Adaptive Signaling Based on Statistical Characterizations of Outdated Feedback in Wireless Communications

Wireless links form a critical component of communication systems that aim to provide ubiquitous access to information. However, the time-varying characteristics (or "state") of wireless channels caused by the mobility of transmitters, receivers, and objects in the environment make it difficult to achieve reliable communication. Adaptive signaling exploits any channel state information (CSI) available at the transmitter to provide the potential to significantly increase the throughput of wireless links and/or greatly reduce the receiver complexity. As such, adaptive signaling has attracted significant research interest in the last decade and has found application in numerous commercial wireless systems, ranging from cellular data systems to wireless local area networks (WLANs). However, one of the great challenges of wireless communications is that it is difficult to obtain perfect CSI due to the inherently noisy and outdated nature of CSI available at the transmitter. Over the last decade, we have championed the idea of choosing the appropriate transmitted signal based on statistical models for the current channel state conditioned on the channel measurements. In this semi-tutorial paper, we first review how this class of methods has been developed for single-antenna systems, and then present novel recent designs for multiple-antenna systems. Key to the development in each case is the development of the error characterization given the outdated estimates and the use of such to allocate data rate and power over time and possibly space. In general, the focus is on rate allocation, while power allocation is done through a pruning method. Numerical results will demonstrate in both the single-antenna and multiple-antenna cases that such an approach provides a robust method for improving system data rate versus the standard practice of employing link margin to compensate for such uncertainties.     

Optimal Electrode Configurations for External Cardiac Pacing and Defibrillation: An Inhomogeneous Study

We have developed an inhomogeneous two-dimensional finite element computer model of the human torso, and have used it to study electrode performance in defibrillation and external cardiac pacing. Gross individual organ effects were assessed first for different electrode configurations by creating models which included one organ at a time, and comparing the results to those obtained with a homogeneous body. Electrode placement on the body was varied in order to determine, within the limitations of the model, optimal electrode configurations for external cardiac pacing and defibrillation. Finally, the electrical and geometric parameters of a previously proposed plate electrode design were optimized for the selected external pacing position. It was found that organs of extreme resistivity, close to the body surface, and within the direct current path between two electrodes, tended to have dominant effects on the surface current density distributions. The optimum pacing position is to place the driven electrode directly over the heart and the receiving electrode on the left lateral chest wall. For defibrillation, the driven electrode is moved to the right of the sternum.     

A fast-acquiring blind predictive DFE

This article presents a blind predictive decision-feedback equalization (DFE) scheme motivated by the work of Labat et al. (see idid., vol.46, p.921-30, July 1998). The proposed scheme outperforms another previously proposed blind predictive DFE scheme, and also eliminates the filter interchange required in the scheme of Labat et al.     

Oncogenes and tumor suppressor genes in photocarcinogenesis

A case of coup de sabre, a linear form of scleroderma, is presented. Treatment consisted of soft-tissue expansion and autologous bone grafting to the forehead, a composite graft for alar reconstruction, and a scalp graft for eyebrow reconstruction. None of the linear scleroderma cases reported in the literature consisted of bony reconstruction.     

Conductometric study of ion—ion and ion-solvent interaction—1. Conductances of lithium acetate in water,methanol and 50, 60 and 70% (W/W) dioxane—water mixtures at 25°C

Conductances of lithium acetate in water, methanol and 50, 60 and 70% (w/w) dioxane—water mixtures were measured. The data was analysed using the Fuoss-Hsia equation in the form of Fernandez—Prini coefficients to find the ion-pair formation constants (K_A) and molar conductances at infinite dilution. The log K_Avs 1/D plot is a straight line passing through the origin in agreement with the Dension—Ramsey theory. The Walden products, solvation numbers and solvodynamic radii were also calculated. The results indicate the formation of solvent separated ion-pairs (SSIP).     

On the design of multidimensional signal sets for OFDM systems

An orthogonal frequency division multiplexing (OFDM) system operating over a wireless communication channel effectively forms a number of parallel frequency-nonselective fading channels, thereby obviating the need for complex equalization and thus greatly simplifying equalization/decoding. However, the OFDM system also exhibits two weaknesses relative to its single-carrier counterparts: (1) the diversity achieved by the OFDM system can be less than a single-carrier system employing the same error control code in a signaling environment rich in diversity and (2) the baseband transmitted signal can exhibit significant amplitude fluctuation over time, thereby precluding efficient transmit amplifier operation. In this paper, nonstandard multidimensional signal sets matched to the OFDM framework are prescribed that address both of these issues. The proposed signal sets are chosen to maximize the diversity achieved by an uncoded system under a constraint to control the peak-to-mean envelope power ratio (PMEPR) of the baseband transmitted waveform. The cost of employing the proposed signal sets is an increase in decoding complexity, as essentially a small amount of controlled equalization has been added to the receiver; thus, the resulting system can be viewed as a hybrid between an OFDM system and a standard single-carrier system. Numerical results are presented which suggest that: (1) the system can provide an attractive alternative to a standard OFDM system in terms of required average transmitted SNR versus receiver complexity and (2) the system yields a modest reduction in PMEPR versus a standard OFDM system