The intricate dependency of networking protocols upon the performance of the wireless channel motivates the investigation of network channel approximations for fading channels. Wireless networking protocols are increasingly being designed and evaluated with the assistance of networking simulators. While evaluating networking protocols such as medium access control, routing, and reliable transport, the network channel model, and its associated capacity, will drastically impact the achievable network throughput. Researcher relying upon simulation results must therefore use extreme caution to ensure the use of similar channel models when performing protocol comparisons. Some channel approximations have been created to mimic the behavior of a fading environment, however there exists little to no justification for these channel approximations. This dissertation addresses the need for a computationally efficient fading channel approximation for use in network simulations. A rigorous flat fading channel model was developed for use in accuracy measurements of channel approximations. The popular two-state Markov model channel approximation is analyzed and shown to perform poorly for low to moderate signal-to-noise ratios (SNR). Three novel channel approximations are derived, with multiple methods of parameter estimation. Each model is analyzed for both statistical performance and network performance. The final model is shown to achieve very accurate network throughput performance by achieving a very close matching of the frame run distributions. This work provides a rigorous evaluation of the popular two-state Markov model, and three novel low complexity channel models in both statistical accuracy and network throughput performance. The novel models are formed through attempts to match key statistical parameters of frame error run and good frame run statistics. It is shown that only matching key parameters is insufficient to achieve an acceptable channel approximation and that it is necessary to approximate the distribution of frame error duration and good frame run duration. The final novel channel approximation, the three-state run-length model, is shown to achieve a good approximation of the desired distributions when some key statistical parameters are matched.

A theory of the carrier transport, optical emission, and photoconductivity from optoelectronic devices based on ambipolar long-channel carbon-nanotube (CNT) field-effect transistors (FETs) is presented in this dissertation. In optical emitters based on ambipolar long-channel CNT FETs, an analytic diffusive-transport model for various recombination mechanisms is provided for the first time. The relationship and the scaling of emitted light-spot size and emitted optical power are clearly depicted for the first time as well. We also implement a numerical diffusive-transport approach for the light emission, in which the focus is on the effects of radiative and nonradiative recombination in the channel, with the movement of the spatial recombination profile in response to the gate and drain voltages. For the first time, we find that the emitted light-spot size and the emitted optical power depend sensitively on the operative nonradiative recombination mechanisms. We implement a numerical diffusive-transport approach including exciton photogeneration as well for photoconductors based on ambipolar long-channel CNT FETs with uniform and near-field photoexcitation. We show that the photocurrents are typically much smaller than the dark currents, and explain some possible reasons. Moreover, the exciton densities in CNTs are calculated and the effect of exciton diffusion is presented.

The definition of polar codes given by Arikan is explicit, but the construction complexity is an issue. This is due to the exponential growth in the size of the output alphabet of the bit-channels as the codeword length increases. Tal and Vardy recently presented a method for constructing polar codes which controls this growth. They approximated each bit-channel with a “better” channel and a “worse” channel while reducing the alphabet size. They constructed a polar code based on the “worse” channel and used the “better” channel to measure the distance from the optimal channel. This thesis considers the knowledge gained from the perspective of the “better” channel. A method is presented using iterative upgrading of the bit-channels which successively results in a channel closer to the original one. It is shown that this approach can be used to obtain a channel arbitrarily close to the original channel, and therefore to the optimal construction of a polar code. / Graduate / 0984 / 0544 / arash.ghayoori@gmail.com