CN106533573B - The asymmetric twin single sideband modulation of one kind, demodulation method and system - Google Patents

Abstract

The invention discloses an asymmetric twin single-sideband modulation and demodulation method and system. The transmitter of the system includes: a transmitter Nyquist filter module for filtering the left band and right band; an up-conversion module for up-converting the signal respectively. After up-conversion, the left band and the right band The baud rate of one side of the band is less than or equal to the bandwidth of the transmitter device, and the baud rate of the other side band is less than or equal to the bandwidth of the transmitter device minus the guard interval; the Hilbert filter module is used to process the up-conversion module The latter signal is filtered to obtain an asymmetric twin single-sideband signal; the receiving end includes: an optical filter module, for the left band: the right band is completely filtered out by optical filtering; for the right band: the left band is basically filtered out by optical filtering to ensure The residual left band width is smaller than the guard interval; the down-conversion module is used to down-convert the signal; the Nyquist filter module at the receiving end is used to perform the Nyquist filter on the signal.

Description

Asymmetric twin single sideband modulation and demodulation method and system

technical field

The invention belongs to the field of optical communication transmission, and relates to a transmitter modulation method based on asymmetric twin single sideband modulation, a receiver demodulation method based on Nyquist matched filtering, and a corresponding system.

Background technique

The direct detection optical communication system has the advantages of low cost, low complexity and easy integration, and is an important solution for short and medium distance optical fiber communication transmission systems. As the transmission distance increases, the two sidebands of the double-sideband signal will experience different dispersion, and the power fading phenomenon will occur when the two sidebands overlap each other after the photodetector (PD) at the receiving end converts them into electrical signals. Single-sideband modulation, on the other hand, can avoid power fading by removing a sideband at the transmitter. Furthermore, in order to fully utilize the bandwidth of the transmitter, a feasible solution is to modulate another independent SSB signal on the removed sideband to become a twin SSB. However, the residual sideband after optical filtering at the receiving end will cause severe inter-channel interference (ICI) to another sideband.

At present, there are mainly two methods to eliminate the mutual crosstalk between the two sidebands of the twin single sideband system. One is to perform joint equalization in the digital signal processing (DSP) of the receiving end, and the other is to set a symmetrical guard interval. Both methods can effectively eliminate crosstalk.

1) Joint equalization is performed in the digital signal processing (DSP) at the receiving end. This method requires a pair of identical optical filters, photodetectors (PDs) and analog-to-digital converters (ADCs) at the receiving end to receive the information of the two sidebands respectively, and the delays of the two paths are also required to be the same. Moreover, performing 2×2 joint equalization requires at least twice the amount of computation compared to two-way independent equalization. Therefore, this method increases the requirements for hardware and the computational complexity of the system.

2) Set a symmetrical guard interval. This method sets the same guard interval as the signal bandwidth symmetrically between the carrier and the signal, so that the optical filter can completely filter out the non-target sidebands. Also, the signal-to-signal beat interference caused by the square-law detection of the photodetector (PD) falls within the guard interval. The disadvantage of this scheme is that half of the spectral efficiency will be sacrificed.

SUMMARY OF THE INVENTION

The invention provides an asymmetric twin sideband modulation and demodulation method and a system for realizing the method, which can eliminate the crosstalk between twin sidebands with a small guard interval without joint equalization.

To achieve the above object, the present invention adopts the following technical solutions:

An asymmetric twin single-sideband modulation method based on Nyquist filtering, including the following steps:

Step 1: Determine the size of the guard interval according to the edge slope of the optical filter. The required optical filter is a general tunable optical filter with variable center wavelength and passband bandwidth;

Step 2: Determine the baud rate of the two sidebands according to the bandwidth of the transmitter device. After that, the two sidebands are Nyquist filtered and upconverted to the left or right (in fact, left to right is equivalent. For the convenience of illustration, it is assumed that the high baud rate sideband is upconverted to the left, and the low frequency is up-converted. The sideband of the bit rate is up-converted to the right), and after the up-conversion, Hilbert filtering is performed to obtain a single-sideband signal; the left and right bands are added to obtain an asymmetric twin single-sideband signal.

Step 3: The left and right sidebands are independently received at the receiving end. For the left band: the right band is completely filtered out by optical filtering; for the right band: the left band is basically filtered out by optical filtering, ensuring that the residual left band width is smaller than the guard interval. Down-conversion and Nyquist-matched filtering are then performed in digital signal processing to completely filter out the left-hand band. 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 the communication channel, including modulator nonlinear pre-compensation, dispersion pre-compensation or fiber Kerr nonlinear pre-compensation.

Further, an iterative algorithm is used in the digital signal processing (DSP) to eliminate the signal-signal beat frequency damage generated by the square-law detection of the photodetector (PD) for the signal at the receiving end.

Further, this asymmetric twin single-sideband structure is not limited to a spectral structure generated by a single laser through electrical domain up-conversion, but can also be a spectral structure in which multiple independent lasers modulate the left and right sidebands and the optical carrier respectively.

Further, the baud rate of the left band is less than or equal to the bandwidth of the transmitter device, and the baud rate of the right band is less than or equal to the bandwidth of the transmitter device minus the guard interval (when the Nyquist filter roll-off coefficient is 0, both take the equal sign. ). Make sure that the spectrum of the Nyquist filter at the transmitter end of the left band does not exceed the bandwidth of the transmitter device, and the right band after filtering does not exceed the bandwidth of the transmitter device minus the guard interval.

A twin single-sideband modulation direct detection system based on Nyquist filtering for realizing the above method, comprising a transmitter and a receiver,

The transmitter includes:

The Nyquist filter module at the transmitting end is used to compress the spectrum to a nearly square shape while ensuring that there is no intersymbol interference (ISI) at the receiving end, so as to improve spectrum utilization;

The up-conversion module is connected to the Nyquist filter module at the transmitting end, and is used to move the baseband signal to a high frequency to prevent the overlap of positive and negative frequency components.

The Hilbert filter module, connected to the up-conversion module, is used to filter the double sideband signal into a single sideband signal, and then send the signal to the communication channel;

The receiving end includes:

Optical filter module to remove unwanted sideband information;

The signal-signal beat interference compensation module is connected to the optical filter module for compensating for the signal-signal beat interference (SSBI);

The down-conversion module is connected to the signal-signal beat frequency interference compensation module, and is used for frequency-converting the electrical signal received by the photodetector (PD) into a baseband signal;

The Nyquist filter module at the receiving end is connected to the down-conversion module for eliminating inter-symbol crosstalk (ISI), improving the signal-to-noise ratio, and removing the other sideband crosstalk remaining after optical filtering.

Further, it also includes:

The transmitter-side modulation module is used to perform quadrature amplitude phase modulation (QAM) format mapping on the original binary sequence, and insert the synchronization sequence and the training sequence as the frame structure preamble sequence.

The transmitter preprocessing module is used to preprocess the transmitter signal, and then send it to the communication channel.

The demodulation module at the receiving end is used to optimize the optimal sampling point and channel equalization of the matched filtered sequence at the receiving end, and decide to demodulate back to the binary sequence.

Further, the preprocessing performed by the front-end data processing module includes: modulator nonlinear compensation, dispersion pre-compensation, and fiber Kerr nonlinear compensation.

Compared with the prior art, the positive effects of the present invention are:

In the case of twin single-sideband modulation signals, the method of the invention combines the characteristics that the left and right sidebands only need to leave a guard interval on one side and Nyquist matched filtering can remove out-of-band interference, and is designed as an asymmetric twin sideband. with a plan. This scheme does not require joint equalization, and can achieve independent reception of the left and right sidebands, increasing the flexibility of the system. In theory, the crosstalk between the left and right sidebands can be completely eliminated, and the same bit error rate performance as the twin single sideband signals with symmetrical guard intervals can be achieved.

Description of drawings

FIG. 1 is a flowchart of a digital signal processing method for asymmetric twin 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 schematic diagram of an experimental result of 80km transmission of a 400Gb/s signal according to an embodiment of the present invention.

Detailed ways

The present invention will be described in further detail below through specific embodiments and accompanying drawings.

The implementation of the technical solution will be specifically described below with reference to the algorithm flowchart 1 of this embodiment. The part shown by the solid line box in FIG. 1 is the main content of the solution of the present invention.

Step 1: Determine the size of the guard interval according to the edge slope of the optical filter. Assuming that the negligible crosstalk threshold is A dB and the edge slope of the optical filter is γdB/nm, the required guard interval size is:

F _gap =A/γnm

Among them, the typical value of A is 20, and the typical value of γ is 500, so the general guard interval is about 0.04nm. Note that the size of the guard interval is only determined by the optical filter, and has nothing to do with the signal rate, and the spectral penalty is constant.

The second step is to perform Nyquist filtering with a roll-off coefficient of α at the transmitting end. The purpose is to compress the signal spectrum into an approximate square. Generally, α can be guaranteed by taking 0.01.

Then the baseband signals are up-converted to the left and right respectively, assuming that the left baseband signal is s ₁ (t) with a baud rate of B ₁ , and the right baseband signal is s ₂ (t) with a baud rate of B ₂ . The up-converted signals are and Then the up-conversion in the digital domain can be expressed as:

Among them, Re{·} represents the operation of taking the real part, and Im{·} represents the operation of taking the imaginary part. k ₁ represents the up-conversion coefficient of the left band, which satisfies k ₁ >0.5+α/2, generally 0.51. k ₂ represents the frequency multiplier of the right band, which satisfies k ₂ >F _gap /B ₂ +0.5+α/2.

Then perform Hilbert filtering separately:

where conj{·} denotes the conjugate operation and Hilbert{·} denotes the Hilbert filter.

The two-band signals are then summed, and a DC carrier component is added by adjusting the modulator bias point. Dispersion pre-compensation is generally performed on the signal in the frequency domain before being sent to the communication channel:

S _pre (f)=S(f)·exp(-β ₂ Lω ² /2),

Among them, S _pre (f) is the frequency domain data after pre-compensation, S(f) is the frequency domain data before pre-compensation, β ₂ is the group velocity dispersion coefficient, L is the fiber length, and ω is the angular frequency relative to the carrier.

Step 3: The left and right sidebands are independently received at the receiving end. Light filtering is performed first. For the left band: the right band is completely filtered out by optical filtering to eliminate crosstalk; for the right band: the left band is basically filtered out by optical filtering to ensure that the residual left band width is smaller than the guard interval.

Then, in the digital signal processing (DSP) stage, the signal-to-signal beat interference is first compensated:

where _ri (t) represents the left and right sideband received signals, respectively, and λ is the amplitude factor depending on the carrier-to-signal power ratio (CSPR) of the signal and the received power. This process can be repeated several times to gradually improve the performance, and it is generally stable after 4 to 6 times. Since the process does not involve decision demodulation, the computational complexity is low.

Then, downconvert the left and right sideband signals separately:

Then, Nyquist matched filtering is performed on the down-converted signal. For right-band reception, the residual left-band crosstalk is completely outside the Nyquist spectrum and can therefore be completely filtered out.

Finally, demodulation at the receiving end, including optimal sampling point optimization, linear equalization, decision and demodulation.

Figure 2 shows the spectral structure of the signal, consisting of three components, from left to right: the left band, the optical carrier, and the right band. The width and spacing parameters of each sideband are given in the above steps.

This spectral structure is not limited to being produced by a single laser, but can also be produced by multiple lasers to produce three parts, which are then combined together by an optical coupler.

FIG. 3 is a schematic diagram of the composition of an equalization system based on frequency domain channel estimation corresponding to the above method, including a transmitter and a receiver.

The transmitter includes: a transmitter modulation module, which is used to map the original binary sequence to a Quadrature Amplitude Phase Modulation (QAM) format, and insert a synchronization sequence and a training sequence as a frame structure preamble sequence; a transmitter Nyquist filter module, connected to The transmitter modulation module is used to compress the spectrum to a nearly square shape without inter-symbol interference (ISI) at the receiver; the up-conversion module is connected to the Nyquist filter module at the transmitter to shift the baseband signal to high frequencies , to prevent the overlap of positive and negative frequency components; the Hilbert filter module, connected to the up-conversion module, is used to filter the double sideband signal into a single sideband signal; the transmitter preprocessing module, connected to the Hilbert filter module, uses It is used to preprocess the signal at the transmitter and then send it to the communication channel.

The receiving end includes: an optical filter module for removing unwanted sideband information; a cyclic prefix/suffix removal module for removing the cyclic prefix/suffix in the training sequence; a signal-signal beat frequency interference compensation module, Connect the optical filter module to compensate for signal-signal beat interference (SSBI); down-conversion module, connect to the signal-signal beat interference compensation module, to convert the electrical signal received by the photodetector (PD) into a baseband signal ; The Nyquist filter module at the receiving end is connected to the down-conversion module to eliminate intersymbol interference (ISI), improve the signal-to-noise ratio, and remove the other sideband crosstalk remaining after optical filtering; the demodulation module at the receiving end is connected to the receiving end The Nyquist filter module is used to optimize the optimal sampling point and channel equalization of the matched filtered sequence at the receiving end, and decide to demodulate it back to the binary sequence.

Figure 4 shows the experimental results of 400Gb/s single-channel transmission, the transmission distance is 80 kilometers, and the horizontal axis is the fiber input power of the signal. The two sidebands have basically the same optimal fiber insertion power and similar bit error rate performance, indicating that the two sidebands with different symbol rates are compatible with each other in the asymmetric twin SSB system.

The above embodiments are only used to illustrate the technical solutions of the present invention rather than limit them. Those of ordinary skill in the art can modify or equivalently replace the technical solutions of the present invention without departing from the spirit and scope of the present invention. The scope of protection shall be subject to what is stated in the claims.

Claims (6)

1. An asymmetric twin single sideband modulation method, the steps of which are: 1) Determine the guard interval required for the asymmetric twin single-sideband signal according to the edge slope of the optical filter; wherein, the guard interval is F _gap =A/γnm; Wherein, A is the acceptable crosstalk threshold of the communication system, and γ is the edge slope of the optical filter; 2) Determine the baud rate of the left band and the right band according to the bandwidth of the transmitter device, then perform Nyquist filtering on one side band and up-convert to the left, and perform Nyquist filtering on the other side band and up-convert to the right; wherein , after up-conversion, the baud rate of one of the left and right bands is less than or equal to the bandwidth of the transmitter device, and the baud rate of the other sideband is less than or equal to the bandwidth of the transmitter device minus the guard interval; set the baseband of the left band The signal is s ₁ (t), the baud rate is B ₁ , the baseband signal of the right band is s ₂ (t), and the baud rate is B ₂ ; after s ₁ (t) is up-converted, it is s ₂ (t) after up-conversion is Among them, Re{·} represents the operation of taking the real part, Im{·} represents the operation of taking the imaginary part, k ₁ represents the up-conversion coefficient of the left band, satisfying k ₁ >0.5+α/2, and k ₂ represents the up-conversion multiplier of the right band, Satisfy k ₂ >F _gap /B ₂ +0.5+α/2, where α is the roll-off coefficient of Nyquist filtering; 3) Hilbert filtering is performed on the left and right bands obtained in step 2) to obtain a single-sideband signal, and the left and right bands are added to obtain an asymmetric twin single-sideband signal. 2 . The method according to claim 1 , wherein the roll-off coefficient α of the Nyquist filter is 0.01. 3 . 3. method as claimed in claim 1 is characterized in that, before sending this asymmetric twin single sideband signal to communication channel, in frequency domain, dispersion pre-compensation is carried out to asymmetric twin single sideband signal: S _pre ( f)=S(f)·exp(-β ₂ Lω ² /2), where S _pre (f) is the frequency domain data after pre-compensation, S(f) is the frequency domain data before pre-compensation, and β ₂ is the group The 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, the steps of which are: 1) The receiver receives the left band and the right band independently; for the left band: the right band is completely filtered out by optical filtering; for the right band: the left band is basically filtered out by optical filtering, ensuring that the remaining left band width is smaller than the guard interval; 2) The receiving end compensates the signal-signal beat frequency interference for the signal processed in step 1), and then down-converts the left and right sidebands respectively. in, is the compensated signal, i=1,2, k ₁ represents the up-conversion coefficient of the left band, satisfying k ₁ >0.5+α/2, k ₂ represents the up-conversion multiplier of the right band, satisfying k ₂ >F _gap /B ₂ + 0.5+α/2, F _gap is the guard interval required for the asymmetric twin single-sideband signal, α is the roll-off coefficient of the Nyquist filter, the baud rate of the left band is B ₁ , and the baud rate of the right band is B ₂ ; then Nyquist matched filtering is performed on the down-converted signal, the left band is completely filtered out, and then demodulation is performed. 5. method as claimed in claim 4, is characterized in that, in step 2), at first to the signal after step 1) processing compensation signal-signal beat frequency interference, then carry out down-conversion and matched filtering; Wherein, utilize Compensation signal-signal beat interference; _ri (t) represents the received SSB signal, λ is the amplitude factor depending on the carrier-to-signal power ratio of the signal and the received power. 6. an asymmetric twin single sideband modulation, demodulation system, is characterized in that, comprises transmitting end and receiving end, wherein, The transmitting end includes: a Nyquist filtering module at the transmitting end, which is used to perform Nyquist filtering on the left band and the right band; an up-conversion module, which is used for respectively Perform up-conversion, wherein, after up-conversion, the baud rate of one of the left and right bands is less than or equal to the bandwidth of the transmitter device, and the baud rate of the other sideband is less than or equal to the bandwidth of the transmitter device minus the guard interval; The Hilbert filtering module is used to perform Hilbert filtering on the signal processed by the up-conversion module to obtain a single-sideband signal, that is, an asymmetric twin single-sideband signal; The receiving end includes: an optical filter module, for the left band: the right band is completely filtered out by optical filtering; for the right band: the left band is basically filtered out by optical filtering, ensuring that the residual left band width is smaller than the guard interval; signal-signal The beat frequency interference compensation module is used to compensate the signal-signal beat frequency interference for the signal processed by the optical filter module; the down-conversion module is used to down-convert the signal processed by the signal-signal beat frequency interference compensation module; The quist filter module is used to filter the signal processed by the down-conversion module.