CN113206705B - Self-interference elimination method based on optical fiber dispersion effect and digital algorithm - Google Patents

Self-interference elimination method based on optical fiber dispersion effect and digital algorithm Download PDF

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CN113206705B
CN113206705B CN202110323030.6A CN202110323030A CN113206705B CN 113206705 B CN113206705 B CN 113206705B CN 202110323030 A CN202110323030 A CN 202110323030A CN 113206705 B CN113206705 B CN 113206705B
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CN113206705A (en
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朱子行
高从芮
赵尚弘
李轩
石磊
李勇军
徐志燕
林涛
王国栋
李�赫
黄蓝锋
周逸潇
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Air Force Engineering University of PLA
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Abstract

The self-interference elimination device based on the optical fiber dispersion effect and the digital algorithm comprises a wavelength tunable laser, a fixed wavelength laser, a double-parallel Mach-Zehnder modulator A, a double-parallel Mach-Zehnder modulator B, an optical combiner, a single mode fiber, an erbium-doped fiber amplifier, an optical detector, a down-conversion module, an ADC and a DSP module. At a base station, a fixed wavelength laser generates incident light to a Mach-Zehnder modulator, a sub-modulator 1 performs carrier suppression double-sideband CS-DSB modulation on the incident light, and a sub-modulator 2 generates a pure optical carrier with phase shift; the fixed wavelength laser carries out the same modulation on the local reference signal, two paths of light are transmitted to the central station, and the erbium-doped fiber amplifier amplifies and outputs the two paths of light. An optical auxiliary self-interference elimination method based on the optical fiber dispersion effect is also provided. The wavelength of the tunable laser is changed, so that the time delay of the reference signal can be adjusted, and the main path strong interference of the self-interference signal is eliminated; by changing the main bias point of the DPMZM, the power fading of a useful signal in optical fiber transmission can be overcome.

Description

Self-interference elimination method based on optical fiber dispersion effect and digital algorithm
Technical Field
The invention belongs to the technical field of microwave photon signal processing, and particularly relates to a self-interference elimination method based on an optical fiber dispersion effect and a digital algorithm under an in-band full-duplex ROF system.
Background
Radio Over Fiber (ROF) has attracted great attention in recent years as a communication technology combining photonic technology and microwave technology, and its characteristics such as low loss, large bandwidth, high mobility and the like are considered as a promising solution. In general, to avoid interference, a base station in an ROF system receives an uplink signal from a mobile user in a frequency band and simultaneously transmits a downlink signal in a different frequency band, limited spectrum resources and the increasing realistic demand for higher data rates have promoted the development of an in-band full-duplex ROF system whose uplink and downlink frequencies are the same, which increases the throughput of information while multiplying spectrum efficiency.
In an in-band full-duplex communication system, self-interference between transmitting and receiving channels is a main problem which needs to be solved urgently at present, a conventional electrical self-interference elimination scheme is limited in working frequency and bandwidth, and an optical auxiliary method can overcome the difficulty encountered by 'electronic bottleneck', so that a powerful support is provided for realizing self-interference elimination in a high frequency band, a large bandwidth, high tuning precision and low loss.
In recent years, self-interference elimination by utilizing a microwave photon technology is widely researched by domestic and foreign research institutes, wherein the scheme based on the double parallel Mach-Zehnder modulator has the advantages of high integration level and large offset depth. 1) Han X, Huo B, Shao Y, et al, optical RF self-interference cancellation by using an integrated dual-parallel MZM [ J ]. IEEE Photonics Journal, 2017, 9 (2): 1-8.2012. The self-interference elimination technology based on the photon predistortion technology realizes the time delay and amplitude modulation through an optical method. 2) Tu Z, Wen A, Li X, et al. A photonic pre-discrimination Technology for RF self-interference cancellation [ J ]. IEEE Photonics Technology Letters, 2018, 30 (14): 1297- & 1300. scheme 3) simulation of multipath interference cancellation based on polarization modulator and dispersive device) Zhou W, Xiang P, Niu Z, et al. 849-. 4Chen Y, Yao J.Photonic-Assisted RF Self-Interference With Improved spectral Efficiency and Fiber Transmission Capability [ J ]. Journal of Lightwave Technology, 2019, 38 (4): 761-768.5Chen Y.A Photonic-Based Wideband RF Self-Interference Cancellation apparatus With Fiber Dispersion Immunity [ J ]. Journal of Lightwave Technology, 2020, PP (99): 1-1.
However, the above scheme has certain limitations. 1) And 2) the problem of long-distance transmission of optical fiber is not considered, and the power fading is generated when useful signals are transmitted in the optical fiber; 3) the dispersion module device is used, the complexity of the system is increased, meanwhile, the PolM modulator is greatly influenced by the polarization state and has poor stability, and the effect of the PolM modulator on counteracting the multipath effect is realized by greatly increasing optical hardware; 4) and 5) long-distance optical fiber transmission of useful signals after self-interference signals are eliminated is realized, but the time delay tuning is carried out on an electrical domain, and the self-interference cancellation depth can be influenced.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention aims to provide a self-interference elimination device based on an optical fiber dispersion effect and a digital algorithm, which is characterized by comprising a wavelength tunable laser 1, a fixed wavelength laser 2, a double-parallel Mach-Zehnder modulator A3, a double-parallel Mach-Zehnder modulator B4, an optical combiner 5, a single-mode optical fiber 6, an erbium-doped optical fiber amplifier 7, an optical detector 8, a down-conversion module 9, an ADC10 and a DSP module; wherein
At the base station a, the wavelength tunable laser 1 outputs laser light to the double-parallel mach-zehnder modulator a3, and the fixed-wavelength laser 2 outputs laser light to the double-parallel mach-zehnder modulator B4; two optical signals are output by the optical combiner 5, transmitted to the central station B through the single-mode fiber 6, amplified by the erbium-doped fiber amplifier 7, beat-frequency by the optical detector 8, down-converted into baseband analog signals by the down-conversion module 9, analog-to-digital converted by the ADC10, and input to the DSP for digital signal processing.
The invention also provides a self-interference elimination method based on the optical fiber dispersion effect and the digital algorithm, and the self-interference elimination device based on the optical fiber dispersion effect and the digital algorithm comprises a wavelength tunable laser 1, a fixed wavelength laser 2, a double parallel Mach-Zehnder modulator A3, a double parallel Mach-Zehnder modulator B4, an optical combiner 5, a single mode fiber 6, an erbium-doped fiber amplifier 7, an optical detector 8, a down-conversion module 9, an ADC10 and a DSP module; at a base station A, a wavelength tunable laser 1 outputs laser to a double-parallel Mach-Zehnder modulator A3, and a fixed wavelength laser 2 outputs laser to a double-parallel Mach-Zehnder modulator B4; two paths of optical signals are output by the optical combiner 5, transmitted to the central station B through the single-mode optical fiber 6, amplified by the erbium-doped optical fiber amplifier 7 and subjected to beat frequency by the optical detector 8 in sequence, then down-converted into baseband analog signals through the down-conversion module 9, subjected to analog-to-digital conversion by the ADC10 and input into the DSP for digital signal processing;
the method specifically comprises the following steps:
for convenience of explanation, first assume that the local reference signal (i) is V1cosωst is a self-interference signal V2cosωs(t+τA) Useful signal of V3cos(ωst+τB) (ii) a Wherein Vi(i is 1, 2, 3) is the voltage of the local reference signal, the interference signal, the useful signal, respectively, τA、τBRespectively, the time delay, omega, generated by the self-interference signal and the useful signal in the space propagationsThe frequencies of the local reference signal, the self-interference signal and the useful signal;
the first step is as follows: at the base station, a wavelength tunable laser modulates a reference signal onto light through an electro-optic modulator;
the wavelength tunable laser 1 outputs optical carriers to a double parallel Mach-Zehnder modulator A3, the double parallel Mach-Zehnder modulator A3 is composed of two sub-modulators and a main modulator, the two sub-modulators include a first sub-MZM-1 and a second sub-MZM-2, and one main modulatorThe device is a main MZM; the first sub MZM-1 electro-optically modulates a local reference signal (i), and sets the first sub MZM-1 as a minimum bias point; the second sub MZM-2 has no input signal and is set as the maximum bias point, and the clean optical carrier output by the second sub MZM-2 is phase-modulated into the main MZM
Figure GSB0000198778010000041
Under small-signal modulation, the double-parallel Mach-Zehnder modulator A3 outputs an optical signal envelope E1(t) is of the form of equation (1):
Figure GSB0000198778010000042
wherein,
Figure GSB0000198778010000043
being an optical carrier of a wavelength-tunable laser 1, EcAmplitude of the optical carrier, ωcIs the input light carrier frequency, omega, at a reference frequencyτLaser 1 tunable for wavelength at a reference frequency omegacOffset frequency of lower, m1＝πV1/VπIs the modulation factor, V, of the sub MZM-1πIs a half-wave voltage, and is,
Figure GSB0000198778010000044
phase shift introduced for bias voltage of main MZM, J1(. 1) is a class 1 Bessel function, t is time;
the second step is that: at the base station, a fixed wavelength laser modulates a receive signal onto light through an electro-optic modulator;
the fixed wavelength laser 2 outputs an optical signal as an optical carrier, the optical carrier is input to a double parallel Mach-Zehnder modulator B4, a first sub MZM-1 electro-optically modulates a received signal which is a self-interference signal plus a useful signal, the modulation mode of the received signal is the same as that of the double parallel Mach-Zehnder modulator A3, and the double parallel Mach-Zehnder modulator B4 modulates an envelope E of the output optical signal2(t), as shown in equation (2):
Figure GSB0000198778010000045
wherein m is2＝πV2/Vπ、m3＝πV3/VπRespectively the modulation indexes of a self-interference signal (c) and a useful signal (c),
Figure GSB0000198778010000046
phase shift introduced for bias voltage of main MZM, J0(. cndot.) is a Bessel function of order 0,
Figure GSB0000198778010000047
is the optical carrier of the fixed wavelength laser 2;
the third step: combining the two optical signals and transmitting the combined optical signals to a central station for amplification; the two optical signals are combined and output at the optical combiner 5; because the two paths of light with different wavelengths are combined, the light is transmitted under different wavelengths after being combined;
due to the dispersion effect of the fiber, the transmission function of the fiber is H (ω) ═ exp (- α L/2+ j β2L(ω-ωc)2/2), where α is the attenuation constant of the fiber, β2Is a group velocity dispersion parameter, L is the fiber length, and omega is the angular frequency; therefore, after the two optical signals are combined by the optical combiner 5 and then transmitted by the single-mode optical fiber 6 and amplified by the erbium-doped optical fiber amplifier 7, the envelope E of the optical signal output by the erbium-doped optical fiber amplifier 7 isSMF(t) is shown in equation (3):
Figure GSB0000198778010000051
wherein G isOAFor the gain of the erbium-doped fiber amplifier 7, θ (ω) ═ β2L(ω-ωc)22, phase shift introduced by the radio frequency signal in the optical fiber dispersion effect;
the fourth step: beat frequency of the optical signal is converted into an electrical signal;
an electrical signal i (t) obtained after the optical signal output by the erbium-doped fiber amplifier 7 is subjected to beat frequency by the optical detector 8 is shown in formula (4):
Figure GSB0000198778010000052
wherein R is the responsivity of the photodetector 8;
the fifth step: the parameters of the tuning device realize self-interference signal elimination and powerless fading transmission of useful signals;
according to the correlation among the signals in the formula (4), self-interference signal cancellation can be realized by satisfying the first three relation formulas in the formula (5), and the fourth relation formula ensures the non-power fading transmission of the useful signals:
Figure GSB0000198778010000061
it can be observed from equation (4) that the reference signal introduces β after transmission through the optical fiber2LωτThe time delay with the time delay tau can be realized by adjusting the wavelength of the tunable laser 1 and changing the wavelength difference of the two lasersAMatching; meanwhile, the tuning step of the tunable laser is an important factor for realizing accurate and adjustable time delay, and the group velocity response parameter beta2＝-20ps2Perm, under the condition that the length of the optical fiber is 20km, the tuning accuracy of 0.25ps can be generated by 100MHz tuning stepping, and the performance index of the self-interference elimination technology in a radio frequency domain can be met;
and a sixth step: further eliminating the residual self-interference signal in a digital domain to obtain a useful signal;
the self-interference signal still has strong residual self-interference components after being canceled in the analog domain, and the radio frequency domain electric signal is converted into a baseband digital signal d (i) through the down-conversion module 9 and the ADC 10; after the digital signal d (i) enters the DSP module, further canceling the residual self-interference component in the DSP module by adopting a digital self-interference elimination means so as to recover a useful signal; the method comprises the following steps of enabling a base band signal of a reference signal c
Figure GSB0000198778010000062
After passing through an M-order self-interference filter, a residual self-interference signal is reconstructed
Figure GSB0000198778010000063
i is a sampling point, the weighting coefficient of each tap of the filter is omega (n), and the reconstructed residual self-interference signal is expressed as
Figure GSB0000198778010000064
M represents a variable from-M/2 to M/2, w (M) is a filter tap weighting coefficient at point M, x (i-M) is a value of the reference signal at (i-M), and the residual digital domain self-interference signal d (i) is subtracted from the reconstructed self-interference signal d (i)
Figure GSB0000198778010000065
And continuously iterating by using a least mean square LMS algorithm to gradually minimize the error so as to eliminate the residual self-interference signal and recover the useful signal.
The invention obtains the corresponding relation between the signal delay and the fiber length, the group velocity dispersion coefficient and the carrier wavelength difference by using the dispersion effect of the optical fiber, and adjusts the optical carrier wavelength of the tunable laser under the condition of determining the fiber length and the fiber group velocity dispersion coefficient, thereby realizing the adjustable delay of the local signal and aligning the self-interference signal.
The invention provides a scheme capable of eliminating self-interference and simultaneously meeting the requirement of long-distance non-fading transmission of signals, and the scheme utilizes the optical fiber dispersion effect to facilitate the nonlinear loss caused by optical fibers and becomes an important means for realizing accurate tuning of signal delay. Meanwhile, the problems that the accuracy of the electric delay line used in a self-interference elimination scheme is poor and the optical delay line is difficult to integrate in an electro-optical device are solved; the invention introduces a clean optical carrier with phase to compensate the power fading of the useful signal during transmission.
The invention provides a self-interference elimination scheme combining an analog domain and a digital domain, wherein a signal after self-interference elimination in the analog domain still has a residual self-interference component and influences the recovery of a useful signal, and the residual self-interference component can be counteracted and the useful signal can be recovered by utilizing the powerful digital signal processing capability by adopting a digital domain self-interference elimination algorithm.
Drawings
FIG. 1 is a schematic structural diagram of a self-interference cancellation apparatus based on fiber dispersion effect and digital algorithm according to the present invention;
FIG. 2(a) (b) is a simulation diagram of self-interference cancellation performance at a single frequency point in the present invention;
FIG. 3(a) (b) is a simulation diagram of the self-interference cancellation performance of the broadband signal in the present invention;
FIG. 4 is a graph illustrating the ability of a test signal to compensate for power fading caused by fiber dispersion during fiber transmission;
fig. 5 is a constellation diagram of a 4QAM useful signal obtained by combining a 1ms digital domain self-interference cancellation algorithm under an analog multipath channel environment.
Detailed Description
The invention is further described below with reference to the accompanying drawings:
as shown in fig. 1, the self-interference cancellation device based on the optical fiber dispersion effect and the digital algorithm in the in-band full-duplex ROF system includes a wavelength tunable laser 1, a fixed wavelength laser 2, a dual-parallel mach-zehnder modulator A3, a dual-parallel mach-zehnder modulator B4, an optical combiner 5, a single-mode fiber 6, an erbium-doped fiber amplifier 7, an optical detector 8, a down-conversion module 9, an ADC10, and a DSP module. At a base station A, a wavelength tunable laser 1 outputs laser light to a double-parallel Mach-Zehnder modulator A3, and a fixed wavelength laser 2 outputs laser light to a double-parallel Mach-Zehnder modulator B4; two optical signals are output by the optical combiner 5, transmitted to the central station B through the single-mode fiber 6, amplified by the erbium-doped fiber amplifier 7, subjected to beat frequency by the optical detector 8, down-converted into baseband analog signals by the down-conversion module 9, subjected to analog-to-digital conversion by the ADC10, and input to the DSP11 for digital signal processing.
For convenience of explanation, first assume that the local reference signal (i) is V1cosωst is a self-interference signal V2cosωs(t+τA) Useful signal of V3cos(ωst+τB). Wherein Vi(i＝1，2，3) Voltages, τ, of local reference signal, interference signal, desired signal, respectivelyA、τBRespectively, the time delay, omega, generated by the self-interference signal and the useful signal in the space propagationsThe frequencies of the local reference signal, the self-interference signal and the useful signal. The wavelength tunable laser 1 outputs optical carriers to a double-parallel Mach-Zehnder modulator A3, the double-parallel Mach-Zehnder modulator A3 is composed of two sub-modulators (a first sub MZM-1 and a second sub MZM-2) and a main modulator (a main MZM), the first sub MZM-1 carries out electro-optical modulation on a local reference signal (I), and the first sub MZM-1 is set as a minimum bias point; the second sub MZM-2 has no input signal and is set as the maximum bias point, and the clean optical carrier output by the second sub MZM-2 is phase-modulated into the main MZM
Figure GSB0000198778010000081
Under small-signal modulation, the double-parallel Mach-Zehnder modulator A3 outputs an optical signal envelope E1(t) is of the form of equation (1):
Figure GSB0000198778010000082
wherein,
Figure GSB0000198778010000083
being an optical carrier of a wavelength-tunable laser 1, EcAmplitude of the optical carrier, ωcIs the input light carrier frequency, omega, at a reference frequencyτAt a reference frequency omega for a wavelength-tunable laser 1cOffset frequency of lower, m1＝πV1/VπIs the modulation factor, V, of the sub MZM-1πIs a half-wave voltage, and is,
Figure GSB0000198778010000084
phase shift introduced for bias voltage of main MZM, J1(. 1) is a class 1 Bessel function, t is time;
the fixed wavelength laser 2 outputs an optical signal as an optical carrier, and the optical carrier is input to a double parallel Mach-Zehnder modulator B4The first sub MZM-1 electro-optically modulates the received signal (self-interference signal + useful signal) in the same way as the double parallel Mach-Zehnder modulator A3, and the double parallel Mach-Zehnder modulator B4 modulates the envelope E of the output optical signal2(t) is shown in equation (2):
Figure GSB0000198778010000091
wherein m is2＝πV2/Vπ、m3＝πV3/VπRespectively the modulation indexes of a self-interference signal (c) and a useful signal (c),
Figure GSB0000198778010000092
phase shift introduced for bias voltage of main MZM, J0(. cndot.) is a Bessel function of order 0,
Figure GSB0000198778010000093
is the optical carrier of the fixed wavelength laser 2.
The two optical signals are combined and output at the optical combiner 5. Because the two paths of light with different wavelengths are combined, the light is transmitted under different wavelengths after being combined.
Due to the dispersion effect of the fiber, the transmission function of the fiber is H (ω) ═ exp (- α L/2+ j β2L(ω-ωc)2/2), where α is the attenuation constant of the fiber, β2The group velocity dispersion parameter, L is the fiber length, and ω is the angular frequency. Therefore, after the two optical signals are combined by the optical combiner 5 and then transmitted by the single-mode optical fiber 6 and amplified by the erbium-doped optical fiber amplifier 7, the envelope E of the optical signal output by the erbium-doped optical fiber amplifier 7 isSMF(t) is shown in equation (3):
Figure GSB0000198778010000094
wherein G isOAFor the gain of the erbium-doped fiber amplifier 7, θ (ω) ═ β2L(ω-ωc)22 is shotThe phase shift introduced by the frequency signal in the effect of fiber dispersion.
An electrical signal i (t) obtained after the optical signal output by the erbium-doped fiber amplifier 7 is subjected to beat frequency by the optical detector 8 is shown in formula (4):
Figure GSB0000198778010000101
where R is the responsivity of the photodetector 8.
According to the correlation among the signals in the formula (4), self-interference signal cancellation can be realized by satisfying the first three relation formulas in the formula (5), and the fourth relation formula ensures the non-power fading transmission of the useful signals:
Figure GSB0000198778010000102
it can be observed from equation (4) that the reference signal introduces β after transmission through the optical fiber2LωτThe delay tau can be realized by adjusting the wavelength of the wavelength tunable laser 1 and changing the wavelength difference of the two lasersAAnd (6) matching. Meanwhile, the tuning step of the tunable laser is an important factor for realizing accurate and adjustable time delay, and the group velocity response parameter beta2＝-20ps2Perkm, with a fiber length of 20km, a tuning step of 100MHz can produce a tuning accuracy of 0.25ps, which is sufficient to meet the performance criteria of the radio frequency domain self-interference cancellation technique.
The self-interference signal has strong residual self-interference component after cancellation in analog domain, and the radio frequency domain electrical signal needs to be converted into the baseband digital signal d (i) by the down-conversion module 9 and the ADC10 (this technique is well known to those skilled in the art and will not be described again). After the digital signal d (i) enters the DSP module, a digital self-interference cancellation means is adopted in the DSP module to further cancel the residual self-interference component to recover the useful signal (the means of self-interference cancellation is divided into three types, respectively in the antenna domain, the analog domain and the digital domain). The method comprises the following steps of enabling a base band signal of a reference signal c
Figure GSB0000198778010000103
After passing through an M-order self-interference filter, a residual self-interference signal is reconstructed
Figure GSB0000198778010000104
i is a sampling point, the weighting coefficient of each tap of the filter is omega (n), and the reconstructed residual self-interference signal is expressed as
Figure GSB0000198778010000105
M represents a variable from-M/2 to M/2, w (M) is a filter tap weighting coefficient at point M, x (i-M) is a value of the reference signal at (i-M), and the residual digital domain self-interference signal d (i) is subtracted from the reconstructed self-interference signal d (i)
Figure GSB0000198778010000111
And continuously iterating by using a classical Least Mean Square (LMS) algorithm to gradually minimize the error so as to eliminate the residual self-interference signal and recover a useful signal. The above embodiments refer to the "simultaneous co-frequency full duplex principle and applications" of friend of Tang, and full-duplex wireless communications systems of Xuemein (Sherman) Shen.
A self-interference elimination scheme based on optical fiber dispersion effect and digital algorithm under an in-band full-duplex ROF system comprises the following steps:
step 1: at a base station, a wavelength tunable laser 1 outputs optical carriers to a double-parallel Mach-Zehnder modulator A3, the double-parallel Mach-Zehnder modulator A3 is composed of two sub-modulators (a first sub MZM-1 and a second sub MZM-2) and a main modulator (a main MZM), the first sub MZM-1 electro-optically modulates a local reference signal (I), and the first sub MZM-1 is set as a minimum bias point; the second sub MZM-2 has no input signal and is set as the maximum bias point, and the clean optical carrier output by the second sub MZM-2 is phase-modulated into the main MZM
Figure GSB0000198778010000112
Under small signal modulation, double parallel MachDeler modulator A3 output optical signal envelope E1(t) is of the form of equation (1):
Figure GSB0000198778010000113
wherein,
Figure GSB0000198778010000114
being an optical carrier of a wavelength-tunable laser 1, EcBeing the amplitude, omega, of the optical carriercIs the input light carrier frequency, omega, at a reference frequencyτLaser 1 tunable for wavelength at a reference frequency omegacOffset frequency of lower, m1＝πV1/VπIs the modulation factor, V, of the sub MZM-1πIs a half-wave voltage, and is,
Figure GSB0000198778010000115
phase shift introduced for bias voltage of main MZM, J1(. 1) is a class 1 Bessel function, t is time;
step 2: at a base station, a fixed wavelength laser 3 outputs an optical signal as an optical carrier, the optical carrier is input to a double parallel Mach-Zehnder modulator B4, a first sub-MZM-1 electro-optically modulates a received signal (a self-interference signal (r) + an active signal (r)), the modulation mode of the first sub-MZM-1 is the same as that of the double parallel Mach-Zehnder modulator A3, and the double parallel Mach-Zehnder modulator B4 modulates an envelope E of the output optical signal2(t) is shown in equation (2):
Figure GSB0000198778010000121
wherein m is2＝πV2/Vπ、m3＝πV3/VπWhich are the modulation indexes of a self-interference signal (c) and a useful signal (c),
Figure GSB0000198778010000122
phase shift introduced for bias voltage of main MZM, J0(. 0) is a Bessel function of the 0 th orderThe number of the first and second groups is,
Figure GSB0000198778010000123
is the optical carrier of the fixed wavelength laser 2.
And step 3: two optical signals pass through the optical combiner 5 and then reach the central station through the single-mode optical fiber, and after being amplified at the erbium-doped optical fiber amplifier 7, the output optical signal envelopes ESMF(t) is shown in equation (3): (ii) a
Figure GSB0000198778010000124
Wherein G isOAFor the gain of the erbium doped fiber amplifier 7, the transfer function of the fiber is
H(ω)＝exp(-αL/2+jβ2L(ω-ωc)2/2), where α is the attenuation constant of the fiber, β2For the group velocity dispersion parameter, L is the fiber length, ω is the angular frequency, and θ (ω) ═ β2L(ω-ωc)2And/2 is the phase shift introduced by the radio frequency signal in the fiber dispersion effect.
And 4, step 4: an electrical signal i (t) obtained after the optical signal output by the erbium-doped fiber amplifier 7 is subjected to beat frequency by the optical detector 8 is shown in formula (4):
Figure GSB0000198778010000125
where R is the responsivity of the photodetector 8.
And 5: the self-interference signal cancellation is realized by adjusting the output wavelength of the tunable light source, the modulation coefficient of the electro-optical modulator and the attenuation coefficient of the electrical attenuator to generate electric signals with equal amplitude and opposite phases, namely the first three terms of the formula (5) are satisfied. In addition, according to the fourth term of the formula (5), the problem of power fading caused by the transmission of useful signals in the optical fiber is solved by adjusting the voltage of the main bias point of the double parallel Mach-Zehnder modulator;
Figure GSB0000198778010000131
step 6: the residual self-interference signal is converted into a baseband digital signal d (i) by the down-conversion module 9 and the ADC10, and then enters the DSP module for further processing, wherein in the DSP module, the baseband signal of the reference signal (c) is
Figure GSB0000198778010000132
After passing through an M-order self-interference filter, a residual self-interference signal is reconstructed
Figure GSB0000198778010000133
i is a sampling point, the weighting coefficient of each tap of the filter is omega (n), and the reconstructed residual self-interference signal is expressed as
Figure GSB0000198778010000134
M represents a variable from-M/2 to M/2, w (M) is the filter tap weighting coefficient at point M, and x (i-M) is the value of the reference signal at (i-M). Subtracting the reconstructed self-interference from the residual digital domain self-interference signal d (i)
Figure GSB0000198778010000135
And continuously iterating by using a classical Least Mean Square (LMS) algorithm to gradually minimize the error so as to eliminate the residual self-interference signal and recover a useful signal.
The self-interference elimination scheme based on the optical fiber dispersion effect and the digital algorithm under the in-band full-duplex ROF system is characterized in that the signal delay is determined by the wavelength difference of carrier frequencies of two optical signals, the corresponding relation between the signal delay and the optical fiber length, the group velocity dispersion coefficient and the carrier wavelength difference is obtained by utilizing the dispersion effect of the optical fiber, and under the condition that the optical fiber length and the optical fiber group velocity dispersion coefficient are determined, the optical carrier wavelength of the tunable laser is adjusted to realize the adjustable delay of a local signal so as to align the self-interference signal.
The self-interference elimination scheme based on the optical fiber dispersion effect and the digital algorithm under the in-band full-duplex ROF system provides a scheme which can realize self-interference elimination and simultaneously meet the requirement of long-distance non-fading transmission of signals under the ROF system, and the utilization of the optical fiber dispersion effect is beneficial to the nonlinear loss caused by optical fibers, so that the scheme becomes an important means for realizing accurate tuning of signal delay. Meanwhile, the problems that the accuracy of the electric delay line used in a self-interference elimination scheme is poor and the optical delay line is difficult to integrate in an electro-optical device are solved; the scheme introduces a clean optical carrier with phase to compensate the power fading of the useful signal during transmission.
According to the self-interference elimination scheme based on the optical fiber dispersion effect and the digital algorithm in the in-band full-duplex ROF system, the signal after the self-interference elimination in the analog domain still has the residual self-interference component, the recovery of the useful signal is affected, and the residual self-interference component can be offset and the useful signal can be recovered by the digital signal processing strong capability through the digital domain self-interference elimination algorithm.
In order to verify the single-frequency-point broadband elimination performance and the useful signal recovery capability of the invention, simulation is carried out by using optisystem14.0 and matlab2019 b.
The wavelength of a fixed wavelength laser is set to 1550nm, the power is 10dBm, the line width is 0.1MHz, the length of a single-mode optical fiber is 20km, the gain of an erbium-doped optical fiber amplifier is 10dB, the noise coefficient is 4.5dB, the wavelength of the tunable laser is set to 1552.67nm under the condition that the time delay of a generated self-interference signal is 0.7ns, and through the process, the self-interference signal elimination performance is respectively as follows:
fig. 2(a) and (b) are diagrams illustrating the suppression depths of the tested single-frequency self-interference signals, which include two frequency points of 16GHz and 20GHz, and the self-interference suppression depths are 52.1dB and 50.8dB, respectively;
FIGS. 3(a) and (b) show the suppression depths of the tested broadband self-interference signals, where the bandwidths are 500MHz and 1GHz respectively, and the self-interference suppression depths are 42.3dB and 40.1dB respectively, when the center frequency is 18 GHz;
FIG. 4 is a graph of the ability of a test signal to compensate for power fading caused by fiber dispersion, as compared to an uncompensated case;
fig. 5 is a useful signal constellation diagram obtained after analog domain cancellation and 1ms digital domain cancellation under a test simulation actual ROE system, where EVM is 7.26%. The signal is 4qam, and the multipath time delay is 10, 37, 48, 53ps, and the multipath attenuation is-5, -32, -36, -43dB through 4 multipath channels.
Fig. 4 shows that the dispersion effect of the optical fiber can be overcome by adjusting the bias voltage of the main bias point of the dual parallel mach-zehnder modulator, so as to realize the power compensation of the useful signal. Fig. 5 shows that the scheme is suitable for the ROF communication system, and can ensure that the radio frequency domain is eliminated and then enters the ADC to realize digital domain elimination, and finally, the useful signal is recovered.

Claims (1)

1. A self-interference elimination method based on fiber dispersion effect and digital algorithm is characterized in that,
the self-interference elimination device based on the optical fiber dispersion effect and the digital algorithm comprises a wavelength tunable laser (1), a fixed wavelength laser (2), a double-parallel Mach-Zehnder modulator A (3), a double-parallel Mach-Zehnder modulator B (4), an optical combiner (5), a single-mode fiber (6), an erbium-doped fiber amplifier (7), an optical detector (8), a down-conversion module (9), an ADC (10) and a DSP module; wherein, at the base station A, the wavelength tunable laser (1) outputs laser light to a double parallel Mach-Zehnder modulator A (3), and the fixed wavelength laser (2) outputs laser light to a double parallel Mach-Zehnder modulator B (4); two paths of optical signals are output by an optical combiner (5), transmitted to a central station B through a single mode fiber (6), amplified by an erbium-doped fiber amplifier (7) and subjected to beat frequency by an optical detector (8), down-converted into baseband analog signals through a down-conversion module (9), subjected to analog-to-digital conversion by an ADC (10), and input to a DSP module for digital signal processing;
the self-interference elimination method specifically comprises the following steps:
for convenience of explanation, first assume that the local reference signal (i) is V1cosωst is a self-interference signal V2cosωs(t+τA) Useful signal of V3cos(ωst+τB) (ii) a Wherein V1、V2、V3Voltages, τ, of local reference signal, interference signal, desired signal, respectivelyA、τBRespectively, the time delay, omega, generated by the self-interference signal and the useful signal in the space propagationsThe frequencies of the local reference signal, the self-interference signal and the useful signal;
the first step is as follows: at the base station, a wavelength tunable laser modulates a reference signal onto light through an electro-optic modulator;
the wavelength tunable laser (1) outputs optical carriers to a double parallel Mach-Zehnder modulator A (3), the double parallel Mach-Zehnder modulator A (3) is composed of two sub-modulators and a main modulator, the two sub-modulators comprise a first sub-MZM-1 and a second sub-MZM-2, and the main modulator is a main MZM; the first sub MZM-1 electro-optically modulates a local reference signal (i), and sets the first sub MZM-1 as a minimum bias point; the second sub MZM-2 has no input signal and is set as the maximum bias point, and the clean optical carrier output by the second sub MZM-2 is phase-modulated into the main MZM
Figure FSB0000198759000000021
Under small-signal modulation, the double-parallel Mach-Zehnder modulator A3 outputs an optical signal envelope E1(t) is of the form of equation (1):
Figure FSB0000198759000000022
wherein,
Figure FSB0000198759000000023
being an optical carrier of a wavelength-tunable laser (1), EcBeing the amplitude, omega, of the optical carriercIs the input light carrier frequency, omega, at a reference frequencyτTunable for wavelength at a reference frequency omega of a laser (1)cOffset frequency of lower, m1＝πV1/VπIs the modulation factor, V, of the sub MZM-1πIs a half-wave voltage, and is,
Figure FSB0000198759000000024
phase shift introduced for bias voltage of main MZM, J1(. 1) is a class 1 Bessel function, t is time;
the second step is that: at the base station, a fixed wavelength laser modulates a receive signal onto light through an electro-optic modulator;
the fixed wavelength laser (2) outputs an optical signal as an optical carrier, the optical carrier is input to a double parallel Mach-Zehnder modulator B (4), a first sub MZM-1 electro-optically modulates a received signal, the received signal is a self-interference signal (a) and a useful signal (c), the modulation mode is the same as that of the double parallel Mach-Zehnder modulator A (3), and the double parallel Mach-Zehnder modulator B (4) modulates an envelope E of the output optical signal2(t), as shown in equation (2):
Figure FSB0000198759000000025
wherein m is2＝πV2/Vπ、m3＝πV3/VπRespectively the modulation indexes of a self-interference signal (c) and a useful signal (c),
Figure FSB0000198759000000026
phase shift introduced for bias voltage of main MZM, J0(. cndot.) is a Bessel function of order 0,
Figure FSB0000198759000000027
is an optical carrier of a fixed wavelength laser (2);
the third step: the two optical signals are combined and then transmitted to a central station for amplification; the two optical signals are combined and output at the optical combiner (5); because the two paths of light with different wavelengths are combined, the light is transmitted under different wavelengths after being combined;
due to the dispersion effect of the fiber, the transmission function of the fiber is H (ω) ═ exp (- α L/2+ j β2L(ω-ωc)2/2), where α is the attenuation constant of the fiber, β2Is a group velocity dispersion parameter, L is the fiber length, and omega is the angular frequency; therefore, after the two optical signals are combined by the optical combiner (5), transmitted by the single-mode optical fiber (6) and amplified by the erbium-doped optical fiber amplifier (7), the optical signal envelope E output by the erbium-doped optical fiber amplifier (7)SMF(t) is shown in equation (3):
Figure FSB0000198759000000031
wherein, GOAFor the gain of the erbium-doped fiber amplifier (7), theta (omega) ═ beta2L(ω-ωc)22, phase shift introduced by the radio frequency signal in the optical fiber dispersion effect;
the fourth step: the beat frequency of the optical signal is converted into an electric signal;
an electrical signal I (t) obtained after the optical signal output by the erbium-doped fiber amplifier (7) is subjected to beat frequency by the optical detector (8) is shown as a formula (4):
Figure FSB0000198759000000032
wherein R is the responsivity of the photodetector (8);
the fifth step: the parameters of the tuning device realize self-interference signal elimination and powerless fading transmission of useful signals;
according to the mutual relation among the signals in the formula (4), self-interference signal cancellation can be realized by meeting the first three relational expressions in the formula (5), and the fourth relational expression ensures the non-power fading transmission of the useful signals:
Figure FSB0000198759000000033
it can be observed from equation (4) that the reference signal introduces β after transmission through the fiber2LωτThe time delay with the time delay tau can be realized by adjusting the wavelength of the wavelength tunable laser (1) and changing the wavelength difference of the two lasersAMatching; meanwhile, the tuning step of the tunable laser is an important factor for realizing accurate and adjustable time delay, and the group velocity response parameter beta2＝-20ps2Perkm, a tuning step of 100MHz with a fiber length of 20km, can produce a tuning accuracy of 0.25ps, sufficientPerformance index of radio frequency domain self-interference elimination technology;
and a sixth step: further eliminating the residual self-interference signal in a digital domain to obtain a useful signal;
the self-interference signal still has strong residual self-interference components after being cancelled in an analog domain, and a radio frequency domain electric signal is converted into a baseband digital signal d (i) through a down-conversion module (9) and an ADC (10); after the digital signal d (i) enters the DSP module, further canceling the residual self-interference component in the DSP module by adopting a digital self-interference elimination means so as to recover a useful signal; the method comprises the following steps of enabling a base band signal of a reference signal c
Figure FSB0000198759000000041
After passing through an M-order self-interference filter, a residual self-interference signal is reconstructed
Figure FSB0000198759000000042
i is a sampling point, the weighting coefficient of each tap of the filter is omega (n), and the reconstructed residual self-interference signal is expressed as
Figure FSB0000198759000000043
M represents a variable from-M/2 to M/2, w (M) is a filter tap weighting coefficient at point M, x (i-M) is a value of the reference signal at (i-M), and the residual digital domain self-interference signal d (i) is subtracted from the reconstructed self-interference signal d (i)
Figure FSB0000198759000000044
And continuously iterating by using a least mean square LMS algorithm to gradually minimize the error so as to eliminate the residual self-interference signal and recover the useful signal.
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