Estimating signal strengths in the design of an indoor wireless network

One issue in the design and implementation of a wireless local area network is the selection of access point (AP) locations. Proper AP placement is necessary to provide adequate signal coverage and also to minimize cochannel coverage overlap. The impact of incorrect placement of APs is significant. Placing APs too far apart can lead to gaps in coverage. On the other hand, placing the units too close together leads to excessive cochannel coverage overlap, degrading system performance. Currently, AP placement involves a "trial and error" technique. When a technician tests the effect of moving an AP from one location to another, it is necessary to spend considerable time manually measuring signal strengths in order to determine how this move affects the AP's coverage area. In this letter, we describe a procedure for estimating the coverage areas of relocated APs. The procedure can be used as part of a manual design process or as part of an automated design tool.     

Conventional Electron Microscopy and Electron Holography of Organic Solar Cells

Organic solar cells are a promising route towards large‐area and low‐price photovoltaic systems. The devices are composed of at least two layers: the hole‐transport layer and the electron‐transport layer. The light absorption can occur in one or both layers. At the interface of the layers the excitons are separated into charge carriers, and every layer deals with one type of carrier. Higher efficiencies of the separation process can be obtained by using a mixed layer containing both materials to obtain a very high interface area. Although the structure of the mixed layers used plays a crucial role for the device performance, until now the morphologies have not been elucidated. In order to correlate physical and optical findings with structure and morphology for the materials in question, electron microscopy experiments were performed on the single components as well as on the layer systems. The conventional electron microscope is a poor phase microscope. As consequence, weak‐phase objects like organic molecules have to be stained or imaged under strong defocus to produce an observable contrast. Artifacts caused by chemical staining and the appearance of Fresnel diffraction using the defocus technique represent the main problems of conventional microscopy. These artifacts can be avoided using electron holography. Holograms of ultrathin sections of thin layers composed of organic dye molecules were recorded. Subsequently, the phase images were reconstructed. In this manner, we succeeded in obtaining high‐contrast electron micrographs without applying staining or defocus. In addition, holograms of crystalline C₆₀ and zinc phthalocyanine were successfully recorded. Holography has been shown to be a useful tool to image beam‐sensitive and weak‐phase objects without artifacts.     

Analogue switch for very low-voltage applications

A new analogue switch suitable for operation at very low-voltage supply in a standard CMOS technology is presented. The proposed switch is based on 'quasi-floating-gate' transistors and has a simple and compact structure. For illustrative purposes, two sample-and-hold circuits operating from a single supply voltage close to the threshold voltage of a transistor, and using the proposed technique, are presented. Experimental results obtained from prototypes in a 1.5 /spl mu/m CMOS technology are provided.     

Algebraic theory of optimal filterbanks

We introduce an optimality theory for finite impulse response (FIR) filterbanks using a general algebraic point of view. We consider an admissible set /spl Lscr/ of FIR filterbanks and use scalability as the main notion based on which performance of the elements in /spl Lscr/are compared. We show that quantification of scalability leads naturally to a partial ordering on the set /spl Lscr/. An optimal solution is, therefore, represented by the greatest element in /spl Lscr/. It turns out that a greatest element does not necessarily exist in /spl Lscr/. Hence, one has to settle with one of the maximal elements that exist in /spl Lscr/. We provide a systematic way of finding a maximal element by embedding the partial ordering at hand in a total ordering. This is done by using a special class of order-preserving functions known as Schur-convex. There is, however, a price to pay for achieving a total ordering: there are infinitely many possible choices for Schur-convex functions, and the optimal solution specified in /spl Lscr/ depends on this (subjective) choice. An interesting aspect of the presented algebraic theory is that the connection between several concepts, namely, principal component filterbanks (PCFBs), filterbanks with maximum coding gain, and filterbanks with good scalability, is clearly revealed. We show that these are simply associated with different extremal elements of the partial ordering induced on /spl Lscr/ by scalability.     

Simultaneous amplification by Er ions and SRS in an Er-doped germano-silica fiber

A flat signal gain over in the entire C- and L-bands by erbium (Er) ions' radiative transition and stimulated Raman scattering in an Er-doped germano-silica fiber can be obtained if proper values of the concentration of Er and background loss in a fiber core are obtained during the fiber fabrication process. The optimized conditions for the flat C- and L-band gain are analyzed as functions of Er concentrations. Even for a low-gain value provided by a germano-silica core fiber with a low Er concentration and an optimum fiber length, a relatively low pump is required to obtain the flat gain band.     

A printer model using signal processing techniques

An accurate printer model that is efficient enough to be used by halftoning algorithms is proposed. The proposed signal processing model (SPM) utilizes a physical model to train adaptive linear combiners (ALCs), after which the average exposure of each subpixel for any input pattern can be calculated using the optimized weight vector. The SPM can be used to model multi-level halftoning and resolution enhancement, as well as traditional halftoning. The SPM is comprised of a single ALC layer followed by a peak-to-average ratio (PAR) correction layer, which serves to produce a PAR of less than 1.5 in the modeled exposure. The PCN (PAR correction network) employs one ALC/pixel and exploits the physics governing the characteristics of exposure in small regions. A relatively small number of training patterns suffices to train the SPM.     

Improvement of AlGaInP light emitting diode by sulfide passivation

The brightness of AlGaInP light emitting diodes (LEDs) has been raised by a factor of 1.12 at 20 mA by sulfide passivation. Meanwhile, the sulfide also can decrease leakage current of AlGaInP LEDs at -2 V to nearly one thousandth of that in the as-fabricated device. The possible causes for the brightness increase of AlGaInP LEDs after sulfide treatment including surface roughness, reduction of Fresnel loss, and effective injection of carriers were demonstrated.     

Ambient Technology — Now You See It, Now You Don't

The continuing progression of Moore's Law has enabled the miniaturisation and dramatic cost reduction in electronics over the last ten years. For a truly pervasive communications environment the challenges of hiding key hardware technologies from the user are rapidly being overcome. This paper reviews the status of these hardware technology developments in the pervasive space and briefly discusses other contributing factors that will enable the pervasive vision to be realised.     

The LHC pre-series dipole cold mass geometry

In order to provide the necessary mechanical aperture for the LHC beam, the main dipole cold masses have to match precisely the nominal circular trajectory of the particles beam. The requirements on the dipole cold mass geometry are dictated by the LHC beam optics and by the allowed limits of mechanical deformation of the interconnection bellows. Keeping the tight tolerances that are imposed necessitates a well controlled bending process and the use of a high accuracy 3D measuring instrument for checking the geometry of the cold mass throughout many manufacturing stages up to the final inspection. The dipole cold mass pre-series production started in 2000. It is almost completed at the three sites. In this paper, we report on the problems encountered to shape correctly the cold masses, their effect on interconnection of the dipole cold masses and on the mechanical aperture. On one side measures to improve the production process in terms of accuracy and reproducibility were taken, on the other side the assembly tolerances could be relaxed following a thorough review of the machine requirements. The summary of the encountered problems, corrective actions and results obtained on the pre-series cold masses are presented and discussed.     

Feasibility analysis of phonon lasers

We identify the main reason impeding coherent generation of phonons in solid state - the inherently high density of phonon states. Based on the results of our analysis we formulate a set of conditions that may make phonon lasing practical and point out the most promising mechanism of phonon lasing: LO/spl rarr/LA+TO in InP. We then develop a complete set of phonon laser equations and evaluate the threshold, output power and efficiency of phonon lasers based on InP MESFET. We show that one can obtain high-power 1.26-THz coherent phonon emission with pump power as small as a few milliwatts and up to 5% slope efficiency. Potential applications are also discussed.