A sp-2048-qam modulation method based on stage-by-stage clipping shaping

Abstract

The application discloses a kind of SP-2048-QAM modulation methods based on stage truncation shaping, specifically: pre-processing is carried out to signal at transmitting end, from the angle of gradually intercepting signal point, pseudo-random bit sequence is shaped by stage truncation, to realize the step-by-step adjustment of signal point modulus and distribution;Subsequently, shaping is combined with four-dimensional modulation, forming a series of intermediate modulation formats;At receiving end, optical signal is detected after being amplified again, the analog signal detected is converted into digital signal by analog-digital converter, and finally corresponding signal equalization, compensation, demapping decision operation is carried out by digital processing algorithm module, and the original data is recovered.The application improves the transmission performance of system, reduces the bit error rate, enhances the adaptive ability of system, so that it can be adjusted signal entropy value flexibly according to the changing channel condition and noise environment, to ensure the stability and reliability of communication.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber communication, and in particular relates to an SP-2048-QAM modulation method based on staged truncation and shaping. Background Technology

[0002] The rapid development of emerging businesses such as big data, the Internet of Things, and virtual reality in recent years has led to an increased demand for high-capacity, high-spectral-efficiency optical transmission. Probabilistic shaping, a modulation optimization technique, offers advantages such as high transmission capacity. It can improve the spectral efficiency of a system without increasing transmission power, thus achieving higher transmission capacity. It has been widely studied in recent years and is gradually becoming a promising new technology. However, as research into probabilistic shaping has deepened, researchers have discovered that in probabilistic shaping systems with significant shaping depth, the digital signal processing algorithm at the receiver exhibits insufficient capability in impairment compensation, resulting in degraded or even unrecoverable signal quality after compensation. To address this issue, academia and industry have contributed two different solutions. One approach involves modifying the digital signal processing (DSP) algorithm itself to improve its robustness in probabilistic shaping systems with varying shaping depths. For example, Beijing University of Posts and Telecommunications proposed a quasi-MB distributed modulation technique. By altering the symbol mapping rules, it can increase the number of constellation points used for phase noise estimation to a limited extent, thereby achieving a carrier phase recovery-friendly probabilistic shaping technique. Huazhong University of Science and Technology has proposed a probability-guided blind phase search algorithm, which improves the carrier phase recovery capability of traditional blind phase search algorithms in probabilistic shaping systems with large shaping depth.

[0003] Another approach is to develop advanced probabilistic shaping modulation schemes based on probabilistic shaping techniques, thereby enhancing the compensation capabilities of the receiver's DSP algorithm. Following this second research approach, the academic community has proposed several probabilistic shaping schemes, among which truncated probabilistic shaping (TPS) is a typical technique. This technique reduces the modulation dimension of the constellation points by forcing the outer circle probability of higher-order modulation formats to zero, effectively reducing the shaping depth and improving the performance of the DSP algorithm. In recent years, TPS technology has been widely researched and applied. In 2022, Fudan University achieved 2000km optical transmission with a single wavelength of 800-Gbps and a total capacity of 4-Tbps based on TPS-64QAM modulation, using Raman-amplified G.654E fiber and a multi-input multi-output Volterra equalizer. In 2023, Bell Labs achieved a 226-GBaud truncated probabilistic shading 256-QAM signal with a single-wavelength net data rate of 2.4-Tbps.

[0004] Although probabilistic truncation shaping (TPS) technology can significantly reduce average transmit power and increase system capacity and transmission distance, it also leads to a significant reduction in signal entropy, which has an adverse effect on system performance.

[0005] In the pursuit of a balance between spectral efficiency and asymptotic power efficiency, four-dimensional modulation schemes have attracted significant attention from researchers both domestically and internationally due to their unique advantages. This modulation scheme forms a four-dimensional modulation space by simultaneously modulating in-phase and quadrature components on two independently orthogonally polarized optical carriers. This not only simplifies the implementation process but also increases the minimum Euclidean distance between constellation points, thus possessing significant research value in the field of optical fiber communication. Therefore, combining a staged truncation and shaping scheme with four-dimensional modulation technology has profound implications for the research and development of optical fiber communication systems. Summary of the Invention

[0006] To address the above problems, this invention provides an SP-2048-QAM modulation method based on staged truncation and shaping.

[0007] The present invention provides an SP-2048-QAM modulation method based on staged truncation and shaping, comprising the following steps:

[0008] Step 1: Perform four-dimensional modulation on the information bits.

[0009] An XOR operation is performed on the initial 11 information bits to generate the 12th parity check bit. Through parity diversity, the X polarization and Y polarization are correlated, and the original signal points are divided into two parts, which increases the minimum Euclidean distance between constellation points to √2 times the original value, thereby realizing four-dimensional modulation.

[0010] Step 2: Extract signal points within specific modulus rings layer by layer to form a series of intermediate modulation formats between the SP-2048-QAM signal and the TPS-SP-2048-QAM signal.

[0011] The SP-2048-QAM signal consists of nine modulus rings, each corresponding to a specific set of signal points. Starting from the outermost modulus ring, signal points are extracted layer by layer inward to generate TPS-4, TPS-12, and TPS-20 signals.

[0012] Step 3: Shape the generated signal using a constant parametric distributed matched network (CCDM). The specific formula for constellation point distribution is as follows:

[0013]

[0014] In the formula: X is the set of constellation points, x i Let be the complex coordinates of the i-th constellation point, λ≥0 be a free parameter, and M be the number of constellation points.

[0015] When λ = 0, the distribution is uniform. The larger λ is, the greater the difference in probability between different energy levels, that is, the lower the energy of the constellation point, the greater the probability of being assigned to it. Given λ, the corresponding P is obtained according to equation (1). X (x i Then, the information entropy H is calculated, and the specific formula is as follows:

[0016]

[0017] The degree of non-uniformity in the probability distribution is measured using the shaping depth, and the specific formula is as follows:

[0018]

[0019] Step 4: Upsample the modulated signal by four times, and then use a root-raised cosine filter with a roll-off factor of 0.01 to perform pulse shaping. The specific formula for the filter is as follows:

[0020]

[0021] In the formula: T is the symbol period, and α is the roll-off factor, which ranges from 0 to 1.

[0022] Step 5: The optical modulator modulates the signal onto the optical domain and transmits it through standard single-mode fiber (SSMF). To enhance the signal, it passes through an erbium-doped fiber amplifier (EDFA) with a noise figure of 4.5 dB; and to suppress out-of-band noise, it passes through an optical filter (OBPF).

[0023] Step 6: At the receiving end, the optical signal is amplified again and then coherently detected. The detected analog signal is converted into a digital signal by an analog-to-digital converter. Finally, the digital processing algorithm module performs corresponding signal equalization, compensation, and demapping decision operations to recover the original data.

[0024] The beneficial technical effects of this invention compared to the prior art are as follows:

[0025] This invention improves the transmission performance of the system by combining staged truncation shaping with four-dimensional modulation: on the one hand, it reduces the bit error rate by increasing the Euclidean distance between signal points in the constellation diagram; on the other hand, it enhances the system's adaptability, enabling it to flexibly adjust the signal entropy value in response to changing channel conditions and noise environments, thus ensuring the stability and reliability of communication. Attached Figure Description

[0026] Figure 1 This is a block diagram of the transmission system of the present invention.

[0027] Figure 2 This is a schematic diagram of the process structure of the present invention.

[0028] Figure 3 This is a constellation diagram of the intermediate modulation format of this invention.

[0029] Figure 4 The bit error rate performance of each modulated signal at different transmit powers when transmitting over a distance of 1440 km is shown. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0031] The present invention provides an SP-2048-QAM modulation method based on staged truncation and shaping, comprising the following steps:

[0032] Step 1: Perform four-dimensional modulation on the information bits.

[0033] An XOR operation is performed on the initial 11 information bits to generate the 12th parity check bit. Through parity diversity, the X polarization and Y polarization are correlated, and the original signal points are divided into two parts, which increases the minimum Euclidean distance between constellation points to √2 times the original value, thereby realizing four-dimensional modulation.

[0034] Step 2: Extract signal points within specific modulus rings layer by layer to form a series of intermediate modulation formats between the SP-2048-QAM signal and the TPS-SP-2048-QAM signal.

[0035] The SP-2048-QAM signal consists of nine modulus rings, each corresponding to a specific set of signal points. Starting from the outermost modulus ring, signal points are extracted layer by layer inward to generate TPS-4, TPS-12, and TPS-20 signals.

[0036] Step 3: Shape the generated signal using a constant parametric distributed matched network (CCDM). The specific formula for constellation point distribution is as follows:

[0037]

[0038] In the formula: X is the set of constellation points, x i Let be the complex coordinates of the i-th constellation point, λ≥0 be a free parameter, and M be the number of constellation points.

[0039] When λ = 0, the distribution is uniform. The larger λ is, the greater the difference in probability between different energy levels, that is, the lower the energy of the constellation point, the greater the probability of being assigned to it. Given λ, the corresponding P is obtained according to equation (1). X (x i Then, the information entropy H is calculated, and the specific formula is as follows:

[0040]

[0041] The degree of non-uniformity in the probability distribution is measured using the shaping depth, and the specific formula is as follows:

[0042]

[0043] Step 4: Upsample the modulated signal by four times, and then use a root-raised cosine filter with a roll-off factor of 0.01 to perform pulse shaping. The specific formula for the filter is as follows:

[0044]

[0045] In the formula: T is the symbol period, and α is the roll-off factor, which ranges from 0 to 1.

[0046] Step 5: The optical modulator modulates the signal onto the optical domain and transmits it through standard single-mode fiber (SSMF). To enhance the signal, it passes through an erbium-doped fiber amplifier (EDFA) with a noise figure of 4.5 dB; and to suppress out-of-band noise, it passes through an optical filter (OBPF).

[0047] Step 6: At the receiving end, the optical signal is amplified again and then coherently detected. The detected analog signal is converted into a digital signal by an analog-to-digital converter. Finally, the digital processing algorithm module performs corresponding signal equalization, compensation, and demapping decision operations to recover the original data.

[0048] Figure 1 The block diagram of the SP-2048-QAM modulation method based on staged truncation and shaping of the present invention is shown. It mainly consists of three parts: 1) SP-2048-QAM transmitter; the signal source mainly consists of signal preprocessing (101), digital-to-analog converter (102), signal light source (103), and IQ modulator (104); 2) Fiber optic transmission link; firstly, through N segments of standard single-mode fiber (1051~105... N The data is transmitted via optical amplifier (1061~106). N Compensation was performed, followed by the use of a tunable filter (1071~107). N ) Optical filtering is performed to suppress spontaneous emission noise generated outside the band; 3) The receiving end coherent detection digital signal processing part; finally, it enters the coherent receiver (108) for corresponding analog-to-digital conversion to obtain digital signals, and finally, the digital processing algorithm module (109) performs corresponding signal equalization, compensation, and demapping decision operations. The present invention combines staged truncation and shaping with four-dimensional modulation in signal preprocessing.

[0049] The specific process of this invention is as follows: Figure 2As shown, in the signal preprocessing stage (101), the first 11 bits of information are XORed to generate the 12th parity bit. These 12 bits are then converted into a four-dimensional SP-2048QAM signal via bit mapping. Next, specific points are selected from the X and Y polarizations based on the modulus value and truncated to form the truncated SP-2048QAM signal. Finally, these truncated signals, along with the MB probability distribution, are input into the CCDM module to obtain the truncated and shaped SP-2048QAM signal.

[0050] Figure 3 This is a constellation diagram of the intermediate modulation format of this invention. By progressively extracting signal points within specific modulus rings, a series of intermediate modulation formats are gradually formed between the 64QAM signal and the TPS-64QAM signal, including TPS-4, TPS-12, and TPS-20 signals. Specifically, the 64QAM signal consists of nine modulus rings, each corresponding to a specific set of signal points. The scheme starts from the outermost modulus ring and progressively extracts signal points inwards, thereby generating TPS-4, TPS-12, and TPS-20 signals. Figure 3 Figures (a), (b), and (c) show the constellation diagrams of TPS-4, TPS-12, and TPS-20 signals, respectively, where X represents the truncated signal and magnitude loop, and the dots represent the retained signal and magnitude loop. Through this multi-stage truncation method, this invention achieves a smooth transition of entropy values ​​from 64QAM to TPS-64QAM. Each stage of truncation affects only a small number of signal points, effectively avoiding a sharp drop in system entropy.

[0051] Figure 4 This study examines the bit error rate (BER) performance of various modulated signals at different transmit powers when transmitting over a distance of 1440 km at the same information rate. Notably, to ensure a consistent information rate for all modulated signals, symbol rates were set to 28 GHz, 30.5 GHz, 31.1 GHz, 32.3 GHz, 33.9 GHz, and 36 GHz, respectively. The results show that as the degree of truncation increases, the signal entropy gradually decreases, leading to improved transmission performance. Specifically, within the transmit power range of -1 dBm to 3 dBm, signals with lower entropy exhibit lower BER and better transmission performance. It is noteworthy that the optimal transmit power of the truncated and shaped four-dimensional signal is approximately 1.0 dBm higher than that of PDM-64QAM, further highlighting its advantages in nonlinear performance.

Claims

1. A SP-2048-QAM modulation method based on staged truncation and shaping, characterized in that, Includes the following steps: Step 1: Perform four-dimensional modulation on the information bits; The initial 11 information bits are XORed to generate the 12th parity check bit. Through parity diversity, the X polarization and Y polarization are correlated, and the original signal points are divided into two parts, which increases the minimum Euclidean distance between constellation points to √2 times the original value, thereby realizing four-dimensional modulation. Step 2: Extract signal points within a specific modulus circle layer by layer to form a series of intermediate modulation formats between the SP-2048-QAM signal and the TPS-SP-2048-QAM signal; The SP-2048-QAM signal consists of nine modulus rings, each of which corresponds to a specific set of signal points. Starting from the outermost modulus ring, signal points are extracted layer by layer inward to generate TPS-4, TPS-12 and TPS-20 signals. Step 3: Shape the generated signal using a constant parametric distributed matched network (CCDM). The specific formula for constellation point distribution is as follows: In the formula: X is the set of constellation points, x i Let be the complex coordinates of the i-th constellation point, λ≥0 be a free parameter, and M be the number of constellation points; When λ = 0, the distribution is uniform. The larger λ is, the greater the difference in probability between different energy levels, that is, the lower the energy of the constellation point, the greater the probability of being assigned to it. Given λ, the corresponding P is obtained according to equation (1). X (x i Then, the information entropy H is calculated, and the specific formula is as follows: The degree of non-uniformity in the probability distribution is measured using the shaping depth, and the specific formula is as follows: Step 4: Upsample the modulated signal by four times, and then use a root-raised cosine filter with a roll-off factor of 0.01 to perform pulse shaping. The specific formula for the filter is as follows: In the formula: T is the symbol period, and α is the roll-off factor, which ranges from 0 to 1; Step 5: The optical modulator modulates the signal onto the optical domain and transmits it through a standard single-mode fiber. To enhance the signal, it passes through an erbium-doped fiber amplifier with a noise figure of 4.5 dB; to suppress out-of-band noise, it passes through an optical filter. Step 6: At the receiving end, the optical signal is amplified again and then coherently detected. The detected analog signal is converted into a digital signal by an analog-to-digital converter. Finally, the digital processing algorithm module performs corresponding signal equalization, compensation, and demapping decision operations to recover the original data.

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