WO2018054236A1 - 一种ofdm系统同步跟踪方法和装置 - Google Patents

一种ofdm系统同步跟踪方法和装置 Download PDF

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WO2018054236A1
WO2018054236A1 PCT/CN2017/101241 CN2017101241W WO2018054236A1 WO 2018054236 A1 WO2018054236 A1 WO 2018054236A1 CN 2017101241 W CN2017101241 W CN 2017101241W WO 2018054236 A1 WO2018054236 A1 WO 2018054236A1
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ofdm symbol
phase rotation
subcarrier
symbol
frequency
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PCT/CN2017/101241
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English (en)
French (fr)
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朱嘉俊
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珠海全志科技股份有限公司
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Priority to EP17852304.9A priority Critical patent/EP3503487B1/en
Priority to US16/333,606 priority patent/US10778496B2/en
Publication of WO2018054236A1 publication Critical patent/WO2018054236A1/zh

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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
    • H04L27/2649—Demodulators
    • H04L27/265—Fourier transform demodulators, e.g. fast Fourier transform [FFT] or discrete Fourier transform [DFT] demodulators
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0014—Carrier regulation
    • 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
    • H04L27/2657—Carrier synchronisation
    • 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
    • H04L27/2657—Carrier synchronisation
    • H04L27/266—Fine or fractional frequency offset determination and synchronisation
    • 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
    • H04L27/2668—Details of algorithms
    • H04L27/2669—Details of algorithms characterised by the domain of operation
    • H04L27/2672—Frequency domain
    • 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
    • H04L27/2668—Details of algorithms
    • H04L27/2673—Details of algorithms characterised by synchronisation parameters
    • H04L27/2675—Pilot or known symbols
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/345—Modifications of the signal space to allow the transmission of additional information
    • H04L27/3455—Modifications of the signal space to allow the transmission of additional information in order to facilitate carrier recovery at the receiver end, e.g. by transmitting a pilot or by using additional signal points to allow the detection of rotations
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0014—Carrier regulation
    • H04L2027/0024—Carrier regulation at the receiver end
    • H04L2027/0026—Correction of carrier offset
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0014—Carrier regulation
    • H04L2027/0044—Control loops for carrier regulation
    • H04L2027/0053—Closed loops
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/0014—Carrier regulation
    • H04L2027/0044—Control loops for carrier regulation
    • H04L2027/0063—Elements of loops
    • H04L2027/0067—Phase error detectors
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00—Arrangements affording multiple use of the transmission path
    • H04L5/0001—Arrangements for dividing the transmission path
    • H04L5/0003—Two-dimensional division
    • H04L5/0005—Time-frequency
    • H04L5/0007—Time-frequency the frequencies being orthogonal, e.g. OFDM(A), DMT

Definitions

  • the present invention relates to the field of communications technologies, and in particular, to a method and apparatus for synchronous tracking of an OFDM system, and more particularly to a method and apparatus for synchronous tracking of an OFDM system based on the 802.11a/g/n/ac protocol.
  • OFDM is an important modulation and demodulation technology that can greatly increase the data transmission rate.
  • OFDM is very sensitive to carrier frequency offset (frequency offset) and timing offset (time offset).
  • frequency offset is caused by the carrier frequency deviation between the sender and the receiver.
  • the frequency offset will destroy the orthogonality and the rotational phase value of the OFDM, resulting in a serious degradation of the demodulation performance of the receiver. Therefore, it is usually added to the frame header during communication.
  • the OFDM training symbol estimates the frequency offset for the receiver and eliminates it.
  • the initial frequency offset estimation of the received frame header under the influence of noise cannot ensure that the frequency offset is accurately estimated and completely eliminated, and only the frequency offset value can be controlled within a certain range.
  • the residual frequency offset will continue to affect the demodulation performance of the receiver. Therefore, subsequent frequency offset tracking is very necessary.
  • the time offset is caused by the crystal frequency difference between the sender and the receiver or the Doppler frequency drift.
  • the difference in crystal oscillator shifts the phase of the sample clock, causing the timing offset to change over time.
  • Time bias is basically inevitable for a system. It causes the subcarrier phase rotation of OFDM, which degrades the demodulation performance of the receiver. Therefore, after timing synchronization at the frame header, time-biased tracking is also very necessary.
  • the protocol specifies that there are several pilots known to each receiver in each OFDM symbol.
  • the pilot is inserted into the information subcarrier to perform synchronous tracking operation on the receiving end.
  • the accuracy of the estimated frequency offset and time offset of a limited number of pilot pilots is quite limited, and the demodulation performance cannot be optimized.
  • the prior art mostly performs frequency in the time domain. Bias estimation and compensation, which requires the use of an inverse Fourier transformer, the complexity of the circuit implementation is very high.
  • the technical problem to be solved by the present invention is to provide a method and device for synchronous tracking of an OFDM system, which overcomes the deficiencies of the prior art, and overcomes the OFDM system of the prior art 802.11 protocol, which uses pilot pilots for synchronous tracking, and the frequency offset estimation time and time.
  • the defect that the partial estimation accuracy is not high, and the frequency offset value estimation and compensation in the time domain lead to defects with high circuit complexity.
  • a method for synchronous tracking of an OFDM system comprising the steps of:
  • step A1 performing OFDM symbol division on the received digital signal, performing fast Fourier transform on the divided OFDM symbols, transforming from time domain to frequency domain, obtaining frequency domain OFDM symbol sequence, and sequentially frequency domain OFDM symbol sequence
  • Each of the frequency domain OFDM symbols performs step A2 to step A5;
  • A2 respectively extracting information subcarrier symbols, pilot symbols, and DC subcarriers in the current frequency domain OFDM symbol, detecting and determining the information subcarrier symbols to generate a restored information subcarrier symbol;
  • A3 Synthesize a pilot symbol, a DC subcarrier, and a reduction information subcarrier symbol in a current OFDM symbol into a restored OFDM symbol.
  • A4. Perform frequency offset estimation and time offset estimation on the restored OFDM symbol, and obtain a corresponding frequency offset estimation phase rotation value and a time offset estimation phase rotation value;
  • phase compensation is performed on the next frequency domain OFDM symbol in the frequency domain OFDM symbol sequence, and the compensated The frequency domain OFDM symbol is set to the current frequency domain OFDM symbol, and returns to step A2.
  • said step A2 comprises the step of generating a reduced information subcarrier symbol using a maximum likelihood hard decision.
  • the step A4 comprises the steps of: correcting the phase offset estimation phase rotation value and the time offset estimation phase rotation value by using a loop filter, and calculating a new frequency offset estimation phase rotation value and a time offset estimation after correction. Phase rotation value.
  • the step A4 comprises the steps of: the loop filter comprises a frequency offset loop filter and a time offset loop filter, and the frequency offset loop filter is used for correcting the phase offset estimation phase rotation value, and the time offset loop filtering The device is used to correct the phase offset estimated phase rotation value.
  • a synchronization tracking device for OFDM system comprising symbol timer, fast Fourier transformer, phase rotation compensator, information subcarrier extractor, channel estimator, detector, pilot subcarrier extractor, subcarrier hard decipher, OFDM Symbol synthesizer, frequency offset and time offset estimator, frequency offset loop filter, time offset loop filter, phase rotation estimation calculator, symbol timer, fast Fourier transformer, phase rotation compensator, information subcarrier extractor
  • the detector, the subcarrier hard discriminator, the OFDM symbol synthesizer, the frequency offset and time offset estimator, the frequency offset loop filter, and the phase rotation estimation calculator are sequentially connected, and the phase rotation compensator, the channel estimator, and the detector are sequentially Connected, the information subcarrier extractor, the pilot subcarrier extractor, and the OFDM symbol synthesizer are sequentially connected, the symbol timer is used for OFDM symbol division of the received digital signal, and the fast Fourier transformer is used to sequentially perform the divided OFDM symbols.
  • Fourier transform transforming it from time domain to frequency domain, obtaining frequency domain OFDM symbol sequence, channel estimator for Channel estimation is performed according to the training sequence transparently transmitted by the phase rotation compensator, and the information subcarrier extractor is used to extract information subcarrier symbols in the current frequency domain OFDM symbol, and the pilot subcarrier extractor is used to extract the current a pilot symbol in the frequency domain OFDM symbol, a DC subcarrier, a detector for detecting the information subcarrier symbol, and a subcarrier hard decider for determining the information subcarrier symbol and generating a restored information subcarrier symbol,
  • An OFDM symbol synthesizer is configured to synthesize pilot symbols, DC subcarriers, and reduction information subcarrier symbols in a current OFDM symbol into a restored OFDM symbol, and a frequency offset and time offset estimator is used to perform frequency offset estimation and time offset estimation on the restored OFDM symbol.
  • the frequency offset loop filter is used for correcting the frequency offset estimation phase rotation value
  • the time offset loop filter is used for correcting the phase offset estimation phase rotation value
  • phase The rotation estimate calculator is configured to calculate a corresponding frequency offset estimation phase rotation value and a time offset estimation phase rotation value
  • the phase rotation compensator is configured to estimate the phase rotation value and the time offset estimation phase rotation value by using the frequency offset to the frequency domain OFDM symbol sequence.
  • the next frequency domain OFDM symbol is phase compensated, and the compensated frequency domain OFDM symbol is set to the current frequency domain OFDM symbol.
  • the subcarrier hard decider generates a reduced information subcarrier symbol using a maximum likelihood hard decision.
  • the present invention uses the information subcarrier hard decision feedback and the pilot subcarrier combination method to estimate the frequency offset and the time offset phase rotation value of the OFDM symbol, thereby greatly improving the accuracy of the estimation, and
  • the phase rotation compensation is provided for the next OFDM symbol, and the entire synchronization tracking process not only improves the tracking accuracy, but also reduces the complexity of the circuit timing implementation.
  • 1 is a schematic diagram of 802.11a/g/n/ac OFDM symbol subcarrier distribution
  • Figure 2 is a schematic view of the apparatus of the present invention.
  • Figure 3 is a schematic diagram of a loop filter of the present invention.
  • the OFDM system synchronization tracking method of the present invention comprises the following steps:
  • step A1 performing OFDM symbol division on the received digital signal, performing fast Fourier transform on the divided OFDM symbols, transforming from time domain to frequency domain, obtaining frequency domain OFDM symbol sequence, and sequentially frequency domain OFDM symbol sequence
  • Each of the frequency domain OFDM symbols performs step A2 to step A5;
  • A2 respectively extracting information subcarrier symbols, pilot symbols, and DC subcarriers in the current frequency domain OFDM symbol, detecting and determining the information subcarrier symbols to generate a restored information subcarrier symbol;
  • A3 Synthesize a pilot symbol, a DC subcarrier, and a reduction information subcarrier symbol in a current OFDM symbol into a restored OFDM symbol.
  • A4. Perform frequency offset estimation and time offset estimation on the restored OFDM symbol, and obtain a corresponding frequency offset estimation phase rotation value and a time offset estimation phase rotation value;
  • Step A5 Using the frequency offset estimation phase rotation value and the time offset estimation phase rotation value, phase compensation is performed on the next frequency domain OFDM symbol in the frequency domain OFDM symbol sequence, and the compensated frequency domain OFDM symbol is set as the current frequency domain OFDM symbol, and the return is performed. Step A2.
  • said step A2 comprises the step of generating a reduced information subcarrier symbol using a maximum likelihood hard decision.
  • the step A4 includes the steps of: correcting the phase offset estimation phase rotation value and the time offset estimation phase rotation value by using a loop filter, and calculating a corrected new frequency offset estimation phase rotation value and a time offset estimation phase rotation value.
  • the loop filter includes a frequency offset loop filter and a time offset loop filter. The frequency offset loop filter is used to correct the phase offset estimation phase rotation value, and the time offset loop filter is used to correct the time offset estimation phase rotation value.
  • the OFDM system synchronization tracking device of the invention comprises a symbol timer, a fast Fourier transformer, a phase rotation compensator, an information subcarrier extractor, a channel estimator, a detector, a pilot subcarrier extractor, a subcarrier hard decipher, an OFDM Symbol synthesizer, frequency offset and time bias estimation Counter, frequency offset loop filter, time offset loop filter, phase rotation estimation calculator, symbol timer, fast Fourier transformer, phase rotation compensator, information subcarrier extractor, detector, subcarrier hard discriminator OFDM symbol synthesizer, frequency offset and time offset estimator, frequency offset loop filter, phase rotation estimation value calculator are sequentially connected, phase rotation compensator, channel estimator and detector are sequentially connected, information subcarrier extractor, guide The frequency subcarrier extractor and the OFDM symbol synthesizer are sequentially connected, the symbol timer is used for performing OFDM symbol division on the received digital signal, and the fast Fourier transform is used to sequentially perform fast Fourier transform on the divided OFDM symbols, and time-lapse thereof The
  • the information subcarrier symbol in the symbol, the pilot subcarrier extractor is used to extract the pilot in the current frequency domain OFDM symbol a symbol, a DC subcarrier, a detector for detecting information subcarrier symbols, a subcarrier hard decider for determining information subcarrier symbols and generating a reduced information subcarrier symbol, and an OFDM symbol synthesizer for guiding the current OFDM symbol
  • the frequency symbol, the DC subcarrier and the restored information subcarrier symbol are synthesized to restore the OFDM symbol
  • the frequency offset and time offset estimator is used for performing frequency offset estimation and time offset estimation on the restored OFDM symbol, and obtaining corresponding frequency offset estimation phase rotation value and time offset.
  • phase rotation value calculator is used to calculate the corresponding frequency offset estimation.
  • Phase rotation value and time offset estimation phase rotation value, the phase rotation compensator is used for phase compensation of the next frequency domain OFDM symbol in the frequency domain OFDM symbol sequence by using the frequency offset estimation phase rotation value and the time offset estimation phase rotation value, after compensation
  • the frequency domain OFDM symbols are set to the current frequency domain OFDM symbols.
  • the subcarrier hard decider generates a reduced information subcarrier symbol using a maximum likelihood hard decision.
  • the demodulation process of the receiver front end is as follows:
  • the received signal is sent to an analog-to-digital converter through an analog circuit to convert from an analog signal to a digital signal.
  • the downsampler downsamples the digital signal from the analog-to-digital converter and reduces its sampling frequency to the frequency required for baseband demodulation.
  • Frequency offset compensation and symbol timing are the initial synchronization effects.
  • the frequency offset compensation reduces the frequency offset effect of the received signal by virtue of the frequency offset value estimated at the frame header training symbol.
  • the symbol timer determines the Fast Fourier Transform (FFT) at the windowing position of each OFDM symbol at the frame header training symbol, that is, performs OFDM symbol division on the received series of digital signals.
  • FFT Fast Fourier Transform
  • the fast Fourier transformer sequentially performs a fast Fourier transform on each of the divided OFDM symbols, and transforms it from the time domain to the frequency domain.
  • the antenna mode is assumed to be SISO; the total number of OFDM symbols tracked is Y. Assuming that the total number of useful subcarriers (information subcarrier, pilot subcarrier (pilot subcarrier), DC subcarrier) is N+1; k represents an index of the useful subcarrier of each OFDM symbol, and the range is
  • the operation process of the device is as follows:
  • y represents the index of the OFDM symbol to be tracked synchronously.
  • Step 2 The phase rotation compensator phase compensates the yth OFDM outputted by the fast Fourier converter, that is, corrects the phase rotation error caused by the frequency offset and the time offset. Its correction algorithm is expressed as:
  • R y (k) represents the kth subcarrier of the yth received OFDM symbol output by the fast Fourier transformer.
  • Equation (1) can be further expressed as:
  • H(k) is expressed as channel estimation information of the kth subcarrier.
  • S y (k) is expressed as the kth subcarrier of the yth transmitted OFDM symbol.
  • ⁇ y (k) a y +k ⁇ b y
  • a y and b y are expressed as a frequency offset phase rotation value and a time offset phase rotation value, respectively.
  • Z y (k) represents the Gaussian white noise of the kth subcarrier.
  • Step 3 After detecting the information subcarrier of the yth OFDM, the detected information subcarrier
  • the subcarrier hard decisioner will be sent to the maximum likelihood hard decision.
  • this communication is single antenna transmission single antenna reception (SISO)
  • SISO single antenna transmission single antenna reception
  • the maximum likelihood method hard decision algorithm is expressed as: Represents the subcarrier symbol that may be sent at the sender; ⁇ is the set of all possible symbols. Restored Expressed as the kth subcarrier symbol of the yth restored OFDM symbol (only information subcarriers).
  • the fourth step the pilot subcarrier generator generates the original pilot subcarrier (pilot subcarrier) of the yth OFDM.
  • the OFDM symbol synthesizer combines the hard-decision information subcarrier with the generated pilot subcarrier to reshape the yth original OFDM symbol (Includes information subcarrier, pilot subcarrier, DC subcarrier).
  • Step 5 The frequency offset and time offset estimator will combine the reshaped original y OFDM symbol Channel estimation information H(k) of the channel estimator, and output of the phase rotation compensator The phase rotation value estimation of the frequency offset and the time offset is performed.
  • the estimation algorithm used here is as follows:
  • a y and b y are expressed as a frequency offset phase rotation amount and a time offset phase rotation amount, respectively.
  • imag ⁇ is expressed as the imaginary part of the value.
  • [ ⁇ ] * is expressed as a complex conjugate of a numerical value.
  • the sixth step the frequency offset phase rotation estimation value and the time offset phase rotation estimation value are respectively sent to respective loop filters for correction.
  • the structure and parameters of the loop filter can be determined according to actual needs.
  • the parameter K p of the second-order loop filter is set to 0.25, and K i is set to be 0.125.
  • Step 7 The frequency offset phase rotation estimated value corrected by the loop filter and the time offset phase estimated rotation value are sent to the phase rotation value calculator for calculation of the kth subcarrier phase rotation value.
  • ⁇ ' y (k) is expressed as a phase rotation estimated value.

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Abstract

本发明公开了一种OFDM系统同步跟踪方法和装置，方法包括步骤：A1、对接收的数字信号进行OFDM符号划分，对划分好的OFDM符号依次进行快速傅里叶变换，依次对频域OFDM符号序列中的每一个频域OFDM符号执行步骤A2到步骤A5；A2、分别提取当前频域OFDM符号里的信息子载波符号、导频符号、直流子载波，对信息子载波符号进行检测和判决生成还原信息子载波符号；A3、还原OFDM符号；A4、对还原OFDM符号进行频偏估计和时偏估计；A5、利用频偏估计相位旋转值和时偏估计相位旋转值对频域OFDM符号序列中的下一个频域OFDM符号进行相位补偿，补偿后的频域OFDM符号设为当前频域OFDM符号，返回步骤A2。

Description

一种OFDM系统同步跟踪方法和装置 技术领域
本发明涉及通信技术领域，具体涉及一种OFDM系统同步跟踪方法和装置，尤其涉及基于802.11a/g/n/ac协议的OFDM系统同步跟踪方法和装置。
背景技术
在802.11a/g/n/ac协议中，OFDM是一种重要的调制解调技术，此技术能够大幅地提升数据传输速率。但是OFDM对于载波频率偏移(频偏)与定时偏移(时偏)十分敏感。频偏和时偏的存在极可能导致接收机的解调性能下降。频偏是由于发送方与接收方载波频率偏差引起的，频偏会破坏OFDM的正交性和旋转相位值，导致接收机的解调性能严重下降，所以在通信过程中通常会在帧头加入OFDM训练符号给接收方估计频偏并消除它。但是在噪声影响下接收帧帧头的初始频偏估计不能保证频偏被准确地估计和被完全地消除，只能把频偏值控制在一定范围内。残余的频偏会继续影响着接收机的解调性能。所以后续的频偏跟踪是非常有必要的。时偏是由于发送方与接收方的晶振频率差异或者多普勒频率漂移所引起的。晶振的差异使抽样时钟的相位漂移，从而导致定时偏差随时间产生变化。时偏对一个系统基本是不可避免的。它会导致OFDM的子载波相位旋转，从而下降接收机的解调性能。所以在帧头进行定时同步后，时偏跟踪也是非常有必要的。在802.11a/g/n/ac OFDM系统里，协议规定了每个OFDM符号里都会有若干个接收机已知的pilot 导频插入到信息子载波当中，以此给接收端做同步跟踪的操作。现有技术利用这些pilot导频进行同步跟踪的方法有不少。但是在噪声影响下，数量有限的pilot导频所估计的频偏值与时偏值的准确性相当有限，不能让解调性能发挥至最佳，另外，现有技术多数是在时域进行频偏值估计和补偿，这就需要使用逆傅里叶变换器，电路实现的复杂度非常高。
发明内容
本发明要解决的技术问题在于，针对现有技术的不足，提供一种OFDM系统同步跟踪方法和装置，克服现有技术802.11协议的OFDM系统采用pilot导频进行同步跟踪，频偏估计值与时偏估计值准确性不高的缺陷，以及在时域进行频偏值估计和补偿，导致电路复杂度高的缺陷。
本发明为解决上述技术问题所采用的技术方案为：
一种OFDM系统同步跟踪方法，包括步骤：
A1、对接收的数字信号进行OFDM符号划分，对划分好的OFDM符号依次进行快速傅里叶变换，将其从时域变换到频域，得到频域OFDM符号序列，依次对频域OFDM符号序列中的每一个频域OFDM符号执行步骤A2到步骤A5；
A2、分别提取当前频域OFDM符号里的信息子载波符号、导频符号、直流子载波，对信息子载波符号进行检测和判决生成还原信息子载波符号；
A3、将当前OFDM符号里的导频符号、直流子载波和还原信息子载波符号合成还原OFDM符号；
A4、对还原OFDM符号进行频偏估计和时偏估计，获取相应的频偏估计相位旋转值和时偏估计相位旋转值；
A5、利用频偏估计相位旋转值和时偏估计相位旋转值对频域OFDM符号序列中的下一个频域OFDM符号进行相位补偿，补偿后的 频域OFDM符号设为当前频域OFDM符号，返回步骤A2。
根据本发明的实施例，所述步骤A2包括步骤：采用最大似然法硬判决生成还原信息子载波符号。
根据本发明的实施例，所述步骤A4包括步骤：利用回路滤波器对频偏估计相位旋转值和时偏估计相位旋转值进行纠正，计算纠正后新的频偏估计相位旋转值和时偏估计相位旋转值。
根据本发明的实施例，所述步骤A4包括步骤：回路滤波器包括频偏回路滤波器和时偏回路滤波器，频偏回路滤波器用于对频偏估计相位旋转值进行纠正，时偏回路滤波器用于对时偏估计相位旋转值进行纠正。
一种OFDM系统同步跟踪装置，包括符号定时器、快速傅立叶变换器、相位旋转补偿器、信息子载波提取器、信道估计器、检测器、导频子载波提取器、子载波硬判决器、OFDM符号合成器、频偏与时偏估计器、频偏回路滤波器、时偏回路滤波器、相位旋转估计值计算器，符号定时器、快速傅立叶变换器、相位旋转补偿器、信息子载波提取器、检测器、子载波硬判决器、OFDM符号合成器、频偏与时偏估计器、频偏回路滤波器、相位旋转估计值计算器依次相连，相位旋转补偿器、信道估计器、检测器依次相连，信息子载波提取器、导频子载波提取器、OFDM符号合成器依次相连，符号定时器用于对接收的数字信号进行OFDM符号划分，快速傅立叶变换器用于对划分好的OFDM符号依次进行快速傅里叶变换，将其从时域变换到频域，得到频域OFDM符号序列，信道估计器用于根据经所述相位旋转补偿器透传的训练序列进行信道估计，得到信道估计值，信息子载波提取器用于提取当前频域OFDM符号里的信息子载波符号，导频子载波提取器用于提取当前频域OFDM符号里的导频符号、直流子载波，检测器用于对信息子载波符号进行检测，子载波硬判决器用于对信息子载波符号进行判决并生成还原信息子载波符号， OFDM符号合成器用于将当前OFDM符号里的导频符号、直流子载波和还原信息子载波符号合成还原OFDM符号，频偏与时偏估计器用于对还原OFDM符号进行频偏估计和时偏估计，获取相应的频偏估计相位旋转值和时偏估计相位旋转值，频偏回路滤波器用于对频偏估计相位旋转值进行纠正，时偏回路滤波器用于对时偏估计相位旋转值进行纠正，相位旋转估计值计算器用于计算相应的频偏估计相位旋转值和时偏估计相位旋转值，相位旋转补偿器用于利用频偏估计相位旋转值和时偏估计相位旋转值对频域OFDM符号序列中的下一个频域OFDM符号进行相位补偿，补偿后的频域OFDM符号设为当前频域OFDM符号。
根据本发明的实施例，子载波硬判决器采用最大似然法硬判决生成还原信息子载波符号。
实施本发明的技术方案，具有以下有益效果：本发明采用信息子载波硬判决反馈和pilot子载波结合的方式估计OFDM符号的频偏与时偏相位旋转值，大大提高了估计的准确性，并在频域处理部分给下一个OFDM符号提供相位旋转补偿，整个同步跟踪流程不只提高了跟踪的准确度，而且降低了电路时序实现的复杂度。
附图说明
下面通过参考附图并结合实例具体地描述本发明，本发明的优点和实现方式将会更加明显，其中附图所示内容仅用于对本发明的解释说明，而不构成对本发明的任何意义上的限制，在附图中：
图1为802.11a/g/n/ac OFDM符号子载波分布示意图；
图2为本发明装置示意图；
图3为本发明回路滤波器示意图。
具体实施方式
如图1、图2和图3所示，本发明OFDM系统同步跟踪方法，包括步骤：
A1、对接收的数字信号进行OFDM符号划分，对划分好的OFDM符号依次进行快速傅里叶变换，将其从时域变换到频域，得到频域OFDM符号序列，依次对频域OFDM符号序列中的每一个频域OFDM符号执行步骤A2到步骤A5；
A2、分别提取当前频域OFDM符号里的信息子载波符号、导频符号、直流子载波，对信息子载波符号进行检测和判决生成还原信息子载波符号；
A3、将当前OFDM符号里的导频符号、直流子载波和还原信息子载波符号合成还原OFDM符号；
A4、对还原OFDM符号进行频偏估计和时偏估计，获取相应的频偏估计相位旋转值和时偏估计相位旋转值；
A5、利用频偏估计相位旋转值和时偏估计相位旋转值对频域OFDM符号序列中的下一个频域OFDM符号进行相位补偿，补偿后的频域OFDM符号设为当前频域OFDM符号，返回步骤A2。
根据本发明的实施例，所述步骤A2包括步骤：采用最大似然法硬判决生成还原信息子载波符号。所述步骤A4包括步骤：利用回路滤波器对频偏估计相位旋转值和时偏估计相位旋转值进行纠正，计算纠正后新的频偏估计相位旋转值和时偏估计相位旋转值。回路滤波器包括频偏回路滤波器和时偏回路滤波器，频偏回路滤波器用于对频偏估计相位旋转值进行纠正，时偏回路滤波器用于对时偏估计相位旋转值进行纠正。
本发明OFDM系统同步跟踪装置，包括符号定时器、快速傅立叶变换器、相位旋转补偿器、信息子载波提取器、信道估计器、检测器、导频子载波提取器、子载波硬判决器、OFDM符号合成器、频偏与时偏估 计器、频偏回路滤波器、时偏回路滤波器、相位旋转估计值计算器，符号定时器、快速傅立叶变换器、相位旋转补偿器、信息子载波提取器、检测器、子载波硬判决器、OFDM符号合成器、频偏与时偏估计器、频偏回路滤波器、相位旋转估计值计算器依次相连，相位旋转补偿器、信道估计器、检测器依次相连，信息子载波提取器、导频子载波提取器、OFDM符号合成器依次相连，符号定时器用于对接收的数字信号进行OFDM符号划分，快速傅立叶变换器用于对划分好的OFDM符号依次进行快速傅里叶变换，将其从时域变换到频域，得到频域OFDM符号序列，信道估计器用于根据经所述相位旋转补偿器透传的训练序列进行信道估计，得到信道估计值，信息子载波提取器用于提取当前频域OFDM符号里的信息子载波符号，导频子载波提取器用于提取当前频域OFDM符号里的导频符号、直流子载波，检测器用于对信息子载波符号进行检测，子载波硬判决器用于对信息子载波符号进行判决并生成还原信息子载波符号，OFDM符号合成器用于将当前OFDM符号里的导频符号、直流子载波和还原信息子载波符号合成还原OFDM符号，频偏与时偏估计器用于对还原OFDM符号进行频偏估计和时偏估计，获取相应的频偏估计相位旋转值和时偏估计相位旋转值，频偏回路滤波器用于对频偏估计相位旋转值进行纠正，时偏回路滤波器用于对时偏估计相位旋转值进行纠正，相位旋转估计值计算器用于计算相应的频偏估计相位旋转值和时偏估计相位旋转值，相位旋转补偿器用于利用频偏估计相位旋转值和时偏估计相位旋转值对频域OFDM符号序列中的下一个频域OFDM符号进行相位补偿，补偿后的频域OFDM符号设为当前频域OFDM符号。根据本发明的实施例，子载波硬判决器采用最大似然法硬判决生成还原信息子载波符号。
下面通过具体的实施例来阐述本发明的技术方案。
接收机前端的解调流程如下：
1.接收信号经过模拟电路被送入模数转换器，从模拟信号转换为数字信号。
2.降采样器会把模数转换器输出的数字信号进行降采样，将其采样频率降至基带解调所需要的频率。
3.频偏补偿与符号定时是起初始同步的作用。频偏补偿凭借在帧头训练符号估计的频偏值降低接收信号的频偏影响。符号定时器则在帧头训练符号确定快速傅里叶变换(FFT)在每个OFDM符号的开窗位置，即对接收的一连串数字信号进行OFDM符号划分。
4.快速傅里叶变换器对每一个划分好的OFDM符号依次进行快速傅里叶变换，将其从时域变换到频域。
同步跟踪算法过程
下面将详细地描述本发明的同步跟踪装置如何在接收机解调过程中运作，给接收机提供可靠的同步跟踪支持，在实施例中假设天线模式为SISO；被跟踪的OFDM符号总数量为Y，假设有用子载波(信息子载波，pilot子载波(导频子载波)，DC子载波)总数为N+1；k表示每个OFDM符号有用子载波的索引，表示范围为
Figure PCTCN2017101241-appb-000001
装置的运作流程如下：
第一步：设索引y＝1。y代表要进行同步跟踪的OFDM符号的索引。y＝1则为第一个被同步跟踪的OFDM符号。
第二步：相位旋转补偿器对快速傅里叶转换器输出的第y个OFDM进行相位补偿，即纠正频偏与时偏造成的相位旋转错误。其纠正算法表示为：
Figure PCTCN2017101241-appb-000002
等式中：(1)Ry(k)表示为快速傅里叶变换器输出的第y个接收的OFDM 符号的第k个子载波。(2)
Figure PCTCN2017101241-appb-000003
是第y个OFDM符号的第k个子载波的相位旋转补偿值。(3)
Figure PCTCN2017101241-appb-000004
是被相位旋转补偿后的第y个OFDM符号的第k个子载波。
等式(1)可再进一步表示为：
Figure PCTCN2017101241-appb-000005
等式中：(1)H(k)表示为第k个子载波的信道估计信息。(2)Sy(k)表示为第y个发送的OFDM符号的第k个子载波。(3)
Figure PCTCN2017101241-appb-000006
表示为第y个OFDM符号的第k个子载波的相位旋转值。(4)θy(k)＝ay+k·by，ay与by分别表示为频偏相位旋转值和时偏相位旋转值。(5)Zy(k)表示第k个子载波的高斯白噪声。
当y＝1，
Figure PCTCN2017101241-appb-000007
因此第一个被跟踪的OFDM符号没有相位旋转补偿。
第三步：检测器检测了第y个OFDM的信息子载波后，被检测的信息子载波
Figure PCTCN2017101241-appb-000008
将被送去子载波硬判决器进行最大似然法硬判决。目前，业界存在的检测方法种类很多，而且不同的通信方式都有各自需要的检测方法。在此，由于我们举例本次通信是单天线发送单天线接收(SISO)，检测方法为LS检测，即：
Figure PCTCN2017101241-appb-000009
最大似然法硬判决的算法表示为：
Figure PCTCN2017101241-appb-000010
Figure PCTCN2017101241-appb-000011
代表在发送端可能发送的子载波符号；ξ为所有可能发送的符号的集合。被还原的
Figure PCTCN2017101241-appb-000012
表示为第y个还原的OFDM符号的第k个子载波符号(只有信息子载波)。
假设所有可能发送的符号为以下四个：
Figure PCTCN2017101241-appb-000013
然后，我们会把
Figure PCTCN2017101241-appb-000014
对ξ集合里的四个可能发送的符号分别进行距离计算：
Figure PCTCN2017101241-appb-000015
然后，分别选取最小距离的dy,l(k)对应的
Figure PCTCN2017101241-appb-000016
作为
Figure PCTCN2017101241-appb-000017
的最大似然判决值。最终，得到还原的
Figure PCTCN2017101241-appb-000018
第四步：pilot子载波生成器生成第y个OFDM原有的pilot子载波(导频子载波)。OFDM符号合成器把已经硬判决的信息子载波和生成的pilot子载波进行组合，重新塑造第y个原有的OFDM符号
Figure PCTCN2017101241-appb-000019
(包含信息子载波，pilot子载波，DC子载波)。
第五步：频偏与时偏估计器将结合重新塑造的原有的第y个OFDM符号
Figure PCTCN2017101241-appb-000020
信道估计器的信道估计信息H(k)，以及相位旋转补偿器输出的
Figure PCTCN2017101241-appb-000021
进行频偏与时偏的相位旋转值估计。这里使用的估计算法如下：
Figure PCTCN2017101241-appb-000022
Figure PCTCN2017101241-appb-000023
以上等式(5)、等式(6)中：(1)ay和by分别表示为频偏相位旋转量和时偏相位旋转量。(2)imag{·}表示为取数值的虚部。(3)[·]*表示为数值的 复数共轭。
第六步：频偏相位旋转估计值与时偏相位旋转估计值分别被送到各自的回路滤波器进行纠正。回路滤波器的结构与参数可根据实际需求而定。在这里，二阶回路滤波器的参数Kp设为0.25，Ki设为和0.125。
第七步：被回路滤波器纠正后的频偏相位旋转估计值与时偏相位估计旋转值被送到相位旋转值计算器进行第k个子载波相位旋转值的计算。计算方式表示为：θ'y(k)＝ay+k·by，θ'y(k)表示为相位旋转估计值。
第八步：第k个子载波相位旋转估计值θ'y(k)将与之前的y-1个OFDM符号的第k个子载波的相位旋转估计值进行累加形成
Figure PCTCN2017101241-appb-000024
即：
Figure PCTCN2017101241-appb-000025
然后，送入相位旋转补偿器通过查表方式产生指数形式的相位旋转补偿值，即：
Figure PCTCN2017101241-appb-000026
准备给第y+1个接收的OFDM符号Ry+1(k)进行相位旋转补偿。
第九步：索引y＝y+1。如果y≤Y，则回到第二步，否则算法结束。
本领域技术人员不脱离本发明的实质和精神，可以有多种变形方案实现本发明，以上所述仅为本发明较佳可行的实施例而已，并非因此局限本发明的权利范围，凡运用本发明说明书及附图内容所作的等效结构变化，均包含于本发明的权利范围之内。

Claims (6)

  1. 一种OFDM系统同步跟踪方法，其特征在于，包括步骤：
    A1、对接收的数字信号进行OFDM符号划分，对划分好的OFDM符号依次进行快速傅里叶变换，将其从时域变换到频域，得到频域OFDM符号序列，依次对频域OFDM符号序列中的每一个频域OFDM符号执行步骤A2到步骤A5；
    A2、分别提取当前频域OFDM符号里的信息子载波符号、导频符号、直流子载波，对信息子载波符号进行检测和判决生成还原信息子载波符号；
    A3、将当前OFDM符号里的导频符号、直流子载波和还原信息子载波符号合成还原OFDM符号；
    A4、对还原OFDM符号进行频偏估计和时偏估计，获取相应的频偏估计相位旋转值和时偏估计相位旋转值；
    A5、利用频偏估计相位旋转值和时偏估计相位旋转值对频域OFDM符号序列中的下一个频域OFDM符号进行相位补偿，补偿后的频域OFDM符号设为当前频域OFDM符号，返回步骤A2。
  2. 根据权利要求1所述的OFDM系统同步跟踪方法，其特征在于，所述步骤A2包括步骤：采用最大似然法硬判决生成还原信息子载波符号。
  3. 根据权利要求2所述的OFDM系统同步跟踪方法，其特征在于，所述步骤A4包括步骤：利用回路滤波器对频偏估计相位旋转值和时偏估计相位旋转值进行纠正，计算纠正后新的频偏估计相位旋转值和时偏估计相位旋转值。
  4. 根据权利要求3所述的OFDM系统同步跟踪方法，其特征在于， 所述步骤A4包括步骤：回路滤波器包括频偏回路滤波器和时偏回路滤波器，频偏回路滤波器用于对频偏估计相位旋转值进行纠正，时偏回路滤波器用于对时偏估计相位旋转值进行纠正。
  5. 一种OFDM系统同步跟踪装置，其特征在于：包括符号定时器、快速傅立叶变换器、相位旋转补偿器、信息子载波提取器、信道估计器、检测器、导频子载波提取器、子载波硬判决器、OFDM符号合成器、频偏与时偏估计器、频偏回路滤波器、时偏回路滤波器、相位旋转估计值计算器，符号定时器、快速傅立叶变换器、相位旋转补偿器、信息子载波提取器、检测器、子载波硬判决器、OFDM符号合成器、频偏与时偏估计器、频偏回路滤波器、相位旋转估计值计算器依次相连，相位旋转补偿器、信道估计器、检测器依次相连，信息子载波提取器、导频子载波提取器、OFDM符号合成器依次相连，符号定时器用于对接收的数字信号进行OFDM符号划分，快速傅立叶变换器用于对划分好的OFDM符号依次进行快速傅里叶变换，将其从时域变换到频域，得到频域OFDM符号序列，信道估计器用于根据经所述相位旋转补偿器透传的训练序列进行信道估计，得到信道估计值，信息子载波提取器用于提取当前频域OFDM符号里的信息子载波符号，导频子载波提取器用于提取当前频域OFDM符号里的导频符号、直流子载波，检测器用于对信息子载波符号进行检测，子载波硬判决器用于对信息子载波符号进行判决并生成还原信息子载波符号，OFDM符号合成器用于将当前OFDM符号里的导频符号、直流子载波和还原信息子载波符号合成还原OFDM符号，频偏与时偏估计器用于对还原OFDM符号进行频偏估计和时偏估计，获取相应的频偏估计相位旋转值和时偏估计相位旋转值，频偏回路滤波器用于对频偏估计相位旋转值进行纠正，时偏回路滤波器用于对时偏估计相位旋转值进行纠正，相位旋转估计值计算器用于计算相应的频偏估计相位旋转值和时偏估计相位旋转值，相位旋转补偿器用于利用频偏估计相位旋转 值和时偏估计相位旋转值对频域OFDM符号序列中的下一个频域OFDM符号进行相位补偿，补偿后的频域OFDM符号设为当前频域OFDM符号。
  6. 根据权利要求5所述的OFDM系统同步跟踪装置，其特征在于：子载波硬判决器采用最大似然法硬判决生成还原信息子载波符号。
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