Performance evaluation of OFDM and single‐carrier systems using frequency domain equalization and phase modulation

In this paper, we study the performance of the continuous phase modulation (CPM)‐based orthogonal frequency division multiplexing (CPM‐OFDM) system. Also, we propose a CPM‐based single‐carrier frequency domain equalization (CPM‐SC‐FDE) structure for broadband wireless communication systems. The proposed structure combines the advantages of the low complexity of SC‐FDE, in addition to exploiting the channel frequency diversity and the power efficiency of CPM. Both the CPM‐OFDM system and the proposed system are implemented with FDE to avoid the complexity of the equalization. Two types of frequency domain equalizers are considered and compared for performance evaluation of both systems; the zero forcing (ZF) equalizer and the minimum mean square error (MMSE) equalizer. Simulation experiments are performed for a variety of multipath fading channels. Simulation results show that the performance of the CPM‐based systems with multipath fading is better than their performance with single path fading. The performance over a multipath channel is at least 5 and 12 dB better than the performance over a single path channel, for the CPM‐OFDM system and the proposed CPM‐SC‐FDE system, respectively. The results also show that, when CPM is utilized in SC‐FDE systems, they can outperform CPM‐OFDM systems by about 5 dB. Copyright © 2010 John Wiley & Sons, Ltd.     

A smart exponential‐threshold‐linear backoff mechanism for IEEE 802.11 WLANs

Based on the standardized IEEE 802.11 Distributed Coordination Function (DCF) protocol, this paper proposes a new backoff mechanism, called Smart Exponential‐Threshold‐Linear (SETL) Backoff Mechanism, to enhance the system performance of contention‐based wireless networks. In the IEEE 802.11 DCF scheme, the smaller contention window (CW) will increase the collision probability, but the larger CW will delay the transmission. Hence, in the proposed SETL scheme, a threshold is set to determine the behavior of CW after each transmission. When the CW is smaller than the threshold, the CW of a competing station is exponentially adjusted to lower collision probability. Conversely, if the CW is larger than the threshold, the CW size is tuned linearly to prevent large transmission delay. Through extensive simulations, the results show that the proposed SETL scheme provides a better system throughput and lower collision rate in both light and heavy network loads than the related backoff algorithm schemes, including Binary Exponential Backoff (BEB), Exponential Increase Exponential Decrease (EIED) and Linear Increase Linear Decrease (LILD). Copyright © 2011 John Wiley & Sons, Ltd.     

Dielectric–Semiconductor Interface Limits Charge Carrier Motion at Elevated Temperatures and Large Carrier Densities in a High‐Mobility Organic Semiconductor

The fundamental nature of charge transport in highly ordered organic semiconductors is under constant debate. At cryogenic temperatures, effects within the semiconductor such as traps or the interaction of charge carriers with the insulating substrate (dipolar disorder or Fröhlich polarons) are known to limit carrier motion. In comparison, at elevated temperatures, where charge carrier mobility often also decreases as function of temperature, phonon scattering or dynamic disorder are frequently discussed mechanisms, but the exact microscopic cause that limits carrier motion is debated. Here, the mobility in the temperature range between 200 and 420 K as function of carrier density is explored in highly ordered perylene‐diimide from 3 to 9 nm thin films. It is observed that above room temperature increasing the gate electric field or decreasing the semiconducting film thickness leads to a suppression of the charge carrier mobility. Via X‐ray diffraction measurements at various temperatures and electric fields, changes of the thin film structure are excluded as cause for the observed mobility decrease. The experimental findings point toward scattering sites or traps at the semiconductor–dielectric interface, or in the dielectric as limiting factor for carrier mobility, whose role is usually neglected at elevated temperatures.     

Scalable Fabrication of Metallic Nanofiber Network via Templated Electrodeposition for Flexible Electronics

Metallic nanofiber networks (MNFNs) are very promising for next‐generation flexible transparent electrodes (TEs) since they can retain outstanding optical and electrical properties during bending due to their ultralong and submicron profile. However, it is still challenging to achieve cost‐effective and high‐throughput fabrication of MNFNs with reliable and consistent performance. Here, a cost‐effective method is reported to fabricate high‐performance MNFN‐TEs via templated electrodeposition and imprint transfer. The fabricated electrodeposition template has a trilayer structure of glass/indium tin oxide/SiO₂ with nanotrenches in the insulating SiO₂ that can be utilized for repeated electrodeposition of the MNFNs, which are then transferred to flexible substrates. The fabricated TEs exhibit excellent optical transmittance (>84%) and electrical conductivity (<0.9 Ω sq^?1) and show desirable mechanical flexibility with a sheet resistance <2 Ω sq^?1 under a bending radius of 3 mm. Meanwhile, the MNFN‐TEs reproduced from the reusable template show consistent and reliable performance. Additionally, this template‐based method can realize the direct patterning of MNFN‐TEs with arbitrary conductive patterns by selective masking of the template. As a demonstration, a flexible dynamic electroluminescent display is fabricated using TEs made by this method, and the light‐emitting pattern is observable from both sides.     

Analysis of Heuristic Graph Partitioning Methods for the Assignment of Packet Control Units in GERAN

Over the last few years, graph partitioning has been recognized as a suitable technique for optimizing cellular network structure. For example, in a recent paper, the authors proposed a classical graph partitioning algorithm to optimize the assignment of cells to Packet Control Units (PCUs) in GSM-EDGE Radio Access Network. Based on this approach, the quality of packet data services in a live environment was increased by reducing the number of cell re-selections between different PCUs. To learn more about the potential of graph partitioning in cellular networks, in this paper, a more sophisticated, yet computationally efficient, partitioning algorithm is proposed for the same problem. The new method combines multi-level refinement and adaptive multi-start techniques with algorithms to ensure the connectivity between cells under the same PCU. Performance assessment is based on an extensive set of graphs constructed with data taken from a live network. During the tests, the new method is compared with classical graph partitioning approaches. Results show that the proposed method outperforms classical approaches in terms of solution quality at the expense of a slight increase in computing time, while providing solutions that are easier to check by the network operator.     

Probabilistic linear models for multiresolution estimation in gray-level images

A multiresolution analysis of digital gray-level images is presented. A gray-level multi-scale framework is determined from two main assumptions: the gray scale is binary at the finest spatial resolution, and the gray levels of composed regions are obtained additively. In order to interrelate the gray-level histograms of the same image at different resolutions, probabilistic linear models are developed, which are then applied for estimation. Linear-optimization theory is used as a way of constructing such models. A general procedure for image processing is sketched, based on gray-level estimation. A versatile algorithm for nonlinear filtering is derived. Some examples of prospective applications are given.This work was partially supported by grant TIC91-646 from the DGYCIT of the Spanish Government.     

High resolution CMOS current comparators: design and applications to current-mode function generation

This paper uses fundamental models to derive design conditions for maximum speed and resolution in CMOS transimpedance comparators. We distinguish two basic comparator architectures depending on whether the input sensing node is resistive or capacitive, and show that each type yields advantages for different ranges of input current. Then, we introduce a class of current comparator structures which use nonlinear sensing and/or feedback to combine the advantages of capacitive-input and resistive-input architectures. Two members of this class are presented demonstrating resolution levels (measured on silicon prototypes) in the range of pAs. They exhibit complementary functional features: one, the current steering comparator, displays better transient response in the very comparison function, while operation of the other, the current switch comparator, is easily extended to support systematic generation of nonlinear transfer functions in current domain. The paper explores also this latter extension, and presents current-mode circuit blocks for systematic generation of nonlinear functions based on piecewise-linear (PWL) approximation. Proposals made in the paper are demonstrated via CMOS prototypes in two single-poly CMOS n-well technologies: 2m and 1.6m. These prototypes show measured input current comparison range of 140 dB, resolution and offset below 10 pA, and operation speed two orders of magnitude better than that of conventional resistive-input circuits. Also, measurements from the PWL prototypes show excellent rectification properties (down to a few pAs) and small linearity errors (down to 0.13%).     

Prediction of subharmonic oscillation in switching regulators: from a slope to a ripple standpoint

New exact critical conditions for predicting subharmonic instability in switching regulators are approximated by simple design-oriented expressions valid under practical conditions. These simplified expressions contain the ripple and slope information of the feedback control signal. Depending on the converter topology, the controller used and values of parasitic parameters, either the slope or the ripple can be dominant in predicting instability. A discussion on the validity of this interpretation is illustrated through six different examples of switching regulators using the concept of the spectral radius and the relative degree of the system loop. Using this approach, the boundary between the desired stable region and the subharmonic instability can be easily obtained. The theoretical results are validated by means of numerical simulations.