Reduced-rank multistage receivers for DS-CDMA in frequency-selective fading channels

Multistage (MS) implementation of the minimum mean-square error (MMSE), minimum output energy (MOE), best linear unbiased estimation (BLUE), and maximum-likelihood (ML) filter banks (FBs) is developed based on the concept of the MS Wiener filtering (MSWF) introduced by Goldstein et al. These FBs are shown to share a common MS structure for interference suppression, modulo a distinctive scaling matrix at each filter's output. Based on this finding, a framework is proposed for joint channel estimation and multiuser detection (MUD) in frequency-selective fading channels. Adaptive reduced-rank equal gain combining (EGC) schemes for this family of FBs (MMSE, MOE, BLUE, and ML) are proposed for noncoherent blind MUD of direct-sequence code-division multiple-access systems, and contrasted with the maximal ratio combining counterparts that are also formed with the proposed common structure under the assumption of known channel-state information. The bit-error rate, steady-state output signal-to-interference plus noise ratio (SINR), and convergence of the output SINRs are investigated via computer simulation. Simulation results indicate that the output SINRs attain full-rank performance with much lower rank for a highly loaded system, and that the adaptive reduced-rank EGC BLUE/ML FBs outperform the EGC MMSE/MOE FBs, due to the unbiased nature of the implicit BLUE channel estimators employed in the EGC BLUE/ML schemes.     

Secrecy Capacity Analysis of MIMO System over Multiple Destinations and Multiple Eavesdroppers

Wireless Personal Communications - In this paper we address physical layer security of multiple input multiple output (MIMO) communication system in presence of multiple destinations and passive...     

MOVPE growth of HgCdTe for high performance 3–5 µm photodiodes operating at 100–180K

New results are reported on the growth of high performance medium wavelength infrared (3–5 μm) (MWIR) HgCdTe photodiodes in the three-layer P-n-N configuration. The detector structures were grown in situ by metalorganic vapor phase epitaxy (MOVPE) on (211)B oriented CdZnTe substrates. The mobilities of the single n-type layers with x-values of ∼0.30 are in the range of (3–4.5)×10⁴ cm²/V-s at 80K. The lifetimes on unpassivated films range from 1–5 and 4–10 μs at 80 and 180K, respectively, which are within a factor of two or less of the lifetimes calculated for Auger-1 and radiative recombination. The P-n-N films were processed into variable-area backside-illuminated diagnostic arrays and tested for quantum efficiency, spectral response, R_DA, I–V curves and 1/f noise in the 120–180K range. The internal one-dimensional quantum efficiencies are in the range of 85–100%. The optical collection lengths are typically ∼25 μm. I–V curves showed that diffusion current is the dominant junction current mechanism for temperatures ≥100K. R₀A values are at the one-dimensional limit for n-side diffusion currents over the 100–180K range. 1/f noise was measured to be very low at 120K and is the same as that measured in similarly processed arrays from recent LPE grown P-on-N heterojunctions. The results demonstrate that MOVPE growth can be used for large area, high performance MWIR HgCdTe detector arrays operating in the 120–180K temperature range.     

Annealing-Induced Morphological Changes in Nanocrystalline Quantum Dots and Their Impact on Charge Transport Properties

The effects of thermal annealing on the morphological and photoconductive properties of cadmium selenide quantum dots coated with zinc sulfide are studied. The results of transmission electron microscopy with in situ annealing show a number of events taking place simultaneously, including aggregation of dots, changes in the size and shape distribution, and reduction in interdot separation. Transient absorption results indicate that there is a small redshift of the spectrum. There is a shortening of the absorption decay lifetimes due to annealing. Higher photocurrents are measured in the annealed compared with unannealed dots at room temperature.     

Photoluminescence and Raman Spectroscopy of Polycrystalline ZnO Nanofibers Deposited by Electrospinning

The technique of electrospinning offers the advantage of growing nanowires in bulk quantities in comparison with traditional methods. We report optical studies of polycrystalline zinc oxide (ZnO) nanofibers (∼100 nm thick and 5 μm long) deposited by electrospinning. Photoluminescence from the nanofibers shows a near-ultraviolet (near-UV) peak corresponding to near-band-edge emission and a strong broad peak in the visible region from oxygen antisite and interstitial defects. Temperature-dependent photoluminescence spectroscopy reveals that different carrier recombination mechanisms are dominant at low temperature. Our Raman spectroscopy results demonstrate that characterization of the quasimodes of longitudinal optical (LO) and transverse optical (TO) phonons present in an ensemble of polycrystalline nanofibers tilted at various angles in addition to the dominant E ₂(high) mode provides a promising technique for assessing the quality of such randomly oriented nanowires.     

Gesture Recognition: A Survey

Gesture recognition pertains to recognizing meaningful expressions of motion by a human, involving the hands, arms, face, head, and/or body. It is of utmost importance in designing an intelligent and efficient human-computer interface. The applications of gesture recognition are manifold, ranging from sign language through medical rehabilitation to virtual reality. In this paper, we provide a survey on gesture recognition with particular emphasis on hand gestures and facial expressions. Applications involving hidden Markov models, particle filtering and condensation, finite-state machines, optical flow, skin color, and connectionist models are discussed in detail. Existing challenges and future research possibilities are also highlighted     

Heterojunction DDR THz IMPATT diodes based on AlxGa1-xN/GaN material system

Simulation studies are made on the large-signal RF performance and avalanche noise properties of heterojunction double-drift region (DDR) impact avalanche transit time (IMPATT) diodes based on Al_xGa_1-xN/GaN material system designed to operate at 1.0 THz frequency. Two different heterojunction DDR structures such as n-Al_0.4Ga_0.6N/p-GaN and n-GaN/p-Al_0.4Ga_0.6N are proposed in this study. The large-signal output power, conversion efficiency and noise properties of the heterojunction DDR IMPATTs are compared with homojunction DDR IMPATT devices based on GaN and Al_0.4Ga_0.6N. The results show that the n-Al_0.4Ga_0.6N/p-GaN heterojunction DDR device not only surpasses the n-GaN/p-Al_0.4Ga_0.6N DDR device but also homojunction DDR IMPATTs based on GaN and Al_0.4Ga_0.6N as regards large-signal conversion efficiency, power output and avalanche noise performance at 1.0 THz.     

Unambiguously Enhanced Ultraviolet Luminescence of AlGaN Wavy Quantum Well Structures Grown on Large Misoriented Sapphire Substrate

High‐quality epitaxy consisting of Al_1?xGa_xN/Al_1?yGayN multiple quantum wells (MQWs) with sharp interfaces and emitting at ≈280 nm is successfully grown on sapphire with a misorientation angle as large as 4°. Wavy MQWs are observed due to step bunching formed at the step edges. A thicker QW width accompanied by a greater accumulation of gallium near the macrostep edge than that on the flat‐terrace is observed on 4° misoriented sapphire, leading to the generation of potential minima with respect to their neighboring QWs. Consequently, a significantly enhanced photoluminescence intensity (at least ten times higher), improved internal quantum efficiency (six times higher at low excitation laser power), and a much longer carrier lifetime are achieved. Importantly, the wafer‐level output‐power of the ultraviolet light emitting diodes on 4° misoriented substrate is nearly increased by 2–3 times. This gain is attributed to the introduction of compositional inhomogeneities in AlGaN alloys induced by gallium accumulation at the step‐bunched region thus forming a lateral potential well for carrier localization. The experimental results are further confirmed by a numerical modeling in which a 3D carrier confinement mechanism is proposed. Herein, the compositional modulation in active region arising from the substrate misorientation provides a promising approach in the pursuit of high‐efficient ultraviolet emitters.     

Design and Characterization of Fabry–Pérot MEMS-Based Short-Wave Infrared Microspectrometers

Microspectrometers based on the monolithic integration of a microelectromechanical system (MEMS) Fabry–Pérot filter and a Hg _x Cd_1–x Te-based infrared detector are discussed and measured results presented. The microspectrometers are designed to operate in the 1.5 μm to 2.6 μm wavelength range. Design equations are presented which account for the mechanical and optical characteristics of the device. Measurements indicate linewidths as narrow as 55 nm, switching times of 40 μs, and a tuning range of 380 nm, which is limited by snap-down. Optical characterization of the distributed Bragg mirrors and the Fabry–Pérot filter are presented, and these are shown to be in good agreement with simple first-order analytical models. Bowing of the movable Fabry–Pérot mirror due to stress gradients is identified as the dominant source of linewidth broadening.