Tribological Properties of TiO<Subscript>2</Subscript> Reinforced Nickel Based MMCs Produced by Pulse Electrodeposition Technique

Nickel–TiO₂ composite coatings were prepared under pulse current conditions by co-deposition of TiO₂ particles and nickel from a Watts type bath. The effect of TiO₂ particle concentration was studied on microhardness, friction coefficient and wear resistance. The morphological features and the structures were studied by scanning electron microscope, X-ray diffraction analysis and 3D profilometry facilities. A wide particle size range (between 95 and 140 nm) was chosen to provide a high dispersion and load bearing ability for the co-deposited layers. It was determined that increasing the particle concentration in the electrolyte dramatically increased the co-deposited TiO₂ particles in the coating. The results showed that the high concentration of TiO₂ particles in the electrolyte yielded the highest amount of particles co-deposited in the plating layer. The influence of the co-deposited TiO₂ volume on microstructure and tribological properties in the coating were investigated. The wear tests were carried out using a constant load by a reciprocating ball-on disk configuration. Wear loss and friction coefficients of Ni/TiO₂ composites were decreased by increasing TiO₂ content in the electrolyte because of the increasing content of TiO₂ in the deposited layer. The change in wear mechanisms by changing TiO₂ content was also determined.     

Preparation of high quality,single domain BaFe12O19 particles by the citrate sol–gel combustion route with an initial Fe/Ba molar ratio of 4

BaFe₁₂O₁₉ particles have been synthesized by citrate sol–gel combustion route in a wide temperature range between 800 and 1200 °C with initial Fe/Ba molar ratios between 12 and 2. Structural, morphological and magnetic properties of the powders have been investigated by XRD, FT-IR, SEM and magnetization measurements. It was observed that both coercivity and specific saturation magnetization increase with annealing at temperatures up to 1100 °C, where a transition from single to multi domain structure occurs. To prevent formation of the hematite phase (α-Fe₂O₃), samples with different Fe/Ba molar ratios between 12 and 2 have been prepared and an intermediate phase, BaFe₂O₄, which may occur in Ba-rich samples has been removed by etching the powders in diluted hydrochloric acid. In this way, it was shown that single domain barium hexaferrite particles having high saturation magnetization, close to the theoretical value, and high coercivity can be synthesized with the initial Fe/Ba molar ratio of 4 in the sol–gel method. The chemical composition of this sample was determined as BaFe_11.80O_19.45 by the EDS analysis and Ba_1.05Fe_11.54O_18.4 using an ICP-MS device. Both are very close to the theoretical formula.     

On relay selection for decode-and-forward relaying

In this letter, we consider a multi-relay network operating in decode-and-forward mode. We propose a novel relay selection method with a low implementation complexity. Unlike the competing schemes, it requires neither error detection methods at relay nodes nor feedback information at the source. We derive a closed-form symbol error rate (SER) expression for multi-relay network under consideration and demonstrate that the proposed selection method is able to extract the full diversity. Extensive Monte Carlo simulations are also presented to confirm the derived SER expressions and to compare the performance of the proposed scheme with its competitors.     

Guest editorial: optical wireless communications

Over the last two decades, wireless communications has gained enormous popularity, offering attractive options for many personal and organizational communication needs due to major intrinsic characteristics such as flexibility, cost effectiveness, and mobility.     

The Value of Snow Depletion Forecasting Methods Towards Operational Snowmelt Runoff Estimation Using MODIS and Numerical Weather Prediction Data

Forecasting streamflow mainly due to snowmelt in the mountainous eastern part of Turkey is important in terms of effective management of water resources at the headwaters of Euphrates River, where large dam reservoirs are located. Monitoring Snow Covered Area (SCA) and modeling snowmelt forms the backbone of the forecasting studies as the snowmelt dominating runoff constitutes approximately 2/3 of total annual volume of runoff during spring and early summer. Two main motivations of the study are; firstly, to assess the methodologies to forecast SCA using Moderate Resolution Imaging Spectroradiometer (MODIS) data and derive Snow Depletion Curve (SDC) for each elevation zone. Secondly, to forecast 1?day ahead daily discharges using the derived SDCs and Numerical Weather Prediction (NWP) data corrected specifically for the area. The Upper Euphrates Basin (10,275?km²) is selected as the pilot basin and MODIS daily snow cover products are analyzed for the snowmelt season. Four different methodologies are proposed and assessed to forecast SDCs; simple averaging, temperature based, stochastic modeling and probabilistic approach. SDCs are derived for the water years 2006?C2010, 4?years data are used to derive the equations of the methodologies and 1?year is used to verify their skills. Forecasting discharges 1?day ahead with Snowmelt Runoff Model using NWP data is the second part of the study. Impact of forecasted SDCs with different methodologies is examined with the model. Model applications provide promising results both for the forecasting of SCA and runoff with an overall Model Efficiency higher than 0.60 and 0.85, respectively.     

Backpressure scheduling in IEEE 802.11 wireless mesh networks: Gap between theory and practice

Achieving efficient bandwidth utilization in wireless networks requires solving two important problems: (1) which packets to send (i.e., packet scheduling) and (2) which links to concurrently activate (i.e., link scheduling). To address these scheduling problems, many algorithms have been proposed and their throughput optimality and stability are proven in theory. One of the most well-known scheduling algorithms is backpressure scheduling which performs both link and packet scheduling assuming a TDMA (Time Division Multiple Access) MAC (Medium Access Control) layer. However, there has been limited work on realizing backpressure scheduling with a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) MAC layer (e.g., IEEE 802.11). In IEEE 802.11 networks, it is expected that the throughput optimality will not be achieved. In this paper, we investigate the extent of this throughput gap between theoretical TDMA-based backpressure scheduling and an approximation of it for IEEE 802.11 WMNs (Wireless Mesh Networks). Through extensive testbed measurements, we verify that there is indeed a non-negligible throughput gap. We present two main reasons behind this gap: Control inaccuracy that results from approximation of link scheduling and information inaccuracy that results from late or incorrect information, for instance, about queue lengths or network topology. Our results show that losses by MAC-layer collisions and backoff, which mainly occur due to control inaccuracy plays a major role for the throughput gap. On the other hand, while losses by queue drops, typically due to information inaccuracy, do occur, their effect can be tolerated. Nevertheless, both types of inaccuracies need to be mitigated in order to improve throughput.     

Bit Interleaved Coded Multiple Beamforming

In this paper, we investigate the performance of bit-interleaved coded multiple beamforming (BICMB). We provide interleaver design criteria such that BICMB achieves full spatial multiplexing of min( N, M) and full spatial diversity of NM with N transmit and M receive antennas over quasi-static Rayleigh flat fading channels. If the channel is frequency selective, then BICMB is combined with orthogonal frequency division multiplexing (OFDM) (BICMB-OFDM) in order to combat ISI caused by the frequency-selective channels. BICMB-OFDM achieves full spatial multiplexing of min(N, M), while maintaining full spatial and frequency diversity of NML for an NtimesM system over L-tap frequency-selective channels when an appropriate convolutional code is used. Both systems analyzed in this paper assume perfect channel state information both at the transmitter and the receiver. Simulation results show that, when the perfect channel state information assumption is satisfied, BICMB and BICMB-OFDM provide substantial performance or complexity gains when compared to other spatial multiplexing and diversity systems.     

Communication Optimizations for Irregular Scientific Computations on Distributed Memory Architectures

This paper describes a number of optimizations that can be used to support the efficient execution of irregular problems on distributed memory parallel machines. These primitives (1) coordinate interprocessor data movement, (2) manage the storage of, and access to, copies of off-processor data, (3) minimize interprocessor communication requirements, and (4) support a shared name space. We present a detailed performance and scalability analysis of the communication primitives. This performance and scalability analysis is carried out using a workload generator, kernels from real applications, and a large unstructured adaptive application (the molecular dynamics code CHARMM).     

Coplanar waveguide edge-coupled bandpass filters with finite groundplanes

Finite ground plane coplanar waveguide edge-coupled bandpass filters which have no parasitic passband and low insertion loss are demonstrated. Two 5-section filters centred at 10 GHz with a 15% bandwidth with identical design parameters but different ground plane widths; are fabricated on alumina substrate; the measured passband insertion loss is <1.5 dB. The filter with wider ground planes has higher attenuation in its stopbands