Interference alignment (IA) has been shown to provide all users of an interference channel with half the capacity achievable in an interference free point-to-point link resulting in linear sum capacity scaling with the number of users in the high SNR regime. The linear scaling is achieved by precoding transmitted signals to align interference subspaces at the receivers, given channel knowledge of all transmit-receive pairs, effectively reducing the number of discernible interferers. The theory of IA was derived under assumptions about the richness of the propagation channel; practical channels do not guarantee such ideal characteristics. This paper presents the first experimental study of IA in measured multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) interference channels. We show that IA achieves the claimed scaling factors in a wide variety of measured channel settings for a 3 user, 2 antennas per node setup. In addition to verifying the claimed performance, we characterize the effect of several realistic system imperfections such as channel estimation error, feedback delay, and channel spatial correlation, on sum rate performance. / text

Two-dimensional (2D) and three-dimensional (3D) periodic-lattice-based microstructures have found multifaceted applications in photonics, microfluidics, tissue engineering, biomedical engineering, and mechanical metamaterials. To fabricate functional periodic microstructures, in particular in 3D, current available technologies have proven to be slow and thus, unsuitable for rapid prototyping or large-volume manufacturing. To address this shortcoming, the new innovative field of pattern-integrated interference lithography (PIIL) was introduced. PIIL enables the rapid, single-exposure fabrication of 2D and 3D custom-modified periodic microstructures through the non-intuitive combination of multi-beam interference lithography and photomask imaging. The research in this thesis aims at quantifying PIIL’s fundamental capabilities and limitations through modeling, simulations, prototype implementation, and experimental demonstrations. PIIL is first conceptualized as a progression from optical interference and holography. Then, a comprehensive PIIL vector model is derived to simulate the optical intensity distribution produced within a photoresist film during a PIIL exposure. Using this model, the fabrication of representative photonic-crystal devices by PIIL is simulated and the performance of the PIIL-produced devices is studied. Photomask optimization strategies for PIIL are also studied to mitigate distortions within the periodic lattice. The innovative field of 3D-PIIL is also introduced. Exposures of photomask-integrated, photomask-shaped, and microcavity-integrated 3D interference patterns are simulated to illustrate the richness and potential of 3D-PIIL. To demonstrate PIIL experimentally, a prototype pattern-integrated interference exposure system is designed, analyzed with the optical design program ZEMAX, and used to fabricate pattern-integrated 2D square- and hexagonal-lattice periodic microstructures. To validate the PIIL vector model, the proof-of-concept results are characterized by scanning-electron microscopy and atomic force microscopy and compared to simulated PIIL exposures. As numerous PIIL underpinnings remain unexplored, research avenues are finally proposed. Future research paths include the design of new PIIL systems, the development of photomask optimization strategies, the fabrication of functional devices, and the experimental demonstration of 3D-PIIL.

Approved for public release; distribution is unlimited / Coherent Sidelobe Cancellation (CSLC) is a coherent processing technique that has the potential of reducing noise jamming through the antenna side lobes. Present CSLCs have the capability of reducing the noise jamming by 25 to 35 dB. The maximum number of side lobe jammers that can be handled by a CSLC is equal to the number of auxiliary antennas. The performance of CSLC is governed by nonlinear stochastic differential equations that are not solvable by analytic means. Therefore this thesis employs simulation techniques to solve these equations. The CSLC becomes saturated as the number of jammers in different directions exceeds the number of loops. Jammer multipath adds an additional degree of freedom for each multipath signal that has a direction different than that of the main jammer. The objective of this thesis was to determine the effect that these multipath or hot clutter signals have on a CSLC. It was found that hot clutter produced substantial degradations on single, double and triple CSLCs. The effect was most pronounced for single cancellers where multipath with a magnitude of 1% of the jamming signal reduced the cancellation ratio by 18 dB. Comparable numbers for double and triple cancellers were 11 dB. / Major, Turkish Air Force / Lieutenant Junior Grade, Turkish Navy

Much of the recent growth in microwave communication systems has occurred within urban areas. Private satellite and point to point microwave services have meant that microwave antennas are a common site on the roofs of offices. With the increase in system density comes the increase in the probability of mutual interference between systems sharing the same frequency band. However, the urban location of these systems means that there is also a greater possibility of a building obstructing the interference path, thus providing protection. Because the introduction of microwave systems into an urban environment on such a scale is a relatively recent event, little information was available regarding the effect of a building on a radio path at microwave frequency. This thesis provides a procedure by which the diffracted field in the shadow of a building may be determined. Diffraction models are developed based on Fresnel Integral methods and tested against results obtained from a measurement campaign conducted at a frequency of 11.2 GHz using real buildings as the diffracting obstacles. The diffraction model developed is extended from originally considering a two-dimensional great circle path to one which considers multiple path diffraction, typically via the roof and around the sides of the building. It has therefore been possible to formulate procedures suitable for incorporation into CCIR recommendations whereby the presence of a building may be considered when determining coordination distance and evaluating practical interference threats on specific paths. Finally methods by which the diffraction mechanism investigated may be incorporated into generalised signal strength prediction procedures that would consider other mechanisms, such as building scatter, are put forward.

Orthogonal frequency division multiple access (OFDMA) systems use the same concept of orthogonal frequency division multiplexing, assign different subcarriers to different users for multiple access requirements. Since the received signals from different users have different carrier frequency offsets (CFOs) induced by Doppler effect and frequency mismatching between transmitter¡¦s and receiver¡¦s oscillators, the system performance will be degraded by the multiple access interference (MAI). In this thesis, we present iterative interference cancellation by signal classification (IICSC) and interference cancellation by user and signal classification (ICUSC) algorithm for QPSK and 16QAM modulation schemes. We investigate the method to select significantly reliable signals and then cancel the corresponding MAI from other signals in frequency domain. In 16QAM modulation scheme, the system is more sensitive to MAI. Therefore, we propose a criterion to choose the users with less MAI and then select reliable signals from these users. Since these reliable signals are not required interference cancellation, we can reduce the total number of interference cancellation operations. From simulation results, the performance of our proposed methods is similar to that of parallel interference cancellation method, but the computational complexity of our proposed method is lower.

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