GO Blockage Technical Note

In SBR+, rays are used to transport fields from the source to the scattering geometry (first bounce) and between different surfaces of the scattering geometry (multi-bounce). However, rays are not used to propagate incident or scattered fields to field observers, whether far-field, near-field, or receiving antennas. Rays are only used to paint equivalent currents on the scattering geometry. Once these currents are determined, SBR+ rays serve no further purpose and are discarded. The painted currents are then radiated in free space to all field observers to establish the scattered field contribution. The scattered field coherently combined with the incident field from the source, also propagated in free space, yields the total (i.e., measurable) field at all defined field observers.

Since propagating the source incident field and radiating the scattered fields to field observers is performed in free space, these operations of the SBR+ algorithm are not mediated by rays. This means that SBR+ applies a physical optics (PO) blockage model: total-field shadows cast by the scattering geometry relative to the field source emerge through destructive interference of the incident and scattered fields. PO blockage not only occurs in connection with the incident field from the source, but also within the scattered field itself. Scattered fields from the previous bounce are diminished behind a blocking surface via destructive interference when the reflecting or transmitting rays from the previous bounce hit that surface and the equivalent currents painted by the incident ray fields at the current bounce are radiated as part of the scattered field contribution.

Because the PO blockage model depends on destructive interference to recover the total-field shadow cast by an obstruction, the PO blockage model requires accurate currents over the extent of the obstruction. This can be hard to achieve in the Geometrical Optics (GO) approximation inherent in a ray propagation model. For example, the obstruction may not be well illuminated by direct rays from the field source or indirect multi-bounce rays after an earlier reflection or transmission. Even small inaccuracies in the painted equivalent currents can spoil the coherent cancellation mechanism needed to yield a robust shadow. An alternative is to invoke the SBR+ GO blockage model. In this model, the scattering contribution of a painted equivalent current to a field observer is included after passing a line-of-sight (LoS) check. This check is performed using the same geometric ray tracing engine that deploys the SBR+ rays. The GO blockage model can be more accurate than the default PO blockage formulation.

In the GO blockage model, it is ambiguous whether a field observer on the opposite side of a thin surface relative to the reflected/transmitted ray, but otherwise having line-of-sight path to the hit point, is blocked. The SBR+ GO blockage model provides an option to resolve this ambiguity. If Surface Self-Block is switched on (true), then any observation points in the opposite half-space of the hit surface with respect to the reflected/transmitted ray will be blocked. If Self-Block is switched off (false), then only a different surface (or different part of the same surface if the surface is curved) can cause blockage.

When GO blockage is used, blockage of the incident field from the field source by directly illuminated surfaces of the scattering geometry is computed by the PO cancellation method, but scattered fields arising from interacting with the geometry are added subject to a line-of-sight blockage check to the field observers. When GO blockage is enabled, PTD, UTD and Creeping Wave radiation are also subject to a line-of-sight blockage check. It is recommended to use the GO blockage feature for SBR-only simulations, as the GO blockage effects for other diffraction mechanisms might not produce accurate results.

The GO blockage model has a non-trivial computational cost relative to the default PO blockage model. This is because many LoS test rays must be shot: one from each ray hit point to each field observer. However, when the GPU is enabled for the analysis, this additional cost is substantially mitigated since the ray tracing operations of these LoS blockage checks are performed very efficiently on the GPU.