Angle clustering in indoor space-time channels based on ray tracing

Clustering of paths in angle in RF multipath scenarios has been observed in measured data, explicitly used in stochastic propagation models, and shown to have a significant impact on the capacity of MIMO links. A 3D ray tracing is used to characterize the path delays and angles in a rectangular building in which all interior walls are parallel and perpendicular to the exterior walls. Path delays and angles are recorded for 200 random transmitter and receiver locations, maintaining constant range and throughout a particular building model. The main contribution of this paper is the observation that when all of the path angles from all 200 trials are measured relative to one building wall and collectively analyzed in a histogram, clustering is clearly apparent. Furthermore, the mean angles of the clusters are a function of the range between the transmitter and receiver. While this angle clustering in the ensemble of channels is not the same as clustering in a single channel trial, it may partially account for the clusters observed in single trials. As a practical matter, these results may impact the use and placement of directional antennas for wireless LANs in buildings of this type. By simply measuring the angles relative to the line-of-sight (LOS) path rather than the building wall, the data is shown to be consistent with the joint probability density function (PDF) of delay and angle based on the geometrically based elliptical model (GBEM).