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

Abstract

The invention discloses an OFDM-IM-LDPC (Orthogonal Frequency Division Multiplexing-Index-Low Density Parity Check) coding and decoding method based on a combination number index, and belongs to the field of According to the method, the symbol bit information and the index position obtained through calculation are independently coded, original direct coding of information source bits is replaced, the misjudgment rate of the index position is reduced, the accuracy of a subcarrier activation mode is improved, meanwhile, the index position is calculated through the combinatorial number method, the application range of the system is expanded, and the system reliability is improved. Meanwhile, resource waste caused by a preset lookup table is reduced, the implementation difficulty is lowered, and higher flexibility and expansibility are provided. According to the method, the high error correction performance of the LDPC code is utilized, the redundancy of bit information and index positions is increased, noise and attenuation in a channel are effectively resisted, and high precision of data decoding is achieved; according to the method, the log-likelihood ratio soft information is selected for decoding, and the robustness and the accuracy of signal demodulation recovery are improved while the judgment complexity is reduced by arranging the soft information in a descending order and dynamically adjusting an illegal subcarrier mode.

Description

Technical Field

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

[0002] With the rapid development of mobile communication technology, wireless communication technology with high efficiency and reliability has received increasing attention. OFDM based on index modulation improves the diversity gain of the system in frequency selective fading channels, especially in low spectrum efficiency scenarios, it can effectively reduce the bit error rate of the system. Therefore, OFDM-IM technology is the forefront of future mobile communication development and has good development prospects.

[0003] The receiving end of OFDM-IM often adopts the log-likelihood ratio detection algorithm based on the subcarrier activation state. However, the LLR detection algorithm may make errors in the subcarrier activation mode judgment or judge an illegal subcarrier activation mode, which not only causes errors in the estimation of index information, but also seriously affects the accuracy of symbol demodulation, resulting in an increase in the overall bit error rate. By encoding the original information at the transmitting end, the corresponding coding gain can be obtained, which can reduce the bit error rate to a certain extent. The index mapping method currently commonly used in the OFDM-IM coding system is the lookup table method. This method constructs the corresponding index structure by searching a preset table, but due to the limitations of complexity and table size, the lookup table method is mainly suitable for situations where the number of subcarriers and active subcarriers is small. Summary of the invention

[0004] In order to solve the problem that the existing coded OFDM-IM system generally adopts the lookup table method, which leads to high complexity and limited scope of application, the purpose of the present invention is to provide an OFDM-IM-LDPC encoding and decoding method based on combination number index. The method uses the characteristics of the wide application range of the combination number method to realize index mapping of a high number of subcarriers, significantly reduce the calculation complexity, and reduce the occupation of system resources; utilize the high error correction performance of the LDPC code to increase the redundancy of the bit information and index position, effectively combat the noise and attenuation in the channel, and achieve high-precision data decoding; through soft information decoding with high robustness, determine the position index of the active subcarrier, reduce the decision error rate of the subcarrier activation mode, and significantly improve the overall performance of the system.

[0005] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] The present invention discloses an OFDM-IM-LDPC encoding and decoding method for the combinatorial number method, which independently encodes the symbol bit information and the calculated index position, replaces the previous scheme of directly encoding the source bit, reduces the error rate of the index position, improves the accuracy of the subcarrier activation mode, and uses the combinatorial number method to calculate the index position, expands the scope of application of the system, reduces the waste of resources caused by the preset lookup table, reduces the difficulty of implementation and provides higher flexibility and expansibility. In addition, the present invention selects log-likelihood ratio soft information for decoding, arranges the soft information in descending order and dynamically adjusts the illegal subcarrier mode, reduces the complexity of the decision, and improves the robustness and accuracy of signal demodulation recovery, so that the performance and reliability of the encoding and decoding system in different application scenarios can be improved.

[0007] The OFDM-IM-LDPC encoding and decoding method based on combination number index disclosed in the present invention comprises 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 an index bit of p1-bits and an information bit of p2-bits, and the symbols carried on the sub-block are finally 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 by the index bit;

[0009] There are n subcarriers in the βth subblock, of which k are active subcarriers. The index positions of the active subcarriers are recorded in set I β In

[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 position to 1, and obtain the subcarrier sequence L β ,Right now

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

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

[0014] Step 2: Subcarrier sequence L obtained in step 1β And the information bits of p2-bits are LDPC coded with rate code rate respectively;

[0015] Step 2.1, subcarrier sequence coding: The subcarrier sequence after LDPC coding is N β :

[0016]

[0017] Among them, the element n β，j Indicates whether the coded subcarrier is active, and n / rate indicates the length of the coded index sequence;

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

[0019] Step 2.2, information bit encoding and mapping: LDPC encoding is performed on the information bits of p2-bits, and the information bits in the encoded information bits are directly mapped into the corresponding M-QAM / PSK symbols, and transmitted using the k active subcarriers before encoding, and the information bits in I β The symbol s carried by the 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, the element s 1，β (γ)∈S represents the symbol corresponding to the information bit, and S represents the set of optional points on the constellation diagram;

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

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

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

[0025] Step 3: According to the N obtained in step 2 β and 1，β 、s 2，β , determine all symbols on the βth sub-block, and then obtain g LDPC All symbols on the sub-blocks, 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 Represents the subcarrier symbol of an OFDM block. There are g sub-blocks before encoding and g sub-blocks after encoding. 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 the number of active subcarriers after encoding is K LDPC =K / rate;

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

[0029]

[0030] x T After serial-to-parallel conversion and digital-to-analog conversion, it is sent into the channel and the received signal y T Expressed as

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

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

[0033] Step 4: The receiving end calculates the LLR soft information of the index bit 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 bit of the transmitted information;

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

[0035] First, calculate the initial deviation and calculate the received frequency domain check bit information The initial deviation of the first symbol with and without phase rotation in M-QAM / PSK modulation is normalized to obtain the deviation without phase rotation. and the deviation when there is phase rotation

[0036]

[0037] in, Refers to the received i-th check bit information; Refers to the first symbol value of M-QAM / PSK modulation when the transmission mode is 1, that is, there is no phase rotation; It refers to the first symbol value of M-QAM / PSK modulation when the transmission mode is 0, that is, there is phase rotation; N0 is the power of the channel Gaussian white noise;

[0038] Then, the new deviations without phase rotation and with phase rotation are recursively calculated for the remaining symbols, and the deviations related to 1 are updated using the Max-log approximation and the deviation from 0

[0039]

[0040] The above formula (11)-formula (13) is the deviation when there is no phase rotation The updating process, 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 deviation after update without phase rotation;

[0041]

[0042] The above formula (14)-formula (16) is the deviation when there is phase rotation The updating process, where T3 and T4 are the updated intermediate values, is the value of the jjth symbol of M-QAM / PSK modulation when the transmission mode is 0, is the deviation after update with phase rotation;

[0043] Final calculation and The difference between the sum of the vectors is used to obtain the LLR soft information value and determine whether the transmission mode is 0 or 1;

[0044] Step 4.2, determine whether the subcarrier is active: select the log-likelihood ratio detection algorithm, and use the posterior probability to determine whether the symbol on the frequency point is active or silent. For any frequency point α, since the probability of active subcarriers and silent subcarriers in each subblock 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 is the ρth possible symbol value in M-QAM / PSK modulation, Y k (α) is 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 judged to be an active subcarrier, thereby obtaining the position of the active subcarrier; 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 judged to be a silent subcarrier, thereby obtaining the position of the silent subcarrier;

[0048] Step 4.3, decode the index position: perform LDPC-BP decoding on the soft information obtained in step 4.1 to obtain the LLR soft information of the corresponding position of the subcarrier, arrange the obtained subcarrier LLR soft information in descending order, and take the first k to reverse the number of combinations; if the calculated number of combinations is greater than 2p 1 , indicating that the decoding and judgment are wrong, and the combination number is an illegal combination number, and then the index value corresponding to the k+1th LLR soft information is selected to replace the index value corresponding to the kth LLR soft information, and the legality of the combination number is verified in turn; after the combination number Z is reversed, it is converted into binary, that is, the transmitted index bit is restored;

[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 bit, that is,

[0050]

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

[0052] The LLR soft information of the sign bit is decoded to restore the transmitted sign bit.

[0053] Beneficial effects:

[0054] 1. The OFDM-IM-LDPC encoding and decoding method based on the combination number index disclosed in the present invention obtains the index position according to the combination number method, performs LDPC encoding on the index information and the symbol bit respectively, and selects the log-likelihood ratio detection algorithm at the receiving end to achieve high-precision recovery of the transmission bits and improve the system reliability.

[0055] 2. The OFDM-IM-LDPC encoding and decoding method based on combination number index disclosed in the present invention adopts the combination number method to calculate the index position. By taking advantage of the wide application range of the combination number method, the application range of OFDM-IM is broadened, and index mapping of a high number of subcarriers is realized. Compared with the traditional mapping method, it can avoid the need to pre-set the lookup table and reduce the waste of index position resources.

[0056] 3. The OFDM-IM-LDPC encoding and decoding method based on the combination number index disclosed in the present invention utilizes the high error correction performance of the LDPC code to increase the redundancy of the bit information and index position, effectively combats the noise and attenuation in the channel, and achieves high-precision data decoding; the LDPC encoding and decoding method is selected to reduce the erroneous judgment of the subcarrier activation mode, and the position index of the active subcarrier is determined through soft information decoding with high robustness, thereby reducing the judgment error rate of the subcarrier activation mode 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 the present invention selects log-likelihood ratio soft information for decoding, and reduces the decision complexity while improving the robustness and accuracy of signal demodulation and recovery by arranging the soft information in descending order and dynamically adjusting the illegal subcarrier mode, thereby improving the performance and reliability of the encoding and decoding system in different application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0059] Figure 1 The figure is a structural diagram of the transmitting end of an OFDM-IM-LDPC encoding and decoding method oriented to the combinatorial number method.

[0060] Figure 2 When data is transmitted with 1 as the parity bit, the QPSK modulation symbol is mapped to the constellation diagram after the active subcarriers.

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

[0062] Figure 4 The index bit error rate curve of OFDM-IM before and after LDPC encoding.

[0063] Figure 5 This is a bit error rate curve of an OFDM-IM-LDPC coding and decoding method based on the combinatorial number method under QPSK modulation. DETAILED DESCRIPTION

[0064] In order to better illustrate the purpose and advantages of the present invention, the invention is further described below with reference to the accompanying drawings and examples.

[0065] Embodiment 1:

[0066] like Figure 1 As shown, the encoding and decoding method of OFDM-IM-LDPC for the combination number method disclosed in this embodiment is specifically implemented as follows:

[0067] Step 1. The transmitter selects QPSK modulation. The total number of OFDM-IM sub-blocks is 128. There are 8 valid sub-carriers in each sub-block, of which 4 are active sub-carriers. The LDPC coding method with a code rate of 1 / 2 is selected. The specific parameters are: modulation index M = 4, number of sub-blocks before coding g = 128, total number of sub-carriers N = n*g = 8*128, number of active sub-carriers K = k*g = 4*128, number of index bits in each sub-block The number of symbol bits p2 = k*log2M = 8, and the information bits transmitted by each sub-block and the system parameters satisfy the relationship: p = p1 + p2. After receiving the source information of m = p*g bits, the index bits are mapped to the index structure using the combination number method. The following is the specific implementation of the combination number method:

[0068] It is known that the number of active subcarriers in each subblock is 4, so the index positions of the active subcarriers can be expressed in descending order as L = {c4, ..., c1}. In the combinatorial number method, for any positive integer Z∈[0, C(8, 4)-1], a unique set of L can always be found so 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 combination number mapping method

[0070]

[0071] At the sending end, the p1-bit information is directly converted into a decimal positive integer Z. For the determined Z, find the largest c k So that C(c k , k)≤Z, then select the largest c k -1 makes C(c k -1, k-1)≤ZC(c k , k), and so on to determine all the index positions, that is, the set I β , the active subcarrier position and subcarrier sequence L are obtained through the known index position β .

[0072] Step 2: For the subcarrier sequence L obtained in step 1 β And the information bits of p2-bits are LDPC encoded with rate code rate respectively.

[0073] Step 2.1, subcarrier sequence encoding: After obtaining the corresponding index structure according to the combination number method in step 1, the index structure is converted into a binary representation of the corresponding index position, and it is coded with 1 / 2 code rate LDPC. The encoded subcarrier sequence is N β ,Right now

[0074] N β ={n β，1 , ..., n β，16} (1)

[0075] Since the number of times 0 and 1 appear in the subcarrier check sequence after LDPC coding no longer satisfies the relationship between (n, k), if the number of times 0 appears 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: Perform 1 / 2 code rate LDPC encoding on the 8-bit information bit, directly map the information bit in the encoded information bit into the corresponding QPSK symbol, and transmit it using the k active subcarriers before encoding, and obtain β The symbol s carried by the 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, the element s 1，β (γ)∈S represents the symbol corresponding to the information bit, and S represents the set of optional points on the constellation diagram.

[0079] The mapping of the parity bits of the coded information bits depends on the transmission mode of the parity bit active subcarriers. If the transmission mode coma=1, then the k2 indexes at 1 are selected as active subcarriers, and the parity bit 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, the k2 indexes at 0 are selected as active subcarriers, and the check bit information bits are mapped into phase rotation The QPSK symbol is mapped to the corresponding position, and the remaining active subcarriers transmit the fixed rotated constellation point. At this time, the transmission constellation diagram is as follows Figure 3 According to the above two transmission modes, the check 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, and then perform IFFT operation to obtain the encoded OFDM-IM symbols. After serial-to-parallel conversion and digital-to-analog conversion, the signal is sent to 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 bit according to different transmission modes and decodes it.

[0084] Step 4.1, the receiving end first determines the transmission mode, that is, the check part uses 0 or 1 to represent the active subcarrier, and adopts the Max-log-map approximation method, using the maximum value instead of the sum to reduce the complexity of the traversal calculation, and determines whether the subcarrier is active based on the obtained transmission mode.

[0085] Step 4.2, determine whether the subcarrier is active: select the log-likelihood ratio detection algorithm, and use the posterior probability to determine whether the symbol on a certain frequency point is active or silent. For any frequency point α, the probability of active subcarriers and silent subcarriers in each subblock is fixed, and the soft information values ​​of the index position information bit and the check bit are index_LLR_info and index_LLR_parity respectively.

[0086] Step 4.3, concatenate the LLR soft information of the information bit and the index bit, and fill the end of the sequence with zeros, and send it to the BP decoding part of the LDPC for decoding. Update the check nodes and variable nodes of the initialized soft information in turn, and finally use whether the product of the decoded signal and the check matrix H is 0 as the termination criterion. Decode the LLR soft information of the corresponding position of the subcarrier, arrange the subcarrier soft information in descending order, and take the first k to reverse the number of combinations. If the calculated number of combinations is greater than 2 p1 , indicating that the decoding and judgment are wrong, and the combination number is an illegal combination number, then the index value corresponding to the k+1th soft information is selected to replace the index value corresponding to the kth soft information, and the legality of the combination number is verified in turn. After the combination number Z is reversed, it is converted into binary, that is, the transmitted index bit is restored.

[0087] Step 5: According to the obtained active subcarrier position, take the symbol at the corresponding position, traverse each bit in the QPSK symbol, calculate the LLR soft information of the symbol bit, and then decode the soft information to restore the transmitted symbol bit.

[0088] according to Figure 4 and Figure 5 From the simulation results, it can be seen 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 not using the LDPC coding method.

[0089] The specific description above further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above 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 in the scope of protection of the present invention.

Claims

1. An OFDM-IM-LDPC coding method based on a combination number index, characterized in that: The following steps are included: Step 1: The transmitter processes the received source bits into sub-blocks to obtain g sub-blocks; each sub-block includes an index bit of p1-bits and an information bit of p2-bits, and the symbols carried on the sub-block are finally 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 by the index bit; There are n subcarriers in the βth subblock, of which k are active subcarriers. The index positions of the active subcarriers are recorded in set I β In I β ={i β，1 ,...,i β，j ,...,i β，k } (1) The element i in the collection β，j Represents the active subcarrier index, satisfying 1≤i β，j ≤n; then set the obtained active subcarrier position to 1, and obtain the subcarrier sequence L β ,Right now L β ＝{l β，1 ，…，l β，j ，…，l β，n} (2) Among them, the element l β，j Indicates whether the jth subcarrier is active, l β，j ∈{0,1}; Step 2: Subcarrier sequence L obtained in step 1 β And the information bits of p2-bits are LDPC coded with rate code rate respectively; Step 2.1, subcarrier sequence coding: The subcarrier sequence after LDPC coding is N β : Among them, the element n β，j Indicates whether the coded subcarrier is active, and n / rate indicates the length of the coded index sequence; Since the relationship between 0 and 1 in the subcarrier check sequence after LDPC coding no longer satisfies the relationship between the subcarrier and the active subcarrier, the case where 0 and 1 appear more frequently in the check sequence is selected as the active subcarrier transmission information; Step 2.2, information bit encoding and mapping: LDPC encoding is performed on the information bits of p2-bits, and the information bits in the encoded information bits are directly mapped into the corresponding M-QAM / PSK symbols, and transmitted using the k active subcarriers before encoding, and the information bits in I β The symbol s carried by the k active subcarriers at position 1，β ,Right now s 1，β ={s 1，β (1),...,s 1，β (γ),...,s 1，β (k)} (4) Among them, the element s 1，β (γ)∈S represents the symbol corresponding to the information bit, and S represents the set of optional points on the constellation diagram; The mapping of the check bits of the encoded information bits depends on the transmission mode of the check bit active subcarriers: when the number of 1s in the subcarrier sequence check bits is greater than the number of 0s, the number of 1s is recorded as k2, and the k2 indexes at 1 are selected as active subcarriers. The check bit information bits are directly mapped to 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 subcarrier sequence check bits is greater than the number of 1s, the number of 0s is recorded as k2, and the k2 indexes at 0 are selected as active subcarriers. The check bit information bits are mapped to phase-rotated M-QAM / PSK symbols, and the symbols are mapped to the corresponding positions, and the remaining active subcarriers transmit fixed rotated constellation points. Based on these two transmission modes, the check bit symbol information s 2，β It is expressed as: s 2,β ={s 2,β (1),...,s 2,β (γ),...,s 2,β (k2)} (5) Among them, the element s 2，β (γ)∈S represents the symbol corresponding to the check bit, and k2 represents the number of active subcarriers in the check sequence; Step 3: According to the N obtained in step 2 β and 1，β 、s 2，β , determine all symbols on the βth sub-block, and then obtain g LDPC All symbols on the sub-blocks, the OFDM block is represented as: x F =[x(1) x(2)…x(N)…x(N LDPC )] (6) x(N) represents the symbol on the Nth subcarrier, x F Represents the subcarrier symbol of an OFDM block. There are g sub-blocks before encoding and g sub-blocks after encoding. 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 the number of active subcarriers after encoding is K LDPC =K / rate; x F Perform IFFT processing to obtain the time domain subcarrier block symbol x T : x T After serial-to-parallel conversion and digital-to-analog conversion, it is sent into the channel and the received signal y T Expressed as y T =x T *h T +w T (8) Among them, h T is the channel impulse response coefficient, w T represents Gaussian white noise; Step 4: The receiving end calculates the LLR soft information of the index bit 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 bit of the transmitted information; Step 5: Take the symbol at the active subcarrier position obtained in step 4, traverse each bit in the symbol, calculate the LLR soft information of the symbol bit, and decode the LLR soft information of the symbol bit, that is, restore the symbol bit of the transmitted information.

2. The OFDM-IM-LDPC encoding and decoding method based on combination number index according to claim 1, characterized in that: Step 4 is implemented as follows: Step 4.1, the receiving end uses the Max-log-map approximation method to determine the transmission mode, that is, the check part uses 0 or 1 to represent the active subcarrier; First, calculate the initial deviation and calculate the received frequency domain check bit information The initial deviation of the first symbol with and without phase rotation in M-QAM / PSK modulation is normalized to obtain the deviation without phase rotation. and the deviation when there is phase rotation in, Refers to the check digit information of the iith received; Refers to the first symbol value of M-QAM / PSK modulation when the transmission mode is 1, that is, there is no phase rotation; It refers to the first symbol value of M-QAM / PSK modulation when the transmission mode is 0, that is, there is phase rotation; N0 is the power of the channel Gaussian white noise; Recursively calculate the new deviations without phase rotation and with phase rotation for the remaining symbols, and use Max-log approximation to update the deviations related to 1 and the deviation from 0 The above formula (11)-formula (13) is the deviation when there is no phase rotation The updating process, 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 deviation after update without phase rotation; The above formula (14)-formula (16) is the deviation when there is phase rotation The updating process, where T3 and T4 are the updated intermediate values, is the value of the jjth symbol of M-QAM / PSK modulation when the transmission mode is 0, is the deviation after update with phase rotation; calculate and The difference between the sum of the vectors is used to obtain the LLR soft information value and determine whether the transmission mode is 0 or 1; Step 4.2, determine whether the subcarrier is active: select the log-likelihood ratio detection algorithm, and use the posterior probability to determine whether the symbol on the frequency point is active or silent. For any frequency point α, since the probability of active subcarriers and silent subcarriers in each subblock is fixed, the log-likelihood ratio λ(α) is expressed as: Where x(α) is the symbol on the subcarrier, s ρ is the ρth possible symbol value in M-QAM / PSK modulation, Y k (α) is the received frequency domain information; 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 judged to be an active subcarrier, thereby obtaining the position of the active subcarrier; 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 judged to be a silent subcarrier, thereby obtaining the position of the silent subcarrier; Step 4.3, decode the index position: perform LDPC-BP decoding on the soft information obtained in step 4.1 to obtain the LLR soft information of the corresponding position of the subcarrier, arrange the obtained subcarrier LLR soft information in descending order, and take the first k to reverse the number of combinations; if the calculated number of combinations is greater than 2p 1 , indicating that the decoding and judgment are wrong, and the combination number is an illegal combination number. Then, the index value corresponding to the k+1th LLR soft information is selected to replace the index value corresponding to the kth LLR soft information, and the legality of the combination number is verified in turn; after the combination number Z is deconstructed, it is converted into binary, that is, the index bit of the transmitted information is restored.

3. The OFDM-IM-LDPC encoding and decoding method based on combination number index according to claim 2, characterized in that: In step 5, the LLR soft information of the sign bit is calculated according to equations (18) and (19), namely: Among them, λ0(j) stores the square Euclidean distance between the received signal and the j-th symbol with bit position 0, λ1(j) stores the square Euclidean distance between the received signal and the j-th symbol with bit position 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 bit.

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