Direct Detection Method, Device and System for Complex-Valued Double-Sideband Signal

Abstract

The present application provides a direct detection method, apparatus and system for complex-valued double-sideband signals. In the embodiments of the present application, a frequency selective phase shifter is used to apply a single-side phase shift to the left sideband signal or the right sideband signal of the information-bearing signal. Thus, even if the frequency of the carrier signal drifts towards the side without phase shift relative to the frequency at the zero point of the phase filter, the detection result will not be affected. Therefore, it is not necessary to accurately align the operating wavelength of the transmitter laser (equivalent to the frequency of the carrier signal) with the wavelength at the phase mutation point of the phase filter (the frequency at the zero point of the phase filter), improving the tolerance to the carrier wavelength drift in complex-valued double-sideband signals. At the same time, since the frequency selective phase shifter applies a single-side phase shift instead of a double-side phase shift, the number of phase mutations to be completed is reduced from two to one, reducing the physical implementation difficulty of the frequency selective phase shifter and providing more options for the physical implementation of the frequency selective phase shifter.

Description

Technical Field

[0001] This application relates to the field of communication technologies, and in particular, to a direct detection method, apparatus, and system for complex-valued double-sideband signals. Background Art

[0002] In recent years, various emerging broadband services have emerged continuously, such as the metaverse, 5G technology, large-scale computing power networks, ultra-high-definition videos, etc. The emergence of new applications has led to a continuous increase in the traffic of metropolitan area networks and data centers, which has promoted the development of optical communication systems.

[0003] Currently, the direct detection receiving device is very sensitive to the carrier wavelength drift in complex-valued double-sideband signals and requires a very stable laser source or a precise temperature control system, which greatly increases the system cost. Moreover, the frequency-selective phase shifter of the direct detection receiving device needs to complete two phase mutations within a small frequency range, and the physical implementation is difficult. Summary of the Invention

[0004] This application provides a direct detection method, apparatus, and system for complex-valued double-sideband signals, which improves the tolerance to carrier wavelength drift in complex-valued double-sideband signals, reduces the physical implementation difficulty of the frequency-selective phase shifter, and provides more options for the physical implementation of the frequency-selective phase shifter.

[0005] To achieve the above object, this application adopts the following technical solutions:

[0006] In the first aspect, a direct detection method for complex-valued double-sideband signals is provided. First, the complex-valued double-sideband signal to be measured is divided into two paths. The complex-valued double-sideband signal to be measured includes a carrier and an information-bearing signal. The information-bearing signal includes a left sideband signal and a right sideband signal. In the frequency domain, the carrier is located between the left sideband signal and the right sideband signal. Then, one path of the complex-valued double-sideband signal to be measured passes through a first photodetector and a first analog-to-digital converter to obtain a first digital signal of the complex-valued double-sideband signal to be measured. The other path of the complex-valued double-sideband signal to be measured first passes through a frequency-selective phase shifter to apply a phase shift to the left sideband signal or the right sideband signal of the information-bearing signal, and then passes through a second photodetector and a second analog-to-digital converter to obtain a second digital signal of the complex-valued double-sideband signal after the phase shift. Finally, the reconstructed information-bearing signal is obtained according to the first digital signal and the second digital signal.

[0007] Based on the above technical solution, in the embodiment of the present application, the direct detection method for complex-valued double-sideband signals uses a frequency-selective phase shifter to apply a unilateral phase shift to the left-sideband signal or the right-sideband signal of the information-bearing signal. In this way, even if the frequency of the carrier signal drifts to the side without phase shift relative to the frequency at the zero point of the phase filter, it will not affect the detection result. Therefore, it is not necessary to accurately align the operating wavelength of the transmitter laser (equivalent to the frequency of the carrier signal) with the wavelength at the phase mutation point of the phase filter (the frequency at the zero point of the phase filter), which improves the tolerance to the carrier wavelength drift in the complex-valued double-sideband signal. At the same time, since the frequency-selective phase shifter applies a unilateral phase shift instead of a bilateral phase shift, the number of phase mutations to be completed is reduced from two to one, which reduces the physical implementation difficulty of the frequency-selective phase shifter and provides more options for the physical implementation of the frequency-selective phase shifter.

[0008] In a possible implementation manner of the first aspect, the transfer function of the frequency-selective phase shifter exhibits all-pass amplitude, and the phase response of the frequency-selective phase shifter is to apply an adjustable unilateral phase shift to the left-sideband signal or the right-sideband signal of the information-bearing signal.

[0009] In a possible implementation manner of the first aspect, the phase response of the frequency-selective phase shifter is to apply an optimal phase shift angle to the left-sideband signal or the right-sideband signal of the information-bearing signal, where the optimal phase shift angle is obtained by continuously changing the phase shift angle to obtain the corresponding bit error rate, and the phase shift angle corresponding to the minimum bit error rate is the optimal phase shift angle.

[0010] In a possible implementation manner of the first aspect, a guard interval is reserved between the left sideband and the right sideband.

[0011] In the second aspect, a direct detection device for complex-valued double-sideband signals is provided. The device includes: a first optical coupler, a first photodetector and a first analog-to-digital converter connected in sequence to the output end of the first optical coupler; a frequency-selective phase shifter, a second photodetector and a second analog-to-digital converter connected in sequence to the output end of the first optical coupler, and a digital signal processing module connected to the output end of the first analog-to-digital converter and the output end of the second analog-to-digital converter;

[0012] The first optical coupler is used to divide the input complex-valued double-sideband signal to be measured into two paths. The complex-valued double-sideband signal to be measured includes a carrier and an information-bearing signal, and the information-bearing signal includes a left-sideband signal and a right-sideband signal. In the frequency domain, the carrier is located between the left-sideband signal and the right-sideband signal;

[0013] The first photodetector is used to obtain a first electrical signal by converting one of the complex-valued double-sideband signals to be measured, and the first analog-to-digital converter is used to convert the first electrical signal into a first digital signal;

[0014] The frequency-selective phase shifter is used to apply a phase shift to the left-sideband signal or the right-sideband signal of the information-bearing signal in the other complex-valued double-sideband signal to be measured. The second photodetector is used to convert the other complex-valued double-sideband signal with the applied phase shift into a second electrical signal, and the second analog-to-digital converter is used to convert the second electrical signal into a second digital signal;

[0015] The digital signal processing module is used to obtain the reconstructed information-bearing signal according to the first digital signal and the second digital signal.

[0016] In a possible implementation manner of the second aspect, the frequency-selective phase shifter is one of a waveform shaper, a silicon photonics integrated high-resolution all-pass phase filter, and a high-quality factor silicon nitride microring resonator.

[0017] In a possible implementation manner of the second aspect, the transfer function of the frequency-selective phase shifter exhibits amplitude all-pass, and the phase response of the frequency-selective phase shifter is to apply an adjustable single-side phase shift to the left-sideband signal or the right-sideband signal of the information-bearing signal.

[0018] In a possible implementation manner of the second aspect, the phase response of the frequency-selective phase shifter is to apply an optimal phase shift angle to the left-sideband signal or the right-sideband signal of the information-bearing signal. Among them, the optimal phase shift angle is obtained by continuously changing the phase shift angle to obtain the corresponding bit error rate, and the phase shift angle corresponding to the minimum bit error rate is the optimal phase shift angle.

[0019] In a third aspect, a signal detection system is provided, which is characterized by including: a transmitter for transmitting a complex-valued double-sideband signal to be measured, and the complex-valued double-sideband signal to be measured includes a carrier and an information-bearing signal; a direct detection device for the complex-valued double-sideband signal as described in the second aspect or any possible implementation manner of the second aspect, which is connected to the transmitter.

[0020] On the basis of the implementation manners provided in the above aspects of the present application, further combinations can be made to provide more implementation manners. Description of the Drawings

[0021] Figure 1 is a schematic structural diagram of the current direct detection receiving device;

[0022] Figure 2(a) is Figure 1 the amplitude-frequency response curve of the phase filter shown in

[0023] Figure 2(b) is Figure 1 the phase-frequency response curve of the phase filter shown in

[0024] Figure 3 a schematic structural diagram of the direct detection device for the complex-valued double-sideband signal provided by the embodiment of the present application;

[0025] Figure 4(a) is Figure 3 the amplitude-frequency response curve of the frequency-selective phase shifter applying a phase shift to the right sideband shown in

[0026] Figure 4(b) is Figure 3 a kind of phase-frequency response curve of the frequency-selective phase shifter applying a phase shift to the right sideband shown in

[0027] Figure 4(c) is Figure 3 another kind of phase-frequency response curve of the frequency-selective phase shifter applying a phase shift to the right sideband shown in

[0028] Figure 5(a) is Figure 3 the amplitude-frequency response curve of the frequency-selective phase shifter applying a phase shift to the left sideband shown in

[0029] Figure 5(b) is Figure 3 a kind of phase-frequency response curve of the frequency-selective phase shifter applying a phase shift to the left sideband shown in

[0030] Figure 5(c) is Figure 3 another kind of phase-frequency response curve of the frequency-selective phase shifter applying a phase shift to the left sideband shown in

[0031] Figure 6 a schematic structural diagram of the direct detection system for the complex-valued double-sideband signal provided by the embodiment of the present application;

[0032] Figure 7 a schematic diagram of the bit error performance of the direct detection device for the complex-valued double-sideband signal provided by the embodiment of the present application;

[0033] Figure 8 a comparison diagram of the tolerance of the carrier wavelength drift in the complex-valued double-sideband signal by the direct detection device provided by the embodiment of the present application adopting two schemes of single-side phase shift and double-side phase shift;

[0034] Figure 9 a schematic flow diagram of the direct detection method for the complex-valued double-sideband signal provided by the embodiment of the present application. Detailed implementation manners

[0035] Next, the technical solutions in the present application will be described in conjunction with the accompanying drawings.

[0036] In the description of the embodiments of the present application, unless otherwise specified, " / " means "or". For example, A / B may mean A or B; herein, "and / or" is merely a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B may mean: A exists alone, A and B exist simultaneously, and B exists alone.

[0037] Hereinafter, the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of this embodiment, unless otherwise specified, the meaning of "a plurality" is two or more.

[0038] First, the proprietary nouns involved in the embodiments of the present application are uniformly explained.

[0039] (1) Complex-valued double-sideband signal

[0040] A complex-valued double-sideband (CV-DSB) signal includes a carrier and an information-bearing signal. Among them, the carrier is a high-frequency signal, whose frequency is usually much higher than that of the information-bearing signal. The carrier itself does not contain information, but it is a medium for information transmission. The information-bearing signal is a signal that contains the information to be transmitted. The information-bearing signal is a complex signal, including a real part and an imaginary part. Among them, the real part is also called the in-phase component, and the imaginary part is also called the quadrature component.

[0041] If x(t) represents the complex-valued double-sideband signal, c represents the carrier, s(t) represents the information-bearing signal, represents the in-phase component of the information-bearing signal, represents the quadrature component of the information-bearing signal, then the complex-valued double-sideband signal x(t) can be expressed as x(t)=c + s(t)=c + +j where j is the imaginary unit.

[0042] In the modulation process, the information-bearing signal will be loaded onto the carrier. Specifically, in the process of double-sideband modulation, the information-bearing signal will generate two sideband signals, a left sideband (LSB) signal and a right sideband (RSB) signal, which respectively correspond to the positive and negative offsets of the carrier frequency. If represents the left sideband signal, With a signal on the right side, the complex-valued double-sideband signal x(t) can also be expressed as . The left-sideband signal and the right-sideband signal are also complex signals. If represents the in-phase component of the left-sideband signal, represents the quadrature component of the left-sideband signal, then the left-sideband signal = + ; if represents the in-phase component of the right-sideband signal, represents the quadrature component of the right-sideband signal, then the right-sideband signal = + .

[0043] Among them, the left-sideband signal refers to the part of the information-bearing signal with a frequency lower than the carrier frequency. In this sideband, the frequency of the modulation signal is obtained by subtracting the carrier frequency. For example, if the carrier frequency of a signal is f c , and the frequency of a modulation signal is f m , then the frequency of the left-sideband signal will be f c - f m . The right-sideband signal refers to the part of the information-bearing signal with a frequency higher than the carrier frequency. In this sideband, the frequency of the modulation signal is obtained by adding the carrier frequency. Continuing with the above example, the frequency of the right-sideband signal will be f c + f m . The frequency of the carrier is in the middle of the frequency of the left-sideband signal and the frequency of the right-sideband signal.

[0044] In some examples, the left-sideband signal is also called the lower sideband signal, and the right-sideband signal is also called the upper sideband signal.

[0045] (2)Baud

[0046] Baud is the abbreviation of Baud, a unit for measuring the modulation rate, which represents the number of signal state changes per unit time. 50 GBaud means that the transmitter can modulate 5 billion symbol states per second, describing the modulation rate of the transmitter.

[0047] (3)16-QAM

[0048] Quadrature Amplitude Modulation (QAM) is a modulation technique that modulates both amplitude and phase simultaneously.

[0049] Hexadecimal QAM means that the modulation scheme uses 16 different symbol states to represent data. These states are usually represented as points in a constellation diagram, with each point representing a different symbol. In 16-QAM, each symbol can carry 4 bits of information. Therefore, the bit rate is the modulation rate (in Baud) multiplied by the number of bits per symbol.

[0050] For a 50 GBaud 16-QAM signal, the bit rate will be 50 GBaud * 4 bits / symbol = 200 Gbps (gigabits per second). Here, 50 GBaud describes the modulation rate of the transmitter, and 16-QAM describes the amount of information that each modulation symbol can carry. These two parameters together define the data transmission rate and efficiency of the communication system.

[0051] (4) Additive White Gaussian Noise

[0052] Additive White Gaussian Noise (AWGN) is a statistical model used to describe the noise characteristics commonly present in many communication systems.

[0053] Among them, additive: means that the noise is superimposed on the signal rather than being mixed with the signal in some way. Therefore, the total received signal is the sum of the original signal and the noise.

[0054] Gaussian: The probability density function of the noise follows a Gaussian distribution (also known as a normal distribution). This means that the amplitude values of the noise are statistically distributed according to a Gaussian distribution, with most noise values concentrated around the mean, and the probability of extreme values occurring is relatively low.

[0055] White noise: This refers to the fact that the noise has the same power spectral density at all frequencies, just like white light contains all the frequencies of visible light. Therefore, white noise is flat in the frequency domain and does not contain any specific frequency components.

[0056] In real-world communication systems, noise is inevitable. Adding AWGN to the transmitted signal can help simulate the noise impact in an actual communication link, thereby evaluating the performance of the system.

[0057] (5) Optical Field Reconstruction

[0058] Optical field reconstruction refers to restoring the complete information of the optical signal, including amplitude and phase, through digital signal processing techniques so that the optical signal transmitted at the sending end can be accurately reproduced at the receiving end.

[0059] (6) Bit Error Rate

[0060] The Bit Error Rate (BER) refers to the ratio of the number of error bits that occur during data transmission to the total number of transmitted bits within a certain period of time. It is an important indicator for measuring the reliability of a data transmission system.

[0061] The process of calculating the BER is as follows: First, a certain amount of data is transmitted. This usually involves sending a known data sequence and receiving this data at the receiving end. At the receiving end, the received data sequence is compared with the original data sequence sent by the sending end. The number of error bits found during the comparison is counted.

[0062] The BER can be calculated using the following formula: BER = number of error bits / total number of bits. Here, the number of error bits refers to the number of bits detected at the receiving end that do not match the original data, and the total number of bits refers to the total number of bits sent during the test.

[0063] (7) Optical Signal-to-Noise Ratio

[0064] The Optical Signal-to-Noise Ratio (OSNR) is an important parameter in optical fiber communication, which is used to measure the quality of an optical signal. The OSNR is defined as the ratio of the optical signal power to the noise power, OSNR = optical signal power / noise power, and the OSNR is usually expressed in decibels (dB). A higher OSNR can reduce the BER and improve the reliability of communication.

[0065] Among them, the optical signal power refers to the useful optical signal power, that is, the optical power of the transmitted data signal; the noise power refers to the total power of the noise introduced during signal transmission. This noise may include Spontaneous Emission Noise (ASE), Shot Noise, Thermal Noise, and noise generated by other non-linear effects.

[0066] (8) Forward Error Correction Threshold

[0067] The Forward Error Correction (FEC) threshold defines the lowest signal quality level at which the FEC algorithm can work reliably. Above this threshold, the FEC algorithm can correct enough errors to keep the transmission error rate within an acceptable range. This threshold is usually expressed in terms of the Optical Signal-to-Noise Ratio (OSNR) or the Bit Error Rate (BER).

[0068] (9) Square Law Detection

[0069] Square-law detection is a non-linear detection process in which a photodetector converts an optical signal into an electric current, and this current is proportional to the square of the amplitude of the input optical signal. In digital communication, square-law detection is usually used to recover the amplitude information of a signal because it can convert the phase and amplitude information of a modulated signal into an electrical signal that can be processed by subsequent circuits.

[0070] Next, the technical solutions involved in the embodiments of the present application will be described.

[0071] In recent years, various emerging broadband services have emerged continuously, such as the metaverse, 5G technology, large-scale computing power networks, ultra-high-definition videos, etc. The emergence of new applications has led to a continuous increase in the traffic of metropolitan area networks and data centers, which has promoted the development of optical communication systems. Receivers in optical communication include two detection methods, coherent detection and direct detection.

[0072] Among them, coherent detection requires a local oscillator light source whose frequency and phase match the optical signal at the transmitting end. The received signal light is mixed with the local oscillator light on a 90-degree optical mixer, and then passes through a photodetector to generate an electrical signal. This

[0073] electrical signal contains the phase and amplitude information of the signal light; the obtained electrical signal is analyzed by subsequent electronic processing circuits to recover the original transmitted data. It can be seen that although coherent detection has superior performance, it requires an expensive narrow linewidth local oscillator laser to generate the local oscillator light, and carrier recovery operations need to be performed to ensure effective mixing between the local oscillator light and the signal light. Therefore, for medium and short-distance transmission scenarios, the system cost is relatively high.

[0074] Direct detection is the simplest and most commonly used signal detection method in optical communication. Its working principle is as follows: when an optical signal is transmitted through an optical fiber and reaches the receiving end, the photodetector at the receiving end converts the optical signal into an electrical signal (such as a current or voltage signal), and the generated electrical signal is then sent to a signal processing circuit for operations such as amplification, filtering, and decision-making to recover the original transmitted data. It can be seen that compared with coherent detection, direct detection does not require a complex local oscillator light source and the phase matching technology required for coherent detection. Therefore, the system design and implementation are simpler, the cost of the optical communication system is lower, and it is suitable for medium and short-distance communication.

[0075] In a direct detection optical communication system for complex-valued double-sideband signals, the transmitter generates a quadrature amplitude modulation (QAM) signal similar to homodyne coherent detection, and uses a low-cost direct detection method to receive the signal, fully combining the advantages of both direct detection and coherent detection.

[0076] The structure of a direct detection receiving device is as Figure 1 shown, with the simplest design, approaching the performance of homodyne coherent detection. Figure 1After the complex-valued double-sideband signal enters the receiver, it is split into two paths by a splitter. One path directly enters the photodetector to detect the in-phase component of the signal. The other path first passes through a phase filter and then enters the photodetector to detect the quadrature component of the signal. The optimal transfer function of the phase filter shows an all-pass amplitude response, as shown in the amplitude-frequency response curve of Fig. 2(a); the phase response shows an adjustable double-sided phase shift applied to the information-bearing signal, as shown in the phase-frequency response curve of Fig. 2(b).

[0077] Since Figure 1 In the receiving device shown, the phase response of the phase filter shows an adjustable double-sided phase shift applied to the information-bearing signal, thereby introducing a certain phase difference between the carrier and the information-bearing signal, causing a conversion between the in-phase component and the quadrature component of the information-bearing signal. However, since the double-sided phase shift needs to satisfy that the carrier has a phase difference with both the left-sideband signal and the right-sideband signal of the information-bearing signal at the same time, it is required that the frequency of the carrier signal is precisely aligned with the frequency at the zero point of the phase filter. That is to say, it is required that the operating wavelength of the transmitter laser (equivalent to the frequency of the carrier signal) is precisely aligned with the wavelength at the phase mutation point of the phase filter (the frequency at the zero point of the phase filter). It is very sensitive to the wavelength drift of the transmitter laser and requires a very stable laser source or a precise temperature control system, which greatly increases the system cost. In addition, the phase filter needs to complete two phase mutations within a small frequency range, and the physical implementation is difficult.

[0078] In view of this, the embodiments of the present application provide a direct detection method, device, and system for complex-valued double-sideband signals. The direct detection method for complex-valued double-sideband signals provided in the embodiments of the present application uses a frequency-selective phase shifter to apply a single-sided phase shift to the left-sideband signal or the right-sideband signal of the information-bearing signal. In this way, even if the frequency of the carrier signal drifts to the side without applying the phase shift relative to the frequency at the zero point of the phase filter, it will not affect the detection result. Therefore, it is not necessary to precisely align the operating wavelength of the transmitter laser (equivalent to the frequency of the carrier signal) with the wavelength at the phase mutation point of the phase filter (the frequency at the zero point of the phase filter), improving the tolerance to the carrier wavelength drift in the complex-valued double-sideband signal. At the same time, since the frequency-selective phase shifter applies a single-sided phase shift instead of a double-sided phase shift, the number of phase mutations that need to be completed is reduced from two to one, reducing the physical implementation difficulty of the frequency-selective phase shifter and providing more choices for the physical implementation of the frequency-selective phase shifter.

[0079] Figure 3 The direct detection device for complex-valued double-sideband signals provided in the embodiments of the present application will be described in detail below.

[0080] The direct detection device for complex - valued double - sideband signals includes: a first optical coupler, a first photodetector and a first analog - to - digital converter that are connected to the output end of the first optical coupler and are connected in sequence; a frequency - selective phase shifter, a second photodetector and a second analog - to - digital converter that are connected to the output end of the first optical coupler and are connected in sequence, and a digital signal processing module that is connected to the output ends of the first analog - to - digital converter and the second analog - to - digital converter.

[0081] The first optical coupler is used to divide the input complex - valued double - sideband signal to be measured into two paths. The complex - valued double - sideband signal to be measured includes a carrier wave and an information - carrying signal. The information - carrying signal includes a left - hand sideband signal and a right - hand sideband signal. In the frequency domain, the carrier wave is located between the left - hand sideband signal and the right - hand sideband signal.

[0082] The complex - valued double - sideband signal to be measured is represented by c + s ( t ) where represents the carrier wave, and s ( t ) represents the information - carrying signal. The information - carrying signal s ( t ) includes the left - hand sideband signal and the right - hand sideband signal , that is, s ( t ) = + . Therefore, the complex - valued double - sideband signal to be measured can be expressed as + + . Since the information - carrying signal s ( t ) is a complex signal, including an in - phase component and a quadrature component, therefore, the information - carrying signal s ( t ) can also be expressed as the sum of the in - phase component and the quadrature component , that is, s ( t ) = + , where j is the imaginary unit.

[0083] The left - hand sideband signal and the right - hand sideband signal are also complex signals. Let represent the in - phase component of the left - hand sideband signal, and represent the quadrature component of the left - hand sideband signal, then the left - hand sideband signal can be expressed as = + ; if represents the in - phase component of the right - hand sideband signal, represents the quadrature component of the right-sideband signal, and the right-sideband signal can be expressed as = + .

[0084] Exemplarily, in the frequency domain, a small guard interval is reserved between the left-sideband signal and the right-sideband signal . This interval does not contain any modulation signals. Since the guard interval can separate the left-sideband signal and the right-sideband signal, during the modulation process, the left-sideband signal and the right-sideband signal will not interfere with each other. For example, a 1 Hz guard band, i.e., -0.5 Hz - 0.5 Hz, can be inserted at the zero frequency of the complex-valued double-sideband signal.

[0085] It is achievable that the carrier may be retained in the guard interval. The guard interval can also avoid interference between the left-sideband signal or the right-sideband signal and the carrier, which helps to improve the efficiency and clarity of signal transmission, and also enables the receiving end to more easily extract the original information from the received signal.

[0086] The first optical coupler is used to receive the input complex-valued double-sideband signal and split the complex-valued double-sideband signal into two paths. The information carried by the optical signals after being split into two paths (referred to as the first optical signal and the second optical signal in this application) is roughly the same, but the intensities of the optical signals after being split into two paths may be different. If the complex-valued double-sideband signal is represented by c + s ( t ), then the optical signals after being split into two paths can both be represented as c + s ( t ).

[0087] The first optical signal is input into the first photodetector. The first photodetector is used to convert the first optical signal into a first electrical signal, and the first analog-to-digital converter is used to convert the first electrical signal into a first digital signal.

[0088] Specifically, the first optical signal c + s ( t ) is input into the first photodetector. The first photodetector converts the first optical signal into a first electrical signal through square-law detection. The first electrical signal is represented by , , and can also be expressed as formula (1):

[0089] (1)

[0090] After that, the first electrical signal enters the first analog-to-digital converter, and the first analog-to-digital converter converts the analog first electrical signal Converted into the first digital signal.

[0091] The second optical signal c + s ( t ) An input frequency selective phase shifter is used to apply a phase shift to the left sideband signal or the right sideband signal of the information-bearing signal in the second optical signal. A second photodetector is used to convert the other complex-valued double sideband signal to be measured with the applied phase shift into a second electrical signal, and a second analog-to-digital converter is used to convert the second electrical signal into a second digital signal.

[0092] Specifically, the time-domain impulse response of the frequency selective phase shifter is represented by h(t) denoted.

[0093] In one way, the transfer function of the frequency selective phase shifter shows an all-pass amplitude, and the phase response of the frequency selective phase shifter is to apply a single-sided phase shift to the right sideband signal of the information-bearing signal.

[0094] Its amplitude-frequency response curve is shown in Figure 4(a), and the amplitude-frequency response shows an all-pass amplitude; its phase-frequency response curve is shown in Figure 4(b) or 4(c), and the phase-frequency response shows a single-sided phase shift applied to the right sideband signal of the information-bearing signal, that is, the frequency selective phase shifter applies a single-sided phase shift to the right sideband signal of the information-bearing signal.

[0095] Assume that the magnitude of the phase shift of the frequency selective phase shifter is represented by denoted, and the phase-frequency response of the frequency selective phase shifter shows a single-sided phase shift applied to the right sideband signal of the information-bearing signal, then the second optical signal c + s(t)= , after passing through the frequency selective phase shifter, can be expressed as . The phase-shifted optical signal is input into the second photodetector, and the second photodetector converts the phase-shifted second optical signal into a second electrical signal , and the second electrical signal , and the second electrical signal can be expressed by formula (2):

[0096] (2)

[0097] In another way, the transfer function of the frequency selective phase shifter shows an all-pass amplitude, and the phase response of the frequency selective phase shifter is to apply a single-sided phase shift to the left sideband signal of the information-bearing signal.

[0098] Its amplitude-frequency response curve is shown in Fig. 5(a), and the amplitude-frequency response shows all-pass amplitude; its phase-frequency response curve is shown in Fig. 5(b) or 5(c), and the phase-frequency response shows applying a single-sided phase shift to the left-sideband signal of the information-bearing signal, that is, the frequency-selective phase shifter applies a single-sided phase shift to the left-sideband signal of the information-bearing signal.

[0099] Assume that the magnitude of the phase shift of the frequency-selective phase shifter is represented by and the phase-frequency response of the frequency-selective phase shifter shows applying a single-sided phase shift to the left-sideband signal of the information-bearing signal. Then the second optical signal c + s(t)= , after passing through the frequency-selective phase shifter, can be expressed as . The phase-shifted optical signal is input into the second photodetector, and the second photodetector converts the phase-shifted second optical signal into a second electrical signal , and the second electrical signal , and the second electrical signal can also be expressed as Equation (3):

[0100] (3)

[0101] In this embodiment, the frequency-selective phase shifter can adopt a frequency-selective phase shifter with step-type phase shift. Its phase-frequency response curve is shown in Fig. 4(b) or 5(b). At specific frequency points, the phase shift amount suddenly jumps from one value to another, and its phase shift process is a sudden change process. Therefore, the physical implementation of this type of frequency-selective phase shifter is more difficult. To reduce the physical implementation difficulty of the frequency-selective phase shifter, the frequency-selective phase shifter can also adopt a frequency-selective phase shifter with continuous phase shift. Its phase-frequency response curve is shown in Fig. 4(c) or 5(c), and the phase shift amount changes continuously with the frequency without sudden jumps. Therefore, the physical implementation of this type of frequency-selective phase shifter is less difficult. In practical applications, different types of frequency-selective phase shifters can be selected according to needs to achieve single-sided phase shift.

[0102] It can be realized that the above single-sided phase shift is an adjustable single-sided phase shift, and the frequency-selective phase shifter can adjust the magnitude of the phase shift applied to the left-sideband signal or the right-sideband signal according to needs.

[0103] It can be realized that the phase response of the frequency-selective phase shifter is to apply an optimal phase shift angle to the left-sideband signal or the right-sideband signal of the information-bearing signal.

[0104] The above optimal phase shift angle is iteratively adjusted based on the actual direct detection device. The optimal phase shift angle is related to the optical signal-to-noise ratio and the carrier signal power ratio. To obtain the optimal phase shift angle, the phase shift angle of the frequency-selective phase shifter can be continuously changed , a reconstructed optical signal is obtained based on an actual direct detection device, and the corresponding bit error rate is calculated according to the reconstructed optical signal and the original optical signal. The phase shift angle corresponding to the minimum bit error rate is the optimal phase shift angle.

[0105] The above process of finding the optimal phase shift angle of the frequency selective phase shifter can be carried out during the test stage of the direct detection device. When the direct detection device is put into use, the phase shift angle of the frequency selective phase shifter in the direct detection device adopts the optimal phase shift angle to ensure the best detection effect in actual application of the direct detection device.

[0106] It is feasible that the frequency selective phase shifter can be one of a waveform shaper, a silicon photonics integrated high-resolution all-pass phase filter, and a high-quality factor silicon nitride microring resonator.

[0107] The first digital signal and the second digital signal are jointly input into the digital signal processing module, and the digital signal processing module is used to obtain a reconstructed information-bearing signal according to the first digital signal and the second digital signal.

[0108] In this embodiment, it is assumed that the expression formulas of the first digital signal and the first electrical signal are the same, and the expression formulas of the second digital signal and the second electrical signal are the same as an example to illustrate the process of the digital signal processing module reconstructing the information-bearing signal:

[0109] In one way, the phase response of the frequency selective phase shifter is to apply a single-side phase shift to the right-side band signal of the information-bearing signal. The first digital signal , the second digital signal .

[0110] To reconstruct the signal, the digital signal processing module first subtracts the first digital signal and the second digital signal , and the following formula (4) can be obtained:

[0111] (4)

[0112] Since is an intermediate frequency signal, and are in a Hilbert transform relationship with each other, then can be expressed as formula (5):

[0113] (5)

[0114] where Hilbert represents the Hilbert transform.

[0115] Combining formula (4) and (5), and Among them and are second-order terms and can be eliminated by an iterative algorithm.

[0116] The reconstructed right-sideband signal can be expressed as the following formula (6):

[0117] (6)

[0118] Then, according to formula (1), we can obtain

[0119] (7)

[0120] Therefore

[0121] (8)

[0122] Then the reconstructed left-sideband signal can be expressed as shown in the following formula (9):

[0123] (9)

[0124] Then the reconstructed information-bearing signal s` = + .

[0125] In another way, the phase response of the frequency-selective phase shifter is to apply a single-sided phase shift to the left-sideband signal of the information-bearing signal, the first digital signal , the second digital signal .

[0126] To reconstruct the signal, the digital signal processing module first subtracts the first digital signal and the second digital signal to obtain the following formula (10):

[0127] (10)

[0128] Since is an intermediate-frequency signal, and are in a Hilbert transform relationship with each other, then can be expressed as formula (11):

[0129] (11)

[0130] where Hilbert represents the Hilbert transform.

[0131] Combining equations (10) and (11), we can solve to obtain and . Among them and are second-order terms, which can be eliminated by an iterative algorithm.

[0132] The reconstructed left-sideband signal can be expressed as the following equation (12):

[0133] (12)

[0134] Then, according to equation (1), we can obtain

[0135] (13)

[0136] Therefore

[0137] (14)

[0138] Then the reconstructed right-sideband signal can be expressed as the following equation (15):

[0139] (15)

[0140] Then the reconstructed information-bearing signal s` = + .

[0141] The digital signal processing module completes the above-mentioned second-order term elimination and signal reconstruction. Specifically, deep learning technology can be used to implement the above signal reconstruction process, thereby optimizing the system performance.

[0142] It is achievable. The implementation carriers of the digital signal processing module include but are not limited to offline signal processing programs, field programmable gate arrays (Field Programmable Gate Array, FPGA), photonic neural networks, etc.

[0143] In the complex-valued double-sideband signal direct detection device provided in the embodiments of the present application, the transfer function of the frequency-selective phase shifter exhibits all-pass amplitude. The frequency-selective phase shifter applies a single-side phase shift to the left-sideband signal or the right-sideband signal of the information-bearing signal. Thus, even if the frequency of the carrier signal drifts towards the side where no phase shift is applied relative to the frequency at the zero point of the phase shifter, it will not affect the detection result. Therefore, it is not necessary to precisely align the operating wavelength of the transmitter laser (equivalent to the frequency of the carrier signal) with the wavelength at the phase mutation point of the phase shifter (the frequency at the zero point of the phase filter), improving the tolerance to the carrier wavelength drift in the complex-valued double-sideband signal. At the same time, since the frequency-selective phase shifter applies a single-side phase shift instead of a double-side phase shift, the number of phase mutations that need to be completed is reduced from two to one, reducing the physical implementation difficulty of the frequency-selective phase shifter and providing more options for the physical implementation of the frequency-selective phase shifter.

[0144] Figure 6 The present application provides a direct detection system for complex-valued double-sideband signals in an embodiment. The direct detection system includes a transmitter and the above-mentioned direct detection device. The transmitter and the direct detection device are connected by an optical fiber. The transmitter is used to emit a complex-valued double-sideband signal to be measured, and the complex-valued double-sideband signal to be measured includes a carrier and an information-bearing signal. The direct detection device is used to receive the complex-valued double-sideband signal to be measured (optical signal) and reconstruct the information-bearing signal in the complex-valued double-sideband signal to be measured. The information-bearing signal in the reconstructed complex-valued double-sideband signal is a digital signal, which is convenient for computer equipment to process.

[0145] It is achievable that in a real-world communication system, noise is inevitable. Therefore, after the complex-valued double-sideband signal to be measured emitted by the transmitter passes through an additive white Gaussian noise channel model, Gaussian white noise will be added to the complex-valued double-sideband signal to be measured to simulate the actual environment of the optical fiber channel.

[0146] It should be noted that the additive white Gaussian noise channel is not an actual physical device but a mathematical model.

[0147] The following specifically analyzes the technical effects that can be achieved in the present application in combination with the above direct detection system.

[0148] The transmitter is set to output a 50 GBaud complex-valued double-sideband 16-QAM (16-QAM) signal. At the zero frequency of the complex-valued double-sideband signal, a 1 Hz guard band is inserted, that is, -0.5 Hz - 0.5 Hz, and the carrier signal power ratio is set to 3 dB. The complex-valued double-sideband 16-QAM signal is introduced with additive white Gaussian noise to simulate the optical fiber channel.

[0149] At the receiving end, an optical signal is converted into an electrical signal by a direct detection device for complex-valued double-sideband signals based on single-sided phase shift. A digital signal processing module is used to realize the optical field reconstruction of the signal. Finally, through the calculation of the bit error rate, the system bit error performance diagram as shown in Figure 7 is obtained. As can be seen from Figure 7 , when the optical signal-to-noise ratio is 24.5 dB, the obtained bit error rate can reach the FEC threshold of 20%. Figure 7 The inset in

[0150] Figure 8 shows the constellation diagram of the recovered signal when the optical signal-to-noise ratio is 36 dB.

[0151] shows a comparison diagram of the tolerance of the carrier wavelength drift in complex-valued double-sideband signals for two schemes of single-sided phase shift and double-sided phase shift adopted by the direct detection device. It can be seen that for the double-sided phase shift scheme, when the FEC threshold is 20%, the tolerance of the carrier wavelength drift in complex-valued double-sideband signals is less than 1 Hz. And when the carrier wavelength drift in complex-valued double-sideband signals is 1 Hz, the bit error rate is extremely high and signal reconstruction can no longer be achieved at all. While for the single-sided phase shift scheme in the embodiment of the present application, when the FEC threshold is 20%, the tolerance of the carrier wavelength drift in complex-valued double-sideband signals is greater than 2 GHz. And when the carrier wavelength drift in complex-valued double-sideband signals is 2 GHz, the bit error rate is still small and signal reconstruction can still be achieved. Therefore, it can be seen that changing the requirement of the phase response of the frequency selective phase shifter from double-sided phase shift to single-sided phase shift in the direct detection device of the embodiment of the present application increases the tolerance of the direct detection device to the carrier wavelength drift in complex-valued double-sideband signals from 0 Hz to more than 2 GHz, improves the tolerance of the direct detection device to the carrier wavelength drift in complex-valued double-sideband signals, and reduces the performance requirements for the transmitter. At the same time, since the double-sided phase shift is changed to single-sided phase shift, the number of phase mutations that the frequency selective phase shifter needs to complete is reduced from two to one, reducing the physical implementation difficulty of the frequency selective phase shifter and providing more choices for the physical implementation of the frequency selective phase shifter.

[0151] In summary, the frequency selective phase shifter described in the embodiment of the present application only needs to apply a single-sided phase shift to the left sideband signal or the right sideband signal of the signal, and does not need to apply double-sided phase shift to both the left and right sidebands simultaneously. For the 20% forward error correction (FEC) threshold, the double-sided phase shift scheme has no tolerance for the carrier wavelength drift in complex-valued double-sideband signals, while in the embodiment of the present application, a tolerance of more than 2 GHz for the carrier wavelength drift in complex-valued double-sideband signals can be achieved. In addition, changing the double-sided phase shift to single-sided phase shift in the embodiment of the present application reduces the number of phase mutations that need to be completed from two to one, reducing the physical implementation difficulty of the frequency selective phase shifter and providing more choices for the physical implementation of the frequency selective phase shifter.

[0152] Figure 9Schematic flowchart of the direct detection method for complex-valued double-sideband signals provided by the embodiments of the present application. The direct detection method is applied to the above direct detection device, and the direct detection method includes the following steps:

[0153] Step S101: Divide the complex-valued double-sideband signal to be measured into two paths.

[0154] Step S102: One path of the complex-valued double-sideband signal to be measured passes through a first photodetector and a first analog-to-digital converter to obtain a first digital signal of the complex-valued double-sideband signal to be measured.

[0155] Step S103: The other path of the complex-valued double-sideband signal to be measured first applies a phase shift to the left sideband signal or the right sideband signal of the information-bearing signal through a frequency-selective phase shifter.

[0156] Step S104: After passing through a second photodetector and a second analog-to-digital converter, a second digital signal of the complex-valued double-sideband signal to be measured after the phase shift is obtained.

[0157] Step S105: Obtain the reconstructed information-bearing signal according to the first digital signal and the second digital signal.

[0158] The complex-valued double-sideband signal to be measured includes a carrier and an information-bearing signal. The information-bearing signal is a complex signal, including an in-phase component and a quadrature component. The information-bearing signal includes a left sideband signal and a right sideband signal. In the frequency domain, the carrier is placed between the left sideband signal and the right sideband signal.

[0159] The first optical coupler receives the input complex-valued double-sideband signal to be measured and divides the complex-valued double-sideband signal to be measured into two paths. The information carried by the optical signals after being divided into two paths (which can be referred to as the first optical signal and the second optical signal in the present application) is approximately the same, but the intensities of the optical signals after being divided into two paths may be different.

[0160] The first optical signal is input into the first photodetector for square-law detection to convert the first optical signal into a first electrical signal, and the first analog-to-digital converter converts the first electrical signal into a first digital signal.

[0161] The second optical signal is input into the frequency-selective phase shifter, and a single-side phase shift is applied to the left sideband signal or the right sideband signal of the information-bearing signal through the frequency-selective phase shifter. Then, after passing through the second photodetector, the other path of the complex-valued double-sideband signal after the phase shift is converted into a second electrical signal, and the second analog-to-digital converter converts the second electrical signal into a second digital signal. Then, the digital signal processing module obtains the reconstructed information-bearing signal according to the first digital signal and the second digital signal.

[0162] In the direct detection method of the complex-valued double-sideband signal provided in the embodiments of the present application, a frequency-selective phase shifter is used to apply a single-side phase shift to the left-sideband signal or the right-sideband signal of the information-bearing signal. In this way, even if the frequency of the carrier signal drifts to the side where no phase shift is applied relative to the frequency at the zero point of the phase filter, it will not affect the detection result. Therefore, it is not necessary to accurately align the operating wavelength of the transmitter laser (equivalent to the frequency of the carrier signal) with the wavelength at the phase mutation of the phase filter (the frequency at the zero point of the phase filter), which improves the tolerance to the carrier wavelength drift in the complex-valued double-sideband signal. At the same time, since the frequency-selective phase shifter applies a single-side phase shift instead of a double-side phase shift, the number of phase mutations to be completed is reduced from two to one, which reduces the physical implementation difficulty of the frequency-selective phase shifter and provides more options for the physical implementation of the frequency-selective phase shifter.

[0163] Among them, the transfer function of the frequency-selective phase shifter shows amplitude all-pass, and the phase response of the frequency-selective phase shifter is to apply an adjustable single-side phase shift to the left-sideband signal or the right-sideband signal of the information-bearing signal.

[0164] It is achievable that the phase response of the frequency-selective phase shifter is to apply an optimal phase shift angle to the left-sideband signal or the right-sideband signal of the information-bearing signal. Among them, the optimal phase shift angle is obtained by continuously changing the phase shift angle to obtain the corresponding bit error rate, and the phase shift angle corresponding to the minimum bit error rate is the optimal phase shift angle.

[0165] It is achievable that a guard interval is reserved between the left sideband and the right sideband.

[0166] It is achievable that the information-bearing signal is obtained according to the first digital signal and the second digital signal, including: performing optical field reconstruction on the first digital signal and the second digital signal through deep learning technology to obtain the reconstructed information-bearing signal.

[0167] It should be noted that the direct detection method of the complex-valued double-sideband signal in the embodiments of the present application is applied to the above-mentioned direct detection device of the complex-valued double-sideband signal, and the technical details in the above-mentioned direct detection device of the complex-valued double-sideband signal can also be applied to this embodiment. For specific details, reference can be made to the content of the above-mentioned embodiments, and no repeated description will be given here.

[0168] It should be understood that the above examples are for helping those skilled in the art to understand the embodiments of the present application, rather than limiting the embodiments of the present application to the specific numerical values or specific scenarios illustrated. Those skilled in the art can obviously make various equivalent modifications or changes according to the above examples, and such modifications or changes also fall within the scope of the embodiments of the present application.

[0169] Those skilled in the art can clearly understand that for the convenience and conciseness of description, the specific working processes of the systems, devices, and units described above can refer to the corresponding processes in the foregoing method embodiments and will not be elaborated herein.

[0170] In several embodiments provided in the embodiments of the present application, it should be understood that the disclosed systems, devices, and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the units is only a logical function division. In actual implementation, there may be other division methods. For example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the displayed or discussed couplings or direct couplings or communication connections between each other can be through some interfaces. The indirect couplings or communication connections of the devices or units can be in electrical, mechanical, or other forms.

[0171] The units described as separate components may or may not be physically separated. The components displayed as units may or may not be physical units. They can be located in one place or distributed to multiple network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0172] In addition, in each embodiment of the embodiments of the present application, each functional unit can be integrated in a processing unit, or each unit can exist physically alone, or two or more units can be integrated in one unit.

[0173] If the function is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the embodiments of the present application, in essence, or the part that contributes to the prior art, or part of this technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for causing a memory (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in the embodiments of the present application. The foregoing storage medium includes: various media such as USB flash drives, mobile hard disks, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical discs that can store program codes.

[0174] As described above, it is only the specific implementation manner of the embodiments of the present application. However, the protection scope of the embodiments of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the embodiments of the present application can easily think of changes or substitutions, which should all be covered within the protection scope of the embodiments of the present application. Therefore, the protection scope of the embodiments of the present application shall be subject to the protection scope of the claims.

Claims

1. A direct detection method for complex-valued double-sideband signals, characterized in that: The method comprises: Divide the complex-valued double-sideband signal to be measured into two paths, wherein the complex-valued double-sideband signal to be measured includes a carrier and an information-bearing signal, wherein the information-bearing signal includes a left-side band signal and a right-side band signal, and in the frequency domain, the carrier is located between the left-side band signal and the right-side band signal; One of the complex-valued double-sideband signals to be measured is passed through a first photodetector and a first analog-to-digital converter to obtain a first digital signal of the complex-valued double-sideband signal to be measured; The other path of the complex-valued double-sideband signal to be measured first applies an adjustable unilateral phase shift to the left-side band signal or the right-side band signal of the information-bearing signal through a frequency selective phase shifter, and then passes through a second photodetector and a second analog-to-digital converter to obtain a second digital signal of the complex-valued double-sideband signal to be measured after the phase shift; The reconstructed information-bearing signal is obtained according to the first digital signal and the second digital signal, and the phase shift in the calculation formula for reconstructing the information-bearing signal is the same as the adjustable unilateral phase shift and changes with the change of the adjustable unilateral phase shift.

2. The method according to claim 1, characterized in that The transfer function of the frequency selective phase shifter is amplitude full pass, and the phase response of the frequency selective phase shifter is to apply an optimal phase shift angle to the left band signal or the right band signal of the information carrying signal, wherein the optimal phase shift angle is obtained by continuously changing the phase shift angle to obtain a corresponding bit error rate, and when the bit error rate is minimized, the corresponding phase shift angle is the optimal phase shift angle.

3. The method according to claim 1 or 2, characterized in that: A guard interval is reserved between the left band signal and the right band signal.

4. The method according to claim 1 or 2, characterized in that: Obtaining the information bearing signal according to the first digital signal and the second digital signal comprises: Light field reconstruction is performed according to the first digital signal and the second digital signal through deep learning technology to obtain the reconstructed information-bearing signal.

5. A direct detection device for complex-valued double-sideband signals, characterized in that: The device comprises: A first optical coupler, a first photodetector and a first analog-to-digital converter connected to the output end of the first optical coupler and connected in sequence; a frequency selective phase shifter, a second photodetector and a second analog-to-digital converter connected to the output end of the first optical coupler and connected in sequence, and a digital signal processing module connected to the output end of the first analog-to-digital converter and the output end of the second analog-to-digital converter; The first optical coupler is used to divide the input complex-valued double-sideband signal to be measured into two paths, the complex-valued double-sideband signal to be measured includes a carrier and an information-bearing signal, the information-bearing signal includes a left-side band signal and a right-side band signal, and in the frequency domain, the carrier is located between the left-side band signal and the right-side band signal; The first photodetector is used to obtain and convert one of the complex-valued double-sideband signals to be measured into a first electrical signal, and the first analog-to-digital converter is used to convert the first electrical signal into a first digital signal; The frequency selective phase shifter is used to apply an adjustable unilateral phase shift to the left band signal or the right band signal of the information-bearing signal in the other channel of the complex-valued double-sideband signal to be measured, the second photodetector is used to convert the other channel of the complex-valued double-sideband signal to be measured after the phase shift is applied to a second electrical signal, and the second analog-to-digital converter is used to convert the second electrical signal into a second digital signal; The digital signal processing module is used to obtain a reconstructed information-bearing signal based on the first digital signal and the second digital signal, and the phase shift in the calculation formula for reconstructing the information-bearing signal is the same as the adjustable unilateral phase shift and changes with the change of the adjustable unilateral phase shift.

6. The device according to claim 5, characterized in that The frequency selective phase shifter is one of a waveform shaper, a silicon photonic integrated high-resolution all-pass phase filter, and a high-quality factor silicon nitride microring resonator.

7. The device according to any one of claims 5 to 6, characterized in that The transfer function of the frequency selective phase shifter is amplitude full pass, and the phase response of the frequency selective phase shifter is to apply an optimal phase shift angle to the left band signal or the right band signal of the information carrying signal, wherein the optimal phase shift angle is obtained by continuously changing the phase shift angle to obtain a corresponding bit error rate, and when the bit error rate is minimized, the corresponding phase shift angle is the optimal phase shift angle.

8. A signal detection system, characterized in that: include: A transmitter, used for transmitting a complex-valued double-sideband signal to be measured, wherein the complex-valued double-sideband signal to be measured includes a carrier and an information-bearing signal; A direct detection device for complex-valued double-sideband signals as claimed in any one of claims 5 to 7 connected to the transmitter.

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