Faizal Karim

A Cross Layer Optimization Mechanism to Improve H.264 Video Transmission over WLANs

Supporting video applications over 802.11 wireless local area networks is a challenging task due to the constant fluctuations in channel error rates and the inefficiency of the MAC layer. New video compression technologies, such as H.264, provide a network adaptation layer for adapting the output of the video encoder to the characteristics of the underlying transport network. In this article we demonstrate that it is possible to improve the performance of H.264 video applications over 802.11 WLANs through a cross-layer design that optimizes the encoded H.264 packet sizes. We propose the use of aggregation and fragmentation mechanisms to create the optimal frame lengths. We also investigate several application layer error

Efficient Simulation of Correlated Dynamics in Quantum-Dot Cellular Automata (QCA)

Many simulations of quantum-dot cellular automata (QCA) rely upon the so-called intercellular Hartree approximation (ICHA), which neglects the possibility of entanglement between cells. While the ICHA is useful for solving many QCA circuits due to its relative simplicity and computational efficiency, its many shortcomings make it prohibitive in accurately modeling the dynamics of large systems of QCA cells. On the other hand, solving a full Hamiltonian for each circuit, while more accurate, becomes computationally intractable as the number of cells increases. This paper aims to find an intermediate solution that exists somewhere in the solution space spanned by the ICHA and the full Hamiltonian. The development of such a solution promises to yield significant results within the QCA research community.

Quantum Mechanical Simulation of QCA with a Reduced Hamiltonian Model

Molecular quantum-dot cellular automata (QCA) is an emerging computing paradigm which utilizes electrostatic coupling between electronic configurations in neighboring molecules to perform information processing. A simulation tool for this technology, QCADesigner, exists and allows designers to quickly layout and simulate QCA circuits constructed with up to thousands of QCA cells. However, in general, large quantum mechanical systems are not suitable for efficient simulation on a classical computer, and as a result, QCADesigner uses the Hartree-Fock approximation to reduce the computational complexity of the simulation. Under certain circumstances, this approximation can lead to the incorrect ground state and hence, produce logically incorrect results at the outputs. In this work, we provide examples of problem circuits and propose a method to identify areas that must be simulated using the full Hamiltonian.

Modeling and Evaluating Errors Due to Random Clock Shifts in Quantum-Dot Cellular Automata Circuits

This paper analyzes the effect of random phase shifts in the underlying clock signals on the operation of several basic Quantum-dot Cellular Automata (QCA) building blocks. Such phase shifts can result from manufacturing variations or from uneven path lengths in the clocking network. We perform numerical simulations of basic building blocks using two different simulation engines available in the QCADesigner tool. We assume that the phase shifts are characterized by a Gaussian distribution with a mean value of

Testing of combinational majority and minority logic networks

i \frac{\pi}{2}

Consequences of Many-Cell Correlations in Clocked Quantum-Dot Cellular Automata

, where i is the clock number and a standard deviation, σ, which is varied in each simulation. Our results indicate that the sensitivity of building blocks to phase shifts depends primarily on the layout while the reliability of all building blocks starts to drop once the standard deviation, σ exceeds 4°. A full adder was simulated to analyze the operation of a circuit featuring a combination of the building blocks considered here. Results are consistent with expectations and demonstrate that the carry output of the full adder is better able to withstand the phase shifts in the clocking network than the Sum output which features a larger combination of the simulated building blocks.

On the Error Effects of Random Clock Shifts in Quantum-dot Cellular Automata Circuits

In this paper, we present an extension to the existing PODEM algorithm to include the ability to generate test patterns for majority and minority networks, specifically targeting quantum-dot cellular automata (QCA), but that is directly applicable to other emergent nanotechnologies such as single electron tunneling (SET) and tunneling phase logic (TPL). A dynamic probability-based controllability technique was developed and used as a guide to make more intelligent decisions on which lines to justify during the automatic test pattern generation (ATPG) process. Lastly, a genetic algorithm was used to fill-in the unspecified values in the test patterns produced by the ATPG in order to achieve compaction on the final test set size. The modified PODEM algorithm was tested on a set of MCNC benchmark circuits when using both fixed polarized cells and external inputs to implement the AND and OR gates. Test set sizes were much smaller when implementing the AND/OR gates using fixed polarized cells, however, the computational times for the latter method were generally shorter.

Characterization of the Displacement Tolerance of QCA Interconnects

Quantum-dot cellular automata (QCA) provides a basis for classical computation without transistors. Many simulations of QCA rely upon the so-called intercellular Hartree approximation (ICHA), which neglects the possibility of entanglement between cells. The ICHA was originally proposed as a solution to the problem of exponential scaling in the computational cost of fully quantum mechanical treatments. However, in some cases, the ICHA predicted errors in QCA operation, and quantum correlations were required for circuits to operate correctly. While quantum correlations can remedy certain problems that present themselves in ICHA calculations, here we present simulations that show that quantum correlations may in fact be problematic in other situations, such as clocked QCA. Small groups of QCA cells are modelled with a Hamiltonian analogous to a quantum mechanical Ising-like spin chain in a transverse field, including the effects of intercellular entanglement completely. When energy relaxation is included in the model, we find that intercellular entanglement changes the qualitative behavior of the system, and new features appear. In clocked QCA, isolated groups of active cells have a tendency to oscillate between polarization states as information propagates. Additionally, energy relaxation tends to bring groups of cells to an unpolarized steady state. This contrasts with the results of previous simulations, which employed the ICHA. The ICHA may in fact be a good approximation in the limit of very low tunneling rates, which can be realized in lithographically defined quantum dots. However, in molecular and atomic implementations of QCA, entanglement will play a greater role. The degree to which intercellular correlations pose a problem for memory, and clocking depends upon implementation-specific details of the interaction of the system with its environment, as well as the systems internal dynamics.

Modelling Techniques for Simulating Large QCA Circuits

This paper analyzes the effect of random phase shifts in the underlying clock signals on the operation of several basic quantum-dot cellular automata (QCA) building blocks. Such phase shifts can result from manufacturing variations or from uneven path lengths in the clocking network. While previous literature has proposed various clock distribution architectures and also provided analysis of manufacturing variations on QCA layouts, so far no literature is available on the characterization of effects resulting from the lack of phase synchronization in the QCA clocks. We perform numerical simulations of these basic building blocks using two different simulation engines available in the QCADesigner tool. We assume that the phase shifts are characterized by a Gaussian distribution with a mean value of ipi/2, where i is the clock number. Our results indicate that the sensitivity of building blocks to phase shifts depends primarily on the layout of the building block, and that most building blocks were able to operate properly under random phase shifts characterized by sigma= 5% pi/2.

Crosstalk in QCA arithmetic circuits

Quantum-dot cellular automata (QCA) is one of several emerging nanoscale devices that is targeted at scalable molecular electronics. In this paper, the tolerance to cell displacements of a QCA interconnect is analyzed in order to determine limits on allowable displacements, as well as to identify the important failure mechanisms. Numerical simulations using the coherence vector formalism are performed for a short length of QCA interconnect under various conditions. Contrary to previous work, our results indicate that wider interconnects display a higher sensitivity to cell displacements due to the formation of cell clusters, which are more prominent in wider interconnects as a result of the increased number of cells.