Add "SIM7600X 4G Cellular"

SIM7600X-4G-Cellular.md

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+# SIM7600X 4G Cellular + GPS
+
+## Overview
+Waveshare SIM7600X 4G HAT provides cellular connectivity and GPS for outdoor remote operation.
+
+## Hardware
+- **Board:** Waveshare SIM7600X 4G HAT
+- **Module:** SIMCom SIM7600A-H
+- **IMEI:** 868002203507276
+- **FCC ID:** UDV-201710
+- **SIM:** Nano-SIM (physical only, no eSIM)
+- **Status:** Already owned, needs SIM card with data plan
+
+### Specs
+| Parameter | Value |
+|-----------|-------|
+| Network | LTE Cat-4 (150/50 Mbps) |
+| Bands | B2/B4/B5/B12/B13/B17/B25/B26 (North America) |
+| GNSS | GPS + GLONASS + BeiDou + Galileo |
+| GPS Accuracy | ±2.5m CEP |
+| Interface | UART + USB-to-UART |
+| Power | 5V via USB or GPIO header |
+| Current | ~200mA idle, ~500mA peak TX |
+| Antenna | u.FL (LTE) + u.FL (GNSS) |
+
+## What It Enables
+1. **Remote telemetry** — monitor SaltyLab from anywhere over 4G
+2. **Remote control** — send commands when out of WiFi range
+3. **GPS tracking** — outdoor position, ride route logging
+4. **Fallback comms** — if WiFi drops during outdoor operation
+5. **MQTT bridge** — relay sensor data to home base over cellular
+
+## Connection to Orin Nano
+- USB cable from HAT USB port → Orin Nano USB
+- Shows as `/dev/ttyUSB0-2` (3 serial ports: AT, PPP, GPS)
+- Or: GPIO header UART (pins TXD/RXD) → Orin Nano UART
+
+## SIM Card
+- Physical nano-SIM required
+- Recommended: Fido/Koodo prepaid data plan (~$15/mo)
+- Insert in slot on underside of HAT
+
+## Software Integration
+- `gpsd` daemon for GPS NMEA parsing
+- ROS2 `saltybot_cellular` package:
+  - GPS → `/gps/fix` (sensor_msgs/NavSatFix)
+  - 4G managed by NetworkManager / `pppd`
+  - Optional: MQTT over cellular for telemetry relay
+
+## References
+- Waveshare wiki: https://www.waveshare.com/wiki/SIM7600X_4G_HAT
+- AT command manual: SIMCom SIM7600 Series
+- Gitea issue: #58
+