Digital fronthaul method and system based on two-level coding modulation

Abstract

The present invention provides a digital fronthaul method and system based on two-stage coded modulation, comprising: a signal processing and transmission step in which continuous orthogonal frequency division multiplexing (OFDM) signals are processed, converted into optical signals, and transmitted into optical fibers; and a signal reception and processing step in which the received optical signals are processed and restored into OFDM signals. The present invention performs forward error correction coding only on high-significant bits, effectively reducing the complexity of coding and decoding. Furthermore, a new modulation mapping scheme is designed to demodulate bits with low error rates after error correction, effectively improving the performance of bits not subjected to forward error correction coding, thereby enhancing overall system performance.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber and wireless converged fronthaul technology, and in particular to a digital fronthaul method and system based on two-stage coding modulation. Background Art

[0002] In 5G base stations, the fronthaul fiber optic communication system connects the baseband processing unit and the remote radio frequency (RF) end, making it a crucial component. Looking ahead, 6G base stations will significantly increase peak rates and deployment density, posing significant challenges to the fronthaul system's carrying capacity. Designing a new fronthaul system with high capacity, low cost, high reliability, and low latency has become a key issue in building future communication base stations.

[0003] In recent years, radio-over-fiber (RoF) technology has become a common method for mobile fronthaul transmission. RoF technologies are generally categorized as analog and digital. Analog RoF modulates the radio waveform directly onto an optical carrier, occupying the same bandwidth as the original signal. This provides a spectrally efficient and low-cost solution, but it is susceptible to transceiver nonlinearities and link distortion. In digital RoF, the radio signal waveform is first quantized at high resolution to maintain signal fidelity. The quantized bits are then remodulated and encoded for transmission in the optical domain. Compared to analog RoF, digital RoF is more robust against system noise and link distortion. However, as data rates increase, digital RoF requires greater bandwidth, and both the throughput and complexity of digital signal processing must be significantly increased.

[0004] Patent document CN110830404A (application number: CN201911056205.0) discloses a digital mobile fronthaul signal quantization method based on vector linear prediction. The method performs IFFT on the OFDM modulated signal to obtain real and imaginary I / Q sampling signals, which are then constructed into a multidimensional vector set. The input multidimensional vector set is differentially vector quantized to output a quantized signal consisting of a codeword-index mapping sequence. The quantized signal is PAM-4 encoded and then converted into an optical signal through electro-optical modulation. The optical signal is input into a single-mode optical fiber and transmitted to the radio remote unit. After photoelectric detection and PAM-4 decoding, the quantized signal is recovered. The recovered quantized signal is subjected to index-codeword mapping and differential demodulation to recover the original differential vector quantized signal. The recovered differential vector quantized signal is demodulated using the vector set to recover the OFDM-IFFT I / Q sampling signals. However, this patent does not fully solve the above technical problems. Summary of the Invention

[0005] In view of the defects in the prior art, the purpose of the present invention is to provide a digital fronthaul method and system based on two-level coding modulation.

[0006] The digital fronthaul method based on two-level coding modulation provided by the present invention includes:

[0007] Signal processing and transmission steps: After processing, the continuous orthogonal frequency division multiplexing (OFDM) signal is converted into an optical signal and transmitted into the optical fiber;

[0008] Signal reception and processing steps: The received optical signal is processed and restored to an OFDM signal;

[0009] The signal processing and transmitting steps include:

[0010] Step S1: quantize the continuous OFDM signal to obtain a bit sequence, and then perform scrambling on the bit sequence;

[0011] Step S2: Divide the bit signal into two parts according to its importance, namely high-significant bits and low-significant bits, and perform corresponding processing on each part;

[0012] Step S3: combining the high-significant bits and the low-significant bits together to perform digital signal modulation to obtain a quadrature amplitude modulation (QAM) symbol;

[0013] Step S4: After the QAM symbol is processed by the digital signal of the optical signal transmitter, it is converted into an electrical signal, and then modulated into an optical signal for transmission;

[0014] The signal receiving and processing steps include:

[0015] Step S5: The optical signal transmitted through the optical fiber undergoes photoelectric conversion to become an electrical signal, which is then converted into a digital signal for digital signal processing at the optical signal receiving end to obtain a symbol with noise;

[0016] Step S6: performing two-stage demodulation and decoding on the noisy symbols to obtain high-significant bits and low-significant bits respectively;

[0017] Step S7: Descramble all bits and then restore the bits to OFDM signals through inverse quantization.

[0018] Preferably, the classification of high-significant bits and low-significant bits is related to the modulation format and the code rate of the forward error correction coding. If only the most significant bit is forward error correction coded and other significant bits are not FEC coded, the ratio of bits coded with FEC to all bits is:

[0019]

[0020] Here, r is the FEC code rate, m=log2(M), and M is the order of the modulation format, that is, for 16QAM, M=16, m=4; for 64QAM, M=64, m=6.

[0021] Preferably, for high-significant bits, FEC encoding and interleaving are performed in sequence; for low-significant bits, only interleaving is performed.

[0022] Preferably, the mapping principle of digital signal modulation is:

[0023] The high-significant bits and their FEC-encoded parity bits are mapped to the least reliable bits (LRBs) of the QAM symbol, and the LRBs are changed between adjacent symbols.

[0024] The other low-significant bits are mapped to bits other than LRBs of the QAM symbol using Gray mapping.

[0025] Preferably, a soft decision is first made on the LRB of the symbol, that is, the log-likelihood ratio LLR of the LRB is calculated, the LLR is sequentially deinterleaved, and FEC decoding is performed to obtain the error-corrected LRB, that is, the corresponding most significant bit;

[0026] Based on the LRB after error correction, hard decisions are made on all bits except the LRB to directly determine whether the bit value is 0 or 1. The specific operation is as follows: interleave the LRB after error correction, demodulate it again as accurate prior information, make hard decisions on all bits except the LRB, and deinterleave the bits after hard decisions to obtain the low-significant bits.

[0027] The digital fronthaul system based on two-level coding modulation provided by the present invention includes:

[0028] Signal processing and transmission module: After processing, the continuous orthogonal frequency division multiplexing (OFDM) signal is converted into an optical signal and transmitted into the optical fiber;

[0029] Signal receiving and processing module: Processes the received optical signal and restores it to OFDM signal;

[0030] The signal processing and transmitting module includes:

[0031] Module M1: quantizes the continuous OFDM signal to obtain a bit sequence, and then scrambles the bit sequence;

[0032] Module M2: divides the bit signal into two parts according to its importance, namely the high-significant bit and the low-significant bit, and processes them respectively;

[0033] Module M3: combines the high-significant bits and the low-significant bits together to perform digital signal modulation to obtain quadrature amplitude modulation (QAM) symbols;

[0034] Module M4: After the QAM symbols are processed by the digital signal of the optical signal transmitter, they are converted into electrical signals, which are then modulated into optical signals for transmission.

[0035] The signal receiving and processing module includes:

[0036] Module M5: After optical fiber transmission, the optical signal undergoes photoelectric conversion, becomes an electrical signal, and then is converted into a digital signal for digital signal processing at the optical signal receiving end to obtain a symbol with noise;

[0037] Module M6: Demodulates and decodes the noisy symbols in two stages to obtain high-significant bits and low-significant bits respectively.

[0038] Module M7: Descrambles all bits and then restores the bits to OFDM signals through inverse quantization.

[0039] Preferably, the classification of high-significant bits and low-significant bits is related to the modulation format and the code rate of the forward error correction coding. If only the most significant bit is forward error correction coded and other significant bits are not FEC coded, the ratio of bits coded with FEC to all bits is:

[0040]

[0041] Here, r is the FEC code rate, m=log2(M), and M is the order of the modulation format, that is, for 16QAM, M=16, m=4; for 64QAM, M=64, m=6.

[0042] Preferably, for high-significant bits, FEC encoding and interleaving are performed in sequence; for low-significant bits, only interleaving is performed.

[0043] Preferably, the mapping principle of digital signal modulation is:

[0044] The high-significant bits and their FEC-encoded parity bits are mapped to the least reliable bits (LRBs) of the QAM symbol, and the LRBs are changed between adjacent symbols.

[0045] The other low-significant bits are mapped to bits other than LRBs of the QAM symbol using Gray mapping.

[0046] Preferably, firstly, a soft decision is made on the LRB of the symbol, that is, the logarithmic natural ratio LLR of the LRB is calculated, the LLR is sequentially deinterleaved, and FEC decoding is performed to obtain the error-corrected LRB, that is, the corresponding most significant bit;

[0047] Based on the LRB after error correction, hard decisions are made on all bits except the LRB to directly determine whether the bit value is 0 or 1. The specific operation is as follows: interleave the LRB after error correction, demodulate it again as accurate prior information, make hard decisions on all bits except the LRB, and deinterleave the bits after hard decisions to obtain the low-significant bits.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] The present invention is based on two-level coded modulation and only performs forward error correction coding on high-significant bits, effectively reducing the complexity of coding and decoding; at the same time, a new modulation mapping scheme is designed to demodulate based on bits with low error rates after error correction, effectively improving the performance of bits that have not undergone forward error correction coding, thereby improving the overall performance of the system; the present invention is easy to implement, easy to use, and has low implementation complexity, and can overcome the shortcomings of the existing technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0051] Figure 1 This is a flow chart of the digital fronthaul method based on two-level coding modulation of the present invention;

[0052] FIG2 is a digital modulation mapping scheme of quaternary amplitude shift keying (4ASK) and octal amplitude shift keying (8ASK) symbols used in the present invention;

[0053] Figure 3 This is a performance diagram of the present invention applied to a digital fronthaul simulation system, where the modulation format is 16QAM;

[0054] Figure 4 This is a performance diagram of the present invention applied to a digital fronthaul simulation system, where the modulation format is 64QAM. DETAILED DESCRIPTION

[0055] The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0056] Example 1

[0057] The embodiment of the present invention provides a digital fronthaul method based on two-level coding modulation, such as Figure 1 As shown, the following steps are included: signal processing and transmission step: after processing, the continuous Orthogonal Frequency Division Multiplexing (OFDM) signal is converted into an optical signal and transmitted into the optical fiber; signal receiving and processing step: after receiving the received optical signal, it is processed and restored to an OFDM signal.

[0058] The signal processing and transmission steps include the following steps:

[0059] Step S1: quantize a continuous OFDM signal to obtain a bit sequence, and then perform scrambling processing on the bit sequence.

[0060] Step S2: Divide the bit signal into two parts according to its importance, namely the most significant bit (MSB) and the least significant bit (LSB), and perform corresponding processing on each part.

[0061] Specifically, the classification of high-significant bits and low-significant bits is related to the modulation format and the code rate of the forward error correction coding. If only the most significant bit is forward error correction (FEC) coded, and the other significant bits are not FEC coded, then the proportion of bits coded with FEC to all bits is

[0062]

[0063] Here, r is the FEC code rate, m=log2(M), and M is the order of the modulation format, that is, for 16QAM, M=16, m=4; for 64QAM, M=64, m=6.

[0064] The high-significant bits and low-significant bits are processed separately. For the high-significant bits, FEC encoding and interleaving are performed in sequence; for the low-significant bits, only interleaving is performed.

[0065] Step S3: Combine the high-significance bits and the low-significance bits together to perform digital signal modulation to obtain a Quadrature Amplitude Modulation (QAM) symbol.

[0066] Specifically, the mapping principles of digital signal modulation are as follows:

[0067] The high-significance bits and their FEC-encoded parity bits are mapped to the least reliable bits (LRBs) of the QAM symbols, and the LRBs are changed between adjacent symbols.

[0068] The other low-significant bits are mapped to bits other than LRBs of the QAM symbol using Gray mapping.

[0069] Step S4: After the QAM symbol undergoes digital signal processing at the optical signal transmitter, it is converted into an electrical signal, which is then modulated into an optical signal for transmission.

[0070] The signal receiving and processing steps include the following steps:

[0071] Step S5: After being transmitted through the optical fiber, the optical signal undergoes photoelectric conversion to become an electrical signal, which is then converted into a digital signal for digital signal processing at the optical signal receiving end to obtain a symbol with noise.

[0072] Step S6: Perform two-stage demodulation and decoding on the noisy symbols to obtain high-significant bits and low-significant bits respectively.

[0073] Specifically, first, a soft decision is made on the LRB of the symbol, that is, the log-likelihood ratio (LLR) of the LRB is calculated, the LLR is deinterleaved in sequence, and FEC decoding is performed to obtain the error-corrected LRB, which is also the corresponding most significant bit.

[0074] Based on the corrected LRBs, a hard decision is made on all bits except the LRBs, directly determining whether the bit value is 0 or 1. Specifically, the corrected LRBs are interleaved and demodulated again using this information as accurate prior information. This means making a hard decision on all bits except the LRBs. The hard-decided bits are then deinterleaved to obtain the least significant bits.

[0075] Step S7: Descramble all bits and then restore the bits to OFDM signals through inverse quantization.

[0076] The embodiment of the present invention also discloses a mapping scheme for digital modulation in this solution, as shown in FIG2 . Figure 2a and Figure 2b The mapping methods for quaternary amplitude shift keying (4ASK) and octal amplitude shift keying (8ASK) symbols are demonstrated. 16QAM and 64QAM symbols are composed of two independent 4ASK and 8ASK symbols, respectively, which are orthogonally superimposed.

[0077] The embodiment of the present invention also provides a designed solution for application in the simulation performance of digital fronthaul system, such as Figure 3 and Figure 4 As shown in the figure, SQNR is the full name of Signal-to-Quantization Noise Ratio, which is translated into Chinese as Signal to Quantization Noise Ratio. It represents the upper limit of system performance due to quantization noise. Uniform quantization is used in the simulation, and the number of quantization bits is 12. Figure 3 The code rate of the Low-Density Parity-Check (LDPC) code used in the 16QAM and Two-Leveling Coding (TLC) is 2 / 3. To achieve the same spectral efficiency, the code rate used in the Bit-Interleaved Coded Modulation (BICM) is 5 / 6. As can be seen from the results, when the complexity of TLC and BICM is similar, that is, when the complexity of 2-iteration TLC is compared with 4-iteration BICM, the performance can be significantly improved by adopting the proposed TLC scheme. The performance of 2-iteration TLC is similar to that of 2-iteration BICM, but the complexity of TLC is reduced by 50% compared to BICM. Figure 4 For 64QAM, the LDPC code rate used in TLC is 2 / 3, while the code rate used in BICM is 8 / 9 to achieve the same spectral efficiency. The simulation results for 64QAM are similar to those for 16QAM. When the complexity of TLC and BICM is similar, that is, when TLC with two iterations is compared to BICM with six iterations, the proposed TLC scheme significantly improves performance. While the performance of TLC with two iterations is similar to that of BICM with two iterations, TLC reduces its complexity by 66.7% compared to BICM.

[0078] Example 2

[0079] The present invention also provides a digital fronthaul system based on two-level coding modulation. The digital fronthaul system based on two-level coding modulation can be implemented by executing the process steps of the digital fronthaul method based on two-level coding modulation, that is, those skilled in the art can understand the digital fronthaul method based on two-level coding modulation as a preferred implementation of the digital fronthaul system based on two-level coding modulation.

[0080] The digital fronthaul system based on two-level coding modulation provided by the present invention includes:

[0081] Signal processing and transmission module: After processing, the continuous orthogonal frequency division multiplexing (OFDM) signal is converted into an optical signal and transmitted into the optical fiber;

[0082] Signal receiving and processing module: Processes the received optical signal and restores it to OFDM signal;

[0083] The signal processing and transmitting module includes:

[0084] Module M1: quantizes the continuous OFDM signal to obtain a bit sequence, and then scrambles the bit sequence;

[0085] Module M2: divides the bit signal into two parts according to its importance, namely the high-significant bit and the low-significant bit, and processes them respectively;

[0086] Module M3: combines the high-significant bits and the low-significant bits together to perform digital signal modulation to obtain quadrature amplitude modulation (QAM) symbols;

[0087] Module M4: After the QAM symbols are processed by the digital signal of the optical signal transmitter, they are converted into electrical signals, which are then modulated into optical signals for transmission.

[0088] The signal receiving and processing module includes:

[0089] Module M5: After optical fiber transmission, the optical signal undergoes photoelectric conversion, becomes an electrical signal, and then is converted into a digital signal for digital signal processing at the optical signal receiving end to obtain a symbol with noise;

[0090] Module M6: Demodulates and decodes the noisy symbols in two stages to obtain high-significant bits and low-significant bits respectively.

[0091] Module M7: Descrambles all bits and then restores the bits to OFDM signals through inverse quantization.

[0092] The classification of most-significant bits and least-significant bits is related to the modulation format and the forward error correction code rate. If only the most-significant bit is forward-error-corrected and the other significant bits are not FEC-encoded, the ratio of FEC-encoded bits to all bits is:

[0093]

[0094] Here, r is the FEC code rate, m=log2(M), and M is the order of the modulation format, that is, for 16QAM, M=16, m=4; for 64QAM, M=64, m=6.

[0095] For high-significant bits, FEC encoding and interleaving are performed in sequence; for low-significant bits, only interleaving is performed.

[0096] The mapping principle of digital signal modulation is:

[0097] The high-significant bits and their FEC-encoded parity bits are mapped to the least reliable bits (LRBs) of the QAM symbol, and the LRBs are changed between adjacent symbols.

[0098] The other low-significant bits are mapped to bits other than LRBs of the QAM symbol using Gray mapping.

[0099] First, a soft decision is made on the LRB of the symbol, that is, the logarithmic natural ratio (LLR) of the LRB is calculated, the LLR is deinterleaved in sequence, and FEC decoding is performed to obtain the error-corrected LRB, that is, the corresponding most significant bit;

[0100] Based on the LRB after error correction, hard decisions are made on all bits except the LRB to directly determine whether the bit value is 0 or 1. The specific operation is as follows: interleave the LRB after error correction, demodulate it again as accurate prior information, make hard decisions on all bits except the LRB, and deinterleave the bits after hard decisions to obtain the low-significant bits.

[0101] like Figure 1 The present invention provides a digital fronthaul system based on multi-level coding, comprising the following modules:

[0102] Quantization: approximate the continuous value of the signal to a finite number of discrete values;

[0103] Dequantization: reconstructing the original continuous signal using a finite number of discrete values ​​(which will introduce quantization noise);

[0104] Scrambling: Pseudo-random sequence scrambling, that is, the input bits are XORed with a pseudo-random uniformly distributed bit sequence to obtain the scrambled bit sequence;

[0105] Descrambling: Recover the original bit sequence by performing an XOR operation on the input data using the same pseudo-random bit sequence used during scrambling.

[0106] Forward error correction coding: Adds redundant information to information bits through specific methods;

[0107] Forward error correction decoding: uses the redundant information in the error correction code to decode and reduce errors in transmission;

[0108] Interleaving: Rearranging the bit sequence in a specific way;

[0109] Deinterleaving: restoring the rearranged sequence to its original order in a specific way;

[0110] Modulation: Mapping the transmitted bit sequence into orthogonal amplitude modulation symbols;

[0111] Demodulation: Demaps the received symbols into a bit sequence;

[0112] Tx-DSP: Performs digital signal processing on quadrature amplitude modulation symbols at the optical transmitter, which may include pulse shaping and digital pre-distortion.

[0113] Rx-DSP: At the optical signal receiving end, it performs digital signal processing on the received digital signal, which may include frequency offset estimation and compensation, equalization, and laser phase noise compensation.

[0114] DAC: At the optical transmitter, performs digital signal processing on the quadrature amplitude modulation symbols and converts the digital signal into an analog signal.

[0115] ADC: converts analog signals into digital signals;

[0116] E / O: electro-optical conversion, converting electrical signals into optical signals;

[0117] O / E: Photoelectric conversion, converting optical signals into electrical signals.

[0118] Those skilled in the art will appreciate that, in addition to implementing the system, device, and various modules provided by the present invention in purely computer-readable program code, it is entirely possible to implement the same program in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, embedded microcontrollers, and the like by logically programming the method steps. Therefore, the system, device, and various modules provided by the present invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; the modules for implementing various functions can also be considered both software programs for implementing the method and structures within the hardware component.

[0119] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. The embodiments of this application and the features in the embodiments may be combined with each other in any manner unless there is a conflict.

Claims

1. A digital fronthaul method based on two-level coding modulation, characterized in that: include: Signal processing and transmission steps: After processing, the continuous orthogonal frequency division multiplexing (OFDM) signal is converted into an optical signal and transmitted into the optical fiber; Signal reception and processing steps: The received optical signal is processed and restored to an OFDM signal; The signal processing and transmitting steps include: Step S1: quantize the continuous OFDM signal to obtain a bit sequence, and then perform scrambling on the bit sequence; Step S2: Divide the bit signal into two parts according to its importance, namely high-significant bits and low-significant bits, and perform corresponding processing on each part; The classification of most-significant bits and least-significant bits is related to the modulation format and the forward error correction code rate. If only the most-significant bit is forward-error-corrected and the other significant bits are not FEC-encoded, the ratio of FEC-encoded bits to all bits is: Where r is the FEC code rate, m = log2(M), where M is the order of the modulation format, i.e., for 16QAM, M = 16, m = 4; for 64QAM, M = 64, m = 6; For high-significant bits, FEC encoding and interleaving are performed in sequence; for low-significant bits, only interleaving is performed; Step S3: combining the high-significant bits and the low-significant bits together to perform digital signal modulation to obtain a quadrature amplitude modulation (QAM) symbol; Step S4: After the QAM symbol is processed by the digital signal of the optical signal transmitter, it is converted into an electrical signal, and then modulated into an optical signal for transmission; The signal receiving and processing steps include: Step S5: The optical signal transmitted through the optical fiber undergoes photoelectric conversion to become an electrical signal, which is then converted into a digital signal for digital signal processing at the optical signal receiving end to obtain a symbol with noise; Step S6: performing two-stage demodulation and decoding on the noisy symbols to obtain high-significant bits and low-significant bits respectively; Step S7: Descramble all bits and then restore the bits to OFDM signals through inverse quantization.

2. The digital fronthaul method based on two-level coding modulation according to claim 1, characterized in that: The mapping principle of digital signal modulation is: The high-significant bits and their FEC-encoded parity bits are mapped to the least reliable bits (LRBs) of the QAM symbol, and the LRBs are changed between adjacent symbols. The other low-significant bits are mapped to bits other than LRBs of the QAM symbol using Gray mapping.

3. The digital fronthaul method based on two-level coding modulation according to claim 1, characterized in that: First, a soft decision is made on the LRB of the symbol, that is, the logarithmic natural ratio (LLR) of the LRB is calculated, the LLR is deinterleaved in sequence, and FEC decoding is performed to obtain the error-corrected LRB, that is, the corresponding most significant bit; Based on the LRB after error correction, hard decisions are made on all bits except the LRB to directly determine whether the bit value is 0 or 1. The specific operation is as follows: interleave the LRB after error correction, demodulate it again as accurate prior information, make hard decisions on all bits except the LRB, and deinterleave the bits after hard decisions to obtain the low-significant bits.

4. A digital fronthaul system based on two-level coded modulation, characterized in that: include: Signal processing and transmission module: After processing, the continuous orthogonal frequency division multiplexing (OFDM) signal is converted into an optical signal and transmitted into the optical fiber; Signal receiving and processing module: Processes the received optical signal and restores it to OFDM signal; The signal processing and transmission module includes: Module M1: quantizes the continuous OFDM signal to obtain a bit sequence, and then scrambles the bit sequence; Module M2: divides the bit signal into two parts according to its importance, namely the high-significant bit and the low-significant bit, and processes them respectively; The classification of most-significant bits and least-significant bits is related to the modulation format and the forward error correction code rate. If only the most-significant bit is forward-error-corrected and the other significant bits are not FEC-encoded, the ratio of FEC-encoded bits to all bits is: Where r is the FEC code rate, m = log2(M), where M is the order of the modulation format, i.e., for 16QAM, M = 16, m = 4; for 64QAM, M = 64, m = 6; For high-significant bits, FEC encoding and interleaving are performed in sequence; for low-significant bits, only interleaving is performed; Module M3: combines the high-significant bits and the low-significant bits together to perform digital signal modulation to obtain quadrature amplitude modulation (QAM) symbols; Module M4: After the QAM symbols are processed by the digital signal of the optical signal transmitter, they are converted into electrical signals, which are then modulated into optical signals for transmission. The signal receiving and processing module includes: Module M5: After optical fiber transmission, the optical signal undergoes photoelectric conversion, becomes an electrical signal, and then is converted into a digital signal for digital signal processing at the optical signal receiving end to obtain a symbol with noise; Module M6: Demodulates and decodes the noisy symbols in two stages to obtain high-significant bits and low-significant bits respectively. Module M7: Descrambles all bits and then restores the bits to OFDM signals through inverse quantization.

5. The digital fronthaul system based on two-level coding modulation according to claim 4, characterized in that: The mapping principle of digital signal modulation is: The high-significant bits and their FEC-encoded parity bits are mapped to the least reliable bits (LRBs) of the QAM symbol, and the LRBs are changed between adjacent symbols. The other low-significant bits are mapped to bits other than LRBs of the QAM symbol using Gray mapping.

6. The digital fronthaul system based on two-level coding modulation according to claim 4, characterized in that: First, a soft decision is made on the LRB of the symbol, that is, the logarithmic natural ratio (LLR) of the LRB is calculated, the LLR is deinterleaved in sequence, and FEC decoding is performed to obtain the error-corrected LRB, that is, the corresponding most significant bit; Based on the LRB after error correction, hard decisions are made on all bits except the LRB to directly determine whether the bit value is 0 or 1. The specific operation is as follows: interleave the LRB after error correction, demodulate it again as accurate prior information, make hard decisions on all bits except the LRB, and deinterleave the bits after hard decisions to obtain the low-significant bits.

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