MIMO Channels - Measurements and Analysis
Sammanfattning: Providing increased transmission speed, quality and an efficient spectrum utilization is crucial for high speed wireless services of today and the future. This thesis is a collection of papers that address an important step in that direction -- the utilization of the spatial domain of the wireless channel. Paper I provides a survey of the most important concepts in channel and radio propagation modeling for spatial wireless channels. A couple of key features of channels and radio propagation, which are not sufficiently included in current MIMO models, are also described. Paper II presents the first conclusive experimental evidence of the keyhole effect in wireless MIMO channels, and concludes that the keyhole effect in real wireless scenarios is not probable. Analysis of the limitations due to measurement imperfections for measurement-based capacity calculations and keyhole investigations are also presented. In addition, a bound for the higher eigenmodes as a function of the finite measurement signal-to-noise ratio and multipath component leakage is derived. Paper III presents a comparison of the waterfilling gain in the different domains, i.e., frequency, spatial and joint spatial-frequency waterfilling, and shows that the largest gain resides in the spatial domain. Paper IV investigates the performance of seven antenna subset selection schemes, with and without pre-processing both in a diversity and capacity sense and the influence of antenna selection on diversity in wideband channels. The evaluations are based on measured wireless personal area network (WPAN) channels, covering both line-of-sight (LOS) and non-line-of-sight (NLOS) channels, and synthetic MIMO channel models. Paper V presents results from one of the first measurement campaigns for the double-directional characterization of outdoor-to-indoor wireless propagation channels. Measurements were performed at 5.2 GHz between 53 different receiver locations in an office building, and three "base station" positions on a nearby rooftop. Results for angular-delay profiles, RMS angular spread, and other statistical parameters characterizing delay and angular dispersion are given. Paper VI studies the influence of measurement random-walk phase noise (in terms of Allan variance of the sum of the phase noise of two free-running local oscillators) on the SAGE estimation algorithm, and the random-walk phase noise effect on capacity estimates. Paper VII analyses the impact of receiver antenna horizontal orientation on the channel capacity of a 6×6 MIMO system. It shows that in a "wave guiding" environment such as a long corridor with the presence of a strong LOS, a significant difference in capacity is observed when the orientation of the uniform linear array at the receiver is changed from parallel to perpendicular (to the LOS).
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