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Antenna And Wave Propagation By Kd Prasad Pdf Free 1370
meanwhile, structured beam patterns are used in quite a few communication systems. for example, all in-line modulation schemes and mimo techniques for optical communications can take advantage of structured beam patterns [ 78 ]. in addition, a structured beam pattern can be used for beam steering to achieve the desired directionality or to provide active beam steering to achieve beam steering while the beam pattern is tilted relative to the front direction of propagation.
in this work, we take advantage of structured oam patterns to enhance capacity in ofdm-based los-free rf communication systems. the receiver structure is based on the use of an array of receiver aperture elements and signal processing for coherent demodulation [ 38 ] and is relevant to the future development of sdm communications.
the main contributions of this paper are summarized as follows: (1) we discuss the detailed structures and dimensions of the transmitter and receiver oam antennas for different oam modes; (2) we demonstrate the application of specially-designed oam mode multiplexing to improve the capacity of a conventional co-polarized communication link based on an array of planar split-ring apertures; (3) we discuss the design of coherent receiver structures for multiplexed communication systems; (4) we present two different antenna structures: (a) the geometrically-optimized lens-like antenna; (b) a planar dipole antenna with the aperture serving as an efficient hybrid reflector and coupling element.
the ability of these multiplexing devices to function over distances of a kilometer or more requires the use of an appropriate signal format. for example, rf communications systems can adopt the oam multiplexing scheme with circular and phased arrays. this approach utilizes the oam property in the em wave to provide orthogonal radio wave propagation, in which the generation, multiplexing and demultiplexing of the oam wave beams are accomplished using the radio transmitter and receiver [ 29, 30, 38, 42, 44, 48, 49, 50, 54, 55, 57, 62, 66, 68, 69, 71, 73, 75, 76, 80, 81, 83 ]. the orthogonality of the oam waves can be used to avoid crosstalk among multiple channels and thus achieve an almost equivalent spectral efficiency to that of an sdm system, where a superposition of signals with orthogonal em waveforms is employed [ 43, 57, 77 ]. this can be achieved by encoding the oam waveforms in a unique order and with different helix unwrapping directions for each data channel [ 48, 55, 58, 72 ]. this has become a popular technology for oam-based radio communications systems [ 41, 49, 54, 57, 60, 61, 62, 65, 66, 67, 68, 69, 71, 73, 75, 76, 77, 80, 81, 83 ].
another example of multiplexing with circular antenna arrays is shown in figure 12 a. in this case, each array element may have an additional phase delay, φ(k), to produce an effective phase along the steering direction. the superposition of the oam beams from all the channels leads to an oam wave that can be generated by a coherent superposition of the phases produced by each array element. in practice, this type of circular array can be implemented using a number of helical antennas that are spatially arranged. for a particular application, it is possible to combine multiple helical antennas and feed them with phase shifters and active elements (for example, a power splitter) to spatially multiplex the channels [ 31, 45, 86, 87 ]. this approach can maintain the beam direction when the arrays are rotated and can also be implemented over a longer range. furthermore, this approach can be implemented with a considerably reduced number of antennas compared with conventional circular arrays [ 31, 45, 86, 87 ].
in this paper, the oam multiplexing technique is briefly outlined and the design considerations and general guidelines for rf oam multiplexing systems and links are explored. as an example, the results from simulations for the l=100m four-oam-mode multiplexed link using a 100m link [ figure rsta20150439f9-rsta20150439f9-figure, rsta20150439f9 class=article__inline-figure>. the transmitted beam size was 3cm and receiver aperture size was 4.5cm.
recent works have focused on exploring the performance of fso systems using oam multiplexing. li et al. [ 72 ] and their follow-up works [ 73, 74 ] have proposed and experimentally demonstrated a novel real-time detection scheme in an experiment at 65ghz. they reported a high-speed 16-qam link of 12gbits1 at a reach distance of 100m. after propagation over 2.1m from the transmitter to receiver, the oam beams were received with different centers and sizes. figure 11 a shows the reconstructed constellations for all modes with measured ber curves for four best modes. they have demonstrated the transmission of a 4gbits1 16-qam signal at 60ghz. similar to the experiment in 2.5, almost all the constellations of four oam beams were recovered and no power leakage into the neighboring beams was observed (except at the edges of constellations). they further extended their previous work by testing an 8gbits1 16-qam signal at 60ghz with ber curves as shown in figure 11 b. since the receiver was only enabled for 16-qam detection, ber curves of all 16-qam signals were included as well. the link with =+1 oam beams and two polarizations exhibits ber curve with a data rate of 10.3bits1 at 30db. the link with =+3 oam beams and two polarizations exhibits ber curves with a data rate of 9.75bits1 at 35db. this work presents the first demonstration of an fso system capable of transmitting and recovering multiple oam beams and detecting multiple constellations at the same time. it also provides the first systematic investigation of the impact of link parameters, including receiver size, physical separation between transmitter and receiver, and misalignment between transmitter and receiver. figure 11. demonstration of a 16gbits1 fso link at 60ghz using four oam beams at a reach distance of 100m [ 72 ]. (a) reconstructed constellations for four oam beams (two polarizations for each oam beam) with measured ber curves for four best oam beams. (b) ber curves for eight oam beams (four polarizations for each beam). (copyright macmillan publishers ltd. 2014.)
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