Scene Geometry¶

Geometry describes the physical place a radio signal travels through when the propagation backend needs an actual three-dimensional scene. It answers questions such as where the transmitter and receiver are, what buildings or walls are nearby, and which antenna arrays and radio materials the ray tracer should use.

In rfgen, geometry is only part of a Scene when the channel uses site-specific ray tracing. Sionna ray tracing (Sionna RT) consumes the 3D scene and computes radio paths through it; rfgen records which scene asset was used and passes it to the channel backend.

Geometry is required for site-specific ray-traced propagation and rejected for statistical propagation presets that do not consume 3D assets. The scene composer fails fast when geometry and propagation backend are incompatible. See Scenes for the compatibility model.

Source: Sionna RT is a differentiable ray tracer for radio propagation. It is built on Mitsuba 3 scene assets and exposes scene, radio-material, antenna-array, path, and radio-map APIs. The Sionna RT technical report covers the channel impulse response and radio-map algorithms.

What Geometry Does¶

Scene geometry provides:

a Sionna RT scene asset,
transmit (TX) and receive (RX) positions and orientations,
TX and RX antenna arrays,
radio-material assignments, meaning Sionna / Mitsuba material properties used by the ray tracer,
optional movement or Doppler inputs, where Doppler is frequency shift caused by relative motion,
a content hash so the generated dataset records which physical scene was used.

The geometry layer does not implement propagation equations. It prepares assets and metadata for Sionna RT; the channel backend computes paths, delays, angles, Doppler, and complex coefficients.

Source: Sionna RT’s technical report documents ray-tracing algorithms for channel impulse responses and radio maps, including material, antenna, array-geometry, transmitter-position, and receiver-position parameters. rfgen therefore treats geometry as backend input, not as an RF propagation implementation owned by the framework.

Minimal Example¶

These are current configuration fragments rather than standalone programs. See Reference / Scene API for the typed schema and complete examples.

Use a shipped ray-tracing scene:

scene:
  channel_application: per_emitter
  tx_pose:
    position_m: [0.0, 0.0, 1.5]
    orientation_rad: [0.0, 0.0, 0.0]
  multi_rx:
    receivers:
      - rx_id: rx0
        rx_pose:
          position_m: [10.0, 0.0, 1.5]
          orientation_rad: [0.0, 0.0, 3.14159]
  geometry:
    backend: sionna_rt
    rt_solver:
      max_depth: 3
      tx_array: {num_rows: 1, num_cols: 1, pattern: iso, polarization: V}
  assets:
    scene_geometry_uri: sionna://builtin/munich

Here sionna://builtin/munich selects a packaged Sionna RT scene and munich names the city asset. The complete scene must also supply its RF frequency, sample rate, and receiver configuration.

Use a custom scene exported from a mature 3D asset path:

scene:
  geometry:
    backend: sionna_rt
  assets:
    scene_geometry_uri: assets/scenes/campus_quad.xml

Available Scene Sources¶

This table compares asset-authoring routes. OpenStreetMap (OSM) is a map-data source, computer-aided design (CAD) and building information modeling (BIM) are engineering asset formats, and Universal Scene Description (USD) is a 3D scene interchange format used by NVIDIA Omniverse.

Source	Status	Best use	Notes
Sionna RT built-in URI	Supported starter path	Generic urban, street-canyon, and test scenes	Zero asset pipeline; good first choice
OSM via Blender-OSM and Mitsuba-Blender	Recommended custom outdoor path	Specific streets, campuses, intersections	Requires manual cleanup; avoids custom OSM conversion code
CAD / BIM / hand-authored Mitsuba XML	Advanced path	Indoor, rooftop, or controlled environments	Higher effort; needs asset discipline
NVIDIA USD / Omniverse path	Deliberately one-way: rfgen writes USD and refuses to read it	NVIDIA ecosystem scene authoring, and viewing an exported scene plan	A USD asset can be named and digested but not ingested; see External Scene Seams
Procedural city generation	Open question	Synthetic city populations	Candidate libraries not yet evaluated

The framework should not write 3D geometry tooling from scratch. Use Sionna RT, Mitsuba, Blender, OSM, CAD/BIM, or another mature asset path.

Source: Sionna RT consumes Mitsuba 3 scene assets, and Mitsuba documents the scene XML format and plugin-based geometry/material model. The detailed asset-path notes are collected in Reference / Scene Geometry Assets.

Sionna RT Ownership¶

rfgen chooses the scene asset, records its content hash and metadata, and passes the geometry configuration to Sionna RT. Sionna RT then computes the ray-traced propagation paths for each transmitter and receiver pair.

A ray-tracing backend loads a scene asset, applies radio materials and antenna arrays, and computes per-path channel information for each TX/RX pair. In RFGen’s SionnaRT path, the returned channel includes solver attenuation over the configured line-of-sight, specular-reflection, optional diffuse-reflection (including Sionna’s optional diffuse scattering), and refraction paths. RFGen does not expose or pass Sionna diffraction, edge-diffraction, or diffraction-lit-region controls. Adding a separate path-loss model would double count the solver attenuation; this is not a field-calibration claim.

Source: Sionna RT’s documentation and technical report describe channel impulse response computation from scene geometry, radio materials, transmitter and receiver positions, and antenna arrays. Channels / Channel records the same delegation rule for the SionnaRT backend.

Asset and Material Summary¶

Ray-tracing scenes typically contain:

Sionna SceneObjects, meaning loaded mesh objects with radio-material assignments,
transmitter and receiver nodes with positions and orientations,
rfgen RTSolverConfig antenna-array payloads for backend antenna definitions,
scene frequency,
radio materials from Sionna’s ITU-R P.2040 library or custom material definitions. ITU-R P.2040 is a recommendation for building-material propagation properties.

Custom array geometry should use the propagation backend’s antenna-array interfaces rather than bespoke RF array math.

Detailed notes on Mitsuba, Blender/OSM ingestion, radio materials, antenna arrays, and comparable ray tracers live in Reference / Scene Geometry Assets.

Source: Sionna RT exposes scene objects, radio materials, transmitters, receivers, tx_array, rx_array, and scene frequency as scene-level inputs. Its radio-material model includes ITU-R P.2040 material data, and ITU-R Recommendation P.2040 defines frequency-dependent building-material electrical properties for radio-wave propagation.

With and Without Geometry¶

Switching from a statistical scene to a geometric scene is a propagation-backend change. Everything else stays conceptually the same: planning, emitter generation, TX impairments, receiver summation, RX capture, RX hardware, labels, and storage.

The following block is a full-contract skeleton. It shows where the channel propagation backend changes, while the surrounding transmitter and receiver hardware transformations remain the same.

from rfgen.core.pipeline import ChannelPipeline
from rfgen.engine.propagation_sionna import SionnaUMi
from rfgen.engine.propagation_sionna_rt import SionnaRT

# Statistical propagation: no geometry asset; complete scene configuration
# supplies Sionna's typed topology and `channel_application: per_emitter`.
statistical_channel = ChannelPipeline([SionnaUMi()])

# Ray tracing: SionnaRT consumes the typed poses and 3D asset shown above.
geometric_channel = ChannelPipeline([SionnaRT()])

The scene-composition flow is identical in both cases. Choose a statistical propagation backend, such as SionnaUMi, when no geometry asset is needed. Choose a ray-tracing backend, such as SionnaRT, when a 3D scene asset should drive propagation. Generated waveforms, receiver capture, receiver hardware, labels, and storage keep the same record contract.

Backend-specific constructors, asset-resolution helpers, and path conventions belong in Reference / Scene Geometry Assets and the config schema reference.

When to Use Geometry¶

Situation	Requires scene geometry asset?	Recommended source
Generic statistical deployment	No	Statistical propagation preset
Generic urban ray-tracing study	Yes	Sionna shipped city scene
Specific campus, street, intersection, or venue	Yes	OSM/Blender or CAD to Mitsuba XML
Indoor layout with walls and rooms	Yes	CAD/BIM or hand-authored Mitsuba XML
Near-field antenna or circuit-level effects	Not with Sionna RT alone	Use a specialized EM/tooling path; treat as open research

Geometry quality often matters more than fine material tuning. Treat this as a validation hypothesis for each scenario, not a universal law.

Source: the OpenGERT Sionna RT sensitivity study reports channel-statistic sensitivity to building-height and position perturbations, compared with smaller material-parameter perturbations when starting materials are roughly accurate. Treat that result as a scenario-specific heuristic, not as a general RF law.

Open Questions¶

Indoor scene authoring. Matterport, Archicad/Revit, and hand-authored XML workflows need a dedicated verification pass for importing indoor walls, rooms, and material assignments.
Vegetation. Sionna’s shipped ITU material list does not include vegetation; foliage material modeling needs a source-backed policy.
USD / Omniverse interop, inbound. This is a settled refusal rather than an unexplored option: a USD asset handed to the shipped ingest is refused by name, because a USD stage carries visual materials from which permittivity and conductivity cannot be derived. Outbound export ships. A qualified converter would reopen the inbound direction on its own merits; see External Scene Seams.
Mobility thresholds. Practical thresholds for when moving transmitters, receivers, or objects require path recomputation need measurement.

References¶

NVIDIA Sionna RT documentation. https://nvlabs.github.io/sionna/rt/
Ait Aoudia, F. et al. Sionna RT: Technical Report. https://nvlabs.github.io/sionna/rt/tech-report/
Mitsuba 3 documentation. Scene format. https://mitsuba.readthedocs.io/en/stable/src/key_topics/scene_format.html
ITU-R Recommendation P.2040. Effects of building materials and structures on radiowave propagation above about 100 MHz. https://www.itu.int/rec/R-REC-P.2040/
Tadik, S. et al. OpenGERT: Open Source Automated Geometry Extraction with Geometric and Electromagnetic Sensitivity Analyses for Ray-Tracing Propagation Models. https://arxiv.org/abs/2501.06945