A full and rigorous treatment of conditional and unconditional stability and instability for an active 3-port microwave circuit, represented at a given frequency by $S$ -parameters, is carried out. By terminating a given port with a variable termination one may study at the input and the output of the resulting 2-port circuit the occurrence of unconditional stability and instability, as well as of special cases of conditional stability, defined as reactive unconditional stability or instability. Such conditions are especially relevant in microwave oscillator design and were exhaustively identified for the first time and treated in previous analysis by the authors. Drawing from its conclusions, one obtains in the plane of the variable termination and with a simple algorithm, a single parametric representation, providing at once, in a closed and direct form, the entire collection of boundaries of the regions, wherein the listed conditions occur at ports of the resulting 2-port circuit. Guidelines for the design of microwave amplifiers and oscillators, both in series and shunt configuration, are sketched. Numerical examples, validated by simulations, are also presented.

Malfatti's problem is more than 200 years old and its solution given by the greedy arrangement has been stated first in 1994. The solution was obtained at that time by considering 14 arrangements and by justifying the exclusion of the non-greedy ones through a complex reasoning relying purely on numerical simulations in several crucial steps. By significantly improving the underlying methods and by filling still extant gaps, a full purely analytical proof of the solution is provided for the first time in the present work, especially based on a mixture of synthetic and analytic Euclidean geometry and on the theory of convex functions.

The purpose of the present work is to introduce two-parameter criteria for unconditional instability and for special cases of conditional stability, which can be particularly interesting for the design of two and three port oscillators. The criteria involve the two-port stability factor K, as was to be expected, and have analogies with the well-known ones for unconditional stability. The mutual relationships existing between the Input and the Output Stability Area, demonstrated in a previous paper, will be used to construct a complete table in which to each of the possible reciprocal positions between stability areas and unit circle, an algebraic criterion corresponds, consisting in the calculation of two simple parameters. Also, limit cases will be analyzed.

In this paper, relationships between conditional stability at the input and at the output of a linear non-unilateral 2-port are analyzed for the first time. For this purpose, the existence of a duality mapping between the two ports is shown and, by using the main properties of Möbius transforms, new mutual relationships between conditional stability configurations at input and output ports are demonstrated. Such new relationships add to the case of unconditional stability for which it is well known that unconditional stability at the input implies unconditional stability at the output (and vice versa). To this end, all the possible cases of reciprocal position between the stability area and the Smith circle have been analyzed showing that, for a given situation at the input, only one corresponding situation can be observed at the output (and vice versa). Limit cases are further considered.

The results of a wide-band indoor measurement campaign are compared with the corresponding simulation results provided by a 3-D ray-tracing field prediction model. A novel comparison criterion based on the "scale level" concept is presented and used in the present work. On the basis of the cited criterion, the validity domain of the ray-tracing tool, the degree of accuracy of the prediction as a function of the scale level, and future necessary improvement of the model have been identified and discussed. The comparisons refer to a variety of propagation parameters such as path loss, delay spread, coherence bandwidth, etc. The influence of environment description (with or without furniture) is also analyzed. The paper shows that the 3-D ray tracing model can be a useful tool for the analysis of indoor propagation and for the design and the implementation of indoor, wide-band applications.

This paper investigates the properties of a method for obtaining Markov models of unspecified order to be applied to narrow-band fading channels with additive white Gaussian noise, The models are obtained by applying the context tree pruning algorithm to experimental or simulated sequences. Fading environments are identified in which the extension from first-order to higher order models is justified. The paper presents, as examples, the evaluation of the covariance function and the packet error distribution.

This paper presents statistical analysis of data obtained by measuring narrow-band path loss at DECT (digital enhanced cordless telecommunications) frequency (1.89 GHz) in an indoor environment. Specific goodness-of-fit tests are applied to the data. The tests assess whether the data generating source belongs to a known family of random variables. Results obtained support that local path-loss distribution can be represented as Weibull or Nakagami in most environments. The close resemblance between such distributions and Rice distribution (with Rayleigh as a special case) confirms that Rice/Rayleigh description of fading can be applied to indoor environments. That similarity leads to a general, simple, approximate expression for the cumulative distribution function of the signal-to-interference ratio. The expression can be used without limitations to the number and the parameters of the Rice interferers.

A finite-state Markov model is fitted to the mobile propagation channel by the use of contingency tables. Narrow-band Rayleigh and Ricean fading are considered in detail, but the techniques extend to other types of fading. Several criteria may indicate that a first-order finite-state Markov model sufficiently characterizes the channel behavior. The parameters of the model are obtained both from theoretical analysis and simulation. Functional dependence between the transition probabilities and the steady-state probabilities is found in the slow fading case.

We present numerical simulations and measurements of shot noise in resonant tunneling structures. We show that when electron-electron interaction through Coulomb force and Pauli exclusion is properly taken into account, the main features of noise behavior of such devices can be correctly predicted. Electron-electron interaction is shown to be responsible for the suppression of shot noise in the positive differential resistance region of the I-V curve, and for the enhancement of shot noise in the negative differential resistance region.

Fading is modeled by applying context tree pruning (CTP) to the sequence of fading states that results from quantizing a sequence of real-valued fading measurements. There results a family of finite state Markov chains of increasing order that represent the channel with increasing fidelity. A fading state may be the union of chain states. A particular model may be chosen from that family according to an application-specific criterion. The resulting models facilitate analysis and simulation of wireless systems that exploit channel memory. An example criterion compares each model's fading state sojourn time statistics with those of the original sequence.

The characterization of the indoor wireless channel is a difficult task owing to the wide variability of indoor environments. It is neccessary to deal with experimental measurements, oriented to determine channel properties related to a specific environment. A flexible set-up is needed to obtain reliable results. In the paper a measurement set-up is presented to acquire digital error streams on a DECT channel (1.152 Mbit/s). Such data are useful in digital wireless system analysis and projects, fitted to a specific environment. The set-up uses two DECT communication testers to modulate the carrier with an externally generated sequence and to demodulate the radio signal. The error stream is obtained by aligning and comparing the demodulator output and the transmitted sequence, and, then, it is stored into the hard disk of a personal computer. A software-driven acquisition board has been built for this purpose. As an example, some channel properties based on data are evaluated, showing a good agreement with theoretical models

The problem of unconditional stability for active two-port networks has been widely discussed in literature. A geometrical condition involving the stability circles and the Smith circle is generally found. Then, depending on the authors, several criteria, involving the two-port S-parameters, and equivalent to the geometrical one, are derived. The equivalence among the different stability criteria has been clearly shown and some complete proof exists in the literature. In this paper, after a critical outlook on the material existing in the literature, a new proof of the unconditional stability criterion is presented. This proof has the advantage of being directly related to the concept of stability circles and to some geometrical conditions on the Smith chart, Moreover, a straightforward proof is given in which the unconditional stability at the input port implies that at the output port and vice versa.

Noise can be a unique probe of electron-electron correlations in nanoscale electron devices. While stationary or small signal transport can often be simply understood in terms of single particle behaviour, noise can be extremely sensitive to many-body effects (Landauer L 1996 Physica B 227 156).We present a new collective phenomenon emerging in electron transport in a resonant tunnelling diode, due to Coulomb repulsion dramatically magnified by the particular shape of the density of states in the quantum well. This phenomenon, for which the name of Coulomb breach is proposed, reveals itself by making shot noise several times greater than that expected in the absence of correlated electron motion. Experimental results showing shot noise enhancement of a factor 6.6 are reported.

A finite state Markov channel (FSMC) model for a digital narrowband mobile LEO satellite channel is presented, which can be used to predict the time-varying behaviour of the channel and to adapt the transmission to the channel state. The model is developed with the help of the so-called context tree pruning (CTP) algorithm. Its effectiveness is demonstrated by comparing theoretical results concerning the digital channel with simulated results obtained from time-continuous channel models.

Several standards for indoor voice and data wireless applications are emerging, and among them DECT is certainly one of the most important. In order to investigate DECT data radio transmission performance in the hostile indoor environment, reliable propagation analysis tools are required. The results of an accurate measurement campaign are compared with the prediction provided by a 3D ray-tracing field prediction model in the paper. The comparisons refer essentially to wideband parameters such as delay spread, delay window etc. Results show a fair agreement between measured and simulated wideband parameters when link distance does not exceed 10-15 m. The influence of environment description (with or without furniture) is also analyzed.

A 3D ray tracing (RT) tool is validated by comparison with measurement in a typical office building. RT shows good reproduction of path loss, even if a slight bias emerges as the radio link distance exceeds 12-14 m. The spatial correlation of the field, which strongly depends on multipath, is accurately predicted by RT independently of distance. This gives confidence in the ability of RT to predict the overall multipath mechanism and the related propagation parameters.

In this paper the applicability of source-sensitivity matched unequal error protection schemes is studied for variable-rate speech transmission on a radio mobile channel. The performance of the unequal error protection scheme is evaluated through simulation taking into account the effects of the channel memory. The simulation of the useful signal and of the interferers is based on a fading model developed by Clarke. The effects of channel memory on different error correction schemes are evaluated and compared

In this paper the applicability of source-sensitivity matched unequal error protection schemes is studied for variable-rate speech transmission on a mobile radio channel. The performance of the unequal error protection scheme is evaluated through simulation taking into account the effects of the channel memory. The simulation of the useful signal and of the interferers is based on the multiplicative fading model developed by Clarke. The effects of channel memory on different error correction and error concealment schemes are evaluated and compared.