JP2009232024A - Orthogonal frequency division multiplexing communication apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide orthogonal frequency division multiplexing communication apparatus and method which can achieve the same dispersion tolerance as that of a conventional technique within a cyclic prefix period shorter than that of the conventional technique and achieve higher dispersion tolerance in the same cyclic prefix period as the conventional technique. <P>SOLUTION: The orthogonal frequency division multiplexing communication apparatus includes first to n-th means for extracting signals of a prescribed period from respective symbols of a received signal and performing discrete Fourier transform, wherein n is an integer ≥2, and a coupling means. The first to n-th means have correspondence relation of one to one with n groups obtained by dividing all sub-carriers on (n-1) frequency positions. The coupling means selects and outputs a signal of a sub-carrier included in each corresponding group from each of outputs from the first to n-th means. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention achieves the same dispersion tolerance as the prior art in a communication system using orthogonal frequency division multiplexing (OFDM) technology with a shorter cyclic prefix (hereinafter referred to as CP; CP: Cyclic Prefix). When using CP having the same length as that of the conventional technique, the present invention relates to a technique for achieving higher dispersion strength.

  The OFDM technology is a method of transmitting transmission data in parallel using a plurality of subcarriers. The symbol rate of each subcarrier is relatively low, so that it is resistant to intersymbol interference, and can be used for digital terrestrial broadcasting or wireless LAN (Local It is already used in the Aera Network) system, and its application to an optical communication system is also being studied (for example, see Non-Patent Documents 1 and 2).

  FIG. 7 is a block diagram schematically showing an OFDM communication system. 7, the OFDM communication system includes an IFFT unit 1, a CP processing unit 2, a transmission unit 3, a reception unit 4, a window processing unit 5, and an FFT unit 6. The IFFT unit 1 receives a complex value corresponding to the data value to be transmitted on each subcarrier in a certain symbol. The IFFT unit 1 performs inverse inverse Fourier transform on the complex value, and the symbol on the time axis of the symbol. Output multiple time samples representing the waveform.

  The CP processing unit 2 adds a CP to a plurality of time samples output from the IFFT unit 1, outputs a sample obtained by combining the time sample output from the IFFT unit 1 and the CP as a signal of one symbol, and a transmission unit 3 Performs processing according to the transmission medium, such as frequency conversion and amplification. In the following description, the time corresponding to the time sample output from the IFFT unit 1 is the FFT period, the period corresponding to the CP time sample is the CP period, and the combined period of the FFT period and the CP period is the symbol period. Call.

  FIG. 8 is a diagram for explaining processing in the CP processing unit 2. According to FIG. 8, 64 time samples, T1 to T64, are obtained from the data transmitted by a certain symbol by inverse discrete Fourier transform, and the CP processing unit 2, for example, copies the first T1 to T16, As a CP, it is added after the T64 sample. Since the discrete Fourier inverse transform outputs a sample of one period of a periodic waveform having an input frequency component, the waveform on the time axis obtained from all 80 samples including the CP is also discontinuous. No points will occur. Therefore, a waveform having a length corresponding to the FFT period is extracted from the waveform on the time axis, the length of which is equal to the symbol period, obtained from the 80 samples, and 64 time samples are obtained from the extracted waveform. The original frequency component can be obtained by performing a discrete Fourier transform.

  Returning to FIG. 7, the reception unit 4 performs processing such as amplification and frequency conversion of a signal received from the transmission medium, and the window processing unit 5 determines, based on the symbol timing, a signal of one symbol having a symbol period length. A signal having a length corresponding to the FFT period described above is taken out, and the FFT unit 6 performs demodulation by performing a discrete Fourier transform on the output of the window processing unit 5. The symbol timing is a signal indicating a symbol boundary of a predetermined subcarrier, and can be recognized by, for example, a receiving communication apparatus by a known synchronization pattern periodically inserted into the subcarrier. Hereinafter, a predetermined subcarrier symbol boundary indicated by the symbol timing is referred to as a reference position.

  The fact that the dispersion tolerance is improved by adding CP will be described below with reference to FIG. FIG. 9A shows a certain symbol output from the communication device on the transmission side. Here, the horizontal direction indicates time, and the vertical direction indicates subcarriers. In FIG. 9A, the FFT period is F and the CP period is C. The window processing unit 5 of the communication apparatus that directly receives the signal shown in FIG. 9A extracts the signal in the FFT period from a position delayed by C / 2 from the reference position indicated by the black circle in FIG. 9B. In FIG. 9B, the subcarrier located at the center on the frequency axis is used for symbol timing. Hereinafter, taking out the signal in the FFT period from the symbol in the symbol period is referred to as window processing, and the range to be taken out by the window processing is indicated by shading in the figure, and the reference position is indicated by a black circle. In the figure, the lower side is the high frequency side. This expression is the same in FIGS.

  For example, an OFDM signal transmitted through an optical fiber transmission line is subjected to a different amount of delay for each subcarrier as shown in FIG. 9C due to the influence of chromatic dispersion of the optical fiber. However, since there is redundancy in the CP period, even if a difference in delay amount occurs only by C by performing window processing from a position delayed by C / 2 from the reference position, as shown in FIG. Demodulation is possible while avoiding intersymbol interference. FIG. 9C shows a limit state where demodulation can be performed correctly. For example, as shown in FIG. 9D, when there is a difference in delay amount due to chromatic dispersion, for example, 2C, the adjacent symbol is a window. Leaking in, and thus affected by intersymbol interference. From FIG. 9, it can be seen that by increasing the CP period, it is possible to avoid inter-symbol interference even when the difference in delay amount increases, that is, the tolerance to chromatic dispersion is improved.

Arthur James Lowry, et al. , “Orthogonal-frequency-division multiplexing for dispersal compensation of long-haul optical systems”, 2006 Optical Society of America ETS E No. 14 6, March 2006 Ivan B. Djordjevic, et al. , “Orthogonal frequency division multiplexing for high-speed optical transmissions”, 2006 Optical Society of America, OPTICS EXPRES. No. 14 9, May 2006

  However, the amount of information that can be transmitted is reduced by increasing the CP period. In other words, the improvement in dispersion tolerance by manipulating the CP period is in a trade-off relationship with the amount of transmission information. Therefore, it is desirable to achieve the same dispersion tolerance with fewer CP periods than in the prior art, and similarly, communication apparatuses and methods that can achieve higher dispersion tolerance with the same CP period are useful. An object of the present invention is to provide a communication apparatus and method for this purpose.

According to the orthogonal frequency division multiplexing communication device of the present invention,
First to n-th means for taking out a signal of a predetermined period from each symbol of the received signal and performing a discrete Fourier transform, where n is an integer equal to or greater than 2, and a combining means, are provided. Means has a one-to-one correspondence with the n groups obtained by dividing all subcarriers at n-1 frequency positions, and the combining means outputs the outputs of the first to nth means respectively. Then, the subcarrier signals included in the corresponding group are selected and output.

According to another embodiment of the communication device according to the invention,
It is also preferable that the first to nth means, each of which determines the extraction start position of a signal to be extracted from each symbol based on information indicating a symbol boundary of a predetermined subcarrier. It is also preferable to use a subcarrier included in the group to be used for determining the extraction start position, where n is 2, and the first means and the second means use the same subcarrier as the extraction start position. It is also preferable that the number of subcarriers included in each group is equal.

According to the orthogonal frequency division multiplex communication method of the present invention,
The received signal is divided into n branches from the first to the nth received signal, where n is an integer greater than or equal to 2, and the first to nth received signals are divided into n−1 frequency positions for all subcarriers. A one-to-one correspondence with the n groups obtained by dividing by step 1, a step of taking out a signal of a predetermined period from each of the symbols of the first to n-th received signals, and performing a discrete Fourier transform; And selecting and outputting a signal of a subcarrier included in the corresponding group from a signal obtained by performing discrete Fourier transform on the nth received signal.

  The received signal is branched, discrete Fourier transform is performed individually, and unnecessary subcarriers, that is, subcarrier signals in which intersymbol interference occurs are removed from the output of each discrete Fourier transform, and intersymbol interference occurs. By outputting a signal of no subcarrier, a higher dispersion tolerance can be achieved in the same CP period, and the same dispersion tolerance as in the prior art can be achieved in a smaller CP period. For the Fourier transform, the signal extraction start position of the FFT period extracted from each symbol is determined based on the symbol boundary of a predetermined subcarrier. However, even if the same subcarrier is used for this determination, different subcarriers are used. May be used for this determination.

  The best mode for carrying out the present invention will be described in detail below with reference to the drawings.

  FIG. 1 is a block diagram of a communication apparatus according to the present invention. FIG. 1 shows only the parts necessary for the description of the present invention. According to FIG. 1, the communication apparatus according to the present invention includes window processing units 11 and 12, FFT units 21 and 22, and a combining unit 30.

  The window processing units 11 and 12 receive the same signal from a receiving unit (not shown), perform window processing based on the symbol timing, and output to the FFT units 21 and 22, respectively. The FFT unit 21 and the FFT unit 22 perform discrete Fourier transform on the input signal and output a complex value corresponding to the data carried by each subcarrier to the combining unit 3. The combining unit 3 selects a complex value corresponding to a half subcarrier from the lowest frequency among the complex values corresponding to the data of each subcarrier output by the FFT unit 21, and outputs each subvalue output by the FFT unit 22. Of the complex values corresponding to the carrier data, the complex value corresponding to the subcarrier with the half higher from the higher frequency is selected, and the complex values corresponding to the selected subcarriers, that is, all the sub-carriers included in the OFDM signal are selected. The complex value corresponding to the carrier is output.

  Hereinafter, the processing in the communication apparatus according to the present invention will be described with reference to FIGS. 2 and 3, the FFT period is F and the CP period is C, and the center subcarrier is used as the reference position. As shown in FIG. 2A, the window processing unit 11 performs window processing from a position that is delayed by C from the reference position indicated by the symbol timing, and the FFT unit 21 displays the shaded portion of FIG. 2A. Discrete Fourier transform the signal. Further, as shown in FIG. 2B, the window processing unit 22 starts the window processing from the reference position indicated by the symbol timing, and the FFT unit 22 applies the signal of the shaded portion in FIG. Convert. The combining unit 30 includes an upper half of the subcarrier output from the FFT unit 21, that is, a subcarrier signal on the low frequency side half, and a lower half of the subcarrier output from the FFT unit 22, that is, a subcarrier on the high frequency side half. The signal is output.

  For example, as shown in FIG. 3A, when the difference in delay amount is 2C, the lower half of the signal extracted by the window processing unit 11 leaks from the adjacent symbol, and thus FFT is performed. Of the subcarriers output by the unit 21, errors due to intersymbol interference occur in the subcarriers on the high frequency side half. However, since no leakage from adjacent symbols occurs in the subcarriers on the low frequency side half, no error due to intersymbol interference occurs. Similarly, as shown in FIG. 3 (b), leakage from adjacent symbols occurs in the upper half of the signal extracted by the window processing unit 12. Therefore, among the subcarriers output from the FFT unit 22, the low frequency An error due to intersymbol interference occurs in the subcarriers on the side half. However, no error due to intersymbol interference occurs in the high frequency side half. Since the combining unit 30 selects and outputs only subcarriers in which symbol interference has not occurred from the outputs of the FFT units 21 and 22, an error occurs in the outputs of the FFT units 21 and 22, The output will not contain any errors. Thereby, in the communication apparatus according to the present invention, it is possible to achieve a dispersion tolerance twice that of the prior art in the same CP period.

  Further, it will be described with reference to FIG. 4 that the CP period required to achieve the same chromatic dispersion tolerance as that of the prior art can be reduced by the communication apparatus according to the present invention. Since the signal extracted by the window processing unit 11 uses only the low frequency side half, and the signal extracted by the window processing unit 12 uses only the high frequency side half, the window processing unit 11 in FIG. The period of the window process performed by the window processing unit 12 is indicated by the upper half-tone, and the period of the window process performed by the window processing unit 12 is indicated by the lower-side halftone. 4 (a) and 4 (b) show the same drawings as FIGS. 9 (a) and 9 (c) for comparison with the window processing in the prior art that uses only one FFT unit. In FIG. 4, the FFT period is F, the CP period in the figure showing the prior art is C, and the CP period in the figure showing the present invention is C / 2. The center subcarrier is used as the reference position.

  FIG. 4A shows a case where no delay amount difference occurs. 4A, the window processing unit 11 performs window processing from the reference position indicated by the symbol timing by C / 2, that is, from a position delayed by the CP period, and the window processing unit 22 Window processing starts from the reference position indicated by the timing. As a result, as shown in FIG. 4 (b), even though the CP period is halved, it can be demodulated correctly avoiding intersymbol interference until the delay amount difference becomes C, as in the prior art. I understand.

  As described above, two sets of window processing units and FFT units are provided, and by shifting the extraction start position of signals extracted from each symbol for discrete Fourier transform, higher chromatic dispersion tolerance can be achieved in the same CP period as the conventional technology. Thus, the same chromatic dispersion tolerance as in the prior art can be achieved with fewer CP periods. Note that the start position is determined based on the same symbol timing.

  Next, another embodiment of the communication device according to the present invention will be described. FIG. 5 is a block diagram of another embodiment of a communication device according to the present invention. FIG. 5 also shows only the portions necessary for explaining the present invention. According to FIG. 5, the communication apparatus includes window processing units 11 to 1n, FFT units 21 to 2n, and a combining unit 30. Note that n is an integer of 2 or more and the number of subcarriers Nc or less, and the window processing unit 1k and the FFT unit 2k (k = 1 to n) are in a correspondence relationship. Further, all subcarriers of the OFDM signal are divided at n-1 frequency positions and grouped into n groups of consecutive subcarriers, and each group is combined with a window processing unit 1k and an FFT unit 2k. Are in a 1: 1 correspondence with each other. Preferably, the number of subcarriers included in each group is made equal.

  In the present embodiment, each of the window processing units 11 to 1n acquires symbol timing from different subcarriers. More specifically, each of the window processing units 11 to 1n uses a subcarrier included in a group associated with each of the window processing units 11 to 1n to determine a reference position, and the FFT units 21 to 2n correspond to each other. Discrete Fourier transform is performed on the output signals of the window processing units 11 to 1n. Further, combining section 30 extracts and outputs only the signal associated with FFT section 2k from the subcarrier signals output by each FFT section 2k.

  Hereinafter, the operation of the communication apparatus according to the present embodiment will be described with reference to FIG. FIG. 6 shows a case in which three of the window processing units 11 to 13 are used. The window processing by the upper half-shaded window processing unit 11 is the same as the window processing by the center-shaded window processing unit 12. Are the window processing by the window processing unit 13 at the bottom of the screen. In FIG. 6, reference numeral 51 is a reference position used by the window processing unit 11, reference numeral 52 is a reference position used by the window processing unit 12, and reference numeral 53 is a reference position used by the window processing unit 13. In FIG. 6, the lowest frequency among the subcarriers included in the corresponding group is used.

  For example, as shown in FIG. 6A, each window processing unit starts window processing from each reference position. Accordingly, for example, as shown in FIG. 6B, even when the difference in delay amount is 3C, intersymbol interference can be avoided. In general, if the CP period is C and the number of sets of the window processing unit and the FFT unit to be used is n, even when the difference in delay amount is C × n, ideally, between symbols It becomes possible to suppress the influence of interference. In addition, although it demonstrated in the form which uses the symbol timing of the lowest frequency among the subcarriers contained in a corresponding group as a reference position, this invention is not limited to this, For example, a plurality of subcarriers Each symbol timing is monitored, and the latest timing may be used as a reference position, and this reference position may be used as a start position for extracting a signal used for discrete Fourier transform.

1 is a block diagram of a communication device according to the present invention. It is a figure explaining operation | movement of the communication apparatus by this invention in case there is no difference in delay amount. It is a figure explaining operation | movement of the communication apparatus by this invention in case there exists a difference in delay amount. It is explanatory drawing which shows that the same dispersion tolerance as the past can be achieved in a shorter CP period. It is a block diagram in other embodiment of the communication apparatus by this invention. It is a figure explaining operation | movement in other embodiment of the communication apparatus by this invention. 1 is a block diagram schematically showing an OFDM communication system. FIG. It is explanatory drawing of a cyclic prefix. It is a figure explaining the improvement of the dispersion tolerance by a cyclic prefix.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 IFFT part 2 CP processing part 3 Transmitting part 4 Receiving part 5, 11, 12, 1n Window processing part 6, 21, 22, 2n Discrete Fourier transform part 30 Coupling part

Claims (6)

An orthogonal frequency division multiplexing communication device,
First to n-th means for extracting a signal of a predetermined period from each symbol of the received signal and performing a discrete Fourier transform, where n is an integer equal to or greater than 2,
A coupling means;
With
The first to n-th means have a one-to-one correspondence with n groups obtained by dividing all subcarriers at n-1 frequency positions,
The combining means selects and outputs the signal of the subcarrier included in the corresponding group from each of the outputs of the first to nth means.
Communication device. 1st to n-th means, each of which determines the extraction start position of a signal extracted from each symbol, based on information indicating a symbol boundary of a predetermined subcarrier.
The communication apparatus according to claim 1. The first to n-th means use subcarriers included in a corresponding group for determining the extraction start position.
The communication apparatus according to claim 2. n is 2, and the first means and the second means use the same subcarrier for determining the extraction start position.
The communication apparatus according to claim 2. The number of subcarriers included in each group is equal,
The communication apparatus according to any one of claims 1 to 4. An orthogonal frequency division multiplex communication method comprising:
N branches the received signal from the first to the nth received signal, where n is an integer greater than or equal to 2,
Associating the first to n-th received signals with n groups obtained by dividing all subcarriers at n-1 frequency positions on a one-to-one basis;
Taking out a signal of a predetermined period from each of the symbols of the 1st to nth received signals and performing a discrete Fourier transform;
Selecting and outputting a signal of a subcarrier included in a corresponding group from signals obtained by discrete Fourier transform of the first to nth received signals;
Including methods.

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