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 resiliency mechanisms and examine their performance under different network conditions.
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 system's internal dynamics.Index Terms-Nanoscale devices, quantum cellular automata, quantum dots.
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.
A simple architecture for data input into a molecular quantum-dot cellular automata (QCA) circuit from an external CMOS circuit is proposed. A "T"-shaped interconnect, utilizing fixed-polarization cells to provide the desired polarization, is controlled via external electrodes connected to a standard CMOS input driver. The applied input signal is used to gate either the propagation of a fixed polarization, P = +1, or that of the complementary fixed polarization, P = −1, into the QCA circuit. The architecture utilizes the field-driven clocking scheme proposed in recent literature to achieve transduction between applied input voltage and a molecular configuration. The system is modelled using the coherence vector formalism with a three-state basis and simulated using the QCADesigner simulation tool.
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