CN110177067B

CN110177067B - Orbital angular momentum index modulation transmission system and method suitable for long-distance transmission

Info

Publication number: CN110177067B
Application number: CN201910411329.XA
Authority: CN
Inventors: 张超; 赵宇飞
Original assignee: Tsinghua University
Current assignee: Tsinghua University
Priority date: 2019-05-17
Filing date: 2019-05-17
Publication date: 2020-09-22
Anticipated expiration: 2039-05-17
Also published as: CN110177067A

Abstract

The invention discloses an orbital angular momentum index modulation transmission system and method suitable for long-distance transmission. At a transmitting end, the transmission system utilizes orbital angular momentum electromagnetic waves of different modes to carry out combination selection to form different mode combinations, and user bit information is transmitted by amplitude-phase modulation and orbital angular momentum electromagnetic wave mode combination index modulation respectively; at a receiving end, on a part of phase planes, judging which group of orbit angular momentum mode combination is received by a phase difference index mapping method, thereby completing the mapping from the electromagnetic wave orbit angular momentum mode combination to the transmitted user bit, and simultaneously recovering the other part of bit information by an amplitude-phase demodulation method, and finally recovering the original information by combining the bit information recovered by the phase difference index mapping. The invention can not only improve the frequency spectrum efficiency of a communication system, but also realize the long-distance partial phase surface receiving of the orbit angular momentum electromagnetic wave with reliability and low complexity.

Description

Orbital angular momentum index modulation transmission system and method suitable for long-distance transmission

Technical Field

The invention relates to the technical field of radio electromagnetic wave communication, in particular to a system and a method for modulating and transmitting an index of orbital angular momentum suitable for long-distance transmission.

Background

The electromagnetic wave Orbital Angular Momentum (OAM) is an inherent property of an electromagnetic wave and is another important physical quantity different from the conventional electromagnetic wave frequency, electric field intensity, and the like. The electromagnetic wave having OAM is also called "vortex electromagnetic wave", and the phase plane is not perpendicular to the propagation direction and shows a spiral distribution. The conventional plane wave phase plane that is usually employed is perpendicular to the propagation direction and therefore does not have OAM. The electromagnetic wave with OAM is obviously different from the conventional plane electromagnetic wave, so that people can develop the electromagnetic wave from a new dimension, and the electromagnetic wave has great value in the fields of information transmission and target detection by applying the electromagnetic wave.

In recent years, research on OAM in the field of low-frequency electromagnetic waves is receiving more and more attention from scholars at home and abroad. In 2007, B, Thid é et al verified that low frequency electromagnetic waves (frequencies below 1GHz) can also generate OAM by numerical simulation of the antenna array. The analysis of the existing electromagnetic wave OAM transmission mode can discover that: as the OAM number increases, the OAM electromagnetic wave divergence angle increases, and the electromagnetic wave beam emitted from the first transmitting/receiving antenna becomes cone-shaped. The longer the transmission distance, the more divergent the beam. In 2012, F, tambourini et al realized a 442m long-distance OAM point-to-point transmission experiment for the first time, and proved that OAM has the potential of improving the spectrum efficiency of a communication system through long-distance multiplexing transmission. The adopted receiving method is two-point difference, namely: phase gradient method. However, the phase gradient method requires that both reception points are located on the same circle perpendicular to the propagation axis, and the center of the circle coincides with the propagation axis. Phase misalignment will reduce the accuracy of OAM mode detection. Moreover, the detection accuracy decreases with decreasing opening angle between the two receiving antennas, namely: when the distance between the two receiving antennas is fixed, the detection accuracy decreases as the transmission distance increases. In 2016 and 12 months, 27.5 km long-distance transmission experiments are completed in an aerospace electronic system laboratory of Qinghua university, in 2018, the subject group further extends the transmission distance to 172 km, and finally four-way four-mode OAM multiplexing transmission is realized.

For review, the previous researches have found that the research results on the transmission of OAM electromagnetic waves in a long distance in a microwave band are few. The OAM wave beam is in an inverted cone shape, the farther the distance is, the larger the annular phase surface is, the more difficult the coaxial full phase surface is to receive, for example, the diameter of the phase surface can even reach several kilometers when the coaxial full phase surface is transmitted over 100 kilometers. At this time, in the actual engineering, only partial phase plane receiving method can be adopted, and the partial phase plane receiving can not obtain the large-amplitude capacity gain caused by OAM dimension orthogonality, and only can obtain an extra channel caused by non-orthogonal OAM multiplexing with small capacity. Therefore, there is still a need to further improve the modulation and detection methods of OAM signal transmission. In addition, the "long distance" in the "long distance transmission" referred to in the present invention generally means a straight distance of more than 100 meters, and the long distance transmission is suitable for long distance wireless communication, mobile relay, microwave relay, vehicle-mounted communication, air-ground communication, satellite communication, space communication, and the like.

Disclosure of Invention

The OAM channel exists as a new dimension, and a novel data chain suitable for receiving the phase plane of the long-distance transmission part can be constructed by combining index modulation. In addition, at a receiving end, because only OAM information on part of phase planes can be received, a joint detection method suitable for receiving on part of phase planes is required to be established to distinguish the OAM bit mapping from the bit mapping of amplitude phase modulation. In order to achieve the above object, the present invention provides an electromagnetic wave orbital angular momentum index modulation transmission system suitable for long-distance transmission, comprising a signal generation subsystem, a transmitting antenna subsystem, and a partial phase plane receiving subsystem, wherein,

the signal generation subsystem includes:

a joint coding module: encoding original information sent by a user into bit information;

a bit decomposition module: decomposing bit information into two parts, wherein one part is input into an amplitude-phase modulation mapping module, and the other part is input into an OAM index modulation mapping module;

an amplitude-phase modulation mapping module: establishing a one-to-one correspondence relationship between input bit information and each amplitude-phase constellation point representing different amplitudes and phases by adopting any one of constellation modulation modes;

an OAM index modulation mapping module: setting a lookup table containing different OAM mode combinations, and establishing a one-to-one mapping relation between bit information and the corresponding selectively transmitted OAM mode combinations;

an arbitrary waveform generator module: the information output by the amplitude-phase modulation mapping module and the OAM index modulation mapping module is synthesized into a joint transmitting symbol through a waveform generator;

the transmitting antenna subsystem is used for feeding the combined transmitting symbols generated by the waveform generator to different OAM generating arrays after up-conversion and power amplification to form multi-path OAM combined transmitting signals, and the multi-path OAM combined transmitting signals are transmitted in a free space after energy is converged by the reflecting surface and the lens;

the partial phase plane receive subsystem includes:

receiving an antenna array: converting electromagnetic waves transmitted in free space into radio frequency signals, converting the radio frequency signals into intermediate frequency signals by a radio frequency link in a down-conversion mode, wherein partial phase planes refer to partial areas of a receiving antenna array arranged on the same phase plane;

a pre-filter: the intermediate frequency signal passes through a pre-filter and then is output into two paths, one path is input into a phase difference index mapping module, and the other path is input into an amplitude-phase modulation detection module;

the phase difference index mapping module: establishing a one-to-one mapping relation between phase differences among receiving antennas on part of phase planes and different OAM mode combinations, and mapping bit information to an index domain;

an index domain filter: after the signals which are mapped by the phase difference index mapping module are filtered and output by the index domain filter, different spectral lines are formed in the index domain, and each OAM mode combination corresponds to one spectral line;

an amplitude-phase modulation detection module: receiving the carrier signal filtered by the pre-filter, thereby demodulating an amplitude-phase constellation point and further obtaining bit information corresponding to the amplitude-phase constellation point;

an OAM mode combination index modulation detection module: detecting each spectral line in the identification index field, and detecting and determining bit information transmitted through OAM mode combination according to the lookup table;

the constellation map combination mapping module: recombining the bit information output by the amplitude-phase constellation points detected by the amplitude-phase modulation detection module and the bit information demodulated by the OAM mode combination index modulation detection module according to the bit decomposition relationship of the transmitting end, thereby forming complete bit information corresponding to the original information;

a joint decoding module: the system is used for receiving the original user bit information output by the constellation map combination mapping module so as to recover the original information transmitted by the user.

Preferably, the transmitting antenna subsystem comprises an antenna feed source for generating different OAM modes and a radio frequency link thereof, the radio frequency link comprises a power divider, a mixer, an amplifier and a radio frequency element, the antenna feed source is in the form of an antenna array or a resonant cavity, and the antenna feed source is in the form of an antenna array or a resonant cavity;

the reflecting surface is a paraboloid with or without metamaterials;

the lens is a convex lens or a metamaterial lens; the receiving antenna comprises a mixer, an amplifier, a power divider and a radio frequency link, and is an antenna array, a resonant cavity or a sensor for OAM receiving detection.

Preferably, the constellation modulation scheme includes BPSK, QPSK, QAM.

Preferably, the electromagnetic wave includes one or more of a light wave, a microwave, a millimeter wave, and a terahertz wave.

Preferably, the antenna feed and/or the receiving antenna is one of a horn antenna, a parabolic antenna, a cassegrain antenna, a patch antenna, and an array antenna.

Preferably, the device for generating the different-mode OAM electromagnetic waves is one or more of a spiral phase plate, a specific reflector antenna, a specific feed antenna, a phased array antenna, a spatial light modulator, a diffraction grating, and a metamaterial.

Preferably, the original information of the user is encoded into bit information to be transmitted;

the bit information is decomposed into two parts, one part is input into an amplitude-phase modulation mapping module and is mapped into amplitude-phase constellation points to be transmitted, the other part enters an OAM index modulation mapping module, and a one-to-one mapping relation is established by combination of a lookup table and different OAM modes;

synthesizing the two parts of information into an intermediate frequency carrier wave to be transmitted, controlling an antenna feed source to transmit OAM electromagnetic waves combined in different modes, and transmitting the OAM electromagnetic waves in a free space after energy convergence through a transmitting surface and a lens;

the receiving antenna receives electromagnetic waves in a free space on a part of phase surfaces, converts the electromagnetic waves into intermediate-frequency signals, the amplitude-phase modulation detection module detects bit information corresponding to output amplitude-phase constellation points, the intermediate-frequency signals form different spectral lines in an index domain after passing through the phase difference index mapping module and the index domain filter, the bit information transmitted by the OAM mode combination is demodulated by judging the positions of the different spectral lines in the index domain, and the two parts of bit information are combined to restore the original information.

Preferably, bit information with a large hamming distance is configured for transmitting OAM mode combinations with small differences, where the small differences of the OAM mode combinations means that the distances between spectral lines in the index domain converted from the OAM mode combinations are smaller than a preset distance threshold.

Preferably, the transmit antenna subsystem generates N_tOAM electromagnetic wave of a mode, a receiving antenna array comprising N_rSelecting M-PSK by amplitude-phase modulation when receiving antenna array transmission, and representing different constellation points as d (M), wherein M ∈ [1,2, …, M]M denotes the Mth constellation point, the transmitting antenna subsystem generates together

OAM mode combination method, corresponding

Bit symbol combination, joint transmission symbol of each OAM combination is expressed as

The ellipses in the formula represent arbitrary 0 or 1 bit information in the transmitted sequence, 1 × N_tRepresents X^(k)Is 1 line N_tVector of columns, where K ∈ [1,2, …, K]Indicating the activated k-th group OAM mode combination index, the position of element 1 indicates the position of the OAM mode to be transmitted in the combination, whereby the combined transmission signal received by the receiving antenna array is simplified to be:

wherein the content of the first and second substances,

power distribution coefficient, X, representing each mode combination^(k)Is a joint transmission of the symbols that,

and when the mode combination k of the transmission OAM index modulation is represented, a channel matrix formed by channel coefficients transmitted between an activated OAM mode of the transmitting antenna subsystem and the receiving antenna array, and z is a Gaussian white noise matrix with the average value of zero.

Preferably, a weighting coefficient is distributed to each OAM mode, an optimization equation is established, under the condition that the total emission power is not changed, the power distribution of all the OAM modes participating in superposition in different modes is adjusted through the AWG, the power is discretized, and the optimal weighting coefficient is determined in a computer search traversal mode, so that the Euclidean distance equality between corresponding spectral lines of all the OAM combined modes in an index domain is adjusted.

The transmission method is not only suitable for transmitting and receiving the OAM electromagnetic wave within 1 kilometer, but also suitable for transmitting and receiving the long-distance OAM electromagnetic wave above 1 kilometer. Due to the limitation of the beam divergence angle, the OAM spiral phase plane is continuously enlarged along with the increase of the distance, kilometer-level transmission is impossible to receive the whole OAM spiral phase plane at the same time, crosstalk among different modes is brought when partial phase planes are received, orthogonality among the modes is damaged, and the degree of freedom is reduced along with the reduction of the receiving phase plane. Therefore, when long-distance transmission is carried out, the OAM multiplexing mode can be degenerated into a low-freedom transmission mode, the invention utilizes the characteristic of OAM as a new freedom degree, utilizes the combination of different modes to form index modulation at a transmitting end, and expands the spectrum efficiency of an OAM communication system from the angle of the index modulation. Because OAM is a new dimension different from other physical quantities of electromagnetic waves, the transmission of the amplitude-phase modulation signal is not influenced after index modulation, and therefore OAM index modulation information and an amplitude-phase modulation constellation can be transmitted at the transmitting end at the same time, and the purpose of OAM long-distance and high-spectrum-efficiency transmission is achieved.

Drawings

The above features and technical advantages of the present invention will become more apparent and readily appreciated from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.

Fig. 1 is a schematic structural diagram of an electromagnetic wave orbital angular momentum index modulation transmission system suitable for long-distance transmission according to an embodiment of the present invention;

fig. 2 is a schematic diagram of an implementation manner of mapping bit information into an OAM mode combination and amplitude-phase modulation constellation according to the embodiment of the present invention;

fig. 3 is a schematic diagram illustrating a principle of vector superposition of different OAM combined mode signals on a receiving antenna according to an embodiment of the present invention;

fig. 4 is a schematic diagram illustrating error rate characteristics of an OAM index modulation system, an MIMO system and a high-order modulation system according to an embodiment of the present invention.

Detailed Description

Embodiments of an electromagnetic wave orbital angular momentum index modulation long-distance transmission system and a transmission method according to the present invention will be described below with reference to the accompanying drawings. Those skilled in the art will recognize that the described embodiments can be modified in various different ways, or combinations thereof, without departing from the spirit and scope of the invention. Accordingly, the drawings and description are illustrative in nature and not intended to limit the scope of the claims. Furthermore, in the present description, the drawings are not to scale and like reference numerals refer to like parts.

In principle, OAM may be received using a specific OAM sensor, or using an antenna. For example, when all-phase plane co-axial reception is performed, the OAM is mapped to an airspace by using an antenna array, and the change of OAM causes the change of the distribution of electromagnetic wave phase planes in space, so that the change of OAM can be estimated by using the antenna array to measure the phase difference of electromagnetic waves at different positions in space. Further, according to analysis, in addition to mapping to a space domain, OAM may form a mapping to other domains, for example, OAM is measured by a rotation method, in practice, a change of OAM is mapped to a time domain, and further mapped to a frequency domain by a spectrometer, and electromagnetic waves of different OAM modes are mapped to several different spectral lines in the frequency domain under the mapping of a rotation function relationship. The Virtual Rotating Antenna (VRA) method disclosed in patent No. 2016109163094 may be configured to construct an index domain through setting of a pre-filter (where a first frequency domain refers to a frequency domain to which a common received signal is mapped after fourier transform, and the index domain refers to a received common frequency signal, because the common frequency signal contains different OAM modes, and after performing fourier transform after performing Virtual interpolation rotation operation, the original common frequency signal may form different spectral lines along with different OAM modes, which is referred to as an index domain herein), and may also construct a one-to-one mapping relationship between different spectral lines and different OAM electromagnetic waves in the index domain through a Virtual interpolation algorithm.

Therefore, as can be seen from the above analysis, this dimension of OAM can be measured by mapping to different transform domains through different mappable relationships, and the set of all mapping functions is the new index domain. There are many methods for constructing OAM index fields, different methods actually correspond to different mapping function relationships, and the same index field may even be obtained by different mapping functions, thereby forming an intricate change space. The mapping method for mapping the OAM to the index field is various, so that the index field of the OAM can be very large, and a large research space is provided for OAM index modulation. The long-distance index modulation transmission system of electromagnetic wave OAM of the present invention is explained below.

As shown in fig. 1, an electromagnetic wave orbital angular momentum index modulation transmission system 10 suitable for long-distance transmission includes a signal generation subsystem 100, a transmitting antenna subsystem 200 and a signal receiving subsystem 300. The signal generation subsystem 100 comprises a joint coding module 101, a bit decomposition module 102, an amplitude and phase modulation mapping module 103, an OAM index modulation mapping module 104 and an arbitrary waveform generator module 105; the transmitting antenna subsystem 200 includes an antenna feed module 201 and a radio frequency link module 202; the partial phase plane receiving subsystem 300 includes a receiving antenna array 301, a pre-filter 302, a phase difference index mapping module 303, an index domain filter 304, an amplitude-phase modulation detection module 305, an OAM mode combination index modulation detection module 306, a constellation combination mapping module 307, and a joint decoding module 308. The following describes the structure of each subsystem.

1. A signal generation subsystem comprising

(1) The joint encoding module 101: the original information of the user is encoded into bit information to be transmitted.

(2) The bit decomposition module 102: decomposing bit information data corresponding to user original information into two parts, wherein one part is input into an amplitude-phase modulation mapping module 103, and the amplitude-phase modulation mapping module 103 represents the input bit information by utilizing electromagnetic waves with different amplitudes and phases, namely the bit information forms a mapping relation with amplitude-phase constellation points representing different amplitudes and phases; the other part is input into an OAM index modulation mapping module 104, and forms a mapping relation with the combination of different OAM modes;

(3) the amplitude-phase modulation mapping module 103: establishing a one-to-one correspondence relationship between the input bit information and each amplitude-phase constellation point by adopting any one of a series of constellation modulation modes such as BPSK (binary phase shift keying), QPSK (quadrature phase shift keying), QAM (quadrature amplitude modulation) and the like;

(4) OAM index modulation mapping module 104: generating combination relations of various OAM modes, forming a lookup table, corresponding input user bit information to the corresponding OAM mode combination selected for transmission, and transmitting different user bit information by adopting different combination modes;

(5) arbitrary waveform generator module 105: and selecting different OAM modes for combining the constellation points mapped by the amplitude-phase modulation, thereby forming a composite joint transmitting symbol. And simultaneously, the transmission power of each group of joint transmission symbols is enabled to be equal by utilizing the amplitude adjusting function in the arbitrary waveform generator and by programming freedom distribution transmission power, and the transmission antenna subsystem is further controlled.

2. Transmitting antenna subsystem

(1) The antenna feed 201: the OAM antenna array with multiple modes is arranged for generating the OAM with multiple modes, and can adopt the forms of a resonant cavity or an antenna array, but is not limited to the typical OAM generation modes, and the radius of a circular ring can be adjusted according to OAM electromagnetic waves with different modes, so that the OAM wave beam divergence angles of different modes are consistent, and the emission energy is concentrated on the same circular ring phase plane;

(2) the radio frequency link module 202: the combined emission signal generating device comprises a power divider, a mixer, a filter and a signal power amplifier submodule, and is used for feeding a combined emission symbol generated by an arbitrary waveform generator to different OAM generating arrays after up-conversion and power amplification to form n paths of OAM combined emission signals, and energy is converged through a reflecting surface and a lens and then is transmitted in a free space.

3. Partial phase plane receiving subsystem

(1) The receive antenna array 301: converting electromagnetic waves transmitted in free space into radio frequency signals, namely guided electromagnetic waves in a transmission line, and converting the radio frequency signals into intermediate frequency signals by a radio frequency link in a down-conversion mode;

(2) the pre-filter 302: the intermediate frequency signal enters a pre-filter 302, the pre-filter 302 can adopt a band-pass filter to filter out-of-band interference information and obtain a modulated carrier signal of a required frequency point, the intermediate frequency signal passes through the pre-filter and then is output in two paths, one path is input into a virtual interpolation rotation module, and the other path is input into an amplitude-phase modulation detection module;

(3) phase difference index mapping module 303: the method can adopt a virtual interpolation method disclosed in patent No. 2016109163094, and virtual interpolation rotation operation is performed on OAM electromagnetic waves on a part of phase surfaces received by an antenna, the OAM electromagnetic waves are converted into an index domain, composite signals of the same frequency point originally can be expanded into different spectral lines in the index domain after Fourier transformation, the different spectral lines correspond to different OAM combination modes, and a one-to-one mapping relation is established between different phase differences and different OAM mode combinations between receiving antennas, as shown in fig. 4, error rate comparison of different transmission modes is performed, wherein OAM indexes are combined with QPSK, namely OAM mode combinations are included;

(4) index domain filter 304: after the virtually interpolated signals are filtered and output through an index domain filter, different spectral lines are formed in an index domain, and each OAM mode combination corresponds to one spectral line to form a one-to-one mapping relation;

(5) amplitude-phase modulation detection module 305: receiving a carrier signal filtered by a pre-filter, and demodulating amplitude-phase modulation constellation points such as BPSK, QPSK, QAM and the like;

(6) OAM mode combination index modulation detection module 306: detecting and identifying each spectral line in the index domain, and detecting and determining user bit information transmitted through OAM index modulation according to a one-to-one mapping lookup table;

(7) constellation map combination mapping module 307: and recombining the information output by the amplitude-phase constellation points detected by the amplitude-phase modulation detection module and the information demodulated by the OAM mode combination index modulation detection module according to the bit decomposition relation of the transmitting end, thereby forming the complete original information.

(8) The joint decoding module 308: the system is used for receiving the original user bit information output by the constellation map combination mapping module so as to recover the original data transmitted by the user.

The signal processing work flow of the whole electromagnetic wave orbital angular momentum index modulation transmission system suitable for long-distance transmission is as follows: as shown in fig. 1, original information of a user is encoded into bit information to be transmitted, the bit information is decomposed into two parts after passing through a bit decomposition module 102, one part of the bit information is input into an amplitude-phase modulation mapping module 103 and is mapped into amplitude-phase constellation points to be transmitted, the other part of the bit information enters an OAM index modulation mapping module 104, a one-to-one mapping relation is established with different OAM mode combinations through a lookup table, then the two parts of information are combined into intermediate frequency carrier waves to be transmitted through an arbitrary waveform generator 105, after frequency mixing and amplification are performed through a radio frequency link 202, an antenna feed source 201 is controlled to transmit OAM electromagnetic waves of different combination modes, and the OAM electromagnetic waves are transmitted in a free space after energy convergence through a transmitting surface. The receiving end antenna array is arranged on the OAM circular ring part phase surface, receives the electromagnetic wave in the free space through the receiving antenna array 301, converts the electromagnetic wave into the guided electromagnetic wave in the transmission line, outputs the band-pass signal to be received after passing through the pre-filter, adopts the method of separation detection and recombination, the amplitude-phase modulation detection module 305 is responsible for detecting and outputting the bit information corresponding to the amplitude-phase constellation point, the signal forms different spectral lines in the index domain after passing through the phase difference index mapping module 303 and the index domain filter module 304, the OAM mode combination index modulation detection module 306 is used for detecting different combination modes of different OAM, recovers the transmission bit information through a one-to-one mapping lookup table, and finally, the two parts of bit data are combined through the constellation map combination module 307 and then output the original bit data of the user. The joint decoding module 308 receives the original user bit information output by the constellation map combination mapping module 307, thereby recovering the original data transmitted by the user.

In summary, the electromagnetic wave orbital angular momentum index modulation transmission system suitable for long-distance transmission of the present invention can achieve OAM information transmission with long-distance and high spectral efficiency, because only part of information of a spiral phase plane can be sampled and received at a receiving end, orthogonality cannot be maintained between OAM signals of each mode, if a transmission multiplexing transmission mode is adopted, mutual crosstalk between channels will be caused, and further, channel capacity is reduced. The invention creatively provides that indexes are formed by adopting OAM combinations in different modes, and a one-to-one mapping relation is established between the indexes and user bit information, so that on the basis of amplitude-phase modulation, due to the introduction of new dimensionality of OAM, the frequency spectrum efficiency of system transmission is further improved under the condition of not increasing original system link resources. At a receiving end, the invention adopts the pre-filter 302 and the index domain filter 304 to separate and detect the mapping signals of the amplitude modulation part and the OAM index modulation part, and realizes the accurate detection of different OAM combination modes on part of phase planes in a virtual interpolation rotation mode, thereby not only reducing the complexity of the maximum likelihood joint detection of the index modulation, but also improving the reliability of information transmission demodulation, and being capable of realizing the transmission requirements of long distance, narrow bandwidth, low complexity, high reliability and quick receiving of a system.

In addition, an electromagnetic wave orbital angular momentum index modulation transmission system suitable for long-distance transmission according to the invention can also have the following additional technical characteristics:

in an alternative embodiment, the electromagnetic waves include one or more of light waves, microwaves, millimeter waves, and terahertz waves.

In an optional embodiment, the antenna in the transmit antenna array and/or the receive antenna array is one of a horn antenna, a parabolic antenna, a cassegrain antenna, a patch antenna, and an array antenna.

Further, the transmitting antenna suitable for long-distance transmission includes an antenna feed source capable of generating different OAM modes and a radio frequency link thereof, the radio frequency link includes a power divider, a mixer, an amplifier and a radio frequency element, and the antenna feed source may be in the form of an antenna element or a resonant cavity, but is not limited to these forms.

In an alternative embodiment, the antenna suitable for long-distance partial phase plane reception includes a mixer, an amplifier, a power divider and other necessary radio frequency links, and the receiving antenna may be an antenna array, a resonant cavity or other sensors for OAM reception detection.

In an alternative embodiment, the converging lens may be one or more of a common optical lens or a metamaterial lens, and the lens and the paraboloid may be selected and matched according to actual needs.

In an alternative embodiment, the reflecting surface may be one or more of a common optical reflecting surface or a metamaterial reflecting surface for converging electromagnetic waves.

In an alternative embodiment, the different-mode OAM electromagnetic waves are generated by one or more of a helical phase plate, a reflector-specific antenna, a feed-specific antenna, a phased array antenna, a spatial light modulator, a diffraction grating, and a metamaterial.

In an optional embodiment, different OAM index modulations correspond to different coded transmission symbols according to a combination mode of different OAM modes, and reliability of system transmission can be further improved by hamming distance between codes in combination with index modulation. Specifically, when different combination modes of the OAM mode are demapped in the index domain, distances between corresponding different spectral lines are not consistent, so that the joint coding module 101 at the transmitting end increases the discrimination degree when the receiving end detects by using different hamming distances between different coding symbols. And codes with large Hamming distance are configured for transmission, so that the reliability of the receiving end joint detection can be further improved. The Hamming distance between two character strings with equal length is the number of different characters at the corresponding positions of the two character strings.

In an alternative embodiment, diversity reception by the receive antenna array is used to further increase the channel capacity of the system. When the number of the receiving antennas is more than or equal to 2, the different receiving antennas can be combined with each other to obtain the multi-antenna diversity gain of the receiving end, meanwhile, the different antennas respectively collect the information of different positions of the OAM phase surface, and the signal-to-noise ratio before the detection of the signal can be improved in a mode of maximum ratio combination, so that the stability of the system is improved when the amplitude-phase modulation constellation points are detected.

In an alternative embodiment, because the OAM of different modes needs to be transformed into an index domain during processing of the received signal and each spectral line after virtual interpolation rotation interpolation forms a one-to-one mapping, however, in the index domain, the distance intervals between the spectral lines corresponding to any OAM mode combination are not equal, and even the phenomenon that the spectral lines corresponding to several mode combinations are very close to each other or even overlap occurs, which brings difficulty to the detection of OAM index modulation. The spectrum interval of different OAM mode combinations in the index domain can be enlarged by power allocation, and the selection of the OAM mode at the transmitting end and the power allocation algorithm are as follows: according to the principle of signal vector superposition, a weighting coefficient is distributed for each OAM mode, an optimization equation is established, under the condition that the total transmitting power is not changed, the power distribution of all OAM modes participating in superposition in different modes is adjusted through an AWG (arbitrary waveform generator), the power is discretized, and the optimal weighting coefficient is determined in a computer search traversal mode, so that the Euclidean distance between corresponding spectral lines of all OAM combined modes in an index domain is adjusted to be equal, and the detection reliability is improved.

The following is a few specific examples to illustrate some specific operations in the transmission system of the present invention and to calculate relevant parameters to verify the reliability of the present invention.

First example

The transmitting end (transmitting antenna subsystem) is capable of generating N_tOAM electromagnetic wave of seed mode, and the receiving end (receiving antenna array) is composed of N_rThe structure schematic diagram of the transmission index modulation is shown in fig. 2, and it is assumed that the amplitude-phase modulation during transmission selects the phase modulation mode of M-PSK (multilevel digital phase modulation, also called polyphase modulation), and the representation of different constellation points (each constellation point corresponds to a signal with a certain amplitude and phase) is denoted as d (M), where M ∈ [1,2, …, M]And M denotes an mth constellation point. The transmitting ends can generate

The OAM mode combination mode can correspond to

Bit symbol combinations are mixed, so that the joint transmission symbol of each OAM combination can be expressed as

The ellipses in the figure represent arbitrary 0 or 1 bit information in the transmission sequence, 1 × N_tRepresents X^(k)Is 1 line N_tVector of columns, where K ∈ [1,2, …, K]Indicating the activated k-th group OAM mode combination index, the position of element 1 indicates the position of the OAM mode to be transmitted in the combination. Thus, the combined transmission signal received at the receiving end can be simply expressed as:

wherein the content of the first and second substances,

when the mode combination k of OAM index modulation is transmitted, a transmitting end activates a channel matrix formed by channel coefficients transmitted between an OAM mode and a receiving end, and z is a Gaussian white noise matrix with the average value of zero. The spectral efficiency of the system can be expressed as:

wherein N is_aIndicating the number of OAM modes activated in index modulation, α indicating the selection coefficient of transmission mode, α being 1 when different information is transmitted in each OAM mode multiplex, α N when the same information is transmitted in each OAM mode _a1. Assuming that the amplitude-phase modulation of the transmitting end adopts a BPSK modulation method, and it is expected that the transmission rate reaches the transmission rate of 4 bits per symbol, the transmitting end may generate 4 different OAM modes for combination, 15 combination methods in total may be used for index modulation transmission, 8 of these combination modes are arbitrarily selected, and combined and mapped with BPSK modulation to form a joint transmission symbol, where each joint transmission symbol transmits 4-bit information, and the specific mapping relationship is shown in table 1:

TABLE 1 mapping relationship between original bit sequence and jointly coded transmitted symbols

After free space transmission, the electromagnetic wave carrying index modulation and M-PSK modulation information reaches the receiving end antenna array, and after passing through the pre-filter, the index modulation is carried outThe system information and the M-PSK modulation information are separately detected. The combination of different OAM modes can be transformed into the index domain on the OAM partial helical phase plane using the virtual interpolation rotation disclosed in patent No. 2016109163094, forming a plurality of non-overlapping spectral lines. Suppose N_rThe signal received by the root receiving antenna at time t can be expressed as:

wherein the content of the first and second substances,

representing the received signals on each antenna, y (t) representing the vector formed by these received signals, and performing autocorrelation operation on the received signals, i.e. obtaining the expectation after multiplying the signals by themselves, can obtain:

where t represents time, θ represents beam divergence angle, φ_iDenotes the radian between the i-th pair of receiving antennas, D denotes the transmission distance, k₁、k₂Denotes the number of the antenna, J₀Representing Bessel functions of order 0, Λ₀Which represents the equivalent wavelength of the OAM electromagnetic wave in the circumferential tangential direction perpendicular to the propagation direction. If an equivalent rotated antenna is to be virtually interpolated at the receiving end, the cross-correlation coefficient P (Φ, t) between the real antenna array and the virtually interpolated equivalent antenna can be expressed as:

where ω represents the rotation speed of the virtual interpolation antenna. Suppose that the weighting coefficient is w^T(φ), for the k OAM mode combination, the result of interpolation output can be obtained by MMSE criterion (minimum mean square error criterion) as follows:

wherein, y^(k)(t) represents the original signal received by the antenna,

representing the signal matrix obtained after processing in the manner of virtual interpolation rotation disclosed in patent No. 2016109163094, assuming that index modulated data can be represented as

The method comprises the steps of representing data transmitted by OAM index modulation, and R (K) representing that the Kth index modulation data and the virtual interpolation spectral lines can form a one-to-one mapping relation, namely, by detecting which spectral line appears in an index domain of a receiving end, which index modulation symbol is transmitted by a transmitting end can be determined.

The M-PSK modulated data can be represented as D ═ { D (1), D (2), …, D (M) }, which is reflected as different constellation points at the receiving end, the constellation points and the data symbols at the transmitting end are also in a one-to-one mapping relationship, the M-PSK modulated data transmitted by the transmitting end can be obtained by demodulating the constellation diagram in a maximum likelihood detection manner, and finally, the two parts of demodulated data are combined to obtain the original user information.

Second example

Based on the electromagnetic wave orbital angular momentum index modulation transmission system suitable for long-distance transmission provided by the invention, the example describes the joint coding and transmitting structure of the transmitting end in detail and the joint decoding process combining Viterbi decoding at the receiving end, namely: the amplitude phase constellation point and the OAM mode are combined and placed in the same constellation space to perform joint modulation, and simultaneously, the receiving end is combined with a viterbi decoder to perform joint decoding, and the structure of the combination is shown in fig. 2.

In two-dimensional constellation optimization, constellation points arranged on a unit circle need to satisfy the following principle: the number of constellation points arranged on the same circle is proportional to the radius of the circle; the constellation points on the same circle need to be evenly distributed. According to these principles, the optimization of constellation points can be obtained by maximizing the euclidean distance with constraint on the average energy, namely:

wherein d is_minRepresenting the minimum Euclidean distance, E, between constellation points_avgRepresents the mean energy of the constellation points, and s.t. refers to the constraint. M represents the total number of constellation points, N_iDenotes a radius R_iThe ith constellation circle of (c). In the optimization process of the three-dimensional constellation diagram in this embodiment, each constellation point may be expanded into a multilayer structure, and the steps of constructing the optimization of the constellation points based on the OAM mode combination and the PSK constellation space are as follows:

(1) optimizing a PSK modulation two-dimensional constellation to ensure that all PSK constellation points are uniformly distributed on a corresponding unit circle, and different PSK constellation diagrams are transmitted by different OAM mode combinations;

(2) through power allocation, spectrum intervals of different OAM mode combinations in an index domain are enlarged, namely, the distance between constellations of each layer is enlarged;

(3) and superposing the PSK constellation points of each layer to form an initial phase, so that the constellation between two adjacent layers is rotated, and the Euclidean distance between the PSK constellation points transmitted by OAM mode combination can be adjusted, so that the minimum Euclidean distance of the whole three-dimensional constellation point is maximum.

As can be seen from fig. 2, different OAM mode combinations may be used to transmit different or the same constellation points, and at the same time, different OAM mode combinations may also be used as indexes to transmit information, and a third three-dimensional modulation space in the z-axis direction is constructed on the basis of the amplitude-phase modulated two-dimensional constellation diagram, thereby forming an OAM-PSK joint constellation space (i.e., a three-dimensional constellation diagram). For the mode combination of OAM and PSK joint modulation, at the transmitting end, the original information is firstly subjected to grid coding, and then the coded symbols are mapped into points in an OAM-PSK joint constellation space. At the receiving end, a two-array-element antenna array can be used to receive signals, the OAM mode combination transmitted by the transmitting end is estimated by adopting the virtual interpolation rotation method disclosed in patent No. 2016109163094, and then the symbols subjected to trellis coding are decoded by adopting the standard viterbi decoding algorithm to obtain PSK signal constellation points, thereby obtaining the original bit data.

OAM transmission is typically only applied in a single path line-of-sight (LOS) transmission environment, and channel noise may be characterized as Additive White Gaussian Noise (AWGN). The received signal may be expressed as:

wherein f is_cRepresenting the carrier frequency, r the received signal, n being zero mean and σ variance²Is the transmitted signal coded by joint modulation, l is the OAM mode number,

for azimuth, k ═ 2 pi/λ is the wave constant, λ is the wavelength, and (x, y, z) represents the coordinates of the constellation points in the three-dimensional constellation.

If d is_minRepresenting Euclidean distances between three-dimensional constellation points of trellis coding, d_minThe method is used as a key index for judging the system performance in a white noise environment. Then, the bit error rate can be expressed as:

wherein Q () is a Gaussian function, E_bRepresenting the symbol energy, N₀Representing the energy of the noise.

Third example

Assuming that a transmitting end can generate a plurality of OAM modes, a receiving end adopts a method of virtual interpolation rotation of two antennas to construct an index domain. Assuming one receiving antenna as a reference position where the initial phases of all OAM modes are aligned, on the second antenna, the combined signal of the different OAM modes can be written as follows:

after simplification, the superimposed signal can be represented in the form of a vector:

is provided with

The received signal can be further written as

As shown in fig. 3, in which,

representing the amplitude of the signal,/_iIndicates the number of OAM patterns, L indicates the total number of OAM patterns,

indicating a phase difference generated between two receiving antennas for each OAM mode, S indicating a superimposed signal, X indicating a projection of the superimposed signal in the X-axis direction, Y indicating a projection of the superimposed signal in the X-axis direction,

the method is characterized in that the phase difference of the superposed signals generated between two receiving antennas is represented, omega represents the angular frequency of the signals, t represents time, and the superposition of the OAM modes in different modes meets the parallelogram vector superposition rule, so that the purpose of controlling and changing the phase of the superposed signals can be realized by changing the amplitudes of the different OAM modes participating in the superposition under the condition that the phase difference formed between the two antennas at the receiving end in different OAM modes is constant.

Suppose that the transmitting end is capable of generating N_tOAM electromagnetic wave, N is selected each time_aMultiple OAM transmits the same M-PSK signals, and all OAM modes implement phase alignment on the first reference antenna at the receiving end, then for the k-th combination mode, the phase difference of the OAM mode combination between the second receiving antenna and the first receiving antenna is:

wherein the content of the first and second substances,

indicating the phase difference between the k-th group of antennas,

represents from N_tSelecting N from root antenna_aAnd in addition, each phase difference corresponds to an OAM transmitting combination mode after virtual interpolation and rotary interpolation, a transmitting end is supposed to be capable of generating 4 OAM signals in different modes, the number of the modes is 1,2,3 and 4 respectively for combined transmission, at most 15 different combinations can be formed, 8 of the combinations can be selected for index modulation transmission, and a BPSK constellation point is combined, so that the combination system achieves the transmission rate of 4 bits of each symbol. In the selected mode combination, if the power of all OAM modes is uniformly distributed, the combination of modes 1 and 3 is substantially the same as the phase difference formed on the two antennas at the receiving end when mode 2 is transmitted alone, i.e. it is not easily distinguished in the index domain due to the close distance between the spectral lines. Therefore, the present example changes the power distribution coefficients of different OAM modes in different combinations

The phase difference of the sum signal is controlled to be changed, that is, the power distribution of different OAM at the transmitting end when the same signal weighting is transmitted is changed, so that the spectral lines corresponding to the mode combination of each OAM in the index domain can be separated, and the one-to-one mapping relation between the different spectral lines and the combination of different OAM modes in the index domain is realized.

In order to achieve the euclidean distances between all spectral lines being as equal as possible, in other words, to maximize the minimum phase difference of the vector superposition signals of all combined modes, and to maximize the euclidean distances between different modes, the power distribution of the transmitting end can be further modeled as the following optimization problem:

wherein the content of the first and second substances,

representing the power distribution coefficients of the different modes,_kindicating a phase difference

The combination of the modes that is the smallest,

indicating that L is selected from L pattern combinations_AThe number of combinations of the seeds is,

representing the phase difference between the two antennas, P_tolRepresenting the total power. And adjusting the power distribution of all the OAM signals participating in superposition in different modes through the AWG under the condition that the total transmitting power is unchanged by optimizing a power distribution model. The specific power allocation steps are as follows:

(1) on the premise that all OAM combined modes are known, firstly, the transmitting power can be discretely quantized into different small intervals, and the specific number setting of the intervals can be determined according to the computing capacity of a transmitting end computer;

(2) for each combination mode, the power distribution coefficients of different OAM modes in the combination mode are changed in sequence

Further calculating the phase difference caused by different OAM combined modes at the receiving end under the distribution coefficient;

(3) under the limiting condition that the total transmitting power is guaranteed to be unchanged, the power combination modes of all the combination modes under the divided distribution coefficients are sequentially traversed in a computer searching and traversing mode, the power distribution modes which enable the phase differences of all the OAM combination modes to be equal are searched and searched, and the power distribution modes are used as the optimal power distribution modes to be implemented at a transmitting end, so that the Euclidean distances between corresponding spectral lines of all the OAM combination modes in an index domain are adjusted to be equal, and the reliability of detection is improved. The corresponding power allocation results of all OAM modes in different index modulation combinations in this embodiment are shown in table 2:

table 2 power allocation results of OAM modes in different index modulation combinations

The method comprises the steps of representing power distribution coefficients of all OAM modes in different combinations, flexibly adjusting the transmitting power of all the modes according to the different mode combinations under the condition that the total power is not changed, and forming a combined transmitting symbol, so that high-reliability detection between the combined modes and spectral lines in an index domain is realized at a receiving end.

In the above, two receiving antennas are taken as an example, the receiving subsystem may actually have more than two receiving antennas, and the method of constructing the index field for any two receiving antennas is not different from the method of constructing the index field for any two receiving antennas, and the description thereof is omitted here.

Fourth example

This embodiment is similar to the first and third embodiments, and this example still assumes that the transmitting end of the OAM index modulation system can generate 4 OAM signals with different modes, and for comparison, it is assumed in the simulation that the transmitting end of the MIMO (which means that a plurality of transmitting antennas and receiving antennas are respectively used at the transmitting end and the receiving end, and signals are transmitted and received through the plurality of antennas at the transmitting end and the receiving end) system is also configured with 4 identical antennas, the total transmitting power is constant, and the receiving end is constantConfiguration N_rAccording to the common receiving antenna, the error rate performance of the transmission system and the change of the transmission capacity of the system along with the receiving diversity are simulated and analyzed.

In the OAM index modulation method provided by the invention, a transmitting end respectively selects and distributes power to combinations of different OAM modes, a virtual interpolation rotation method is adopted at a receiving end to determine the transmitted OAM combination mode, BPSK constellation points transmitted by the combination mode are detected at the same time, and user data are combined and demodulated. The comparative multi-antenna transmission system is a line-of-sight MIMO (LoS-MIMO) communication system, the receiving end determines the channel characteristics by an open-loop channel estimation method, i.e., does not need pilot information, estimates the channel information directly according to the characteristics of the signals, and the simulation process calculates the error rate performance of different transmission systems under the condition of known channel characteristics, as shown in fig. 4.

In the figure, AWGN in the notation represents additive white gaussian noise, OAM:4 represents that the transmitting end transmits 4 OAM signals of different modes, Rx:2 represents that the receiving end has two antennas, and Tx:4 represents that the transmitting end has 4 antennas.

As can be seen from fig. 4, 4 OAM modes are adopted in combination with QPSK amplitude-phase modulation mode for joint index modulation transmission, and three-dimensional space constellation points are formed by joint coding, and the receiving end N is_rThe system can achieve 4 bits/s/Hz communication efficiency per symbol transmission, while achieving the same communication transmission rate 2. The amplitude-phase modulation method usually adopts a high-order modulation transmission mode, and can compare with the error code performance of constellation point modulation by adopting 16 QAM. In the LoS-MIMO transmission system adopting 4 same antennas, to achieve the same symbol transmission rate, it is also necessary to multiplex and transmit QPSK constellation points on each antenna. Simulation shows that compared with a LoS-MIMO transmission system with known channel characteristic CSI, the OAM index modulation system provided by the invention has a lower error rate, because the OAM index modulation system can better avoid inter-carrier interference ICI and introduces space gain, the error code performance can be further reduced compared with a common MIMO system under the condition of realizing the same spectrum efficiency. Meanwhile, the OAM index modulation system only activates part of radio frequency links each time, so a complex conflict avoidance algorithm is not neededIt is used. The receiver design is simpler than V-BLAST, and therefore has lower system implementation complexity, which can reduce link overhead. Compared with a high-order modulation mode under the condition of the same spectral efficiency, the OAM index modulation method provided by the invention has obviously reduced error code performance, because the OAM index modulation system provided by the invention adopts a joint coding method, a common two-dimensional constellation map is converted into a three-dimensional space constellation map, and the Euclidean distance between constellation points is further enlarged by utilizing phase rotation between different plane constellation maps, so that the reliability of the system is greatly enhanced.

The invention also provides an electromagnetic wave orbit angular momentum index modulation transmission method suitable for long-distance transmission, which is not only suitable for transmitting and receiving the OAM electromagnetic wave within 1 kilometer, but also suitable for transmitting and receiving the long-distance OAM electromagnetic wave above 1 kilometer, and comprises the following steps:

encoding original information of a user into bit information to be transmitted;

the receiving antenna of the partial phase surface receives electromagnetic waves in a free space and converts the electromagnetic waves into intermediate frequency signals, the amplitude-phase modulation detection module detects and outputs bit information corresponding to amplitude-phase constellation points, the intermediate frequency signals form different spectral lines in an index domain after passing through the phase difference index mapping module and the index domain filter, the bit information transmitted by OAM mode combination is demodulated by judging the positions of the different spectral lines in the index domain, and the two parts of bit information are combined to recover the original information.

Preferably, the OAM mode combination with small difference configures bit information with large hamming distance for transmission.

Preferably, the method for detecting different phase differences between the receiving antennas and establishing a one-to-one mapping relationship with different OAM mode combinations includes one or more of a virtual interpolation rotation method, a two-point phase gradient direct detection method, a single-point approximation detection method, and a phase accumulation detection method.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. An orbital angular momentum index modulation transmission system suitable for long-distance transmission is characterized by comprising a signal generation subsystem, a transmitting antenna subsystem and a partial phase plane receiving subsystem, wherein,

the signal generation subsystem includes:

the partial phase plane receive subsystem includes:

the phase difference index mapping module: establishing a one-to-one mapping relation between phase differences among receiving antennas on part of phase planes and different OAM mode combinations, and mapping bit information to an index domain through virtual interpolation rotation operation;

2. An orbital angular momentum index modulation transmission system suitable for long distance transmission as claimed in claim 1,

the transmitting antenna subsystem comprises an antenna feed source for generating different OAM modes and a radio frequency link thereof, wherein the radio frequency link comprises a power divider, a mixer, an amplifier and a radio frequency element, the antenna feed source is in the form of an antenna array or a resonant cavity, and the antenna feed source is in the form of an antenna array or a resonant cavity;

the reflecting surface is a paraboloid with or without metamaterials;

3. The system according to claim 1, wherein the constellation modulation scheme comprises BPSK, QPSK, QAM.

4. An orbital angular momentum index modulation transmission system suitable for long-distance transmission according to claim 1,

the electromagnetic wave comprises one or more of light wave, microwave, millimeter wave and terahertz wave.

5. An orbital angular momentum index modulation transmission system suitable for long-distance transmission according to claim 2,

the antenna feed source and/or the receiving antenna is one of a horn antenna, a parabolic antenna, a Cassegrain antenna, a patch antenna and an array antenna.

6. An orbital angular momentum index modulation transmission system suitable for long-distance transmission according to claim 2,

the equipment for generating the OAM electromagnetic waves with different modes is one or more of a spiral phase plate, a specific reflector antenna, a specific feed source antenna, a phased array antenna, a spatial light modulator, a diffraction grating and a metamaterial.

7. An orbital angular momentum index modulation transmission method suitable for long-distance transmission is characterized by comprising the following steps:

encoding original information of a user into bit information to be transmitted;

the receiving antenna receives electromagnetic waves in a free space on a part of phase surfaces and converts the electromagnetic waves into intermediate-frequency signals, the amplitude-phase modulation detection module detects and outputs bit information corresponding to amplitude-phase constellation points, the intermediate-frequency signals pass through the phase difference index mapping module and the index domain filter and then are mapped to the index domain through virtual interpolation rotation operation, different spectral lines are formed in the index domain, the bit information transmitted by OAM mode combination is demodulated by judging the positions of the different spectral lines in the index domain, and the two parts of bit information are combined to restore the original information.

8. The method according to claim 7, wherein the transmission method comprises the steps of,

and configuring bit information with large Hamming distance for transmission by using the OAM mode combination with small difference, wherein the small difference of the OAM mode combination means that the distance between spectral lines in the index domain converted from the OAM mode combination is smaller than a preset distance threshold.

9. The method according to claim 7, wherein the transmission method comprises the steps of,

transmit antenna subsystem generates N_tOAM electromagnetic wave of a mode, a receiving antenna array comprising N_rSelecting M-PSK by amplitude-phase modulation when receiving antenna array transmission, and representing different constellation points as d (M), wherein M ∈ [1,2, …, M]M denotes the Mth constellation point, the transmitting antenna subsystem generates together

OAM mode combination method, corresponding

wherein the content of the first and second substances,

represents eachPower distribution coefficient, X, of individual mode combinations^(k)Is a joint transmission of the symbols that,

10. The method according to claim 7, wherein the transmission method comprises the steps of,

distributing a weighting coefficient for each OAM mode, establishing an optimization equation, adjusting the power distribution of all the OAM modes participating in superposition in different modes through an AWG (arrayed waveguide grating) under the condition that the total transmitting power is not changed, discretizing the power, and determining the optimal weighting coefficient in a computer search traversal mode, thereby realizing the adjustment of equal Euclidean distances between corresponding spectral lines of all the OAM combined modes in an index domain.