A RoF system suitable for few-mode transmission

Abstract

The application discloses a RoF system suitable for few-mode transmission, which comprises four parts of a few-mode RoF signal source unit, a few-mode transmission unit, a wireless transmission unit and a signal processing unit. In order to reduce the signal crosstalk problem caused by the coupling between the optical modes in the few-mode transmission process, the in-mode displacement modulation and the inter-mode selective spectrum overlapping modulation of the wireless signal optical spectrum are carried out by using the few-mode optical modulation; the diversity of the overall channel of the RoF system is considered, and in order to reduce the complexity of the signal processing process, the scheme of separate equalization of the few-mode transmission and the wireless transmission is proposed. Compared with the existing RoF few-mode transmission system, the application directly operates on the optical spectrum of the RoF signal, and reduces the mutual coupling degree on the spectrum of the optical modes. Meanwhile, the system is reduced in the realization difficulty and the calculation complexity by the separate equalization of the few-mode equalization and the wireless equalization.

Description

Technical Field

[0001] This invention belongs to the field of optical communication technology, specifically relating to a RoF system suitable for few-mode transmission. Background Technology

[0002] In recent years, with the rapid development of wireless communication technology, wireless spectrum resources have become increasingly scarce. Furthermore, high transmission loss and radio frequency interference in high-frequency communication have constrained the development of long-distance, high-quality wireless communication. In contrast, RoF (Robotic Free) technology, with its advantages of wide bandwidth, low loss, and no electromagnetic interference, has attracted widespread attention and research in the field of communications. Meanwhile, with the rise of spatial division multiplexing (SDM) technology in optical communication, researchers worldwide have demonstrated the rationality and necessity of combining SDM with RoF through their research findings. Modular division multiplexing (MDM), as a type of SDM, is gradually becoming a key application consideration for small-area wireless access network coverage.

[0003] Most current RoF few-mode transmission systems focus on a hard integration of the two: the wireless signal is modulated externally to obtain the optical wireless signal, which is then directly multiplexed / demultiplexed and converted to photoelectric signals. Finally, the signal transmission quality is evaluated using technical means. Due to coupling between different modes of light, some solutions propose MIMO equalization after the photoelectric conversion of the RoF signal to improve interference caused by few-mode transmission. Another approach uses wavelength division multiplexing (WDM), where each mode uses a different wavelength of light for direct modulation of the wireless signal. This improves the isolation between modes and reduces signal interference. The former method requires significant computational resources and does not fundamentally improve signal transmission quality, nor does it consider the actual wireless channel conditions during communication. Furthermore, the processing speed increases dramatically with system size, making it unsuitable for large-scale wireless access network applications and flexible processing. The solution that combines wavelength division multiplexing (WDM) and mode division multiplexing (MDM) technologies does not directly address the problem of mode coupling. However, its implementation involves a complex system composition, high cost, and is not suitable for large-scale wireless access network coverage scenarios. Furthermore, it introduces factors such as four-wave mixing in WDM, increasing the difficulty of subsequent signal processing.

[0004] Therefore, in RoF few-mode transmission systems, a more general and flexible system design must be proposed to address the coupling problem between different modes of light, ensuring high-quality wireless signal transmission while combining RoF and mode division multiplexing technologies. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a RoF system suitable for few-mode transmission, overcoming the issues of high mode crosstalk, implementation difficulty, and mismatch with the high-quality transmission requirements of large-scale wireless access networks in current RoF few-mode transmission systems. This system reduces the impact of mode coupling while also optimizing the equalization problem in signal processing, reducing module complexity and improving signal processing speed.

[0006] To achieve the above-mentioned technical objectives, the present invention is implemented through the following technical solution:

[0007] A RoF system suitable for few-mode transmission includes a few-mode RoF signal source unit, a few-mode transmission unit, a wireless transmission unit, and a signal processing unit;

[0008] The few-mode RoF signal source unit first generates the wireless signal required for communication by a wireless signal generator. This electrical signal is then sent to the few-mode optical modulation module to perform RoF optical spectrum misalignment modulation using a dual-arm Mach-Zehnder modulator according to the corresponding optical mode. The modulated optical signal is then converted to the corresponding optical mode by a mode converter.

[0009] The RoF signal, after spectrum misalignment modulation and mode conversion, is multiplexed in the few-mode transmission unit by the mode multiplexer and sent into the few-mode fiber for transmission; the few-mode amplifier compensates for the loss during signal transmission; the remote communication end uses the mode demultiplexer to demultiplex the signal to obtain the RoF signal of each mode.

[0010] The optical signal after passing through the few-mode transmission unit is converted into an electro-optical signal by the photoelectric conversion module to obtain the original wireless signal; the wireless signal obtained by the photoelectric conversion module is processed by the transmitting antenna and sent into short-range free space and received by the receiving antenna of the receiving end;

[0011] The signal processing unit first reduces crosstalk during wireless transmission by performing wireless equalization, then performs few-mode equalization to reduce crosstalk and noise caused by mode coupling and amplifiers in few-mode transmission, and finally the demodulation module completes the quality assessment of the wireless signal.

[0012] Preferably, the few-mode RoF signal source unit includes a wireless signal generator with specified parameters and an optical modulator for few-mode transmission; wherein, the wireless signal generator generates a wireless signal conforming to the specified parameters; and the few-mode optical modulator performs spectral misalignment control during the optical modulation process of the wireless signal according to the degree of crosstalk between modes.

[0013] Preferably, the wireless signal with the specified parameters includes bandwidth, data rate, and carrier frequency;

[0014] Preferably, the few-mode optical modulator changes the sideband position of the wireless signal spectrum at the optical carrier frequency by adjusting the bias voltage of the dual-arm Mach-Zehnder modulator and the phase difference of the input electrical signals of the two arms; the intra-mode spectral misalignment can reduce crosstalk between signals due to mode coupling in the frequency domain, providing a higher crosstalk margin for communication under conditions of large intra-mode crosstalk; for inter-mode coupling with relatively low crosstalk, the traditional same spectral position is used to minimize system complexity; the RoF signal after special modulation is converted into the corresponding mode light and processed by the few-mode transmission unit;

[0015] Preferably, the few-mode transmission unit includes a mode multiplexer, a few-mode fiber, a few-mode amplifier, and a mode demultiplexer; the mode demultiplexer multiplexes the few-mode RoF signal generated by the few-mode RoF signal source unit into modes and sends it into a few-mode fiber of a specified length for long-distance transmission; wherein the few-mode amplifier performs power compensation on the few-mode signal during transmission; at the far end, the mode demultiplexer completes the mode demultiplexing of the few-mode light to obtain the light of each mode, and the photoelectric conversion of the RoF signal is completed by the photoelectric conversion module respectively;

[0016] Preferably, the wireless transmission unit includes a transmitting antenna and a receiving antenna; the transmitting antenna, after performing radio frequency preparation work such as amplification and filtering of the wireless signals corresponding to each mode generated by the photoelectric conversion module, sends the wireless signals into short-range free space for transmission, and the receiving antennas of each user receive, convert, and filter the wireless signals.

[0017] Preferably, the signal processing unit includes two main modules: equalization and demodulation. Considering the channel sequence and system complexity during signal transmission, the equalization part of the signal is divided into wireless equalization and few-mode equalization. The former is used to reduce the influence of various crosstalk during wireless transmission, while the latter is used to reduce inter-mode crosstalk and intra-mode crosstalk caused by mode coupling, thereby improving the efficiency of subsequent signal demodulation and thus improving the overall signal transmission quality of the system.

[0018] The beneficial effects of this invention are:

[0019] This invention provides a system design suitable for RoF few-mode transmission. By controlling the optical spectrum position of the wireless signal, the crosstalk effect of mode coupling on the signal spectrum is reduced, thereby improving the signal transmission quality. At the same time, it proposes to reduce the complexity of the equalization system at the receiver by performing wireless equalization first and then few-mode equalization.

[0020] This system utilizes a dual-arm Mach-Zehnder modulator and phase shifter to control the spectral position of the optically modulated wireless signal, achieving intra-mode signal spectral misalignment and selective inter-mode spectral overlap. This reduces crosstalk caused by mode coupling from the spectral dimension of the signal. Simultaneously, a few-mode equalization and wireless equalization module are introduced at the signal receiver to reduce both few-mode crosstalk and wireless channel crosstalk during signal transmission. This system design reduces the coupling between signal spectra while proposing an equalization sequence after few-mode transmission in RoF (Royalty over Fire) technology, reducing the algorithm complexity of traditional single equalization modules and providing a feasible solution for future large-scale wireless access network coverage. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the present invention applicable to RoF few-mode transmission systems;

[0023] Figure 2 This is a schematic diagram of the RoF few-mode modulation module of the present invention;

[0024] Figure 3 This is a schematic diagram of the spectrum misalignment modulation effect of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1

[0027] Figure 1 This is a schematic diagram of the RoF few-mode transmission system design proposed in this invention. The design includes a few-mode RoF signal source unit, a few-mode transmission unit, a wireless transmission unit, and a signal processing unit.

[0028] The few-mode RoF signal source first generates the radio frequency signal required for communication using a wireless signal generator, and then... Figure 2The few-mode optical modulation module shown completes the optical spectrum misalignment modulation of the wireless radio frequency signal, and then the mode conversion module converts the fundamental mode spectrum misalignment RoF optical signal into a corresponding mode few-mode spectrum misalignment RoF optical signal for use by the subsequent few-mode transmission section. Figure 2 The few-mode optical modulation module shown mainly includes a 90° phase shifter, a bias voltage source, a dual-arm Mach-Zehnder modulator, a mode converter, single-mode fiber, and few-mode fiber. Multiple signals generated by the wireless signal generator are split into two paths and sent to the electrical signal input terminals of the dual-arm Mach-Zehnder modulator corresponding to each mode. Based on the principle of optical single-sideband modulation, left or right single-sideband modulation can be achieved by adjusting the phase difference between the two input electrical signals of the dual-arm Mach-Zehnder modulator and the bias voltage of the two arms. Therefore, one path, according to the corresponding mode spectrum requirements, is phase-shifted by a 90° phase shifter before the electrical signal input terminal of the upper or lower arm of the dual-arm Mach-Zehnder modulator. Simultaneously, the bias voltage of the upper and lower arms is adjusted to control the spectral sideband position of the RoF signal. Then, the spectrally misaligned RoF signals obtained by each modulator are converted into corresponding few-mode spectrally misaligned RoF signals through mode conversion.

[0029] The few-mode transmission section includes a mode multiplexer, a few-mode fiber, a few-mode amplifier, and a mode demultiplexer. The mode multiplexer converts the few-mode spectral misalignment RoF signal generated by the few-mode RoF signal source into a beam of light and sends it into the few-mode fiber for transmission. Considering the loss during transmission, the few-mode RoF signal needs to be amplified. After passing through the mode demultiplexer, the individual few-mode RoF signals are obtained.

[0030] The wireless transmission section includes a transmitting antenna unit and a receiving antenna unit. The spectrum-misaligned RoF signals transmitted through few-mode transmission are converted into initial communication radio frequency signals by the photoelectric conversion module. These signals are then processed by the wireless signal transmitting antenna unit and transmitted into free space. After free space transmission, the wireless signals are received by the receiving antenna unit and processed accordingly.

[0031] The signal processing section includes wireless equalization, few-mode equalization, and signal demodulation modules. Because the wireless signal undergoes electro-optical-electrical conversion and transmission through the few-mode wireless channel in this system design, the signal equalization section must consider not only interference from the wireless channel but also appropriate few-mode equalization for the few-mode transmission. During few-mode transmission, due to mode coupling in the optical fiber, signals of different modes may couple with each other. Considering the difference between inter-mode and intra-mode coupling, a specific method is employed... Figure 2 The spectral peak shifting modulation shown above produces the following spectral effect on the optical signal: Figure 3 As shown. The equilibrium approach is based on the following inferences.

[0032] Using matrix form, let the wireless signal be denoted as X(t), the few-mode transmission system response as M(t), and the wireless channel system response as W(t). Then, the system transmission matrix H(t) for the entire few-mode transmission and wireless transmission process can be written as:

[0033] H(t)=M(t)W(t)

[0034] Therefore, neglecting noise, the received signal Y(t) can be written as:

[0035] Y(t)=X(t)H(t)=X(t)M(t)W(t)

[0036] The purpose of an equilibrium system:

[0037] Y(t)=X(t)

[0038] Therefore, theoretically, the system matrix of an equilibrium system should be:

[0039] E(t) = W -1 (t)M -1 (t)

[0040] So that,

[0041] Y(t)E(t)=X(t)M(t)W(t)W -1 (t)M -1 (t)=X(t)

[0042] in,(·) -1 Let represent the inverse matrix. From the above equation, we can see that the equalization order is first wireless equalization, then few-mode equalization.

Claims

1. A RoF system suitable for few-mode transmission, characterized in that, It includes a few-mode RoF signal source unit, a few-mode transmission unit, a wireless transmission unit, and a signal processing unit; The few-mode RoF signal source unit first generates the wireless signal required for communication by a wireless signal generator. The wireless signal is sent to the few-mode optical modulation module to perform RoF optical spectrum misalignment modulation using a dual-arm Mach-Zehnder modulator according to the corresponding optical mode. The modulated optical signal is then converted to the corresponding optical mode by a mode converter. The RoF signal, after spectral misalignment modulation and mode conversion, is mode multiplexed by the mode multiplexer in the few-mode transmission unit and then sent into the few-mode fiber for transmission; the few-mode amplifier compensates for the loss during signal transmission. At the remote communication end, a mode demultiplexer is used to perform mode demultiplexing to obtain RoF signals for each mode; The optical signal after passing through the few-mode transmission unit is converted into an electro-optical signal by the photoelectric conversion module to obtain the original wireless signal; the wireless signal obtained by the photoelectric conversion module is processed by the transmitting antenna and sent into short-range free space and received by the receiving antenna of the receiving end; The signal processing unit first reduces crosstalk during wireless transmission by performing wireless equalization, then performs few-mode equalization to reduce crosstalk and noise caused by mode coupling and amplifiers in few-mode transmission, and finally the demodulation module completes the quality assessment of the wireless signal. The few-mode RoF signal source unit includes a wireless signal generator with specified parameters and an optical modulator for few-mode transmission; wherein, the wireless signal generator generates a wireless signal that conforms to the specified parameters; and the few-mode optical modulator performs spectral misalignment control during the optical modulation process of the wireless signal according to the degree of crosstalk between each mode. The few-mode optical modulator changes the sideband position of the wireless signal spectrum at the optical carrier frequency by adjusting the bias voltage of the dual-arm Mach-Zehnder modulator and the phase difference of the input electrical signals of the two arms. The intra-mode spectral misalignment can reduce crosstalk between signals caused by mode coupling in the frequency domain, providing a high crosstalk margin for communication under conditions of large intra-mode crosstalk. For inter-mode coupling with relatively low crosstalk, the traditional same spectral position is used to minimize system complexity. The RoF signal after special modulation is converted into the corresponding mode light and processed by the few-mode transmission unit.

2. The RoF system suitable for few-mode transmission according to claim 1, characterized in that, The specified parameters for the wireless signal include bandwidth, speed, and carrier frequency.

3. The RoF system suitable for few-mode transmission according to claim 1, characterized in that, The few-mode transmission unit includes a mode multiplexer, a few-mode fiber, a few-mode amplifier, and a mode demultiplexer. The mode demultiplexer remultiplexes the few-mode RoF signal generated by the few-mode RoF signal source unit into modes and sends it into a few-mode fiber of a specified length for long-distance transmission. The few-mode amplifier performs power compensation on the few-mode signal during transmission. At the remote end, the mode demultiplexer completes the mode demultiplexing of the few-mode light to obtain the light of each mode, and the photoelectric conversion module completes the photoelectric conversion of the RoF signal.

4. The RoF system suitable for few-mode transmission according to claim 1, characterized in that, The wireless transmission unit includes a transmitting antenna and a receiving antenna. After the transmitting antenna performs radio frequency preparation work by amplifying and filtering the wireless signals corresponding to each mode generated by the photoelectric conversion module, it sends the wireless signals into short-range free space for transmission. The receiving antennas of each user receive, convert, and filter the wireless signals.

5. The RoF system suitable for few-mode transmission according to claim 1, characterized in that, The signal processing unit includes two main modules: equalization and demodulation. Considering the channel sequence and system complexity during signal transmission, the equalization part of the signal is divided into wireless equalization and few-mode equalization. The former is used to reduce the impact of various crosstalk during wireless transmission, while the latter is used to reduce inter-mode crosstalk and intra-mode crosstalk caused by mode coupling, thereby improving the efficiency of subsequent signal demodulation and thus improving the overall signal transmission quality of the system.

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