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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.

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.

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.

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.

This paper describes the use of two-dimensional (2-D) pattern measurements as a tool for evaluating new antenna configurations for reducing power absorbed in the head. The estimation is from comparisons of the total power in measured cuts of the power pattern in a plane containing the head (the principal cut) for various handheld antenna configurations. For the estimate to be reliable, the power in the cuts should be proportional to the total radiated power. In practice this may not always be the case, but useful results for guidance of antenna design can still be obtained. Configurations with parasitic reflecting elements for directing the power away from the head are demonstrated to be effective in reducing the absorbed power in the head. The results are consistent with those from existing forward differences in the time domain (FDTD) analysis for directive antennas. The resulting directive patterns are more efficient and so the in situ terminal can be expected to have an improved performance in a homogeneous multipath environment.

Revisions to the formula for secondary emission presented in a previous paper are suggested in relation to higher voltage applications. The revisions include the separation of the ksv and ksdelta smoothness factors for V and delta, assigned a 1.0 default value. This applies to typical dull srufaces in conventional working tube environments. Lower values are best for roughened surfaces while higher values are suited to smooth and oxide-free surfaces.