OFDM-IM-LDPC coding and decoding method based on combination number index

Abstract

The application discloses a combination number index-based OFDM-IM-LDPC coding and decoding method, and belongs to the field of high-frequency wireless communication. The application independently encodes symbol bit information and the calculated index position, replaces the direct encoding of the source bit in the past, reduces the misjudgment rate of the index position, improves the accuracy of the subcarrier activation mode, simultaneously adopts the combination number method to calculate the index position, expands the application range of the system, simultaneously reduces the resource waste caused by the preset lookup table, reduces the implementation difficulty, and provides higher flexibility and expansibility. The application utilizes the high error correction performance of the LDPC code, increases the redundancy of the bit information and the index position, effectively resists the noise and attenuation in the channel, and realizes high precision of data decoding; the application selects the log-likelihood ratio soft information for decoding, arranges the soft information in descending order and dynamically adjusts the illegal subcarrier mode, reduces the judgment complexity, and improves the robustness and accuracy of signal demodulation recovery.

Description

Technical Field

[0001] This invention relates to an OFDM-IM-LDPC encoding and decoding method based on combinatorial index, belonging to the field of high-frequency wireless communication. Background Technology

[0002] With the rapid development of mobile communication technology, highly efficient and reliable wireless communication technologies are receiving increasing attention. Indexed modulation (OFDM) improves the diversity gain of systems in frequency-selective fading channels, especially effectively reducing the bit error rate in scenarios with low spectral efficiency. Therefore, OFDM-IM technology represents a cutting-edge direction for future mobile communication development and possesses promising prospects.

[0003] OFDM-IM receivers often employ a log-likelihood ratio (LLR) detection algorithm based on subcarrier activation states. However, the LLR detection algorithm can lead to incorrect subcarrier activation mode determination or the identification of illegal subcarrier activation modes. This not only causes errors in index information estimation but also severely impacts the accuracy of symbol demodulation, resulting in an increased overall bit error rate (BER). Encoding the raw information from the transmitter can yield a coding gain, thus reducing the BER to some extent. Currently, the lookup table method is commonly used for index mapping in OFDM-IM encoding and decoding systems. This method constructs the corresponding index structure by searching a pre-defined table. However, due to limitations in complexity and table size, the lookup table method is primarily suitable for situations where the number of subcarriers and active subcarriers is relatively small. Summary of the Invention

[0004] To address the issues of high complexity and limited applicability inherent in existing OFDM-IM coding systems that commonly employ lookup table methods, this invention aims to provide an OFDM-IM-LDPC encoding and decoding method based on combinatorial indexing. This method leverages the wide applicability of combinatorial indexing to achieve index mapping for high subcarrier numbers, significantly reducing computational complexity and system resource consumption. Utilizing the high error correction performance of LDPC codes, it increases the redundancy of bit information and index positions, effectively combating noise and attenuation in the channel and achieving high-precision data decoding. Furthermore, through robust soft-information decoding, it determines the position index of active subcarriers, reducing the error rate in subcarrier activation mode decisions and significantly improving the overall system performance.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] This invention discloses an OFDM-IM-LDPC encoding and decoding method based on the combinatorial method. It replaces the previous scheme of directly encoding source bits by independently encoding symbol bit information and calculated index positions, reducing the false judgment rate of index positions and improving the accuracy of subcarrier activation modes. Simultaneously, the use of the combinatorial method to calculate index positions expands the system's applicability, reduces resource waste caused by pre-set lookup tables, lowers implementation difficulty, and provides greater flexibility and scalability. Furthermore, this invention uses log-likelihood ratio soft information for decoding. By arranging the soft information in descending order and dynamically adjusting illegal subcarrier modes, it reduces decision complexity while improving the robustness and accuracy of signal demodulation and recovery, thereby improving the performance and reliability of the encoding and decoding system in different application scenarios.

[0007] The OFDM-IM-LDPC encoding and decoding method based on combinatorial index disclosed in this invention includes the following steps:

[0008] Step 1: The transmitter processes the received source bits into sub-blocks to obtain g sub-blocks; each sub-block includes p1-bits of index bits and p2-bits of information bits, and the symbols carried on the sub-block are ultimately determined by p1-bits and p2-bits; according to the combination number method, the position of the active subcarrier in each sub-block is calculated from the index bits.

[0009] The β-th sub-block contains n subcarriers, of which k are active subcarriers. The index positions of the active subcarriers are recorded in set I. β In, that is

[0010] I β ={i β，1 , ..., i β，j , ..., i β，k} (1)

[0011] The element i in the set β，j Represents the active subcarrier index, satisfying 1≤i β，j ≤n; then set the obtained active subcarrier positions to 1 to obtain the subcarrier sequence L. β ,Right now

[0012] L β ={l β，1 , ..., l β，j , ..., l β，n} (2)

[0013] Among them, element l β，j Indicates whether the j-th subcarrier is active, l β，j ∈{0,1};

[0014] Step 2, the subcarrier sequence L obtained in Step 1β The information bits of p2-bits are respectively LDPC encoded at rate rate;

[0015] Step 2.1, Subcarrier Sequence Encoding: The subcarrier sequence after LDPC encoding is N β :

[0016]

[0017] Where, element n β，j This indicates whether the encoded subcarrier is active, and n / rate represents the length of the encoded index sequence.

[0018] Since the relationship between 0 and 1 in the subcarrier check sequence after LDPC encoding no longer satisfies the relationship between the subcarrier and the active subcarrier, the case in the check sequence where 0 and 1 appear more frequently is selected as the active subcarrier to transmit information.

[0019] Step 2.2, Information Bit Encoding and Mapping: LDPC encoding is performed on the p2-bit information bits. The information bits in the encoded information bits are directly mapped to the corresponding M-QAM / PSK symbols. The k active subcarriers before encoding are used for transmission, resulting in the information bit encoding in I... β Symbols s carried by k active subcarriers at position 1，β ,Right now

[0020] s 1，β ={s 1，β (1), ..., s 1，β (γ), ..., s 1，β (k)} (4)

[0021] Among them, element s 1，β (γ)∈S represents the symbol corresponding to the information bit, and S represents the set of points that can be selected on the constellation diagram;

[0022] The mapping of the parity bits in the encoded information bits depends on the transmission mode of the active subcarriers: When the number of 1s in the parity bits of the subcarrier sequence is greater than the number of 0s, the number of 1s is denoted as k2, and the k2 indices at the 1s are selected as active subcarriers. The parity bit information bits are directly mapped to the corresponding M-QAM / PSK symbols, and the symbols are mapped to the corresponding positions. The remaining active subcarriers transmit fixed constellation points. When the number of 0s in the parity bits of the subcarrier sequence is greater than the number of 1s, the number of 0s is denoted as k2, and the k2 indices at the 0s are selected as active subcarriers. The parity bit information bits are mapped to phase-rotated M-QAM / PSK symbols, and the symbols are mapped to the corresponding positions. The remaining active subcarriers transmit fixed rotated constellation points. Based on these two transmission modes, the parity bit symbol information s 2，β It can be represented as:

[0023] s 2,β ={s 2,β (1), ..., s 2,β (γ), ..., s 2,β (k2)} (5)

[0024] Among them, element s 2，β (γ)∈S represents the sign corresponding to the parity bit, and k2 represents the number of active subcarriers in the parity sequence;

[0025] Step 3: Based on N obtained in Step 2 β and s 1，β s 2，β Determine all symbols on the β-th sub-block, and then obtain g respectively. LDPC All symbols on each sub-block, at this time the OFDM block is represented as:

[0026] x F =[x(1)x(2)…x(N)…x(N) LDPC (6)

[0027] x(N) represents the symbol on the Nth subcarrier, x F This represents the subcarrier symbols of an OFDM block. Before encoding, there are g sub-blocks; after encoding, the number of sub-blocks is g. LDPC =g / rate, there are N subcarriers before encoding, and the total number of subcarriers after encoding is N. LDPC = N / rate, there are K active subcarriers before encoding, and K active subcarriers after encoding. LDPC =K / rate;

[0028] For x F Perform IFFT processing to obtain the time-domain subcarrier block symbol x. T :

[0029]

[0030] x T After undergoing serial-to-parallel conversion and digital-to-analog conversion, the signal is sent into the channel, resulting in the received signal y. T Represented as

[0031] y T =x T *h T +W T (8)

[0032] Among them, h T It is the channel impulse response coefficient, w T Indicates Gaussian white noise;

[0033] Step 4: The receiving end calculates the LLR soft information of the index bits according to different transmission modes, decodes the LLR soft information, selects the log-likelihood ratio detection algorithm to determine whether the subcarrier is active, and obtains the position of the active subcarrier; performs LDPC-BP decoding on the index position to recover the index bits of the transmitted information.

[0034] Step 4.1: The receiver uses the Max-log-map approximation method to determine the transmission mode, that is, whether the check part uses 0 or 1 to represent the active subcarrier;

[0035] First, calculate the initial deviation, then calculate the received frequency domain parity bit information. The initial deviation of the first symbol of M-QAM / PSK modulation with or without phase rotation is obtained by normalization to get the deviation without phase rotation. Deviation when phase rotation is involved

[0036]

[0037] in, This refers to receiving the i-th parity bit information; This refers to the value of the first symbol in M-QAM / PSK modulation when the transmission mode is 1, i.e., without phase rotation; This refers to the value of the first symbol in M-QAM / PSK modulation when the transmission mode is 0, i.e., when there is phase rotation; N0 is the power of the channel's white Gaussian noise.

[0038] Then, the biases for the new phaseless rotation and phased rotation are recursively calculated for the remaining symbols, and the biases associated with 1 are updated using a Max-log approximation. and the deviation related to 0

[0039]

[0040] Formulas (11) to (13) above represent the deviations when there is no phase rotation. The update process, where T1 and T2 are intermediate values ​​for updating. For the jj-th symbol of M-QAM / PSK modulation when transmission mode is 1, This is the deviation after the phase-free rotation has been updated;

[0041]

[0042] Formulas (14) to (16) above represent the deviations when there is phase rotation. The update process, where T3 and T4 are intermediate values ​​for updating. The value of the jj-th symbol in M-QAM / PSK modulation when the transmission mode is 0. This is the deviation after the phase rotation has been updated;

[0043] Final calculation and The difference between the sums of the vectors yields the LLR soft information value, which determines the transmission mode as 0 or 1.

[0044] Step 4.2: Determine if a subcarrier is active: The log-likelihood ratio detection algorithm is used to determine whether a symbol at a frequency point is active or silent based on the posterior probability. For any frequency point α, since the probability of active and silent subcarriers appearing in each sub-block is fixed, the log-likelihood ratio λ(α) can be expressed as:

[0045]

[0046] Where x(α) is the symbol on the subcarrier, s ρ Y represents the ρth possible symbol value during M-QAM / PSK modulation. k (α) represents the received frequency domain information;

[0047] When λ(α) is greater than 0, the probability of an active subcarrier is greater than that of a silent subcarrier, so the subcarrier is determined to be an active subcarrier, and the position of the active subcarrier can be obtained from this. When λ(α) is less than 0, the probability of an active subcarrier is less than that of a silent subcarrier, so the subcarrier is determined to be a silent subcarrier, and the position of the silent subcarrier can be obtained from this.

[0048] Step 4.3: Decode the index positions: Perform LDPC-BP decoding on the soft information obtained in Step 4.1 to obtain the LLR soft information at the corresponding positions of the subcarriers. Sort the obtained subcarrier LLR soft information in descending order, and take the first k to calculate the number of combinations. If the calculated number of combinations is greater than 2p... 1 This indicates an error in decoding and decision-making, and the combination number is an illegal combination number. Therefore, the index value corresponding to the (k+1)th LLR soft information is selected to replace the index value corresponding to the kth LLR soft information, and the validity of the combination number is verified in turn. After the combination number Z is decomposed, it is converted into binary, that is, the transmitted index bits are recovered.

[0049] Step 5: Take the symbol at the active subcarrier position obtained in Step 4.2, traverse each bit in the symbol, and calculate the LLR soft information of the symbol bits, i.e.

[0050]

[0051] Where λ0(j) stores the squared Euclidean distance between the received signal and the j-th symbol with bit 0, λ1(j) stores the squared Euclidean distance between the received signal and the j-th symbol with bit 1, L0 and L1 are the log-likelihood ratios, ∈ is a small value used to ensure numerical stability, and LLR is the LLR soft information of the symbol bits.

[0052] The LLR soft information of the symbol bits is decoded to recover the transmitted symbol bits.

[0053] Beneficial effects:

[0054] 1. The OFDM-IM-LDPC encoding and decoding method based on combinatorial index disclosed in this invention calculates the index position using the combinatorial method, performs LDPC encoding on the index information and symbol bits respectively, and selects the log-likelihood ratio detection algorithm at the receiving end to achieve high-precision recovery of transmitted bits and improve system reliability.

[0055] 2. The OFDM-IM-LDPC encoding and decoding method based on combinatorial index disclosed in this invention uses the combinatorial method to calculate the index position. Taking advantage of the wide applicability of the combinatorial method, it broadens the applicability of OFDM-IM and realizes index mapping for high subcarrier numbers. Compared with traditional mapping methods, it can avoid the need to pre-set a lookup table and reduce the waste of index position resources.

[0056] 3. The OFDM-IM-LDPC encoding and decoding method based on combination number index disclosed in this invention utilizes the high error correction performance of LDPC codes to increase the redundancy of bit information and index positions, effectively combating noise and attenuation in the channel and achieving high precision in data decoding. By using the LDPC encoding and decoding method, erroneous decisions of subcarrier activation modes are reduced. Through highly robust soft information decoding, the position index of active subcarriers is determined, reducing the decision error rate of subcarrier activation modes and significantly improving the overall performance of the encoding and decoding system.

[0057] 4. The OFDM-IM-LDPC encoding and decoding method based on combination number index disclosed in this invention uses log-likelihood ratio soft information for decoding. By arranging the soft information in descending order and dynamically adjusting the illegal subcarrier mode, the decision complexity is reduced while improving the robustness and accuracy of signal demodulation and recovery, thereby improving the performance and reliability of the encoding and decoding system in different application scenarios. Attached Figure Description

[0058] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0059] Figure 1 This is a diagram of the transmitter structure for an OFDM-IM-LDPC encoding and decoding method oriented towards combinatorial methods.

[0060] Figure 2 When data is transmitted using 1 for the parity bit, the QPSK modulation symbols are mapped to the constellation diagram after the active subcarriers.

[0061] Figure 3 When data is transmitted using 0 for the parity bit, the QPSK modulation symbols are mapped to the constellation diagram after the active subcarriers.

[0062] Figure 4 The graph shows the bit error rate curves of the index bits before and after OFDM-IM is encoded by LDPC.

[0063] Figure 5 This is a bit error rate curve for a combinatorial method-based OFDM-IM-LDPC encoding and decoding method under QPSK modulation. Detailed Implementation

[0064] To better illustrate the purpose and advantages of the present invention, the invention will be further described below in conjunction with the accompanying drawings and examples.

[0065] Example 1:

[0066] like Figure 1 As shown in the figure, this embodiment discloses an OFDM-IM-LDPC encoding and decoding method oriented towards combinatorial methods. The specific implementation steps are as follows:

[0067] Step 1: The transmitter uses QPSK modulation. The total number of OFDM-IM subblocks is 128, with 8 effective subcarriers in each subblock, including 4 active subcarriers. LDPC encoding with a 1 / 2 code rate is used. Specific parameters are: modulation index M = 4, number of subblocks before encoding g = 128, total number of subcarriers N = n*g = 8*128, number of active subcarriers K = k*g = 4*128, and the number of index bits in each subblock... The number of symbol bits is p2 = k * log2M = 8. The information bits transmitted in each sub-block satisfy the relationship with the system parameters: p = p1 + p2. After receiving the source information of m = p * g bits, the index bits are mapped to an index structure using the combination number method. The specific implementation of the combination number method is as follows:

[0068] Given that the number of active subcarriers in each sub-block is 4, the index positions of the active subcarriers, arranged in descending order, can be represented as L = {c4, ..., c1}. In the combinational method, for any positive integer Z ∈ [0, C(8, 4) - 1], there can always be a unique set of L such that Z and L are mapped one-to-one. The mapping relationship can be expressed as: Z = C(c4, 4) + C(c3, 3) + C(c2, 2) + C(c1, 1), where L = {c4, c3, c2, c1}. The specific mapping method is as follows:

[0069] Table 1 OFDM-IM Combinatorial Number Mapping Method

[0070]

[0071] At the sending end, the information in p1-bits is directly converted into a positive decimal integer Z. For a given Z, the maximum value of c is found. k Make C(c) k If k) ≤ Z, then choose the largest c. k -1 makes C(c k -1, k-1)≤ZC(c k By following this process, all index positions can be determined, i.e., set I. β The active subcarrier positions and subcarrier sequence L are obtained from the known index positions. β .

[0072] Step 2: Process the subcarrier sequence L obtained in Step 1. β The information bits of p2-bits are respectively LDPC encoded at rate.

[0073] Step 2.1, Subcarrier Sequence Encoding: After obtaining the corresponding index structure using the combinational method in Step 1, the index structure is converted into a binary representation of the corresponding index position, and then LDPC encoding at a 1 / 2 code rate is performed. The encoded subcarrier sequence is N. β ,Right now

[0074] N β ={n β，1 , ..., n β，16} (1)

[0075] Since the number of occurrences of 0 and 1 in the subcarrier test sequence after LDPC encoding no longer satisfies the relationship between (n, k), if the number of occurrences of 0 is greater than 1, the default transmission mode coma = 0; otherwise, the transmission mode coma = 1.

[0076] Step 2.2, Information Bit Encoding and Mapping: The 8-bit information bits are encoded using LDPC at a 1 / 2 code rate. The information bits in the encoded information bits are directly mapped to the corresponding QPSK symbols. The k active subcarriers from before encoding are used for transmission, resulting in the QPSK symbol in I... β Symbols s carried by k active subcarriers at position 1,β ,Right now

[0077] s 1，β ={s 1，β (1), ..., s 1，β (4)} (2)

[0078] Among them, element s 1，β (γ)∈S represents the symbol corresponding to the information bit, and S represents the set of points that can be selected on the constellation diagram.

[0079] The mapping of the parity bits in the encoded information bits depends on the transmission mode of the active subcarriers. If the transmission mode coma = 1, then k2 indices at position 1 are selected as active subcarriers. The parity information bits are directly mapped to the corresponding QPSK symbols, and the symbols are mapped to the corresponding positions. The remaining active subcarriers transmit fixed constellation points. At this time, the transmission constellation diagram is as follows: Figure 2 As shown; if the transmission mode coma = 0, then k2 indices at position 0 are selected as active subcarriers, and the parity information bits are mapped to phase rotation. The QPSK symbols are mapped to their corresponding positions, and the remaining active subcarriers transmit the fixed, rotated constellation points. The transmission constellation diagram is then as follows: Figure 3 As shown. Based on the two transmission modes above, the parity bit symbol information s is obtained. 2，β ,Right now

[0080] s 2，β ={s 2，β (1), ..., s 2，β (4), ..., s 2，β (k2)} (3)

[0081] Step 3: Map the generated QPSK symbols to the corresponding active subcarrier index positions, then perform IFFT operations to obtain the encoded OFDM-IM symbols. After serial-to-parallel conversion and digital-to-analog conversion, the signal is sent into the channel to obtain the received signal.

[0082] y T =x T *h T +W T (4)

[0083] Step 4: The receiving end calculates the soft information of the index bits according to different transmission modes and performs decoding.

[0084] Step 4.1: The receiver first determines the transmission mode, that is, whether the check part uses 0 or 1 to represent the active subcarrier. The Max-log-map approximation method is used to replace the summation with the maximum value to reduce the complexity of traversal calculation. The receiver then determines whether the subcarrier is active based on the obtained transmission mode.

[0085] Step 4.2: Determine if the subcarrier is active: The log-likelihood ratio detection algorithm is used to determine whether a symbol at a certain frequency point is active or silent by using the posterior probability. For any frequency point α, the probability of active and silent subcarriers appearing in each sub-block is fixed, and the soft information values ​​of the index position information bit and the parity bit are obtained as index_LLR_info and index_LLR_parity, respectively.

[0086] Step 4.3: Concatenate the LLR soft information of the information bits and index bits, padding the end of the sequence with zeros, and feed it into the BP decoding part of the LDPC for decoding. Update the check node and variable node sequentially for the initialized soft information. The final termination criterion is whether the product of the decoded signal and the check matrix H is 0. The LLR soft information at the corresponding position of the subcarrier is obtained through decoding. The subcarrier soft information is sorted in descending order, and the first k elements are used to calculate the number of combinations. If the calculated number of combinations is greater than 2... p1 If the decoding and decision are incorrect, and the combination number is invalid, then the index value corresponding to the (k+1)th soft information is selected to replace the index value corresponding to the kth soft information, and the validity of the combination number is verified sequentially. After the combination number Z is deduced, it is converted into binary, thus recovering the transmitted index bits.

[0087] Step 5: Based on the obtained active subcarrier positions, take the symbols at the corresponding positions, traverse each bit in the QPSK symbol, calculate the LLR soft information of the symbol bits, and then decode the soft information to recover the transmitted symbol bits.

[0088] according to Figure 4 and Figure 5 The simulation results show that the OFDM-IM system using the present invention has a significant improvement in index bit error rate and overall bit error rate performance compared to the OFDM-IM system without LDPC encoding and decoding.

[0089] The above detailed description further illustrates the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1.A method for OFDM-IM-LDPC encoding and decoding based on combination number index, characterized in that: The method comprises the following steps: Step 1, the transmitter carries out sub-block processing on the received source bits to obtain g sub-blocks; each sub-block comprises p1-bits of index bits and p2-bits of information bits, and the symbols carried on the sub-block are determined by the p1-bits and the p2-bits; the positions of active sub-carriers in each sub-block are calculated from the index bits according to the combination number method; There are n subcarriers in the βth subblock, in which k active subcarriers, the index position of the active subcarriers is recorded in set I β In other words, I β = {i β，1 ,..., i β，j ,..., i β，k} (1) Element i in the set β，j represents an active subcarrier index, satisfying 1≤i β，j ≤n; then the obtained active subcarrier position is set to 1, and a subcarrier sequence L β , that is L β ＝{l β，1 ，…，l β，j ，…，l β，n} (2) wherein the elements l β，j denote whether the jthsubcarrier is active, l β，j ∈ {0, 1}; Step 2, the sub-carrier sequence L obtained in Step 1 is multiplied by a sequence of length Np2, which is generated by a pseudo-random sequence generator with a seed of 0 and a length of Np2, and the multiplied sequence is added to the sub-carrier sequence L obtained in Step 1 to obtain a sequence L2. β and the information bits of p2-bits Step 2.1, Subcarrier sequence encoding: The LDPC encoded subcarrier sequence is N β : wherein the element n β，j represents whether the coded subcarriers are active or not, n / rate represents the length of the coded index sequence; Since the relationship between 0 and 1 in the LDPC coded sub-carrier check sequence no longer satisfies the relationship between the sub-carrier and the active sub-carrier, the case where 0 and 1 appear more frequently in the check sequence is selected as the active sub-carrier to transmit information; Step 2.2, information bit encoding and mapping: LDPC encode the p2-bits of information bits, directly map the information bits in the encoded information bits into corresponding M-QAM / PSK symbols, transmit using the k active subcarriers before encoding, obtain the symbols carried by the k active subcarriers in the I β position 1，β i.e. s 1，β = {s 1，β (1),..., s 1，β (γ),..., s 1，β (k)} (4) wherein the element s 1，β (γ) ∈ S denotes the symbol corresponding to the information bit, and S represents the set of points selectable on the constellation. The mapping of the check bits of the coded information bits depends on the transmission mode of the check bit active subcarriers: when the number of 1s in the check bit subcarrier sequence is more than the number of 0s, the number of 1s is denoted as k2, the k2 indexes of 1 are selected as the active subcarriers, the check bit information bits are directly mapped into the corresponding M-QAM / PSK symbols, and the symbols are mapped to the corresponding positions, and the remaining active subcarriers transmit fixed constellation points; when the number of 0s in the check bit subcarrier sequence is more than the number of 1s, the number of 0s is denoted as k2, and the k2 indexes of 0 are selected as the active subcarriers, the check bit information bits are mapped into M-QAM / PSK symbols with phase rotation, and the symbols are mapped to the corresponding positions, and the remaining active subcarriers transmit fixed constellation points after rotation, based on the two transmission modes, the check bit symbol information s 2，β is represented as: s 2,β = {s 2,β (1),..., s 2,β (γ),..., s 2,β (k2)} (5) wherein the element s 2，β (γ)∈S denotes the symbol corresponding to the parity check bit, and k2 denotes the number of active subcarriers in the parity check sequence. Step 3, determining all the symbols on the βthsub-block according to the N symbols obtained in Step 2, and then obtaining g β and s 1，β , respectively 2，β , determining all the symbols on the βthsub-block according to the N symbols obtained in Step 2, and then obtaining g LDPC symbols on the βthsub-block, at this time the OFDM block is represented as: x F = [x(1) x(2)... x(N)... x(N LDPC )] (6) x(N) denotes a symbol on the Nth subcarrier, x F x(N) denotes a symbol on the Nth subcarrier, x LDPC x(N) denotes a symbol on the Nth subcarrier, x LDPC x(N) denotes a symbol on the Nth subcarrier, x LDPC x(N) denotes a symbol on the Nth subcarrier, x x F IFFT processing is performed to obtain time-domain subcarrier block symbols x T : x T After serial-parallel conversion and digital-analog conversion, the received signal y T is expressed as y T = x T h T w T (8) where h T is the channel impulse response coefficient, w T denotes a Gaussian white noise; Step 4, the receiving end calculates the LLR soft information of the index bits according to different transmission modes, decodes the LLR soft information, judges whether the sub-carrier is active by using a log-likelihood ratio detection algorithm, and obtains the position of the active sub-carrier; LDPC-BP decoding is performed on the index position to recover the index bits of the transmitted information; Step 5, the symbols in the position of the active sub-carrier obtained in step 4 are taken, each bit in the symbols is traversed, the LLR soft information of the symbol bits is calculated, and the LLR soft information of the symbol bits is decoded, that is, the symbol bits of the transmitted information are recovered. 2.The OFDM-IM-LDPC coding and decoding method based on combination number index according to claim 1, wherein: The implementation method of step 4 is as follows: Step 4.1, the receiving end judges the transmission mode by using a Max-log-map approximation method, that is, whether 0 or 1 in the check part represents an active sub-carrier; First, the initial offset is calculated, calculating the received frequency domain check bit information The initial offset of the first symbol with and without phase rotation of the M-QAM / PSK modulation is calculated, and normalized to obtain the offset without phase rotation and the offset with phase rotation wherein denotes the reception of the check bit information of the second ii; denotes the value of the first symbol of the M-QAM / PSK modulation without phase rotation for transmission mode 1; denotes the value of the first symbol of the M-QAM / PSK modulation with phase rotation for transmission mode 0; N0is the power of the channel Gaussian white noise; the extrinsic information for the remaining symbols is updated with the max-log approximation for the extrinsic information for the remaining symbols and the extrinsic information for the remaining symbols is updated with the max-log approximation for the extrinsic information for the remaining symbols Equations (11) - (13) above are the deviations without phase rotation where T1 and T2 are the updated intermediate values, is the value of the jjth symbol of M-QAM / PSK modulation when the transmission mode is 1, is the updated deviation without phase rotation; Equations (14) - (16) above are the deviations with phase rotation where T3 and T4 are the updated intermediate values, is the value of the jjth symbol of M-QAM / PSK modulation with transmission mode 0, is the updated deviation with phase rotation. Computing and The difference of the sum of the vectors gives the LLR soft information value and determines the transmission mode as 0 or 1. Step 4.2, judging whether the sub-carrier is active: a log-likelihood ratio detection algorithm is selected, and the posterior probability is used to judge whether the symbol on the frequency point is active or silent; for any frequency point α, since the probabilities of the active sub-carrier and the silent sub-carrier appearing in each sub-block are fixed, the log-likelihood ratio λ(α) is represented as: where x(a) is the symbol on the subcarrier, s ρ is the ρth possible symbol value for M-QAM / PSK modulation, Y k (a) is the received frequency domain information; When λ(α) is greater than 0, the probability of the active sub-carrier is greater than that of the silent sub-carrier, so the sub-carrier is judged to be an active sub-carrier, and thus the position of the active sub-carrier can be obtained; when λ(α) is less than 0, the probability of the active sub-carrier is less than that of the silent sub-carrier, so the sub-carrier is judged to be a silent sub-carrier, and thus the position of the silent sub-carrier can be obtained; Step 4.3, decoding the index position: LDPC-BP decoding is performed on the soft information obtained in step 4.1 to obtain LLR soft information of the subcarrier corresponding position, the obtained subcarrier LLR soft information is arranged in descending order, and the first k are combined to obtain the inverse of the number; if the calculated combination number is greater than 2p 1 , it indicates that the decoding and decision are wrong, the combination number is an illegal combination number, and then the index value corresponding to the (k+1)th LLR soft information is selected to replace the index value corresponding to the kth LLR soft information, and the verification of whether the combination number is legal is sequentially performed; after the inverse solution of the combination number Z is obtained, it is converted into binary, that is, the index bits of the transmitted information are recovered. 3.The OFDM-IM-LDPC coding and decoding method based on combination number index according to claim 2, characterized in that: In step 5, the LLR soft information of the symbol bits is calculated according to formula (18) (19), that is: Wherein, λ0(j) stores the square Euclidean distance between the received signal and the jth symbol with bit 0, λ1(j) stores the square Euclidean distance between the received signal and the jth symbol with bit 1, L0 and L1 are log-likelihood ratios, ∈ is a small value used to ensure numerical stability, and LLR is the LLR soft information of the symbol bits.

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