A phase recovery-free subcarrier joint modulation and detection method and apparatus

Abstract

This invention discloses a phase recovery-free subcarrier joint modulation and detection method and apparatus. The apparatus includes a transmitter processing module and a receiver processing module. The transmitter processing module groups the digital signal or bit sequence to be transmitted; maps the grouped digital signal or bit sequence into a joint modulation symbol carried by at least two digital subcarriers, including a common phase degree of freedom and a common phase invariant degree of freedom. The common phase invariant degree of freedom remains unchanged when the common phase of the joint modulation symbol undergoes a common phase rotation, and carries the information to be transmitted; multiplexes the subcarriers to generate an electrical domain modulation signal to be optically modulated; the receiver processing module constructs a joint received symbol based on the received signals corresponding to at least two digital subcarriers, performs joint detection on the joint received symbol based on a common phase invariant decision metric, and recovers the digital signal or bit sequence to be transmitted based on the joint detection result.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber communication and coherent optical communication technology, specifically relating to a phase recovery-free subcarrier joint modulation and detection method, apparatus and coherent optical communication system, and particularly to a subcarrier joint modulation and detection method that reduces the receiver's dependence on explicit common carrier phase recovery by combining at least two digital subcarriers to form a high-dimensional modulation symbol and carrying the information to be transmitted in a common phase-invariant degree of freedom. Background Technology

[0002] As optical fiber communication systems evolve towards higher transmission rates, higher spectral efficiency, and lower system costs, coherent optical communication technology has gained widespread attention in applications such as short-distance interconnects, data center interconnects, optical access networks, and passive optical networks. Coherent detection can simultaneously acquire the amplitude and phase information of optical signals, making it suitable for carrying high-order modulation formats and multi-subcarrier modulation signals, which is beneficial for improving system sensitivity and spectral efficiency.

[0003] However, in coherent optical communication systems, phase noise in the transmitting and receiving lasers causes random phase rotation in the received signal. For high-order quadrature amplitude modulation signals, this phase rotation significantly affects the accuracy of constellation decision-making. This is especially true in coherent optical communication systems using low-cost, high-linewidth lasers, where the laser phase changes rapidly, making the impact of phase noise on system performance even more pronounced.

[0004] To reduce the impact of laser phase noise, traditional coherent optical communication receivers typically require a carrier phase recovery module to estimate and compensate for the common carrier phase in the received signal. Common methods include decision-assisted phase recovery, blind phase search, pilot-assisted phase recovery, and residual carrier-assisted phase recovery. These methods can improve the system's tolerance to phase noise to some extent, but they usually require additional phase estimation, phase tracking, or phase compensation steps. Under conditions of large-linewidth lasers, they may also face problems such as high tracking complexity, parameter sensitivity, error propagation, and insufficient robustness. Therefore, relying solely on increasingly complex explicit carrier phase recovery algorithms is not necessarily suitable for low-cost, large-linewidth coherent optical communication systems.

[0005] Multidimensional modulation and digital subcarrier multiplexing offer new technical solutions to alleviate these problems. By jointly designing multiple dimensions or multiple subcarriers, the geometric structure and decision-making methods of the signal can be optimized at the modulation format level. However, existing high-order coherent modulation schemes typically still treat the common carrier phase as an unknown quantity that needs to be recovered, and the receiver still needs to rely on an explicit carrier phase recovery module to complete stable decision-making.

[0006] Therefore, there is an urgent need for a new subcarrier joint modulation and detection scheme that can reduce the dependence on explicit common carrier phase recovery at the modulation symbol design and receiver decision level, so that the information to be transmitted is carried on degrees of freedom that are insensitive to common phase rotation, and the signal recovery is completed through joint detection metric that is invariant to common phase, thereby improving the robustness and practicality of high-order modulation signals in large-linewidth coherent optical communication systems. Summary of the Invention

[0007] To address the technical problems existing in the prior art, the present invention aims to provide a phase recovery-free subcarrier joint modulation and detection method and apparatus. This method combines at least two digital subcarriers to form a high-dimensional modulation symbol, maps the information to be transmitted to a common phase-invariant degree of freedom (CDR) instead of a common phase CDR, and performs joint detection at the receiving end based on a common phase-invariant decision metric. This eliminates the need for explicit common carrier phase recovery as a necessary step in joint detection, thereby improving the phase noise tolerance of coherent optical communication systems under large-linewidth laser conditions.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A phase recovery-free subcarrier joint modulation method includes the following steps: Acquire the digital signal or bit sequence to be transmitted; The digital signal or bit sequence to be transmitted is mapped into a joint modulation symbol carried by at least two digital subcarriers; The joint modulation symbol is represented as a high-dimensional complex vector, which includes a common phase degree of freedom and a common phase invariant degree of freedom, wherein the common phase invariant degree of freedom remains unchanged when the joint modulation symbol undergoes a common phase rotation; The information of the digital signal or bit sequence to be transmitted is carried in the common phase invariant degree of freedom, and not in the common phase degree of freedom; Multiplexing, optical modulation, and coherent transmission are performed on at least two digital subcarriers carrying the joint modulation symbols.

[0009] Preferably, the number of the at least two digital subcarriers is K, where K is greater than or equal to 2; the joint modulation symbol is composed of complex modulation symbols carried by the K digital subcarriers respectively.

[0010] Preferably, the common phase invariant degree of freedom includes one or more of the following: the total energy of the joint modulation symbol, the energy distribution relationship between the at least two digital subcarriers, and the relative phase relationship between the at least two digital subcarriers.

[0011] Preferably, under the common phase rotation, the joint modulation symbol is transformed into an equivalent symbol that has a different common phase but the same degree of freedom as the common phase; the digital signal or bit sequence to be transmitted is mapped to a common phase equivalence class composed of the equivalent symbols, rather than to a specific common carrier phase.

[0012] Preferably, the at least two digital subcarriers are modulated by the same signal optical carrier and coherently received by the same local oscillator, so that the phase noise of the transmitting laser and / or the phase noise of the local oscillator laser are approximately common phase rotations on the at least two digital subcarriers.

[0013] Preferably, the joint modulation symbols belong to a preset high-dimensional constellation set, and the constellation points in the high-dimensional constellation set are determined by common phase-invariant degrees of freedom. The high-dimensional constellation set can be one of the following: a multi-shell high-dimensional constellation set, a spherical high-dimensional constellation set, a hyperspherical high-dimensional constellation set, a lattice-type high-dimensional constellation set, a codebook-type high-dimensional constellation set, or an optimized high-dimensional constellation set.

[0014] Preferably, the at least two digital subcarriers are two digital subcarriers that together constitute a four-dimensional joint modulation symbol. The information of the four-dimensional joint modulation symbol is carried by one or more of the following: the total energy of the joint symbol, the energy distribution relationship between the two digital subcarriers, and the relative phase relationship between the two digital subcarriers. The common phase does not carry the information to be transmitted.

[0015] Preferably, the four-dimensional joint modulation symbols belong to a two-shell or multi-shell four-dimensional constellation set.

[0016] Preferably, global symbol flipping, global phase perturbation, global phase rotation, or global phase scrambling are jointly applied to the joint modulation symbols; wherein, the global symbol flipping, global phase perturbation, global phase rotation, or global phase scrambling does not carry information to be transmitted.

[0017] This invention also provides a phase recovery-free subcarrier joint detection method, the steps of which include: Acquire the received symbols corresponding to at least two digital subcarriers; The received symbols corresponding to the at least two digital subcarriers are combined to form a joint received symbol; For candidate joint modulation symbols in a preset high-dimensional constellation set, calculate the common phase invariance decision metric between the joint received symbol and the candidate joint modulation symbol; The transmitted joint modulation symbol is determined from the candidate joint modulation symbols based on the common phase invariance decision metric; Recover the corresponding digital signal or bit sequence based on the determined joint modulation symbols; The information of the candidate joint modulation symbols is carried in the common phase invariant degree of freedom, but not in the common phase degree of freedom.

[0018] Preferably, the common phase invariance decision metric is obtained by eliminating, optimizing, or marginalizing the unknown common phase between the joint received symbol and the candidate joint modulation symbol, so that the common phase invariance decision metric is independent of the common carrier phase.

[0019] Preferably, the common phase invariant decision metric includes a decision metric obtained based on one or more of the inner product modulus between the joint received symbol and the candidate joint modulated symbol, the distance minimization result, the likelihood function, or the posterior probability.

[0020] Preferably, the joint detection includes one or more of hard-decision detection, soft-decision detection, maximum likelihood detection, approximate maximum likelihood detection, lookup table detection, parallel metric computation detection, or spherical decoding detection.

[0021] Preferably, the receiving end processes the received signals corresponding to the at least two digital subcarriers using one or more of the following methods: synchronization, frequency offset compensation, dispersion compensation, channel equalization, subcarrier demultiplexing, matched filtering, and downsampling. The receiving end may also include one or more of the following methods: phase noise suppression, equalization-enhanced phase noise effect compensation, residual phase drift correction, or phase correlation auxiliary compensation; wherein, the joint detection does not require explicit common carrier phase recovery as a necessary step.

[0022] The present invention also provides a phase recovery-free subcarrier joint modulation and detection device, including a transmitter processing module and a receiver processing module; The transmitter processing module includes a bit packetization module, a subcarrier joint mapping module, and a subcarrier multiplexing module; The bit grouping module is used to acquire the digital signal or bit sequence to be transmitted and to group the digital signal or bit sequence to be transmitted. The subcarrier joint mapping module is used to map the grouped digital signal or bit sequence into a joint modulation symbol jointly carried by at least two digital subcarriers, and to carry the information of the digital signal or bit sequence in the common phase invariant degree of freedom of the joint modulation symbol, rather than in the common phase degree of freedom. The subcarrier multiplexing module is used to multiplex at least two digital subcarriers carrying the joint modulation symbol to generate an electrical domain modulation signal to be optically modulated. The receiving end processing module includes a joint receiving symbol construction module, a common phase invariant joint detection module, and a bit recovery module; The joint reception symbol construction module is used to construct joint reception symbols based on the received signals corresponding to at least two digital subcarriers. The common phase invariant joint detection module is used to perform joint detection on the joint received symbols based on the common phase invariant decision metric. The bit recovery module is used to recover the digital signal or bit sequence to be transmitted based on the result of the joint detection.

[0023] The present invention also provides a coherent optical communication system, including a transmitter, an optical fiber transmission link, and a receiver; The transmitting end is used to map the digital signal or bit sequence to be transmitted into a joint modulation symbol carried by at least two digital subcarriers, and to carry the information of the digital signal or bit sequence to be transmitted in the common phase invariant degree of freedom of the joint modulation symbol, rather than in the common phase degree of freedom; The optical fiber transmission link is used to transmit optical signals carrying the joint modulation symbols; The receiving end is used to construct a joint receiving symbol based on the received signals corresponding to at least two digital subcarriers, and to perform joint detection on the joint receiving symbol based on the common phase invariance decision metric to recover the digital signal or bit sequence to be transmitted.

[0024] Preferably, when the coherent optical communication system includes multiple digital subcarriers, the multiple digital subcarriers are divided into one or more subcarrier groups, each subcarrier group including at least two digital subcarriers; the joint modulation and the joint detection are performed on each subcarrier group respectively.

[0025] Compared with the prior art, the positive effects of the present invention are as follows: This invention combines at least two digital subcarriers to form a high-dimensional modulation symbol and maps the information to be transmitted to a common phase-invariant degree of freedom instead of a common phase degree of freedom. This ensures that the rotation of the common carrier phase does not directly change the degree of freedom carrying the information, thereby reducing the system's dependence on explicit common carrier phase recovery from the modulation symbol design level.

[0026] This invention performs joint detection of jointly received symbols using a common phase invariant decision metric. This decision metric can be obtained by eliminating, optimizing, or marginalizing unknown common phases. Therefore, joint detection does not require explicit common carrier phase recovery as a necessary step, which helps to reduce the complexity of phase recovery at the receiver and improve the system robustness under large linewidth laser conditions.

[0027] This invention can be extended from two digital subcarriers to K digital subcarriers, and can also perform grouped joint modulation and grouped joint detection on multiple digital subcarriers, making it suitable for digital subcarrier multiplexing coherent optical communication systems.

[0028] This invention is compatible with existing coherent optical communication receivers' processing procedures such as synchronization, frequency offset compensation, dispersion compensation, channel equalization, subcarrier demultiplexing, matched filtering, and downsampling. Furthermore, it does not exclude auxiliary processing such as phase noise suppression, equalization enhancement of phase noise effect compensation, and residual phase drift correction. It can further improve system performance while maintaining the core mechanism of joint detection with unchanged common phase. Attached Figure Description

[0029] Figure 1 This is a flowchart of the phase recovery-free subcarrier joint modulation method of the present invention.

[0030] Figure 2 This is a flowchart of the detection method of the present invention.

[0031] Figure 3 This is a schematic diagram illustrating the principle of parameter decomposition and common phase invariance of K digital subcarriers and joint high-dimensional symbols in this invention.

[0032] Figure 4 This is a schematic diagram of a specific embodiment of the two-subcarrier four-dimensional joint modulation symbol of the present invention.

[0033] Figure 5 This is a schematic diagram of the device of the present invention.

[0034] Figure 6 This is a diagram of the coherent optical communication system of the present invention. Detailed Implementation

[0035] To make the technical features, advantages, and effects of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0036] like Figure 1 As shown, an optional embodiment of the present invention provides a phase recovery-free subcarrier joint modulation method, the steps of which include: The digital signal or bit sequence to be transmitted is mapped into a joint modulation symbol carried by at least two digital subcarriers; The joint modulation symbol is represented as a high-dimensional complex vector, including a common phase degree of freedom and a common phase invariant degree of freedom; wherein, the common phase invariant degree of freedom remains unchanged when the joint modulation symbol undergoes a common phase rotation; The information of the digital signal or bit sequence to be transmitted is carried using the common phase-invariant degree of freedom. Multiplexing, optical modulation, and coherent transmission are performed on at least two digital subcarriers carrying the joint modulation symbols.

[0037] like Figure 2As shown, an optional embodiment of the present invention provides a method for detecting a modulated signal generated based on the above modulation method, the steps of which include: Obtain the received symbols corresponding to the at least two digital subcarriers and combine them into a joint received symbol; Based on candidate joint modulation symbols in a preset high-dimensional constellation set, a common phase invariance decision metric is calculated between the joint received symbol and the candidate joint modulation symbols; wherein, the information of the candidate joint modulation symbols is carried in the common phase invariance degree of freedom; The transmitted joint modulation symbol is determined from the candidate joint modulation symbols based on the common phase invariance decision metric; The corresponding digital signal or bit sequence is recovered based on the determined joint modulation symbols.

[0038] In the above process, the digital signal processing at the receiving end may include one or more of the following: synchronization, frequency offset compensation, dispersion compensation, channel equalization, subcarrier demultiplexing, matched filtering, and downsampling. Furthermore, the receiving end may also include phase noise suppression, equalization-enhanced phase noise effect compensation, residual phase drift correction, or other phase-related auxiliary compensation processing. It should be noted that the auxiliary processing does not change the core mechanism of the invention; that is, the joint detection does not require explicit common carrier phase recovery as a necessary step.

[0039] like Figure 3 As shown, in an optional embodiment, the number of digital subcarriers participating in joint modulation is ,in The complex modulation symbol carried by the kth digital subcarrier is denoted as Then by The joint modulation symbol composed of multiple digital subcarriers can be represented as a high-dimensional complex vector: in, Represents a joint modulation symbol, They represent the first to the second, respectively. The complex modulation symbols carried by each digital subcarrier, This indicates transpose.

[0040] In a coherent optical communication system, if the If multiple digital subcarriers are modulated by the same signal optical carrier and coherently received by the same local oscillator, then the phase noise of the transmitting laser and / or the phase noise of the receiving local oscillator laser will be... The main characteristic on each digital subcarrier is an approximately common phase rotation. This common phase rotation can be expressed as: in, This indicates the phase of an unknown common carrier, where j is the imaginary unit.

[0041] This invention will combine modulation symbols The degrees of freedom are decomposed into two categories: common phase degrees of freedom and common phase invariant degrees of freedom. The common phase degrees of freedom change under common phase rotation and are not used to carry the information to be transmitted; the common phase invariant degrees of freedom remain unchanged under common phase rotation and are used to carry the information to be transmitted.

[0042] In an optional embodiment, the common phase-invariant degrees of freedom include one or more of the following: joint symbol total energy, energy distribution relationship between subcarriers, and relative phase relationship between subcarriers. Specifically, the joint symbol total energy can be expressed as: The normalized energy distribution relationship of the k-th digital subcarrier can be expressed as: The relative phase relationship between the i-th digital subcarrier and the j-th digital subcarrier can be expressed as: in, Indicates complex conjugation. This indicates taking a complex argument. Due to the effect of common phase rotation, Become Therefore, the total energy of the joint symbol, the normalized energy distribution, and the relative phase relationship between subcarriers do not change with the common phase. It changes with the changes.

[0043] Specifically, for the total energy, we have: For normalized energy distribution, we have: Regarding the relative phase relationship, we have: Therefore, it can be seen that the aforementioned degrees of freedom remain unchanged under common phase rotation. Thus, this invention maps the digital signal or bit sequence to be transmitted to a set of joint modulation symbols determined by the common phase-invariant degrees of freedom, rather than mapping it to a specific common carrier phase. In other words, joint symbols with different common phases but the same common phase-invariant degrees of freedom can be considered as the same common phase equivalence class, and the information to be transmitted is mapped to this equivalence class, rather than to a specific absolute common phase.

[0044] In an optional embodiment, when the coherent optical communication system includes multiple digital subcarriers, the multiple digital subcarriers can be divided into one or more subcarrier groups, each subcarrier group including at least two digital subcarriers. For each subcarrier group, the above-described subcarrier joint mapping, common phase invariant joint modulation symbol construction, and common phase invariant joint detection are performed respectively. The subcarrier group can be composed of adjacent digital subcarriers, or it can be composed of non-adjacent digital subcarriers, symmetrically distributed digital subcarriers, or digital subcarriers selected according to a preset frequency interval.

[0045] like Figure 4 As shown, in one specific embodiment, the number of digital subcarriers participating in joint modulation is 2, and the two digital subcarriers each carry complex modulation symbols. and Together they constitute a four-dimensional joint modulation symbol: The four-dimensional joint modulation symbol can be parameterized by the total energy of the joint symbol, the energy distribution between the two subcarriers, the relative phase between the two subcarriers, and the common phase. Specifically, it can be expressed as: in, Represents the total energy of the joint symbol. This represents the energy distribution parameter between two digital subcarriers. This indicates the relative phase between two digital subcarriers. Indicates the common phase.

[0046] Among the parameters mentioned above, , and It remains unchanged under common phase rotation, and therefore can be used as a degree of freedom for information carrying; The common phase degree of freedom changes under common phase rotation and therefore does not carry the information to be transmitted. Thus, the four-dimensional joint modulation symbol can be constructed as follows: That is, setting the common phase to a fixed value, a preset value, or a random value that does not carry information, so that the information to be sent is only generated by... , and Sure.

[0047] In one optional embodiment, the four-dimensional joint modulation symbols belong to a two-shell or multi-shell four-dimensional constellation set. Different shells correspond to different total joint symbol energies, and constellation points within the same shell are determined by the energy distribution relationship and relative phase relationship between the two digital subcarriers. Taking a two-shell constellation as an example, the total joint symbol energy can be selected from two preset energy values, corresponding to the inner shell and outer shell respectively; within each shell, the corresponding energy distribution parameters and relative phase parameters can be determined through a preset codebook, spherical point mapping, grid construction, or optimization algorithms.

[0048] In an alternative embodiment, points may first be generated on a unit sphere. Then, it is mapped to the energy distribution parameters and relative phase parameters between the two subcarriers: in, This represents the arctangent function in the four quadrants. This method can generate a relatively uniformly distributed four-dimensional joint constellation of points under a given shell energy. It should be noted that the spherical point mapping is only one possible implementation; the four-dimensional joint constellation can also be obtained through lattice construction, codebook search, geometric shaping, probabilistic shaping, or machine optimization.

[0049] In another alternative embodiment, global symbol flipping, global phase perturbation, global phase rotation, or global phase scrambling may be jointly applied to the joint modulation symbols. For example, it can be set as follows: in, The global symbol flip is equivalent to a global phase change and does not carry the information to be transmitted, therefore it does not change the information carrying mechanism of this invention. Similarly, other preset or random global phase perturbation methods can also be used to improve signal statistical characteristics, spectral characteristics, or achieve other system design goals.

[0050] At the receiving end, after coherent reception and digital signal processing, the received symbols corresponding to the at least two digital subcarriers are obtained and combined to form a joint received symbol. For In the case of multiple digital subcarriers, the joint reception symbol can be represented as: in, This represents the received symbol corresponding to the k-th digital subcarrier.

[0051] When the phase of the common carrier is unknown, the joint reception symbol can be represented as: in, Indicates the unknown common carrier phase. Indicates candidate joint modulation symbols, This represents noise terms and other residual damage terms.

[0052] To avoid For explicit estimation, this invention employs a common-phase-invariant decision metric for joint detection of jointly received symbols. In a maximum likelihood or near-maximum likelihood detection implementation, the decision criterion can be expressed as: in, Represents the set of candidate joint modulation symbols. The joint modulation symbol obtained from the decision is represented.

[0053] Expanding the above distance metric yields: in, This represents the conjugate transpose. For a given candidate joint modulation symbol... The one that minimizes this distance Make To obtain the maximum value is... Therefore, the above judgment criteria can be equivalently expressed as: Because this judgment metric depends on Without relying on The phase of the common carrier is such that it is insensitive to phase rotation. Therefore, the receiver can perform joint detection without requiring explicit common carrier phase recovery as a necessary step.

[0054] In another alternative embodiment, the common phase invariant joint detection can also be implemented using a soft decision method. Assuming the value corresponding to the l-th bit is 0 or 1, soft information can be constructed based on the common phase invariant distance. Definition: The soft information of the l-th bit can then be represented as: in, and These represent the candidate joint modulation symbol subsets where the l-th bit is 0 and 1, respectively. This represents noise power-related parameters. The soft decision result can be further input into the forward error correction decoding module to improve the system's error correction performance.

[0055] In practical implementation, the common phase invariant joint detection can be achieved through lookup tables, parallel metric computation, approximate maximum likelihood detection, spherical decoding, or other low-complexity search methods. When the number of candidate constellation points is small, the candidate joint modulation symbol set can be directly traversed; when the number of candidate constellation points is large or the number of subcarriers is large, the implementation complexity can be reduced through hierarchical search, group detection, pre-screening of candidate sets, or hardware parallel computation.

[0056] like Figure 5 As shown, the present invention also provides a phase recovery-free subcarrier joint modulation and detection device. In an optional embodiment, the device includes a transmitter processing module and a receiver processing module. The transmitter processing module includes a bit grouping module, a subcarrier joint mapping module, and a subcarrier multiplexing module. The bit grouping module is used to acquire the digital signal or bit sequence to be transmitted and divide it into bit groups corresponding to joint modulation symbols; the subcarrier joint mapping module is used to map the grouped bit sequence into joint modulation symbols carried by at least two digital subcarriers; the subcarrier multiplexing module is used to load the joint modulation symbols onto the corresponding digital subcarriers and generate an electrical domain modulation signal to be optically modulated.

[0057] The receiver processing module includes a joint received symbol construction module, a common phase invariant joint detection module, and a bit recovery module. The joint received symbol construction module is used to construct joint received symbols based on the received signals corresponding to at least two digital subcarriers; the common phase invariant joint detection module is used to perform joint detection on the joint received symbols based on the common phase invariant decision metric; and the bit recovery module is used to recover the corresponding digital signal or bit sequence based on the joint detection result.

[0058] This invention can also be applied to coherent optical communication systems. For example... Figure 6 As shown, the coherent optical communication system includes a transmitter, an optical fiber transmission link, and a receiver. The transmitter performs bit packetization, subcarrier joint mapping, common-phase-invariant joint modulation symbol construction, and subcarrier multiplexing, and generates an optical signal carrying the joint modulation symbols through a light source, a digital-to-analog converter, and an optical modulation module. The optical signal is transmitted to the receiver via the optical fiber transmission link. The receiver obtains the digital received signal through a coherent detection module and an analog-to-digital converter, and performs front-end processing, joint received symbol construction, common-phase-invariant joint detection, and bit recovery through a receiver digital signal processing module.

[0059] In an optional embodiment, the coherent optical communication system may employ a distributed feedback laser, an external cavity laser, an integrated narrow linewidth laser, a semiconductor laser, or other light sources with phase noise. For systems employing large linewidth lasers, since the information to be transmitted is not carried in the common phase degree of freedom, the rotation of the common carrier phase does not directly change the information-carrying degree of freedom; therefore, the method can improve the system's tolerance to laser phase noise.

[0060] In an optional embodiment, the receiver may further incorporate auxiliary processing such as phase noise suppression, equalization-enhanced phase noise effect compensation, or residual phase drift correction. This auxiliary processing can be used to mitigate phase noise coupling effects caused by dispersion compensation, channel equalization, or other digital signal processing, thereby improving the amplitude distribution, radial distribution, or shell separability of the jointly received symbols. It should be noted that this type of auxiliary processing is used to further improve system performance and does not change the core mechanism of this invention, which relies on information carried by common phase-invariant degrees of freedom and joint detection based on common phase-invariant decision metrics.

[0061] In one performance verification embodiment, the two-subcarrier four-dimensional joint modulation method was applied to a coherent optical communication transmission system. Results show that, even with laser phase noise, since common phase rotation only changes the common phase degree of freedom without altering the total joint symbol energy, the energy distribution relationship between the two subcarriers, or the relative phase relationship between the two subcarriers, the receiver can complete joint detection based on the common phase invariance decision metric. Furthermore, when the receiver incorporates phase equalization to enhance phase noise compensation, system performance can be further improved.

[0062] The above are preferred embodiments of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A phase recovery-free subcarrier joint modulation method, comprising the following steps: The digital signal or bit sequence to be transmitted is mapped into a joint modulation symbol carried by at least two digital subcarriers; The joint modulation symbol is represented as a high-dimensional complex vector, including a common phase degree of freedom and a common phase invariant degree of freedom; wherein, the common phase invariant degree of freedom remains unchanged when the joint modulation symbol undergoes a common phase rotation; The information of the digital signal or bit sequence to be transmitted is carried using the common phase-invariant degree of freedom. Multiplexing, optical modulation, and coherent transmission are performed on at least two digital subcarriers carrying the joint modulation symbols.

2. The method according to claim 1, characterized in that, The common phase invariant degree of freedom includes one of the following: the total energy of the joint modulation symbol, the energy distribution relationship between the at least two digital subcarriers, and the relative phase relationship between the at least two digital subcarriers.

3. The method according to claim 1, characterized in that, Under the common phase rotation, the joint modulation symbol is transformed into an equivalent symbol that has a different common phase but the same degree of freedom as the common phase; the digital signal or bit sequence to be transmitted is mapped to a common phase equivalence class composed of the equivalent symbols.

4. The method according to claim 1, 2, or 3, characterized in that, The number of the at least two digital subcarriers is K, where K is greater than or equal to 2; the joint modulation symbol is composed of complex modulation symbols carried by the K digital subcarriers respectively.

5. The method according to claim 1, 2, or 3, characterized in that, The at least two digital subcarriers are modulated by the same signal optical carrier and coherently received by the same local oscillator, so that the phase noise of the transmitting laser and / or the phase noise of the local oscillator laser are approximately common phase rotations on the at least two digital subcarriers.

6. The method according to claim 1, 2, or 3, characterized in that, The joint modulation symbols belong to a preset high-dimensional constellation set, and the constellation points in the high-dimensional constellation set are determined by the common phase-invariant degree of freedom; the high-dimensional constellation set is a multi-shell high-dimensional constellation set, a spherical high-dimensional constellation set, a hyperspherical high-dimensional constellation set, a lattice-type high-dimensional constellation set, or a codebook-type high-dimensional constellation set.

7. A modulation signal detection method according to claim 1, comprising the following steps: Obtain the received symbols corresponding to the at least two digital subcarriers and combine them into a joint received symbol; Based on candidate joint modulation symbols in a pre-defined high-dimensional constellation set, a common phase-invariant decision metric is calculated between the joint received symbol and the candidate joint modulation symbols; wherein... The information of the candidate joint modulation symbols is carried in the common phase-invariant degree of freedom; The transmitted joint modulation symbol is determined from the candidate joint modulation symbols based on the common phase invariance decision metric; The corresponding digital signal or bit sequence is recovered based on the determined joint modulation symbols.

8. The method according to claim 7, characterized in that, The common phase invariance decision metric is obtained by eliminating, optimizing, or marginalizing the unknown common phase between the joint received symbol and the candidate joint modulation symbol, so that the common phase invariance decision metric is independent of the common carrier phase. The common phase invariant decision metric includes a decision metric obtained based on one or more of the inner product modulus between the joint received symbol and the candidate joint modulated symbol, the distance minimization result, the likelihood function, or the posterior probability.

9. A phase recovery-free subcarrier joint modulation and detection device, characterized in that, Includes a transmitter processing module and a receiver processing module; The transmitter processing module includes a bit packetization module, a subcarrier joint mapping module, and a subcarrier multiplexing module; The bit grouping module is used to group the digital signal or bit sequence to be transmitted; The subcarrier joint mapping module is used to map the grouped digital signal or bit sequence into a joint modulation symbol jointly carried by at least two digital subcarriers. The joint modulation symbol includes a common phase degree of freedom and a common phase invariant degree of freedom. The common phase invariant degree of freedom remains unchanged when the common phase of the joint modulation symbol is rotated. The information of the digital signal or bit sequence to be transmitted is carried by the common phase invariant degree of freedom. The subcarrier multiplexing module is used to multiplex at least two digital subcarriers carrying the joint modulation symbol to generate an electrical domain modulation signal to be optically modulated. The receiving end processing module includes a joint receiving symbol construction module, a common phase invariant joint detection module, and a bit recovery module; The joint reception symbol construction module is used to construct joint reception symbols based on the received signals corresponding to at least two digital subcarriers. The common phase invariant joint detection module is used to perform joint detection on the joint received symbols based on the common phase invariant decision metric. The bit recovery module is used to recover the digital signal or bit sequence to be transmitted based on the result of the joint detection.

10. A coherent optical communication system, comprising a transmitter, an optical fiber transmission link, and a receiver; The transmitting end is used to map the digital signal or bit sequence to be transmitted into a joint modulation symbol carried by at least two digital subcarriers; the joint modulation symbol includes a common phase degree of freedom and a common phase invariant degree of freedom; wherein, The common phase invariant degree of freedom remains unchanged when the joint modulation symbol undergoes a common phase rotation; the common phase invariant degree of freedom is used to carry the information of the digital signal or bit sequence to be transmitted; The optical fiber transmission link is used to transmit optical signals carrying the joint modulation symbols; The receiving end is configured to construct a joint receiving symbol based on the received signals corresponding to at least two digital subcarriers, and to perform joint detection on the joint receiving symbol based on a common phase invariance decision metric to recover the digital signal or bit sequence to be transmitted.

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