High-performance P-n-p heterojunction bipolar transistor design

The performance of P-n-p AlGaAs/GaAs heterojunction bipolar transistors (HBTs) for analog and digital circuit applications is compared to that of N-p-n HBTs. The theoretical analysis shows that the P-n-p HBTs are comparable with N-p-n HBTs in high-speed digital operation, while the N-p-n HBTs exhibit higher cut-off frequency in microwave and millimeter-wave operation. Analytical equations and SPICE circuit simulation are used in support of the comparison between the P-n-p and N-p-n HBTs. Optimization of device doping profile design for P-n-p and N-p-n heterojunction bipolar transistors in analog and digital circuits are presented.     

1-Min Rain Rate Statistics Predictions From 1-Hour Rain Rate Statistics Measurements

A new method is presented, which allows the recovery of the cumulative distribution function (CDF) of point rain rate with 1-min integration time (P(R)₁) from rain rate CDFs with longer integration time (in this case, 1-hour time average rain rate statistics P(R)₆₀)- The method generates the scaling factors between P(R)₆₀) and P(R)₁) as a function of the probability level and of the site geographical coordinates. From the scaling factors, a scaling law of the form proposed by Chebil and Raman is devised. The method is applicable worldwide and produces predictions far more accurate than the ones provided by general P(R) estimation techniques.     

Bandwidth guaranteed call admission in TDMA/CDMA ad hoc wireless networks

This paper first studied the timeslot assignment problem in time division multiple access/code division multiple access (TDMA/CDMA) wireless ad hoc networks. Given a path P, we prove that a timeslot assignment providing one unit of bandwidth on P can be found in O(P) time if such an assignment exists. The results have been extended to the case that P can provide two units of bandwidth. Based on the timeslot assignment for the special cases, an efficient slot assignment algorithm with O(P²k) is proposed for general cases, where k is the number of slots in a TDMA frame. Then, the timeslot assignment algorithm is integrated into a quality of service (QoS) call admission scheme for QoS call requests. Extensive simulations are conducted and the results have demonstrated the superior performance of our method.     

Zero-Rate Feedback Can Achieve the Empirical Capacity

The utility of limited feedback for coding over an individual sequence of discrete memoryless channels is investigated. This study complements recent results showing how limited or noisy feedback can boost the reliability of communication. A strategy with fixed input distribution P is given that asymptotically achieves rates arbitrarily close to the mutual information induced by P and the state-averaged channel. When the capacity-achieving input distribution is the same over all channel states, this achieves rates at least as large as the capacity of the state-averaged channel, sometimes called the empirical capacity.     

The Effectiveness and Efficiency of Hybrid Transform/DPCM Interframe Image Coding

We seek to evaluate the efficiency of hybrid transform/ DPCM interframe image coding relative to an optimal scheme that minimizes the mean-squared error in encoding a stationary Gaussian image sequence. The stationary assumption leads us to use the asymptotically optimal discrete Fourier transform (DFT) on the full frame of an image. We encode an actual image sequence with full-frame DFT/DPCM at several rates and compare it to previous interframe coding results with the same sequence. We also encode a single frame at these same rates using a full-frame DFT to demonstrate the inherent coding gains of interframe transform DPCM over intraframe coding. We then generate a pseudorandom image sequence with precise Gauss-Markov statistics and encode it by hybrid full-frame DFT/DPCM at various rates. We compare the signal-to-noise ratios (SNR's) of these reconstructions to the optimal ones calculated from the rate-distortion function. We conclude that in a medium rate range below 1 bit/pel/frame where reconstructions for hybrid transform/ DPCM may be unsatisfactory, there is enough margin for improvement to consider more sophisticated coding schemes.     

Transport of electrons in monolithic hot electron Si transistors

The transport of electrons across the base of monolithic hot electron transistors is studied using a simplified model. The base is assumed to be limited by abrupt barriers and no account is taken of backscattering from the collector region. The collisions in the base are considered to be of only one type which represents an average between interactions with optical and acoustic phonons. A fundamental part in the analysis is played by the function P_EX(i, x), (i = 1, 2, …) which gives the total probability of exit into the collector of an electron whose ith collision occurs at a point of abscissa x within the base. The function P_EX(i, x) is determined iteratively for decreasing values of i, using the theorem of compound probabilities, and from there the transport factor across the base is derived. Programs have been written to this effect, and the results are illustrated by means of examples which demonstrate the effect on the transport factor of the various parameters of the device, and show some comparisons with a previous theory[4].     

柠檬酸锌二水合物太赫兹光谱及失水动力学研究

利用太赫兹时域光谱技术(THz-TDS)对柠檬酸锌二水合物晶体及其失水动力学进行准确、无标记检测。实验结果表明,室温下柠檬酸锌二水合物有2个明显的吸收峰：一个主峰在1.91 THz,一个次峰在1.77 THz。随着温度升高,柠檬酸锌二水合物吸收主峰强度不断减少,直至消失。根据柠檬酸锌二水合物在1.91 THz处的特征吸收峰面积在不同温度、不同加热时间的变化关系,利用阿伦尼乌斯方程得到柠檬酸锌二水合物转变成无水合物的焓变为381.13 J/g(232.6kJ/mol),与传统差示扫描量热法(DSC)相比测量误差为4.2%。这些结果表明,THz-TDS可以提供一种快速、有效的结晶水合物焓变检测方法。     

Study and implementation of a low conduction loss zero-current resonant switch

Zero-current (ZC) resonant switches allow one to reduce the switching losses in high-frequency DC/DC switched mode power supplies. ZC resonant switches can be either unidirectional (half-wave) or bidirectional (full-wave). If a conventional power MOSFET is chosen to implement the ZC resonant switch, the turn-on of the slow intrinsic diode has to be avoided. This is usually done with a fast blocking diode, which is connected in series with the MOSFET. Furthermore, an antiparallel fast diode is added when a FW ZC resonant switch is required. The conduction losses are relevant in this implementation, owing to the threshold voltage and to the series resistances of the two diodes. In this paper, a low-conduction-loss FW ZC resonant switch has been proposed. Its implementation is based on a power MOSFET and a single antiparallel Schottky diode. The possibility of an implementation with a power MOSFET alone is also discussed. A control circuit suitable for the proposed ZC resonant switch has been described. The experimental results obtained from a ZCS-QR buck converter are discussed.<>     

Alias-Component Matrices of Multirate Systems

We consider a multirate system, which is a generalization of linear time-invariant systems. Such a system is invariant to a certain shift in the input sequence. In particular, assume that p and q are coprime. A multirate system with the property that a delay of mq samples in its input sequence results in a delay of mp samples in its output sequence is called an (mp, mq)-periodic system. This multirate system can be obtained by cascading an upsampler, followed by a linear periodically time-varying (LPTV) kernel system, then followed by a downsampler. Here, we study the alias-component matrices of multirate systems. We show that they can be obtained from the alias-component matrices of their LPTV kernels by some row and column additions. An example shows the use of the method to design rate changers for a specified frequency band swap.     

A New Base—6 FFT Algorithm

A new FFT algorithm has been deduced, which is called the base-6 FFT algorithm. The amount for calculating the DFT of complex sequence of N =2 r by the base-6 FFT algorithm is M r( N )=14/3· N log 6 N -4 N +4 for multiplication operation of real number and A r( N )=23/3· N log 6 N -2 N +2 for addition operation of real number. The amount for calculating the DFT of real sequence is a half of it with the complex sequence.     

A Hardware-Efficient Algorithm for Real-Time Computation of Zadoff–Chu Sequences

A hardware-efficient algorithm and architecture for computing Zadoff–Chu (ZC) sequence elements on-line using the CORDIC algorithm are proposed. Zadoff–Chu sequences possess good correlation properties that are essential in a variety of engineering applications, such as establishing timing synchronization between a mobile terminal and a base station in the emerging 3GPP long-term evolution (LTE) physical layer standard for cellular communications. The proposed algorithm computes ZC-sequence elements both in time domain and frequency domain using a simple duality relationship. Algorithm transforms are employed to compute the elements recursively and eliminate the need for multipliers with non-constant terms. A reconfigurable hardware architecture was implemented and applied in a searcher block for detecting the physical random access channel (PRACH) in LTE. The PRACH provides a mechanism for a mobile to establish initial access along with uplink synchronization by transmitting a preamble that is constructed from ZC sequences. The proposed architecture is capable of generating these preambles on the fly with high accuracy, eliminating the need for storing a large number of long complex-valued ZC sequence elements. Simulation results demonstrate that the proposed architecture is capable of achieving detection error rates for LTE PRACH that are close to ideal rates achieved using floating point precision. (The work has been presented in part in Mansour (2009).)     

Orthogonalization of circular stationary vector sequences and itsapplication to the Gabor decomposition

Certain vector sequences in Hermitian or in Hilbert spaces, can be orthogonalized by a Fourier transform. In the finite-dimensional case, the discrete Fourier transform (DFT) accomplishes the orthogonalization. The property of a vector sequence which allows the orthogonalization of the sequence by the DFT, called circular stationarity (CS), is discussed in this paper. Applying the DFT to a given CS vector sequence results in an orthogonal vector sequence, which has the same span as the original one. In order to obtain coefficients of the decomposition of a vector upon a particular nonorthogonal CS vector sequence, the decomposition is first found upon the equivalent DFT-orthogonalized one and then the required coefficients are found through the DFT. It is shown that the sequence of discrete Gabor (1946) basis functions with periodic kernel and with a certain inner product on the space of N-periodic discrete functions, satisfies the CS condition. The theory of decomposition upon CS vector sequences is then applied to the Gabor basis functions to produce a fast algorithm for calculation of the Gabor coefficients     

On the stability of paths, Steiner trees and connected dominating sets in mobile ad hoc networks

We propose algorithms that use the complete knowledge of future topology changes to set up benchmarks for the minimum number of times a communication structure (like paths, trees, connected dominating sets, etc.) should change in the presence of a dynamically changing topology. We first present an efficient algorithm called OptPathTrans that operates on a simple greedy principle: whenever a new source–destination (s–d) path is required at time instant t, choose the longest-living s–d path from time t. The above strategy when repeated over the duration of the s–d session yields a sequence of long-lived stable paths such that number of path transitions is the global minimum. We then propose algorithms to determine the sequence of stable Steiner trees and the sequence of stable connected dominating sets to illustrate that the principle behind OptPathTrans is very general and can be used to find the stable sequence of any communication structure as long as there is a heuristic or algorithm to determine that particular communication structure in a given network graph. We study the performance of the three algorithms in the presence of complete knowledge of future topology changes as well as using models that predict the future locations of nodes. Performance results indicate that the stability of the communication structures could be considerably improved by making use of the knowledge about locations of nodes in the near future.     

Quadtree and symmetry in FFT computation of digital images

The discrete Fourier transform (DFT) of a real sequence f[x, y] of size N×N, where N=2ⁿ, can be computed by a two-dimensional (2-D) FFT of size N/4, or smaller if f[x, y] is known to have certain symmetries. This paper presents theorems that identify the symmetry in f[x, y] based on the depth of the quadtree to expedite 2-D FFT computation of coherent digital images. In principle, it establishes that if the quadtree of f[x, y] has maximum depth kn , then the DFT can be computed by a 2-D FFT of size K/2. An algorithm is given, and its performance analyzed. Finally, applications are considered in transform coding systems and lossy compression of images     

On Cyclic Autocorrelation and the Walsh-Hadamard Transform

It is shown that the complete set of circular shift invariants called the Q-spectrum, of the Walsh-Hadamard transform (WHT) of a periodic sequence is related to the cyclic autocorrelation of the given sequence through the Hadamard matrices. It is also shown that the modified WHT (MWHT) of the cyclic autocorrelation yields the Q-spectrum within some scale factors. This is analogous to the discrete Fourier transform (DFT) case, i.e., the DFT of the autocorrelation of a sequence yields the shift invariant power spectrum. The Q-spectrum can be computed efficiently using the MWHT rather than the WHT. A physical interpretation for the Q-spectrum is also provided. The motivation for this study is to show that while the WHT is inherently associated with the notion of dyadic time shifts, it does have analogous properties with respect to cyclic time shifts.     

DFT/LMS算法在DSSS中的应用及性能分析

本文分析了直接序列扩频(DSSS)系统中最小错误概率(MPE)意义下的最优滤波器,并依据矩阵求逆引理证明最小均方误差(MMSE)意义下的最优滤波——维纳滤波也是MPE意义下的最优滤波。在DSSS中应用自适应滤波,无须先验已知扩频码的码型和干扰的统计特性,就能一并完成解扩以及有效抑制干扰。离散傅立叶变换/最小均方(DFT/LMS)算法的收敛速度远快于LMS算法,而运算量、稳健性与LMS算法基本相同。基于DFT/LMS算法的自适应滤波大大简化DSSS系统接收机的设计,显著增强系统抗干扰能力,具有很强的实用性。     

A Stochastic Model for a Pseudo Affine Projection Algorithm

This paper presents a statistical analysis of a Pseudo Affine Projection (PAP) algorithm, obtained from the Affine Projection algorithm (AP) for a step size alpha < 1 and a scalar error signal in the weight update. Deterministic recursive equations are derived for the mean weight and for the mean square error (MSE) for a large number of adaptive taps N compared to the order P of the algorithm. Simulations are presented which show good to excellent agreement with the theory in the transient and steady states. The PAP learning behavior is of special interest in applications where tradeoffs are necessary between convergence speed and steady-state misadjustment.     

类奈曼-皮尔逊检验加权lp相关的干扰鲁棒同步算法

为实现正交频分复用（Orthogonal Frequency Division Multiplexing,OFDM）系统的时域同步,本文利用ZC（Zadoff-Chu）序列提出了类奈曼-皮尔逊检验加权的l_p（p=1或2）相关（Neyman-Pearson-like test Weighted l_p-Correlation,l_p-NPWC）同步算法.分析表明：该算法利用类奈曼-皮尔逊检验能有效抑制多径效应对l_p相关同步的影响,特别地,当取p=1时,它对重尾分布噪声及循环前缀导致的伪峰具有极强的鲁棒性.实验及仿真结果均证明了理论分析的正确性和有效性,并表明本文算法相较于现有算法在各种干扰环境中均具有更高的同步精度和性能.     

一种基于DDST的频域能量最小化OFDM频偏估计方法

提出一种基于数据依赖叠加训练序列的OFDM载波频偏估计方法。叠加训练序列的周期性使其DFT能量间隔分布在特定的频点上,为了减小信息序列对叠加训练序列的影响,引入数据依赖序列使信息序列DFT在这些特定频点上为零,利用这个特性可进行频偏估计,只有得到正确的频偏估计时,这些旋转后的信息序列在这些特定频点的能量才得到最小值。仿真表明,该方法在不降低传输速率的情况下,有着较好的频偏估计性能。     

Analog DFT Processors for OFDM Receivers: Circuit Mismatch and System Performance Analysis

An N-symbol discrete Fourier transform (N -DFT) processor based on analog CMOS current mirrors that operate in the strong inversion region is presented. It is shown that transistor mismatch can be modeled as an input-referred noise source that can be used in system-level studies. Simulations of a radix-2, 256-symbol fast Fourier transform (FFT) show that the model produces equivalent results to those of a model that incorporates a mismatch term into each current mirror. It is shown that in general, high-radix FFT structures and specifically the full-radix DFT have reduced sensitivity to mismatch and a reduced number of current mirrors compared to radix-2 structures and have some key advantages in terms of transistor count with respect to comparable digital implementations. Simulations of an orthogonal frequency-division multiplexing system with forward error control coding, that take into account current mirror nonidealities such as mismatch, show that an analog DFT front end loses only 0.5 dB with respect to an ideal circuit.