Signal transmission method and system based on polarization multiplexing radio frequency-free space light

Abstract

The invention provides a signal transmission method and system based on polarization multiplexing radio frequency-free space light, relates to the technical field of radio communication, and adopts an integrated design of a single-line carrier photodiode and a lens to realize RF / FSO signal synchronous generation and polarization multiplexing transmission through a photon-assisted technology. The photoelectric conversion incomplete characteristic of UTC-PD is utilized, an optical signal is converted into a millimeter wave / terahertz radio frequency signal and a leakage optical signal, focusing transmission of the two signals is achieved in combination with a lens, and the hardware complexity is reduced. A dual-polarization dual-channel framework is constructed through a polarization beam splitter, the optical fiber polarization multiplexing coherent technology is compatible, and spectrum efficiency multiplication is achieved in combination with an MIMO algorithm. And a receiving end adopts separated photoelectric dual-channel receiving and is fused with a digital signal processing algorithm. The system has an atmospheric environment adaptive capability, is suitable for scenes with strict requirements on light weight, high reliability and large capacity, such as a satellite-ground link and a 6G network, and provides an innovative solution for next-generation all-environment high-speed communication.

Description

Technical Field

[0001] The present invention relates to the field of radio communication technology, and further to free-space optical communication technology, hybrid radio frequency-free-space optical communication technology, polarization multiplexing technology, coherent detection technology, and digital signal processing (DSP) algorithms. Specifically, the present invention proposes a signal transmission method and system based on polarization multiplexing radio frequency-free-space optical. Background Art

[0002] Free-space optical (FSO) technology, with its unlicensed spectrum resources, highly directional beam propagation characteristics, and excellent spectral efficiency, has become a core solution for high-speed satellite-to-ground links and next-generation wireless access networks. Compared to traditional RF technologies, FSO can utilize near-infrared bands such as 1550nm for ultra-wideband signal transmission. Its theoretical bandwidth can reach hundreds of GHz and is not constrained by spectrum licensing policies. This makes it particularly suitable for high-bandwidth scenarios such as intersatellite laser communications and the interconnection between high-altitude platforms (HAPs) and ground base stations. However, the practical deployment of FSO systems still faces significant challenges. First, the optical intensity scintillation effect caused by atmospheric turbulence can cause fluctuations in the signal-to-noise ratio at the receiver. This is particularly true in satellite-to-ground links, where temperature gradients and wind speed fluctuations in the near-Earth boundary layer exacerbate this interference. Second, meteorological conditions such as rain and fog significantly attenuate optical signals. For example, dense fog (visibility <50m) can cause optical power attenuation exceeding 300dB / km, leading to complete interruption of the FSO link. Furthermore, beam alignment errors require extremely high precision from the mechanical tracking system, further increasing system complexity and maintenance costs. To address these issues, existing research attempts to improve system robustness by hybridizing radio frequency (RF) and FSO architectures. For example, millimeter wave (MMW) bands, with their low attenuation in rain and fog and wide bandwidth, can serve as redundant backup channels for FSO links. Experimental results show that in dense fog and attenuating environments, mmW bands can increase FSO link availability to 100%.

[0003] In recent years, hybrid radio frequency-free-space optical (RF / FSO) systems have become a key research frontier in next-generation network technology, attracting extensive research investment, and many researchers have made progress in fusion of RF and FSO transmission. However, traditional hybrid RF / FSO systems mostly use independent transmitter architectures, requiring separate transmitters for the RF link and the FSO link, which significantly increases system complexity and weight. To reduce system complexity, Wang et al. implemented an optoelectronic integrated transmission system with a shared transmitter based on the light leakage mechanism of a single-line carrier photodiode (UTC-PD). This shared structure can better adapt to the lightweight and highly reliable transmission requirements of future satellite-to-ground links.

[0004] As we all know, current high-speed fiber-optic communication systems generally adopt polarization multiplexing coherent detection schemes (PDM), that is, transmitting two independent signals with orthogonal polarization states (such as horizontal / vertical polarization) through a single optical fiber to achieve spectral efficiency doubling and compatibility with high-speed coherent optical modules. However, traditional hybrid radio frequency-free space optical (RF-FSO) systems face significant bottlenecks when designing compatibility with existing PDM architectures: First, existing hybrid systems mostly adopt independent transmitter architectures, and polarization controllers and modulators need to be configured for RF and FSO links respectively, which makes the optical path structure of the transmitter complicated and makes it difficult to achieve synchronous generation and transmission of dual-polarization signals; second, although some improved schemes simplify the complexity of the transmitter, due to the special structure, they are temporarily unable to meet the simultaneous transmission of two polarization state signals. Therefore, the present invention proposes a multi-input multi-output (MIMO) hybrid RF-FSO transmission system based on photon-assisted millimeter-wave signal generation technology. The system supports polarization-multiplexed RF / FSO fused signal transmission, while avoiding the hardware redundancy of independent transmitters and ensuring compatibility with the PDM architecture. This solution confirms the synergistic advantages of shared transmitter structure and PDM-MIMO technology, and provides a feasible path for the deep integration of hybrid systems and fiber optic networks. Summary of the Invention

[0005] In view of this, the present invention proposes a signal transmission method and system based on polarization multiplexing radio frequency-free space light to solve the problems existing in the above-mentioned prior art.

[0006] On one hand, to achieve the above-mentioned object, the present invention proposes a signal transmission method based on polarization multiplexing radio frequency-free space light, characterized by comprising:

[0007] Two polarization signals are generated through a polarization beam splitter and a delay line;

[0008] After the polarization signal is coupled with the optical signal generated by the laser, a single-line carrier photodiode is used to perform photoelectric conversion on the coupled signal to generate an RF / FSO fusion signal;

[0009] Focusing the RF / FSO fusion signal through a lens and then transmitting the signal;

[0010] Using two optical receivers and an electrical receiver to receive the RF / FSO fusion signal;

[0011] The received signal sampled by the oscilloscope is sent to the digital signal processing module for offline digital signal processing, and the corresponding carrier recovery algorithm is used for signal recovery according to different signals.

[0012] Furthermore, during the generation of the RF / FSO fusion signal, based on the incomplete photoelectric conversion characteristics of a single-line carrier photodiode, while generating the electrical signal, the leakage light portion is regarded as a free-space optical link, thereby generating the RF / FSO fusion signal.

[0013] Furthermore, in the process of receiving the RF / FSO fusion signal using two optical receivers and an electrical receiver, the FSO signal is received based on the optical receiver, and the RF signal is received based on the electrical receiver;

[0014] For the optical receiver, the FSO signal is received by two optical receivers respectively, coupled and sent to the integrated coherent receiver together;

[0015] For an electrical receiver, one receiving antenna is set in the horizontal direction and the other in the vertical direction, and the two polarization directions are received separately.

[0016] Furthermore, the process of using corresponding carrier recovery algorithms for different signals to perform signal recovery includes:

[0017] For RF signals, the signal recovery process includes digital down-conversion, downsampling, orthogonal imbalance compensation algorithm, CMA / CMMA equalization algorithm, frequency offset compensation algorithm, phase offset compensation algorithm, DDLMS blind equalization algorithm, and MIMO-VLNE algorithm. For FSO signals, except that digital down-conversion is not required, the remaining steps are the same as the RF signal processing flow.

[0018] On the other hand, to achieve the above-mentioned purpose, the present invention proposes a signal transmission system based on polarization multiplexing radio frequency-free space light, characterized in that it includes a transmitter, a receiver, and a digital signal processing module, wherein the transmitter includes a polarization multiplexing module and a fusion signal generation module; the receiver includes an optical receiver and an electrical receiver;

[0019] Generate a polarization signal by the polarization multiplexing module, and generate an RF / FSO fusion signal according to the polarization signal by the fusion signal generation module and send the fusion signal;

[0020] The RF / FSO fusion signal is received by the optical receiver and the electrical receiver, and the signal is restored by the digital signal processing module.

[0021] Furthermore, the polarization multiplexing module uses a polarization beam splitter (PBS) and a delay line (DL) to construct a dual-polarization dual-channel system, and uses the polarization beam splitter (PBS) to separate the optical signal into horizontal and vertical polarization states; inserts a delay line (PBS) in at least one polarization channel to decorrelate the two polarization signals; and independently amplifies the two optical signals through a polarization-maintaining erbium-doped fiber amplifier.

[0022] Furthermore, the fusion signal generation module includes a single-line carrier photodiode and a lens. After the polarization signal is coupled with the optical signal generated by the laser, the coupled signal is photoelectrically converted by the single-line carrier photodiode to generate an RF / FSO fusion signal, which is focused by the lens and then sent to free space.

[0023] Furthermore, the two optical signals in the RF / FSO fused signal are received by two optical receivers respectively, combined into one signal, and sent to an integrated coherent receiver and an oscilloscope. The receiving antenna of the electrical receiver is set to one horizontal direction and the other vertical direction, and the two polarization directions are received separately. The two electrical signals in the RF / FSO fused signal are converged by a lens, amplified by the electrical receiver and an electrical amplifier, and then sent to the oscilloscope for sampling.

[0024] Furthermore, the digital signal processing module uses a Gram-Schmidt orthogonalization algorithm, a CMA / CMMA equalization algorithm, a frequency offset compensation algorithm, a phase offset compensation algorithm, a DDLMS blind equalization algorithm, and a MIMO-VLNE algorithm to perform signal recovery on the optical signal and the electrical signal in the RF / FSO fusion signal respectively.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The proposed polarization-multiplexed RF-free-space optical (RF / FSO) integrated transmission method and system utilizes polarization multiplexing technology in RF / FSO systems and combines it with a shared transmitter design, thereby reducing system complexity while increasing system capacity. This method is an important foundation for next-generation communications and satellite-to-ground communications.

[0027] The present invention is applicable to a variety of RF signal generation methods and RF frequency bands, including but not limited to photon-assisted generation of millimeter wave signals, photon-assisted generation of terahertz signals, etc.

[0028] The present invention is applicable to a variety of communication system architectures, including but not limited to photon-assisted millimeter-wave terahertz systems, direct modulation and direct detection systems, coherent transmission systems, etc.

[0029] The present invention is applicable to various MIMO architectures and can be combined with multiple-input multiple-output MIMO system algorithms, including but not limited to MIMO-CMA algorithm, MIMO-MRC algorithm, etc.

[0030] The present invention is applicable to various signal formats, including but not limited to phase modulation of high-order modulation formats such as QPSK, 8PSK, 16QAM, 64QAM and 256QAM, amplitude modulation of PAM format, multi-carrier modulation of OFDM and DMT, new hybrid modulation, modulation combined with probability shaping algorithm (PS), and physical layer network coding. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present invention. The same reference symbols are used throughout the drawings to represent the same components. In the drawings:

[0032] Figure 1 Schematic diagram of the structure of the signal transmission system based on polarization multiplexing radio frequency-free space light of the present invention; Figure 1: ECL: laser, AWG: arbitrary waveform generator, EA: electrical amplifier, I / Q Mod: IQ modulator, PM-EDFA: polarization maintaining erbium-doped fiber amplifier, PM-OC: polarization maintaining optical coupler, DL: delay line, PBS: polarization beam splitter, EDFA: erbium-doped fiber amplifier, UTC-PD: single-line carrier photodiode, Lens: lens, HA: horn antenna, RF: radio frequency, FSO: free space light, FSO Receiver: free space optical receiver, LO: local oscillator, ICR: integrated coherent receiver, OSC: oscilloscope, DSP: digital signal processing;

[0033] Figure 2 The figure is a flow chart of the signal transmission method based on polarization multiplexing radio frequency-free space light of the present invention. DETAILED DESCRIPTION

[0034] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art. It should be noted that, unless there is a conflict, the embodiments of the present disclosure and the features described in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0035] This embodiment proposes a signal transmission system based on polarization multiplexing radio frequency-free space light. It uses a combination of two UTC-PDs and a lens to achieve the fusion transmission of optical and electrical signals. It combines the multi-input multi-output architecture and polarization multiplexing technology. The experimental device design block diagram is shown in the attached figure. Figure 1 shown.

[0036] At the transmitter, an arbitrary waveform generator (AWG) generates a baseband signal. After being amplified by two electrical amplifiers (EAs), the signal is modulated by an I / Q modulator onto an optical carrier generated by laser ECL1. After amplification by a PM-EDFA, the signal is split into two paths by a PM-OC. One path is decorrelated by a delay line (DL), combined by a polarization beam splitter (PBS), amplified by an erbium-doped fiber amplifier (EDFA), and then split into two polarization signals by the PBS. The two signals are then coupled to the light generated by laser ECL2 and converted to optical by a UTC-PD. The dual-polarization signals are focused by a lens and transmitted into free space.

[0037] At the receiving end, for the optical receiver, the two polarized optical signals are coupled and sent together to an integrated coherent receiver (ICR). For the electrical receiver, one antenna is in the H direction and the other in the V direction, and the two polarization directions are received separately. Specifically, after being received by the two FSO receivers, the optical signals are combined into one signal and sent to the integrated coherent receiver (ICR), where the local oscillator (LO) is used for homodyne detection and ultimately sent to an oscilloscope for sampling. The electrical receiver can be placed within 0.2m of the optical receiver. The two electrical signals are converged by a lens, received by a horn antenna, amplified by an electrical amplifier, and finally sent to an oscilloscope for sampling.

[0038] The digital signal processing (DSP) process for RF signals includes digital downconversion, downsampling, quadrature-scaling imbalance compensation (GSOP), CMA / CMMA equalization, frequency offset compensation (FOE), phase offset compensation (CPE), DDLMS blind equalization, and MIMO-VLNE. For FSO signals, the digital signal processing process is identical to that for RF signals, except that digital downconversion is not required.

[0039] Compared to traditional millimeter-wave / terahertz or free-space optical transmission systems, this invention leverages the unique characteristics of UTC-PD and combines polarization multiplexing technology to achieve converged RF and FSO transmission, reducing transmitter complexity and weight. Combined with multiple-input, multiple-output (MIMO) technology, the polarization-multiplexed signal transmission mode increases transmission capacity and broadens the physical architecture of future RF / FSO converged transmission. By integrating optical and electrical transmission, this approach provides a powerful solution for future high-speed, broadband communications in all environments.

[0040] Example 2

[0041] This embodiment proposes a signal transmission method based on polarization multiplexing radio frequency-free space optical, which reduces system complexity and weight while increasing system capacity. By complementing the advantages of RF and FSO, high-speed broadband communication can be achieved in all environments. Figure 2 As shown, the following steps are included:

[0042] The transmitting end generates a signal by photon-assisted means; specifically, the signal generated by photon-assisted technology can be a millimeter wave signal or a terahertz signal.

[0043] Generate polarization multiplexed signals through polarization beam splitters and delay lines;

[0044] By utilizing the incomplete optical-to-electrical conversion characteristics of a single-line carrier photodiode (UTC-PD), while the UTC-PD generates an electrical signal, the leaked light can be treated as a free-space optical link. Furthermore, both the optical and electrical signals generated by the UTC-PD can be focused by the same lens.

[0045] The basic architecture uses two UTC-PDs at the transmitter end to send RF / FSO fusion signals in two polarization directions;

[0046] For the optical receiver, the two polarized optical signals are received by two optical receivers respectively, coupled and sent to the integrated coherent receiver together;

[0047] For an electrical receiver, one receiving antenna is set to the horizontal direction and the other to the vertical direction, and the two polarization directions are received separately;

[0048] Use an oscilloscope to sample the signal;

[0049] The sampled signal is sent to the digital signal processing (DSP) module for offline digital signal processing. For different signals, the corresponding carrier recovery algorithm is used to recover the signal;

[0050] Calculate the signal-to-noise ratio and bit error rate to evaluate the performance of the solution.

[0051] This invention uses photon-assisted technology to generate electrical signals, using the light leakage mechanism of the UTC-PD to generate optical signals. Polarization multiplexing technology is also used to ensure that the signal contains two polarization paths. The fused RF / FSO signal is emitted by the UTC-PD, focused by a lens, and then transmitted via a wireless link. The receiving end receives the signal using both optical and electrical receivers.

[0052] The present invention utilizes photon-assisted technology to generate RF signals, a method capable of supporting frequencies from millimeter waves to terahertz. A mixed optical signal, consisting of a 1550nm optical signal and a local oscillator (LO) light, is fed into a unidirectional carrier photodiode (UTC-PD). Due to the incomplete photoelectric conversion of the UTC-PD, while the beat frequency generates an electrical signal, a portion of the optical signal passes directly through the UTC-PD to form an FSO signal. Placing a lens behind the UTC-PD simultaneously focuses both the optical and electrical signals, enabling the fusion of RF and FSO signal transmission, with both signals sharing the same transmitter.

[0053] In terms of architecture, a polarization beam splitter (PBS) and delay line are used to build a dual-polarization dual-channel system, which is compatible with fiber polarization multiplexing (PDM) coherent detection to achieve capacity doubling.

[0054] The UTC-PD and lens combination is a key component for generating fused RF / FSO signals. The core innovations of this combination lie in three key areas: functional integration, signal synchronization, and system compatibility. First, the UTC-PD utilizes a unique optoelectronic conversion mechanism to physically transform a single input optical signal into a dual-mode RF and FSO signal. Part of the optical energy is mixed to generate millimeter-wave to terahertz radio frequencies, while the remaining unconverted optical energy directly transmits to form the FSO beam. This eliminates the separate optical paths and circuits required for traditional standalone transmitters, reducing hardware complexity by over 50%. This makes it particularly suitable for future satellite-to-ground links, where system weight and complexity requirements are stringent. Experimental testing shows that when the input optical power of the UTC-PD is 9dBm, the output electrical signal power after optoelectronic conversion is -29dBm, with a leakage optical power of -9dBm. Second, through the collimation and focusing design of a single lens, the RF electromagnetic wave and the FSO beam share the same transmission path, maintaining spatial synchronization of the signals in dynamic environments. Finally, this combination is well-suited to the PDM architecture of existing fiber-optic communication systems. By leveraging the coherence-preserving properties of the UTC-PD and the wide-spectrum compatibility of the lens, it can directly reuse the polarization diversity technology of optical fiber networks without adding additional modulation modules, doubling system capacity and meeting the stringent requirements for lightweight, anti-interference, and high compatibility in scenarios such as satellite-to-ground links and 6G networks. This device combination fundamentally overcomes the performance bottlenecks of traditional hybrid system discrete designs and becomes the physical layer cornerstone for highly reliable converged communications.

[0055] The generated RF / FSO fusion signal is transmitted over a wireless channel. Both the electrical and optical signals are dual-polarized. The receiver utilizes an RF-FSO dual-channel architecture to receive signals, receiving the optical and electrical signals separately. The optical signal link must be strictly aligned with the FSO transmission path using a precision optical alignment mechanism, while the millimeter-wave RF receiver module can be deployed near the optical receiver (typically ≤ 0.2 meters).

[0056] Electrical reception techniques can be employed, but are not limited to, the following: Reception using a combination of a lens and a horn antenna, with one horn antenna positioned in the H direction and the other in the V direction, allowing separate reception for the two polarizations. A low-noise amplifier (LNA) can be used to amplify the signal, followed by down-conversion using a local oscillator (LO)-driven mixer. The signal is then amplified using an electrical amplifier (EA). Finally, an oscilloscope (OSC) is used to capture the signal and perform offline digital signal processing (DSP). Alternatively, after receiving the signal at the horn antenna, the EA can directly amplify the signal and feed it into the OSC for direct down-conversion at the DSP. The need for an amplifier depends on the actual signal strength.

[0057] Optical receiving technologies, including but not limited to the following, can be used: the received optical signal is amplified by an erbium-doped fiber amplifier (EDFA) and sent to an integrated coherent receiver (ICR). A local oscillator (LO) signal is used for homodyne detection. Finally, an oscilloscope (OSC) is used to capture the signal and perform offline DSP processing.

[0058] As a preferred embodiment, the present method can be applied to a variety of communication system architectures, including but not limited to photon-assisted millimeter-wave terahertz systems, direct modulation and direct detection systems, coherent transmission systems, etc., and is applicable to various signal formats, including but not limited to phase modulation of high-order modulation formats such as QPSK, 8PSK, 16QAM, 64QAM and 256QAM, amplitude modulation of PAM format, multi-carrier modulation of OFDM and DMT, new hybrid modulation, modulation combined with probability shaping algorithm, and physical layer network coding. In addition, the present invention is applicable to a variety of multiple-input multiple-output (MIMO) architectures and can be combined with MIMO system algorithms, including but not limited to MIMO-CMA algorithm, MIMO-MRC algorithm, etc.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A signal transmission method based on polarization multiplexing radio frequency-free space light, characterized in that: include: Two polarization signals are generated through a polarization beam splitter and a delay line; After the polarization signal is coupled with the optical signal generated by the laser, a single-line carrier photodiode is used to perform photoelectric conversion on the coupled signal to generate an RF / FSO fusion signal; Focusing the RF / FSO fusion signal through a lens and then transmitting the signal; Using two optical receivers and an electrical receiver to receive the RF / FSO fusion signal; The received signal sampled by the oscilloscope is sent to the digital signal processing module for offline digital signal processing, and the corresponding carrier recovery algorithm is used for signal recovery according to different signals.

2. The signal transmission method according to claim 1, wherein: During the generation of the RF / FSO fusion signal, based on the incomplete photoelectric conversion characteristics of a single-line carrier photodiode, while generating an electrical signal, the leaked light portion is regarded as a free-space optical link, thereby generating the RF / FSO fusion signal.

3. The signal transmission method according to claim 1, wherein: In the process of receiving the RF / FSO fusion signal using two optical receivers and an electrical receiver, the FSO signal is received based on the optical receiver, and the RF signal is received based on the electrical receiver; For the optical receiver, the FSO signal is received by two optical receivers respectively, coupled and sent to the integrated coherent receiver together; For an electrical receiver, one receiving antenna is set in the horizontal direction and the other in the vertical direction, and the two polarization directions are received separately.

4. The signal transmission method according to claim 1, wherein: The process of using corresponding carrier recovery algorithms for different signals to recover signals includes: For RF signals, the signal recovery process includes digital down-conversion, downsampling, orthogonal imbalance compensation algorithm, CMA / CMMA equalization algorithm, frequency offset compensation algorithm, phase offset compensation algorithm, DDLMS blind equalization algorithm, and MIMO-VLNE algorithm. For FSO signals, except that digital down-conversion is not required, the remaining steps are the same as the RF signal processing flow.

5. A signal transmission system based on polarization multiplexing radio frequency-free space light, characterized in that: It includes a transmitting end, a receiving end and a digital signal processing module, wherein the transmitting end includes a polarization multiplexing module and a fusion signal generation module; the receiving end includes an optical receiver and an electrical receiver; Generate a polarization signal by the polarization multiplexing module, and generate an RF / FSO fusion signal according to the polarization signal by the fusion signal generation module and send the fusion signal; The RF / FSO fusion signal is received by the optical receiver and the electrical receiver, and the signal is restored by the digital signal processing module.

6. The signal transmission system according to claim 5, characterized in that: The polarization multiplexing module uses a polarization beam splitter (PBS) and a delay line (DL) to construct a dual-polarization dual-channel system. The polarization beam splitter (PBS) is used to separate the optical signal into horizontal and vertical polarization states; a delay line (PBS) is inserted into at least one polarization channel to decorrelate the two polarization signals; and the two optical signals are independently amplified by a polarization-maintaining erbium-doped fiber amplifier.

7. The signal transmission system according to claim 5, wherein: The fusion signal generation module includes a single-line carrier photodiode and a lens. After the polarization signal is coupled with the optical signal generated by the laser, the coupled signal is photoelectrically converted by the single-line carrier photodiode to generate an RF / FSO fusion signal, which is focused by the lens and then sent into free space.

8. The signal transmission system according to claim 5, wherein: The two optical signals in the RF / FSO fusion signal are received by two optical receivers respectively, combined into one signal, and sent to an integrated coherent receiver and an oscilloscope. The receiving antenna of the electrical receiver is set to one horizontal direction and the other vertical direction, and receives signals in two polarization directions separately. The two electrical signals in the RF / FSO fusion signal are converged by a lens, amplified by the electrical receiver and an electrical amplifier, and then sent to the oscilloscope for sampling.

9. The signal transmission system according to claim 5, wherein: The digital signal processing module uses the Gram-Schmidt orthogonalization algorithm, CMA / CMMA equalization algorithm, frequency offset compensation algorithm, phase offset compensation algorithm, DDLMS blind equalization algorithm, and MIMO-VLNE algorithm to respectively perform signal recovery on the optical signal and electrical signal in the RF / FSO fusion signal.

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