📡 Distributed Antenna System (DAS) Design for In-Building Wireless

Why DAS Is Needed

Modern buildings present a hostile environment for outdoor macro-cell radio signals. Reinforced concrete, low-E glass, and metallic HVAC structures collectively attenuate signals by 20–35 dB. Underground parking garages, tunnels, and basements may receive no usable outdoor signal whatsoever. A Distributed Antenna System (DAS) solves this by distributing RF energy from a head-end signal source through a network of cables and small indoor antennas, achieving uniform coverage where a single macro antenna cannot.

Passive DAS Architecture

A passive DAS uses only passive RF components — coaxial cable, splitters, combiners, directional couplers, and antennas — to distribute signal from a BTS (base transceiver station) or signal booster. No active amplification exists in the distribution network itself. Components:

• Signal source: Dedicated BTS, small cell, or bi-directional amplifier (BDA)
• Trunk cable: Low-loss 7/8" or 1-5/8" HELIAX from head-end to riser
• Splitters/combiners: 2-way (3.5 dB insertion loss), 4-way (7 dB), 8-way (10.5 dB)
• Distribution cable: LMR-400 or 1/2" HELIAX to each antenna
• Indoor antennas: Omni ceiling-mount, typically 2–3 dBi gain, 50Ω

Passive DAS link budget example (800 MHz, single floor):

• BTS output power: +33 dBm
• Trunk cable loss (20 m × 0.065 dB/m LMR-400): −1.3 dB
• 4-way splitter: −7.0 dB
• Branch cable loss (15 m × 0.065 dB/m): −0.98 dB
• Antenna port power: 33 − 1.3 − 7.0 − 0.98 = +23.7 dBm

FCC Part 90 and carrier agreements typically limit radiated power to +24 dBm or less per antenna port to control interference. The radiated EIRP adds the antenna gain: 23.7 + 2.15 = 25.85 dBm EIRP. At 15 m from an omnidirectional antenna in free space (FSPL ≈ 58 dB at 800 MHz), the received signal would be approximately −32 dBm — far above any LTE or public safety receiver sensitivity.

Active DAS Architecture

Active DAS introduces amplification at distributed remote units (RUs), enabling transport over optical fiber rather than RF coaxial cable. Signal chain:

1. Head-end unit (HEU) / Master unit (MU): Accepts RF input from BTS, converts to optical or digital baseband for fiber transport
2. Fiber backbone: Single-mode or multimode fiber; digital DAS uses Ethernet transport (CPRI, eCPRI, or proprietary)
3. Remote unit (RU): Converts optical/digital back to RF; contains power amplifiers (typically +24 to +33 dBm output); connects to antennas via short coax runs

Active DAS decouples coverage area from signal-source location. RUs can be placed hundreds of metres from the MU with negligible signal degradation. Enterprise-scale active DAS platforms (CommScope ION-E, JMA TEKO, Corning ONE) support 4–12 simultaneous carriers across multiple operators and frequency bands from a single fiber plant.

Multi-Carrier, Multi-Band Design

Modern neutral-host DAS must support simultaneous operation of multiple mobile network operators (MNOs) on different frequency bands. A typical mid-size venue (500,000 sq ft) might require:

• Band 14 (758–768/788–798 MHz) — FirstNet public safety
• Band 12/17 (700 MHz) — AT&T, T-Mobile LTE
• Band 5 (850 MHz) — legacy 3G voice
• Band 4/66 (AWS, 1700/2100 MHz) — LTE data
• Band 25/41 (1900 MHz / 2.5 GHz) — Sprint/T-Mobile legacy
• n77/n78 (3.5 GHz) — 5G NR mid-band

Passive splitters and directional couplers are broadband (typically 380–2700 MHz or with separate 3.5 GHz paths), so a single coax plant can simultaneously carry all bands. Active DAS platforms handle band combining digitally at the MU.

Neutral-Host DAS

A neutral-host DAS is owned and operated by a venue or third-party infrastructure provider (e.g., ExteNet, Boingo, Tillman Infrastructure) and leases capacity to multiple MNOs. The MNO connects its BTS or small cell to the head-end and pays a capacity fee. Neutral-host arrangements require careful RF isolation between operators — typically > 30 dB port-to-port isolation in the combining network — and SLA-driven performance monitoring. The FCC does not require individual neutral-host DAS licenses, but each MNO's BTS must operate under the MNO's existing spectrum license.

DAS for 5G NR

5G NR introduces new challenges for DAS. Sub-6 GHz 5G (n77/n78 at 3.3–3.8 GHz) is the most DAS-compatible. mmWave 5G (n258/n261 at 26/28 GHz) cannot be practically distributed over coax beyond a few metres and requires purpose-built mmWave RUs at very close antenna spacing (every 10–15 m). The high bandwidth of 5G NR (100–400 MHz channel bandwidth) demands wideband amplifiers and antennas. Some active DAS platforms support 5G NR via eCPRI (Enhanced CPRI) transport, carrying IQ samples over standard Ethernet at 25 Gbps per RU.

Design Tools: iBwave

iBwave Design is the industry-standard software for DAS and small-cell in-building planning. It imports building floor plans (CAD/BIM), places antenna nodes, and runs a 3D ray-tracing propagation model to predict coverage. The tool generates bill of materials, cable schedules, and RSSI/SINR coverage maps. iBwave Unity extends this to multi-floor, multi-building campus environments. Commissioning engineers use iBwave Site (mobile app) for walk-test data collection and map overlay. Key outputs from iBwave: per-antenna port power levels, predicted RSSI and SINR maps, active DAS configuration files, and DAS link budget reports compliant with carrier acceptance criteria.

Commissioning and Acceptance Testing

DAS commissioning involves: (1) Passive sweep — measure port-to-port insertion loss and VSWR across all frequency bands before connecting active equipment; (2) Passive intermodulation (PIM) test at each antenna port at ≥ +43 dBm two-carrier stimulus, verifying < −153 dBc PIM products (3GPP threshold); (3) Drive test / walk test with test UE or scanner to verify RSSI ≥ −85 dBm (typical carrier requirement) throughout the coverage zone; (4) Data throughput verification (LTE PDSCH throughput ≥ carrier minimum). For public safety DAS subject to IFC Chapter 51, AHJ acceptance test requires ≥ −95 dBm in all critical areas with uplink SNR verification on the donor network.