Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item APPLICATION SPECIFIC PRECISION ANALYSIS OF CHOLESKY DECOMPOSITION IN MMSE MIMO RECEIVER SYSTEMS(2010) Ikram, Muhammad Umer; Petrov, Peter D; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We conduct an exploration study of various bit precisions for Cholesky decomposition. This research focuses on obtaining the minimum required signal to noise ratio (SNR) in Cholesky decomposition by reducing the internal precision of the computation. Primary goal of this research is to minimize resources and reduce power by performing calculations at a lower internal precision than the full 32-bit fixed or floating point. Cholesky decomposition is a key component in minimum mean square error (MMSE) multiple-input multiple-output (MIMO) receiver systems. It is used to calculate inverse of a matrix in many modern wireless systems. Cholesky decomposition is a very computation heavy process. We have investigated the effects of internal bit precisions in Cholesky decomposition. This is an exploration study to provide a benchmark for system designers to help decide on the internal precision of their system given SNRline, signal and noise variances, required output SNR and symbol error rate. Using pseudo floating point to control internal bit precision we have simulated Cholesky decomposition at various internal bit precisions with variable signal and noise variances, and SNRline values. These simulations have provided SNR for lower triangular matrix L, its inverse L-1, and the solution vector x (from the matrix equation Ax = b). In order to observe the effects of various bit precisions on SNR and symbol error probability, SNR in L and L-1 are plotted against condition number for 2x2, 4x4, 8x8, and 16x16 input matrices, and loss in symbol error probability (Psym) is plotted against condition number for 4x4 matrices for QPSK, 16QAM and 64QAM constellations. We find that as the internal precision is lowered there is a loss in SNR for L and L-1 matrices. It is further observed that loss in symbol error rate is negligible for internal bit precisions of 28 bits and 24 bits in all constellations. The loss in symbol error rate begins to show at 20 bits of precision and then increases drastically, especially for higher SNRline. These results provide an excellent resource for system designers. With these benchmarks, designers can decide on the internal precision of their systems according to their specifications.Item DIFFERENTIAL MODULATION FOR BROADBAND SPACE-TIME/COOPERATIVE WIRELESS COMMUNICATIONS(2006-10-24) Himsoon, Thanongsak; Liu, K. J. Ray; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Among various diversity techniques to combat fading in wireless channels, spatial diversity through MIMO coding scheme is an effective way to increase link capacity and system reliability without sacrificing bandwidth efficiency. Recently, cooperative diversity has been introduced as an efficient alternative to improve system performance without the requirement of additional antennas. However, most of existing works on MIMO and cooperative communications are based on an assumption that the destination has perfect knowledge of channel state information of all transmission links and hence introduces high complexity to the receiver. To overcome such problems, this thesis proposes differential modulation schemes for space-time coded MIMO and cooperative communications. By exploiting spatial/cooperative diversity without the requirement of channel state information, the proposed schemes provide an excellent tradeoff between receiver complexity and system performance. First, a matrix rotation based signal design for differential space-time modulation is investigated to minimize the union bound on block error probability. Next, a robust differential scheme for MIMO-OFDM systems is proposed by which the signal transmission of each differentially encoded signal is completed within one OFDM block rather than multiple blocks as in existing works. Then, a differential scheme for UWB systems employing MIMO multiband OFDM is proposed to explore all available diversities by jointly encoding across spatial, temporal, and frequency domains. To exploit cooperative diversity, an amplify-and-forward differential cooperative scheme and a threshold-based decode-and-forward differential cooperative scheme are proposed. The proposed differential cooperative schemes are first considered in a two-node cooperation system, and the proposed works are extended to a general multi-node scenario. Finally, a general framework to improve lifetime of battery-operated devices by exploiting cooperative diversity is proposed such that the device lifetime can be greatly improved by efficiently taking advantages of both different locations and energy levels among distributed nodes in wireless networks.Item Cross-Layer Design for Multi-Antenna Ultra-Wideband Systems(2005-11-28) Siriwongpairat, Wipawee; Liu, K. J. Ray; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ultra-wideband (UWB) is an emerging technology that offers great promises to satisfy the growing demand for low cost and high-speed digital wireless home networks. The enormous bandwidth available, the potential for high data rates, as well as the potential for small size and low processing power long with low implementation cost, all present a unique opportunity for UWB to become a widely adopted radio solution for future wireless home-networking technology. Nevertheless, in order for UWB devices to coexist with other existing wireless technology, the transmitted power level of UWB is strictly limited by the FCC spectral mask. Such limitation poses significant design challenges to any UWB system. This thesis introduces various means to cope with these design challenges. Advanced technologies including multiple-input multiple-output (MIMO) coding, cooperative communications, and cross-layer design are employed to enhance the performance and coverage range of UWB systems. First a MIMO-coding framework for multi-antenna UWB communication systems is developed. By a technique of band hopping in combination with jointly coding across spatial, temporal, and frequency domains, the proposed scheme is able to exploit all the available spatial and frequency diversity, richly inherent in UWB channels. Then, the UWB performance in realistic UWB channel environments is characterized. The proposed performance analysis successfully captures the unique multipath-rich property and random-clustering phenomenon of UWB channels. Next, a cross-layer channel allocation scheme for UWB multiband OFDM systems is proposed. The proposed scheme optimally allocates subbands, transmitted power, and data rates among users by taking into consideration the performance requirement, the power limitation, as well as the band hopping for users with different data rates. Also, an employment of cooperative communications in UWB systems is proposed to enhance the UWB performance and coverage by exploiting the broadcasting nature of wireless channels and the cooperation among UWB devices. Furthermore, an OFDM cooperative protocol is developed and then applied to enhance the performance of UWB systems. The proposed cooperative protocol not only achieves full diversity but also efficiently utilizes the available bandwidth.