| 1. |
- Castillo Tapia, Pilar, et al.
(author)
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Modulated Geodesic Lens Antenna Array
- 2021
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In: 2021 15Th European Conference On Antennas And Propagation (Eucap). - : IEEE.
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Conference paper (peer-reviewed)abstract
- We present the design of a geodesic Luneburg lens array antenna. The water drop lens is a compact and highly-efficient alternative to the planar Luneburg lens. Our proposed lens antenna is able to steer 110 deg in azimuth with negligible scan losses. On the other hand, the linear array of four elements increases 6 dB of directivity in elevation. A feeding network composed by a 1:4 power splitter and four different phase shifters allows to steer 60 deg in elevation.
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| 2. |
- Fonseca, Nelson J. G., et al.
(author)
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Compact parallel‐plate waveguide half‐Luneburg geodesic lensin the Ka‐band
- 2020
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In: IET Microwaves, Antennas & Propagation. - : Wiley-Blackwell. - 1751-8725 .- 1751-8733.
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Journal article (peer-reviewed)abstract
- A parallel-plate waveguide half-Luneburg geodesic lens is designed and experimentally validated in the Ka-band. The geodesic lens profile is modulated using spline functions to reduce its height, while the symmetry halving the lens enables reduction of its in-plane dimensions. This design provides high gain, equivalent to that of a full-Luneburg lens, over a reduced angular range of ±20° to ±30° in azimuth. The specific design reported here has a maximum realized gain of 23.3 dBi in measurement, compared with 23.8 dBi for a full-Luneburg lens twice the size of this design. Very wideband operation is demonstrated both in simulations and measurements. This design is of interest for applications having limited space like millimetre-wave systems on board cubesats and small satellites.
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| 3. |
- Fonseca, Nelson J. G., et al.
(author)
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Equivalent Planar Lens Ray-Tracing Model to Design Modulated Geodesic Lenses Using Non-Euclidean Transformation Optics
- 2020
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In: IEEE Transactions on Antennas and Propagation. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0018-926X .- 1558-2221. ; 68:5, s. 3410-3422
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Journal article (peer-reviewed)abstract
- This article describes a design procedure that enables a time-efficient evaluation of the focusing properties of modulated geodesic lenses using ray tracing on the equivalent gradient-index planar lens. The method uses transformation optics to define the equivalent planar relative permittivity distribution of axially symmetric surfaces and a ray-tracing model to evaluate the phase distribution in the aperture of the lens. This approach is of interest to optimize modulated geodesic lenses having polynomial profiles, reducing their height while preserving their wideband behavior and wide angular focusing properties. The approach is validated with a specific lens design. The profile is optimized at 30 GHz, while the focusing properties are monitored over the complete Ka up-link frequency band allocated to satellite communications (i.e., 27.5 & x2013;31 GHz). The manufactured prototype produces 21 beams equally spaced every 7.5 & x00B0; over the extended angular range of & x00B1;75 & x00B0;. The ray-tracing model results are compared in detail with the corresponding full-wave model results and experimental data. The manufactured design has return loss better than 15 dB over a fractional frequency bandwidth larger than 30 & x0025;, in line with the predictions. Excellent scanning properties are demonstrated over an angular range of & x00B1;60 & x00B0; with scan losses below 1 dB and good pattern stability, including on sidelobe levels. A height reduction by a factor of 4, when compared to a conventional geodesic lens, is demonstrated with this specific design.
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| 4. |
- Fonseca, Nelson J. G., et al.
(author)
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The Water Drop Lens : A Modulated Geodesic Lens Antenna Based on Parallel Curves
- 2018
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In: ISAP 2018 - 2018 International Symposium on Antennas and Propagation. - : Institute of Electrical and Electronics Engineers (IEEE).
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Conference paper (peer-reviewed)abstract
- This paper discusses the design of parallel plate waveguide geodesic nses based on parallel curves enabling low-profile beamformers with a ry wide angular scanning range. The proposed approach provides smooth ns profiles with additional degrees of freedom for the optimization of odesic lenses, enabling Gaussian beam but also shaped patterns. The tational symmetry of the lens results in a shape looking like water op ripples, also known as capillary waves, at the surface of a fluid, nce the water drop lens appellation.
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| 5. |
- Liao, Qingbi, et al.
(author)
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Channel Capacity Analysis of Different Mobile Antenna Array Topologies in the Outdoor Urban Environment for 5G mm Wave System
- 2017
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In: 2017 IEEE Antennas and Propagation Society International Symposium, Proceedings. - : Institute of Electrical and Electronics Engineers (IEEE). - 9781538632840 ; , s. 1473-1474
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Conference paper (peer-reviewed)abstract
- The 15 and 28 GHz frequency bands have been identified as interesting candidates for future 5G cellular communication systems. In this paper, based on ray-tracing simulations, we investigate the channel capacities at 15 and 28 GHz for a number of different array processing techniques, including beamforming, spatial multiplexing, and two types of hybrid beamforming. This is done for an 8 x 32 multiple input multiple output system, for the single user case. The hybrid beamforming techniques combines the beamforming and spatial multiplexing techniques, in order to reduce the number of analog-to-digital converters and thereby reducing the system complexity, while still maintaining multiplexing capabilities for improved system capacities. The achieved channel capacities the hybrid beamforming techniques with up to four parallel streams outperforms pure beamforming, but does not reach capacities close to that of full spatial multiplexing. This indicates that the outdoor channel can support multiplexing of more than four parallel data streams to a single user.
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| 6. |
- Liao, Qingbi, et al.
(author)
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Compact Multibeam Fully Metallic Geodesic Luneburg Lens Antenna Based on Non-Euclidean Transformation Optics
- 2018
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In: IEEE Transactions on Antennas and Propagation. - : IEEE. - 0018-926X .- 1558-2221. ; 66:12, s. 7383-7388
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Journal article (peer-reviewed)abstract
- Non-Euclidean transformations have been recently proposed to produce a link between 3-D homogeneous surfaces and 2-D dielectric lenses. Therefore, the propagation in a geometrical surface has the same response of an equivalent refractive index distribution. By using this concept, we propose here a fully metallic Luneburg lens where the propagation is only in the air. Two metallic plates, following a curved shape, are employed to support the propagation mimicking the designed curvature. To reduce the height of the required curvature, the surface has been mirrored twice with respect to two z constant planes. The lens is fed by 11 waveguide ports spaced with an angle of 12.5° providing 1-D beam scanning over an angular range of ±62.5°. A prototype is manufactured and measured with a good agreement with the simulated results between 25 and 36 GHz to demonstrate the concept.
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| 7. |
- Liao, Qingbi
(author)
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Fully metallic antennas for millimeter wave applications
- 2021
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Doctoral thesis (other academic/artistic)abstract
- Modern societal demands for a high data throughput, short time latency and low energy consumption are difficult to satisfy using current wirelesscommunication techniques. The carrier frequency of previous wireless communication, such as broadcast, the global system for mobile communication, and wireless local area networks, are in the sub-3 GHz spectrum. Electromagnetic waves in the sub-3 GHz spectrum possess a long wavelength and small free space path loss (FSPL), but with a narrow absolute bandwidth. This absolute bandwidth limits the channel capacity according to Shannon theory. Under the assumption that relative bandwidth is fixed, the absolute bandwidth is proportional to the carrier frequency. That means, wireless communications with high carrier frequency can provide wide bandwidth and large channel capacity. Besides, the sub-3 GHz spectrum is already too crowded to have future advanced wireless communications. Nowadays, it is essential to move carrier frequencies to higher frequency spectrum. The millimeter wave (mmWave) frequency band can provide an extensive bandwidth but suffers high atmospheric attenuation and FSPL. The highattenuation and loss limit the propagation distance of mmWave to a few kilometers. Additionally there is a high attenuation due to precipitation, as the wavelength of mmWaves are of the same order in size as rain drops. Due to these losses, there are restricted applications in the mmWave band used for wireless communications. However, the electromagnetic spectrumshortage encourages new researches to look for solutions overcoming the drawbacks of mmWave.Specific requirements on antenna designs are imposed by using mmWave communication, including manufacturing costs, integration, efficiency, scanningrange, and directivity. Antennas designed for the mmWave have a small physical size, which requires finer manufacturing resolution and increases manufacturing costs. To compensate for the high FSPL and attenuations, high directive antennas with low side lobe level are favorable. To improve the radiation efficiency, it is preferred to use fully metallic structuresas opposed to structures containing dielectrics for antennas operating in the mmWave range. This thesis investigates the innovative techniques for designing high performance fully metallic antennas in mmWave. Antennas made in gap waveguides and geodesic lens antennas have low manufacturing costs, low loss, and high directivity. The gap waveguide technology can be used to manufacture antennas in separated pieces. These pieces are united together afterwards. The manufacturing cost is reduced in this way. In gap waveguides, the radiation leakages from gaps between separated pieces are prevented using metasurfaces. The research emphasis is placed on the properties of glide-symmetric metasurfaces. Comparing with non-glide metasurfaces, glide-symmetric metasurfaces have an extended electromagnetic bandgap. On the other side, the geodesic lens antenna is designed based on geometrical optics (GO). The graded index lenses can be transformed to geodesic shapes through GO. Since the mmWave presents optical propagation characteristics, GO can be used as a good approximation. A ray-tracing model is developed to calculate the radiation patterns of geodesic lenses and its performance is verified by full wave simulations. Geodesic lens antennas implemented in parallel plate waveguides are in full metal and allow waves to propagate in vacuum or air.
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| 8. |
- Liao, Qingbi, et al.
(author)
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Generalized Ray-tracing Model for Modified Geodesic Luneburg Lens Antennas
- 2021
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In: 2020 international symposium on antennas and propagation (isap). - : IEEE. ; , s. 193-194
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Conference paper (peer-reviewed)abstract
- In this paper, we present an accurate and efficient ray-tracing model for calculating the radiation pattern of rotationally symmetric geodesic lenses. The performance of the ray-tracing model is compared to full-wave simulations, CST and HFSS, through the radiation patterns of a modified geodesic Luneburg lens antenna. The modified geodesic Luneburg lens profile has two parts, one is the original geodesic curve using geometrical optics, and one is a transition for reducing reflections. The radiation pattern from the ray-tracing model has a good agreement with the results from the full-wave simulations. However, the ray-tracing is significantly faster than the full-wave simulators.
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| 9. |
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| 10. |
- Liao, Qingbi, et al.
(author)
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Global Energy Efficiency Optimization of a Ka-Band Multi-Beam LEO Satellite Communication System
- 2021
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In: IEEE Access. - : Institute of Electrical and Electronics Engineers (IEEE). - 2169-3536. ; 9, s. 55232-55243
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Journal article (peer-reviewed)abstract
- We investigate the issue of global energy efficiency optimization in a multi-beam low Earth orbit (LEO) satellite communication system. Current terrestrial networks provide high-quality and low cost communication mainly in densely populated areas. However, the cost of extending wideband coverage to remote areas is unaffordable. LEO satellites provide a low cost solution for offering global coverage by supporting terrestrial networks. We consider downlink transmissions in Ka-band, where a LEO satellite transmits to access points or users directly. Impairments due to Ka-band channels, inter-beam interference as well as Doppler effects are taken into account in our mathematical model. We formulate the problem of jointly optimizing beam assignment and power allocation for maximizing global energy efficiency. Given the intractability of this problem, we propose to divide it into two subproblems: first, beam assignment optimization under fixed power per beam, and second, power allocation optimization under fixed beam assignment. We devise two algorithms, beam-wise power optimization and equal power optimization. These two algorithms solve the subproblems, beam assignment and power allocation, in a different way. Numerical results show that our proposed methods can greatly improve the global energy efficiency compared to the baseline method with a fixed power per beam.
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