Efficient ray-tracing algorithms for radio wave propagation in urban environments

摘要

Recent times have seen a significant increase in capacity demands resulting in small cells deployment. The application of standard radio channel models is not straightforward in such environments. The radio wave propagation becomes site-specific as it is greatly affected by the objects present in the environment. udA ray-tracing model computes the dominant paths through which radio waves propagate considering the geometry of the environment. The aim of this work is to develop efficient ray-tracing acceleration techniques for radio wave propagation in urban environments. udA radio wave that interacts with a wall or an edge of a building undergoes reflection or diffraction respectively. The horizontal area where the reflected or diffracted rays can possibly propagate represents the lit region of the illuminated wall or edge.The buildings present in the lit region block the rays to form shadow regions. A valid ray exists at a given receiver location only if it is located inside the lit region and outside of all the shadow regions of an illuminated wall or edge. The lit and shadow polygons are represented by four sides so that a geometrical test requires four computations to validate if a receiver is located inside or outside of a polygon. A recursive algorithm has been developed to validate the rays at all receiver locations. udA mobile receiver enters and leaves the lit regions and the associated shadow regions as it moves along a linear route. The entry and exit points of the receiver through the lit polygons and the associated shadow polygons are pre-computed to produce a database of relative location of polygons intersection along the route. A novel method has been developed that makes use of this database to accelerate the ray computations. The geometrical test to validate a ray segment is replaced by a simple check that compares the receiver location with the polygons intersections along the route.udThe database of visible walls and edges, known as visibility list, is always required for ray tracing computations. The visibility checks must therefore be performed independently each time the transmitter location is changed. This increases the pre-processing time for the ray tracing model and limits its application for mobile transmitter scenarios. A database that pre-computes the list of visible walls and edges to each wall and edge in the environment, or so-called the intra-visibility matrix is proposed. A novel algorithm is developed that determines the visibility list for a mobile transmitter moving along a linear trajectory using the intra-visibility matrix. This work presents ray tracing acceleration techniques for both a mobile receiver and transmitter. The algorithm for mobile receiver has been validated by comparing the path loss against measured data available from COST 231 project. The ray tracing acceleration algorithm for mobile transmitter can reduce the pre-processing time by up to 90% and accurately predicts the ray paths as validated against the COST 231 measured data using the reciprocity principle.