1100  LoRaWAN Review: Physical Layer and Modulation

1100.1 Learning Objectives

By the end of this chapter, you will be able to:

  • Explain LoRa Modulation: Understand chirp spread spectrum and its advantages
  • Analyze Spreading Factor Trade-offs: Calculate airtime, range, and battery impact for SF7-SF12
  • Select Optimal Bandwidth: Choose between 125, 250, and 500 kHz for different applications
  • Calculate Link Budget: Determine required SF based on distance and environment
  • Distinguish LoRa from LoRaWAN: Clearly separate physical layer from MAC layer concepts

1100.2 Prerequisites

Required Chapters:

Related Review Chapters:

Chapter Focus
Architecture & Classes Review Network topology, device classes
Security & ADR Review Encryption, adaptive data rate
Deployment Review Regional parameters, TTN, troubleshooting

Estimated Time: 15 minutes

What is LoRa? LoRa (Long Range) is a physical layer modulation technique using “chirps” - signals that sweep across frequencies. Think of it like a slide whistle that goes from low to high pitch.

Why Chirps? - Chirp signals are very resistant to interference - Can be received even when the signal is weaker than the noise floor - Multiple chirp “speeds” (spreading factors) allow range/speed trade-offs

Simple Analogy: Imagine shouting across a canyon. You can whisper quickly (high data rate, short distance) or yell slowly (low data rate, long distance). LoRa lets you choose how to “shout” based on your needs.

1100.3 Quick Reference Card

Quick Reference | Review Topic

1100.3.1 Essential LoRaWAN Parameters

Parameter Typical Value Notes
Frequency Bands 868 MHz (EU), 915 MHz (US), 433 MHz (Asia) Region-specific ISM bands
Range 2-15 km (urban), 15-45 km (rural) Line of sight dependent
Data Rate 0.3 - 50 kbps Spreading factor dependent
Battery Life 5-10+ years With duty cycling and Class A
Payload Size 51-222 bytes SF and region dependent
Max TX Power 14-27 dBm Region regulations apply
Gateway Capacity 1000s of devices Per gateway, SF orthogonality
Security AES-128 End-to-end encryption

1100.3.2 LoRa vs LoRaWAN

Aspect LoRa LoRaWAN
Layer Physical (PHY) MAC/Network
Function Modulation technique Protocol stack
Defines Radio parameters, chirp spread spectrum Device classes, security, network topology
Proprietary Yes (Semtech IP) No (LoRa Alliance standard)
Use Point-to-point or mesh Star-of-stars network

1100.4 Spreading Factor Trade-offs

1100.4.1 Spreading Factor Progression

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graph LR
    SF7[SF7<br/>5.5 kbps<br/>41 ms airtime<br/>Shortest range]
    SF8[SF8<br/>3.1 kbps<br/>72 ms<br/>↓]
    SF9[SF9<br/>1.8 kbps<br/>144 ms<br/>↓]
    SF10[SF10<br/>980 bps<br/>247 ms<br/>↓]
    SF11[SF11<br/>537 bps<br/>494 ms<br/>↓]
    SF12[SF12<br/>293 bps<br/>988 ms<br/>Longest range]

    SF7 -->|Better Data Rate| SF8
    SF8 --> SF9
    SF9 --> SF10
    SF10 --> SF11
    SF11 -->|Better Range| SF12

    style SF7 fill:#27AE60,color:#fff
    style SF8 fill:#2ECC71,color:#fff
    style SF9 fill:#F39C12,color:#fff
    style SF10 fill:#E67E22,color:#fff
    style SF11 fill:#E74C3C,color:#fff
    style SF12 fill:#C0392B,color:#fff

Figure 1100.1: LoRaWAN Spreading Factor Progression: SF7 to SF12 Trade-offs

{fig-alt=“LoRaWAN spreading factor trade-off progression from SF7 to SF12. SF7 offers highest data rate (5.5 kbps) and shortest airtime (41 ms) but shortest range shown in green. Progresses through SF8 (3.1 kbps, 72 ms), SF9 (1.8 kbps, 144 ms), SF10 (980 bps, 247 ms), SF11 (537 bps, 494 ms), to SF12 with lowest data rate (293 bps) and longest airtime (988 ms) but longest range shown in red.”}

This chart shows energy consumption per byte: SF12 uses 24x more energy than SF7, making SF selection critical for battery-powered devices.

1100.4.2 Detailed Spreading Factor Comparison

SF Data Rate (EU868) Airtime (51B) Range Factor Battery Impact Capacity Impact
SF7 5470 bps 41 ms 1x (baseline) Best High capacity
SF8 3125 bps 72 ms 1.6x Good Good
SF9 1757 bps 144 ms 2.5x Fair Fair
SF10 980 bps 247 ms 4x Poor Low
SF11 537 bps 494 ms 6x Very Poor Very Low
SF12 293 bps 988 ms 10x Worst Severely Limited
NoteUnderstanding Spreading Factors

Key Principle: Higher SF = more chips per symbol = better noise immunity = longer range BUT slower data rate and longer airtime.

Orthogonality: Different SFs can coexist on the same frequency channel without interfering, allowing gateway multiplexing.

Trade-off Example: A message taking 41ms at SF7 takes 988ms at SF12 (24x longer airtime), consuming 24x more battery per transmission.

1100.4.3 Bandwidth Options

Bandwidth Common Use Data Rate Impact Range Impact
125 kHz Standard LoRaWAN Baseline Maximum range
250 kHz Higher throughput 2x faster Reduced ~10%
500 kHz Low latency 4x faster Reduced ~20%

1100.5 Chirp Spread Spectrum Explained

1100.5.1 How CSS Works

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graph TB
    subgraph "Chirp Spread Spectrum Encoding"
        SYMBOL["Data Symbol<br/>(e.g., '5')"]
        CHIRP["Chirp Signal<br/>(Frequency Sweep)"]
        START["Start Frequency<br/>(Based on symbol)"]
        SWEEP["Sweep Up to<br/>Maximum Frequency"]
        DECODE["Receiver Correlates<br/>Chirp Position"]
    end

    SYMBOL --> START
    START --> CHIRP
    CHIRP --> SWEEP
    SWEEP --> DECODE

    subgraph "Key Properties"
        PROP1["Noise Immunity:<br/>Signal below noise floor"]
        PROP2["Multipath Resistance:<br/>Chirps don't interfere"]
        PROP3["Doppler Tolerance:<br/>Good for motion"]
    end

    style SYMBOL fill:#2C3E50,color:#fff
    style CHIRP fill:#16A085,color:#fff
    style DECODE fill:#E67E22,color:#fff

Figure 1100.2: Chirp Spread Spectrum Encoding Process and Key Properties

{fig-alt=“Chirp spread spectrum encoding diagram showing how data symbols are converted to frequency sweeps. Each symbol determines the starting frequency of an upward chirp. The receiver correlates the chirp position to decode data. Key properties include noise immunity (signal can be below noise floor), multipath resistance, and Doppler tolerance for mobile applications.”}

1100.5.2 Why CSS is Ideal for IoT

Property Benefit for IoT Technical Explanation
Sub-noise Reception Extreme range Processing gain recovers signals 20+ dB below noise
Multipath Immunity Urban deployment Time-spread chirps avoid destructive interference
Low Power TX Battery life Lower transmit power needed for same range
Doppler Tolerance Mobile devices Frequency shift affects all chirp parts equally
Jamming Resistance Security Spread spectrum makes narrowband jamming ineffective

1100.7 Knowledge Check: Physical Layer

Question 1: What is the primary trade-off when using higher spreading factors (SF) in LoRa?

Explanation: B. Higher SF improves sensitivity and range but increases time-on-air, lowering throughput and raising collision risk in dense networks.

Question 2: You’re deploying soil moisture sensors in a vineyard covering 5 km radius. Sensors transmit once per hour. Which spreading factor should you use?

Explanation: C. With 5 km range requirement and infrequent transmission (hourly), higher SF is appropriate. The battery impact is minimal since airtime is a small fraction of total time. SF10-11 provides excellent range without the extreme battery penalty of SF12.

Question 3: A device sends 20-byte messages at SF10 (247ms airtime) every 15 minutes. What percentage of time is it transmitting?

Explanation: A. 247ms every 15 minutes (900,000ms) = 247/900,000 = 0.027% duty cycle. This demonstrates LoRaWAN’s extremely low airtime usage, enabling long battery life.

Question 4: When would you use 500 kHz bandwidth instead of standard 125 kHz?

Explanation: B. 500 kHz bandwidth provides 4x faster data rate but reduces range by ~20%. Only use when latency matters and range reduction is acceptable, typically with mains-powered devices.

1100.8 Summary

This chapter reviewed LoRa physical layer fundamentals:

  • LoRa vs LoRaWAN: LoRa is the physical layer modulation using chirp spread spectrum; LoRaWAN is the MAC layer protocol
  • Spreading Factors: SF7-SF12 provide range-data rate trade-offs, with each SF doubling airtime and increasing range by ~1.6x
  • CSS Benefits: Chirp spread spectrum enables sub-noise floor reception, multipath immunity, and Doppler tolerance
  • Link Budget: Every 6 dB of additional loss halves range; SF12 provides 14 dB more sensitivity than SF7
  • Bandwidth Options: 125/250/500 kHz trade speed for range, with 125 kHz standard for maximum coverage

1100.9 What’s Next

Continue your LoRaWAN review:

Prerequisites: - LoRaWAN Overview - Start here if new to LoRaWAN - LoRaWAN Architecture - Network structure and device classes - LPWAN Fundamentals - Core LPWAN concepts

Deep Dives: - LoRaWAN Comprehensive Review - Full technical review - LoRaWAN Quiz Bank - Practice questions