Long-Range Communication

Introduction

When a robot's operating range exceeds WiFi/Bluetooth coverage, long-range communication solutions are needed. 4G/5G cellular networks provide high-bandwidth wide-area coverage, LoRa offers ultra-long-range low-power transmission, and RC transmitters provide low-latency real-time control.

4G/5G Cellular Communication

4G LTE Modules

Module	Manufacturer	Download Speed	Bands	Interface	Features
SIM7600CE	SIMCom	150 Mbps	All China carriers	UART/USB	Common in China
SIM7600G-H	SIMCom	150 Mbps	Global bands	UART/USB	Global roaming
EC20	Quectel	150 Mbps	All China carriers	USB/UART	Stable and reliable
EC25	Quectel	150 Mbps	All China carriers	USB	Compact module

Connection Method

    SIM card
      │
 ┌────┴────┐
 │ 4G Module│
 │(SIM7600) │
 └─┬──┬────┘
   │  │
 UART USB ──→ MCU / Linux SBC
   │
   └── AT command control

Basic AT Command Operations

# Check module status
AT          → OK
AT+CPIN?    → +CPIN: READY  (SIM card ready)
AT+CSQ      → +CSQ: 25,0    (Signal strength, 25/31)
AT+CREG?    → +CREG: 0,1    (Registered on local network)
AT+CGATT?   → +CGATT: 1     (GPRS attached)

# Establish PPP connection or use module's built-in TCP/IP stack
AT+CIPOPEN=0,"TCP","server.example.com",8080
AT+CIPSEND=0,12
Hello Robot!

# Provide network interface via USB (recommended)
# Module acts as USB NIC, Linux auto-recognizes as eth1/wwan0

Usage on Linux SBC

# Raspberry Pi / Jetson using 4G module via USB
# Module typically recognized as /dev/ttyUSB0-3 and a NIC interface

# Manage with NetworkManager
sudo nmcli connection add type gsm ifname '*' con-name '4G' apn 'cmiot'
sudo nmcli connection up '4G'

# Check connection
ip addr show wwan0
ping -I wwan0 8.8.8.8

5G Modules

Feature	4G LTE	5G NR
Peak download	150 Mbps	1–10 Gbps
Peak upload	50 Mbps	500 Mbps
Latency	20–50 ms	1–10 ms
Power consumption	Medium	Higher
Module cost	Low	High
Coverage	Wide	Developing

5G's low-latency capability (uRLLC mode can reach 1 ms) opens the possibility for cloud-based robot control.

Application Scenarios

• Remote monitoring: Streaming robot video and status over 4G
• Cloud control: Leveraging cloud computing power for path planning and decision-making
• OTA upgrades: Remote firmware updates
• Data collection: Real-time field test data upload
• Multi-robot coordination: Coordinating multiple robots via a cloud server

LoRa

Principle

LoRa (Long Range) uses CSS (Chirp Spread Spectrum) modulation to achieve ultra-long-range communication at extremely low power:

\[ \text{Chirp signal}: f(t) = f_0 + \frac{BW}{T_s} \cdot t \]

Frequency varies linearly over time (up-chirp/down-chirp), trading spreading gain for transmission distance.

Key Parameters

Parameter	Description	Typical Value
Frequency band	ISM license-free band	433 MHz / 868 MHz / 915 MHz / 470 MHz (China)
Spreading factor SF	Higher = longer range, lower rate	7–12
Bandwidth BW	Signal frequency span	125/250/500 kHz
Coding rate CR	Forward error correction redundancy	4/5, 4/6, 4/7, 4/8
Transmit power	Output power	Max 20 dBm (100 mW)

Data Rate vs. Range

SF	Rate @ BW=125 kHz	Sensitivity	Approximate Range
7	5.47 kbps	-123 dBm	2–5 km
9	1.76 kbps	-129 dBm	5–8 km
12	0.29 kbps	-137 dBm	10–15 km

\[ \text{Link budget} = P_{tx} + G_{ant} - L_{path} > S_{rx} \]

SX1262/SX1268

The Semtech SX1262 is the latest generation LoRa transceiver chip:

Parameter	Value
Frequency range	150–960 MHz
Transmit power	+22 dBm (SX1262) / +15 dBm (SX1261)
Sensitivity	-148 dBm (SF12, 125 kHz)
Current	TX: 118 mA, RX: 4.6 mA, Sleep: 0.6 uA
Interface	SPI
Package	4x4 mm QFN

LoRa Modules

Module	Chip	Band	Interface	Price
EBYTE E22-400T30D	SX1268	410–493 MHz	UART	~30 RMB
EBYTE E22-900T30D	SX1262	850–930 MHz	UART	~30 RMB
RAK4630	SX1262+nRF52840	Multi-band	BLE+LoRa	~60 RMB
Heltec LoRa32 V3	SX1262+ESP32-S3	Multi-band	WiFi+BLE+LoRa	~50 RMB

Arduino LoRa Example

// Using RadioLib library for SX1262
#include <RadioLib.h>

// SX1262 pin definitions
SX1262 radio = new Module(8, 14, 12, 13);  // CS, DIO1, RST, BUSY

void setup() {
    Serial.begin(115200);

    // Initialize LoRa
    int state = radio.begin(
        470.0,    // Frequency MHz
        125.0,    // Bandwidth kHz
        9,        // Spreading factor
        7,        // Coding rate 4/7
        0x12,     // Sync word
        22,       // TX power dBm
        8,        // Preamble length
        0,        // TCXO voltage
        false     // Use LDO
    );

    if (state == RADIOLIB_ERR_NONE) {
        Serial.println("LoRa initialized successfully!");
    }
}

// Sender
void sendRobotStatus() {
    // Pack robot status (lat/lon + battery + status code)
    struct {
        float lat;
        float lon;
        uint8_t battery;
        uint8_t status;
    } data = {39.9042, 116.4074, 85, 0x01};

    int state = radio.transmit((uint8_t*)&data, sizeof(data));
    if (state == RADIOLIB_ERR_NONE) {
        Serial.printf("Sent successfully, RSSI=%.1f dBm\n", radio.getRSSI());
    }
}

// Receiver
void receiveData() {
    uint8_t buf[64];
    int state = radio.receive(buf, sizeof(buf));
    if (state == RADIOLIB_ERR_NONE) {
        Serial.printf("Received %d bytes, RSSI=%.1f, SNR=%.1f\n",
            radio.getPacketLength(), radio.getRSSI(), radio.getSNR());
    }
}

LoRa Application Scenarios

Scenario	Data Content	Frequency	Description
Agricultural patrol vehicle	GPS + status	Every 10 s	Long range, low frequency
Environmental monitoring station	Temp/humidity + pressure	Every minute	Solar powered
Search and rescue robot	Location + distress signal	Event-triggered	Emergency scenario
Drone swarm	Simple commands	1–10 Hz	Beyond visual range control

RC Transmitters

2.4 GHz RC Systems

Traditional hobby RC transmitters are widely used for robot teleoperation:

Brand	Model	Channels	Protocol	Latency	Range
FlySky	FS-i6X	10	AFHDS 2A	~10 ms	1.5 km
FrSky	X9D+	16	ACCESS	~8 ms	2 km
RadioLink	AT9S	12	FHSS	~10 ms	1.5 km
TBS	Crossfire	12	LoRa hybrid	~5 ms	40 km+

Receiver Outputs

Output Type	Description	Application
PWM	One wire per channel, 1–2 ms pulse	Direct servo driving
PPM	All channels multiplexed on one wire	MCU parsing
SBUS	Serial protocol (inverted UART)	Flight controller/MCU
CRSF	High-speed bidirectional serial	TBS Crossfire

SBUS Protocol

SBUS (Serial Bus) was developed by Futaba and has become the standard output for RC receivers:

Parameter	Value
Baud rate	100000 (non-standard)
Data bits	8-bit, even parity, 2 stop bits
Signal level	Inverted (requires inverter or software inversion)
Frame length	25 bytes
Channels	16 (11-bit/channel) + 2 digital channels
Frame rate	14 ms or 7 ms (high-speed mode)

// ESP32 SBUS parsing
#define SBUS_BAUDRATE 100000
#define SBUS_FRAME_SIZE 25

uint16_t channels[16];

void parseSBUS(uint8_t* buf) {
    if (buf[0] != 0x0F) return;  // Frame header

    channels[0]  = ((buf[1]      | buf[2]<<8)                   & 0x07FF);
    channels[1]  = ((buf[2]>>3   | buf[3]<<5)                   & 0x07FF);
    channels[2]  = ((buf[3]>>6   | buf[4]<<2  | buf[5]<<10)     & 0x07FF);
    channels[3]  = ((buf[5]>>1   | buf[6]<<7)                   & 0x07FF);
    // ... continue parsing remaining channels

    // Channel value range: 172-1811, center 992
    // Map to -100%~+100%
}

void setup() {
    // SBUS is inverted UART; ESP32 can use uart_set_line_inverse
    Serial2.begin(SBUS_BAUDRATE, SERIAL_8E2, 16, -1, true);  // Inverted
}

Telemetry Links

Overview

A telemetry link provides bidirectional data transmission between the robot and a ground station, commonly used in drones and unmanned ground vehicles.

Common Solutions

Solution	Band	Range	Bandwidth	Application
Telemetry radio	433/915 MHz	5–20 km	100 kbps	MAVLink telemetry
WiFi relay	2.4/5 GHz	1–5 km	High	Video + telemetry
4G	Cellular	Unlimited	High	Remote monitoring
Video TX	5.8 GHz	1–10 km	20–50 Mbps	FPV video

MAVLink Protocol

MAVLink (Micro Air Vehicle Link) is the standard telemetry protocol in the drone/robot domain:

• Lightweight message protocol
• Supports 256 message types
• CRC checksum
• Compatible with multiple transport layers (UART, UDP, TCP)
• Widely used by flight controllers such as PX4 and ArduPilot

V2X (Vehicle-to-Everything)

Overview

V2X communication is used for outdoor mobile robots (autonomous vehicles, delivery robots) to interact with their surroundings:

Type	Description
V2V	Vehicle-to-Vehicle, collision avoidance coordination
V2I	Vehicle-to-Infrastructure, traffic signals
V2P	Vehicle-to-Pedestrian, safety alerts
V2N	Vehicle-to-Network, cloud services

Technical Standards

Standard	Technology	Latency	Range
DSRC (IEEE 802.11p)	WiFi variant	<10 ms	300 m
C-V2X (3GPP)	Cellular technology	<20 ms	500 m
NR-V2X (5G)	5G sidelink	<3 ms	500 m

Delivery Robot Applications

Long-range communication capabilities needed by outdoor delivery robots:

• Report location and status over 4G/5G
• Receive dispatch commands and route updates
• Remote takeover (teleoperation) upon anomalies
• OTA firmware upgrades

Comprehensive Communication Comparison

Solution	Range	Bandwidth	Latency	Power	Cost	Monthly Fee
4G LTE	Coverage area	150M	30 ms	High	Medium	Yes
5G NR	Coverage area	1G+	5 ms	High	High	Yes
LoRa	2–15 km	50k	100 ms+	Very low	Low	No
RC transmitter	1–2 km	100k	10 ms	Low	Medium	No
Telemetry radio	5–20 km	100k	20 ms	Medium	Low	No

Summary

• 4G/5G provides wide-area high-bandwidth connectivity, suitable for remote monitoring and cloud collaboration
• 5G's low-latency mode opens possibilities for cloud-based real-time control
• LoRa achieves ultra-long-range communication at extremely low power, suitable for low-data-rate scenarios
• RC transmitters provide reliable low-latency real-time control
• SBUS is the standard data protocol from RC receiver to MCU
• MAVLink is the de facto standard for drone/robot telemetry
• Practical systems typically combine multiple communication methods to cover different requirements

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