809  Quiz: Indoor Deployments & Link Budgets

809.1 Introduction

This chapter focuses on link budget calculations and indoor RF propagation for IoT deployments. You’ll work through scenarios involving multi-floor office buildings, wall penetration losses, and technology selection based on coverage requirements.

NoteLearning Objectives

By completing this chapter, you will be able to:

  • Calculate link budgets for indoor Wi-Fi and sensor deployments
  • Analyze multi-floor coverage with floor and wall penetration losses
  • Evaluate fade margin requirements for reliable indoor operation
  • Design coverage solutions for challenging indoor environments

809.2 Prerequisites

Before attempting these assessments, you should have completed:

809.4 Scenario-Based Assessment: Smart Building RF Deployment

Scenario: You’re designing a wireless sensor network for a 5-story commercial building with reinforced concrete construction. The facility houses 200 environmental sensors (temperature, humidity, occupancy) distributed across offices, conference rooms, and hallways. Each sensor must report data every 5 minutes and operate on coin-cell batteries for 5+ years.

Building characteristics: - Concrete walls: 20cm thick, rebar-reinforced - Maximum sensor distance from gateway: 30-40 meters - Multiple walls between sensors and gateways (typically 2-4 walls) - Dense office furniture and metal HVAC ducts - Existing 2.4 GHz Wi-Fi network (15 access points)

Available technology options:

Option A: 2.4 GHz Protocol (Zigbee/Thread) - Free space path loss at 30m: 69.6 dB - Wall penetration loss: 10-15 dB per wall - Device availability: Excellent (many vendors) - Spectrum congestion: High (Wi-Fi, Bluetooth coexistence) - Typical transmit power: +3 to +10 dBm

Option B: Sub-GHz Protocol (868/915 MHz proprietary) - Free space path loss at 30m: 60.8 dB - Wall penetration loss: 5-7 dB per wall - Device availability: Limited (fewer vendors) - Spectrum congestion: Low (ISM band, but less crowded) - Typical transmit power: +10 to +14 dBm

Analysis Questions:

  1. Link Budget Calculation: Calculate total path loss for a sensor 30m away through 3 concrete walls for both options. Which frequency band provides better link budget margin?

  2. Battery Life Trade-off: If 868 MHz requires 28 dB less transmit power than 2.4 GHz, estimate the battery life improvement. (Hint: Radio transmission typically accounts for 60-80% of sensor energy budget)

  3. Interference Risk: The building’s Wi-Fi network operates on channels 1, 6, and 11. If you choose 2.4 GHz for sensors, which Zigbee channels (11-26) would minimize Wi-Fi interference?

  4. Cost-Benefit Analysis: 2.4 GHz modules cost $3/unit while 868 MHz modules cost $8/unit. Gateway costs are $200 (2.4 GHz) vs $400 (868 MHz). For 200 sensors and 10 gateways, calculate total hardware cost difference.

  5. Design Decision: Given the link budget, battery life, interference, and cost trade-offs, which technology would you recommend and why?

1. Link Budget Calculation:

868 MHz Path Loss: - Free space: 60.8 dB - Wall losses: 3 walls × 6 dB = 18 dB - Total: 78.8 dB

2.4 GHz Path Loss: - Free space: 69.6 dB - Wall losses: 3 walls × 12.5 dB = 37.5 dB - Total: 107.1 dB

Link Budget Margin (assuming -100 dBm receiver sensitivity): - 868 MHz: Requires -21.2 dBm transmit power → Margin: ~31 dB (if using +10 dBm) - 2.4 GHz: Requires +7.1 dBm transmit power → Margin: ~3 dB (if using +10 dBm)

Winner: 868 MHz provides 28 dB better link budget

2. Battery Life Improvement:

Power consumption scales exponentially with transmit power: - 28 dB difference = ~630× less power (10^(28/10) = 631) - If radio uses 70% of energy budget: Battery life improvement = 4-6× longer - 2.4 GHz: ~1-2 year battery life - 868 MHz: 5-8 year battery life ✓ Meets 5-year requirement

3. Interference Mitigation:

Wi-Fi channels 1, 6, 11 map to 2.4 GHz as follows: - Wi-Fi Ch 1: 2.412 GHz (±11 MHz) = 2.401-2.423 GHz - Wi-Fi Ch 6: 2.437 GHz (±11 MHz) = 2.426-2.448 GHz - Wi-Fi Ch 11: 2.462 GHz (±11 MHz) = 2.451-2.473 GHz

Zigbee channels in clear zones: - Zigbee Ch 25 (2.475 GHz) - Above Wi-Fi Ch 11 - Zigbee Ch 26 (2.480 GHz) - Highest available - Avoid Zigbee Ch 11-20 (overlap with Wi-Fi)

4. Cost Analysis:

2.4 GHz System: - Sensors: 200 × $3 = $600 - Gateways: 10 × $200 = $2,000 - Total: $2,600

868 MHz System: - Sensors: 200 × $8 = $1,600 - Gateways: 10 × $400 = $4,000 - Total: $5,600

Cost difference: $3,000 more for 868 MHz

But over 5 years: - Battery replacement cost: ~$2 per sensor per change - 2.4 GHz: 200 sensors × 2-3 replacements × $2 = $800-$1,200 - 868 MHz: 200 sensors × 0 replacements = $0 - True total cost difference: ~$1,800-$2,200

5. Recommended Decision:

Choose 868 MHz if: - Budget allows $2-3K higher initial investment - 5-year battery life is mandatory (avoids maintenance) - Concrete walls require robust penetration - Minimal interference tolerance

Choose 2.4 GHz if: - Budget constrained (<$3K total) - Willing to replace batteries every 1-2 years - Can deploy more gateways to reduce distance - Can coordinate with Wi-Fi team for channel planning

For this scenario: 868 MHz is recommended because: 1. Meets 5-year battery requirement (2.4 GHz likely fails) 2. 28 dB link budget advantage handles concrete walls reliably 3. Lower maintenance cost (no battery replacements) 4. Less interference from crowded 2.4 GHz band

Key Engineering Insight: The 28 dB link budget difference translates to either 3× range OR 600× power savings. In battery-powered IoT, this difference determines project success or failure. The $3K upfront cost premium is recovered through eliminated battery replacements and reduced maintenance truck rolls.

Verification Questions: 1. If walls had only 5 dB loss at 2.4 GHz, would the recommendation change? (Calculate new link budget) 2. What happens to 868 MHz advantage if you need 100 kbps data rates? (Higher rates reduce sub-GHz benefits) 3. How many gateways would 2.4 GHz need to match 868 MHz coverage? (Estimate from range difference)

809.5 Quick Reference: Electromagnetic Properties Quiz

Question: An IoT device transmits at 868 MHz while another transmits at 2.4 GHz. If both transmit the same power, which statement correctly describes their electromagnetic properties?

Explanation: Using the relationships c = f × λ and E = h × f, we know that higher frequency results in shorter wavelength and higher energy. At 2.4 GHz (2400 MHz), the wavelength is λ = 3×10⁸ m/s ÷ 2.4×10⁹ Hz = 0.125 m (12.5 cm). At 868 MHz, the wavelength is λ = 3×10⁸ m/s ÷ 868×10⁶ Hz = 0.346 m (34.6 cm). The 2.4 GHz signal also carries more energy per photon (E = h × f), making it 2.4/0.868 ≈ 2.8× more energetic. However, higher frequency signals experience more path loss and absorption, which is why sub-GHz bands like 868 MHz are preferred for long-range IoT applications despite carrying less energy per photon.

Question: Calculate the free space path loss (FSPL) for a 2.4 GHz signal traveling 50 meters. Use the formula: FSPL(dB) = 20log10(d_km) + 20log10(f_MHz) + 32.45

Explanation: Using FSPL(dB) = 20log10(d_km) + 20log10(f_MHz) + 32.45 with d = 50m = 0.05 km and f = 2400 MHz: - FSPL = 20log10(0.05) + 20log10(2400) + 32.45 - FSPL = 20(-1.301) + 20(3.380) + 32.45 - FSPL = -26.02 + 67.60 + 32.45 = 74.03 dB

This means the signal strength decreases by 74 dB over 50 meters in free space. In practice, indoor environments add 10-30 dB additional loss due to walls, furniture, and multipath interference. This is why a Wi-Fi router with +20 dBm transmit power might produce a -54 dBm received signal at 50m in an office environment.

809.6 Summary

This quiz covered critical link budget concepts for indoor IoT deployments:

  1. Multi-floor Coverage: Floor penetration (15 dB) significantly impacts adjacent-floor coverage; 6.6 dB excess margin is insufficient for production deployments
  2. Smart Building Design: Sub-GHz (868 MHz) provides 28 dB better link budget than 2.4 GHz through concrete walls
  3. Cost-Benefit Analysis: Higher initial cost for 868 MHz modules is offset by eliminated battery replacements

Key Takeaways:

  • Always calculate link budget with fade margin for worst-case scenarios
  • Floor penetration loss (10-20 dB) often exceeds wall penetration loss
  • The 28 dB sub-GHz advantage translates to 3× range OR 600× power savings
  • Total cost of ownership must include maintenance and battery replacements

809.7 What’s Next

Continue testing your wireless knowledge:

Related Chapters: - Mobile Wireless Fundamentals - Deeper dive into link budgets - Mobile Wireless Labs and Implementation - Hands-on RF measurements