6 Spectrum & Propagation
Key Concepts
- Spectrum Licensing: Government-administered allocation of radio frequency bands to operators; ensures interference-free operation
- Type Approval: Regulatory certification confirming a device meets technical standards for radio emissions in a jurisdiction
- CE Marking (Europe): Conformity marking required for wireless devices in European Economic Area; includes RED directive compliance
- FCC Part 15: US regulations for unlicensed devices; limits on conducted power and field strength at specified test distances
- Propagation Model: Mathematical description of path loss vs distance; indoor models add building-specific attenuation factors
- Okumura-Hata Model: Empirical propagation model for urban/suburban cellular coverage prediction based on frequency and antenna height
- ITU-R Models: International Telecommunication Union propagation models for various environments (indoor, urban, suburban)
- SRD (Short Range Device): Category of low-power radio devices operating under license-exempt regulations; includes most IoT devices
6.1 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.
Learning Objectives
By the end of this chapter, you will be able to:
- Differentiate licensed spectrum from unlicensed ISM bands based on access rights, cost, and interference guarantees
- Evaluate regional spectrum allocations (EU 868 MHz, US 915 MHz, Asia-Pacific 920 MHz) and their regulatory constraints
- Calculate free-space path loss using the FSPL formula for any frequency and distance combination
- Analyse the trade-offs between frequency, range, bandwidth, and wall penetration for IoT deployments
- Justify spectrum selection decisions by constructing link budgets that compare licensed and unlicensed options
For Beginners: Spectrum and Propagation
Radio signals behave differently at different frequencies – lower frequencies travel farther and penetrate walls better, while higher frequencies carry more data but are easily blocked. Governments regulate who can use which frequencies through licensing. This chapter connects these physical realities with the regulatory framework that governs wireless IoT.
6.2 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
6.3 Spectrum Licensing
6.3.1 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.
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
6.3.2 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 |
6.4 Wireless Propagation Characteristics
6.4.1 Frequency vs Range Trade-off
The choice of frequency band involves fundamental trade-offs between range, bandwidth, and penetration:
6.4.2 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 6.4 dB more path loss than a 2.4 GHz signal at the same distance.
Practical 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.
Putting Numbers to It
The FSPL formula \(FSPL(dB) = 20\log_{10}(d_{km}) + 20\log_{10}(f_{MHz}) + 32.45\) shows why frequency matters. The \(20\log_{10}(f)\) term means doubling frequency adds ~6 dB loss. From 868 MHz to 2.4 GHz (2.77× frequency increase):
\[\Delta FSPL = 20\log_{10}(2400/868) = 20\log_{10}(2.77) = 20(0.44) = 8.8 \text{ dB}\]
This 8.8 dB = \(10^{0.88} \approx 7.6\times\) power ratio. For same range, 2.4 GHz needs 7.6× more TX power. Or for same power, 868 MHz reaches \(\sqrt{7.6} \approx 2.8\times\) farther!
6.5 Interference and Coexistence
6.5.1 Sources of Interference
Understanding potential interference sources helps in selecting the appropriate frequency band:
6.5.2 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
Sensor Squad: The Spectrum Playground Rules!
Sammy Sensor: “Licensed spectrum is like having your own private playground – nobody else can use it, but you have to pay rent. Unlicensed spectrum is like a public park – free for everyone, but sometimes it gets really crowded!”
Lila the Light Sensor: “Path loss is like shouting across a field. The farther away you are, the harder it is to hear. And shouting in a high-pitched voice (high frequency) fades faster than a deep voice (low frequency)!”
Max the Motion Detector: “Here is a cool trick: if you know the formula FSPL = 20log(d) + 20log(f) + 32.45, you can predict how weak a signal will be before you even turn on your device. It is like predicting the weather for radio waves!”
Bella the Button: “Different countries have different wireless rules. In Europe, you can use 868 MHz but only transmit 1% of the time. In the US, you use 915 MHz with no time limit. Always check your local rules before deploying!”
6.6 Worked Example: Campus IoT Network — Licensed vs Unlicensed Spectrum Decision
Scenario: MedPark Health, a 6-building hospital campus in Manchester, UK, deploys 3 distinct IoT systems:
- System A: 2,000 environmental sensors (temperature, humidity) in patient rooms — readings every 15 minutes
- System B: 400 asset tracking tags on wheelchairs, infusion pumps, portable monitors — real-time location every 10 seconds
- System C: 50 critical patient telemetry monitors — continuous ECG/SpO2 streaming requiring <500 ms latency
6.6.1 Link Budget Comparison Across Buildings
The longest path crosses from Building 1 (main hospital) to Building 6 (research annex): 280 m outdoor, passing through 2 external brick walls (each 30 cm).
| Parameter | 868 MHz (LoRaWAN) | 2.4 GHz (BLE mesh) | LTE Band 20 (800 MHz) |
|---|---|---|---|
| FSPL at 280 m | 81.2 dB | 90.0 dB | 80.4 dB |
| 2 brick walls (30 cm each) | 2 x 6 dB = 12 dB | 2 x 10 dB = 20 dB | 2 x 6 dB = 12 dB |
| Indoor path (40 m corridor) | 8 dB | 12 dB | 8 dB |
| Total path loss | 101.2 dB | 122.0 dB | 100.4 dB |
| Transmit power | +14 dBm | +4 dBm | +23 dBm |
| Receiver sensitivity | -137 dBm (SF12) | -96 dBm | -141 dBm (NB-IoT) |
| Link margin | 49.8 dB | -22.0 dB | 63.6 dB |
BLE cannot span buildings without repeaters. LoRaWAN and cellular both have ample margin.
6.6.2 Technology-to-System Mapping
| System | Technology | Why |
|---|---|---|
| A (2,000 env sensors) | LoRaWAN (868 MHz, unlicensed) | Low data rate (50 bytes/15 min), 5+ year battery, 3 gateways cover entire campus. No subscription fees for 2,000 devices saves GBP 36,000/year |
| B (400 asset tags) | BLE + Wi-Fi hybrid (2.4 GHz, unlicensed) | Hospital already has 180 Wi-Fi APs providing location infrastructure. BLE beacons on assets; Wi-Fi APs as receivers. 3 m accuracy sufficient for room-level tracking |
| C (50 patient monitors) | Private LTE (Band 20, licensed) | Continuous ECG streaming at 4 kbps per monitor needs guaranteed <500 ms latency. Shared unlicensed spectrum cannot guarantee QoS during Wi-Fi congestion peaks. 50 devices justifies GBP 1,200/year cellular cost for clinical safety |
6.6.3 5-Year Cost Analysis
| Cost Component | LoRaWAN (System A) | BLE/Wi-Fi (System B) | Private LTE (System C) |
|---|---|---|---|
| Infrastructure | 3 gateways: GBP 2,400 | 0 (uses existing APs) | 0 (carrier network) |
| Device hardware | 2,000 x GBP 8 = GBP 16,000 | 400 x GBP 12 = GBP 4,800 | 50 x GBP 45 = GBP 2,250 |
| Subscription (5 yr) | GBP 0 (private network) | GBP 0 | 50 x GBP 2/mo x 60 = GBP 6,000 |
| Battery replacement (5 yr) | 0 (7-year life) | 400 x GBP 3 x 2 = GBP 2,400 | 0 (mains powered) |
| Total | GBP 18,400 | GBP 7,200 | GBP 8,250 |
Key Decision: Match Spectrum to Requirements
MedPark Health uses all three spectrum types because each system has different requirements:
- Unlicensed sub-GHz for high-volume, low-rate sensors where cost per device matters most
- Unlicensed 2.4 GHz for asset tracking where existing Wi-Fi infrastructure eliminates deployment cost
- Licensed cellular only for the 50 critical monitors where guaranteed latency is a patient safety requirement
The common mistake is choosing one technology for everything. Using cellular for all 2,450 devices would cost GBP 294,000 in subscriptions alone over 5 years — 9x more than the mixed approach (GBP 33,850 total).
6.7 Concept Relationships
| Concept | Relationship | Key Insight |
|---|---|---|
| Frequency ↔︎ Path Loss | Higher frequency = Higher path loss | 5 GHz suffers 16 dB more loss than sub-GHz at 100m |
| Licensed ↔︎ Cost | Exclusive spectrum = Recurring fees | $1-3/device/month vs free ISM bands |
| Duty Cycle ↔︎ Capacity | 1% duty cycle = 36 sec/hour | EU 868 MHz limits packet rate more than US 915 MHz |
| Wavelength ↔︎ Penetration | Longer wavelength = Better penetration | Sub-GHz penetrates concrete 2-3× better than 2.4 GHz |
Common Pitfalls
1. Skipping Type Approval for New Markets
Selling wireless devices in a new country without local type approval is illegal and can result in product recalls, fines, and market bans. FCC certification does not cover European markets; CE RED certification does not cover North America. Budget for type approval in each target market.
2. Using a Propagation Model Without Verifying Its Applicability
Okumura-Hata is valid for 150-1500 MHz over distances of 1-20 km with specific antenna height assumptions. Applying it to indoor 2.4 GHz deployments with 3-meter ceilings violates its validity range and produces incorrect predictions. Always check model assumptions against your scenario.
3. Assuming Module Certification Covers Final Product
A certified Wi-Fi module provides FCC/CE certification for the module itself, but the final product incorporating the module may still require additional testing if the host design affects the module’s RF performance. Confirm with your certification body whether modular certification covers your integration.
4. Not Accounting for Regulatory Updates
Radio regulations change regularly — new bands are opened, power limits change, and new certification requirements are added. IoT devices with 10-year lifetimes may face regulatory changes mid-deployment. Monitor regulatory updates and build in firmware update capability for frequency or power adjustments.
6.8 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
6.9 See Also
- IoT Wireless Frequency Bands - Detailed coverage of 2.4 GHz, 5 GHz, sub-GHz bands
- Design Considerations and Labs - Frequency selection frameworks
- LoRaWAN Overview - Sub-GHz LPWAN deep dive
6.10 What’s Next
| If you want to… | Read this |
|---|---|
| Apply frequency selection to IoT technology choices | IoT Wireless Frequency Bands |
| Learn link budget and range calculation | Quiz: Indoor & Link Budgets |
| Understand cellular spectrum specifically | Cellular Spectrum for IoT |
| Practice with real measurement labs | Lab: Cellular Modem |
| Review all mobile wireless concepts | Mobile Wireless Review |
6.11 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/