CN106533573B - The asymmetric twin single sideband modulation of one kind, demodulation method and system - Google Patents
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- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/67—Optical arrangements in the receiver
- H04B10/671—Optical arrangements in the receiver for controlling the input optical signal
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/60—Receivers
- H04B10/66—Non-coherent receivers, e.g. using direct detection
- H04B10/69—Electrical arrangements in the receiver
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Abstract
The invention discloses a kind of asymmetric twin single sideband modulation, demodulation method and systems.The transmitting terminal of this system includes: transmitting terminal nyquist filtering module, for being filtered to left side band, the right band；Up-converter module; for carrying out up-conversion respectively to signal; wherein, after up-conversion, the baud rate of the sideband in left side band, the right band is less than or equal to transmitting terminal bandwidth of a device, the baud rate of another side band is less than or equal to transmitting terminal bandwidth of a device and subtracts protection interval；Hilbert filter module, for treated that signal is filtered to obtain asymmetric twin single sideband singal to up-converter module；Receiving end includes: optical filter module, for left side band: filtering out the right band completely by light filtering；For the right band: being substantially filtered out left side band by light filtering, guarantee that remaining left side bandwidth is less than protection interval；Down conversion module, for carrying out down coversion to signal；Receiving end nyquist filtering module, for carrying out nyquist filtering to signal.
Description
Technical Field
The invention belongs to the field of optical communication transmission, relates to a transmitting end modulation method based on asymmetric twin single sideband modulation, and relates to a receiving end demodulation method based on Nyquist matching filtering and a corresponding system.
Background
The direct detection optical communication system has the advantages of low cost, low complexity and easy integration, and is an important solution for medium-short distance optical fiber communication transmission systems. As the transmission distance increases, the two sidebands of the double sideband signal experience different dispersion, and the phenomenon of power fading occurs when the two sidebands overlap each other after the Photoelectric Detector (PD) at the receiving end is converted into an electric signal. Whereas single sideband modulation can avoid power fading by removing one sideband at the transmitting end. Further, in order to fully utilize the bandwidth of the transmitting end, one possible solution is to modulate another independent single sideband signal on the removed sideband into a twin single sideband. But the side band remaining after optical filtering at the receiving end may cause severe inter-channel interference (ICI) to the other side band.
At present, there are two methods for eliminating mutual crosstalk between two sidebands of a twin single sideband system, one is to perform joint equalization in Digital Signal Processing (DSP) at a receiving end, and the other is to set a symmetric guard interval. Both methods can effectively eliminate crosstalk.
1) And performing combined equalization in Digital Signal Processing (DSP) at a receiving end. This method requires a pair of identical optical filters, a Photodetector (PD) and an analog-to-digital converter (ADC) at the receiving end for receiving the information of the two sidebands, respectively, and the time delays of the two paths are also required to be identical. And performing 2 x 2 joint equalization may have at least twice the computational effort compared to two-way independent equalization. This approach increases the hardware requirements and computational complexity of the system.
2) Symmetrical guard intervals are provided. This approach symmetrically sets a guard interval between the carrier and the signal that is as wide as the signal bandwidth, so that the optical filter can completely filter out the non-target sidebands. Also, signal-to-signal beat interference caused by Photodetector (PD) square-law detection may fall within the guard interval. The disadvantage of this approach is that half of the spectral efficiency is sacrificed.
Disclosure of Invention
The invention provides an asymmetric twin single sideband modulation and demodulation method and a system for realizing the method, which can eliminate crosstalk between twin sidebands with a smaller guard interval on the premise of not carrying out joint equalization.
In order to achieve the purpose, the invention adopts the following technical scheme:
an asymmetric twin single sideband modulation mode based on Nyquist filtering comprises the following steps:
the first step is as follows: the size of the guard interval is determined according to the slope of the edge of the optical filter. The required optical filter is a common tunable optical filter, and the center wavelength and the passband bandwidth are variable;
the second step is that: and determining the baud rates of the two sidebands according to the bandwidth of the device at the transmitting end. Then, nyquist filtering and left or right up-conversion are respectively carried out on the two sidebands (actually, the left and right are equivalent, the left and right up-conversion is assumed to be carried out on the sidebands with high baud rate, and the right and left up-conversion is assumed to be carried out on the sidebands with low baud rate, for convenience of explanation), and the hilbert filtering is respectively carried out after the up-conversion to obtain single-sideband signals; and adding the left and right side bands to obtain an asymmetric twin single sideband signal.
The third step: and the receiving end receives the left and right sidebands independently. For the left sideband: completely filtering out the right sideband by optical filtering; for the right sideband: the left sideband is basically filtered through optical filtering, and the width of the residual left sideband is ensured to be smaller than the guard interval. Then, down-conversion and Nyquist matching filtering are carried out in digital signal processing, and left side bands are thoroughly filtered. Optical filtering can be achieved by adjusting the center wavelength and passband bandwidth of the optical filter.
Further, the signal at the transmitting end is preprocessed before entering a communication channel, wherein the preprocessing comprises modulator nonlinear precompensation, dispersion precompensation or fiber Kerr nonlinear precompensation.
Further, signal-signal beat frequency damage generated by square law detection of a Photoelectric Detector (PD) is eliminated for the receiving end signal in Digital Signal Processing (DSP) by using an iterative algorithm.
Further, the asymmetric twin single sideband structure is not limited to a spectrum structure generated by a single laser through electrical domain up-conversion, and can also be a spectrum structure formed by recombining a left sideband, a right sideband and an optical carrier wave through modulating by a plurality of independent lasers respectively.
Further, the baud rate of the left sideband is equal to or less than the bandwidth of the device at the transmitting end, and the baud rate of the right sideband is equal to or less than the bandwidth of the device at the transmitting end minus the guard interval (equal sign is taken when the nyquist filter roll-off coefficient is 0). The frequency spectrum of the Nyquist filter of the left sideband at the transmitting end does not exceed the bandwidth of the device at the transmitting end, and the frequency spectrum of the right sideband after filtering does not exceed the bandwidth of the device at the transmitting end minus a protection interval.
A twin single sideband modulation direct detection system based on Nyquist filtering for realizing the method comprises a transmitting end and a receiving end,
the transmitting end includes:
the transmitting end Nyquist filtering module is used for compressing the frequency spectrum to be close to a square under the condition of ensuring that the receiving end has no intersymbol interference (ISI), and improving the frequency spectrum utilization rate;
and the up-conversion module is connected with the transmitting terminal Nyquist filtering module and is used for moving the baseband signal to high frequency and preventing the overlapping of positive and negative frequency components.
The Hilbert filtering module is connected with the upper frequency conversion module and used for filtering the double-sideband signal into a single-sideband signal and then sending the signal to a communication channel;
the receiving end includes:
an optical filter module for removing unwanted sideband information;
the signal-signal beat frequency interference compensation module is connected with the optical filter module and is used for compensating signal-signal beat frequency interference (SSBI);
the down-conversion module is connected with the signal-signal beat frequency interference compensation module and is used for converting the electric signal received by the Photoelectric Detector (PD) into a baseband signal;
and the receiving end Nyquist filtering module is connected with the down-conversion module and used for eliminating intersymbol interference (ISI), improving the signal-to-noise ratio and removing the residual sideband crosstalk after optical filtering.
Further, still include:
and the transmitting end modulation module is used for carrying out quadrature amplitude phase modulation (QAM) format mapping on the original binary sequence and inserting the synchronization series and the training sequence as a frame structure leader sequence.
And the transmitting terminal preprocessing module is used for preprocessing the transmitting terminal signal and then sending the signal to the communication channel.
And the receiving end demodulation module is used for carrying out optimal sampling point optimization and channel equalization on the sequence after the receiving end matched filtering and judging and demodulating the sequence back to a binary sequence.
Further, the preprocessing performed by the front-end data processing module includes: nonlinear compensation of a modulator, dispersion pre-compensation and Kerr nonlinear compensation of optical fibers.
Compared with the prior art, the invention has the following positive effects:
the method disclosed by the invention is designed into an asymmetric twin single sideband scheme by combining the characteristics that only one side of a left sideband and a right sideband needs to be reserved with a guard interval and Nyquist matching filtering to remove out-of-band interference under the condition of modulating signals by the twin single sideband. The scheme does not need to carry out joint equalization, can realize independent reception of the left and right sidebands, and increases the flexibility of the system. In theory left and right side interband crosstalk can be completely eliminated and the same bit error rate performance can be achieved as a twin single sideband signal with a symmetrical guard interval.
Drawings
Fig. 1 is a flow chart of a digital signal processing method of asymmetric twinned single sideband modulation based on nyquist filtering according to an embodiment of the present invention.
Fig. 2 is a schematic diagram of a signal spectrum structure according to an embodiment of the present invention.
Fig. 3 is a schematic structural diagram of an asymmetric twin single sideband modulation system based on nyquist filtering according to an embodiment of the present invention.
FIG. 4 is a diagram illustrating the experimental results of 80km transmission of 400Gb/s signals according to an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail below with reference to specific examples and the accompanying drawings.
The following describes the implementation of the technical solution in detail with reference to the algorithm flowchart 1 of this embodiment, and the part shown by the solid line box in fig. 1 is the main content of the solution of the present invention.
The first step is as follows: the size of the guard interval is determined according to the slope of the edge of the optical filter, and assuming that the negligible crosstalk threshold is a dB and the slope of the edge of the optical filter is γ dB/nm, the required size of the guard interval is:
Fgap＝A/γnm
where a is typically 20 and gamma is typically 500, the typical guard interval is about 0.04 nm. Note that here the guard interval size is determined only by the optical filter, independent of the signal rate, and the spectral cost is constant.
And secondly, performing Nyquist filtering with roll-off coefficient α at the transmitting end, and compressing the signal spectrum into an approximate square shape, wherein generally α can be ensured to be 0.01.
Then the baseband signals are up-converted left and right respectively, and the left baseband signal is assumed to be s1(t) baud rate B1And the right baseband signal is s2(t) baud rate B2. After up-conversion the signals are respectivelyAndthe up-conversion in the digital domain can be expressed as:
wherein Re {. is used for representing the operation of the real part, and Im {. is used for representing the operation of the imaginary part. k is a radical of1Representing the left upper band up-conversion coefficient, satisfying k1>0.5+ α/2, generally 0.51. k2Representing the upper frequency conversion multiple of the right side band, and satisfying k2>Fgap/B2+0.5+α/2。
Then, Hilbert filtering is performed:
where conj {. cndot } represents a conjugate operation and Hilbert {. cndot } represents a Hilbert filter.
The two sideband signals are then summed and added to the dc carrier component by adjusting the modulator bias point. Before being transmitted to a communication channel, the signal is typically dispersion pre-compensated in the frequency domain:
Spre(f)＝S(f)·exp(-β2Lω2/2),
wherein S ispre(f) Is the frequency domain data after pre-compensation, S (f) is the frequency domain data before pre-compensation, β2For group velocity dispersion coefficient, L is the fiber length and ω is the angular frequency relative to the carrier.
The third step: and the receiving end receives the left and right sidebands independently. First, optical filtering is performed. For the left sideband: the right side band is completely filtered out through optical filtering to eliminate crosstalk; for the right sideband: the left sideband is basically filtered through optical filtering, and the width of the residual left sideband is ensured to be smaller than the guard interval.
Then, in the Digital Signal Processing (DSP) stage, the signal-signal beat interference is first compensated:
wherein r isi(t) denotes the left and right sideband received signals, respectively, and λ is the carrier-to-signal power ratio (CSPR) and the received power of the signal with which the amplitude factor depends. This process may be iterated multiple times to gradually improve performance, typically substantially stabilizing after 4 to 6 times. Since this process does not involve decision demodulation, the computational complexity is low.
Then, down-converting the left and right side band signals respectively:
the signal after down conversion is then nyquist matched filtered. For reception of the right sideband, the residual left band crosstalk is completely outside the nyquist spectrum and can therefore be filtered out completely.
And finally, carrying out receiving end demodulation, including optimal sampling point optimization, linear equalization, judgment and demodulation.
Fig. 2 shows the spectral structure of a signal, which comprises three components, from left to right: left sideband, optical carrier, right sideband. The width of each sideband and the spacing parameters are selected as given in the above step.
The spectrum structure is not limited to be generated by a single laser, but three parts can be generated by a plurality of lasers respectively and then combined together through an optical coupler.
Fig. 3 is a schematic diagram of the equalization system based on frequency domain channel estimation corresponding to the above method, including a transmitting end and a receiving end.
The transmitting end includes: the transmitting terminal modulation module is used for carrying out quadrature amplitude phase modulation (QAM) format mapping on the original binary sequence and inserting a synchronization series and a training sequence as a frame structure leader sequence; the transmitting end Nyquist filtering module is connected with the transmitting end modulating module and used for compressing the frequency spectrum to be close to a square under the condition of ensuring that the receiving end does not have intersymbol interference (ISI); the up-conversion module is connected with the transmitting terminal Nyquist filtering module and is used for shifting the baseband signal to high frequency and preventing the overlapping of positive and negative frequency components; the Hilbert filtering module is connected with the upper frequency conversion module and used for filtering the double-sideband signal into a single-sideband signal; and the transmitting terminal preprocessing module is connected with the Hilbert filtering module and used for preprocessing the transmitting terminal signals and then sending the signals to a communication channel.
The receiving end includes: an optical filter module for removing unwanted sideband information; a cyclic prefix/suffix removal module for removing a cyclic prefix/suffix in the training sequence; the signal-signal beat frequency interference compensation module is connected with the optical filter module and is used for compensating signal-signal beat frequency interference (SSBI); the down-conversion module is connected with the signal-signal beat frequency interference compensation module and is used for converting the electric signal received by the Photoelectric Detector (PD) into a baseband signal; the receiving end Nyquist filtering module is connected with the down-conversion module and used for eliminating intersymbol interference (ISI), improving the signal-to-noise ratio and removing the residual other sideband crosstalk after optical filtering; and the receiving end demodulation module is connected with the receiving end Nyquist filtering module and used for carrying out optimal sampling point optimization and channel equalization on the sequence after the receiving end matched filtering and judging and demodulating the sequence back to a binary sequence.
Fig. 4 shows the experimental result of 400Gb/s single channel transmission, the transmission distance is 80km, and the horizontal axis shows the incoming fiber power of the signal. The two sidebands have basically the same optimal fiber entering power and similar error rate performance, which shows that the two sidebands with different symbol rates are compatible in the asymmetric twinborn single sideband system.
The above embodiments are only intended to illustrate the technical solution of the present invention and not to limit the same, and a person skilled in the art can modify the technical solution of the present invention or substitute the same without departing from the spirit and scope of the present invention, and the scope of the present invention should be determined by the claims.
Claims (6)
1. An asymmetric twin single sideband modulation method comprises the following steps:
1) determining a protection interval required by the asymmetric twin single sideband signal according to the edge slope of the optical filter; wherein the protection interval is FgapA/γ nm; wherein, A is the acceptable crosstalk threshold of the communication system, and gamma is the edge slope of the optical filter;
2) determining baud rates of a left sideband and a right sideband according to the bandwidth of a transmitting end device, then carrying out Nyquist filtering and left up-conversion on one sideband, and carrying out Nyquist filtering on the other sidebandPerforming special filtering and performing frequency up-conversion to the right; after up-conversion, the baud rate of one of the left sideband and the right sideband is less than or equal to the bandwidth of a transmitting end device, and the baud rate of the other sideband is less than or equal to the bandwidth of the transmitting end device minus a guard interval; let the baseband signal of the left sideband be s1(t) Baud rate B1The baseband signal of the right sideband is s2(t) Baud rate B2；s1(t) after upconversion tos2(t) after upconversion to
Wherein Re {. is used for real part operation, Im {. is used for imaginary part operation, and k is used for real part operation1Representing the left upper band up-conversion coefficient, satisfying k1>0.5+α/2，k2Representing the upper frequency conversion multiple of the right side band, and satisfying k2>Fgap/B2+0.5+ α/2, α is the roll-off coefficient of the nyquist filtering;
3) respectively carrying out Hilbert filtering on the left side band and the right side band obtained in the step 2) to obtain single-side band signals, and adding the left side band and the right side band to obtain asymmetric twin single-side band signals.
2. The method of claim 1, wherein the nyquist filtered roll-off coefficient α is 0.01.
3. The method of claim 1, wherein the asymmetric twinned single sideband signal is transmittedBefore the communication channel, carrying out dispersion pre-compensation on the asymmetric twin single sideband signal in a frequency domain: spre(f)＝S(f)·exp(-β2Lω2/2) wherein Spre(f) Is the frequency domain data after pre-compensation, S (f) is the frequency domain data before pre-compensation, β2For group velocity dispersion coefficient, L is the fiber length and ω is the angular frequency relative to the carrier.
4. An asymmetric twin single sideband demodulation method comprises the following steps:
1) the receiving end respectively and independently receives the left side band and the right side band; for the left sideband: completely filtering out the right sideband by optical filtering; for the right sideband: the left side band is basically filtered through optical filtering, and the width of the residual left side band is ensured to be smaller than a guard interval;
2) the receiving end compensates the signal-signal beat frequency interference processed in the step 1), and then down-conversion is respectively carried out on the left sideband and the right sidebandWherein,for the compensated signal, i is 1,2, k1Representing the left upper band up-conversion coefficient, satisfying k1>0.5+α/2，k2Representing the upper frequency conversion multiple of the right side band, and satisfying k2>Fgap/B2+0.5+α/2，FgapThe guard interval required for the asymmetric twinned single sideband signal, α is the roll-off coefficient of the Nyquist filtering, and the baud rate of the left sideband is B1The baud rate of the right sideband is B2(ii) a And then carrying out Nyquist matching filtering on the down-converted signal, thoroughly filtering out a left sideband, and then demodulating.
5. The method of claim 4, wherein in step 2), the processed signal of step 1) is first compensated for signal-to-signal beat interference, and then down-converted and matched filtered; wherein use is made ofCompensating for signal-to-signal beat interference; r isi(t) represents the received single sideband signal, and λ is the carrier-to-signal power ratio and the received power of the signal for which the amplitude factor depends.
6. An asymmetric twin single sideband modulation and demodulation system is characterized by comprising a transmitting end and a receiving end, wherein,
the transmitting end includes: the transmitting end Nyquist filtering module is used for carrying out Nyquist filtering on the left side band and the right side band; the up-conversion module is used for respectively up-converting the signals filtered by the Nyquist filtering module of the transmitting terminal, wherein after the up-conversion, the baud rate of one sideband of the left sideband and the right sideband is less than or equal to the bandwidth of a device of the transmitting terminal, and the baud rate of the other sideband is less than or equal to the bandwidth of the device of the transmitting terminal minus a protection interval; the Hilbert filtering module is used for performing Hilbert filtering on the signal processed by the up-conversion module to obtain a single-sideband signal, namely an asymmetric twin single-sideband signal;
the receiving end includes: optical filter module, for left sideband: completely filtering out the right sideband by optical filtering; for the right sideband: the left side band is basically filtered through optical filtering, and the width of the residual left side band is ensured to be smaller than a guard interval; the signal-signal beat frequency interference compensation module is used for compensating the signal-signal beat frequency interference processed by the optical filtering module; the down-conversion module is used for performing down-conversion on the signal processed by the signal-signal beat frequency interference compensation module; and the receiving end Nyquist filtering module is used for filtering the signals processed by the down-conversion module.
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| CN108566250B (en) * | 2018-03-02 | 2021-03-12 | 北京大学 | Modulation and demodulation method and system based on carrier quadrature bias single sideband signal |
| CN108599862A (en) * | 2018-03-27 | 2018-09-28 | 北京邮电大学 | a kind of passive optical network uplink transmission method and optical line terminal |
| CN111953424B (en) * | 2020-09-17 | 2021-11-30 | 上海交通大学 | Residual edge band optical signal modulation method and system and direct detection optical fiber communication method |
| CN112152956B (en) * | 2020-09-17 | 2022-06-17 | 中电科思仪科技股份有限公司 | Single-sideband demodulation method for baseband signal tracking |
| CN114465669B (en) * | 2022-03-21 | 2023-11-07 | 网络通信与安全紫金山实验室 | Intermediate frequency signal and millimeter wave signal mixed transmission system and method |
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| CN115134005B (en) * | 2022-06-29 | 2023-10-13 | 聊城大学 | Direct detection method and system for bias double-sideband signal based on CADD receiver |
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