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New designs of slot elements and a new configuration for array antennas for synthetic aperture radar (SAR) at L- and X-bands are presented. The L-band elements are cavity-backed crossed slots for dual polarization, and for X-band, interleaved ridged waveguide slot subarrays are used. The height of the L-band cavity is reduced by adopting an inverted-T configuration. Each of the crossed slots is fed by a pair of differential probes to ensure the correct modal excitation with high isolation. For evaluation, the X-band slot subarray is center fed by a coax probe although slot excitation is also possible. An initial prototype array comprising $4 \times 3$ L-band elements interleaved with X-band subarrays was fabricated to allow initial measurements. This element design is applicable for larger arrays and can be readily configured to frequency ratios other than L- and X-bands. A construction advantage of the slot design over SAR patch arrays is the use of all-metallic surfaces. [ABSTRACT FROM AUTHOR]

Multiple element antennas (MEAs) support multiple-input multiple-output (MIMO) communications and other antenna diversity techniques, resulting in better communications performance than available from single antenna systems. The space occupied by MEAs becomes important when the antennas are integrated into small or portable devices, and a metric would be helpful for the design of compact MEAs. A figure of merit for the spatial efficiency is presented here for evaluating the MEA compactness with respect to the MEA bandwidth and the potential communications performance. A spherical volume is used and different propagation scenario models can be incorporated. The impact of element efficiencies, mutual coupling, correlation, diversity scheme, and propagation scenario are included. [ABSTRACT FROM PUBLISHER]

A desirable characteristic of a multiple element antenna (MEA) is to be compact, but a smaller size tends to lead to higher ohmic and mutual coupling losses. A metric for the efficiency of the MEA would help clarify the tradeoffs between compactness and performance. In a MIMO/diversity antenna, the total efficiency seen at each port directly affects the signal-to-noise ratio (SNR) in the diversity branch. The SNR after diversity combining governs the performance of the diversity antenna system. In this paper, MEA efficiencies is therefore discussed and formulated in the context of mutual coupling and diversity combining. The impact of MEA efficiency on the diversity gain and the information theoretical capacity is also formulated and demonstrated using measurements of example MEAs. With these formulations, an equivalent number of idealized (lossless, uncorrelated, uncoupled, equal power) branches can be found for an MEA, and this defines the diversity order and the capacity order of the MEA. With this metric, the performance of different MEAs can be compared. [ABSTRACT FROM PUBLISHER]

MIMO systems, including those for mobile channels, are well studied as combinations of antennas, other microwave components, and sophisticated digital signal processing. The communications performance is based on many assumptions, including that of linear channels. But in practice, key components such as microwave amplifiers are non-linear. This paper addresses MIMO compensation of non-linear amplifiers at both the transmitter and the receiver. The in-band distortion is modeled using Busgang's theorem. Based on this model, a decision algorithm obtains an optimized initial estimate of the transmitted symbols which are used for subsequent channel estimation. The final channel matrix estimate is obtained through two alternative methods: statistical linearization; or analytical linearization. The approach is blind in the sense that no pilots are used for sounding the channel. Performance is evaluated by simulation, allowing comparison with the benchmark of coherent detection with perfect channel knowledge. We establish the maximum SNR value for which the nonlinearities are essentially compensated. The performances of the two methods converge as the MIMO system becomes linear. The linear case allows fair comparison with known results of the conventional decision-directed Kalman filtering, and two pilot-aided systems. Finally we compare the performance with a look-up table technique for compensating the non-linearity. [ABSTRACT FROM PUBLISHER]

A new blind channel estimation technique is presented for uncoded orthogonal frequency-division multiplexing (OFDM) systems. Instead of using pilots to sound the channel, a decision algorithm first makes primary estimates of the data symbol for each subcarrier based on a constrained linear minimum mean square error (MMSE) criterion. Then, these estimates are applied to optimal MMSE channel estimation. The technique requires only one value from the time–frequency correlation of the channel transfer function. Performance is evaluated by simulation so that comparison can be made with known optimal coherent/differential detection. Compared with known decision-directed Kalman-based estimation and two pilot-aided OFDM schemes (block pilots and comb pilots), the presented technique performs better for regions with mid to high signal-to-noise ratios (SNRs). Its robustness to the time variation of the channel is also quantified by simulation, showing only small degradation in performance relative to the quasistatic case of wireless local area network (WLAN) systems. Finally, the impact of covariance assumptions in the channel modeling is quantified using simulation, offering a feel for the performance with mismatch between the channel model and the receiver assumptions. [ABSTRACT FROM PUBLISHER]

We address the optimization of the geometric design of two classes of multifaceted antenna arrays, namely, pyramids and pyramidal frusta. The optimization involves choosing the face elevation and the number of arrays. The optimal face elevation minimizes the maximum scan angle encountered by the arrays, given a general hemispherical coverage requirement. Firstly, an approach is presented to compute the face elevation of the pyramidal frustum, based on equalizing the maximum scan angles. It is demonstrated that the equalization-based approach is not always optimal. Secondly, a new optimal approach based on minimax optimization is presented and used for both pyramids and pyramidal frusta, along with a comparison of the two approaches. Finally, the choice of the number of arrays is addressed using different cost functions including scan angles, scan losses, antenna dimensions, geometric directivity, and total number of antenna elements. Quantitative guidelines on how to choose among planar, pyramidal, and pyramidal frustum antenna systems are outlined. [ABSTRACT FROM AUTHOR]

An approach is presented for optimizing the geometry of pyramidal antenna arrays, given a full azimuth and a full or limited elevation scan requirement. It involves choosing the best face elevation and number of faces of the pyramid, which minimize the maximum scan angle while maximizing the geometric directivity. The analysis shows that the 4-face pyramid is the best choice. The derivation assumes that the distance to the antenna is infinite (the true far-field case). However, when finite distances are considered, the maximum scan angle increases only slightly at the minimum far-field distance used for low sidelobe pattern measurement. [ABSTRACT FROM AUTHOR]

Presents information on a study which reviewed the spaced directive antennas for mobile communication channel using the Fourier transform method. Spacing requirements for directional antennas; Frequency and space diversity for general directive antennas; Conclusions.

Presents a class of simple antennas that produce diversity action and offer performance improvement over conventional single-port (nondiversity) antennas. Advantage of the system over conventional antenna; Patterns for antenna diversity; Antenna examples.

Presents information on a study on super-resolution of pulsed multipath channels for delay spread characterization. Use of simple rectangular pulses for channel characterization; Demonstration of super-resolution using a subtractive deconvulsion and an inverse filter technique; Details on channel sounding; Conclusion.

Presents information on a study which introduced a beam partitioning for a mobile communication system surrounded uniformly by dense scatterers using the Fourier method. Derivation of angular partitioning; Beam synthesis; Analysis of fading rate reduction scheme; Conclusions.

Presents information on a study which described the use of two-dimensional pattern measurements as a tool for evaluating an antenna configuration for reducing power absorbed in the head. Principal pattern cut technique and experiment; Experimental results and discussion; Conclusions.

Discusses the power relations in a dielectric slab for complex dielectric constant modelling of biological tissue at microwave frequencies. Investigation of the microwaves absorbed by the human head from handheld communication antenna; Function of the system as a two-dimensional set-up; Absorption reaction to increase slab conductivity; Identification of alternative complex dielectric constants.

In a recent paper, Blumenfeld [1] outlined the continuous modeling approach to the description of the joint distribution of homes and workplaces in a city. In this note, the circularly symmetric form of the quarivariate normal model is further analyzed, and distributions of the desire line lengths and other variables are derived. [ABSTRACT FROM AUTHOR]

Presents a study on conditions for angle diversity for antenna beams. Correlation operations in a circular support; Rotation of an elliptic beam; Conclusions.

This paper develops formulas for estimating average distances associated with zones. The average distance between two points, one located at random in a zone and another located at random in another, is first investigated. The average distance from a fixed point to a point located at random in a zone, and the average distance between two points located at random in the same zone, are also examined. The accuracy of the approximations for these average distances are found by examining some simple cases. [ABSTRACT FROM AUTHOR]

Formulas are given for determining the cost and accuracy of a given sampling scheme. Home-based, work-based and roadside surveys are examined. As in any estimation problem, one may optimize either cost or accuracy but not both. For any required accuracy, formulas are given to minimize the survey cost. It is difficult to obtain a measure of accuracy that leads to tractable mathematics; however, a formula is given that will minimize a linear combination of the sample variances of the estimators of the traffic flows. Some approximations are made that lead to simple formulas for determining the sample size. An example for a home-based survey is worked assuming preliminary estimates given by the gravity model. [ABSTRACT FROM AUTHOR]

The distribution of hourly volumes is described and its form and characteristics are discussed. Theoretical models of the distribution are derived so that an estimate of the distribution may be obtained if only a fraction of the traffic data for the year is known. Depending on the size of this sample different estimation procedures are recommended. The hour has long been the conventional counting interval used for traffic counts over long periods. Other counting intervals are considered to determine if the hour is the best unit. The distribution of daily traffic volumes is found to be best represented by a displaced gamma distribution. [ABSTRACT FROM AUTHOR]

Coverage in mobile communications systems requires an acceptable link gain over a maximum service area, with areas of low link gain being problematic. A standard approach to maximize link gain is to deploy multiple-input–multiple-output (MIMO) antennas that adapt to the changing multipath. There is also recent interest in using active surfaces for adaptive reflections within the multipath environments, and this calls for new ideas at the system and component levels. A critical mechanism in urban propagation is corner diffraction, which illuminates shadow areas but with low link gain. We present a simple add-on system for corners, comprising a simple scattering dipole or scattering array, which can significantly improve link gain in the shadow region over a wide bandwidth. The concept is demonstrated through analysis, simulation, and physical measurement. Only shadowed areas are boosted, and the line-of-sight coverage is not significantly affected. Our demonstration is for a passive, fixed configuration, which is suitable for retrofitting to existing systems. [ABSTRACT FROM AUTHOR]

Waveguide arrays comprise slot radiators cut into hollow, single-mode, standing wave structures. Their impedance bandwidth and compactness are limited by the waveguide cutoff. By using the TEM mode of a coaxial waveguide, a more compact structure with a larger bandwidth is possible at the expense of increased ohmic loss in the waveguide. Using this basis, a new type of slotted coaxial waveguide antenna is presented. A compact waveguide array means that the element and array aspects are not independent, and with no accurate transmission line models for the structure, the design is from a parametric study by simulation. Two basic designs are demonstrated: the first with no constraint on the azimuthal extent of the slots, and the second where some mechanical strength of the waveguide is maintained by constraining the azimuthal extent of the slots to be 90° or less. The parametric studies provide insight for realizing the high bandwidth, and measurements of prototypes confirm the performance. These antennas are useful for integration with cylindrical structures such as masts or bicycle and drone frames. [ABSTRACT FROM AUTHOR]

New microelectronic fabrication techniques enable the creation of high-performance on-chip antennas by using new or modified antenna designs. A system's performance is normally improved by a high antenna gain which requires an electrically large aperture typically supporting an array. Consequently, array design is of particular interest for high-performance on-chip systems. The feed for an array is a challenge because it must be low loss while extending over the large aperture. A solution is the waveguide slot array which integrates antenna elements with a low-loss feeding structure. This study presents a new slot array structure which is motivated by advanced microelectronic fabrication. It comprises a triangular waveguide as the array feed, and slots for the elements. This allows fabrication by a self-assembly technique which lithographically controls the shape of a three-dimensional structure. This process is complementary metal-oxide-semiconductor compatible, enabling radio and radar system-on-chip solutions. For production, the low mass of the structure makes it inherently robust, although hermetic packaging with a radio window akin to UV-erasable eproms would be required (not included here). The design concept is demonstrated with a physical prototype operating at a low frequency. The fabrication concept is demonstrated by a physical onchip prototype, and its performance is assessed by simulation. [ABSTRACT FROM AUTHOR]

The article discusses various reports published within the issue, including one on antenna design and characterization, and another on innovations in MIMO systems.

Presents a study which discussed a method for evaluating antenna diversity systems with an example of a handset using antennas in indoor mobile communication. Issues on antenna matching; Conclusion.

Examines the empirical justifications for the lognormal, Rayleigh and Suzuki probability density functions in multipath fading channels. Statistical descriptions of fading processes; Quantification of the rate of convergence of the central limit theorem (CLT) for the lognormal and Rayleigh distributions; Conclusion.

A micro-electro-mechanical systems fabrication process has been developed to create self-assembled on-chip high efficiency antennas. The self-assembly creates out-of-plane antenna structures, which can have good radiation efficiency on low resistivity substrates. The structural material is SU-8 and the self-assembly curvatures are defined lithographically. The losses are reduced by having the antennas, transmission lines, and an isolating conducting ground plane placed on top of the supporting (and lossy) dielectric layer. The fabrication process includes deposition of a thick metal layer, which envelopes the antenna structure. This paper presents the fabrication process for a self-assembled monopole antenna, and numerical and physical experiments for the antenna pattern characteristics. The physical measurements show a realized gain (includes mismatch loss) of −1.29 dBi at 59 GHz. [ABSTRACT FROM AUTHOR]

Parasitic elements are rotated around a monopole on a groundplane to attain rotated patterns for diversity. A parametric study of the geometry using simulation is used to demonstrate how the parasitic rotation can attain uncorrelated patterns in an idealized multipath propagation environment. Various parasitic designs such as wires and loops, and other shapes can be used for realizing a low- or medium-gain pattern, and high bandwidth including multiband operation. A proof-of-concept prototype is implemented with the rotation using an electro-active polymer actuator and a triangular parasitic element, and physical experiments confirm low correlations (inner products) between the patterns. [ABSTRACT FROM PUBLISHER]

A beam steering antenna is described which uses bending parasitic elements controlled by bending fluidic actuators (BFAs). The structure is a monopole antenna surrounded by four parasitic elements on a groundplane. The BFAs are controlled by linear controllers located below the groundplane. In this way, the parasitic elements comprise BFAs and the action of bending them away from the monopole to alter the mutual impedances is analogous to the reactance variation of switched parasitic antenna. When a parasitic element is not bent, it can act as a reflector or director for low-gain beam formation, while the other parasitic element is bent away from the monopole and hence not affecting the beam. Two configurations have been realized with different numbers of bent parasitic elements, and experimental results for these configurations are presented. Patterns with low cross correlations can be attained while keeping the return loss below a reasonable limit. [ABSTRACT FROM AUTHOR]

A pattern reconfigurable ring-patch antenna is presented. It comprises a bendable parasitic plate located in the middle of a fixed square ring. The fixed ring supports the feed, and the plate is effectively a parasitic structure. The plate's bending action is from a bisecting hinge between the rectangular halves (flaps) that can be actuated separately. The actuation is by extending a shape memory alloy spring. The restoration is by decompressing the shape memory spring action using a parallel standard spring. The parasitic plate thus reconfigures with rectangular flaps which bend up, level, and down, relative to the ring plane. As a result of the reconfiguration, the pattern of the antenna is changed while the impedance match is maintained. Simulated and experimental results are presented for the pattern and impedance of a 100 mm\,\times\,100 mm square ring patch antenna prototype operating at 1.15 GHz. The effects of some geometric parameters on the pattern and impedance are investigated in order to analyze the pattern changing mechanism. A two-element array of the reconfigurable ring-patch antenna is presented that provides further de-correlation of the patterns. [ABSTRACT FROM PUBLISHER]

A frequency-reconfigurable microstrip patch antenna is implemented using a stretchable silicone TC5005 substrate. The stretchable patch is fabricated by injecting liquid metal alloy Galinstan into a square reservoir fabricated in the silicone elastomer substrate. An aperture-coupled feeding method is presented to enhance the strain by separating the fixed feed element from the stretched radiating element. The implemented prototype is demonstrated to be stretchable by a strain of up to 300% (and the material has the potential for more). The electrical length of the patch antenna varies with the stretching, and we demonstrate frequency tuning from 1.3 to 3 GHz. A maximum radiation efficiency of 80% is measured for the antenna. The fabrication process of the antenna system and experimental results for impedance and radiation pattern are presented. [ABSTRACT FROM AUTHOR]

Reconfigurable structures based on smart materials offer a potential solution to realize adaptive antennas for emerging communication devices. In this paper, a reconfigurable axial mode helix antenna is studied. A shape memory alloy spring actuator is used to adjust the height of a helix antenna. With the total length of the helix wire fixed, the pitch spacing and pitch angle are varied as the height is varied. This in turn can alter the antenna pattern in order to adjust to altered operating conditions. In order to undertake the design, the Kraus equations for the axial mode helix are compared with simulation results, and their range of applicability is clarified. It is shown that based on these equations, antenna gain variation is possible by varying the height of the antenna, while keeping its conductor length fixed. We then show that a pattern can be reconfigured using a two-helix structure. Finally, a proof-of-concept helix antenna is implemented using a shape memory alloy actuator. Measurement results confirm that the pattern can reconfigure while maintaining a reasonable impedance match. [ABSTRACT FROM AUTHOR]

The design, fabrication and measurement of self-assembled vertical and oblique monopole antennas, are presented. Vertical on-chip antennas offer advantages over conventional on-chip planar antennas, most notably potentially higher efficiency with associated superior coupling between distant and adjacent ports along the direction of the chip plane. The fabrication method enables lithographical specification of the monopole profile and its sloping angle so that the orientation and shape of monopoles can be controlled. The fabrication process uses SU-8 material and is performed under 200^\circC and is therefore compatible with many commercial microelectronic fabrication processes such as complementary metal-oxide silicon (CMOS) technology. This allows the integration of the antennas with CMOS front ends and other signal processing stages for communications or sensing. [ABSTRACT FROM PUBLISHER]

The theory of propagating waves near a surface is reviewed with an eye to gain insight into the mechanisms involved, and to provide analytical-based models, for power-efficient on-body propagation. The Zenneck wave, and in particular the Norton wave, are appraised as candidate mechanisms for the propagation. For flush-mounted ("band aid") antennas, desired for on-body sensors, the Norton wave is the only direct propagation mechanism between the sensors. The Norton wave fits very well to simulation results presented here, and comparisons are also made with available published physical experiments, although these measurements typically feature the optical paths of elevated, or non-flush, antennas. [ABSTRACT FROM AUTHOR]