WO2013010484A1

WO2013010484A1 - Dispersion compensation method and device

Info

Publication number: WO2013010484A1
Application number: PCT/CN2012/078819
Authority: WO
Inventors: 周伟勤; 陈雪; 刘思聪; 王会涛; 赵勇; 周娴; 樊洋洋
Original assignee: 中兴通讯股份有限公司; 北京邮电大学
Priority date: 2011-07-21
Filing date: 2012-07-18
Publication date: 2013-01-24
Also published as: CN102255667A; CN102255667B

Abstract

A method and device for dispersion compensation comprising: a transmitter inserting pilot information on N_pilot (N_pilot > 1) orthogonal frequency division multiplexing (OFDM) symbols causing the pilot information to be carried on a sub-carrier of the same frequency; a receiver extracting the pilot information from the N_pilot OFDM symbols, calculating the phase of each pilot information, then calculating the dispersion phase of the pilot information on the basis of the phase of each pilot information; on the basis of the relationship between dispersion phase frequency and sub-carrier frequency, using the dispersion phase of the pilot information to calculate the dispersion phase of each sub-carrier in the OFDM symbol, and performing compensation for the dispersion phase. The present solution enables the dispersion compensation of high-speed fiber optic communication, reducing training overhead for dispersion compensation to 1/N_sc of the overhead of a traditional dispersion compensation processing solution, where 1/N_sc is the number of sub-carriers carrying data within an OFDM symbol.

Description

Method and device for performing dispersion compensation

Technical field

The present invention relates to orthogonal frequency division multiplexing and optical communication technologies, and more particularly to a method and apparatus for performing dispersion compensation.

Background technique

Orthogonal Frequency Division Multiplexing (OFDM) is one of the multi-carrier modulation techniques. It divides high-speed information data into N low-speed data streams and modulates them into N mutually orthogonal subcarriers for parallel transmission. Its spectrum utilization can be nearly doubled compared to traditional single-carrier modulation techniques. At the same time, since the data is divided into N ways, the symbol period is expanded by N times compared with the original high-speed signal period, so that the OFDM system can reduce the influence of delay spread.

When an OFDM system is applied to an optical transmission system, due to the characteristics of the optical fiber, the signal is affected by the dispersion effect during transmission, so that different subcarrier transmission rates of the OFDM signal are different, resulting in signal time domain waveform broadening and intersymbol interference. - Inter-symbol interference (ISI). At the same time, within the same symbol, the orthogonality between different subcarriers is also destroyed, forming inter-subcarrier interference (ICI). From the frequency point of view, the ICI introduced by the dispersion causes the data carried on each subcarrier to be added with a dispersion phase, and the magnitude of the dispersion phase is related to the subcarrier frequency, and the lower frequency subcarrier of the high frequency subcarrier will be subjected to greater dispersion. Phase effect.

In order to eliminate the influence of ISI introduced by the dispersion, a cyclic prefix is usually inserted between adjacent symbols as a guard interval, and the length of the guard interval should be greater than the maximum delay spread of the time domain signal, so that the time domain expansion of any one symbol does not affect Its adjacent symbols.

In order to eliminate the influence of ICI, the commonly used method is to insert a training sequence at the transmitting end, and use the training sequence to obtain the frequency response of the channel, so that the tap coefficient of the equalizer is equal to the inverse of the channel frequency response, and then the product of the received data is used for dispersion compensation.

Since the dispersion coefficient of the fiber channel changes slowly, it can be considered that the influence of the dispersion effect on the signal remains substantially unchanged during a time period corresponding to a certain number of OFDM symbols. Estimating the dispersion characteristics of the fiber channel through the training sequence at the receiving end, and correspondingly introducing the OFDM symbol into the dispersion Phase distortion compensation eliminates ICI. At the same time, in order to estimate the excellent scattered phase damage more accurately under the influence of the ASE noise introduced by the amplifier, multiple OFDM training sequence symbols (usually OFDM training symbols > 5) need to be inserted in the initial stage of the transmitting end, one of the OFDM training sequence symbols. The length is equal to the number of subcarriers of one OFDM symbol, which makes the training sequence take up a large overhead in the process of one dispersion estimation and compensation. Summary of the invention

The technical problem to be solved by the present invention is to provide a method and apparatus for performing dispersion compensation, which can reduce the training overhead of dispersion compensation when performing dispersion compensation on an optical communication system.

In order to solve the above technical problem, a method for performing dispersion compensation according to the present invention includes: transmitting end at N _pil . _Transmitting pilot information in _t (N _pil _t > 1) orthogonal frequency division multiplexing (OFDM) symbols, and carrying the pilot information on subcarriers of the same frequency;

The receiving end is from the N _pil . Extracting the pilot information from _t OFDM symbols, calculating a phase of each pilot information, and calculating a dispersion phase of the pilot information according to a phase of the pilot information; according to a dispersion phase and a frequency of a subcarrier The relationship is calculated by using the dispersion phase of the pilot information, calculating the dispersion phase on each subcarrier in the OFDM symbol, and performing compensation of the dispersion phase.

Preferably, said from said N _pil . Extracting the pilot information from the _t OFDM symbols, and calculating the phase of each pilot information includes:

Performing a fast Fourier transform (FFT) on the received optical signal to obtain FFT-after data; extracting the N _pil from the FFT-after data. _The pilot information in the _t OFDM symbols, the angle of the pilot information is obtained, and the phase of each pilot information is calculated according to the amplitude of each pilot information.

Preferably, the calculating the dispersion phase of the pilot information according to the phase of the pilot information comprises:

The phase of the pilot information is removed from the random phase and the modulation phase of the pilot information to obtain a dispersion phase of the pilot information.

Preferably, the pilot information is a fixed real value, and the modulation phase is zero;

Decoding a phase of the pilot information to remove a random phase and a modulation phase packet of the pilot information Included: the extracted N _pil . The phase of the pilot information of the _t OFDM symbols is averaged, and the obtained average value is used as the dispersion phase of the pilot information.

Preferably, the transmitting end is at N _pil . Inserting pilot information into _t OFDM symbols, and carrying the pilot information on subcarriers of the same frequency includes:

The original bit information is multi-ary modulated to obtain a complex symbol sequence, and each N _se (N _s _≥ 0) complex symbols are divided into a group, and N _{pil is} selected. _t group complex symbol;

For the selected N _pil . _{The t-} group complex symbol performs an inverse fast Fourier transform (IFFT), and when IFFT is performed, an input of the IFFT is selected to insert the pilot information for the N _pil . _{The t-} group complex symbol selects the same input terminal to insert the pilot information.

Preferably, according to the relationship between the dispersion phase and the frequency of the subcarrier, using the dispersion phase of the pilot information, calculating the dispersion phase on each subcarrier in the OFDM symbol, and performing compensation of the dispersion phase includes:

Calculating a dispersion phase on each subcarrier of the OFDM symbol by using a dispersion phase meter of the pilot information according to a relationship between a dispersion phase and a frequency of the subcarrier;

Converting the dispersion phase on each subcarrier into an exponential form and performing conjugate processing to obtain a compensation value for each subcarrier;

The compensation values of the subcarriers are correspondingly multiplied to the FFT-after data of each channel.

Preferably, the subcarrier carrying the pilot information is an intermediate frequency subcarrier.

Preferably, a device for performing dispersion compensation comprises: an inverse fast Fourier transform (IFFT), an extraction pilot module, a framing angle module, an average denoising module, a dispersion phase estimation module, and a compensation module, wherein:

The IFFT is set to: at N _pil . Transmitting pilot information in _t (N _{pil ·} _t > 1) Orthogonal Frequency Division Multiplexing (OFDM) symbols, and carrying the pilot information on subcarriers of the same frequency;

The extraction pilot module is set to: from the N _pil . Extracting the pilot information from _t OFDM symbols;

The framing angle module is configured to: calculate a phase of each pilot information;

The average denoising module is configured to: calculate the pilot according to a phase of the pilot information Dispersion phase of information;

The dispersion phase estimation module is configured to: estimate a dispersion phase of each subcarrier in the OFDM symbol by using a dispersion phase of the pilot information according to a relationship between a dispersion phase and a frequency of a subcarrier;

The compensation module is configured to: compensate for the dispersion phase on each subcarrier in the OFDM symbol. Preferably, a fast Fourier transform (FFT) device is further included, wherein:

The FFT device is configured to: perform FFT on the received optical signal to obtain FFT data; the framing angle module is configured to: align an angle of the pilot information, according to an angle of each pilot information Calculate the phase of each pilot information.

The average denoising module is for the extracted N _pil . The phase of the pilot information of the _t OFDM symbols is averaged, and the obtained average value is used as the dispersion phase of the pilot information.

In summary, the embodiment of the present invention effectively implements dispersion compensation of a high-speed optical communication system, and can reduce the training overhead of dispersion compensation to 1/N _SC of the training overhead of the conventional dispersion compensation processing scheme, where N _sc is an OFDM symbol. The number of subcarriers carrying data within, and the implementation complexity is low. BRIEF abstract

1 is a block diagram of an apparatus for inserting pilot information in an apparatus for performing dispersion compensation according to an embodiment of the present invention;

2 is a schematic diagram showing a correspondence relationship between an IFFT input point and a frequency of a subcarrier in the generated OFDM according to the embodiment;

3 is a schematic diagram showing a relationship between a frequency of a subcarrier in an OFDM signal and a dispersion phase added to the bearer data in the embodiment;

4 and FIG. 5 are flowcharts of a method for performing dispersion compensation according to the embodiment;

6 is a block diagram of an apparatus for performing dispersion compensation in an apparatus for performing dispersion compensation according to an embodiment of the present invention. Preferred embodiment of the invention

In this embodiment, the pilot information is used to perform dispersion compensation, and the transmitting end inserts pilot information on multiple OFDM symbols, and the receiving end extracts pilot information in multiple OFDM symbols after performing Fast Fourier Transform (FFT). According to the principle that the same subcarrier is affected by the dispersion, the dispersion phase and the averaging operation can be used to obtain the dispersion phase after the influence on the subcarrier, and the dispersion phase of the adjacent subcarrier is estimated by the relationship between the subcarrier frequency and the dispersion phase. The size, completes the dispersion compensation of the entire OFDM symbol.

The present embodiment will be further described in detail below with reference to the accompanying drawings.

FIG. 5 shows a method for performing dispersion compensation according to the embodiment, which includes the following steps:

Step 501: Selecting OFDM symbols from the initial OFDM symbol sequence to insert pilot information, selecting an intermediate frequency subcarrier to carry pilot information, and modulating the OFDM base charged signal into an optical signal by using a Mach-Zehnder modulator (MZM), and transmitting the optical signal to the optical fiber channel. Transmission

In this step, one pilot information can be inserted in each OFDM symbol.

Step 501 specifically includes the following steps:

Step a: Performing multi-ary modulation on the original bit information to obtain a complex symbol sequence, and each N _sc complex symbols is a group, and the former N _{pil is} selected. _t (N _pil .t > 1) sets the complex symbols (corresponding to the pre-N _{pil of the} transmitting end. _t OFDM symbols) to insert pilot information;

A set of complex symbols is data carried by subcarriers in one OFDM symbol, and the number of subcarriers carrying data in the OFDM symbol is N _sc .

Due to the random noise introduced by amplifiers such as EDFA, the system needs to average the pilot phase information at the receiving end to eliminate the effects of noise. This requires the selection of multiple OFDM symbols to be inserted into the pilot information at the transmitting end. In general, selecting five OFDM symbols to insert pilot information can eliminate the influence of random noise on the pilot information, that is, N _pil at this time. _t = 5;

Step b: For the selected front N _pil . _{The t-} group complex symbol is subjected to inverse fast Fourier transform (IFFT), and the pilot information is inserted in the process to obtain N _pil . _t time domain samples of OFDM symbols containing pilot information;

As shown in Figure 1, an input of the IFFT can be set to a fixed real value as the pilot value of the OFDM symbol, thereby completing the insertion of the pilot information. Since this pilot information is a real number, Therefore, its initial phase is 0. Figure 2 shows the correspondence between the position of the IFFT input point and the subcarrier frequency carried by the input data at this point, for N _pil . _{The t-} group complex symbol selects the same input to insert pilot information.

In this step, the pilot information should be selected to be carried on the intermediate frequency subcarrier, and low frequency or high frequency subcarriers should not be selected. If the high frequency subcarrier is used to carry the pilot information, since the dispersion phase of the high frequency subcarrier carrying data is large, the phase blur phenomenon is likely to occur, so that the dispersion phase compensation range is reduced. If the low frequency subcarrier is used to carry the pilot information, since the phase of the low frequency subcarrier carrying data is small, it is susceptible to random influence and error. The influence of this error will be an index as the subcarrier frequency increases when estimating the dispersion phase. When the dispersion is estimated, when the dispersion phase of the high-frequency subcarrier data is estimated, the influence of this error becomes very large, resulting in poor dispersion compensation and lowering the OSNR tolerance of the system. Therefore, considering the dispersion compensation range and compensation effect, the IF subcarrier bearer pilot information should be selected.

Step c: N _{pil of the} IFFT output. _The time-domain samples of the _t- group OFDM symbols are subjected to a digital-to-analog converter (DAC) to obtain an OFDM time domain signal, and are modulated into an optical signal by MZM, and then sent to the optical fiber channel for transmission.

Step 502: The receiving end performs a fast Fourier transform (FFT) on the optical signal, and extracts N _pil from the FFT data. _The pilot information in the _t OFDM symbols is obtained by taking the amplitude of the pilot information, and calculating the phase of each pilot information according to the amplitude; after the transmission of the optical channel, the signal is affected by the dispersion, so that the data carried by the different subcarriers is attached. The dispersion phase is different, and the dispersion phase of the subcarrier data of different frequencies is different. As shown in FIG. 3, the dispersion value is CD=800 ps/nm, the OFDM spectral bandwidth is 28 GHz, and the subcarrier frequency and its dispersion phase are plotted when the number of subcarriers is 512, and the dispersion phase and subcarrier of the subcarrier data of different frequencies are shown. The carrier frequency is in a quadratic exponential relationship. In addition, the information is subject to random effects, mainly from optical amplifiers such as EDFAs.

Since the FFT completes the conversion of the signal time domain sample value to the frequency domain sample value, the receiving end

The output after FFT is the symbol data carried by each subcarrier of the OFDM signal, and the phase includes the data modulation phase, the dispersion phase and the random phase, as shown in the following equation:

Φ = Φ ₀ +Φ _Ο + Φ^ (1) Where Φ represents the phase of the data after FFT, Φ. Represents the modulation phase, _ot represents the dispersion phase, and Φ represents the random phase.

The expression of the dispersion phase ^ _D is as follows:

Where, represents the optical carrier center frequency (Hz), c represents the speed of light (m/s), represents the dispersion of the fiber (ps/nm), and represents the frequency of the kth subcarrier of the OFDM baseband signal.

It can be seen from Equation 2 that the dispersion phase on different subcarriers tends to increase exponentially with the subcarrier frequency.

In Formula 2, D _t = D * L. L represents the transmission distance and D represents the dispersion coefficient of the fiber. The size of D has different values than the subcarriers of different frequencies of the OFDM optical signal, and the relative relationship between them is from a certain law, as shown in the following equation:

Among them, the wavelength corresponding to different frequency components of the optical signal, Α is the material coefficient of the optical fiber, and different fiber properties for different optical fibers.

Step 503: averaging the phase of the pilot information, removing the random influence, and obtaining the dispersion phase of the estimated pilot information;

For the former N _pil . _{For t} OFDM symbols containing pilot information, the insertion position of the pilot information, that is, the frequency of the subcarriers carrying the pilot information is the same, which means the former N _pil . _In the phase of the pilot information of the _t OFDM symbols, the magnitude of the dispersion phase is the same; and since the initial phase (ie, the modulation phase) of the pilot information is 0, the phase of the extracted pilot information includes only the dispersion phase. And random phase, so for N _pil . The phase of the _t pilot information is averaged to eliminate the influence of the random phase, and an estimated value of the dispersion phase in the phase of the pilot information is obtained.

Step 504: Estimate and compensate the dispersion phase on each subcarrier signal according to the relationship between the dispersion phase and the subcarrier frequency.

Step A: estimating, according to the magnitude of the dispersion phase of the pilot information, the dispersion phase of the data carried by the other subcarriers according to Equation 2;

It can be seen from FIG. 3 that the dispersion phase of the data on different subcarriers is represented by the subcarrier frequency. The law of number growth, and obeys the quadratic exponential function. From this relationship, the magnitude of the dispersion phase attached to the other subcarrier data can be estimated by the magnitude of the dispersion phase of the pilot information.

Step B: Converting the estimated dispersion phase of the data on each subcarrier into an exponential form exp[ ' )], and performing conjugate processing on the dispersion phase of the exponential form to obtain a compensation value exp [__/ for each subcarrier data. '( )]; Step C: Return the obtained compensation value to the FFT output, and multiply the corresponding output values of the FFT to compensate the dispersion phase of each subcarrier data.

As shown in FIG. 1 and FIG. 6, the architecture diagram of the apparatus for performing dispersion compensation according to the present embodiment includes:

An inverse fast Fourier transformer for N _pil . _{The t-} group complex symbol is IFFT, and the pilot information is inserted during this process, for N _pil . _{The t} group complex symbol selects the same input terminal to insert pilot information to obtain N _pil . _t time-domain samples of OFDM symbols containing pilot information, complete data frequency domain to time domain transformation,

P/S, used to complete the parallel and parallel conversion of data.

A digital-to-analog converter (DAC) for N _pil . The time domain sample values of the OFDM symbol are digital-to-analog converted to obtain an OFDM time domain signal.

A Mach-Zehnder Modulator (MZM) for modulating a baseband OFDM electrical signal into an OFDM optical signal.

A fast Fourier transform (FFT) device is used to perform FFT on the optical signal to complete the transform of the data from the time domain to the frequency domain.

Extract the pilot module from the selected N _pil . Pilot information is extracted from _t OFDM symbols.

The framing angle module is configured to take a pilot angle of the pilot information, and calculate a phase of each pilot information according to the amplitude of each pilot information.

The average denoising module, since the pilot information is a fixed real value, its modulation phase is zero, and the average denoising module is the extracted N _pil . The phase of the _t pilot information is averaged to eliminate the influence of random noise on the pilot phase, and the obtained average value is used as the dispersion phase of the pilot information.

The dispersion phase estimation module is configured to estimate a dispersion phase included in each subcarrier carrier data of the OFDM symbol according to a relationship between a dispersion phase and a frequency of the subcarrier, and a dispersion phase of the pilot information. The compensation module is configured to phase compensate according to the estimated dispersion phase of each subcarrier data, thereby eliminating the influence of the dispersion phase.

Obviously, those skilled in the art should understand that the above modules and steps of the present invention can be implemented by a general computing device, which can be concentrated on a single computing device or distributed over a network composed of multiple computing devices. Alternatively, they may be implemented by program code executable by the computing device, such that they may be stored in the storage device by the computing device, or they may be separately fabricated into individual integrated circuit modules, or their Multiple modules or steps are implemented as a single integrated circuit module. Thus, the invention is not limited to any particular combination of hardware and software.

The above is only the embodiment of the present invention, and is not intended to limit the present invention, and various modifications and changes can be made to the present invention. All modifications, equivalents, improvements, etc., made within the spirit and scope of the invention are intended to be included within the scope of the invention.

INDUSTRIAL APPLICABILITY The embodiment of the present invention effectively implements dispersion compensation of a high-speed optical communication system, and can reduce the training overhead of dispersion compensation to 1/N _SC of the training overhead of the conventional dispersion compensation processing scheme, where N _sc is an intra-OFDM carrier. The number of subcarriers of data, and the implementation complexity is low.

Claims

Claim

1. A method for performing dispersion compensation, comprising:

The transmitting end is at N _pil . _Transmitting pilot information in _t (N _pil _t > 1) orthogonal frequency division multiplexing (OFDM) symbols, and carrying the pilot information on subcarriers of the same frequency;

2. The method of claim 1 wherein said from said N _pil . Extracting the pilot information from the _t OFDM symbols, and calculating the phase of each pilot information includes:

The method according to claim 1, wherein the calculating the dispersion phase of the pilot information according to the phase of the pilot information comprises:

4. The method of claim 3, wherein:

The pilot information is a fixed real value, and the modulation phase is zero;

The phase-removing random phase of the pilot information and the modulation phase of the pilot information include: the extracted N _pil . The phase of the pilot information of the _t OFDM symbols is averaged, and the obtained average value is used as the dispersion phase of the pilot information.

5. The method of claim 1, wherein the transmitting end is at N _pil . Inserting pilot information into _t OFDM symbols, and carrying the pilot information on subcarriers of the same frequency includes:

The original bit information is modulated in multiple numbers to obtain a complex symbol sequence, each N _se (N _s _≥ 0) The complex symbols are divided into a group, and N _{pil is} selected. _t group complex symbol;

The method according to claim 2, wherein, according to the relationship between the dispersion phase and the frequency of the subcarrier, the dispersion phase on each subcarrier in the OFDM symbol is calculated by using the dispersion phase of the pilot information, And compensation for dispersion phase includes:

7. The method of claim 1, wherein the subcarriers carrying the pilot information are intermediate frequency subcarriers.

8. A device for performing dispersion compensation, comprising: an inverse fast Fourier transform (IFFT), an extraction pilot module, a framing angle module, an average denoising module, a dispersion phase estimation module, and a compensation module, wherein:

The average denoising module is configured to: calculate a dispersion phase of the pilot information according to a phase of the pilot information;

The dispersion phase estimation module is configured to: estimate a dispersion phase of each subcarrier in the OFDM symbol by using a dispersion phase of the pilot information according to a relationship between a dispersion phase and a frequency of a subcarrier; The compensation module is configured to: compensate for the dispersion phase on each subcarrier in the OFDM symbol.

9. The apparatus according to claim 8, further comprising a fast Fourier transform (FFT), the FFT device configured to: perform FFT on the received optical signal to obtain FFT data; The argument module is configured to: align the pilot information, and calculate the phase of each pilot information according to the amplitude of each pilot information.

10. The apparatus according to claim 8, wherein

The pilot information is a fixed real value, and the modulation phase is zero;