Joseph Micallef

Body area network for wireless patient monitoring

Patient data monitoring is a key issue for health and disease management. The use of wireless sensors within a body area network (BAN) makes this task seamless and easy. A BAN system is presented, which allows the connectivity of a wide range of heterogeneous body sensors to a portable hub device that is connectable to external networks (IEEE 802.11, GPRS). This BAN is based on the use of Zigbee/IEEE 802.15.4 standard technology and off-the-shelf modules. It is currently being used at the European level for the detection and the prediction of the human physiological state in relation to wakefulness, fatigue, and stress applications in which users carrying out daily activities are monitored in an unobtrusive and comfortable way. Characterised by its low power consumption, low cost, and ability to connect a wide range of heterogeneous sensors, this system can substantially improve the performance of different services, especially those that are health related.

Effects of interdiffusion on the sub‐band‐edge structure of In0.53Ga0.47As/InP single quantum wells

The disordering of In0.53Ga0.47As/InP single quantum wells has been studied using an error function distribution to model the compositional profile after interdiffusion. When considering interdiffusion on the group‐III sublattice only, a large strain buildup results during the early stages of disordering. Details are presented showing how this interdiffusion and the effects of strain lead to an interesting carrier confinement profile which differs from that of disordered AlGaAs/GaAs and InGaAs/GaAs quantum‐well structures. An abrupt confinement profile is maintained even after significant interdiffusion, with a well width equal to that of the as‐grown quantum well. The combined effects of strain with the unstrained band‐gap profile results in a potential buildup in the barrier near the interface, while it gives rise to two ‘‘miniwells’’ inside the potential wells. The sub‐band‐edge structure shows that the potential buildup can result in quasibound subband states, while the heavy‐hole well can support the ground state within the miniwells. In contrast, when identical interdiffusion on both group‐III and group‐V sublattices is considered, the structure remains lattice matched, the confinement profile changes to that of a graded profile, and the ground‐state transition energy shifts to shorter wavelengths.

Comparative study of automatic speech recognition techniques

Over the past decades, extensive research has been carried out on various possible implementations of automatic speech recognition (ASR) systems. The most renowned algorithms in the field of ASR are the mel-frequency cepstral coefficients and the hidden Markov models. However, there are also other methods, such as wavelet-based transforms, artificial neural networks and support vector machines, which are becoming more popular. This review article presents a comparative study on different approaches that were proposed for the task of ASR, and which are widely used nowadays.

Quantum‐confined Stark effect in interdiffused AlGaAs/GaAs quantum well

The quantum‐confined Stark effect is analyzed in an interdiffused Al0.3Ga0.7As/GaAs single quantum well (QW) with an as‐grown well width of 100 A, where the confinement profile is modeled by an error function. Results indicate a twofold enhancement of the Stark shift for the interdiffused quantum well over that of the square quantum well for a 50 kV/cm applied field. The fundamental exciton absorption peak also shows a much larger reduction with increasing applied field in the more extensively interdiffused QW. These characteristics may be used to realize optical modulators with higher on/off ratios and lower drive voltages.

Polarization dependent refractive index of an interdiffusion induced AlGaAs/GaAs quantum well

The polarization dependent refractive index, nR, at room temperature is calculated for interdiffusion‐induced Al0.3Ga0.7As/GaAs single quantum well (QW) structures for the wavelength range 0.75–2 μm. The confinement profile is modeled by an error function and nR is determined using the real and imaginary parts of the dielectric function, including contributions from the Γ, X, and L Brillouin zones. Results show that at longer wavelengths nR decreases with increasing interdiffusion, which normally provides a positive index step with respect to a less interdiffused QW. For shorter wavelengths (around the QW band edge), the wavelength range for a positive refractive index step increases as the extent of disordering between two interdiffused QWs is increasing.

Effects of Different Cation and Anion Interdiffusion Rates in Disordered In0.53Ga0.47As/InP Single Quantum Wells

The effects of different cation and anion interdiffusion rates when disordering In0.53Ga0.47As/InP single quantum wells are investigated using an error function distribution to model the compositional profile after interdiffusion. The early stages of disordering result in a spatially dependent strain buildup, which can be either compressive or tensile. The effects of this strain profile and the compositional distribution give rise to interesting carrier confinement profiles after disordering. A significantly faster cation interdiffusion rate produces a red shift of the ground-state transition energy, which with prolonged interdiffusion saturates and then decreases. A significantly higher anion interdiffusion rate causes a blue shift in the ground state transition energy, and shifts the light hole ground state above the heavy hole ground state. The results from the model are compared with reported experimental results which have been interpreted in terms of different interdiffusion rates on the two sublattices.

Confinement Subbands in an InGaAs/GaAs Non-Square Quantum Well

Calculations of the confinement subbands energy levels, interband transitions energy and related overlapping wavefunctions in a single In0.2Ga0.8As/GaAs non-square strained quantum well structure have been carried out for an error function confinement profile. The results indicate a squeezing of subband states during the latter stages of diffusion, and an enhancement of the off diagonal transitions overlapping wavefunction, during the mid stages of diffusion, which give rise to a relaxed selection rule.

On the design of low-voltage, low-power CMOS analog multipliers for RF applications

Novel low-voltage, low-power techniques in the design of portable wireless communication systems are required. Two system examples of low-power analog multipliers operating from a 1.2 V supply are presented. These proposed structures achieve the required multiplication function by using current processing. The circuits were fabricated using standard double-poly CMOS processes for a 900 MHz application. Measurement results of the prototypes are comparable to other higher voltage designs.

Analysis of the Spur Characteristics of Edge-Combining DLL-Based Frequency Multipliers

A complete analysis of the spur characteristics of edge-combining delay-locked loop (DLL)-based frequency multipliers is presented in this brief. The novelty of this analysis is the fact that it can be used to estimate the effect of both the in-lock error and the delay-stage mismatch on the spurious level of the frequency multiplier with low computational complexity. In addition, a way to reduce the mismatch between the delay cells in the delay line is discussed via an analytic model and verified by the implementation of a delay cell in a 65-nm CMOS process.

Low voltage SC TDM correlator for the extraction of time delay

This paper proposes a novel technique for the computation of interaural time delay cues which are useful for sound localization. A switched capacitor implementation is used in order to achieve good accuracy. Furthermore, to reduce the area requirement and minimize the accumulation of offset and noise, a time-division multiplexed approach has been used requiring just three fast op amps, one analogue multiplier and a comparator. These blocks have been optimized to operate at a supply voltage of 1.8 V.