CN112291013B - Sampling clock frequency offset blind estimation method of optical OFDM system - Google Patents
Sampling clock frequency offset blind estimation method of optical OFDM system Download PDFInfo
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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
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
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- H04B10/40—Transceivers
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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
- H04L27/2662—Symbol synchronisation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
- H04L27/2695—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation with channel estimation, e.g. determination of delay spread, derivative or peak tracking
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Abstract
The invention discloses a blind estimation method for sampling clock frequency offset of an optical OFDM system, which comprises the following steps: the receiving end performs symbol synchronization and channel equalization on the baseband OFDM signal; respectively extracting data symbols on the kth subcarrier of the mth OFDM symbol of the ith OFDM frame; performing square inversion operation on the extracted data symbols for four times, then obtaining a phase and dividing the phase by 4 to obtain a phase rotation amount; iterating i from 1 to N f, repeating the two steps to obtain N f phase rotation amounts, and obtaining an average phase rotation amount; the sampling clock frequency offset is calculated from the average phase rotation. The invention does not need extra pilot frequency or training symbols, and reduces the complexity of the OFDM frame structure and the system cost.
Description
Technical Field
The invention relates to the technical field of optical communication, in particular to a blind estimation method for sampling clock frequency offset of an optical OFDM system.
Background
An optical Orthogonal Frequency Division Multiplexing (OFDM) system has the advantages of frequency selective fading resistance, high spectrum utilization rate and the like, and has been widely applied to the field of optical communication. In an optical OFDM system, an OFDM signal is generated at a transmitting end through a digital-to-analog converter (DAC), and is sampled at a receiving end through an analog-to-digital converter (ADC). In practical applications, the clock signals driving the DAC and ADC are often derived from asynchronous clock sources, so there is a Sampling Clock Frequency Offset (SCFO) in the system. The sampling clock frequency deviation will cause amplitude attenuation and phase rotation of the modulation symbols on the sub-carriers of the optical OFDM system. Amplitude attenuation can be compensated by channel equalization, and phase rotation introduced by sampling clock frequency deviation will cause damage to the performance of the optical OFDM system.
Methods for estimating and compensating for sampling clock frequency offset fall into two general categories: one is realized by hardware, such as a precise adjustable clock circuit is adopted to reduce the sampling clock frequency offset of the system, and the other is realized by a DSP scheme, so that additional hardware cost is not needed, and the cost is lower. Among them, sampling clock frequency offset estimation algorithms based on DSP schemes can be divided into two categories. One is the traditional pilot-assisted sampling clock frequency offset estimation, however pilot-based assistance typically requires sacrificing several subcarriers in the system, thus reducing spectral efficiency, especially for OFDM systems with smaller FFT sizes, which is more costly. The other is based on training sequence assisted sampling clock frequency offset estimation. Compared with a pilot frequency auxiliary estimation method, the sampling clock frequency offset estimation method based on the training sequence can effectively improve the frequency spectrum efficiency of the system.
Through literature search of the prior art, rui Deng et al found Optical Fiber Communications Conference and Exhibition (OFC): 1-3 (optical fiber communication conference 2017), real-Time VLLC-OFDM HD-SDI Video transmission SYSTEM WITH TS-based SFO estimation (laser visible light communication Real-time high-definition video transmission system based on training sequence sampling clock frequency deviation estimation), and estimating the sampling clock frequency deviation by using OFDM frame with a plurality of training symbols at the head and tail, wherein the method does not need pilot frequency symbols, thus improving the frequency spectrum benefit. However, the method needs to use a plurality of training symbols, and the calculation operation needs to be performed on the plurality of training symbols, so that the complexity is high.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a blind estimation method for sampling clock frequency offset of a low-complexity optical OFDM system. According to the invention, the phase rotation caused by the sampling clock frequency offset is estimated by extracting the data symbols on the subcarriers of the single OFDM symbol of the OFDM frame, so that the sampling clock frequency offset of the optical OFDM system is estimated, no additional pilot frequency or training symbol is needed, and the complexity of the system is reduced.
In order to achieve the above purpose, the method of the invention comprises the following specific steps:
step one, a transmitting end continuously transmits an OFDM frame, wherein the OFDM frame is composed of a training symbol S TS and N sym OFDM symbols with QPSK modulation formats;
The OFDM symbol in the first step consists of data subcarriers with the FFT point number of N FFT, N CP cyclic prefixes and N CS cyclic suffixes, and the training symbol consists of a training sequence with the FFT point number of N FFT, N CP cyclic prefixes and N CS cyclic suffixes;
Secondly, at a receiving end, performing symbol synchronization on the received OFDM frame by using the training symbol S TS in the first step, and determining a symbol synchronization point;
Step three, extracting training symbols of the ith OFDM frame from the symbol synchronization point position determined in the step two Channel estimation is carried out, and channel response estimated value/>, is calculated
Step four, extracting the mth OFDM symbol R i,m of the ith OFDM frame, and utilizing the channel response estimated value of step threeChannel equalization is performed on the OFDM symbol R i,m, and the equalized OFDM symbol/>
Step five, the OFDM symbol after equalization from the step fourExtracting data symbols on the kth subcarrierPerforming the fourth power and the inverse operation, and dividing the phase angle by 4 to estimate the phase rotation of the data symbol on the kth subcarrier of the mth OFDM symbol of the ith OFDM frame caused by the sampling clock frequency offsetWherein arg {.cndot. } represents the phase angle;
The phase rotation amount of the step five The following conditions need to be satisfied: /(I)
Step six, the i takes values from 1 to N f in sequence, and repeatedly executing the step three to the step five to obtain N f phase differences, and averaging the N f phase differences to obtain an average phase difference caused by sampling clock frequency offset
Step seven, utilizing the average phase differenceCalculating the frequency offset of the sampling clock/>Which is expressed as
N f in the step six takes on a value to meet the frequency offset of the sampling clockThe error range is not more than 2ppm;
The beneficial effects of the invention are as follows: the estimation of the sampling clock frequency offset is realized by extracting the data symbols on the subcarriers of a single OFDM symbol, and the estimation of the sampling clock frequency offset is carried out without additional pilot symbols or training sequences, so that the frequency spectrum efficiency of the optical OFDM system is improved, and the frame structure complexity and the system cost of the optical OFDM system are reduced.
Drawings
Fig. 1 is a schematic diagram of an OFDM frame structure in an embodiment of the present invention
FIG. 2 is a flow chart of a blind estimation method of sampling clock frequency offset according to the present invention
FIG. 3 is a graph of an estimated OFDM frame number versus sampling clock frequency offset in an embodiment of the method of the present invention
Detailed Description
The following description of the embodiments of the invention is presented in conjunction with the accompanying drawings to provide a better understanding of the invention to those skilled in the art.
The OFDM frame structure used in this example is mainly composed of one training symbol and OFDM symbol as shown in fig. 1. Both training symbols and OFDM symbols contain cyclic prefixes and cyclic suffixes, wherein the training symbols are used to achieve channel estimation and channel equalization.
In this example, the method comprises the following specific implementation steps:
Step one, a transmitting end generates continuous OFDM frames, each OFDM frame comprises a BPSK training sequence and OFDM symbols, the training symbols are positioned at the head of the OFDM frame, the number of IFFT points is 256 points, IFFT input data of the OFDM symbols meet the requirements of hermitian symmetry, data subcarriers are 100, and a modulation format is QPSK. The lengths of the cyclic prefix and the cyclic suffix are respectively 8, the peak-to-average power of the OFDM is reduced by adopting a limiting method, and the limiting ratio is 12dB;
Converting the OFDM frame into a baseband analog OFDM signal through a DAC (digital-to-analog converter), wherein the sampling rate of the DAC is 10.001GSa/s, the baseband analog OFDM signal inhibits a high-frequency part through a 5GHz low-pass filter, and then the baseband analog OFDM signal is amplified through a 14GHz electric amplifier and modulated to be directly modulated into a laser, and the laser of the direct modulator generates an optical OFDM signal with the center wavelength of 1557.92nm, and the optical OFDM signal is sent into a 20km standard single-mode fiber and transmitted to a receiving end;
Step three, the receiving end converts the optical OFDM signal into a baseband analog OFDM signal through a photoelectric detector, and then converts the baseband analog OFDM signal into an OFDM frame through an ADC (analog-to-digital converter), the ADC is set to double oversampling, and the sampling rate is 20GSa/s;
Fourthly, performing symbol synchronization on the OFDM frame by using the BPSK training symbols of the transmitting end, and finding out a synchronization point;
Step five, extracting an ith OFDM frame from the synchronous point position, performing channel estimation by using BPSK training symbols, and calculating channel response;
step six, extracting the 15 th OFDM symbol of the OFDM frame of the ith frame, and carrying out channel equalization by utilizing the channel response obtained in the step five;
step seven, extracting the data symbol on the 25 th subcarrier from the 15 th OFDM symbol after equalization Calculating the phase rotation amount/>, caused by the frequency offset of the sampling clockWherein arg {.cndot. } represents the phase angle
Step eight, the i takes values from 1 to 1e3 in sequence, and repeatedly executing the steps five to seven to obtain 1e3 phase differences, and averaging the 1e3 phase differences to obtain an average phase difference caused by sampling clock frequency offset
Step nine, utilizing the average phase differenceCalculating the frequency offset of the sampling clock/>Which is expressed asAgain, N FFT＝256、NCP＝NCS =8, k=25, and therefore,
In the example of the method of the present invention, the sampling rate of the DAC is 10.001GSa/s, the ADC is 2 times oversampling, and the sampling rate is 20GSa/s, so the theoretical value of the sampling clock frequency offset is (10.001-10)/10×1e6=100 ppm. As shown in FIG. 3, which shows the OFDM frame number-sampling clock frequency offset estimation curve, it is easy to see that when i gets to 200 th frame, the deviation of the sampling clock frequency offset estimation does not exceed 2ppm, and thereafter the deviation of the estimation is kept at 2ppm all the time, so as to achieve the ideal sampling clock frequency offset estimation effect.
In summary, the above embodiments are only preferred examples of the present invention, and are not intended to limit the scope of the present invention. It should be noted that several equivalent modifications and substitutions can be made by those skilled in the art based on the disclosure of the present invention, and these equivalent modifications and substitutions should also be considered as the protection scope of the present invention.
Claims (7)
1. The blind estimation method for the sampling clock frequency offset of the optical OFDM system is characterized by comprising the following steps of:
Step one, a transmitting end continuously transmits an OFDM frame, wherein the OFDM frame is composed of a training symbol S TS and N sym OFDM symbols;
Secondly, at a receiving end, performing symbol synchronization on the received OFDM frame by using the training symbol S TS in the first step, and determining a symbol synchronization point;
Step three, extracting training symbols of the ith OFDM frame from the symbol synchronization point position determined in the step two Channel estimation is carried out, and channel response estimated value/>, is calculated
Step four, extracting the mth OFDM symbol R i,m of the ith OFDM frame, and utilizing the channel response estimated value of step threeChannel equalization is performed on the OFDM symbol R i,m, and the equalized OFDM symbol/>
Step five, the OFDM symbol after equalization from the step fourExtracting data symbols on the kth subcarrierFor the extracted data symbol/>Performing the fourth-order sum and the inverse operation, and dividing the phase angle by 4 to estimate the phase rotation/>, of the data symbol on the kth subcarrier of the mth OFDM symbol of the ith OFDM frame caused by the sampling clock frequency offsetWherein arg {.cndot. } represents the phase angle;
step six, the i takes values from 1 to N f in sequence, and repeatedly executes the steps three to five, thereby obtaining N f phase rotation amounts, and averaging the N f phase rotation amounts, namely obtaining the average phase rotation amount caused by the frequency deviation of the sampling clock
Step seven, utilizing the average phase rotation quantityCalculating the frequency offset of the sampling clock/>Which is expressed as。
2. The method for blind estimation of sampling clock frequency offset in an optical OFDM system according to claim 1, wherein the modulation format of the OFDM frame in the first step is QPSK.
3. The blind estimation method of sampling clock frequency offset of optical OFDM system according to claim 1, wherein the OFDM symbol in the first step is composed of subcarriers with FFT point number of N FFT, N CP cyclic prefixes and N CS cyclic suffixes.
4. The method according to claim 1, wherein the training symbol S TS of the first step is located at the head of the OFDM frame.
5. The blind estimation method of sampling clock frequency offset of optical OFDM system according to claim 1, wherein the training symbol S TS in the first step is composed of a training sequence with FFT point number N FFT, N CP cyclic prefixes and N CS cyclic suffixes.
6. The method for blind estimation of sampling clock frequency offset in an optical OFDM system according to claim 1, wherein the phase rotation amount in the fifth step isThe following conditions need to be satisfied: /(I)。
7. The blind estimation method of sampling clock frequency offset in an optical OFDM system according to claim 1, wherein said N f in said step six is valued to satisfy the sampling clock frequency offsetThe error range is not more than 2ppm.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN1310528A (en) * | 2000-02-22 | 2001-08-29 | 汤姆森特许公司 | Fast Fourier transformation window synchronization with decreased complexity for orthogonal frequency division multiplexing system |
| CN106559370A (en) * | 2016-11-05 | 2017-04-05 | 上海大学 | A kind of method of low complexity OFDM PON system sample clock frequency deviation compensation |
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| US5732113A (en) * | 1996-06-20 | 1998-03-24 | Stanford University | Timing and frequency synchronization of OFDM signals |
| GB2480311A (en) * | 2010-05-13 | 2011-11-16 | Univ Bangor | Optical OFDM synchronisation using clock signal transmitted outside OFDM symbol frequency band |
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| CN1310528A (en) * | 2000-02-22 | 2001-08-29 | 汤姆森特许公司 | Fast Fourier transformation window synchronization with decreased complexity for orthogonal frequency division multiplexing system |
| CN106559370A (en) * | 2016-11-05 | 2017-04-05 | 上海大学 | A kind of method of low complexity OFDM PON system sample clock frequency deviation compensation |
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Effective date of registration: 20211223 Address after: 412007 No. 88 Taishan Road, Zhuzhou City, Hunan Province Applicant after: HUNAN University OF TECHNOLOGY Applicant after: HUNAN GREAT WALL COMPUTER SYSTEM CO.,LTD. Address before: 412007 No. 88 Taishan Road, Tianyuan District, Zhuzhou City, Hunan Province Applicant before: HUNAN University OF TECHNOLOGY |
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