The objective of this dissertation is to design new architectures for multiple antenna wireless communication systems operating in a mobile-to-mobile environment and to develop a theoretical framework according to which these systems can be analyzed. Recent information theory has demonstrated that the wireless channel can support enormous capacity if the multipath is properly exploited by using multiple antennas. Future communication systems will likely evolve into a variety of combinations encompassing mobile-to-mobile and mobile-to-fixed-station communications. Therefore, we explore the use of multiple antennas for mobile-to-mobile communications. Based on the characteristics of mobile-to-mobile radio channels, we propose new architectures that deploy directional antennas for multiple antenna systems operating in a mobile-to-mobile environment. The first architecture consists of multiple input and multiple output (MIMO) systems with directional antennas, which have good spatial correlation properties, and provides higher capacities than conventional systems without requiring a rich scattering environment. The second one consists of single input and multiple output (SIMO) systems with directional antennas, which improve signal-to-interference-plus-noise ratio (SINR) over conventional systems. We also propose a new combining scheme to select the outputs of optimal combing (SOOC) in this architecture. Optimal combining (OC) is the key technique for multiple antenna systems to suppress interference and mitigate the fading effects. Based on the complex random matrix theory, we develop an analytical framework for the performance analysis of OC. We derive several important closed-form solutions such as the moment generating function (MGF) and the joint eigenvalue distributions of SINR with arbitrary-power interferers and thermal noise. We also analyze the effects of spatial correlations on MIMO OC systems with arbitrary-power interferers in an interference environment. Our novel multiple antenna architectures and the theoretical framework according to which they can be analyzed would provide other researchers with useful tools to analyze and develop future MIMO systems.