806 Spectrum Licensing and Wireless Propagation
806.1 Introduction
This chapter covers spectrum licensing models (licensed vs unlicensed) and wireless propagation characteristics. Understanding these concepts is essential for making informed decisions about wireless technology deployment, regulatory compliance, and performance prediction.
NoteLearning Objectives
By the end of this chapter, you will be able to:
- Distinguish between licensed and unlicensed spectrum
- Understand regional spectrum variations and regulatory requirements
- Calculate free space path loss for different frequencies
- Compare frequency vs range vs bandwidth trade-offs
- Predict wireless performance based on propagation characteristics
806.2 Prerequisites
Before diving into this chapter, you should be familiar with:
- Electromagnetic Waves and the Spectrum: Understanding wave properties and frequency relationships
- IoT Wireless Frequency Bands: Knowledge of 2.4 GHz, 5 GHz, and sub-GHz bands
- Basic mathematics: Logarithmic calculations (dB) and formulas
806.3 Spectrum Licensing
806.3.1 Licensed vs Unlicensed Spectrum
Radio frequency spectrum is a finite resource regulated by governmental bodies. Understanding the distinction between licensed and unlicensed spectrum is crucial for IoT deployment.
%% fig-cap: "Licensed vs unlicensed spectrum characteristics and trade-offs"
%% fig-alt: "Comparison diagram showing licensed spectrum (exclusive use, requires fees, protected from interference, used by cellular operators for 4G/5G) versus unlicensed ISM bands (free to use, shared spectrum, subject to power limits, used by Wi-Fi/Bluetooth/LoRa) with respective advantages and limitations"
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graph LR
A["RF Spectrum<br/>Allocation"] --> B["Licensed Spectrum<br/>Cellular Operators"]
A --> C["Unlicensed ISM<br/>Anyone can use"]
B --> B1["✓ Exclusive rights<br/>✓ No interference<br/>✓ Guaranteed QoS<br/>✗ Expensive fees"]
C --> C1["✓ Free to use<br/>✓ Global standards<br/>✗ Shared/crowded<br/>✗ Interference risk"]
B --> B2["4G LTE<br/>5G NR<br/>NB-IoT (licensed)"]
C --> C2["Wi-Fi<br/>Bluetooth<br/>LoRa, Zigbee"]
B1 --> D["Regulatory<br/>Approval<br/>Required"]
C1 --> E["Follow<br/>Power/Duty<br/>Limits"]
style A fill:#2C3E50,stroke:#16A085,stroke-width:3px,color:#fff
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style C fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
style D fill:#E67E22,stroke:#16A085,stroke-width:2px
style E fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
Licensed Spectrum: - Requires regulatory approval and fees - Exclusive use rights (cellular operators) - Protected from interference - Examples: 4G LTE (700-2600 MHz), 5G (3.5 GHz, mmWave)
Unlicensed Spectrum (ISM Bands): - Free to use (within regulatory limits) - Shared among many users and devices - Subject to power and duty cycle restrictions - Examples: 2.4 GHz, 5 GHz, 868/915 MHz
806.3.2 Regional Variations
Different countries allocate spectrum differently. IoT devices must comply with regional regulations:
| Region | Sub-GHz | 2.4 GHz ISM | 5 GHz | Notes |
|---|---|---|---|---|
| Europe | 868 MHz | 2.400-2.483 GHz | 5.150-5.875 GHz | ETSI regulations |
| North America | 915 MHz | 2.400-2.483 GHz | 5.150-5.875 GHz | FCC Part 15 |
| Asia-Pacific | 920-925 MHz | 2.400-2.483 GHz | 5.150-5.875 GHz | Varies by country |
| Global | 433 MHz | Universal | Limited | Check local rules |
806.4 Wireless Propagation Characteristics
806.4.1 Frequency vs Range Trade-off
The choice of frequency band involves fundamental trade-offs between range, bandwidth, and penetration:
%% fig-cap: "Frequency vs range vs bandwidth trade-offs for IoT"
%% fig-alt: "Trade-off diagram showing inverse relationship between frequency, range, and bandwidth for IoT wireless technologies: sub-GHz bands offer longest range (10+ km) but lowest bandwidth (1-50 kbps), 2.4 GHz balances range (100-300m) and bandwidth (250 kbps - 11 Mbps), 5 GHz provides highest bandwidth (54-1200 Mbps) but shortest range (50-100m), with penetration capability decreasing as frequency increases"
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graph TB
A["Frequency Band<br/>Selection"] --> B["Sub-GHz<br/>433/868/915 MHz"]
A --> C["2.4 GHz<br/>ISM Band"]
A --> D["5 GHz<br/>Wi-Fi Band"]
B --> B1["Range: 10+ km<br/>Bandwidth: 1-50 kbps<br/>Penetration: Excellent"]
C --> C1["Range: 100-300m<br/>Bandwidth: 250k-11M<br/>Penetration: Good"]
D --> D1["Range: 50-100m<br/>Bandwidth: 54M-1.2G<br/>Penetration: Poor"]
B1 --> E["Use Case:<br/>Rural sensors<br/>Smart agriculture"]
C1 --> F["Use Case:<br/>Smart home<br/>Building automation"]
D1 --> G["Use Case:<br/>Video streaming<br/>High-speed data"]
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806.4.2 Free Space Path Loss
Signal strength decreases with distance according to the free space path loss formula:
\[ FSPL(dB) = 20\log_{10}(d) + 20\log_{10}(f) + 32.45 \]
Where: - \(d\) = distance in kilometers - \(f\) = frequency in MHz
Key insight: Path loss increases with both distance AND frequency. A 5 GHz signal experiences approximately 7 dB more path loss than a 2.4 GHz signal at the same distance.
TipPractical Example: Path Loss Comparison
For a device 10 meters away:
At 868 MHz (sub-GHz): \[FSPL = 20\log_{10}(0.01) + 20\log_{10}(868) + 32.45 = 51.2 \text{ dB}\]
At 2.4 GHz: \[FSPL = 20\log_{10}(0.01) + 20\log_{10}(2400) + 32.45 = 60.0 \text{ dB}\]
At 5 GHz: \[FSPL = 20\log_{10}(0.01) + 20\log_{10}(5000) + 32.45 = 66.4 \text{ dB}\]
The sub-GHz signal has 8.8 dB less path loss than 2.4 GHz, meaning it requires less transmit power or achieves greater range.
806.5 Interference and Coexistence
806.5.1 Sources of Interference
Understanding potential interference sources helps in selecting the appropriate frequency band:
%% fig-cap: "Common interference sources by frequency band"
%% fig-alt: "Interference source diagram showing 2.4 GHz band impacted by Wi-Fi routers, Bluetooth devices, Zigbee networks, microwave ovens, cordless phones, and baby monitors; 5 GHz band has less interference from Wi-Fi-only and weather radar (DFS); sub-GHz bands have minimal interference from garage doors, simple remotes, and industrial equipment"
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graph TB
A["Interference<br/>Sources"] --> B["2.4 GHz Band<br/>⚠ High Congestion"]
A --> C["5 GHz Band<br/>✓ Lower Congestion"]
A --> D["Sub-GHz<br/>✓ Minimal Interference"]
B --> B1["Wi-Fi routers<br/>Bluetooth devices<br/>Zigbee networks"]
B --> B2["Microwave ovens<br/>Cordless phones<br/>Baby monitors"]
C --> C1["Wi-Fi 5/6 only<br/>Weather radar DFS<br/>Satellite comms"]
D --> D1["Garage doors<br/>Simple RF remotes<br/>Industrial equipment"]
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806.5.2 Coexistence Strategies
IoT protocols employ various techniques to coexist in crowded spectrum:
- Frequency Hopping (Bluetooth): Rapidly switches between channels
- Channel Selection (Wi-Fi): Chooses less congested channels
- CSMA/CA (Wi-Fi, Zigbee): Listen before transmit
- Spread Spectrum (LoRa): Spreads signal across wide bandwidth
- Time Division (WirelessHART): Allocates specific time slots
806.6 Summary
This chapter explored spectrum licensing and wireless propagation:
Spectrum Licensing: - Licensed Spectrum: Exclusive use, guaranteed QoS, requires regulatory fees (cellular operators) - Unlicensed ISM Bands: Free to use, shared spectrum, power/duty cycle limits (Wi-Fi, LoRa, Bluetooth) - Trade-off: Licensed offers interference-free operation but costs $1/month/device; unlicensed is free but subject to interference
Regional Variations: - Europe: 868 MHz (1% duty cycle limit), 2.4/5 GHz ISM (ETSI regulations) - North America: 915 MHz (no duty cycle limit), 2.4/5 GHz ISM (FCC Part 15) - Asia-Pacific: 920-928 MHz (varies by country) - Global: 433 MHz, 2.4 GHz ISM
Wireless Propagation: - Free Space Path Loss (FSPL): Increases with both distance AND frequency - FSPL formula: FSPL(dB) = 20log₁₀(d) + 20log₁₀(f) + 32.45 - At 100m: Sub-GHz has ~9 dB less path loss than 2.4 GHz, ~16 dB less than 5 GHz - Lower frequency → better range, better penetration, lower path loss
Frequency vs Range Trade-offs: - Sub-GHz: 10+ km range, excellent penetration, 1-50 kbps bandwidth - 2.4 GHz: 100-300m range, good penetration, 250 kbps - 11 Mbps bandwidth - 5 GHz: 50-100m range, poor penetration, 54 Mbps - 1.2 Gbps bandwidth
806.7 What’s Next
Continue your wireless fundamentals journey:
- Design Considerations and Labs: Frequency selection frameworks, interference analysis, and hands-on spectrum analysis labs
- Knowledge Checks and Assessments: Scenario-based problems testing your understanding
Related Chapters: - Mobile Wireless Fundamentals - Deeper dive into link budgets and propagation models - Wi-Fi Fundamentals and Standards - 802.11 WLAN details - LoRaWAN Overview - Sub-GHz LPWAN deep dive
806.8 References
Books: - “Wireless Communications: Principles and Practice” by Theodore S. Rappaport - “RF and Microwave Wireless Systems” by Kai Chang
Regulatory Bodies: - FCC (US): https://www.fcc.gov/ - ETSI (Europe): https://www.etsi.org/ - ITU (International): https://www.itu.int/