Free space optics (FSO) technology is proving to be an exceptionally beneficial supplement to conventional Fiber Optics and radio frequency (RF) links. FSO and RF links are greatly affected by atmospheric conditions. Hybrid FSO/RF systems have emerged as a promising solution for high data rate wireless communication. FSO technology can be used effectively in multi-user scenarios to support Point-to-Multi-Point (P2MP) networks. In this work we present and analyse a P2MP Hybrid FSO/RF network that uses a number of FSO links for data transmission from the central node to different remote nodes of the network. A common backup RF link is used by the central node to transmit data to any of the remote nodes in case of failure of any FSO links. Each remote node is assigned a transmit buffer at the central node for the downlink transmission. We deploy a non-equal priority protocol and p-persistent strategy for nodes accessing the RF link and consider the back up RF transmission link with lower frame transmission rates as compared to the FSO link. Under different atmospheric conditions, we also study various performance metrics of the network. We study the throughput from the central node to the remote nodes individually as well as the following: the average transmit buffer size, the frame queuing delay in the transmit buffers, the efficiency of the queuing systems and the frame loss probability. / Graduate

This thesis comprises of two parts. In the first, we improve the performance of existing hybrid FSO/RF communication systems. Conventional hybrid RF and optical wireless communication systems make use of independent and parallel Free Space Optical (FSO) and RF channels to achieve higher reliability than individual channels. This thesis is based on the idea that true hybridization can be accomplished only when both channels collaboratively compensate the shortcomings of each other and thereby, improve the performance of the system as a whole. We believe that optimization on the combined channel capacities instead of the individual channel capacities of the FSO and RF channels can increase the system availability by a large amount. Using analysis and simulation, we show that, by using Hybrid Channel Codes, we can obtain more than two orders of magnitude improvement in bit error rates and many-fold increase in system availability over the currently existing best systems. Simulations also show that the average throughput obtained using the new system is over 35% better when compared to the present systems. The goodput is much higher because of the elimination of data repetition. Also by avoiding data duplication, we preserve to a great extent the crucial security benefits of FSO communications. The second half of the thesis deals with magnetic recording systems. Due to the insatiable and ever-increasing needs of data storage, novel techniques have to be developed to improve the capacity of magnetic recording channels. These capacity requirements translate to improving storage densities and using higher recording rates. For these channels, improvements even in the order of a tenths of a dB have a big impact on the storage densities of the recording device. Recently, LDPC codes have been constructed to achieve the independent and uniformly distributed (i.u.d.) capacity of partial response (PR) channels. The “guess algorithm” has been proposed for memoryless channels, to improve the performance of iterative belief propagation decoding to that of Maximum Likelihood (ML) decoding. In the second part of this thesis, the “guess algorithm” is extended to channels with memory. It is shown using asymptotic density evolution analysis that the gains obtained using this algorithm on these channels are more than those obtained over memoryless channels. The “guess algorithm” is further extended to magnetic recording channels which are characterized by ISI and additive white gaussian noise (AWGN). Simulations show that gains of upto one dB are possible on magnetic recording channels.