A Frequency-Reconfigurable Monopole Antenna with Switchable Stubbed Ground Structure

A frequency-reconfigurable coplanar-waveguide (CPW) fed monopole antenna using switchable stubbed ground structure is presented. Four PIN diodes are employed in the stubs stretching from the ground to make the antenna reconfigurable in three operating modes: a single-band mode (2.4-2.9 GHz), a dual-band mode (2.4-2.9 GHz/5.09-5.47 GHz) and a triple-band mode (3.7-4.26 GHz/5.3-6.3 GHz/8.0-8.8 GHz). The monopole antenna is resonating at 2.4 GHz, while the stubs produce other operating frequency bands covering a number of wireless communication systems, including WLAN, WiMAX, C-band, and ITU. Furthermore, an optimized biasing network has been integrated into this antenna, which has little influence on the performance of the antenna. This paper presents, compares and discusses the simulated and measured results

Effects of Rate Adaption on the Throughput of Random Ad Hoc Networks

The capacity of wireless ad hoc networks has been studied in an excellent treatise by Gupta and Kumar [1], assuming a fixed transmission rate. By contrast, in this treatise we investigate the achievable throughput improvement of rate adaptation in the context of random ad hoc networks, which have been studied in conjunction with a fixed transmission rate in [1]. Our analysis shows that rate adaptation has the potential of improving the achievable throughput compared to fixed rate transmission, since rate adaptation mitigates the effects of link quality fluctuations. However, even perfect rate control fails to change the scaling law of the per-node throughput result given in [1], regardless of the absence or presence of shadow fading. This result is confirmed in the context of specific adaptive modulation aided design examples

Exact BER Calculation of Asynchronous DS-CDMA Systems Communicating over Hoyt Channels

An asynchronous binary DS-CDMA system using random spreading sequences is considered in flat Hoyt fading channels. A new closed-form expression is derived for the conditional characteristic function of the multiple access interference. The exact average BER is expressed as a single numerical integration based on the characteristic function approach. The numerical results obtained from our exact BER analysis are verified by our simulation results and also compared to those obtained by the standard Gaussian approximation

Accurate BER Analysis of QPSK Modulated Asynchronous DS-CDMA Systems Communicating over Rayleigh Channels

The accurate average BER calculation of an asynchronous DS-CDMA system using random spreading sequences is studied in Rayleigh fading channels. An accurate closed-form expression is derived for the conditional characteristic function of the multiple access interference. An accurate BER expression is provided, which only requires a single numerical integration. Our numerical simulation results verify its accuracy, and also demonstrate the relative inaccuracy of the Gaussian approximation

Exact BER Performance of Asynchronous DS-CDMA Systems using Quadriphase Spreading and QPSK Modulation over Rayleigh Channels

An accurate closed-form expression is derived for calculating the average BER in an asynchronous DS-CDMA system using random complex-valued spreading sequences for transmission over Rayleigh channels. This accurate solution is based on the Characteristic Function (CF) approach and only requires a single numerical integration. Our numerical simulation results verify its accuracy, and also demonstrate the relative inaccuracy of the Gaussian approximation

Precise BER Formulas for Asynchronous QPSK-Modulated DS-CDMA Systems Using Random Quaternary Spreading Over Rayleigh Channels

Precise bit-error-ratio (BER) analysis of an asynchronous QPSK-modulated direct-sequence code-division multiple-access system using random quaternary spreading sequences for transmission over Rayleigh channels is performed based on the characteristic-function approach. Its accuracy is verified by our numerical simulation results and also compared with those of the Gaussian approximation. Index Terms—Asynchronous direct-sequence code-division multiple-access (DS-CDMA), bit-error-ratio (BER), precise, QPSK, quarternary spreading, Rayleigh

Disorder effects on the spin-Hall current in a diffusive Rashba two-dimensional heavy-hole system

We investigate the spin-Hall effect in a two-dimensional heavy-hole system with Rashba spin-orbit coupling using a nonequilibrium Green's function approach. Both the short- and long-range disorder scatterings are considered in the self-consistent Born approximation. We find that, in the case of long-range collisions, the disorder-mediated process leads to an enhancement of the spin-Hall current at high heavy-hole density, whereas for short-range scatterings it gives a vanishing contribution. This result suggests that the recently observed spin-Hall effect in experiment is a result of the sum of the intrinsic and disorder-mediated contributions. We have also calculated the temperature dependence of spin-Hall conductivity, which reveals a decrease with increasing the temperature.Comment: 5 pages, 2 figures, Typos in the values of hole density correcte