CN108696470B - DMT modulation method for visible light communication - Google Patents

DMT modulation method for visible light communication Download PDF

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CN108696470B
CN108696470B CN201810604041.XA CN201810604041A CN108696470B CN 108696470 B CN108696470 B CN 108696470B CN 201810604041 A CN201810604041 A CN 201810604041A CN 108696470 B CN108696470 B CN 108696470B
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visible light
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CN108696470A (en
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梁晴晴
王瑾
曾福江
胡煌
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China University of Geosciences
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2626—Arrangements specific to the transmitter only
    • H04L27/2627—Modulators
    • H04L27/2628—Inverse Fourier transform modulators, e.g. inverse fast Fourier transform [IFFT] or inverse discrete Fourier transform [IDFT] modulators
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
    • H04B10/114—Indoor or close-range type systems
    • H04B10/116—Visible light communication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2668—Details of algorithms
    • H04L27/2673—Details of algorithms characterised by synchronisation parameters
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    • H04L27/2678—Blind, i.e. without using known symbols using cyclostationarities, e.g. cyclic prefix or postfix
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/345—Modifications of the signal space to allow the transmission of additional information
    • H04L27/3461—Modifications of the signal space to allow the transmission of additional information in order to transmit a subchannel
    • H04L27/3483—Modifications of the signal space to allow the transmission of additional information in order to transmit a subchannel using a modulation of the constellation points

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Abstract

A DMT modulation method and system of visible light communication, carry on serial-parallel conversion to form Q group of parallel binary data stream in serial data stream that irregular LDPC encodes after sending information first; secondly, mapping each group of binary data streams to a constellation diagram through QAM modulation; then performing IFFT modulation on the signal formed after mapping; adding a cyclic prefix to a packet formed by every 2Q IFFT output points in the IFFT modulated signal; finally, performing parallel-serial conversion on the signals added with the cyclic prefixes to form serial data streams; wherein each group of binary data streams is QAM-modulated and mapped to the signal formed on the constellation diagram as the first half signal C of the IFFT-modulated input signal0To CQ‑1The latter half of the signal satisfies C2Q‑q‑1＝Cq *In the formula, Q is 0,1, …, or Q-1, and represents a conjugate. The invention reduces the ISI of the VLC system, improves the frequency spectrum utilization rate and the data transmission rate of the system, and simultaneously improves the reliability and the accuracy of the system transmission information.

Description

DMT modulation method for visible light communication
Technical Field
The present invention relates to the field of optical communications, and more particularly, to a DMT modulation method and system for visible light communications.
Background
Visible Light Communication (VLC), a new wireless access technology that has been developed in recent years, has attracted wide attention worldwide due to its advantages of low power consumption, wide frequency band, high security, and no electromagnetic pollution. In a conventional VLC system, as shown in fig. 1, first, an electrical signal is converted into a light signal by a transmitter and transmitted through an LED1-4, and then a receiving-end Photodiode (PD) converts the received light power into a current. In VLC systems, LEDs may transmit data and illuminate simultaneously.
In a visible light communication system, DMT modulation can efficiently utilize the modulation bandwidth to achieve high spectrum utilization while being adaptive to ISI caused by multipath propagation. The multi-carrier modem technique can be implemented through DMT based on Fast Fourier Transform (FFT) algorithm. The invention aims to encode information transmitted in a VLC system by using QC-LDPC codes on the basis of FFT, and aims to reduce ISI of the VLC system, further improve the spectrum utilization rate and the data transmission rate of the system and simultaneously improve the reliability and the accuracy of information transmission of the system.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a technical scheme of a visible light communication DMT modulation method and system which is formed by coding information transmitted in a VLC system by using QC-LDPC codes on the basis of FFT (fast Fourier transform algorithm), and aims to reduce ISI (inter-symbol interference) of the VLC system, further improve the spectrum utilization rate and the data transmission rate of the system, and simultaneously improve the reliability and the accuracy of information transmission of the system.
According to the first aspect of the present invention, the technical solution adopted by the present invention to solve the technical problem is: a DMT modulation method for visible light communication is constructed, comprising the following steps:
(1) serial data streams formed after irregular QC-LDPC coding is carried out on the transmitted information are subjected to serial-parallel conversion to form a plurality of groups of parallel binary data streams;
(2) each group of binary data streams is mapped to a constellation diagram through QAM modulation;
(3) carrying out IFFT modulation on the signal formed after mapping;
(4) adding a cyclic prefix to a group formed by every 2Q IFFT output points in the IFFT modulated signal;
(5) carrying out parallel-serial conversion on the signals added with the cyclic prefixes to form serial data streams;
wherein, each group of binary data stream is mapped to the signal formed on the constellation diagram through QAM modulation, the signal is used as the first half signal of the input signal modulated by IFFT, and the latter half signal satisfies the following conditions:
C2Q-q-1＝Cq *,
wherein Q is 0,1, …, Q-1, C0To CQ-1For the first half, denotes the conjugate, CQTo C2Q-1For the latter half of the signal, Q is the number of groups of binary data streams in parallel in step (1).
Further, in the DMT modulation method for visible light communication of the present invention, C0And CQIs set to 0.
Further, in the DMT modulation method for visible light communication according to the present invention, the serial data stream formed in step (5) is used for digital-to-analog conversion, and then converted into an analog signal, and then transmitted through visible light.
Further, in the DMT modulation method for visible light communication of the present invention, during the encoding process of the irregular QC-LDPC code, the random information bits generated by the source are multiplied by the generator matrix to obtain the encoded codeword; wherein, the check matrix corresponding to the generating matrix is:
Figure GDA0002577262640000021
in the formula, H denotes a check matrix, I is an identity matrix having a size of L × L, P is an identity matrix after cyclic shift, a superscript of P denotes the number of cyclic shift, the size of the identity matrix of the check matrix H is m × n, and L is a prime number satisfying L > n > m.
Further, in the DMT modulation method for visible light communication of the present invention, the decoding method of the irregular QC-LDPC code includes the steps of:
s1, initialization: calculating to obtain an initial probability likelihood ratio message L (P) transmitted to each variable node i by the channeli) (ii) a Initializing an initial message L that a variable node i passes to its neighboring check node j according to the following formula (1)(0)(qij)：
L(0)(qij)＝L(Pi) (1)；
Wherein i is 1, 2, …, N, j is 1, 2, …, M, N is L N, M is L M;
s2, iterative processing; wherein, when carrying out the first iteration number l, the method comprises the following steps which are carried out in sequence:
s21, check node information processing:
for all check nodes j and variable nodes i ∈ R (j) adjacent to the check nodes j, in the L iteration, the message L transmitted to the check nodes by the variable nodes is calculated according to the formula (2)(l)(rji)：
Figure GDA0002577262640000031
S22, variable node information processing:
for all variable nodes i and check nodes j ∈ C (i) adjacent to the variable nodes i, at the time of the L iteration, a message L transmitted to the variable nodes by the check nodes is calculated according to the formula (3)(l)(qij)：
Figure GDA0002577262640000032
S23, decoding judgment: calculating hard decision information L of all variable nodes according to formula (4)(l)(qi)：
Figure GDA0002577262640000033
Then according to the hard decision information L(l)(qi) Deriving a decoding result
Figure GDA0002577262640000034
If L is(l)(qi) If greater than 0, then
Figure GDA0002577262640000035
Otherwise
Figure GDA0002577262640000036
S24, judgment of iteration stop: if it satisfies
Figure GDA0002577262640000037
Or the preset maximum iteration number is reached, stopping decoding and outputting the decoding result of the last decoding as the final decoding result, otherwise, returning to the step S21 to continue iteration; in the formula (I), the compound is shown in the specification,
Figure GDA0002577262640000038
is the transposition of the decoding result of this time;
wherein the irregular QC-LDPC code is a binary irregular QC-LDPC code, rji(b) Represented is the external probability information, q, passed from check node j to variable node iij(b) Representing the extrinsic probability information passed from variable node i to check node j, c (i) representing the set of all check nodes connected to variable node i, r (j) representing the set of all variable nodes connected to check node j, c (i) \\ j representing the set of all check nodes connected to variable node i except check node j, r (j) \\ i representing the set of all variable nodes connected to check node j except variable node i, and b representing binary codes 0 and 1.
Further, in the DMT modulation method for visible light communication of the present invention, in step S24, the decoding result
Figure GDA0002577262640000039
Is equal to [ c1c2… cN]。
Further, in the DMT modulation method for visible light communication according to the present invention, after performing IFFT modulation in step (3), the signals at the output end corresponding to each set of 2Q IFFT points are real-valued signals:
Figure GDA0002577262640000041
where, u (g) is a real value signal, g ═ 0,1, … 2Q-1.
Further, in the DMT modulation method for visible light communication according to the present invention, in step (4), the signal obtained by adding a cyclic prefix to the real-valued signal is:
Figure GDA0002577262640000042
according to another aspect of the present invention, there is provided a DMT modulation system for visible light communication, which performs DMT modulation on visible light communication using any one of the above DMT modulation methods for visible light communication.
The DMT modulation method and the DMT modulation system for the optical communication have the following beneficial effects: the ISI of the VLC system is reduced, the frequency spectrum utilization rate and the data transmission rate of the system are improved, and meanwhile, the reliability and the accuracy of system transmission information are improved. The QC-LDPC code is further used for coding, the decoding is simple, the ISI of a VLC system is reduced, the spectrum utilization rate and the data transmission rate of the system are further improved, and the reliability and the accuracy of system transmission information are improved.
Drawings
The invention will be further described with reference to the accompanying drawings and examples, in which:
fig. 1 is a conventional visible light communication system model;
fig. 2 is a flowchart of a method for decoding a QC-LDPC code based on DMT modulation;
fig. 3 is a block diagram of a VLC system based on DMT modulation;
FIG. 4 is a graph comparing BER for VLC and AWGN channels;
figure 5 is a graph comparing the BER of DMT-VLC systems for different length cyclic prefixes;
FIG. 6 is a graph comparing BER of DMT-VLC system for different code rates;
figure 7 is a graph comparing the BER of DMT-VLC system for different number of iterations.
Detailed Description
For a more clear understanding of the technical features, objects and effects of the present invention, embodiments of the present invention will now be described in detail with reference to the accompanying drawings.
In the present invention, a DMT modulation technique based on a VLC system is proposed to reduce ISI, while applying a QC-LDPC code to the DMT-VLC system for efficient coding.
In the invention, a DMT modulation system of QC-LDPC coding is mainly utilized to code the information to be transmitted through the QC-LDPC code, and meanwhile, an LLR-BP decoding algorithm based on log-likelihood ratio is utilized to decode the signal at the output end. And the performance of the QC-LDPC coded DMT modulation system under two different channels was analyzed.
The VLC system architecture is introduced as follows.
A. Channel analysis
A conventional optical wireless system, such as a conventional LED-based indoor VLC system (as shown in fig. 1), can be expressed by the following equation
Figure GDA0002577262640000051
Where y (t) and x (t) represent the amplitudes of the received signal and the transmitted signal, respectively, R represents a photoelectric conversion coefficient of the receiving end, h (t) represents an impulse response of a visible light channel, and n (t) represents Additive White Gaussian Noise (AWGN), assuming that the noise is independently and equally distributed.
DMT modulation is robust to scattered signals introduced by multipath effects due to the presence of a Cyclic Prefix (CP). The reflected light power is negligible and only the direct link is considered. The optical channel fading of the direct channel can be modeled according to the lambertian radiation pattern of the LED by:
Figure GDA0002577262640000052
the order m of the lambertian model may be represented by the following formula, i ═ ln2/ln (cos Φ)1/2) Wherein phi1/2Is the half power angle of the LED. A represents the physical area of the photodetector, d represents the distance between the emitting end and the receiving end, and phi represents the included angle between the emitted light and the normal direction of the emitted lightAnd ψ denotes an angle between the incident light and the receiver plane, the amplitude of the output signal in the time domain of each subcarrier at the receiving end can be expressed by the following equation:
Figure GDA0002577262640000053
B. code structure
Currently, research on visible light communication systems mainly focuses on modulation techniques, channel equalization techniques, and fast transmission rates. With the rapid development of key technologies and chips of visible light communication, future visible light communication will develop towards ultra-wideband, ultra-high speed, large coverage area and high reliability, including multimedia services with high requirements on error code performance, such as mobile big data and high-definition images. Therefore, a channel coding scheme of a VLC system, which has a low error floor, high-rate communication, and excellent performance, must be found. In this context, some simple block codes such as Reed-solomon (rs) codes or LDPC codes and turbo codes with strong error correction capability can be applied to VLC systems. LDPC codes are a class of linear block error correcting codes that can be described by a sparse Check Matrix h (parity Check Matrix h) or a Bipartite Graph (Bipartite Graph). The minimum hamming distance of an LDPC code increases linearly with increasing code length. When the posterior probability iterative decoding is carried out, the BER of the system is reduced along with the increase of the length of the code word; and the performance of the LDPC code is very close to the Shannon limit when the iterative decoding algorithm is adopted. Therefore, the LDPC code can be constructed by using a random structure or an algebraic method, but when the code length is short, the performance of the randomly constructed LDPC code is not ideal. And in this case a higher error floor is more likely to occur. In other words, a randomly constructed code makes decoding difficult for the system. In order to solve the problem, the invention provides a QC-LDPC code for effectively coding the transmission information of a system, wherein a check matrix of the QC-LDPC code is shown as the following correction array:
Figure GDA0002577262640000061
this is an irregular QC-LDPC code, I is an identity matrix of size L × L (x is the number of rows before and the number of columns after, and other positions are the same), and P is the identity matrix after cyclic shift. The size of the base matrix of the check matrix is m × n, and the size of the check matrix H is Lm × Ln. Wherein L ═ i, m ═ j, n ═ k, and i is a prime number satisfying i > k > j. It is clear that the check matrix H (i, j, k) is a full rank matrix because it is an upper triangular matrix and the elements on the diagonal are non-zero elements. So its code rate can be expressed as
Figure GDA0002577262640000062
Since the check matrix H (i, j, k) is in the form of an upper triangle, it can be efficiently encoded. And it can be easily verified because there is no 4-ring in its corresponding bipartite graph.
In the encoding process of the QC-LDPC code, the random information bit generated by the information source is multiplied by the generation matrix G to obtain the encoded code word, namely
c＝s·G (5)
Where s denotes a random information bit, c denotes an encoded codeword, and G denotes a generator matrix. Any one generator matrix can be permutated to reduce to:
G＝[I0|Q0](6)
this fixed form of the matrix is referred to as the generator matrix of the system. Wherein I0Is an identity matrix. Usually, the parity check matrix of the LDPC code is subjected to LU decomposition or gaussian elimination to obtain a generator matrix. Permuting the check matrix so that it is converted into: h1＝[P1|I1]Wherein is to P1Transposing to obtain Q0I.e. Q0＝P1’。
According to the expression of system transmission information, Belief Propagation (BP) decoding algorithms can be divided into probability-based BP decoding algorithms and log-likelihood ratio-based BP decoding algorithms. In the invention, the LLR-BP decoding algorithm is used for decoding the information at the receiving end of the system, the information transmitted among nodes of the LLR-BP decoding algorithm is a log-likelihood ratio, a large amount of operation is changed into addition operation, the operation time is reduced to a great extent, and the decoding speed is accelerated. Referring to fig. 2, the decoding steps are as follows:
the invention adopts binary LDPC code, firstly defining the following variables: r isji(b) (b is 0,1) represents external probability information transferred from the check node j to the variable node i, q isij(b) Representing the extrinsic probability information passed from variable node i to check node j, c (i) representing the set of all check nodes connected to variable node i, r (j) representing the set of all variable nodes connected to check node j, c (i) \\ j representing the set of all check nodes connected to variable node i except check node j, and r (j) \\ i representing the set of all variable nodes connected to check node j except variable node i.
S1, initialization
Firstly, an initial probability likelihood ratio message L (P) transmitted to each variable node i by a channel is calculatedi) I 1, 2, …, n, then initializing, for each variable node i and its adjacent check node j ∈ c (i), an initial message L passed by the variable node to the check node according to equation (7)(0)(qij)：
L(0)(qij)＝L(Pi) (7)
S2, iterative processing; wherein, the steps which are carried out in sequence as follows are included when the first iteration is carried out:
s21: check node information processing
Also called horizontal step, for all check nodes j and variable nodes i ∈ R (j) adjacent to the check nodes j, at the L-th iteration, the message L transmitted to the check nodes by the variable nodes is calculated according to the formula (8) or (9)(l)(rji)：
Figure GDA0002577262640000081
Or
Figure GDA0002577262640000082
S22: variable node information processing
Also called vertical step, for all variable nodes i and check nodes j ∈ C (i) adjacent to the variable nodes i, at the time of the L iteration, a message L transmitted to the variable nodes by the check nodes is calculated according to the formula (10)(l)(qij)：
Figure GDA0002577262640000083
S23: decoding decision
Hard decision information of all variable nodes is obtained by calculation
Figure GDA0002577262640000084
If L is(l)(qi) If greater than 0, then
Figure GDA0002577262640000085
Otherwise
Figure GDA0002577262640000086
S24: iteration stop decision
If it satisfies
Figure GDA0002577262640000087
Or the preset maximum iteration number is reached, stopping decoding and outputting the decoding result of the last decoding as the final decoding result, otherwise returning to S21 to continue the iteration. Wherein
Figure GDA0002577262640000088
Is equal to [ c1c2… cN]。
DMT modulation analysis
As described above, DMT modulation is a special multi-carrier modulation technique, which can divide a high-speed serial data stream into a plurality of parallel low-speed data streams and modulate the data streams onto sub-carriers with different frequencies, and the sub-carriers need to be orthogonal to each other, so that each sub-carrier can be ensured not to interfere with each other when transmitting data in a channel, thereby reducing the influence of multipath fading, and effectively suppressing the ISI problem caused by multipath effect, and further improving the spectrum utilization rate and data transmission rate of the system.
A block diagram of a VLC system based on DMT modulation is shown in fig. 3. The input high-speed serial data stream is converted into Q sets of parallel low-speed binary data streams through serial-to-parallel conversion. Each group of bit streams is mapped onto a constellation diagram by Quadrature Amplitude Modulation (QAM). The data after QAM constellation mapping is a complex-valued signal, which can be represented as Cq＝Aq+jBqWherein Q is 0,1, …, Q-1. These signals are then Fast Fourier Transform (FFT) modulated onto the corresponding carriers at the DMT transmitter. The baseband DMT modulated transmission sequence contains Q subcarriers, and 2Q IFFT points are required to obtain a real-valued signal. For 2Q input signals of the IFFT module, the first half of the signals are assigned to CqThe latter half is assigned a value of CqSatisfies the following formula:
C2Q-q-1＝Cq *(12)
wherein Q is 0,1, … Q-1. Normally, the first signal and the Q-th signal are set to 0, i.e., C0＝CQThen the final DMT sequence does not contain any dc signal. The signals at the output of the 2Q IFFT points are real-valued signals, which can be expressed as:
Figure GDA0002577262640000091
wherein g is 0,1, … 2Q-1.
Adding a cyclic prefix with the length of P into each 2Q IFFT point group is used for reducing the influence of ISI. In practice, the cyclic prefix is typically a portion of the signal u (g) that is copied and then inserted at the front of the signal u (g). Then the signal may now be represented as
Figure GDA0002577262640000092
Wherein g is 0,1, … 2Q + P-1.
Discrete time sampling values of the DMT time domain signal corresponding to each unit sequence with the length of (Q + P) can be obtained at the output end of the IFFT module through parallel-serial conversion. The output serial data is then passed through digital to analog conversion to a channel. In the receiving process, the signals pass through a photosensitive diode, an amplifier and a filter, and can be restored to binary data through analog-to-digital conversion. Assuming that the impulse response of the digital-to-analog converter and the filter response before the analog-to-digital conversion are not considered, the DMT frame signal received by the receiving end (before the analog-to-digital conversion) can be expressed as:
Figure GDA0002577262640000093
and transmitting the signal subjected to serial-parallel conversion and cyclic prefix deletion to an FFT module. Ideally, the signal at the output of the FFT module should be the original signal sent from the transmitting end to the IFFT module. In addition, the equalization processing of the channel is utilized to achieve the purpose of eliminating or reducing intersymbol interference caused by parameter changes of active or passive devices in a transmitter and a receiver. The resulting signal is demapped and then the signal is transmitted in a DMT-VLC system by parallel-to-serial conversion.
Results of the experiment
In this section, the effect of cyclic prefix length, iteration number and code rate on the performance of DMT-VLC system is mainly studied. The invention uses the bit error rate of the received signal under the condition of different signal-to-noise ratios as the judgment standard of the quality of the received signal. Meanwhile, the performance of the QC-LDPC code under two different channels is researched by utilizing an LLR-BP decoding algorithm.
According to the QC-LDPC code constructed as described above, a check matrix of size Lm × Ln is constructed, where L is 31, m is 50, and n is 70. The signal at the receiving end can be expressed as:
1. when the signal at the transmitting end is 0, the conditional probability density function of the received signal can be expressed as:
Figure GDA0002577262640000101
2. when the signal at the transmitting end is 1, the conditional probability density function of the received signal can be expressed as:
Figure GDA0002577262640000102
wherein H0And H1Representing the transmitted signals as 0 and 1, respectively. Sigma2Representing the log amplitude variance. I represents light intensity. Because a binary QC-LDPC code is selected, the LLR-BP algorithm is adopted to decode the optical signal which is subjected to QC-LDPC coding.
And simulating the encoding performance of the QC-LDPC code under two different channel conditions, namely a VLC channel and an AWGN channel. The experimental results are shown in fig. 4, and it can be seen that, when the QC-LDPC coding is not performed, the performance of the VLC channel is significantly worse than that of the AWGN channel. This is because there is more background noise, ISI, and multipath interference in the VLC channel than in the AWGN channel. Therefore, it is necessary to adopt QC-LDPC coding in VLC channels to improve the performance of VLC systems. As expected, it can be seen from fig. 4 that the VLC system using the QC-LDPC coding can achieve a higher coding gain than the system not using the QC-LDPC coding.
As can be seen from fig. 5, when the length of the cyclic prefix is set to 10, the reliability of the system is significantly improved. But when the length of the cyclic prefix is increased to 30, the BER of the system is increased instead. According to the simulation result and numerical analysis, the situation that in a VLC system, the arrangement of the cyclic prefix in the DMT modulation module plays a crucial role in improving the anti-interference capability of the system can be obtained. However, the cyclic prefix is too long, which may result in an increase in complexity of system hardware, and further affect the data transmission rate of the system, and may occupy more hardware resources. Therefore, for different channel environments, it is necessary to select a cyclic prefix with a suitable length to ensure that the system is in a state of optimal performance, while avoiding the problem of excessive hardware complexity caused by DMT modulation.
As an important parameter of the LDPC code, the selection of the code rate has a large influence on the encoding performance of the LDPC code. FIG. 6 is a diagram showing the performance of four LDPC codes with the same code length but different code rates in a DMT-VLC system. According to simulation results, the performance of the QC-LDPC code is reduced with the increase of the code rate, because when the code rate is increased, the check bits in the code are gradually reduced, and the error correction capability of the code is also reduced. Therefore, an appropriate code rate is selected to obtain the best coding performance. In the DMT-VLC system proposed in the present invention, the code rate for achieving the optimal performance should be set to 0.4.
The number of iterations is an important factor affecting the decoding quality. In the DMT-VLC system, the decoding algorithms with different iteration numbers are simulated, and the influence of the iteration numbers on the decoding performance is checked, and the simulation result is shown in fig. 7. As can be seen from fig. 7, the accuracy of the system to transmit data increases as the number of iterations increases. But when the number of iterations increases to a certain value, the performance of the system does not improve. Considering that the larger the number of decoding iterations is, the higher the computational complexity of the system becomes, and efficient decoding cannot be performed, the number of decoding iterations is set to 50 in the present invention.
While the present invention has been described with reference to the embodiments shown in the drawings, the present invention is not limited to the embodiments, which are illustrative and not restrictive, and it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (7)

1. A DMT modulation method for visible light communication, comprising the steps of:
(1) serial data streams formed after irregular QC-LDPC coding is carried out on the transmitted information are subjected to serial-parallel conversion to form a plurality of groups of parallel binary data streams;
(2) respectively mapping each group of binary data streams to a constellation diagram through QAM modulation;
(3) carrying out IFFT modulation on the signal formed after mapping;
(4) adding a cyclic prefix to a group formed by every 2Q IFFT output points in the IFFT modulated signal;
(5) carrying out parallel-serial conversion on the signals added with the cyclic prefixes to form serial data streams;
wherein, each group of binary data stream is mapped to the signal formed on the constellation diagram through QAM modulation, the signal is used as the first half signal of the input signal modulated by IFFT, and the latter half signal satisfies the following conditions:
C2Q-q-1＝Cq *,
wherein Q is 0,1, …, Q-1, C0To CQ-1For the first half, denotes the conjugate, CQTo C2Q-1For the latter half of the signals, Q is the number of groups of binary data streams in parallel in step (1);
in the encoding process of the irregular QC-LDPC encoding, multiplying random information bits generated by an information source by a generating matrix to obtain encoded code words; wherein, the check matrix corresponding to the generating matrix is:
Figure FDA0002577262630000011
in the formula, H denotes a check matrix, I is an identity matrix having a size of L × L, P is an identity matrix after cyclic shift, a superscript of P denotes the number of cyclic shift, the size of the identity matrix of the check matrix H is m × n, and L is a prime number satisfying L > n > m.
2. The DMT modulation method for visible light communication of claim 1, wherein C is0And CQIs set to 0.
3. The DMT modulation method for visible light communication of claim 1, wherein the serial data stream formed in step (5) is used for digital-to-analog conversion, converted into analog signals, and then transmitted through visible light.
4. The DMT modulation method for visible light communication according to claim 1, wherein the decoding method of the irregular QC-LDPC code comprises the steps of:
s1, initialization: calculating to obtain an initial probability likelihood ratio message L (P) transmitted to each variable node i by the channeli) (ii) a Initializing an initial message L that a variable node i passes to its neighboring check node j according to the following formula (1)(0)(qij)：
L(0)(qij)＝L(Pi) (1)；
Wherein i is 1, 2, …, N, j is 1, 2, …, M, N is L N, M is L M;
s2, iterative processing; wherein, when carrying out the first iteration number l, the method comprises the following steps which are carried out in sequence:
s21, check node information processing:
for all check nodes j and variable nodes i ∈ R (j) adjacent to the check nodes j, in the L iteration, the message L transmitted to the check nodes by the variable nodes is calculated according to the formula (2)(l)(rji)：
Figure FDA0002577262630000021
S22, variable node information processing:
for all variable nodes i and check nodes j ∈ C (i) adjacent to the variable nodes i, at the time of the L iteration, a message L transmitted to the variable nodes by the check nodes is calculated according to the formula (3)(l)(qij)：
Figure FDA0002577262630000022
S23, decoding judgment: calculating hard decision information L of all variable nodes according to formula (4)(l)(qi)：
Figure FDA0002577262630000023
Then according to the hard decision information L(l)(qi) Deriving a decoding result
Figure FDA0002577262630000024
If L is(l)(qi) If greater than 0, then
Figure FDA0002577262630000025
Otherwise
Figure FDA0002577262630000026
S24, judging iteration stop: if it satisfies
Figure FDA0002577262630000027
Or the preset maximum iteration number is reached, stopping decoding and outputting the decoding result of the last decoding as the final decoding result, otherwise, returning to the step S21 to continue iteration; in the formula (I), the compound is shown in the specification,
Figure FDA0002577262630000028
is the transposition of the decoding result of this time;
wherein the irregular QC-LDPC code is a binary irregular QC-LDPC code, rji(b) Represented is the external probability information, q, passed from check node j to variable node iij(b) Representing the extrinsic probability information passed from variable node i to check node j, c (i) representing the set of all check nodes connected to variable node i, r (j) representing the set of all variable nodes connected to check node j, c (i) \\ j representing the set of all check nodes connected to variable node i except check node j, r (j) \\ i representing the set of all variable nodes connected to check node j except variable node i, and b representing binary codes 0 and 1.
5. According to claimThe DMT modulation method for visible light communication as claimed in claim 4, wherein the decoding result is obtained in step S24
Figure FDA0002577262630000031
Is equal to [ c1c2…cN]。
6. The DMT modulation method for visible light communication according to claim 2, wherein after IFFT modulation in step (3), the signals at the corresponding output terminals of each group of 2Q IFFT points are real-valued signals:
Figure FDA0002577262630000032
where, u (g) is a real value signal, g ═ 0,1, … 2Q-1.
7. The DMT modulation method for visible light communication according to claim 6, wherein in step (4), the real-valued signal added with a cyclic prefix is:
Figure FDA0002577262630000033
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