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Performance Comparison of Near-Field Focused and Conventional Phased Antenna Arrays at 140 GHz
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The use of large arrays might be the solution to the capacity problems in wireless communications. The signal-to-noise ratio (SNR) grows linearly with the number of array elements N when using Massive MIMO receivers and half-duplex relays. Moreover, intelligent reflecting surfaces (IRSs) have recently attracted attention since these can relay signals to achieve an SNR that grows as N2, which seems like a major benefit. In this paper, we use a deterministic propagation model for a planar array of arbitrary size, to demonstrate that the mentioned SNR behaviors, and associated power scaling laws, only apply in the far-field. They cannot be used to study the regime where N∞. We derive an exact channel gain expression that captures three essential near-field behaviors and use it to revisit the power scaling laws. We derive new finite asymptotic SNR limits but also conclude that these are unlikely to be approached in practice. We further prove that an IRS-aided setup cannot achieve a higher SNR than an equal-sized Massive MIMO setup, despite its faster SNR growth. We quantify analytically how much larger the IRS must be to achieve the same SNR. Finally, we show that an optimized IRS does not behave as an “anomalous” mirror but can vastly outperform that benchmark.
A scheme for wireless power transfer (WPT) in the radiative near-field (Fresnel) region is presented. The proposed Fresnel WPT scheme is designed to focus microwaves to a diffraction-limited region where a detector can be positioned, achieving reasonably high power transfer efficiency over moderate distances. The configuration consists of transmit and receive microstrip patch array antennas, with the receiving antenna connected to a power-harvesting half-wave rectifier (rectenna). Fresnel region operation enables the fields radiated by the transmitting aperture to be localized both in range and cross-range. Using Fresnel region focusing, we achieve an increase of 66.8% in the amount of received power when compared to the performance of a conventional beam-forming array. We also demonstrate the efficiency improvement by powering an LED using the on-axis and off-axis focusing configurations.
A large electronically reconfigurable reflectarray antenna that has 160 × 160 reflecting elements was designed, fabricated, and evaluated so that it could be applied to a millimeter-wave imaging system operating in the 60-GHz band. To make it feasible to construct such a large reflectarray, the reflecting element structure had to be simple and easily controlled; therefore, a reflecting element consisting of a microstrip patch and a single-bit digital phase shifter using a p-i-n diode was employed. A large reflectarray antenna was fabricated using the reflecting elements. The measured radiation patterns and antenna gain were in good agreement with those that were calculated. Furthermore, the near-field beam focusing capabilities, which was required to image near-field objects, were also verified through an experiment. Finally, the response time for beamforming was measured, which was far less than the system requirements.
An antenna is a transducer between electromagnetic waves that are radiated through space and the electromagnetic waves that are contained by a transmission line. It is a two-port transducer where one port represents space, but it sometimes involves several waveguide or coaxial line ports. The transfer function of the antenna might contain an efficiency factor because of energy dissipated in the antenna, and would contain the factors governing the spatial distribution of intensity radiated or received by the antenna. Because of the vector character of the field, the spatial portion of the transfer function is multipole in nature. An isotropic antenna is impossible. There would be a function describing the spatial distribution of field strength produced by the antenna. This is called the antenna pattern and is represented by one or more cross sections in the principal planes. The antenna pattern in a specified plane is a plot of the field strength amplitude versus a space coordinate, which is usually an angle. Sometimes, field intensity rather than field strength is plotted. Linear antennas, like linear networks, obey the reciprocity law. Although large antennas are sometimes used with a fixed pattern in space, the more interesting and difficult cases allow the pattern or beam to move in space.
Very large MIMO is a technique that potentially can offer large network capacities in multi-user scenarios where the users are equipped only with single antennas. In this paper we are investigating channel properties for a realistic, though somewhat extreme, outdoor base station scenario using a large array. We present measurement results using a 128 element linear array base station and 26 different user position in line-of-sight (LOS) and 10 different user position in non line-of-sight (NLOS). We analyze the Ricean K-factor, received power levels over the array, antenna correlation and eigenvalue distributions. We show that the statistical properties of the received signal vary significantly over the large array. Near field effects and the non-stationarities over the array help decorrelating the channel for different users, thereby providing a favorable channel conditions with stable channels and low interference for the considered single antenna users.
A detailed performance analysis of near-field-focused planar arrays is addressed. Design curves and performance data are shown for planar square arrays as a function of the array size, the inter-element distance, and the focal distance. The performance curves are compared with results presented in earlier studies relevant to continuous-aperture antennas, as well as with numerical results obtained from a full-wave analysis of planar microstrip arrays.
Some focusing properties of Fresnel zone plate (FZP) lens antennas in the near-field region are presented at the ka-band. Simulated and measured results of the FZP antenna show displacement of the maximum intensity of the electric field along the axial direction from the focal point toward the antenna aperture. This displacement increases as the antenna's focal length increases. In addition, the focused beam scanning of the FZP lens antennas in the radiation near-field is examined.
Antenna arrays are normally characterized in terms of their far
field radiation patterns, beam width and directive gain. However, for
some applications the array is focused in the near field. The focus size
is determined in terms of the near field properties of the array. The
focusing is done through adjusting the phase of the elements of the
array. The simulations are done using the method of moments (MoM). Two
methods for adjusting the phase are compared. The focusing techniques
analyzed here have several applications including imaging apertures, for
which high spatial resolution is of great importance
The diffraction field of continuous, rectangular apertures is analyzed when the system is focused in the Fresnel region. The Fresnel region is defined by phase and amplitude considerations, and the boundary separating the near and Fresnel regions is given as well as the conventional boundary distinguishing between the Fresnel region and far field. Curves have been plotted for several square apertures of varying size for both uniform amplitude illumination and tapered illumination while the focused condition is maintained. A theorem is given proving that the far-field pattern occurs in the focal plane of a continuous, focused aperture. Also included is the expression for gain of a focused rectangular antenna, and a discussion of the concept of gain where strict 1/R^{2} dependence does not exist. A comparison of focused circular and rectangular apertures is made in regard to beamwidth, energy between 3-db points, and energy in the main beam. Consideration is also given to the problem of maximizing the field at the point of focus, and as a corollary the "depth of focus" is examined.
This paper presents a novel antenna having a beam focused to a small spot in the near-field zone which is intended for use in microwave industrial inspection. The array is implemented using broadband U-slotted microstrip patch-antenna elements and an integrated microstrip feeding network. Analogy with a dielectric lens is used to generate appropriate feeding coefficients for the elements of the array. The concept is demonstrated with two practical implementations having 300- and 126-mm focal distances. The measured results are presented and shown to be in good agreement with the predictions, validating the development concept.
Antenna theory: analysis and design
- C A Balanis