Hybrid transmission light forward transmission system and related method

Abstract

The invention relates to the technical field of radio over fiber communication, and provides a hybrid transmission optical forward transmission system and a related method. The method comprises: separating an input bit stream into index bits and communication modulation bits; generating an unmodulated AFDM waveform with a corresponding chirp slope according to the index bit control, and processing the unmodulated AFDM waveform into a first baseband signal; modulating the communication modulation bit to an AFDM subcarrier to generate a second baseband signal, and carrying out up-conversion on the second baseband signal to obtain a first radio frequency signal; performing frequency division coupling on the first baseband signal and the first radio frequency signal to generate a mixed radio frequency driving signal; receiving the optical carrier frequency division mixed signal and converting the optical carrier frequency division mixed signal into a mixed radio frequency signal of an electric domain; separating the mixed radio frequency signal into a digital baseband signal and an analog radio frequency signal; performing discrete affine Fourier transform on the digital baseband signal, and determining chirp slope of the digital baseband signal through peak detection so as to demodulate index bits; and controlling the switching network to gate the analog radio frequency signal to the corresponding transmitting antenna according to the index bit.

Description

Technical Field

[0001] This application relates to the field of optical wireless communication, and in particular to a hybrid transmission optical fronthaul system and related methods. Background Technology

[0002] In 5G and future 6G mobile communication networks, the fronthaul network between the centralized unit and the remote radio unit undertakes the critical task of high-speed data transmission. To meet the requirements of high bandwidth and low latency, optical-on-fiber (ORF) technology has become the mainstream solution, with analog ORF systems and digital ORF transmission being two basic architectures. Analog ORF systems directly modulate the radio frequency signal onto the optical carrier for transmission, offering advantages such as simple structure and low latency. However, the signal is susceptible to nonlinear impairments and noise accumulation in the optical fiber, resulting in poor transmission quality. Digital ORF transmission digitizes the signal before transmission, offering strong anti-interference capabilities and high reliability. However, the digital processing is complex, requiring signal sampling and quantization, leading to lower spectral efficiency and higher hardware costs and power consumption.

[0003] To balance transmission quality and efficiency, hybrid digital-analog optical radio frequency (HRF) technology has emerged. This technology decomposes the signal to be transmitted into digital and analog components for joint transmission. However, most existing typical solutions employ time-division multiplexing, where the digital and analog components are transmitted alternately in the time domain. While this approach combines the advantages of both transmission modes, it essentially means that the digital and analog streams share the same time slot. To accommodate both, the system's effective data transmission rate or bandwidth utilization is inherently reduced, typically resulting in an overall spectral efficiency that is halved, failing to fully utilize valuable fiber optic spectrum resources. Summary of the Invention

[0004] This application provides a hybrid transmission optical fronthaul system and related methods, which transmits digital index and analog service signals through frequency division multiplexing, avoiding halving of spectral efficiency, and utilizes AFDM slope mapping to achieve highly reliable and low-complexity optical control antenna selection.

[0005] On the one hand, this application provides a hybrid transmission optical fronthaul system, including a centralized unit and a radio frequency remote unit. The centralized unit includes a signal generation module and an optical modulation module, and the radio frequency remote unit includes a photoelectric conversion module, a signal separation module, an index demodulation module, and a line control module. The signal generation module is used to separate the input bit stream into index bits and communication modulation bits, control the generation of an unmodulated AFDM waveform with a corresponding chirp slope according to the index bits, and digitize it into a first baseband signal. The communication modulation bits are modulated onto an AFDM subcarrier to generate a second baseband signal, and upconverted to a first radio frequency signal. The first baseband signal and the first radio frequency signal are frequency-division coupled to generate a hybrid radio frequency drive signal. The optical modulation module is used to modulate the optical carrier using the hybrid radio frequency drive signal, generate an optical carrier frequency division hybrid signal, and transmit it through an optical fiber link; The photoelectric conversion module is used to convert the received optical carrier frequency division mixed signal into a mixed radio frequency signal in the electrical domain; The signal separation module is used to separate the mixed radio frequency signal into a digital baseband signal carrying index bits and an analog radio frequency signal carrying communication modulation bits; The index demodulation module is used to perform discrete affine Fourier transform on the digital baseband signal and determine its chirp slope through peak detection in order to demodulate the index bits. The line control module is used to control the switch network to select the analog radio frequency signal to the corresponding transmit antenna according to the demodulated index bits.

[0006] Optionally, the signal generation module mentioned above includes: A bit separation unit is used to separate the input bit stream into an index bit stream and a communication modulation bit stream; An index mapping unit is used to map the index bitstream to specific time-domain chirp slope parameters. ; The first AFDM modulation unit is used to adjust the time-domain chirp slope parameter. and preset frequency domain pre-tuning factor Perform an inverse discrete affine Fourier transform on the all-1 sequence to generate the unmodulated AFDM waveform with the corresponding chirp slope. ; A digital-to-analog converter unit is used to convert the unmodulated AFDM waveform into digital-to-analog converter. It is converted into an analog signal and then up-converted to generate the first baseband signal.

[0007] Optionally, the above-mentioned index bitstream and the time-domain chirp slope parameter The mapping relationship is as follows: , Represents the index bit sequence, k The number of index bits, For the mapped first k A specific time-domain chirp slope parameter, a candidate slope set C = .

[0008] Optionally, the above frequency division coupling specifically involves placing the first baseband signal in the baseband and placing the first radio frequency signal in a radio frequency band higher than the baseband signal, so that the spectra of the two are orthogonal in the frequency domain.

[0009] Optionally, the signal separation module includes a power divider, a low-pass filter, and a band-pass filter; the power divider splits the mixed radio frequency signal into two paths, one of which is filtered out by the low-pass filter to extract the digital baseband signal, and the other of which is filtered out by the band-pass filter to extract the analog radio frequency signal.

[0010] Optionally, the above-mentioned index demodulation module is specifically used for: demodulating the digital baseband signal. Using different candidate time-domain chirp slope parameters Perform discrete affine Fourier transforms on each subcarrier; calculate the signal power at each subcarrier position in the output sequence after each transform; find the maximum power value corresponding to each candidate slope; and generate the candidate time-domain chirp slope parameters that produce the maximum power value. The slope of the received signal is determined, and then the corresponding index bits are demodulated.

[0011] Optionally, the above-mentioned index demodulation module demodulates the index bits by solving the following expression: The For the estimated time domain Chirp slope, where C is the set of candidate slopes. Indicates the use of candidate slopes The discrete affine Fourier transform was performed.

[0012] Optionally, the antenna control module includes a single-pole multi-throw switch, whose input is connected to the analog radio frequency signal, and whose multiple outputs are respectively connected to multiple transmitting antennas. The control terminal of the switch receives the index bits output by the index demodulation module.

[0013] Optionally, the aforementioned digital baseband signal and the analog radio frequency signal are transmitted simultaneously in the optical fiber link, and the transmission of the digital baseband signal does not occupy the bandwidth resources of the analog radio frequency signal used to carry communication modulation bits.

[0014] On the other hand, this application provides a signal processing method applied to the above-mentioned hybrid transmission optical fronthaul system, comprising: The input bitstream is separated into index bits and communication modulation bits; The unmodulated AFDM waveform with the corresponding chirp slope is generated according to the index bits and processed into a first baseband signal. The communication modulation bits are modulated onto an AFDM subcarrier to generate a second baseband signal, and then upconverted to a first radio frequency signal. The first baseband signal and the first radio frequency signal are frequency-division coupled to generate a hybrid radio frequency drive signal, which is used to modulate the optical carrier. It receives optical carrier frequency division multiplexing (OCM) mixed signals and converts them into mixed radio frequency signals in the electrical domain; The hybrid radio frequency signal is separated into a digital baseband signal carrying index bits and an analog radio frequency signal carrying communication modulation bits; The digital baseband signal is subjected to a discrete affine Fourier transform, and its chirp slope is determined by peak detection in order to demodulate the index bits. Based on the demodulated index bits, the control switch network selects the analog radio frequency signal to the corresponding transmit antenna.

[0015] As can be seen from the technical solution provided in this application, on the one hand, by orthogonally multiplexing the digital baseband signal carrying control information and the analog radio frequency signal carrying service data in the frequency domain, the two signals can be transmitted simultaneously on different frequency bands without interference. This overcomes the problem of halving the spectral efficiency caused by the digital and analog signals having to occupy the same frequency band in a time-division hybrid transmission scheme, thus significantly improving the overall spectral utilization of the hybrid transmission optical fronthaul system. On the other hand, by mapping the antenna index information to a specific chirp slope of the AFDM waveform and transmitting it through the digital channel, at the receiving end, by utilizing the energy focusing characteristic of DAFT transform, the slope can be accurately recovered from the digital baseband signal through simple peak detection, and then the index bits can be demodulated. This optical indexing mechanism utilizes the anti-interference capability of digital transmission to ensure... To ensure the reliability of control signaling transmission, complex digital signal processing or high-order demodulation algorithms are not required on the RF remote unit side; index recovery can be completed solely through transform domain operations, reducing the processing complexity and implementation cost of remote equipment. Thirdly, highly reliable digital transmission is used for index information with low data volume and high reliability requirements, while high-bandwidth-efficiency analog transmission is used for user service information with high data volume. This allows both transmission methods to leverage their respective strengths and avoid their weaknesses. The digital part provides reliable assurance for the correct transmission (antenna selection) of the analog part, while the analog part achieves efficient transmission of service data under the assurance of the digital part. The two are organically combined through frequency division multiplexing, enabling the system to possess both the reliability of a digital system and the efficiency of an analog system at the architectural level. This provides a fronthaul solution with both high performance and low complexity for scenarios such as large-scale antenna arrays. In summary, the technical solution of this application avoids halving the spectral efficiency by transmitting digital index and analog service signals through frequency division multiplexing, and achieves highly reliable and low-complexity optically controlled antenna selection using AFDM slope mapping. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a simplified diagram of the hybrid transmission optical fronthaul system provided in the embodiments of this application; Figure 2 This application describes the overall architecture and signal flow of the hybrid transmission optical fronthaul system provided in the embodiments of this application. It depicts the complete process from the central unit separating the input bits, generating digital and analog AFDM signals respectively, frequency division coupling, optical modulation, to the radio frequency remote unit performing photoelectric conversion, signal separation, index demodulation and antenna selection. Figure 3 The embodiments of this application provide a visual representation of the signal in the form of a spectrum diagram. Figure 1 The morphological changes of the key processing nodes (① to ⑦) of the system shown reveal the process of orthogonal multiplexing and separation of digital baseband signals and analog radio frequency signals in the frequency domain. Detailed Implementation

[0018] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0019] In this specification, adjectives such as "first" and "second" are used only to distinguish one element or action from another, without necessarily requiring or implying any actual such relationship or order. Where circumstances permit, reference to an element or component or step (etc.) should not be construed as being limited to only one of the elements, components, or steps, but may be one or more of the elements, components, or steps, etc.

[0020] For ease of description, the dimensions of the various parts shown in the accompanying drawings are not drawn to actual scale.

[0021] Analog optical radio frequency (RF) systems directly modulate RF signals onto optical carriers for transmission, offering advantages such as simple structure and low latency. However, the signal is susceptible to nonlinear impairments and noise accumulation in the optical fiber, resulting in poor transmission quality. Digital RF transmission digitizes the signal before transmission, offering strong anti-interference capabilities and high reliability. However, the digital processing is complex, requiring signal sampling and quantization, leading to lower spectral efficiency and higher hardware costs and power consumption. To balance transmission quality and efficiency, hybrid digital-analog RF technology has emerged. This technology decomposes the signal to be transmitted into digital and analog components for joint transmission. However, most existing solutions employ time-division multiplexing, where the digital and analog components are transmitted alternately in the time domain. While this combines the advantages of both transmission modes, the digital and analog streams essentially share the same time slot. To accommodate both, the system's effective data transmission rate or bandwidth utilization is inherently reduced, typically halving the overall spectral efficiency and failing to fully utilize valuable fiber optic spectrum resources. Therefore, maintaining the reliability of digital transmission and the efficiency of analog transmission while avoiding spectral efficiency loss has become a key issue in improving fronthaul network performance.

[0022] To address the aforementioned problems in the prior art, this application proposes a hybrid transmission optical fronthaul system, the structure of which is shown in the attached figure. Figure 1 As shown. Figure 1 The example hybrid transmission optical fronthaul system includes a central unit (CU) 101 and a remote radio unit (RRU) 102. The central unit 101 includes a signal generation module and an optical modulation module, and the remote radio unit 102 includes a photoelectric conversion module, a signal separation module, an index demodulation module, and a line control module, as detailed below: The signal generation module is used to separate the input bit stream into index bits (IB) and communication modulated bits (CMB), control the generation of an unmodulated affine frequency division multiplexing (AFDM) waveform with a corresponding chirp slope based on the index bits, and digitize it into a first baseband signal. The communication modulated bits are modulated onto the AFDM subcarrier to generate a second baseband signal, and upconverted to a first radio frequency signal. The first baseband signal and the first radio frequency signal are frequency-division coupled to generate a hybrid radio frequency drive signal. The optical modulation module is used to modulate the optical carrier using a hybrid radio frequency drive signal, generate a hybrid optical carrier frequency division signal, and transmit it through an optical fiber link. The photoelectric conversion module is used to convert the received optical carrier frequency division mixed signal into a mixed radio frequency signal in the electrical domain; The signal separation module is used to separate the mixed radio frequency signal into a digital baseband signal carrying index bits and an analog radio frequency signal carrying communication modulation bits; The index demodulation module is used to perform Discrete Affine Fourier Transform (DAFT) on the digital baseband signal and determine its chirp slope through peak detection in order to demodulate the index bits. The line control module is used to control the switch network to select the analog radio frequency signal to the corresponding transmit antenna based on the demodulated index bits.

[0023] from Figure 1 As seen in the example of the hybrid transmission optical fronthaul system, on the one hand, by orthogonally multiplexing the digital baseband signal carrying control information and the analog radio frequency signal carrying service data in the frequency domain, the two signals can be transmitted simultaneously on different frequency bands without interference. This overcomes the problem of halved spectral efficiency caused by the digital and analog signals having to occupy the same frequency band in a time-division hybrid transmission scheme, thus significantly improving the overall spectral utilization of the hybrid transmission optical fronthaul system. On the other hand, by mapping the antenna index information to a specific chirp slope of the AFDM waveform and transmitting it through the digital channel, at the receiving end, utilizing the energy focusing characteristic of DAFT transform, the slope can be accurately recovered from the digital baseband signal through simple peak detection, and then the index bits can be demodulated. This optical indexing mechanism utilizes the anti-interference capability of digital transmission to ensure... To ensure the reliability of control signaling transmission, complex digital signal processing or high-order demodulation algorithms are not required on the RF remote unit side; index recovery can be completed solely through transform domain operations, reducing the processing complexity and implementation cost of remote equipment. Thirdly, highly reliable digital transmission is used for index information with low data volume and high reliability requirements, while high-bandwidth-efficiency analog transmission is used for user service information with high data volume. This allows both transmission methods to leverage their respective strengths and avoid their weaknesses. The digital part provides reliable assurance for the correct transmission (antenna selection) of the analog part, while the analog part achieves efficient transmission of service data under the assurance of the digital part. The two are organically combined through frequency division multiplexing, enabling the system to possess both the reliability of a digital system and the efficiency of an analog system at the architectural level. This provides a fronthaul solution with both high performance and low complexity for scenarios such as large-scale antenna arrays. In summary, the technical solution of this application avoids halving the spectral efficiency by transmitting digital index and analog service signals through frequency division multiplexing, and achieves highly reliable and low-complexity optically controlled antenna selection using AFDM slope mapping.

[0024] Please see Figure 2 , is corresponding to Figure 1 The overall architecture diagram of a hybrid transmission optical fronthaul system. Logically, a hybrid transmission optical fronthaul system includes... Figure 1The example demonstrates a functionally coordinated centralized unit and a radio frequency remote unit connected via a fiber optic link. The core concept of this application lies in the orthogonal separation and fusion transmission of control information and service data in the frequency domain. Specifically, low-rate, high-reliability control information, such as antenna selection (i.e., index bits, IB), is transmitted via a digital channel, while high-rate user service data (communication modulation bits) is transmitted via an analog channel. This achieves high-spectral-efficiency data transmission while ensuring control reliability. The following, in conjunction with the appendix... Figure 2 and Figure 3 The signal flow and spectral changes shown provide a detailed explanation of the system: The centralized unit is used for signal generation, mixing, and optical modulation. It is the signal processing and transmission center of the system, and its core task is to generate mixed signals according to the above concept. For example... Figure 2 As shown in the "CU" section on the left, the centralized unit mainly includes a signal generation module, namely the Frequency-divided Digital and Analogue RoF RF Signal Generation (FDARG) module and an optical modulation module. The signal generation module is responsible for receiving the raw bitstream and completing all key baseband and RF processing, ultimately outputting a frequency-divided multiplexed hybrid RF drive signal. Its internal processing flow forms a clear causal chain: Step S101: Bit stream separation.

[0025] The input high-speed data bitstream first enters the bit splitting unit. This unit performs a fundamental and crucial operation: separating the input bitstream into index bits and communication modulation bits. Here, the index bits are used for subsequent spatial modulation to select a specific transmit antenna, while the communication modulation bits are the actual user data to be transmitted. This separation is a prerequisite for all subsequent differential processing.

[0026] Step S102: The index signal processing chain (digital channel generation) processes the index bits.

[0027] The index signal processing chain aims to generate a highly reliable and easily detectable control signal. For example... Figure 1 The upper half of the FDARG path is shown, and its process includes waveform generation and digitization, detailed below: 1) Waveform Generation: An unmodulated AFDM waveform with a corresponding chirp slope is generated based on the index bits. Specifically, the index bits IB are input to a mapping unit, which maps k bits to a predetermined rule (e.g., k bits are mapped to...). A discrete chirp slope value maps a bit sequence to a specific time-domain chirp slope parameter. Subsequently, this time-domain chirp slope parameter The input is an all-one sequence to the Inverse Discrete Affine Fourier Transform (DAFT) module. The DAFT module outputs an AFDM waveform whose chirp slope is uniquely determined by the index bits, mathematically expressed as: ………………. Formula (1) Where N is the total number of subcarriers, and n and m are the time-domain and transform-domain indices, respectively. This is the preset frequency domain pre-tuning factor. This waveform... Its characteristic is that it does not carry traditional modulation symbols; all its information is contained in the shape of the waveform itself (i.e., the chirp slope).

[0028] 2) Digitization: The generated AFDM waveform x_2(n) then enters the analog-to-digital converter and is digitized into the first baseband signal. This signal is... Figure 3 The blue square "IB" shown in step ① is positioned in the baseband frequency band. Transmitted digitally, this signal achieves strong anti-interference capabilities, ensuring the reliability of control information.

[0029] Step S103: Communication Signal Processing Chain (Analog Channel Generation) This chain processes communication modulation bits, aiming to efficiently carry large-capacity service data. For example... Figure 2 The lower half of the path of FDARG is shown below: 1) AFDM modulation: The communication modulation bits (CMB) are first mapped to a constellation (e.g., QPSK, 16QAM) to become symbols. These symbols are then fed into another inverse discrete affine Fourier transform module, using a pre-defined, fixed time-domain chirp slope parameter. and the same The signal is modulated onto the AFDM subcarrier to generate a second baseband signal. : ………………. Formula (2) Where N is the total number of AFDM subcarriers, and n and m represent the subcarriers and the time-domain sample sequence, respectively. and These represent the time-domain chirp slope and the frequency-domain pre-tuning factor, respectively.

[0030] 2) Up-conversion: Second baseband signal After passing through the up-converter, it is up-converted into the first radio frequency signal. This signal is... Figure 2The red square Analogue AFDM shown in step ② has a spectrum located at a relatively high radio frequency.

[0031] Step S104: Frequency division coupling.

[0032] At this point, the system has generated two signals with different spectral positions. Next, the first baseband signal and the first radio frequency signal are frequency-division coupled to generate a hybrid radio frequency drive signal. For example... Figure 2 As shown, the two signals are combined in the "RF Mixer". Specifically, this is achieved by placing the first baseband signal in the baseband and the first RF signal in an RF band higher than the baseband signal, making their spectra orthogonal in the frequency domain. The spectrum of the combined signal is shown below. Figure 3 As shown in step ③, the digital “IB” and the analog “Analogue AFDM” do not overlap in frequency band. This design is key to this application, as it enables the digital and analog components to be transmitted simultaneously and independently, fundamentally avoiding the halving of spectral efficiency caused by traditional time-division hybrid architectures.

[0033] The task of the optical modulation module is to load a hybrid radio frequency (RF) signal from the electrical domain onto an optical wave for transmission in the optical fiber. The hybrid RF drive signal is used to modulate the optical carrier, generating a hybrid optical carrier frequency division multiplexing (HCM) signal, which is then transmitted via the optical fiber link. In a preferred embodiment, as shown... Figure 2 As shown, the optical modulation module includes a directly modulated laser and a Mach-Zehnder modulator. The optical carrier generated by the directly modulated laser serves as the optical input to the Mach-Zehnder modulator (MZM), while the mixed radio frequency drive signal serves as the radio frequency input to the MZM. The MZM converts the amplitude change of the radio frequency signal into a change in light intensity through the electro-optic effect, thereby generating light like... Figure 3 The optical carrier frequency division multiplexing (OCM) signal shown in step ④ is then injected into the optical fiber and transmitted to the radio frequency remote unit at the end of the forward transmission link.

[0034] The radio frequency remote unit (RF remote unit) is located at the end of the fiber optic link. Its core mission is to accurately recover the control and data information transmitted by the centralized unit and to implement multi-antenna signal transmission with minimal hardware. For example... Figure 2 As shown in the "RRU" section on the right, its processing flow corresponds to that of the centralized unit, forming a complete communication link. After the optical signal transmitted in the optical fiber arrives at the RRU, it is first processed by the photoelectric conversion module. The core of this module is a photodetector (PD), whose function is to convert the received optical carrier frequency division multiplexing (FCM) signal into a mixed radio frequency (RF) signal in the electrical domain. The PD directly detects the optical signal, restoring the linear change in light intensity to a change in current, thereby obtaining... Figure 3The electrical signal shown in step ⑤ perfectly reproduces the characteristics of the transmitter's spectrum ③, namely, it contains digital components located in the baseband and analog radio frequency components located at specific frequency points. The recovered electrical signal is a mixture of digital and analog signals and needs to be separated for independent processing. The signal separation module undertakes this task, separating the mixed radio frequency signal into a digital baseband signal carrying index bits and an analog radio frequency signal carrying communication modulation bits. Its physical implementation includes a power divider, a low-pass filter, and a band-pass filter. The power divider splits the mixed RF signal into two identical signals, one of which enters a low-pass filter (LPF). This LPF has a carefully designed cutoff frequency that allows the baseband digital signal to pass through without distortion while completely suppressing high-frequency analog RF components. Therefore, this signal, after being filtered by the LPF, outputs the digital baseband signal. Figure 2 The "IB" signal shown in step ⑥. The other path from the power divider is connected to a bandpass filter. The center frequency of this bandpass filter is the same as the carrier frequency of the first RF signal, and its bandwidth is sufficient to pass through the entire analog AFDM signal. This signal is filtered out by the bandpass filter to produce the analog RF signal, i.e. Figure 2 The “Analogue AFDM” signal shown in step ⑦. Through this frequency domain orthogonality-based filtering operation, the control flow and data flow are clearly and without interference, paving the way for the distinctly different subsequent processing flows.

[0035] The index demodulation module is a key component of optically controlled spatial modulation in this application. Its design goal is to minimize the processing complexity of the RRU while ensuring high reliability. The function of the index demodulation module is to perform a discrete affine Fourier transform on the digital baseband signal and determine its chirp slope through peak detection to demodulate the index bits. Its working principle deeply utilizes the energy concentration characteristics of the AFDM waveform in the transform domain. 1) Signal preparation: Assume the digital baseband signal output from the LPF is It is essentially a transmitted waveform. The channeled version contains a specific chirp slope hidden within it. 2) Matching Transform and Energy Detection: The receiver knows the set of all possible candidate slopes C= The demodulator handles digital baseband signals. Using different candidate time-domain chirp slope parameters Perform Discrete Affine Fourier Transform (DAFT) on each candidate slope. For each candidate slope, calculate the signal power at each position in the transformed output sequence. The DAFT transform is equivalent to the inverse operation of the transmitted waveform if and only if the candidate slope used perfectly matches the slope actually embedded in the signal. The signal energy will be highly concentrated at a specific subcarrier position in the transform domain, producing a significant peak. If the slopes mismatch, the energy will be dispersed. 3) Peak Decision: Therefore, the algorithm finds the maximum power value corresponding to each candidate slope, and then calculates the candidate time-domain chirp slope parameter that produces the maximum power value. The slope is determined to be the true slope of the received signal. This decision-making process can be fully described by a concise and rigorous mathematical expression: , It is equivalent to ,That middle, It is the estimated time-domain chirp slope. Indicates the use of a specific candidate slope DAFT Transformation. This represents the power value at the point with the highest energy in the output sequence after the DAFT transform. When using... When the output signal's energy is perfectly matched to its true slope, the energy will be most concentrated, and this maximum value will be reached. 4) Index recovery: Based on the estimated time-domain chirp slope... By querying the transmitter mapping function The inverse function of the index bit can be used to uniquely demodulate the corresponding index bit. The entire demodulation process does not require complex channel equalization or high-order demodulation algorithms, but only relies on DAFT transform and peak comparison, achieving a balance between high reliability and low complexity.

[0036] The system needs to perform spatial modulation based on the recovered index information. The antenna control module's role is to control the switch network to select the analog RF signal to the corresponding transmit antenna based on the demodulated index bits. A typical implementation uses a single-pole multi-throw (SPMD) switch. The SPMD switch's input is connected to the analog RF signal output from a band-pass filter (BPF), and its multiple outputs are connected to multiple transmit antennas. The switch's control terminal receives the index bits output from the index demodulation module. These index bits are directly decoded into the address code of the antenna port. At each transmission moment, the control switch connects the shared analog RF front-end to a designated antenna for radiation. This design eliminates the need for the RRU to equip each antenna with a separate and expensive RF link, achieving multi-antenna transmission with a simplified "RF front-end + switch" structure, significantly reducing hardware cost, complexity, and power consumption.

[0037] This application also provides a signal processing method applied to the above-mentioned hybrid transmission optical fronthaul system, including: S1: Separate the input bit stream into index bits and communication modulation bits.

[0038] S2: Generate an unmodulated AFDM waveform with a corresponding chirp slope according to the index bit control, and process it into a first baseband signal, specifically including mapping, IDAFT modulation (formula (1)), digital-to-analog conversion, and up-conversion processing.

[0039] S3: Modulate the communication modulation bits onto the AFDM subcarrier to generate a second baseband signal, and upconvert it to a first radio frequency signal. Specifically, this includes constellation mapping, IDAFT modulation (formula (2)) and upconversion.

[0040] S4: Frequency-division coupling of the first baseband signal and the first radio frequency signal generates a hybrid radio frequency drive signal, which is used to modulate the optical carrier, that is, merging the two signals with orthogonal spectra.

[0041] S5: Receives the optical carrier frequency division mixed signal and converts it into a mixed radio frequency signal in the electrical domain (photoelectric conversion).

[0042] S6: Separate the mixed radio frequency signal into a digital baseband signal carrying index bits and an analog radio frequency signal carrying communication modulation bits (power splitting and filtering).

[0043] S7: Perform discrete affine Fourier transform on the digital baseband signal and determine its chirp slope through peak detection in order to demodulate the index bits. The specific process is the aforementioned matching transform, energy calculation, peak search and slope decision (corresponding to the aforementioned mathematical expression).

[0044] S8: Based on the demodulated index bits, control the switching network to select the analog radio frequency signal to the corresponding transmit antenna.

[0045] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application. The specific embodiments described above further illustrate the purpose, technical solutions, and beneficial effects of this application. It should be understood that the above descriptions are merely specific embodiments of this application and are not intended to limit the protection scope of this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A hybrid transmission optical fronthaul system, comprising a central unit and a radio frequency remote unit, characterized in that, The centralized unit includes a signal generation module and an optical modulation module, and the radio frequency remote unit includes a photoelectric conversion module, a signal separation module, an index demodulation module, and a line control module; The signal generation module is used to separate the input bit stream into index bits and communication modulation bits, control the generation of an unmodulated AFDM waveform with a corresponding chirp slope according to the index bits, and digitize it into a first baseband signal. The communication modulation bits are modulated onto an AFDM subcarrier to generate a second baseband signal, and upconverted to a first radio frequency signal. The first baseband signal and the first radio frequency signal are frequency-division coupled to generate a hybrid radio frequency drive signal. The optical modulation module is used to modulate the optical carrier using the hybrid radio frequency drive signal, generate an optical carrier frequency division hybrid signal, and transmit it through an optical fiber link; The photoelectric conversion module is used to convert the received optical carrier frequency division mixed signal into a mixed radio frequency signal in the electrical domain; The signal separation module is used to separate the mixed radio frequency signal into a digital baseband signal carrying index bits and an analog radio frequency signal carrying communication modulation bits; The index demodulation module is used to perform discrete affine Fourier transform on the digital baseband signal and determine its chirp slope through peak detection in order to demodulate the index bits. The line control module is used to control the switch network to select the analog radio frequency signal to the corresponding transmit antenna according to the demodulated index bits.

2. The hybrid transmission optical fronthaul system according to claim 1, characterized in that, The signal generation module includes: A bit separation unit is used to separate the input bit stream into an index bit stream and a communication modulation bit stream; An index mapping unit is used to map the index bitstream to specific time-domain chirp slope parameters. ; The first AFDM modulation unit is used to adjust the time-domain chirp slope parameter. and preset frequency domain pre-tuning factor Perform an inverse discrete affine Fourier transform on the all-1 sequence to generate the unmodulated AFDM waveform with the corresponding chirp slope. ; A digital-to-analog converter unit is used to convert the unmodulated AFDM waveform into digital-to-analog converter. It is converted into an analog signal and then up-converted to generate the first baseband signal.

3. The hybrid transmission optical fronthaul system according to claim 2, characterized in that, The index bitstream and the time-domain chirp slope parameter The mapping relationship is as follows: The Represents the index bit sequence, k The number of index bits, For the mapped first k A specific time-domain chirp slope parameter, candidate slope set C= .

4. The hybrid transmission optical fronthaul system according to claim 1, characterized in that, The frequency division coupling specifically involves placing the first baseband signal in the baseband and placing the first radio frequency signal in a radio frequency band higher than the baseband signal, so that the spectra of the two are orthogonal in the frequency domain.

5. The hybrid transmission optical fronthaul system according to claim 1, characterized in that, The signal separation module includes a power divider, a low-pass filter, and a band-pass filter; the power divider splits the mixed radio frequency signal into two paths, one of which is filtered out by the low-pass filter to obtain the digital baseband signal, and the other of which is filtered out by the band-pass filter to obtain the analog radio frequency signal.

6. The hybrid transmission optical fronthaul system according to claim 1, characterized in that, The index demodulation module is specifically used for: For the digital baseband signal Using different candidate time-domain chirp slope parameters Perform discrete affine Fourier transforms on each; Calculate the signal power at each subcarrier position in the output sequence after each transformation; Find the maximum power value corresponding to each candidate slope; Candidate time-domain chirp slope parameters that will produce the maximum power value The slope of the received signal is determined, and then the corresponding index bits are demodulated.

7. The hybrid transmission optical fronthaul system according to claim 6, characterized in that, The index demodulation module demodulates the index bits by solving the following expression: The For the estimated time domain Chirp slope, where C is the set of candidate slopes. Indicates the use of candidate slopes The discrete affine Fourier transform was performed.

8. The hybrid transmission optical fronthaul system according to claim 1, characterized in that, The antenna control module includes a single-pole multi-throw switch, whose input is connected to the analog radio frequency signal, and whose multiple outputs are respectively connected to multiple transmitting antennas. The control terminal of the switch receives the index bits output by the index demodulation module.

9. The hybrid transmission optical fronthaul system according to claim 1, characterized in that, The digital baseband signal and the analog radio frequency signal are transmitted simultaneously in the optical fiber link, and the transmission of the digital baseband signal does not occupy the bandwidth resources of the analog radio frequency signal used to carry communication modulation bits.

10. A signal processing method applied to the hybrid transmission optical fronthaul system according to any one of claims 1 to 9, characterized in that, include: The input bitstream is separated into index bits and communication modulation bits; The unmodulated AFDM waveform with the corresponding chirp slope is generated according to the index bits and processed into a first baseband signal. The communication modulation bits are modulated onto an AFDM subcarrier to generate a second baseband signal, and then upconverted to a first radio frequency signal. The first baseband signal and the first radio frequency signal are frequency-division coupled to generate a hybrid radio frequency drive signal, which is used to modulate the optical carrier. It receives optical carrier frequency division multiplexing (OCM) mixed signals and converts them into mixed radio frequency signals in the electrical domain; The hybrid radio frequency signal is separated into a digital baseband signal carrying index bits and an analog radio frequency signal carrying communication modulation bits; The digital baseband signal is subjected to a discrete affine Fourier transform, and its chirp slope is determined by peak detection in order to demodulate the index bits. Based on the demodulated index bits, the control switch network selects the analog radio frequency signal to the corresponding transmit antenna.

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