CN116094893B - Ocean buoy OFDM opportunity communication method based on code-rate-free code - Google Patents
Ocean buoy OFDM opportunity communication method based on code-rate-free code Download PDFInfo
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Abstract
The invention relates to a ocean buoy OFDM opportunity communication method based on code-rate-free codes, which generates code-rate-free code information according to the number of source symbols; dividing original data into a plurality of source data blocks, for one source data block, obtaining a precoding matrix by each module without code rate code coding information, carrying out matrix multiplication on an inverse matrix of the precoding matrix and a source symbol in a binary domain to generate an intermediate symbol, and further generating a coding symbol; then forming a data packet and transmitting the data packet in a marine buoy channel through an OFDM communication system; and acquiring a precoding decoding matrix according to the code symbol number of the received data packet, performing row transformation and element elimination in a binary domain, performing binary domain operation on code symbols of corresponding rows, combining the obtained different source data blocks, and recovering the information source data. The invention can utilize limited communication opportunities in complex ocean channel environment, and improve communication reliability and effectiveness.
Description
Technical Field
The invention belongs to the technical field of wireless communication, and particularly relates to an ocean buoy OFDM opportunity communication method based on code-rate-free codes.
Background
Orthogonal frequency division multiplexing (Orthogonal Frequency Division Multiplexing, OFDM) is a special multi-carrier modulation technique that allows the spectrum of the individual sub-carrier channels to overlap each other, making full use of the spectrum resources. In the time domain, OFDM can effectively reduce the problem of inter-symbol interference (Inter Symbol Interference, ISI) caused by multi-path delay spread of a marine wireless channel by adding a cyclic prefix. In the frequency domain, the frequency selective fading of the marine wireless channel due to multipath can affect part of OFDM subcarriers, but the fading on each subchannel is not frequency selective any more, and can be regarded as flat fading. In addition, the OFDM modulation and demodulation technology can be conveniently realized by using inverse fast fourier transform (Inverse Fast Fourier Transform, IFFT) and fast fourier transform (Fast Fourier Transformation, FFT) algorithms, and is very suitable for the requirements of the ocean buoy communication system for low power consumption and miniaturization.
The code rate-free code technology is used as a code coding technology without fixed code rate, adopts a random coding scheme based on probability distribution, can continuously generate any number of coding symbols, and can successfully decode only by receiving a certain number of coding symbols by a receiving end. Because only the number of the received code symbols is concerned, but not the specific code symbols, the code rate-free code can adaptively adjust the code rate under the condition of unknown channel state information, fully utilizes the communication opportunity and improves the reliability and the effectiveness of communication. Common code-rate-free code techniques are LT codes and Raptor codes.
Chinese patent CN 111917517A discloses an ultra-long offshore communication method, in which in order to overcome the problem of bit errors caused by small-scale fading in the ocean, an OFDM communication method based on low-density parity check codes (Low Density Parity Check, LDPC) is adopted, where the LDPC code is a forward error correction technique (Forward Error Correction, FEC) that can correct a certain number of bit errors in the physical layer. The disadvantage of the communication method is that firstly, when large-scale fading caused by complex ocean environment is encountered, the signal-to-noise ratio of communication is lower, the accuracy of synchronization of the communication is reduced, the bit error rate is increased, when the number of the error bits exceeds the error correction threshold of the LDPC code, the data transmission fails, the receiving end cannot receive the information of the source, and the communication reliability cannot be guaranteed. And secondly, the LDPC code is a physical layer error correction code with a fixed code rate, and the change of the ocean channel state cannot be automatically adapted by adjusting the code rate.
Through the above analysis, the problems and defects existing in the prior art are as follows: the ocean buoy communication channel has complex environment, and wireless communication can be affected by small-scale fading such as frequency selective fading and time selective fading, and random large-scale fading due to the seawater film covering antenna. The traditional FEC technology has limited error correction capability, is easy to cause data loss when suffering from large-scale fading, and cannot guarantee reliability. And the traditional FEC technology is fixed code rate coding, can not automatically adapt to the complex and changeable channel environment of the ocean, and automatically adjusts the code rate. The conventional HARQ technology causes that the transmitting end continuously retransmits some data packets overtime when the state of the ocean channel is poor for a long time, and the effectiveness of the communication system is low. Secondly, when the ocean channel state is poor, the reliability of the feedback link cannot be guaranteed by the receiving end, and the ACK feedback information can be lost, so that the transmitting end repeatedly transmits the successfully received data packet, and the channel resource and the communication opportunity are wasted. The traditional OFDM communication system is not designed for burst communication in marine complex channel environment, and the accuracy of OFDM symbol transmission is lower under the conditions of time-varying channel and low signal-to-noise ratio.
The difficulty of solving the problems and the defects is as follows: in a channel environment of a marine complex environment, a burst OFDM communication scheme needs to be provided at a physical layer, so that the accuracy of time synchronization can be ensured under a lower signal-to-noise ratio, carrier frequency synchronization can be ensured by compensating frequency offset under the condition of Doppler frequency shift, the orthogonality of OFDM subcarriers is protected, and more reliable physical layer bit transmission is provided. Under the condition that ocean buoy communication resources and opportunities are limited, a coding and transmission scheme capable of guaranteeing reliability with error correction capability and automatically adapting to channel state change and adjusting code rate is needed.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention aims to provide a ocean buoy OFDM opportunity communication method based on code-rate-free codes, so that limited communication opportunities are utilized in a complex ocean channel environment, and the communication reliability and effectiveness are improved.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
an ocean buoy OFDM opportunity communication method based on code-rate-free codes comprises the following steps:
step 1, generating code rate-free code information according to the number of source symbols at a transmitting end; the code rate-free code coding information is an LDPC coding matrix, an HDPC coding matrix and an LT coding matrix;
step 2, dividing the original data into a plurality of source data blocks, wherein each source data block comprises K source symbols;
step 3, for a source data block, combining the LDPC coding matrix and the HDPC coding matrix of the source symbol and the first K rows of the LT coding matrix to obtain a precoding matrix A, and performing matrix multiplication on the inverse matrix of the precoding matrix A and the source symbol in a binary domain to generate an intermediate symbol C;
step 4, generating a code symbol, wherein the first K bits of the code symbol are input source symbols, the K+n bits are repair symbols, the code symbol is obtained by multiplying an intermediate symbol C and an LT code matrix in a binary domain, and when the confirmation of a receiver to the source data block is received, the generation is stopped, wherein n is a natural number;
step 5, forming a data packet and transmitting the data packet in a marine buoy channel through an OFDM communication system;
step 6, at the receiving end, according to the code symbol number of the received data packet, finding out the corresponding LT code matrix information, and combining with the LDPC code matrix and the HDPC code matrix to form a pre-code decoding matrix;
step 7, performing row transformation and cancellation on the received pre-coding decoding matrix in a binary domain, performing binary domain operation on coding symbols of corresponding rows, if decoding is successful, performing confirmation feedback on the source data block number of the partial data packet, and then coding the next source data block;
and 8, combining the obtained different source data blocks to recover the information source data.
Compared with the prior art, the OFDM communication system is used as a physical layer bit transmission system, and improved methods such as time synchronization, frequency synchronization, channel equalization and the like are adopted. And meanwhile, the information source information is subjected to block coding and sending by adopting a code rate-free code coding technology systemizing a Raptor coding and inactivating decoding algorithm, and is recovered at a receiver after decoding. It has the following advantages:
first, the OFDM physical layer transmission system has higher spectrum utilization rate, fully utilizes spectrum resources, and has higher effectiveness in ocean buoy channel environment.
Secondly, the OFDM physical layer transmission system is designed aiming at ocean buoy channel parameters, has higher adaptability to Doppler frequency shift and multipath effect influence of buoy-ship communication environment, can ensure certain reliability under the condition of lower signal-to-noise ratio of a received signal caused by the coverage of a seawater film and the shielding of sea waves, and has higher reliability under the ocean buoy channel environment.
Thirdly, the systematic Raptor code coding method is a code rate-free code scheme, and when the signal to noise ratio of a received signal is randomly changed due to the coverage of a seawater film and the shielding of ocean waves on a ocean buoy, the code rate can be adjusted by self-adapting to the channel change, so that a communication opportunity window is fully utilized, and the efficiency is improved. And because of its systematic code characteristics, the receiver can recover the source symbol directly without decoding if the first K code symbols it transmits, i.e., the source symbol itself, can be received without errors.
Furthermore, the time synchronization algorithm adopted by the OFDM physical layer transmission system is a dimensional iterative trapezoidal search time synchronization algorithm, and has higher time synchronization accuracy under the condition that the signal-to-noise ratio of a received signal is low when the ocean buoy is covered by a seawater film or is blocked by ocean waves. Meanwhile, the device has one-dimensional search characteristics, is simple in structure and is easier to realize by hardware.
Furthermore, the inactivation decoding algorithm of the Raptor code adopted by the invention can ensure that the source symbols can be successfully decoded and recovered with very high success rate under the condition that the number of the source symbols is slightly more than that of the source symbols. And the advantages of BP and GE decoding methods are combined, so that the method has lower decoding complexity and accelerates the decoding speed.
In summary, the physical layer OFDM communication system of the ocean buoy OFDM opportunistic communication method based on the code-rate-free code has strong adaptability to the ocean buoy channel environment, can effectively resist small-scale fading caused by Doppler shift and multipath effect of a ship-buoy channel, has certain transmission capacity when the ocean buoy is covered by a seawater film and has lower signal to noise ratio due to wave shielding, and has higher effectiveness and reliability as a whole; the code rate-free code coding and decoding transmission scheme based on the systematic Raptor code can ensure the data transmission reliability only by a small amount of confirmation feedback to the source data block, and can adapt to the change of the ocean buoy channel environment to change the coding code rate at the same time, thereby fully utilizing the communication opportunity. The method has strong adaptability to the ocean buoy communication environment as a whole, and the buoy communication has high reliability and effectiveness.
Drawings
Fig. 1 is a schematic block diagram of an ocean buoy OFDM opportunistic communication system based on code-rate-free codes provided by an embodiment of the invention.
Fig. 2 is a schematic diagram of source block segmentation of source data according to an embodiment of the present invention.
Fig. 3 is a schematic diagram of generating intermediate symbols by Raptor precoding according to an embodiment of the present invention.
Fig. 4 is a schematic diagram of generating a repair symbol by the Raptor encoder according to the embodiment of the present invention.
Fig. 5 is a schematic diagram of an OFDM physical layer transmission frame structure according to an embodiment of the present invention.
Fig. 6 is a schematic diagram of a time synchronization algorithm according to an embodiment of the present invention.
Fig. 7 is a schematic diagram of a Raptor decoding process according to an embodiment of the present invention.
Fig. 8 is a flowchart of a marine buoy OFDM opportunistic communication system based on code-rate-less codes provided by an embodiment of the invention.
Detailed Description
The present invention will be described in further detail with reference to the following examples in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
As a wireless communication technology for providing data transmission to a marine buoy, a marine buoy communication technology is being developed toward high reliability, high efficiency, miniaturization, low power consumption, and the like. However, due to the complex marine environment in which the ocean buoy wireless communication channel is located, not only multipath delay spread and Doppler frequency shift are caused by the scattering of sea waves and the relative movement of the buoy, and small-scale fading such as frequency selective fading and time selective fading is generated, but also random large-scale fading is generated due to the coverage of the antenna by the sea water film caused by the sea waves, so that the ocean buoy communication environment is very bad, the communication opportunity is limited, and the data transmission with high reliability and high effectiveness is very difficult. Therefore, a need exists for a marine buoy opportunistic communication system that can effectively resist small-scale fading and random large-scale fading of a marine buoy channel, and that fully exploits the communication "opportunities".
The invention provides a ocean buoy OFDM opportunistic communication method based on code-rate-free codes, wherein a physical layer of the system adopts an OFDM modulation mode to transmit information bits, and adopts modes such as channel coding, a time-frequency synchronization algorithm, channel estimation and the like to reduce the bit error rate of an OFDM communication system under the influence of small-scale fading such as multipath effect, doppler frequency shift and the like. Meanwhile, the system Raptor code is adopted as a transmission scheme of code-rate-free code to ensure the reliability and the effectiveness of communication, so that the system Raptor code can automatically adapt to the random channel change of the ocean buoy caused by the influence of seawater film coverage, sea wave shielding and the like, the code rate is passively adjusted, the communication opportunity of the ocean buoy is fully utilized, decoding can be completed only by receiving a bit of code symbols with the number of source symbols at a receiving end, and the effectiveness of the system is improved. The system has higher communication reliability and effectiveness under the complex ocean channel environment.
Specifically, as shown in fig. 1, the ocean buoy OFDM opportunistic communication method based on no code rate codes of the invention is characterized in that a transmitting end transmits signals to a receiving end through an ocean buoy wireless channel, the receiving end decodes and recovers information source data, and mainly comprises the following steps,
and step 1, generating code rate-free code information.
And generating code-rate-free code coding information such as an LDPC coding matrix, an HDPC coding matrix, an LT coding matrix and the like which are needed by coding in advance according to the number of source symbols.
Illustratively, according to the number K of source symbols, the number S of rows of the LDPC encoding matrix and the number H of rows of the HDPC encoding matrix may be obtained by table lookup, where the LDPC encoding matrix is an s×h-dimensional low-density parity check matrix, and is composed of K/S cyclic submatrices with columns S, and the weight of the submatrices is 3. The HDPC encoding matrix is a high density parity check matrix of dimension H x (s+k), each column of the matrix is generated from an enumerated binary reflective gray sequence, and each column has a column weight of H/2. The LT encoding matrix is randomly generated from the source symbol number K and the degree distribution function. Thus, each different K value corresponds to a unique LDPC coding matrix, HDPC coding matrix, and LT coding matrix, respectively.
Illustratively, the present invention may employ Raptor coding.
And 2, source data block segmentation.
The source data block dividing mode is shown in fig. 2, the transmitting end calculates the block number N of the original data according to the bit number S, the source symbol number K and the symbol length T of the source data, divides the original data into a plurality of source data blocks, and fills zero at the tail part of the data which is not full. Each source data block contains K source symbols.
The specific formula is as follows:
the resulting source symbol may be represented as x= (X) 1 ,x 2 ,…,x K ) T 。
And 3, pre-coding by a raptor to generate an intermediate symbol C.
Referring to fig. 3, for a source data block, a transmitting end reads a source symbol thereof and combines the source symbol with a first K rows of an LT coding matrix according to an LDPC coding matrix and an HDPC coding matrix to obtain a precoding matrix a, and performs matrix multiplication on an inverse matrix of the precoding matrix a and the source symbol in a binary domain to generate an intermediate symbol C.
Specifically, the LDPC code matrix ends up with the unit matrix I of S dimension S And 0 matrix Z of S x H 1 Unit array I of H dimension is complemented at end of HDPC coding matrix H S is the number of lines of the LDPC coding matrix, and H is the number of lines of the HDPC coding matrix; the intermediate symbol C is expressed as:
C＝A -1 D
wherein D is an input symbol consisting of S+H all 0 symbols Z 2 And K coded symbols, the number L of the generated intermediate symbols C is equal to the sum of S, H, K.
And 4, generating a Raptor coding symbol.
The first K bits of the coding symbol output by the encoder are input source symbols, the K+n bits are repair symbols, the first K bits are obtained by multiplying the intermediate symbols C and LT coding matrixes in a binary domain by the matrixes, and the generation is stopped when the confirmation of the source data block of the receiver is received; where n is a natural number, i.e., n=1, 2,3 ….
Specifically, the Raptor code symbol of the present invention can be expressed as:
E＝(x 1 ,x 2 ,…,x K ,e K+1 ,e K+2 ,…,e K+n ) T
wherein e K+1 ,e K+2 ,…,e K+n For repair symbols, the generation is shown in fig. 4, expressed as:
e K+1 ,e K+2 ,…,e K+n ＝E (K+1,K+2,…,K+n) ＝G LT(K+1,K+2,…,K+n) C
G LT(K+1,K+2,…,K+n) a generator matrix encoded for LT.
After the encoded symbols are generated by the above steps, the data packets may be assembled and transmitted via the ocean buoy channel, which is described in detail below in steps 5-21.
And 5, filling the packet header, taking the source data block number and the Raptor coding symbol number as packet header information, and filling the packet header information before the corresponding coding symbols to form a data packet.
And 6, checking the codes. In this embodiment, CRC encoding is used, the encoded symbol after filling the packet header is CRC encoded, and the obtained CRC check code is filled into the tail.
Specifically, the generator polynomial of the CRC check code can be expressed as follows:
G(x)＝x 15 +x 11 +x 5 +1
the data after the step 6 is an OFDM system data packet, and the following steps 7, 8, 9 and 10 are all performed with the OFDM system data packet as a data processing unit.
And 7, the physical layer channel codes, the coding gain is obtained, and the reliability of physical layer bit transmission is improved.
In the embodiment of the present invention, there are various ways of channel coding, such as RS code, convolutional code, polar code, etc., but the code rate needs to be set to 1/2.
And 8, QPSK mapping, namely mapping the bits after channel coding into QPSK symbols of an IQ plane.
In step 9, scrambling, in order to avoid that the QPSK mapping points caused by long connection "0" or "1" are too concentrated and affect the time synchronization performance of the receiver, the QPSK symbols are multiplied by the locally fixed scrambling sequence, so that the phases of the QPSK symbols are uniformly distributed.
In the embodiment of the invention, the scrambling mode is to multiply QPSK symbols with the same length by QPSK symbols with each IFFT point number, the phases of the QPSK symbols after scrambling are uniformly distributed on a constellation diagram, excessive aggregation of constellation points is avoided, and the scrambling sequence is as follows, wherein N is IFFF point number.
Step 10, ifft transformation. After converting serial data stream into parallel data, through IFFT conversion, frequency domain signal is converted into OFDM symbol of time domain, and cyclic prefix can be added simultaneously so as to resist influence of ISI and ICI.
And 11, framing. After a certain number of OFDM symbols are generated in step 10, they are combined with the preamble training sequence into frames, and zero padding is performed at the end of the frames to prevent the influence of frame data smearing on the next frame.
A frame format of the invention is shown in figure 5, wherein a preamble sequence is generated by two sections of identical frequency domain CAZAC sequences through N-point IFFT transformation, and is marked as A (N), a cyclic prefix CP and a cyclic suffix CS form a preamble training sequence, the length of the preamble training sequence is identical to the number of IFFT points, the number of the preamble training sequence is N, and N OFDM symbols and zero padding at the tail of the frame are arranged after the preamble training sequence.
Step 12, radio frequency transmission. The baseband signal is multiplied by the carrier wave to become a radio frequency signal, so that wireless transmission is facilitated.
And step 13, ocean buoy channel transmission. When the buoy is not covered by a seawater film and is shielded by sea waves, the ship-buoy channel is mainly influenced by Doppler frequency shift effect and multipath effect, wherein the Doppler frequency shift effect is generated by relative motion of the ship-buoy, and the multipath mainly comprises a direct path, a sea surface reflection path and a scattering path of rough sea surface, and is a communication opportunity. When the buoy is covered by sea water or is influenced by sea wave shielding, the signal to noise ratio of the received signal can be greatly reduced, communication is interrupted, and almost all the transmitted signals can not be received.
And step 13, radio frequency receiving. The radio frequency signal is multiplied by the carrier wave to be reduced into a baseband signal, and the baseband signal affected by the channel is sent to a receiving end.
Step 14, time synchronization, for detecting the arrival of the frame and locating the OFDM symbols.
The invention can adopt cross-correlation synchronization or autocorrelation synchronization, and in one embodiment, adopts a one-dimensional iterative trapezoidal search time synchronization method, which comprises the following steps:
1. the received signal is subjected to N-point delayed autocorrelation operation, and the autocorrelation value of the preamble sequence part shows a trapezoidal relationship in time due to the cyclic prefix CS and the cyclic suffix CP of the preamble sequence. P (P) d The autocorrelation value at this point, r (n), is the received signal, and the formula is as follows:
2. initial positions of A, B, C at which 3 points are set are D respectively A 、D B 、D C At the start of the autocorrelation window.
3. Dividing intoThe autocorrelation values P of the 3 points of A, B, C are recorded separately A 、P B 、P C Autocorrelation value LP of last iteration of point A A Autocorrelation value LP of last iteration of point C C The autocorrelation values of these 3 points satisfy P A ＝0.75*P B ＝P C 、LP C ≥P C And LP (LP) A ≤P A However, since the "false peak" may be caused by noise, it is necessary to determine the positional relationship of these 3 points, provided that 0.125×l.ltoreq.d C -D B Less than or equal to 2 x N+0.625 x L and less than or equal to 0.125 x L and less than or equal to D B And +.2+0.625×l, where L is the leader length. The delayed autocorrelation result and A, B, C relationship is shown in fig. 6.
4. If all the conditions are met, the synchronization is successful, and the time synchronization position is D 1 ＝D B 。
If the above conditions are not fully satisfied, updating the C point position D C ＝D C +1 and correlation value LP C ＝P C 。
If the correlation value of the point C is larger than that of the point B, the point C is still in the ascending stage, and the point D of the point B is updated B ＝D C Correlation value P B ＝P C The method comprises the steps of carrying out a first treatment on the surface of the Otherwise, not updating.
If the correlation value of the point B is smaller than the point A, the point B is in a descending stage, and the position D of the point A is updated A ＝D B Correlation value P A ＝P B The method comprises the steps of carrying out a first treatment on the surface of the Otherwise, not updating.
Step 15, frequency synchronization, the purpose of which is to estimate and compensate for the frequency offset between the received signal and the transmitted signal. In one embodiment of the present invention, the frequency deviation may be calculated in the time domain or the frequency domain, and frequency synchronization may be performed according to the frequency deviation. For example, the frequency synchronization method may employ a Moose algorithm, an SC algorithm, an M & M algorithm, and the like.
And step 16, FFT, performing symbol positioning, performing FFT conversion on the OFDM symbol serial-parallel conversion after the CP removal in the frame, and converting the time domain signal back to the frequency domain. When there is a cyclic prefix, it is also necessary to remove the cyclic prefix first.
Step 17, channel estimation and equalization. And dividing the FFT frequency domain training symbols by the local frequency domain training symbols to obtain an estimated channel matrix, dividing the signal by the channel matrix, and eliminating the influence of the channel on the signal.
And 18, descrambling. The signal is divided by the locally fixed scrambling sequence to recover the original QPSK symbol.
Step 19, QPSK demapping. And restoring the QPSK symbol according to the quadrant position.
Step 20, channel decoding. The information bits are sent to a channel decoder corresponding to the channel code to recover the data packet.
And step 21, checking.
When CRC check coding is employed, a CRC check is correspondingly also required. Dividing the data packet by the generating polynomial of the CRC code, judging whether the data packet is wrong or not according to whether the remainder is zero, discarding if the data packet is wrong, and sending the data packet into a Raptor decoding stage if the data packet is wrong.
And 22, generating a raptor decoding matrix.
At the receiving end, according to the code symbol number of the received data packet, look up a table to find out the corresponding LT code matrix information, and combine with LDPC code matrix and HDPC code matrix to form a pre-code decoding matrix.
Illustratively, the precoding decoding matrix synthesis is performed only after receiving more data packets than the number of source symbols. And when all source data block numbers from 1 to K are received, the partial code symbols are directly subjected to source data block combination without Raptor decoding.
Step 23, raptor decoding.
And sending the received precoding decoding matrix and the received coded symbols to a Raptor decoder, specifically, carrying out row transformation and cancellation on the precoding decoding matrix in a binary domain, carrying out binary domain operation on the coded symbols of the corresponding row, carrying out confirmation feedback on the source data block number of the partial data packet if the decoding is successful, and then starting to encode the next source data block by the Raptor encoder.
In one embodiment of the present invention, the Raptor decoding process is shown in fig. 7, and the steps are as follows:
first, the precoding decoding matrix A combined in step 22 receive And carrying out Gaussian elimination change to recover the middle symbol C.
Wherein, for A receive Performing column-row transformation on the received code symbol and all-zero symbol Z 3 The combined matrix F performs the same transformation; to record the transformation process, two arrays c [ L ] can be created]And d [ M ]]Record line location, initialize cI]＝i(i＝1,2,…,L-1),f[i]=i (i=1, 2, …, M-1), where m=s+h+n; during the transformation, if A receive I rows and added to j rows, then F [ F [ i ]]]Is also added to F [ F [ j ]]]On the row; if A receive When column i is exchanged with column j, c [ i ]]And c [ j ]]Exchange is also performed; final A receive Unit array I transformed to a size of L L And an all zero matrix Z 4 And C1]],…,C[c[L]]＝F[f[1]],…,F[f[L]]The intermediate symbol C is recovered;
second, the intermediate symbol C is combined with G used for precoding LT(1,2,…K) And (5) performing binary field matrix multiplication to finally translate the source symbol X.
And step 24, combining the source data blocks.
And combining different source data blocks output by the Raptor decoder to recover the information source data.
The complete flow of the present invention can be seen with reference to fig. 8.
Claims (10)
1. The ocean buoy OFDM opportunistic communication method based on the code-rate-free code is characterized by comprising the following steps of:
step 1, generating code rate-free code information according to the number of source symbols at a transmitting end; the code rate-free code coding information is an LDPC coding matrix, an HDPC coding matrix and an LT coding matrix;
step 2, dividing the original data into a plurality of source data blocks, wherein each source data block comprises K source symbols;
step 3, for a source data block, combining the LDPC coding matrix and the HDPC coding matrix of the source symbol and the first K rows of the LT coding matrix to obtain a precoding matrix A, and performing matrix multiplication on the inverse matrix of the precoding matrix A and the source symbol in a binary domain to generate an intermediate symbol C;
step 4, generating a code symbol, wherein the first K bits of the code symbol are input source symbols, the K+n bits are repair symbols, the code symbol is obtained by multiplying an intermediate symbol C and an LT code matrix in a binary domain, and when the confirmation of a receiver to the source data block is received, the generation is stopped, wherein n is a natural number;
step 5, forming a data packet and transmitting the data packet in a marine buoy channel through an OFDM communication system;
step 6, at the receiving end, according to the code symbol number of the received data packet, finding out the corresponding LT code matrix information, and combining with the LDPC code matrix and the HDPC code matrix to form a pre-code decoding matrix;
step 7, performing row transformation and cancellation on the received pre-coding decoding matrix in a binary domain, performing binary domain operation on coding symbols of corresponding rows, if decoding is successful, performing confirmation feedback on the source data block number of the partial data packet, and then coding the next source data block;
and 8, combining the obtained different source data blocks to recover the information source data.
2. The ocean buoy OFDM opportunistic communication method based on code-rate-free codes of claim 1, wherein step 1, firstly, according to the number K of source symbols, look-up table is used to obtain the number S of LDPC coding matrix lines, the number H of HDPC coding matrix lines, wherein the LDPC coding matrix is an S x H-dimensional low-density parity check matrix, and is composed of K/S cyclic submatrices with number S of columns, and the weight of the submatrices is 3; the HDPC coding matrix is a high-density parity check matrix in H (S+K) dimension, each column of the matrix is generated according to an enumerated binary reflective Gray sequence, and the column weight of each column is H/2; the LT coding matrix is randomly generated according to the number K of the source symbols and the degree distribution function, and each different K value corresponds to a unique LDPC coding matrix, an HDPC coding matrix and an LT coding matrix respectively.
3. The ocean buoy OFDM opportunistic communication method based on code-rate-free codes according to claim 1, wherein the step 2 is to calculate the block number N of the original data according to the bit number S of the source data, the number K of the source symbols and the symbol length T, and the specific formula is as follows:
zero padding is added at the tail for data that is not full.
4. The ocean buoy OFDM opportunistic communication method based on code-rate-free codes of claim 1, wherein in step 3, the LDPC coding matrix ends up supplementing the unit matrix I of the S dimension S And 0 matrix Z of S x H 1 The HDPC coding matrix is a unit matrix I for ending and supplementing H dimension H S is the number of lines of the LDPC coding matrix, and H is the number of lines of the HDPC coding matrix; the intermediate symbol C, expressed as:
C＝A -1 D
wherein D is an input symbol consisting of S+H all 0 symbols Z 2 And K coding symbols, wherein the number of generated intermediate symbols C is equal to the sum of S, H, K;
the raptor code symbol is expressed as:
E＝(x 1 ,x 2 ,…,x K ,e K+1 ,e K+2 ,…,e K+n ) T
wherein e K+1 ,e K+2 ,…,e K+n For repair symbols, expressed as:
e K+1 ,e K+2 ,…,e K+n ＝E (K+1,K+2,…,K+n) ＝G LT(K+1,K+2,…,K+n) C
G LT(K+1,K+2,…,K+n) a generator matrix encoded for LT.
5. The ocean buoy OFDM opportunistic communication method based on code-rate-less codes of claim 1, wherein the step 5 comprises:
step 501, filling a packet header to form a data packet;
step 502, checking the code;
step 503, physical layer channel coding;
step 504, QPSK mapping;
step 505, scrambling;
step 506, ifft transformation;
step 507, framing;
step 508, radio frequency transmission;
step 509, ocean buoy channel transmission;
step 510, radio frequency reception;
step 511, time synchronization;
step 512, frequency synchronization;
step 513, fft transformation;
step 514, channel estimation and equalization;
step 515, descrambling;
step 516, QPSK demapping;
step 517, channel decoding;
and 518, checking.
6. The ocean buoy OFDM opportunistic communication method based on code-rate-less codes of claim 5, wherein in step 501, a source data block number and a Raptor code symbol number are used as header information, and are filled into corresponding code symbols to form a data packet;
step 502, performing CRC encoding on the encoded symbol after filling the packet header, and filling the obtained CRC check code to the tail;
in the step 503, the code rate is set to 1/2;
step 504, mapping the bits after channel coding into QPSK symbols in IQ plane;
step 505, multiplying the QPSK symbol with a fixed scrambling code sequence to uniformly distribute the phases thereof;
step 506, converting the serial data stream into parallel data, converting the frequency domain signal into the time domain OFDM symbol through IFFT, and adding a cyclic prefix at the same time;
step 507, combining the generated OFDM symbols with the preamble training sequence into a frame, and zero padding at the end of the frame;
step 508, multiplying the baseband signal with the carrier wave to become a radio frequency signal;
step 510, multiplying the radio frequency signal with the carrier wave to reduce the radio frequency signal to a baseband signal, and sending the baseband signal affected by the channel to the receiving end;
step 511, using cross-correlation synchronization or auto-correlation synchronization;
step 512, calculating a frequency deviation in the time domain or the frequency domain, and performing frequency synchronization according to the frequency deviation;
step 513, performing symbol positioning, performing FFT conversion on the OFDM symbol after CP removal in the frame, and transforming the time domain signal back to the frequency domain;
step 514, dividing the frequency domain training symbol after FFT with the local frequency domain training symbol to obtain an estimated channel matrix, dividing the signal with the channel matrix, and eliminating the influence of the channel on the signal;
step 515, dividing the signal with the locally fixed scrambling code sequence to recover the original QPSK symbol;
step 516, recovering the information bits of the QPSK symbol according to the quadrant positions;
step 517, sending the information bits to a channel decoder corresponding to the channel code, and recovering the data packet;
and 518, dividing the data packet by the generating polynomial of the CRC code, judging whether the data packet has errors or not according to whether the remainder is zero, discarding if yes, and sending to a Raptor decoding stage if no.
7. The ocean buoy OFDM opportunistic communication method based on code-rate-free codes of claim 6, wherein the data obtained through the step 502 is an OFDM system data packet, and the steps 503 to 506 use the OFDM system data packet as a data processing unit;
in step 507, the preamble sequence is generated by two identical frequency domain CAZAC sequences through N-point IFFT transformation, and denoted as a (N), where a (N), its cyclic prefix CP and cyclic suffix CS form a preamble training sequence, and the length of the preamble training sequence is N identical to the number of IFFT points, followed by N OFDM symbols and zero padding at the end of the frame.
8. The method for OFDM opportunistic communication of marine buoy based on code without code rate according to claim 6, wherein the step 511 adopts a one-dimensional iterative trapezoidal search time synchronization method, and the procedure is as follows:
1) The received signal is subjected to N-point delay autocorrelation operation, P d The autocorrelation value at this point, r (n), is the received signal, and the formula is as follows:
2) Initial positions of the 3 points A, B, C are set to be D respectively A 、D B 、D C Is positioned at the initial position of the autocorrelation window;
3) Recording the autocorrelation values P of A, B, C A 、P B 、P C Autocorrelation value LP of last iteration of point A A Autocorrelation value LP of last iteration of point C C When the autocorrelation value satisfies P A ＝0.75*P B ＝P C 、LP C ≥P C And LP (LP) A ≤P A When the position relation of the 3 points is judged, the condition is that 0.125 x L is less than or equal to D C -D B Less than or equal to 2 x N+0.625 x L and less than or equal to 0.125 x L and less than or equal to D B Less than or equal to 2 x n+0.625 x L, wherein L is the length of the leader sequence;
4) If all the conditions are met, the synchronization is successful, and the time synchronization position is D 1 ＝D B ；
If the above conditions are not fully satisfied, updating the C point position D C ＝D C +1 and correlation value LP C ＝P C ；
If the correlation value of the point C is larger than that of the point B, the point C is still in the ascending stage, and the point D of the point B is updated B ＝D C Correlation value P B ＝P C The method comprises the steps of carrying out a first treatment on the surface of the Otherwise, not updating;
if the correlation value of the point B is smaller than the point A, the point B is in a descending stage, and the position D of the point A is updated A ＝D B Correlation value P A ＝P B The method comprises the steps of carrying out a first treatment on the surface of the Otherwise, not updating.
9. The ocean buoy OFDM opportunistic communication method based on code-rate-free codes of claim 1, wherein step 6, precoding decoding matrix synthesis is performed after receiving more than the number of source symbols of the data packet; when all source data block numbers from 1 to K are received, the received source symbols are indicated, and the partial code symbols are directly combined with the source data blocks without Raptor decoding.
10. The ocean buoy OFDM opportunistic communication method based on code-rate-less codes of claim 1, wherein the decoding process of step 7 and step 8 is as follows:
first, for the pre-coding decoding matrix A receive Performing Gaussian elimination change to recover an intermediate symbol C;
wherein, for A receive Performing column-row transformation on the received code symbol and all-zero symbol Z 3 The combined matrix F performs the same transformation; establishing two arrays c L]And d [ M ]]Record line location, initialize cI]＝i(i＝1,2,…,L-1),f[i]=i (i=1, 2, …, M-1), where m=s+h+n; during the transformation, if A receive I rows and added to j rows, then F [ F [ i ]]]Is also added to F [ F [ j ]]]On the row; if A receive When column i is exchanged with column j, c [ i ]]And c [ j ]]Exchange is also performed; final A receive Unit array I transformed to a size of L L And an all zero matrix Z 4 And C1]],…,C[c[L]]＝F[f[1]],…,F[f[L]]The intermediate symbol C is recovered;
second, the intermediate symbol C is combined with G used for precoding LT(1,2,…K) And (5) performing binary field matrix multiplication to finally translate the source symbol X.
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