CN106849963B - Method and device for reducing peak-to-average power ratio of three-dimensional coherent light orthogonal frequency division multiplexing system - Google Patents
Method and device for reducing peak-to-average power ratio of three-dimensional coherent light orthogonal frequency division multiplexing system Download PDFInfo
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- H04B1/0007—Software-defined radio [SDR] systems, i.e. systems wherein components typically implemented in hardware, e.g. filters or modulators/demodulators, are implented using software, e.g. by involving an AD or DA conversion stage such that at least part of the signal processing is performed in the digital domain wherein the AD/DA conversion occurs at radiofrequency or intermediate frequency stage
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- H04L27/2626—Arrangements specific to the transmitter only
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Abstract
The invention discloses a method and a device for reducing the peak-to-average power ratio of a three-dimensional coherent light orthogonal frequency division multiplexing system, which comprises the following steps: (1) Converting the binary sequence to obtain a subcarrier index signal; (2) Mapping to obtain 3N real number form subcarriers a frequency domain orthogonal frequency division multiplexing signal of the wave component; (3) Obtaining 2N frequency domain orthogonal frequency division multiplexing signals of complex form subcarrier components after recombination; (4) Modulating an orthogonal frequency division multiplexing signal from a frequency domain to a time domain; (5) Introducing a K sequence from a frequency domain, performing zero padding, modulating and then overlapping with a time domain orthogonal frequency division multiplexing signal; (6) Searching a K sequence combination which enables the peak-to-average power ratio of the signal to be minimum; (7) Dividing the time domain orthogonal frequency division multiplexing signal into a real part and an imaginary part corresponding to the real part, and converting the real part and the imaginary part into an optical signal; and (8) transmitting the optical signal. The invention effectively reduces the peak-to-average power ratio, relieves the influence of the nonlinear effect of the optical fiber channel on the signal and improves the overall performance.
Description
Technical Field
The present invention relates to the field of optical communications, and in particular, to a method and an apparatus for reducing a peak-to-average power ratio (PAPR) in a three-dimensional coherent optical Orthogonal Frequency Division Multiplexing (OFDM) system.
Background
In the field of optical communication, the coherent optical orthogonal frequency division multiplexing technology has been widely used in long-distance optical fiber transmission systems, and has the advantages of large capacity, high spectrum utilization rate, effective dispersion resistance and the like. However, the problem of too high peak-to-average power ratio exists when the orthogonal frequency division multiplexing technology is used, so that the orthogonal frequency division multiplexing signal is easily interfered by the nonlinear effect of the optical fiber, and the performance of the system is reduced.
At present, the orthogonal frequency division multiplexing technology based on constant envelope and the amplitude keying orthogonal frequency division multiplexing technology based on coding are generally adopted to improve the problem of high peak-to-average power ratio of the coherent light orthogonal frequency division multiplexing system, however, the constellation diagrams used for signal mapping adopted by the method are limited in a two-dimensional signal space, and the improvement of the system performance is limited.
Disclosure of Invention
In view of this, an embodiment of the present invention provides a method for reducing a peak-to-average power ratio of a three-dimensional coherent optical orthogonal frequency division multiplexing system, including the following steps:
(1) Binary sequences are input to the radio frequency transmitter, the binary sequences being grouped in the radio frequency transmitter by serial-to-parallel conversion modules, each k = log 2 M bits are a group of subcarrier index signals, and M represents the size of a three-dimensional signal constellation diagram;
(2) Inputting the subcarrier index signal obtained in the step (1) into a three-dimensional signal mapping module, and obtaining a frequency domain orthogonal frequency division multiplexing signal of 3N subcarrier components in a real number form through the three-dimensional signal mapping module, wherein N represents the subcarrier number of the orthogonal frequency division multiplexing signal;
(3) Mapping and recombining the orthogonal frequency division multiplexing signal obtained in the step (2) to obtain the orthogonal frequency division multiplexing signal 2N frequency domain orthogonal frequency division multiplexing signals of complex form subcarrier components;
(4) Modulating the orthogonal frequency division multiplexing signal obtained in the step (3) from a frequency domain to a time domain through a two-dimensional inverse Fourier transform module;
(5) Introduction of K = (K) from frequency domain 0 ,K 1 ,…,K N-1 ) Performing zero padding on the sequence, converting the sequence into a two-dimensional matrix, and overlapping the two-dimensional matrix with the time domain orthogonal frequency division multiplexing signal obtained in the step (4) after two-dimensional inverse Fourier transform;
(6) Finding out the K sequence combination which enables the peak-to-average power ratio of the orthogonal frequency division multiplexing signal to be minimum through a search algorithm by combining the steps (4) and (5);
(7) Dividing the time domain orthogonal frequency division multiplexing signal obtained in the step (6) into a real number part and an imaginary number part corresponding to the real number part through a parallel-serial conversion and cyclic prefix insertion module, inputting the real number part and the imaginary number part into an upper digital-to-analog conversion module and a lower digital-to-analog conversion module which correspond to each other, respectively inputting the real number part and the imaginary number part into an upper low-pass filter and a lower low-pass filter after the real number part and the imaginary number part are converted by the upper digital-to-analog conversion module and the lower digital-to-analog conversion module, respectively inputting the real number part and the imaginary number part into the upper low-pass filter and the lower low-pass filter, processing the real number part and the imaginary number part by the upper digital-to-analog conversion module and the lower digital-to-analog conversion module, inputting the real number part and the imaginary number part into the upper digital-to-analog conversion module and the lower digital conversion module, respectively inputting the real number part and the imaginary number part into the lower digital to-analog conversion module, and inputting the real number part into the upper low-digital conversion module, respectively, and inputting the upper low-pass filter, and the upper low-pass filter to process;
(8) And (4) transmitting the optical signal obtained in the step (7) through an optical fiber link.
Further, in the step (2), the frequency domain orthogonal frequency division multiplexing signal is represented as:
in the formula: s. the m ＝(X m Y m Z m ) T M is more than or equal to 0 and less than or equal to M-1 represents a two-dimensional matrix S 3×N Represents a three-dimensional signal, X, Y, Z represents the coordinates of the signal in a three-dimensional signal constellation, and T represents a transpose.
Further, in the step (3), the recombined frequency domain ofdm signal is represented as:
in the formula:
further, in the steps (4) and (5), the time domain orthogonal frequency division multiplexing signal after introducing the K sequence is represented as:
s＝ifft2(S 2×N )+ifft2(μK 2×N )
in the formula:
μ is a constant for adjusting the inserted K-sequence signal power, ifft2 represents a two-dimensional inverse fourier transform, and the orthogonal frequency division multiplexing signal component of the time domain has a complex form.
Further, in the step (6), the search algorithm includes the following steps:
6.1 First, setting the inserted K sequence as an all-zero sequence, and calculating the peak-to-average power ratio of the orthogonal frequency division multiplexing signal after two-dimensional inverse Fourier transform;
6.2 A subcarrier index variable u is set, and the number of post-selection levels of each component in a u =0,K sequence is set as C, and a vector E = (E) for post-selection levels 1 ,E 2 ,…,E C ) Represents;
6.3 With each of the post-selection levels E) C Substitute K u Comparing the peak-to-average power ratios of the OFDM signals to select a level E which minimizes the peak-to-average power ratio C ；
6.4 Add u = u +1 and repeat step 6.3) until u = L-1, the search procedure is ended, L representing the length of the K sequence that needs to be searched.
The peak-to-average power ratio reduction device of the three-dimensional coherent optical orthogonal frequency division multiplexing system comprises a radio frequency transmitter and an optical signal converter, wherein the radio frequency transmitter is connected with the optical signal converter and comprises a serial-parallel conversion module, the binary sequence is input into the serial-parallel conversion module and is grouped by the serial-parallel conversion module, each plurality of bits are a group and used as a subcarrier index signal, the serial-parallel conversion module is connected with a three-dimensional signal mapping module, the subcarrier index signal is processed by the three-dimensional signal mapping module to obtain 3N frequency domain orthogonal frequency division multiplexing signals of real number form subcarrier components, the three-dimensional signal mapping module is connected with the three-dimensional signal mapping and recombining module to obtain 2N frequency domain orthogonal frequency division multiplexing signals of complex number form subcarrier components after recombining, the three-dimensional signal mapping and recombining module is connected with the peak-to-average power ratio reducing module, the peak-to-average power ratio reduction module is connected with the parallel-to-serial conversion and cyclic prefix insertion module and comprises a two-dimensional inverse Fourier transform module, a zero padding and two-dimensional inverse Fourier transform module, an orthogonal frequency division multiplexing signal selection module with the minimum peak-to-average power ratio and a K sequence optimization module, wherein the two-dimensional inverse Fourier transform module modulates the recombined frequency domain orthogonal frequency division multiplexing signals into the time domain to obtain 2N time domain orthogonal frequency division multiplexing signals of complex subcarrier components, the zero padding and two-dimensional inverse Fourier transform module converts the frequency domain K sequence signals subjected to zero padding into the time domain to also obtain 2N time domain orthogonal frequency division multiplexing signals of the complex subcarrier components, and then the time domain orthogonal frequency division multiplexing signals are superposed with the recombined time domain orthogonal frequency division multiplexing signals to combine the orthogonal frequency division multiplexing signal selection module with the minimum peak-to-average power ratio and the K sequence optimization module, the method comprises the steps of obtaining an orthogonal frequency division multiplexing signal with the minimum peak-to-average power ratio, dividing the orthogonal frequency division multiplexing signal with the minimum peak-to-average power ratio into a real part and an imaginary part corresponding to the real part through a parallel-serial conversion and cyclic prefix insertion module, connecting an upper digital-to-analog conversion module and a lower digital-to-analog conversion module to the output end of the parallel-serial conversion and cyclic prefix insertion module, connecting an upper low-pass filter to the upper digital-to-analog conversion module, connecting a lower low-pass filter to the lower digital-to-analog conversion module, connecting the output ends of the upper low-pass filter and the lower low-pass filter to an optical signal converter, processing the real part through the upper digital-to-analog conversion module and the upper low-pass filter, inputting the optical signal converter into the imaginary part after the processing of the imaginary part through the lower digital-to-analog conversion module and the lower low-pass filter, and completing the conversion of a radio frequency to an optical signal in the optical signal converter.
Further, the zero padding and two-dimensional inverse Fourier transform module finds the best K sequence combination by using the time domain orthogonal frequency division multiplexing signals after superposition and recombination through a search algorithm to obtain the orthogonal frequency division multiplexing signals with the minimum peak-to-average power ratio.
Further, the optical signal converter is connected to an optical fiber link, and the optical signal is transmitted through the optical fiber link.
Further, the optical signal converter is an optical I/Q modulator.
The optical signal converter comprises an upper branch Mach-Zehnder modulator, a lower branch Mach-Zehnder modulator, a laser diode and a phase shifter, wherein the upper branch Mach-Zehnder modulator and the lower branch Mach-Zehnder modulator are both connected with the laser diode, the upper branch Mach-Zehnder modulator is connected with an upper low-pass filter, the lower branch Mach-Zehnder modulator is connected with a lower low-pass filter, the upper branch Mach-Zehnder modulator and the lower branch Mach-Zehnder modulator convert input radio frequency signals into optical signals, the laser diode provides light sources for the upper branch Mach-Zehnder modulator and the lower branch Mach-Zehnder modulator, and the phase shifter shifts the phase pi/2 of output signals of the lower branch Mach-Zehnder modulator to enable the output signals of the upper branch Mach-Zehnder modulator and the lower branch Mach-Zehnder modulator to be orthogonal and output in a superposition mode.
Compared with the prior art, the invention has the following beneficial effects: the K sequence is introduced as the fourth dimension of the three-dimensional signal, 2N complete complex data are obtained in the frequency domain and the time domain through three-dimensional signal mapping and recombination, an optimal K sequence combination is found by utilizing a search algorithm, the peak-to-average power ratio of the three-dimensional coherent light orthogonal frequency division multiplexing system is effectively reduced, and the search algorithm has very low complexity. Meanwhile, compared with the traditional mapping system based on two-dimensional signals, the three-dimensional signal mapping provides larger Euclidean distance, and the error performance of the system is effectively improved. Through frequency domain subcarrier signal recombination, the frequency band utilization rate of the system is effectively improved. At the receiving end, the inserted K sequence does not carry information, so the K sequence can be directly removed and then a simple solution and recombination is utilized.
Drawings
Fig. 1 is a schematic diagram of a peak-to-average power ratio reduction apparatus of a three-dimensional coherent optical orthogonal frequency division multiplexing system according to the present invention.
Fig. 2 is a schematic diagram of the papr reduction module of fig. 1.
Fig. 3 is a schematic diagram of the performance improvement of the peak-to-average power ratio according to an embodiment of the present invention.
Fig. 4 is a diagram illustrating the improvement of the peak-to-average power ratio performance after the search length of the K sequence in fig. 3 is reduced.
Fig. 5 is a diagram illustrating the performance of the systematic symbol error rate in accordance with an embodiment of the present invention.
Fig. 6 is a graph showing the performance of the systematic symbol error rate after the search length of the K sequence in fig. 5 is reduced.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be further described with reference to the accompanying drawings.
Referring to fig. 1, an embodiment of the invention provides a peak-to-average power ratio reduction apparatus for a three-dimensional coherent optical orthogonal frequency division multiplexing system, including a radio frequency transmitter 1 and an optical signal converter 2, where the radio frequency transmitter 1 is connected to the optical signal converter 2, and the optical signal converter 2 is connected to an optical fiber link 3.
The radio frequency transmitter 1 comprises a serial-to-parallel conversion module 11, a three-dimensional signal mapping module 12, a three-dimensional signal mapping recombination module 13, a peak-to-average power ratio reduction module 14, and a parallel-to-serial conversion and cyclic prefix insertion module 15, wherein the output end of the parallel-to-serial conversion and cyclic prefix insertion module 15 is connected with an upper digital-to-analog conversion module 16 and a lower digital-to-analog conversion module 17, the upper digital-to-analog conversion module 16 is connected with an upper low-pass filter 18, the lower digital-to-analog conversion module 17 is connected with a lower low-pass filter 19, and the output ends of the upper low-pass filter 18 and the lower low-pass filter 19 are both connected with the optical signal converter 2.
Referring to fig. 2, the papr reduction module 14 includes a two-dimensional inverse fourier transform module 141, a zero padding and two-dimensional inverse fourier transform module 142, an ofdm signal selection module 143 with minimum papr, and a K sequence optimization module 144.
The binary sequence is input into a serial-parallel conversion module 11 and is grouped by the serial-parallel conversion module 11, each K bits are a group as a subcarrier index signal, the serial-parallel conversion module 11 is connected with a three-dimensional signal mapping module 12, the subcarrier index signals are processed by the three-dimensional signal mapping module 12 to obtain frequency domain orthogonal frequency division multiplexing signals of 3N subcarrier components in real form, the three-dimensional signal mapping module 12 is connected with a three-dimensional signal mapping and recombining module 13 to obtain the frequency domain orthogonal frequency division multiplexing signals of 2N subcarrier components in complex form after recombination, the three-dimensional signal mapping and recombining module 13 is connected with a peak-to-average power ratio reducing module 14, the peak-to-average power ratio reducing module 14 is connected with a parallel-serial conversion and cyclic prefix insertion module 15, and a two-dimensional inverse fourier transform module 141 modulates the recombined frequency domain orthogonal frequency division multiplexing signals to the time domain, obtaining 2N time domain orthogonal frequency division multiplexing signals of complex form subcarrier components, the zero padding and two-dimensional inverse Fourier transform module 142 converting the frequency domain K sequence signals after zero padding to the time domain, also obtaining 2N time domain orthogonal frequency division multiplexing signals of complex form subcarrier components, then superposing the time domain orthogonal frequency division multiplexing signals obtained after recombination, combining the orthogonal frequency division multiplexing signal selection module 143 with the minimum peak-to-average power ratio and the K sequence optimization module 144 to obtain the orthogonal frequency division multiplexing signals with the minimum peak-to-average power ratio, then dividing the orthogonal frequency division multiplexing signals with the minimum peak-to-average power ratio into a real part and an imaginary part corresponding to the real part through a parallel-serial conversion and cyclic prefix insertion module 15, and connecting an upper digital-to-analog conversion module 16 and a lower digital-to-analog conversion module 17 with the output end of the cyclic prefix insertion module 15 through the parallel-to-serial conversion and cyclic prefix insertion module 15, the upper digital-to-analog conversion module 16 is connected with an upper low pass filter 18, the lower digital-to-analog conversion module 17 is connected with a lower low pass filter 19, the output ends of the upper low pass filter 18 and the lower low pass filter 19 are both connected with the optical signal converter 2, the real part is processed by an upper digital-to-analog conversion module 16 and an upper low-pass filter 18 and then input to the optical signal converter 2, and the imaginary part is processed by a lower digital-to-analog conversion module 17 and a lower low-pass filter 19 and then input to the optical signal converter 2, and conversion from radio frequency to optical signals is completed in the optical signal converter 2.
The optical signal converter 2 is an optical I/Q modulator, in the first embodiment, the upper branch mach-zehnder modulator (MZM) 24 and the lower branch mach-zehnder modulator 21 included in the optical signal converter 2 are both connected to the laser diode 23, the upper branch mach-zehnder modulator 24 is connected to the upper low-pass filter 18, the lower branch mach-zehnder modulator 21 is connected to the lower low-pass filter 19, the upper branch mach-zehnder modulator 24 and the lower branch mach-zehnder modulator 21 convert the input radio frequency signal into an optical signal, the laser diode 23 provides a light source for the upper branch mach-zehnder modulator 24 and the lower branch mach-zehnder modulator 21, the phase shifter 22 phase-shifts the output signal of the lower branch mach-zehnder modulator 21 by pi/2, and the output signals of the upper branch mach-zehnder modulator 24 and the lower branch mach-zehnder modulator 21 are orthogonal and are superposed and output.
The method for reducing the peak-to-average power ratio of the three-dimensional coherent light orthogonal frequency division multiplexing system comprises the following steps:
(1) Binary sequence input radio frequency transmitter, said binary sequence input radio frequency transmitterThe system sequences are grouped in the radio frequency transmitter via serial-to-parallel conversion modules, with each k = log 2 M bits are a group of subcarrier index signals, and M represents the size of a three-dimensional signal constellation diagram;
(2) Inputting the subcarrier index signal obtained in the step (1) into a three-dimensional signal mapping module, and obtaining 3N frequency domain orthogonal frequency division multiplexing signals of subcarrier components in a real number form through the three-dimensional signal mapping module;
the frequency domain orthogonal frequency division multiplexing signal is represented as:
in the formula: s. the m ＝(X m Y m Z m ) T M is more than or equal to 0 and less than or equal to M-1 represents a two-dimensional matrix S 3×N Which represents a three-dimensional signal, X, Y, Z which represents the coordinates of the signal in a three-dimensional signal constellation, and T which represents a transpose.
(3) Mapping and recombining the orthogonal frequency division multiplexing signals obtained in the step (2) to obtain frequency domain orthogonal frequency division multiplexing signals of 2N complex subcarrier components;
the recombined frequency domain orthogonal frequency division multiplexing signal is expressed as:
in the formula:
(4) Modulating the orthogonal frequency division multiplexing signal obtained in the step (3) from a frequency domain to a time domain through a two-dimensional inverse Fourier transform module;
(5) Introduction of K = (K) from frequency domain 0 ,K 1 ,…,K N-1 ) Performing zero padding on the sequence, converting the sequence into a two-dimensional matrix, and superposing the two-dimensional matrix with the time domain signal obtained in the step (4) after two-dimensional inverse Fourier transform;
the time domain orthogonal frequency division multiplexing signal after introducing the K sequence is expressed as:
s＝ifft2(S 2×N )+ifft2(μK 2×N )
in the formula:
μ is a constant for adjusting the inserted K-sequence signal power, ifft2 represents a two-dimensional inverse fourier transform, and the orthogonal frequency division multiplexing signal component of the time domain has a complex form.
(6) Finding out the K sequence combination which enables the peak-to-average power ratio of the orthogonal frequency division multiplexing signal to be minimum by combining the steps (4) and (5) through a search algorithm;
the search algorithm comprises the following steps:
6.1 First, setting the inserted K sequence as an all-zero sequence, and calculating the peak-to-average power ratio of the orthogonal frequency division multiplexing signal after two-dimensional inverse Fourier transform;
6.2 A subcarrier index variable u is set, and the number of post-selection levels of each component in a u =0,K sequence is set as C, and a vector E = (E) for post-selection levels 1 ,E 2 ,…,E C ) Represents;
6.3 With each post-select level E) C In place of K u Comparing the peak-to-average power ratios of the OFDM signals to select a level E which minimizes the peak-to-average power ratio C ；
6.4 Add u = u +1, repeat step 6.3) until u = L-1, the search procedure ends, L representing the length of the K sequence that needs to be searched.
(7) Dividing the time domain orthogonal frequency division multiplexing signal obtained in the step (6) into a real part and an imaginary part corresponding to the real part through a parallel-serial conversion and cyclic prefix insertion module, inputting the real part and the imaginary part into a corresponding upper digital-to-analog conversion module and a corresponding lower digital-to-analog conversion module, after the conversion of the upper digital-to-analog conversion module and the lower digital-to-analog conversion module, the input signals are respectively input into an upper low-pass filter and a lower low-pass filter, and after the processing of the upper low-pass filter and the lower low-pass filter, the input signals are input into an optical signal converter, and the conversion from radio frequency to optical signals is completed in the optical signal converter;
(8) The optical signal obtained in the step (7) is processed the transmission is over a fiber optic link.
Example parameters:
the method comprises the steps of setting the number of subcarriers of an orthogonal frequency division multiplexing signal to be 64, verifying the influence of the method on the system performance by adopting an Additive White Gaussian Noise (AWGN) channel, setting the average power of a three-dimensional signal constellation diagram to be 1, setting the length of a K sequence to be searched to be L, and setting the post-selection level of each component in the K sequence to be { -1,1} or { -3, -1,1,3}.
The verification results are as follows:
in fig. 3, L = N, in CCDF (complementary cumulative distribution function) =10 -4 When the number of post-selected levels C =2 and μ =0.5774, the peak-to-average power is improved by about 3.5dB as compared with the original system, and the average power of the transmission signal is increased by 9.63%. When μ =2, the peak-to-average power ratio performance can be improved by 1.8dB, with an average power increase of only 1.94%. When the post-selection level number C =4, the corresponding peak-to-average power ratio performance can be improved by about 1 dB.
The performance of the system after the search length reduction of the K sequence is shown in fig. 4, and it can be seen from the figure that when C =2, μ =1, and L = N/2, the system is at Pr =10 -4 The peak-to-average power ratio can be improved by about 3 dB.
Fig. 5 and 6 show the performance of the systematic symbol error rate, which is very close to the original system as the inserted K sequence power decreases and the search length is reduced, because the insertion of the K sequence causes the original three-dimensional signal to be interfered by more powerful noise.
The invention introduces the K sequence as the fourth dimension of the three-dimensional signal, obtains 2N complete complex data in the frequency domain and the time domain through three-dimensional signal mapping and recombination, finds an optimal K sequence combination by utilizing a search algorithm, effectively reduces the peak-to-average power ratio of the three-dimensional coherent light orthogonal frequency division multiplexing system, and the search algorithm has very low complexity. Meanwhile, compared with the traditional mapping system based on two-dimensional signals, the three-dimensional signal mapping provides larger Euclidean distance, and the error performance of the system is effectively improved. Through frequency domain subcarrier signal recombination, the frequency band utilization rate of the system is effectively improved. At the receiving end, the inserted K sequence does not carry information, so the K sequence can be directly removed and then a simple solution and recombination is utilized.
The features of the embodiments and embodiments described herein above may be combined with each other without conflict.
The above description is only exemplary of the present invention and should not be taken as limiting the invention, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (8)
1. The method for reducing the peak-to-average power ratio of the three-dimensional coherent light orthogonal frequency division multiplexing system is characterized by comprising the following steps of:
(1) Binary sequences are input to the radio frequency transmitter, the binary sequences being grouped in the radio frequency transmitter by serial-to-parallel conversion modules, each k = log 2 M bits are a group of subcarrier index signals, and M represents the size of a three-dimensional signal constellation diagram;
(2) Inputting the subcarrier index signal obtained in the step (1) into a three-dimensional signal mapping module, and obtaining 3N frequency domain orthogonal frequency division multiplexing signals of subcarrier components in a real number form through the three-dimensional signal mapping module, wherein N represents the subcarrier number of the orthogonal frequency division multiplexing signals;
(3) Mapping and recombining the orthogonal frequency division multiplexing signals obtained in the step (2) to obtain frequency domain orthogonal frequency division multiplexing signals of 2N complex subcarrier components;
(4) Modulating the orthogonal frequency division multiplexing signal obtained in the step (3) from a frequency domain to a time domain through a two-dimensional inverse Fourier transform module;
(5) Introduction of K = (K) from frequency domain 0 ,K 1 ,…,K N-1 ) Performing zero padding on the sequence, converting the sequence into a two-dimensional matrix, and overlapping the two-dimensional matrix with the time domain signal obtained in the step (4) after two-dimensional inverse Fourier transform;
(6) Finding out the K sequence combination which enables the peak-to-average power ratio of the orthogonal frequency division multiplexing signal to be minimum through a search algorithm by combining the steps (4) and (5);
(7) Dividing the time domain orthogonal frequency division multiplexing signal obtained in the step (6) into a real number part and an imaginary number part corresponding to the real number part through a parallel-serial conversion and cyclic prefix insertion module, inputting the real number part and the imaginary number part into an upper digital-to-analog conversion module and a lower digital-to-analog conversion module which correspond to each other, respectively inputting the real number part and the imaginary number part into an upper low-pass filter and a lower low-pass filter after the real number part and the imaginary number part are converted by the upper digital-to-analog conversion module and the lower digital-to-analog conversion module, respectively inputting the real number part and the imaginary number part into the upper low-pass filter and the lower low-pass filter, processing the real number part and the imaginary number part by the upper digital-to-analog conversion module and the lower digital-to-analog conversion module, inputting the real number part and the imaginary number part into an optical signal converter, and completing the conversion from radio frequency to optical signals in the optical signal converter;
(8) And (4) transmitting the optical signal obtained in the step (7) through an optical fiber link.
2. The method for peak-to-average power ratio reduction in the three-dimensional coherent optical orthogonal frequency division multiplexing system according to claim 1, wherein in the step (2), the frequency-domain orthogonal frequency division multiplexing signal is represented as:
in the formula: s m ＝(X m Y m Z m ) T M is more than or equal to 0 and less than or equal to M-1 represents a two-dimensional matrix S 3×N Represents a three-dimensional signal, X, Y, Z represents the coordinates of the signal in a three-dimensional signal constellation, and T represents a transpose.
3. The method for peak-to-average power ratio reduction of a three-dimensional coherent optical orthogonal frequency division multiplexing system according to claim 2, in the step (3), the frequency domain ofdm signal after the recombination is represented as:
in the formula:
4. the method for peak-to-average power ratio reduction of a three-dimensional coherent optical orthogonal frequency division multiplexing system according to claim 1, in the steps (4) and (5), the time domain orthogonal frequency division multiplexing signal after introducing the K sequence is represented as:
s＝ifft2(S 2×N )+ifft2(μK 2×N )
formula (II) the method comprises the following steps:
μ is a constant for adjusting the inserted K-sequence signal power, ifft2 represents a two-dimensional inverse fourier transform, and the orthogonal frequency division multiplexing signal component of the time domain has a complex form.
5. The method for peak-to-average power ratio reduction of the three-dimensional coherent optical orthogonal frequency division multiplexing system according to claim 4, wherein the searching algorithm in the step (6) comprises the steps of:
6.1 First, setting the inserted K sequence as an all-zero sequence, and calculating the peak-to-average power ratio of the orthogonal frequency division multiplexing signal after two-dimensional inverse Fourier transform;
6.2 Set a subcarrier index variable u, and set the number of post-selection levels of each vector in a sequence u =0,K as C, the post-selection level uses a vector E = (E) 1 ,E 2 ,…,E C ) Represents;
6.3 With each post-select level E) C In place of K u Comparing the peak-to-average power ratios of the OFDM signals to select a level E which minimizes the peak-to-average power ratio C ；
6.4 Add u = u +1, repeat step 6.3) until u = L-1, the search procedure ends, L representing the length of the K sequence that needs to be searched.
6. The device for reducing the peak-to-average power ratio of the three-dimensional coherent optical orthogonal frequency division multiplexing system is characterized by comprising a radio frequency emitter and an optical signal converter, wherein the radio frequency emitter is connected with the optical signal converter and comprises a serial-parallel conversion module, a binary sequence is input into the serial-parallel conversion module and is grouped by the serial-parallel conversion module, each bit is a group serving as a subcarrier index signal, the serial-parallel conversion module is connected with a three-dimensional signal mapping module, the subcarrier index signals are processed by the three-dimensional signal mapping module to obtain frequency domain orthogonal frequency division multiplexing signals of 3N subcarrier components in real number form, N represents the number of subcarriers of the orthogonal frequency division multiplexing signals, the three-dimensional signal mapping module is connected with a three-dimensional signal mapping and recombining module to obtain the frequency domain orthogonal frequency division multiplexing signals of 2N subcarrier components in complex number form, the three-dimensional signal mapping and recombining module is connected with a peak-to-average power ratio reducing module, the peak-to-average power ratio reducing module is connected with a parallel-serial conversion and cyclic prefix inserting module, the peak-to-average power ratio reducing module comprises a two-dimensional inverse Fourier transform module, a zero-padding and two-dimensional inverse Fourier transform module, an orthogonal frequency division multiplexing signal selecting module with the minimum peak-to-average power ratio and a K sequence optimizing module, the two-dimensional inverse Fourier transform module modulates the recombined frequency domain orthogonal frequency division multiplexing signal into the time domain to obtain 2N time domain orthogonal frequency division multiplexing signals of complex form subcarrier components, the zero-padding and two-dimensional inverse Fourier transform module converts the zero-padded frequency domain K sequence signal into the time domain to also obtain 2N time domain orthogonal frequency division multiplexing signals of complex form subcarrier components, and the time domain orthogonal frequency division multiplexing signals are superposed with the recombined time domain orthogonal frequency division multiplexing signals, combining the orthogonal frequency division multiplexing signal selection module with the minimum peak-to-average power ratio and the K sequence optimization module to obtain an orthogonal frequency division multiplexing signal with the minimum peak-to-average power ratio, dividing the orthogonal frequency division multiplexing signal with the minimum peak-to-average power ratio into a real part and an imaginary part corresponding to the real part through a parallel-serial conversion and cyclic prefix insertion module, connecting an upper digital-to-analog conversion module and a lower digital-to-analog conversion module at the output end of the parallel-serial conversion and cyclic prefix insertion module, connecting the upper digital-to-analog conversion module with an upper low-pass filter, connecting the lower digital-to-analog conversion module with a lower low-pass filter, connecting the output ends of the upper low-pass filter and the lower low-pass filter with an optical signal converter, processing the real part through the upper digital-to analog conversion module and the upper low-pass filter and then inputting the optical signal converter, and completing the conversion from radio frequency to optical signals in the optical signal converter;
the zero padding and two-dimensional inverse Fourier transform module finds the best K sequence combination by using the superposed and recombined time domain orthogonal frequency division multiplexing signals through a search algorithm to obtain the orthogonal frequency division multiplexing signals with the minimum peak-to-average power ratio; the optical signal converter is connected with an optical fiber link, and the optical signal is transmitted through the optical fiber link.
7. The apparatus of claim 6, wherein the optical signal converter is an optical I/Q modulator.
8. The device for reducing the peak-to-average power ratio of the three-dimensional coherent optical orthogonal frequency division multiplexing system according to claim 6, wherein the optical signal converter comprises an upper branch Mach-Zehnder modulator, a lower branch Mach-Zehnder modulator, a laser diode and a phase shifter, the upper branch Mach-Zehnder modulator and the lower branch Mach-Zehnder modulator are both connected with the laser diode, the upper branch Mach-Zehnder modulator is connected with the upper low-pass filter, the lower branch Mach-Zehnder modulator is connected with the lower low-pass filter, the upper branch Mach-Zehnder modulator and the lower branch Mach-Zehnder modulator convert the input radio frequency signals into optical signals, the laser diode provides light sources for the upper branch Mach-Zehnder modulator and the lower branch Mach-Zehnder modulator, and the phase shifter shifts the phase of the output signals of the lower branch Mach-Zehnder modulator by pi/2, so that the output signals of the upper branch Mach-Zehnder modulator and the lower branch Mach-Zehnder modulator are orthogonal and are superposed and output.
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