A BPSK direction modulation device and method based on microwave photons

Abstract

This invention discloses a BPSK directional modulation device and method based on microwave photonics, belonging to the fields of optical fiber communication and microwave technology. The device mainly includes a laser, two DPMZMs, two optical filters, an optical beamsplitter, two PDs, two power dividers, and two transmitting antennas. The output port of the laser is connected to the input port of the optical beamsplitter. The two output ports of the optical beamsplitter are connected to the input ports of DPMZM1 and DPMZM2, respectively. The output ports of DPMZM1 and DPMZM2 are connected to the input ports of optical filters 1 and 2, respectively. After filtering, optical signals 1 and 2 are input to PD1 and PD2, respectively, and then output as directional modulation signals 1 and 2. Finally, the modulated BPSK directional modulation signals are transmitted by transmitting antennas 1 and 2, respectively. This invention uses DPMZMs to achieve the phase setting function of baseband signals. The device does not involve electrical phase shifters and uses a microwave photonic system, realizing long-distance transmission and processing of communication signals, with the characteristics of low loss, high speed, and strong stability.

Description

Technical Field

[0001] This invention relates to the fields of optical fiber communication and microwave technology, and in particular to a binary phase shift keying (BPSK) directional modulation device and method based on microwave photons. Background Technology

[0002] Wireless communication technology has been widely applied in various fields with the development of science and technology. However, its inherent openness, mobility, and instability pose a risk of eavesdropping during information transmission, especially in the military field where information security requirements are even more stringent. Physical layer security of wireless communication systems is an important research direction. Directional modulation is achieved by changing the phase of the antenna's transmitted signal using a phase shifter, ensuring correct information reception in the desired direction at the receiver. Eavesdropping, unable to demodulate and recover the signal's phase information, cannot receive accurate information. However, traditional directional modulation techniques, due to their use of electrical scanning, suffer from slow scanning speed, limited bandwidth, high loss, and large size. Furthermore, beam scanning exhibits drawbacks such as beam skew, small bandwidth, and low frequency. The proposed method for directional modulation using microwave photonics technology not only effectively solves these problems but also gives the system advantages such as strong electromagnetic interference resistance, compatibility with higher operating frequency bands, more flexible tuning, long-distance transmission, and lightweight and compact size. Therefore, directional modulation technology based on microwave photonics has significant research value. Summary of the Invention

[0003] This invention proposes a device and method for achieving BPSK directional modulation using a laser, a dual parallel-Mach Zehnder modulator (DPMZM), an optical filter, an optical beam splitter, a photodetector (PD), and a transmitting antenna.

[0004] The technical solution adopted in this invention is:

[0005] A microwave photonics-based BPSK directional modulation device includes a laser, two DPMZMs, two optical filters, an optical beamsplitter, two PDs, two power dividers, and two transmitting antennas. The output port of the laser is connected to the input port of the optical beamsplitter. The two output ports of the optical beamsplitter are connected to the input ports of DPMZM1 and DPMZM2, respectively. The output ports of DPMZM1 and DPMZM2 are connected to the input ports of optical filters 1 and 2, respectively. After filtering, optical signals 1 and 2 are input to PD1 and PD2, respectively, and directional modulation signals 1 and 2 are output. Finally, the modulated BPSK directional modulation signals are transmitted by transmitting antennas 1 and 2, respectively.

[0006] Each of the two DPMZMs consists of a Y-type optical splitter, two parallel sub-Mach Zehnder Modulators (MZMs), and an optical combiner. DPMZM1 corresponds to sub-modulators MZM1 and MZM2, while DPMZM2 corresponds to sub-modulators MZM3 and MZM4. The local oscillator signal, generated by the signal source, is split into two paths by power divider 1, and input to the RF ports of sub-modulators MZM1 and MZM3 respectively. Binary information symbols are split into two paths by power divider 2, and input to the RF ports of sub-modulators MZM2 and MZM4 respectively. Here, MZM1 and MZM3 operate at the quadrature point, while MZM2 and MZM4 operate at the minimum point. The DC bias angle of the main modulator of DPMZM1 is θ1, and the DC bias angle of the main modulator of DPMZM2 is θ2.

[0007] Based on the device described above, the method for generating BPSK directional modulation signals based on microwave photonics is as follows:

[0008] Step 1: The optical signal E generated by the laser c (t) is input to the DPMZM1 and DPMZM2 modulators at equal power after passing through the optical splitter; the local oscillator signal V LO sin(ω LO t) The signal is split into two paths by power divider 1, and input to the RF ports of sub-modulators MZM1 and MZM3 respectively; binary information code element V C (t)=V C s(t) is split into two paths by power divider 2, and input to the RF ports of sub-modulators MZM2 and MZM4 respectively;

[0009] Among them, V LO and V C The amplitudes ω of the radio frequency signal and the binary information symbol are respectively. LO Let t be the frequency of the radio frequency signal, and s(t) be the binary information.

[0010] The expressions for the output optical signals of DPMZM1 and DPMZM2 are as follows:

[0011]

[0012]

[0013] Among them, E c (t) represents the laser output signal, μ represents the modulator loss, m and β represent the modulation index of the radio frequency signal and binary information symbols on the modulator, respectively, and J n (·) denotes the first-order n-th order Bessel function, with the modulation index being a local minimum. Under small-signal modulation, only the low-order sidebands are considered.

[0014] Step 2: The output optical signal of DPMZM1 is filtered by optical filter 1 to extract the carrier and negative first-order sideband; the output optical signal of DPMZM2 is filtered by optical filter 2 to extract the carrier and positive first-order sideband. The output expressions after filtering are as follows:

[0015]

[0016]

[0017] Step 3: After the two signals enter PD1 and PD2 respectively, the beat frequency output signals are as follows:

[0018]

[0019]

[0020] Where, γ pd The response of PD1 and PD2.

[0021] Step 4: The two directional modulation signals output by PD1 and PD2 are transmitted using antenna 1 and antenna 2 respectively. At the same time, the DC bias angles θ1 and θ2 of the main modulators of DPMZM1 and DPMZM2 are adjusted to generate BPSK directional modulation signals.

[0022] The values ​​of the DC bias angles θ1 and θ2 that need to be adjusted are calculated as follows:

[0023] Assume antennas 1 and 2 are placed parallel to each other, with the normal direction of the two antennas (i.e., 0 degrees) being the desired communication direction, while the eavesdropping direction is α degrees to the right of the normal direction of the two antennas. To achieve directional modulation, the signal phase needs to be adjusted so that the BPSK phase information can be correctly recovered in the desired direction, but the BPSK phase information cannot be correctly recovered in the eavesdropping direction.

[0024] To avoid signal grating lobe interference, the wavelength is related to the distance between the transmitting antennas as d = λ / 2. Since the eavesdropping direction is at α degrees of the normals to antennas 1 and 2, the resulting optical path difference is Δd = dsinα. Assuming the transmitted signal is a single-frequency signal, at the eavesdropping location, the relative phase shift of the received signal from antenna 1 relative to the signal from antenna 2...

[0025] Let θ 1,0 and θ 1,1 θ represents the phase of the signal emitted by antenna 1 when the symbol is "0" and "1", respectively. 2,0 and θ 2,1 These represent the phases of the signal emitted by antenna 2 when the symbol is "0" and "1", respectively. Since the phase difference between the signals when the symbol is "0" and "1" in BPSK modulation is π, we obtain...

[0026]

[0027] To meet the requirements of directional modulation and make it impossible to distinguish between "0" and "1" in the direction of eavesdropping, the received signal symbols must have the same phase when they are "0" and "1", which can be represented as:

[0028]

[0029] Where k is an integer.

[0030] On the other hand, the signal received in the desired direction (main lobe direction) needs to be expressible as:

[0031]

[0032] In order to effectively demodulate the signal, the two symbols must be 180 degrees out of phase. According to equations (8) and (9), the following conditions must be met:

[0033]

[0034] Combining equations (8) and (10), the signal phase relationship that satisfies directional modulation can be obtained, as shown in Table 3-1:

[0035] Table 1 Signal Phase Relationship

[0036]

[0037]

[0038] Taking a 30° deviation of the eavesdropping direction from the normal direction of the two transmitting antennas as an example, and combining the directional modulation signal obtained by the beat frequency, i.e., formulas (5) and (6), we can obtain the phase introduced by the main bias angle of the two DPMZMs as follows:

[0039]

[0040] Finally, the normalized directional modulation signal is represented as:

[0041]

[0042] Where the modulation index β = π, and s(t) is a binary symbol sequence.

[0043] The advantages of this invention are that it uses DPMZM to realize the phase setting function of baseband signal, and the system does not involve an electrical phase shifter, thus avoiding the shortcomings of electrical phase shifters; this invention uses a microwave photonic system to realize long-distance transmission and processing of communication signals, and the system has the characteristics of low loss, high speed and strong stability; at the same time, compared with electrical phase shifters, this invention has a simple principle and is easy to implement. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the BPSK direction modulation device based on microwave photons proposed in this invention;

[0045] Figure 2 The receiver time-domain signal in the embodiment;

[0046] Figure 3 The phase of the modulated signal to be recovered in the desired direction in the embodiment;

[0047] Figure 4 The phase of the modulated signal recovered from the eavesdropping direction (α = 30 degrees) in the embodiment. Detailed Implementation

[0048] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments:

[0049] In this example, the device includes: a laser, an RF signal source, an AWG signal source, DPMZM1, DPMZM2, an optical beam splitter, an optical filter 1, an optical filter 2, PD1, PD2, a power divider, an antenna 1, and an antenna 2.

[0050] Step 1: Device Connection. The laser's output port is connected to the input port of the optical beamsplitter. The two output ports of the optical beamsplitter are connected to the input ports of DPMZM1 and DPMZM2, respectively. After the radio frequency signal is generated by the signal source, it is connected to the power divider. The two power-divided paths are input to the sub-modulators MZM1 and MZM3 of DPMZM1 and DPMZM2, respectively. After the binary information code is generated by the signal source, it is connected to the power divider. The two power-divided paths are input to the sub-modulators MZM3 and MZM4 of DPMZM1 and DPMZM2, respectively. The two signals output from the modulation module are input to optical filter 1 and optical filter 2, respectively. The output optical signals are input to photodetector 1 and photodetector 2, respectively. Finally, the two electrical signals output from the photodetectors enter the transmitting antenna.

[0051] Step 2: The laser generates an optical carrier with a working wavelength of 1550nm and an optical power of 16dBm; the radio frequency signal source generates a radio frequency signal with a frequency of 10GHz and a power of 0dBm; the half-wave voltage of the DPMZM is 3.5V and the extinction ratio is 35dB; the pseudo-random code (PBRS) is used to simulate binary information symbols, the PBRS signal voltage is -3.5V, and the AWG outputs the PBRS signal; the responsivity of PD1 and PD2 is 0.7A / W.

[0052] Step 3: Set the sub-modulators MZM1 and MZM3 in the two DPMZMs to work at the quadrature modulation point, and MZM2 and MZM4 to work at the minimum modulation point. Set the bias voltage of the main modulator according to the calculation to achieve the desired bias angles θ1 and θ2. Set optical filter 1 to filter out the upper carrier and negative first-order sideband respectively, and optical filter 2 to filter out the lower carrier and positive first-order sideband respectively.

[0053] Step 4: PD1 and PD2 perform photoelectric detection to recover the directional modulation signal, which is then input to transmitting antenna 1 and transmitting antenna 2 respectively.

[0054] Step 5: The signal data from transmitting antenna 1 and transmitting antenna 2 are processed by MATLAB. Based on the free space transmission model, the antenna propagation is simulated. Communication information is received in the desired direction and the eavesdropping direction, respectively. The information phase is recovered using Hilbert transform, and the directional modulation functionality is verified.

[0055] Step Six: Observe and analyze the phase information, such as Figure 3 As shown, the phase waveform recovered from the information received in the desired receiving direction is basically consistent with the waveform of the transmitted binary information symbols, meaning the information can be correctly recovered. Figure 4 As shown, the phase waveform recovered from the information received in the eavesdropping direction oscillates within a small range and does not correspond to the waveform of the binary information code, meaning the information cannot be correctly recovered. This verifies the feasibility of achieving microwave photon-based directional modulation by introducing the above two sets of phase values ​​using the principal bias angle of DPMZM.

[0056] In summary, this scheme utilizes a parallel dual-DPMZM modulation module to perform directional modulation and transmission of binary signals. An antenna was simulated using MATLAB, ultimately achieving BPSK directional modulation. In this process, the problems of low frequency, small bandwidth, and high loss in the electrical phase shifter were effectively solved. This scheme can obtain high-performance, high-bandwidth modulated signals, is easy to implement, and offers flexible operation. It has significant inspirational and potential application value in directional modulation for modern radar, electronic warfare, and wireless communication systems.

[0057] In summary, the above-described embodiments are merely examples of the present invention and are not intended to limit the scope of protection of the present invention. It should be noted that those skilled in the art can make several equivalent modifications and substitutions based on the content disclosed in the present invention, such as changing the radio frequency signal frequency, symbol signal frequency, optical carrier wavelength, optical carrier power, radio frequency signal power, and the DC bias angle of the modulator. These equivalent modifications and substitutions, as well as adjustments to the frequency range, should also be considered within the scope of protection of the present invention.

Claims

1. A BPSK directional modulation method based on microwave photons, comprising a modulation device including a laser, two DPMZMs, two optical filters, an optical beamsplitter, two PDs, two power dividers, and two transmitting antennas; the output port of the laser is connected to the input port of the optical beamsplitter, the two output ports of the optical beamsplitter are respectively connected to the input ports of DPMZM1 and DPMZM2, the output ports of DPMZM1 and DPMZM2 are respectively connected to the input ports of optical filters 1 and 2, the filtered optical signals 1 and 2 are input to PD1 and PD2 respectively, and directional modulation signals 1 and 2 are output, respectively, and finally the modulated BPSK directional modulation signals are transmitted by transmitting antennas 1 and 2 respectively. The signal consists of two DPMZMs, each composed of a Y-type optical splitter, two parallel sub-modulators (MZMs) and an optical combiner. DPMZM1 corresponds to sub-modulators MZM1 and MZM2, and DPMZM2 corresponds to sub-modulators MZM3 and MZM4. The local oscillator signal, generated by the signal source, is split into two paths by power divider 1, and input to the RF ports of sub-modulators MZM1 and MZM3 respectively. The binary information code is split into two paths by power divider 2, and input to the RF ports of sub-modulators MZM2 and MZM4 respectively. MZM1 and MZM3 operate at the quadrature point, while MZM2 and MZM4 operate at the minimum point. The DC bias angle of the main modulator of DPMZM1 is [value missing]. The DC bias angle of the DPMZM2 master modulator is Its characteristics are, The process is as follows: Step 1: The optical signal generated by the laser The signal is input to the DPMZM1 and DPMZM2 modulators at equal power after passing through an optical splitter; the local oscillator signal... The signal is split into two paths by power divider 1, and input to the RF ports of sub-modulators MZM1 and MZM3 respectively; binary information code elements The power divider splits the signal into two paths, which are then input to the RF ports of sub-modulators MZM2 and MZM4, respectively. in, and These represent the amplitudes of the radio frequency signal and the binary information code, respectively. The frequency of the radio frequency signal. It is binary information; The expressions for the output optical signals of DPMZM1 and DPMZM2 are as follows: （1） （2） in, For laser output signal, For modulator losses, , These represent the modulation indices of the radio frequency signal and the binary information symbols to the modulator, respectively. Indicates the first type n The Bessel function of order 1 has a modulation index that is a local minimum. Under small-signal modulation, only the low-order sidebands are considered. Step 2: The output optical signal of DPMZM1 is filtered by optical filter 1 to extract the carrier and negative first-order sideband; the output optical signal of DPMZM2 is filtered by optical filter 2 to extract the carrier and positive first-order sideband. The output expressions after filtering are as follows: （3） （4） Step 3: After the two signals enter PD1 and PD2 respectively, the beat frequency output signals are as follows: （5） （6） in, The response of PD1 and PD2; Step 4: The two directional modulation signals output from PD1 and PD2 are transmitted using antenna 1 and antenna 2 respectively, while simultaneously adjusting the DC bias angle of the main modulators of DPMZM1 and DPMZM2. and , generating BPSK directional modulation signals; DC bias angle to be adjusted and The value is calculated as follows: Assuming antenna 1 and antenna 2 are placed parallel to each other, with the normal directions of the two antennas pointing in the desired communication direction, while the eavesdropping direction is to the right of the normal directions of the two antennas. Directional modulation requires adjusting the signal phase to correctly recover the BPSK phase information in the desired direction, but it cannot correctly recover the BPSK phase information in the eavesdropping direction. To avoid interference from signal grating lobes, the relationship between wavelength and transmit antenna spacing is as follows: Since the eavesdropping direction is along the normals of antenna 1 and antenna 2 At a certain degree, the resulting optical path difference is Assuming the transmitted signal is a single-frequency signal, at the eavesdropping location, the relative phase shift of the signal received from antenna 1 relative to the signal received from antenna 2... ; set up and These represent the phases of the signal emitted by antenna 1 when the code symbols are "0" and "1", respectively. and These represent the phases of the signals emitted by antenna 2 when the symbol is "0" and "1", respectively; since in BPSK modulation, the phase difference between the signals when the symbol is "0" and "1" is... Therefore, we get （7） To meet the requirements of directional modulation and make it impossible to distinguish between "0" and "1" in the direction of eavesdropping, the received signal symbols must have the same phase when they are "0" and "1", which is represented as: （8） in It is an integer; On the other hand, the signal received in the desired direction, i.e., the main lobe direction, needs to be represented as: （9） In order to effectively demodulate the signal, the two symbols must be 180 degrees out of phase. According to equations (8) and (9), the following conditions must be met: （10） Combining equations (8) and (10), the signal phase relationship that satisfies directional modulation can be obtained, as shown in Table 1: Table 1 Signal Phase Relationship Taking a 30° deviation of the eavesdropping direction from the normal direction of the two transmitting antennas as an example, and combining the directional modulation signal obtained by the beat frequency, i.e., formulas (5) and (6), we can obtain that the phase introduced by the main bias angle of the two DPMZMs is: （11） Finally, the normalized directional modulation signal is represented as: （12） Among them, modulation index , It is a binary code sequence.

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