Differentially-Encoded Di-Symbol Time-Division Multiuser Impulse Radio in UWB Channel

Ultra-wideband (UWB) communication systems are expected to operate in a highly frequency-selective multipath fading environment. To exploit multipath diversity gains in a multiuser scenario, we developed a differentially-encoded, di-symbol time-division multiuser impulse radio (d²TD-IR) system with delay-sum autocorrelation receivers. In traditional time-division multiple access systems, each user transmits a single pulse during a symbol duration in a pre-assigned chip which is longer than maximum excess delay of the channel. However, due to the exponential decay property of UWB channel, we proposed the use of much shorter chip duration, which significantly increases the transmission rate. Because dense pulse transmission will induce multiuser interference, two time-hopping access sequences, which alternately encode the odd- and even-index symbols, are employed with delay-sum autocorrelation receivers to maximally suppress the interference. It was shown that when the chip duration is properly chosen, the proposed system outperforms the conventional time-hopping impulse radio system at high signal-to-noise ratio. This paper also proposed a method to estimate the optimal chip duration when only the average power decay profile of the UWB channel is known.     

An efficient numerical approach, for determining the dispersioncharacteristics of dual-mode elliptical-core optical fibers

Based on vectorial formulations which combine the surface integral equation method and the finite-element method, a novel numerical approach is proposed for calculating the dispersion coefficients of dual-mode elliptical-core fibers with arbitrary refractive index profiles. By differentiating the original formulations involving the propagation constant β and the guided mode fields H_x and H_y once and twice with respect to the normalized frequency V, the new formulations for {dβ/dV, dH_x/dV, dH_y/dV} and for {d²β/dV², d² H_x/dV², d²H_y/dV² } are obtained respectively. Once {β, H_x, H_y } is solved through the eigenvalue procedure which dominates the computing time, only a few matrix manipulations are required to obtain {dβ/dV, dH_x/dV, dH_y/dV} and {d²β/dV², d²H_x/dV² , d²H_y/dV²}. Some numerical examples are examined to see the influence of different refractive index distributions with dips on the dispersions of the four nondegenerate LP ₁₁ modes for elliptical-core fibers     

On using a blackbody source to pump a Cd+photo-ionization laser

We report laser oscillations in Cd II on the 4d⁹ 5s^{2 2}D_5/2-4d¹⁰ 5_p ²P_3/2 transition at 441.6 nm using a laser-produced tungsten plasma as a pumping source. The axial electron density is estimated using a Mach-Zehnder interferometer     

A two-wavelength, passively self-injection locked, CW Ti3+:Al2O3 laser

We have demonstrated that passive self-injection control is very effective in a CW Ti³⁺:Al₂O₃ laser to produce high efficiency (η~15%), narrow-line, two-wavelength emission. Light reinjection is achieved with an original high-resolution two-wavelength selector combining an interference wedge and a grazing incidence grating. The device operates outside the main nonselective laser cavity; thus avoiding insertion losses and optical damage problems. A 1.5 W, background free, total output power has been obtained simultaneously at two narrow laser lines (fwhm: 8 pm), independently tunable over 50 mn. The two laser lines can be emitted in a single beam or in two separated beams. Self-injection wavelength control has been demonstrated both for linear and for ring laser cavities. In the latter case, unidirectional behavior is also obtained in the same way. Single-step (5s²S_1/2-5P²P_1/2,3/2 ) and two-step (5s²S_1/2-5p²P_3/2-5d² D_3/2,5/2) excitation of atomic rubidium vapor has been performed with our original laser device     

New laser oscillations in the N atom quartet manifold

Three new laser oscillations have been observed in a pulsed discharge of He and N2. These oscillations at 1.0568, 1.0611, and 1.0623 μm are fine-structure components of the 3d⁴P → 3p⁴P⁰transition of N(I). The three observed transitions (out of a possible seven) are those with the largest transition matrix elements that do not share a common lower state.     

叠加弱能量PN序列DVB-T帧同步算法

本文提出了一种DVB-T 2K系统的帧同步新算法以改进高速移动环境同步性能.该算法利用叠加的训练帧的自相关性和保护间隔的循环特性获得同步信号,其中训练帧的能量仅为发射端总能量的1.41%.仿真表明:该算法在150km/h到450km/h的高速移动DVB-T系统能获得良好的帧同步性能;同时,该叠加的PN序列对系统性能的影响可以忽略不计.     

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.     

Radiative autoionization--A proposal for the construction of short-wavelength lasers

Radiatsive autoionization (RA) is a spontaneous decay mechanism of highly excited states, whereby the atom or molecule makes a radiative bound- (or quasi-bound-) free transition, emitting a photon and an electron at the same time. Calculations show that, for each transition, the intensity of the emitted radiation peaks on the short-wavelength side. Since RA involves a final empty state, population inversion is achieved once the initial state is populated. The basic lasing condition can thus be satisfied automatically. The phenomenon is analogous to the radiative dissociation process which is the physical mechanism for the excimer laser. Examples are given for the H- and He 2p²³p, He- and Li 2p³⁴S0, Li and Be+ 1s2p²²p, He 2p3p¹P, and 2p3d¹D⁰, as well as for the Li 1s2s2p⁴P⁰state which undergoes relativistic RA.     

电力线通信中OFDM同步改进算法研究

正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)是一种能够有效对抗电力线通信中的频率选择性衰落、多径和窄带干扰的技术,而OFDM技术对同步偏差十分敏感,微小的同步偏差也会影响OFDM系统的性能.针对电力线通信中OFDM系统的同步问题,给出一种基于恒包络零自相关(CAZAC)序列的时频同步的方法.MATLAB仿真表明,所提算法在减少运算量的基础上能很好地完成符号同步和频偏同步,明显改善电力线中OFDM的同步性能.     

New List Sphere Decoding (LSD) and Iterative Synchronization Algorithms for MIMO-OFDM Detection With LDPC FEC

New sphere decoding and synchronization algorithms for multiple-input–multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems are proposed in this paper. In particular, an iterative list branch-and-bound (BB) algorithm based on the basic BB algorithm is described to obtain a candidate list to compute soft information that is used in the iterative detector. Furthermore, an improved algorithm that uses prior information from the preceding iteration to calculate the lower bound is proposed, and the candidate list is updated every iteration. To obtain a complete modem architecture, we propose an efficient expectation–maximization (EM)-based iterative algorithm for synchronization and channel estimation to interface with the proposed list-sphere-decoding detector, and we investigate the performance of the designed MIMO-OFDM modem on a realistic fading channel. The obtained performance results show that it is possible to practically design a performing MIMO-OFDM modem with high spectral efficiency, i.e., 8 bit/s/Hz with a 4 $times$ 4 16-QAM MIMO-OFDM system.      

DFIG-Based Power Generation System With UPS Function for Variable-Speed Applications

The power generation system with a doubly fed induction generator (DFIG), which can be used as an autonomous power system after the loss of mains in a distributed generation network, is described. After the mains outage, a fixed frequency and an amplitude of the output voltage are obtained, despite the variable rotor speed. For this reason, it can be successfully applied in the variable-speed wind turbines, adjustable speed water plants, or diesel engines. Moreover, the stand-alone operation of DFIG is useful in a flywheel-based high-energy rotary uninterruptible power supply system. An output voltage is controlled directly by the synchronization of an actual voltage vector with the reference vector represented in a synchronously rotating polar frame. The rotor current angular speed is obtained as a result of vectorial phase-locked loop operation. Any sensors or estimators of the rotor speed or position are unnecessary. Both amplitude and angle control loops are linear. The use of stand-alone operation in grid-connected systems requires mains outage detection. Also, the grid voltage recovery requires a method of synchronization and soft connection of a generator to the grid. The proposed methods of output voltage control, synchronization, and detection of mains loss were tested in a laboratory system.      

Research on Time Synchronization Algorithm of High Precision and Low Power Consumption Based on IRBRS WSNs

Based on the study of RBS algorithm in wireless sensor network, an improved algorithm named RBRS is presented, to solve the time synchronization problem of multi hop networks. On the basis of RBS, the broadcast group and least square linear regression methods were used to realize entire network time synchronization in the algorithm. Synchronization error and overhead with the existing improved BRS are compared in the article. The result shows that the algorithm has certain superiority in many algorithms, which is suitable for light and low power consumption network load. Through an emulation with the matlab software, the result indicates that error accumulation is lower in the optimized algorithm, the synchronization overhead is significantly reduced and can realize the time synchronization of the whole network. A time synchronization algorithm based on RBRS algorithm is proposed for multi-hop and the low power consumption. A variable period synchronization method is introduces in the algorithm: in line with Bias maximum a posterior estimation principle, the maximum phase offset is estimated to determine the synchronization period, which can reduce the number of node synchronization, and with the least square linear regression method, the periodic fitting clock offset. The Simulation in matlab indicate that RBRS algorithm can improve the synchronization accuracy and reduce energy consumption significantly, which is conducive to extend the life of the network.

     

基于DSP的4/πQPSK调制解调器的设计

QPSK调制解调器与具有强大数字信号处理功能的DSP芯片相结合,实现了调制解调的高效实时处理。系统采用TI公司的TMS320C5409为核心处理器,设计了硬件电路系统的3个主要模块,包括A/D转换模块,程序存储器模块和数据存储器模块。将一种简单高效的定时同步算法——最大平均功率定时同步算法引入到系统的解调设计中,以提高定时同步的运算速度和准确性。系统软件采用汇编语言编写,并通过CCS软件仿真,结果证明该算法能够完成4/πQPSK调制解调的全部功能,且具有体积小、功耗低、集成度高、软件可移植性强等优点。     

Adjacent cell interference in FH-MFSK cellular mobile radio system

The effect of adjacent cell interference in cellular mobile system using FH-MFSK transmission is evaluated quantitatively. The performance of base to mobile communication in the system is analyzed, assuming perfect synchronization between users in all the cells. Analysis of the system employing no power control shows that the number of simultaneous users possible at average bit error probability Pbof less than 1 × 10^-3is reduced greatly from the corresponding figure for the isolated cell (which is about 170). It is then shown that a simple power control strategy could reduce the adjacent cell interference significantly. A reasonable knowledge of the distribution of users within a cell allows the optimization of the receiver threshold with respect to distance from the base. With this optimization, each cell could accomodate ≳ 115 users at Pb< 10^-3, the exact figure being dependent on the user distribution. The power control also helps to reduce the average power transmitted from a base.     

Tunneling effect in CdxHg1-xTe photodiodes

The current-voltage I-U characteristics and those of log I-U, dU/dI-U, and d²U/dI²-U of CdxHg1-xTe (x = 0.20-0.27) photodiodes were measured in the temperature range of 4.2-77 K. The data analysis indicates the backward-type behavior and both the elastic and inelastic resonant-tunneling effects. The resonant tunneling itself seems to originate from resonance-energy states (defect states) located within the junction area.     

Choice of Regulator for Mutual Network Synchronization

This paper compares time-discrete regulators used for mutual single-ended synchronization of digital networks. The regulators control the phase differences between clocks in the network nodes. These clocks are disturbed mainly by short-term phase and frequency fluctuations and long-term linear frequency drift. The characteristics of the regulators under study are stability, engineering, and implentation. The main results of the stability analysis are that thePregulator brings about a constant slip rate, thePlregulator is not slip-free, the Pll2regulator is slip-free. With nonzero initial phase differences and jitter, the stability of the regulators containing integrators is secured by not regulating one of the nodes. In conclusion, the Pll2regulator is the best choice for single-ended mutual network synchronization.     

Low-complexity centralized joint power and admission control in cognitive radio networks

This letter introduces a centralized joint power and admission control algorithm for cognitive radio networks. Its novelty lies in the proposed admission metric. Unlike those in existing algorithms, our metric predetermines the admission order of N secondary users which intend to access the network. This allows us to search a group of admitted secondary users with the bisection method. The proposed algorithm is shown by simulation to achieve a comparable performance to existing algorithms, and the computational complexity is reduced from O(N³) to O(N² log₂ N).     

Asymptotic Critical Total Power for $k$ -Connectivity of Wireless Networks

An important issue in wireless ad hoc networks is to reduce the transmission power subject to certain connectivity requirement. In this paper, we study the fundamental scaling law of the minimum total power (termed as critical total power) required to ensure k -connectivity in wireless networks. Contrary to several previous results that assume all nodes use a (minimum) common power, we allow nodes to choose different levels of transmission power. We show that under the assumption that wireless nodes form a homogeneous Poisson point process with density lambda in a unit square region [0, 1]², the critical total power required to maintain k-connectivity is Theta((Gamma(c/2 + k)/(k - 1)!) lambda^1-c/2) with probability approaching one as lambda goes to infinity, where c is the path loss exponent. If k also goes to infinity, the expected critical total power is of the order of k^c/2 lambda^1-c/2. Compared with the results that all nodes use a common critical transmission power for maintaining k-connectivity, we show that the critical total power can be reduced by an order of (log lambda)^c/2 by allowing nodes to optimally choose different levels of transmission power. This result is not subject to any specific power/topology control algorithm, but rather a fundamental property of wireless networks.     

Robust and Accurate Frequency and Timing Synchronization Using Chirp Signals

We propose a new robust and accurate synchronization procedure using a training sequence composed of chirp signals. We use a new integer frequency estimation algorithm and propose a new combination of a known fractional frequency offset estimation algorithm and timing synchronization algorithm. The training sequence is composed of one up and two down chirp symbols, also known as Newman phases. The integer frequency offset estimation algorithm uses the effect of timing and frequency offsets on the matched filter outputs of the chirp signals. Autocorrelation and reversed autocorrelation are used to acquire the timing instant and the fractional frequency offset. We present the complete timing and frequency synchronization procedure and study the output signals of the autocorrelation and reversed autocorrelation algorithms. Finally, we check the performance of the synchronization procedure via Monte Carlo simulation in several multipath channels. Our algorithms are accurate and more robust compared to previously published state-of-the art algorithms.      

基于分簇的多跳无线传感网络时间同步算法

针对典型的时间同步算法应用到多跳拓扑网络时存在时间同步精确度差、收敛速度慢、功耗较大等问题,文章提出一种基于分簇的多跳无线传感网络时间同步算法。该算法结合了TPSN算法和RBS算法的同步思想,摒弃了TPSN算法同步周期长的缺点而保留了其同步精确度高的优点,也解决了RBS同步开销大和难于应用到多跳网络的问题。该算法具有相当高的同步精度,并有效降低了同步功耗,具有一定的实用性。