Digital subcarrier multiplexing coherent optical communication system and method

Abstract

The invention relates to the technical field of coherent light communication, and provides a digital subcarrier multiplexing coherent light communication system and method, the system divides a single carrier signal into a plurality of subcarrier signals for respective modulation, and the system comprises a transmitting module, a channel unit and a receiving module; the transmitting module is used for modulating and transmitting a subcarrier multiplexing signal based on a DSP (Digital Signal Processor) algorithm, and comprises a wide-area complex recursive least square algorithm; the transmitting module performs hybrid modulation on the plurality of subcarrier signals; half of the subcarrier signals are configured with preposed pilots. According to the invention, based on the wide-area complex number recursive least square algorithm, the subcarrier signal is converted from a real number to a wide-area complex number, a significant modulation effect is still achieved under high-frequency attenuation, and the bit error rate performance is improved; the transmitting module performs hybrid modulation on a plurality of subcarrier signals, and configures different code rates of forward error correction codes according to the order of each subcarrier signal modulation format, so that the spectrum efficiency is improved; and only half of the subcarrier signals need to be configured with the preposed pilot frequency, so that the pilot frequency overhead is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of coherent optical communication, and in particular to a digital subcarrier multiplexing coherent optical communication system and method. Background Art

[0002] In the field of optical communications, digital subcarrier multiplexing (DSCM) technology plays an important role and its applications cover many aspects.

[0003] DSCM technology significantly improves the spectrum utilization of optical fiber communication systems by dividing optical signals into multiple sub-signals and transmitting them at different frequencies, allowing more data to be transmitted within limited spectrum resources. The DSCM system independently modulates each sub-carrier and configures the modulation order, shaping parameters, and forward error correction (FEC) code rate accordingly to meet different user traffic requirements.

[0004] However, since the signal is transmitted within a limited bandwidth, the system will cause severe high-frequency attenuation, causing the signal-to-noise ratio (SNR) of each subcarrier to vary greatly, severely reducing the effect of the traditional real domain equalizer. Summary of the invention

[0005] To this end, the technical problem to be solved by the present invention is that the optical communication system in the prior art has the problem of real domain equalizer modulation failure under high-frequency attenuation. A digital subcarrier multiplexing coherent optical communication system and method are provided, which still have significant modulation effect under high-frequency attenuation, improve the error performance and spectrum efficiency, and reduce the pilot overhead.

[0006] In a first aspect, in order to solve the above technical problems, the present invention provides a digital subcarrier multiplexing coherent optical communication system, which divides a single carrier signal into multiple subcarrier signals for modulation respectively, and the coherent optical communication system comprises:

[0007] A transmitting module, which modulates and transmits a digital subcarrier multiplexed signal based on a DSP algorithm;

[0008] A channel unit, which is used to transmit the digital subcarrier multiplexed signal;

[0009] A receiving module, used for receiving and demodulating the digital subcarrier multiplexed signal;

[0010] Among them, the DSP algorithm includes a wide-area complex recursive least squares algorithm, which is used to convert the subcarrier signal from a real number to a wide-area complex number; the transmitting module performs mixed modulation on the multiple subcarrier signals, and configures different forward error correction code rates according to the order of the modulation format of each subcarrier signal; the transmitting module configures half of the multiple subcarrier signals with a pre-pilot, and the subcarrier signal configured with the pre-pilot and the subcarrier signal not configured with the pre-pilot have a symmetrical relationship in waveform; the subcarrier signals are multiplexed to obtain the digital subcarrier multiplexed signal.

[0011] In one embodiment of the present invention, the tap coefficients of the wide-area linear model are iterated based on the wide-area complex recursive least squares algorithm; the tap coefficients of the wide-area linear model are twice the tap coefficients of the adaptive filter;

[0012] The exponentially weighted least squares cost function in the iterative process for:

[0013] ;

[0014] in, is the output signal of the wide area linear model, p(k) is the pilot symbol, λ is the forgetting factor, n represents the nth iteration, and k represents the time k.

[0015] In one embodiment of the present invention, the wide-area complex recursive least squares algorithm is combined with a recursive least squares algorithm, and its tap update formula is:

[0016] ;

[0017] ;

[0018] ;

[0019] ;

[0020] in, are the tap coefficients of the wide area linear model, a(n) is the gain vector, P(n) is the covariance matrix of the input signal, and I is the identity matrix; , is the input signal at time k, For the input signal conjugate signal.

[0021] In one embodiment of the present invention, the modulation process of the transmitting module includes performing QAM mapping and subcarrier frequency domain addition;

[0022] After the QAM mapping is performed, a QAM signal is obtained, and after the power of the QAM signal is normalized, the subcarrier frequency domain addition is performed to keep the frequency spectrum of the subcarrier signal at the same height.

[0023] In one embodiment of the present invention, the order of the QAM signal is positively correlated with the spectrum efficiency and data rate of the coherent optical communication system.

[0024] In one embodiment of the present invention, the modulation process of the transmitting module further includes probability shaping; the QAM signal conforms to the Maxwell-Boltzmann distribution after the probability shaping.

[0025] In one embodiment of the present invention, the forward error correction code is SD-LDPC coding based on the DVB-S.2 standard, and its code rate is 9 / 10 or 4 / 5; the SD-LDPC coding with a code rate of 4 / 5 is applied to the subcarrier signal of the high-order modulation format, and the SD-LDPC coding with a code rate of 9 / 10 is applied to the subcarrier signal of the low-order modulation format.

[0026] In one embodiment of the present invention, the pilot ratio of the coherent optical communication system is 5%.

[0027] In one embodiment of the present invention, the algorithms used by the receiving module include a polarization demultiplexing algorithm, a matched filtering algorithm, a carrier recovery algorithm and an equalization algorithm.

[0028] In a second aspect, in order to solve the above technical problems, the present invention also provides a digital subcarrier multiplexing coherent optical communication method, which divides a single carrier signal into multiple subcarrier signals and modulates them separately. The coherent optical communication method includes:

[0029] Step S1, modulating and transmitting a digital subcarrier multiplexed signal based on a DSP algorithm;

[0030] Step S2, transmitting the digital subcarrier multiplexed signal;

[0031] Step S3, receiving and demodulating the digital subcarrier multiplexed signal;

[0032] Among them, the DSP algorithm includes a wide-area complex recursive least squares algorithm, which is used to convert the subcarrier signal from a real number to a wide-area complex number; the step S1 performs mixed modulation on the multiple subcarrier signals, and configures different forward error correction code rates according to the order of the modulation format of each subcarrier signal; the step S1 configures half of the multiple subcarrier signals with a pre-pilot, and the subcarrier signal configured with the pre-pilot and the subcarrier signal not configured with the pre-pilot have a symmetrical relationship in waveform; the subcarrier signals are multiplexed to obtain the digital subcarrier multiplexed signal.

[0033] The above technical solution of the present invention has the following beneficial effects compared with the prior art:

[0034] The present invention provides a digital subcarrier multiplexing coherent optical communication system and method. Based on a wide-area complex recursive least squares algorithm, an input sequence is converted from a real number to a wide-area complex number, and a significant modulation effect is still achieved under high-frequency attenuation, thereby improving the error code performance. The subcarrier signals are mixed modulated, and different forward error correction code rates are configured according to the order of the subcarrier signal modulation format, thereby improving the spectrum efficiency. Only half of the subcarrier signals need to be configured with a pre-pilot, thereby reducing the pilot overhead. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to make the contents of the present invention more clearly understood, the present invention is further described in detail below based on specific embodiments of the present invention in conjunction with the accompanying drawings.

[0036] Figure 1 It is a module diagram of a digital subcarrier multiplexing coherent optical communication system in an embodiment of the present invention;

[0037] Figure 2 Flow chart of the DSP algorithm of the transmitting end of the DSCM system in an embodiment of the present invention;

[0038] Figure 3 Spectrum diagrams of signals with 1, 2, 4 and 8 subcarriers respectively under the same bandwidth in an embodiment of the present invention;

[0039] Figure 4 Flow chart of the DSP algorithm of the receiving end of the DSCM system in an embodiment of the present invention;

[0040] Figure 5 A structural block diagram of a DSCM signal generation structure with a modulation format of 4-16-16-4QAM in an embodiment of the present invention;

[0041] Figure 6 It is a structural framework diagram of the pre-pilot in the four-subcarrier multiplexed signal in an embodiment of the present invention;

[0042] Figure 7 It is a simulation structure diagram of a single polarization AWGN channel in an embodiment of the present invention;

[0043] Figure 8 Graph showing the relationship between the number of iteration symbols and mean square error of three iterative algorithms in an embodiment of the present invention;

[0044] Fig. 9 DSCM signal constellation diagrams restored by real RLS algorithm and wide-area complex RLS algorithm respectively in the embodiment of the present invention;

[0045] Fig.10 Result diagram of MSE and constellation diagram comparison using three iterative algorithms for an X-polarized subcarrier in an embodiment of the present invention;

[0046] Fig.11 is a graph showing the variation of BER with SNR of a four-subcarrier multiplexing signal in an embodiment of the present invention;

[0047] Fig.12 NGMI threshold diagrams of SD-LDPC with two code rates in an embodiment of the present invention;

[0048] Fig.13 is a curve diagram showing the change of NGMI of a four-subcarrier multiplexing signal with a shaping parameter β in an embodiment of the present invention;

[0049] Fig.14 It is a curve diagram showing the variation of the maximum SE of the transmission channels of the DSCM system with pilot optimization and the DSCM system without pilot optimization with SNR in the embodiment of the present invention. DETAILED DESCRIPTION

[0050] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0051] In the field of optical communications, digital subcarrier multiplexing (DSCM) technology significantly improves the spectrum utilization of optical fiber communication systems by dividing optical signals into multiple sub-signals and transmitting them at different frequencies, allowing more data to be transmitted within limited spectrum resources. The DSCM system independently modulates each subcarrier and configures the modulation order, shaping parameters, and forward error correction code (FEC) rate accordingly to meet different user traffic requirements.

[0052] However, since the signal is transmitted within a limited bandwidth, the system will cause severe high-frequency attenuation, causing the signal-to-noise ratio (SNR) of each subcarrier to vary greatly, seriously reducing the effect of the traditional real domain equalizer. In this regard, although the use of probability shaping PS combined with FEC can adjust the transmission information entropy of a single carrier, it cannot reduce the overhead of the pilot symbol in each code block.

[0053] Embodiment 1

[0054] This embodiment provides a digital subcarrier multiplexing coherent optical communication system, which divides a single carrier signal into multiple subcarrier signals for modulation. Figure 1 As shown, including:

[0055] The transmitting module 100 modulates and transmits a digital subcarrier multiplexing DSCM signal based on a digital signal processing (DSP) algorithm;

[0056] A channel unit 200, which is used to transmit the DSCM signal;

[0057] A receiving module 300, which is used to receive and demodulate the DSCM signal;

[0058] Among them, the DSP algorithm includes a wide-area complex recursive least squares algorithm, which is used to convert the subcarrier signal from a real number to a wide-area complex number; the transmitting module performs mixed modulation on the multiple subcarrier signals, and configures different forward error correction code rates according to the order of the modulation format of each subcarrier signal; the transmitting module configures half of the multiple subcarrier signals with a pre-pilot, and the subcarrier signal configured with the pre-pilot and the subcarrier signal not configured with the pre-pilot have a symmetrical relationship in waveform; the subcarrier signals are multiplexed to obtain the DSCM signal.

[0059] This embodiment is based on a wide-area complex recursive least squares algorithm to convert an input sequence from a real number to a wide-area complex number, and still has a significant modulation effect under high-frequency attenuation, thereby improving the error performance; hybrid modulation is performed on the subcarrier signal, and different forward error correction code rates are configured according to the order of the subcarrier signal modulation format, thereby improving the spectrum efficiency; only half of the subcarrier signals need to be configured with a pre-pilot, thereby reducing the pilot overhead.

[0060] Next, a digital subcarrier multiplexing coherent optical communication system described in this embodiment is introduced in detail, which is divided into two parts: principle description and simulation verification. It should be noted that descriptions such as "principle description 1", "principle description 2", "simulation verification 1", and "simulation verification 2" are not limitations on the order of time, and their role is only to show the difference.

[0061] The following is a principle description of the digital subcarrier multiplexing coherent optical communication system:

[0062] Principle Description 1. Subcarrier Multiplexing and Demultiplexing

[0063] The DSCM coherent optical system requires DSP algorithms in both the transmitting module 100 and the receiving module 300 to respectively complete subcarrier multiplexing and demultiplexing.

[0064] Specifically, the DSP algorithm of the transmitting module 100 mainly includes shaping filtering and subcarrier multiplexing. The specific process of the subcarrier multiplexing is as follows: Figure 2 As shown, subcarrier signals S1, S2, ..., S kAn independent data sequence is used and QAM mapping is performed; then the baud rate of each subcarrier signal is determined according to the number of subcarrier signals and the bandwidth size, and up-sampling is performed; then, the subcarrier signals are shaped and filtered by a square root raised cosine (SRRC) filter to obtain K independent baseband signals; each baseband signal is transformed into the frequency domain and moved to a different position of the spectrum, and the modulation frequency is set according to its bandwidth, for example, the spectrum of 1, 2, 4 and 8 subcarrier signals under the same bandwidth is as follows Figure 3 As shown; after all the subcarrier signals are added in the frequency domain, they are transformed to the time domain through Inverse Fast Fourier Transform (IFFT), and then pre-equalized and digital-to-analog converted to obtain the DSCM signal.

[0065] Optionally, in order to reduce interference between the subcarrier signals, a spectrum protection band may be appropriately increased to ensure that a certain gap is left between the spectra of two adjacent subcarrier signals.

[0066] Specifically, the subcarrier demultiplexing process is as follows: Figure 4 As shown, the receiving module 300 receives the DSCM signal, transforms it into the frequency domain after analog-to-digital conversion; moves each of the received DSCM signals to the baseband in turn, and filters it using a matched filter; downsamples the low baud rate signal in each of the DSCM signals to obtain output signals R1, R2, ..., R k , and then converted to the time domain and processed in parallel. The subsequent DSP algorithm is similar to the traditional single-carrier DSP algorithm.

[0067] Principle Description 2: Wide Area Complex Recursive Least Squares Algorithm

[0068] In coherent optical transmission systems, linear equalization is actually an application of adaptive filters in communication systems. When there are two signal sets (input signal and expected response signal), adaptive filters can implement time-varying linear filtering to model the signal relationship to remove unwanted noise. The adaptive filter is used to process real signals, but there are also some complex extensions of adaptive algorithms, that is, the input-output relationship is regarded as a complex linear relationship.

[0069] Specifically, in the pilot-based linear equalization module, the tap coefficients are {g1(n),…,g M The output signal of the adaptive filter at time k is for:

[0070] (2-1);

[0071] Where n represents the nth iteration, x i (k) is the value of the ith input signal at time k, and M is the order of the adaptive filter.

[0072] Furthermore, combined with the conjugate property of complex signals, another output signal can be expressed as:

[0073] (2-2);

[0074] Among them, {h1(n),…,h M (n)} are the tap coefficients of the conjugate input filter.

[0075] Specifically, for a given set of {g i (n)}, usually there is no {h i (n)} is such that y 1,n (k)=y 2,n (k), Formula 2-1 and Formula 2-2 represent two different input-output relationships, indicating that different output signals are generated for the same complex input signal.

[0076] Furthermore, using and its conjugate The model used as the input signal is a wide-area linear model, and the relationship between the input signal and the output signal is:

[0077] (2-3);

[0078] It can be seen that the tap coefficient of this model is twice the tap coefficient of the adaptive filter, and its processing capability for input signal characteristics is stronger.

[0079] Furthermore, this embodiment uses the wide-area complex recursive least squares algorithm to iterate the tap coefficients in Formula 2-3, and the exponentially weighted least squares cost function in the iterative process can be expressed as:

[0080] (2-4);

[0081] Among them, p(k) is the pilot symbol, that is, the expected signal corresponding to the input signal; λ is the forgetting factor; its input-output relationship is expressed in matrix form as follows:

[0082] (2-5);

[0083] (2-6);

[0084] (2-7);

[0085] Furthermore, the wide-area complex recursive least squares algorithm is combined with a recursive least squares (RLS) algorithm, and its tap update formula is:

[0086] (2-8);

[0087] (2-9);

[0088] (2-10);

[0089] (2-11);

[0090] Where a(n) is the gain vector and P(n) is the input signal The covariance matrix of , I is the identity matrix.

[0091] Specifically, the wide-area complex RLS algorithm optimizes the RLS algorithm and the pilot-based linear equalization algorithm, has better equalization capability, and improves the bit error performance of the DSCM system.

[0092] Principle 3: Spectral efficiency

[0093] In the field of communications, spectral efficiency (SE) is used to evaluate the effectiveness of a system. It describes the capacity that a system can provide and is defined as the bit transmission rate per unit frequency band: (3-1); in, is the effective bandwidth, is the data rate; the unit of the spectrum efficiency SE is .

[0094] Specifically, assume that the reception is performed with the minimum bandwidth, that is, the bandwidth is equal to the signal symbol rate R s , taking into account the roll-off factor α of the SRRC filter and the guard band between adjacent subcarrier signals, the effective bandwidth is for: (3-2);

[0095] Wherein, ρ is the protection ratio of two adjacent subcarrier signal bands, and its calculation formula is: (3-3);

[0096] Among them, D i is the frequency difference between the center frequencies of two adjacent subcarrier signal spectra, Bi It is the bandwidth occupied by a single subcarrier signal.

[0097] Specifically, a single carrier system is divided into a DSCM system having n subcarrier signals through the subcarrier multiplexing, and it is defined that each subcarrier signal has the same symbol rate, and the total symbol rate is still R s , then the symbol rate of each subcarrier signal for:

[0098] (3-4);

[0099] Furthermore, the data rate R b for:

[0100] (3-5);

[0101] Formula (3-5) is applied to signals that have undergone polarization division multiplexing (PDM); where β is the shaping parameter, is the pilot ratio of the ith subcarrier signal, is the FEC code rate of the i-th subcarrier signal.

[0102] The spectrum efficiency SE of the entire DSCM system can be expressed as:

[0103] (3-6);

[0104] Optional, such as Figure 5 As shown, taking the generation of a DSCM signal with a modulation format of 4-16-16-4QAM as an example, in the shaping filtering stage, after the initial QAM mapping is completed, the average powers of the QPSK signals (a special QAM signal, namely, 4-QAM signal) S1, S4 and the 16QAM signals S2, S3 are inconsistent, and power normalization is required before the subcarrier frequency domain addition is performed to ensure that the frequency spectra of the subcarrier signals S1, S2, S3 and S4 remain at the same height, thereby avoiding the performance degradation of the DSCM system due to the power loss of the subcarrier signals close to the outer layer.

[0105] Specifically, a low-order modulation signal with high robustness to noise is placed in a high-frequency region of the frequency band, while a high-order modulation signal is placed in a middle region of the frequency band.

[0106] Specifically, the DSCM signal with flexible spectrum efficiency and data rate can be obtained through formulas 3-5 and 3-6.

[0107] Specifically, without considering the pilot overhead, Table 1 shows the spectrum efficiency SE and data rate of a 32GBd signal through various subcarrier hybrid QAM modulations in a 37.5GHz bandwidth channel; the order of the QAM signal is positively correlated with the spectrum efficiency and data rate of the coherent optical communication system; as the order of the hybrid modulation format increases, the spectrum efficiency SE and data rate also increase accordingly, but the maximum transmission distance in the optical fiber will decrease accordingly. For example, when the subcarrier hybrid modulation format is 4-4-4-4QAM, the maximum transmission distance under the current FEC code rate is 1045km; when the subcarrier hybrid modulation format is 16-16-16-16QAM, the maximum transmission distance under the current FEC code rate is 380km.

[0108] Table 1 SE and data rate of subcarrier hybrid QAM modulation

[0109] It can be seen that as the order of the modulation format continues to increase, under the condition of being able to achieve error-free transmission over a fixed optical fiber distance, designing a subcarrier hybrid modulation technology with higher SE and data rate has a strong application prospect. At the same time, the high-order modulation signal requires a pilot symbol to assist the DSP algorithm of the DSCM system, which can further improve the spectrum efficiency SE and data rate in combination with the wide-area complex PLS algorithm.

[0110] Principle 4: FEC Overhead and Pilot Overhead

[0111] exist Figure 6 In the embodiment, subcarrier signals S1 and S4 use the PS-64QAM modulation format, while subcarrier signals S2 and S3 use the PS-256QAM modulation format; an entire code block satisfies the Maxwell-Boltzmann MB distribution.

[0112] Optionally, if the channel SNR is higher, a higher order debugging format may be used.

[0113] Optionally, since the subcarrier signals S1 and S2 use different modulation formats and are affected by high-frequency fading to different degrees, the ratios of the parity bits and information bits are different, that is, the FEC code rates may be different;

[0114] Specifically, the low-order modulation signal uses FEC with a higher code rate, and the high-order modulation signal uses FEC with a lower code rate.

[0115] Optionally, the FEC is SD-LDPC coding based on the DVB-S.2 standard, and its code rate is 9 / 10 or 4 / 5; the SD-LDPC coding with a code rate of 4 / 5 is applied to the subcarrier signals S2 and S3 of the high-order modulation format; the SD-LDPC coding with a code rate of 9 / 10 is applied to the subcarrier signals S1 and S4 of the low-order modulation format.

[0116] It can be seen that the hybrid modulation method can reduce the FEC overhead.

[0117] exist Figure 6 In the figure, only subcarrier signals S1 and S2 are configured with pre-pilots, and the red dots in the PS constellation diagram on the right represent pilot symbols.

[0118] Specifically, the taps and estimated frequency offset phase shift obtained by pilot training of the subcarrier signals S1 and S2 can be used in the subcarrier signals S3 and S4 that are symmetrical with their waveforms; the pre-pilot is a universal pilot and is applied to the DSP algorithm of the receiving module 300 with the same pilot ratio, such as polarization demultiplexing algorithm, matched filtering algorithm, carrier recovery algorithm and equalization algorithm.

[0119] It can be seen that the pilot insertion structure can save half of the pilot overhead.

[0120] The following is a simulation verification of the digital subcarrier multiplexing coherent optical communication system:

[0121] Simulation Verification 1. Wide Area Complex Recursive Least Squares Method

[0122] like Figure 7 As shown, this embodiment builds a single polarization AWGN channel simulation platform. In this platform, the transmitting module 100 is composed of a transmitting end TxDSP, a digital-to-analog conversion unit DAC and an I / Q modulator; the modulated signal is a four-subcarrier uniform 64QAM signal, which enters the channel unit 200 after passing through the I / Q modulator. The channel unit 200 is a high-frequency fading AWGN channel with an SNR of 22dB; in order to simulate the high-frequency fading problem existing in the real channel, an exponential high-frequency attenuation with an attenuation coefficient of 0.3 is added to the high-frequency fading AWGN channel. The receiving module 300 is composed of a coherent receiver, an analog-to-digital conversion unit ADC and a receiving end RxDSP; the modulated signal is received by the coherent receiver and sent to the receiving end RxDSP for processing after passing through the analog-to-digital conversion unit ADC. For a relatively ideal simulation signal, the algorithm of the receiving end RxDSP only includes a matched filtering algorithm, a carrier recovery algorithm and a linear equalization algorithm; the linear equalization algorithm uses 2000 pilot symbols for tap iteration, and is applied to the entire symbol code block after the iteration is completed.

[0123] Figure 8 The relationship between the number of iteration symbols Iterations n and the mean square error (MSE) is shown when using three iterative algorithms: wide-area complex recursive least squares W-RLS, real recursive least squares RLS, and least mean square error (LMS).

[0124] It can be seen that the real RLS algorithm cannot converge when applied to a signal with high-frequency fading, while the wide-area complex RLS algorithm can converge quickly, and the asymmetry of the complex signal spectrum can partially compensate for the high-frequency fading loss, producing a certain degree of linear equalization effect; in contrast, the amplitude-frequency response of the real signal is symmetrical, and splitting the complex signal into real and imaginary parts and applying the real RLS algorithm separately cannot train frequency-domain asymmetric taps and achieve equalization effects. At the same time, although the LMS algorithm can also converge to the same level, it requires nearly 1,500 iterative symbols to complete convergence, while the wide-area complex RLS can quickly complete convergence when the number of iterative symbols is 100.

[0125] Fig. 9 SC1, SC2, SC3 and SC4 are DSCM signals restored by algorithm equalization. Fig. 9 (a) is the DSCM signal constellation diagram restored by the real RLS algorithm, Fig. 9 (b) is the constellation diagram of the DSCM signal restored by the wide-area complex RLS algorithm.

[0126] It can be seen that the wide-area complex RLS algorithm can restore the 64QAM constellation diagram more clearly, and the overall bit error rate is reduced from 2.92×10-2 to 2.88×10-3, which is reduced by one order of magnitude.

[0127] Combination Figure 8 and Fig. 9 It can be seen that the wide-area complex RLS algorithm can use fewer pilot training symbols to achieve a good equalization effect, it still has a significant modulation effect under high-frequency attenuation, and improves the bit error performance.

[0128] Furthermore, the experimental transmission signal is a PS-PDM-64QAM four-subcarrier signal with a β of 1.7, a baud rate of 1.6GBd×4, a roll-off factor of 0.05, and a spectrum protection band ratio set to 20%. Since the use of an optical filter in the receiving module 300 will introduce optical filtering loss, and the frequency difference between two adjacent subcarriers is too small, subcarrier demultiplexing is selected in the electrical domain. The same experimental structure as the single-carrier system is built, and the DSP algorithms of the transmitting module 100 and the receiving module 300 include a subcarrier multiplexing algorithm, a subcarrier demultiplexing algorithm, and a DSP algorithm used in a conventional single-carrier system.

[0129] like Fig.10 As shown, further verification is performed on the wide-area complex RLS algorithm. The subcarrier signal in the X polarization cannot converge using the real RLS algorithm, but can converge using both the wide-area complex RLS algorithm and the LMS algorithm, and the convergence speed of the wide-area complex RLS algorithm is much faster than that of the LMS algorithm.

[0130] It can be seen that the wide-area complex RLS algorithm can indeed use fewer pilot training symbols to achieve a good equalization effect, it still has a significant modulation effect under high-frequency attenuation, and improves the bit error performance.

[0131] Simulation Verification 2: FEC Overhead and Pilot Overhead

[0132] Fig.11 Graphs showing how the overall bit error rate (BER) of subcarrier signals SC1 and SC2 and the overall bit error rate BER of subcarrier signals SC3 and SC4 vary with SNR.

[0133] Specifically, only the subcarrier signals SC1 and SC2 are configured with the pre-pilot; the subcarrier signals SC3 and SC4 use the pre-pilot of the subcarrier signals SC1 and SC2 to recover the signal, and their bit error performance is only slightly lower than that of the subcarrier signals SC1 and SC2, with a maximum reduction of 0.28dB.

[0134] It can be seen that the loss of error performance caused by saving half of the pilot overhead is very small.

[0135] Fig.12The Normalized Generalized Mutual Information (NGMI) thresholds of two SD-LDPC codes with different code rates applied to uniform QAM signals are shown. NGMI represents the number of information bits carried by each transmitted bit and can be used to evaluate the performance of PS signals. When the final calculated NGMI exceeds the threshold of the corresponding LDPC code rate, it can be regarded as a reliable transmission for the PS signal.

[0136] Specifically, the test signal using 9 / 10 LDPC rate is a uniform 64QAM signal, and the vertical coordinate Post BER is the bit error probability measured after the data passes through the encoder, that is, the bit error rate, which is used to evaluate the error correction capability of the encoder. When the NGMI of the received subcarrier signal exceeds 0.883, the transmission can be considered to be error-free. Similarly, the test signal using 4 / 5 LDPC rate is a uniform 256QAM, and its NGMI threshold for error-free transmission is 0.778.

[0137] Furthermore, when the pilot ratio is 5%, the transmission system can obtain a higher achievable information rate (Achievable Information Rate, referred to as AIR).

[0138] Simulation Verification III. Spectral Efficiency

[0139] Fig.13 The curve of NGMI of each subcarrier of the four-subcarrier multiplexing signal changing with the shaping parameter β is shown; simulation parameters: SNR is fixed at 18dB, the pilot ratio is 5%, the roll-off coefficient of the SRRC filter is 0.01, and the spectrum protection band ratio is set to 5%.

[0140] Fig.13 The blue line in the middle represents the NGMI curve of subcarriers SC1 and SC4 with a modulation format of PS-64QAM and a FEC code rate of 9 / 10. When β is 1.80, the NGMI is just above the threshold of 0.883. In order to leave some redundancy space under the current SNR, the β value of 1.80 is selected as the maximum β that can be transmitted when the SNR is 18dB. Fig.13 As shown in the black circle on the left.

[0141] Fig.13 The red line in the middle represents the NGMI curve of subcarriers SC2 and SC3 with a modulation format of PS-256QAM. The FEC code rate used is 4 / 5, and its maximum β is 2.75. Fig.13 As shown in the black circle on the right.

[0142] According to the formula 3-6, it can be calculated that the maximum SE of the current DSCM system under the condition of SNR of 18dB is 10.150bit / s / Hz. Similarly, according to the above simulation method, the corresponding maximum SE can be determined under each SNR condition.

[0143] Furthermore, Fig.14 The relationship between SNR and maximum SE for transmission channels with and without pilot optimization in the DSCM system is shown. It can be observed that the SE of the signal w pilot optimization after pilot optimization is improved by 0.220 bit / s / Hz at an SNR of 15dB compared with the signal w / o pilot optimization without pilot optimization, and the SE gain gradually increases with the increase of SNR; the pilot optimization includes configuring pre-pilots for half of the subcarrier signals and setting the pilot ratio to 5%.

[0144] In summary, this embodiment provides a digital subcarrier multiplexing coherent optical communication system, which converts the input sequence from real numbers to wide-area complex numbers based on the wide-area complex recursive least squares algorithm, and still has a significant modulation effect under high-frequency attenuation, thereby improving the error performance; hybrid modulation is performed on the subcarrier signal, and different forward error correction code rates are configured according to the order of the subcarrier signal modulation format, thereby improving the spectrum efficiency; only half of the subcarrier signals need to be configured with a pre-pilot, thereby reducing the pilot overhead.

[0145] Embodiment 2

[0146] This embodiment provides a digital subcarrier multiplexing coherent optical communication method, which divides a single carrier signal into multiple subcarrier signals for modulation respectively. The coherent optical communication method includes:

[0147] Step S1, modulating and transmitting a digital subcarrier multiplexed signal based on a DSP algorithm;

[0148] Step S2, transmitting the digital subcarrier multiplexed signal;

[0149] Step S3, receiving and demodulating the digital subcarrier multiplexed signal;

[0150] Among them, the DSP algorithm includes a wide-area complex recursive least squares algorithm, which is used to convert the subcarrier signal from a real number to a wide-area complex number; the step S1 performs mixed modulation on the multiple subcarrier signals, and configures different forward error correction code rates according to the order of the modulation format of each subcarrier signal; the step S1 configures half of the multiple subcarrier signals with a pre-pilot, and the subcarrier signal configured with the pre-pilot and the subcarrier signal not configured with the pre-pilot have a symmetrical relationship in waveform; the subcarrier signal is multiplexed to obtain the subcarrier multiplexed signal.

[0151] This embodiment provides a digital subcarrier multiplexing coherent optical communication method, which has the same beneficial effects as the above-mentioned digital subcarrier multiplexing coherent optical communication system.

[0152] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present application may adopt the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program codes.

[0153] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0154] These computer program instructions may also be stored in a computer readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer readable memory produce an article of manufacture including an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0155] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0156] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A digital subcarrier multiplexing coherent optical communication system, which divides a single carrier signal into multiple subcarrier signals for modulation respectively, characterized in that: The coherent optical communication system comprises: A transmitting module, which modulates and transmits a digital subcarrier multiplexed signal based on a DSP algorithm; A channel unit, which is used to transmit the digital subcarrier multiplexed signal; A receiving module, used for receiving and demodulating the digital subcarrier multiplexed signal; Among them, the DSP algorithm includes a wide-area complex recursive least squares algorithm, which is used to convert the subcarrier signal from a real number to a wide-area complex number; the transmitting module performs mixed modulation on the multiple subcarrier signals, and configures different forward error correction code rates according to the order of the modulation format of each subcarrier signal; the transmitting module configures half of the multiple subcarrier signals with a pre-pilot, and the subcarrier signal configured with the pre-pilot and the subcarrier signal not configured with the pre-pilot have a symmetrical relationship in waveform; the subcarrier signals are multiplexed to obtain the digital subcarrier multiplexed signal.

2. A digital subcarrier multiplexing coherent optical communication system according to claim 1, characterized in that: Iterating the tap coefficients of the wide-area linear model based on the wide-area complex recursive least squares algorithm; The tap coefficient of the wide area linear model is twice the tap coefficient of the adaptive filter; The exponentially weighted least squares cost function in the iterative process for: ； in, is the output signal of the wide area linear model, p(k) is the pilot symbol, λ is the forgetting factor, n represents the nth iteration, and k represents the time k.

3. A digital subcarrier multiplexing coherent optical communication system according to claim 2, characterized in that: The wide-area complex recursive least squares algorithm combines the recursive least squares algorithm, and its tap update formula is: ； ； ； ； in, are the tap coefficients of the wide area linear model, a(n) is the gain vector, P(n) is the covariance matrix of the input signal, and I is the identity matrix; , is the input signal at time k, For the input signal conjugate signal.

4. A digital subcarrier multiplexing coherent optical communication system according to claim 1, characterized in that: The modulation process of the transmitting module includes QAM mapping and subcarrier frequency domain addition; After the QAM mapping is performed, a QAM signal is obtained, and after the power of the QAM signal is normalized, the subcarrier frequency domain addition is performed to keep the frequency spectrum of the subcarrier signal at the same height.

5. A digital subcarrier multiplexing coherent optical communication system according to claim 4, characterized in that: The order of the QAM signal is positively correlated with the spectrum efficiency and data rate of the coherent optical communication system.

6. A digital subcarrier multiplexing coherent optical communication system according to claim 4, characterized in that: The modulation process of the transmitting module also includes probability shaping; after the probability shaping, the QAM signal conforms to the Maxwell-Boltzmann distribution.

7. A digital subcarrier multiplexing coherent optical communication system according to claim 1, characterized in that: The forward error correction code is an SD-LDPC code based on the DVB-S.2 standard, and its code rate is 9 / 10 or 4 / 5; the SD-LDPC code with a code rate of 4 / 5 is applied to the subcarrier signal of the high-order modulation format, and the SD-LDPC code with a code rate of 9 / 10 is applied to the subcarrier signal of the low-order modulation format.

8. A digital subcarrier multiplexing coherent optical communication system according to claim 1, characterized in that: The pilot ratio of the coherent optical communication system is 5%.

9. A digital subcarrier multiplexing coherent optical communication system according to claim 1, characterized in that: The algorithms used by the receiving module include polarization demultiplexing algorithm, matched filtering algorithm, carrier recovery algorithm and equalization algorithm.

10. A digital subcarrier multiplexing coherent optical communication method, which divides a single carrier signal into multiple subcarrier signals and modulates them separately, characterized in that: The coherent optical communication method comprises: Step S1, modulating and transmitting a digital subcarrier multiplexed signal based on a DSP algorithm; Step S2, transmitting the digital subcarrier multiplexed signal; Step S3, receiving and demodulating the digital subcarrier multiplexed signal; Among them, the DSP algorithm includes a wide-area complex recursive least squares algorithm, which is used to convert the subcarrier signal from a real number to a wide-area complex number; the step S1 performs mixed modulation on the multiple subcarrier signals, and configures different forward error correction code rates according to the order of the modulation format of each subcarrier signal; the step S1 configures half of the multiple subcarrier signals with a pre-pilot, and the subcarrier signal configured with the pre-pilot and the subcarrier signal not configured with the pre-pilot have a symmetrical relationship in waveform; the subcarrier signals are multiplexed to obtain the digital subcarrier multiplexed signal.

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