CN108599775B - Construction method of hybrid check LDPC code - Google Patents

Abstract

The invention provides a method for constructing a hybrid check LDPC code, which belongs to the technical field of communication channel coding. The method first constructs a quasi-cyclic LDPC code as a basic LDPC code; then determines the check nodes to be replaced in the basic LDPC code. According to the hierarchical structure of the check matrix of the basic LDPC code, only one layer of check nodes is selected for replacement; the optimal sub-code is selected according to the EXIT function; finally, the selected layer of check nodes in the basic LDPC code to be replaced is replaced As the check node constrained by the optimal subcode, the hybrid check LDPC code is constructed. The invention constructs a hybrid check LDPC code with strong practicability suitable for complex interference channels, which can effectively reduce the bit error rate of data transmission on complex interference channels and improve communication reliability.

Description

A Construction Method of Hybrid Check LDPC Code

technical field

The invention belongs to the technical field of communication channel coding, and particularly relates to a method for constructing a hybrid check LDPC code.

Background technique

With the growth of transmission data, the increase of transmission terminals, and the extension of transmission distance, limited spectrum resources are increasingly crowded, and future wireless communication systems need to achieve reliable information transmission under complex interference channels. The complex application environment and endless interference methods constitute the main interference source of complex interference channels: due to the multipath interference caused by complex terrain such as mountain forests and dense buildings, the transmission signal is greatly attenuated; due to the continuous increase in the number of communication devices , there are a large number of different system interference and co-frequency interference of other equipment; there are more and more types of communication systems, and some communication systems introduce interference due to their own characteristics, such as the periodic attenuation of signals due to rotor occlusion in helicopter satellite communication systems; in special communication, The malicious interference of the other party is more likely to directly interrupt the communication of the other party.

In complex interference channels, channel coding techniques must be used to recover the original information. Low Density Parity Check (LDPC) code is a channel coding with excellent performance that has received extensive attention in recent years. An LDPC code can be defined by a sparse check matrix, and the number of "1" in the check matrix is far less than the number of "0". An LDPC code can also be represented by a Tanner graph, and all nodes on the Tanner graph are divided into check nodes and variable nodes. Different types of nodes are connected by the edges in the graph according to the following rules: when the i-th row and the j-th column of the LDPC code check matrix is 1, the check node c _i is connected to the variable node v _j . In LDPC codes, each check node can be regarded as a single parity check code constraint, and each variable node can be regarded as a repetition code constraint.

Quasi-cyclic LDPC codes are a class of LDPC codes constructed using algebraic structures, and are an important branch of practical LDPC codes. The parity check matrix of the quasi-cyclic LDPC code consists of a series of small square matrices, each of which is a cyclic shift matrix of a zero matrix or an identity matrix. The quasi-cyclic characteristics of quasi-cyclic LDPC codes make them have the advantages of efficient encoding and decoding, so they have been widely used in practical communication systems. The new generation digital satellite broadcasting standard DVB-S2, the International Space Data System Advisory Committee standard CCSDS and the wireless local area network standard 802.11ac all incorporate the quasi-cyclic LDPC code into the channel coding scheme. However, most quasi-cyclic LDPC codes are optimized for the traditional additive white Gaussian noise channel and cannot cope with a large number of bit errors in complex interference channels, and their direct use in complex interference channels will lead to performance degradation.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose a method for constructing a hybrid check LDPC code in order to overcome the deficiencies of the prior art. The present invention replaces part of the check nodes constrained by a single parity check code in the basic LDPC code with check nodes constrained by sub-codes such as Simplex code with stronger error correction capability, and designs a method to remove the short quasi-cyclic LDPC code. Loop algorithm, and a subcode optimization selection algorithm based on EXIT function, construct a hybrid check LDPC code with strong practicability suitable for complex interference channels, which can effectively reduce the error of data transmission on complex interference channels. code rate and improve communication reliability.

The present invention proposes a method for constructing a hybrid check LDPC code, which is characterized in that the method comprises the following steps:

1) construct quasi-cyclic LDPC code as basic LDPC code; Concrete steps are as follows:

1-1) On the GF(p) field, construct a quasi-cyclic LDPC code with column weight a and row weight b, where p is a prime number, and b>a>0;

1-2) remove the quasi-cyclic LDPC code short loop constructed in step 1-1), and the specific steps are as follows:

初始化条件下令

1-2-1) Set the target girth as g, and set the optimized basic LDPC code check matrix as

initialization condition command

中是否存在长度为2l的环：若存在，则转入步骤1-2-3)；若不存在，则转入步骤1-2-8)；1-2-2) Inspection

Whether there is a ring with a length of 21 in: if there is, then go to step 1-2-3); if not, then go to step 1-2-8);

1-2-3) Calculate the value of each cyclic shift matrix with a ring length of 2l, and update the calculation result to the count matrix n; where, n={n _0,0 ,n _0,1 , .. .n _0,b-1 ;n _1,0 ,n _1,1 ,...n _1,b-1 ;...;n _a-1,0 ,n _a-1,1 ,...n _a-1,b-1 }, n _{s, t} represents the value of the ring with the s-th row and t-th column cyclic shift matrix participation length 2l, 0≤s≤a-1, 0≤t≤b-1;

1-2-4) Count the number of values in the count matrix n, there are M different values in the design number matrix n, and arrange the M values from large to small as n ₀ , n ₁ ,...,n _{M -1} ; let the subscript value of n _k =0;

1-2-5) find all the cyclic shift matrices corresponding to n _k in the count matrix n, and use the set formed by the cyclic shift matrices as the set to be selected Ψ;

中所有约束节点的度数不低于2：若存在，则在Ψ中随机选取一个满足度数约束条件的循环移位阵，并转入步骤1-2-7)；若不存在，则令n_k的下标指示值k＝k+1，重新返回步骤1-2-5)；1-2-6) Check whether there is a cyclic shift matrix that satisfies the degree constraint in Ψ, where the degree constraint refers to replacing the selected cyclic shift matrix with a zero matrix.

The degree of all constrained nodes in is not less than 2: if it exists, randomly select a cyclic shift matrix that satisfies the degree constraint in Ψ, and go to step 1-2-7); if it does not exist, let n _k The subscript indicated value of k=k+1, return to step 1-2-5);

1-2-7) replace the cyclic shift matrix selected in step 1-2-6) with a zero matrix;

和构造完毕的基础LDPC码，进入步骤2)；若不成立，则令l＝l+1，重新返回步骤1-2-2)；1-2-8) Determine whether l=g/2-1 is established: if so, the short loop of the quasi-cyclic LDPC code is removed, and the optimized basic LDPC code check matrix is obtained

and the constructed basic LDPC code, enter step 2); if not established, then make l=l+1, and return to step 1-2-2);

2) determine the check node to be replaced in the basic LDPC code constructed in step 1);

中选取校验节点度数为d_C的一层校验节点作为待替换的校验节点；若

中存在多层校验节点度数为d_C，则优先选取靠中间的一层的校验节点作为待替换的校验节点；The degree of check node selected by the design plan is d _C , 2≤d _C ≤b, from

Select a layer of check nodes with a check node degree of d _C as the check node to be replaced; if

If there are multiple layers of check nodes, the degree of which is d _C , then the check node of the middle layer is preferentially selected as the check node to be replaced;

3) Select the optimal subcode according to the EXIT function; the specific steps are as follows:

3-1) Determine the set of subcodes to be selected Ω;

From the RM codes, BCH codes and Simplex codes that satisfy the single parity check constraint, the legal subcodes with the information sequence length of d _C -1 are included as the candidate subcodes into the candidate subcode set Ω;

3-2) the basic LDPC code that the fixed step 1) constructs, calculates the decoding threshold value that each candidate sub-code and basic LDPC code combination in Ω is calculated according to EXIT function;

The EXIT function expression of the variable node is as follows:

In the formula, I _{E, VND} represent the external information output by the variable node, I _{A, V} represent the average prior mutual information of the input variable node, λ _i represents the proportion of the edge connected to the variable node of degree _dvi , I _{E, REP} represents the variable node EXIT function under complex interference channel, E _b /N ₀ represents the signal-to-noise ratio;

The EXIT function of the check node consists of two parts: the check node EXIT function constrained by the single parity check code and the check node EXIT function constrained by the subcode, and the expression is as follows:

In the formula, I _{E, CND} represent the external information output by the check node, I _{A, C} represent the average prior information of the input check node, ρ _i represents the check node constrained by the single parity check code with degree dc _i The proportion of connected edges, I _{E, SPC} represents the check node EXIT function constrained by a single parity check code in a complex interference channel, ρ _C represents the proportion of edges connected to the check node constrained by the subcode, I _{E, Cmpt} represents the check node EXIT function of subcode constraints under complex interference channels;

3-3) According to the result of step 3-2), the sub-code to be selected with the lowest decoding threshold in Ω is determined as the optimal sub-code;

4) Replace the check node to be replaced determined according to step 2) in the basic LDPC code with the check node constrained by the optimal subcode selected according to step 3), and the hybrid check LDPC code is constructed.

The characteristics and beneficial effects of the present invention are:

The invention first removes the short loop in the quasi-cyclic LDPC code by designing a computer search algorithm, increases the girth of the quasi-cyclic LDPC code, and improves the decoding performance of the basic LDPC code. By replacing the check nodes constrained by a layer of single parity check codes in the basic LDPC code with check nodes constrained by sub-codes with stronger error correction capabilities, the sub-code redundancy is introduced while reducing the code rate loss, so that It can effectively deal with a large number of errors in complex interference channels, improve communication reliability, and has strong practical value.

Description of drawings

FIG. 1 is a flowchart of an algorithm for removing short loops of quasi-cyclic LDPC codes in the present invention.

FIG. 2 is a bidirectional Tanner diagram of the hybrid check LDPC code in the present invention.

FIG. 3 is a diagram of a periodic occlusion channel model according to an embodiment of the present invention.

FIG. 4 is a schematic diagram of performance simulation comparison between a hybrid check LDPC code and two quasi-cyclic LDPC codes according to an embodiment of the present invention.

Detailed ways

The present invention proposes a method for constructing a hybrid check LDPC code. The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

A construction method of a hybrid check LDPC code proposed by the present invention, the method comprises the following steps:

1) construct quasi-cyclic LDPC code as basic LDPC code; Concrete steps are as follows:

1-1) On the GF(p) field, construct a quasi-cyclic LDPC code with column weight a and row weight b, where p is a prime number and b>a>0. The check matrix H _b of the quasi-cyclic LDPC code can be expressed as:

Among them, I(p _i,j ) (i=0,1,...,a-1, j=0,1,...,b-1) means that each row of the p-order identity matrix is cyclically moved to the right Circular shift matrix generated by p _i,j bits. Shift factor p _i,j =mod(α ⁱ β ^j ,p), where α and β are two different elements on the GF(p) field, and the order of α is equal to a, and the order of β is equal to b .

1-2) Remove the short loop of the quasi-cyclic LDPC code constructed in step 1-1), the flow is as shown in Figure 1, and the specific steps are as follows:

初始化条件下令

l＝2。1-2-1) Set the target girth as g, and set the optimized basic LDPC code check matrix as

initialization condition command

l=2.

中是否存在长度为2l的环：若存在，则转入步骤1-2-3)；若不存在，则转入步骤1-2-8)。1-2-2) Inspection

Whether there is a ring of length 2l in : if it exists, go to step 1-2-3); if not, go to step 1-2-8).

1-2-3) Calculate the value of each cyclic shift matrix with a ring length of 2l, and update the calculation result into the count matrix n. where, n={n _0,0 ,n _0,1 ,...n _0,b-1 ;n _1,0 ,n _1,1 ,...n _1,b-1 ;...;n _a-1,0 ,n _a-1,1 ,...n _a-1,b-1 }, n _s,t represents the value of the s-th row, t-th column cyclic shift matrix participation length of the ring of 2l , 0≤s≤a-1, 0≤t≤b-1.

1-2-4) Count the number of values in the count matrix n, there are M different values in the design number matrix n, and arrange the M values from large to small as n ₀ , n ₁ ,...,n _{M -1} . Let the subscript of n _k indicate the value k=0.

1-2-5) Find all the cyclic shift matrices corresponding to the numerical value equal to n _k in the counting matrix n, and use the set formed by the cyclic shift matrices as the set to be selected Ψ.

中所有约束节点的度数不低于2：若存在，则在Ψ中随机选取一个满足度数约束条件的循环移位阵，并转入步骤1-2-7)；若不存在，则令n_k的下标指示值k＝k+1，重新返回步骤1-2-5)。1-2-6) Check whether there is a cyclic shift matrix that satisfies the degree constraint in Ψ, where the degree constraint refers to replacing the selected cyclic shift matrix with a zero matrix.

The degree of all constrained nodes in is not less than 2: if it exists, randomly select a cyclic shift matrix that satisfies the degree constraint in Ψ, and go to step 1-2-7); if it does not exist, let n _k The subscript indicates the value k=k+1, and returns to step 1-2-5).

1-2-7) Replace the cyclic shift matrix selected in step 1-2-6) with a zero matrix.

和构造完毕的基础LDPC码，进入步骤2)；若不成立，则令l＝l+1，重新返回步骤1-2-2)。1-2-8) Determine whether l=g/2-1 is established: if so, the short loop of the quasi-cyclic LDPC code is removed, and the optimized basic LDPC code check matrix is obtained

and the constructed basic LDPC code, go to step 2); if not, set l=l+1, and return to step 1-2-2).

2) determine the check node to be replaced in the basic LDPC code constructed in step 1);

具有分层结构，即

可分为a层。同一层内，变量节点的度数都相同，校验节点的度数也都相同；由于步骤1)的去除短环操作，位于不同层的校验节点度数则不完全相同。根据

的分层结构，只选取其中一层校验节点以待在步骤4)中完成节点替换，从而保证在引入子码的同时降低码率损失。设计划选定的校验节点度数为d_C(2≤d_C≤b)，若有多层校验节点度数为d_C，优先选取靠中间的一层，使得子码参与基础码Tanner图中环的数目达到最大，从而最大限度地纠正环上的错误。Step 1) Optimized basic LDPC code check matrix

has a hierarchical structure, i.e.

Can be divided into a layer. In the same layer, the degrees of variable nodes are the same, and the degrees of check nodes are also the same; due to the short-loop removal operation in step 1), the degrees of check nodes located in different layers are not exactly the same. according to

In the hierarchical structure of , only one layer of check nodes is selected to complete node replacement in step 4), thereby ensuring that the code rate loss is reduced while introducing subcodes. The degree of the selected check node is d _C (2≤d _C ≤ b) in the design plan. If there are multiple layers of check node degree of d _C , the middle layer is preferentially selected, so that the subcode participates in the cycle in the Tanner graph of the basic code. to maximize the number of errors in the ring.

3) Select the optimal subcode according to the EXIT function; the specific steps are as follows:

3-1) Determine the set of subcodes to be selected Ω.

All legal subcodes form a set of candidate subcodes. The legal subcode must first satisfy the single parity check constraint, that is, there is a column of values in the generator matrix in the systematic form that are all "1". Except for the known first-order RM codes and BCH codes, all Simplex codes also satisfy the above constraints. On GF(2), the Simplex code of ( ^2r -1,r) is the dual code of the ( ^2r -1,2r- ^r -1) Hamming code. From the characteristics of the dual code, the generator matrix of the Simplex code is the parity check matrix of the corresponding dual Hamming code. The check matrix H _Ham of the (2 ^r -1, 2 ^r -r-1) system Hamming code is composed of all r-dimensional column vectors that are not all 0, so H _Ham must contain 1 column and all 1 column. Therefore, according to the duality property, the generator matrix of all Simplex codes contains all 1 columns, that is to say, all Simplex codes conform to the single-parity check constraint. Second, the legal subcode must satisfy the information sequence length of d _C -1. The legal subcodes that meet the above conditions are included as candidate subcodes into the candidate subcode set Ω.

3-2) Fix the basic LDPC code constructed in step 1), and calculate the decoding threshold of each candidate subcode in Ω combined with the basic LDPC code according to the EXIT function.

The EXIT function of a variable node can be expressed as:

In the formula, I _{E, VND} represent the external information output by the variable node, I _{A, V} represent the average prior mutual information of the input variable node, λ _i represents the proportion of the edge connected to the variable node of degree _dvi , I _{E, REP} represents the variable node EXIT function under complex interference channel, and E _b /N ₀ represents the signal-to-noise ratio.

The EXIT function of the check node consists of two parts: the check node EXIT function constrained by the single parity check code and the check node EXIT function constrained by the subcode, which can be expressed as:

In the formula, I _{E, CND} represent the external information output by the check node, I _{A, C} represent the average prior information of the input check node, ρ _i represents the check node constrained by the single parity check code with degree dc _i The proportion of connected edges, I _{E, SPC} represents the check node EXIT function constrained by a single parity check code in a complex interference channel, ρ _C represents the proportion of edges connected to the check node constrained by the subcode, I _{E, Cmpt} represents the check node EXIT function constrained by subcodes under complex interference channels.

3-3) According to the result of step 3-2), determine the sub-code to be selected with the lowest decoding threshold in Ω as the optimal sub-code, and set the code length of the optimal sub-code to be n _C bits.

4) Replace the one-layer check node selected according to step 2) in the basic LDPC code with the check node constrained by the optimal subcode selected according to step 3), and the hybrid check LDPC code is constructed.

The specific replacement process can be represented by the bidirectional Tanner graph structure of the hybrid check LDPC code. The bidirectional Tanner graph of the hybrid check LDPC code is shown in Figure 2. There are two types of nodes: variable nodes and check nodes:

The variable nodes of the hybrid check LDPC code are the same as the variable nodes of the basic LDPC code, the number of variable nodes is (b-a)·p, and all the variable nodes correspond to the basic LDPC code encoding sequence one-to-one.

The check node of the hybrid check LDPC code is different from the basic LDPC code. In addition to the check nodes constrained by (a-1)·p single parity check codes, it also includes check nodes constrained by p subcodes. The check node constrained by the p subcodes is composed of the optimal subcode with a code length of n _C bits and an information sequence length of d _C -1 bits selected according to step 3) and a selected one of the basic LDPC codes according to step 2). It is obtained by replacing the check node with the layer degree d _C. After replacement, the check node implies the existing single parity check code constraint in the basic LDPC code while satisfying the subcode constraint. The sub-code constraint means that the basic LDPC code encoding sequence corresponding to the variable node connected to the check node of each sub-code constraint has d _C bits, and the first d _C -1 bits are selected as the sub-code information sequence for sub-code encoding to generate a length of It is a check sequence of n _C -d _C +1 bits. The single-parity check code constraint means that the check bits corresponding to all 1-column positions in the subcode generation matrix and the information sequence of d _C -1 bits satisfy the existing basic LDPC code single-parity check equation, so this check Bits are not transmitted over the communication channel. If p _C bits are punctured, the remaining n _C -d _C -p _C bits of the check sequence after subcode encoding are transmitted in the communication channel, forming n _C -d _C -p _C subcode variable nodes with degree 1. In the hybrid check LDPC code, all check nodes constrained by a single parity check code only participate in the parity check, and there is no direct corresponding coding sequence; the check nodes constrained by each subcode have a length of n _C -d _C -p _C bits. The subcode coding sequence of .

So far, the construction of the hybrid check LDPC code is completed. The length of the information sequence of the hybrid check LDPC code constructed from the (a, b, p) basic LDPC code and the (n _C , d _C -1) subcode is (ba)·p bits. If p _C bits are punctured, the code length is (b+n _C -d _C -p _C )·p bits. Then, the code rate of the hybrid check LDPC code can be expressed as:

Example

As an example of a complex interference channel, the embodiment of the present invention considers a transmission application under a periodic occlusion channel, and the channel model is shown in FIG. 3 . In this model, the communication signal is attenuated by a rectangular window: in a blocking period, the signal is only interfered by additive white Gaussian noise during the unblocked period, and the relative amplitude is 0; the signal is greatly attenuated during the blocking period, and the attenuation amplitude and blocking ratio are It depends on the specific application scenario. This model can be used for channel modeling in practical communication scenarios such as helicopter satellite communication and multi-rotor UAV communication.

A quasi-cyclic LDPC code is constructed, with parameters p=61, a=3, b=5, α=13, β=9, then the quasi-cyclic LDPC information sequence length is 122 bits, and the code length is 305 bits. Set the target girth g=14, and align the cyclic LDPC code to remove short loops (loops with a loop length less than or equal to 12), the obtained basic LDPC code check matrix is as follows:

The middle layer of the basic LDPC code check matrix is selected for expansion, and the selected single parity check node has a degree of 5. The subcodes are optimally selected from first-order RM codes, BCH codes and Simplex codes. According to the optimization result of EXIT diagram, if first-order RM code or BCH code is used for extension, the decoding convergence threshold is 2.101dB, and if Simplex code is used for extension, the decoding convergence threshold is 1.995dB. Therefore, for the (122, 305) basic LDPC code, the Simplex code is the optimal subcode. The check node constrained by each subcode transmits 2 check bits on the communication channel, and a hybrid check LDPC code with an information sequence length of 122 bits, a code length of 427 bits and a code rate of 2/7 can be constructed.

Figure 4 shows the decoding performance of the designed (122, 427) hybrid-check LDPC code in a periodically occluded channel, and compares it with the (142, 497) quasi-cyclic LDPC code and the (200, 900) quasi-cyclic LDPC code. The decoding performance was compared. Figure 4 shows that when the channel occlusion ratio is 10% and 25%, and the frame error rate is ^10-4 , the decoding performance of the hybrid-check LDPC code is better than that of the quasi-cyclic LDPC code with the same code rate by 1.2dB. Figure 4 also shows that the decoding performance of the hybrid check LDPC code with a code rate of 2/7 is better than that of the (200, 900) quasi-cyclic LDPC code with a code rate of 2/9. The hybrid check LDPC code is used in the above, and the bandwidth efficiency is improved by 28.6% compared with the use of the quasi-cyclic LDPC code.

Claims (1)

1. A construction method of a hybrid check LDPC code is characterized by comprising the following steps:

1) constructing a quasi-cyclic LDPC code as a basic LDPC code; the method comprises the following specific steps:

1-1) constructing a quasi-cyclic LDPC code with the column weight of a and the row weight of b on a GF (p) field, wherein p is a prime number, and b > a > 0;

1-2) removing the short loop of the quasi-cyclic LDPC code constructed in the step 1-1), which comprises the following specific steps:

1-2-1) setting the target girth as g and setting the optimized check matrix of the basic LDPC code as

Order under initialization condition

l＝2；

1-2-2) examination

Whether or not a loop of length 2l is present: if yes, switching to the step 1-2-3); if not, switching to the step 1-2-8);

1-2-3) calculating the value of each cyclic shift array participating in a ring with the length of 2l, and updating the calculation result into a counting matrix n; wherein n ═ { n ═ n_0,0,n_0,1,...n_0,b-1；n_1,0,n_1,1,...n_1,b-1；...；n_a-1,0,n_a-1,1,...n_a-1,b-1}，n_s,tRepresenting the values of the loops with the length of 2l participated by the s-th row and the t-th column cyclic shift array, wherein s is more than or equal to 0 and less than or equal to a-1, and t is more than or equal to 0 and less than or equal to b-1;

1-2-4) counting the number of numerical values in the counting matrix n, designing M different numerical values in the number matrix n, and arranging the M numerical values from large to small into n₀,n₁,...,n_M-1(ii) a Let n be_kThe subscript index value k of (a) is 0;

1-2-5) finding a numerical value of n in the count matrix equal to n_kAll corresponding cyclic shift arrays, and taking a set formed by the cyclic shift arrays as a set psi to be selected;

1-2-6) checking if there is any in ΨThe cyclic shift array satisfying degree constraint condition, wherein the degree constraint condition refers to that the selected cyclic shift array is replaced by zero matrix

The degrees of all constraint nodes in the node are not lower than 2: if yes, randomly selecting a cyclic shift array meeting degree constraint conditions from psi, and turning to the step 1-2-7); if not, let n_kThe subscript indicated value k is k +1, and the step 1-2-5) is returned again;

1-2-7) replacing the cyclic shift array selected in the step 1-2-6) with a zero matrix;

1-2-8) judging whether l ═ g/2-1 holds: if yes, removing the short loop of the quasi-cyclic LDPC code to obtain an optimized basic LDPC code check matrix

And the constructed basic LDPC code enters the step 2); if not, making l equal to l +1, and returning to the step 1-2-2);

2) determining check nodes to be replaced in the basic LDPC code constructed in the step 1);

the degree of the check node selected by the plan is set as d_C，2≤d_CB is less than or equal to b, from

The degree of the selected check node is d_CThe first layer of check nodes is used as the check nodes to be replaced; if it is

In which there are multiple layers of check nodes with degree d_CPreferentially selecting the check node at the middle layer as the check node to be replaced;

3) selecting an optimal subcode according to an EXIT function; the method comprises the following specific steps:

3-1) determining a set omega of the sub-codes to be selected;

the information sequence is long as d from RM codes, BCH codes and Simplex codes meeting the single parity check constraint condition_C-1 legal subcodes are taken as candidate subcodes and are brought into a candidate subcode set omega;

3-2) fixing the basic LDPC codes constructed in the step 1), and calculating a decoding threshold value of each sub-code to be selected in omega and the combination of the basic LDPC codes according to an EXIT function;

the EXIT function expression of the variable node is as follows:

in the formula I_E,VNDExtrinsic information representing the output of a variable node, I_A,VRepresenting the average a priori mutual information, λ, of the nodes of the input variables_iIs expressed in degree dv_iThe ratio of edges connected to the variable nodes of (1)_E,REPRepresenting the EXIT function of a variable node under a complex interference channel, E_b/N₀Representing the signal-to-noise ratio;

the EXIT function of the check node consists of a check node EXIT function constrained by the single parity check code and a check node EXIT function constrained by the subcode, and the expression is as follows:

in the formula I_E,CNDExtrinsic information representing the output of check nodes, I_A,CMean prior information, p, representing the input check nodes_iIs expressed and degree is dc_iThe ratio of edges connected to check nodes constrained by the single parity check code, I_E,SPCCheck node EXIT function, rho, representing single parity check code constraint under complex interference channel_CIndicating the proportion of edges connected to subcode-constrained check nodes, I_E,CmptA check node EXIT function representing subcode constraint under a complex interference channel;

3-3) determining the sub-code to be selected with the lowest decoding threshold value in omega as the optimal sub-code according to the result of the step 3-2);

4) and replacing the check node to be replaced determined according to the step 2) in the basic LDPC code with the check node constrained by the optimal subcode selected according to the step 3), and finishing the construction of the hybrid check LDPC code.

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