Sample path properties of timed discrete event systems

The basic problem of constructing a perturbed sample path (given a parameter perturbation) from information contained in a nominal sample path is considered. Two conditions, observability and constructability, which have to be satisfied for this to be feasible are identified. One approach for accomplishing this task is to develop an augmented system model which captures both nominal and perturbed system behaviour. For the case of systems with Markov properties, an explicit methodology is presented for constructing such models. It is also shown that by an observability transformation it is generally possible to satisfy constructability conditions allowing the performance of a perturbed discrete event system to be estimated by observing only a nominal sample path. In practice, a variety of techniques may be used to accomplish this goal, depending on issues such as parameter value availability and convergence speed of various performance sensitivity, estimates     

A Hybrid Multichannel Hardware Simulator for Frequency-Selective Mobile Radio Paths

We present the analysis and design of an efficient hybrid digital/analog multichannel hardware simulator for frequencyselective Rayleigh-fading mobile radio paths. The simulator has eight RF outputs and is linear and reciprocal. All outputs exhibit mutually independent Rayleigh fading over a 45 dB range and have identical time delay spreads. Analog RF techniques are combined with a digital signal processor (DSP) to generate random path modulation. Frequency selectivity from time delay spread is created using several RF path delay elements. Simulated vehicle velocity is continuously variable from 6 to 60 mph. The simulator is thus a flexible tool for the study of mobile radio transmission techniques.     

Performance of Multipath Routing for On-Demand Protocols in Mobile Ad Hoc Networks

Mobile ad hoc networks are characterized by multi-hop wireless links, absence of any cellular infrastructure, and frequent host mobility. Design of efficient routing protocols in such networks is a challenging issue. A class of routing protocols called on-demand protocols has recently found attention because of their low routing overhead. The on-demand protocols depend on query floods to discover routes whenever a new route is needed. Such floods take up a substantial portion of network bandwidth. We focus on a particular on-demand protocol, called Dynamic Source Routing, and show how intelligent use of multipath techniques can reduce the frequency of query floods. We develop an analytic modeling framework to determine the relative frequency of query floods for various techniques. Our modeling effort shows that while multipath routing is significantly better than single path routing, the performance advantage is small beyond a few paths and for long path lengths. It also shows that providing all intermediate nodes in the primary (shortest) route with alternative paths has a significantly better performance than providing only the source with alternate paths. We perform some simulation experiments which validate these findings.     

A Computer Model to Evaluate Airborne Receiver Performance in Interference Environments

A computer model is described to evaluate airborne radio performance in a mixed environment of friendly and hostile emitters. A new performance criterion, the communications operability volume, is introduced. This criterion permits operational, scenario-based comparisons of various radio designs over a range of interference environments. Communications operability volumes can be calculated for different waveforms, receiver antenna patterns (including adaptive arrays), and detection thresholds. An empirical propagation algorithm is used to calculate path losses and/or maximum communications range. The computer model provides several types of graphic outputs that are used to compare antijam or electromagnetic compatibility performance for different radio designs at various locations relative to the sources of interference in a scenario.     

Flip-flop resolving time test circuit

Integrated circuit flip-flop resolving time parameters, required for calculation of synchronizer performance and reliability, are measured by wafer probing, without the need for dicing or bonding, by incorporation of test structures on an IC along with the flip-flop to be measured. The circuit has five digital inputs, five digital outputs, including one for frequency measurements, and two analog inputs plus power and ground connections. Several delays that are fabricated as part of the test circuit, including a voltage controlled delay with a few picosecond resolution, are calibrated as part of the test procedure by grating them into and out of the delay path of a ring oscillator. Each of the delay values is calculated by subtracting the period of the ring oscillator with the delay omitted, from the period of the ring oscillator with the delay included. A frequency divider is fabricated as part of the test structure to reduce the output of the ring oscillator to less than 200 kHz so no high-frequency inputs of outputs from the IC are required.     

Investigation of a Single-Sideband Mixer Anomaly

Measurements on a 6-GHz single-sideband (SSB) balun-coupled mixer revealed a feedthrough of RF signals between the two mixer sections that caused the IF outputs to be unbalanced at the +-90° local oscillator (LO) phase differences when using a ring diode quad. Using a bridge diode quart in this same mixer eliminated this IF output unbalance. These measurements also give conclusive evidence that the balun-coupled mixer has a short-circuited image frequency voltage with the ring diode quad and an open-circuited image frequency voltage with the bridge diode quad. These two image frequency impedance conditions are independent of circuit terminating impedances and solely depend on the image frequency current path being completed or interrupted by the ring or bridge diode quads, respectively.     

Cancellation of Amplifier Offset and 1/f Noise: An Improved Chopper Stabilized Technique

In this brief, a theoretical analysis on the limits of conventional chopper stabilization technique is presented and a solution conceived to improve the amplifier performance is proposed. The expected replicas of the 1/f noise at the chopping frequency and its multiples are attenuated by a modified chopping control. Simulations done by using records of real 1/f noise outputs showed that, by following the proposed approach, the spectrum of the signal is not affected while the 1/f replicas are reduced by more than 40 dB with respect to conventional techniques. The required circuit for the generation of the chopping signal is also described. The resulting overhead with respect to conventional techniques is negligible and fully acceptable.     

On the application of forking nodes to product-form queueing networks

We define a ‘forking node’ as a service centre with one input feeding two outputs (each served by its own queue) under the control of an internal path-selection (PS) policy. We assume that both outputs lead to paths through which a packet reaches its final destination. However, the mean downstream delays on the two paths may be different and the PS policy should favour the path with the lower downstream delay. Using simulation, we compare the performance of this system under a variety of random, deterministic, state-dependent PS policies, including threshold-based and join-shortest-queue with bias (JSQ + b). We show that JSQ + b has better performance than the other alternatives. Moreover, if the input process to the forking node is Poisson, standard time series analysis techniques show that its two outputs are very close to being independent Poisson processes. Thus, if we find an accurate and efficient ‘offline’ analytical performance model for JSQ + b forking node, we can extend the applicability of product-form queueing networks to include such forking nodes. For this reason, we present several ways of modelling the performance of a JSQ + b node, using bounds, and compare their results on example networks. We establish a closed-form expression relating the bias b and the delays of the downstream paths. Copyright © 2007 John Wiley & Sons, Ltd.     

Fundamentals of dynamic frequency hopping in cellular systems

We examine techniques for increasing spectral efficiency of cellular systems by using slow frequency hopping (FH) with dynamic frequency-hop (DFH) pattern adaptation. We first present analytical results illustrating the improvements in frequency outage probabilities obtained by DFH in comparison with random frequency hopping (RFH). Next, we show simulation results comparing the performance of various DFH and RFH techniques. System performance is expressed by cumulative distribution functions of codeword error rates. Systems that we study incorporate channel coding, interleaving, antenna diversity, and power control. Analysis and simulations consider the effects of path loss, shadowing, Rayleigh fading, cochannel interference, coherence bandwidth, voice activity, and occupancy. The results indicate that systems using DFH can support substantially more users than systems using RFH     

A Simplified Method for Prediction of Multipath Fading Outage of Digital Radio

We propose a simplified method based on the correlated Rayleigh probability distribution of in-band radio signals. It can be efficiently applied to the calculation of the outage for various radio path length, propagation terrain, radio frequency, bandwidth, and digital modulation techniques. Predicted results are compared to various propagation data.     

Application of open-loop uplink power control in Ka-band satellitelinks

Propagation impairments impose a limit on the use of the 20/30-GHz frequency band for satellite communication applications. Power control is one of the techniques that can be used to mitigate such impairments. Results of an experiment conducted using the Advanced Communications Technology Satellite (ACTS) to evaluate the efficacy of open-loop uplink power control are presented. A power control system is required to maintain the power flux received at the satellite at a constant level irrespective of the fading along the propagation path. The control parameter for the power controller was derived from the fading and enhancements observed on a downlink beacon signal thus requiring frequency translation of the propagation effects to the uplink frequency. In this scheme, the controller performance is largely a function of the frequency translation accuracy, which is determined by the prevailing propagation conditions. In addition, equipment-induced variations in the control parameter can produce power control errors. To maintain the control accuracy within reasonable limits, an algorithm that can differentiate various propagation factors as well as equipment effects was devised. It was found that under most conditions, the power control accuracy could be maintained within ±2.5 dB     

Studies on the performance characteristics and improvements of the piezoelectrically-driven micro gas compressors

This study presents the main characteristics of a micro gas compressor produced by microfabrication techniques on silicon wafers. The compressor consists of a compression chamber, check valves and a silicon membrane where the piezoelectric bimorph actuator is installed. Compressor performance was investigated under various working conditions of input voltage and frequency to the actuator at several downstream back pressures. Volume stroke ratio is a critical parameter for gas compressors. However, micro actuators do not generally produce large displacement, so the volume stroke ratio of the micro compressor is expected to be significantly less than that of conventional mechanical compressors. Therefore, the possibility of using dual compression was also investigated in order to improve micro compressor performance. The performance of the micro compressor is evaluated in this study through experiments and simulation.     

一种改进的多约束最佳路径算法研究

本文描述了MPLS网络中的多约束最佳路径问题,以及该问题的各种变型.分析了现有的解决这些问题的算法存在的各种缺陷,并针对一般性的多约束最佳路径问题的解法,提出了改进的具体措施.通过具体的实例分析和计算机仿真,验证了新算法在性能上的改善,主要的性能指标包括路径计算的成功比率和路径的平均代价等.结合仿真结果讨论了算法中涉及到的关键参数对算法性能的影响.     

Hybrid slow frequency hopping/ direct sequence spread spectrum indoor communication systems with QPSK modulation

This paper analyses the performance of a spread-spectrum multiple access system based on hybrid direct sequence/slow frequency hopping techniques with QPSK modulation. The performance is analyzed considering multipath and multiple access interferences and an indoor channel having Rician distributed path gains. The system performance is investigated for 2 diversity techniques and compared, for given delay spread, bandwidth and bit rate, with the performance provided by bpsk modulation. It is shown that the larger symbol duration of QPSK is favorable to combat the effect of multipath propagation.     

Channel‐estimate‐based frequency‐domain equalization (CE‐FDE) for broadband single‐carrier transmission

A channel‐estimate‐based frequency‐domain equalization (CE‐FDE) scheme for wireless broadband single‐carrier communications over time‐varying frequency‐selective fading channels is proposed. Adaptive updating of the FDE coefficients are based on the timely estimate of channel impulse response (CIR) to avoid error propagation that is a major source of performance degradation in adaptive equalizers using least mean square (LMS) or recursive least square (RLS) algorithms. Various time‐domain and frequency‐domain techniques for initial channel estimation and adaptive updating are discussed and evaluated in terms of performance and complexity. Performance of uncoded and coded systems using the proposed CE‐FDE with diversity combining in different time‐varying, multi‐path fading channels is evaluated. Analytical and simulation results show the good performance of the proposed scheme suitable for broadband wireless communications. For channels with high‐Doppler frequency, diversity combining substantially improves the system performance. For channels with sparse multi‐path propagation, a tap‐selection strategy used with the CE‐FDE systems can significantly reduce the complexity without sacrificing the performance. Copyright © 2004 John Wiley & Sons, Ltd.     

Performance of interleaved trellis-coded differential 8-PSKmodulation over fading channels

The performance of trellis-coded differential octal phase-shift keying (coded 8-DPSK) with differentially coherent detection and soft-decision Viterbi decoding is investigated. A suitable receiver is presented whose signal processing is based on Nyquist signaling, requiring only one complex sample per modulation interval. Symbol synchronization and automatic frequency control are performed in a decision-directed way. Bit-error-rate (BER) performance over Gaussian, Rayleigh, and Rician channels is determined by means of computer simulations. The performance of coded 8-DPSK on the Gaussian channel is shown for a four-state convolutional trellis code. The unquantized outputs of up to three symbol detectors with delays of 1, 2, and 3 symbol periods are used for metric computation. The coding gain which includes losses due to timing and frequency synchronization errors is found to be 2.5 dB at BER=10^-5 with respect to uncoded 4-DPSK. Much larger gains are achieved for fading channels if interleaving is applied. Using an eight-state trellis code the performance is determined on Rayleigh and Rician channels for various Doppler spreads and interleaver sizes     

Data processing techniques for the detection and interpretation of teleseismic signals

This paper is a collection of six papers describing recent developments in automated detection and identification of teleseismic earthquakes and explosions in a seismic noise background. The first paper evaluates the assumption that the outputs of seismometer arrays can be added since the signals will reinforce while the noise is cancelled. Signal and noise correlations vs. distance and frequency are presented for an array of 1600 km in extent. The second paper describes a method utilizing orthogonal expansions of the Kautz type in an effort to determine spectral and temporal differences between both types of signals and noise. Theory and measurements indicate that the seismic noise background is largely composed of fundamental and higher mode Rayleigh waves. The third paper describes a thermal equilibrium analogy to estimate the noise energies in each mode to account for the observed depth and frequency behavior. The use of multiple and partial coherence functions for resolving noise backgrounds into their propagation components is described in the fourth paper. Compressional, shear, and surface wave components of signals can be separated from seismic noise backgrounds by recognizing their differing polarization properties, as shown in the fifth paper. The source mechanisms can theoretically be identified from their radiation patterns provided instrument and travel path distortions are removed. A final paper describes this theory and how these various methods of detecting and isolating the signals can be integrated into an automated signal analysis system.     

Implementation and evaluation of a dual parallel linearizationsystem for AM-SCM video transmission

An externally modulated transmitter was linearized for AM subcarrier multiplexing (AM-SCM) video transmission. Linearization was accomplished by combining the outputs of two lithium niobate Mach-Zehnder interferometers which were simultaneously modulated. By controlling the optical and RF power at each modulator, more than 20 dB of third-order distortion cancellation was demonstrated for a multitone video spectrum. Results are presented for both NTSC and PAL B/G frequency plans. The effects of modulator nonideality on system performance are analyzed and confirmed experimentally. In particular, overall system performance is shown to be very sensitive to acoustic effects and to optical and electrical multipath interference. System modeling is used to provide tolerances on various critical parameters     

A Balun-Bandpass filter with good in-band performance and wide stopband

A high performance Balun BandPass Filter (BPF) with capacitively loaded multiple coupled lines with very simple structure is proposed in this paper, this structure realizes simultaneous size reduction and superior harmonic response suppression performance in bandpass filtering meanwhile good differential performance of the Balun. The theory of this structure for unbalanced input into balanced output has been studied in this paper and a proper Balun and BPF characteristic by the symmetric feeding and skew symmetric feeding have been obtained to prove the theory. The enter frequency of the fabricated Balun-BPF is operated at 2.45 GHz with 6.93% Fractional Band Width (FBW), and this frequency is used for Bluetooth and some other communication systems. The differences between the two outputs are 180° ± 1.92° in phase and within 0.33 dB in magnitude. At f ₀, the amplitude imbalance and phase difference are within 0.25 dB and 180.86°, respectively. The measured frequency responses agree well with the simulated ones. With the theoretical analyses and practical results, it is shown that the proposed one has the advantages of simple structure, convenient analysis and good performance of both BPF and Balun.     

A Subsampling Quadrature Σ∆ Modulator Based on Distributed Resonators for Use in Radio Receiver

The receiver architecture proposed in this brief seizes the subsampling properties of continuous-time sigma-delta (SigmaDelta) modulators based on distributed resonators to construct a quadrature receiver. The proposed architecture is based on a low-pass SigmaDelta modulator that subsamples an intermediate frequency signal around the sampling frequency and does not require quadrature mixers. Instead, the quadrature mixing is replaced by suitably choosing the sampling instants inside the loop. Two practical circuit implementations are proposed. The first one uses separate circuitry for the I and Q paths. The second architecture introduces an innovative way to produce the I and Q outputs that is immune to path mismatch due to the sharing of all the analog circuitry for both paths. The proposed modulator may be feasible for the typical IF frequencies used in cellular base stations.