Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/02—Channels characterised by the type of signal
  • H04L5/04—Channels characterised by the type of signal the signals being represented by different amplitudes or polarities, e.g. quadriplex

Definitions

• the present invention relates to a method for generating microwave or RF electromagnetic wave beams with non-zero orbital angular momentum and with intensity distribution concentrated in a limited angular region.
• the radio transmissions are limited by the fact that, for each carrier frequency, at most two signals may be transmitted, said signals being respectively associated with one of the two mutually orthogonal polarization states of the electromagnetic field which is propagated at said carrier frequency.
• OAM Orbital Angular Momentum
• data transmission channel (or in short “data channel”) is used here to indicate any signal which carries information, without any limitation as to the type of said information (data, audio, video, multimedia, etc.) or the technology used for the (analog or digital) generation/encoding thereof and for its transmission (modulation method) .
• the Orbital Angular Momentum (OAM) of an electromagnetic wave is a parameter which uniquely characterizes, in a generic plane perpendicular to the direction in which the wavefront advances, a rotational motion of the wavefront (or the locus of the equiphase points of the field of an electromagnetic wave) centred on the axis along which the said wave propagates.
• Orderal Angular Momentum is currently understood as meaning the number of complete rotations (i.e. through 360 degrees) performed by the wavefront as it advances along the direction of propagation over a distance equal to the wavelength ⁇ . It is a dimensionless quantity, also known as “topological charge”.
• LG Laguerre-Gauss
• beam indicates a set of paraxial waves, i.e. waves which have directions of propagation which diverge only slightly from the direction, z, along which the entire beam is propagated. Said beam is monochromatic (formed by waves which are sinusoids with the same frequency) and coherent (said sinusoids have phases in a deterministic relationship with each other) .
• the electromagnetic field of an LG beam has a progression in space centred on the propagation axis z and completely characterized by two integer indices, respectively defined as azimuthal index i and radial index p:
• - I azimuthal index indicates the number of times the phase of the EM field is periodically repeated when moving along a closed curve which embraces the axis z (a simple example: a circumference centred on the axis z) . It coincides with the Orbital Angular Momentum OAM of the beam itself, which, as mentioned, is the number of complete helical turns performed by the wavefront of the LG beam, when it advances along the axis of propagation z over a distance equal to the wavelength ⁇ ;
• p+1 is the number of relative maximums (alternately, positive and negative) of the field amplitude, succeeding each other along the generic radial half-line .
• OAM are conserved during propagation of a given LG beam, including the far field regions in relation to the source generating it, on which the applicational interest for radiocommunications is concent ated .
• Fluor field is understood as meaning a distance very far from the source which irradiates the EM field with respect to the greatest value from among the wavelength ⁇ , the geometrical dimension d of the source and the ratio d 2 /A.
• I oik 1 2 intensity of the beam
• phase profile which, if measured, may be used for the discrimination of wave beams with different OAM.
• exp(-ilO) describes in fact the dependency of the phase on the azimuthal coordinate ⁇ ; it therefore represents the phase profile of the LG beam.
• Superposition of various electromagnetic fields may generate a beam with a phase which depends on the azimuthal coordinate ⁇ and the radial coordinate r and which may be described by a non- integer OAM value.
• the rotational motion of the wavefront around its propagation axis is substantially independent of the polarization state of the wave itself.
• two independent data transmission channels may be associated with two electromagnetic wave beams which have the same carrier frequency and the same Orbital Angular Momentum, but have two orthogonal polarization states .
• the telecommunication systems able to make use of the Orbital Angular Momentum in order to discriminate between different radio signals envisage transmitting, in a given region of the space, electromagnetic wave beams with different Orbital Angular Momentum and associating, by means of a suitable encoding and modulation system, a different data channel with each of them.
• the aforementioned electromagnetic wave beams are suitably demodulated and decoded so as to acquire the information transmitted via the data channels associated with them.
• a particularly major drawback arises from the fact that a monochromatic electromagnetic wave or a coherent beam of paraxial and isofrequential waves, with non-zero integer Orbital Angular Momentum has an electromagnetic field with zero or substantially zero amplitude (in particular as regards its components perpendicular to the direction of propagation) in a zone around the axis of propagation z on which the rotational motion of the wavefront is centred.
• the wave beams with OAM having a non-zero integer value I have, in the generic plane perpendicular to the direction of propagation z, an intensity distribution in the form of a ring with a central hole; in the case where the radial index p is also a non-zero integer, the intensity distribution takes the form of several concentric rings, all with a central hole.
• the phase of the electromagnetic field has a singularity, namely a zone in which the contributions of the EM field with different phases interfere destructively so that the beam intensity is zero and the phase indeterminate.
• the technical problem which is posed, therefore, is that of providing a method for generating microwave or RF electromagnetic wave beams with non-zero OAM, able to concentrate in a predictive manner a significant fraction of the intensity transported by the beam itself in a limited area of the generic plane perpendicular to the direction of propagation of said beam, it being desirable at the same time to concentrate in this area also the maximum possible amount of information concerning the progression of the beam phase, so as to concentrate in limited areas all the information able to identify, during reception, a single OAM beam superposed on other beams and be able to perform the correct reception of the data flow transported by it, by means of receiving antennas which have smaller dimensions and/or are fewer in number and/or are concentrated in limited areas.
• this method should allow simple control over the location and configuration of this area where there is a maximum power and concentration of information about the phase profile of the beam generated .
• said N ⁇ 2 selected wave beams carry the same power.
• said N beams are Laguerre-Gauss beams and are selected with a respective amplitude
• I ⁇ I where A is a real and positive constant the same for all the N beams selected and where l—, 2 fc ' — is a normalization factor such as to ensure that the powers carried by said N beams are the same.
• the angular aperture ⁇ in the plane of intersection ( ⁇ ) of said circle segment in which the main lobe is included is preferably the angular aperture at - 3dB.
• the step of selecting the N wave beams to be superposed comprises the further steps of:
• the step of selecting the N paraxial and coherent wave beams, with non-zero integer 0AM to be superposed comprises the following steps:
• the step of selecting the N paraxial and coherent wave beams to be superposed, with non-zero integer OAM values comprises the following further steps: a2.0) selecting a value £ eff of the effective Orbital
• each of said N>2 wave beams to be superposed has a reference phase arg(a k ) phase-shifted by n rad with respect to the reference phase arg(3 ⁇ 4+i) of the immediately preceding and/or successive beam with
• the present invention also relates to a method for far-field reception and discrimination of an electromagnetic wave beam with non-zero OAM generated according to the method according to one of Claims 1-10 and transmitted via RF or microwaves towards a designated reception area situated at a great distance from the transmitter, comprising the steps of:
• reception means arranged in the designated reception area at least in the main lobe with angular aperture ⁇ of the OAM beam transmitted; - uniquely determining via said reception means the effective OAM value ff of the limited main lobe of the beam received;
• said unique determination of the effective 0AM value ff of the limited main lobe of the beam received comprises a step of mapping the phase profile associated with the wavefront of the 0AM beam in said main lobe of angular aperture ⁇ ; alternatively, said unique determination may comprise the following steps
• Preferred uses of beams generated according to the invention comprise the use in microwave or RF broadcasting or point-to-point telecommunication systems, for example according to the method of Claims 13 and 14, in directional radio links and in radar systems.
• Figure la shows a schematic illustration of a monochromatic electromagnetic wave with non-zero 0AM inserted in a reference system with cylindrical coordinates ;
• Figure lb shows a schematic illustration of the wave projection of Fig. la in a plane perpendicular to the direction of propagation z;
• Figures 2a, 2b show a graphical illustration, respectively, of the intensity distribution and phase profile of a beam generated according to the prior art
• Figures 3a, 3b show a graphical illustration, respectively, of the intensity distribution and phase profile of a first example of a beam, according to the invention
• Figures 4a, 4b show a graphical illustration, respectively, of the intensity distribution and phase profile of a second example of a beam, according to the invention
• Figure 5 shows a graphical illustration, respectively, of the intensity distribution and phase profile of a third example of a beam, according to the invention.
• the electromagnetic wavefront with non-zero Orbital Angular Momentum rotates about the axis of propagation z and thus traces in space a helical type trajectory which extends along the direction of propagation z (axis of the helix) .
• the Orbital Angular Momentum I expresses the number of complete rotations performed by the wavefront as it advances along the direction of propagation z over a distance equal to the wavelength ⁇ .
• the speed and direction of rotation of the wavefront of the electromagnetic wave are therefore expressed, respectively, by the modulus and sign of the corresponding Orbital Angular Momentum i, which, in a generic plane ⁇ perpendicular to the direction of propagation z (Fig. lb) and positioned at a great distance from a source for transmission of the said electromagnetic wave, may be schematically represented by the number of rotations of a suitable vector V about the axis z and the direction of rotation - clockwise or anticlockwise - of the said vector.
• electromagnetic wave beams with non-zero integer angular momentum which are generated according to the prior art, have a dark zone in an area around the axis of propagation z, namely a zone in which the electromagnetic field has a zero or substantially zero amplitude; in which dark zone, however, there is maximum information about the characteristic phase profile of the said wave (Fig. 2b) .
• the method for generating microwave or RF electromagnetic wave beams with non-zero Orbital Angular Momentum, which are propagated in a direction z and the intensity distribution of which is mainly concentrated in a limited angular region, that is deliberately restricted, also referred to below as main lobe comprises the following steps: a) selection of N>2 paraxial, isofrequential and coherent wave beams, said N beams being substantially coaxial and having:
• a necessary condition for the phase profile of the resultant beam to be a function corresponding to a unique OAM value is that said superposition should be coherent, namely that the EM fields which are superposed should have phases in a deterministic relationship with each other.
• said N beams selected for coherent superposition should all have the same polarization state of the electromagnetic field.
• the N wave beams selected carry the same power .
• the N>2 selected beams are Laguerre-Gauss beams
• said beams are selected with amplitudes
• «>- 1 ⁇ ( 2 3 ⁇ 4 ! , where A is a constant which is real, positive and the same for all the beams selected and is
• the N LG beams to be superposed are therefore preferably selected such that the respective normalization factors ensure
• the N>2 wave beams selected for superposition have the same reference phases arg (a k ) .
• the wave beams selected, with consecutive OAM values 4 each have a reference phase arg((3 ⁇ 4) phase-shifted by n rad with respect to the reference phase arg(c3 ⁇ 4 + i ) of the beam with OAM value +i which is immediately preceding and/or successive.
• ⁇ is defined as being the angular aperture at -3dB, namely between the two points, in the generic plane ⁇ perpendicular to the direction of propagation z, whereat the intensity of the electromagnetic field at a great distance from the source which irradiates it is half the absolute maximum of said intensity;
• step a) of selecting the paraxial, isofrequential and coherent wave beams with non- zero integer OAM to be superposed comprises the following further steps:
• Said angular aperture ⁇ in degrees is negative or positive depending on the sign of the aforementioned values of the angular momentum; a2.1) selecting the maximum (modulus) OAM value l iaxl of the wave beams to be superposed, based on the equation
• the intensity distribution of the beam generated by the coherent superposition according to the method of the present invention is characterized by a single irradiation lobe, whose angular aperture ⁇ at -3dB is, in the far field, defined by:
• said ratio defining the value ff of the effective angular momentum inside the main lobe with concentrated intensity of the beam generated by means of coherent superposition according to the invention .
• the Applicant has first surmised and then established by means of numerous experimental tests that coherent superposition of N (preferably coaxial) beams of paraxial, isofrequential, coherent waves with consecutive, integer, same-sign Orbital Angular Momentum values gives rise to a wave beam with a phase profile having a maximum number of singularities equal to the maximum value I i maK I from among the Angular Momentum values
• one of said singularities is situated along the axis of propagation z (coinciding with the vertex of the circle segment which contains the concentrated intensity region of angular aperture ⁇ in the plane Y perpendicular to the axis z of propagation of the beam generated) . It has also been observed that said central singularity corresponds to an 0AM value equal to a
• Lin I each of which corresponds to a unitary
• the impure beam generated according to the invention has in fact a total 0AM which is non-zero and equal to the sum of the 0AM values of the wave beams which are used during superposition, but intensity transported by the field concentrated in a restricted region inside which a phase profile corresponding to an 0AM value ff is present.
• Said Effective Angular Momentum ff / as defined above is a parameter useful for evaluating in quantitative terms the possibility of performing suitable characterization of the spatial progression of the phase of the field received, while remaining on a small surface (such as that of a receiving antenna used in practice), and therefore being able to discriminate in a small space between several superposed 0AM beams transporting various data channels, this effect being particularly useful for applications in the microwave or RF telecommunication sector.
• ⁇ 1 2, . ⁇ may be performed using a series of arrays of dipoles arranged concentrically.
• N arrays of dipoles each formed as a circular ring, the N circular rings being arranged concentrically and each being designed to generate one of said N selected wave beams.
• D k is the total number of dipoles which form the k-th circular ring
• min[D k ] is the minimum number of dipoles necessary for forming the k-th circular ring
• Acp k is the phase-shift in radians of two consecutive dipoles of the k-th ring
• all the dipoles should be fed with a signal with normalized amplitude cc k - p k !/ ⁇ ( ⁇ t k ⁇ +p k ) ⁇ ; therefore all the dipoles of a same k-th ring must be supplied with the same normalized amplitude a k .
• Each dipole of a same k-th ring must also be supplied with a signal phase-shifted by 2nf k /D k with respect to the signal supplying the adjacent dipole in the k-th ring.
• the diameter of the central ring and the distance between one ring and the next one are preferably not less than the wavelength X.
• the internal circumference should generate the beam with lowest topological charge (OAM) , a lower minimum number of dipoles being necessary.
• the phase profile of the beam obtained has a singularity centred on the axis z, while inside the concentrated-intensity main lobe there is a phase variation corresponding to an effective OAM ;
• Fig. 4 shows in fact an overall phase variation of between 0 and 4n situated within the aperture ⁇ .
• reception means arranged in the designated reception area at least within the limited region of angular aperture ⁇ of the 0AM beam transmitted; -) uniquely determining via said reception means the effective OAM value ff of the limited angular region of the beam received;
• the angular aperture ⁇ is known a priori (for example calibrated during installation) on the receiver side.
• microwave or RF broadcasting or point-to-point telecommunication systems which operate by implementing the following steps : - generation of at least one OAM beam according to the method of Claim 1, each generated beam having a respective effective OAM value ff inside the limited angular region different from the effective
• each of the said beams generated according to the invention has a reference phase non-coherent with the reference phases of the other beams .
• phase information which characterizes the OAM beam generated according to the method of the present invention allows the beam to be discriminated from other fields with the same frequency superposed on it and may be concentrated in a region with angular aperture much more restricted than the 360° of the peripheral angular region typical of the EM field of the beams with integer OAM values obtainable according to the prior art, thus enabling the design of OAM wave telecommunication systems which have receiving antennas that can be concentrated within a limited region of space and towards which, in the case of point-to-point communications, the maximum of the irradiated intensity of the beam generated will be directed.
• the invention is also suitable for broadcasting communications where it is sufficient for the main radiation lobe to be sufficiently broad to strike the entire area to be covered. Still with reference to broadcasting, but not only thereto, the invention is suitable also for those cases where the power irradiated by an antenna which transmits OAM beams assumes very low values in one or more directions which, for various reasons, must be protected from the antenna radiation.

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Citations (1)

• 2013
  • 2013-04-19 IT IT000641A patent/ITMI20130641A1/it unknown
• 2014
  • 2014-04-17 WO PCT/IB2014/060815 patent/WO2014170869A1/en active Application Filing

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