A Quasi-coherent Communication Method and System Based on Dispersion Pre-compensation

Abstract

This invention discloses a quasi-coherent communication method and system based on dispersion pre-compensation, relating to the field of simplified coherent communication technology. The quasi-coherent communication method based on dispersion pre-compensation includes the following steps: performing dispersion pre-compensation on the generated original intensity modulation signal using a compensation function, wherein the original intensity modulation signal is a real signal and the compensation function is an odd function, so that the compensated intensity modulation signal is still a real signal; modulating the compensated intensity modulation signal to obtain an optical signal, and transmitting it to the receiving end for quasi-coherent reception, including coherent demodulation, single-sideband filtering, and envelope detection. This invention can effectively compensate for the effects of dispersion while continuing to use the traditional transmitter intensity modulation architecture.

Description

Technical Field

[0001] This invention relates to the field of simplified coherent communication technology, and specifically to a quasi-coherent communication method and system based on dispersion pre-compensation. Background Technology

[0002] Commercially available coherent optical communication technology is essentially a combination of coherent lightwave communications and digital signal processing (DSP). DSP generally consists of the following parts: dispersion compensation → clock sampling error extraction → adaptive equalization (completing polarization demultiplexing and polarization mode dispersion compensation) → carrier recovery (frequency difference estimation and compensation) → carrier recovery (phase noise estimation and compensation) → symbol decision → differential decoding. This digital coherent optical communication technology is widely used in 100G, 400G, and 800G optical communication systems, and has become the dominant technology in long-distance networks and metropolitan area networks.

[0003] With the rise of internet companies, data communication between data centers has grown exponentially. However, existing digital coherent optical communication technology is too expensive and consumes a lot of power, resulting in huge electricity costs and heat dissipation requirements. Meanwhile, due to the short distances between data centers, traditional direct-mode and direct-detection technologies are widely used for interconnection. However, for data center interconnections of 80km or more, coherent optical communication technology is required.

[0004] At the same time, access networks also have significant requirements for increased capacity and transmission distance. Using coherent optical communication technology can greatly improve signal reception sensitivity. However, due to the low-cost requirements of access networks, the complexity of existing digital coherent optical communication technologies is unacceptable.

[0005] The main energy consumption of existing coherent optical communication comes from analog-to-digital conversion (ADC) and digital signal processing (DSP). Therefore, replacing the digital signal processing part with analog signal processing or optical signal processing can significantly reduce the power consumption of optical modules and meet the needs of developing a green economy.

[0006] Therefore, the industry demands a coherent optical communication mode that is lower in cost and lower in power consumption. One direction is to replace the digital signal processing used in large-scale commercial coherent optical communication with analog signal processing. Optional technologies include optical phase-locked loop (PLL) technology, which was once a competitor to digital coherent communication. Figure 1 ), electronic lock phase loop technology ( Figure 2 Delay differential detection () Figure 3 However, each has its inherent drawbacks; including the analog coherent circuit currently under research, which attempts to accomplish all the functions of current digital signal processing using analog circuits, but the implementation of analog coherent circuits is quite difficult and is still in the technological exploration stage.

[0007] Another option is to adopt a simplified and lower-cost coherent optical communication mode. Among these, the quasi-coherent scheme ( Figure 4 The proposal has attracted widespread attention in the industry. Its method is as follows: the transmitter transmits a direct-modulated signal, such as an OOK signal or a PAM signal; the receiver performs coherent reception, uses an intrinsic laser to beat the optical signal, down-converts the direct-modulated signal, and then performs envelope detection on the signal.

[0008] The signal demodulated using intrinsic laser light can be considered an intermediate frequency (IF) electrical signal. Then, envelope detection technology is used to detect the IF signal envelope, which is then filtered and sent to the decision circuit for final determination. The advantages of this technique are that signal processing can be performed using relatively simple analog electronics, making it easy to implement; and because the effect of phase noise is negligible, relatively inexpensive lasers can be used. The quasi-coherent scheme also facilitates smooth upgrades to existing networks.

[0009] Traditional wavelength division multiplexing (WDM) systems operate in the C-band. When the signal symbol rate reaches 25 GHz or higher, dispersion becomes the main factor limiting the transmission distance of the communication system. Existing quasi-coherent solutions initially attempted to use equalization techniques at the receiving end. However, for quasi-coherent receiver systems, if equalization is used to compensate for dispersion after envelope detection, the compensation effect is not ideal because envelope detection is a quadratic process, which results in the loss of field information. A 25 GHz quasi-coherent system using this technique can only transmit 20 km in the C-band. Subsequently, the industry proposed single-sideband (SSB) technology, which uses a low-pass filter to remove the high-frequency sidebands of the intermediate frequency (IF) signal before performing envelope detection on the SSB IF signal. This SSB technology only eliminates the frequency attenuation effect caused by dispersion; it does not completely eliminate dispersion. When a 25 GHz quasi-coherent system transmits more than 35 km in the C-band, the system can no longer operate normally. Therefore, using only SSB technology is insufficient to further increase the transmission distance. If the intermediate frequency (IF) signal is equalized before envelope detection, although the IF signal still retains complete optical field information, only analog or radio frequency (RF) circuits can be used, which is technically very difficult. Achieving the adjustment of different dispersion values ​​required in practical engineering is even more technically challenging. Summary of the Invention

[0010] To address the shortcomings of existing technologies, the first aspect of this invention provides a quasi-coherent communication method based on dispersion pre-compensation, which can effectively compensate for the effects of dispersion while continuing to use the traditional transmitter intensity modulation architecture.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0012] A quasi-coherent communication method based on dispersion pre-compensation, characterized by the following steps:

[0013] Dispersion pre-compensation is performed on the generated original intensity modulation signal using a compensation function. The original intensity modulation signal is a real signal, and the compensation function is an odd function, so that the compensated intensity modulation signal is still a real signal.

[0014] The optical signal is modulated based on the compensated intensity modulation signal to obtain an optical signal, which is then transmitted to the receiving end for quasi-coherent reception.

[0015] In some embodiments,

[0016] when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth.

[0017] The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

[0018] In some embodiments,

[0019] when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth.

[0020] The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

[0021] In some embodiments, based on a compensation function, an FIR equalizer is used in the time domain to perform dispersion pre-compensation on the generated original intensity modulation signal.

[0022] In some embodiments, the spectrum of the original intensity modulated signal is multiplied by a compensation function in the frequency domain to perform dispersion pre-compensation on the generated original intensity modulated signal.

[0023] The second aspect of the present invention provides another quasi-coherent communication method based on dispersion pre-compensation, which can effectively compensate for the effects of dispersion while continuing to use the traditional transmitter intensity modulation architecture.

[0024] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0025] A quasi-coherent communication method based on dispersion pre-compensation, comprising the following steps:

[0026] The optical signal is received and modulated by the original intensity modulation signal after dispersion pre-compensation, so that the dispersion in the upper or lower sideband of the channel's total transfer function is completely compensated. The original intensity modulation signal is a real signal, and the dispersion pre-compensation function is an odd function, ensuring that the compensated intensity modulation signal remains a real signal.

[0027] The optical signal is demodulated to obtain the intermediate frequency signal, and then the baseband is filtered to obtain the single-sideband intermediate frequency signal. The retained single-sideband corresponds to the upper or lower sideband where the dispersion is fully compensated.

[0028] Envelope detection is performed on the single-sideband intermediate frequency signal to recover the original intensity modulation signal.

[0029] In some embodiments,

[0030] when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth.

[0031] The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

[0032] In some embodiments,

[0033] when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth.

[0034] The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

[0035] In some embodiments, the receiving end uses a bandwidth of The low-pass filter filters the intermediate frequency signal.

[0036] The second aspect of the present invention provides another quasi-coherent communication method based on dispersion pre-compensation, which can effectively compensate for the effects of dispersion while continuing to use the traditional transmitter intensity modulation architecture.

[0037] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0038] A quasi-coherent communication system based on dispersion pre-compensation includes:

[0039] The transmitting device is used to perform dispersion pre-compensation on the generated original intensity modulation signal using a compensation function. The original intensity modulation signal is a real signal, and the compensation function is an odd function, so that the compensated intensity modulation signal is still a real signal. The device modulates the signal based on the compensated intensity modulation signal to obtain an optical signal, and sends it to the receiving end for demodulation.

[0040] The receiving end equipment demodulates the optical signal and filters it to obtain the intermediate frequency (IF) signal; it then performs envelope detection on the IF signal to recover the original intensity-modulated signal.

[0041] Compared with the prior art, the advantages of the present invention are as follows:

[0042] The quasi-coherent communication method based on dispersion pre-compensation in this invention pre-compensates the dispersion of the generated original intensity modulation signal using a compensation function. The original intensity modulation signal is a real signal, and the compensation function is an odd function. The compensated intensity modulation signal is then modulated to obtain an optical signal, which is sent to the receiving end for demodulation. This invention utilizes the single-sideband filtering characteristic of the receiving end, ensuring that the pre-compensated modulation signal remains a real signal, thus allowing direct intensity modulation of the laser and reducing technical difficulty and cost. Attached Figure Description

[0043] Figure 1 This is a block diagram of a coherent optical receiver based on an optical phase-locked loop in the prior art;

[0044] Figure 2 This is a block diagram of a coherent optical receiver based on an electronically locked phase loop in the prior art;

[0045] Figure 3 A block diagram of the existing technology for coherent optical receivers based on delayed coherence;

[0046] Figure 4 This is a schematic diagram of a quasi-coherent receiver structure;

[0047] Figure 5 This is a flowchart of a quasi-coherent communication method based on dispersion pre-compensation at the origin in an embodiment of the present invention;

[0048] Figure 6 This is a flowchart of a quasi-coherent communication method based on dispersion pre-compensation at the receiving end in an embodiment of the present invention. Detailed Implementation

[0049] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0050] See Figure 5 As shown in the figure, an embodiment of the present invention discloses a quasi-coherent communication method based on dispersion pre-compensation, which includes the following steps:

[0051] S1. Use a compensation function to perform dispersion pre-compensation on the generated original intensity modulation signal. The original intensity modulation signal is a real signal, and the compensation function is an odd function, so that the compensated intensity modulation signal is still a real signal.

[0052] S2. Modulate the compensated intensity modulation signal to obtain an optical signal, and send it to the receiving end for quasi-coherent reception.

[0053] It is worth noting that in optical fiber transmission, the dispersion effect can be expressed in the frequency domain as:

[0054] (1)

[0055] Where ω is the angular frequency and z is the transmission distance. is the dispersion coefficient.

[0056] If classical dispersion compensation is used, the frequency domain expression of the compensation function is:

[0057] (2)

[0058] Using equation (2) for compensation can ensure the entire transmission process. However, due to For even functions, use After dispersion pre-compensation at the transmitter, the real signal used for modulation will become a complex signal. The transmitter must use complex IQ modulation, that is, adjust the MZM modulator with the real part signal and the imaginary part signal respectively, and then combine the two modulated optical signals.

[0059] To address the aforementioned issues, this invention proposes a method that ensures the modulated signal remains a real signal after dispersion pre-compensation by utilizing the characteristics of single-sideband filtering at the receiving end. This approach allows for direct intensity modulation of the laser or the use of an external modulator for intensity modulation.

[0060] The following specific examples will further illustrate this point:

[0061] For ease of description, one end will be named the X end and the other end the Y end. The X end can be the sending end and the Y end the receiving end; alternatively, the X end can be the receiving end and the Y end the sending end.

[0062] Using X as the transmitting end and Y as the receiving end, without loss of generality, the demodulation laser frequency at the Y end is set to be lower than the transmitting laser frequency at the X end, and ,in The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth.

[0063] First, an intensity-modulated signal x1(t) to be transmitted is generated at the X terminal. x1(t) is a real signal with the following spectrum function: It is understandable. The positive and negative spectra are conjugate and equal, that is:

[0064] (3)

[0065] Here, conj() is used for conjugate calculation.

[0066] Then, a compensation function is used to perform dispersion pre-compensation on the signal. The function is:

[0067] (4)

[0068] Where sgn() is a symbolic function, it can be seen that It is an odd function and satisfies that the positive and negative spectra are conjugate, i.e.

[0069] (5)

[0070] The compensated signal spectrum is as follows:

[0071] (6)

[0072] because satisfy:

[0073] (7)

[0074] Therefore, the time-domain signal x2(t) after using the dispersion pre-compensation method in this embodiment of the invention is still a real signal, so the traditional intensity modulation architecture can be used.

[0075] Specific pre-compensation methods can include using an FIR equalizer in the time domain, or directly multiplying the signal spectrum in the frequency domain. .

[0076] Subsequently, the optical signal is modulated using x2(t), transmitted through optical fiber, and then coherently demodulated. The overall transfer function of the channel is:

[0077] (8)

[0078] As can be seen, the dispersion of the lower sideband is fully compensated. Although the dispersion of the upper sideband is actually doubled, in this embodiment of the invention, it will not have much impact due to the single-sideband filtering at the receiving end.

[0079] At the receiving end, after demodulation of the optical signal, its intermediate frequency signal spectrum is:

[0080] (9)

[0081] because The upper band, after demodulation, constitutes the high-frequency portion of the intermediate frequency (IF) signal, and the lower band, after demodulation, constitutes the low-frequency portion of the IF signal.

[0082] Using bandwidth The low-pass filter filters the signal, and the spectrum of the filtered intermediate frequency signal is as follows:

[0083] (10)

[0084] Finally, envelope detection is performed on the filtered intermediate frequency signal to recover the original signal x1(t).

[0085] It is worth noting that, in another embodiment, in order to reuse the laser, when the Y end is used as the transmitting end and the X end as the receiving end, then... At this time, due to The upper sideband demodulation forms the low-frequency part of the intermediate frequency signal, and the lower sideband demodulation forms the high-frequency part of the intermediate frequency signal. The low-pass filter at the X end will filter out the lower sideband of the transmitted signal. At this time, the dispersion pre-compensation only needs to ensure the elimination of dispersion of the upper sideband signal. At this time, the pre-compensation at the transmitting end of equation (4) is modified as follows:

[0086] (11)

[0087] The overall transfer function of the channel becomes:

[0088] (12)

[0089] After filtering and envelope detection, the original signal can be recovered, and the dispersion is effectively compensated.

[0090] In summary, the quasi-coherent communication method based on dispersion pre-compensation in this invention pre-compensates the dispersion of the generated original intensity modulation signal using a compensation function. The original intensity modulation signal is a real signal, and the compensation function is an odd function. The compensated intensity modulation signal is then modulated to obtain an optical signal, which is sent to the receiving end for demodulation. This utilizes the single-sideband filtering characteristic of the receiving end, ensuring that the pre-compensated modulation signal remains a real signal, thus allowing direct intensity modulation of the laser and reducing technical difficulty and cost.

[0091] See Figure 6 As shown in the figure, this invention discloses another quasi-coherent communication method based on dispersion pre-compensation, which includes the following steps:

[0092] S1 ， The optical signal is received and modulated by the original intensity modulation signal after dispersion pre-compensation, so that the dispersion in the upper or lower sideband of the channel's total transfer function is completely compensated. The original intensity modulation signal is a real signal, and the dispersion pre-compensation function is an odd function, ensuring that the compensated intensity modulation signal remains a real signal.

[0093] S2 ， The optical signal is demodulated to obtain an intermediate frequency signal, and then baseband filtering is performed to obtain a single-sideband intermediate frequency signal. The retained single-sideband corresponds to the upper or lower sideband where the dispersion is fully compensated.

[0094] S3 ， Envelope detection is performed on the single-sideband intermediate frequency signal to recover the original intensity modulation signal.

[0095] In some embodiments,

[0096] when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth.

[0097] The compensation function is Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

[0098] when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth.

[0099] The compensation function is Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

[0100] In some embodiments, based on a compensation function, an FIR equalizer is used in the time domain to perform dispersion pre-compensation on the generated original intensity modulation signal.

[0101] In some embodiments, the spectrum of the original intensity modulated signal is multiplied by a compensation function in the frequency domain to perform dispersion pre-compensation on the generated original intensity modulated signal.

[0102] This invention also discloses a quasi-coherent communication system based on dispersion pre-compensation, comprising: a transmitting device and a receiving device.

[0103] The transmitting device is used to perform dispersion pre-compensation on the generated original intensity modulation signal using a compensation function. The original intensity modulation signal is a real signal, and the compensation function is an odd function, so that the compensated intensity modulation signal is still a real signal. The device modulates the signal based on the compensated intensity modulation signal to obtain an optical signal, which is then sent to the receiving end for demodulation. The receiving device demodulates the optical signal and filters it to obtain an intermediate frequency (IF) signal. It also performs envelope detection on the IF signal to recover the original intensity modulation signal.

[0104] In some embodiments, when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth.

[0105] The compensation function is Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

[0106] when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth.

[0107] The compensation function is Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

[0108] In some embodiments, the transmitting device uses an FIR equalizer in the time domain based on a compensation function to perform dispersion pre-compensation on the generated original intensity modulated signal.

[0109] In some embodiments, the transmitting device multiplies the spectrum of the original intensity modulated signal by a compensation function in the frequency domain to perform dispersion pre-compensation on the generated original intensity modulated signal.

[0110] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A quasi-coherent communication method based on dispersion pre-compensation, characterized in that, The method includes the following steps: Dispersion pre-compensation is performed on the generated original intensity modulation signal using a compensation function. The original intensity modulation signal is a real signal, and the compensation function is an odd function, so that the compensated intensity modulation signal is still a real signal. The optical signal is modulated based on the compensated intensity modulation signal to obtain an optical signal, which is then transmitted to the receiving end for quasi-coherent reception. when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth. The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. The dispersion coefficient; when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth. The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

2. The quasi-coherent communication method based on dispersion pre-compensation according to claim 1, characterized in that: Based on the compensation function, an FIR equalizer is used in the time domain to perform dispersion pre-compensation on the generated original intensity modulation signal.

3. The quasi-coherent communication method based on dispersion pre-compensation according to claim 1, characterized in that: In the frequency domain, the spectrum of the original intensity modulated signal is multiplied by a compensation function to perform dispersion pre-compensation on the generated original intensity modulated signal.

4. A quasi-coherent communication method based on dispersion pre-compensation, characterized in that, The method includes the following steps: The optical signal is received and modulated by the original intensity modulation signal after dispersion pre-compensation, so that the dispersion in the upper or lower sideband of the channel's total transfer function is completely compensated. The original intensity modulation signal is a real signal, and the dispersion pre-compensation function is an odd function, ensuring that the compensated intensity modulation signal remains a real signal. The optical signal is demodulated to obtain the intermediate frequency signal, and then the baseband is filtered to obtain the single-sideband intermediate frequency signal. The retained single-sideband corresponds to the upper or lower sideband where the dispersion is fully compensated. Envelope detection is performed on the single-sideband intermediate frequency signal to recover the original intensity modulation signal; when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth. The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. The dispersion coefficient; when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth. The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

5. A quasi-coherent communication method based on dispersion pre-compensation according to claim 4, characterized in that: The receiving end uses bandwidth of The low-pass filter filters the intermediate frequency signal.

6. A quasi-coherent communication system based on dispersion pre-compensation, characterized in that, include: The transmitting device is used to perform dispersion pre-compensation on the generated original intensity modulation signal using a compensation function. The original intensity modulation signal is a real signal, and the compensation function is an odd function, so that the compensated intensity modulation signal is still a real signal. The device modulates the signal based on the compensated intensity modulation signal to obtain an optical signal, and sends it to the receiving end for demodulation. The receiving end equipment is used to demodulate the optical signal and filter it to obtain the intermediate frequency signal; and to perform envelope detection on the intermediate frequency signal to recover the original intensity modulation signal. when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth. The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. The dispersion coefficient; when At that time, among them The carrier frequency of the demodulated intermediate frequency signal. The carrier frequency of the transmitting laser. 2B represents the carrier frequency of the receiving laser and the signal bandwidth. The compensation function is: Where sgn() is the sign function, ω is the angular frequency, and z is the transmission distance. is the dispersion coefficient.

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