Apparatus and method for frequency synchronization in ofdm system

The present invention relates to methods and devices for transmitting data in an orthogonal frequency division multiplexing system, and in particular, methods and devices for obtaining frequency synchronization using orthogonal frequency division multiplexing symbols for data transmission. Regarding.

Orthogonal Frequency Division Multiplexing (hereinafter referred to as OFDM) technology is a 4th generation (4G) modulation technology that is expected to be adopted as the standard for digital TV in Europe, Japan and Australia. .. This OFDM technology was initially encouraged as a wireless LAN (Local Area Network) technology, and in recent years it has been developed as a cellular system for wireless Internet services by adding mobility to the OFDM-based wireless LAN technology. ..

OFDM band-spreading technology distributes data across multiple subcarriers at exactly the same frequency interval. Such frequency intervals provide "orthogonality" within the technique of preventing the demodulator from referencing another frequency that is not his own. In addition, OFDM technology is a type of multi-carrier modulation method, and exhibits excellent performance in a multi-path mobile reception environment. Therefore, OFDM technology has been attracting attention as a modulation method suitable for terrestrial digital TV and digital audio broadcasting. This OFDM technology has been studied mainly in the field of communication, but it was adopted as a modulation method for the Digital Audio Broadcast (DAB) system proposed by the EBU (European Broadcasting Union), and accordingly in the field of broadcasting. Has been researched and developed.

FIG. 1 is a block diagram showing the structure of a transmitter / receiver in the physical layer of a general OFDM system. Referring to FIG. 1 above, the input bitstream to be transmitted is first transmitted through the encoder 111 to the Serial / Parallel: S / P converter 112. Then, the serial / parallel transformer 112 collects N symbols and transmits them to the inverse fast Fourier transform (hereinafter referred to as IFFT) 113, and the transformor transfers the above symbols from the symbols in the frequency region. It is converted into a time region symbol and transmitted to the parallel / serial converter 114, and the parallel / serial converter 114 converts the parallel time region symbol into a serial symbol. Here, the N data symbols collected above are referred to as "OFDM symbols". The time domain symbols obtained by the parallel / serial converter 114 are cyclic prefixes (Cyclic) in units of OFDM symbols in order to eliminate the influence of multi-path channels. Prefix: CP) CP is added by the adder 115, converted from the digital region to the analog region signal by the digital / analog converter (116), and then transmitted to the receiving side via the channel 120.

When the transmission signal is received via the channel 120, the receiver 130 first converts the received signal from the analog region to the digital region signal by the analog / digital converter 131, and the CP remover 132 multipaths. Remove CP from OFDM symbols contaminated by the effects of. The received signal from which the CP has been removed in this way is converted into a signal in the frequency domain by passing through the serial / parallel converter 133 and then the fast Fourier transform (FFT) device 134. The received symbol converted to the frequency domain passes through the equalizer 135 for canceling the influence of channel interference, then through the parallel / serial converter 136, and finally through the decoder 137, and then the output bitstream at the receiving end. Is output as.

FIG. 2 is a diagram showing data symbols transmitted in a general OFDM system in time and frequency units. As shown in FIG. 1 above, in the OFDM system, the data symbols in one OFDM symbol are transmitted on N carrier waves in units of N. The N data symbols transmitted through the N carrier waves constitute one OFDM symbol 201, and the M OFDM symbols form one frame 202. The start symbol of the frame 202 generally includes a pilot symbol for frequency synchronization, channel estimation, and the like, and preambles, control information, and the like are transmitted through the pilot symbol.

This type of OFDM system exhibits excellent performance in a mobile reception environment and also has high frequency band utilization efficiency, but the distance between subcarriers that are orthogonal to each other is kept narrow, and a single system is used. It is relatively vulnerable to frequency offsets compared to carrier systems.

Hereinafter, an example of orthogonality between subcarriers in such an OFDM system will be described with reference to the waveform diagram of FIG. By referring to the waveform diagram of FIG. 3 showing the three subcarriers, it can be seen that the data information is transmitted using the frequencies fn-1301, the frequency fn302, and the frequency fn + 1303 that are adjacent to each other. .. The data information transmitted using the respective frequencies 301 to 303 is transmitted as a sinusoidal signal, and the first frequency signal 304, the second frequency signal 305, and the third frequency signal 306 are accurate secondary signals. Since they are located at the frequency position of the carrier wave, they do not interfere with each other.

FIG. 4 is a diagram illustrating interference that occurs between subcarriers when a frequency offset is present in a general OFDM system. If there is a frequency offset of only Δf401 with respect to the exact frequency of each subcarrier, the receiver will not be able to capture the exact frequency position of each subcarrier from the received signal and will demodulate at a position offset by Δf401. Take a data sample for. Therefore, interference occurs between the first subcarrier signal 402, the second subcarrier signal 403, and the third subcarrier signal 404 of the three subcarriers shown in FIG. For example, the signal sample 405 having a frequency offset of only Δf401 with respect to the second subcarrier signal 403 is interfered with by the first subcarrier signal 407 and the third subcarrier signal 406 at the frequency position. As described above, the OFDM system has a narrow frequency interval between the subcarriers due to the orthogonality between the subcarriers, and is affected by a relatively large interference with respect to the frequency offset described above.

In the conventional initial frequency synchronization method for correcting the frequency offset in such an OFDM system, the initial frequency synchronization is synchronized using two pilot OFDM symbols. The initial frequency synchronization in the conventional method corresponds to the first stage of performing detailed frequency synchronization (correcting the frequency offset within twice the subcarrier band) first and a multiple of the band having a band twice wider than the subcarrier band. It is carried out by the second step of resolving the frequency ambiguity for the part to be used.

FIG. 5 is a diagram showing a configuration example of a pilot OFDM symbol by a conventional acquisition method of initial frequency synchronization in an OFDM system. The first pilot OFDM symbol 501 is for acquiring detailed frequency synchronization (hereinafter, first frequency synchronization) as the first process of frequency synchronization acquisition, and is a value only on the even-numbered subcarriers in the frequency domain. And has a "0" value on the odd-numbered subcarriers. The first pilot OFDM symbol 501 is formed by repeating pilot signals having the same half-symbol length in the time domain. The first frequency synchronization acquisition process is a process of obtaining a fractional part of the frequency offset.

The second pilot OFDM symbol 502 is for resolving frequency ambiguity, which is the second process of initial frequency synchronization acquisition, and this process will be referred to as the second frequency synchronization acquisition process. The second pilot OFDM symbol 502 has values on all subcarriers. The second frequency synchronization acquisition process is a process of obtaining an integer part of the frequency offset.

That is, the total frequency offset is composed of a fractional part of the frequency offset within twice the subcarrier band and an integer part of the frequency offset represented by a multiple of twice the subcarrier band. It can be expressed as.<maths num="1"><img file="JP4672014B2_D0001.tif" /></maths> In Equation 1 above, Δf represents the total frequency offset, φ represents the fractional part of the frequency offset, and T represents the symbol length. Further, g represents an integer part of a frequency offset corresponding to an integral multiple of twice the subcarrier band.

If one data symbol is represented by an FFT of magnitude N, the received symbol signal w (t) having a frequency offset can be expressed by the following equation 2.<maths num="2"><img file="JP4672014B2_D0002.tif" /></maths>

If one received data symbol located in the first half OFDM symbol of the first pilot symbol is w (t0), the received data located in the second half OFDM symbol at the same position as the first half OFDM symbol above. If the symbol is w (t0 + T / 2), the following relation is established.<maths num="3"><img file="JP4672014B2_D0003.tif" /></maths>

Taking the phase of the correlation coefficient of the half OFDM symbol length repeated in the above equation 3, the relation between the fractional part φ of the frequency offset and the total frequency offset Δf is established as shown in the following equation 4.<maths num="4"><img file="JP4672014B2_D0004.tif" /></maths> That is, the fractional part of the frequency offset can be estimated by estimating the phase by taking the correlation coefficient of the repeating part of the first pilot symbol.

In the conventional frequency offset estimation method, it is suggested to use the function of the following equation 5 in order to improve the estimation accuracy.<maths num="5"><img file="JP4672014B2_D0005.tif" /></maths>

If it can be guaranteed that the absolute value of the initial frequency offset is within the range smaller than the subcarrier band, the total frequency offset can be estimated by the following equation 6.<maths num="6"><img file="JP4672014B2_D0006.tif" /></maths>

However, in reality, the initial value of the frequency offset cannot be guaranteed to be within the range smaller than the subcarrier band, and there is ambiguity that is a multiple of twice the subcarrier band.

Therefore, in order to finally determine the frequency offset, it is necessary to resolve the ambiguity of the integer part of the frequency offset corresponding to the integer g in the above equation 1, and for that purpose, the second pilot symbol is used. First, in the first frequency synchronization acquisition process, estimation is performed for a portion corresponding to a fractional portion of the frequency offset. Then, only the portion corresponding to 2 g / T at the frequency offset remains.

X the frequency conversion value of each of the 1st pilot symbol and the 2nd pilot symbol_{1, l}And X_{2, k}Then, the second pilot symbol is determined so that the frequency conversion value of the even-numbered subcarrier of the second pilot symbol and the differentially modulated value of the frequency conversion value of the first pilot symbol have a specific pattern. Then, in order to determine the g value which is an integer part of the frequency offset, in the conventional method, if possible, the difference between the first and second pilot symbols and the correlation coefficient of the predetermined pattern are obtained with respect to the g value, and the correlation coefficient is obtained. The final value is "g", which represents the value with the maximum correlation coefficient. Through such a process, the frequency offset is finally estimated.

FIG. 6 is a block diagram showing a transmitter / receiver configuration in the physical layer for obtaining initial frequency synchronization in a conventional OFDM system. In transmitter 610 of FIG. 6, pilot bits are converted to time domain by IFFT 613 through serial / parallel converter 612 for transmission through the first and second OFDM symbols in each frame as described above. , Furthermore, it is converted to serial by the parallel / serial converter 614. The pilot bits converted in this way are cyclically prefixed by the cyclic prefix (CP) inserter 615, converted from a digital signal to an analog signal by the digital / analog converter 616, and received via channel 620. It is sent to the machine 630.

The signal received via the channel 620 is first converted from an analog signal to a digital signal by the analog / digital converter 631 and transmitted to the correlator 638. Then, the correlator 638 finds the repeating pattern of the first pilot OFDM symbol for any received signal and calibrates the fractional part of the frequency offset in order to obtain the first frequency synchronization. When the fractional part of the frequency offset is thereby calibrated, the received signal is cyclically stripped by the cyclic prefix remover 632 and converted to a parallel signal by passing through the serial / parallel converter 633 to the FFT device 634. By passing through, it is converted into a signal in the frequency domain. The ambiguity resolution unit 640 resolves the ambiguity of the frequency offset by investigating the correlation coefficient of the differential values of the first and second pilot symbols in the frequency domain in order to obtain the second frequency synchronization, that is, the frequency offset. Calibrate the integer part of to get the final initial frequency synchronization.

The conventional frequency synchronization acquisition method described above is known to be a method capable of obtaining an accurate frequency initial offset. However, the overhead is very high because two OFDM symbols in one frame are used for pilot transmission to calibrate the initial frequency offset in the OFDM-based radio system. Therefore, to solve this, a method has been developed that allows the initial frequency offset to be obtained while using only one pilot OFDM symbol to calibrate the initial frequency offset.

In the method using one pilot OFDM symbol, only the first pilot OFDM symbol is transmitted without transmitting the second pilot OFDM symbol as shown in Fig. 5 above, so the symbol corresponding to the second pilot OFDM symbol has data. Can be transmitted, resulting in reduced overhead compared to the method using two pilot OFDM symbols. In this way, even with the method of acquiring the initial frequency synchronization using only one pilot OFDM symbol, the fractional part of the frequency offset is found. The first frequency fluttering period acquisition process and the integer part of the frequency offset are found. Both of the second frequency synchronization acquisition processes to resolve the ambiguity are carried out. That is, the process of finding the frequency offset in the method of obtaining the initial frequency synchronization using one pilot OFDM symbol is the same as the method using the above equation 6.

However, the method using one pilot OFDM symbol can reduce the system overhead as compared with the method using two pilot OFDM symbols, but when resolving the ambiguity for determining the integer part of the frequency offset, a predetermined number of lines are used. It is assumed that the channel does not change with respect to the subcarrier of. Therefore, the method using one pilot OFDM symbol has a disadvantage that the performance is deteriorated when the initial frequency synchronization is acquired for the channel environment having selectivity in the frequency domain. Therefore, there is an urgent need to develop an alternative that can guarantee the acquisition performance of the initial frequency synchronization while reducing the system overhead.

<p> Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and is a frequency synchronization acquisition method and apparatus capable of reducing the overhead of the system in the process of acquiring the initial frequency synchronization in the OFDM system. The purpose is to provide.</p><p> Another object of the present invention provides methods and devices capable of acquiring frequency synchronization in an OFDM system without transmitting a pilot OFDM symbol.</p><p> Yet another object of the present invention provides methods and devices capable of acquiring frequency synchronization using cyclic prefixes in an OFDM system.</p><p> A further object of the present invention is to provide a method and apparatus capable of obtaining frequency synchronization in the time domain without transmitting a preamble in an OFDM system.</p><p> Yet another object of the present invention provides methods and devices capable of adjusting the data size of a data OFDM symbol to obtain frequency synchronization in an OFDM system.</p>

<p> A method for obtaining frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system according to an embodiment of the present invention to achieve the above object is to set the size of the data symbol of a particular OFDM symbol within the same frame. The size of the data symbol of the general OFDM symbol is set smaller than the size of the data symbol of the general OFDM symbol, and the process of transmitting the specific OFDM symbol and the time interval of the data symbol and the data symbol of the general OFDM symbol after receiving the specific OFDM symbol. Corresponds to the process of obtaining the correlation coefficient of the OFDM symbol based on both the time intervals, the process of estimating the fractional part of the frequency offset by taking each phase of the correlation coefficient, and the fractional part of the frequency offset. It is characterized by including the process of determining the integer portion of the frequency offset, estimating the total frequency offset, and obtaining frequency synchronization.</p><p> In the orthogonal frequency division multiplexing system according to another embodiment of the present invention, the method of transmitting the OFDM symbol of the transmitter for obtaining frequency synchronization is such that the size of the data symbol of a specific OFDM symbol is set in the same frame in each frame. The process of performing inverse fast Fourier transform by setting the size smaller than the size of the data symbol of a general OFDM symbol in the above, and the process of inserting a cyclic prefix (CP) into the data symbol of the above specific OFDM symbol and transmitting it. It is characterized by including.</p><p> In the orthogonal frequency division multiplexing system according to still another embodiment of the present invention, the method of receiving the OFDM symbol of the receiver for obtaining frequency synchronization is a data symbol size smaller than the data symbol size of a general OFDM symbol. The process of receiving a specific OFDM symbol having the above and obtaining the correlation coefficient based on both the time interval of the data symbol and the time interval of the general OFDM symbol, and taking the respective phases of the above correlation coefficient. It is characterized by including a process of estimating a fractional portion of a frequency offset and a process of estimating an entire frequency offset by determining an integer portion of the frequency offset corresponding to the fractional portion of the frequency offset and obtaining frequency synchronization. ..</p><p> An Orthogonal Frequency Division Multiplexing (OFDM) system that obtains frequency synchronization and communicates according to a further embodiment of the present invention sets the data symbol size of a specific OFDM symbol to the data of a general OFDM symbol in the same frame. Set smaller than the size of the symbol and send the specific OFDM symbol, and receive the specific OFDM symbol and set both the time interval of the data symbol and the time interval of the data symbol of the general OFDM symbol. Based on this, the correlation coefficient of the OFDM symbol is obtained, the phase of each of the above correlation coefficients is taken to estimate the fractional part of the frequency offset, and the integer portion of the frequency offset corresponding to the fractional portion of the frequency offset is determined. It is characterized by including a receiver that estimates a frequency offset and acquires frequency synchronization.</p><p> A transmitter that transmits an OFDM symbol to obtain frequency synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) system according to yet another embodiment of the present invention determines the size of the data symbol of a particular OFDM symbol in each frame. A transform unit having at least one IFFT device that is set smaller than the size of the data symbol of a general OFDM symbol in the same frame and performs an inverse Fast Fourier Transform (IFFT), and the data symbol of the specific OFDM symbol described above. It is characterized by including a transmission unit that inserts a cyclic prefix into the data and transmits the data.</p><p> In an Orthogonal Frequency Division Multiplexing (OFDM) system according to yet another embodiment of the present invention, a receiving device that receives an OFDM symbol for obtaining frequency synchronization is a data symbol smaller than the size of the data symbol of a general OFDM symbol. A specific OFDM symbol having a size is received, the correlation coefficient is obtained based on both the time interval of the data symbol and the time interval of the general OFDM symbol, and the respective phases of the correlation coefficient are taken. When the correlation part having at least one correlator for estimating the fractional part of the frequency offset and the fractional part of the frequency offset are estimated, the integer part of the frequency offset corresponding to the fractional part of the frequency offset is determined and all frequencies are determined. It is characterized by comprising an estimation means for estimating an offset and acquiring frequency synchronization.</p>

<p> According to the present invention, the overhead of the system can be reduced in the process of acquiring the initial frequency synchronization in the OFDM system, and the initial frequency synchronization can be acquired without transmitting the pilot OFDM symbol.</p><p> Also according to the present invention, initial frequency synchronization in the time domain is performed by using cyclic prefixes in an OFDM system and by transmitting a specific OFDM symbol having a size smaller than the data size of a general OFDM symbol in each frame. Can be acquired.</p>

Hereinafter, a detailed description will be given with reference to the drawings to which preferred embodiments of the present invention are attached. Then, in explaining the present invention, if it is determined that a specific description of the related known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted.

First, to explain the basic concept of the present invention, the present invention does not use two pilot OFDM symbols to obtain initial frequency synchronization, but one OFDM symbol per frame. Is divided into, for example, an N1 size FFT smaller than the FFT size N of a general OFDM symbol, or two (or two or more) FFTs having N1 and N2 sizes. It is a proposal of a new method that can acquire.

That is, the transmitter in the present invention is arranged so that the FFT size of the OFDM symbol that transmits data is different in each frame, and when the receiver receives the OFDM symbol, the receiver is based on the time interval of the N data symbol. Find the correlation coefficient between the click prefix and the data in the OFDM symbol, and in the cyclic prefix and the OFDM symbol, for example, based on the time interval of the N1 data symbol or both based on the time interval of the N1 and N2 data symbols. Find the correlation coefficient between the data of. Then, after obtaining the first frequency synchronization by taking the phase with respect to the correlation coefficient exceeding a predetermined threshold, the second frequency synchronization for resolving the ambiguity of the frequency offset is acquired to capture the initial frequency synchronization. It is possible to capture the initial frequency synchronization without transmitting a special preamble or pilot OFDM symbol.

Here, the process of acquiring the first frequency synchronization is the process of determining the fractional part of the frequency offset, and the process of acquiring the second frequency synchronization is the process of determining the integer part of the frequency offset. ..

Hereinafter, a detailed description will be given with reference to the drawings attached with embodiments of the present invention for obtaining initial frequency synchronization. FIG. 7 is a block diagram showing a configuration of a transmitter / receiver for obtaining initial frequency synchronization in an OFDM system according to an embodiment of the present invention. Referring to FIG. 7, the transmitter 710 includes a commonly configured encoder 711, a serial / parallel converter (S / P) 712, a parallel / serial converter (P / S) 714, and cyclic prefix insertion. It is equipped with a device 715 and a digital / analog converter (D / A) 716. In the conventional method, the pilot bit is transmitted for frequency synchronization, but in the transmitter 710 of the present invention, the data bit is transmitted without transmitting a separate pilot bit. The transmitter 710 can apply an IFFT to the data bits and applies an IFFT to the OFDM symbol in a size smaller than N (for example, N1 and / or N2) of the IFFT size of a general OFDM symbol. It is equipped with an IFFT unit 713 that can be used. The serial / parallel converter 712 converts the data bits encoded by the encoder 711 into a parallel signal and transmits it to the IFFT unit 713, and the output of the IFFT unit 713 is transmitted to the parallel / serial converter 714. Will be done.

In the present invention, the IFFT unit 713 includes, for example, a large number of IFFT devices 713a, 713b, 713c having N, N1, and N2 sizes as shown in FIG. The N-size IFFT device 713a performs an N-size IFFT so as to process general OFDM symbols on the input data bits, and the N1 size IFFT device 713b is arranged in each frame. Perform an N1 magnitude IFFT to process OFDM symbols with an FFT magnitude less than the N to be. Further, when at least one OFDM symbol arranged in each frame is divided into two parts having a size of N1 and N2, as shown in FIG. 7, the IFFT unit 713 is divided into N1 and N2 by the IFFT devices 713b and 713c. Perform size IFFT at the same time.

On the other hand, in the present embodiment, two cases of N1 or N1 and N2 smaller than N as the FFT size of the OFDM symbol are illustrated, but the above-mentioned IFFT portion 713 has an OFDM having two or more FFT portions smaller than N. It can also be configured to handle symbols.

The parallel / serial converter 714 converts the IFFTed transmission data into serial data and inputs it to the cyclic prefix inserter 715. The cyclic prefix inserter 715 inserts a cyclic prefix into the input serial data and inputs it to the digital / analog converter 716 so that it can be converted into an analog signal. The analog signal thus converted is transmitted to the receiver via channel 720.

The receiver 730, which receives the signal transmitted from the transmitter via the channel 720 in this way, converts the received signal into an analog / digital converter (A / D) 731 that converts the received signal into a digital signal, and a digital signal. Resolve the ambiguity of the output correlation value with the correlation unit 738 that receives the converted OFDM symbol, finds the correlation value between the cyclic prefix and the data in the OFDM symbol, and obtains the first frequency synchronization. It is equipped with an ambiguity resolution unit 740 that acquires the second frequency synchronization. It should be noted that in FIG. 7, general components such as the CP remover and the FFT device provided in the receiver of the OFDM system are not shown for convenience of explanation.

Then, when the above-mentioned correlation unit 738 receives the OFDM symbol, the correlation coefficient between the cyclic prefix and the data in the OFDM symbol is based on the time interval of the N data symbol and based on the time interval of the data symbol smaller than N. To obtain the first frequency synchronization that determines the fractional part of the frequency offset by taking the phase with respect to the correlation coefficient exceeding a predetermined threshold.

Referring to FIG. 7, the correlation unit 738 receives an OFDM symbol converted into a digital signal by the analog-to-digital converter (A / D) 731, and the received OFDM symbol has an N magnitude IFFT. The 0th correlator 738a for the applied OFDM symbol, the 1st correlator 738b for the N1 size IFFT-applied OFDM symbol, and N2 when one symbol is divided into two FFT parts. Simultaneously pass through the second correlator 738c for magnitude IFFT. Here, a large number of correlators 738a, 738b, and 738c obtain the correlation coefficient between the cyclic prefix and the data in the OFDM symbol based on the data symbol time intervals of N, N1, and N2, respectively, and the obtained phase. It is confirmed whether the relational number is larger than the preset threshold value, and the fractional part of the frequency offset is estimated by taking the phase with respect to the correlation coefficient exceeding the threshold value.

When one or more fractional parts of the frequency offset are estimated from the correlation unit 738, the correlation unit 738 transmits the correlation coefficient to the ambiguity resolution unit 740, and the ambiguity resolution unit 740 next The frequency synchronization estimation algorithm of the present invention is provided as described above, and the integer part of the frequency offset is determined to obtain the second frequency synchronization. Therefore, the receiver 730 that has acquired both the first and second frequency synchronizations with the above configuration can determine the frequency offset and acquire the initial synchronization without receiving the pilot OFDM symbol.

Hereinafter, the frequency synchronization estimation algorithm of the present invention will be described in more detail. In the embodiment of the present invention, the correlation coefficient between the cyclic prefix and the data in the OFDM symbol is obtained for one OFDM symbol in each frame having an FFT size different from that of a general OFDM symbol, and the initial frequency is determined. Estimate synchronization. Equation 7 can be derived from the OFDM symbol having an FFT magnitude of N.<maths num="7"><img file="JP4672014B2_D0007.tif" /></maths> In the above formula 7,<maths num="8"><img file="JP4672014B2_D0008.tif" /></maths>Represents the fractional part of the frequency offset of an OFDM symbol with an FFT magnitude of N<maths num="9"><img file="JP4672014B2_D0009.tif" /></maths>Represents an integer portion corresponding to a frequency offset that is a multiple of the subcarrier band. Moreover, the estimation of the frequency offset of the OFDM symbol whose FFT size is reduced to N1 can be expressed by the following equation 8.<maths num="10"><img file="JP4672014B2_D0010.tif" /></maths> In the above formula 8,<maths num="11"><img file="JP4672014B2_D0011.tif" /></maths>Represents the fractional part of the frequency offset of an OFDM symbol with an FFT magnitude of N1<maths num="12"><img file="JP4672014B2_D0012.tif" /></maths>Represents the portion corresponding to the frequency offset that is a multiple of the subcarrier band, and T1 represents the time interval corresponding to the data excluding the length of the cyclic prefix of the partial symbol having the FFT magnitude of N1.

Unlike the above equation 1, the above equation 8 does not repeat the pilot data of the half OFDM symbol length, but the correlation coefficient between the OFDM symbol data having one OFDM symbol length difference and the cyclic prefix of the repetition. Since the frequency offset is estimated using, the range of the fractional part of the frequency offset is also estimated within the absolute value of the half length of the subcarrier band, and the ambiguity is also an integer of the subcarrier band that is not an integral multiple of the subcarrier band. Occurs at double.

If one OFDM symbol in each frame is divided into two FFT parts having N1 and N2 magnitudes, and the data portion subjected to N2 magnitude FFT by the second FFT part is used, the frequency offset estimation is as follows. It can be expressed as Equation 9.<maths num="13"><img file="JP4672014B2_D0013.tif" /></maths> In the above formula 9,<maths num="14"><img file="JP4672014B2_D0014.tif" /></maths>Represents the fractional part of the frequency offset of an OFDM symbol with an FFT magnitude of N2.<maths num="15"><img file="JP4672014B2_D0015.tif" /></maths>Represents the integer part corresponding to the frequency offset that is a multiple of the subcarrier band, and T2 represents the time interval corresponding to the data excluding the length of the cyclic prefix of the partial symbol having the FFT size of N2.

Using Equations 7-9 above, the only frequency offset Δf can be found by selecting N1 and N2 for N.

Cyclic prefix by time and frequency coordinates when one OFDM symbol in each frame is transmitted by FFT of N1 size or divided into two FFTs of N1 and N2 size in this way. Various examples of the method of transmitting the OFDM data and the method of transmitting the OFDM data will be specifically described with reference to FIGS. 8 to 10 below.

FIG. 8 is a diagram showing a method of transmitting cyclic prefixes and OFDM data according to an embodiment of the present invention. Referring to FIG. 8, the pilot OFDM symbol 801 with the N-size FFT repeats the sample at the rear end of the L-length OFDM symbol in the time domain, and the cyclic prefix 802 is added to the front end of the OFDM symbol. And sent. Then, the N1 and N2 size FFT-applied OFDM symbols transmit the N1 size first FFT part 803 and the cyclic prefix 805, and the N2 size second FFT part 804 and the cyclic prefix 806. To do. Here, if a frequency offset of Δf occurs, the fractional part and the integer part of the frequency offset are shown differently according to the magnitude of the FFT.

For example, as shown in FIG. 7, assuming that N = 16, N1 = 9, and N2 = 7, T / T1 = 1.178 and T / T2 = 2.286. At this time, the frequency offset<maths num="16"><img file="JP4672014B2_D0016.tif" /></maths>If is 2.5, then the fractional part of the frequency offset in the OFDM symbol with an N magnitude FFT, as shown in Table 1 below.<maths num="17"><img file="JP4672014B2_D0017.tif" /></maths>Is 1.0π, which is the fractional part of the frequency offset in an OFDM symbol with FFTs of magnitude N1 and N2.<maths num="18"><img file="JP4672014B2_D0018.tif" /></maths>Are 1.444π and 0.428π, respectively. Therefore, in theory, the fractional part of the frequency offset where Δf matches<maths num="19"><img file="JP4672014B2_D0019.tif" /></maths>And g representing the integer part₀, G₁, G₂The value of can be obtained.

<tables num="1"><img file="JP4672014B2_D0020.tif" /></tables>

Then, the integer part corresponding to the decimal part of the frequency offset may be provided in the receiver as table information, for example.

Here, in the actual frequency offset estimation process, an estimation error may be added and it may be difficult to estimate an accurate frequency offset. For such an estimation error, the values of N1 or N1 and N2 are set appropriately. It can be reduced by doing. In addition, the frequency offset can be found more clearly through the iterative process of estimating and correcting the frequency offset and then re-estimating the frequency offset in the next frame.

In an OFDM system according to an embodiment of the present invention, one OFDM symbol consisting of N data samples is transmitted together with L cyclic prefixes. The embodiment shown in FIG. 8 is a case where the cyclic prefix is divided into two parts of the FFT, and the total length of the cyclic prefix is the same as that of the OFDM symbol having an FFT of N magnitude. is there. However, in such a case, the reception performance may deteriorate due to the delay length of multi-path fading.

FIG. 9 is a diagram showing a method of transmitting cyclic prefixes and OFDM data according to another embodiment of the present invention. In the figure, one OFDM symbol in each frame is of N1 and N2 size. Another embodiment in which the two FFTs are divided and transmitted is shown.

With reference to Figure 9 above, the L-sized cyclic prefixes 903 and 904, which are the same as general OFDM symbols with N-sized FFTs in front of the two N1 and N2-sized parts 901 and 902, respectively. To place. Instead, the N1 and N2 sizes of both data parts to match one OFDM symbol with the total number of samples of the cyclic prefix, and the cyclic prefix size to insert additional cyclic prefixes. Can be reduced to a suitable size.

FIG. 10 is a diagram showing a method of transmitting cyclic prefixes and OFDM data according to another embodiment of the present invention. In the same diagram as well, the FFT size of one OFDM symbol in each frame is calculated from N. An example of sending with a small N1 is shown. Referring to FIG. 10, one of the data OFDM symbols in each frame is subjected to an N1 magnitude FFT and the remaining data OFDM symbols 1001 are subjected to an N magnitude FFT. , They and the L-sized cyclic prefix are sent together. The cyclic prefix 1004 for the N1 sized FFT-fed OFDM symbol 1003 can also send the cyclic prefix 1004 extended to N by reducing N1.

As described above, FIGS. 8 to 10 show initial frequency synchronization by providing an OFDM symbol with an FFT of N size and an OFDM symbol with an FFT of smaller size than N in each frame. It is a figure which showed the example which tries to improve the performance of the acquisition method of the above, and the present invention is not limited to the said embodiment.

Hereinafter, the operation of the transmitter / receiver according to the present invention will be described with reference to FIGS. 11 and 12. In the following operation description, an example is given in which the OFDM symbol for acquiring the initial frequency synchronization is transmitted separately in the FFT part of N1 and N2 size.

FIG. 11 is a flowchart showing the operation of the transmitter in the OFDM system according to the embodiment of the present invention. Referring to FIG. 11, when the transmission of data is started, the transmitter determines whether the FFT of N magnitude or the FFT of smaller magnitude than N is selected in the S1101 stage. As a result of this judgment, if an OFDM symbol corresponding to an N-sized FFT is transmitted, the transmitter takes N data symbols in the S1102 stage and performs an N-sized IFFT on the above data symbols in the S1103 stage. , Shift the symbol into the time domain. Then, in the S1104 stage, the transmitter adds an L-sized cyclic prefix to the data symbol in the time domain, transmits the OFDM symbol in the S1105 stage, and then proceeds to the S1101 stage again.

On the other hand, if the result of the judgment in the above S1101 stage is that the OFDM symbol corresponding to the FFT having a size smaller than N is transmitted, the transmitter takes N1 data symbols or N1 + N2 data symbols in the S1110 stage. , S11 Perform N1 size IFFT in 12 steps. After that, N2 size IFFT is performed at the S1113 stage. Here, the above S1113 step may not be carried out in some cases. That is, when the above S1113 step is carried out, after the IFFT of the N2 size data symbol is carried out, as shown in FIGS. 9 and 10 above, the N size OFDM symbol is changed to N1 and N2 smaller than N. This is a case where FFT is performed in two parts. However, in the example shown in FIG. 11 above, only IFFT is performed for N1 data symbols. Then, in the S1114 stage, the transmitter inserts a cyclic prefix into the IFFT-applied data symbol and outputs an OFDM symbol to be transmitted, and then proceeds to the S1105 stage, and IFFT is performed to the receiver with a size smaller than the above N. Send the OFDM symbol with.

The operation of the receiver that has received the OFDM transmitted from the transmitter that operates as described above will be described with reference to FIG. 12 attached. FIG. 12 is a flowchart showing the operation of the receiver in the OFDM system according to the embodiment of the present invention. At the S1201 stage, the receiver receives and stores the OFDM symbol containing the cyclic prefix. The receiver continuously adds a new OFDM symbol for each OFDM symbol reception and updates the received OFDM symbol. Then, in the S1202 stage, the correlation coefficient between the Iclick prefix and the data in the OFDM symbol is obtained for each received OFDM symbol based on the time interval of N data symbols, and in the S1203 stage, N1 pieces. The correlation coefficient between the cyclic prefix and the data in the OFDM symbol is calculated based on the time interval of the data symbol of. Here, when one OFDM symbol is divided into two parts in the S1204 stage, the correlation coefficient is obtained based on the time interval of N1 data symbols and based on the time interval of N2 data symbols. To find the correlation coefficient.

After that, in the S1205 step and the S1206 step, the receiver sets the calculation result of the correlation coefficient of the time interval of the N and N1 data symbols as the preset reference values of the first threshold value (Th1) and the second threshold value (Th1). Th2) Determine if it is greater than. Then, as shown in FIGS. 9 and 10 above, when the receiver divides one OFDM symbol into two parts and performs FFT in the S1207 stage, the correlation coefficient based on the time interval of the N2 data symbol. It is determined whether or not the calculation result of is larger than the third threshold value (Th3) as a preset reference value.

If the calculation result of the correlation coefficient satisfies the predetermined reference value in the steps S1205 to S1207, the receiver takes the phase of the correlation coefficient in the step S1208 and estimates the fractional part of the frequency offset. Acquire 1 frequency synchronization. On the other hand, if the calculation result of the correlation coefficient does not meet the predetermined reference value, the process proceeds to step S1209.

Once the fractional part of the frequency offset has been estimated, in step S1209 the receiver checks to see if at least two fractional parts of the frequency offset have been estimated through OFDM symbols containing FFT parts of different sizes. At this time, if the fractional part of the frequency offset is not estimated or only one is estimated, the process after the S1202 stage is repeated again. On the other hand, if at least two fractional parts of the frequency offset are estimated in the S1209 stage, the receiver resolves the ambiguity in order to estimate the total frequency offset in the S1210 stage and determines the integer part of the frequency offset. 2 Perform the frequency synchronization acquisition process.

The total frequency offset estimated through the process of FIG. 12 above is used to correct the frequency offset, and is repeated after the frequency offset is corrected to reduce the error in the initial frequency synchronization acquisition process. Estimates can be made.

In the detailed description of the present invention, specific embodiments have been described, but it goes without saying that various modifications can be made without departing from the scope of the present invention. Therefore, the scope of the present invention is not limited to the embodiments described, and should be defined by the scope of claims as well as the scope of claims.

<figref num="1">It is a block diagram which showed the transmission / reception block in the physical layer of a general OFDM system.</figref><figref num="2">It is a figure which showed the data symbol transmitted in the general OFDM system in time and frequency units.</figref><figref num="3">It is a figure which showed an example of orthogonality between subcarriers in a general OFDM system.</figref><figref num="4">It is a figure which showed an example of the interference which occurs between subcarriers when a frequency offset exists in a general OFDM system.</figref><figref num="5">It is a figure which showed the configuration example of the pilot OFDM symbol by the acquisition method of the conventional initial frequency synchronization in the conventional OFDM system.</figref><figref num="6">It is a block diagram which showed the structure of the transmitter / receiver in the physical layer for the acquisition of the initial frequency synchronization in the conventional OFDM system.</figref><figref num="7">It is a block diagram which showed the structure of the transmitter / receiver for the acquisition of the initial frequency synchronization in the OFDM system by embodiment of this invention.</figref><figref num="8">It is a figure which showed the cyclic prefix and the method of transmitting OFDM data by one Embodiment of this invention.</figref><figref num="9">It is a figure which showed the cyclic prefix and the method of transmitting the OFDM data by another embodiment of this invention.</figref><figref num="10">It is a figure which showed the method of transmitting a cyclic prefix and OFDM data by another Embodiment of this invention.</figref><figref num="11">It is a flowchart which showed the operation of the transmitter in the OFDM system by embodiment of this invention.</figref><figref num="12">It is a flowchart which showed the operation of the receiver in the OFDM system by embodiment of this invention.</figref>

Code description

710: Transmitter 711: Encoder 712: Serial / Parallel Converter (S / P) 713: IFFT section 713a, 713b, 713c: IFFT device 714: Parallel / serial converter (P / S) 715: Cyclic prefix inserter (C / P) 716: Digital / Analog Converter (D / A) 720: Channel 730: Receiver 731: Analog / Digital Converter (A / D) 738: Correlation part 738a, b, c: 0th, 1st, 2nd correlator 740: Ambiguity Resolution Department