JP6043837B2 - Method for synchronizing OFDM symbols - Google Patents

Description

  The present embodiment relates to a method for time-synchronizing OFDM symbols in an OFDM system.

  The contents described in this part merely provide background information for this embodiment, and do not constitute the prior art.

  FIG. 1 is a diagram illustrating OFDM symbols transmitted and received in an OFDM (Orthogonal Frequency Division Multiplexing) system.

  The OFDM symbol 110 includes a CP (Cyclic Prefix) 120 and a symbol 130.

  The CP 120 corresponds to a guard interval (GI) located before the symbol 130. Usually, when an OFDM symbol is transmitted from an OFDM transmitter to an OFDM receiver, a plurality of OFDM symbols are transmitted using one or more channels. In the case where a plurality of OFDM symbols are transmitted, if a plurality of OFDM symbols are transmitted one after another in a state where there is no protection interval in each OFDM symbol, interference between OFDM symbols, in particular, between symbols within the OFDM symbol ( Inter Symbol Interference (ISI) is generated. Therefore, the CP 120 is positioned before the symbol 130 in order to prevent such interference. The CP 120 has the same data as the end 140 of the symbol having the same length as the CP.

  The symbol 130 corresponds to a part including data to be transmitted.

  The OFDM receiver receives the OFDM symbol transmitted from the OFDM transmitter. The OFDM receiver performs fast Fourier transform (FFT) on the received OFDM symbol, and extracts desired data by demodulating the fast Fourier transformed data. However, a frequency offset is generated due to the Doppler effect in the process of transmitting the OFDM symbol or in the process in which the OFDM receiver receives the OFDM symbol and performs a fast Fourier transform, so the OFDM receiver is transformed by the fast Fourier transform. Separately from the process of demodulating the received data, the generated frequency offset is removed by using frequency synchronization of the received OFDM symbol.

  The OFDM receiver grasps the boundary between the CP of the received OFDM symbol and the symbol by using time synchronization. However, conventional OFDM receivers do not consider whether an OFDM symbol is an OFDM symbol that is fast Fourier transformed or an OFDM symbol that is frequency-synchronized to remove the frequency offset. The OFDM symbol has been time-synchronized like the OFDM symbol to be converted. As a result, there is a problem in that, when removing a frequency offset through frequency synchronization in a multipath environment, there are many errors from the expected result.

  The present embodiment provides a method for time synchronization in different manners depending on whether an OFDM symbol is an OFDM symbol that is fast Fourier transformed or an OFDM symbol that is precisely frequency synchronized in an OFDM receiver. Main purpose.

  According to one aspect of the present embodiment, a plurality of OFDM symbols (hereinafter referred to as “multiple supply OFDM symbols”) received from an OFDM (Orthogonal Frequency Division Multiplexing) transmitter and fast Fourier transformed or modulated. ) And a second input through a Fine Frequency Synchronization process, or a first input OFDM symbol through the Fast Fourier Transform process using the plurality of supplied OFDM symbols. And providing a protection period of the first and second input OFDM symbols in a different form depending on whether it is an OFDM symbol or not. The

  According to another aspect of the present embodiment, in a method for precisely time-synchronizing OFDM symbols input to a precision frequency conversion process, a plurality of OFDM symbols (hereinafter referred to as “a plurality of supply OFDM symbols”) that have been fast Fourier transformed or demodulated. Abbreviated to each of the plurality of supplied OFDM symbols, and a channel impulse response having a maximum power among the plurality of supplied OFDM symbols. And providing a point of time of two supplied OFDM symbols as a starting point of a protection interval of the OFDM symbol input to the precision frequency conversion process.

  In addition, according to another aspect of the present embodiment, in a method for time-synchronizing OFDM symbols input to a precision frequency conversion process, a plurality of OFDM symbols (hereinafter referred to as “a plurality of supply OFDM symbols”) that have been fast Fourier transformed or demodulated. Abbreviated as ”), knowing the power of the channel impulse response of each of the plurality of supplied OFDM symbols, and at the time of the channel impulse response of each of the plurality of supplied OFDM symbols Correlation index (hereinafter referred to as “correlation index”) corresponding to the power size of each channel impulse response of the symbol and having a numerical value that decreases arithmetically before and after each channel impulse response time of the plurality of supplied OFDM symbols. ”) And a step of setting a point of time when the sum of the numerical values of the correlation index is maximized as a starting point of a protection interval of the OFDM symbol input to the precise frequency conversion process. Provide a time synchronization method.

  As described above, according to one aspect of the present embodiment, time synchronization is performed in a different manner depending on whether an OFDM symbol is subjected to fast Fourier transform or an OFDM symbol is subjected to precise frequency synchronization. Even if the symbol is fast Fourier transformed or precision frequency synchronized, it has the advantage of having the result of maximizing the error.

It is the figure which showed the OFDM symbol transmitted / received by an OFDM system. 1 is a block diagram illustrating an OFDM receiver according to an embodiment of the present invention. 4 is a timing diagram schematically illustrating a method of time-synchronizing OFDM symbols subjected to fast Fourier transform according to an embodiment of the present invention. 2 is a timing diagram schematically illustrating a method for time-synchronizing OFDM symbols that are frequency-synchronized according to an embodiment of the present invention. 4 is a timing diagram schematically illustrating a method for time-synchronizing an OFDM symbol that is frequency-synchronized according to another embodiment of the present invention. 3 is a flowchart illustrating a time synchronization method according to an embodiment of the present invention.

  Hereinafter, some embodiments of the present invention will be described in detail through exemplary drawings. In adding a reference numeral to each component in each drawing, it is noted that the same component is given the same symbol as much as possible even if it is displayed on another drawing. There must be. In the description of the present invention, if it is determined that a specific description of a related known configuration or function will obscure the gist of the present invention, a detailed description thereof will be omitted.

  In describing the constituent elements of the present invention, terms such as first, second, A, B, (a), and (b) can be used. Such terms are merely for distinguishing the constituent elements from other constituent elements, and the essence, order or order of the corresponding constituent elements are not limited by the terms. Throughout the specification, when any part “includes” or “comprises” any component, this does not exclude other components unless specifically stated to the contrary, It is meant to further include other components. Further, terms such as “... part” and “module” described in the specification mean a unit for processing at least one function or operation, and this means that hardware, software, or hardware and software are connected. It can be realized by combination.

  FIG. 2 is a block diagram illustrating an OFDM receiver according to an embodiment of the present invention.

  Referring to FIG. 2, an OFDM receiver according to an embodiment of the present invention includes an RF unit 210, an ADC unit 220, a digital front end unit 230, a time synchronization unit 240, a guard interval removal unit 250, and a precise frequency synchronization. , A fast Fourier transform unit 270, a precise time synchronization unit 280, and other decoding units 290.

  The RF unit 210 serves to convert an RF signal received from the OFDM transmitter into an analog baseband baseband signal.

  The ADC unit 220 serves to convert the analog baseband signal converted by the RF unit into a digital baseband signal.

  A digital front end (Digital Front End) unit 230 performs various functions. Specifically, the digital front end (Automatic Gain Control, AGC), DC offset removal, IQ mismatch (Imbalance) compensation, and adjacent channel interference. Functions such as (Adjacent Channel Interference, ACI) or co-channel interference (CCI) removal, carrier frequency error compensation, and sampling frequency error compensation can be performed.

  The coarse symbol synchronization unit 240 generally serves to confirm the start position of each frame when receiving a plurality of OFDM symbols from the digital front end unit. At this time, since it is not a step for precise confirmation, many time errors occur.

  The guard interval removing unit 250 serves to remove a CP that exists as a guard interval in the OFDM symbol received from the time synchronization unit or the precise time synchronization unit.

  The precision frequency synchronization unit 260 serves to remove frequency offset using a precision frequency synchronization process. A frequency offset is generated by the Fourier transform or the Doppler effect. Such a frequency offset induces adjacent channel interference after the fast Fourier transform, and thus needs to be removed. The precision frequency synchronization unit removes such a frequency offset through a precision frequency synchronization process.

  The fast Fourier transform unit 270 receives a symbol in the OFDM symbol from which the guard interval is removed from the guard interval removing unit, and performs a fast Fourier transform on the received symbol.

  A fine time synchronization unit 280 receives a plurality of OFDM symbols received from the fast Fourier transform unit or other decoding units through different paths, and performs time synchronization. The input OFDM symbol is output to the guard interval removal unit. The precise time synchronization unit outputs an input OFDM symbol in which a boundary between a protection interval and a symbol is divided using a plurality of received OFDM symbols. However, the precise time synchronization unit performs a guard interval and a symbol in a different manner depending on whether the input OFDM symbol is input to the fast Fourier transform unit through the protection interval removal unit or the precise frequency conversion unit. Divide boundaries. A specific description thereof will be described with reference to FIGS.

  The other decoding unit 290 serves to decode the data Fourier-transformed by the fast Fourier transform unit and output desired data.

  Hereinafter, a scheme in which an OFDM receiver according to an embodiment of the present invention operates will be schematically described.

  When receiving an OFDM symbol for the first time from an OFDM transmitter, it operates the same as a conventional OFDM receiver. After the signal received from the OFDM transmitter is converted into an analog signal, it is converted again into a digital signal. The OFDM symbol thus converted is generally signal-synchronized, and after each frame is divided, the protection interval is removed. The OFDM symbol from which the guard interval has been removed in this way is input to the fast Fourier transform unit and the precise frequency synchronization unit without being specifically divided. When an OFDM symbol is input to the precision frequency synchronization unit, a frequency error is compensated by the digital front end unit through a frequency synchronization process. After the OFDM symbol is input to the fast Fourier transform unit and subjected to Fourier transform, desired data is obtained through decoding by other decoding units. The OFDM symbol that has undergone the decoding process in the decoding unit is input again to the digital front end unit, and compensates for the error that has occurred in the above-described approximately time synchronization process.

  However, in the OFDM receiver according to an embodiment of the present invention, when the OFDM symbol is received from the fast Fourier transform unit or other decoding unit by the precise time synchronization unit, the time synchronization is performed collectively without dividing them. Instead, the time synchronization is made different depending on whether the OFDM symbol is input to the fast Fourier transform unit or another decoding unit. The OFDM symbol divided and time-synchronized in this way is subjected to the protection interval removal unit, the protection interval is removed, and is input to the fast Fourier transform unit or the precise frequency synchronization unit to repeat the above-described process. In this way, in the time synchronization by the precise time synchronization unit, since the time synchronization is performed by classifying which configuration is the OFDM symbol to be input, the generated error can be significantly reduced compared to the conventional case. .

  Each component included in the OFDM receiver shown in FIG. 2 is connected to a communication path that connects software modules or hardware modules inside the apparatus, and operates organically between them. Such components communicate using one or more communication buses or signal lines.

  FIG. 3a is a timing diagram that schematically illustrates a method for time-synchronizing OFDM symbols that are fast Fourier transformed according to one embodiment of the present invention.

  When the time domain signal is viewed, there are a plurality of signals received from the fast Fourier transform unit or other decoding units through different paths. When any one of the signals is viewed, the portions indicated as 0, 1, and 2 each indicate an OFDM symbol. In the description of the time synchronization method, only OFDM symbols (310, 315, 320) indicated as 1 out of OFDM symbols included in each signal will be described, and the same for all other OFDM symbols. Can be applied. In FIG. 3a, three signals received through different paths are shown, and three OFDM symbols are included in each signal. However, the number is only one example. However, the present invention is not necessarily limited to this.

The first OFDM symbol 310 has the smallest channel impulse response at t ₁ , and the second and third OFDM symbols 320 and 330 are also the largest at t ₂ and t ₃ , respectively. Power and medium sized power channel impulse response. Here, the power of the channel impulse response is arbitrarily set for convenience of explanation, and is not limited to this, and may be different from that shown in FIG.

In this case, the protection interval is set so that the OFDM symbol input to the fast Fourier transform unit via the protection interval removal unit includes the most component components of each channel in the protection interval. Since the guard interval has a length that has already been set, it cannot always be set to include all channel components. Therefore, the protection interval must be set so that all channel components are included, and in order to include the largest number of components, many channel impulse responses with high power are included in the protection interval. The protection interval must be set so that Referring to FIG. 3a, the guard interval of the input OFDM symbol is started at t ₄ so as to include the components of all channels, including the channel impulse response with the highest power and the channel impulse response with medium power. Set as follows. Wherein the components of all the channels by setting the start point of the guard interval to t _4, the power is included the channel impulse response are all larger.

  As described above, in the precise time synchronization of the OFDM symbols input to the fast Fourier transform unit through the protection interval removal unit, the protection interval is set so that the constituent components of each channel are contained most. When a protection interval having a length that has already been set is set in this way, a symbol having a length that has already been set is also set.

  FIG. 3b is a timing diagram that schematically illustrates a method for time-synchronizing a precision frequency-synchronized OFDM symbol according to one embodiment of the present invention.

  When the time domain signal is viewed, there are a plurality of signals received from the fast Fourier transform unit or other decoding units through different paths.

Each OFDM symbol 340, 350, 360 has a channel impulse response with different sizes of power to each other at _{t 5, t 6, t 7} respectively.

In the OFDM symbol input to the precision frequency synchronization unit through the protection interval removal unit, the protection interval is set so that the correlation between the CP in the OFDM symbol and the same part as the CP of the symbol is maximized. When setting the correlation between the CP in the OFDM symbol and the same part as the CP of the symbol to be the largest, the protection interval is set by using a simple method with the time of the channel impulse response having the largest power as a starting point. This is because the correlation between the CP in the OFDM symbol and the same part as the CP of the symbol is proportional to the power of the channel impulse response. Referring to FIG. 3b, since the power size of the channel impulse response is the largest in the second OFDM symbol 350, the guard interval of the input OFDM symbol is a time point when the channel impulse response of the second OFDM symbol exists. ₆ is set as the starting point. The guard interval of t ₆ as a starting point is set to be set to be present in succession in the guard interval symbols.

  As described above, when the OFDM symbol input to the precision frequency synchronization unit through the protection interval removal unit is precisely time synchronized, protection is performed from the channel impulse response time having the largest power as a starting point by a simple method. Set the interval.

  FIG. 4 is a timing diagram schematically illustrating a method of time-synchronizing OFDM symbols that are frequency-synchronized according to another embodiment of the present invention.

  When the time domain signal is viewed, there are a plurality of signals received from the fast Fourier transform unit or other decoding units through different paths.

Each OFDM symbol 410, 420 and 430 has a channel impulse response having different sizes of the power at _{t 1,} t 2, _{t 3,} respectively.

  A correlation index for each OFDM symbol is obtained based on the power of the channel impulse response. Here, the correlation index is an index that can grasp the correlation between the CP in the OFDM symbol and the same part as the CP of the symbol, and the time point and the numerical value correspond to the channel impulse response time point and power. The correlation index has a numerical value corresponding to the power size of the channel impulse response at the time of the channel impulse response, and the numerical value of the correlation index decreases arithmetically before and after the time of the channel impulse response.

  In the OFDM symbol input to the precision frequency synchronization unit through the protection interval removal unit, the protection interval is set so that the correlation between the CP in the OFDM symbol and the same part as the CP of the symbol is maximized. In this case, the correlation between the CP in the OFDM symbol and the same part as the CP of the symbol can be grasped by using the correlation index acquired previously.

The obtained correlation indices are added together, and the part with the largest correlation index is searched. This is because when the correlation index is the largest, it can be grasped that the correlation between the CP in the OFDM symbol and the same part as the CP of the symbol is the largest. Therefore, the OFDM symbol input to the precise frequency synchronization unit through the protection interval removal unit is set with a protection interval starting from the point in time where the correlation index is the largest. Referring to FIG. 4, t ₂ , which is the time when the channel impulse response of the second OFDM symbol exists, is the time when the correlation index is the largest, so the protection interval is set starting from this time. In this way, the protection interval is set, and the symbols are set so that the symbols are successively located.

  As described above, in the precise time synchronization of the OFDM symbol input to the precision frequency synchronization unit through the guard interval removal unit, the correlation index of each OFDM symbol is calculated by another method, The protection interval is set starting from the point in time when the sum of the numerical values of the correlation indices is the largest.

  FIG. 5 is a flowchart illustrating a time synchronization method according to an embodiment of the present invention.

  A plurality of OFDM symbols are received from the fast Fourier transform unit or other decoding units (S510). The precise time synchronization unit receives a plurality of OFDM symbols received from the fast Fourier transform unit or other decoding units through different paths.

  It is determined whether or not the precise time synchronized OFDM symbol is input to the fast Fourier transform unit (S520). The precision time synchronization unit synchronizes time using a plurality of received OFDM symbols, and the output OFDM symbol is input to the fast Fourier transform unit through the protection interval removal unit or input to the precise frequency synchronization unit. Judge whether or not.

  The protection period of the input OFDM symbol is set so that the component components of the respective channels of the plurality of OFDM symbols received are contained most in the protection period of the input OFDM symbol (S530). When setting the guard interval so as to include the most component components of each of the plurality of OFDM symbols received by the precise time synchronization unit, the power of each channel impulse response in the plurality of OFDM symbols is considered. In order to include the largest number of component components of each channel in a plurality of OFDM symbols, the protection interval of the input OFDM symbol is set so that the sum of the power of each channel impulse response is maximized.

  The protection interval of the input OFDM symbol is set so that the correlation between the CP in the OFDM symbol and the same portion as the CP of the symbol is maximized (S540). As an example, the power of each channel impulse response in a plurality of OFDM symbols received by the precise time synchronization unit is grasped, and a protection interval is set starting from a point in time when a channel impulse response having the largest power exists. May be. As another example, a correlation interval of each of a plurality of OFDM symbols received by the precise time synchronization unit may be grasped, and a protection interval may be set starting from a point in time when the sum of the correlation indexes is the largest. . Here, the correlation index is an index that can grasp the correlation between the CP in the OFDM symbol and the same part as the CP of the symbol, and the time point and the numerical value correspond to the channel impulse response time point and power. The correlation index has a numerical value corresponding to the power size of the channel impulse response at the time of the channel impulse response, and the numerical value of the correlation index decreases arithmetically before and after the time of the channel impulse response.

  In FIG. 5, it is described that the steps S <b> 510 to S <b> 540 are sequentially performed. However, this is merely an example of the technical idea of the embodiment of the present invention. That is, if the person has ordinary knowledge in the technical field to which the embodiment of the present invention belongs, the order shown in FIG. 5 is changed and executed without departing from the essential characteristics of the embodiment of the present invention. However, FIG. 5 is limited to a chronological order because it can be modified and modified in various ways to execute one or more of the processes S510 to S540 in parallel. is not.

  On the other hand, each process shown in FIG. 5 can be embodied as a computer-readable code on a computer-readable recording medium. Computer readable recording media include all types of recording devices that can store data that can be read by a computer system. In other words, computer-readable recording media include magnetic storage media (eg, ROM, floppy disk, hard disk, etc.), optical interpretation media (eg, CD-ROM, DVD, etc.) and carrier waves (eg, via the Internet). Storage media). Further, the computer-readable recording medium is distributed in a computer system connected via a network, and the computer-readable code can be stored and executed in a distributed manner.

  The above description is merely illustrative of the technical idea of the present embodiment. Any person having ordinary knowledge in the technical field to which the present embodiment belongs will depart from the essential characteristics of the present embodiment. Various modifications and variations are possible without departing from the scope. Therefore, the present embodiment is not intended to limit the technical idea of the present embodiment, but for explanation, and the scope of the technical idea of the present embodiment is limited by such an embodiment. There is no. The protection scope of this embodiment must be construed according to the following claims, and all technical ideas within the equivalent scope must be construed as being included in the scope of rights of this embodiment. right.

110: OFDM symbol, 120: protection section, 130: symbol, 140: end of symbol, 210: RF section, 220: ADC section, 230: digital front end section, 240: roughly time synchronization section, 250: protection section Removal unit 260: Precision frequency synchronization unit 270: Fast Fourier transform unit 280: Precision time synchronization unit 290: Other decoding unit

Claims (12)

Receiving a plurality of OFDM symbols that have been fast Fourier transformed or demodulated (hereinafter abbreviated as "multiple supplied OFDM symbols");
Distinguishing between a first input OFDM symbol that has undergone a fast Fourier transform process or a second input OFDM symbol that has undergone a precision frequency synchronization process; and Setting the protection interval of the first or second input OFDM symbol differently depending on whether it is one input OFDM symbol or the second input OFDM symbol;
An OFDM symbol time synchronization method comprising: The protected section is
The OFDM symbol time synchronization method according to claim 1, wherein the length of the guard interval is already set. Setting a guard interval for the first or second input OFDM symbol,
The OFDM symbol according to claim 1, wherein a protection interval of the first or second input OFDM symbol is set using a channel impulse response (CIR) of the plurality of supplied OFDM symbols. Time synchronization method. The guard interval of the first input OFDM symbol is
4. The OFDM symbol time synchronization method according to claim 2, wherein the OFDM symbol time synchronization method is set so as to include the maximum power of a component of a channel of the plurality of supplied OFDM symbols. 5. The guard interval of the first input OFDM symbol is
In order to maximize the power of the constituent components of the channels of the plurality of supplied OFDM symbols, the sum of the channel impulse responses of the plurality of supplied OFDM symbols existing during the already set length of the guard interval is maximized The OFDM symbol time synchronization method according to claim 4, wherein the OFDM symbol time synchronization method is set as follows. The guard interval of the second input OFDM symbol is
The same interval as the protection interval of the second input OFDM symbol and the protection interval of the second input OFDM symbol existing in the symbol of the second input OFDM symbol (hereinafter, “the same interval as the protection interval in the symbol”) The OFDM symbol time synchronization method according to claim 2, wherein the correlation is set to maximize. The guard interval of the second input OFDM symbol is
Any of the plurality of supplied OFDM symbols having a maximum channel impulse response so that the correlation between the protection interval of the second input OFDM symbol and the same interval as the protection interval in the symbol is maximized. [7] The OFDM symbol time synchronization method according to claim 6, wherein a time point of one supplied OFDM symbol is set as a starting point of a protection interval of the second input OFDM symbol. The guard interval of the second input OFDM symbol is
[7] The method according to claim 6, wherein the correlation is set so that the correlation between the protection interval of the second input OFDM symbol and a protection interval in the symbol is the same as that of the protection interval in the symbol is maximized. The described OFDM symbol time synchronization method. The correlation index is
The numerical value of the correlation index corresponds to the size of the power of the channel impulse response of each of the plurality of supplied OFDM symbols at the time of the channel impulse response of each of the plurality of supplied OFDM symbols, and each of the plurality of supplied OFDM symbols 9. The OFDM symbol time synchronization method according to claim 8, wherein the time is reduced arithmetically before and after the channel impulse response time. The guard interval of the second input OFDM symbol is
10. The OFDM symbol time synchronization method according to claim 9, wherein a time point at which a sum of values of the correlation index becomes maximum is set as a starting point. In a method for precise time synchronization of OFDM symbols input to a precision frequency conversion process,
Receiving a plurality of OFDM symbols that have been fast Fourier transformed or demodulated (hereinafter abbreviated as "multiple supplied OFDM symbols");
Determining a power of a channel impulse response of each of the plurality of supplied OFDM symbols; and performing precision frequency conversion on a time point of any one of the plurality of supplied OFDM symbols having a maximum channel impulse response power Setting as a starting point of the protection interval of the OFDM symbol input into the process;
An OFDM symbol precise time synchronization method comprising: In a method for time-synchronizing OFDM symbols input to a precision frequency conversion process,
Receiving a plurality of OFDM symbols that have been fast Fourier transformed or demodulated (hereinafter abbreviated as "multiple supplied OFDM symbols");
Ascertaining the power of the channel impulse response of each of the plurality of supplied OFDM symbols;
Corresponding to the channel impulse response power size of each of the plurality of supplied OFDM symbols at each channel impulse response time of each of the plurality of supplied OFDM symbols, and before and after each channel impulse response time of the plurality of supplied OFDM symbols Obtaining a correlation index having a numerical value that decreases arithmetically (hereinafter, abbreviated as “correlation index”); and a time at which the sum of the numerical values of the correlation index is maximized. Setting as a starting point of the protection interval of the OFDM symbol input to
An OFDM symbol precise time synchronization method comprising: