CN106559370A - A kind of method of low complexity OFDM PON system sample clock frequency deviation compensation - Google Patents

Abstract

The present invention provides a kind of method of low complexity OFDM PON system sample clock frequency deviation compensation, the phase rotation coefficient that sample clock frequency deviation causes is eliminated using subcarrier sampling frequency offset increment of rotation, the data collected to ADC in receiving terminal carry out sign synchronization, find the position of FFT windows, training sequence frequency domain value and local training sequence frequency domain value that training sequence and data symbol are obtained using extraction are extracted respectively, estimate the channel response initial value of system, using the channel response initial value Jing recursion and reponse system of system obtain channel response at each symbol more on the occasion of, the phase place that systematic sampling clock frequency deviation causes is compensated according to the system channel response coefficient for updating.The present invention is easy to hardware and realizes, economizes on resources, while significantly reducing the computation complexity of OFDM PON sample clock frequency deviation backoff algorithms.

Description

A kind of method of low complexity OFDM-PON system sample clock frequency deviation compensation

Technical field

The present invention relates to a kind of low complexity OFDM-PON (Orthogonal Frequency Division Multiplexing-Passive Optical Network, orthogonal frequency division multiplexing passive optical network) system sampling clock frequency The method of deviation compensation, belongs to digital technology.

Background technology

OFDM-PON technologies are the fusions of ofdm system and PON.Because which has transmission capacity, (up-downgoing speed can reach greatly 40Gbit/s even 100Gbit/S), using the teaching of the invention it is possible to provide the business of all kinds, various broadbands and various qos requirements, traffic scheduling And broadband management very flexibly the advantages of, in -2 (Next Generation Passive of next-generation passive optical network Optical Network 2, NG-PON2) in show huge potentiality.

The general principle of light OFDM is similar to electricity OFDM, and the two difference is that signal is changed into from the wireless channel transmission of electrical domain The fibre channel transmission of area of light, therefore OFDM-PON haves the shortcomings that OFDM technology, is mainly reflected in OFDM-PON systems Symbol timing error (Symbol Time Offset, STO), carrier frequency shift (Carrier Frequency Offset, ) and sampling clock deviation (Sampling Frequency Offset, SFO) CFO.Sampling clock deviation is mostly derived from transmitting The D/A converter (Digital-to-Analog Converter, DAC) and the A/D converter (Analog- of receiver of machine To-Digital Converter, ADC) crystal oscillator can not possibly have identical clock cycle and phase place, cause both Deviation can be produced between sampling interval, so that serious impact is brought to the performance of whole system：One be through sampling son Orthogonality is no longer kept between carrier wave, interchannel interference is produced；Another is the time-varying timing offset that sampling frequency offset is produced, and is passed through The phase place change of time-varying can be caused after discrete Fourier transform (Discrete Fourier Transformation, DFT).

In the OFDM-PON data transmission systems of high speed, for transmitting terminal DAC and the sample clock frequency of receiving terminal ADC Generally in several GSps, and the system clock for being used for the FPGA of real time signal processing only has hundreds of MHz, be much smaller than during sampling Clock, therefore the Digital Signal Processing of OFDM-PON needs to use parallel processing technique.The parallel processing of multichannel means that logic is provided The increase that source consumes, therefore the signal processing algorithm using low complex degree is particularly important in OFDM-PON.In great majority In the OFDM-PON system sampling clock deviation compensation methods of open report, it is most of be all use for reference Wireless OFDM System in adopt By Shafiee et al. propose classical synchronized algorithm, i.e., frequency departure is estimated using pilot frequency information.This synchronized algorithm Need to calculate phase rotation coefficient exp (the jd θ of each OFDM symbol_l,k), need to use when FPGA hardware is realized to be configured to cos＆ The CORDIC cores of sin patterns, by cause resource significant wastage and realization complexity it is higher.

The content of the invention

For the defect that existing sampling clock compensation technique resource consumption is larger, it is an object of the invention to provide a kind of The method of low complexity OFDM-PON system sample clock frequency deviation compensation.Eliminated using subcarrier sampling frequency offset increment of rotation Fall the phase rotation coefficient that sample clock frequency deviation causes, it is hard so as to improve FPGA in traditional sampling Clock Frequency Offset Compensation method Part calculates phase rotation coefficient exp (the jd θ of each OFDM symbol_l,k) when the wasting of resources that causes and the shortcomings of big time delay；Profit The frequency channels response of initial OFDM symbol subcarrier is obtained with the algorithm of recurrence thought, and then each OFDM symbol is adopted Sample frequency deviation of clock is compensated, and will greatly reduce computation complexity.

Inventive method assumes that ofdm system has been realized in correct sign synchronization, and sampling clock frequency deviation is obtained Estimate.For achieving the above object, the present invention is adopted the following technical scheme that：

A kind of method of low complexity OFDM-PON system sample clock frequency deviation compensation, comprises the following steps that：

1) the sampling clock offset estimation value for obtaining is estimated by OFDM-PON system receiving terminals, each subcarrier is obtained adjacent Phase rotation increment between OFDM symbol；

2) the twiddle factor increment of frequency domain between each subcarrier adjacent OFDM symbol is launched by Euler's formula, and approximately Represented with phase rotation increment；

3) with reference to channel response initial value, according to the approximate twiddle factor increment between adjacent OFDM symbol by the way of recursion Obtain the channel response after each subcarrier is extended at each OFDM symbol；

4) sampling clock frequency deviation is compensated according to the channel response after extension.

The step 1) in adjacent OFDM symbol k-th subcarrier phase rotation increment θ_kObtained by equation below Go out：

In formula, Δ is sample clock frequency relative deviation, and N is that the calculating of ofdm system FFT/IFFT is counted, N_sFor one The length of OFDM data symbol, including OFDM data and data cyclic prefix.

The step 2) between adjacent OFDM symbol k-th subcarrier phase rotation increment θ_kMuch smaller than 0, and it is solid Definite value, the phase rotation coefficient that the sampling frequency offset between adjacent OFDM symbol causesBy the phase place between adjacent OFDM symbol Increment of rotation θ_kApproximate representation：

The step 3) in channel response at OFDM symbol be given by previous OFDM symbol channel response, according to channel Response initial value and recurrence formula obtain the channel response of each OFDM symbol, are specifically given by equation below,

Wherein l is the call number of the OFDM symbol for receiving, and H (0, k) it is system channel response initial value.

The step 4) the system channel response coefficient that updates mended to the phase place that systematic sampling clock frequency deviation causes Repay, be specifically given by equation below：

Wherein Y₁(l, k) is that the receives frequency of uncompensated frequency departure is responded, and H (l, k) is l-th OFDM symbol kth The frequency channels response of individual subcarrier, this completes sampling frequency offset compensation.

The present invention compared with prior art, obviously projects substantive distinguishing features and marked improvement with following：

(1) present invention proposes one kind and is easy to hard-wired sampling clock frequency compensation method, more particularly to using son Carrier wave sampling frequency offset increment of rotation eliminates the phase rotation coefficient that sample clock frequency deviation causes, and utilizes recurrence thought Algorithm obtain the frequency channels response of initial OFDM symbol subcarrier, and then sampling clock frequency deviation is carried out to each OFDM symbol Compensation, has greatly saved resource, has reduced complexity.

(2) the compatible existing OFDM-PON systems of sample clock frequency deviation compensation method proposed by the present invention, use this The signal processing algorithm of low complex degree avoids the increase of the logical resource consumption that the parallel processing of multichannel in OFDM-PON is caused.

Description of the drawings

Fig. 1 is OFDM frame structure schematic diagrames in the embodiment of the present invention.

Fig. 2 is sampling clock frequency bias compensation method flow chart of the present invention.

Fig. 3 is that the algorithm in the embodiment of the present invention realizes structural representation.

Fig. 4 is that channel response of the present invention updates schematic flow sheet.

Fig. 5 is the experimental result bit error rate figure in the embodiment of the present invention.

Specific embodiment

Below in conjunction with the accompanying drawings, the specific embodiment of the present invention is described further.

The frame structure adopted by this example is as shown in figure 1, be mainly made up of training sequence and OFDM symbol.Training sequence and OFDM symbol all includes Cyclic Prefix, and wherein training sequence is used for the channel equalization and channel compensation for realizing ofdm system.This The FFT/IFFT that example is adopted counts as 64, and the length of Cyclic Prefix is 16 points, and OFDM symbol number N included per frame is 100.

The flow chart that the present invention implements described ofdm system sampling clock frequency bias compensation method is illustrated in figure 2, Fig. 3 is this Structural representation is realized in invention example.With reference to Fig. 2 and Fig. 3, detailed process is as follows：

1st, method assumes that having been obtained for the estimate Δ of sampling clock frequency deviation.Obtained according to sampling clock frequency deviation value Δ first To the phase rotation increment of each subcarrier, implement according to equation below：

2nd, the data collected to ADC in receiving terminal carry out sign synchronization, find the position of FFT windows, extract respectively Training sequence and data symbol.Training sequence and OFDM data symbol to extracting does FFT, obtains the training sequence of frequency domain Train value R_trsWith data X_l,k.Wherein l represents the call number of the OFDM data symbol for receiving, subcarriers of the k for OFDM data.

3rd, local training sequence is stored in the training sequence of transmitting terminal, the training sequence frequency domain value obtained using extraction R_trsWith local training sequence frequency domain value L_trsEstimate system channel response initial value H (0, k).Implement according to following public Formula：

4th, using the mode of the channel response initial value Jing recursion of system obtain channel response at each symbol more on the occasion of.Mould The structure of block is implemented according to equation below as shown in figure 4, be made up of a multiplier and delayer：

5th, equilibrium is carried out to data according to the system channel response coefficient for updating, is also to systematic sampling clock while balanced The phase place that frequency deviation causes is compensated, and is implemented as follows according to formula：

It is illustrated in figure 5 the compensation performance verification result of this example, systematic sampling intensity modulated Direct-detection Optical OFDM system System.The data modulation format of system is 16QAM, and transmitting luminous power is 7.5dBm, after 25Km single-mode fibers and optical attenuation After device, received optical power is -13dBm.Under different sampling clock offset frequency situations, the error code before system balance and after compensation is forthright Can be as shown in figure 5, it can be seen that without compensation, the bit error rate of system be with the increasing of sampling clock frequency deviation Plus sharply increase, and to sampling clock frequency deviation compensation after, the performance of system is effectively improved, when sampling clock frequency deviation- When between 150ppm to 150ppm, the bit error rate after compensation is almost equal with during 0 frequency deviation.Compensated using the inventive method and preferable The effect of compensation is about the same.

Above example is merely to illustrate technical scheme, is not intended to limit the scope of the present invention, all at this Any modification, equivalent and improvement for being made within spirit and principle etc., are contained within protection scope of the present invention.

Claims (5)

1. the method that a kind of low complexity OFDM-PON system sample clock frequency deviation is compensated, it is characterised in that concrete steps It is as follows：

1) the sampling clock offset estimation value for obtaining is estimated by OFDM-PON system receiving terminals, each subcarrier adjacent OFDM is obtained and is accorded with Phase rotation increment between number；

2) the twiddle factor increment of frequency domain between each subcarrier adjacent OFDM symbol is launched by Euler's formula, and approximately uses phase Position increment of rotation is represented；

3) with reference to channel response initial value, obtained by the way of recursion according to the approximate twiddle factor increment between adjacent OFDM symbol Channel response of each subcarrier after extension at each OFDM symbol；

4) sampling clock frequency deviation is compensated according to the channel response after extension.

2. the method that low complexity OFDM according to claim 1-PON system sample clock frequency deviation is compensated, which is special Levy and be, the step 1) in adjacent OFDM symbol k-th subcarrier phase rotation increment θ_kDrawn by equation below：

${\mathrm{\θ}}_k=\frac{2{\mathrm{\πkN}}_s\mathrm{\Δ}}{N}$

In formula, Δ is sample clock frequency relative deviation, and N is that the calculating of ofdm system FFT/IFFT is counted, N_sFor an OFDM number According to the length of symbol, including OFDM data and data cyclic prefix.

3. the method that low complexity OFDM according to claim 1-PON system sample clock frequency deviation is compensated, which is special Levy and be, the step 2) between adjacent OFDM symbol k-th subcarrier phase rotation increment θ_kMuch smaller than 0, and it is solid Definite value, the phase rotation coefficient that the sampling frequency offset between adjacent OFDM symbol causesBy the phase place between adjacent OFDM symbol Increment of rotation θ_kApproximate representation：

$e^{-{\mathrm{j\θ}}_k}\≈1+{\mathrm{j\θ}}_k.$

4. the method that low complexity OFDM according to claim 1-PON system sample clock frequency deviation is compensated, which is special Levy and be, the step 3) in channel response at OFDM symbol be given by previous OFDM symbol channel response, according to channel Response initial value and recurrence formula obtain the channel response of each OFDM symbol, are specifically given by equation below,

$H(l,k)=H(l-1,k)*e^{-{\mathrm{j\θ}}_k}\≈H(l-1,k)*(1+j\mathrm{\θ}k)$

Wherein l is the call number of the OFDM symbol for receiving, and H (0, k) it is system channel response initial value.

5. the method that low complexity OFDM according to claim 1-PON system sample clock frequency deviation is compensated, which is special Levy and be, the step 4) the system channel response coefficient that updates mended to the phase place that systematic sampling clock frequency deviation causes Repay, be specifically given by equation below：

$Y(l,k)=\frac{Y_1(l,k)}{H(l,k)}$

Wherein Y₁(l, k) is the receives frequency response of uncompensated frequency departure, and H (l, k) is l-th OFDM symbol, k-th sub- load The frequency channels response of ripple, this completes sampling frequency offset compensation.