CN110138438B - Modulation method for improving satellite communication spectrum efficiency - Google Patents
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Abstract
A modulation method for improving the efficiency of satellite communication frequency spectrum, use the communication system of a transmitting antenna many receiving antennas, the transmitting end equips a traveling wave ring resonator antenna, this kind of antenna produces OAM wave of different modes; mapping one part of information to be transmitted into an OAM wave of a mode of an orbital angular momentum signal, namely a certain mode, modulating the other part of the information to be transmitted into a traditional digital modulation symbol through a P-point constellation diagram, transmitting the digital modulation symbol by using the orbital angular momentum signal of a specific mode at each transmission time, wherein the modulation process or method comprises three processes of orbital angular momentum mode modulation mapping, orbital angular momentum mode detection and digital modulation demodulation; the invention adds the new dimension of the orbital angular momentum in the satellite communication transmission, improves the spectrum efficiency, and can enhance the safety of information transmission because of the orthogonality among orbital angular momentum signals of different modes and no influence of interference among channels.
Description
Technical Field
The invention relates to the field of wireless communication, in particular to a modulation method for improving the spectrum efficiency of satellite communication, which provides a new degree of freedom for wireless communication in a satellite communication scene by utilizing orbital angular momentum.
Background
The contradiction between the increasing demand for wireless communication and the limited spectrum resources is increasingly prominent due to the rapid development of wireless communication technology. How to improve spectrum efficiency under the condition of limited spectrum resources becomes a hot spot of wireless communication research. According to classical electromagnetic theory, an electromagnetic wave possesses angular momentum perpendicular to the direction of propagation, in addition to linear momentum parallel to the direction of propagation. Orbital Angular Momentum (OAM) is a part of the Angular Momentum of an electromagnetic wave, which is derived from the helical phase front of the electromagnetic wave and has the expression e-ilφl, where φ is the azimuth angle, l is the OAM mode, and can be any integer. The OAM waves in different modes are mutually orthogonal and present as a plurality of concentric rings with different radiuses in spatial distribution, so that transmission information can be loaded on different OAM waves, and the correction of the OAM waves is utilizedAnd multiplexing or modulation is realized alternately, so that the communication capacity is improved. OAM technology has been applied to the fields of optical communication and wireless communication at present.
In the field of wireless communication, OAM electromagnetic waves can be generated by using a circular array antenna, a spiral phase plate, etc., and the existing research on OAM waves focuses on constructing a typical spatial multiplexing system similar to a V-BLAST (Vertical-Bell Laboratories layerwise Space-Time) system to realize channel multiplexing by using its orthogonality, however, it has been proved by research that a wireless communication system for realizing spatial multiplexing on a channel by using OAM waves cannot achieve a higher channel capacity than a conventional spatial multiplexing multiple-input multiple-output wireless communication system, and the former is still a subset of the latter. It follows that there is no significant benefit in using OAM signals in terms of spatial multiplexing. In a mimo wireless communication system, in addition to parallel transmission of multiple input signals of the system by using spatial multiplexing, spatial modulation techniques for transmitting information by using spatially distributed bearers of multiple transmitting antennas are also a new research direction.
In the spatial modulation technology, only one antenna is used for transmitting digital modulation symbols in each transmission process, input signals are transmitted in a staggered mode at different transmission time to avoid the problems of signal interference and antenna synchronization in a spatial multiplexing system, and meanwhile, compared with the spatial multiplexing system, the complexity of a receiving end is reduced. However, this also means that the transmitting end needs to provide as many transmitting antennas as possible to improve the spectral efficiency of transmission, which also makes the transmitting end need a larger antenna space to satisfy the safety distance of at least half wavelength between the antennas. However, for a special satellite communication scene, because the physical space resources on the satellite are very limited, reasonable arrangement of two or more antennas cannot be provided, and at this time, if the traditional spatial modulation is adopted, the gain is greatly reduced, so it is necessary to research how to provide the modulation freedom of the cooling space for the satellite communication by using new physical quantity in such an environment so as to improve the spectral efficiency of the satellite communication, and the orthogonality among different orbital angular momentum modes can just meet the requirement.
Disclosure of Invention
The purpose of the invention is as follows: in a satellite communication scene, if a plurality of antennas are arranged on a satellite, the physical characteristics of at least half-wavelength safety distance required between the antennas can be contradicted with the small physical space on the satellite and the hardware size of the antennas, and the diversity gain brought by the plurality of antennas can be greatly reduced due to the influence of fading channels because the satellite-ground propagation distance is too long, so that the communication system only uses one transmitting antenna and a plurality of receiving antennas in the scene; in order to overcome the defect that the traditional spatial modulation technology cannot be used for providing extra gain in satellite communication, the invention provides a modulation method for improving the spectrum efficiency of satellite communication, and the spectrum efficiency of satellite communication is improved by utilizing the orthogonality among orbital angular momentum signals of multiple modes.
The technical scheme of the invention is as follows: a modulation method for improving the efficiency of satellite communication frequency spectrum, use the communication system of a transmitting antenna many receiving antennas, the transmitting end equips a traveling wave ring resonator antenna, this kind of antenna produces OAM wave of different modes; mapping one part of information to be transmitted into an OAM wave of a certain mode of an orbital angular momentum signal, modulating the other part of the information to be transmitted into a traditional digital modulation symbol through a P-point constellation diagram, and transmitting the digital modulation symbol by using the orbital angular momentum signal of a specific mode at each transmission time, wherein the modulation process or method comprises three processes of orbital angular momentum mode modulation mapping, orbital angular momentum mode detection and digital modulation demodulation:
mapping the first l information bits of a group of random information bit sequences q to be transmitted into a selected orbital angular momentum mode, wherein l is log2L and L are the number of orbital angular momentum modes generated by the traveling wave annular resonant cavity antenna; mapping the remaining m information bits into a P-point modulation constellation, wherein m is log2M and M are the size of the P point modulation constellation diagram;
the detection of the orbital angular momentum signal mode is realized, because only one mode of orbital angular momentum signal is used for transmitting information at each transmission moment, a receiving antenna array is used at a receiving end, and the receiving antenna arrays are respectively arranged at corresponding circular receiving ends of the orbital angular momentum signals under different modesOn the region, it is assumed that in each receiving region, the two receiving antennas are in the same tangent plane of the OAM wave of the corresponding mode, and the included angle between the two antennas relative to the OAM wave propagation axis is β, and
(Lmaxthe maximum number of OAM modes); then only two receiving antennas in the receiving area of the corresponding mode can receive the transmission signal at each transmission moment, so that at each transmission moment, assuming that the orbital angular momentum signal mode of the transmission information is l, the detection data y received by the receiving end in the corresponding circular receiving area can be represented as
Wherein y is [ y ═ y1，y2]，y1And y2Two receiving antennas in the receiving area corresponding to the transmission signal mode
And
p represents the total transmission power of the orbital angular momentum modulation system, xpIs a symbol modulated by P point constellation diagram, omega is additive white Gaussian noise of a receiving end, hlThe channel response when the signal is transmitted with orbital angular momentum of mode l is h because of two receiving antennaslCan be written as hl＝[hl，1，hl，2]. The detection utilizes the phase difference of the receiving antenna to calculate the transmitted orbital angular momentum signal mode by the phase gradient method, and the calculation formula can be expressed as
Wherein phi1And phi2For receiving antennas
And
respectively receiving orbital angular momentum phases, wherein beta is an included angle of the two antennas relative to an orbital angular momentum signal propagation axis;
based on the detection result of the orbital angular momentum mode, the maximum likelihood detection method is utilized to realize the demapping of the digital symbols, and the calculation formula can be expressed as
Where χ is the set of modulation symbols for all P-point constellations, y1For the corresponding orbital angular momentum signal mode is
Time-of-day receiving antenna in receiving area
In the received signal of (a) the received signal,
for receiving antennas
The corresponding signal mode is
The response of the channel in time is,
the characteristics of the channel according to orbital angular momentum are expressed as
Where Δ is a constant, representing propagationCoefficient of signal strength and phase change in the process, d1For receiving antennas
Distance from the transmitting antenna, λ is wavelength, k is wave number, and
corresponding receiving antenna
The change in phase during the propagation process,
for receiving antennas
The helical orbital angular momentum phase of the received signal.
The transmitting antenna uses a traveling wave ring resonant cavity antenna, and the antenna generates orbital angular momentum signals of different modes and uses the orthogonality to carry part of information to be transmitted.
The receiving antenna array is used at the receiving end, and the orbital angular momentum signals of different modes are in concentric circular rings with different radiuses in the spatial distribution of the receiving intensity; therefore, the receiving antennas are respectively arranged on the corresponding annular receiving areas of the orbital angular momentum signals in different modes, and in each annular receiving area corresponding to the orbital angular momentum mode, two common receiving antennas are used to judge the mode of the OAM wave of the transmission signal through the phase difference value of the OAM between the receiving antennas.
The receiving antennas are respectively arranged on the corresponding circular receiving areas of the orbital angular momentum signals in different modes, so that only one mode of orbital angular momentum signal transmission information is available at each transmission moment, and only the receiving antennas in the corresponding receiving areas can receive the transmission signals, and the sub-area corresponding mode receiving of the signals not only enhances the reliability of information receiving, but also can ensure the safety of information transmission to a certain extent; in each circular receiving area of the orbital angular momentum signal corresponding to the mode, since correct detection of the orbital angular momentum mode can be ensured by using the phase difference of the two receiving antennas, the number of the receiving antennas of the system can be set to 2L, wherein L is the number of the orbital angular momentum modes generated by the transmitting antennas.
The invention provides a modulation method for improving the satellite communication spectrum efficiency, in a satellite communication scene, if a plurality of antennas are arranged on a satellite, the physical characteristics of at least half-wavelength safety distance between the antennas can be contradicted with a small physical space on the satellite, and because the satellite-ground propagation distance is too long, the diversity gain caused by the plurality of antennas can be greatly reduced due to the influence of channel fading. Aiming at the scene, the invention provides a modulation method for improving the spectral efficiency by utilizing the orbital angular momentum based on a wireless communication system which is provided with a traveling wave annular resonant cavity antenna at a transmitting end and uses a receiving antenna array at a receiving end.
Has the advantages that: the invention adds the new dimension of the orbital angular momentum in the satellite communication transmission, improves the spectrum efficiency, has no influence of interference between channels due to the orthogonality among orbital angular momentum signals of different modes, and can enhance the safety of information transmission because receiving end antennas are respectively arranged in receiving areas of the orbital angular momentum signals under different modes. Compared with the prior art, the invention adopts the physical quantity of OAM to provide a new degree of freedom for satellite communication which can not utilize the space distribution of a plurality of antennas to bear transmission information, utilizes the orthogonality among the OAM signals of different modes to bear the transmission information of a part, improves the spectrum efficiency, does not bring the influence of interference among channels due to the orthogonality and the modulation characteristic that only one mode of OAM wave transmission signals is used at any transmission moment, and simultaneously, the design of the sub-region corresponding mode receiving of the receiving end antenna not only enhances the reliability of information receiving, but also can ensure the safety of information transmission to a certain extent.
Drawings
Fig. 1 is a system block diagram of the present modulation method.
Fig. 2 is a system scene diagram of the modulation method taking satellite-to-ground communication as an example.
Fig. 3 is three processes of the present modulation method.
Fig. 4 is an example of the present modulation method mapping criteria.
Detailed Description
In order to better understand the technical content of the present invention, specific embodiments are described below with reference to the accompanying drawings.
The invention provides a modulation method for improving spectral efficiency, which is based on a satellite communication scene, wherein if a plurality of antennas are arranged on a satellite, the physical characteristic that at least half-wavelength safe distance is needed between the antennas can generate contradiction with a small physical space on the satellite and the hardware size of the antennas, and because the satellite-ground propagation distance is too long, the diversity gain caused by the antennas can be greatly reduced due to the influence of channel fading. A system block diagram is shown in fig. 1.
The communication system of fig. 1 is described as follows: the transmitting end adopts a traveling wave annular resonant cavity antenna to generate the OAM signals of L modes; the receiving end uses a receiving antenna array, because the orbital angular momentum signals of different modes are presented as a plurality of concentric rings with different radiuses in the spatial distribution of the receiving intensity, the receiving antennas are respectively arranged on the corresponding rings of the orbital angular momentum signals under different modes, and in each circular ring-shaped receiving area corresponding to the orbital angular momentum mode, two common receiving antennas are used for judging the mode of the OAM wave of the transmission signal through the phase difference value of OAM between the receiving antennas. Taking the inter-satellite communication as an example, assuming that the transmitting end can generate 4 different modes of OAM waves, a system scenario diagram is shown in fig. 2. The modulation principle is to divide the information to be transmitted into two parts, one part is mapped into OAM waves of one mode, and the other part is modulated into a traditional digital modulation symbol through a P point constellation diagram. And at each transmission moment, transmitting the digital modulation result through the mapped OAM wave, thereby improving the spectral efficiency of the system. The modulation method comprises three processes as shown in fig. 3 and is performed in the order described:
OAM mode modulation mapping process: in each transmission time, mapping one part of information to be transmitted into an OAM signal in one mode, modulating the other part of the information to be transmitted into a digital modulation symbol, and transmitting the digital modulation symbol through the OAM signal in the selected mode;
OAM mode detection process: calculating an OAM mode of a transmitted signal by using a phase gradient method through a phase difference of a received signal between receiving antennas in a receiving area of the orbital angular momentum signal corresponding to the mode;
c. digital modulation and demodulation process: and according to the detected OAM mode result, utilizing a maximum likelihood detection method to realize the demapping of the digital modulation symbols.
The present invention has been particularly shown and described with reference to the system block diagram of FIG. 1. The modulation method comprises the following three processes:
1) OAM mode modulation mapping process: a set of random bit sequence information q to be transmitted q ═ q1，q2，q3，…，qn]Entering an OAM modulation mapping module, wherein the first l (l is log)2L) bits are mapped to the selected OAM mode, leaving m (m-n-L) logs2M) bit mapping to P-point modulation constellation, assuming M-QAM modulation is used, the modulation symbol is xp. The total amount of transmitted information is determined by the number of OAM modes that the transmitting end can generate and the number of symbols of the modulation constellation.
For example, 3-bit information is transmitted, a transmitting antenna capable of generating 4 OAM modes is selected, i.e., L is 4, L is 2, BPSK modulation is adopted, i.e., M is 2, M is 1, the first two bits in each transmission are mapped into the OAM mode, BPSK symbols are transmitted through the selected OAM signals, and fig. 4 shows a mapping rule of such modulation. After OAM mode modulation mapping, the information to be sent is transmitted into a transmission channel.
2) OAM mode detection process: after receiving the transmission signal, the receiving end first needs to detect the OAM mode. Because the number of the selectable OAM modes is L, and the receiving-end antennas are arranged in the corresponding receiving ring regions of the orbital angular momentum signals under different modes, only the antennas in the receiving region of the corresponding mode will receive the transmission information at each transmission time, wherein there are two receiving antennas in each receiving region, therefore, at each transmission time, it is assumed that the orbital angular momentum signal mode of the transmission information is L, and the two receiving antennas in the corresponding receiving region are L
And
and
the OAM wave in the corresponding mode has the same section, and the included angle between the OAM wave and the OAM wave propagation axis is beta
(LmaxThe maximum number of OAM modes), then at each transmission instant the transmission channel can theoretically be considered to extend to one lx 2 channel.
Setting transmission information to x ═ x1，x2，x3，…，xL]TSince there is only one mode of OAM signal transmission per transmission time, only one element among L elements of x is xpAnd the remaining elements are 0. The transmitted symbols are transmitted over an L x 2 channel, the received symbols are y, are 2 x 1 vectors, and are represented as
Where ρ is the total transmission power of the transmission signal, ω is additive white gaussian noise at the receiving end, H is a 2 × L channel matrix, and each element corresponds to the transmission function of the OAM wave of the L-th mode and the corresponding receiving antenna, and is expressed as
H＝[h1，h2，h3，…，hL] (2)
Wherein h isl＝[hl，1，hl，2]And respectively represent the channel responses of the two receiving antennas. So that at each transmission instant, equation (1) can be written again as
Wherein h islA channel matrix corresponding to the OAM signal with pattern l is represented. It can be seen that after the OAM is added with this degree of freedom, the information transmission rate of the system is n-log2L+log2M, the spectral efficiency of which grows logarithmically according to the number of OAM modes. And calculating the mode of the transmitted OAM wave according to the phase difference between the two receiving antennas corresponding to the mode by a phase gradient method:
wherein phi1And phi2Is an antenna
And
the received OAM phases, respectively.
3) Digital modulation and demodulation process: after the OAM mode detection is realized at the receiving end, the demapping of the digital symbols is realized by utilizing a maximum likelihood detection method according to the OAM mode detection result, and the calculation formula can be expressed as
Where χ is the set of modulation symbols for all P-point constellations, y1For receiving antennas
In the received signal of (a) the received signal,
for receiving antennas
The corresponding OAM wave mode is
The channel response of time. The formula of the channel response can be modeled according to the characteristics of the OAM channel: because the received signal strength is distributed in the ring area, the attenuation corresponds to the Frans equation of free space, and the attenuation strength is the power of-2 of the propagation distance, the channel modeling formulas of the two receiving antennas are respectively
Where Δ is a constant, representing the coefficient of variation of signal strength and phase during propagation, d1And d2Respectively, the distances of the receiving antennas from the transmitting antennas of the corresponding modes, λ is the wavelength, k is the wave number, and
and
respectively correspond to
And
the change in phase during the propagation process,
and
respectively correspond to
And
the spiral OAM phase of the signal. Due to the fact that
And
only OAM phases of received signals are different, so that only one antenna in a receiving area of a corresponding mode is selected to be used for estimating a digital modulation symbol in a formula for realizing demapping by a maximum likelihood detection method. By combining the result of OAM mode detection with the result of digital modulation and demodulation, the complete detection result of the transmitted signal can be obtained.
The modulation method is applied to channel capacity analysis of a satellite communication system:
suppose that a transmitting antenna transmits a symbol x modulated by a P-point constellation through an OAM wave with a mode of lpAccording to the above, the first antenna at the receiving end in the receiving area corresponding to the mode
The received signal is
Where ρ is the total transmit power, ω is the mean 0 and the variance is
White additive gaussian noise. Therefore, at a known modulation symbol xpAnd OAM mode l, y1Can be expressed as
Assuming that in OAM modulation, the transmission information is subject to a uniform distribution of independent and equal distributions, the probability of each OAM mode being selected is
The probability of each constellation symbol being selected is
(M is the total number of symbols), and a reception antenna according to the formula (9)
The probability density function of the received signal is
According to the characteristic of OAM modulation, input information bits are respectively mapped into an OAM mode and a digital modulation symbol, so that for a Gaussian channel, an input stream is a discrete input symbol, a receiving end receives a continuous output signal under the influence of noise, namely, the information input and output mode is consistent with the characteristic comparison of a discrete input continuous output channel, and the channel capacity formula based on the channel can express the channel capacity of OAM modulation as the channel capacity of the channel capacity formula based on the channel
Wherein
The maximum value in equation (11) is achieved when the input information is uniformly distributed, and the transmission information is uniformly distributed independently and uniformly in OAM modulation as described above, so that the channel capacity result can be obtained by substituting equations (9) and (10) for equation (11).
Claims (4)
1. A modulation method for improving the efficiency of satellite communication frequency spectrum is characterized in that a communication system with a transmitting antenna and a plurality of receiving antennas is used, the transmitting end is provided with a traveling wave annular resonant cavity antenna, and the antennas generate OAM waves with different modes; mapping one part of information to be transmitted into OAM waves in a mode of an orbital angular momentum signal, modulating the other part of the information to be transmitted into a traditional digital modulation symbol through a P-point constellation diagram, and transmitting the digital modulation symbol by using the orbital angular momentum signal in a specific mode at each transmission time, wherein the modulation method comprises three processes of orbital angular momentum mode modulation mapping, orbital angular momentum mode detection and digital modulation and demodulation:
mapping the first l information bits of a group of random information bit sequences q to be transmitted into a selected orbital angular momentum mode, wherein l is log2L and L are the number of orbital angular momentum modes generated by the traveling wave annular resonant cavity antenna; mapping the remaining m information bits to a P-point modulation constellation, where m is log2M and M are the size of the P point modulation constellation diagram;
the detection of the orbital angular momentum signal mode is realized, because only one mode of orbital angular momentum signal transmission information exists at each transmission moment, a receiving antenna array is used at a receiving end, the receiving antenna arrays are respectively arranged on corresponding circular receiving areas of the orbital angular momentum signals under different modes, and if in each receiving area, two receiving antennas are positioned on the same tangent plane of the OAM wave of the corresponding mode, the included angle between the two receiving antennas and the OAM wave propagation axis is beta, and
Lmaxthe maximum number of OAM modes; then only two receiving antennas in the receiving area of the corresponding mode can receive the transmission signal at each transmission moment, so that at each transmission moment, assuming that the orbital angular momentum signal mode of the transmission information is l, the detection data y received by the receiving end in the corresponding circular receiving area can be represented as
Wherein y is [ y ═ y1,y2]，y1And y2Two receiving antennas in the receiving area corresponding to the transmission signal mode
And
p represents the total transmission power of the orbital angular momentum modulation system, xpIs a symbol modulated by P point constellation diagram, omega is additive white Gaussian noise of a receiving end, hlThe channel response when the signal is transmitted with orbital angular momentum of mode l is h because of two receiving antennaslCan write hl＝[hl,1,hl,2]Detecting the signal mode of orbital angular momentum transmitted by phase gradient method and calculating the phase difference of receiving antenna, the calculation formula can be expressed as
Wherein phi1And phi2For receiving antennas
And
respectively receiving orbital angular momentum phases, wherein beta is an included angle of the two antennas relative to an orbital angular momentum signal propagation axis;
based on the detection result of the orbital angular momentum mode, the maximum likelihood detection method is utilized to realize the demapping of the digital symbols, and the calculation formula can be expressed as
Where χ is the set of modulation symbols for all P-point constellations, y1For the corresponding orbital angular momentum signal mode is
Time-of-day receiving antenna in receiving area
In the received signal of (a) the received signal,
for receiving antennas
The corresponding signal mode is
The response of the channel in time is,
the characteristics of the channel according to orbital angular momentum are expressed as
Where Δ is a constant representing the strength of the signal during propagationCoefficient of degree and phase change, d1For receiving antennas
Distance from the transmitting antenna, λ is wavelength, k is wave number, and
corresponding receiving antenna
The change in phase during the propagation process,
for receiving antennas
The helical orbital angular momentum phase of the received signal.
2. The modulation method according to claim 1 for improving spectral efficiency of satellite communications,
and
the received signals are only different in OAM phase, so only one antenna needs to be selected for estimating the digital modulation symbol in the formula for realizing the demapping by the maximum likelihood detection method.
3. The modulation method according to claim 1 for improving spectral efficiency of satellite communications, wherein the transmitting antenna uses a traveling wave ring resonator antenna, such that the antenna generates different modes of orbital angular momentum signals, which by their orthogonality carry part of the information to be transmitted.
4. The modulation method according to claim 1 for improving spectral efficiency of satellite communications, wherein a receiving antenna array is used at a receiving end, and the different modes of orbital angular momentum signals are presented as a plurality of concentric circles with different radii on the spatial distribution of the receiving intensity; therefore, the receiving antennas are respectively arranged on the corresponding annular receiving areas of the orbital angular momentum signals in different modes, and in each annular receiving area corresponding to the orbital angular momentum mode, two common receiving antennas are used to judge the mode of the OAM wave of the transmission signal through the phase difference value of the OAM between the receiving antennas.
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| CN111464226B (en) * | 2020-03-06 | 2021-09-28 | 哈尔滨工业大学 | Block-by-block detection method of SEFDM (sequence-orthogonal frequency division multiplexing) system based on direct sequence spread spectrum under low compression factor |
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| US20240031007A1 (en) * | 2021-02-24 | 2024-01-25 | Qualcomm Incorporated | Mode division duplex for orbital angular momentum communications |
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