A simplified homogenous self-coherent system based on alamouti encoding and digital subcarrier multiplexing

Abstract

The application relates to the technical field of optical communication systems, and more particularly to a simplified homogenous self-coherent system based on Alamouti coding and digital subcarrier multiplexing. By adopting Alamouti coding between polarizations, the architecture level is simplified. The signal after Alamouti coding can realize polarization-insensitive reception at the receiving end, so that the receiving end can save half the number of 90-degree mixers and BPDs, and does not need a polarization beam splitter to separate the carrier and the signal into two orthogonal polarization states. In addition, compared with the existing homogenous self-coherent scheme, the application does not need to perform automatic polarization tracking and control on the carrier transmission branch, thereby saving the additional control module cost. In addition, the application adopts the DSCM technology to reduce the complexity of dispersion compensation at the receiving end, thereby further reducing the algorithm cost of dispersion compensation in addition to saving the frequency offset and phase compensation cost.

Description

Technical Field

[0001] This invention relates to the field of optical communication system technology, and more specifically, to a simplified coherent system based on Alamouti coding and digital subcarrier multiplexing. Background Technology

[0002] To meet the ever-increasing demand for data traffic, the need for high-capacity transmission systems in short-distance data center optical interconnects is becoming increasingly urgent. While traditional intensity modulation direct detection (IM / DD) systems offer advantages in cost, power consumption, and system complexity, they can only achieve one-dimensional modulation and are susceptible to frequency-selective fading caused by dispersion, limiting their ability to handle high-capacity, long-distance transmission. Coherent reception technology, on the other hand, offers better receiver sensitivity and linearity, and allows for modulation of more dimensions. However, traditional coherent systems suffer from high system and DSP complexity, making them unsuitable for direct application in short-distance optical transmission scenarios. To simplify the complexity of coherent systems, self-coherent systems based on co-source coherence have been proposed, avoiding the need for frequency offset and phase noise compensation in receiver-side digital signal processing (DSP), thus simplifying the receiver-side DSP.

[0003] In coherent systems with the same source, the local oscillator (LO) at the receiver originates from the same laser used for signal modulation at the transmitter, and transmission is achieved via an additional fiber optic link. However, the carrier wave transmitted from the transmitter to the receiver undergoes a change in polarization state after transmission, leading to power fading at a certain polarization at the receiver and affecting normal signal reception. To address this issue, an automatic polarization controller is needed in the carrier transmission branch to track and control the carrier's polarization state in real time. However, this approach adds an extra automatic polarization control module, adjusting the carrier by feedback on its state, which increases the system's complexity to some extent.

[0004] To avoid the use of automatic polarization controllers, a complementary polarization diversity coherent receiver has been proposed. This scheme, in addition to the two receiving units included in a traditional coherent receiver—two 90° optical mixers and four balanced detectors (BPDs)—introduces a third receiving unit for LO and signal mixing to provide complementary polarization diversity, thereby solving the polarization fading problem of the transmitted LO. To address the rapid polarization changes of the LO, a polarization tracking algorithm is also required in the receiver-side DSP. While this scheme avoids the use of an automatic polarization controller, it adds an extra receiving unit and increases the complexity of the receiver-side DSP. In the coherent scheme based on the complementary polarization diversity coherent receiver, compared to the traditional coherent receiver, a third receiving unit for LO and signal mixing is introduced to avoid polarization fading, increasing the receiver's complexity. Simultaneously, the need to employ a polarization tracking algorithm in the receiver-side DSP further increases the DSP's complexity. Furthermore, compensation for dispersion-induced impairments in fiber optic transmission will also constitute a significant portion of the power consumption in the receiver-side DSP. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this invention provides a simplified coherent system based on Alamouti coding and digital subcarrier multiplexing, which simplifies the architecture of the coherent receiver and the complexity of the receiver's DSP.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] A simplified coherent method based on Alamouti coding and digital subcarrier multiplexing is proposed. In the transmitter's digital signal processing (DSP), a pseudo-random bit sequence is first mapped to a set of QAM symbols, which are then used to generate digital subcarrier signals for each polarization. Next, Alamouti coding is performed on the subcarrier signals between the two polarizations, and a root-raised cosine filter is used to perform Nyquist shaping. Then, the subcarriers on each polarization are digitally multiplexed, and finally, the signal is resampled. In the receiver's DSP, the acquired signal is first resampled and orthogonalized, then each subcarrier is demultiplexed and matched filtering is performed. Next, synchronization, Alamouti decoding, and equalization are performed, and finally, the bit error rate (BER) is calculated.

[0008] Based on the aforementioned technical means, this invention proposes a simplified coherent method based on Alamouti coding and digital subcarrier multiplexing to simplify the coherent system architecture, solve the polarization fading problem in coherent systems with the same origin, and simplify the fiber dispersion compensation process, further reducing the complexity of the receiver DSP. This invention performs Alamouti coding on signals with two orthogonal polarizations at the transmitter, enabling the receiver to achieve polarization insensitivity, thus avoiding the polarization fading problem. Therefore, polarization tracking and control of the transmission carrier branches are unnecessary. Simultaneously, since the encoded transmission enables polarization-insensitive reception, the receiver only needs to receive either polarization to recover the transmitted signal. Therefore, compared to traditional coherent receivers, its coherent reception structure can be simplified to requiring only one 90° optical mixer and two BPDs, saving half the number of receiver units and eliminating the need for a polarization beam splitter, significantly simplifying coherent reception at the architectural level. Furthermore, by employing DSCM technology, the dispersion compensation algorithm in the receiver DSP is simplified, further reducing the complexity of the receiver DSP algorithm.

[0009] Furthermore, the transmitter splits the laser-generated light source into two paths: one for optical signal modulation and the other for transmission to the receiver as a local oscillator (LO). The transmitter's offline DSP generates two DSCM signals, each containing four subcarriers (SC). After sampling by an arbitrary waveform generator (AWG), these signals are modulated onto orthogonal X and Y polarizations by a dual-polarization IQ modulator, amplified by an erbium-doped fiber amplifier (EDFA), and then transmitted through a single-mode fiber (SMF). The receiver amplifies the transmitted signal using another EDFA and controls the received signal power using an adjustable optical attenuator (VOA). In the carrier transmission branch, the carrier is also amplified by an EDFA after transmission through the fiber. A coupler then couples the carrier and signal into a 90° optical mixer, where two balanced detectors (BPDs) detect the real and imaginary parts of the signal. The detected signals are acquired by an oscilloscope (Osc) and then processed in the offline DSP.

[0010] Furthermore, the Alamouti encoding includes: at the second time, the information from the first time is repeated alternately between the two polarizations; the signal on the X polarization at the second time is the negative conjugate of the signal on the Y polarization at the first time, and the signal on the Y polarization at the second time is the conjugate of the signal on the X polarization at the first time; after the signal is transmitted through the optical fiber, the polarization state will change; the signal's appearance at the receiving end includes that the original signal on the X polarization will be superimposed on the Y polarization, and similarly, the signal on the Y polarization will also be superimposed on the X polarization, forming crosstalk between the X and Y polarizations.

[0011] Furthermore, at the receiving end, the channel transmission matrix is ​​represented as: H = [h xx ,h xy h yx ,h xx The received signal is represented as follows:

[0012] s' x1 =h xx s1+h xy s2

[0013]

[0014] In the formula, s' x1 and s' x2 The odd and even sequences of the received signal are s1 and s2, respectively; s1 and s2 are the signals modulated by the transmitting end on X-polarization and Y-polarization, respectively; h xx h xy h yx h yy For X-channel response, XY cross-channel response, YX cross-channel response, and Y-channel response; * indicates conjugate operation.

[0015] Furthermore, since the received signal contains inter-polarization crosstalk, equalization is used to resolve polarization crosstalk and achieve channel equalization; the equalization process is expressed as:

[0016]

[0017]

[0018] In the formula, s' xo and s' xe w represents the odd and even sequences of the received signal, respectively. xx w xy w yx w yy These are four FIR filters with finite impulse responses, where p is a one-tap phase factor used to compensate for phase noise introduced by the mismatch between the signal and optical carrier transmission distances; * indicates conjugate operation; s xo and s xe These are the equalization results for the odd-numbered and even-numbered sequences of the received signal, respectively. For s' xe . conjugate.

[0019] Furthermore, the tap coefficient update of the FIR filter is expressed as:

[0020]

[0021]

[0022]

[0023]

[0024] Where μ is the step size for adjusting the tap convergence speed. For s' xo conjugate, e o and e e The error signal is obtained from the following formula:

[0025] e o =d o -s xo

[0026] e e =d e -s xe

[0027] Where, d o and d e The standard constellation points obtained from the judgment;

[0028] The update rule for the phase factor p is as follows:

[0029] p1=p1+μ p e o (w xx s' xo ) *

[0030]

[0031]

[0032] Where, μ p The step size is used to control the convergence speed of the phase factor; * indicates the conjugate operation.

[0033] Furthermore, the digital subcarrier multiplexing divides a high baud rate signal into multiple low baud rate signals, while eliminating the overlap between spectra and eliminating the need to maintain a guard interval between each subcarrier.

[0034] The present invention also provides a simplified coherent source system based on Alamouti coding and digital subcarrier multiplexing, including a transmitter digital signal processing (DSP) module and a receiver digital signal processing (DSP) module;

[0035] The transmitter's digital signal processing (DSP) module is used to map pseudo-random bit sequences into a set of QAM symbols, which are then used to generate digital subcarrier signals for each polarization. Next, the subcarrier signals between the two polarizations are Alamouti encoded, and a root-raised cosine filter is used to perform Nyquist shaping on the signals. Then, the subcarriers on each polarization are digitally multiplexed, and finally, the signals are resampled.

[0036] The receiver's digital signal processing (DSP) module is used to resample and orthogonalize the acquired signal, then demultiplex each subcarrier and perform matched filtering; next, it performs synchronization, Alamouti decoding and equalization, and finally calculates the bit error rate (BER).

[0037] Based on the aforementioned technical means, compared to currently proposed coherent coherent schemes originating from the same source, this invention can further simplify the architecture of the coherent receiver and the complexity of the receiver-end DSP, promoting the application of coherent technology in short-range optical communication scenarios. Architecture-level simplification is achieved by employing Alamouti coding between polarizations. The signal after Alamouti coding can achieve polarization-insensitive reception at the receiver, thus saving half the number of 90° mixers and BPDs at the receiver, and eliminating the need for polarization beam splitters to separate the carrier and signal into two orthogonal polarization states. Furthermore, compared to existing coherent coherent schemes originating from the same source, this invention does not require automatic polarization tracking and control of the carrier transmission branch, saving additional control module overhead. Moreover, this invention reduces the complexity of dispersion compensation at the receiver by employing DSCM technology, further reducing the algorithmic overhead of dispersion compensation in addition to saving on frequency offset and phase compensation overhead.

[0038] Furthermore, the transmitter's digital signal processing (DSP) module splits the laser-generated light source into two paths: one for optical signal modulation and the other for transmission to the receiver as a local oscillator (LO). The transmitter's offline DSP generates two DSCM signals, each containing four subcarriers (SC). After sampling by an arbitrary waveform generator (AWG), these signals are modulated onto orthogonal X and Y polarizations by a dual-polarization IQ modulator, amplified by an erbium-doped fiber amplifier (EDFA), and then transmitted through a single-mode fiber (SMF). The receiver's DSP module uses another EDFA to amplify the transmitted signal and controls the received power using an adjustable optical attenuator (VOA). In the carrier transmission branch, the carrier is also amplified by an EDFA after transmission through the fiber. A coupler then couples the carrier and signal into an optical mixer, where two balanced detectors (BPDs) detect the real and imaginary parts of the signal. The detected signals are acquired by an oscilloscope (Osc) and then processed in the offline DSP.

[0039] Furthermore, the Alamouti encoding includes: at the second time, the information from the first time is repeated alternately between the two polarizations; the signal on the X polarization at the second time is the negative conjugate of the signal on the Y polarization at the first time, and the signal on the Y polarization at the second time is the conjugate of the signal on the X polarization at the first time; after the signal is transmitted through the optical fiber, the polarization state will change; the signal's appearance at the receiving end includes that the original signal on the X polarization will be superimposed on the Y polarization, and similarly, the signal on the Y polarization will also be superimposed on the X polarization, forming crosstalk between the X and Y polarizations.

[0040] Compared to existing technologies, the advantages of this invention are as follows: By combining Alamouti coding and DSCM (Digital Subcarrier Multiplexing) technology, the coherent system is simplified at both the architecture and receiver DSP levels. At the architecture level, by performing Alamouti coding on two orthogonally polarized signals at the transmitter, polarization-insensitive reception is achieved at the receiver. Therefore, the receiver does not need a polarization beam splitter to separate the signal and carrier into two orthogonal polarization states; only a 90° optical mixer and two BPDs are needed for signal reception, saving half the components compared to traditional coherent receiver structures. Furthermore, since the receiver can achieve polarization-insensitive reception, no additional automatic polarization control module is needed for the carrier transmission branch, resulting in significant simplification of the system architecture. Compared to traditional coherent systems, in coherent systems with the same source, the LO and the carrier of the modulation signal originate from the same laser, thus effectively avoiding compensation for frequency offset and laser phase noise in the receiver DSP. In addition, compensation for fiber dispersion in the receiver DSP would account for a significant portion of the overhead, and its complexity is proportional to the square of the baud rate. This invention employs DSCM technology, which reduces the complexity of dispersion compensation by dividing a high baud rate signal into several low baud rate signals, thereby further reducing the complexity of the receiver DSP. Attached Figure Description

[0041] Figure 1 This is a system device diagram and transceiver DSP flow chart of the present invention.

[0042] Figure 2 This is a schematic diagram of the dual-polarization signal encoded by Alamouti at the transmitter of this invention.

[0043] Figure 3 This is a schematic diagram of the signal received by the present invention.

[0044] Figure 4 This is a schematic diagram of the equalizer structure of the present invention.

[0045] Figure 5These are schematic diagrams of a conventional single-carrier signal and a DSCM signal, where (a) represents a schematic diagram of a single-carrier signal and (b) represents a schematic diagram of a DSCM signal.

[0046] Figure 6 This is a schematic diagram illustrating the effect of transmission carrier polarization rotation on BER performance.

[0047] Figure 7 This is a diagram illustrating the BER performance under different OSNR values ​​during back-to-back transmission.

[0048] Figure 8 This is a comparison of the transmission performance of 40km fiber optic cables. (a) represents 50Gbaud 4SC-16QAM, and (b) represents traditional 50Gbaud 16QAM. Detailed Implementation

[0049] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. The present invention will be described in one embodiment below with reference to specific embodiments. The accompanying drawings are for illustrative purposes only and represent schematic diagrams, not actual pictures, and should not be construed as limiting the present patent. In order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged, or reduced, and do not represent the actual product size. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0050] In the description of this invention, it should be understood that if terms such as "upper," "lower," "left," and "right" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, they are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms describing positional relationships in the accompanying drawings are only for illustrative purposes and should not be construed as limiting this patent. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances. In addition, if the embodiments of this invention involve descriptions of "first," "second," etc., such descriptions are only for descriptive purposes and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one of those features. Furthermore, the meaning of "and / or" throughout the text is to include three parallel solutions. Taking "A and / or B" as an example, it includes solution A, or solution B, or a solution that simultaneously satisfies A and B.

[0051] Example 1:

[0052] A simplified coherent method based on Alamouti coding and digital subcarrier multiplexing is proposed. In the transmitter's digital signal processing (DSP), a pseudo-random bit sequence is first mapped to a set of QAM symbols, which are then used to generate digital subcarrier signals for each polarization. Next, Alamouti coding is performed on the subcarrier signals between the two polarizations, and a root-raised cosine filter is used to perform Nyquist shaping. Then, the subcarriers on each polarization are digitally multiplexed, and finally, the signal is resampled. In the receiver's DSP, the acquired signal is first resampled and orthogonalized, then each subcarrier is demultiplexed and matched filtering is performed. Next, synchronization, Alamouti decoding, and equalization are performed, and finally, the bit error rate (BER) is calculated.

[0053] The transmitter splits the laser-generated light source into two paths: one for optical signal modulation and the other for transmission to the receiver as the local oscillator (LO). The transmitter's offline DSP generates two DSCM signals, each containing four subcarriers (SC). These signals are sampled by an arbitrary waveform generator (AWG) and modulated onto orthogonal X and Y polarizations by a dual-polarization IQ modulator. They are then amplified by an erbium-doped fiber amplifier (EDFA) and transmitted through a single-mode fiber (SMF). The receiver amplifies the transmitted signal using another EDFA and controls the received signal power using an adjustable optical attenuator (VOA). In the carrier transmission branch, the carrier is also amplified by an EDFA after transmission through the fiber. A coupler then couples the carrier and signal into a 90° optical mixer. The real and imaginary parts of the signal are detected by two balanced detectors (BPD). The detected signals are acquired by an oscilloscope (Osc) and processed in the offline DSP.

[0054] like Figure 2 and Figure 3 As shown, a schematic diagram of the Alamouti-encoded signal is presented, where the information from the first moment is repeated alternately between two polarizations at the second moment, as shown below. Figure 2 As shown, the signal on the X-polarized side at the second moment is the negative conjugate of the signal on the Y-polarized side at the first moment; the asterisk (*) in the figure indicates the conjugation operation. The signal on the Y-polarized side at the second moment is the conjugate of the signal on the X-polarized side at the first moment. After the signal is transmitted through the optical fiber, the polarization state changes. The signal's appearance at the receiving end is as follows: Figure 3 As shown, the signal originally on the X polarization will be superimposed on the Y polarization, and similarly, the signal on the Y polarization will be superimposed on the X polarization, forming crosstalk between the X and Y polarizations.

[0055] Consider the channel transmission matrix as: H = [h xx ,h xy h yx ,h xxAt the receiving end, the received signal is represented as follows:

[0056]

[0057] In the formula, s' x1 and s' x2 The odd and even sequences of the received signal are s1 and s2, respectively; s1 and s2 are the signals modulated by the transmitting end on X-polarization and Y-polarization, respectively; h xx h xy h yx h yy For X-channel response, XY cross-channel response, YX cross-channel response, and Y-channel response; * indicates conjugate operation.

[0058] As can be seen from formula (1), no matter how the polarization state changes, the transmitted signal can be recovered by receiving the signal of any one polarization. Therefore, there is no need to track and control the polarization state of the transmission carrier. At the same time, it can simplify the coherent receiver, requiring only half the number of devices to receive the signal.

[0059] Since the received signal contains inter-polarity crosstalk, equalization is used to resolve polarization crosstalk and achieve channel equalization; the structure of the equalizer is as follows: Figure 4 As shown, the equilibrium process is represented as follows:

[0060]

[0061] In the formula, s' xo and s' xe w represents the odd and even sequences of the received signal, respectively. xx w xy w yx w yy These are four FIR filters with finite impulse responses, where p is a one-tap phase factor used to compensate for phase noise introduced by the mismatch between the signal and optical carrier transmission distances; * indicates conjugate operation; s xo and s xe These are the equalization results for the odd-numbered and even-numbered sequences of the received signal, respectively. For s' xe . conjugate.

[0062] Furthermore, the tap coefficient update of the FIR filter is expressed as:

[0063]

[0064] Where μ is the step size for adjusting the tap convergence speed. For s' xo conjugate, e o and e eThe error signal is obtained from the following formula:

[0065]

[0066] Where, d o and d e The standard constellation points obtained from the judgment;

[0067] The update rule for the phase factor p is as follows:

[0068]

[0069] Where, μ p The step size is used to control the convergence speed of the phase factor; * indicates the conjugate operation.

[0070] DSCM (Digital Subcarrier Multiplexing) technology divides a high-baud-rate signal into multiple low-baud-rate signals without overlapping frequencies and without requiring guard intervals between each subcarrier, thus ensuring the spectral efficiency of the signal. Figure 5 Schematic diagrams of conventional single-carrier signals and DSCM signals are given. Since the complexity of dispersion compensation at the receiver is quadratically related to the signal baud rate, DSCM technology can simplify the complexity of dispersion compensation in the receiver's DSP.

[0071] Example 2

[0072] This embodiment provides a simplified coherent source system based on Alamouti coding and digital subcarrier multiplexing, including a transmitter digital signal processing (DSP) module and a receiver digital signal processing (DSP) module.

[0073] The transmitter's digital signal processing (DSP) module is used to map pseudo-random bit sequences into a set of QAM symbols, which are then used to generate digital subcarrier signals for each polarization. Next, the subcarrier signals between the two polarizations are Alamouti encoded, and a root-raised cosine filter is used to perform Nyquist shaping on the signals. Then, the subcarriers on each polarization are digitally multiplexed, and finally, the signals are resampled.

[0074] The receiver's digital signal processing (DSP) module is used to resample and orthogonalize the acquired signal, then demultiplex each subcarrier and perform matched filtering; next, it performs synchronization, Alamouti decoding and equalization, and finally calculates the bit error rate (BER).

[0075] Based on the aforementioned technical means, compared to currently proposed coherent coherent schemes originating from the same source, this invention can further simplify the architecture of the coherent receiver and the complexity of the receiver-end DSP, promoting the application of coherent technology in short-range optical communication scenarios. Architecture-level simplification is achieved by employing Alamouti coding between polarizations. The signal after Alamouti coding can achieve polarization-insensitive reception at the receiver, thus saving half the number of 90° mixers and BPDs at the receiver, and eliminating the need for polarization beam splitters to separate the carrier and signal into two orthogonal polarization states. Furthermore, compared to existing coherent coherent schemes originating from the same source, this invention does not require automatic polarization tracking and control of the carrier transmission branch, saving additional control module overhead. Moreover, this invention reduces the complexity of dispersion compensation at the receiver by employing DSCM technology, further reducing the algorithmic overhead of dispersion compensation in addition to saving on frequency offset and phase compensation overhead.

[0076] The transmitter's digital signal processing (DSP) module splits the laser-generated light source into two paths: one for optical signal modulation and the other for transmission to the receiver as a local oscillator (LO). The transmitter's offline DSP generates two DSCM signals, each containing four subcarriers (SC). After sampling by an arbitrary waveform generator (AWG), these signals are modulated onto orthogonal X and Y polarizations by a dual-polarization IQ modulator, amplified by an erbium-doped fiber amplifier (EDFA), and then transmitted through a single-mode fiber (SMF). The receiver's DSP module uses another EDFA to amplify the transmitted signal and controls the received power using an adjustable optical attenuator (VOA). In the carrier transmission branch, the carrier is also amplified by an EDFA after transmission through the fiber. A coupler then couples the carrier and signal into a 90° optical mixer. The real and imaginary parts of the signal are detected by two balanced detectors (BPD). The detected signals are acquired by an oscilloscope (Osc) and then processed in the offline DSP.

[0077] Example 3

[0078] like Figure 1 The system diagram and DSP flowchart of the transceiver terminal of the present invention are shown. Based on the above system, simulation and experimental verification were performed. Figure 1As shown, the transmitter splits the laser-generated light source into two paths: one for optical signal modulation and the other for transmission to the receiver as the LO (Local Optical Array). At the transmitter, an offline DSP generates two 50Gbaud 16QAM (4SC-16QAM) signals with four subcarriers (SC). After passing through an arbitrary waveform generator (AWG) with a sampling rate of 64GSa / s, these signals are modulated onto orthogonal X and Y polarizations by a dual-polarized IQ modulator (DP-IQM), amplified by an erbium-doped fiber amplifier (EDFA), and then transmitted over 40km of single-mode fiber (SMF). At the receiver, another EDFA amplifies the transmitted signal, and a variable optical attenuator (VOA) controls the received signal power. In the carrier transmission branch, the carrier, after transmission through the fiber, is also amplified to 13dBm by an EDFA. A 3dB coupler then couples the carrier and signal into a 90° optical mixer, where the real and imaginary parts of the signal are detected by two BPDs. The detected signals were acquired by an oscilloscope (Osc) with a sampling rate of 80 GSa / s and then processed in an offline DSP.

[0079] (1) Transmitter DSP

[0080] In the transmitter DSP, the pseudo-random bit sequence is first mapped into a set of eight 16QAM symbols, which are used to generate four digital subcarrier signals for each polarization, each subcarrier being 12.5 Gbaud. Then, the subcarrier signals between two polarizations are Alamouti encoded, and the signals are Nyquist shaped using a root-raised cosine filter with a roll-off factor of 0.1. Next, the four subcarriers on each polarization are multiplexed to synthesize a 50 Gbaud 4SC-16QAMDSCM signal, which is then resampled to match the sampling rate of the AWG.

[0081] (2) Receiver DSP

[0082] At the receiving end, the acquired signal is first resampled and orthogonalized, then each subcarrier is demultiplexed and matched filtering is performed. Next, synchronization, Alamouti decoding, and equalization are performed, and finally the bit error rate (BER) is calculated.

[0083] Results Analysis

[0084] based on Figure 1 The system setup was first tested, and the polarization-insensitive receiving characteristics of the proposed scheme were verified through simulation. The simulation was performed jointly by MATLAB and VPI. Figure 6 The BER performance obtained by changing the polarization rotation angle of the carrier branch is presented. As can be seen from the figure, the BER performance of the 50Gbaud 4SC-16QAM signal remains almost unchanged regardless of the polarization rotation angle.

[0085] Then, the experimental transmission performance of the 50Gbaud 4SC-16QAM signal was studied. First, the BER under different optical signal-to-noise ratios (OSNR) was tested in back-to-back optical transmission. The experimental results are as follows: Figure 6 As shown in the figure, the transmission performance of a 50Gbaud 4SC-16QAM signal is compared with that of a traditional 50Gbaud 16QAM signal. Figure 7 It can be seen that the BER curves of the 50Gbaud 4SC-16QAM signal and the traditional 50Gbaud 16QAM signal are basically the same, showing almost identical performance.

[0086] Next, we will discuss the performance of transmitting 40km of optical fiber. Figure 8 The BER performance of 50Gbaud 4SC-16QAM and 50Gbaud 16QAM signals after transmission over 40km of single-mode fiber is presented. Figure 8 (a) The performance of a 50Gbaud 4SC-16QAM signal with and without frequency-domain dispersion compensation was compared. Experimental results show that for a 50Gbaud 4SC-16QAM signal, after frequency-domain dispersion compensation, an equalizer with approximately 9 taps is needed to achieve optimal performance. Without frequency-domain dispersion compensation, an equalizer with 11 taps is required. This means that only two additional taps are needed to eliminate dispersion-related damage, significantly reducing the complexity of the receiver's DSP. Furthermore, Figure 8 (b) also shows the equalizer tap lengths required for a traditional 50 Gbaud 16QAM signal with and without frequency-domain dispersion compensation. As can be seen from the figure, with frequency-domain dispersion compensation, approximately 35 taps are needed to achieve optimal performance, which is longer than the required tap length for a 50 Gbaud 4SC-16QAM signal. Without frequency-domain dispersion compensation, the required tap length is even longer, and the performance cannot be achieved as with frequency-domain dispersion compensation. Therefore, it can be concluded that using DSCM technology can significantly reduce the complexity of the receiver's DSP.

[0087] In summary, through simulation and experimental verification, the proposed simplified coherent system based on Alamouti coding and DSCM technology can effectively reduce the complexity of the coherent receiver architecture and the receiver DSP, while ensuring system performance.

[0088] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A simplified coherent method based on Alamouti coding and digital subcarrier multiplexing, characterized in that, In the transmitting end digital signal processing (DSP), the pseudo-random bit sequence is first mapped into a set of QAM symbols, which are used to generate digital subcarrier signals for each polarization. Then, the subcarrier signals between the two polarizations are Alamouti encoded, and the signals are Nyquist shaped using a root-raised cosine filter. Next, the subcarriers on each polarization are digitally multiplexed, and finally, the signal is resampled. In the receiving end digital signal processing (DSP), the acquired signal is first resampled and orthogonalized, then the subcarriers are demultiplexed and matched filtering is performed. Next, synchronization, Alamouti decoding, and equalization are performed, and finally, the bit error rate (BER) is calculated. The transmitter splits the laser-generated light source into two paths: one for optical signal modulation and the other for transmission to the receiver as the local oscillator (LO). The transmitter's offline DSP generates two DSCM signals, each containing four subcarriers (SC). These signals are sampled by an arbitrary waveform generator (AWG) and modulated onto orthogonal X and Y polarizations by a dual-polarization IQ modulator. They are then amplified by an erbium-doped fiber amplifier (EDFA) and transmitted through a single-mode fiber (SMF). The receiver amplifies the transmitted signal using another EDFA and controls the received signal power using an adjustable optical attenuator (VOA). In the carrier transmission branch, the carrier is also amplified by an EDFA after transmission through the fiber. A coupler then couples the carrier and signal into a 90° optical mixer. The real and imaginary parts of the signal are detected by two balanced detectors (BPD). The detected signals are acquired by an oscilloscope (Osc) and processed in the offline DSP.

2. The simplified coherent source method based on Alamouti coding and digital subcarrier multiplexing according to claim 1, characterized in that, The Alamouti encoding includes: at a second time, the information from the first time is repeated alternately between the two polarizations; the signal on the X polarization at the second time is the negative conjugate of the signal on the Y polarization at the first time, and the signal on the Y polarization at the second time is the conjugate of the signal on the X polarization at the first time; after the signal is transmitted through the optical fiber, the polarization state will change; the signal at the receiving end will be represented by the original X polarization signal being superimposed on the Y polarization, and similarly, the Y polarization signal will also be superimposed on the X polarization, forming crosstalk between X and Y polarizations.

3. The simplified coherent source method based on Alamouti coding and digital subcarrier multiplexing according to claim 2, characterized in that, The channel transmission matrix is ​​represented as: At the receiving end, the received signal is represented as follows: In the formula, and For the received signal, there are odd and even sequences; and The signal is modulated onto the X-polarization and Y-polarization of the transmitting end; , , , For X-channel response, XY cross-channel response, YX cross-channel response, and Y-channel response; * indicates conjugate operation.

4. The simplified coherent source method based on Alamouti coding and digital subcarrier multiplexing according to claim 3, characterized in that, Since the received signal contains inter-polarity crosstalk, equalization is used to resolve polarization crosstalk and achieve channel equalization; the equalization process is expressed as follows: In the formula, and These are the odd and even sequences of the received signal, respectively. , , , These are four FIR filters with finite impulse responses. p A phase factor with one tap, used to compensate for phase noise introduced by the mismatch between signal and optical carrier transmission distances; * indicates conjugate operation; and These are the equalization results for the odd-numbered and even-numbered sequences of the received signal, respectively. for . conjugate.

5. The simplified coherent source method based on Alamouti coding and digital subcarrier multiplexing according to claim 4, characterized in that, The tap coefficient update of the FIR filter is expressed as: in, To adjust the step size of the tap convergence speed, for conjugate, and The error signal is obtained from the following formula: in, and The standard constellation points obtained from the judgment; The update rule for the phase factor p is as follows: in, The * denotes the conjugate operation to control the convergence speed of the phase factor.

6. The simplified coherent source method based on Alamouti coding and digital subcarrier multiplexing according to any one of claims 1 to 5, characterized in that, The digital subcarrier multiplexing described above divides a high baud rate signal into multiple low baud rate signals, while eliminating the overlap between spectra and eliminating the need to maintain a guard interval between each subcarrier.

7. A simplified coherent system based on Alamouti coding and digital subcarrier multiplexing, characterized in that, Includes a transmitter digital signal processing (DSP) module and a receiver digital signal processing (DSP) module; The transmitter's digital signal processing (DSP) module is used to map pseudo-random bit sequences into a set of QAM symbols, which are then used to generate digital subcarrier signals for each polarization. Next, the subcarrier signals between the two polarizations are Alamouti encoded, and a root-raised cosine filter is used to perform Nyquist shaping on the signals. Then, the subcarriers on each polarization are digitally multiplexed, and finally, the signals are resampled. The receiver's digital signal processing (DSP) module is used to resample and orthogonalize the acquired signal, then demultiplex each subcarrier and perform matched filtering; next, synchronization, Alamouti decoding and equalization are performed, and finally the bit error rate (BER) is calculated. The transmitter's digital signal processing (DSP) module splits the laser-generated light source into two paths: one for optical signal modulation and the other for transmission to the receiver as a local oscillator (LO). The transmitter's offline DSP generates two DSCM signals, each containing four subcarriers (SC). After sampling by an arbitrary waveform generator (AWG), these signals are modulated onto orthogonal X and Y polarizations by a dual-polarization IQ modulator, amplified by an erbium-doped fiber amplifier (EDFA), and then transmitted through a single-mode fiber (SMF). The receiver's DSP module uses another EDFA to amplify the transmitted signal and controls the received power using an adjustable optical attenuator (VOA). In the carrier transmission branch, the carrier is also amplified by an EDFA after transmission through the fiber. A coupler then couples the carrier and signal into a 90° optical mixer. The real and imaginary parts of the signal are detected by two balanced detectors (BPD). The detected signals are acquired by an oscilloscope (Osc) and then processed in the offline DSP.

8. The simplified coherent system based on Alamouti coding and digital subcarrier multiplexing according to claim 7, characterized in that, The Alamouti encoding includes: at a second time, the information from the first time is repeated alternately between the two polarizations; the signal on the X polarization at the second time is the negative conjugate of the signal on the Y polarization at the first time, and the signal on the Y polarization at the second time is the conjugate of the signal on the X polarization at the first time; after the signal is transmitted through the optical fiber, the polarization state will change; the signal at the receiving end will be represented by the original X polarization signal being superimposed on the Y polarization, and similarly, the Y polarization signal will also be superimposed on the X polarization, forming crosstalk between X and Y polarizations.

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