CN101277290B - A frequency synchronization method and device for an orthogonal frequency division multiplexing system - Google Patents

Abstract

The present invention is a method and device for frequency synchronization of an OFDM system, wherein: the transmitting end forms a training symbol with a cyclic prefix and a plurality of data symbols added with a cyclic prefix and inserted with a pilot code into a normal Cross-frequency division multiplexing signal frame, and send the OFDM signal; the receiving end correlates the cyclic prefix of the training symbol of the received OFDM signal with the part of the training symbols corresponding to the cyclic prefix to obtain Fractional frequency offset; remove the cyclic prefix of each OFDM symbol in the received OFDM signal, and slide the training symbol of the received OFDM signal with the original training symbol The differential correlation operation obtains the integer multiple frequency offset; the correlation operation is performed on the pilot codes of at least two consecutive data symbols of the received OFDM signal to obtain the residual frequency offset.

Description

A frequency synchronization method and device for an orthogonal frequency division multiplexing system

technical field

The invention relates to the field of broadband wireless access, in particular to a frequency synchronization technology of an Orthogonal Frequency Division Multiplexing (OFDM) system, specifically a frequency synchronization method and device for an Orthogonal Frequency Division Multiplexing (OFDM) system.

Background technique

Orthogonal Frequency Division Multiplexing (OFDM) system has been widely used as a modulation and multiple access technique for more than 20 years, and it is a well-known transmission technique with high spectral efficiency, which is able to counteract the severe channel degradation. OFDM technology achieves the elimination of high-dispersion wireless channels at a relatively low cost by dividing a single high-rate bit stream into multiple lower-rate bit streams modulated on different subcarriers. Most of the inter-symbol interference caused by high-speed transmission is eliminated.

However, there are several outstanding issues that need special consideration when implementing the OFDM system, and one of the most critical issues is the synchronization issue at the receiving end of the OFDM system. Due to the characteristics of OFDM, the system must satisfy both time synchronization and frequency synchronization. It is well known that OFDM systems are very sensitive to carrier frequency offsets caused by inconsistencies between the transmitter crystal oscillator and the receiver crystal oscillator. Integer multiples) and fractional frequency offsets (less than half the subcarrier spacing). The offset of the carrier frequency destroys the orthogonality between the carriers, so the performance of the system is greatly degraded. The purpose of frequency synchronization is to reduce the frequency deviation between the crystal oscillator of the transmitter and the crystal oscillator of the receiver to within the range allowed by the OFDM system.

The method of frequency synchronization is generally divided into two parts: integer multiple frequency offset estimation and fractional multiple frequency offset estimation. The integer multiple frequency offset can only be estimated after the multiple frequency offset compensation. There are many technical documents related to frequency synchronization in the prior art, and US Patent 6,959,050, US Patent 6,993,094 and the following documents are hereby specially cited:

Shrenik Patel, Leonard J. Cimini, Jr. and Bruce McNair, "Comparison of Frequency Offset Estimation Techniques for Burst OFDM", IEEE55th VTC Spring, Vol.2, 6-9May2002, pp.772-776,

Michael Speth, Stefan A. Fechtel, "Optimum Receiver Design for Wireless Broad-bandsystems using OFDM-Part I", IEEE Trans.On Comm., Vo.47, No.11, Nov.1999,

Michael Speth, Stefan A. Fechtel, "Optimum Receiver Design for Wireless Broad-bandsystems using OFDM-Part II", IEEE Trans.On Comm., Vo.49, No.4, Apr.2001,

Thierry Pollet, Paul Spruyt and Marc Moeneclaey, "The BER performance of OFDMsystems using Non-Synchronized Sampling", IEEE Globe Telecommunications Conference, Vol.1, 28Nov.-2Dec.pp.253-257, 1994,

Hanli Zhou, Bruce McNair, "Anintegrated OFDM receiver for high speed mobile datacommunication", the disclosed content is incorporated here as the prior art document of the present invention.

Contents of the invention

The object of the present invention is to provide a frequency synchronization method and device for an OFDM system, to overcome the deterioration of system performance caused by frequency out-of-synchronization, and to provide an Frequency synchronization scheme for frequency offset caused by inconsistency. Technical scheme of the present invention is:

An OFDM frequency synchronization method, the transmitting end forms an OFDM signal frame together with a training symbol with a cyclic prefix and a plurality of data symbols added with a cyclic prefix and inserted with a pilot code, and Send out the OFDM signal; the receiving end performs a correlation operation on the cyclic prefix of the training symbol of the received OFDM signal and the part of the training symbol corresponding to the cyclic prefix to obtain a fractional frequency offset; remove the received The cyclic prefix of each OFDM symbol in the OFDM signal, and the training symbol of the received OFDM signal and the original training symbol are subjected to a sliding differential correlation operation to obtain an integer multiple frequency offset ; Perform a correlation operation on the pilot codes of at least two consecutive data symbols of the received OFDM signal to obtain a residual frequency offset.

The training symbol is a modulation sequence with good cross-correlation and autocorrelation; and, when a training symbol and a data symbol added with a cyclic prefix are jointly framed, the training symbol is at the beginning of the frame beginning part.

The OFDM signal carrying the pilot code refers to inserting the pilot code into the sub-carrier of the data symbol.

The related operations of fractional frequency offset include: normalized estimated frequency offset value

in:

${\stackrel{<span\; class="notranslate">\; ^</span>}{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}}_{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arg</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; fft</span>}}}\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; fft</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; fft</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; *</span><span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}$

where N _fft refers to the size of the FFT;

G is the ratio of the cyclic prefix length to the useful symbol length;

x(n) refers to the nth sample point of the training symbol;

arg(x) refers to the angle at which the complex variable x is computed.

Fast Fourier transform is performed on the OFDM signal with the cyclic prefix removed.

The related operations of integer multiple frequency offset include: the estimated value of integer multiple frequency offset

in:

${\stackrel{<span\; class="notranslate">\; ^</span>}{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}}_{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; I</span>}}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; \&Delta;\; f</span>}<span\; class="notranslate">\; *</span>\underset{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; arg</span>}\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; |</span>\underset{\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; x</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; Y</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; Y</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span>$

Wherein, X(l) refers to the original training symbols of the transmitter framing;

Y(m, l) refers to the received training symbols with m sample point offset;

m=0, ±1, ±2, L, ±f _I . f _I refers to the estimated range of the normalized integer multiple frequency offset;

l=0...P-2.P represents the length of the training symbols;

Δf refers to the subcarrier spacing.

The correlation operation of residual frequency offset includes: normalized estimated frequency offset value

in:

${\stackrel{<span\; class="notranslate">\; ^</span>}{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}}_{\mathrm{<span\; class="notranslate">\; fr</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arg</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}\underset{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}$

Wherein, N _p is the number of pilot codes used for frequency tracking in each data symbol;

p _l (k) and p _m (k) refer to the pilot code on the mth data symbol and the kth subcarrier of the first data symbol respectively.

The present invention also provides an OFDM frequency synchronization device, which includes: a transmitter and a receiver; the transmitter is used to combine a training symbol with a cyclic prefix with a plurality of symbols added with a cyclic prefix and the data symbols inserted with the pilot code jointly form an OFDM signal frame, and send the OFDM signal; the receiver receives the OFDM signal; the receiving The machine includes: a fractional multiple frequency offset estimation unit, which is used to correlate the cyclic prefix of the training symbol of the received OFDM signal with the part of the training symbols corresponding to the cyclic prefix to obtain the fractional multiple frequency offset; The prefix removal unit is used to remove the cyclic prefix of each OFDM symbol in the received OFDM signal; the integer multiple frequency offset estimation unit is used to use the received OFDM signal A sliding differential correlation operation is performed between the training symbol and the original training symbol to obtain an integer multiple frequency offset; the residual frequency offset estimation unit is used to perform a correlation operation on the pilot codes of at least two consecutive data symbols of the received OFDM signal The residual frequency offset is obtained; the frequency synchronization scheme in the present invention includes two processes: coarse frequency synchronization and fine frequency synchronization. Coarse frequency synchronization refers to frequency acquisition to eliminate integer multiple frequency offset and fractional multiple frequency offset. Fine frequency synchronization refers to frequency tracking to eliminate phase errors caused by residual frequency errors and sampling frequency offsets. Specifically, it can be divided into three parts: the estimation of fractional frequency offset, the estimation of integer frequency offset and the tracking of residual frequency offset. The present invention avoids the limitations of the prior art and provides a new frequency synchronization solution.

Description of drawings

Fig. 1 is the structural block diagram of the OFDM system of the specific embodiment of the present invention;

Fig. 2 is a structural block diagram of a receiver according to a specific embodiment of the present invention;

Fig. 3 is a schematic diagram of the frame structure of the present invention;

Fig. 4 is the structural block diagram of the fractional frequency offset estimation unit of the specific embodiment of the present invention;

Fig. 5 is a structural block diagram of an integer multiple frequency offset estimation unit according to a specific embodiment of the present invention;

Fig. 6 is a schematic diagram of frequency tracking in a specific embodiment of the present invention.

Detailed ways

The specific implementation manner of the present invention will be described below in conjunction with the accompanying drawings. The invention provides a method and device for estimating and compensating the frequency offset caused by the inconsistency between the transmitter crystal oscillator and the receiver crystal oscillator in an OFDM communication system. The signal of the OFDM communication system has a frame structure composed of a training symbol and a plurality of OFDM data symbols, and the OFDM data symbols are inserted with pilot codes on some subcarriers.

The schematic diagram of a typical OFDM system shown in FIG. 1 is a preferred embodiment of the present invention. In the OFDM transmitter 100, the data source 101 produces a series of constellation point symbols, and the output of the data source unit 101 performs data mapping in the data mapping and pilot code insertion unit 102, and performs subcarrier allocation and pilot code insertion . Inverse Fast Fourier Transformer 103 performs inverse Fast Fourier Transform (IFFT) on the input signal and generates OFDM symbols. In order to avoid inter-symbol interference (ISI) caused by multipath fading, the cyclic prefix module 104 is added to each OFDM symbol. A fixed-length cyclic prefix is added to the beginning part. Usually, the last part of the OFDM symbol is used to generate a cyclic prefix. The length of the cyclic prefix generally depends on the characteristics of the channel where the OFDM signal is transmitted.

In order to generate the frame structure shown in FIG. 3 , a complete OFDM frame is generated in the framing module 105 , and then the signal is sent to the analog-to-digital converter 106 for D/A conversion, and then sent to the transmitter front end 107 . Thereafter, the OFDM signal is transmitted through the antenna and travels through the wireless channel 110 .

Fig. 3 is applicable to the frame structure of the typical OFDM system of the present invention, described frame is made up of a plurality of OFDM symbols 302, and each described symbol 302 is made up of useful information data 304 and fixed-length cyclic prefix 303, Usually, the cyclic prefix is generated from the last part of its corresponding OFDM symbol. As shown in FIG. 3 , a training symbol is added to the beginning of each frame, and the training symbol is a training sequence with good cross-correlation and auto-correlation.

119 in Fig. 1 is a receiver. First, receive the OFDM signal at the front end 111 of the receiver, then carry out analog-to-digital conversion in the analog-to-digital converter 112 for the OFDM signal received, and deframe the received signal in the deframing module 113. Before deframing, Time synchronization must first be realized, and this step is implemented in unit 114 and the synchronization information is fed back to the deframing module 113, and the training symbols are taken out from the deframing module 113 for performing in the fractional multiplier frequency offset estimation unit 109 Estimation of fractional octave frequency offset. The cyclic prefix removal operation is performed on each received OFDM symbol in the cyclic prefix removal unit 114 . The signal is then subjected to a Fast Fourier Transform 115 . The symbol after transformation is first carried out the estimation of integer multiple frequency offset in integer multiple frequency offset estimating unit 108, meanwhile, extracts the pilot code of at least two continuous described OFDM data symbols and carries out frequency tracking 120, and this frequency tracking method will A detailed explanation is given in FIG. 6 . Finally, the frequency-corrected OFDM symbols are sent to the data demapping unit 116 to recover a series of constellation point symbols and sent to the data output unit 117 for output.

As shown in Figure 2, frequency synchronization includes frequency offset estimation and compensation. The data received from the RF front end 111 is first sent to the analog/digital conversion unit 112 for analog/digital conversion, and the time synchronization unit 118 is used to realize symbol/frame synchronization. After time synchronization is realized, the fractional frequency offset The estimation unit 201 uses the cyclic prefix of the training symbol to realize the estimation of the fractional octave frequency offset, and the specific details of the fractional octave frequency offset estimation are introduced in FIG. 4 . Then, after fast Fourier transform is performed on the received signal, the integer multiple frequency offset estimation unit 202 uses the training symbols to realize integer multiple frequency offset estimation. The specific details of integer multiple frequency offset estimation are introduced in FIG. 5 . Then, the outputs of unit 201 and unit 202 are combined in a combining unit 205 to compensate the frequency offset, and finally the residual frequency offset is tracked 203 to eliminate the residual frequency offset.

The estimation of the fractional octave frequency offset will be described in detail below. In the present invention, the fractional octave frequency offset is estimated by using the training symbols in the frame structure shown in FIG. 3 . As shown in Figure 4, first the training symbol extraction unit 401 extracts the training symbol, assuming that the nth sample point of the training symbol is x(n). The unit 402 is a delay device, and the signal is delayed by N _fft after passing through the 402 unit N _fft here refers to the size of the Fast Fourier Transform (FFT), and then the cyclic prefix of the training sequence is correlated with the training sequence delayed by N _fft samples in the correlator 403, And calculate angle in seeking angle module 404, obtain the estimated angle value as follows:

${\stackrel{<span\; class="notranslate">\; ^</span>}{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}}_{\mathrm{<span\; class="notranslate">\; f</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arg</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; fft</span>}}}\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; G</span>}<span\; class="notranslate">\; \&times;</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; fft</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; fft</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; *</span><span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}$

Wherein, δ _f =Δf/fs, Δf is the true fractional frequency offset, f _s is the subcarrier spacing, and G is the ratio of the cyclic prefix to the useful OFDM symbol length. Based on the above conditions, the normalized fractional octave frequency offset can be estimated. The estimation range of this fractional multiple frequency offset estimation method is ±f _s /2, and the frequency offset exceeding this range is regarded as an integer multiple frequency offset and obtained through integer multiple frequency offset estimation.

The estimation of integer times frequency offset is introduced below. As shown in FIG. 5 , the initial training symbol of each frame is usually used to estimate the integer multiple frequency offset. First, the training symbol extraction module 501 extracts training symbols from the received signal, the training symbols contain the frequency offset to be estimated, and the sliding window control unit 502 performs sliding extraction on the above-mentioned extracted training symbols at a certain interval and outputs a set of For sequences with different starting positions, the sliding interval is usually one subcarrier. The range of the sliding window generally depends on the required integer multiple frequency offset range. The starting position of the extraction is not fixed, and generally can be set to the required integer multiple frequency offset range. point. Each group of sequences after sliding extraction is respectively carried out differential correlation operation in differential correlator 503 in order to eliminate the influence of multipath fading, and the operation in differential correlator 503 can be represented by following formula:

R _Y (m, l) = Y (m, l)Y ^* (m, (l+1))

Wherein, Y(m, l) is the extracted sequence with m sample point offset;

l=0...P-2.P refers to the length of the training symbols;

The original training symbol generator 504 produces the original training sequence sent, and its output is sent to another differential correlator 505 to perform a differential correlation operation, which can be represented by the following formula:

T _X (l)=X(l)X ^* (l+1)

Among them, X(l) is the original training sequence.

Afterwards, both the differential correlator 503 and the output of the differential correlator 503 are sent to the correlator 506 to perform a correlation operation and generate a series of data C _m , which can be expressed as follows:

${\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; P</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; Y</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; )</span>$

Among them, m=0, ±1, ±2, L, ±f _I , f _I refers to the range of the estimated integer multiple frequency offset;

The selector 507 selects the maximum value in C _m as an estimated value and uses the following formula to perform an integer multiple frequency offset The calculation of:

${\widehat{\mathrm{\&delta;}}}_{f_I}=\mathrm{\&Delta;f}*S,$ in $S=\underset{m}{\arg \max }\left|C_m\right|,$ Δf refers to the subcarrier spacing.

The purpose of frequency tracking is to further eliminate the residual frequency offset after fractional and integer frequency offset compensation. The pilot code in OFDM symbols is very suitable for frequency offset tracking because tracking requires continuous execution in real time, while in OFDM Of the data symbols, only the pilot code meets this requirement. The frequency tracking method will be specifically introduced below in conjunction with FIG. 6 .

As shown in Figure 6, at first, the pilot code extracting unit 601 extracts the pilot code in the OFDM data symbol, and the basis of extraction is to realize according to the preset position in the pilot pattern table 602, and the extracted pilot code After passing through the delayer 603, m*N _fft samples are delayed, wherein, m is an integer greater than or equal to 1. Correlating the current pilot code and the pilot code delayed by m*N _fft samples in the correlator 604 for N _p samples, and converting it into a specific angle value through the angle calculation module 605 . In the frequency offset compensation module 606, the estimated angle value is used to perform frequency compensation.

可以表示如下：Assuming that the pilot codes on the kth subcarrier of the mth OFDM symbol and the first OFDM symbol are p _l (k) and p _m (k) respectively, then the estimated residual frequency offset

Can be expressed as follows:

${\stackrel{<span\; class="notranslate">\; ^</span>}{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}}_{\mathrm{<span\; class="notranslate">\; fr</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; arg</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}\underset{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{{\mathrm{<span\; class="notranslate">\; N</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}$

Wherein, N _p is the total number of pilot subcarriers available for frequency offset tracking in each OFDM symbol.

The frequency synchronization scheme in the present invention includes two processes: coarse frequency synchronization and fine frequency synchronization. Coarse frequency synchronization refers to frequency acquisition to eliminate integer multiple frequency offset and fractional multiple frequency offset. Fine frequency synchronization refers to frequency tracking to eliminate phase errors caused by residual frequency errors and sampling frequency offsets. Specifically, it can be divided into three parts: the estimation of fractional frequency offset, the estimation of integer frequency offset and the tracking of residual frequency offset. The present invention avoids the limitations of the prior art and provides a new frequency synchronization solution.

The above specific embodiments are only used to illustrate the present invention, but not to limit the present invention.

Claims (14)

1. OFDM frequency synchronization method, transmitting terminal has the training symbol of Cyclic Prefix and a plurality of data symbol mutual group that has increased Cyclic Prefix and the inserted pilot code frequency-division multiplex singal frame that is orthogonal with one, and sends this orthogonal frequency-division multiplex singal; It is characterized in that:

Receiving terminal carries out related operation with the Cyclic Prefix that part of training symbol corresponding with this Cyclic Prefix of the training symbol of the orthogonal frequency-division multiplex singal that receives, and to obtain the mark frequency multiplication inclined to one side;

The Cyclic Prefix of each OFDM symbol in the orthogonal frequency-division multiplex singal that remove to receive, and the training symbol of the orthogonal frequency-division multiplex singal that receives and the original training symbol difference related operation that slides obtained integer frequency offset;

The pilot code of at least two continuous data symbols of the orthogonal frequency-division multiplex singal that receives is carried out related operation obtain residual frequency deviation.

2. method according to claim 1 is characterized in that, described training symbol is the modulation sequence with good cross correlation and autocorrelation; And,

When having increased the common framing of data symbol of Cyclic Prefix, described training symbol is at the start-up portion of this frame with a training symbol.

3. method according to claim 1 is characterized in that, the described orthogonal frequency-division multiplex singal that is loaded with pilot code is meant: described pilot code is inserted on the subcarrier of described data symbol.

4. method according to claim 1 is characterized in that, the inclined to one side related operation of mark frequency multiplication comprises: the frequency deviation value of the estimation of normalization

Wherein:

${\widehat{\mathrm{\&delta;}}}_f=\arg (\frac{1}{G\&times;N_{\mathrm{fft}}}\underset{n=0}{\overset{G\&times;N_{\mathrm{fft}}-1}{\mathrm{\&Sigma;}}}x(n+N_{\mathrm{fft}})\&CenterDot;x{\left(n\right)}^{*})/2\mathrm{\&pi;}$

N wherein _FftBe meant the size of FFT;

G is meant the ratio of circulating prefix-length and useful symbol lengths;

X (n) is meant n sampling point of training symbol;

Arg (x) is meant the angle of calculated complex variable x.

5. method according to claim 1 is characterized in that, the orthogonal frequency-division multiplex singal of having removed Cyclic Prefix is carried out fast fourier transform.

6. method according to claim 1 is characterized in that, the related operation of integer frequency offset comprises: the estimated value of integer frequency offset

Wherein:

${\widehat{\mathrm{\&delta;}}}_{f_I}=\mathrm{\&Delta;f}*\underset{m}{\arg \max }\left|\underset{l=0}{\overset{P-2}{\mathrm{\&Sigma;}}}X\left(l\right)X^{*}(l+1)Y(m,l)Y^{*}(m,(l+1))\right|$

Wherein, X (l) is meant the original training symbol of transmitting terminal framing;

(m l) is meant the training symbol that the skew of m sampling point is arranged of reception to Y;

M=0, ± 1, ± 2 ..., ± f _If _IBe meant the estimation range of the integer frequency offset of normalization;

L=0...P-2; P represents the length of training symbol;

Δ f is meant subcarrier spacing.

7. method according to claim 1 is characterized in that, the related operation of residual frequency deviation comprises: the frequency deviation value of the estimation of normalization

Wherein:

${\widehat{\mathrm{\&delta;}}}_{\mathrm{fr}}=\arg \left(\frac{1}{N_p}\underset{k=0}{\overset{N_p-1}{\mathrm{\&Sigma;}}}{p_l}^{*}\left(k\right)p_m\left(k\right)\right)/2\mathrm{\&pi;}$

Wherein, N _pIt is the number that is used for the pilot code of frequency-tracking in described each data symbol;

p _l(k) and p _m(k) be meant pilot code on k the subcarrier of m data symbol and the 1st data symbol respectively.

8. OFDM frequency synchronization device, this device comprises: transmitter and receiver; Described transmitter is used for having the training symbol of Cyclic Prefix and a plurality of data symbol mutual group that has increased Cyclic Prefix and the inserted pilot code frequency-division multiplex singal frame that is orthogonal with one, and sends this orthogonal frequency-division multiplex singal; Described receiver receives this orthogonal frequency-division multiplex singal; It is characterized in that: described receiver comprises:

The inclined to one side estimation unit of mark frequency multiplication, the Cyclic Prefix that part of training symbol corresponding with this Cyclic Prefix that is used for the training symbol of the orthogonal frequency-division multiplex singal that will receive carry out related operation, and to obtain the mark frequency multiplication inclined to one side;

Cyclic prefix removal unit is used for removing the Cyclic Prefix of each OFDM symbol of the orthogonal frequency-division multiplex singal of reception;

The integer frequency offset estimation unit is used for the training symbol of the orthogonal frequency-division multiplex singal that will receive and the original training symbol difference related operation that slides and obtains integer frequency offset;

The residual frequency deviation estimation unit, the pilot code that is used at least two continuous data symbols of the orthogonal frequency-division multiplex singal that will receive is carried out related operation and is obtained residual frequency deviation.

9. device according to claim 8 is characterized in that, described training symbol is the modulation sequence with good cross correlation and autocorrelation; And,

When having increased the common framing of data symbol of Cyclic Prefix, described training symbol is at the start-up portion of this frame with a training symbol.

10. device according to claim 8 is characterized in that, described transmitter comprises: pilot code is inserted the unit, is used for pilot code is inserted into the subcarrier of described data symbol.

11. device according to claim 8 is characterized in that, the inclined to one side estimation unit of described mark frequency multiplication comprises: the frequency deviation value of the estimation of normalization

Wherein:

${\widehat{\mathrm{\&delta;}}}_f=\arg (\frac{1}{G\&times;N_{\mathrm{fft}}}\underset{n=0}{\overset{G\&times;N_{\mathrm{fft}}-1}{\mathrm{\&Sigma;}}}x(n+N_{\mathrm{fft}})\&CenterDot;x{\left(n\right)}^{*})/2\mathrm{\&pi;}$

N wherein _FftBe meant the size of FFT;

G is meant the ratio of circulating prefix-length and useful symbol lengths;

X (n) is meant n sampling point of training symbol;

Arg (x) is meant the angle of calculated complex variable x.

12. device according to claim 8 is characterized in that, described receiver also comprises: fast Fourier transform unit, the orthogonal frequency-division multiplex singal that is used for after cyclic prefix removal unit is handled carries out fast fourier transform.

13. device according to claim 8 is characterized in that, described integer frequency offset estimation unit comprises: the estimated value of integer frequency offset

Wherein:

${\widehat{\mathrm{\&delta;}}}_{f_I}=\mathrm{\&Delta;f}*\underset{m}{\arg \max }\left|\underset{l=0}{\overset{P-2}{\mathrm{\&Sigma;}}}X\left(l\right)X^{*}(l+1)Y(m,l)Y^{*}(m,(l+1))\right|$

Wherein, X (l) is meant the original training symbol of transmitting terminal framing;

(m l) is meant the training symbol that the skew of m sampling point is arranged of reception to Y;

M=0, ± 1, ± 2 ..., ± f _If _IBe meant the estimation range of the integer frequency offset of normalization;

L=0...P-2; P represents the length of training symbol;

Δ f is meant subcarrier spacing.

14. device according to claim 8 is characterized in that, described residual frequency deviation estimation unit comprises: the frequency deviation value of the estimation of normalization

Wherein:

${\widehat{\mathrm{\&delta;}}}_{\mathrm{fr}}=\arg \left(\frac{1}{N_p}\underset{k=0}{\overset{N_p-1}{\mathrm{\&Sigma;}}}{p_l}^{*}\left(k\right)p_m\left(k\right)\right)/2\mathrm{\&pi;}$

Wherein, N _pIt is the number that is used for the pilot code of frequency-tracking in described each data symbol;

p _l(k) and p _m(k) be meant pilot code on k the subcarrier of m data symbol and the 1st data symbol respectively.

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