LTE Breakout

Owned by Anthony Luo
Last updated: 2024-09-14 by Daniel Puratich
2 min read

Introduction

Motivation

Provide a lighter weight package that allows for command+control over LTE.

Analysis of Existing Solutions & POR design

For the 2024 competition season, WARG’s quadcopter PEGASUS had ELRS @ 2.4GHz for control (RC link), and LTE over a cellphone for telemetry. For 2025, the Plan of Record setup is to use ELRS 2.4 GHz for control + telemetry; the phone will be removed. This decision was made in order to lighten Pegasus, allowing for longer flight times and greater payload capacity; and the deciding factor was largely the observed “good” Link Stats (Quality + RSSI) at Alma, QC for the 2024 competition and in the testing leading up to it.

Problems with POR design

Although link stats observed from competition + testing may be nominal, ELRS is still a P2P connection with finite range + power. Relays for ELRS are possible, but difficult to implement infrastructure wise. Since competition conditions will always be variable, it is desirable to maintain “options” for connectivity, allowing the most reliable operation even in areas with heavy noise. This is particularly true as the competition is slated to change to new airfields with noise that we have not yet characterized.

Adding a phone or LTE hat for a raspberry pi is difficult because COTS solutions either require complicated driver/encoding/decoding of MAVLink messages, or add substantial amounts of weight.

Common Industry Solutions

It is common in the industry to connect robots to wifi or the internet. Many commercial drones and vehicles are able to have LTE or cellular connectivity using cots modems. Quectel provides relatively low-cost modems that are common in industry, with integrated protocols/interfaces allowing easy cellular connectivity. It is possible even to use quectel modems as a fully integrated endpoint, where all we have to do is send / receive data from the modem and the modem manages all of the networking infrastructure.

Part Selection

Modem:

Quectel EC25A series. (North America bands).

Microprocessor

  • Choose STM for same reason we use STM everywhere else.

  • QFN package or LQFP64

EE Requirements:

  • LQFP64 (easy to solder)

  • SIM Card

EFS Requirements:

  • Prefer SWD over JTAG.

  • 3 Uarts (Ideally 4)

    • 1 for Ardu Comms

    • 1 for EC25 debug UART ← main communication channel for rev1

    • 1 for comms to other devices (if applicable)

    • 1 spare (debug)

  • CAN FD support

    • Migrating our entire product to CAN. CAN FD just faster, why not support if we can?

  • USB OTG FS + HS

    • STM must act as master, support FS / HS.

    • From the Quectel docs:

    • EC25 series module contains one integrated Universal Serial Bus (USB) interface which complies with the

      USB 2.0 specification and supports high-speed (480 Mbps) and full-speed (12 Mbps) modes. The USB

      interface can only serve as a slave device.

stm32 F415 seems to have teh most stock.

 

STM32 F415 pinout configuration

 

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