Multi-residual carrier modulation method, demodulation method and system

Abstract

The invention belongs to the technical field of optical fiber communication and coherent optical communication, and discloses a multi-residual carrier modulation method, demodulation method and system. The system comprises a transmitting end and a receiving end, the transmitting end comprises a mapping module and a modulator; the mapping module is used for modulating and mapping a to-be-sent digital signal and generating a corresponding modulation signal; the modulator modulates an optical carrier according to the modulation signal to obtain a modulated optical signal; introducing an optical carrier component comprising a plurality of optical carriers into the modulated optical signal to form a composite optical signal comprising the modulated optical signal and a residual carrier component; the receiving end comprises a detection module, a compensation module and a signal recovery module; the detection module performs beat frequency on the received composite optical signal and local oscillation light to realize coherent detection to obtain an electric signal; the compensation module compensates the electric signal; and the signal recovery module filters the compensated signal, and performs phase noise compensation on the signal by using any residual carrier component in the residual carrier components obtained by filtering to obtain a signal after phase recovery.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber communication and coherent optical communication technology, specifically relating to a multi-residual carrier modulation method, demodulation method and system, and particularly to a multi-residual carrier generation and phase noise compensation method that is insensitive to modulator bias drift and receiver DC isolation. Background Technology

[0002] As optical fiber communication systems evolve towards higher transmission rates and higher spectral efficiency, coherent optical communication systems based on high-order quadrature amplitude modulation (QAM) have been widely adopted. However, in coherent detection systems, laser phase noise causes random rotation of the received signal constellation, severely limiting the system performance of high-order modulation formats. The impact of phase noise is particularly significant in systems employing low-cost, high-linewidth lasers (such as distributed feedback lasers, DFB).

[0003] To reduce the impact of phase noise, researchers have proposed a phase noise compensation method based on a residual carrier. This method introduces a residual carrier of a certain power by offsetting the modulator bias point from the extinction point at the transmitter. This residual carrier is then extracted at the receiver as a phase reference, thereby achieving frequency offset estimation and phase noise compensation. This type of method does not require the introduction of additional time-domain or frequency-domain pilot signals and can improve the tolerance to laser linewidth without significantly increasing system complexity, making it highly valuable for engineering applications.

[0004] However, in practical engineering systems, the above-mentioned scheme based on a single residual carrier still has limitations. On the one hand, the modulator's bias point is easily affected by factors such as temperature changes and device aging, causing the residual carrier power to change over time, thus affecting the stability and reliability of phase noise compensation. On the other hand, the front end of a practical receiver usually includes AC coupling or DC-block circuits to suppress DC components and low-frequency noise. This can cause significant attenuation of the residual carrier when the frequency difference between the signal light and the local oscillator light is small or close to zero, and may even lead to the residual carrier not being effectively detected, causing the phase noise compensation method based on the residual carrier to fail.

[0005] Therefore, there is an urgent need for a new residual carrier generation and phase noise compensation scheme that can stably generate a residual carrier that can be used for phase reference even under engineering conditions such as modulator bias drift and DC isolation circuit at the receiver, thereby improving the robustness and practicality of the residual carrier-based phase noise compensation method in actual coherent optical communication systems. Summary of the Invention

[0006] To address the technical problems existing in the prior art, the present invention aims to provide a multi-residual carrier modulation method, demodulation method, and system. By generating a residual carrier that is insensitive to modulator bias drift and receiver DC isolation and performing phase noise compensation and demodulation, the present invention can stably generate a residual carrier that can be used for phase reference without increasing the complexity of traditional coherent optical communication systems, tolerate phase noise introduced by large-linewidth lasers, and achieve coherent detection of high-order orthogonal amplitude modulation signals.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A multi-residual carrier modulation method, comprising the following steps: The digital signal to be transmitted is modulated to generate the corresponding modulated signal. The optical carrier is modulated according to the above modulation signal to obtain a modulated optical signal carrying the above digital signal information; An optical carrier component containing multiple optical carriers is introduced into the modulated optical signal to form a composite optical signal containing the modulated optical signal and the residual carrier component.

[0008] Preferably, a low-frequency sinusoidal jitter signal is introduced into the modulated optical signal by introducing a low-frequency sinusoidal jitter signal into the bias terminal voltage of the modulator, thereby introducing an optical carrier component containing multiple optical carriers; the frequency of the low-frequency sinusoidal jitter signal is higher than the cutoff frequency of the DC isolation or AC coupling circuit at the receiving end.

[0009] Preferably, the residual carrier component is a three-residual carrier structure, including a central residual carrier and two frequency-shifted residual carrier components symmetrically distributed to its left and right.

[0010] Preferably, the bias point of the modulator is set to deviate from the extinction point.

[0011] Preferably, the modulator is an IQ modulator.

[0012] A multi-residual carrier demodulation method, comprising the following steps: The received composite optical signal is beat-frequency with the local oscillator light to achieve coherent detection and obtain an electrical signal; The electrical signal is resampled and dispersion compensated after analog-to-digital conversion; the frequency offset is estimated for the dispersion-compensated signal to obtain a frequency offset estimate; and digital frequency offset compensation is performed on the dispersion-compensated signal based on the frequency offset estimate. The digital frequency offset compensation signal is filtered, and any one of the residual carrier components obtained from the filtering is used to compensate for the phase noise of the digital frequency offset compensation signal to obtain the phase-recovered signal.

[0013] Preferably, one residual carrier component is selected from the residual carrier components obtained by filtering; the complex conjugate of the selected residual carrier component is multiplied by the digital frequency offset compensation signal to obtain the phase-recovered signal.

[0014] A multi-residual carrier modulation and demodulation system, characterized in that it includes a transmitter and a receiver; The transmitter includes a mapping module and a modulator; The mapping module is used to modulate and map the digital signal to be transmitted, and generate a corresponding modulated signal to be input into the modulator; The modulator is used to modulate an optical carrier according to the above-mentioned modulation signal to obtain a modulated optical signal carrying the above-mentioned digital signal information; and to introduce an optical carrier component containing multiple optical carriers into the modulated optical signal to form a composite optical signal containing the modulated optical signal and a residual carrier component. The receiving end includes a detection module, a compensation module, and a signal recovery module; The detection module is used to beat the received composite optical signal with the local oscillator light to achieve coherent detection and obtain an electrical signal; The compensation module is used to resample and perform dispersion compensation on the electrical signal after analog-to-digital conversion; to estimate the frequency offset of the dispersion-compensated signal to obtain a frequency offset estimate; and to perform digital frequency offset compensation on the dispersion-compensated signal based on the frequency offset estimate. The signal recovery module is used to filter the digital frequency offset compensation signal, and use any one of the residual carrier components in the filtered residual carrier components to perform phase noise compensation on the digital frequency offset compensation signal to obtain the phase-recovered signal.

[0015] Preferably, a low-frequency sinusoidal jitter signal is introduced into the bias terminal voltage of the modulator to introduce an optical carrier component containing multiple optical carriers into the modulated optical signal. The frequency of the low-frequency sinusoidal jitter signal is higher than the cutoff frequency of the DC isolation or AC coupling circuit at the receiving end.

[0016] Preferably, the signal recovery module selects one residual carrier component from the filtered residual carrier components; multiplies the complex conjugate of the selected residual carrier component with the digital frequency offset compensation signal to obtain the phase-recovered signal.

[0017] Compared with the prior art, the positive effects of the present invention are as follows: This invention utilizes the approximately linear response of the modulator near the extinction point. By introducing a DC bias and low-frequency jitter at the bias end, the residual carrier is expanded from a single DC component to a component located between 0 and... The three narrowband components (3RC) are mainly composed of jitter amplitude. This invention significantly reduces the sensitivity of the residual carrier amplitude to modulator bias drift; simultaneously, even with DC isolation or AC coupling at the receiver, stable phase noise compensation can still be achieved using the frequency-shifted residual carrier. This invention requires no additional pilot signal, does not increase system complexity, is fully compatible with existing coherent or simplified coherent optical communication system structures, and enables reliable transmission of high-order modulation format signals using low-cost, high-linewidth lasers. Attached Figure Description

[0018] Figure 1 This is a flowchart of the multi-residual carrier modulation method of the present invention.

[0019] Figure 2 This is a schematic diagram of the principle and structure of multi-residual carrier generation and phase noise compensation based on bias jitter according to the present invention.

[0020] Figure 3 System block diagram of the present invention.

[0021] Figure 4 The figure shows the experimental results of the electrical spectrum of the receiver under the multiple residual carrier scheme of the present invention; (1) is the overall spectrum of the electrical domain signal at the receiving end when the laser linewidth at the transmitting end is 100 kHz; (2) is the overall spectrum of the electrical domain signal at the receiving end when the laser linewidth at the transmitting end is 1 MHz; (3) is the local amplified spectrum of the electrical domain signal at the receiving end when the laser linewidth at the transmitting end is 100 kHz; (4) is the local amplified spectrum of the electrical domain signal at the receiving end when the laser linewidth at the transmitting end is 1 MHz.

[0022] Figure 5 The figure shows the experimental results of phase noise tracking using different residual carrier components in the multi-residual carrier scheme of the present invention. (1) The phase tracking trajectories corresponding to different residual carrier components when the laser linewidth at the transmitting end is 100 kHz; (2) The phase tracking trajectories corresponding to different residual carrier components when the laser linewidth at the transmitting end is 1 MHz. Detailed Implementation

[0023] To make the various technical features, advantages, or effects of the present invention more apparent and understandable, a detailed description is provided below in conjunction with the accompanying drawings.

[0024] like Figure 1 As shown, an optional embodiment of the present invention provides a multi-residual carrier modulation method, the steps of which include: The digital signal to be transmitted is modulated to generate the corresponding modulated signal. The optical carrier is modulated according to the above modulation signal to obtain a modulated optical signal carrying the above digital signal information; An optical carrier component containing multiple optical carriers is introduced into the modulated optical signal to form a composite optical signal containing the modulated optical signal and the residual carrier component.

[0025] In an optional embodiment, the residual carrier generation method provided by the present invention, which is insensitive to modulator bias drift and receiver DC isolation, includes the following steps: At the transmitting end, the digital signal to be transmitted is first modulated and mapped to generate a corresponding complex baseband modulated signal. After digital-to-analog conversion, the IQ modulator is driven to modulate the amplitude and phase of the continuous wave optical carrier output by the laser, thereby obtaining a modulated optical signal carrying data information.

[0026] Meanwhile, by setting the bias point of the IQ modulator to a position away from the extinction point, and by making the bias point contain a certain AC component, a certain power optical carrier component containing multiple optical carriers is artificially introduced into the modulated optical signal, forming a composite optical signal containing the modulated optical signal and the residual carrier component.

[0027] Let the light field output by the laser be represented as: in The optical carrier angular frequency, This represents the phase noise introduced by the signal laser. In an IQ modulator, a residual carrier component is introduced by setting the modulator bias point away from the extinction point. For a Mach-Zehnder modulator, the response of the carrier component in its output optical field to the bias voltage can be approximated by a cosine function. in The bias voltage is... Half-wave voltage. If the bias corresponding to the extinction point (null point) is set to... and order When the modulator operates near the extinction point and satisfies the small signal condition When, for the above cosine function in Performing a first-order Taylor expansion in the vicinity, we find that the carrier component has an approximately linear relationship with the bias voltage: Thus obtain .

[0028] Furthermore, to ensure stable detection of the residual carrier even under DC isolation or AC coupling conditions at the receiver, and to reduce the sensitivity of the residual carrier amplitude to bias drift, this invention introduces a DC component and a low-frequency sinusoidal jitter component into the bias voltage of the modulator. The equivalent can be written as in Used to set the average residual carrier strength The amplitude of the shaking. The jitter frequency, and It is higher than the cutoff frequency of the DC isolation or AC coupling circuit at the receiving end.

[0029] Furthermore, the above Substituting into the linear approximation, we can obtain the carrier component approximation as follows: in This is the proportionality constant. Write the sine term in exponential form. The aforementioned carrier components can then be decomposed into a superposition of three terms. Therefore, it can be seen that the present invention forms three residual carrier components in the optical domain, respectively located in , and At this point, a three-residual-carrier (3RC) structure is formed; the amplitudes of the frequency-shifted residual carriers on both sides are mainly determined by the jitter amplitude. This decision makes it more robust to modulator bias drift.

[0030] An optional embodiment of the present invention provides a phase noise compensation method based on three residual carriers, comprising the following steps: The signal light containing the three residual carrier structure is transmitted to the receiving end via an optical fiber link. The receiving end performs coherent detection by beating the signal light with the local oscillator light to obtain an electrical signal. The electrical signal is then converted from analog to digital and resampled to obtain a digital signal.

[0031] Furthermore, let the local oscillator field be... Then, after the three residual carrier components beat at the local oscillator frequency, a signal will be generated in the electrical domain located at... as well as The three narrowband components.

[0032] Furthermore, when the receiver has a DC isolation or AC coupling structure, components near the zero frequency will be suppressed; or when the components near the zero frequency are suppressed due to bias point drift, the components located near the zero frequency will be suppressed. The frequency-shifted residual carrier components can still be completely preserved, thus ensuring that the phase reference remains usable.

[0033] Furthermore, the digital signal is subjected to narrowband filtering, and a phase reference signal is extracted from any frequency-shifted residual carrier component (or the center residual carrier component) among the three residual carriers. Phase extraction is then performed to obtain a phase estimate, which is used to perform complex exponential rotation compensation on the received signal to achieve phase noise compensation. Furthermore, since the three residual carrier components originate from the same optical carrier, their phase evolution is consistent. Therefore, the central residual carrier or any frequency-shifted residual carrier can be selected for phase noise compensation, or the phase estimation results of multiple residual carriers can be jointly processed to improve the stability and robustness of phase noise compensation.

[0034] like Figure 2 As shown, this invention discloses a multi-residual carrier modulation method based on bias jitter, which is applicable to the transmitter end of a coherent optical communication system.

[0035] In coherent optical communication systems, the transmitting end typically uses an IQ modulator to modulate the signal light. Let the output light field of the laser incident on the modulator be represented as: Where j is the imaginary unit, The optical carrier angular frequency, The phase noise of the signal laser at time t, which generates the digital signal, can be described by the Wiener process model.

[0036] Ideally, the bias point of the IQ modulator is set at the extinction point, which allows the optical carrier component to be sufficiently suppressed, retaining only the sideband components corresponding to the modulated signal. However, in practical engineering systems, in order to introduce a residual carrier for phase noise compensation, the modulator bias point is usually set slightly off from the extinction point.

[0037] Within the small-signal operating range near the extinction point, the relationship between the carrier component and the bias voltage in the modulator output optical field can be approximated as linear. Therefore, when the modulator bias voltage changes with time, the amplitude of the carrier component in the output optical field also changes accordingly.

[0038] Based on the above characteristics, the present invention introduces a low-frequency sinusoidal dithering signal at the modulator bias terminal, so that the modulator bias voltage is expressed as: in, This is the DC bias voltage. This is the bias jitter amplitude. The frequency of the low-frequency sinusoidal jitter signal. The frequency is higher than the cutoff frequency of the DC isolation or AC coupling circuit at the receiving end, so that the frequency-shifted residual carrier component generated by the bias jitter can avoid the suppression frequency band of the DC isolation or AC coupling circuit, thereby being stably detected at the receiving end and used for phase noise compensation. It should be noted that the low-frequency sinusoidal jitter signal only acts on the bias end of the modulator to control the time variation of the optical carrier suppression degree. It does not carry data information, nor does it change the modulation mode of the IQ modulator for the signal to be transmitted, ultimately resulting in a composite optical signal containing the modulated optical signal and the residual carrier component.

[0039] Under this bias jitter, the residual carrier component in the modulator output optical field no longer appears as a single DC carrier component, but is modulated into multiple frequency components in the frequency domain. After spectral expansion, the output optical field can be represented as containing a central residual carrier and two frequency-shifted residual carrier components symmetrically distributed to its left and right, thus forming a multi-residual carrier structure.

[0040] In an optional embodiment, when a single low-frequency sinusoidal bias jitter is used, the generated residual carrier mainly includes the carrier located at the center frequency and its... The three carrier components at that point form a three residual carrier (3RC) structure.

[0041] After digital-to-analog conversion and electro-optic modulation, under bias jitter, the signal light output by the transmitter can be expressed as: in, The digital signal to be transmitted This represents the multiple residual carrier components generated by offset jitter, whose spectrum includes the central residual carrier and its symmetrically shifted residual carrier components. Since the amplitude of the multiple residual carrier components is mainly determined by the offset jitter amplitude... The decision is made, and does not entirely depend on whether the modulator bias point is precisely maintained near the extinction point, therefore this structure It has stronger robustness to modulator bias drift.

[0042] like Figure 2 As shown, the composite optical signal output from the transmitter (i.e., the signal light containing the data signal and multiple residual carrier components) is transmitted to the receiver via an optical fiber link. At the receiver, it beats with the local oscillator light generated by the local oscillator laser to achieve coherent detection, obtaining the electrical signal r(t). Let the received signal light... The frequency deviation between the light and the local oscillator is Then the electrical signal at the receiving end can be expressed as: in, This represents the phase of the local oscillator light.

[0043] After analog-to-digital conversion, the electrical signal r(t) is first resampled and subjected to dispersion compensation. Subsequently, frequency offset estimation is performed on the dispersion-compensated signal based on a multi-residual carrier structure. The frequency offset estimate is obtained by detecting the position of the residual carrier components in the signal spectrum, and digital frequency offset compensation is then performed on the dispersion-compensated signal.

[0044] After frequency offset compensation, the digitally offset compensated signal is subjected to narrowband digital low-pass filtering. One residual carrier component is randomly selected from the multiple residual carrier components for extraction, resulting in a residual carrier signal carrying phase noise. Since the three residual carrier components originate from the same laser, their phase noise evolution is highly consistent. Therefore, using the center residual carrier or any side residual carrier for phase noise extraction can yield equivalent phase noise estimation results.

[0045] The phase noise compensation is completed by taking the complex conjugate of the extracted residual carrier signal and multiplying it with the compensated received signal, thus obtaining the phase-recovered signal. in, This is the signal after frequency offset compensation. The extracted residual carrier phase reference signal is used. After phase noise compensation, the signal is then subjected to frame synchronization, subcarrier demodulation, channel equalization, Nyquist matched filtering, and downsampling processing in sequence to finally recover the original transmitted digital signal S(t).

[0046] like Figure 3 As shown, an optional embodiment of the present invention also provides a multi-residual carrier modulation and demodulation system, characterized in that it includes a transmitter and a receiver; The transmitter includes a mapping module and a modulator; The mapping module is used to modulate and map the digital signal to be transmitted, and generate a corresponding modulated signal to be input into the modulator; The modulator is used to modulate an optical carrier according to the above-mentioned modulation signal to obtain a modulated optical signal carrying the above-mentioned digital signal information; and to introduce an optical carrier component containing multiple optical carriers into the modulated optical signal to form a composite optical signal containing the modulated optical signal and a residual carrier component. The receiving end includes a detection module, a compensation module, and a signal recovery module. The detection module is used to beat the received composite optical signal with the local oscillator light to achieve coherent detection and obtain an electrical signal; The compensation module is used to resample and perform dispersion compensation on the electrical signal after analog-to-digital conversion; to estimate the frequency offset of the dispersion-compensated signal to obtain a frequency offset estimate; and to perform digital frequency offset compensation on the dispersion-compensated signal based on the frequency offset estimate. The signal recovery module is used to filter the digital frequency offset compensation signal, and use any one of the residual carrier components in the filtered residual carrier components to perform phase noise compensation on the digital frequency offset compensation signal to obtain the phase-recovered signal.

[0047] In an alternative embodiment, a low-frequency sinusoidal jitter signal is introduced into the bias voltage of the modulator, thereby introducing an optical carrier component containing multiple optical carriers into the modulated optical signal. The frequency of the low-frequency sinusoidal jitter signal is higher than the cutoff frequency of the DC isolation or AC coupling circuit at the receiving end.

[0048] In one optional embodiment, the signal recovery module selects one residual carrier component from the filtered residual carrier components; multiplies the complex conjugate of the selected residual carrier component with the digital frequency offset compensation signal to obtain the phase-recovered signal.

[0049] Performance testing: Figure 4 and Figure 5 This is the experimental result of transmitting 25 kilometers using a 64G baud rate and a 16QAM signal. (For example...) Figure 4 As shown, under different laser linewidths at the transmitting end, multiple residual carrier components generated by bias jitter can be clearly observed in the electrical domain spectrum at the receiving end, verifying the feasibility of this method in engineering systems. Figure 5 As shown, when using different residual carrier components for phase noise tracking, the obtained phase trajectories are highly consistent, indicating that the multi-residual carrier structure provides a redundant and equivalent phase reference source for phase noise compensation. In summary, this invention, by introducing low-frequency bias jitter at the modulator bias end, realizes a multi-residual carrier generation and phase noise compensation method that is insensitive to modulator bias drift and receiver DC isolation conditions. Without increasing system hardware complexity, it significantly improves the stability and robustness of the residual carrier-based phase noise compensation method in practical engineering systems.

[0050] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Those skilled in the art can modify or make equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention. The scope of protection of the present invention should be determined by the claims.

Claims

1. A multi-residual carrier modulation method, comprising the following steps: The digital signal to be transmitted is modulated to generate the corresponding modulated signal. The optical carrier is modulated according to the above modulation signal to obtain a modulated optical signal carrying the above digital signal information; An optical carrier component containing multiple optical carriers is introduced into the modulated optical signal to form a composite optical signal containing the modulated optical signal and the residual carrier component.

2. The method according to claim 1, characterized in that, A low-frequency sinusoidal jitter signal is introduced into the modulated optical signal by introducing an optical carrier component containing multiple optical carriers into the modulated optical signal; the frequency of the low-frequency sinusoidal jitter signal is higher than the cutoff frequency of the DC isolation or AC coupling circuit at the receiving end.

3. The method according to claim 1, characterized in that, The residual carrier component is a three-residual carrier structure, including a central residual carrier and two frequency-shifted residual carrier components symmetrically distributed to its left and right.

4. The method according to claim 1, 2, or 3, characterized in that, The bias point of the modulator is set to deviate from the extinction point.

5. The method according to claim 1, 2, or 3, characterized in that, The modulator is an IQ modulator.

6. A multi-residual carrier demodulation method, comprising the following steps: The received composite optical signal is beat-frequencyd with the local oscillator light to obtain an electrical signal; The electrical signal is resampled and dispersion compensated after analog-to-digital conversion; Frequency offset estimation is performed on the dispersion-compensated signal to obtain the frequency offset estimate; Digital frequency offset compensation is performed on the dispersion-compensated signal based on the frequency offset estimate; The digital frequency offset compensation signal is filtered, and any one of the residual carrier components obtained from the filtering is used to compensate for the phase noise of the digital frequency offset compensation signal to obtain the phase-recovered signal.

7. The method according to claim 6, characterized in that, Select any one residual carrier component from the filtered residual carrier components; multiply the complex conjugate of the selected residual carrier component with the digital frequency offset compensation signal to obtain the phase-recovered signal.

8. A multi-residual carrier modulation and demodulation system, characterized in that, Includes the transmitter and receiver; The transmitter includes a mapping module and a modulator; The mapping module is used to modulate and map the digital signal to be transmitted, and generate a corresponding modulated signal to be input into the modulator; The modulator is used to modulate an optical carrier according to the above-mentioned modulation signal to obtain a modulated optical signal carrying the above-mentioned digital signal information; and to introduce an optical carrier component containing multiple optical carriers into the modulated optical signal to form a composite optical signal containing the modulated optical signal and a residual carrier component. The receiving end includes a detection module, a compensation module, and a signal recovery module; The detection module is used to beat the received composite optical signal with the local oscillator light to achieve coherent detection and obtain an electrical signal; The compensation module is used to resample and perform dispersion compensation on the electrical signal after analog-to-digital conversion; and to perform frequency offset estimation on the dispersion-compensated signal to obtain a frequency offset estimate. Digital frequency offset compensation is performed on the dispersion-compensated signal based on the frequency offset estimate; The signal recovery module is used to filter the digital frequency offset compensation signal, and use any one of the residual carrier components in the filtered residual carrier components to perform phase noise compensation on the digital frequency offset compensation signal to obtain the phase-recovered signal.

9. The system according to claim 8, characterized in that, A low-frequency sinusoidal jitter signal is introduced into the bias terminal voltage of the modulator to introduce an optical carrier component containing multiple optical carriers into the modulated optical signal. The frequency of the low-frequency sinusoidal jitter signal is higher than the cutoff frequency of the DC isolation or AC coupling circuit at the receiving end.

10. The system according to claim 8, characterized in that, The signal recovery module selects one residual carrier component from the filtered residual carrier components; it then multiplies the complex conjugate of the selected residual carrier component with the digital frequency offset compensation signal to obtain the phase-recovered signal.

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