Highly Accurate Closed-Form Approximations to the Sum of α-μVariates and Applications

Sums of fading envelopes occur in several wireless communications applications, such as equal-gain combining, signal detection, outage probability, intersymbol interference, etc. The exact evaluation of the sum statistics is known to be very intricate. One of the purposes of this Letter is to provide highly accurate closed-form approximations to the probability density function and cumulative distribution function of the sum of independent identically distributed (i.i.d.)α-μ(generalized gamma) variates. Based on and as an extension of such an approach, simple precise approximations for the performance metrics of equal-gain combining and maximal-ratio combining receivers operating on i.i.d. /spl alpah/-μ fading channels are proposed. Samples examples are given to illustrate that, for practical purposes, exact and approximate solutions are indistinguishable from each other.     

The multivariate α-μ distribution

An infinite series formulation for the multivariate α-μ joint probability density function with arbitrary correlation matrix and non-identically distributed variates is derived. The expression is exact and general and includes all of the results previously published in the literature concerning the distributions comprised by the α-μ distribution. The general expression is then particularized to an indeed very simple, approximate closed-form solution. In addition, a multivariate joint cumulative distribution function is obtained, again in simple, closed-form manner. As an application example, the exact and approximate performances of the selection combining scheme given in terms of the outage probability is shown. Approximate and exact results are very close to each other for small as well as medium values of correlation.     

Switching rates of two-branch selection diversity in κ-μ and α-μ distributed fadings

An analytical expression for the switching rate of a dual branch selection diversity combiner in κ-μ and α-μ distributed fadings is derived. Independent and identically distributed (i.i.d.) κ-μ as well as α-μ fading channels are considered. The switching rates for dual i.i.d. Rayleigh, Rician and Nakagami-m fading channels are special cases of the switching rate for dual i.i.d. κ-μ fading channels. Similarly, the general switching rates for dual α-μ fading channels also include special cases such as Rayleigh, Nakagami-m, and Weibull fading channels.     

Asymptotic SEP for M-PSK Signals overα-μFading Channels

The α-μfading is a general physical model recently rewritten from the Stacy (or generalized gamma) distribution, encompassing a variety of fading environments such as Rayleigh, Nakagami-m, and Weibull fading. We characterize the asymptotic average symbol error probability (SEP) at a high signalto- noise ratio (SNR)?specifically, the high-SNR slope and power gain?in terms of two physical fading parametersαandμ. We further extend the asymptotic SEP analysis to a multiple-input multiple-output diversity system employing orthogonal space? time block codes inα-μfading channels.     

Microfabrication of ultra-long reinforced silicon neural electrodes

The authors describe a simple dry-etch silicon microfabrication process to develop an array of electrodes with multiple recording sites suitable for neural recording applications. This new high-yield fabrication process uses commercially available ultra-thin silicon wafers as substrate material. A xenon difluoride system is used to etch the silicon substrate to form the electrode structures. The novel concept of structural reinforcement to produce elongated and reliable probe electrodes is introduced. The authors demonstrate recording silicon electrodes that can reach lengths longer than 10 mm having only 50 μm thicknesses and an 100 μm average width. This new microfabrication process illustrates a simple, cost-effective and mass-producible method for developing ultralong silicon probes for deep brain implantation and neural recording.     

Linewidth enhancement factor of quantum cascade lasers with single phonon resonance-continuum depopulation structure on Peltier cooler

The linewidth enhancement factor (α-factor) of a commercially available 6.1 μm InGaAs/InAlAs distributed-feedback quantum cascade laser capable of continuous-wave operation with a Peltier cooler and its current dependence were investigated. α tended to increase slowly with an increase in current. α, probably affected by detuning, was still smaller than that of conventional 1.55 μm semiconductor laser diodes.     

Lead chalcogenide VECSEL on Si emitting at 5 μm

A mid-infrared vertical external cavity surface emitting laser (VECSEL) on a Si substrate has been realised. It is optically pumped and emits around 5 μm wavelength. Maximum output power of 26 mW_p (limited by the 1.5μm wavelength pump laser) was observed at 100 K operating temperature with 3μs pulse widths. The active part is just a 1.3μm-thick PbTe layer.     

Microcavity-enhanced electron emission from lead zirconate-titanate cathodes

Enhanced electron emission has been observed from lead zirconatetitanate (PZT) surfaces into which arrays of microcavities have been fabricated by nanopowder blasting. Arrays of microcavities, each having an elliptical cross-sectional geometry with major and minor axis lengths of 800 and 600 μm, respectively, and depths adjustable from 40 to 300 μm, exhibit a pronounced dependence of the emitter electron current on microcavity depth. For electric field strengths at the emitter surface of ∼5-12 V/μm, the RMS current density generated by an array of 250 μm-deep microcavities with a packing density of 214±2 cm^-2 ranges from ∼3 mA cm^-2 to beyond 8 mA cm^-2, or more than a factor of five larger than that produced from a planar PZT surface.     

Mid-infrared GaSb-based EP-VCSELl emitting at 2.63 μm

Electrically-pumped GaSb-based vertical-cavity surface-emitting lasers emitting up to 2.63 μm at room temperature are reported. The whole structure was grown monolithically in one run by solid-source molecular beam epitaxy. This heterostructure is composed of two n-doped AlAsSb/GaSb DBRs, a type-I GaInAsSb/AlGaAsSb multiquantum- well active region and an InAsSb/GaSb tunnel junction. A quasi-CW (1 μs, 5 %) operation was obtained at room temperature for 35 μm-diameter devices with threshold current of 85 mA.     

Dynamic characteristics of micro-optothermal expansion and optothermal microactuators

The dynamic characteristics of micro-optothermal (OT) expansion and novel OT microactuators are theoretically analysed using a heat transient model and finite element modelling thermal simulation. Three different-shaped microactuators are machined by an excimer laser micromachining system using a single layer material; first with a single OT expansion arm; second with a bi-direction microactuator (BDMA); and third with a bi-direction microswitch (BDMS). A red laser diode (650 nm) with a maximum power output of 30 mW and an adjustable frequency of 0?20 Hz is used as an irradiation light source. The experimental results show that the maximum response frequency of the OT microactuators can be at least 15 Hz, and the deflections of the BDMA (about 13 μm at 10 mW, 2 Hz) and the BDMS (about 10 μm at 8 mW, 2 Hz) are significantly magnified compared with the OT expansion (1.32 μm at 10 mW, 2 Hz) of the single arm, proving the feasibility of the enlarged bi-direction deflection/vibration of the microactuators. The expansion and deflection amplitude of the microactuators decrease as the laser frequency increases, and the experimental data and curves greatly agree with the theoretical predictions. The new method of OT microactuators can be widely applied in the fields where simple structure, easy fabrication, large displacement and wireless controlling are required.     

Cavity-length dependent thermal characteristics of InGaN blue laser diodes

Thermal characteristics of InGaN-based blue laser diodes (LDs) emitting around 450 nm are investigated with varying cavity length of the LDs. Characteristic temperature, junction temperature, and thermal resistance are measured for the cavity length from 650 to 1450 μm. It is found that the characteristic temperature increases slowly with cavity length, and the thermal resistance is significantly improved from .30 W to ~17 K/W as the cavity length increases length from 650 to 1450 μm.     

On the true Cramer-Rao lower bound for data-aided carrier-phase-independent frequency offset and symbol timing estimation

In this letter we present new and simple closed form expressions for the true Cramer-Rao lower bound (CRB) for data-aided (DA) joint and individual carrier frequency offset and symbol timing estimation from a linearly modulated waveform transmitted over an AWGN channel. The bounds are derived under a carrier-phase-independent (CPI) estimation strategy wherein the carrier phase is viewed as a nuisance parameter and assumed to have a worst-case noninformative uniform distribution over [-ππ]. The computation of these CRBs requires only a single numerical integration. In addition, computationally simpler yet highly accurate asymptotic lower bounds are presented. As particularizations, new bounds for individual CPI frequency estimation with known symbol timing from M-PSK and continuous wave (CW) signals are also reported.     

Suppression of polarisation-induced signal fluctuation in optic distributed sensing system based on stimulated Brillouin scattering

A new scheme capable of suppressing polarisation sensitivity in the Brillouin optical time domain analysis distributed sensing system is demonstrated. The polarisation-induced signal fluctuation is suppressed from >+300 μϵ(which causes measurement to fail) to +50 μϵ by introducing a cost-effective polarisation diversity scheme on the pump light. Stable distributed strain measurement along a 1.2 km singlemode fibre is achieved. This scheme needs no active component and the strain measurement time is only 3s.     

10 gbit/s 0.0065 mm2 6 mw analogue adaptive equaliser utilising negative capacitance

An area- and power-efficient analogue adaptive equaliser (AEQ) is realised in a 0.13 μm CMOS technology. The negative capacitance circuits are exploited at the equalisation filter to achieve wider bandwidth and larger high-frequency boosting, instead of using passive inductors that lead to a large chip area. Measured results demonstrate the data rate of 10 Gbit/s for 20 and 34 inch FR4 traces as channels, while dissipating only 6 mW from a single 1.2 V supply. The chip core occupies an extremely small area of 50 x 130 μm². To the best of the authors' knowledge, this chip achieves the lowest power consumption and the smallest chip area among the recently reported AEQs.     

In situ spatially-resolved thermal and brillouin diagnosis of high-power ytterbium-doped fibre laser by brillouin optical time domain analysis

An in situ spatially-resolved diagnosis of thermal and Brillouin characteristics of a double-clad ytterbium-doped fibre (YDF) laser operating at 1.09 μm is demonstrated. For this a Brillouin optical time domain analysis technique based on 1.55 μm Brillouin pump and probe beams is utilised. A 2.4 K temperature difference across the YDF laser when it was running with 6.5 W pump power has been measured and resolved. Based on the measured thermal and Brillouin characteristics of it is expected the YDF, that the effective Brillouin gain coefficient would decrease by 20% for 1.09 μm radiation if an 80 K temperature variation is built up across it, as a result of the quantumheating by the pump power.     

Single-chip A/D converter for digital microphones with on-chip preamplifier and time-domain noise isolation

A 1-bit sigma-delta modulator (ΣΔM) with an on-chip preamplifier for digital electret microphones has been implemented. A differential gm-opamp-RC preamplifier eliminates the traditional single-ended JFET interface and is integrated with an on-chip ΣΔM by removing all external components. The proposed time-domain noise isolation technique preserves circuit performance under a single power supply condition. The prototype implemented in a 0.18 μm CMOS technology achieves a 78 dB dynamic range and 62 dB peak signal-to-noiset distortion ratio (both A-weighted) with a current consumption of 450 μmA under a 1.8 V supply.     

Improved p- and n-type thin-film microstructures for thermoelectricity

Improved microstructures based on thin-films of n-type bismuth telluride (Bi_{2Te3) and p-type antimony telluride (Sb2Te3) to convert temperature differences in electricity are presented. The microstructures are obtained by thin-film deposition, applying the co-evaporation method to the bismuth/antimony and telluride materials. Measurements demonstrated the superior thermoelectric features of the obtained films when compared with the most recent thermoelectric thin-film deposition. Measurements show that the deposited films present thermoelectric properties comparable to those reported for the same materials in bulk form, as is the case of the materials used in conventional macro-scale Peltier modules. The absolute values of the Seebeck coefficient are in the range 150?250 μVK^-1 and the in-plane electrical resistivity is in the 7-15 μΩm range. Measurements for the Bi2Te3 and Sb2Te3 films also show figures of merit at room temperatures of 0.84 and 0.5, and power factors of 4.87 and 2.81 mWK^-2 m^-1, respectively. These microstructures are used for the fabrication of thermoelectric microgenerators to supply stand-alone microsystems with power consumption that ranges from cents of μW to a few mW.}     

Common-path optical coherence tomography guided fibre probe for spatially precise optical nerve stimulation

A simple and effective common-path optical coherence tomography guided fibre probe for optical nerve stimulation has been developed and tested. The probe is capable of real-time monitoring of the fibre-to-nerve distance up to more than 1 mm at an axial resolution of approximately 3 μm, thus improving the precision and safety of stimulation laser power delivery.     

Susceptibility of lithium-niobate modulator to high-power microwave pulses

The first measurement of the susceptibility of an off-the-shelf lithiumniobate intensity modulator to damage and disruption from high-power microwave pulses is reported. The device tested survived 1 kHz repetition rate pulses at 2.5 GHz centre frequency and 40 μs width up to 200 W peak power. The results are discussed in terms of material parameters and device characteristics.     

IaAsP/InGaAsP quantum-well 1.3 μm vertical-cavity surface-emitting lasers

1.3 μm vertical-cavity surface-emitting lasers based on a novel gain media consisting of InAsP/InGaAsP strain-compensated multiple quantum wells are reported. SiO₂/TiO₂/ dielectric thin-film pairs and wafer-bonded GaAs/Al(Ga)As distributed Bragg reflectors are used as the top and bottom cavity mirrors, respectively. The device with a 5 μm-diameter selectively etched tunnel-junction aperture exhibits submilliampere threshold current as low as 0.54 mA and single-transverse mode emission. Maximum output optical power of 1.9 mW was observed in multimode lasing devices.