CN109547387B - Method and system for suppressing nonlinear effect of CO-OFDM (CO-orthogonal frequency division multiplexing) system - Google Patents
Method and system for suppressing nonlinear effect of CO-OFDM (CO-orthogonal frequency division multiplexing) system Download PDFInfo
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- H04L27/00—Modulated-carrier systems
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Abstract
The invention discloses a method and a system for suppressing nonlinear effect of a CO-OFDM system, which comprises the following steps: at a system sending end, for any one packet, obtaining a conjugate symbol which is in phase conjugation with a pilot frequency symbol so as to obtain a corresponding adjacent estimation group; carrying out carrier mapping on the pilot frequency symbol, the conjugate symbol and the data symbol, and sending the carrier to a system receiving end; at a system receiving end, carrying out frequency domain signal reduction on a carrier wave sent from a system sending end, and extracting a pilot frequency symbol, a conjugate symbol and a data symbol; coherently superposing the extracted pilot symbols and corresponding conjugate symbols to compensate for nonlinear phase shift in the pilot symbols; and estimating the nonlinear phase shift of the data symbols in the same adjacent estimation group by using the extracted pilot symbols and the corresponding conjugate symbols, and compensating the nonlinear phase shift of the data symbols. The invention can effectively, comprehensively and real-timely compensate the damage caused by the nonlinear effect in the CO-OFDM system.
Description
Technical Field
The invention belongs to the field of optical communication, and particularly relates to a method and a system for suppressing a nonlinear effect of a CO-OFDM system.
Background
The sub-carrier Frequency intervals in the CO-OFDM (Coherent Optical Orthogonal Frequency division multiplexing) system are strictly equal, which is easy to satisfy the phase matching condition of the nonlinear four-wave mixing effect, and further causes serious inter-carrier interference. Meanwhile, the CO-OFDM signal has high peak-to-average power ratio, so that the CO-OFDM signal can easily reach a Kerr nonlinear threshold and is further influenced by self-phase modulation and cross-phase modulation. How to efficiently suppress nonlinear effects in fiber links is a key issue in CO-OFDM technology.
The existing channel monitoring scheme based on carrier pilot frequency can only carry out approximate estimation on random phase noise, can not eliminate phase deviation related to carrier frequency spectrum position caused by nonlinear effect, and can reduce frequency spectrum efficiency; the traditional back transmission algorithm is difficult to have a good compensation effect in the real-time compensation of the nonlinear damage of the CO-OFDM system due to the extremely high calculation complexity. Therefore, an effective and comprehensive compensation method for the nonlinear damage of the optical fiber of the CO-OFDM system does not exist at present.
Disclosure of Invention
Aiming at the defects and improvement requirements of the prior art, the invention provides a method for suppressing the nonlinear effect of a CO-OFDM system, which aims to calculate and compensate the nonlinear phase shift related to the carrier frequency spectrum position in pilot frequency and data respectively, thereby effectively, comprehensively and real-timely compensating the damage caused by the nonlinear effect in the CO-OFDM system.
To achieve the above object, according to a first aspect of the present invention, there is provided a method for suppressing nonlinear effects in a CO-OFDM system, comprising the steps of:
(1) at a system sending end, for any packet, obtaining a conjugate symbol which is phase-conjugated with a pilot symbol, thereby obtaining an adjacent estimation group which is composed of the conjugate symbol and the pilot symbol and data symbol in the packet;
(2) carrying out carrier mapping on the pilot frequency symbol, the conjugate symbol and the data symbol, and sending the carrier to a system receiving end;
(3) at a system receiving end, carrying out frequency domain signal reduction on a carrier wave at a transmitting end, and extracting a pilot frequency symbol, a conjugate symbol and a data symbol;
(4) coherently superposing the extracted pilot symbols and corresponding conjugate symbols to compensate for nonlinear phase shift in the pilot symbols; estimating the nonlinear phase shift of the data symbols in the same adjacent estimation group by using the extracted pilot symbols and the corresponding conjugate symbols, and compensating the nonlinear phase shift of the data symbols according to the estimation result;
wherein each packet comprises one channel of pilot symbols and one or more channels of data symbols.
Further, in step (2), the carrier mapping of the pilot symbols, the conjugate symbols, and the data symbols is performed according to a controllable carrier mapping pair of IFFT (Inverse Fast Fourier Transform), and in step (3), the frequency domain signal reduction of the carrier transmitted from the system transmitting end is performed by FFT (Fast Fourier Transform).
Further, in step (4), estimating the nonlinear phase shift of the data symbols in the same neighboring estimation group by using the extracted pilot symbols and the corresponding conjugate symbols, including:
for any path of data symbol DiThe nonlinear phase shift is estimated using the following equation:
wherein i is a data symbol DiCarrier number of thetaiFor non-linear phase shift of data symbols loaded in the ith carrier, arg is angle finding operation, RkAnd RhAre respectively a data symbol DiPilot symbols and conjugate symbols in the adjacent estimation group, k and h are pilot symbols RkAnd the conjugate symbol Rhη carrier numberikhThe coefficient of mixing the four waves of the ith carrier wave affected by the nonlinearity of the kth carrier wave and the h carrier wave.
According to a second aspect of the present invention, there is provided a CO-OFDM system comprising: the device comprises a carrier mapping module, a frequency domain signal restoring module, a coherent superposition module and a proximity estimation module;
the carrier mapping module is used for obtaining a conjugate symbol which is phase-conjugated with a pilot symbol in a packet for any packet at a system sending end so as to obtain an adjacent estimation group consisting of the conjugate symbol, the pilot symbol in the packet and a data symbol; the carrier mapping module is also used for carrying out carrier mapping on the pilot frequency symbol, the conjugate symbol and the data symbol and sending the carrier to a system receiving end;
the frequency domain signal reduction module is used for carrying out frequency domain signal reduction on a carrier wave sent from a system sending end at a system receiving end and extracting a pilot frequency symbol, a conjugate symbol and a data symbol;
the coherent superposition module is used for carrying out coherent superposition on the pilot symbols extracted by the frequency domain signal restoration module and the corresponding conjugate symbols so as to compensate the nonlinear phase shift in the pilot symbols;
the adjacent estimation module is used for estimating the nonlinear phase shift of the data symbols in the same adjacent estimation group by using the pilot frequency symbols and the conjugate symbols extracted by the frequency domain signal restoration module and compensating the nonlinear phase shift of the data symbols according to the estimation result;
wherein each packet comprises one channel of pilot symbols and one or more channels of data symbols.
Further, the carrier mapping module performs carrier mapping on the pilot symbols, the conjugate symbols and the data symbols according to the controllable carrier mapping pair of IFFT, and the frequency domain signal reduction module performs frequency domain signal reduction on the carrier transmitted from the system transmitting end, which is performed by FFT.
Further, the proximity estimation module estimates the nonlinear phase shift of the data symbols in the same proximity estimation group by using the pilot symbols and the conjugate symbols extracted by the frequency domain signal recovery module, and comprises:
for any path of data symbol DiThe nonlinear phase shift is estimated using the following equation:
wherein i is a data symbol DiCarrier number of thetaiFor non-linear phase shift of data symbols loaded in the ith carrier, arg is angle finding operation, RkAnd RhAre respectively a data symbol DiPilot symbols and conjugate symbols in the adjacent estimation group, k and h are pilot symbols RkAnd the conjugate symbol Rhη carrier numberikhThe coefficient of mixing the four waves of the ith carrier wave affected by the nonlinearity of the kth carrier wave and the h carrier wave.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) the nonlinear effect suppression method of the CO-OFDM system provided by the invention comprises the steps that at a sending end of the system, carrier mapping is carried out on a conjugate symbol which is in phase conjugation with a pilot symbol, and the carrier loaded with the conjugate symbol and the carrier loaded with the pilot symbol and a data symbol are sent to a receiving end together; and carrying out frequency domain signal reduction on the carrier at a receiving end, extracting a conjugate symbol, a pilot symbol and a data symbol, then carrying out coherent superposition on the pilot symbol and the conjugate symbol to compensate the nonlinear phase shift in the pilot symbol, and estimating and compensating the nonlinear phase shift of the data symbol in the same group by using the pilot symbol and the conjugate symbol. The nonlinear effect suppression method of the CO-OFDM system provided by the invention can respectively compensate the nonlinear phase shift of the pilot frequency symbol and the data symbol by using the conjugate symbol which is phase-conjugated with the pilot frequency symbol, so that the nonlinear effect of the CO-OFDM system can be effectively suppressed, and the method has higher flexibility.
(2) The nonlinear effect suppression method of the CO-OFDM system provided by the invention compensates the nonlinear phase shift of the pilot symbol by coherent superposition of the pilot symbol and the conjugate symbol, estimates the nonlinear phase shift of the data symbol in the same group by using the pilot symbol and the conjugate symbol and compensates the nonlinear phase shift, and all involved calculations are conventional and simple.
(3) According to the nonlinear effect suppression method of the CO-OFDM system, on one hand, the pilot frequency symbol and the conjugate symbol are subjected to coherent superposition to compensate the nonlinear phase shift of the pilot frequency symbol, on the other hand, the correlation of nonlinear damage in the same group is constructed, so that the nonlinear phase shift of the data symbol in the same group can be estimated by using the pilot frequency symbol and the conjugate symbol, corresponding compensation is carried out, and no additional pilot frequency symbol is introduced in the process of compensating the nonlinear phase shift in the system, so that the original spectral efficiency of the system can be kept while the nonlinear resistance of the CO-OFDM system is improved.
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Fig. 1 is a flowchart of a method for suppressing nonlinear effect in a CO-OFDM system according to an embodiment of the present invention;
fig. 2 is a schematic carrier spectrum diagram according to an embodiment of the present invention;
FIG. 3 is a schematic diagram of a CO-OFDM system according to an embodiment of the present invention;
the same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
the device comprises a carrier mapping module 1, a frequency domain signal restoring module 2, a coherent superposition module 3 and a proximity estimation module 4.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
In a CO-OFDM system, the total subcarrier number is 128, the effective subcarrier number is 102, the symbol rate is 10GS/s, 16QAM modulation is performed, the data rate is 29Gb/s, and the transmission distance is 10 spans (700 kilometers); at a sending end of the system, 85 paths of symbols input in parallel are grouped into 17 groups, and each group comprises 5 paths of symbols, wherein one path of symbols is a pilot symbol, and the other 4 paths of symbols are data symbols.
In the above application example, the method for suppressing the nonlinear effect of the CO-OFDM system, as shown in fig. 1, includes the following steps:
(1) at a system sending end, for any packet, obtaining a conjugate symbol which is phase-conjugated with a pilot symbol, thereby obtaining an adjacent estimation group which is composed of the conjugate symbol and the pilot symbol and data symbol in the packet;
in this embodiment, each neighboring estimation group includes a pilot symbol, a conjugate symbol phase-conjugated with the pilot symbol, and 4 data symbols;
taking one of the packets as an example, with a pilot symbol SkConjugate sign S being phase-conjugatedhThe concrete form is as follows:
Sh＝Ahexp(-jφh)＝Sk *＝Akexp(-jφk)；
wherein h is a conjugate symbol carrier label, k is a pilot symbol carrier label, AkIs the pilot symbol amplitude, AhIs the conjugate sign amplitude, phikIs the pilot symbol phase, phihIs the conjugate sign phase, j represents the imaginary number, which represents the sign conjugate;
(2) according to the controllable carrier mapping pair of IFFT, carrying out carrier mapping on the pilot frequency symbol, the conjugate symbol and the data symbol, and sending the carrier to a system receiving end;
after carrier mapping, the signal spectrum structure is shown in fig. 2, in which carriers in two adjacent estimation groups are shown, wherein the k-th carrier1Individual carrier and h1The carriers are respectively pilot symbol carrier and conjugate symbol carrier in the first adjacent estimation group, kth2Individual carrier and h2The carrier waves are respectively a pilot frequency symbol carrier wave and a conjugate symbol carrier wave in the second adjacent estimation group; it should be understood that, with different grouping modes at the receiving end of the system, after carrier mapping, the spectrum structure in each neighboring estimation group will also be different;
(3) at a system receiving end, carrying out frequency domain signal reduction on a carrier wave sent from a system sending end through FFT (fast Fourier transform), and extracting a pilot frequency symbol, a conjugate symbol and a data symbol;
pilot symbol SkAnd a conjugate symbol ShAfter FFT reduction at the receiving end, a pilot symbol R is obtained by extractionkAnd the conjugate symbol RhThe concrete form is as follows:
Rk＝Ar,kexp(jφk+θk)；
Rh＝Ar,hexp(jφh+θh)；
wherein A isr,kFor the receiving end pilot symbols RkAmplitude, Ar,hFor the receiving end conjugate symbol RhAmplitude, θkAnd thetahRespectively, phase shifts caused by non-linearity on the corresponding carriers, and theta can be considered within an error tolerancek≈θh；
(4) Coherently superposing the extracted pilot symbols and corresponding conjugate symbols to compensate for nonlinear phase shift in the pilot symbols; estimating the nonlinear phase shift of the data symbols in the same adjacent estimation group by using the extracted pilot symbols and the corresponding conjugate symbols, and compensating the nonlinear phase shift of the data symbols according to the estimation result;
compensating for non-linear phase shift in pilot symbols R by coherent superpositionkCorresponding pilot symbols are
The concrete form of (A) is as follows:
wherein the content of the first and second substances,
pilot symbols after nonlinear suppression;
in an alternative embodiment, in step (4), estimating the nonlinear phase shift of the data symbols in the same neighboring estimation group by using the extracted pilot symbols and the corresponding conjugate symbols, includes:
for any path of data symbol DiUsing the following formulaEstimating its nonlinear phase shift:
wherein i is a data symbol DiCarrier number of thetaiFor non-linear phase shift of data symbols loaded in the ith carrier, arg is angle finding operation, RkAnd RhAre respectively a data symbol DiPilot symbols and conjugate symbols in the adjacent estimation group, k and h are pilot symbols RkAnd the conjugate symbol Rhη carrier numberikhThe method comprises the following steps of taking four-wave mixing coefficients of an ith carrier affected by the nonlinearity of a kth carrier and an h-th carrier;
finally, with the data symbol DiCorrespondingly, the data symbol after the nonlinear phase shift compensation of the receiving end
Can be obtained by the following formula:
the present invention also provides a CO-OFDM system, as shown in fig. 3, including: a sending end parallel-to-serial module (P/S), a sending end digital-to-analog conversion module (DAC), a sending end IQ modulation module, a transmission optical fiber, a receiving end analog-to-digital conversion module (ADC), a receiving end serial-to-parallel module (S/P) and a receiving end CP removing module, wherein the sending end parallel-to-serial module converts parallel data of a sending end into serial data, the sending end digital-to-analog conversion module converts serial digital signals output by the sending end parallel-to-serial module into analog signals, the sending end IQ modulation module modulates the analog signals output by the sending end digital-to-analog conversion module onto optical signals, the transmission optical fiber completes signal transmission from the sending end to the receiving end, the receiving end analog-to-digital conversion module converts the received analog data into digital signals, the receiving end serial-to-parallel, the receiving end CP removing module removes the cyclic prefix of the parallel data output by the receiving end serial-parallel module; the CO-OFDM system shown in fig. 3, further comprising: the system comprises a carrier mapping module (IFFT)1, a frequency domain signal restoring module (FFT)2, a coherent superposition module 3 and a proximity estimation module 4;
the carrier mapping module 1 is arranged at the upstream of the parallel-serial module at the transmitting end and used for obtaining a conjugate symbol which is phase-conjugated with a pilot symbol in a packet for any packet at the transmitting end of the system, so as to obtain an adjacent estimation group which is composed of the conjugate symbol, the pilot symbol in the packet and a data symbol; the carrier mapping module is also used for carrying out carrier mapping on the pilot frequency symbol, the conjugate symbol and the data symbol and sending the carrier to a system receiving end;
the frequency domain signal reduction module 2 is arranged at the downstream of the receiving end CP removing module and is used for carrying out frequency domain signal reduction on the carrier wave sent from the system sending end at the system receiving end and extracting a pilot frequency symbol, a conjugate symbol and a data symbol;
a coherent superposition module 3, configured to coherently superpose the pilot symbols extracted by the frequency domain signal recovery module 2 and the corresponding conjugate symbols to compensate for the nonlinear phase shift in the pilot symbols;
a proximity estimation module 4, configured to estimate a nonlinear phase shift of a data symbol in the same proximity estimation group by using the pilot symbol and the conjugate symbol extracted by the frequency domain signal recovery module 2, and compensate the nonlinear phase shift of the data symbol according to an estimation result;
wherein each packet comprises one channel of pilot symbols and one or more channels of data symbols.
In this embodiment, the specific implementation manners of the carrier mapping module 1, the frequency domain signal restoring module 2, the coherent addition module 3, and the proximity estimation module 4 may refer to the specific implementation manners in the above method embodiments, and are not described in detail herein.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (2)
1. A method for suppressing the nonlinear effect of a CO-OFDM system is characterized by comprising the following steps:
(1) at a system sending end, for any packet, obtaining a conjugate symbol which is phase-conjugated with a pilot symbol, thereby obtaining an adjacent estimation group which is composed of the conjugate symbol and the pilot symbol and data symbol in the packet;
(2) carrying out carrier mapping on the pilot frequency symbol, the conjugate symbol and the data symbol, and sending the carrier to a system receiving end;
(3) at a system receiving end, carrying out frequency domain signal reduction on a carrier wave sent from a system sending end, and extracting a pilot frequency symbol, a conjugate symbol and a data symbol;
(4) coherently superposing the extracted pilot symbols and corresponding conjugate symbols to compensate for nonlinear phase shift in the pilot symbols; estimating the nonlinear phase shift of the data symbols in the same adjacent estimation group by using the extracted pilot symbols and the corresponding conjugate symbols, and compensating the nonlinear phase shift of the data symbols according to the estimation result;
in the step (4), estimating the nonlinear phase shift of the data symbols in the same neighboring estimation group by using the extracted pilot symbols and the corresponding conjugate symbols, including:
for any path of data symbol DiThe nonlinear phase shift is estimated using the following equation:
2. A CO-OFDM system comprising: the device comprises a carrier mapping module, a frequency domain signal restoring module, a coherent superposition module and a proximity estimation module;
the carrier mapping module is used for obtaining a conjugate symbol which is phase-conjugated with a pilot symbol in a packet for any packet at a system sending end, thereby obtaining an adjacent estimation group consisting of the conjugate symbol, the pilot symbol in the packet and a data symbol; the carrier mapping module is also used for carrying out carrier mapping on the pilot frequency symbol, the conjugate symbol and the data symbol and sending the carrier to a system receiving end;
the frequency domain signal reduction module is used for carrying out frequency domain signal reduction on a carrier wave sent from a system sending end at a system receiving end and extracting a pilot frequency symbol, a conjugate symbol and a data symbol;
the coherent superposition module is used for carrying out coherent superposition on the pilot symbols extracted by the frequency domain signal restoration module and the corresponding conjugate symbols so as to compensate the nonlinear phase shift in the pilot symbols;
the adjacent estimation module is used for estimating the nonlinear phase shift of the data symbols in the same adjacent estimation group by using the pilot frequency symbols and the conjugate symbols extracted by the frequency domain signal restoration module and compensating the nonlinear phase shift of the data symbols according to the estimation result;
the proximity estimation module estimates a nonlinear phase shift of data symbols in the same proximity estimation group using the pilot symbols and conjugate symbols extracted by the frequency domain signal recovery module, including:
for any path of data symbol DiThe nonlinear phase shift is estimated using the following equation:
each group comprises one path of pilot frequency symbols and one or more paths of data symbols; i is the data symbolNumber DiCarrier number of thetaiFor non-linear phase shift of data symbols loaded in the ith carrier, arg is angle finding operation, RkAnd RhRespectively being said data symbols DiPilot symbols and conjugate symbols in the adjacent estimation group, k and h are the pilot symbols R respectivelykAnd said conjugate symbol Rhη carrier numberikhThe coefficient of mixing the four waves of the ith carrier wave affected by the nonlinearity of the kth carrier wave and the h carrier wave.
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| CN110971307B (en) * | 2019-11-08 | 2021-07-27 | 华中科技大学 | Method for compensating interference between subcarriers inherent to SEFDM system |
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| CN106464375A (en) * | 2014-04-08 | 2017-02-22 | 阿斯顿大学 | Method of non-linearity compensation in optical fiber communications |
| CN108199778A (en) * | 2018-01-31 | 2018-06-22 | 华中科技大学 | A kind of CO-OFDM system phase noise compensation method and system based on RF pilot tones |
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| CN106464375A (en) * | 2014-04-08 | 2017-02-22 | 阿斯顿大学 | Method of non-linearity compensation in optical fiber communications |
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