Frequency synchronizing device for ofdm/cdma system

Description: TECHNICAL FIELD [Detailed description of the invention]

[Technical field to which the invention belongs]

【0001】

The present invention relates to a modulation / demodulator in an Orthogonal Frequency Division Multiplexing / Code Division Multiple Access (OFDM / CDMA) system, and particularly to a frequency synchronizer in the time domain in an OFDM / CDMA system. is there.

【0002】

[Conventional technology]

In general, OFDM technology includes digital transmission such as Digital Audio Broadcastiong (DAB) and digital television, Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). Widely applied in technology. The OFDM method is a multi-carrier technology in which the data to be transmitted is divided into several pieces, modulated, and then transmitted in parallel. However, although not widely used due to structural complexity, recently Fast Fourier Transfom (FFT) and Inverse FFT: Hereafter, it will be possible with the development of various digital signal processing technologies including IFFT). The OFDM method is similar to the conventional FDM, but above all, it has a feature that the optimum transmission efficiency can be obtained at the time of high-speed data transmission by maintaining the orthogonality between the subcarriers and transmitting the data. Recently, various realization technologies such as OFDM / TDMA and OFDM / CDMA that use the OFDM method when transmitting high-speed data such as WATM have been proposed as such advantages.

【0003】

FIG. 1 is a block diagram of a general OFDM / CDMA system, and then the MC (Multi-Carrier) -CDMA system of the OFDM / CDMA system will be briefly described with reference to the same diagram.

【0004】

Reference number 100 is the transmitter of the MC-CDMA system and reference number 120 is the receiver. These transmitter 100 and receiver 120 are applied equally not only in the forward direction but also in the reverse direction.

【0005】

First, the configuration and operation of the transmitter 100 will be described. The spreader 101 spreads and outputs transmission data using a orthogonal code of length N and a PN spreading sequence. Usually, in an OFDM / CDMA system, the N is 256. If the transmitter 100 is a forward transmitter, the diffuser 101 includes a diffuser for subscriber classification and a diffuser for base station classification, and if it is a reverse transmitter, the diffuser 101. Is a diffuser that includes a channel diffuser and a diffuser for user classification. Hereinafter, each of N pieces of data is defined as chip data. Each chip data diffused by each of the diffusers 101 is input to the adder 102 (not shown) by inserting a pilot signal before inputting to the adder 102. The chip data that is added in units of chip data by the adder 102 and output in series is input to the series / parallel converter 103. The series / parallel converter 103 receives the chip data output from the adder 102 and outputs it in parallel. At this time, the number of chip data output in parallel is N, or it may not be. Hereinafter, it will be described as N in the description. Then, each of the parallelized sample data is input to the inverse fast Fourier transform (IFFT) 104. IFFT104, which receives the input of N parallelized sample data, outputs the chip data in OFDM modulation. In other words, the IFFT104 performs an inverse fast Fourier transform on each of the chip data and outputs them in parallel on different subcarriers having orthogonality in the frequency domain. The subcarrier is output in the time domain by IFFT104. The data output from IFFT104 is defined as sample data, and N sample data are defined as OFDM symbols.

【0006】

The sample data output in parallel is input to the parallel / series converter 105. The parallel / series converter 105 that receives this outputs the sample in series. The parallel / series converter 105 inserts and outputs a guard interval in units of N sample data, that is, in units of one OFDM symbol. This protection section is data obtained by copying a part of the sample data at the end of the OFDM symbol composed of N sample data, and is inserted at the tip of the OFDM symbol. The length of this protection section must be longer than the impulse response length. The transmission filter 106 filters the data output from the parallel / series converter 105 and transmits it to the radio channel 107 through the RF unit (not shown). This radio channel 107 shows the case of a white Gaussian channel, whereby white Gaussian noise is added by the adder 109.

【0007】

The receiver 120 receives a carrier wave containing white Gaussian noise through the white Gaussian channel. The received carrier wave is converted into a baseband signal through the RF unit and input to the multiplier 110. The multiplier 110 inputs a predetermined frequency correction signal, compensates for a frequency error generated on the channel 107, and outputs the signal. The ADC (Analog to Digital Converter) 115 receives an analog signal compensated by the multiplier 110, converts it into an OFDM symbol of digital data, and outputs it. The serial / parallel converter 111 receives the OFDM symbols in series and outputs N sample data constituting the OFDM symbols in parallel. The Fast Fourier Transform Device (FFT) 112 inputs each sample data in parallel, converts each subcarrier to the original sample data in the frequency domain, and outputs it to the parallel / series converter 113. The parallel / series converter 113 converts the sample data input in parallel in series and outputs it to the despreader 114. The despreader 114 receives the input of the sample data in series, despreads it, restores the original received data, and outputs it.

【0008】

Generally, in a transmission system using such an OFDM, if the local oscillator of the transmitter / receiver is not tuned, a frequency offset will occur, and such a frequency offset will lose the orthogonality between the subcarriers. To. In such cases, even a small offset can be a serious cause of poor performance of the receiving system. Therefore, it is indispensable to realize a frequency synchronization technology that maintains orthogonality between subcarriers in OFDM / CDMA WATM transmission technology that uses OFDM.

【0009】

The frequency synchronization technique normally used in the receiver of an OFDM system is carried out in two stages, wide frequency synchronization (Coarse Synchronization) and fine frequency synchronization (Fine Synchronaization). The first wide frequency synchronization technology is the process of eliminating the initial frequency offset corresponding to an integral multiple of the subcarrier spacing, and the second fine frequency synchronization removes the residual frequency offset of an integral multiple or less remaining after the initial synchronization. It is a process to do.

【0010】

Among the technologies related to this, there is a method proposed by Classen & Myer and Nogammi & Nagashima for wide frequency synchronization technology.

【0011】

Figures 2 to 4 show the frequency synchronization device at the receiving end using the wide frequency synchronization technology and the fine frequency synchronization technology.

【0012】

First, among the wide frequency synchronization technologies, the technology proposed by Classen & Myer will be described with reference to FIG. Classen & Myer uses the test correction frequency to estimate the frequency offset by obtaining the correlation value between the data already known at the time of transmission and the received data while moving the test correction frequency by a certain frequency interval. It utilizes the property that the correlation value is maximized when the test correction frequency is closest to the actual frequency offset transitioned in the actual channel. FIG. 2 is a block diagram for a method for detecting a test correction frequency offset.

【0013】

Explaining with reference to FIG. 2 below, the multiplier 128 inputs the test correction frequency and compensates for the frequency offset of the received signal and outputs it. The ADC 129 converts the received data received in the form of an analog signal into the form of a digital signal and outputs it. The protection section remover 122 is a protection section (Guard) from the received received data. Output excluding Interval). This protection section removal method sets the length of two OFDM symbols and one protection section in a window, and while moving this window one sample at a time with two OFDM symbols and one protection section, the correlation value is calculated and the maximum is obtained. Only the length of the protected section is removed with the point where the value starts to be output as the start point of the protected section. The FFT124 performs a fast Fourier transform to demodulate the sample data output from the multiplier 128 and outputs it to the chip data in stream form. The chip data is input to the despreader, the delayer 125, and the estimator 127. The delay device 125 delays this chip data for a time of one chip data length and then outputs it to the estimation unit 127. The reference tone pattern generation unit 126 generates a reference tone having a predetermined pattern that the base station and the terminal know each other, and outputs the reference tone to the estimation unit 127.

【0014】

The estimation unit 127 inputs the chip data output from the FFT 124, the chip data delayed through the delay device 125, and the reference tone of a predetermined pattern output from the reference tone pattern generation unit 126, and the estimated frequency offset f._e^ (Hereafter, f_e^ Is f_eIt is assumed that a symbol with "^" is added above "" is output. That is, the estimation unit 127 uses the chip data of two consecutive sub-channels and the correlation value of the reference tone to perform an estimated frequency offset. f_eOutput ^. This estimated frequency offset f_e^ Is an element that determines the test correction frequency.

【0015】

This estimation unit 127 obtains the estimated frequency offset by the following mathematical formula 4.

[Number 4]

<img file="JPP3428965B2_D0001.tif" />f_e^ Indicates the estimated frequency offset amount, Z_{l, k}And Z_{l + 1, k}Indicates the chip data of consecutive subcarriers, X_{l, k}Is a data symbol string already known at the time of transmission, and s is the frequency shift amount for synchronous estimation. l indicates the index of the sample data, and k indicates the index of the OFDM symbol. It can be seen that two consecutive chip data in the above mathematical formula 4 are in the same OFDM symbol.

【0016】

Second, the method proposed by Nogammi &amp; Nagashima will be explained with reference to Fig. 3. The ADC 131 converts the received data received in the form of an analog signal into sample data in the digital form and outputs the data. The protection section remover 133 divides the frame of the received sample data, removes the protection section for preventing interference between the sample data from the received data, and outputs the frame. The FFT135 performs a fast Fourier transform on the sample data output from the ADC 131 and outputs it as chip data in the form of a stream. The received data output from the FFT135 is input to the despreader and correlator 139. The reference tone pattern generator 137 generates a reference tone in a predetermined pattern and outputs it to the correlator 139. The correlator 139 receives the input of the chip data output from the FFT 135 and the reference tone output from the reference tone pattern generator 137, and the estimated frequency offset f._eOutput ^.

【0017】

The wide synchronization technology proposed by Nogammi &amp; Nagashima does not use the correlation value of the reference tone for two consecutive sample data, but uses the correlation value of one sample data and the reference tone that the receiver already knows. So, it has a difference from Classen &amp; Myer's technology.

【0018】

The estimated frequency offset by this Nogammi &amp; Nagashima technique is calculated by the following mathematical formula 5.

[Number 5]

<img file="JPP3428965B2_D0002.tif" />【0019】

Then, there are the fine frequency synchronization technology proposed by Dafara &amp; Adami and the fine frequency synchronization technology proposed by Moose.

【0020】

The technology proposed by Dafara &amp; Adami proposed a method of acquiring fine frequency synchronization using the transmitted signal property. That is, when there is no frequency offset, the signal in the protected section of the received signal and the original signal are the same, and when there is a frequency offset, the signal in the protected section and the original signal have different phases due to the frequency offset. , When the signal in the protected section is multiplied by the original signal, the resulting imaginary part takes advantage of the property of having information about the frequency offset. The proposed method is a technique for removing the residual frequency offset by using such a property.

【0021】

A block configuration for such fine frequency synchronization will be described with reference to FIG. The band filter 141 filters the received data input in analog form into the band desired by the system and outputs it. The multiplier 143 inputs the filtered reception data and a predetermined test correction frequency, corrects the fine frequency offset, and outputs the data. The ADC 145 converts the received data in the analog form in which the frequency offset output from the multiplier 143 is corrected into the OFDM frame data in the digital form and outputs the data. The protection section remover 153 receives the OFDM frame output from the ADC 145, removes the protection section included in the OFDM frame, and outputs the OFDM symbol. The FFT155 parallelizes the OFDM symbols output from the protection section remover 153 into N sample data, and executes the fast Fourier transform to output the N sample data as N chip data.

【0022】

The frequency detection unit 147 detects a frequency error in order to compensate for the minute frequency offset. The frequency detection unit 147 can use the following two methods for detecting a frequency error.

【0023】

The first method is to detect the frequency error through the a path by using the protected section. More specifically, the frequency detection unit 147 receives the OFDM frame output from the ADC 145 and detects the protection section from the OFDM frame. The detected protection section is compared with the sample data section copied to generate the protection section in the pure sample data, and a frequency error is detected.

【0024】

The second method using the b-path uses fast Fourier-transformed chip data. If frequency errors are detected by this method, a carrier extractor 157 must be provided. The carrier wave extractor 157 detects the pilot chip data inserted according to a certain rule in the chip data string output from the FFT 155 and provides it to the frequency detector 147. Then, the frequency detector 147 compares the signal known in advance with the pilot chip data to detect the frequency error.

【0025】

Method a is a method proposed by Dafara &amp; Adami, which detects a protection section from the received data in digital form output from ADC145 and outputs a fine estimated frequency offset, which is obtained by mathematical formula 6.

[Number 6]

<img file="JPP3428965B2_D0003.tif" />Here, N is the number of samples of the OFDM symbol, and I is the number of samples of the protected interval.

【0026】

Then, b is a method proposed by Moose, which inputs a pilot signal from the FFT155 and the carrier extractor 157 and outputs a fine estimated frequency offset, and is calculated by mathematical formula 7.

[Number 7]

<img file="JPP3428965B2_D0004.tif" />Where L is the number of samples used to estimate the frequency error.

【0027】

The fine estimated frequency offset obtained by the frequency detector 147 through the a path or the b path is input to the Voltage Controlled Oscillator (VCO) 151 through the low frequency filter 149. This VCO 151 receives a fine estimated frequency offset, generates a test correction frequency, and outputs it to the multiplier 143.

【0028】

Guard Interval Based (GIB) The fine frequency synchronization technology through the a path is performed at the tip of the FFT of the receiver, and the b path is based on the pilot signal contained in the original received data. The difference is that the fine frequency synchronization technique (or Maximum Likelihood Optimization: MLE) through is performed at the trailing edge of the FFT.

【0029】

And the test correction frequency offset is f_eIf ^, the baseband received signal Z (t) is [Number 8]

<img file="JPP3428965B2_D0005.tif" />In this case, the GIB algorithm always has a phase difference between two samples. [Number 9]

<img file="JPP3428965B2_D0006.tif" />However, the MLE algorithm has a phase difference between two samples. [Number 10]

<img file="JPP3428965B2_D0007.tif" />Is affected by the length of the protected section.

【0030】

As described above, the conventional wide frequency synchronization technique has a disadvantage that it is difficult to guarantee the performance at the time of realization because it is sensitive to channel noise.

【0031】

In the case of fine frequency synchronization technology, it is locked to the subcarrier closest to the position of the residual frequency offset. However, when the residual frequency offset amount is approximately ± 0.5 of the subcarrier spacing, the conventional fine frequency synchronization technique has a fatal problem that synchronization is not acquired.

【0032】

In the conventional method using the data at the rear end of the fast Fourier transform, the delay time for synchronization acquisition becomes longer due to the processing at the rear end of the fast Fourier transform, and the time required for synchronization acquisition becomes longer after all. It has the problem of becoming.

【0033】

[Problems to be Solved by the Invention]

Therefore, an object of the present invention is to provide a frequency synchronization device that performs frequency synchronization using only time domain signals in a frequency synchronization device of an orthogonal frequency division multiple access / code division multiple access system.

【0034】

Another object of the present invention is to provide a frequency synchronization device capable of performing frequency synchronization in three stages of wide, narrow, and fine in the time domain of an orthogonal frequency division multiple access / code division multiple access system to obtain accurate synchronization. To do.

【0035】

[Means for solving problems]

To achieve such an object, the present invention uses an OFDM symbol composed of a plurality of data samples and an OFDM frame including a protection section inserted at the tip of each symbol to prevent interference between the symbols. In the frequency synchronizer of the orthogonal frequency division multiplex / code division multiplex connection (OFDM / CDMA) system that converts the data, the frequency corrector that compensates for the frequency offset of the analog data received by the frequency correction signal and the reception An analog / digital converter for converting the obtained analog data into the OFDM frame, wide, narrow, and fine frequency offset values are estimated, and the frequency correction signal is output to the frequency corrector based on the estimated frequency offset values. Provided is a frequency synchronization device for an OFDM / CDMA system, which comprises a frequency synchronization unit.

【0036】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In adding reference numerals to the components of each drawing shown below, the same reference numerals are used for the same components as much as possible. In the description of the present invention, if there is a risk that the specific description of the related known function or configuration may obscure the gist of the present invention, the detailed description thereof will be omitted.

【0037】

FIG. 5 is a block configuration diagram of the frequency synchronization device of the OFDM / CDMA system according to the embodiment of the present invention.

【0038】

Referring to FIG. 5, the band filter 160 inputs the received data of the analog form signal to perform band filtering. The frequency correction unit 161 receives the band-filtered received data and the first frequency correction signal or the second frequency correction signal, compensates the received data with a predetermined control signal, and compensates the frequency offset with the first and second frequency correction signals. Output. The ADC 162 receives the received data in the analog form in which the frequency offset is compensated, converts it into the sample data in the digital form, and outputs the data. The sample data in this digital form is input to the protected section remover 163. The protection section remover 163 inputs this sample data and outputs it excluding the protection section inserted in OFDM frame units. The frequency synchronization unit 200 inputs the signal output from the ADC 162 and estimates the estimated frequency offsets for wide, narrow, and fine frequency synchronization, and the first and second frequency correction signals based on the estimated estimated frequency offsets. Is output to the frequency correction unit 161.

【0039】

Hereinafter, each frequency synchronization method of wide (coarse), narrow (regular), and fine (fine) will be specifically described.

【0040】

First, to explain the wide frequency synchronization method, the control unit 195 controls the general operation according to the present invention. In particular, the control unit 195 outputs a wide frequency delay signal in order to carry out the initial wide frequency synchronization, outputs a narrow delay signal when the wide frequency synchronization occurs, and outputs a fine delay signal when the narrow frequency synchronization occurs. The delay device 164 delays the OFDM frame output from the ADC 162 for a certain period of time and outputs it. This delay time is the time of one OFDM frame length. The protected section / carrier extractor 166 inputs the OFDM frame output from the ADC 162 and the OFDM frame output from the delay device 164, and the portion of the protected section and the original OFDM symbol copied to generate the protected section. (Hereinafter, copy data) is extracted. The mobile index generator 165 inputs an initial wide delay signal from the control unit 195 and outputs an integer mobile index, and outputs a 1/10 unit mobile index when a small delay signal is input. The wide frequency synchronization unit 180 includes a correlation value detector 167, a MIN / MAX (Minimum / Maximum) detector 168, and an adder 169. The correlation value detector 167 inputs the protection section and the copy data from the protection section / carrier extractor 166, receives the movement index value from the movement index generator 165, and moves the protection section and the copy data to an integral multiple, that is, a sample. The correlation value is detected and output to the MIN / MAX detector 168 while moving in data units. The MIN / MAX detector 168 receives a delay signal from the control unit 195, detects the maximum value or the minimum value of the correlation value input by the MIN / MAX detector 168 from the delay signal, and outputs the delay signal. In wide frequency synchronization, it receives a wide delay signal from the control unit 195, detects the minimum value, and outputs it. At this time, the output signal is a wide estimation signal. Further, when the minimum value is detected, the MIN / MAX detector 168 notifies the control unit 195 of this.

【0041】

This means that if there is a frequency offset, the received signal will move as a whole, and noise will be injected into the protection band due to this effect. Using this property, the wide frequency offset is estimated by the protection section / carrier extractor 166, the correlation value detector 167, and the MIN / MAX detector 168. The estimated frequency offset calculation method for wide frequency synchronization using the power detection in the protection band in the time domain according to the present invention is calculated by the following mathematical formula 11.

[Number 11]

<img file="JPP3428965B2_D0008.tif" />Where i is the size of the sliding window, K_MINAnd K_MAXIs the minimum and maximum number of subcarriers of the FFT, Z_lIndicates a symbol.

【0042】

Secondly, to explain the narrow frequency synchronization method, the protection section / carrier wave extractor 166 inputs the OFDM frame output from the ADC 162, extracts the protection section and the copy data, and outputs it to the correlation value detector 167. To do. The correlation value detector 167 receives the protection section and the copy data from the protection section / carrier extractor 166, and inputs the movement index at 1/10 interval of the sample data length from the movement index 165 to sample the detected protection section. The correlation value is detected and output while moving the data and the sample data of the same copy data as this sample data to 1/10 width. The MIN / MAX detector 168 receives a small delay signal from the control unit 195, detects the correlation value having the maximum power among the correlation values detected by the correlation value detector 167, and sets the frequency at this time to the small estimated frequency offset. Is output as a small-width estimation signal. The estimated frequency offset by narrow frequency synchronization is calculated by mathematical formula 12.

[Number 12]

<img file="JPP3428965B2_D0009.tif" />Where f_trialIs the forced test correction frequency, the range of which is greater than 0 and less than 1. For the time being, if the interval of the compulsory test correction frequency is selected, the small frequency offset amount is determined while increasing the interval in unit intervals. Z indicates sample data in the protected section, and X indicates copy data.

【0043】

When the wide frequency offset and the narrow frequency offset are estimated, the MIN / MAX detector 168 outputs the wide estimation signal and the narrow estimation signal. The adder 169 adds the wide estimation signal and the narrow estimation signal and outputs them to the frequency correction unit 161. Hereinafter, the signal obtained by adding the wide estimation signal and the narrow estimation signal will be referred to as a first frequency correction signal.

【0044】

Thirdly, to explain the fine frequency synchronization method, the protection section / carrier wave extractor 166 inputs the OFDM frame output from the ADC 162, extracts the protection section and the copy data from this OFDM frame, and outputs the protection section / copy data to the frequency detector 170. Output. The fine frequency synchronization method is performed after wide and narrow frequency synchronization, and the GIB frequency synchronization method described in FIG. 4 is used. The fine frequency synchronization unit 190 includes a frequency detector 170, a low frequency filter 171, and a VCO 172. That is, the frequency detector 170 inputs the sample data of the protection section output from the protection section / carrier wave extractor 166 under the control of the control unit 195 and the sample data of the same copy data as this sample data. The phase difference of the sample data is detected, this is determined as a fine frequency offset, and it is output to the VCO 172 through the low frequency filter 171. The low frequency filter 171 and VCO172 operate under the control of the control unit 195. The VCO172 inputs a fine frequency offset, generates a second frequency correction signal, and outputs the second frequency correction signal to the frequency correction unit 161. The mathematical formula for calculating the fine frequency offset is achieved by the following mathematical formula 13.

[Number 13]

<img file="JPP3428965B2_D0010.tif" />【0045】

At this time, the frequency correction unit 161 receives the control of the control unit 195 and receives the first frequency correction signal output from the adder 169 and the second frequency correction signal output from the VCO 172 to correct the frequency offset of the received signal. ..

【0046】

As described above, the present invention is strong in noise characteristics because it carries out OFDM transmission, and performs frequency synchronization in three stages of wide, narrow, and fine in the time domain, and is approximately 1/2 of the width carrier interval in the frequency domain. Since the frequency synchronization of the above can be obtained, more accurate synchronization can be guaranteed, and there is an effect that the performance of the receiver can be improved.

[A brief description of the drawing] [Figure 1]

It is a block block diagram of an OFDM / CDMA system.

[Figure 2]

It is a block block diagram of the wide frequency synchronization apparatus in a general OFDM / CDMA system.

[Fig. 3]

It is a block block diagram which shows another Embodiment of the wide frequency synchronization apparatus in a general OFDM / CDMA system.

[Fig. 4]

It is a block block diagram of the fine frequency synchronization apparatus in a general OFDM / CDMA system.

[Fig. 5]

It is a block block diagram of the frequency synchronization apparatus of the OFDM / CDMA system receiver by embodiment of this invention.

[Explanation of symbols]

160 band filter 161 Frequency correction section 162 ADC 163 Protected section remover 164 Delayer 165 Mobile index generator 166 Protected section / carrier extractor 167 Correlation value detector 168 MIN / MAX detector 169 adder 170 frequency detector 171 Low frequency filter 172 VCO 180 Wide frequency synchronizer 190 Fine frequency synchronizer 195 Control unit 200 frequency synchronizer