A polarization multiplexing direct detection system and method based on CADD receiver

Abstract

This invention discloses a polarization multiplexing direct detection system and method based on a CADD receiver. The system includes a transmitter and a receiver. At the transmitter, four independent signals form two pairs of asymmetric double-sideband signals, serving as X-polarization and Y-polarization, respectively. An asymmetric guard band is reserved in the middle of the signal spectrum. The spectra of the right-sideband signal of X-polarization and the left-sideband signal of Y-polarization overlap. At the receiver, the signal is split into two polarizations by a coupler, and then each polarization is filtered by an optical bandpass filter to suppress unwanted orthogonal carrier components. The filtered X-polarized and Y-polarized signals are then input into two identical CADD receivers for optical field recovery. After digital signal processing at the receiver, the final binary bit sequence is obtained. This invention achieves optical field recovery of polarization-multiplexed complex double-sideband signals, making it more suitable for cost-sensitive short-distance optical transmission.

Description

Technical Field

[0001] This invention relates to the field of short-distance optical fiber communication technology, and in particular to a polarization multiplexing direct detection system and method based on a CADD receiver. Background Technology

[0002] Coherent detection dominates long-distance transmission because it enables optical field recovery of dual-polarized complex signals. Furthermore, various linear impairments in signal transmission, such as dispersion and polarization mode dispersion, can be mitigated through digital signal processing (OSNR). However, coherent detection requires a local oscillator laser, placing high demands on the laser's wavelength stability and linewidth. Direct detection, on the other hand, can be performed without a local oscillator laser. Therefore, direct detection is typically preferred for cost-sensitive short-distance transmission. Traditional intensity-modulated direct detection is limited in transmission distance and spectral efficiency due to the lack of optical field recovery. Self-coherent detection, however, has attracted widespread research attention because it can achieve optical field recovery of signals with lower complexity.

[0003] To date, researchers have proposed various self-coherent detection schemes capable of recovering the optical field of a signal. Among them, carrier-assisted differential detection (CADD) has attracted widespread research attention. This scheme can achieve field recovery of complex double-sideband signals through direct detection, achieving electrical spectral efficiency similar to that of single-polarization coherent detection systems. Furthermore, to further simplify the structure of the CADD receiver, researchers have proposed an S-CADD scheme that does not use a single-ended photodiode and conducted simulation studies.

[0004] To further improve spectral efficiency and system capacity, polarization division multiplexing (PDM) technology is increasingly being applied to self-coherent detection. The difference between PDM coherent detection and PDM self-coherent detection lies in the latter's susceptibility to polarization fading. This is primarily due to the random polarization rotation of the transmitted optical carrier within the fiber optic link. At the receiver, without active polarization control, a polarization beamsplitter alone cannot uniformly split the optical carrier into X and Y polarizations. Since self-coherent detection relies on a strong carrier for optical field recovery, optical field recovery of the polarized signal is impossible without a sufficiently powerful carrier. To address this issue, researchers have reported a filter-based PDM single-sideband (SSB) scheme. At the transmitter, the generated PDM-SSB signal has two orthogonal carriers located on opposite sides. At the receiver, polarization diversity is achieved through a pair of filters. However, this is merely a polarization division multiplexing single-sideband scheme with relatively low spectral efficiency. Summary of the Invention

[0005] This invention provides a polarization multiplexing direct detection system and method based on a CADD receiver to solve the technical problem of low spectral efficiency in the prior art.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0007] On one hand, the present invention provides a polarization multiplexing direct detection system based on a CADD receiver, the polarization multiplexing direct detection system based on a CADD receiver including a transmitting end and a receiving end; wherein,

[0008] At the transmitting end, four independent signals form two pairs of asymmetric twin-single-sideband (TSB) signals, serving as X-polarization and Y-polarization respectively. After adding virtual carriers to the two pairs of asymmetric TSB signals, IQ (In-phase / quadrature) modulation is performed to obtain the final optical signal, which is then sent into the optical fiber and transmitted to the receiving end. An asymmetric guard band is reserved in the middle of the signal spectrum of the asymmetric TSB signals. The spectra of the right-sideband signal of X-polarization and the left-sideband signal of Y-polarization overlap.

[0009] The receiving end includes a coupler, a pair of optical bandpass filters, and two CADD (Carrier-assisted Differential Detection) receivers. At the receiving end, the received signal is split into X-polarized and Y-polarized signals by the coupler. The X-polarized and Y-polarized signals are then respectively input to an optical bandpass filter for filtering to suppress unwanted orthogonal carrier components. The filtered X-polarized signal is input to one of the CADD receivers, and the filtered Y-polarized signal is input to the other CADD receiver. The two CADD receivers have identical structures and are used to reconstruct the X-polarized and Y-polarized complex signals, respectively. Finally, the reconstructed X-polarized and Y-polarized signals are sent to the receiving end for digital signal processing to obtain the final binary bit sequence.

[0010] Furthermore, the process of the transmitting end generating the final optical signal and sending it into the optical fiber specifically includes:

[0011] Four independent pseudo-random binary sequences are mapped to four 16-QAM (Quadrature Amplitude Modulation) symbol sequences, then upsampled, and subsequently pulse-shaped using a raised cosine root filter with a roll-off factor of 0.01. The four pulse-shaped independent signals are then upconverted and combined into two asymmetric twin-generated single-sideband signals, which are used for X-polarization and Y-polarization, respectively. An asymmetric guard band is reserved in the middle of the signal spectrum through upconversion to mitigate polarization crosstalk. Finally, after adding a virtual carrier, IQ modulation is performed to obtain the final optical signal, which is then sent into the optical fiber.

[0012] Furthermore, the CADD receiver at the receiving end is either an asymmetric CADD (A-CADD) or a symmetric CADD (S-CADD). Therefore, it can be divided into polarization multiplexing systems based on A-CADD receivers and polarization multiplexing systems based on S-CADD receivers; abbreviated as PDM A-CADD and PDM S-CADD.

[0013] Furthermore, the A-CADD includes: a first coupler, a second coupler, a first 90-degree mixer, a first photodetector, a first balanced photodiode, and a second balanced photodiode.

[0014] Furthermore, the A-CADD's processing procedure for the received signal includes:

[0015] First, the signal is evenly split into two outputs by the first coupler. The first output signal from the first coupler is delayed using an optical delay line. The delayed signal is then split into two outputs by the second coupler. The first output signal from the second coupler is sent to the first photodetector for photoelectric conversion to obtain the photocurrent I0.

[0016]

[0017] Among them, C x Indicates an X-polarized optical carrier; S x (t-τ) represents the delayed X-polarized complex double-sideband signal; S y,l (t-τ) represents the delayed Y-polarized left-band signal;

[0018] The second signal output from the first coupler and the second signal output from the second coupler are fed together into the first 90-degree mixer. Then, the output signals of the first 90-degree mixer are fed into the first balanced photodiode and the second balanced photodiode, respectively, to obtain photocurrents I1 and I2.

[0019]

[0020]

[0021] Where Re{·} and Im{·} represent the real and imaginary parts, respectively, and * denotes conjugate; Sx(t) represents the X-polarized complex double-sideband signal; S y,l (t) represents the left-side band signal of Y-polarization;

[0022] The first complex signal R1(t) is then constructed using photocurrents I0, I1, and I2:

[0023]

[0024] Where j represents the imaginary unit.

[0025] Furthermore, the S-CADD includes a third coupler, a second 90-degree mixer, a third balanced photodiode, and a fourth balanced photodiode.

[0026] Furthermore, the S-CADD's processing of the received signal includes:

[0027] First, the signal is split into two outputs by the third coupler. The first output signal from the third coupler is delayed using an optical delay line. Then, the delayed signal and the second output signal from the third coupler enter the second 90-degree mixer together. The output signals from the second 90-degree mixer are photoelectrically converted using the third and fourth balanced photodiodes, respectively, to obtain photocurrents I3 and I4.

[0028]

[0029]

[0030] Where τ' represents optical delay; S x(t-τ') represents the X-polarized signal after a delay of τ'; S y,l (t-τ') represents the Y-polarized left-sideband signal delayed by τ'; Re{·} and Im{·} represent the real and imaginary parts, respectively, and * denotes conjugate; C x Indicates an X-polarized optical carrier; S x (t) represents the X-polarized complex double-sideband signal; S y,l (t) represents the left-side band signal of Y-polarization;

[0031] A complex signal R(t) is constructed using photocurrents I3 and I4:

[0032]

[0033] Delaying R(t) twice yields the signal R(t-τ′), which is then combined to construct the second complex signal R2(t):

[0034]

[0035] Among them, S x (t-2τ') represents the X-polarized signal after a delay of 2τ'; S y,l (t-2τ') represents the Y-polarized left-sideband signal delayed by 2τ'.

[0036] Furthermore, the signal processing procedure of the receiving end digital signal processing module includes: inputting the reconstructed X-polarized and Y-polarized complex signals into the SSBI (Signal-signal beat interference) joint iterative elimination module for SSBI joint iterative elimination; then performing down-conversion and matched filtering on the X-polarized and Y-polarized signals after SSBI iterative elimination; and finally performing symbol decision to obtain the final binary bit sequence; the signal processing procedure of the SSBI joint iterative elimination module includes:

[0037] The reconstructed X-polarized and Y-polarized signals are downconverted, matched, and symbol decided. The resulting bit sequence is used to reconstruct the inter-polarization and intra-polarization SSBI. Then, the reconstructed SSBI is subtracted from the input signals of the two polarizations to eliminate SSBI interference. After several iterations, SSBI mitigation is achieved.

[0038] On the other hand, the present invention also provides a polarization multiplexing direct detection method based on a CADD receiver, implemented using the above-mentioned polarization multiplexing direct detection system based on a CADD receiver, comprising:

[0039] At the transmitting end, four independent signals form two pairs of asymmetric twin-single-sideband (TSB) signals, serving as X-polarization and Y-polarization respectively. After adding virtual carriers to the two pairs of asymmetric TSB signals, IQ (In-phase / quadrature) modulation is performed to obtain the final optical signal, which is then sent into the optical fiber and transmitted to the receiving end. The asymmetric TSB signals have an asymmetric guard band reserved in the middle of their signal spectrum. The spectra of the right-sideband signal of X-polarization and the left-sideband signal of Y-polarization overlap.

[0040] At the receiving end, the received signal is split into X-polarized and Y-polarized signals by a coupler. Then, the X-polarized and Y-polarized signals are each input to an optical bandpass filter for filtering to suppress unwanted orthogonal carrier components. The filtered X-polarized signal is input to one CADD receiver, and the filtered Y-polarized signal is input to another CADD receiver. The two CADD receivers have identical structures and are used to reconstruct the two complex X-polarized and Y-polarized signals, respectively. Finally, the reconstructed X-polarized and Y-polarized signals are sent to the receiver's digital signal processing module to obtain the final binary bit sequence.

[0041] The beneficial effects of the technical solution provided by this invention include at least the following:

[0042] This invention achieves polarization diversity by using a pair of optical bandpass filters at the receiver, and then combines this with a CADD receiver to recover the optical field of polarization-multiplexed complex double-sideband signals. Furthermore, this invention proposes a joint SSBI cancellation scheme, which, after several iterations, effectively mitigates inter-polarization and intra-polarization SSBI distortion. Compared to previous CADD schemes, this invention achieves higher electrical spectral efficiency by incorporating polarization multiplexing technology. Finally, the proposed PDM S-CADD (Polarization Division Multiplexing Symmetric-CADD) system requires a lower CSPR (Carrier-to-Signal Power Ratio) than PDM A-CADD (Polarization Division Multiplexing Asymmetric-CADD), and has a simpler receiver hardware structure, making it more suitable for cost-sensitive short-distance optical transmission. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying 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.

[0044] Figure 1 This is a block diagram of a polarization multiplexing direct detection system based on a CADD receiver provided in an embodiment of the present invention;

[0045] Figure 2 This is a receiver structure diagram provided in an embodiment of the present invention; wherein, (a) is a structural schematic diagram of A-CADD; and (b) is a structural schematic diagram of S-CADD;

[0046] Figure 3 This is a schematic diagram of the SSBI joint iterative elimination algorithm provided in an embodiment of the present invention;

[0047] Figure 4 This is a schematic diagram of the simulation settings provided in an embodiment of the present invention;

[0048] Figure 5 This is a schematic diagram illustrating the relationship between bit error rate (BER) and the number of iterations provided in an embodiment of the present invention. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0050] First, it should be noted that in the embodiments of the present invention, the words "exemplarily," "for example," etc., are used to indicate that they are examples, illustrations, or descriptions. Any embodiment or design scheme described as "exemplary" in the present invention should not be construed as being more preferred or advantageous than other embodiments or design schemes. Specifically, the use of the term "exemplarily" is intended to present the concept in a specific manner. Furthermore, in the embodiments of the present invention, the meaning expressed by "and / or" can be both, or it can be either one or the other.

[0051] Furthermore, in the embodiments of the present invention, the terms "image" and "picture" may sometimes be used interchangeably. It should be noted that, without emphasizing the distinction between them, their intended meanings are consistent. Similarly, the terms "of," "corresponding (relevant)," and "corresponding" may sometimes be used interchangeably. It should be noted that, without emphasizing the distinction between them, their intended meanings are consistent.

[0052] Furthermore, in embodiments of the present invention, sometimes a subscript (such as W1) may be mistakenly written as a non-subscript form (such as W1). Without emphasizing the difference, the meaning they express is the same.

[0053] To achieve higher spectral efficiency in CADD receivers, this embodiment provides a polarization multiplexing direct detection system based on a CADD receiver and a polarization multiplexing direct detection method implemented using this system. The optical field recovery of polarization multiplexed complex double-sideband signals is achieved based on the CADD receiver. Specifically, it includes polarization division multiplexing asymmetric-CADD (PDM A-CADD) and polarization division multiplexing symmetric-CADD (PDM S-CADD). Polarization diversity of the PDM asymmetric double-sideband signal is achieved by using a pair of optical bandpass filters, and the optical field of the complex signal is recovered using A-CADD and S-CADD. Furthermore, this embodiment proposes a joint iterative mitigation algorithm for signal-signal beat interference (SSBI), which can effectively alleviate inter-polarization and intra-polarization SSBI distortion.

[0054] like Figure 1 As shown, the system in this embodiment includes a transmitter and a receiver; wherein,

[0055] At the transmitting end, four independent signals form two pairs of asymmetric twin-single-sideband signals, representing X-polarization and Y-polarization respectively. To increase spectral efficiency, the right-sideband signal of X-polarization and the left-sideband signal of Y-polarization are overlaid. Furthermore, the asymmetric guard band of the asymmetric twin-single-sideband signals is primarily used to mitigate polarization crosstalk.

[0056] Specifically, the PDM signal has a baud rate of 30 Gbaud and a bit rate of 120 bit / s. At the transmitter, four independent pseudo-random binary sequences (PRBS) are mapped into four 16-QAM symbol sequences, then upsampled, and subsequently pulse-shaped using a root raised cosine (RRC) filter with a roll-off factor of 0.01. The four pulse-shaped independent signals are then upconverted and combined into two asymmetric twin-generated single-sideband (SSB) signals, used for X-polarization and Y-polarization, respectively. An asymmetric guard band is reserved in the middle of the signal spectrum during upconversion to mitigate polarization crosstalk. Finally, after adding a virtual carrier, IQ modulation is performed to obtain the final optical signal, which is then fed into SSMF (Standard Single-Mode Fiber).

[0057] The receiving end includes a coupler, a pair of optical bandpass filters, and two identical CADD (Carrier-assisted differential detection) receivers. At the receiving end, the signal transmitted through the optical fiber is first uniformly split into two polarizations by the coupler. Then, a pair of optical bandpass filters are used to suppress unwanted orthogonal carrier components (one optical bandpass filter for X-polarization and another for Y-polarization). Taking X-polarization as an example, the signal after passing through the optical bandpass filters can be represented as C. x +S x (t)+S y,l (t), where C x It is an X-polarized optical carrier, S x S represents an X-polarized complex double-sideband signal. y,l (t) represents the left-side band signal of Y-polarization. Then, two CADD receivers are used to recover the optical field for X-polarization and Y-polarization respectively. CADD receivers can be divided into A-CADD receivers and S-CADD receivers based on their structure. Therefore, they can be classified into polarization multiplexing systems based on A-CADD receivers (both CADD receivers are A-CADD receivers) and polarization multiplexing systems based on S-CADD receivers (both CADD receivers are S-CADD receivers). These are abbreviated as PDM A-CADD and PDM S-CADD. The specific structures of A-CADD and S-CADD receivers are as follows... Figure 2 As shown.

[0058] For the A-CADD receiver: First, the input signal is evenly split into two paths by the first coupler. The first path uses an optical delay line for delay. Then, the delayed signal is split into two paths by the second coupler. Path 1 is sent to the first photodetector for photoelectric conversion to obtain the photocurrent I0.

[0059]

[0060] Where τ represents the optical delay, set to 35 ps.

[0061] The second path of the first coupler and the second path 2 of the second coupler output are fed together into the first 90-degree mixer, and then the output signal enters the first and second balanced photodiodes to obtain photocurrents I1 and I2:

[0062]

[0063]

[0064] Where Re{·} and Im{·} represent the real part and the imaginary part, respectively, and * represents conjugation.

[0065] The first complex signal R1 is then constructed using photocurrents I0, I1, and I2:

[0066]

[0067] For the S-CADD receiver: First, the input signal is split into two paths by the third coupler. The first path is delayed using an optical delay line. Then, the delayed signal and the second signal from the third coupler enter the second 90-degree mixer together. Subsequently, the third and fourth balanced photodiodes are used for photoelectric conversion to obtain photocurrents I3 and I4.

[0068]

[0069]

[0070] Where τ' represents the optical delay, set to 17.5 ps, and S x (t-τ') represents the X-polarized signal after a delay of τ'; S y,l (t-τ') represents the Y-polarized left-side band signal delayed by τ';

[0071] A complex signal R is constructed using photocurrents I3 and I4:

[0072]

[0073] The signal R is delayed twice to obtain the signal R(t-τ′), which is then combined to construct R2:

[0074]

[0075] Among them, S x (t-2τ') represents the X-polarized signal after a delay of 2τ'; S y,l (t-2τ') represents the Y-polarized left-side band signal delayed by 2τ';

[0076] When using an A-CADD receiver, the corresponding scheme is PDM A-CADD. Two A-CADD receivers in PDM A-CADD can obtain two complex polarization signals R1, and the two constructed polarization signals R1 are sent together to... Figure 4 The receiver digital signal processing shown in the diagram has a first module called SSBI joint iterative cancellation, the specific structure of which is as follows: Figure 3 As shown. First, one of the two polarization signals R1 is used as the X-polarization input signal, and the other as the Y-polarization input signal. Then, after down-conversion, matched filtering, and symbol decision, the two signals are used to obtain a bit sequence. The bit sequence is then used to construct S. x S y S x,rS y,l The first two values ​​are used to reconstruct the intra-polarization SSBI, and the latter two are used to reconstruct the inter-polarization SSBI. Subtracting the SSBI interference from the transmitted signals R1 and R2 effectively mitigates SSBI. According to... Figure 5 It can be seen that after two iterations, performance convergence can be achieved, eliminating most of the SSBI interference. Then, the signal after SSBI elimination undergoes a final down-conversion and matched filtering to recover the corresponding sideband signal. Finally, sign decision is performed on the sideband signal to obtain the final binary bit sequence. Similarly, for PDM S-CADD, two polarization signals R2 are obtained, and the same operation is performed to obtain the final bit sequence of the PDM S-CADD system.

[0077] Furthermore, this embodiment verifies the feasibility of the proposed scheme using the optical communication simulation software VPI transmission Makers 11.1 and MATLAB. The simulation results are shown below. Figure 4 As shown. Figure 5 The relationship between the bit error rate and the number of iterations for the proposed PDM A-CADD and PDMS-CADD receivers at an optical signal-to-noise ratio (OSNR) of 35 dB is shown. It can be observed that the performance converges sufficiently after several iterations. Furthermore, the PDM S-CADD receiver exhibits even better performance.

[0078] In summary, this embodiment provides a polarization multiplexing direct detection scheme based on a CADD receiver. Polarization diversity is achieved by using a pair of optical bandpass filters at the receiver, and then optical field recovery of the polarization multiplexed complex double-sideband signal is achieved by combining it with the CADD receiver. Furthermore, this invention proposes a joint SSBI cancellation scheme, which effectively alleviates inter-polarization and intra-polarization SSBI distortion after several iterations. Compared with previous CADD schemes, this invention achieves higher electrical spectral efficiency by combining polarization multiplexing technology. Finally, the proposed PDM S-CADD system requires a lower CSPR than PDM A-CADD, and the receiver hardware structure is simpler, making it more suitable for cost-sensitive short-distance optical transmission.

[0079] Furthermore, it should be noted that the present invention can be provided as a method, apparatus, or computer program product. Therefore, embodiments of the present invention can take the form of a completely or partially hardware embodiment, a completely or partially software embodiment, or an embodiment combining software and hardware aspects. Moreover, when implemented in software, embodiments of the present invention can take the form of a computer program product implemented on one or more computer-usable storage media containing computer-usable program code. The computer program product includes one or more computer instructions or computer programs. When the computer instructions or computer program are loaded or executed on a computer, all or part of the processes or functions described in the embodiments of the present invention are generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, the computer instructions can be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium can be any usable medium accessible to a computer or a data storage device such as a server or data center containing one or more sets of usable media. The available medium can be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape), an optical medium (e.g., DVD), or a semiconductor medium. A semiconductor medium can be a solid-state drive (SSD).

[0080] Embodiments of the present invention are described with reference to flowchart illustrations and / or block diagrams of methods, terminal devices (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, embedded processor, or other programmable data processing terminal device to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing terminal device, generate instructions for implementing the flowchart illustrations. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0081] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing terminal device to operate in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1The functions specified in one or more boxes. These computer program instructions may also be loaded onto a computer or other programmable data processing terminal equipment to cause a series of operational steps to be performed on the computer or other programmable terminal equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable terminal equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0082] It should also be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. The terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or terminal device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal device. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or terminal device that includes said element. Furthermore, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, and B alone, where A and B can be singular or plural. Additionally, the character " / " in this text generally indicates an "or" relationship between the preceding and following objects, but it can also indicate an "AND / OR" relationship. Please refer to the context for specific interpretations. "At least one" refers to one or more items, while "more than" refers to two or more items. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, at least one of a, b, or c can be represented as: a, b, c, ab, ac, bc, or abc, where a, b, and c can be single or multiple.

[0083] Furthermore, it is understood that in various embodiments of the present invention, the order of the above-mentioned process numbers does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0084] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0085] In the several embodiments provided by this invention, it should be understood that the disclosed devices, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative. For instance, the division of functional modules / units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another device, or some features may be ignored or not executed. Furthermore, the shown or discussed mutual couplings or direct couplings or communication connections may be through some interfaces; indirect couplings or communication connections between devices or units may be electrical, mechanical, or other forms. Units described as separate components may or may not be physically separate, and components shown as units may or may not be physical units, i.e., they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs. Additionally, the functional units in the various embodiments of this invention may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit.

[0086] If the method is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0087] Finally, it should be noted that the above description represents a preferred embodiment of the present invention. It should be pointed out that although preferred embodiments have been described, those skilled in the art, once they understand the basic inventive concept of the present invention, can make various improvements and modifications without departing from the principles described herein. These improvements and modifications should also be considered within the scope of protection of the present invention. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the embodiments of the present invention.

Claims

1. A polarization multiplexing direct detection system based on a CADD receiver, characterized in that, The polarization multiplexing direct detection system based on a CADD receiver includes a transmitter and a receiver; wherein, At the transmitting end, four independent signals form two pairs of asymmetric twin-single-sideband (TSB) signals, serving as X-polarization and Y-polarization respectively. After adding virtual carriers to the two pairs of asymmetric TSB signals, IQ (In-phase / quadrature) modulation is performed to obtain the final optical signal, which is then sent into the optical fiber and transmitted to the receiving end. An asymmetric guard band is reserved in the middle of the signal spectrum of the asymmetric TSB signals. The spectra of the right-sideband signal of X-polarization and the left-sideband signal of Y-polarization overlap. The receiving end includes a coupler, a pair of optical bandpass filters, and two CADD (Carrier-assisted Differential Detection) receivers. At the receiving end, the received signal is split into X-polarized and Y-polarized signals by the coupler. The X-polarized and Y-polarized signals are then respectively input to an optical bandpass filter for filtering to suppress unwanted orthogonal carrier components. The filtered X-polarized signal is input to one of the CADD receivers, and the filtered Y-polarized signal is input to the other CADD receiver. The two CADD receivers have identical structures and are used to reconstruct the X-polarized and Y-polarized complex signals, respectively. Finally, the reconstructed X-polarized and Y-polarized signals are sent to the receiving end's digital signal processing module to obtain the final binary bit sequence.

2. The polarization multiplexing direct detection system based on a CADD receiver as described in claim 1, characterized in that, The process of the transmitting end generating the final optical signal and sending it into the optical fiber specifically includes: Four independent pseudo-random binary sequences are mapped to four 16-QAM (Quadrature Amplitude Modulation) symbol sequences, then upsampled, and subsequently pulse-shaped using a raised cosine root filter with a roll-off factor of 0.

01. The four pulse-shaped independent signals are then upconverted and combined into two asymmetric twin-generated single-sideband signals, which are used for X-polarization and Y-polarization, respectively. An asymmetric guard band is reserved in the middle of the signal spectrum through upconversion to mitigate polarization crosstalk. Finally, after adding a virtual carrier, IQ modulation is performed to obtain the final optical signal, which is then sent into the optical fiber.

3. The polarization multiplexing direct detection system based on a CADD receiver as described in claim 1, characterized in that, The CADD receiver at the receiving end is either an asymmetric CADD or a symmetric CADD.

4. The polarization multiplexing direct detection system based on a CADD receiver as described in claim 3, characterized in that, The asymmetric CADD includes: a first coupler, a second coupler, a first 90-degree mixer, a first photodetector, a first balanced photodiode, and a second balanced photodiode.

5. The polarization multiplexing direct detection system based on a CADD receiver as described in claim 4, characterized in that, The asymmetric CADD's processing of the received signals includes: First, the signal is evenly split into two outputs by the first coupler. The first output signal from the first coupler is delayed using an optical delay line. The delayed signal is then split into two outputs by the second coupler. The first output signal from the second coupler is sent to the first photodetector for photoelectric conversion to obtain the photocurrent I0. Among them, C x Indicates an X-polarized optical carrier; S x (t-τ) represents the delayed X-polarized complex double-sideband signal; S y,l (t-τ) represents the delayed Y-polarized left-band signal; The second signal output from the first coupler and the second signal output from the second coupler are fed together into the first 90-degree mixer. Then, the output signals of the first 90-degree mixer are fed into the first balanced photodiode and the second balanced photodiode, respectively, to obtain photocurrents I1 and I2. Where Re{·} and Im{·} represent the real and imaginary parts, respectively, and * denotes conjugate; Sx(t) represents the X-polarized complex double-sideband signal; S y,l (t) represents the left-side band signal of Y-polarization; The first complex signal R1(t) is then constructed using photocurrents I0, I1, and I2: Where j represents the imaginary unit.

6. The polarization multiplexing direct detection system based on a CADD receiver as described in claim 3, characterized in that, The symmetrical CADD includes a third coupler, a second 90-degree mixer, a third balanced photodiode, and a fourth balanced photodiode.

7. The polarization multiplexing direct detection system based on a CADD receiver as described in claim 6, characterized in that, The symmetrical CADD's processing of the received signal includes: First, the signal is split into two outputs by the third coupler. The first output signal from the third coupler is delayed using an optical delay line. Then, the delayed signal and the second output signal from the third coupler enter the second 90-degree mixer together. The output signals from the second 90-degree mixer are photoelectrically converted using the third and fourth balanced photodiodes, respectively, to obtain photocurrents I3 and I4. Where τ' represents optical delay; S x (t-τ') represents the X-polarized signal after a delay of τ'; S y,l (t-τ') represents the Y-polarized left-sideband signal delayed by τ'; Re{·} and Im{·} represent the real and imaginary parts, respectively, and * denotes conjugate; C x Indicates an X-polarized optical carrier; S x (t) represents the X-polarized complex double-sideband signal; S y,l (t) represents the left-side band signal of Y-polarization; A complex signal R(t) is constructed using photocurrents I3 and I4: Delaying R(t) twice yields the signal R(t-τ′), which is then combined to construct the second complex signal R2(t): Among them, S x (t-2τ') represents the X-polarized signal after a delay of 2τ'; S y,l (t-2τ') represents the Y-polarized left-sideband signal delayed by 2τ'.

8. The polarization multiplexing direct detection system based on a CADD receiver as described in claim 1, characterized in that, The digital signal processing module at the receiving end performs the following signal processing steps: inputting the reconstructed X-polarized and Y-polarized complex signals into the SSBI (Signal-signal beat interference) joint iterative elimination module for SSBI joint iterative elimination; then performing down-conversion and matched filtering on the X-polarized and Y-polarized signals after SSBI iterative elimination; and finally performing symbol decision to obtain the final binary bit sequence. The signal processing procedure of the SSBI joint iterative elimination module includes: The reconstructed X-polarized and Y-polarized signals are downconverted, matched, and symbol decided. The resulting bit sequence is used to reconstruct the inter-polarization and intra-polarization SSBI. Then, the reconstructed SSBI is subtracted from the input signals of the two polarizations to eliminate SSBI interference. After several iterations, SSBI mitigation is achieved.

9. A method for direct polarization multiplexing detection based on a CADD receiver, implemented using the polarization multiplexing direct detection system based on a CADD receiver as described in any one of claims 1-8, characterized in that, The polarization multiplexing direct detection method based on a CADD receiver includes: At the transmitting end, four independent signals form two pairs of asymmetric twin-single-sideband (TSB) signals, serving as X-polarization and Y-polarization respectively. After adding virtual carriers to the two pairs of asymmetric TSB signals, IQ (In-phase / quadrature) modulation is performed to obtain the final optical signal, which is then sent into the optical fiber and transmitted to the receiving end. The asymmetric TSB signals have an asymmetric guard band reserved in the middle of their signal spectrum. The spectra of the right-sideband signal of X-polarization and the left-sideband signal of Y-polarization overlap. At the receiving end, the received signal is split into X-polarized and Y-polarized signals by a coupler. Then, the X-polarized and Y-polarized signals are respectively input into an optical bandpass filter for filtering to suppress unwanted orthogonal carrier components. The filtered X-polarized signal is input into one CADD receiver, and the filtered Y-polarized signal is input into another CADD receiver. The two CADD receivers have the same structure and are used to reconstruct the two complex signals, X-polarized and Y-polarized, respectively. Finally, the reconstructed X-polarized and Y-polarized signals are sent to the receiver's digital signal processing module to obtain the final binary bit sequence.

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