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Pilot-aided orthogonal frequency-division multiplexing (OFDM) channel estimation has, so far, been optimized only for open-loop systems for which the uniform spacing of pilots is optimal. In this paper, we examine closed-loop OFDM systems and show how uniform pilots may no longer be optimal. In doing so, we propose a feedback technique in which the pilot allocation mechanism is adapted to the mobile channel. After deriving a general expression for the symbol error rate as a function of pilot allocation, we optimize the pilot locations and propose two practical solutions using bounds on the objective function. We proceed to formulate pilot allocation based on the maximal average OFDM channel capacity. We examine the feedback overhead that is associated with our method and make comparisons with the conventional uniform framework. Finally, we suggest the use of vector quantization in the context of the generalized Lloyd algorithm to reduce feedback and extend our method to a multiple-input-multiple-output (MIMO)-OFDM system based on the Alamouti space-time block code. Monte-Carlo simulations illustrate the error-rate/capacity performance gains via nonuniform pilots and illustrate the effects of Doppler frequency and carrier frequency offset. Index Terms--Feedback communications, least squares estimation, orthogonal frequency-division multiplexing (OFDM), pilot-symbol-aided modulation (PSAM).

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. Index Terms--Geometric directivity, hemispherical scan coverage, maximum scan angle, multibeam antennas, pyramidal antenna arrays.

In this paper, a beam partitioning is introduced for a mobile surrounded uniformly by dense scatterers. The beams generate signals with equal mean Doppler shift contributions. The equal Doppler contribution of each signal is desirable in antenna combining or diversity systems. Beams that correspond to the partitioning are synthesized using the Fourier method, and a fading rate reduction scheme is described. The fading rate reduction is quantified as an increased correlation distance relative to omnidirectional antennas.

A new concept in single-port adaptive antennas using parasitic elements with switched terminating impedances is presented including results from a concept prototype. Each parasitic element can be effectively terminated in three impedance values. The antenna concept provides multiple radiation patterns with a single RF signal port without the need for RF switches or phase shifters in the direct RF signal path. Impedance variations in the active antenna element are minimized by use of only rotationally symmetric configurations. Measured patterns are used to demonstrate the performance improvement expected using switched diversity combining in a simulated uniform scattering scenario. The concept prototype having one active element and four parasitic elements, is shown to have equivalent diversity performance to between three and four uncorrelated branches. Index Terms - Adaptive, averaged uniform scattering scenario, diversity gain, single-port diversity antenna, switched parasitic antenna elements, wire monopole antenna.

Frequency selective channels can be measured by either a continuous wave frequency sweep, which gives directly the transfer function, or by sounding the channel with pulses, which results in the impulse response. When the sounding pulse becomes a chirp-type waveform, the two approaches become similar. Simple rectangular pulses can be used for partial, but usually sufficient, channel characterization. In this paper, the process is described for resolving impulse response components into bins smaller than the duration of the sounding pulse and smaller than the reciprocal of the channel bandwidth. From such "super-resolution," the delay spread of the propagation channel can be established accurately even when it is much less than the sounding pulse duration, allowing longer, higher energy pulses to be used for channel characterization. The process is demonstrated using subtractive deconvolution where a loop gain of unity is shown to be stable; and a modified inverse filter technique, in which the modification caters for the spectral zeros of the sounding pulse. Index Terms - Deconvolution, delay estimation, multipath, radio propagation.

Switched parasitic elements provide a useful implementation of antenna pattern diversity. The basic principle is presented with some examples of wire antennas computed using the method of moments. The modeled diversity gain available from selection combining of uncorrelated signals is used to quantify the expected improvement relative to nondiversity antennas. The advantage of the switched parasitic concept is that it is a relatively simple system, which can give the adaptive antenna performance of many branch selection or switched diversity. Index Terms - Antenna arrays, antenna radiation patterns, diversity, mobile communications, multibeam antennas, personal communications.

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]

In the uncorrelated scenario model, a continuous source distribution illuminates a receiver with waves bearing signals that are uncorrelated with respect to their angle of arrival. This model is used for multipath situations such as scintillating atmospheres, but also can be applied to optical beams for indoor communications, acoustic beamforming, and, of particular interest here, mobile communications. For diversity antennas operating in such a scenario, patterns that produce uncorrelated signals are required. An angular separation of directive beams in multipath scenarios acts to decorrelate the received signals. It is of interest to quantify the minimum angular spacing required of beams in order to provide a framework for the design and configuration of the antennas. The approach is to consider only the main lobe of the antenna pattern and to take it as a real function. This is reasonable as long as most of the energy is conveyed via the main beam. In practice, the sidelobe structure and nonuniformity in the phase of an actual pattern act to improve the situation in the sense that the decorrelation angles become smaller. The conditions for angular diversity result in a simple rule-of-thumb for the minimum beam separation requirement, which is essentially independent of the directivity. Finally, both scalar (acoustic case and circularly polarized case) and vector (linearly polarized case) elliptic beams rotated about their boresight axes are analyzed for the decorrelation rotation angle as a function of the ratio of the ellipticity. The resulting design curves offer a guideline to beam configuration for multipath scenarios. Index Terms - Multibeam antennas.

Empirical justifications for the lognormal, Rayleigh and Suzuki probability density functions in multipath fading channels are examined by quantifying the rates of convergence of the central limit theorem (CLT) for the addition and multiplication of random variables. The accuracy of modeling the distribution of rays which experience multiple reflections/diffractions between transmitter and receiver as lognormal is quantified. In addition, it is shown that the vector sum of lognormal rays, such as in a narrow-band signal envelope, may best be approximated as being either Rayleigh, lognormal or Suzuki distributed depending on the fading channel conditions. These conditions are defined statistically. Index Terms - Central limit theorem, lognormal distribution, shadow fading.

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]

A signal combination method unites the phase and amplitude relationships and uses second-order bandpass sampling for partial phase control of component signals. It relies upon a spectral replication sampling property. The performance of the technique for interception of slow frequency hopping spread spectrum, antenna combination and a switchable beam linear antenna array is discussed.

Bandpass sampling using suitable interpolants enables the frequency-shift in bandpass signals. The bandpass sampling technique phase shifts the second order sampled signals. The variation in the separation of sample streams enables the generation of different magnitudes of phase shifts. The phase shifts and frequency shifts are determined digitally.

The automated detection of people having a fall is particularly important for the elderly in indoor care situations. Privacy concerns, and regulations that prohibit cameras in indoor environments, mean that optical sensing must give way, at least in some situations, to less explicit sensing such as radar. Currently, fall detection using radar Doppler signatures has limitations. We demonstrate a radar-based technique that detects breathing and other movements seamlessly, and can detect a fall after it has happened, i.e., even when the person is static. Using a low-cost, ceiling-mounted radar at low microwave frequencies (sub-6 GHz), our results show it is possible to remotely localize a person within a few cm. The sensor system includes a small neural network model that can distinguish a person from other moving objects in an indoor environment. This can reduce false alarms of the fall detection and hence improve system reliability in real-world deployment. In our experiments, the neural network differentiates a person from a pet (an example of a complex moving entity) with an accuracy exceeding 95%. This sensor system demonstration paves a way forward for general indoor fall detection, extending to the well-being of our elderly through real-time, ongoing monitoring of their breathing and other activities of daily living. [ABSTRACT FROM AUTHOR]

The implementation of high-speed wireless networks, such as currently used fourth generation (4G) systems and future 5G systems, feature challenging processing. Field programmable gate arrays (FPGAs) can straddle research and development for these current and future networks since they provide scaling through reconfigurable logic, high parallelism, and low power consumption. This brief demonstrates an FPGA circuit implementation, with measurements, of a minimal system (a 5G element or “unit cell”): a single-user mobile with antenna diversity and a distributed antenna system (DAS) at the base station. The demonstration system has a bandwidth of 20 MHz, runs at 2.4 GHz, and has two antennas at both the transmitting base station and at the receiving mobile. The modulation is orthogonal frequency division multiplexing (OFDM) with space-time block coding (STBC). The FPGA is a Virtex-6, used for software defined radio (SDR), and this can readily be scaled to handle larger-dimensioned, higher-capacity systems. The receiver has time-offset synchronization, frequency-offset, and channel estimation. The high-level algorithm design (Xilinx System Generator) for these functions and the OFDM-STBC, and the resources consumed on the FPGA during real-time implementation, are included. We also compare the use of coax and fiber for linking the distributed antennas, using off-the-shelf components. The approach used here of combining simulations with physical measurement of a minimal system is a practical way forward for assessing candidate systems for 5G. [ABSTRACT FROM AUTHOR]

A method for evaluating antenna diversity systems is presented with an example of a handset using up to four antennas in indoor mobile communication. The method includes a 'two-at-a-time' measurement test setup that uses maximum likelihood estimation of the covariance matrix and compensation for the imperfect match, which typically occurs during the design and evaluation process for compact antennas. The diversity performance of the antenna system can be presented as an effective order of diversity, which is the equivalent number of equal power, uncorrelated diversity branches for the same diversity gain at a given probability. Index Terms--Antenna arrays, covariance matrices, diversity methods, estimation, fading channels, mobile antennas, signal processing.

The Fourier relations between channel transfer function and scattering distribution can apply to personal and mobile communications where multipath is a prevalent phenomena. In this paper, the transform relations are reviewed and interpreted for the mobile radio channel. The effective scattering distribution is the vector product of the antenna pattern and the incident waves and is a scalar function of angle and delay time. The space base-band frequency correlation function transforms with the averaged power of the effective scattering distribution. If the angular power density marginal of the effective scattering distribution is known, then the transform relations can be used for configuring antennas for spatial diversity. Similarly, if the delay time marginal is known, then conditions for frequency diversity are available. The two-dimensional (2-D) transform gives a convenient route for assessing tradeoffs between combined frequency and space diversity. Using modeled distributions, solutions are given for spaced directive antennas and an example is discussed for the space-frequency tradeoff. Index Terms--Land mobile radio, land mobile radio diversity systems, multipath interference.

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]

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]

This paper presents three beamforming designs for multiuser multiple-input and multiple-output with orthogonal frequency-division multiplexing, where the transmit and receive beamformers are obtained iteratively with closed-form steps. In the first case, the transmit (Tx) beamformers are set and the receive (Rx) beamformers are calculated. It works by projecting the Tx beamformers into a null space of appropriate channels. This eliminates one interference term for each user. Then the Rx beamformer for each user maximizes its instantaneous signal-to-noise ratio (SNR) while satisfying an orthogonality condition to eliminate the remaining interferences. The second case is jointly optimizing the Tx and Rx beamformers from constrained SNR maximization. It uses the results from the first case. The third case is also for joint optimization of Tx–Rx beamformers but combines constrained SNR and signal-to-interference-plus-noise ratio maximization. The minimum number of antennas required is derived as part of the formulation. All cases can include a linear constellation precoder for extracting multipath diversity. Finally, the required feedback rates are derived and compared to existing beamforming methods. Using the standardized statistical channel model for IEEE 802.11n, our simulations demonstrate faster beamforming, improved error performance, and the ability to extract multipath diversity which is not possible in the least-square approach. [ABSTRACT FROM PUBLISHER]

Water-filled eigenchannels offer the highest multi-input multi-output (MIMO) information-theoretic capacity, but digital techniques such as quadrature amplitude modulation and finite block lengths will degrade the capacity from the Shannon limit to the capacity of a digital link. Furthermore, eigen-MIMO requires channel overheads, such as estimating the channel state information (CSI) and feeding it back to the transmitter, which further compromise the capacity. In this paper, the joint influence of channel estimation and imperfect feedback on the information-theoretic capacity and the practicable capacity is analyzed. The channel is modeled as static over a MIMO channel block. In each block, the forward channel is used for CSI estimation and for the payload data transmission. In the back direction, the channel is used to feed back a quantized form of the CSI to the transmitter with a throughput constraint. These three channel usages are combined into an effective simplex channel simplifying the capacity analysis. The capacities are formulated as functions of the link parameters, enabling optimization of the number of training symbols, the feedback duration, and the power allocation for training and data transfer, with the criterion of maximum capacity. The results presented are subject to the usual approximations used in communications theory. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]