WO2008038985A1

WO2008038985A1 - Apparatus and method of recording optical information and method of data encoding thereof and method of reproducing optical information

Info

Publication number: WO2008038985A1
Application number: PCT/KR2007/004686
Authority: WO
Inventors: Bi Woong Chung
Original assignee: Daewoo Electronics Corporation
Priority date: 2006-09-25
Filing date: 2007-09-21
Publication date: 2008-04-03
Also published as: KR20080027990A

Abstract

A holographic optical information recording apparatus includes: an encoder that encodes original data in an error correcting manner by the use of a low-density parity check code and that converts data, which has been encoded in the error correcting manner by the use of the low- density parity check code, into double-sized data; a special light modulator that modulates a first beam emitted from a light source by representing the data, which has been encoded by the use of the low-density parity check code and converted into double-sized data, into a pixel array so as to record a hologram on a recording medium by allowing the first beam to overlap with a second beam emitted from the light source. When recording optical information by the use of the LDPC, it is possible to improve acquisition efficiency and accuracy of likelihood information of the LDPC by converting the data into double-sized data and then mapping the double-sized data to pixels.

Description

APPARATUS AND METHOD OF RECORDING OPTICAL INFORMATION AND METHOD OF DATA ENCODING THEREOF AND METHOD OF REPRODUCING OPTICAL INFORMATION

Technical Field

[1] The present invention relates to optical information recording apparatus and method, a data encoding method, and an optical information reproducing method, and more particularly, to optical information recording apparatus and method, a data encoding method, and an optical information reproducing method, which use a hologram. Background Art

[2] A holographic optical information processing apparatus is a page-oriented memory and uses an input and output method of a parallel signal processing type. Accordingly, the holographic optical information processing apparatus can process data faster than a CD (Compact Disk) or a DVD (Digital Versatile Disk) that records and reproduces data in the unit of bit.

[3] The holographic optical information processing apparatus performs a recording operation by projecting an information beam including image information of recording data and a reference beam to an optical information recording medium so as to overlap with each other. The holographic optical information processing apparatus reproduces the original data by projecting the reference beam to the optical information recording medium, detecting a reproduced beam, which is diffracted and reproduced from the recording medium, by the use of a light-receiving array element such as a CMOS (Complementary Metal-Oxide Semiconductor) or a CCD (Charge Coupled Device), and performing a signal processing operation and a decoding process on the reproduced beam.

[4] However, due to a variation in channel characteristic such as contraction of the optical information recording medium, a detected data page may have errors at the time of detecting an image of a data page. For example, pixels of the data page (hereinafter, referred to as 'data pixels') and pixels of the light-receiving array element (hereinafter, referred to as 'detection pixels') may not match with each other due to misalignment therebetween. These errors may cause a fairly high bit error rate (BER).

[5] In order to reduce the BER, an error correction code (ECC) may be used. As the

ECC, a low-density parity check (LDPC) code that has performance approaching the Shannon's theoretical limitation of the channel capacity is known.

[6] The LDPC code is a linear block code in which most elements of a parity check matrix are "0". In general, a parity check code has a block which contains information symbols and parity check symbols which are a modulo sum of specific information symbols to constitute a codeword.

[7] The relations between the parity check symbols and the information symbols can be represented by a parity check matrix "H". The parity check matrix "H" can be represented by a set of linear homogeneous equations. That is, the LDPC code is a kind of parity check code and has a parity check matrix "H" of which most elements are "0" and the other elements are a small number of weights which are randomly distributed.

[8] A process of encoding the LDPC code with the parity check matrix "H" is as follows. When the parity check matrix "H" is obtained, a generator matrix "G" cor- responding to the parity check matrix "H" is obtained using the relation "GH =0". A codeword "C" corresponding to the information symbol block X can be obtained from the relation "C=XG" If the number of "1" per column of the matrix "H" (H=MxN) is "W" and the number of "1" per row is "Wx(NVM) "are constant, the matrix is called a regular LDPC code.

[9] If the number of " 1 " per each column is not constant and the number of " 1 " per row is not exactly equal to Wx(NM), the matrix is called an irregular LDPC code. In general, the irregular LDPC code has better error correction ability, but it is harder to embody the irregular LDPC code by hardware.

[10] Meanwhile, in a process of decoding the LDPC code, the most probably approximate codeword satisfying the relation where the product of the codeword and the parity check matrix "H" is equal to "0" is detected from received signal vectors.

[11] That is, the sum-product algorithm of the methods of decoding the LDPC code performs a soft decision iterative decoding process using probability values. In the sum-product algorithm, a decoding operation is repeatedly performed in which massages of probability are transmitted between nodes in the codeword graph so as to converge to the code word satisfying the maximum likelihood.

[12] As another method of decoding the LDPC code, an LLR algorithm using a log- likelihood ratio (LLR) is known. The LLR algorithm is described, for example, in Korean Patent No. 10-0538281.

[13] The LLR algorithm is described in brief as follows. The initial LLR of a LDPC decoder is obtained from the data pixel by calculating the probability when the data pixel is "0" and the probability when the data pixel is " 1". The obtained LLR is used as information for calculating the probability of pixel by partially inserting a specific mark disposed between the data pixels into the data image.

[14] On the other hand, when the data pixel corresponding to one LDPC code block has many noises or large darkness, the probability for acquiring accurate data from the date pixel is reduced. On the contrary, when the data pixel has many noises or small darkness, the probability for calculating accurate data is enhanced. Disclosure of Invention

Technical Problem

[15] The LDPC code can be decoded by accurate likelihood information on the data pixels in a specific block, but cannot be decoded by inaccurate likelihood information on the data pixel exceeding the error correction ability. Technical Solution

[16] The invention provides a data encoding method of encoding data in an error correction manner by the use of a low-density parity check code, optical information recording apparatus and method is used for the data encoding method, and an optical information reproducing method of reproducing data from a hologram recorded in a recording medium.

Advantageous Effects

[17] As described above, the method of decoding a low density check code and the optical information processing apparatus according to the invention have such an effect that when recording optical information by the use of the LDPC, it is possible to improve acquisition efficiency and accuracy of likelihood information of the LDPC by converting the data into double-sized data and then mapping the double-sized data to pixels.

Brief Description of the Drawings

[18] Fig. 1 is a block diagram illustrating an optical information processing apparatus according to an embodiment of the invention.

[19] Fig. 2 is a diagram schematically illustrating a 2:4 conversion in the optical information processing apparatus according to the embodiment of the invention.

[20] Figs. 3 to 5 are exemplary diagrams illustrating pixels corresponding to input bits in the 2:4 conversion according to the embodiment of the invention.

[21] Fig. 6 is a block diagram illustrating a process of encoding optical information in the optical information processing apparatus according to the embodiment of the invention.

[22] Fig. 7 is a block diagram illustrating a process of decoding optical information in the optical information processing apparatus according to the embodiment of the invention.

[23] Fig. 8 is a graph illustrating a bit error rate due to the 2:4 conversion according to the embodiment of the invention. Best Mode for Carrying Out the Invention

[24] A holographic optical information recording apparatus includes: an encoding unit that encodes data in an error correcting manner by the use of a low-density parity check code and that converts the data, which has been encoded in the error correcting manner by the use of the low-density parity check code, into double-sized data; and a spatial light modulator that modulates a first beam emitted from a light source by representing the data, which has been encoded by the use of the low-density parity check code and converted into double-sized data, into a pixel array so as to record a hologram on a recording medium by allowing the first beam to overlap with a second beam emitted from the light source.

[25] The encoding unit may include a low-density parity check encoder that encodes the original data in the error correcting manner by the use of the low-density parity check code and a converter for converting the low-density parity check code, which has been generated by the low-density parity check encoder, into double-sized code.

[26] The spatial light modulator may represent the low-density parity check code into a two-dimensional data page form. The size may be the number of bits.

[27] A holographic optical information recording method includes: encoding data in an error correcting manner by the use of a low-density parity check code and converting the data, which has been encoded in the error correcting manner by the use of the low- density parity check code, into double-sized data; modulating a first beam emitted from a light source by representing the data, which has been encoded by the use of the low-density parity check code and converted into the double-sized data, into a pixel array; and recording a hologram on a recording medium by allowing the first beam to overlap with a second beam emitted from the light source on the recording medium.

[28] The converted data represented in a pixel array form may have a two dimensional data page form. The size may be the number of bits.

[29] A data encoding method of a holographic recording apparatus includes: encoding original data in the error correcting manner by the use of a low-density parity check code; and converting the low-density parity check code into double-sized data. The size may be the number of bits. The converted data may be represented in a two- dimensional data page form by a spatial light modulator.

[30] A holographic optical information reproducing method includes: applying a beam to a recording medium having a recorded hologram formed by encoding data in an error correcting manner by the use of a low-density parity check code, converting the encoded data into double-sized data, and recording the converted data; and detecting a reproduced beam, which is reproduced from the hologram, in a two-dimensional data page form by the use of an optical information detector.

[31] Four times pixels of the data page detected by the optical information detector may be deconverted into two times data and then an error detection is performed on the de- converted data. The likelihood information may be calculated from the data page and is then decoded by the use of the low-density parity check code. The likelihood in- formation may be calculated by the use of information on the brightest pixel and the third brightest pixel in a four-pixel array unit of the data page.

[32] The likelihood information may satisfy the following Math Figure 1 :

[33] MathFigure 1

[34] where F denotes likelihood information of a pixel, ml denotes intensity of the brightest pixel, m3 denotes intensity of the third brightest pixel, and σ denotes an intensity variation of a pixel.

Mode for the Invention

[35] Now, preferred embodiments of the invention will be described in detail with reference to the attached drawings.

[36] Fig. 1 is a block diagram illustrating an optical information processing apparatus according to an embodiment of the invention.

[37] As shown in Fig. 1, an optical information processing apparatus 100 according to an embodiment of the invention includes a light source 110, a beam splitter 120, a multiplexer 130, a spatial light modulator 140, an optical information detector 150, an equalizer 160, an encoding unit 170, and a decoding unit 180.

[38] Light emitted from the light source 110 is split into a first beam S and a second beam R by the beam splitter 120. The first beam S goes through a first shutter 190a, is changed in optical path by a reflecting mirror, and is then incident on the spatial light modulator 140. The first beam S will be called a signal beam hereinafter. The second beam R goes through a second shutter 190b, is reflected by the multiplexer 130, and is incident on an optical information recording medium 200 at a determined angle. The second beam R will be called a reference beam hereinafter.

[39] The spatial light modulator 140 represents the encoded binary data sent from the data encoding unit 170 into data page in the unit of a twodimensional page. The data encoding unit 170 encodes original data (source data, user data, or input data) by the use of the lowdensity parity check (LDPC) code and provides the encoded data to the spatial light modulator 140 in the unit of a page.

[40] The spatial light modulator 140 optically modulates data page information sent from the data encoding unit 170 to represent twodimensional data pages and projects the data page to the signal beam S traveling to the optical information recording medium 200. When the reference beam R and the signal beam S are incident on the optical information recording medium 200, a hologram is generated in the optical in- formation recording medium 200 by means of interference of the reference beam R and the information beam S with each other.

[41] The multiplexer 130 controls the angle of the reference beam R incident on the optical information recording medium 200. That is, the multiplexer performs an angle multiplexing operation. The multiplexer 130 can be embodied by a rotating mirror like a galvanic mirror.

[42] Meanwhile, in order to reproduce recorded data, only the reference beam R is applied to the optical information recording medium 200. At the time of reproduction, the second shutter 190b allows the reference beam R split by the beam splitter 120 to travel to the recording medium. The first shutter 190a blocks the traveling of the signal beam S to the recording medium.

[43] The reference beam R incident on the recording medium through the second shutter

190b is diffracted by the interference pattern recorded in the optical information recording medium 200 to generate the reproduced beam which has an image of a data page. The reproduced beam is detected by the optical information detector 150 in the unit of the data page. The detected data page is equalized by the equalizer 160 and is decoded by the data decoding unit 180. The optical information detector 150 can be embodied by a lightreceiving array element such as a CMOS (Complementary Metal Oxide Semiconductor) or a CCD (Charge Coupled Device). An MMSE (Minimum Mean Square Error) equalizer or the like can be used as the equalizer 160.

[44] The data decoding unit 180 is a device for decoding the LDPC code. The data decoding unit 180 decodes the LDPC code output from the equalizer 160 to finally output the original data.

[45] On the other hand, the generation of the data page in the spatial light modulator 140 will be described in more detail.

[46] Fig. 2 is a diagram schematically illustrating the 2:4 conversion in the optical information processing apparatus according to the embodiment of the invention. The 2:4 conversion serves to convert input 2bit data into doublesized pixels, that is, 4 pixels. 2 bits are converted into 4 bits, which are mapped to an array of 4 pixels. The 4pixel array is represented by one ON pixel and three OFF pixels (see Fig. 2).

[47] The 2:4 conversion can be used to acquire likelihood information of the LDPC code at the time of reproducing optical information by comparing intensity values of 4 pixels with each other.

[48] First, in case of BPSK (Binary Phase Shift Keying) modulation with an AWGN

(Additive White Gaussian Noise) channel, the likelihood information can be obtained from Math Figure 2.

[49] MathFigure 2 F- ^2y 2 σ

[50] Here, F denotes the likelihood information of a pixel, y denotes a signal level of the

AWGN (Additive White Gaussian Noise) channel, and σ denotes a variation of noise.

[51] In this embodiment, the likelihood information can be obtained using Math Figure 2 and the intensity difference between pixels. That is, the likelihood information can be calculated using the intensity difference between pixels instead of y as a signal level of the AWGN channel, which can be expressed by Math Figure 3. [52] MathFigure 3

_ 2(?w l -ml)

Jr¹ 2 σ

[53] Here, F denotes the likelihood information of a pixel, ml denotes the intensity of the brightest pixel among 4 pixels, m2 denotes the intensity of the second brightest pixel among 4 pixels, and σ denotes the intensity variation of a pixel.

[54] That is, the likelihood information of the input 2-bit data can be always calculated constant, since the calculated likelihood information varies depending on the intensity ml of the brightest pixel among 4 pixels and the intensity m2 of the second brightest pixel among 4 pixels.

[55] On the other hand, by changing the pixel to be compared among 4 pixels, which are obtained by the 2:4 conversion, the likelihood information of a specific bit of the 2-bit data can be enhanced.

[56] Figs. 3 to 5 are exemplary diagrams showing the pixels for the 2:4 conversion according to this embodiment of the invention. The numerals assigned to the pixels indicate the order of brightness of the pixels.

[57] Fig. 3 shows a case where the first bit of the 2-bit data to be subjected to the 2:4 conversion is "0", Fig.4 shows a case where the other bit of the 2-bit data to be subjected to the 2:4 conversion is "0" and Fig. 5 shows a case where the bits of the 2-bit data to be subjected to the 2:4 conversion are always different from each other.

[58] First, as shown in Fig. 3, when the first bit of the 2-bit data is 0, the first pixel has the brightest intensity and the second pixel has the second brightest intensity commonly in the conversion of 2:4. The first-bit data of the 2-bit data to be subjected to the 2:4 conversion has the same value at positions corresponding to the intensity of the third pixel and the fourth pixel. That is, the first-bit data of the 2-bit data is not affected by the third and fourth pixels.

[59] As shown in Fig. 4, when the second bit of the 2-bit data is 0, the first pixel has the brightest intensity and the third pixel has the second brightest intensity in the 2:4 conversion. The second-bit data of the 2-bit data to be subjected to the 2:4 conversion has the same value at positions corresponding to the intensity of the second pixel and the fourth pixel. That is, the second-bit data of the 2-bit data is not affected by the second and fourth pixels.

[60] Accordingly, when the first bit and the second bit of the 2-bit data input under the

2:4 conversion condition are identical to each other, the likelihood information can be calculated by Math Figure 4.

[61] MathFigure 4

Jr¹ 2 σ

[62] Here, F denotes the likelihood information of a pixel, ml denotes the intensity of the brightest pixel, m3 denotes the intensity of the third brightest pixel, and σ denotes the intensity variation of a pixel.

[63] On the other hand, as shown in Fig. 5, when there is no common bit in the 2-bit data, the first pixel has the brightest intensity and the fourth pixel has the second brightest intensity in the 2:4 conversion. However, the likelihood information can be calculated by the known method, since the 2-bit data has no common data.

[64] Hereinafter, processes of encoding and decoding original data in the optical information processing apparatus 100 according to this embodiment will be described in detail with reference to the attached drawings. In the following description, the same elements are denoted by the same reference numerals as described above and shown in the figures.

[65] Fig. 6 is a block diagram illustrating an optical information encoding process of the optical information processing apparatus according to this embodiment, Fig. 7 is a block diagram illustrating an optical information decoding process of the optical information processing apparatus according to this embodiment, and Fig. 8 is a graph illustrating a bit error rate of the 2:4 modulation code encoding process according to this embodiment.

[66] As shown in Fig. 6, the optical information encoding process in the optical information processing process according to this embodiment can be performed by the data encoding unit 170 to record the optical information.

[67] Then, the data encoding unit 170 will be described in brief. The data encoding unit 170 includes an LDPC encoder 172 for encoding the input data in the error correction manner by the use of the LDPC code and a converter for converting the LDPC code generated by the LDPC encoder 172 into double-sized data. The converter serves to convert the 2-bit data into the 4-bit data in this embodiment.

[68] In the data encoding unit 170, in response to the input of optical information, the

LDPC code corresponding to data is generated from the LDPC encoder 172 and the generated LDPC code is converted by the 2:4 converter 174. At this time, the LDPC code which is converted by the 2:4 converter 174 is as described above.

[69] The data page having been subjected to the 2:4 conversion is projected to the signal beam S by the spatial light modulator 140. The signal beam S to which the data page has been projected is incident on the recording medium 200 to generate a hologram on the recording medium by means of interference with the reference beam R.

[70] On the other hand, as shown in Fig. 7, the decoding process for reproducing the optical information in the optical information processing apparatus is performed by the data decoding unit 180 of the optical information processing apparatus.

[71] The data decoding unit 180 will be described in brief now. The data decoding unit

180 includes a 2:4 deconverter 184 for acquiring the likelihood information for decoding from the data page detected by the optical information detector 150 and de- converting the data in the 2:4 manner and an LDPC decoder 182 for performing an LDPC decoding process using the likelihood information and the data acquired by the deconverter 184. The 2:4 decon version indicates that two data as the original data are obtained from the four pixels.

[72] In the data decoding unit 180, at the time of reproducing the optical information recorded in the recording medium 200, the likelihood information for decoding the LDPC code by means of the 2:4 deconversion and the optical information stored in the recording medium 200 can be reproduced as the original data.

[73] Fig. 8 shows a simulation result of the likelihood information resulting from the 2:4 conversion of the LDPC code in the optical information processing apparatus according to this embodiment, where a pixel spreading quantity of 1.4 Nyquist size and a BER (bit error rate) for decoding data by the use of the likelihood information obtained from the 2:4 deconversion using the LDPC code, which has a length of 1800 and a code encoding rate of 0.9, in the channel to which the AWGN(Additive White Gaussian Noise) is added. In this case, similarly, when the 2:4 conversion and the LDPC code are used, the capability is enhanced by 0.25 dB in comparison with the case where only the LDPC is used.

[74] Another element having another additional function may be added or any element may be replaced with another element. However, it should be considered that any modification belongs to the technical scope of the invention, as long as it includes the necessary elements of the invention. Industrial Applicability

[75] In the method of decoding a low-density parity check code and the optical information processing apparatus using the method according to the embodiments of the invention, when recording optical information by the use of the LDPC, it is possible to improve acquisition efficiency and accuracy of likelihood information of the LDPC by converting the data into double-sized data and then mapping the double-sized data to pixels. Accordingly, it is possible to enhance data processing efficiency of an apparatus for recording and reproducing data into a hologram of a twodimensional data page type.

[76]

Claims

[1] A holographic optical information recording apparatus, comprising: an encoding unit that encodes data in an error correcting manner by the use of a low-density parity check code and that converts the data, which has been encoded in the error correcting manner by the use of the low-density parity check code, into double-sized data; and a spatial light modulator that modulates a first beam emitted from a light source by representing the data, which has been encoded by the use of the low-density parity check code and converted into double-sized data, into a pixel array so as to record a hologram on a recording medium by allowing the first beam to overlap with a second beam emitted from the light source.

[2] The holographic optical information recording apparatus according to claim 1, wherein the encoding unit includes a low-density parity check encoder that encodes the original data in the error correcting manner by the use of the low- density parity check code and a converter for converting the low-density parity check code, which has been generated by the low-density parity check encoder, into double-sized code.

[3] The holographic optical information recording apparatus according to claim 2, wherein the spatial light modulator represents the low-density parity check code into a two-dimensional data page form.

[4] The holographic optical information recording apparatus according to claim 1, wherein the size is the number of bits.

[5] A holographic optical information recording method, comprising: encoding data in an error correcting manner by the use of a low-density parity check code and converting the data, which has been encoded in the error correcting manner by the use of the low-density parity check code, into double- sized data; modulating a first beam emitted from a light source by representing the data, which has been encoded by the use of the low-density parity check code and converted into the double-sized data, into a pixel array; and recording a hologram on a recording medium by allowing the first beam to overlap with a second beam emitted from the light source on the recording medium.

[6] The holographic optical information recording method according to claim 5, wherein the converted data represented in a pixel array form has a two dimensional data page form.

[7] The holographic optical information recording method according to claim 5, wherein the size is the number of bits. [8] A data encoding method of a holographic recording apparatus, the data encoding method comprising: encoding data in the error correcting manner by the use of a low-density parity check code; and converting the low-density parity check code into double-sized data. [9] The data encoding method according to claim 8, wherein the size is the number of bits. [10] The data encoding method according to claim 8, wherein the converted data is represented in a two-dimensional data page form by a spatial light modulator. [11] A holographic optical information reproducing method, comprising: applying a beam to a recording medium having a recorded hologram formed by encoding data in an error correcting manner by the use of a low-density parity check code, converting the encoded data into double-sized data, and recording the converted data; and detecting a reproduced beam, which is reproduced from the hologram, in a two- dimensional data page form by the use of an optical information detector. [12] The holographic optical information reproducing method according to claim 11, wherein four times pixels of the data page detected by the optical information detector are deconverted into two times data and then an error detection operation is performed on the deconverted data. [13] The holographic optical information reproducing method according to claim 11, wherein the likelihood information is calculated from the data page and is then decoded by the use of the low-density parity check code. [14] The holographic optical information reproducing method according to claim 13, wherein the likelihood information is calculated by the use of information on the brightest pixel and the third brightest pixel in a four-pixel array unit of the data page. [15] The holographic optical information reproducing method according to claim 13, wherein the likelihood information satisfies the following expression:

σ where F denotes likelihood information of a pixel, ml denotes intensity of the brightest pixel, m3 denotes intensity of the third brightest pixel, and σ denotes an intensity variation of a pixel.