Hybrid coding system of harq and diversity combining fusion for fso / rf cooperative communication

Abstract

The application belongs to the technical field of FSO / RF cooperative communication, and particularly relates to a hybrid coding system of HARQ and diversity combining fusion of FSO / RF cooperative communication. The hybrid coding system comprises a signal coding module, a bit data division module, a signal transmission and reception module, a bit data combining / replacing module, a signal decoding module and an automatic retransmission module. The application is applicable to the FSO / RF cooperative communication system, can effectively avoid the link idle problem caused by traditional hard switching, and continuously increase the information amount obtained by the receiving end in the high-performance RF link through the HARQ process, solve the rate mismatch problem, and increase the HARQ efficiency. The application has potential for future free space optical and millimeter wave cooperative communication, utilizes the complementary link performance of free space optical and millimeter wave in different weather, and meets the high reliability requirement of wideband transmission.

Description

Technical Field

[0001] This invention belongs to the field of free space optical (FSO) and radio frequency (RF) cooperative communication technology, specifically involving a free space optical and radio frequency cooperative communication system that integrates settling at different rates with Hybrid Automatic Repeat reQuest (HARQ) encoding. Background Technology

[0002] With the deployment of 5G and the rapid development of 6G communication networks, providing high-data-rate and high-reliability data streams for communication networks has become an important research topic in the field of communication. Free-space optical communication (FSO) features high data rates, spectrum freedom, ease of deployment, and high security, making it a current research hotspot in the field of wireless backhaul networks. However, free-space optical communication is susceptible to atmospheric influences, limiting system reliability. Cooperative communication systems with radio frequency (RF) have become a primary solution. Addressing the issues of space laser communication's susceptibility to atmospheric channel influences and low link reliability, as well as the low transmission rate and susceptibility to electromagnetic interference in wireless RF communication links, this research focuses on integrated space RF-laser data transmission technology. It designs reasonable integrated RF-laser data transmission schemes to suit different RF-laser rate matching levels and studies retransmission schemes adapted to fused channels. This allows the composite communication system to combine the high reliability of RF communication links with the high transmission rate of laser communication links, significantly improving system reliability and application prospects.

[0003] In communication systems, data validity is affected by interference such as channel noise, turbulence, and obstructions, which may lead to errors or loss of received data. HARQ, or Hybrid Automatic Repeat Request, is a technology that combines FEC (Fault-Oriented Error Correction) and Automatic Repeat Request (ARQ). It can detect erroneous data during transmission and has a certain error correction capability, effectively improving the transmission efficiency and reliability of the system. HARQ technology has proven to play a crucial role in the reliable and efficient transmission of data in communication systems under noisy environments.

[0004] The basic principle of diversity merging is to use multiple branches to receive signals. Each branch receives signals carrying the same message, and the correlation between the receiving branches is very small. Merging techniques combine the signals from each branch, thereby reducing the probability of large fading. Diversity reception technology can also achieve diversity gain, improving receiver sensitivity. Traditional diversity merging techniques combine waveform-level radio frequency signals with free-space optical signals, but they do not consider the additional coding gain that HARQ retransmission can bring when performing bit-level merging and the radio frequency signal rate is low. Summary of the Invention

[0005] The purpose of this invention is to propose a novel hybrid coding system that integrates HARQ and diversity merging in FSO / RF cooperative communication. Compared with the traditional HARQ and diversity merging schemes in current FSO / RF cooperative communication systems, this system can better utilize the high signal-to-noise ratio of the wireless channel and the coding gain of the wireless channel brought by HARQ retransmission, thereby solving the rate mismatch problem and increasing HARQ efficiency.

[0006] The hybrid coding system for FSO / RF cooperative communication, which combines HARQ and diversity merging, uses OFDM as the signal modulation scheme. It corrects bit errors introduced during transmission through FEC encoding and decoding. Before FEC decoding, the bit data from the RF and FSO channels are merged to obtain good gain for the RF channel. A CRC check is used to verify the correctness of the corrected bit data; if it passes, transmission ends; otherwise, a retransmission request is made according to rules. During retransmission, if the buffer of data to be retransmitted does not exceed a set threshold, only RF is used for retransmission. After OFDM demodulation to the bit level, the bit data from the RF channel with better channel conditions replaces the corresponding bit data in the FSO channel according to the sampling position. If retransmission is required, the data is replaced according to the retransmission data position. If the buffer of data to be retransmitted overflows, both RF and FSO are used for retransmission simultaneously. The unreplaced FSO channel bit data is then combined using the maximum ratio based on multiple FSO retransmission results to improve the signal-to-noise ratio and obtain greater gain. This invention is applicable to FSO / RF cooperative communication systems. First, it can effectively avoid the link idleness problem caused by traditional hard handover. Second, it can continuously increase the amount of information obtained by the receiver in the high-performance RF link through the HARQ process, thus solving the rate mismatch problem and increasing HARQ efficiency.

[0007] The hybrid coding system for FSO / RF cooperative communication, which combines HARQ and diversity merging, specifically includes a signal encoding module, a bit data partitioning module, a signal transmission and reception module, a bit data merging / replacing module, a signal decoding module, and an automatic retransmission module, wherein:

[0008] The signal encoding module is used to perform CRC check encoding and FEC error correction encoding on the signal;

[0009] The bit data partitioning module partitions the bit data of the RF channel according to the ratio of the FSO and RF channel rates, in preparation for HARQ retransmission.

[0010] The signal transmission and reception module uses OFDM modulation to modulate the processed bit data into a communication signal that can be transmitted in the communication system and transmits it. The receiving end receives the signal and demodulates it into bit data, and determines the retransmission method according to the number of data to be retransmitted in the buffer.

[0011] The bit data merging / replacing module merges or replaces bit sequences based on the retransmission results.

[0012] The signal decoding module performs FEC decoding and CRC verification on the processed bit data;

[0013] The automatic retransmission module terminates transmission if the CRC check passes, and if it fails, it requests a retransmission according to the rules and merges the results of multiple retransmissions.

[0014] In this invention, the signal encoding module and CRC check utilize linear coding theory. At the transmitting end, based on the k-bit binary code sequence to be transmitted, a check code (i.e., CRC code) of r bits is generated according to certain rules and appended to the information, forming a new binary code sequence of (k+r) bits. The original data stream length is K, and after FEC encoding, the data length is M. Therefore, the code rate is K / M. The overhead of FEC encoding, OH, is OH = (MK) / K. Different overheads correspond to different coding gains. The output bit sequence is denoted as A. bit .

[0015] In this invention, the bit data partitioning module samples and groups the bit information multiple times for the RF link according to its rate ratio with that of the FSO link. Assuming the rate ratio of the FSO to the RF link is n, the bit sequence A after FEC encoding... bit Perform downsampling by n times to obtain the bit sequence B. bit And sequence A bit The sampling rate of sequence B bit n times, A bit The modulated signal sequence is assigned to the FSO link, B bit The modulated signal sequence is distributed to the RF link for transmission.

[0016] In this invention, the signal transmission and reception module uses OFDM signals for communication, expressed as:

[0017]

[0018] Where N represents the number of OFDM subcarriers, a n The information modulated on each subcarrier, f n ∈{f0,…,f N-1}, and f n =nf v f v The interval for each subcarrier. The transmitter to A bit and B bit OFDM modulation is performed separately, and transmission is carried out using FSO and RF links.

[0019] After receiving the data, the receiving end uses OFDM demodulation to demodulate the FSO and RF signals and places them in a buffer. The method of retransmission is determined based on the number of bits in the buffer.

[0020] In this invention, the bit data merging / replacement module determines the FSO channel conditions. If the number of data to be retransmitted does not exceed a set threshold, it indicates that the FSO channel conditions are good, and only RF is used for retransmission (Scenario 1). After OFDM demodulation to the bit level, the RF channel bit data with better channel conditions replaces the corresponding bit data in the FSO channel according to the sampling position. If retransmission is required, the replacement is performed according to the retransmission data position. If the number of data to be retransmitted exceeds the set threshold, it indicates that the FSO channel conditions are poor (Scenario 2). In this case, both RF and FSO are used for retransmission. The unreplaced FSO channel bit data is then combined using the maximum ratio based on the results of multiple FSO retransmissions to improve the signal-to-noise ratio.

[0021] In this invention, the signal decoding module first uses FEC for error correction decoding, and then performs CRC verification.

[0022] In this invention, the automatic retransmission module works as follows: after sending data, the sending end waits for a feedback signal from the receiving end. If the receiving end detects that the received data is error-free (verified by CRC check), it sends an ACK signal to the sending end and continues transmission; if there is an error, it sends a NACK signal, allowing retransmission up to the maximum number of times. The sending end sends the same data bits and redundant bits each time, and the receiving end does not discard the data that erroneously transmitted in the previous transmission, but instead merges it with the data received during retransmission.

[0023] The hybrid coding system for HARQ and diversity merging in FSO / RF cooperative communication provided by this invention can be made into an integrated adaptive sensing communication system with fiber optic wireless integration. Its workflow is as follows:

[0024] (1) Perform CRC encoding and FEC encoding on the bit data to be transmitted;

[0025] (2) The bit data is sampled and grouped multiple times according to the rate ratio of the RF link and the FSO link. The original signal is allocated to the FSO link for transmission, and the sampled signal is allocated to the RF link for transmission.

[0026] (3) Perform OFDM modulation and transmit the signal. At the receiving end, demodulate the FSO and RF signals respectively and determine the retransmission mode based on the number of data buffers to be retransmitted.

[0027] (4) The bit data of FSO and RF link are merged or replaced according to the sampling position. In scenario one, only the bit data of RF channel with better channel conditions is used to replace the bit data of FSO channel at the corresponding position according to the sampling position. In scenario two, the bit data of RF channel with better channel conditions is still used to replace the bit data of FSO channel at the corresponding position according to the sampling position. The FSO channel bit data that is not replaced is merged according to the maximum ratio based on the results of multiple FSO retransmissions to improve the signal-to-noise ratio.

[0028] (5) Perform FEC decoding and CRC verification. If the verification passes, the transmission ends. If the verification fails, return to process (2) and retransmit according to the retransmission method determined in process (3).

[0029] As can be seen from the free-space optical and radio frequency cooperative communication technology provided by the present invention, the present invention has the following advantages by adopting a transmission mechanism that integrates diversity with hybrid automatic repeat request coding:

[0030] (1) Compared with hard handover HARQ, which uses only one link for data transmission, this invention can make full use of the advantages of dual links and does not waste transmission resources.

[0031] (2) This invention integrates diversity merging technology. Compared with traditional HARQ, which can only obtain time diversity gain through multiple retransmissions, diversity gain can be obtained from the radio frequency branch.

[0032] (3) When the FSO channel conditions are good, there is little data to be retransmitted. Only RF is used to retransmit FSO to continuously transmit new information to ensure transmission efficiency. When the FSO channel conditions are poor, there is a lot of data to be retransmitted. In this case, FSO and RF are retransmitted at the same time to improve retransmission efficiency and accuracy.

[0033] This invention is applicable to the field of free-space optical and radio frequency (RF) cooperative communication. It can fully utilize dual-link resources and additionally obtain diversity gain from the radio frequency branch. Simultaneously, thanks to a unique radio link retransmission mechanism, this method can maintain continuous transmission of the FSO link even when it is slightly unstable, achieving the effect of gaining different bits of data from the RF link only through RF retransmission, thus improving retransmission benefits. If the channel deteriorates further (the number of buffered retransmission data exceeds a threshold), FSO is added to the retransmission, simultaneously obtaining the retransmission benefits of both FSO and RF, improving retransmission accuracy.

[0034] This invention has the potential for future free-space light and millimeter-wave collaborative communication. The transmission of free-space light and millimeter-wave signals in the atmosphere is affected by various factors in the atmospheric environment, such as absorption, scattering, turbulence, and weather. This invention utilizes the complementary link performance of free-space light and millimeter waves under different weather conditions to design a suitable integrated link retransmission and transmission scheme, which can meet the high reliability requirements of wideband transmission. Attached Figure Description

[0035] Figure 1 This is a block diagram illustrating the principle of the hybrid coding system of FSO / RF cooperative communication, which combines HARQ and subset fusion.

[0036] Figure 2 This is a flowchart of the transmission and reception process of the hybrid coding system for FSO / RF cooperative communication using HARQ and diversity merging.

[0037] Figure 3 This diagram illustrates the implementation method of the hybrid coding system for FSO / RF cooperative communication using HARQ and diversity merging in the present invention, under scenario one: when the FSO link quality is relatively good.

[0038] Figure 4 This diagram illustrates the implementation method for a hybrid coding system combining HARQ and diversity merging in FSO / RF collaborative communication, based on scenario two: when the FSO link quality is poor.

[0039] Figure 5 This is a simulation comparison diagram between the present invention and the traditional solution.

[0040] The diagram is labeled as follows: 101 - Signal encoding module, 102 - Bit data partitioning module, 103 - Signal transmission and reception module, 104 - Bit data merging / replacing module, 105 - Signal decoding module, 106 - Automatic retransmission module. Detailed Implementation

[0041] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0042] The hybrid coding system for FSO / RF cooperative communication that combines HARQ and diversity fusion, as proposed in this invention, is as follows: Figure 1 As shown, it includes a signal encoding module 101, a bit data partitioning module 102, a signal transmission and receiving module 103, a bit data merging / replacing module 104, a signal decoding module 105, and an automatic retransmission module 106.

[0043] Signal encoding module 101 performs CRC check encoding and FEC error correction encoding on the signal;

[0044] The bit data partitioning module 102 partitions the bit data of the RF channel according to the ratio of the FSO and RF channel rates, in preparation for HARQ retransmission.

[0045] The signal transmission and reception module 103 uses OFDM modulation to modulate the processed bit data into a communication signal that can be transmitted in the communication system and transmits it. The receiving end receives the signal and demodulates it into bit data, and determines the retransmission method according to the number of data to be retransmitted in the buffer.

[0046] The bit data merging / replacing module 104 merges or replaces bit sequences based on the retransmission results.

[0047] The signal decoding module 105 performs FEC decoding and CRC check on the processed bit data.

[0048] Automatic retransmission module 106: If the CRC check passes, the transmission ends; if it fails, the transmission is retransmitted according to the retransmission method determined in module 103.

[0049] The specific workflow of the FSO / RF cooperative communication system HARQ and the hybrid coding system of diversity merging proposed in this invention is as follows:

[0050] 1. The signal encoding module 101 performs CRC encoding and FEC encoding on the bit data to be transmitted;

[0051] CRC checksums utilize linear coding theory. At the sending end, based on the k-bit binary code sequence to be transmitted, an r-bit checksum (CRC code) is generated according to certain rules and appended to the information, forming a new binary code sequence of (k+r) bits. If the original data stream length is K, after FEC encoding, the data length is M, then the code rate is K / M. The overhead OH of FEC encoding is OH = (MK) / K. Different overheads correspond to different coding gains, and the output bit sequence is A. bit ;

[0052] Here, the encoding length of each bit block is 8000, the CRC encoding bit length is 24, and the code rate of CRC and FEC is 0.8333; CRC check encoding is performed first, followed by FEC error correction encoding.

[0053] Second: The bit data partitioning module 102 partitions the bit data of the RF channel according to the ratio of the FSO and RF channel rates to prepare for HARQ retransmission.

[0054] For the RF link, the bit information is sampled and grouped multiple times according to its rate ratio with that of the FSO link. Assuming the rate ratio of the FSO to the RF link is n, the bit sequence A after FEC encoding... bit Perform downsampling by n times to obtain the bit sequence B. bit And sequence A bit The sampling rate of sequence B bit n times, A bit The modulated signal sequence is assigned to the FSO link, B bit The modulated signal sequence is allocated to the RF link for transmission;

[0055] Here we assume that the RF rate is 1 / 4 of the FSO rate and the maximum number of retransmissions is 3. Then the RF channel is sampled 3 times, from the 1st, 2nd and 3rd bit positions respectively, with a sampling interval of 4.

[0056] Third: The signal transmission and receiving module 103 uses OFDM modulation to modulate the processed bit data into a communication signal that can be transmitted in the communication system and transmits it. The receiving end receives the signal and demodulates it into bit data. The retransmission method is determined according to the amount of data to be retransmitted in the buffer.

[0057] The communication signal is an OFDM signal, and its expression is written as:

[0058]

[0059] Where N represents the number of OFDM subcarriers, a n The information modulated on each subcarrier, f n ∈{f0,…,f N-1}, and f n =nf v f v The interval for each subcarrier. The transmitter to A bit and B bit OFDM modulation is performed separately, and transmission is carried out using FSO and RF links.

[0060] During signal transmission, we use additive white Gaussian noise channels with different signal-to-noise ratios to simulate the RF channel with better channel conditions and the FSO channel with slightly worse channel conditions. During demodulation, we only need to perform the reverse processing steps as in modulation.

[0061] After receiving the data, the receiving end uses OFDM demodulation to demodulate the FSO and RF signals and places them in a buffer. The method of retransmission is determined based on the number of bits in the buffer.

[0062] 4. The bit sequence is merged or replaced based on the retransmission result through the bit data merging / replacing module 104.

[0063] If the number of data to be retransmitted does not exceed the set threshold, it indicates that the FSO channel conditions are good, and only RF can be used for retransmission (Scenario 1). After OFDM demodulation to the bit level, the bit data of the RF channel with better channel conditions is used to replace the corresponding bit data of the FSO channel according to the sampling position. If retransmission is required, the replacement is performed according to the retransmission data position. If the number of data to be retransmitted exceeds the set threshold, it indicates that the FSO channel conditions are poor (Scenario 2). In this case, both RF and FSO are used for retransmission. The FSO channel bit data that has not been replaced is combined using the maximum ratio based on the results of multiple FSO retransmissions to improve the signal-to-noise ratio.

[0064] 5. The processed bit data is FEC decoded, CRC checked and retransmitted through the signal decoding module 105 and the automatic retransmission module 106.

[0065] First, FEC is used for error correction decoding, followed by CRC verification. If the CRC verification passes, an ACK signal is sent to the sender to continue transmission; if the CRC verification fails, a NACK signal is sent, and retransmission is performed according to the retransmission method determined in step three. Retransmissions can be repeated up to the maximum number of retransmissions. The receiving end does not discard the data that erroneously occurred in the previous transmission; instead, it is placed in a buffer and merged with the data received during retransmission.

[0066] Figure 5 The figure shows a comparison of the bit error rate performance of the system of the present invention under different signal-to-noise ratios, wherein the signal-to-noise ratio gain of the radio frequency channel is set to 5 and the maximum number of transmissions is three.

[0067] Figure 5In (c), four curves are plotted. The first curve shows the bit error rate without HARQ technology. The second curve shows HARQ using only FEC error correction without retransmission. The third curve shows HARQ using only RF for retransmission. The fourth curve shows HARQ using both RF and FSO for retransmission. It can be seen that HARQ technology can effectively improve transmission stability. When channel conditions are poor, simultaneous retransmission using both RF and FSO can further improve stability but also increases retransmission costs and reduces transmission efficiency. Therefore, when channel conditions are good, only RF retransmission is needed. In practical applications, we can use the number of data packets to be retransmitted in the buffer as a basis for deciding whether to switch retransmission methods. For example, if there are more than n groups of data to be retransmitted, it is considered that the channel conditions are poor, and RF retransmission alone is insufficient; therefore, both FSO and RF should be used for retransmission.

[0068] Figure 5 (a) and (b) present the relationship between the FSO link outage probability and throughput and the receiver SNR under different HARQ rounds and HARQ strategies when the FEC overhead is 5%. Figure 5 (a) It can be seen that although the symbol rate and transmission rate of the RF link are one-quarter of those of the FSO link, when M=2 and the RF link performs HARQ, the outage probability is still significantly lower compared to a single FSO link. The system performance of two retransmissions on the RF link is similar to that of a system retransmitting once together on both the RF and FSO links. Regarding throughput, when the signal-to-noise ratio (SNR) at the FSO link receiver is low, the FSO link needs to participate in retransmissions to achieve higher throughput due to the higher rate of the RF link. However, when the SNR increases, the FSO link can transmit the next data frame while the RF link is performing HARQ, thus a strategy of RF-only retransmission can achieve higher throughput. Figure 5 As shown in (b), under high signal-to-noise ratio conditions, the throughput of the single RF link HARQ strategy is better than that of the RF / FSO hybrid HARQ strategy.

[0069] In summary, the verification results demonstrate that the hybrid coding system for FSO / RF cooperative communication, which integrates HARQ and diversity merging, proposed in this invention, can be applied to the field of free-space optical and radio frequency cooperative communication. It can fully utilize dual-link resources and additionally obtain diversity gain from the radio frequency branch. Simultaneously, thanks to the unique radio link retransmission mechanism, this invention can maintain continuous transmission of the FSO link even when the FSO link is slightly unstable, obtaining the effect of different bit sequence gains from the radio frequency link only through RF retransmission, thus improving the retransmission benefit. If the channel deteriorates further (the data to be retransmitted exceeds the threshold), FSO is added to the retransmission, simultaneously obtaining the retransmission benefits of both FSO and RF, improving the retransmission accuracy.

[0070] The division of steps in this embodiment is only for clarity of description. In implementation, they can be combined into one step or some steps can be split into multiple steps. As long as they contain the same logical relationship, they are all within the protection scope of this invention.

Claims

1. A hybrid coding system for FSO / RF cooperative communication using HARQ and diversity union fusion, characterized in that, Specifically, it includes a signal encoding module, a bit data partitioning module, a signal transmission and reception module, a bit data merging / replacing module, a signal decoding module, and an automatic retransmission module; among which: The signal encoding module is used to perform CRC check encoding and FEC error correction encoding on the signal; The bit data partitioning module partitions the bit data of the RF channel according to the ratio of the FSO and RF channel rates, in preparation for HARQ retransmission. The signal transmission and reception module uses OFDM modulation to modulate the processed bit data into a communication signal that can be transmitted in the communication system and transmits it. The receiving end receives the signal and demodulates it into bit data, and determines the retransmission method according to the number of data to be retransmitted in the buffer. The bit data merging / replacing module merges or replaces bit sequences based on the retransmission results. The signal decoding module performs FEC decoding and CRC verification on the processed bit data; The automatic retransmission module ends transmission if the CRC check passes, and if it fails, it requests a retransmission according to the rules and merges the results of multiple retransmissions. Orthogonal Frequency Division Multiplexing (OFDM) is used as the signal modulation scheme. Forward Error Correction (FEC) encoding and decoding correct bit errors introduced during transmission. Before FEC decoding, the bit data of the RF channel and the FSO channel are merged. Cyclic Redundancy Check (CRC) is used to verify the correctness of the bit data after error correction. If the verification is successful, the transmission ends; otherwise, a retransmission request is made according to the rules. If the data buffer to be retransmitted does not exceed the set threshold during retransmission, only RF is used for retransmission. After OFDM demodulation to the bit level, the RF channel bit data is used to replace the corresponding bit data in the FSO channel according to the sampling position. If retransmission is required, the data is replaced according to the retransmission data position. If the number of data buffers to be retransmitted overflows, both RF and FSO are used for retransmission simultaneously. The unreplaced FSO channel bit data is then combined using the maximum ratio based on the results of multiple FSO retransmissions.

2. The hybrid coding system according to claim 1, characterized in that, In the signal encoding module, CRC check utilizes linear coding theory. At the transmitting end, based on the k-bit binary code sequence to be transmitted, an r-bit check code (CRC code) is generated according to certain rules and appended to the k-bit binary code sequence, forming a new binary code sequence of (k+r) bits. The original data stream length is K, and after FEC encoding, the data length is M. Therefore, the code rate is K / M, and the overhead of FEC encoding is OH = (MK) / K. Different overheads correspond to different encoding gains, and the output bit sequence is... .

3. The hybrid coding system according to claim 2, characterized in that, In the bit data partitioning module, for the RF link, the bit information is sampled and grouped multiple times according to its rate ratio with that of the FSO link. Assuming the rate ratio of the FSO to the RF link is n, the bit sequence after FEC encoding is... Perform downsampling by n times to obtain the bit sequence And sequence The sampling rate is the sequence n times, The modulated signal sequence is assigned to the FSO link. The modulated signal sequence is distributed to the RF link for transmission.

4. The hybrid coding system according to claim 3, characterized in that, In the signal transmission and reception module, the communication signal is an OFDM signal, expressed as: ， （1） Where N represents the number of OFDM subcarriers, The information modulated on each subcarrier, ,and , The interval between each subcarrier; the transmitter pairs and OFDM modulation is performed separately, and transmission is carried out using FSO and RF links; After receiving the data, the receiving end uses OFDM demodulation to demodulate the FSO and RF signals and places them in a buffer. The method of retransmission is determined based on the number of bits in the buffer.

5. The hybrid coding system according to claim 4, characterized in that, In the bit data merging / replacement module, after OFDM demodulation to the bit level, the RF channel bit data is used to replace the corresponding bit data in the FSO channel according to the sampling position; if retransmission is required, the data is replaced according to the retransmission data position; wherein: If the buffer of data to be retransmitted does not exceed the set threshold, it means that only RF is used for retransmission, which is called scenario one; If the number of data to be retransmitted exceeds the set threshold, RF and FSO are used for retransmission simultaneously. The FSO channel bit data that has not been replaced is combined by the maximum ratio based on the results of multiple FSO retransmissions to improve the signal-to-noise ratio. This is called Scenario 2.

6. The hybrid coding system according to claim 5, characterized in that, In the signal decoding module, FEC is first used for error correction decoding, and then CRC verification is performed.

7. The hybrid coding system according to claim 6, characterized in that, In the automatic retransmission module, after the sending end sends data, it waits for the feedback signal from the receiving end. The receiving end detects the received data. If it is an error-free transmission, it sends an ACK signal to the sending end and continues transmission; if it is an erroneous transmission, it sends a NACK signal and retransmits up to the maximum number of times. The sending end sends the same data bits and redundant bits each time. The receiving end does not discard the data that was erroneous in the previous transmission, but merges it with the data received during retransmission.

8. The hybrid coding system according to claim 7, characterized in that, The workflow is as follows: (1) Perform CRC encoding and FEC encoding on the bit data to be transmitted; (2) The bit data is sampled and grouped multiple times according to the rate ratio of the RF link and the FSO link. The high-speed signal is allocated to the FSO link for transmission, and the low-speed signal is allocated to the RF link for transmission. (3) Perform OFDM modulation, transmit the signal, demodulate the FSO and RF signals at the receiving end respectively, and determine the retransmission mode based on the number of data buffers to be retransmitted; (4) The bit data of FSO and RF link are merged or replaced according to the sampling position. If it is scenario one, only the bit data of RF channel is used to replace the bit data of FSO channel corresponding position according to the sampling position. If it is scenario two, the bit data of RF channel is still used to replace the bit data of FSO channel corresponding position according to the sampling position. The bit data of FSO channel that is not replaced is merged according to the maximum ratio based on the results of multiple FSO retransmissions to improve the signal-to-noise ratio. (5) Perform FEC decoding and CRC verification. If the verification passes, the transmission ends. If it fails, return to process (2) and retransmit according to the retransmission method determined in process (3).

Images