Indoor propagation models simplify a complicated electromagnetic environment. The point is not to predict every multipath null. The point is to make assumptions explicit enough that an engineer can compare placements, identify risk, and decide what must be measured.
Start with a link budget
A basic received-power estimate is:
Received power = transmit power + transmitter gain + receiver gain − path loss − other losses
All terms are in dB-family units, so they add and subtract. Use the link-budget calculator to work through a specific case.
Free-space path loss is a reference, not an indoor building
ITU‑R P.525 gives free-space basic transmission loss as 20 log10(4πdf/c), where d is distance, f is frequency, and c is the speed of light. Doubling distance adds about 6.02 dB of free-space loss. Doubling frequency also adds about 6.02 dB.
Free space contains no walls, furniture, people, diffraction around corners, or building-specific multipath. It is useful as a reference and for clear outdoor paths, but it should not be described as a complete indoor model.
A clear-path indoor model changes the distance term
The 3GPP indoor-hotspot office LOS expression in TR 38.901 uses 32.4 + 17.3 log10(d3D) + 20 log10(fc), with distance in metres and frequency in GHz. Its distance exponent is lower than free space, representing the corridor-like waveguiding present in the underlying scenario.
SiteRF uses clear-path expressions separately from geometry. This separation matters: a model that already includes generic obstruction or clutter loss would double-count walls if explicit boundary losses were added again.
Walls should be explicit
For a ray that crosses a modeled boundary, SiteRF subtracts the material’s band-resolved transmission loss. An opening replaces a span of wall and can carry its own material. This makes the geometry inspectable: you can identify which boundary affected a path rather than hiding all obstruction inside one empirical coefficient.
Material loss is not a universal constant. Construction, moisture, reinforcement, incidence angle, frequency, and installation details matter. Treat library values as planning inputs and record site-specific overrides. The RF attenuation reference shows how widely published measurements of the same material disagree, with the source, thickness, and frequency of every specimen.
Three-dimensional distance still matters
A plan is two-dimensional, but transmitter and receiver heights contribute to the three-dimensional path length. Near an access point, height separation can dominate horizontal distance. Use a receiver height that represents the actual device class and a mounting height that represents the installation.
What the model omits
A practical planning model may omit or simplify fine-grained multipath, dynamic occupancy, diffraction, polarization mismatch, cable and connector variation, interference from unmodeled transmitters, and device-specific receiver behavior. That is not a reason to avoid prediction; it is a reason to state the limits and validate consequential designs.
A defensible workflow
- Calibrate the plan against a known dimension.
- Enable only real RF boundaries.
- Assign materials and openings explicitly.
- Use realistic radio, antenna, height, and receiver inputs.
- Inspect model-range diagnostics.
- Compare alternatives with the same assumptions.
- Validate the chosen design in the field.
For a quick reference calculation, use the free-space path loss calculator. To interpret the result visually, continue with How to read RF coverage heatmaps.