CN114710204A - Single-polarization coherent detection system and method for avoiding polarization fading - Google Patents
Single-polarization coherent detection system and method for avoiding polarization fading Download PDFInfo
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Abstract
The invention provides a single polarization coherent detection system and a method for avoiding polarization fading, which relate to the technical field of optical communication and signal processing, and are based on the frame of the traditional single polarization coherent detection system, aiming at tolerating and resisting the polarization fading in the single polarization coherent detection system and further improving the transmission performance of the system, introducing a low-cost polarization scrambler, forward error correction coding and time sequence interweaving matching scheme, accelerating the polarization change of a single polarization optical signal by using the low-cost polarization scrambler, obtaining a highly random polarization state, thereby avoiding the lasting overlong polarization fading time, reducing the continuous code error number caused by the polarization fading, further reducing the requirement on the time sequence interweaving length, and finally correcting the code error caused by the polarization fading by the randomization coding technology by using the forward error correction coding technology, wherein the scheme has low cost and simple structure, effectively reduces the requirement on the polarization fading speed of the low-cost polarization scrambler, and the requirement of the time sequence interleaving length is effectively reduced, thereby ensuring the feasibility of practical application.
Description
Technical Field
The present invention relates to the field of optical communication and signal processing technologies, and in particular, to a single-polarization coherent detection system and method for avoiding polarization fading.
Background
In an optical fiber communication system, compared with an intensity modulation-direct detection system with sensitive cost and simple structure, coherent modulation-coherent detection is widely applied due to the advantages of high transmission rate, high transmission capacity and high spectrum utilization rate. Coherent detection is a signal demodulation mechanism, and a detection mode of mixing continuous light emitted by a local oscillator with received signal light and then demodulating a modulated signal through a receiver end is utilized, so that the coherent detection can improve the spectrum efficiency and the receiving sensitivity, but frequency/wavelength synchronization, phase synchronization and polarization alignment of a local laser and a detected signal are required, and the above preconditions are all realized through a digital signal processing algorithm at the receiver end.
In a conventional single-polarization coherent detection system, if no special device is used to process polarization, when the polarization states of the received signal light and the local oscillator light are not consistent, the amplitude of the coherently detected signal will be faded, i.e. a polarization fading phenomenon, and especially when the polarization states of the two beams of light are orthogonal, the amplitude of the detected signal will be faded to zero. Generally, the polarization state of the signal light needs to be matched with the polarization state of the local oscillator light to ensure that the coherent detection system can demodulate the signal well, and the instantaneous polarization state mismatch will cause the signal to be damaged or even lost, resulting in short-time system breakdown. Furthermore, as is known, compared with the conventional direct detection, the coherent detection system is complex and has higher cost, and some application fields are sensitive to the price of communication system equipment, so how to avoid polarization fading and reduce the complexity of polarization-dependent hardware becomes a problem to be urgently alleviated and overcome.
Disclosure of Invention
In order to reduce and avoid system performance deterioration caused by polarization fading and realize reduction of solution complexity, the invention provides a single-polarization coherent detection system and a method for avoiding polarization fading, which avoid overlong polarization fading duration time, time sequence interweave and randomize continuous burst error codes caused by polarization fading through a low-cost polarization scrambler so as to improve the error correction performance of a front item, correct and recover the error codes at a receiving end by a forward error correction technology, realize resistance and tolerance to the polarization fading phenomenon in a single-polarization coherent system, have simple structure, are compatible with the traditional scheme and have high cost.
In order to achieve the technical effects, the technical scheme of the invention is as follows:
a single polarization coherent detection system for avoiding polarization fading comprises an optical transmitter, an optical fiber link and an optical receiver comprising a low-cost polarization scrambler, wherein the optical transmitter comprises a transmitter DSP, an electrical amplifier, a single polarization vector modulator and a laser, the transmitter DSP comprises forward error correction coding and time sequence interleaving, the optical receiver comprises an optical filter, a low-cost polarization scrambler, a local oscillator, a coherent receiver and a receiver DSP, and the receiver DSP comprises time sequence de-interleaving and forward error correction decoding; in a transmitter DSP at the end of an optical transmitter, after a signal source is subjected to serial/parallel conversion, channel coding operation including prefix error correction coding and time sequence interleaving, a baseband electric signal is obtained through digital/analog conversion, the baseband electric signal is output to a single polarization vector modulator through an electric amplifier, the single polarization vector modulator is loaded on a continuous optical carrier generated by a laser, and a single polarization optical signal is generated and then is sent to an optical fiber link for transmission;
completing optical fiber link transmission, filtering out single polarization optical signals with out-of-band noise by an optical filter, accelerating random change of polarization by a low-cost polarization scrambler, inputting the signals and local oscillator light emitted by a local oscillator into a coherent receiver, and performing frequency mixing detection in the coherent receiver to generate coherent detection light;
let the threshold of the polarization fading amplitude of coherent detection light be AthWhen the amplitude of the coherent detection light is lower than AthWhen the time is short, the continuous burst error code at the corresponding time is lower than A due to polarization fadingthIs the polarization decay time DeltaTthIntroduction and application of low-cost polarization scrambler for accelerating polarization change of single-polarization optical signal and reducing polarization fading time delta Tth；ΔTthThe inner single polarization optical signal generates continuous error code, and the time sequence interweaving in the transmitter DSP converts the continuous burst error code caused by polarization fading into random error codeIn order to improve the error correction capability of the previous item, the forward error correction coding is used for correcting the error code at the receiving end;
coherent detection optical signals are sent to a receiver DSP of the optical receiver, and information sink is obtained through analog/digital conversion, channel decoding containing time sequence de-interleaving and forward error correction decoding and parallel/serial conversion operation;
in the technical scheme, based on the processing of the optical transmission signals at the transmitter end and the receiver end in the traditional single-polarization coherent detection system, the polarization fading in the single-polarization coherent detection system is tolerated as the target, a low-cost polarization scrambler is introduced to be matched with forward error correction coding and time sequence interleaving, the polarization change of the single-polarization optical signals is accelerated by using the low-cost polarization scrambler, and a highly random polarization state is obtained, so that the situation that the polarization fading time lasts too long is avoided, the number of continuous error codes is reduced, the requirement on the time length of time sequence interleaving is reduced, and finally the error codes are corrected through forward error correction, and the feasibility of practical application is ensured.
Preferably, the optical fiber link comprises an optical fiber and an optical amplifier for transmitting a single-polarization optical signal. Preferably, the low cost polarization scrambler is an extruded fiber.
Preferably, the low-cost polarization scrambler is a deflection optical fiber coil and has a simple structure.
Here, it is considered that an optical device for polarization tracking or polarization maintaining is expensive, and compared to this, a polarization scrambler in which polarization is randomly changed is implemented in a simple form such as a pressed optical fiber, which is low in cost.
Preferably, the time-sequence interleaving time is set to TinThe number of time-sequence interleaving bits is n, and the duration of each bit in transmission is TbThen, the following conditions are satisfied:
Tin＝n*Tb；
time-sequential interleaving time TinAnd a polarization decay time Δ TthShould satisfy the following relationship:
i.e. the time-sequential interleaving time TinCovering when the polarization is fadingInterval delta TthThe minimum requirement that continuous bit errors caused by polarization fading are converted into random bit errors through time sequence interweaving in the time dimension is met; in that
In the process, burst continuous error codes are randomized at the cost of time sequence interleaving time, and the error code correcting capability is improved by matching with a forward error correction coding technology so as to realize tolerance and resistance to polarization fading.
Here, the time-sequence interleaving time TinSetting of (D) and polarization decay time DeltaTthClosely related, when the time-sequential interleaving is not sufficient in the time dimension to cover the polarization-fading time Δ TthWhen is at time
When the error correction is carried out, the capability of correcting continuous error codes is limited due to the forward error correction technology, and the error codes cannot be effectively corrected no matter whether the time sequence interweaves or not, otherwise, the time sequence interweaves randomly distribute the continuous error codes caused by polarization fading in a larger time dimension, so that the improvement of the performance of the forward error correction technology is facilitated, and the error correction is realized. Preferably, the deflection time interval of the low-cost deflection scrambler is TscramTime interval T of low cost offset scramblerscramAnd polarization fading time DeltaTthShould satisfy the following relationship:
that is, the introduction of a low-cost polarization scrambler is intended to reduce the polarization decay time Delta TthAt intervals of time T of deflectionscramFor reference, a finer division is made in the time dimension and at each time interval T of the perturbationsscramIn the method, the amplitude of coherent detection light is not less than the threshold value of polarization fading amplitude A through the random polarization state caused by polarization disturbancethReducing the polarization decay time DeltaTthThereby reducing or even avoiding bit errors caused by the same.
Preferably, the deflection time interval T of the low-cost deflection scramblerscramThe smaller the polarization decay time Δ TthAnd then decreases, thereby making the time sequence interleaving time TinAnd also decreases. When in use
Time-sequence interleaving time TinAnd the optimization can be effectively realized, and more register resources are saved.
The application of the low-cost polarization scrambler effectively improves the requirement on the time sequence interleaving length, so the polarization scrambling time interval T of the low-cost polarization scramblerscramIs satisfied at a time reasonably less than the polarization decay time Δ TthOn the premise of realizing the length of time sequence interleaving (time sequence interleaving time T)in) And (4) reasonably optimizing.
Preferably, the time-series interleaving represents that the code words after forward error correction coding are randomly arranged in the time dimension when the continuous bit errors are converted into random bit errors, so that the forward error correction performance is improved.
Preferably, the channel coding in the transmitter DSP includes forward error correction coding and timing interleaving, and the channel decoding in the receiver DSP includes timing deinterleaving and forward error correction decoding for cooperating with the timing interleaving and forward error correction operations, both of which have an inherent logical association with the low-cost scrambler, and should be considered jointly.
The invention also provides a single-polarization coherent detection method for avoiding polarization fading, which comprises the following steps:
s1, in a transmitter DSP of an optical transmitter, after an information source is subjected to operations such as serial/parallel conversion, channel coding and the like, a baseband electric signal is obtained through digital/analog conversion, and the baseband electric signal is output to a single polarization vector modulator through an electric amplifier; wherein, the channel coding process comprises forward error correction coding and time sequence interleaving;
s2, loading the amplified baseband electric signal to a continuous optical carrier generated by a laser through a single polarization vector modulator, and sending the baseband electric signal to an optical fiber link for transmission after generating a single polarization optical signal; wherein, the optical fiber link comprises an optical fiber and an optical amplifier;
s3, completing optical fiber link transmission at an optical receiver end, filtering out a single polarized light signal with out-of-band noise by an optical filter, accelerating random change of polarization by a low-cost polarization scrambler, inputting the single polarized light signal and local oscillator light emitted by a local oscillator into a coherent receiver, and performing frequency mixing detection in the coherent receiver to generate coherent detection light;
s4, setting a polarization fading amplitude threshold A of coherent detection lightthWhen the amplitude of the coherent detection light is lower than the threshold AthWhen the time is longer, the corresponding time continuous burst error code is caused by the polarization fading, and the time lower than the threshold value is the polarization fading time Delta Tth(ii) a The introduction and application of the low-cost polarization scrambler can accelerate the polarization change of a single-polarization optical signal and reduce the polarization fading time delta Tth；
S5, in the polarization fading time delta TthIn the method, a single polarization optical signal generates continuous error codes, channel coding comprises forward error correction coding and time sequence interleaving, wherein the time sequence interleaving converts continuous burst error codes caused by polarization fading into random error codes so as to improve the capacity of foreword error correction, and the forward error correction coding is used for correcting the error codes at a receiving end;
s6, sending the coherent detection optical signal into a receiver DSP, and obtaining an information sink through operations such as analog/digital conversion, channel decoding, parallel/serial conversion and the like; wherein, the channel decoding comprises time sequence de-interleaving and forward error correction decoding.
Compared with the prior art, the technical scheme of the invention has the beneficial effects that:
the invention provides a single polarization coherent detection system and a method for avoiding polarization fading, which are based on the processing of light transmission signals at the transmitter end and the receiver end in the traditional single polarization coherent detection system, aim at tolerating and resisting the polarization fading in the single polarization coherent detection system, introduce a low-cost polarization scrambler, forward error correction coding and time sequence interweaving, accelerate the polarization change of a single polarization light signal by using the low-cost polarization scrambler, and obtain a highly random polarization state, thereby avoiding overlong polarization fading duration time, effectively reducing the requirement on time sequence interweaving time length, and ensuring the feasibility of practical application; time sequence interweaving randomizes continuous burst error codes caused by polarization fading so as to improve the performance of the previous error correction; finally, the forward error correction technology corrects and recovers the error code at a receiving end, resistance and tolerance to the polarization fading phenomenon in a single-polarization coherent system are realized, the scheme has a simple structure, is compatible with the traditional scheme structure, is high in cost efficiency, and the low-cost polarization scrambler is more favorable for integration based on the current mainstream system structure.
Drawings
Fig. 1 is a schematic structural diagram of a single-polarization coherent detection system for avoiding polarization fading according to embodiment 1 of the present invention;
fig. 2 is a schematic diagram of an optical signal transmission code stream without applying a low-cost polarization scrambler according to embodiment 2 of the present invention;
fig. 3 is a schematic diagram of an optical signal transmission code stream when a low-cost polarization scrambler is applied in embodiment 2 of the present invention;
fig. 4 is a schematic diagram of a binary code stream when polarization fading occurs without time-sequence interleaving according to embodiment 2 of the present invention;
fig. 5 is a schematic diagram of a binary code stream when a polarization fading phenomenon occurs during time-sequence interleaving according to embodiment 2 of the present invention;
fig. 6 is a flowchart illustrating a single-polarization coherent detection method for avoiding polarization fading according to embodiment 3 of the present invention.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent;
for better illustration of the present embodiment, certain parts of the drawings may be omitted, enlarged or reduced, and do not represent actual dimensions;
it will be understood by those skilled in the art that certain descriptions of well-known structures in the drawings may be omitted.
The technical solution of the present invention is further described below with reference to the accompanying drawings and examples.
The positional relationships depicted in the drawings are for illustrative purposes only and are not to be construed as limiting the present patent;
example 1
In this embodiment, as shown in fig. 1, a single polarization coherent detection system for avoiding polarization fading is proposed, the system includes an optical transmitter, an optical fiber link, and an optical receiver including a low-cost scrambler, the optical transmitter includes a transmitter DSP, an electrical amplifier, a single polarization vector modulator, and a laser, wherein the transmitter DSP includes forward error correction coding and time sequence interleaving, and the optical receiver includes an optical filter, a low-cost scrambler, a local oscillator, a coherent receiver, and a receiver DSP, wherein the receiver DSP includes time sequence de-interleaving and forward error correction decoding; in a transmitter DSP at the end of an optical transmitter, after a signal source is subjected to serial/parallel conversion, channel coding operation including prefix error correction coding and time sequence interleaving, a baseband electric signal is obtained through digital/analog conversion, the baseband electric signal is output to a single polarization vector modulator through an electric amplifier, the single polarization vector modulator is loaded on a continuous optical carrier generated by a laser, and a single polarization optical signal is generated and then is sent to an optical fiber link for transmission;
completing optical fiber link transmission, filtering out single polarization optical signals with out-of-band noise by an optical filter, accelerating random change of polarization by a low-cost polarization scrambler, inputting the signals and local oscillator light emitted by a local oscillator into a coherent receiver, and performing frequency mixing detection in the coherent receiver to generate coherent detection light;
let the threshold of the polarization fading amplitude of coherent detection light be AthWhen the amplitude of the coherent detection light is lower than AthWhen the time is short, the continuous burst error code at the corresponding time is lower than A due to polarization fadingthIs the polarization decay time DeltaTthThe introduction and application of the low-cost polarization scrambler can accelerate the polarization change of the single-polarization optical signal and reduce the polarization fading time delta Tth；ΔTthThe inner single polarization optical signal generates continuous error codes, the continuous burst error codes caused by polarization fading are converted into random error codes in the time sequence interweaving in the DSP of the transmitter so as to improve the capacity of the previous error correction, and the forward error correction codes are used for correcting the error codes at the receiving end;
coherent detection optical signals are sent to a receiver DSP of the optical receiver, and information sink is obtained through analog/digital conversion, channel decoding containing time sequence de-interleaving and forward error correction decoding and parallel/serial conversion operation;
in this embodiment, the whole system is based on the processing of the optical transmission signal at the transmitter end and the receiver end in the conventional single-polarization coherent detection system, and aims to tolerate the polarization fading in the single-polarization coherent detection system, and introduces a low-cost polarization scrambler, forward error correction coding and time sequence interleaving, and accelerates the polarization change of the single-polarization optical signal by using the low-cost polarization scrambler, so as to obtain a highly random polarization state, thereby avoiding the polarization fading time from lasting too long, reducing the number of continuous error codes, reducing the requirement on the time sequence interleaving time length, and ensuring the feasibility of practical application.
In this embodiment, referring to fig. 1, the optical fiber link comprises an optical fiber and an optical amplifier for transmitting a single-polarization optical signal. In this embodiment, the low-cost polarization scrambler may be a simple extruded fiber or a deflection fiber coil, which is low in cost, or other polarization scramblers may be used to generate random rather than deterministic polarization changes, but the polarization fading resistance in the single-polarization coherent detection system can be realized with low complexity and low cost compared with a complex and expensive polarization controller or other devices.
Referring to fig. 1, channel coding in the transmitter DSP includes forward error correction coding and timing interleaving, and channel decoding in the receiver DSP includes timing deinterleaving and forward error correction decoding for cooperating with the timing interleaving and forward error correction operations, both of which have an inherent logical association with the low-cost scrambler and should be applied jointly.
Example 2
The embodiment specifically explains the realization of polarization fading resistance in a single-polarization coherent detection system, compared with a high-speed optical modulation code stream (A, B, C)>10Gb/s), the change of the polarization state of the light can be regarded as a slow-changing process (usually in the millisecond order, occasionally appearing in the microsecond order under the special environment), and the threshold of the polarization fading amplitude of the coherent detection light is set as athReferring to fig. 2, the abscissa represents time, and the ordinate represents the polarization fading, and when the amplitude of the coherent detection light is lower than the threshold value of the amplitude of the polarization fading, the corresponding time (duration randomly varies, and for the sake of simplicity, the average value Δ T thereof may be usedthTo discuss), as shown in "diagonal" code stream block in fig. 2, when fig. 2 shows the code stream schematic diagram without applying the low cost polarization scrambler, after introducing the low cost polarization scrambler, the code stream schematic diagram refers to fig. 3, from fig. 3 and fig. 3In comparison with FIG. 2, the application of the low-cost polarization scrambler effectively reduces the polarization of the polarization-scrambled signal below the polarization fading threshold AthThe continuous error code number caused by polarization fading is effectively reduced, however, in order to ensure practical feasibility, when the low-cost polarization scrambler is combined with the time sequence interleaving technology, the polarization scrambling rate of the low-cost polarization scrambler is reasonable, and finally the error code is corrected by the forward error correction technology.
Generally, to convert continuous bit errors into random bit errors, time-series interleaving is introduced to rearrange bit positions in a binary code stream sequence to improve forward error correction performance, in other words, continuous bits originally constituting a symbol are discretely distributed over a wider time dimension after time-series interleaving, taking fig. 4 as an example, forward error correction coding is assumed as (7,4) hamming code, it is known that 1 (7,4) hamming code word can correct 1 bit error in the code word at most, and if the coded binary code stream causes 2 continuous bit errors due to polarization fading at a certain moment, as indicated by an arrow in fig. 4. Without time-series interleaving, 2 consecutive errors correspond to the same hamming codeword N, at which point forward error correction coding fails because the erroneous bits cannot be corrected, as shown in fig. 5, the bits in the 2 consecutive hamming codewords are reordered using time-series interleaving. When the sequential interleaving auxiliary forward error correction coding converts continuous error codes into random error codes, randomly arranging code word bit positions after the forward error correction coding in a time dimension, wherein if the original positions of 2 continuous error bits after interleaving before interleaving are shown as 5, the sequential interleaving operation converts 2 continuous error bits appearing in 1 Hamming code word in figure 4 into 2 Hamming code words respectively transmitting 1 bit by error, which are indicated by arrows in figure 5, so that (7,4) Hamming codes can correct respective error bits in 2 groups, error codes caused by polarization fading are avoided, and under the premise of fixed forward error correction coding, the sequential interleaving time is optimized by combining a low-cost scrambler, and the sequential interleaving time is set as TinThe number of time-sequence interleaving bits is n, and the duration of each bit in transmission is TbThen, the following conditions are satisfied:
Tin＝n*Tb；
in general, the timing interleaving time TinAnd polarizationFading time Δ TthShould substantially satisfy the following relationship:
i.e. the time T of interleavinginCovering a polarization decay time Δ TthIs the minimum requirement for converting continuous bit errors caused by polarization fading into random bit errors through time sequence interleaving in a time dimension.
Here, the time-sequence interleaving time TinSetting of (D) and polarization decay time DeltaTthClosely related, when the time-sequential interleaving is not sufficient in the time dimension to cover the polarization-fading time Δ TthWhen is at time
At this time, because the forward error correction technique has a limited capability of correcting continuous bit errors, no matter whether the time sequence interleaving is adopted, the bit errors cannot be effectively corrected, and conversely, as shown in fig. 5, the time sequence interleaving is beneficial to forward error correction coding to correct continuous bit errors caused by polarization fading. When in use
The error correction capability of forward error correction coding tends to be ideal. However, the too long bit sequence for time sequence interleaving will greatly occupy storage and computational resources, so that the forward error correction coding and time sequence interleaving technique face a serious challenge in overcoming slow polarization fading, and is lack of practical value.
Therefore, the introduction of the low-cost polarization scrambler accelerates the change speed of the polarization state of the signal light by a certain rule to obtain a highly random polarization state, thereby avoiding overlong polarization fading duration, namely effectively reducing the amplitude of coherent detection light lower than a threshold AthTime Δ T ofthThe application of the low-cost polarization scrambler effectively improves the requirement on the time sequence interleaving length and reduces Tin。
Therefore, in practical implementation, the deflection time interval of the low-cost deflection scrambler is set as TscramTime interval T of low cost offset scramblerscramAnd polarization fading time DeltaTthShould satisfy the following relationship:
that is, the introduction of a low-cost polarization scrambler is intended to reduce the polarization decay time Delta TthAt intervals of time T of deflectionscramFor reference, a finer division is made in the time dimension and at each time interval T of the perturbationsscramIn the method, the amplitude of coherent detection light is not less than the threshold value of polarization fading amplitude A through the random polarization state caused by polarization disturbancethReducing the polarization decay time DeltaTthThereby reducing or even avoiding bit errors caused by the same.
Deflection disturbing time interval T of low-cost deflection disturbing devicescramThe smaller, the polarization decay time Δ TthAnd then decreases, thereby making the time sequence interleaving time TinAnd also decreases. When in use
Time-sequence interleaving time TinAnd the optimization can be effectively realized, and more register resources are saved. Low cost deflection scrambler and deflection scrambling time interval T of low cost deflection scramblerscramSmaller, i.e. low cost, rate requirements of the scrambler
The higher. In practical implementation, the deflection disturbing time interval T of the low-cost deflection scramblerscramShould satisfy the condition that the time is reasonably less than the polarization fading time Delta TthOn the premise of realizing the length of time sequence interleaving (time sequence interleaving time T)in) And (4) reasonably optimizing.
The low-cost polarization scrambler is different from the polarization mode dispersion used for processing polarization multiplexing signals in a coherent detection system in the existing scheme, and the low-cost polarization scrambler is based on the problem of polarization fading in a cooperative processing single-polarization coherent detection system.
Example 3
Referring to fig. 6, the present invention further provides a single-polarization coherent detection method for avoiding polarization fading, the flow chart of the method is shown in fig. 6, and the method includes the following steps:
s1, in a transmitter DSP of an optical transmitter, after an information source is subjected to operations such as serial/parallel conversion, channel coding and the like, a baseband electric signal is obtained through digital/analog conversion, and the baseband electric signal is output to a single polarization vector modulator through an electric amplifier; wherein, the channel coding process comprises forward error correction coding and time sequence interleaving;
s2, loading the amplified baseband electric signal to a continuous optical carrier generated by a laser through a single polarization vector modulator, and sending the baseband electric signal to an optical fiber link for transmission after generating a single polarization optical signal; wherein, the optical fiber link comprises an optical fiber and an optical amplifier;
s3, completing optical fiber link transmission at an optical receiver end, filtering out a single polarized light signal with out-of-band noise by an optical filter, accelerating random change of polarization by a low-cost polarization scrambler, inputting the single polarized light signal and local oscillator light emitted by a local oscillator into a coherent receiver, and performing frequency mixing detection in the coherent receiver to generate coherent detection light;
s4, setting a polarization fading amplitude threshold A of coherent detection lightthWhen the amplitude of the coherent detection light is lower than the threshold AthWhen the time is longer, the corresponding time continuous burst error code is caused by the polarization fading, and the time lower than the threshold value is the polarization fading time Delta Tth(ii) a The introduction and application of the low-cost polarization scrambler can accelerate the polarization change of a single-polarization optical signal and reduce the polarization fading time delta Tth；
S5, in the polarization fading time delta TthIn the method, a single polarization optical signal generates continuous error codes, channel coding comprises forward error correction coding and time sequence interleaving, wherein the time sequence interleaving converts continuous burst error codes caused by polarization fading into random error codes so as to improve the capacity of foreword error correction, and the forward error correction coding is used for correcting the error codes at a receiving end;
s6, sending the coherent detection optical signal into a receiver DSP, and obtaining an information sink through operations such as analog/digital conversion, channel decoding, parallel/serial conversion and the like; wherein, the channel decoding comprises time sequence de-interleaving and forward error correction decoding.
It should be understood that the above-described embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. This need not be, nor should it be exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.
Claims (10)
1. A single polarization coherent detection system for avoiding polarization fading is characterized by comprising an optical transmitter, an optical fiber link and an optical receiver comprising a low-cost polarization scrambler, wherein the optical transmitter comprises a transmitter DSP, an electric amplifier, a single polarization vector modulator and a laser, the transmitter DSP comprises forward error correction coding and time sequence interleaving, the optical receiver comprises an optical filter, a low-cost polarization scrambler, a local oscillator, a coherent receiver and a receiver DSP, and the receiver DSP comprises time sequence de-interleaving and forward error correction decoding; in a transmitter DSP at an optical transmitter end, after a signal source passes through serial/parallel conversion, channel coding operation including forepart error correction coding and time sequence interleaving, a baseband electric signal is obtained through digital/analog conversion, the baseband electric signal is output to a single polarization vector modulator through an electric amplifier, the single polarization vector modulator is loaded on a continuous optical carrier generated by a laser, and a single polarization optical signal is generated and then sent to an optical fiber link for transmission;
completing optical fiber link transmission, filtering out single polarization optical signals with out-of-band noise by an optical filter, accelerating random change of polarization by a low-cost polarization scrambler, inputting the signals and local oscillator light emitted by a local oscillator into a coherent receiver, and performing frequency mixing detection in the coherent receiver to generate coherent detection light;
let the threshold of the polarization fading amplitude of coherent detection light be AthWhen the amplitude of the coherent detection light is lower than AthWhen the time is short, the corresponding time continuous burst error code caused by polarization fading is lower than AthIs the polarization decay time DeltaTthThe introduction and application of the low-cost polarization scrambler can accelerate the polarization change of the single-polarization optical signal and reduce the polarization fading time delta Tth；ΔTthInternal single polarized lightThe signal generates continuous error codes, the continuous burst error codes caused by polarization fading are converted into random error codes in the time sequence interweaving in the DSP of the transmitter so as to improve the error correction capability of the previous item, and the forward error correction coding is used for correcting the error codes at the receiving end;
coherent detection optical signals are sent to a receiver DSP of the optical receiver, and information sink is obtained through analog-to-digital conversion, channel decoding including time sequence de-interleaving and forward error correction decoding, and parallel/serial conversion operation.
2. The single-polarization coherent detection system for avoiding polarization fading of claim 1, wherein the optical fiber link comprises an optical fiber and an optical amplifier for transmitting single-polarization optical signals.
3. The single-polarization coherent detection system for avoiding polarization fading of claim 2, wherein the low-cost polarization scrambler is an extruded fiber.
4. The single-polarization coherent detection system for avoiding polarization fading of claim 2, wherein the low-cost polarization scrambler is a deflection fiber coil.
5. The single-polarization coherent detection system for avoiding polarization fading of claim 1, wherein the time-sequence interleaving time is TinThe number of time-series interleaving bits is n, and the duration of each bit in transmission is TbThen, the following conditions are satisfied:
Tin＝n*Tb；
time-sequential interleaving time TinAnd polarization fading time DeltaTthShould satisfy the following relationship:
i.e. the time-sequential interleaving time TinCovering a polarization decay time Δ TthContinuous bit errors caused by polarization fading are interleaved in time dimension by time sequenceMinimum requirements for conversion to random bit errors; in that
In the process, burst continuous error codes are randomized at the cost of time sequence interleaving time, and the error code correcting capability is improved by matching with a forward error correction coding technology so as to realize tolerance and resistance to polarization fading.
6. The single-polarization coherent detection system for avoiding polarization fading of claim 5, wherein the polarization-disturbing time interval of the low-cost polarization scrambler is TscramTime interval T of low cost offset scramblerscramAnd polarization fading time DeltaTthShould satisfy the following relationship:
that is, the introduction of a low-cost polarization scrambler is intended to reduce the polarization decay time Delta TthAt intervals of time T of deflectionscramFor reference, a fine division is made in the time dimension and at each time interval T of the perturbationsscramIn the method, the amplitude of coherent detection light is not less than a polarization fading amplitude threshold A through a random polarization state caused by polarization disturbancethReducing the polarization decay time DeltaTthThereby reducing or even avoiding bit errors caused by the same.
7. The single-polarization coherent detection system for avoiding polarization fading of claim 6, wherein the polarization-disturbing time interval T of the low-cost polarization scrambler is TscramThe smaller, the polarization decay time Δ TthAnd then decreases, thereby making the time sequence interleaving time TinIs also reduced; when in use
Time-sequence interleaving time TinThe optimization can be effectively realized, and the register resource is saved; deflection disturbing time interval T of low-cost deflection scramblerscramSmaller, low cost scrambler rates
The larger the size is, the higher the requirement on a low-cost polarization scrambler is; deflection disturbing time interval T of low-cost deflection disturbing devicescramIs to satisfy a reasonable smaller than polarization decay time Delta TthOn the premise of realizing the time sequence interleaving time TinAnd (4) reasonably optimizing.
8. The single-polarization coherent detection system for avoiding polarization fading of claim 7, wherein the time-series interleaving represents randomly arranged forward error correction coded codewords in a time dimension when converting continuous bit errors into random bit errors, thereby improving forward error correction performance.
9. The single-polarization coherent detection system for avoiding polarization fading of claim 8, wherein the channel coding in the transmitter DSP comprises forward error correction coding and timing interleaving, and the channel decoding in the receiver DSP comprises timing deinterleaving and forward error correction decoding for cooperating with the timing interleaving and forward error correction operations, both of which have an inherent logical association with the low-cost scrambler and should be considered jointly.
10. A single-polarization coherent detection method for avoiding polarization fading, the method comprising:
s1, in a transmitter DSP of an optical transmitter, after an information source is subjected to operations such as serial/parallel conversion, channel coding and the like, a baseband electric signal is obtained through digital/analog conversion, and the baseband electric signal is output to a single polarization vector modulator through an electric amplifier; wherein, the channel coding process comprises forward error correction coding and time sequence interleaving;
s2, loading the amplified baseband electric signal to a continuous optical carrier generated by a laser through a single polarization vector modulator, and sending the baseband electric signal to an optical fiber link for transmission after generating a single polarization optical signal; wherein, the optical fiber link comprises an optical fiber and an optical amplifier;
s3, completing optical fiber link transmission at an optical receiver end, filtering out a single polarized light signal with out-of-band noise by an optical filter, accelerating random change of polarization by a low-cost polarization scrambler, inputting the single polarized light signal and local oscillator light emitted by a local oscillator into a coherent receiver, and performing frequency mixing detection in the coherent receiver to generate coherent detection light;
s4, setting a polarization fading amplitude threshold A of coherent detection lightthWhen the amplitude of the coherent detection light is lower than the threshold AthWhen the time is longer, the corresponding time continuous burst error code is caused by the polarization fading, and the time lower than the threshold value is the polarization fading time Delta Tth(ii) a The introduction and application of the low-cost polarization scrambler can accelerate the polarization change of a single-polarization optical signal and reduce the polarization fading time delta Tth；
S5, in the polarization fading time delta TthIn the method, a single polarization optical signal generates continuous error codes, channel coding comprises forward error correction coding and time sequence interleaving, wherein the time sequence interleaving converts continuous burst error codes caused by polarization fading into random error codes so as to improve the capacity of foreword error correction, and the forward error correction coding is used for correcting the error codes at a receiving end;
s6, sending the coherent detection optical signal into a receiver DSP, and obtaining an information sink through operations such as analog/digital conversion, channel decoding, parallel/serial conversion and the like; wherein, the channel decoding comprises time sequence de-interleaving and forward error correction decoding.
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