Hazard-rate tolerance method for an opportunistic-replacementpolicy

A model for a system with several types of units is presented. A unit is replaced at failure or when its hazard (failure) rate exceeds limit L, whichever occurs first. When a unit is replaced because its hazard rates reaches L, all the operating units with their hazard rate falling in the interval (L-u, L) are replaced. This policy allows joint replacements and avoids the disadvantages resulting from the replacement of new units, down time, and unrealistic assumptions for distributions of unit life. The long-run cost rate is derived. Optimal L and u are obtained to minimize the average total replacement cost rate. Application and analysis of results are demonstrated through a numerical example     

Light-to-input nonlinearities in an R-LED series network

The light-to-current (L-I) and light-to-voltage (L-V) differential nonlinearities in the simple network of a customary LED and an external resistor R in series are analyzed and calculated theoretically and compared with experimental data. Particular emphasis is placed on the influence of the log-arithmetic slope ν of the L-I characteristic and the bias current I upon the ratio of the corresponding nonlinearity parameters. It is thus deduced that, for a given optical power P, over superlinear portions of the L-I curve (ν>1) the L-I linearity is typically better than its corresponding L-V linearity. On the contrary, when the L-I dependence is sublinear (ν<1) the voltage driving scheme may ensure for the R-LED network, or the LED alone, a local L-V response much more linear than the L-I response, provided that appropriate (optimum) I and/or R values are chosen     

Integrated reactive components in power electronic circuits

Methods of integrating capacitive and inductive components into new compact devices are presented. Configurations for integrating various combinations of L-C networks are shown. A example of the construction of an integrated L-C for a series-resonant converter is evaluated both practically and experimentally     

Transverse mode control of vertical-cavity top-surface-emittinglasers

Transverse mode characteristics and control for vertical-cavity top-surface-emitting lasers (VCSELs) are discussed. A spatial filtering concept for the control of VCSEL transverse modes that allows over 1.5-mW single TEM₀₀ transverse mode emission to be routinely achieved from continuous-wave electrically excited VCSELs is introduced. Without spatial filtering, L-I and V-I kinks are observed     

Identities and approximations for the weight distribution of q-ary codes

An explicit formula is derived that enumerates the complete weight distribution of an (n, k, d) linear code using a partially known weight distribution. An approximation formula for the weight distribution of q-ary linear (n, k , d) codes is also derived. It is shown that, for a given q-ary linear (n, k, d) code, the ratio of the number of codewords of weight u to the number of words of weight u approaches the constant Q=q ^-(n-k) as u becomes large. The error term is a decreasing function of the minimum weight of the dual. The results are also valid for nonlinear (n, M, d) codes with the minimum weight of the dual replaced by the dual distance     

Performance of gain-guided surface emitting lasers withsemiconductor distributed Bragg reflectors

The performance limitations of gain-guided vertical cavity surface emitting lasers (VCSELs) which use epitaxially grown semiconductor distributed Bragg reflectors (DBRs) are discussed. The light-current ( L-I) characteristics and emission wavelength of such lasers are examined as a function of temperature and time under continuous wave (CW) and pulsed operation. The authors observed a sharp roll-over in the CW L-I characteristics which limits the maximum output power. The threshold current under CW operation is found to be lower than that obtained under pulsed conditions. Several microseconds long delay in lasing turn-on is also observed. It is shown quantitatively that these anomalies are a consequence of severe heating effects. It is shown that reduction of the series resistance and threshold current density can lead to significant improvements in the power performance of VCSELs     

Error-correcting codes for list decoding

In the list-of-L decoding of a block code the receiver of a noisy sequence lists L possible transmitted messages, and is in error only if the correct message is not on the list. Consideration is given to (n,e,L) codes, which correct all sets of e or fewer errors in a block of n bits under list-of-L decoding. New geometric relations between the number of errors corrected under list-of-1 decoding and the (larger) number corrected under list-of-L decoding of the same code lead to new lower bounds on the maximum rate of (n,e,L) codes. They show that a jammer who can change a fixed fraction p<1/2 of the bits in an n-bit linear block code cannot prevent reliable communication at a positive rate using list-of- L decoding for sufficiently large n and an L⩽n. The new bounds are stronger for small n , but weaker for fixed e/n in the limit of large n and L than known random coding bounds     

Distributed feedback laser diode and module for CATV systems

Harmonic distortion of distributed feedback laser diodes (DFB-LDs) for analog transmission systems is investigated. It is shown that, under a modulation frequency of less than 1 GHz, the harmonic distortion depends on the nonlinearity of the light output power-current (P-I) curve under the continuous wave (CW) condition, which is determined by the coupling constant κ L, and that the distortion can be minimized at κ L~1. A 1.3 μm wavelength InGaAsP DFB-PPIBH (p-substrate partially inverted buried heterostructure) LD and its module, with low distortion by the control of a coupling constant, have been developed     

A replacement policy maximizing MTTF of a system with several spareunits

A replacement policy is considered that maximizes mean time-to-failure (MTTF) of a system with N spare units. The optimum replacement time of a system with k spares (k=1, 2, ..., N) is derived successively from MTTF with k-1 spares by induction. The maximum MTTF is approximately given by a reciprocal of the hazard rate     

More on the decoder error probability for Reed-Solomon codes

A combinatorial technique similar to the principle of inclusion and exclusion is used to obtain an exact formula for P_E (u), the decoder error probability for Reed-Solomon codes. The P_E(u) for the (255, 223) Reed-Solomon code used by NASA and for the (31, 15) Reed-Solomon code (JTIDS code) are calculated using the exact formula and are observed to approach the Qs of the codes rapidly as u gets large. An upper bound for the expression |P_E(u)/ Q-1| is derived and shown to decrease nearly exponentially as u increases     

Real-time multi-DSP control of three-phase current-source unitypower factor PWM rectifier

The design of a real-time multi-DSP controller for a high-quality six-valve, three-phase current-source unity power factor pulse-width-modulated (PWM) rectifier is discussed. With the decoupler preprocessor and the dynamic trilogic PWM trigger scheme, each of the three input currents can be controlled independently. The pole-placement control method implemented, which is based on the a-b- c frame system model and fast parallel computer control, achieves fast AC current response. The low-frequency resonance in the AC filter L-C networks is damped effectively. The experimental results were obtained for a 1 kVA bipolar transistor current-source PWM rectifier with a real-time controller using three TMS320C25 DSPs     

Periodic-replacement models with threshold levels

The authors suggest five replacement policies where a unit is replaced at periodic times, jT(j=1,2, . . .), and the replacement cost is expensive when some number of events occurring in (0,t) is greater than a threshold level. The usual models for inspection, periodic replacement, block replacement, parallel systems, and cumulative damage can be transformed into replacement models with threshold levels. The mean cost-rate of each model is obtained, using well-known results of reliability theory. The optimum replacement time which minimizes the cost-rate of an inspection model is discussed and shown to exist uniquely     

Replacement policies for a cumulative damage model with minimalrepair at failure

The authors apply periodic replacement with minimal repair at failure to cumulative damage models: a unit is replaced at time T , at shock N, or at damage Z and undergoes minimal repair between replacements. The mean cost-rate is obtained, and each optimal T*, N*, and Z* to minimize the cost-rate is discussed. A numerical example is given for an exponential case     

Performance analysis of nonblocking packet switch with input andoutput buffers

The performance of nonblocking packet switches such as the knockout switch and Batcher banyan switch for high-speed communication networks can be improved as the switching capacity L per output increases; the switching capacity per output refers to the maximum number of packets transferred to an output during a slot. The N×N switch with L=N was shown to attain the best possible performance by M.J. Karol et al. (1987). Here a N×N nonblocking packet switch with input and output buffers is analyzed for an arbitrary number of L such that 1⩽L⩽N. The maximum throughput and packet loss probability at input are obtained when N=∞     

A new model for calculating the input impedance of coax-fedcircular microstrip antennas with and without air gaps

A model is presented for calculating the input impedance of a probe-fed circular antenna with and without air gaps between the substrate and the ground plane. It is based on the cavity method, the dynamic permittivity constant (to take into account the influence of the fringing field at the edge of radiating element), and the resonant parallel R-L-C circuit with an inductive reactance. Numerical results are shown to agree well with experimental ones. The model is well suited for computer-aided design     

SPICE macro model for the simulation of zener diodeI-V characteristics

The problems encountered when using the existing SPICE diode model to represent the I-V characteristics of a Zener diode in the reverse region are examined. A Zener diode macro model that has accurate I-V simulation characteristics and can be easily constructed using SPICE-provided primitives is presented. The static I-V characteristics and temperature response of the diode are reviewed. The performance of the model is discussed, and its main enhancements as compared to the SPICE model are identified     

Effect of parameter variations on the performance of GaAs/AlGaAsmultiple-quantum-well electroabsorption modulators

A theoretical investigation is presented of the dependence of electroabsorption in GaAs/Al_xGa_1-xAs multiple-quantum-well (MQW) structures on the MQW parameters (Al mole fraction x, well thickness L_z barrier thickness L_b and interface quality) and on the applied electric field studied. The on/off ratio of a modulator using MQWs with x=0.45, L_z=75 Å, and L _b=78 Å is predicted to increase by 20% compared to that of a modulator using MQWs with x=0.3, L_z =100 Å, and L_b=100 Å, when the MQW total active region thickness is 1 μm     

Inverted thin-film transistors with a simple self-aligned lightlydoped drain structure

The I-V characteristics of inverted thin-film transistors (TFT) are studied. A simple lightly doped drain (LDD) structure is utilized to control the channel electric field at the drain junction and to improve the performance of the TFTs. The LDD region is self-aligned to the channel and the source/drain regions. It is created by a spacer around an oxide mask which exclusively defines the channel length L_ch. Experimental data show that the leakage current, subthreshold swing SS, saturation current, and on/off current ratio of the inverted TFTs are closed related to L_ch, L_LDD, the drain bias, gate voltage, and LDD dose. With a gate deposited at low temperature, a saturation current of ~1.25 μA at 5 V and a leakage current of ~0.03 pA per micrometer of channel width were achieved. The current ratio therefore exceeds seven orders of magnitude, with an SS of 380 mV/decade. At 3.3 V, the current ratio is ~7×10⁶     

In the optimum design of the block adaptive FIR digital filter

A general optimum block adaptive (GOBA) algorithm for adaptive FIR (finite impulse response) filtering is presented. In this algorithm, the correction terms for the filter coefficients in each block, instead of the convergence factors, are optimized in a least squares sense. There are no constraints on the block length L and the filter tap number N. It is shown that the GOBA algorithm is reduced to the normalized LMS algorithm when L⩾N. The convergence of the GOBA algorithm can be assured if the correlation matrix of the input signal is positive definite. Computer simulations based on an efficient computing procedure confirm that the GOBA algorithm achieves faster convergence with slightly degraded convergence accuracy in stationary environments and better weight tracking capability in nonstationary environments as compared to existing block adaptive algorithms with no constraints on L and N     

Sequential imperfect preventive maintenance policies

Improvement factors in hazard rate and age for a sequential preventative maintenance (PM) policy are introduced. Two imperfect PM models are analyzed: (1) PM reduces the hazard rate while it increases with the number of PMs, and (2) PM reduces the age. The PM is done at intervals x_k (k=1,2, . . . , N) and is imperfect. The optimal policies to minimize the mean cost rates are discussed. The optimal PM sequences {x_k} are computed for a Weibull distribution