Femtocells [Guest Editorial]

Momentum is building rapidly around femtocells, as several operators worldwide have commercially launched 3G femtocells. This will further accelerate during the course of 2010, with many more operators expected to deploy. As explained in the first part of this series, which appeared in the September 2009 issue of IEEE Communications Magazine, femtocells are small, inexpensive base stations operating in the licensed cellular bands; they are meant to be placed in individual homes or businesses, and backhauled onto the operator's network via wireline broadband access.     

Femtocells [Guest Editorial]

Femtocells are destined to transform the way mobile operators build their cellular networks and grow their coverage and capacity. Femtocells are small base stations operating in the licensed cellular bands. They are so small and inexpensive, and transmit at such low power, that they are meant to be placed in individual homes and backhauled onto the operator?s network via conventional digital subscriber line (DSL) or cable broadband access (or, when available, fiber).     

Excimer laser chemical ammonia patterning on PET film

Laser is a promising technique used for biopolymer surface modification with micro and/or nano features. In this work, a 193 nm excimer laser was used for poly (ethylene terephthalate) (PET) surfaces chemical patterning. The ablation threshold of the PET film used in the experiments was 62 mJ/cm(2) measured before surface modification. Surface chemical patterning was performed by irradiating PET film in a vacuum chamber filled with ammonia at the flux of 10, 15, 20, 25 ml/min. Roughness of the surface characterized by profilometry showed that there were no significant observed change after modification comparing original film. But the hydrophilicity of the surface increased after patterning and a minimum water contact angle was obtained at the gas flux of 20 ml/min. FT-IR/ATR results showed the distinct amino absorption bands presented at 3352 cm(-1)and 1613 cm(-1) after modification and XPS binding energies of C(1s) at 285.5 eV and N(1s) at 399.0 eV verified the existence of C-N bond formation on the PET film surface. Tof-SIMS ions mapping used to identify the amine containing fragments corroborates that amino grafting mainly happened inside the laser irradiation area of the PET surface. A hypothesized radical reaction mechanism proposes that the collision between radicals in ammonia and on the PET surface caused by the incident laser provokes the grafting of amino groups.     

Scalable electronic packet switches

Due to the changed economic environment, the rush to implementing packet switches with switching capacities above 1 Tb/s, which had proceeded at a frantic pace for some years, has slowed down considerably. Most service providers do not foresee the deployment of switches and routers with gigantic capacities in the near future. The immediate interest does now rarely go beyond the subterabit range, with a sweet spot between 120-640 Gb/s, where the emphasis is on feature-rich systems that enable the convergence of legacy services with new emerging data services. Although the current focus is on smaller switches, it is still relevant to understand their evolution path to multiterabit capacities. The scalability issues are also critical to reduce complexity and simplify implementation, in order to push the limits of what can be achieved in the switches within current economic and market constraints. We analyze the current state of the art of practical large packet switches and routers, and discuss the issues affecting their scalability. Our approach is pragmatic, with most of our attention devoted to three major scalability aspects: implementation, support of quality of service, and multicasting. After a general discussion of these issues, we show their impact on the most popular switch architectures.     

Correlation between thermal resistance, channel temperature, infrared thermal maps and failure mechanisms in low power MESFET devices

Channel temperature T_ch of GaAs MESFETs, determined by means of electrical measurements (ΔV_gs), has been compared and correlated with thermal maps obtained by high resolution infrared microscopy. Results show that in low power devices with a small number of gates, the value of T_ch derived from ΔV_gs measurements is close to the maximum values of temperature measured on the hottest junctions on the chip. Local thermal inhomogeneities, in particular those observed in devices characterized by high values of R_th, can explain and confirm results of failure analyses of MESFETs submitted to accelerated life tests in operating conditions where degradation phenomena occur in localized areas of the device.     

A growth rate, structure and surface morphology study of Si1-x-yGexCy films deposited by ArF-LCVD in tilted geometry

Ge, SiGe, SiC and SiGeC films were grown by ArF-Excimer laser induced chemical vapor deposition. The results demonstrate that in ArF-LCVD a fine and effective control of both the deposition rate, film properties and surface morphology is possible without altering the gas composition and pressure, by changing exclusively the distance between laser beam and substrate surface or by the fact of irradiating or not the films. Different distances have been achieved by tilting the sample 30° with respect to the beam, a geometry simultaneously producing both, an irradiated and a no irradiated zone in the same film.The extensive characterisation of these films was carried out through different techniques in order to see the influence of the irradiation geometry on composition, microstructure and roughness. The evaluation of the deposition rate and the XPS results revealed different growth rate behaviour along the film without considerable variations in composition. AFM proved the small roughness of the films and its strong dependence on the laser beam to substrate distance. Raman spectroscopy and XRD were used to determine the main structural properties. Additional information about surface morphology was also obtained through SEM.For pure Ge, some more studies have been performed due to the tendency of these films to show significant changes, especially in growth rate and roughness, both when the laser irradiated the samples and when temperature, pressure and laser power were varied.     

Pulsed laser deposition of silicon-substituted hydroxyapatite coatings

Thin films of Si-substituted hydroxyapatite (Si-HA) were deposited on Si and Ti substrates by pulsed laser deposition (PLD), in the presence of a water vapour atmosphere. The PLD ablation targets were made with different mixtures of commercial carbonated HA and Si powder, in order to produce the Si-HA thin films. The physicochemical properties of the coatings and the incorporation of the Si into the HA structure was studied by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The Si atoms were successfully incorporated into the HA structure, and were found to be in the form of SiO₄⁴⁻ groups, principally displacing carbonate groups off the HA structure.     

Structural characterization of bioceramics and mineralized tissues based on Raman and XRD techniques

The design of new bioceramics requires a deep understanding of their structural characteristics using a combination of different characterization techniques. This paper offers an exhaustive Raman spectroscopy and X-ray diffraction study of two groups of bioceramics natural and synthetic, as well as of living tissues with different degrees of mineralization. Based on these results, two Raman-XRD correlation charts are proposed. Using a single Raman measurement, these charts are valuable tools for the identification of a number of structural parameters, such as the apatite phase percentage or crystallite size, of different calcium phosphate-based bioceramics and mineralized tissues.     

Laser assisted formation of binary and ternary Ge/Si/Sn alloys

Binary and ternary Si/Ge/Sn alloys were epitaxially grown on virtual Germanium buffer layers using pulsed laser induced epitaxy with a 193 nm Excimer laser source. The role of the processing parameters on the intermixing of the components (Sn, Ge and Si) has been studied. Characterization of the resulting Ge_1 − xSn_x and Si_{1 − y − x}Ge_ySn_x alloys yield up to 1% Sn concentration in substitutional sites of the Ge or SiGe matrix.     

Performance of shared-memory switches under multicast burstytraffic

We study shared-memory switches under multicast bursty traffic and characterize the relation between their performance and the multicast distribution that defines the mix of multicast traffic arriving at the switches. We consider two schemes that have been used in practical realizations of these switches to replicate multicast cells: (1) replication-at-receiving (RAR), where multiple copies of a multicast cell are stored in the buffer and served independently, and (2) replication-at-sending (RAS), where a single instance of a multicast cell is stored in the buffer, and the cell is replicated as it is transmitted to the output ports. For each scheme, we study two configurations: (1) the shared-memory-only (SMO) configuration, where the bandwidth of the replication mechanism is sufficient to accommodate even the worst-case replication requirements, and (2) the shared-memory-with-replication-first-in-first-out (SM+RFIFO) configuration, where the bandwidth of the replication mechanism is lower than that required by the worst case, and thus an additional buffer is used in front of the shared memory to temporarily store cells while they are replicated. For all cases, using simulation, we find upper bounds for the buffer requirements to achieve a desired cell-loss rate. We show that these upper bounds are significantly larger than the buffer requirements under unicast traffic and are approached even for very small volumes of multicast traffic; thus, these upper bounds should be used in practice to size the buffers to achieve the desired performance under traffic with general multicast distributions. We also study shared-memory switches with output demultiplexers and characterize and compare the different multicasting schemes that are used in these switches