A simple self-interference cancellation bidirectional in-band full-duplex RoF system and method

Abstract

This invention provides a simple self-interference cancellation bidirectional in-band full-duplex RoF system and a self-interference cancellation bidirectional in-band full-duplex RoF method: The transmitted signal is electro-optically modulated and transmitted to a central station; the signal is transmitted via a transmitting antenna and received along with a useful signal; the received signal is modulated onto light and transmitted back to the central station; the reference optical signal and the received optical signal are combined and beat; the wavelength of a tunable laser is tuned to achieve deep self-interference cancellation. This invention utilizes the dispersion effect of optical fiber to achieve precise delay tuning by changing the wavelength difference between the two lasers; placing the tunable laser at the central station allows for tuning of self-interference delay and amplitude parameters, simplifying the base station structure; the transmit branch and reference branch are shared, avoiding the return of the reference signal; furthermore, the optical fiber of the reference branch can be replaced by a dispersion module, improving equipment integration and reducing equipment size.

Description

Technical Field

[0001] This invention belongs to the field of microwave photonic signal processing technology, specifically relating to a simple self-interference cancellation bidirectional in-band full-duplex RoF system and method. Background Technology

[0002] Radio-over-fiber (ROF) systems are an emerging wireless access technology integrating wireless and optical systems, offering advantages such as long transmission distance, large information capacity, light weight, and resistance to electromagnetic interference. With the increasing congestion of wireless channel frequency bands and the diversification of application scenarios, in-band full-duplex (IBFD) technology combined with ROF links can double spectral efficiency and has received widespread attention in recent years. However, at the base station, the co-location of transmit and receive antennas generates strong self-interference, affecting the reception of useful signals.

[0003] Compared with traditional RF electrical self-interference cancellation schemes, optically assisted self-interference cancellation technology can achieve greater bandwidth and higher tuning accuracy, and when combined with ROF systems, it can simultaneously achieve signal transmission over longer distances. In recent years, scholars at home and abroad have proposed a series of optically assisted self-interference cancellation schemes for ROF systems. ([1]Chen YA photonic-based wideband RF self-interference cancellation approach with fiberdispersion immunity[J]. Journal of Lightwave Technology, 2020, 38(17): 4618-4624. [2] Chen Y, Yao J. Photonic-assisted RF self-interference cancellation with improved spectrum efficiency and fiber transmission capability[J]. Journal of Lightwave Technology, 2019, 38(4): 761-768. [3] Zhu Z, Gao C, Zhao S, et al. Photonics-Assisted Ultrawideband RF Self-Interference Cancellation WithSignal of Interest Recovery and Fiber Transmission[J]. Journal of Lightwave Technology, 2022, 40(3): 655-663.)

[0004] However, the above solutions have certain limitations. In References 1 and 2, the delay is adjusted in the electrical domain, which affects the depth of self-interference elimination. Reference 3 utilizes the fiber dispersion effect to achieve high-precision delay tuning, but both lasers must be placed in the base station, increasing the complexity of the base station structure. Furthermore, none of the above solutions consider the downlink portion of the signal. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a simple self-interference cancellation bidirectional in-band full-duplex RoF system, comprising a wavelength-tunable laser, a first Mach-Zehnder modulator, an optical splitter, a first circulator, a first optical fiber, a second circulator, a first optical amplifier, a first photodetector, an electrical amplifier, a transmitting antenna, a receiving antenna, a fixed-wavelength laser, a second Mach-Zehnder modulator, a second optical fiber, an optical combiner, a second optical amplifier, and a second photodetector;

[0006] The system is divided into a signal transmission section and a self-interference cancellation section, which are connected by a first optical fiber.

[0007] (1) Signal transmission section:

[0008] At the central station, a wavelength-tunable laser generates an optical signal that is injected into the first Mach-Zehnder modulator;

[0009] The first Mach-Zehnder modulator modulates the transmitted signal onto the light to form the transmitted light signal, and adjusts the first Mach-Zehnder modulator to a positive quadrature bias point;

[0010] The transmitted optical signal is split into two paths by an optical splitter; one path enters port 1 of the first circulator and is output from port 2 of the first circulator; the other path is output after passing through the second optical fiber.

[0011] The optical signal output from port 2 of the first circulator enters port 1 of the base station through the first optical fiber and is output from port 2 of the second circulator.

[0012] The optical signal output from port 2 of the second circulator is amplified by the first optical amplifier;

[0013] The amplified optical signal is beat at the first photodetector to form an electrical signal to be transmitted.

[0014] The electrical signal to be transmitted is amplified by an amplifier and finally emitted by the transmitting antenna.

[0015] (2) Self-interference cancellation part:

[0016] The electrical signal emitted by the transmitting antenna forms a self-interference signal, which, together with the useful signal from the far end, forms a mixed signal, which is then received by the local receiving antenna.

[0017] Fixed-wavelength laser outputs optical carrier wave;

[0018] The second Mach-Zehnder modulator modulates the mixed signal onto the optical carrier to form the received optical signal. The second Mach-Zehnder modulator is adjusted to a negative quadrature bias point.

[0019] The received optical signal enters port 3 of the second circulator and is output from port 1 of the second circulator;

[0020] The optical signal output from port 1 of the second circulator is transmitted back to the first circulator of the central station through the first optical fiber, enters from port 2 of the first circulator, and is output from port 3 of the first circulator.

[0021] The optical signal output from port 3 of the first circulator is combined with the optical signal output from the second optical fiber in the "signal transmission section" at the optical combiner.

[0022] After the combined optical signal is amplified by the second optical amplifier, it beats at the second photodetector and outputs a useful electrical signal.

[0023] A self-interference cancellation bidirectional in-band full-duplex RoF method is also provided, which is based on the aforementioned self-interference cancellation bidirectional in-band full-duplex RoF system with a simple structure. The specific process of this method is as follows:

[0024] Step 1: Electro-optically modulate the transmitted signal and transmit it to the central station.

[0025] First, at the central station, the wavelength-tunable laser outputs an optical carrier wave, which is injected into the first Mach-Zehnder modulator. The first Mach-Zehnder modulator modulates the transmitted signal onto the light, forming the transmitted optical signal. The Mach-Zehnder modulator is then adjusted to a positive quadrature bias point. Assuming the transmitted signal is V1sinω... s t, where V1 is the amplitude of the transmitted signal, ω s Let ω be the angular frequency of the transmitted signal and t be time. Under small-signal modulation, the envelope E1(t) of the output optical signal modulated by the first Mach-Zehnder modulator is shown in Equation 1:

[0026] E1(t)=E c1 exp(j(ω c +ω τ )t)((1+j)J0(m1)+(1-j)J1(m1)exp(jω s t)-(1-j)J1(m1)exp(-jω s t)) (1)

[0027] Among them, E c1 To provide the amplitude of the optical carrier for a tunable laser, ω c The input optical carrier frequency at the reference frequency, ω τ For a tunable laser at the reference frequency ω c The offset frequency is as follows: J0() is a 0th-order Bessel function of the kind, J1() is a 1st-order Bessel function of the kind, and m1=πV1 / V π V is the modulation index of the first Mach-Zehnder modulator; π It is the half-wave voltage of the modulator;

[0028] The optical signal E1(t) output from the first Mach-Zehnder modulator is split by an optical splitter. One path serves as a reference signal for subsequent self-interference cancellation, while the other path is input from port 1 of the first circulator, output from port 2 of the first circulator, and transmitted to the base station via the first optical fiber. Inside the base station, the split optical signal E1(t) is input from port 1 of the second circulator, output from port 2 of the second circulator, and output from port 12 of the second circulator.

[0029]

[0030] Due to the dispersion effect of optical fibers, the transfer function of optical fibers is H(ω)=exp(-αL / 2+jβ2L(ω-ω)). c ) 2 / 2), where ω is the angular frequency, α is the attenuation constant, L is the fiber length, β2 is the group velocity dispersion parameter; τ1 is the delay caused by the optical signal propagating in the fiber, θ(ω)=β2L(ω-ω c ) 2 / 2 represents the phase shift introduced by the radio frequency signal in the fiber dispersion effect;

[0031] The optical signal output from port 12 of the second circulator is amplified by the first optical amplifier and then beats at the first photodetector to form the electrical signal to be transmitted. This electrical signal is then amplified by another amplifier, and the resulting amplified electrical signal I(t) is denoted as...

[0032]

[0033] Where R1 is the responsivity of the first photodetector, A is the amplification factor of the electrical amplifier, and τ2=β2Lω r The delay caused by fiber dispersion;

[0034] Step 2: The signal is transmitted via the transmitting antenna and received along with the useful signal.

[0035] After the amplified electrical signal I(t) is transmitted through the transmitting antenna, a self-interference signal is generated. This self-interference signal, together with the useful signal from the far end, forms a mixed signal, which is received by the local receiving antenna. Assuming that the attenuation of the self-interference signal in the self-interference channel is α1 and the delay is τ2, then the received self-interference signal can be simplified as I2(t) = V2sin(ωt). s (t+τ1+τ2)); where The received useful signal can be expressed as I3(t) = V3sin(ω) s (t+τ3)), where V3 and τ3 are the amplitude and delay of the useful signal, respectively;

[0036] Step 3: Modulate the received signal onto optical fiber and transmit it back to the central station.

[0037] A fixed-wavelength laser outputs an optical carrier wave, which is injected into a second Mach-Zehnder modulator. The second Mach-Zehnder modulator is adjusted to a negative quadrature bias point and modulates the mixed signal received by the receiving antenna onto the optical carrier wave. The optical signal output from the second Mach-Zehnder modulator enters through port 3 of the second circulator, exits through port 1 of the second circulator, is transmitted through the first optical fiber to port 2 of the first circulator, and is then output from port 3 of the first circulator. The envelope E of the optical signal output from port 3 of the first circulator... SMF2 (t) is shown in Equation 4:

[0038]

[0039] In the formula, G OA E represents the amplification gain of the optical amplifier. c2 For the amplitude of a fixed-wavelength laser, m2 = πV2 / V π m3=πV3 / V π These are the modulation indices of the second Mach-Zehnder modulator relative to the self-interference signal and the useful signal, respectively.

[0040] Step 4: Combining the reference optical signal and the received optical signal to capture the frequency.

[0041] The reference signal passes through the second optical fiber. To match the delay introduced by the signal transmission in the second optical fiber, it is assumed that the length of the second optical fiber is 2L, and the group velocity dispersion parameter of the second optical fiber is consistent with that of the first optical fiber. After passing through the second optical fiber, the reference signal is combined with the optical signal output from port 3 of the first circulator in the optical combiner. The combined optical signal is amplified by the optical amplifier and then beats at the second photodetector. The output beat frequency electrical signal is:

[0042]

[0043] In the formula, R2 is the responsivity of the second photodetector;

[0044] Step 5: Tuning the wavelength of the tunable laser to achieve deep elimination of self-interference.

[0045] According to equation (5), by tuning the power and wavelength of the optical attenuator, the following relationship can be satisfied to achieve self-interference signal cancellation:

[0046]

[0047] The advantages of this invention are as follows:

[0048] 1. By utilizing the dispersion effect of optical fiber, precise tuning of the delay was achieved by changing the wavelength difference between the two lasers.

[0049] 2. By placing the tunable laser at the central station, the self-interference delay and amplitude parameters can be tuned at the central station, while simplifying the structure of the base station;

[0050] 3. The transmit branch and the reference branch are shared, which avoids the backhaul of the reference signal. In addition, the optical fiber of the reference branch can be replaced by a dispersion module, which can improve the integration of the equipment and reduce the size of the equipment. Attached Figure Description

[0051] Figure 1 A simplified schematic diagram of an optical dispersion delay self-interference cancellation device and method for base stations;

[0052] Figure 2 This is a simulation diagram of the single-frequency signal cancellation performance of the present invention;

[0053] Figure 3 This is a simulation diagram of the broadband signal cancellation performance of the present invention. Detailed Implementation

[0054] This invention provides a simple self-interference cancellation bidirectional in-band full-duplex RoF system, such as... Figure 1 As shown, it includes a wavelength-tunable laser, a first Mach-Zehnder modulator, an optical splitter, a first circulator, a first optical fiber, a second circulator, a first optical amplifier, a first photodetector, an electrical amplifier, a transmitting antenna, a receiving antenna, a fixed-wavelength laser, a second Mach-Zehnder modulator, a second optical fiber, an optical combiner, a second optical amplifier, and a second photodetector.

[0055] The system can be divided into a signal transmission section and a self-interference cancellation section, which are connected by a first optical fiber.

[0056] 1. Signal transmission section:

[0057] At the central station, a wavelength-tunable laser generates an optical signal that is injected into a first Mach-Zehnder modulator. The first Mach-Zehnder modulator modulates the transmitted signal onto the light to form a transmitted optical signal, adjusting the first Mach-Zehnder modulator to a positive quadrature bias point. The transmitted optical signal is split into two paths by an optical splitter. One path enters port 1 of the first circulator and is output from port 2 of the first circulator; the other path is output after passing through a second optical fiber. The optical signal output from port 2 of the first circulator passes through the first optical fiber into port 1 of the second circulator at the base station and is output from port 2 of the second circulator. Then, the optical signal output from port 2 of the second circulator is amplified by a first optical amplifier. Subsequently, the amplified optical signal is beat-frequencyd at a first photodetector to form an electrical signal to be transmitted. Subsequently, the electrical signal to be transmitted is amplified by an electrical amplifier and finally emitted by the transmitting antenna.

[0058] 2. Self-interference cancellation section:

[0059] The electrical signal emitted by the transmitting antenna forms a self-interference signal, which, together with the useful signal from the far end, forms a mixed signal. This mixed signal is received by the local receiving antenna. A fixed-wavelength laser outputs an optical carrier. A second Mach-Zehnder modulator modulates the mixed signal onto the optical carrier to form a received optical signal. The second Mach-Zehnder modulator is adjusted to a negative quadrature bias point. Subsequently, the received optical signal enters port 3 of the second circulator and is output from port 1 of the second circulator. The optical signal output from port 1 of the second circulator is transmitted back to the first circulator at the central station via the first optical fiber, enters from port 2 of the first circulator, and is output from port 3 of the first circulator. The optical signal output from port 3 of the first circulator is combined with the optical signal output from the second optical fiber in the "signal transmission section" at the optical combiner. After being amplified by the second optical amplifier, the combined optical signal beats at the second photodetector. By tuning the wavelength and power of the tunable laser, the delay and amplitude alignment of the self-interference signal can be achieved, the self-interference signal is eliminated, and the useful electrical signal is output.

[0060] use Figure 1 The structure shown implements a self-interference cancellation bidirectional in-band full-duplex RoF method, the specific process of which is as follows:

[0061] Step 1: Electro-optic modulation of the transmitted signal and transmission to the central station.

[0062] First, at the central station, the wavelength-tunable laser outputs an optical carrier wave, which is injected into the first Mach-Zehnder modulator. The first Mach-Zehnder modulator modulates the transmitted signal onto the light, forming the transmitted optical signal. The Mach-Zehnder modulator is then adjusted to a positive quadrature bias point. Assume the transmitted signal is V1sinω. s t, where V1 is the amplitude of the transmitted signal, ω s Let ω be the angular frequency of the transmitted signal, and t be time. Under small-signal modulation, the envelope E1(t) of the output optical signal modulated by the first Mach-Zehnder modulator is shown in Equation 1:

[0063] E1(t)=E c1 exp(j(ω c +ω τ )t)((1+j)J0(m1)+(1-j)J1(m1)exp(jω s t)-(1-j)J1(m1)exp(-jω s t)) (1)

[0064] Among them, E c1 To provide the amplitude of the optical carrier for a tunable laser, ω c The input optical carrier frequency at the reference frequency, ω τ For a tunable laser at the reference frequency ω cThe offset frequency is given by J0(), where J0() is a 0th-order Bessel function of the kind and J1() is a 1st-order Bessel function of the kind, and m1 = πV1 / V. π V is the modulation index of the first Mach-Zehnder modulator; π It is the half-wave voltage of the modulator.

[0065] The optical signal E1(t) output from the first Mach-Zehnder modulator is split by an optical splitter. One path serves as a reference signal for subsequent self-interference cancellation, while the other path is input from port 1 of the first circulator, output from port 2 of the first circulator, and transmitted to the base station via the first optical fiber. Inside the base station, the split optical signal E1(t) is input from port 1 of the second circulator and output from port 2 of the second circulator. The output from port 12 of the second circulator is...

[0066]

[0067] Due to the dispersion effect of optical fibers, the transfer function of optical fibers is H(ω)=exp(-αL / 2+jβ2L(ω-ω)). c ) 2 / 2)(The transmission functions expressed by the two optical fibers are the same), where ω is the angular frequency, α is the attenuation constant, L is the fiber length, and β2 is the group velocity dispersion parameter. τ1 is the delay caused by the optical signal propagating in the optical fiber, θ(ω)=β2L(ω-ω c ) 2 / 2 represents the phase shift introduced by the radio frequency signal in the fiber dispersion effect.

[0068] The optical signal output from port 12 of the second circulator is amplified by the first optical amplifier and then beats at the first photodetector to form the electrical signal to be transmitted. This electrical signal is then amplified by an electrical amplifier, and the resulting amplified electrical signal I(t) is denoted as...

[0069]

[0070] Where R1 is the responsivity of the first photodetector, A is the amplification factor of the electrical amplifier, and τ2=β2Lω τ The delay caused by fiber dispersion.

[0071] Step 2: The signal is transmitted via the transmitting antenna and received along with the useful signal.

[0072] Amplified electrical signal I(t) is transmitted via a transmitting antenna, generating a self-interference signal. This self-interference signal, along with the useful signal from a distant location, forms a mixed signal, which is then received by the local receiving antenna. Assume the attenuation of the self-interference signal in the self-interference channel is α1, and the delay is τ2. Then, the received self-interference signal can be simplified to I2(t) = V2sin(ωt). s (t+τ1+τ2)). Wherein The received useful signal can be expressed as I3(t) = V3sin(ω) s (t+τ3)), where V3 and τ3 are the amplitude and delay of the useful signal, respectively.

[0073] Step 3: Modulate the received signal onto the optical fiber and transmit it back to the central station.

[0074] A fixed-wavelength laser outputs an optical carrier wave, which is injected into a second Mach-Zehnder modulator. The second Mach-Zehnder modulator is adjusted to a negative quadrature bias point and modulates the mixed signal received by the receiving antenna onto the optical carrier wave. The optical signal output from the second Mach-Zehnder modulator enters through port 3 of the second circulator, exits through port 1 of the second circulator, is transmitted through the first optical fiber to port 2 of the first circulator, and is then output from port 3 of the first circulator. The envelope E of the optical signal output from port 3 of the first circulator is... SMF2 (t) is shown in Equation 4:

[0075]

[0076] In the formula, G OA E represents the amplification gain of the optical amplifier. c2 For the amplitude of a fixed-wavelength laser, m2 = πV2 / V π m3=πV3 / V π These are the modulation indices of the second Mach-Zehnder modulator relative to the self-interference signal and the useful signal, respectively.

[0077] Step 4: Combine the reference optical signal and the received optical signal and then beat the frequency.

[0078] The reference signal passes through the second optical fiber. To compensate for the delay introduced by the signal propagation in the second fiber, it is assumed that the length of the second fiber is 2L, and the group velocity dispersion parameter of the second fiber is consistent with that of the first fiber. After passing through the second fiber, the reference signal is combined with the optical signal output from port 3 of the first circulator in an optical combiner. The combined optical signal is amplified by an optical amplifier and then beats at the second photodetector. The output beat frequency electrical signal is:

[0079]

[0080] In the formula, R2 is the responsivity of the second photodetector.

[0081] Step 5: Tune the wavelength of the tunable laser to achieve deep elimination of self-interference.

[0082] As can be seen from equation (5), the following relationship can be satisfied by tuning the power and wavelength of the optical attenuator to achieve self-interference signal cancellation:

[0083]

[0084] Thus, by tuning the wavelength and power of the tunable laser and adjusting the bias of the Mach-Zehnder modulator, the three key factors for self-interference cancellation have been achieved: delay, amplitude, and inversion.

[0085] The features of this invention are:

[0086] 1. Due to the dispersion effect of optical fiber, the reference signal introduces β2Lω. τ The delay can be precisely matched with the delay and amplitude of the self-interference signal by tuning the wavelength and power of the tunable laser.

[0087] 2. An integrated transceiver structure is designed, placing the tunable laser at the central station and extracting the reference optical signal at the transmitting branch. This avoids the backhaul of the reference signal at the base station and simplifies the base station structure to some extent. Furthermore, the optical fiber in the reference branch can be replaced with a dispersion module, improving equipment integration and reducing equipment size.

[0088] 3. By placing the tunable laser at the central station, the self-interference delay and amplitude parameters can be tuned only at the central station, making the scheme more operable.

[0089] To verify the elimination performance of the present invention, simulation was performed using OptiSystem 14.0.

[0090] Transmitter section: A single-frequency signal with a center frequency of 3GHz and a power of 16.5dBm is used. The signal is modulated by a first Mach-Zehnder modulator with a half-wave voltage of 4V, then transmitted to the base station via a first circulator, a first optical fiber with a length of 10km, and a second circulator. After amplification by an erbium-doped fiber amplifier, it is beat-frequencyd by a first photodetector with a responsivity of 0.9A / W, and then amplified by an electrical amplifier. The erbium-doped fiber amplifier is set to power control mode, with a maximum output optical power of 10dBm, a noise figure of 4.5dB, and an electrical amplifier gain of 46dB.

[0091] The transmitted signal experiences a delay of 0.4 ns during transmission in the channel.

[0092] Self-interference cancellation section: The laser frequency is fixed at 193.7186 THz and the power is 13.23 dBm. The self-interference signal is modulated onto the light by a second Mach-Zehnder modulator, and then transmitted back to the central station via a second circulator, a 10 km long first optical fiber, and a first circulator. After being split by an optical splitter, it is rejoined via a 20 km long second optical fiber, and then output via a beat frequency output from an erbium-doped fiber amplifier and a second photodetector with a responsivity of 0.9 A / W to a spectrum analyzer for observation. The wavelength-tunable laser frequency is adjusted to 193.4 THz and the power is 16 dBm. After the above process, the single-frequency self-interference signal cancellation performance is 59.8 dB. Figure 2As shown. The self-interference signal is changed to a broadband signal with a bandwidth of 400MHz. Simultaneously, a single-frequency useful signal of 3GHz with a power of -3.5dBm is introduced. At this point, the self-interference cancellation depth is 43.2dB. Figure 3 As shown.

Claims

1. A self-interference cancellation bidirectional in-band full-duplex RoF system with simple architecture, characterized in that, The system comprises a wavelength tunable laser, a first Mach-Zehnder modulator, an optical splitter, a first optical circulator, a first optical fiber, a second optical circulator, a first optical amplifier, a first photodetector, an electrical amplifier, a transmitting antenna, a receiving antenna, a fixed wavelength laser, a second Mach-Zehnder modulator, a second optical fiber, an optical combiner, a second optical amplifier, and a second photodetector. The system is divided into a signal transmitting part and a self-interference cancellation part, and the two parts are connected through a first optical fiber. (1) The signal transmitting part: At the central station, a wavelength tunable laser generates an optical signal which is injected into a first Mach-Zehnder modulator. The first Mach-Zehnder modulator modulates the transmitting signal on the light to form a transmitting optical signal, and the first Mach-Zehnder modulator is adjusted to a positive quadrature bias point. The transmitting optical signal is split into two paths by an optical splitter; one path enters a first optical circulator 1 port and is output from a first optical circulator 2 port; the other path is output after passing through a second optical fiber. The optical signal output from the first optical circulator 2 port enters a base station second optical circulator 1 port through a first optical fiber and is output from a second optical circulator 2 port. The optical signal output from the second optical circulator 2 port is amplified by a first optical amplifier. The amplified optical signal is frequency-mixed at a first photodetector to form a to-be-transmitted electrical signal. The to-be-transmitted electrical signal is power-amplified by an electrical amplifier and is finally emitted by a transmitting antenna. (2) The self-interference cancellation part: The electrical signal emitted by the transmitting antenna forms a self-interference signal, which, together with a remote useful signal, forms a mixed signal, and the mixed signal is received by a local receiving antenna. A fixed wavelength laser outputs an optical carrier. A second Mach-Zehnder modulator modulates the mixed signal onto the optical carrier to form a receiving optical signal, and the second Mach-Zehnder modulator is adjusted to a negative quadrature bias point. The receiving optical signal enters a second optical circulator 3 port and is output from a second optical circulator 1 port. The optical signal output from the second optical circulator 1 port is returned to the central station first optical circulator through a first optical fiber, enters from a first optical circulator 2 port, and is output from a first optical circulator 3 port. The optical signal output from the first optical circulator 3 port is combined with the optical signal output from the second optical fiber in the signal transmitting part at an optical combiner. The combined optical signal is amplified by a second optical amplifier and is frequency-mixed at a second photodetector to output a useful electrical signal.

2. A self-interference cancellation bidirectional in-band full-duplex RoF method based on a simple structure self-interference cancellation bidirectional in-band full-duplex RoF system as claimed in claim 1, characterized in that, The specific process is as follows: Step one: electro-optical modulation of the transmitting signal and transmission to the central station First, at the central station, a wavelength tunable laser outputs an optical carrier, which is injected into a first Mach-Zehnder modulator. The first Mach-Zehnder modulator modulates a transmission signal onto the light to form a transmission optical signal, and the Mach-Zehnder modulator is adjusted to a positive quadrature bias point. Assuming that the transmission signal is V1sinωt, where V1is the amplitude of the transmission signal, ω is the angular frequency of the transmission signal, and t is time; under small signal modulation, the output light signal envelope E1(t) after the first Mach-Zehnder modulator is shown in Equation 1: s E1(t) = V1sinωt cosωct (1) s where V1is the amplitude of the transmission signal, ω is the angular frequency of the transmission signal, t is time, and ωc is the angular frequency of the light carrier. E1(t) = E c1 exp(j(ω c +ω τ )t)((1+j)J0(m1)+(1-j)J1(m1)exp(jω s t)-(1-j)J1(m1)exp(-jω s t)) (1) where E c1 is the amplitude of the optical carrier provided by the tunable laser, ω c is the reference frequency, which is also the input optical carrier frequency, ω τ is the offset frequency of the tunable laser at the reference frequency ω c ; J0() is the 0th order Bessel function of the first kind, J1() is the 1st order Bessel function of the first kind, and m1= πV1 / V π is the modulation index of the first Mach-Zehnder modulator; V π is the half-wave voltage of the modulator; The optical signal E1(t) output from the first Mach-Zehnder modulator is split by an optical splitter, one path of which is provided as a reference signal for subsequent self-interference cancellation, and the other path is input from a first optical circulator 1 port, output from a first optical circulator 2 port, and transmitted to a base station through a first optical fiber; in the base station, the split optical signal E1(t) is input from a second optical circulator 1 port and output from a second optical circulator 2 port; the second optical circulator 12 port output is Due to the dispersion effect of optical fibers, the transfer function of optical fibers is H(ω)=exp(-αL / 2+jβ2L(ω-ω)). c ) 2 / 2), where ω is the angular frequency, α is the attenuation constant, L is the fiber length, β2 is the group velocity dispersion parameter; τ1 is the delay caused by the optical signal propagating in the fiber, θ(ω)=β2L(ω-ω c ) 2 / 2 represents the phase shift introduced by the radio frequency signal in the fiber dispersion effect; The optical signal output from the second optical circulator 12 port is amplified by a first optical amplifier and is frequency-mixed at a first photodetector to form a to-be-transmitted electrical signal; the to-be-transmitted electrical signal is amplified by an electrical amplifier, and the amplified electrical signal I(t) is represented as wherein R1 is the responsivity of the first photodetector, A is the amplification factor of the electrical amplifier, τ2= β2Lω τ is the delay due to fiber dispersion; Step two: signal transmission through a transmitting antenna and reception together with a useful signal The amplified electrical signal I(t) generates a self-interference signal after being transmitted by the transmitting antenna, and the self-interference signal and the remote useful signal jointly form a mixed signal, which is received by the local receiving antenna; assuming that the attenuation of the self-interference signal in the self-interference channel is α1 and the delay is τ2; then the received self-interference signal can be simplified as I2(t) = V2sin(ω s (t+τ1+τ2)); wherein The received useful signal can be expressed as I3(t) = V3sin(ω s (t+τ3)), wherein V3 and τ3 are the amplitude and delay of the useful signal, respectively. Step three: modulate the received signal on light and backhaul to the central station The fixed wavelength laser output light carrier is injected into the second Mach-Zehnder modulator, the second Mach-Zehnder modulator is adjusted to a negative quadrature bias point, and the second Mach-Zehnder modulator modulates the mixed signal received by the receiving antenna onto the light carrier; the optical signal output by the second Mach-Zehnder modulator enters the second circulator 3 port, and after output from the second circulator 1 port, it is transmitted to the first circulator 2 port through the first optical fiber, and is output from the first circulator 3 port; the envelope E of the optical signal output from the first circulator 3 port SMF2 (t) as shown in formula 4: where G OA is the amplification gain of the optical amplifier, E c2 is the amplitude of the fixed-wavelength laser, m2= πV2 / V π , m3= πV3 / V π are the modulation indices of the second Mach-Zehnder modulator with respect to the self-interference signal and the useful signal, respectively; Step four: beat frequency after the reference light signal and the received light signal are combined The reference signal passes through the second optical fiber, in order to match the delay introduced by the signal in the second optical fiber, assuming the second optical fiber length is 2L, the group velocity dispersion parameter of the second optical fiber is consistent with the first optical fiber; the reference signal passes through the second optical fiber and combines with the light signal output from the first circulator 3 port in the optical combiner, the combined light signal is amplified by the optical amplifier, and then beats at the second photodetector, and the output beat frequency electric signal is: In the formula, R2 is the responsivity of the second photodetector; Step five: tune the wavelength of the tunable laser to achieve deep elimination of self-interference It is known from formula (5) that by tuning the power and wavelength of the optical attenuator, the following relationship can be achieved to eliminate the self-interference signal:

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