Connectivity properties of a packet radio network model

A model of a packet radio network in which transmitters with range R are distributed according to a two-dimensional Poisson point process with density D is examined. To ensure network connectivity, it is shown that πR²D, the expected number of nearest neighbors of a transmitter, must grow logarithmically with the area of the network. For an infinite area there exists an infinite connected component with nonzero probability if π R²D>N₀, for some critical value N₀. It is shown that 2.195<N ₀<10.526     

Scanning properties of large dual-shaped offset and symmetricreflector antennas

The scanning properties of shaped reflectors, both offset and circularly symmetric, are examined and compared to conic section scanning characteristics. Scanning of the pencil beam is obtained by lateral and axial translation of a single point source feed. The feed is kept pointed toward the center of the subreflector. The effects of power spillover and aperture phase error as a function beam scanning are examined for several different types of large reflector design including dual-offset, circularly symmetric large f/D, and smaller f/D dual reflector antenna system. It is shown that the Abbe-sine condition for improved scanning of an optical system cannot, inherently, be satisfied in a dual-shaped reflector system that is shaped for high gain and low feed spillover. The gain loss, with scanning, of a high-gain shaped reflector pair is demonstrated to be due to both aperture phase error loss and power spillover loss     

1/f noise in hot-carrier damaged MOSFET's: effects ofoxide charge and interface traps

The 1/f noise in the drain current of hot-carrier damaged MOSFETs biased in weak inversion has been studied. By the use of a biased annealing treatment to simultaneously decrease the density of oxide trapped charge (N_ot) and increase the density of interface traps (D_it), the authors have separated the contributions of these two kinds of defects. The results clearly indicate that, while the low-frequency 1/f noise is correlated with N_ot, the high-frequency 1/f noise is correlated with D_it     

The Zak transform and some counterexamples in time-frequencyanalysis

It is shown how the Zak transform can be used to find nontrivial examples of functions f, g∈L²(R) with f×g≡0≡F×G, where F, G are the Fourier transforms of f, g, respectively. This is then used to exhibit a nontrivial pair of functions h, k∈L²(R), h≠k, such that |h|=|k|, |H |=|K|. A similar construction is used to find an abundance of nontrivial pairs of functions h, k∈L² (R), h≠k, with |A_h |=|A_k| or with |W_h|=|W _k| where A_h, A_k and W_h, W_k are the ambiguity functions and Wigner distributions of h, k, respectively. One of the examples of a pair of h, k∈L²(R), h≠k , with |A_h|=|A_k| is F.A. Grunbaum's (1981) example. In addition, nontrivial examples of functions g and signals f₁≠f₂ such that f₁ and f₂ have the same spectrogram when using g as window have been found     

Sample converses in source coding theory

The rate and distortion performance of a sequence of codes along a sample sequence of symbols generated by a stationary ergodic information source are studied. Two results are obtained: (1) the source sample sequence is encoded by an arbitrary sequence of block codes which operate at a fixed rate level R, and a sample converse is obtained which states that, with probability one, the lower limit of the code sample distortions is lower bounded by D(R), the value of the distortion rate function at R; (2) the source sample sequence is encoded by an arbitrary sequence of variable-rate codes which operate at a fixed distortion level D, and a sample converse is obtained which states that, with probability one, the lower limit of the code sample rates is lower bounded by R(D), the value of the rate distortion function at D. A novel ergodic theorem is used to obtain both sample converses     

Bias in a stress-strength problem

The bias of the maximum likelihood estimator for R≠Pr{ X<Y} where X and Y are independent normal random variables with unknown parameters is discussed. The bias is an odd function with respect to δ=gauf^-1 (R), where gauf(·) is the Cdf of the standard normal distribution, so the study is restricted to R ⩾0.5, or equivalently, δ⩾0. There exists δ₀>0 such that the bias is positive in the interval 0<δ<δ₀. R has a positive bias at least in the interval 0.84<R<0.94     

Repeated convolutional codes for high-error-rate channels

An error-correction scheme for an M-ary symmetric channel (MSC) characterized by a large error probability p_e is considered. The value of p_e can be near, but smaller than, 1-1/M, for which the channel capacity is zero, such as may occur in a jamming environment. The coding scheme consists of an outer convolutional code and an inner repetition code of length m that is used for each convolutional code symbol. At the receiving end, the m inner code symbols are used to form a soft-decision metric, which is passed to a soft-decision decoder for the convolutional code. The effect of finite quantization and methods to generate binary metrics for M>2 are investigated. Monte Carlo simulation results are presented. For the binary symmetric channel (BSC), it is shown that the overall code rate is larger than 0.6R₀, where R₀ is the cutoff rate of the channel. New union bounds on the bit error probability for systems with a binary convolutional code on 4-ary and 8-ary orthogonal channels are presented. For a BSC and a large m, a method is presented for BER approximation based on the central limit theorem     

Sequential decoding based on an error criterion

An analysis of sequential decoding is presented that is based on the requirement that a set probability error P_e be achieved. The error criterion implies a bounded tree or trellis search region: the shape of this is calculated for the case of a binary symmetric channel with crossover probability P and random tree codes of rate R. Since the search region is finite at all combinations of p and R below capacity, there is no cutoff rate phenomenon for any P_e>0. The decoder delay (search depth), the path storage size, and the number of algorithm steps for several tree search methods are calculated. These include searches without backtracking and backtracking searches that are depth- and metric-first. The search depth of the non-backtracking decoders satisfies the Gallager reliability exponent for block codes. In average paths searched, the backtracking decoders are much more efficient, but all types require the same peak storage allocation. Comparisons are made to well-known algorithms     

Design and experimental characteristics of strained In0.52Al0.48As/InxGa1-xAs (x>0.53)HEMTs

Strained In_0.52Al_0.48 As/In_xGa _1-xAs (x>0.53) HEMTs (high electron mobility transistors) are studied theoretically and experimentally. A device design procedure is reported that is based on band structure and charge control self-consistent calculations. It predicts the sheet carrier density and electron confinement as a function of doping and thickness of layers. The DC performance at 300 K is presented. Wafer statistics demonstrate improvement of device characteristics with excess indium in the channel (g¯_{m, intr}=500 and 700 mS/mm for x=0.60 and 0.65). Microwave characterization shows the f_T improvement (f_T=40 and 45 GHz for x=0.60 and 0.65, respectively) and the R_ds limitations of the 1-μm-long-gate HEMTs     

On the performance of an ATM switch with multichannel transmissiongroups

A routing architecture applying the concept of multichannel transmission groups (MCTGs) for ATM systems is proposed. A queuing analysis of an internally nonblocking ATM switch employing this MCTG concept with partially shared output buffers is presented. The analysis is based on the discrete-time DA///D/c /B queuing model. Both bulk input traffic bulk-size distribution (A) and deterministic traffic (D₁ +. . .+D_N) are considered. The impact of switch speedup on the performance is also taken into account. It is shown that the MCTG architecture yields better performance in terms of delay and cell loss probability than its single channel counterpart. It is also found that the switch speedup required to closely approximate the optimal performance obtained by having the switch fabric run N times as fast as the input and output channels, where N is the size of the switch, is rather small compared to N. This makes the practical realization of the proposed switch architecture feasible     

Phase-difference distribution for a narrow-band non-Gaussian noiseand related statistics

A probability density function _Pm(R₁,R₂,Δ) is presented for a narrowband noise process in which R₁ and R₂ are two envelope samples and Δ is the phase difference. For m=1 the process is Gaussian, but for m=2,3, etc., it is non-Gaussian. New second-order statistical properties are identified for it as well as the density function for the resulting envelope when a signal is added to the noise. These results are given, though the major concern is with the density of the phase difference Δ and the density of &thetas;, the response of an FM detector fed with the noise     

Bayes estimation of reliability under a random environment governedby a Dirichlet prior

The reliability function of a component whose lifetime is exponentially distributed with a known parameter λ>0 is R (t|λ)=exp (-λt). If an environmental effect multiplies the parameter by a positive factor η, then the reliability function becomes R(t|η,λ)=exp(-ηλt). The authors assume that η itself is random, and its uncertainty is described by a Dirichlet process prior D(α) with parameter α=MG₀, where M>O represents an intensity of assurance in the prior guess, G₀, of the (unknown) distribution of η. Under squared error loss, the Bayes estimator of R(t|η,λ) is derived both for the no-sample problem and for a sample of size n. Using Monte Carlo simulation, the effects of n, M, G₀ on the estimator are studied. These examples show that: (a) large values of n lead to estimates where the data outweigh the prior, and (b) large values of M increase the contribution of the prior to the estimates. These simulation results support intuitive ideas about the effect of environment and lifetime parameters on reliability     

Balanced codes and nonequiprobable signaling

The problem of shaping signal constellations that are designed for the Gaussian channel is considered. The signal constellation consists of all points from some translate of a lattice Λ that lie within a region R. The signal constellation is partitioned into T annular subconstellations Ω_o,…,Ω_T-1, by scaling the region R. Signal points in the same subconstellation are used equiprobably, and a shaping code selects region Ω_i with frequency f_i. If the signal constellation is partitioned into annular subconstellations of unequal size. then the transmission rate should vary with the choice of codeword in the shaping code. and it will be necessary to queue the data in buffers. It is described how the balanced binary codes constructed by D. E. Knuth (1986) can be used to avoid a data rate that is probabilistic. The basic idea is that if symbols 0 and 1 represent constellations of unequal size. and if all shaping codewords have equally many 0's and 1's, then the data rate will be deterministic     

Experimental values for the hole diffusion coefficient andcollector transit velocity in P-n-p AlGaAs/GaAs HBTs

The diffusion coefficient (D_h) and a value for the collector velocity (v_h) of holes in AlGaAs/GaAs P-n-p HBTs (heterojunction bipolar transistors) were obtained from high-frequency measurements on structures with different base and collector widths. Quantities for D_h and v _h of 5.6 cm²/s and 5.5×10⁶ cm/s, respectively, were obtained by plotting the total emitter-collector delay versus inverse emitter current and extrapolating the data to infinite emitter current to obtain the base and collector transit delays. An f_t and f_max as high as 15 and 29 GHz, respectively, were obtained for non-self-aligned (1-μm emitter mesa/base contact separation) devices with a 2.6-μm×10-μm emitter     

Nonequiprobable signaling on the Gaussian channel

Signaling schemes for the Gaussian channel based on finite-dimensional lattices are considered. The signal constellation consists of all lattice points within a region R, and the shape of this region determines the average signal power. Spherical signal constellations minimize average signal power, and in the limit as N →∞, the shape gain of the N-sphere over the N-cube approaches πe/6≈1.53 dB. A nonequiprobable signaling scheme is described that approaches this full asymptotic shape gain in any fixed dimension. A signal constellation, Ω is partitioned into T subconstellations Ω₀ , . . ., Ω_τ-1 of equal size by scaling a basic region R. Signal points in the same subconstellation are used equiprobably, and a shaping code selects the subconstellation Ω_i with frequency f_i. Shaping codes make it possible to achieve any desired fractional bit rate. The schemes presented are compared with equiprobable signaling schemes based on Voronoi regions of multidimensional lattices. For comparable shape gain and constellation expansion ratio, the peak to average power ratio of the schemes presented is superior. Furthermore, a simple table lookup is all that is required to address points in the constellations. It is also shown that it is possible to integrate coding and nonequiprobable signaling within a common multilevel framework     

Optimization of signal sets for partial-response channels. II.Asymptotic coding gain

For Pt. I see ibid., vol.37, no.5, p.1327-141 (1991). For a linear, time-invariant, discrete-time channel with a given transfer function H(f), and information rate R bits/ T, where T is the symbol interval, an optimal signal set of length K is defined to be a set of 2^RK inputs of length K that maximizes the minimum l₂ distance between pairs of outputs. The author studies the minimum distance between outputs, or equivalently, the coding gain of optimal signal sets as K→∞. He shows how to estimate the coding gain, relative to single-step detection, of an optimal signal set length K when K is large     

Wavelength dependence of 1/f noise in the light output oflaser diodes: an experimental study

The optical power emitted by a monomode GaAlAs laser is filtered with a monochromator. The 1/f noise in the filtered emission is found to be directly dependent on the noncoherent emission, such as S_pαP_nc^m. Here s_p is the spectral density of the 1/f fluctuations, P_nc is the average noncoherent power, m=3/2 under spontaneous emission, and m=4 in the superradiation and laser regions. Study of the 1/f noise in the optical power in a band centered at the laser wavelength and with variable bandwidth shows three operating regions. (1) LED region (at low currents): the fluctuations with a 1/f spectrum are uncorrelated in wavelength. (2) Superradiation region (at currents close to the threshold): the fluctuations are correlated. (3) Laser region: the 1/f noise apparently is dominated by noncoherent emission within a small optical band around the laser wavelength     

Analysis of joint rainfall rate and duration statistics: microwavesystem design implications

The joint statistical analysis of duration D and intensity R of rain exceedances relevant to the design of microwave communication systems is considered. The data analyzed are derived from point precipitation-rate observations recorded over a period of 49 years in the city of Barcelona, Spain, using a Jardi gauge. The concept of event and the duration of a single exceedance of a threshold R within that event is introduced. The analysis shows the parametric R dependence of the annual average number of exceedances and of the average duration     

GaAs/AlGaAs quantum-well, long-wavelength infra-red (LWIR) detectorwith a detectivity comparable to HgCdTe

The authors achieved the first high responsitivity R_v=30000 V/W, high detectivity D*=1×10¹⁰ cm √(Hz)/W GaAs/AlGaAs multiquantum-well superlattice detector which is sensitive in the long wavelength infra-red (LWIR) spectral region. This detector operates at λ=8.3 μm and at a temperature of T=77 K