OFDM peak power reduction with simple amplitude predistortion

In this paper, we propose a new peak-to-average power ratio (PAPR) reduction algorithm for OFDM transmission. The algorithm is based on simple amplitude predistortion of a subset of the input symbol blocks, the predistorted symbols being selected according to a metric which measures their contribution to the output signal samples of large. magnitude. This metric-based algorithm is simple, highly flexible, and can be implemented as a one-shot process, although its performance can be further improved by iterating the process one or more times.     

Modal approach to scattering of electromagnetic waves by a conducting tape helix

Electromagnetic wave scattering by a tape helix of infinite extent is studied by using Floquet wave expansions for the guided modes and scattered fields. The solution reduces to earlier results as a special limiting case for normal incidence on a sheath helix. The current induced on an infinite helix computed by the presented technique bears close resemblance to the current induced on a long but finite helix as computed by Galerkin's method. The spatial frequency spectrum of the induced current is plotted to show the dominance of the spatial harmonics that are phase matched with the guided modes of the helix. Azimuthal patterns of the scattered field are included to illustrate that interference increases as the diameter of the helix is increased.     

Asymptotically Efficient Reduced Complexity Frequency Offset and Channel Estimators for Uplink MIMO-OFDMA Systems

In this paper, we address the joint data-aided estimation of frequency offsets and channel coefficients in uplink multiple-input multiple-output orthogonal frequency-division multiple access (MIMO-OFDMA) systems. As the maximum-likelihood (ML) estimator is impractical in this context, we introduce a family of suboptimal estimators with the aim of exhibiting an attractive tradeoff between performance and complexity. The estimators do not rely on a particular subcarrier assignment scheme and are, thus, valid for a large number of OFDMA systems. As far as complexity is concerned, the computational cost of the proposed estimators is shown to be significantly reduced compared to existing estimators based on ML. As far as performance is concerned, the proposed suboptimal estimators are shown to be asymptotically efficient, i.e., the covariance matrix of the estimation error achieves the Cramer-Rao bound when the total number of subcarriers increases. Simulation results sustain our claims.     

Peak-to-average power ratio reduction in CDMA systems using metric-based symbol predistortion

In this paper, we propose a simple peak-to-average power ratio (PAPR) reduction algorithm for code-division multiple access (CDMA) systems. The described algorithm is based on predistorting a set of symbols using an appropriate metric and does not need sending any side information to the receiver. Using binary phase-shift keying (BPSK) modulation, we show that performance of this algorithm approaches that of the optimum PAPR reduction procedure while avoiding its excessive complexity.     

Full-rate full-diversity 2 × 2 space-time codes of reduced decoder complexity

Multiple-input multiple-output (MIMO) techniques have become an essential part of broadband wireless communications systems. For example, the recently developed IEEE 802.16e specifications for broadband wireless access include three MIMO profiles employing 2times2 space-time codes (STCs) and two of those are mandatory on the downlink of Mobile WiMAX systems. Conventional approaches to STC design are based on performance criteria such as coding gain, diversity gain, multiplexing gain, and ignore the decoder complexity. In this paper, we take an alternative approach and present a full-rate full-diversity 2times2 STC design leading to substantially lower complexity of the optimum detector than existing schemes. This makes the implementation of high performance full-rate codes realistic in practical systems.     

On the design of NMR sensor for well-logging applications

The magnetic fields of antenna and magnet used in inside-out nuclear magnetic resonance (NMR) well-logging tool are computed using a finite-element method (FEM). A typical operating frequency of such tools is 2 MHz, at which the skin depth is about 47 μm for copper conductor. A direct application of FEM to evaluate power loss at such frequency, therefore, requires very fine discretization of the conductors, which, in turn, makes the problem numerically ill-conditioned. A perturbation technique along with FEM is used to evaluate the power loss in conductors that avoids the need for small discretization steps along the conductor thickness. The design of the magnet, on the other hand, is complicated by the fact that the model is nonlinear in nature because of the presence of ferrites and steel materials surrounding the magnet and because the size of the problem is usually quite large; quarter of a million unknowns is fairly common. A typical nonlinear FEM model requires about 35 h of central processing unit (CPU) time on a Sun Ultra 60 296 MHz workstation with one gigabyte of RAM. The magnet is built by stacking several magnet segments along the axial direction and the objective of the design is to magnetize these segments in such a way so as to produce a desired field profile in front of the magnet. It generally requires many executions of the nonlinear FEM model. An optimal control technique is used in conjunction with the FEM to speed up the design process. Very good agreement between the measured and computed antenna efficiency and magnetic field is obtained thus validating the numerical model     

Image of static current loop over a superconducting sphere

It is shown that the magnetic field of a static current loop with its axis radial to a superconducting sphere can be computed by replacing the sphere by a single image loop. This image solution is derived from the inversion of a point of charge by an equivalent sphere     

Closed form expression of the Green's function for the time-domain wave equation for a lossy two-dimensional medium

The Green's function that relates the electric field to a line current source in a conductive, homogeneous medium is evaluated in the space-time domain in closed form. Early and late time asymptotes are supplied.