Light and millimeter wave fusion two-way transmission system and method based on optical heterodyne

Abstract

The invention relates to a light and millimeter wave fusion two-way transmission system and method based on optical heterodyne, and belongs to the technical field of space optical communication. According to the invention, a single-sideband optical signal is generated by means of an optical heterodyne method, a single-sideband light source with carrier waves is provided for a spatial optical link and a millimeter wave link, and multiplexing of related devices is realized between transmitting ends of the two links, between receiving ends of the two links and between a receiving end of the spatial optical link and a transmitting end of the millimeter wave link. And the structural complexity of the fused bidirectional system is reduced. The transmission system can be divided into an isomorphic two-way transmission mode and a heterogeneous two-way transmission mode, in the isomorphic two-way transmission mode, the photoelectric detector is subjected to time division multiplexing by a spatial optical link and a millimeter wave link, and in the heterogeneous two-way transmission mode, two paths of optical signals sharing the photoelectric detector are completely overlapped in a frequency domain. And decomposition of two paths of signals is realized through a prior information method. According to the invention, the complexity of the system structure is reduced through the device multiplexing design, and the development of fusion transmission in space-based and air-based networks is promoted.

Description

Technical Field

[0001] The present invention belongs to the technical field of space optical communication, and relates to a light and millimeter wave fusion bidirectional transmission system and method based on optical heterodyne. Background Art

[0002] With the rise of data services that require a lot of bandwidth, such as cloud computing, online video, and virtual reality, users' demand for high-speed wireless connections is growing exponentially, while existing traditional wireless networks are gradually unable to meet the increasing frequency band requirements. In this context, free space optical (FSO) communication and millimeter wave communication have shown great development potential. FSO communication transmits information in free space through lasers, and can achieve high-speed and secure data transmission without spectrum licenses. Therefore, it is widely regarded as an important way to promote the development of satellite Internet and has attracted the attention of academia and industry. On the other hand, with the increasing shortage of radio spectrum resources, millimeter wave communication provides greater bandwidth and brings opportunities for the upgrade of mobile communication systems. Therefore, it is generally regarded as a key technology for expanding the capacity of next-generation mobile networks. The development of optical millimeter wave generation technology has not only promoted the development of millimeter wave communication systems towards lightweight and high bandwidth, but also created the possibility of combining FSO with millimeter waves. Among them, the optical heterodyne method has the advantages of avoiding the influence of light wave dispersion and being able to flexibly generate millimeter wave electrical signals of various frequencies. At the same time, the characteristic of the optical heterodyne method that does not require the introduction of a radio frequency source when generating millimeter wave electrical signals is more conducive to the advancement of system lightweighting, and has become one of the research hotspots in recent years.

[0003] In different application scenarios, FSO and millimeter wave are both effective wireless communication solutions with high capacity, and they have complementary advantages in terms of channel characteristics. In FSO systems, optical signals are easily affected by atmospheric turbulence when passing through the atmosphere, and fog is also one of the main factors affecting the quality of FSO signal transmission. In contrast, millimeter wave signals are less sensitive to atmospheric turbulence and fog, so they are more stable in this regard, but millimeter wave electrical signals are more significantly affected by raindrops. The complementary channel characteristics of these two technologies have prompted them to be integrated in practical applications to form a more extensive and efficient communication solution, thereby improving the overall performance and reliability of the system. At present, the device functions of optical and millimeter wave links at the transmitting and receiving ends in traditional fusion transmission systems are repeated, and the structural integration is low, which is not conducive to the application of fusion transmission in space-based and air-based networks. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a bidirectional transmission system and method for optical and millimeter wave fusion based on optical heterodyne, which generates a single-sideband optical signal by means of an optical heterodyne method, provides a single-sideband with a carrier for a spatial optical link and a millimeter wave link, and realizes the multiplexing of related devices between the transmitting ends and the receiving ends of the two links, as well as between the receiving end of the spatial optical link and the transmitting end of the millimeter wave link, thereby reducing the structural complexity of the fused bidirectional system.

[0005] To achieve the above objectives, one aspect of the present invention provides a bidirectional transmission system for optical and millimeter wave fusion based on optical heterodyne, the system comprising a transmission link in a first direction and a transmission link in a second direction, and the transmission links in the two directions have the same link structure.

[0006] Among them, the link structure includes a signal transmitting end and a signal receiving end; the signal transmitting end includes a signal modulation unit for generating a single-sideband optical signal, a millimeter wave generating unit for generating a millimeter wave electrical signal, and an optical switch, one end of the optical switch is connected to the signal modulation unit, and the other end is used to select a spatial optical antenna or a millimeter wave generating unit; the signal receiving end includes an electrical switch, and a demodulation unit connected to the electrical switch for demodulating the millimeter wave electrical signal.

[0007] In the transmission link in the first direction, the signal transmitting end selects the spatial optical link or the millimeter wave link for signal transmission through the optical switch, and the millimeter wave generating unit of the signal transmitting end is connected to the electrical switch of the receiving end in the transmission link in the second direction;

[0008] In the transmission link in the second direction, the signal transmitting end selects the spatial optical link or the millimeter wave link for signal transmission through an optical switch, and the millimeter wave generating unit of the signal transmitting end is connected to the electrical switch of the receiving end in the transmission link in the first direction.

[0009] Furthermore, the signal modulation unit includes a pulse generator, a Mach-Zehnder modulator, a first laser source and a second laser source. The pulse generator receives a source codeword and generates a pulse signal; the Mach-Zehnder modulator receives the pulse signal and the optical signal output by the first laser source to load information; the optical signal output by the second laser source is frequency-domain superimposed with the optical signal output by the Mach-Zehnder modulator to form a single-sideband optical signal. The first laser source and the second laser source output optical signals of different wavelengths.

[0010] Furthermore, the millimeter wave generating unit includes a combiner, a photodetector and a bandpass filter. The input end of the combiner is connected to the optical switch to receive the single-sideband optical signal output by the signal modulation unit; the input end of the photodetector is connected to the output end of the combiner; the input end of the bandpass filter is connected to the output end of the photodetector, and the output end of the bandpass filter outputs a millimeter wave electrical signal. In addition, the input end of the combiner in the first direction transmission link also receives an optical signal transmitted from the space optical link of the second direction transmission link; the input end of the combiner in the second direction transmission link also receives an optical signal transmitted from the space optical link of the first direction transmission link.

[0011] Furthermore, the demodulation unit includes a square law detector, a low pass filter and a decision device. The input end of the square law detector is connected to the electric switch; the input end of the low pass filter is connected to the output end of the square law detector; the input end of the decision device is connected to the output end of the low pass filter, and the decision device performs digital signal processing (DSP) on the input signal to demodulate the source signal.

[0012] Another aspect of the present invention provides a bidirectional transmission method of optical and millimeter wave fusion based on optical heterodyne, which reduces the use of devices and reduces the volume of the transmission system by multiplexing devices in the spatial optical link and millimeter wave link in the first direction and the spatial optical link and millimeter wave link in the second direction. The specific design is as follows:

[0013] At the transmitting end of the transmission link in any direction, a signal modulation unit is multiplexed into an optical signal generating unit of the spatial optical link and the millimeter wave link in that direction, so as to generate a single sideband optical signal;

[0014] At the receiving end of the transmission link in any direction, a demodulation unit is multiplexed as the receiving end of the spatial optical link and the millimeter wave link in that direction, and the demodulation unit selects to receive the millimeter wave electrical signal transmitted by the millimeter wave link in that direction or the spatial optical signal transmitted by the spatial optical link in another direction through an electrical switch;

[0015] In both the first direction and the second direction transmission links, a millimeter wave generating unit is provided, and the millimeter wave generating unit is multiplexed as the receiving end of the spatial optical link and the transmitting end of the millimeter wave link. In any direction, for example, in the first direction, the millimeter wave generating unit receives the single sideband optical signal of the signal modulation unit in the first direction through an optical switch, and at the same time, receives the spatial optical signal transmitted from the second direction spatial optical link and converts it into a millimeter wave electrical signal, and then transmits it to the demodulation unit of the receiving end of the second direction for signal demodulation.

[0016] Through the optical switch and the electrical switch, the first and second transmission directions can use the same type of link or different types of links for transmission. When the two transmission directions use the same type of link (spatial optical link or millimeter wave link), the system is in a homogeneous bidirectional transmission mode. Since the two links will not work at the same time, the photodetector and the bandpass filter can be time-division multiplexed between the two links. When the two transmission directions use different types of links (i.e., one direction is a spatial optical link and the other direction is a millimeter wave link), the system is in a heterogeneous bidirectional transmission mode.

[0017] In the heterogeneous bidirectional transmission mode, the millimeter wave generating units in both directions will simultaneously receive the single-sideband optical signal from the signal modulation unit and the spatial optical signal from the spatial optical link. In order to avoid signal crosstalk caused by the multiplexing of photodetectors and bandpass filters in the two links, the two signals need to be decoupled using the prior information method. In the heterogeneous bidirectional transmission mode, the wavelength difference between the first laser source and the second laser source in the signal modulation units in the first and second directions is the same.

[0018] The specific way to avoid signal crosstalk is:

[0019] The optical field of a single sideband optical signal in a spatial optical link and the optical field of a single sideband optical signal in a millimeter wave link are defined as:

[0020]

[0021] Where, S1(t), ω1, ω2, θ1(t) are the amplitude information of the left band of E1, the carrier frequency of the left band, the carrier frequency of the right band, and the phase information of the left band. S2(t), ω1, ω2, θ2(t) are the amplitude information of the left band of E2, the carrier frequency of the left band, the carrier frequency of the right band, and the phase information of the left band.

[0022] The superimposed optical signal enters the photodetector and generates a beat frequency. The electrical signal generated after the beat frequency is:

[0023] I=2[S1(t)+S2(t)] 2 +[S1(t)+S2(t)] 2 cos[(ω11-ω2)t]

[0024] Among them, the first half on the right side of the equal sign is the generated baseband electrical signal, and the second half is the millimeter-wave electrical signal with mixed information. After passing through a bandpass filter with a center frequency of |ω1-ω2|, the millimeter-wave electrical signal with mixed information at a center frequency of |ω1-ω2| can be retained:

[0025] I hybrid-MMW =[S1(t)+S2(t)] 2cos[(ω1-ω2)t]

[0026] The mixed millimeter wave electrical signal obtained by the bandpass filter is divided into two parts, one part is processed by DSP at the receiving end of the millimeter wave link, and the other part is transmitted at the transmitting end of the spatial optical link. According to the expression of the mixed millimeter wave electrical signal, there is a certain relationship between the amplitude of the mixed millimeter wave electrical signal and the amplitude of the optical signal in the spatial optical link and the millimeter wave link. The mixed information is separated by the prior information method, and the information of the signals in the two links is restored respectively:

[0027]

[0028] Among them, S konwn (t) is a known signal on one side of the two-way system, and H(t) is a channel response obtained by channel estimation based on the known signal.

[0029] The beneficial effects of the present invention are as follows: the present invention uses two laser sources with different wavelengths to generate single-sideband optical signals and uses MZM to modulate the signal on light, so that the space optical link and the millimeter wave link can reuse laser sources and MZM and other devices at the transmitting end. At the receiving end, the space optical link and the millimeter wave link will perform a series of processing such as square-law detection, low-pass filtering and symbol judgment, so the relevant electrical domain devices can be reused. The present invention can reuse photodetectors and bandpass filters in the time domain and frequency domain for the receiving end of the space optical link and the transmitting end of the millimeter wave link. The present invention solves the problem of repeated device functions and low structural integration at the transmitting and receiving ends of the optical and millimeter wave links in traditional hybrid transmission systems, which is not conducive to the development of space and air network communications.

[0030] Other advantages, objectives and features of the present invention will be described in the following description to some extent, and to some extent, will be obvious to those skilled in the art based on the following examination and study, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below in conjunction with the accompanying drawings, wherein:

[0032] Figure 1 A structural diagram of a bidirectional transmission system for optical and millimeter wave fusion based on optical heterodyne provided in one embodiment of the present invention;

[0033] Figure 2 It is a traditional bidirectional transmission system structure of light and millimeter wave. Figure 2 (a) is the FSO link, Figure 2(b) is a millimeter wave link;

[0034] Figure 3 Schematic diagram of the average bit error rate of the FSO link receiving optical power change under heterogeneous bidirectional transmission;

[0035] Figure 4 Schematic diagram of the average bit error rate of the millimeter wave link signal-to-noise ratio change under heterogeneous bidirectional transmission;

[0036] Figure 5 Schematic diagram of the average bit error rate of the FSO link receiving optical power change under homogeneous bidirectional transmission;

[0037] Figure 6 Schematic diagram of the average bit error rate of the millimeter wave link signal-to-noise ratio change under homogeneous bidirectional transmission; DETAILED DESCRIPTION

[0038] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0039] like Figure 2 As shown in FIG, the traditional optical and millimeter wave bidirectional transmission system is mainly composed of FSO links and millimeter wave links. Figure 2 (a) shows the FSO link in a bidirectional transmission system. Figure 2 (b) shows the mmWave link in a conventional bidirectional transmission system. Figure 2 (a) Deployment in the forward and backward directions can form a traditional FSO bidirectional transmission system. Figure 2 (b) Deployment in the forward and backward directions can form a traditional millimeter wave bidirectional transmission system. However, in these traditional hybrid transmission systems, the functions of the optical and millimeter wave links at the transmitting and receiving ends are repeated, and the structural integration is low, which is not conducive to the challenges of the development of space and air network communications.

[0040] Therefore, the present invention proposes a bidirectional transmission system that combines light and millimeter waves based on optical heterodyne, which multiplexes related devices between the transmitting ends of the two links of spatial light and millimeter waves, between the receiving ends of the two links, and between the receiving end of the spatial light link and the transmitting end of the millimeter wave link, thereby reducing the structural complexity of the fused bidirectional system. The fused bidirectional system constructed based on the three-level multiplexing design can be further divided into homogeneous bidirectional transmission and heterogeneous bidirectional transmission according to whether the link types used for bidirectional transmission are the same, so as to meet specific usage purposes. In the case of homogeneous bidirectional transmission, the photodetector is shared in the time domain. In the case of heterogeneous bidirectional transmission, the two optical signals of the shared photodetector completely overlap in the frequency domain, and can be decomposed by the method of prior information.

[0041] Specifically, Figure 1 The figure shows a bidirectional transmission system for fusion of light and millimeter wave provided by an embodiment of the present invention, in which the transmitting end of the space optical link and the transmitting end of the millimeter wave link multiplex the laser source and MZM and other devices; the receiving ends of the two links multiplex the devices such as square law detection, low-pass filtering and symbol decision; the receiving end of the space optical link and the transmitting end of the millimeter wave link multiplex the photodetector and bandpass filter. In the homogeneous bidirectional transmission mode, since the space optical link and the millimeter wave link will not work at the same time in the two transmission directions, the two links can perform time division multiplexing on the photodetector and the bandpass filter. In the heterogeneous bidirectional transmission mode, the single sideband optical signals of the space optical link and the millimeter wave link pass through the photodetector at the same time, and are completely aliased in the frequency domain. Since in the heterogeneous bidirectional transmission mode, one side uses the integration of the millimeter wave transmitting end and the space optical receiving end, and the other end uses the integration of the millimeter wave receiving end and the space optical transmitting end, and the transmitting ends on both sides record the transmitted symbol information, the receiving ends on both sides can use the prior characteristics of the symbol information to extract the effective signal from the aliased signal of the photodetector.

[0042] According to the parameter settings in Table 1, an optical and millimeter-wave fusion bidirectional transmission system based on optical heterodyne, a traditional optical and millimeter-wave hybrid bidirectional transmission system, an FSO bidirectional transmission system, and a millimeter-wave bidirectional transmission system were constructed through experiments, and the system performances were compared and analyzed.

[0043] Table 1

[0044]

[0045] Figure 3The figure shows the average bit error rate of the FSO link of the two bidirectional transmission systems in the heterogeneous bidirectional transmission mode as the received optical power changes. It can be observed that with the increase of the received optical power, the average bit error rate of the FSO link of the two systems shows a gradually decreasing trend. In the fused bidirectional transmission system proposed in the present invention, when the received optical power is about -6.4dBm, the average bit error rate reaches the FEC threshold value; while in the traditional bidirectional transmission system, the average bit error rate also touches the FEC threshold when the atmospheric attenuation reaches -6.8dBm. Under the same received optical power conditions, the average bit error rate of the FSO link of the two bidirectional transmission systems is slightly different. Figure 4 The figure shows the average bit error rate of the MMW link of the two bidirectional transmission systems in the heterogeneous bidirectional transmission mode as the signal-to-noise ratio changes. When the signal-to-noise ratio of the fusion bidirectional transmission system proposed in the present invention is about 18.8dB, the average bit error rate reaches the FEC threshold value; while the traditional bidirectional transmission system reaches the corresponding FEC threshold value when the signal-to-noise ratio is about 18.3dB. Under the same signal-to-noise ratio, the average bit error rate of the MMW link of the two systems shows relatively consistent characteristics. In addition, Figure 5 The average bit error rate of the FSO link in the two systems is shown as the received optical power changes under homogeneous bidirectional transmission. Figure 5 It can be seen that when the received optical power changes, the average bit error rate change trends of the FSO links of the two systems are basically the same. Figure 6 The average bit error rate of the MMW link in the two systems under homogeneous bidirectional transmission as the signal-to-noise ratio changes is presented. It can be seen that under the same signal-to-noise ratio conditions, the average bit error rate of the MMW link in the two systems shows similar characteristics. The above results show that the system structure of the present invention can realize homogeneous bidirectional transmission and heterogeneous bidirectional transmission while saving a large number of devices and maintaining performance close to that of the traditional system.

[0046] In summary, the present invention uses an FSO analog transmission system and an optically generated millimeter wave system that generate single sideband by optical heterodyne to multiplex related devices between the transmitting ends of the two links, between the receiving ends of the two links, and between the receiving end of the spatial optical link and the transmitting end of the millimeter wave link, thereby reducing the structural complexity of the fused bidirectional system. The fused bidirectional system constructed based on the three-level multiplexing design can be further divided into homogeneous bidirectional transmission and heterogeneous bidirectional transmission according to whether the link types used for bidirectional transmission are the same, so as to meet specific usage purposes. In the case of homogeneous bidirectional transmission, the photodetector is shared in the time domain. In the case of heterogeneous bidirectional transmission, the two optical signals of the shared photodetector completely overlap in the frequency domain, and can be decomposed by the method of prior information.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A bidirectional transmission system based on optical heterodyne fusion of light and millimeter wave, characterized in that: The system includes a transmission link in a first direction and a transmission link in a second direction, and the transmission links in the two directions have the same link structure; The link structure includes a signal transmitting end and a signal receiving end; the signal transmitting end includes a signal modulation unit for generating a single-sideband optical signal, a millimeter wave generating unit for generating a millimeter wave electrical signal, and an optical switch, one end of the optical switch is connected to the signal modulation unit, and the other end is used to select a spatial optical antenna or a millimeter wave generating unit; the signal receiving end includes an electrical switch, and a demodulation unit connected to the electrical switch for demodulating the millimeter wave electrical signal; In the transmission link in the first direction, the signal transmitting end selects the spatial optical link or the millimeter wave link for signal transmission through an optical switch, and the millimeter wave generating unit of the signal transmitting end is connected to the electrical switch of the receiving end in the transmission link in the second direction; In the transmission link in the second direction, the signal sending end selects the spatial optical link or the millimeter wave link for signal transmission through an optical switch, and the millimeter wave generating unit of the signal sending end is connected to the electrical switch of the receiving end in the transmission link in the first direction.

2. The system according to claim 1, characterized in that The signal modulation unit includes a pulse generator, a Mach-Zehnder modulator, a first laser source and a second laser source; the pulse generator receives a source codeword and generates a pulse signal; the Mach-Zehnder modulator receives the pulse signal and the optical signal output by the first laser source to load information; the optical signal output by the second laser source is superimposed in the frequency domain with the optical signal output by the Mach-Zehnder modulator to form a single-sideband optical signal; wherein the first laser source and the second laser source output optical signals of different wavelengths.

3. The system according to claim 1, characterized in that The millimeter wave generating unit includes a combiner, a photodetector and a bandpass filter; the input end of the combiner is connected to the optical switch to receive the single-sideband optical signal output by the signal modulation unit; the input end of the photodetector is connected to the output end of the combiner; the input end of the bandpass filter is connected to the output end of the photodetector, and the output end of the bandpass filter outputs a millimeter wave electrical signal; The input end of the combiner in the first direction transmission link also receives an optical signal transmitted by the space optical link of the second direction transmission link; the input end of the combiner in the second direction transmission link also receives an optical signal transmitted by the space optical link of the first direction transmission link.

4. The system according to claim 1, characterized in that The demodulation unit includes a square-law detector, a low-pass filter and a decision device; the input end of the square-law detector is connected to the electric switch; the input end of the low-pass filter is connected to the output end of the square-law detector; the input end of the decision device is connected to the output end of the low-pass filter, and the output end of the decision device outputs a demodulated signal.

5. A method for bidirectional transmission of light and millimeter wave fusion based on optical heterodyne, characterized in that: The method reduces the use of devices and reduces the volume of the transmission system by multiplexing devices in a spatial optical link and a millimeter wave link in a first direction and in a spatial optical link and a millimeter wave link in a second direction, including: At the transmitting end of the transmission link in any direction, a signal modulation unit is multiplexed into an optical signal generating unit of the spatial optical link and the millimeter wave link in that direction, so as to generate a single sideband optical signal; At the receiving end of the transmission link in any direction, a demodulation unit is multiplexed as the receiving end of the spatial optical link and the millimeter wave link in that direction, and the demodulation unit selects to receive the millimeter wave electrical signal transmitted by the millimeter wave link in that direction or the spatial optical signal transmitted by the spatial optical link in another direction through an electrical switch; In both the first direction and the second direction transmission links, a millimeter wave generating unit is provided, and the millimeter wave generating unit is multiplexed as the receiving end of the spatial optical link and the transmitting end of the millimeter wave link; in either direction, the millimeter wave generating unit receives the single sideband optical signal of the signal modulation unit in that direction through an optical switch, and at the same time, receives the spatial optical signal transmitted from the spatial optical link in the other direction and converts it into a millimeter wave electrical signal, which is then transmitted to the demodulation unit at the receiving end in the other direction for signal demodulation.

6. The method according to claim 5, characterized in that In the signal modulation unit, a laser source is used to generate an optical signal, and a Mach-Zehnder modulator is used to load information; another laser source with a different wavelength is used to generate an unmodulated optical signal, and the unmodulated optical signal and the optical signal loaded with information are superimposed in the frequency domain to form a single-sideband optical signal.

7. The method according to claim 5, characterized in that The millimeter wave generating unit includes a combiner, a photodetector and a bandpass filter; the input end of the combiner is connected to the optical switch to receive the single-sideband optical signal output by the signal modulation unit; the input end of the photodetector is connected to the output end of the combiner; the input end of the bandpass filter is connected to the output end of the photodetector, and the output end of the bandpass filter outputs a millimeter wave electrical signal; wherein the combiner simultaneously receives a spatial optical signal transmitted from a spatial optical link.

8. The method according to any one of claims 5 to 7, characterized in that: When one direction is transmitted through a spatial optical link and the other direction is transmitted through a millimeter wave link, the millimeter wave generating units in both directions will simultaneously receive the single-sideband optical signal from the signal modulation unit and the spatial optical signal from the spatial optical link; the single-sideband optical signal and the spatial optical signal are superimposed by the combiner. At this time, the two signals need to be decoupled through the prior information method to avoid signal crosstalk.

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