Optical frequency conversion and digital self-interference cancellation device and method for reference signal return

Abstract

A digital self-interference cancellation device for optical frequency conversion and simultaneous reference signal return in an in-band full-duplex ROF system is provided, comprising a signal source (1), an electrical amplifier (2), a first power divider A (3), a transmitting antenna (4), a receiving antenna (5), a local oscillator (6), a second power divider B (7), a laser (8), a polarization controller (9), a dual-polarization dual-drive Mach-Zehnder modulator (10) located at base station A; a single-mode fiber (15); and an erbium-doped fiber amplifier (16), a polarization beam splitter (17), a first photodetector A (18), a second photodetector B (19), and a digital signal processor (DSP) (20) located at central station B. A digital self-interference cancellation method for optical frequency conversion and simultaneous reference signal return in an in-band full-duplex ROF system is also provided. This invention transforms a nonlinear problem into a linear problem for processing, reducing the processing difficulty in the digital domain; it eliminates the need for parameter tuning in the optical domain, instead using a high-bit ADC for digital domain processing; and it significantly improves the algorithm's computation speed by utilizing FTRLS multiplication.

Description

Technical Field

[0001] This invention belongs to the field of microwave photonic signal processing technology, specifically relating to a digital self-interference cancellation device and method for simultaneous optical frequency conversion and reference signal return in an in-band full-duplex ROF system. Background Technology

[0002] Optical over-the-air (RoF) radio frequency systems are an emerging wireless access technology that combines photonics and wireless communication to meet the demands of high-speed, high-capacity wireless communication. Optical fiber, with its low loss, high bandwidth, and resistance to electromagnetic interference, makes RoF technology a promising candidate for future applications in wireless broadband communication, satellite communication, and intelligent transportation systems. Given the increasingly scarce spectrum resources, in-band full-duplex RoF systems ensure that base stations can simultaneously receive uplink signals from mobile users and transmit downlink signals on the same frequency band, thus significantly increasing both information throughput and spectral efficiency.

[0003] In in-band full-duplex ROF systems, self-interference between the transmit and receive channels is a problem that urgently needs to be solved. Simple optical domain self-interference cancellation schemes are limited in dealing with multipath and nonlinear problems, while effective combination with digital algorithms provides strong support for achieving high-frequency, large-bandwidth, and low-loss self-interference cancellation.

[0004] In recent years, the use of microwave photonics combined with digital algorithms to achieve self-interference cancellation has been widely studied by scientific research institutions at home and abroad. Among them, the focus is on the joint self-interference cancellation scheme of microwave photonics analog domain and nonlinear adaptive filtering digital domain for nonlinear and multipath effects (1. Xiaolei, Yucheng, Zhang, et al. Optimized self-interference cancellation based on optical dual-parallel MZM for co-frequency and co-timefull duplex wireless communication under nonlinear distortion and emulated multipath effect[J]. Optics express, 2019, 27(26): 37286-37297). Self-interference cancellation scheme based on deep learning algorithm for optical wireless millimeter-wave transmission system and simultaneous recovery of useful signal (2, Zhou Q, Shen S, Chen YW, et al. Simultaneous nonlinear self-interference cancellation and signal of interest recovery using dual input deep neural network in new radio access networks[J]. Journal of Lightwave Technology, 2020, 39(7): 2046-2051. : 1297-1300). Reference (3, Zheng L, Liu Z, Xiao S, et al. Hybrid wideband multipath self-interference cancellation with an LMS pre-adaptive filter for in-band full-duplex OFDM signal transmission[J]. Optics Letters, 2020, 45(23): 6382) uses LMS as a pre-filter to match wireless multipath self-interference signals and achieves cancellation in the optical domain.Reference (4, Han M, Shi T, Chen Y. Digital-assisted photonic analog wideband multipath self-interference cancellation[J].IEEE Photonics Technology Letters,2022) uses RLS as a pre-filter to match weak multipath residual self-interference signals. After being synthesized with the main path reference signal, self-interference cancellation and frequency down-conversion are achieved in the optical domain.

[0005] However, the above schemes have certain limitations. Reference 1 does not consider fiber optic transmission and down-conversion, and the nonlinear adaptive filtering algorithm RLS-Vollterra has high complexity when the nonlinear component of the residual self-interference signal is strong; Reference 2 limits the self-interference cancellation depth and the recovery capability of the useful signal when the power difference between the self-interference signal and the useful signal is large; References 3 and 4 both use electrical delay lines for precise delay matching, and the non-flat frequency response of the electrical tuning device for broadband signals will affect the algorithm performance, and the pre-matching scheme involving high frequency requires the use of a high-performance arbitrary waveform generator. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides a digital self-interference cancellation device for optical frequency conversion and simultaneous reference signal return in an in-band full-duplex ROF system. The device includes a signal source 1, an electrical amplifier 2, a first electrical power divider A3, a transmitting antenna 4, a receiving antenna 5, a local oscillator 6, a second electrical power divider B7, a laser 8, a polarization controller 9, and a dual-polarization dual-drive Mach-Zehnder modulator 10 located at base station A; a single-mode optical fiber 15; and an erbium-doped fiber amplifier 16, a polarization beam splitter 17, a first photodetector A18, a second photodetector B19, and a digital signal processor (DSP) 20 located at central station B. The dual-polarization dual-drive Mach-Zehnder modulator 10 includes a first dual-drive Mach-Zehnder modulator A11, a second dual-drive Mach-Zehnder modulator B12, a 90-degree polarization rotator 13, and a polarization combiner 14.

[0007] At base station A:

[0008] The optical carrier generated by laser 8 is input to dual-polarization dual-drive Mach-Zehnder modulator 10 via polarization controller 9;

[0009] The local oscillator 6 generates a local oscillator signal and outputs it to the second power divider B7. The second power divider B7 performs power distribution on the local oscillator signal ④, with one output to the first dual-drive Mach-Zehnder modulator A11 and the other output to the second dual-drive Mach-Zehnder modulator B12.

[0010] The carrier modulation signal generated by signal source 1 is amplified by power amplifier 2 and output to the first power divider A3. The first power divider A3 distributes the power, and one output is used as a reference signal ②. This signal, along with the local oscillator signal ④ generated by local oscillator source 6 via the second power divider B7, is input to the first dual-drive Mach-Zehnder modulator A11 for electro-optic modulation, generating the X-polarized light signal E. x The other output goes to transmitting antenna 4 and is emitted from there;

[0011] The self-interference signal ① transmitted through the multipath self-interference channel and the useful signal ③ at the far end are received by the receiving antenna 5 together to form a mixed signal. Then, the mixed signal and another local oscillator signal ④ generated by the local oscillator source 6 through the second power divider B7 are input to the second dual-drive Mach-Zehnder modulator B12 for electro-optic modulation. The resulting × polarization state light signal is output after changing the polarization state through the 90-degree polarization rotator 13.

[0012] The optical carrier modulation signal output from the first dual-drive Mach-Zehnder modulator A11 and the optical carrier modulation signal output from the 90-degree polarization rotator 13 are combined at the polarization combiner 14 to form a polarization-combined optical signal; the polarization-combined optical signal is transmitted to the central station B via the single-mode fiber 15.

[0013] At Central Station B:

[0014] After being amplified by the erbium-doped fiber amplifier 16, the beam is split by the polarization beam splitter 17 and output to the first photodetector A18 and the second photodetector B19 respectively. After being beat at the first photodetector A18, an intermediate frequency reference signal with optical frequency conversion is obtained. After being beat at the second photodetector B19, an intermediate frequency received signal with optical frequency conversion is obtained. Both the intermediate frequency reference signal and the intermediate frequency received signal are input to the digital signal processor DSP 20 for digital signal processing.

[0015] A digital self-interference cancellation method for simultaneous optical frequency conversion and reference signal return in an in-band full-duplex ROF system is also provided. This method is based on the aforementioned digital self-interference cancellation device for simultaneous optical frequency conversion and reference signal return in an in-band full-duplex ROF system, assuming the self-interference signal ① is... Reference signal ② is Useful signal ③ is The local oscillator signal ④ is Where V SI V R V SOI V LO The voltages ω of the self-interference signal ①, the local reference signal ②, the useful signal ③, and the local oscillator signal ④ are respectively. SI ω R ω SOI ω LOThese are the angular frequencies of the self-interference signal ①, the local reference signal ②, the useful signal ③, and the local oscillator signal ④, respectively. These are the phases of the self-interference signal ①, the local reference signal ②, the useful signal ③, and the local oscillator signal ④, respectively.

[0016] The method specifically includes the following steps:

[0017] At base station A:

[0018] Step 1: The carrier modulation signal generated by signal source 1 is amplified by electrical amplifier 2 and output to the first power divider A3. The first power divider A3 distributes the power of the signal, one of which is output as reference signal ②; the other is the self-interference signal ① formed after transmission through the multipath self-interference channel and the far-end useful signal ③ is received by receiving antenna 5 together to form a mixed signal (①+③).

[0019] Step 2: The laser 8 generates an optical carrier, which is then used by the polarization controller 9 to generate linearly polarized light. The linearly polarized light is then input to the dual-polarization dual-drive Mach-Zehnder modulator 10.

[0020] Reference signal ② and a local oscillator signal ④ generated by the second power divider B7 from local oscillator source 6 are both input to the first dual-drive Mach-Zehnder modulator A11 for electro-optic modulation. The first dual-drive Mach-Zehnder modulator A11 is set to the minimum bias point, and the X-polarized light signal E is output. x The other output of the first power divider A3 is sent to the transmitting antenna 4 and emitted from it.

[0021] The mixed signal (①+③) and the other local oscillator signal ④ generated by the local oscillator source 6 via the second power divider B7 are both input to the second dual-drive Mach-Zehnder modulator B12 for electro-optic modulation. Similarly, the second dual-drive Mach-Zehnder modulator B12 is set to the minimum bias point. The X-polarized light signal output from the second dual-drive Mach-Zehnder modulator B12 is rotated into a Y-polarized light signal E by the 90-degree polarization rotator 13. y ;

[0022] These two signals E with different polarization states x E y The beam is combined at polarization combiner 14 to form a polarized light signal. The envelope E1(t) of the polarized light signal is shown in Equation 1.

[0023]

[0024] in, For optical carrier, E c ω is the amplitude of the optical carrier wave. c For the input optical carrier frequency, m SI =πV SI / V π mR =πV R / V π m SOI =πV SOI / V π m LO =πV LO / V π The modulation coefficients V for the self-interference signal ①, reference signal ②, useful signal ③, and local oscillator signal ④, respectively. π J0() and J1() are the half-wave voltages of the dual-polarization dual-drive Mach-Zehnder modulator 10, respectively, and t is time.

[0025] Step 3: The combined signal is transmitted to central station B via single-mode fiber 15; due to the dispersion effect of the fiber, the transmission function of the fiber is H(ω)=exp(-αL / 2+jβ2L(ω-ω)). c ) 2 / 2), where α is the attenuation constant, β2 is the group velocity dispersion parameter, and L is the fiber length; therefore, the optical signal envelope E output after the signal passes through single-mode fiber 15 SMF (t) is shown in Formula 2:

[0026]

[0027] Among them, E SMFx E SMFy E respectively x E y Different polarization states of light signals are obtained after transmission through single-mode fiber 15;

[0028] At Central Station B:

[0029] Step 4: The optical signal output from the single-mode fiber 15 is amplified by the erbium-doped fiber amplifier 16, then split by the polarization beam splitter 17, and output to the first photodetector A18 and the second photodetector B19 respectively. After beat frequency conversion at the first photodetector A18, an optically frequency-converted intermediate frequency (IF) reference signal is obtained. After beat frequency conversion at the second photodetector B19, an optically frequency-converted IF received signal is obtained. The IF reference signal i BPF1 (t), intermediate frequency received signal i BPF2 The formula for (t) is as follows:

[0030]

[0031] Among them, G OA R1 and R2 are the amplification factor of the erbium-doped fiber amplifier, and the responsivity of photodetector A18 and photodetector B19, respectively. As the amplitude term of the useful signal, when the frequencies of the local oscillator signal and the useful signal are not much different, it can ensure that the intermediate frequency useful signal is less affected by the periodic power fading.

[0032] Step 5: Both the intermediate frequency (IF) reference signal and the IF received signal are input to the digital signal processor (DSP) 20 for digital signal processing. At this point, the IF received signal, including the self-interference signal, the useful signal, and the IF reference signal, is transmitted to the central station. In the DSP 20, the signal is sampled by a high-bit ADC and quantized to the digital domain for further processing.

[0033] In one embodiment of the present invention, during actual processing, the useful signal is first stopped according to the agreement, and only the self-interference signal ① and the reference signal ② are received at the digital end. Adaptive filtering is implemented using Fast Recursive Least Squares (FTRLS) to minimize the error between the intermediate frequency (IF) self-interference signal and the IF reference signal, thereby obtaining the filter parameter values. Subsequently, the useful signal transmitted from the remote end is received according to the agreement. The IF received signal is subtracted from the self-interference reconstructed signal obtained by filtering the IF reference signal, and finally the IF useful signal is obtained. The useful signal is then down-converted, frequency offset and phase offset are recovered, and after equalization, the useful signal is recovered.

[0034] The advantages of this invention are as follows:

[0035] 1. Self-interference signals, after passing through nonlinear devices such as amplifiers and modulators and multipath linear channels, have complex linear / nonlinear components. Using nonlinear filtering or deep learning algorithms to eliminate nonlinear components using digital baseband signals would bring great processing difficulty and computational load. However, the reference signal feedback method transmits the nonlinear components of the reference signal back to the central station at the same time, transforming the nonlinear problem into a linear problem for processing, thus reducing the processing difficulty in the digital domain.

[0036] 2. Compared to self-interference cancellation schemes that combine optical and digital domains, this method does not require parameter tuning in the optical domain. Instead, it uses a high-bit ADC, such as a 12-bit ADC, which can achieve a dynamic range of 72.25dB in quantization error, making it possible to achieve deep self-interference cancellation of large signals in the digital domain.

[0037] 3. Because adaptive filtering of self-interference signals with nonlinear components requires a relatively long filter length N to ensure the elimination depth of the self-interference signal, the traditional recursive least squares (RLS) algorithm requires O(N) time complexity. 2 The multiplication operation of FTRLS is only O(7N+14), which greatly improves the operation speed of the algorithm. Its most significant feature is that it does not require matrix multiplication. Attached Figure Description

[0038] Figure 1 This is a diagram of a digital self-interference cancellation scheme based on optical frequency conversion and simultaneous return of reference signal in an in-band full-duplex ROF system.

[0039] Figure 2(a) is a comparison diagram before and after the elimination of the 800MHz bandwidth self-interference signal, Figure 2(b) is a comparison diagram before the elimination of the self-interference signal and the recovery of the useful signal with the 400MHz bandwidth, and Figure 2(c) is a constellation diagram of the recovered useful signal. Detailed Implementation

[0040] The present invention will now be described in detail with reference to the accompanying drawings.

[0041] This invention provides a digital self-interference cancellation device for optical frequency conversion and simultaneous reference signal return in an in-band full-duplex ROF system. It includes a signal source 1, an electrical amplifier 2, a first electrical power divider A3, a transmitting antenna 4, a receiving antenna 5, a local oscillator 6, a second electrical power divider B7, a laser 8, a polarization controller 9, and a dual-polarization dual-drive Mach-Zehnder modulator 10 located at base station A; a single-mode optical fiber 15; and an erbium-doped fiber amplifier 16, a polarization beam splitter 17, a first photodetector A18, a second photodetector B19, and a digital signal processor (DSP) 20 located at central station B. The dual-polarization dual-drive Mach-Zehnder modulator 10 includes a first dual-drive Mach-Zehnder modulator A11, a second dual-drive Mach-Zehnder modulator B12, a 90-degree polarization rotator 13, and a polarization combiner 14.

[0042] At base station A:

[0043] The optical carrier generated by laser 8 is input to dual-polarization dual-drive Mach-Zehnder modulator 10 via polarization controller 9.

[0044] The local oscillator 6 generates a local oscillator signal and outputs it to the second power divider B7. The second power divider B7 performs power distribution on the local oscillator signal ④, with one output to the first dual-drive Mach-Zehnder modulator A11 and the other output to the second dual-drive Mach-Zehnder modulator B12.

[0045] The carrier modulation signal generated by signal source 1 is amplified by power amplifier 2 and output to the first power divider A3. The first power divider A3 distributes the power, and one output is used as a reference signal ②. This signal, along with the local oscillator signal ④ generated by local oscillator source 6 via the second power divider B7, is input to the first dual-drive Mach-Zehnder modulator A11 for electro-optic modulation, generating the X-polarized light signal E. x The other output goes to transmitting antenna 4 and is emitted from there.

[0046] The self-interference signal ① transmitted through the multipath self-interference channel and the useful signal ③ at the far end are received by the receiving antenna 5 together to form a mixed signal. Then, the mixed signal and another local oscillator signal ④ generated by the local oscillator source 6 through the second power divider B7 are input to the second dual-drive Mach-Zehnder modulator B12 for electro-optic modulation. The resulting X-polarized light signal is output after changing its polarization state through the 90-degree polarization rotator 13.

[0047] The optical carrier modulation signal output from the first dual-drive Mach-Zehnder modulator A11 and the optical carrier modulation signal output from the 90-degree polarization rotator 13 are combined at the polarization combiner 14 to form a polarization-combined optical signal. The polarization-combined optical signal is transmitted to the central station B via single-mode fiber 15.

[0048] At Central Station B:

[0049] After being amplified by the erbium-doped fiber amplifier 16, the beam is split by the polarization beam splitter 17 and output to the first photodetector A18 and the second photodetector B19 respectively. After being beat at the first photodetector A18, an intermediate frequency reference signal with optical frequency conversion is obtained. After being beat at the second photodetector B19, an intermediate frequency received signal with optical frequency conversion is obtained. Both the intermediate frequency reference signal and the intermediate frequency received signal are input to the digital signal processor DSP 20 for digital signal processing.

[0050] A digital self-interference cancellation method is provided for optical frequency conversion and reference signal simultaneous return in an in-band full-duplex ROF system.

[0051] In this invention, the digital domain self-interference cancellation adopts the Fast Recursive Least Squares (FTRLS) algorithm, which uses the intermediate frequency (IF) reference signal to filter out the self-interference signal in the IF received signal, and finally achieves useful signal recovery.

[0052] To facilitate the explanation of signal frequency conversion and transmission in the optical domain, nonlinearity, multipath effects, and other factors are not considered. We assume the self-interference signal ① is... Reference signal ② is Useful signal ③ is The local oscillator signal ④ is Where V SI V R V SOI v LO The voltages ω of the self-interference signal ①, the local reference signal ②, the useful signal ③, and the local oscillator signal ④ are respectively. SI ω R ω SOI ω LO These are the angular frequencies of the self-interference signal ①, the local reference signal ②, the useful signal ③, and the local oscillator signal ④, respectively. These are the phases of the self-interference signal ①, the local reference signal ②, the useful signal ③, and the local oscillator signal ④, respectively.

[0053] At base station A:

[0054] Step 1: The carrier modulation signal generated by signal source 1 is amplified by electrical amplifier 2 and output to the first power divider A3. The first power divider A3 distributes the power, with one output serving as the reference signal ②. The other output, a self-interference signal ① formed after transmission through a multipath self-interference channel, is received by the receiving antenna 5 along with the distant useful signal ③, forming a mixed signal (①+③).

[0055] Step 2: The laser 8 generates an optical carrier, which is then used by the polarization controller 9 to generate linearly polarized light. The linearly polarized light is then input to the dual-polarization dual-drive Mach-Zehnder modulator 10.

[0056] Reference signal ② and a local oscillator signal ④ generated by the second power divider B7 from local oscillator source 6 are both input to the first dual-drive Mach-Zehnder modulator A11 for electro-optic modulation. The first dual-drive Mach-Zehnder modulator A11 is set to the minimum bias point, and the X-polarized light signal E is output. x The other output of the first power divider A3 is sent to the transmitting antenna 4 and emitted from it.

[0057] The mixed signal (①+③) and the other local oscillator signal ④ generated by the local oscillator source 6 via the second power divider B7 are both input to the second dual-drive Mach-Zehnder modulator B12 for electro-optic modulation. Similarly, the second dual-drive Mach-Zehnder modulator B12 is set to the minimum bias point. The X-polarized light signal output from the second dual-drive Mach-Zehnder modulator B12 is rotated into a Y-polarized light signal E by the 90-degree polarization rotator 13. y .

[0058] These two signals E with different polarization states x E y The beam is combined at polarization combiner 14 to form a polarized light signal. The envelope E1(t) of the polarized light signal is shown in Equation 1.

[0059]

[0060] in, For optical carrier, E c ω is the amplitude of the optical carrier wave. c For the input optical carrier frequency, m SI =πV SI / V π m R =πV R / V π m SOI =πV SOI / V πm LO =πV LO / V π The modulation coefficients V for the self-interference signal ①, reference signal ②, useful signal ③, and local oscillator signal ④, respectively. π J0() and J1() are the half-wave voltages of the dual-polarization dual-drive Mach-Zehnder modulator 10, respectively, and t is time.

[0061] Step 3: The combined signal is transmitted to central station B via single-mode fiber 15. Due to the dispersion effect of the optical fiber, the transmission function of the optical fiber is H(ω)=exp(-αL / 2+jβ2L(ω-ω)). c ) 2 / 2), where α is the attenuation constant, β2 is the group velocity dispersion parameter, and L is the fiber length. Therefore, the optical signal envelope E output after the signal passes through single-mode fiber 15 SMF (t) is shown in Formula 2:

[0062]

[0063] Among them, E SMFx E SMFy E respectively x E y Different polarization states of light signals are obtained after transmission through single-mode fiber 15.

[0064] At Central Station B:

[0065] Step 4: The optical signal output from the single-mode fiber 15 is amplified by the erbium-doped fiber amplifier 16, then split by the polarization beam splitter 17, and output to the first photodetector A18 and the second photodetector B19 respectively. After beat frequency conversion at the first photodetector A18, an optically frequency-converted intermediate frequency (IF) reference signal is obtained. After beat frequency conversion at the second photodetector B19, an optically frequency-converted IF received signal is obtained. IF reference signal i BPF1 (t), intermediate frequency received signal i BPF2 The formula for (t) is as follows:

[0066]

[0067] Among them, G OA R1 is the amplification factor of the erbium-doped fiber amplifier, and R2 is the responsivity of the first photodetector A18 and the second photodetector B19, respectively. As the amplitude term of the useful signal, when the frequencies of the local oscillator signal and the useful signal are not significantly different, it can be ensured that the intermediate frequency useful signal is less affected by the periodic power fading.

[0068] Step 5: Both the intermediate frequency (IF) reference signal and the IF received signal are input to the digital signal processor (DSP) 20 for digital signal processing. At this point, the IF received signal, including self-interference signals, useful signals, and the IF reference signal, is transmitted to the central station B, where it is sampled and quantized in the digital domain by a high-bit ADC in the DSP 20 for further processing.

[0069] In the actual processing, the useful signal is first stopped as agreed, and only the self-interference signal ① and the reference signal ② are received at the digital end. Adaptive filtering is implemented using Fast Recursive Least Squares (FTRLS) (Dechene D J. "Fast transversal recursive least-squares (FT-RLS) algorithm [C]" IEEE Trans Signal Proc. Citeseer, 2007.) to minimize the error between the intermediate frequency (IF) self-interference signal and the IF reference signal, thus obtaining the filter parameter values. Subsequently, the useful signal transmitted from the remote end is received as agreed. The IF received signal is subtracted from the self-interference reconstructed signal obtained by filtering the IF reference signal, and finally the IF useful signal is obtained. The useful signal is then down-converted, frequency offset and phase offset recovered, and equalized to recover the useful signal. The down-conversion, frequency offset, phase offset recovery, and equalization techniques are well known to those skilled in the art and will not be described in detail here.

[0070] The advantages of this invention are as follows:

[0071] 1. Self-interference signals, after passing through nonlinear devices such as amplifiers and modulators and multipath linear channels, have complex linear / nonlinear components. Using nonlinear filtering or deep learning algorithms to eliminate nonlinear components using digital baseband signals would bring great processing difficulty and computational load. However, the reference signal feedback method transmits the nonlinear components of the reference signal back to the central station at the same time, transforming the nonlinear problem into a linear problem for processing, thus reducing the processing difficulty in the digital domain.

[0072] 2. Compared to self-interference cancellation schemes that combine optical and digital domains, this method does not require parameter tuning in the optical domain. Instead, it uses a high-bit ADC, such as a 12-bit ADC, which can achieve a dynamic range of 72.25dB in quantization error, making it possible to achieve deep self-interference cancellation of large signals in the digital domain.

[0073] 3. Because adaptive filtering of self-interference signals with nonlinear components requires a relatively long filter length N to ensure the elimination depth of the self-interference signal, the traditional recursive least squares (RLS) algorithm requires O(N) time complexity. 2The multiplication operation of FTRLS is only O(7N+14), which greatly improves the operation speed of the algorithm. Its most significant feature is that it does not require matrix multiplication.

[0074] To verify the multipath and nonlinear self-interference signal cancellation capability and useful signal recovery capability of the present invention, simulations were performed using OptiSystem 14.0 and MATLAB 2021b.

[0075] The self-interference signal is set as a QAM modulated signal with a frequency of 12.5 GHz, a bandwidth of 800 MHz, and a power of 30 dBm. The local oscillator signal has a frequency of 10 GHz and a power of 27 dBm. The useful signal is a 16QAM signal with a frequency of 12.5 GHz, a bandwidth of 400 MHz, and a power of -3 dBm. The self-interference channel consists of 5 paths with different delays and attenuations. The delays are [3, 30, 70, 80, 103] ps, and the attenuations are [8, 10, ..., ...]. [12,13,16]dB, light source wavelength is 1550nm, power is 16dBm, linewidth is 0.1MHz, Mach-Zehnder modulator extinction ratio is 30dB, single-mode fiber length is 10km, reference wavelength is 1552nm, erbium-doped fiber amplifier gain is 10dB, noise figure is 4.5dB, polarization beam splitter offset is 2°, photodetector A responsivity is 0.9A / W, photodetector B responsivity is 0.85A / W. In the FTRLS algorithm, the filter order N is 800, the forgetting factor λ = 1, and the forward / backward prediction minimum weighted square error is... The initial values ​​are all 0.00001. After the above process, the elimination performance of the schemes is as follows:

[0076] Figure 2(a) shows the spectrum before and after self-interference cancellation. When no useful signal is transmitted, the self-interference signal can achieve a cancellation depth of 48dB, and the nonlinear components in the self-interference signal are also eliminated. Figure 2(b) shows the spectrum of the self-interference signal and the recovered useful signal. When the useful signal is transmitted, the useful signal can be recovered from the received signal. It can be seen from the figure that the cancellation performance is slightly reduced. This is mainly because the amplitude of the self-interference signal obtained after the self-interference signal and the useful signal beat through the optical link is slightly smaller than the amplitude obtained by the self-interference signal beating through the optical link alone. This causes an error when recovering the useful signal from the received signal in the digital domain. Secondly, as shown in Figures 2(a) and 2(b), the self-interference signal, the useful signal, and the reference signal can be optically converted to an intermediate frequency with a center frequency of 2.5GHz. Figure (c) shows the constellation diagram of the recovered 16QAM useful signal, and the EVM reaches 7.18%.

Claims

1. A digital self-interference cancellation device for optical frequency conversion, reference signal simultaneous backhaul in an in-band full-duplex ROF system, characterized in that, The system comprises a signal source (1), an electrical amplifier (2), a first electrical power divider A (3), a transmitting antenna (4), a receiving antenna (5), a local oscillator (6), a second electrical power divider B (7), a laser (8), a polarization controller (9), a dual-polarization dual-drive Mach-Zehnder modulator (10), a single-mode optical fiber (15), a doped fiber amplifier (16), a polarization beam splitter (17), a first photoelectric detector A (18), a second photoelectric detector B (19), a digital signal processor DSP (20) at a central station B; wherein the dual-polarization dual-drive Mach-Zehnder modulator (10) comprises a first dual-drive Mach-Zehnder modulator A (11), a second dual-drive Mach-Zehnder modulator B (12), a 90-degree polarization rotator (13), and a polarization beam combiner (14). At the base station A: The optical carrier generated by the laser (8) is input to the dual-polarization dual-drive Mach-Zehnder modulator (10) through the polarization controller (9); The local oscillator signal generated by the local oscillator (6) is output to the second electrical power divider B (7), which divides the power of the local oscillator signal (4) and outputs one to the first dual-drive Mach-Zehnder modulator A (11) and the other to the second dual-drive Mach-Zehnder modulator B (12); The carrier modulation signal generated by the signal source (1) is amplified by the electric amplifier (2) and then output to the first electric power divider A (3), which divides the power of the signal. One of the output signals is used as the reference signal ② and input to the first double-drive Mach-Zehnder modulator A (11) together with the local oscillator signal ④ generated by the local oscillator source (6) through the second electric power divider B (7) to generate the X polarization state optical signal E x ; the other output signal is output to the transmitting antenna (4) and emitted by the antenna. The self-interference signal (1) transmitted through the multipath self-interference channel and the remote useful signal (3) are received by the receiving antenna (5) to form a mixed signal, and then the mixed signal and the other local oscillator signal (4) generated by the local oscillator (6) through the second electrical power divider B (7) are input to the second dual-drive Mach-Zehnder modulator B (12) for electro-optical modulation, and the generated X-polarization state optical signal is output after changing the polarization state through the 90-degree polarization rotator (13); The optical carrier modulation signal output by the first dual-drive Mach-Zehnder modulator A (11) and the optical carrier modulation signal output by the 90-degree polarization rotator (13) are combined at the polarization beam combiner (14) to form a polarization combined optical signal; the polarization combined optical signal is transmitted to the central station B through the single-mode optical fiber (15); At the central station B: After being amplified by the doped fiber amplifier (16), the polarization beam splitter (17) splits the beam, and outputs it to the first photoelectric detector A (18) and the second photoelectric detector B (19), respectively. The first photoelectric detector A (18) obtains an intermediate frequency reference signal after frequency mixing, and the second photoelectric detector B (19) obtains an intermediate frequency received signal after frequency mixing. The intermediate frequency reference signal and the intermediate frequency received signal are input into the digital signal processor DSP (20) for digital signal processing.

2. A digital self-interference cancellation method for optical frequency conversion and reference signal simultaneous backhaul under in-band full-duplex ROF system based on the digital self-interference cancellation device for optical frequency conversion and reference signal simultaneous backhaul under in-band full-duplex ROF system as claimed in claim 1, assuming that the self-interference signal ① is the reference signal ② is the useful signal ③ is the local oscillator signal ④ is wherein V SI , V R , V SOI , V LO are the voltages of the self-interference signal ①, the local reference signal ②, the useful signal ③, the local oscillator signal ④, respectively, ω SI , ω R , ω SOI , ω LO are the angular frequencies of the self-interference signal ①, the local reference signal ②, the useful signal ③, the local oscillator signal ④, respectively, are the phases of the self-interference signal ①, the local reference signal ②, the useful signal ③, the local oscillator signal ④, respectively. characterized in that The method specifically comprises the following steps: At the base station A: Step 1: The carrier modulation signal generated by the signal source (1) is amplified by the electrical amplifier (2) and then output to the first electrical power divider A (3), which divides the power of the carrier modulation signal. One of the divided signals is the reference signal (2); the other signal is the self-interference signal (1) formed after being transmitted through the multipath self-interference channel, and the remote useful signal (3) is received by the receiving antenna (5) to form a mixed signal (1+3); Step 2: the laser (8) generates an optical carrier, which is input into a polarization controller (9) to generate linearly polarized light, and the linearly polarized light is input into a double-polarization double-drive Mach-Zehnder modulator (10); The reference signal ② and a local oscillator signal ④ generated by the second electric power divider B (7) from the local oscillator source (6) are input to the first double-drive Mach-Zehnder modulator A (11) for electro-optical modulation. The first double-drive Mach-Zehnder modulator A (11) is set to the minimum bias point, and an X polarization state optical signal E is output x ; the other output of the first electric power divider A (3) is output to the transmitting antenna (4) and emitted by the transmitting antenna (4); The mixed signal (①+③) and another local oscillator signal ④ generated by the second electric power divider B (7) from the local oscillator source (6) are input to the second double-drive Mach-Zehnder modulator B (12) for electro-optical modulation. The second double-drive Mach-Zehnder modulator B (12) is also set to the minimum bias point. The X polarization state optical signal output by the second double-drive Mach-Zehnder modulator B (12) is rotated to a Y polarization state optical signal E by the 90-degree polarization rotator (13) y ; The two signals E x , E y are combined at the polarization beam combiner (14) to form a polarized light signal with envelope E1(t) given by equation 1: wherein, is the optical carrier, E c is the amplitude of the optical carrier, ω c is the input optical carrier frequency, m SI = πV SI / V π , m R = πV R / V π , m SOI = πV SOI / V π , m LO = πV LO / V π are the modulation coefficients of the self-interference signal ①, the reference signal ②, the useful signal ③, the local oscillator signal ④, respectively, V π is the half-wave voltage of the dual-polarization dual-drive Mach-Zehnder modulator (10), J0(), J1() are the 0th-order and 1st-order Bessel functions of the first kind, respectively, and t is time. Step 3: The combined signal is transmitted to the central station B through a single-mode fiber (15). Due to the dispersion effect of the fiber, the transmission function of the fiber is H(ω) = exp(-αL / 2 + jβ2L(ω-ω0 c ) 2 / 2), where α is the attenuation constant, β2 is the group velocity dispersion parameter, and L is the fiber length. Therefore, the optical signal envelope E SMF (t) output after the signal passes through the single-mode fiber (15) is as shown in Equation 2: wherein E SMFx , E SMFy , E x , E y are different polarization state optical signals after transmission through a single mode optical fiber (15); At the central station B: Step 4: The optical signal output by the single-mode optical fiber (15) is amplified by the doped fiber amplifier (16), split by the polarization beam splitter (17), and output to the first photodetector A (18) and the second photodetector B (19) respectively. After beating at the first photodetector A (18), the optical frequency conversion intermediate frequency reference signal is obtained, and after beating at the second photodetector B (19), the optical frequency conversion intermediate frequency receiving signal is obtained. BPF1 (t), the intermediate frequency receiving signal i BPF2 (t) is as follows: wherein G OA R1, R2 are the responsivity of the first photodetector A (18), second photodetector B (19) respectively; As the amplitude term of the useful signal, when the frequency difference between the local signal and the useful signal is small, the intermediate frequency useful signal can be ensured to be less affected by the power periodic fading. Step 5: the intermediate frequency reference signal and the intermediate frequency received signal are input into a digital signal processor (20) for digital signal processing; at this point, the intermediate frequency received signal and the intermediate frequency reference signal are jointly transmitted to the central station B, and are sampled and quantized to a digital domain by a high-bit ADC in the digital signal processor (20) for further processing.

3. The method of claim 2, wherein the digital self-interference cancellation of the in-band full-duplex ROF system down optical frequency conversion, reference signal simultaneous backhaul is characterized by, In the actual processing process, first, according to the agreement, the useful signal is stopped from being sent, and only the self-interference signal ① and the reference signal ② are received at the digital end, and a fast recursive least square (FTRLS) is used to realize adaptive filtering, so that the error of the self-interference signal and the intermediate frequency reference signal is minimized, and the filter parameter value is obtained; Subsequently, according to the agreement, the useful signal transmitted by the far end is received, the intermediate frequency received signal is subtracted from the self-interference reconstructed signal obtained by filtering the intermediate frequency reference signal, and finally the intermediate frequency useful signal is obtained, the useful signal is frequency offset, phase offset recovered, and equalized to recover the useful signal.

Images