A robust asynchronous multiuser STBC-OFDM transmission scheme for frequency-selective channels

In this paper, we propose a robust STBC transmission scheme to combat timing synchronization errors over frequency-selective multiple-access channels. We start by deriving the equivalent channel model in the presence of timing synchronization errors. Based on this correlated equivalent channel model, the statistical channel power gain profile is analyzed and shown to exhibit unequal channel power gains on different subcarriers. Furthermore, a robust statistical bit loading algorithm is proposed to optimize the BER performance in scenarios where link adaptation based on instantaneous channel information is infeasible or undesirable. Simulation results show that our proposed scheme is robust to timing synchronization errors, providing around 7.5dB SNR advantage at the BER of 10_-4 over the conventional scheme in various channel environments.     

General Formula for Transmit Power Rise in CDMA Systems With Fast Power Control

Transmit power rise (TPR) in code-division multiple-access (CDMA) systems can be defined as the ratio of the average transmit power to the average receive power, where the average total channel power attenuation is assumed to be one. The TPR has been practically used in the universal mobile telecommunications system radio network planning for modeling of the joint impact of the fast power control, the propagation channel power attenuations, and the antenna diversity. In this letter, we present a general analytical formula for the calculation of TPR in CDMA systems with fast power control, for an arbitrary multipath power profile, arbitrary numbers of transmit and receive antennas, and arbitrary maximum transmit power.     

An analytical calculation of power delay profile and delay spread with experimental verification

Realistic system and link level simulations require specific information about the radio channel. A key role is played by the power delay profile which is widely agreed to be a negative exponential characterized by the delay spread. Since this finding is based on empirical evidence only, the delay spread must be derived from measured data. The power delay profile is rigorously derived by combining ray optical and statistical properties of wave propagation in a single room. We show that the power delay profile is indeed a negative exponential and provide a decay constant which serves as an analytical upper bound for experimentally obtained delay spreads of indoor environments. To the best knowledge of the author, this is the first derivation of the power delay profile from first principles.     

Thermal management using synthetic jet ejectors

The utility of a synthetic jet ejector for thermal management at low flow rates is discussed. A synthetic jet ejector typically consists of a primary "zero-mass-flux" unsteady jet driving a secondary airflow through a low profile, high aspect ratio channel. A simple configuration of a nominally two-dimensional jet ejector in a rectangular channel is used to investigate the effects of channel width on the induced flow rate, power dissipated, heat transfer coefficient and thermal efficiency. An active heat sink for high power microprocessors is developed using the jet ejector concept and its performance is discussed.     

Channel‐power profile estimation for orthogonal frequency‐division multiplexing systems

In this work, a channel‐power profile estimation for orthogonal frequency‐division multiplexing systems, based on the cyclic prefix (CP), is introduced. By knowing the delay of each path, the time‐dispersion information can be derived. The proposed method, considering long intersymbol interference (ISI) fading channels, requires only the coarse symbol timing information. More specifically, quasi‐stationary fading channels are considered. The basic contribution is to obtain the maximum‐likelihood estimation of the correlation coefficient based on the CP. Subsequently, the relationship between the correlation coefficient and the channel‐tap powers is explored. With the estimate of correlation coefficient, the least‐square solution of the channel‐tap powers can be determined. The proposed method is suitable for both short and long ISI channels. Furthermore, the Cramér–Rao lower bound of the channel‐power profile estimation is analyzed, and simulations confirm the advantages of the proposed estimator. Copyright © 2011 John Wiley & Sons, Ltd.     

Semiconductor Structures with a one-dimensional quantum channel and in-plane side gates fabricated by pulse force nanolithography

Quasi-one-dimensional semiconductor structures with a variable longitudinal potential profile are fabricated by pulse power nanolithography, which is carried out using an atomic force microscope. Structures are fabricated on the basis of AlGaAs/GaAs heterostructures with a deep (130 nm from the surface) twodimensional electron gas. The channel potential profile is formed with the help of sectioned in-plane gates formed on both channel sides. The electrical parameters of the structures measured at temperatures down to 1.5 K confirmed the efficiency of the applied method to fabricate insulating regions with lateral sizes of ~10 nm.     

Nonuniform displacement of MOSFET channel pinchoff

The combined effects of the fixed-field oxide change and of the edge contour and impurity profile on MOSFET characteristics are considered, and results are presented which give new insight into the performance of MOSFET devices. It was found that this oxide charge lowers the threshold potential, resulting in an increase in conductivity towards and the two edges of the channel along the width direction. As a consequence, the geometric channel pinchoff locus shifts towards the drain as the channel edge is approached. The oxide charge and the net impurity profile under the field region adjacent to the channels causes the current density to increase gradually towards the edges of the channel, in the channel width direction. At high power densities, this may lead to drain-induced corner breakdown. Further, in the subthreshold region, the electric field at the corner of the drain junction is increased, leading possibly to corner breakdown.     

An improved procedure to calculate the refractive index profilefrom the measured near-field intensity

Refractive index profile of optical waveguides is reconstructed from the measured transmitted near-field intensity. A Butterworth low-pass digital filter is employed in the frequency domain to remove impulsive and high frequency fluctuations which have severe effects on the procedure to calculate the index profile from the measured power intensity. The proposed method has been applied to measure the index profile of monomode optical fiber, Ti:LiNbO₃ and buried MgTi:LiNbO₃ channel waveguides     

Adaptive rate and power DS/CDMA communications in fading channels

We investigate the average information rates attained by adapting the transmit power and the information rate relative to channel variations in code division multiple access communication systems. Our results show that the rate adaptation provides a higher average information rate than the power adaptation for a given average transmit power, and the rate increase when using rate adaptation is more significant for channels with a faster decaying multipath intensity profile and weaker line-of-sight component     

Analysis and optimization of pilot symbol-assisted Rake receivers for DS-CDMA systems

The effect of imperfect channel estimation (CE) on the performance of pilot-symbol-assisted modulation (PSAM) and MRC Rake reception over time- or frequency-selective fading channels with either a uniform power delay profile (UPDP) or a nonuniform power delay profile (NPDP) is investigated. For time-selective channels, a Wiener filter or linear minimum mean square error (LMMSE) filter for CE is considered, and a closed-form asymptotic expression for the mean square error (MSE) when the number of pilots used for CE approaches infinity is derived. In high signal-to-noise ratio (SNR), the MSE becomes independent of the channel Doppler spectrum. A characteristic function method is used to derive new closed-form expressions for the bit error rate (BER) of Rake receivers in UPDP and NPDP channels. The results are extended to two-dimensional (2-D) Rake receivers. The pilot-symbol spacing and pilot-to-data power ratio are optimized by minimizing the BER. For UPDP channels, elegant results are obtained in the asymptotic case. Furthermore, robust spacing design criteria are derived for the maximum Doppler frequency.     

OFDM系统中基于EM迭代的时变信道估计与数据检测

针对OFDM系统中难以实时获取时变信道统计信息的问题,提出了一种基于期望最大化(EM, expectation maximization)迭代的多符号联合信道估计和数据检测算法。为了获取合适的迭代初始值,利用梳状导频的信息,设计了基于最小二乘算法的低复杂度的初始化方案。通过分析算法中的信道统计量与信噪比的关系,提出了忽略信道自相关矩阵的简化算法,避免了获取信道统计信息的操作。仿真结果表明,提出的算法对信道的功率延迟分布和多普勒功率谱等统计信息不敏感,在未知信道多普勒功率谱的条件下,仍然具有较低的估计误差和误码率。      

Design optimization of high-performance low-temperature 0.18 μmMOSFETs with low-impurity-density channels at supply voltage below 1 V

A 0.18 μm nMOS structure with a vertically nonuniform low-impurity-density channel (LIDC) at 77 K has been studied at supply voltage below 1 volt. An abrupt Gaussian profile is used in the channel. The investigation is based on two-dimensional (2-D) energy transport simulation with appropriate models to account for quantum and low-temperature freeze-out effects. The study focuses on achieving high driving capability and low off-current at low supply voltage and on minimizing short-channel effects. Some guidelines are proposed for improving device performance and suppressing short-channel effects of the LIDC MOS devices. It is shown that at 77 K the optimized nonuniform LIDC 0.18 μm nMOS structure with an abrupt impurity channel profile at supply voltage as low as 0.9 V is able to provide a saturation drain current comparable to that of a room-temperature LIDC 0.1 μm nMOS device at 1.5 V. Furthermore, the 77 K LIDC 0.18 μm nMOS consumes considerably lower dynamic and standby power than the room-temperature 0.1 μm nMOS. These results suggest that the LIDC MOS structure with an abrupt channel profile is very suitable for low-power and high-speed ULSI applications at low temperature     

A vertical channel JFET fabricated using silicon planar technology

A vertical channel JFET with a new structure was fabricated using a self-aligned process and doped polysilicon technology. This structure is suitable for a high power device, since many channels are easily integrated on a single chip. It is also suitable for a high frequency device, because two essential conditions for high frequency operation, sufficiently low gate resistance and small channel length, can be realized without difficulty. This device shows triode-like I-V characteristics, which are determined by the channel impurity concentration and gate diffusion profile. Typical performances of an n-channel, 4 mm/spl times/4 mm, 5520 channel power FET, designed for an audio amplifier, are a voltage amplification factor of 5, a source-to-gate breakdown voltage of 60 V, a drain-to-gate breakdown voltage of 200 V, and I/SUB DSS/=4 A at V/SUB DS/=7 V.     

Buried channel GaAs MESFET's—Scattering parameter and linearity dependence on the channel doping profile

The influence of the epitaxial structure (doping profile) in the channel region of a GaAs MESFET on its small signal scattering parameters is investigated both theoretically and experimentally. The large signal frequency behavior of FET's with a buried channel is compared with that of uniformally doped FET's. In the case of buried channel devices a much smaller variation of the s parameters with frequency is observed. This phenomenon can be understood by considering the inner transistor to consist of a two-dimensional transmission line. The principle of this new model, which is mainly based on technological data, is presented. Furthermore, the improvement of the nonlinear distortion is investigated. The FET's with a graded doping profile show very small intermodulation products, -42 dBm for an input power level of 4 dBm (Pout= 9 dBm: 1-dB gain compression) at a relatively small drain-source voltage of only 4 V.     

Investigation of power Trench MOSFETs with retrograde body profile

We present here a power Trench MOSFET (T-MOS) with retrograde body doping profile. The channel length and trench depth are both shortened compared with conventional T-MOS. High energy implantation is used to form retrograde body profile. Electronic parameters of the new structure have been obtained by process and device simulation. The results show that the new structure has much lower specific on-resistance (R_ds,on) because of its shorter channel when compared with conventional T-MOS. As the trench depth is shallowed, the gate charge density Q_g is also reduced.     

High-resolution 3-D direction-of-arrival determination for urbanmobile radio

The in-depth knowledge of the mobile radio channel is particularly important for radio communication modeling and advanced technology system design. We propose an accurate method to determine jointly the azimuth and elevation angle and the delay of waves incoming at the receiver. The method is applied to measurements of the complex impulse response of the mobile radio channel, performed on a planar array placed on a mobile in an urban cellular environment. The directions-of-arrival (DOA) were obtained by the means of a direction finding algorithm-two-dimensional (2-D) unitary ESPRIT. Two-dimensional spatial smoothing as an extension of ordinary spatial smoothing is utilized to decorrelate coherent waves. The application of 2-D unitary ESPRIT increases the angular resolution over conventional Fourier analysis or the scattering function by an order of magnitude and overcomes difficulties due to secondary lobes. The time delay is determined from wideband channel sounder measurements. The results confirm some assumptions on propagation mechanisms: (1) the wave-guiding property of streets (canyon effect), which is especially pronounced for long-delayed paths; (2) the variation of the number of incoming waves with their excess delay-the larger the excess delay, the lower the number of paths comprising an echo in the power delay profile; (3) if a single path remains, the privileged DOA is the direction of the street; (4) the exponential part of the power delay profile due to scatterers all around the receiver; and (5) the elevation dependence or the impinging power. In the tested receiver locations, paths with elevations between 0° and 40° dominate, containing about 90% of the received power     

Cooperative Communications in Ultra-Wideband Wireless Body Area Networks: Channel Modeling and System Diversity Analysis

In this paper, we explore the application of cooperative communications in ultra-wideband (UWB) wireless body area networks (BANs), where a group of on-body devices may collaborate together to communicate with other groups of on-body equipment. Firstly, time-domain UWB channel measurements are presented to characterize the body-centric multipath channel and to facilitate the diversity analysis in a cooperative BAN (CoBAN). We focus on the system deployment scenario when the human subject is in the sitting posture. Important channel parameters such as the pathloss, power variation, power delay profile (PDP), and effective received power (ERP) crosscorrelation are investigated and statistically analyzed. Provided with the model preliminaries, a detailed analysis on the diversity level in a CoBAN is provided. Specifically, an intuitive measure is proposed to quantify the diversity gains in a single-hop cooperative network, which is defined as the number of independent multipaths that can be averaged over to detect symbols. As this measure provides the largest number of redundant copies of transmitted information through the body-centric channel, it can be used as a benchmark to access the performance bound of various diversity-based cooperative schemes in futuristic body sensor systems.     

Channel Estimation in OFDM Systems With Unknown Power Delay Profile Using Transdimensional MCMC

This paper considers the problem of channel estimation for orthogonal-frequency-division multiplexing (OFDM) systems, where the number of channel taps and their power delay profile are unknown. Using a Bayesian approach, we construct a model in which we estimate jointly the coefficients of the channel taps, the channel order and decay rate of the power delay profile (PDP). In order to sample from the resulting posterior distribution we develop three novel Trans-dimensional Markov chain Monte Carlo (TDMCMC) algorithms and compare their performance. The first is the basic birth and death TDMCMC algorithm. The second utilizes Stochastic Approximation to develop an adaptively learning algorithm to improve mixing rates of the Markov chain between model subspaces. The third approximates the optimal TDMCMC proposal distribution for between-model moves using conditional path sampling proposals. We assess several aspects of the model in terms of sensitivities to different prior choices. Next we perform a detailed analysis of the performance of each of the TDMCMC algorithms. This allows us to contrast the resulting computational effort required under each approach versus the estimation performance. Finally, using the TDMCMC algorithm which produces the best performance in terms of exploration of the model subspaces, we assess its performance in terms of channel estimation mean-square error (MSE) and bit error rate (BER). It is shown that the proposed algorithm can achieve results very close to the case where both the channel length and the PDP decay rate are known.      

Delay Spread Statistics in a Two-Path Distributed Antenna System with Gamma-Lognormal Fading Channels

This letter analyzes the statistical properties of the root-mean-square (RMS) delay spread in a two-path distributed antenna system, where the received signal power from each path is subject to composite gamma fading and lognormal shadowing. The channel power delay profile considered consists of two delta functions with correlated power levels. We present analytical expressions for the probability density function of the RMS delay spread, as well as the cumulative distribution function, mean value and standard deviation. These analyses are validated through comparison with Monte-Carlo simulation results.     

The inhomogeneous channel FET: ICFET

Improvement in the operation of field-effect transistors is predicted through the use of various longitudinal inhomogeneous channel resistivity profiles with heavier doping concentration near the source than the drain end of the channel. The inhomogeneous channel field-effect transistor or ICFET is shown to have a marked improvement in cut-off frequency, device gain, output power and gain-bandwidth product regardless of the actual profile providing only that the doping concentration is higher near the source than the drain end of the channel. The various ICFETs analyzed and compared with the homogeneous FET are the following: linear, Gaussian, complementary error function and exponential. Solutions for each of these cases with doping concentration changes of 10 to 1 and 100 to 1 between the source and drain ends were obtained. The linear case with a 100 to 1 change in doping concentration is seen to provide tremendous device operation improvement, i.e. gain or power is improved by a factor of 30·7 and gain bandwidth by a factor of 955.