electrical signal is accessed to a CDMA decoder through an interface;
6) electrical signal and orthogonal spreading code for transmission to CDMA decoderW L And despreading, recovering the data signal of the user by utilizing the code division multiplexing principle, and finally recovering the original transmission data after QAM demodulation.
Further, in step 1), the pair of QAM-modulated pseudo-random sequences and orthogonal spreading codesW L The direct spread spectrum is specifically performed as follows: for uplink signals, a PRBS sequence is firstly mapped into a data symbol to be spread by a QAM constellation, and then for ONU1~NRespectively using orthogonal spreading codesW L Performing a spread spectrum in whichNRepresenting the number of users.
Further, the orthogonal spread spectrum codeW L Is generated by Hadamard matrix, the rows and columns in the matrix are mutually orthogonal, and orthogonal spread spectrum codesW L Expressed as:
wherein the content of the first and second substances,Lrepresents a positive integer and is an even number;W 1 representing a row and a column.
Further, in step 5), the frequency difference generated in the optical line terminal forms optical beat noise at the user receiving end, and the optical beat noise is generatedN beat Maximum value of
Wherein the content of the first and second substances,G c the gain of the code is increased by the gain of the code,Bis the bandwidth of the baseband signal and,γis the modulation index. The spread signal has high intersymbol gain, and beat frequency noise can be obtained by the above formulaN beat It will be lowered.
Further, in step 2), the modulation of the optical signal modulation module is performed by using an orthogonal mode multiplexing method, and the SLM is used to modulate the optical signal of the one path.
Further, in step 3), the optical field distribution coupled after mode multiplexing by the orthogonal mode multiplexing module is ring-shaped, and there is no overlap between rings.
Has the advantages that: compared with the prior art, the chip and mode coherence based uplink beat frequency noise suppression method provided by the invention is an uplink transmission method for suppressing beat frequency noise caused by frequency difference generated by different lasers. The method allocates each ONU with a designated chip, and the chips are mutually orthogonal. The uplink data is firstly subjected to constellation mapping, then each uplink data is subjected to spread spectrum operation by using a chip respectively, and then the uplink data is converted into an optical signal by an optical transceiver and then transmitted back to the OLT; due to the orthogonality of the chips, the mutual interference of the uplink signals of the ONUs is greatly reduced. In addition, the coding gain of the chips can also inhibit the uplink beat noise. Meanwhile, multimode optical fiber is used as a transmission medium, each mode can independently transmit signals from a user to a terminal, and an optical orthogonal mode division multiplexing method is provided, so that crosstalk between the modes is eliminated, and the information transmission quality of the system is greatly improved.
Drawings
Fig. 1 is a schematic diagram of an ECDM PON uplink transmission scheme based on orthogonal mode division multiplexing;
FIG. 2 is a schematic diagram of orthogonal pattern generation;
FIG. 3 is a graph of intensity distribution of an optical signal in an orthogonal mode;
FIG. 4 is a diagram of a user spreading preamble pattern;
fig. 5 is a diagram of a code pattern after spreading of a user.
Detailed Description
The present invention will be further described with reference to the following embodiments.
Description of abbreviations referred to in this application: ECDMA, electrical code division multiple access, representing code division multiple access; ECDM, electrical code division multiplexing, representing code division multiplexing; PON, passive optical network, representing a passive optical network; PRBS, Pseudo Random Binary Sequence, representing a Pseudo Random Binary Sequence; ONU, Optical Network Unit, represents the Optical Network Unit; QAM, Quadrature Amplitude Modulation, representing Quadrature Amplitude Modulation; SLM, Spatial Light Modulator, representing a Spatial Light Modulator; the OLT, optical line terminal, represents an optical line terminal.
An uplink beat frequency noise suppression method based on chip and mode coherence introduces ECDMA into an optical access network system and simultaneously realizes the capacity expansion of the optical access system based on orthogonal mode division multiplexing. The method adopts the code division multiple access technology to spread the spectrum of the uplink transmission data, optical beat frequency noise is formed at a user receiving end due to the frequency difference generated by a terminal laser, and code gain caused by the spread spectrum can well inhibit the beat frequency noise, and meanwhile, the orthogonal mode division multiplexing technology is adopted, so that the crosstalk between modes is eliminated.
An uplink beat frequency noise suppression method based on chip and mode coherence comprises the following steps:
1) adopts the principle of code division multiplexing to carry out the treatment on the QAM modulated pseudo-random sequence and the orthogonal spread spectrum codeW L Go on straightReceiving spread spectrum;
2) the electrical signal is transmitted to the optical transceiver module 13 through the interface, the electrical signal is converted into an optical signal, the optical signal sent by each user is converted into a path of optical signal through the power combining module 14, and the optical signal is modulated into an orthogonal mode through the optical signal modulation module 15;
3) each orthogonal mode is mode-multiplexed by an orthogonal mode multiplexing module 16 and then coupled into a multimode fiber 17, and is transmitted into an orthogonal mode demultiplexing module 19 after passing through an erbium-doped fiber amplifier 18;
4) each mode demultiplexed by the orthogonal mode demultiplexing module 19 is converted into a basic mode light by an orthogonal mode after passing through the optical signal demodulation module 20, and the basic mode light signal is transmitted to the optical line terminal;
5) in the optical line terminal, an optical signal is converted into an electrical signal by an optical transceiver module 21, and is accessed to a CDMA decoder 22 through an interface;
6) the electrical signal and the orthogonal spreading code transmitted to the CDMA decoder 22W L And despreading, recovering the data signal of the user by utilizing the code division multiplexing principle, and finally recovering the original transmission data after QAM demodulation.
In step 1), the QAM modulated pseudo-random sequence and the orthogonal spread spectrum code are subjected toW L The direct spread spectrum is specifically performed as follows: for uplink signals, a PRBS sequence is firstly mapped into a data symbol to be spread by a QAM constellation, and then for ONU1~NRespectively using orthogonal spreading codesW L Performing a spread spectrum in whichNRepresenting the number of users.
Orthogonal spread spectrum codeW L Is generated by Hadamard matrix, the rows and columns in the matrix are mutually orthogonal, and orthogonal spread spectrum codesW L Expressed as:
wherein L represents a positive integer and is an even number;W 1 representing a row and a column.
GetL=4 getW 4 Is composed of
For two paths of random sequences, any two lines of the random sequences are selected to carry out correlation operation to obtain orthogonal spread spectrum codesW L And (4) sequencing.
Step 5), the frequency difference generated in the optical line terminal forms optical beat noise at the user receiving end, and the optical beat noiseN beat Maximum value of
Wherein the content of the first and second substances,G c the gain of the code is increased by the gain of the code,Bis the bandwidth of the baseband signal and,γis the modulation index. The spread signal has high intersymbol gain, and beat frequency noise can be obtained by the above formulaN beat It will be lowered.
In step 2), the optical signal modulation module 15 modulates one optical signal by using an SLM (selective laser modulation) by using an orthogonal mode multiplexing method.
In the step 3), the optical field distribution coupled after mode multiplexing by the orthogonal mode multiplexing module 16 is annular, and the rings are not overlapped.
In order to increase transmission capacity and suppress beat noise during uplink transmission, the invention provides an ECDMPON uplink transmission scheme based on orthogonal mode division multiplexing. Code Division Multiplexing (ECDM) is a multiplexing mode for distinguishing original signals of each path by different codes, has the advantages of high inter-code gain, good confidentiality, low signal mutual interference and the like, and is widely used in wireless access, such as a CDMA system. The invention introduces the ECDM technology into the optical access network and firstly provides an ECDM PON uplink transmission scheme based on orthogonal mode division multiplexing.
In the conventional mode multiplexing, mode fields of a fundamental mode signal and a high-order mode signal are seriously overlapped, so that coupling between modes occurs, and the transmission performance of a system is seriously influenced. The invention adopts an orthogonal mode multiplexing method, modulates the fundamental mode Gaussian light by using the SLM, couples the modulated fundamental mode Gaussian light into the multimode fiber after multiplexing, and the coupled light field is distributed in a ring shape without overlapping the ring and the mode without crosstalk. And the light of the three orthogonal modes at the demultiplexing end is demodulated back to the light of the basic mode after passing through the spatial light modulator.
Examples
Taking a three-mode fiber optic communication system as an example, each mode is used as an independent channel for three users. Fig. 1 shows an embodiment of the system of the present invention.
In the system, 11 is a CDMA encoder, which performs spread spectrum on a user data sequence, 12 is a multiplier, a spread spectrum code is directly multiplied by a constellation mapped sequence, 13 is an optical transceiver module, which converts an electrical signal into an optical signal, 14 is a power combining module, which combines three optical signals into one optical signal, 15 is an optical signal modulation module, which modulates a fundamental mode light into an orthogonal mode light, 16 is an orthogonal mode multiplexing module, 19 is an orthogonal mode demultiplexing module, 17 is a multimode optical fiber, 18 is an erbium-doped optical fiber amplifier, 20 is an optical signal demodulation module, which recovers the orthogonal mode light into the fundamental mode light, 21 is an optical transceiver module, which converts the optical signal into the electrical signal, 22 is a CDMA decoder, which recovers the spread spectrum data sequence of a corresponding user into a mapped initial sequence.
The principle of generating the orthogonal mode is shown in fig. 2, where 21 is an optical splitting module that splits an input light beam into N gaussian light beams, 22 is an optical signal modulation module, and 23 is an optical multiplexing module that combines N orthogonal mode optical signals into one orthogonal mode optical signal. And respectively modulating the N paths of Gaussian beams by using the SLM to generate the annular orthogonal mode light with different radiuses.
As shown in fig. 3, the light field intensity distribution diagram of three orthogonal modes is generated, and the light of each mode is in a ring shape, the radius is increased sequentially, and the light does not overlap with each other.
Taking three users corresponding to one mode as an example, each user randomly generates a binary bit stream and converts the binary bit stream into a bipolar non-return-to-zero code, a spreading code is generated by using a Hadamard matrix, fig. 4 is a code pattern before user spreading, and fig. 5 is a code pattern after user spreading. And simultaneously, selecting code elements in the same period of time, finding that the number of the code elements of each user is multiplied after spreading, and the stronger the capability of inhibiting beat frequency noise along with the increase of code gain.
The working process of the system is as follows, firstly, each user is allocated to a designated code chip for spread spectrum after QAM mapping, electric signals after spread spectrum are converted into optical signals after passing through an optical transceiver module, and the optical signals of a group of three users are transmitted on one path of orthogonal mode after power combination. The three orthogonal modes are coupled into the multimode fiber after mode multiplexing, and are transmitted to a demultiplexing end after being amplified by the EDFA, the demultiplexing end restores optical signals transmitted in the multimode fiber into the three orthogonal modes, and each orthogonal mode is restored into a basic mode optical signal after passing through the optical signal modulation module. At the terminal, the optical transceiver module converts the optical signal into an electrical signal, each user is decoded by a corresponding unique code chip, and then the binary bit stream is restored by QAM demapping after decoding.
The above description is only a preferred embodiment of the present invention, and it will be apparent to those skilled in the art that various modifications and variations can be made without departing from the technical principles of the present invention, and these modifications and variations should also be construed as the scope of the present invention.
Claims (6)
1. An uplink beat frequency noise suppression method based on chip and mode coherence is characterized in that: the method comprises the following steps:
1) adopts the principle of code division multiplexing to carry out the treatment on the QAM modulated pseudo-random sequence and the orthogonal spread spectrum codeW L Performing direct spread spectrum;
2) the electrical signals are transmitted to an optical transceiver module (13) through an interface, the electrical signals are converted into optical signals, the optical signals sent by each user are converted into a path of optical signals through a power combination module (14), and the optical signals are modulated into an orthogonal mode through an optical signal modulation module (15);
3) each orthogonal mode is coupled into a multimode fiber (17) after mode multiplexing through an orthogonal mode multiplexing module (16), and is transmitted into an orthogonal mode demultiplexing module (19) after passing through an erbium-doped fiber amplifier (18);
4) each mode demultiplexed by the orthogonal mode demultiplexing module (19) is changed into basic mode light from an orthogonal mode after passing through the optical signal demodulation module (20), and the basic mode optical signal is transmitted to the optical line terminal;
5) in the optical line terminal, an optical signal is converted into an electrical signal through an optical transceiver module (21), and is accessed to a CDMA decoder (22) through an interface;
6) electrical signal and orthogonal spreading code for transmission to a CDMA decoder (22)W L And despreading, recovering the data signal of the user by utilizing the code division multiplexing principle, and finally recovering the original transmission data after QAM demodulation.
2. The method of claim 1 for chip-based, pattern-coherent uplink beat noise suppression, wherein: in step 1), the pair of QAM modulated pseudo-random sequence and orthogonal spread spectrum codeW L The direct spread spectrum is specifically performed as follows: for uplink signals, a PRBS sequence is firstly mapped into a data symbol to be spread by a QAM constellation, and then for ONU1~NRespectively using orthogonal spreading codesW L Performing a spread spectrum in whichNRepresenting the number of users.
3. The method of claim 2 for chip-based, pattern-coherent uplink beat noise suppression, wherein: the orthogonal spread spectrum codeW L Is generated by Hadamard matrix, the rows and columns in the matrix are mutually orthogonal, and orthogonal spread spectrum codesW L Expressed as:
wherein L represents a positive integer and is an even number;W 1 representing a row and a column.
4. Code-based according to claim 1The method for suppressing the uplink beat frequency noise with chip and mode coherence is characterized in that: in step 5), the frequency difference generated in the optical line terminal forms optical beat frequency noise at the user receiving end, and the optical beat frequency noiseN beat Maximum value of
Wherein the content of the first and second substances,G c the gain of the code is increased by the gain of the code,Bis the bandwidth of the baseband signal and,γis the modulation index.
5. The method of claim 1 for chip-based, pattern-coherent uplink beat noise suppression, wherein: in the step 2), the modulation of the optical signal modulation module (15) is performed by using an orthogonal mode multiplexing method, and the SLM is used to modulate the optical signal of the one path.
6. The method of claim 1 for chip-based, pattern-coherent uplink beat noise suppression, wherein: in the step 3), the optical field distribution coupled after mode multiplexing by the orthogonal mode multiplexing module (16) is annular, and the rings are not overlapped.
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| CN112235070A (en) * | 2020-12-17 | 2021-01-15 | 南京信息工程大学 | Loop control method of orthogonal mode division multiplexing system |
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