Browsing by Subject "Channel capacity"
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Item Multiple-Input Multiple-Output (MIMO) for multimode optical fiber communication channels(2012-12) Zisman, Sagi; Vishwanath, Sriram; Bank, SethThis thesis evaluates the benefits of Multiple Input Multiple Output (MIMO) techniques on the capacity of Multimode Fiber (MMF) links. Optical MMF MIMO systems take advantage of the spatial diversity present in the multiple propagating paths in multimode fibers. By using multiple lasers at the input facet of the fiber and multiple photodetectors at the output, we show that the capacity of the link is improved from the single device link, hence demonstrating the usefulness of MIMO in such optical systems. An initial simulation of butt-coupling a Vertical Cavity Surface Emitting Laser (VCSEL) to multimode fiber reveals that the placement position of the laser axis with respect to the fiber axis is critical in exciting a large number of modes. More specifically, we show that there exists a tradeoff between total power coupled into the fiber and the number of modes launched. We then consider a mathematical description of the fiber channel and use it to simulate the capacity of a 1x1, 2x2, and 3x3 MIMO links over a statistical ensemble of channel realizations. This simulation reveals that a 2x2 system is capable of approximately a 50% increase in capacity over the 1x1 case while the 3x3 system is capable of approximately an 80% increase. Moreover, we show that the choice of the placement positions on the facets of the fiber affects the channel capacity, thereby implying that an optimal device position exists. We find the optimal device geometry by an exhaustive search and compare the capacities for the optimal geometry and that of a suboptimal one. A capacity tolerance study is then developed that considers perturbations about the optimal device locations and shows that the capacity of a rotated laser plane is over 90% of the capacity of the original device locations. A second perturbation study considers lateral offsets and shows that systems with a higher number of devices show good tolerance with poorer lateral tolerances for systems with less devices. When small lasers and a large grid of possible device locations are used, an exhaustive search for the optimal device location becomes computationally infeasible. We show that the problem of searching for the optimal detector locations while holding the laser positions fixed is submodular. This property allows a greedy algorithm to select the device positions at a small fraction of the computational complexity, however, only guaranteeing that the capacity of the resulting configuration is greater than a (1 - e^-1) fraction of the optimal configuration. We use this technique to compare the exhaustive search and the greedy search for coarse grids, and then exclusively use the greedy algorithm to select a device configuration for a fine grid whereby an exhaustive search is computationally infeasible.Item On the capacity of multi-terminal systems : the interference and fading broadcast channels(2011-05) Jafarian, Amin; Vishwanath, Sriram; de Veciana, Gustavo; Caramanis, Constantine; Voloch, Felipe; Erez, UriA central feature of wireless networks is multiple users sharing a common medium. Cellular systems are among the most common examples of such networks. The main phenomenon resulting from this inter-user interaction is interference, and thus analyzing interference networks is critical to determine the capacity of wireless networks. The capacity region of an interference network is defined as the set of rates that the users can simultaneously achieve while ensuring arbitrarily small probability of decoding error. It is an inherently hard problem to find the capacity region of interference networks. Even the capacity region of a general 2-user interference channel is a prominent open problem in information theory. This work's goal is to derive achievable regions that are improved over known results, and when possible, capacity theorems, for K user interference networks. Another multiuser channel that is commonly found in wireless systems is a broadcast channel. Broadcast channels stand side by side with Interference channels as the two of the most important channels for which capacity results are still not completely known. In this work we develop inner and outer bounds on the capacity region of fading broadcast channels, using which we find a part of the capacity region under some conditions. In summary, this work first presents coding arguments for new achievable rate regions and, where possible, capacity results for K-user interference networks. Second, it provides inner and outer-bounds for a class of fading broadcast channels.