ADR-0002 — WiFi (not BLE) as the field transport

Status: Accepted · Date: 2026-06-14

Context

The naive design is "wearable advertises over BLE → coach's phone reads it." But BLE range is ~10–30 m and a pitch is ~100×64 m. A coach on the touchline can't receive BLE from a player 80 m away.

Decision

Use WiFi (2.4 GHz) as the field transport: wearables push packets (MQTT) to a broker via one outdoor AP. (4G is the alternative long-range transport.)

Consequences

• + Covers the whole pitch from one AP (line-of-sight ~100 m), handles 10 devices.
• + Enables a true real-time push pipeline to the backend.
• − Requires a live field network (AP + broker + backend) during the session.

Alternatives considered

• BLE to phone — rejected: range can't cover the pitch.
• 4G + IoT SIM (Path B / Traccar) — works at range but adds per-device SIM cost and ~1 Hz / 1–3 s latency; reserved as a fallback.