Finite Signal-set Capacity of Two-user Gaussian Multiple Access Channel

The capacity region of a two-user Gaussian Multiple Access Channel (GMAC) with complex finite input alphabets and continuous output alphabet is studied. When both the users are equipped with the same code alphabet, it is shown that, rotation of one of the user’s alphabets by an appropriate angle can make the new pair of alphabets not only uniquely decodable, but will result in enlargement of the capacity region. For this set-up, we identify the primary problem to be finding appropriate angle(s) of rotation between the alphabets such that the capacity region is maximally enlarged. It is shown that the angle of rotation which provides maximum enlargement of the capacity region also minimizes the union bound on the probability of error of the sumalphabet and vice-verse. The optimum angle(s) of rotation varies with the SNR. Through simulations, optimal angle(s) of rotation that gives maximum enlargement of the capacity region of GMAC with some well known alphabets such as M-QAM and M-PSK for some M are presented for several values of SNR. It is shown that for large number of points in the alphabets, capacity gains due to rotations progressively reduce. As the number of points N tends to infinity, our results match the results in the literature wherein the capacity region of the Gaussian code alphabet doesn’t change with rotation for any SNR.

Geotechnical Characterization of Some Indian Bentonites for their Use as Buffer Material in Geological Repository

The present study examines the geotechnical properties of Indian bentonite clays for their suitability as buffer material in deep geological repository for high-level nuclear wastes. The bentonite samples are characterized for index properties, compaction, hydraulic conductivity and swelling characteristics. Evaluation of geotechnical properties of the compacted bentonite-sand admixtures, from parts of NW India reveals swelling potentials and hydraulic conductivities in the range of 55 % - 108 % and 1.2 X 10 –10 cm/s to 5.42x 10 –11 cm/s respectively. Strong correlation was observed between ESP (exchangeable sodium percentage) and liquid limit/swell potential of tested specimens. Relatively less well-defined trends emerged between ESP and swell pressure/hydraulic conductivity. The Barmer-1 bentonite despite possessing relatively lower montmorillonite content of 68 %, developed higher Atterberg limit and swell potential, and exhibited comparable swelling pressure and hydraulic conductivity as other bentonites with higher montmorillonite contents (82 to 86 %). The desirable geotechnical properties of Barmer clay as a buffer material is attributed to its large ESP (63 %) and, EMDD (1.17 Mg/m3) attained at the experimental compactive stress(5 MPa).

Influence of the highest occupied molecular orbital energy level of the donor material on the effectiveness of the anode buffer layer in organic solar cells

Efficiency of organic photovoltaic cells based on organic electron donor/organic electron acceptor junctions can be strongly improved when the transparent conductive Anode is coated with a Buffer Layer (ABL). Here, the effects of a metal (gold) or oxide (molybdenum oxide) ABL are reported, as a function of the Highest Occupied Molecular Orbital (HOMO) of different electron donors. The results indicate that a good matching between the work function of the anode and the highest occupied molecular orbital of the donor material is the major factor limiting the hole transfer efficiency. Indeed, gold is efficient as ABL only when the HOMO of the organic donor is close to its work function Phi(Au). Therefore we show that the MoO(3) oxide has a wider field of application as ABL than gold. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Reducing Buffer Requirements in Core Routers using Dynamic Buffering

Earlier studies have exploited statistical multiplexing of flows in the core of the Internet to reduce the buffer requirement in routers. Reducing the memory requirement of routers is important as it enables an improvement in performance and at the same time a decrease in the cost. In this paper, we observe that the links in the core of the Internet are typically over-provisioned and this can be exploited to reduce the buffering requirement in routers. The small on-chip memory of a network processor (NP) can be effectively used to buffer packets during most regimes of traffic. We propose a dynamic buffering strategy which buffers packets in the receive and transmit buffers of a NP when the memory requirement is low. When the buffer requirement increases due to bursts in the traffic, memory is allocated to packets in the off-chip DRAM. This scheme effectively mitigates the DRAM access bottleneck, as only a part of the traffic is stored in the DRAM. We build a Petri net model and evaluate the proposed scheme with core Internet like traffic. At 77% link utilization, the dynamic buffering scheme has a drop rate of just 0.65%, whereas the traditional DRAM buffering has 4.64% packet drop rate. Even with a high link utilization of 90%, which rarely happens in the core, our dynamic buffering results in a packet drop rate of only 2.17%, while supporting a throughput of 7.39 Gbps. We study the proposed scheme under different conditions to understand the provisioning of processing threads and to determine the queue length at which packets must be buffered in the DRAM. We show that the proposed dynamic buffering strategy drastically reduces the buffering requirement while still maintaining low packet drop rates.

End-bearing capacity of driven piles in sand using the stress characteristics method: analysis and implementation

The method of stress characteristics has been employed to compute the end-bearing capacity of driven piles. The dependency of the soil internal friction angle on the stress level has been incorporated to achieve more realistic predictions for the end-bearing capacity of piles. The validity of the assumption of the superposition principle while using the bearing capacity equation based on soil plasticity concepts, when applied to deep foundations, has been examined. Fourteen pile case histories were compiled with cone penetration tests (CPT) performed in the vicinity of different pile locations. The end-bearing capacity of the piles was computed using different methods, namely, static analysis, effective stress approach, direct CPT, and the proposed approach. The comparison between predictions made by different methods and measured records shows that the stress-level-based method of stress characteristics compares better with experimental data. Finally, the end-bearing capacity of driven piles in sand was expressed in terms of a general expression with the addition of a new factor that accounts for different factors contributing to the bearing capacity. The influence of the soil nonassociative flow rule has also been included to achieve more realistic results.

Capacity Optimizing Hop Distance in a Mobile Ad Hoc Network with Power Control

In a dense multi-hop network of mobile nodes capable of applying adaptive power control, we consider the problem of finding the optimal hop distance that maximizes a certain throughput measure in bit-metres/sec, subject to average network power constraints. The mobility of nodes is restricted to a circular periphery area centered at the nominal location of nodes. We incorporate only randomly varying path-loss characteristics of channel gain due to the random motion of nodes, excluding any multi-path fading or shadowing effects. Computation of the throughput metric in such a scenario leads us to compute the probability density function of random distance between points in two circles. Using numerical analysis we discover that choosing the nearest node as next hop is not always optimal. Optimal throughput performance is also attained at non-trivial hop distances depending on the available average network power.

Phase relations in the system Sr–Ir–O and thermodynamic measurements on SrIrO3, Sr2IrO4 and Sr4IrO6 using solid-state cells with buffer electrodes

The standard Gibbs energies of formation of SrIrO3, Sr2IrO4 and Sr4IrO6 have been determined in the temperature range from 975 to 1400 K using solid-state cells with (Y2O3) ZrO2 as the electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. For the design of appropriate working electrodes, phase relations in the ternary system Sr–Ir–O were investigated at 1350 K. The only stable oxide detected along the binary Ir–O was IrO2. Three ternary oxides, SrIrO3, Sr2IrO4 and Sr4IrO6, compositions of which fall on the join SrO–IrO2, were found to be stable. Each of the oxides coexisted with pure metal Ir. Therefore, three working electrodes were prepared consisting of mixtures of Ir+SrO+Sr4IrO6, Ir+Sr4IrO6+Sr2IrO4, and Ir+Sr2IrO4+SrIrO3. These mixtures unambiguously define unique oxygen chemical potentials under isothermal and isobaric conditions. Used for the measurements was a novel apparatus, in which a buffer electrode was introduced between reference and working electrodes to absorb the electrochemical flux of oxygen through the solid electrolyte. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. The standard Gibbs energies of formation of the compounds, obtained from the emf of the cells, can be represented by the following equations: View the MathML sourcem View the MathML source View the MathML source where Δf (ox)Go represents the standard Gibbs energy of formation of the ternary compound from its component binary oxides SrO and IrO2. Based on the thermodynamic information, chemical potential diagrams for the system Sr–Ir–O were developed.

Solid-state cells with buffer electrodes for accurate thermodynamic measurements: system Nd-Ir-O

A new design for the solid-state cell incorporating a buffer electrode for high-temperature thermodynamic measurements is presented. The function of the buffer electrode, placed between the reference and working electrodes, is to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevents polarization of the measuring electrode and ensures accurate data. The application of this novel design and its advantages are demonstrated by measurement of the standard Gibbs energies of formation of Nd6Ir2O13 (low-temperature form) and Nd2Ir2O7 in the temperature range from 975 to 1450 K. Yttria-stabilized zirconia is used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. For the design of appropriate working electrodes, phase relations in the ternary system NdIrO were investigated at 1350 K. The two ternary oxides, Nd6Ir2O13 and Nd2Ir2O7, compositions of which fall on the join Nd2O3IrO2, were found to coexist with pure metal Ir. Therefore, two working electrodes were prepared consisting of mixtures of Ir+Nd2O3+Nd6Ir2O13 and Ir+Nd6Ir2O13+ Nd2Ir2O7. These mixtures unambiguously define unique oxygen chemical potentials under isothermal and isobaric conditions. The standard Gibbs energies of formation (ΔG°f (ox)) of the compounds from their component binary oxides Nd2O3 and IrO2, obtained from the emf of the cells, can be represented by the equations:View the MathML source View the MathML source Based on the thermodynamic information, chemical potential diagrams for the system NdIrO are developed.

Refinement of thermodynamic properties of ReO2

The standard Gibbs energy of formation of ReO2 in the temperature range from 900 to 1200 K has been determined with high precision using a novel apparatus incorporating a buffer electrode between reference and working electrodes. The role of the buffer electrode was to absorb the electrochemical flux of oxygen through the solid electrolyte from the electrode with higher oxygen chemical potential to the electrode with lower oxygen potential. It prevented the polarization of the measuring electrode and ensured accurate data. The Re+ReO2 working electrode was placed in a closed stabilized-zirconia crucible to prevent continuous vaporization of Re2O7 at high temperatures. The standard Gibbs energy of the formation of ReO2 can be represented by the equation View the MathML source Accurate values of low and high temperature heat capacity of ReO2 are available in the literature. The thermal data are coupled with the standard Gibbs energy of formation, obtained in this study, to evaluate the standard enthalpy of formation of ReO2 at 298.15 K by the ‘third law’ method. The value of standard enthalpy of formation at 298.15 K is: View the MathML source(ReO2)/kJ mol−1=−445.1 (±0.2). The uncertainty estimate includes both random (2σ) and systematic errors.

Thermodynamic Data for Mn3O4, Mn2O3 and MnO2

Thermodynamic properties of Mn3O4, Mn2O3 and MnO2 are reassessed based on new measurements and selected data from the literature. Data for these oxides are available in most thermodynamics compilations based on older calorimetric measurements on heat capacity and enthalpy of formation, and high-temperature decomposition studies. The older heat capacity measurements did not extend below 50 K. Recent measurements have extended the low temperature limit to 5 K. A reassessment of thermodynamic data was therefore undertaken, supplemented by new measurements on high temperature heat capacity of Mn3O4 and oxygen chemical potential for the oxidation of MnO1-x, Mn3O4, and Mn2O3 to their respective higher oxides using an advanced version of solid-state electrochemical cell incorporating a buffer electrode. Because of the high accuracy now achievable with solid-state electrochemical cells, phase-equilibrium calorimetry involving the ``third-law'' analysis has emerged as a competing tool to solution and combustion calorimetry for determining the standard enthalpy of formation at 298.15 K. The refined thermodynamic data for the oxides are presented in tabular form at regular intervals of temperature.

Vertical uplift capacity of a group of two coaxial anchors in a general c-phi soil

The vertical uplift resistance of a group of two horizontal coaxial strip anchors, embedded in a general c-phi soil (where c is the unit cohesion and phi is the soil friction angle), has been determined by using the lower bound finite element limit analysis. The variation of uplift factors F-c and F-gamma, due to the components of soil cohesion and unit weight, respectively, with changes in depth (H)/width (B) has been established for different values of vertical spacing (S)/B. As compared to a single isolated anchor, the group of two anchors provides a significantly greater magnitude of F-c for phi <= 20 degrees and with H/B >= 3. The magnitude of F-c becomes almost maximum when S/B is kept closer to 0.5H/B. On the other hand, with the same H/B, as compared to a single anchor, hardly any increase in F-gamma occurs for a group of two anchors.

Mutual inhibition and capacity sharing during parallel preparation of serial eye movements

Many common activities, like reading, scanning scenes, or searching for an inconspicuous item in a cluttered environment, entail serial movements of the eyes that shift the gaze from one object to another. Previous studies have shown that the primate brain is capable of programming sequential saccadic eye movements in parallel. Given that the onset of saccades directed to a target are unpredictable in individual trials, what prevents a saccade during parallel programming from being executed in the direction of the second target before execution of another saccade in the direction of the first target remains unclear. Using a computational model, here we demonstrate that sequential saccades inhibit each other and share the brain's limited processing resources (capacity) so that the planning of a saccade in the direction of the first target always finishes first. In this framework, the latency of a saccade increases linearly with the fraction of capacity allocated to the other saccade in the sequence, and exponentially with the duration of capacity sharing. Our study establishes a link between the dual-task paradigm and the ramp-to-threshold model of response time to identify a physiologically viable mechanism that preserves the serial order of saccades without compromising the speed of performance.

Adaptation of small capacity natural gas engine for producer gas operation

This article addresses the adaptation of a low-power natural gas engine for using producer gas as a fuel. The 5.9 L natural gas engine with a compression ratio of 10.5:1, rated at 55 kW shaft power, delivered 30 kW using producer gas as fuel in the naturally aspirated mode. Optimal ignition timing for peak power was found to be 20 degrees before top dead centre. Air-to-fuel ratio (A/F) was found to be 1.2 +/- 0.1 over a range of loads. Critical evaluation of the energy flows in the engine resulted in identifying losses and optimizing the engine cooling. The specific fuel consumption was found to be 1.2 +/- 0.1 kg of biomass per kilowatt hour. A reduction of 40 per cent in brake mean effective pressure was observed compared with natural gas operation. Governor response to load variations has been studied with respect to frequency recovery time. The study also attempts to adopt a turbocharger for higher power output. Preliminary results suggest a possibility of about 30 per cent increase in the output.

Study of n-ZnO/p-Si (100) thin film heterojunctions by pulsed laser deposition without buffer layer

ZnO/Si heterojunctions were fabricated by growing ZnO thin films on p-type Si (100) substrate by pulsed laser deposition without buffer layers. The crystallinity of the heterojunction was analyzed by high resolution X-ray diffraction and atomic force microscopy. The optical quality of the film was analyzed by room temperature (RT) photoluminescence measurements. The high intense band to band emission confirmed the high quality of the ZnO thin films on Si. The electrical properties of the junction were studied by temperature dependent current-voltage measurements and RT capacitance-voltage (C-V) analysis. The charge carrier concentration and the barrier height (BH) were calculated, to be 5.6x10(19) cm(-3) and 0.6 eV respectively from the C-V plot. The BH and ideality factor, calculated using the thermionic emission (TE) model, were found to be highly temperature dependent. We observed a much lower value in Richardson constant, 5.19x10(-7)A/cm(2) K-2 than the theoretical value (32 A/cm(2) K-2) for ZnO. This analysis revealed the existence of a Gaussian distribution (GD) with a standard deviation of sigma(2)=0.035 V. By implementing the GD to the TE, the values of BH and Richardson constant were obtained as 1.3 eV and 39.97 A/cm(2) K-2 respectively from the modified Richardson plot. The obtained Richardson constant value is close to the theoretical value for n-ZnO. These high quality heterojunctions can be used for solar cell applications. (C) 2012 Elsevier B.V. All rights reserved.