US-12627373-B2 - Equalization apparatus, optical communication system, and equalizing method of optical signals

US12627373B2US 12627373 B2US12627373 B2US 12627373B2US-12627373-B2

Abstract

Channel monitoring means monitors channel quality information of a plurality of optical signals of a plurality of channels transmitted from a transmitting apparatus and received by a receiving apparatus, and outputting first information including the monitored channel quality information, the transmitting apparatus processing a plurality of data signals to transform them into a plurality of mixed data signals using a transfer matrix and converting the plurality of mixed signals into the plurality of optical signals. Estimation means estimates transmission a first objective between the transmitting apparatus and the receiving apparatus based on the first information and second information received from the transmitting apparatus, and outputting an estimation result. Matrix configurating means configures the transfer matrix based on the estimation result to optimize the first objective between the transmitting apparatus and the receiving apparatus, and outputting the configured transfer matrix to the transmitting apparatus for updating the transfer matrix therein.

Inventors

Mingqi Wu

Assignees

NEC CORPORATION

Dates

Publication Date: 20260512
Application Date: 20231221
Priority Date: 20221228

Claims (17)

1 . An equalization apparatus comprising: channel monitoring means for monitoring channel quality information of a plurality of optical signals of a plurality of channels transmitted from a transmitting apparatus and received by a receiving apparatus, and outputting first information including the monitored channel quality information, the transmitting apparatus processing a plurality of data signals to transform them into a plurality of mixed data signals using a transfer matrix and converting the plurality of mixed signals into the plurality of optical signals; estimation means for estimating transmission a first objective between the transmitting apparatus and the receiving apparatus based on the first information and second information received from the transmitting apparatus, and outputting an estimation result; and matrix configurating means for configuring the transfer matrix based on the estimation result to optimize the first objective between the transmitting apparatus and the receiving apparatus, and outputting the configured transfer matrix to the transmitting apparatus for updating the transfer matrix therein.
2 . The equalization apparatus according to claim 1 , wherein the first objective is a performance factor that includes one of a transmission capacity, mutual information, and a spectral efficiency.
3 . The equalization apparatus according to claim 1 , wherein the channel quality information of each channel includes one of a noise power level, a signal to noise ratio, an optical signal to noise ratio, a bit error rate and an error vector magnitude.
4 . The equalization apparatus according to claim 3 , wherein the second information includes at least one of a forward error coding rate, a pulse shaping type, a roll-off factor, a symbol rate, an oversampling rate, a channel spacing, a polarization type, a shaping factor when the signal is probabilistically constellation shaped, a geometric mapping when the signal is geometric constellation shaped.
5 . The equalization apparatus according to claim 1 , wherein the receiving apparatus converts the plurality of optical signals into the plurality of mixed data signals and processes the plurality of mixed data signals to transform them into the plurality of data signals by using an inverse transfer matrix that is an inverse of the transfer matrix, the estimation means estimates the first objective between the transmitting apparatus and the receiving apparatus based on the first and second information, and third information received from the receiving apparatus, and outputs the estimation result; and the matrix configuration means configures the inverse transfer matrix based on the first to third information to optimize the first objective between the transmitting apparatus and the receiving apparatus, and outputs the configured inverse transfer matrix to the receiving apparatus for updating the inverse transfer matrix therein.
6 . The equalization apparatus according to claim 5 , wherein the transfer matrix is a square matrix having the same number of columns and rows as the number of the plurality of channels, and the square matrix has a first configuration and a second configuration switched by the matrix configuration means according to a predetermined rule.
7 . The equalization apparatus according to claim 6 , wherein the first configuration of the transfer matrix is an identity matrix, and the second configuration of the transfer matrix is configured to achieve more signal power to a channel with lower channel quality.
8 . The equalization apparatus according to claim 7 , wherein the second configuration H 2 of the transfer matrix is an element-wise product of an N×N complex Hadamard matrix F 2 and an N×N coefficient matrix K 2 as described by the following equation: [Equation 1] H ⁢ 2 = F ⁢ 2 ∘ K ⁢ 2 = F ⁢ 2 ∘ [ σ 1 σ 1 + … + σ N … σ 1 σ 1 + … + σ N ⋮ ⋱ ⋮ σ N σ 1 + … + σ N … σ N σ 1 + … + σ N ] [ 1 ] where N denotes the number of the plurality of channels, and when j denotes a column number and k denotes a row number, an element F jk of the complex Hadamard matrix F 2 for j, k=1, 2, . . . , N is described by the following Equation: [Equation 2] f jk = exp [ 2 ⁢ π ⁢ i ⁡ ( j - 1 ) ⁢ ( k - 1 ) N ] [ 2 ]
9 . The equalization apparatus according to claim 7 , wherein, when i denotes a channel number and noise power measured in a channel Ci is represented as NPi, the coefficient matrix K 2 may be determined by the following Equation: [Equation 17] σ i = NPi [ 3 ] or when a signal to noise ratio measured in the channel Ci is represented as SNRi, the coefficient matrix K 2 may be determined by the following Equation: [Equation 18] σ i = 1 SNRi [ 4 ]
10 . The equalization apparatus according to claim 7 , wherein the estimation means estimates an indicator of transmission system with the first configuration and the second configuration, and the indicator includes one of a capacity, a spectral efficiency, mutual information, and a data rate.
11 . The equalization apparatus according to claim 5 , wherein the matrix configurating means determines the transfer matrix based on the estimation result, and the configuration with higher indicator from the estimation means is configured for the transfer matrix.
12 . The equalization apparatus according to claim 11 , wherein the estimation means estimates the indicator of a transmission system for the first configuration and the second configuration using the first and second information.
13 . The equalization apparatus according to claim 5 , wherein the estimation means: refers to information indicating channel quality to obtain desired equalized channel quality according to the first to third information, refers to information indicating a desired first objective corresponding to the desired equalized channel quality to obtain the desired first objective, and outputs the estimation result based on the desired first objective.
14 . The equalization apparatus according to claim 13 , wherein the information indicating channel quality and the information indicating the desired first objective are provided as predetermined look-up tables.
15 . The equalization apparatus according to claim 13 , wherein the information indicating channel quality and the information indicating the desired first objective as predetermined simulation models.
16 . An optical communication system comprising: a transmitting apparatus configured to process a plurality of data signals to transform them into a plurality of mixed data signals using a transfer matrix and to convert the plurality of mixed signals into a plurality of optical signals; a receiving apparatus configured to receive the plurality of optical signals transmitted from the transmitting apparatus; and an equalization apparatus, wherein the equalization apparatus comprises: channel monitoring means for monitoring channel quality information of the plurality of optical signals of a plurality of channels transmitted from the transmitting apparatus and received by the receiving apparatus, and outputting first information including the monitored channel quality information; estimation means for estimating a first objective between the transmitting apparatus and the receiving apparatus based on the first information and second information received from the transmitting apparatus, and outputting an estimation result; and matrix configurating means for configuring the transfer matrix based on the estimation result to optimize the first objective between the transmitting apparatus and the receiving apparatus, and outputting the configured transfer matrix to the transmitting apparatus for updating the transfer matrix therein.
17 . An equalizing method of optical signals comprising: monitoring channel quality information of a plurality of optical signals of a plurality of channels transmitted from a transmitting apparatus and received by a receiving apparatus, and outputting first information including the monitored channel quality information, the transmitting apparatus processing a plurality of data signals to transform them into a plurality of mixed data signals using a transfer matrix and converting the plurality of mixed signals into the plurality of optical signals; estimating a first objective between the transmitting apparatus and the receiving apparatus based on the first information and second information received from the transmitting apparatus, and outputting an estimation result; and configuring the transfer matrix based on the estimation result to optimize the first objective between the transmitting apparatus and the receiving apparatus, and outputting the configured transfer matrix to the transmitting apparatus for updating the transfer matrix therein.

Description

INCORPORATION BY REFERENCE This application is based upon and claims the benefit of priority from Japanese patent application No. 2022-211666, filed on Dec. 28, 2022, the disclosure of which is incorporated herein in its entirety by reference. TECHNICAL FIELD The present disclosure relates to an equalization apparatus, an optical communication system, and an equalizing method of optical signals. BACKGROUND ART To meet the increasing demand for higher communication capacity in optical fiber transmission, technology development leads to multiplexing techniques in many perspectives of resources such as wavelength division multiplexing (WDM) and space division multiplexing (SDM). Signal modulation techniques such as high-order quadrature amplitude modulation (QAM), probabilistic constellation shaping (PCS), and geometric constellation shaping (GCS) are also developed to approach the Shannon limit. When multiplexing technique is applied, signals are transmitted simultaneously in a plurality of channels. In transmission using the WDM, channels are frequency bands and corresponding elements during transmission. In transmission using the SDM, channels are cores in a multi-core fiber (MCF) and corresponding elements during transmission. In particular, when the SDM is used with an uncoupled MCF, performance differences between channels occur due to a plurality of causes. Additionally, the performance differences accumulate in long-distance transmission due to the noise figure and the gain control by Erbium-Doped Fiber Amplifiers (EDFA) in repeaters. The performance distance becomes the bottleneck of the overall capacity of the SDM transmission. Thus, it is necessary to eliminate the performance differences between channels in multiplexing transmission. It is noted that the performance differences can be quantified by a plurality of indicators such as Quality factor (Q-factor), signal-to-noise ratio (SNR), bit error rate (BER), and error vector magnitude (EVM). Japanese Patent No. 6786404 discloses an equalization method that can eliminate Q-factor differences between channels in SDM transmission with an uncoupled MCF. On the transmitter side, two channels of data vectors are equally separated and mixed by a matrix before transmitting. On the receiver side, two channels of restored vectors are equivalent to suffering the average of distortions from both cores during the transmission. As a result, the Q-factor difference between two channels can be eliminated. Although the method disclosed in PTL1 can eliminate the Q-factor difference in the SDM, the disclosed transfer matrix introduces a penalty to the channels. This is due to the equal separation and mixing of signal vectors. International Patent Application No. PCT/JP2021/022600 discloses another equalization method that can eliminate the Q-factor difference between channels in SDM transmission with an uncoupled MCF. Compared to PTL1, a channel monitor is placed on the receiver side and the coefficients of matrix are determined by channel condition information from the channel monitor. As a result, the Q-factor difference can be eliminated with lower penalty compared with PTL1. Further other equalization methods have been also proposed in International Patent Application Publication No. WO 2021/019620, Japanese Unexamined Patent Application Publication No. 2020-136831, and Published Japanese Translation of PCT International Publication for Patent Application, No. 2016-519857, for example. SUMMARY The abovementioned methods can eliminate the Q-factor difference in the SDM to improve overall capacity in transmission using a conventional QAM signal. However, in a transmission system using constellation-shaped signals, these methods may not be able to maximize total spectral efficiency (SE). In other words, the total transmission capacity may decrease with these methods when the constellation-shaped signals are used in the transmission. The reason for a decrease in overall capacity will be explained as follows. Traditional coherent signal modulation based on QAM has limitations related to granularity and flexibility in transmission. To address these limitations while pushing the limit closer to the Shannon limit, constellation shaping technologies have been developed and commercialized including PCS and GCS. In QAM, each constellation point has the same probability of being used. In PCS, lower-energy constellation points are more frequently used, and higher-energy constellation points are less frequently used. The probability distribution is determined and realized by an encoder. By configuring the shape of probability distribution in the encoder, PCS can provide closer increments of data rate in transmission than traditional QAM. Considering a two-channel SDM transmission system, huge benefits in data rate can be achieved from PCS in the better-quality channel before equalization. The data rate merit may overtake the merit of equalizing the better channel and worse cha