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Optics and Raytracing

by Jose Alves

A number of notable ray-tracing techniques exist, particularly for solving problems where the structure is very electrically large (Structure dimension >1000λ). In the following sub sections Iterative Physical Optics, Uniform Theory of Diffraction, Intelligent Ray tracing and Shooting Bouncing Ray will be explained, however many more techniques exist.

Iterative Physical Optics (IPO)

IPO is an iterative high-frequency technique suitable for the evaluation of the interaction between a radiating source and electrically large and very large scattering structures (i.e. whose dimensions are larger than the field wavelength).

Typical applications are: reflector antenna performance assessment (including compact antenna test range), antenna co-siting analysis on large platforms (ships, aircrafts, vehicles), radome performance verification, radar cross section evaluation of complex targets (such as tanks, aeronautical platforms, combat vessels), etc.

IPO was originally developed for analysing the scattering from arbitrarily shaped cavities with perfectly electrically conducting (PEC) walls. It was then extended to the case of impedance boundary conditions and dielectric thin slabs.

The IPO algorithm is based on the application of the equivalence theorem. The equivalent currents on the scattering objects surface are estimated by using the Physical Optics (PO) approximation for both impenetrable (PEC or impedance boundary condition) and penetrable (multilayered electrically thin slabs) objects.

An iterative process reconstructs the interactions between objects without resorting to ray-tracing: equivalent currents backward scattering is used to compute reflection from an object while the forward scattering is used to compute the masking from the same object (i.e. the shadow behind it). At each iteration, further reflection/masking are introduced.

The equivalent currents estimation process is similar to the Geometrical Optics (GO) ray tracking algorithm but the ray tracking operation is replaced by the calculation of the scattered field from the surfaces. Moreover, the diffractive contributions in PO approximation, although not asymptotically correct, avoid the sharp boundaries which are present in the estimate of GO ray-based current.

The computational time can be reduced using GPU acceleration and Fast Far-Field Approximation (FaFFA) algorithm with proper preconditioner. The figure below demonstrates an application of the IPO solver for a Helix antenna installed on an electrically large satellite platform.

Uniform Theory of Diffraction (UTD)

The typical application field of the UTD technique is the evaluation of the performance, such as the pattern distortion, antenna coverage, coupling levels, near-field levels and identification of EMI/EMC problems of installed antennas on larger structures, accounting for the interaction with surrounding structures and with other radiating/receiving systems.

The UTD technique can be used if the interaction between the platform and the antenna is weak. Examples of this are antennas mounted on cars, naval platforms, aeronautical platforms and insatellite communication. The technique is well suited at high frequency regimes where the antenna–platform interaction is small.

UTD is a ray-tracing based technique. The delay, the unit vectors of departure and arrival and the components of the electric field are available for each ray.Therefore, it is suitable for any kind of application which requires the evaluation of the frequency response and of the channel impulse response such as is the case of simulating wireless communications. Other parameters such as the Power Delay Profile, the Delay Spread, the Angle of Arrival (AOA) and the Angle of Departure (AoD) can be derived from the computed responses. The Spreading function can be derived for those applications requiring to account for the relative motion between the transmitting antenna and receiving antenna, applicable in Vehicle to Vehicle (V2V), Vehicle to Infrastructure (V2I) and Vehicle to Everything (V2X) communication. The following figure shows the typical analysis performed by a UTD solver, where a 5G antenna radiation is visualised over a large rural area.

Intelligent Ray tracing

Intelligent Ray tracing (IRT) algorithm tries to combat the excessive computational time that can be associated with computation of multiple ray paths by considering that the deterministic modelling of pathfinding generates a large number of rays, but only a few of them deliver the main part of the received electromagnetic energy. Principle of IRT is that Preprocessing can be performed on the database of the structure which leads to acceleration of the ray path finding for more efficient simulation. Such techniques as well as others like Dominant Path are often deployed to investigate network planning applications, including virtual test drive for cellular, V2X or Radar, where modelling of internal buildings or external streets is required.

Shooting Bouncing Ray

Asymptotic Shooting Bouncing ray (SBR) is another ray tracing method - it combines geometric optics and physical optics, usually to investigate radiation andscattering scenarios. SBR uses geometric optics to describe multiple reflections of electromagnetic waves on the surface of an object, and finally uses physical optics to solve the scattering field when the electromagnetic wave leaves the object. Such techniques are often used to capture large target Radar Cross section information and range profiles. They can also be a useful visualisation tool to understand the length and path of a particular signals launched from the excitation source and subsequently received at a given point, for example the following figure shows the installed automotive radar performance at 77GHz.

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