Indoor Radio Network Planning:
DAS, Small Cells, and 5G

Why Indoor Coverage Is Critical

The outdoor-to-indoor penetration problem is getting worse with every building generation. Modern energy-efficient buildings are designed with low-emissivity (Low-E) glass, metal cladding, and concrete/steel structural elements that attenuate radio signals by 15–35 dB. A 5G NR signal from an outdoor macro at 3.5 GHz loses roughly 20–30 dB passing through the façade of a modern office tower — reducing an excellent outdoor signal to marginal or unusable coverage at depth inside the building.

The business case for dedicated indoor coverage systems is straightforward: enterprise customers and high-value subscribers spend the majority of their day indoors (offices, shopping centers, hospitals, hotels). Providing excellent indoor coverage is directly correlated with subscriber retention in the enterprise and premium consumer segments.

Indoor Radio Architectures

Three primary architectures address indoor coverage, each with different cost profiles, deployment complexity, and performance characteristics:

Distributed Antenna Systems (DAS)

DAS distributes a base station's signal through a network of passive (coaxial cable + splitters + antennas) or active (fiber + remote radio heads + antennas) components across a building. The base station — typically co-located in a basement equipment room — provides the radio interface, while the distributed antenna network brings the signal to every floor and zone.

Passive DAS (coaxial) is cost-effective for small to medium buildings but suffers from high cable loss over long runs — limiting antenna density at range. Active DAS (fiber) eliminates cable loss and supports much larger buildings (hospitals, airports, stadiums) with hundreds of antenna ports, but requires active remote units at each distribution point, increasing cost and maintenance complexity. Multi-operator DAS shares a single antenna infrastructure across multiple MNOs, reducing per-operator cost in neutral-host deployments.

Small Cells and Femtocells

A small cell is a self-contained base station — integrating the radio, baseband, and antenna in a single unit — deployed indoors on ceilings or walls. Unlike DAS (which extends an outdoor macro's signal), small cells are independent radio nodes with their own cell IDs, requiring proper frequency planning and inter-cell coordination to avoid interference with neighboring small cells and the overlying outdoor macro layer.

Enterprise-grade small cells (Femto/Enterprise Pico) connect via the building's IP network to a small cell gateway (HeNB-GW for LTE, ng-eNB for 5G NSA). Deployment is straightforward — one power + Ethernet cable per unit — but the backhaul capacity and reliability of the building's IP network becomes a critical dependency. Each small cell typically covers 1,000–3,000 m² at 100–500 mW transmit power.

Indoor Base Stations (IBS)

An IBS (Indoor Base Station) is a full-capability base station — identical in radio functionality to an outdoor macro — installed indoors at higher power than small cells (up to 2 W per antenna port). IBS systems use dedicated fiber fronthaul to a central baseband unit and are deployed in very large venues (airports, convention centers, stadiums) where small cell coverage range is insufficient and DAS cable installation is impractical. Modern 5G IBS systems support massive MIMO (32T32R or 64T64R) indoors, enabling spatial multiplexing in high-density crowd scenarios.

Indoor coverage floor plan showing three access points, their coverage radials, and a dead zone in the server room that needs an additional AP.

Indoor Propagation Characteristics

Wall and Floor Penetration Loss

Every wall or floor between transmitter and receiver adds penetration loss on top of the distance-based path loss. The magnitude depends on the material: plasterboard partitions add 3–5 dB, brick walls 8–12 dB, reinforced concrete 15–20 dB, and Low-E glass 20–30 dB. Multi-floor loss is even higher: a concrete floor with steel reinforcement between transmitter and receiver adds 10–15 dB per floor penetrated. Indoor link budgets must explicitly account for the number and type of obstacles on the dominant propagation path.

Multi-path and Reflections

Inside buildings, the propagation environment is rich in reflections and diffraction paths from walls, furniture, and equipment. Unlike outdoor propagation where the line-of-sight path dominates, indoor propagation is often dominated by reflected and diffracted paths — particularly in long corridors (waveguide effect, path loss exponent ~1.7) and around corners (diffraction-dominated). This multi-path richness also enables spatial multiplexing in MIMO systems — indoor 5G MIMO typically achieves better spatial multiplexing gain than outdoor because the diverse scattering environment creates well-separated spatial channels.

Indoor Link Budget

An indoor link budget extends the standard terrestrial link budget with additional loss terms specific to the indoor environment. The key additions: floor penetration loss (if the transmitter is on a different floor), wall penetration loss (per material type and count), indoor shadowing margin (typically 5–8 dB for 95% indoor location probability — lower than outdoor because the environment is more predictable).

Link budget structure. For indoor planning, Building Penetration Loss is replaced by Wall/Floor Penetration Loss terms specific to the path to each coverage zone.

Indoor link budgets drive antenna density decisions: given a target RSRP at the worst-case coverage point (typically the center of the most distant room on the most attenuated path), the maximum allowable path loss determines the maximum antenna spacing. In a typical office environment at 3.5 GHz, antennas must be spaced no more than 15–20 m apart to maintain −90 dBm RSRP with 95% location probability.

Indoor Planning Methodology

A structured indoor planning project follows a defined sequence:

5G Indoors: Challenges and Opportunities

5G NR at FR1 (3.5 GHz) faces more severe indoor propagation challenges than LTE at 1800 MHz: path loss at 3.5 GHz is 6 dB higher than at 1800 MHz at the same distance, and penetration loss through modern building materials is 5–10 dB higher. This means 5G indoor systems require higher antenna density than equivalent LTE systems — roughly 1.5–2× more antenna points for the same coverage quality.

The 5G advantage indoors comes from capacity rather than coverage: 5G NR's wider spectrum bandwidths (100 MHz at 3.5 GHz vs 20 MHz for LTE), spatial multiplexing gains from MIMO, and network slicing for enterprise use cases (URLLC for manufacturing automation, eMBB for AR/VR) create a compelling case for dedicated indoor 5G investment in high-value enterprise buildings. A single 5G IBS sector with 64T64R serving a 2,000 m² open-plan office can provide multi-Gbps aggregate throughput to hundreds of simultaneous users.

NEXT GIS Indoor Planning

NEXT GIS supports indoor planning through floor plan import (CAD DXF, GeoTIFF, PDF raster), per-room material assignment (wall types, floor types), and indoor propagation modeling using empirical multi-wall models (modified COST 231 Multi-Wall) or ray-tracing-based models for complex geometries. Antenna placement is optimized iteratively: the planner positions antennas manually on the floor plan, runs the coverage calculation, and adjusts until all coverage targets are met.