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Session:Wireless Communications over Rapidly Time-Varying Channels
Time:Tuesday, May 18, 16:38 - 16:55
Presentation: Special Session Lecture
Topic: Special Sessions: Wireless Communications over Rapidly Time-Varying Channels
Authors: Akbar M. Sayeed; University of Wisconsin 
Abstract: The degrees of freedom in a wireless channel determine itsstatistics and capacity. The degrees of freedom in turndepend on the number and spatial distribution of physicalpropagation paths in the environment as well as the signalspace dimensions (number of antennas, bandwidth). In this paper, we use a virtual representation of time- and frequency-selectiveMIMO channels to relate the degrees of freedom to the physical propagation paths. The virtual representation is characterized by virtual channel coefficients in angle-delay-Doppler space. Itinduces a partitioning of paths that reveals the contribution of each path to the degrees of freedom as well as fundamental dependencies in angle, delay and Doppler that constrain the degrees of freedom. As the signal space dimensions increase, the degrees of freedom increase whereas the statistics of the virtual channel coefficients deviate from Gaussian (Rayleigh) to ``specular'' in which the amplitude of the coefficients is deterministic and the channel variations are manifested solely in the phase. Implications of the angle-delay-Doppler dependencies on channel structure and capacity are discussed, which are particularly relevant to ultra-wideband systems.

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