23  Quiz: Indoor & Link Budgets

In 60 Seconds

This quiz focuses on link budget calculations for indoor IoT deployments. You will learn that same-floor Wi-Fi coverage provides excellent margin (31.7 dB), but adding a single concrete floor (15 dB penetration loss) makes adjacent-floor coverage marginal. Sub-GHz (868 MHz) provides 28 dB better link budget than 2.4 GHz through concrete walls, translating to either 3x range or 600x power savings – making it the preferred choice for battery-powered building sensors.

Key Concepts

  • Link Budget: System accounting of signal power from transmitter to receiver: TX power + gains - losses ≥ RX sensitivity + margin
  • Receiver Sensitivity: Minimum signal power for acceptable demodulation at specified BER; e.g., -130 dBm for LoRa SF12
  • System Gain: TX EIRP - RX sensitivity; maximum allowable path loss before adding margins
  • Fade Margin: Extra signal budget beyond minimum required; compensates for fading, shadowing, and multipath
  • Link Margin: Total system gain - actual path loss; positive margin means link is closed
  • MCL (Maximum Coupling Loss): Total path loss the system can tolerate while maintaining connectivity
  • Antenna Gain: Directional gain of antenna pattern in dBi; adds directly to TX EIRP or RX sensitivity
  • Cable Loss: Signal attenuation in coaxial cables; ~3 dB per 10 meters at 2.4 GHz for standard RG-58

23.1 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.

Learning Objectives

By completing this chapter, you will be able to:

  • Calculate complete link budgets for indoor Wi-Fi and IoT sensor deployments using the FSPL formula and wall/floor attenuation values
  • Differentiate between same-floor and multi-floor coverage scenarios by quantifying floor penetration losses (15 dB per concrete floor)
  • Evaluate whether a given fade margin is sufficient for reliable indoor operation under worst-case propagation conditions
  • Justify the selection of sub-GHz (868 MHz) versus 2.4 GHz for battery-powered building sensors using link budget and power consumption trade-offs

This quiz tests your understanding of link budgets and indoor wireless propagation. Link budgets determine whether your wireless signal will be strong enough to reach its destination. Questions cover path loss calculations, wall attenuation, and receiver sensitivity – essential skills for planning reliable IoT deployments.

23.2 Prerequisites

Before attempting these assessments, you should have completed:

23.4 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)

23.5 Quick Reference: Electromagnetic Properties Quiz

Sammy Sensor: “A link budget is like a money budget for radio signals. You start with your transmit power (your income), subtract path loss and wall losses (your expenses), and see if you have enough left over (margin) to get the message through!”

Lila the Light Sensor: “Floors are the biggest signal blockers in buildings! One concrete floor eats up 15 dB of signal – that is like losing 97% of your signal power. Always check if your sensor can still be heard through the floor!”

Max the Motion Detector: “The 28 dB advantage of 868 MHz over 2.4 GHz through concrete walls is HUGE. In power terms, that means a sub-GHz sensor can use 600 times less battery power to send the same message through the same wall. That is the difference between a 1-year and a 5-year battery!”

Bella the Button: “Fade margin is like bringing an umbrella on a cloudy day. You might not need it, but if it rains (interference, furniture, people moving), you will be glad you planned ahead. Always design with at least 10 dB extra!”

Scenario: You’re deploying Wi-Fi-based occupancy sensors across a 3-story office building. Calculate link budgets to determine AP placement and validate coverage.

Building Specifications:

  • Dimensions: 50m × 30m × 12m (3 floors, 4m per floor)
  • Construction: Concrete floors (15 dB loss), drywall partitions (5 dB loss)
  • Layout: Open-plan offices with 2-3 drywall partitions per path
  • Sensor specs: ESP32 with Wi-Fi (2.4 GHz), RX sensitivity -85 dBm, TX power +17 dBm

AP Specifications:

  • Model: Enterprise Wi-Fi 6 (802.11ax)
  • TX power: +20 dBm (100 mW, regulatory limit)
  • Antenna gain: 5 dBi (omnidirectional)
  • Placement: Ceiling-mounted (4m high)
  • RX sensitivity: -90 dBm

Required Margin:

  • Fade margin: 10 dB (accounts for people moving, furniture, multipath fading)
  • Interference margin: 3 dB (2.4 GHz band congestion)
  • Total required margin: 13 dB

Scenario 1: Same-Floor Corner Sensor

Path Description:

  • AP at ceiling center (25m, 15m from walls)
  • Sensor in corner office (diagonal corner)
  • Distance: √(25² + 15²) = 29.15 meters
  • Obstacles: 2 drywall partitions (5 dB each)

Link Budget Calculation:

Downlink (AP → Sensor):

TX Power (AP):        +20 dBm
TX Antenna Gain:       +5 dBi
EIRP:                 +25 dBm

Free-Space Path Loss (FSPL):
  FSPL = 20log₁₀(0.02915) + 20log₁₀(2400) + 32.45    [d in km, f in MHz]
       = -30.7 + 67.6 + 32.45
       = 69.35 dB

Additional Losses:
  Drywall (2 walls):    -10 dB
  Cable loss:            -0.5 dB
  Total Loss:           -79.85 dB

RX Antenna Gain:       +2 dBi (sensor)
RX Power:              25 - 79.85 + 2 = -52.85 dBm
RX Sensitivity:        -85 dBm
Link Margin:           -52.85 - (-85) = 32.15 dB
Required Margin:       -13 dB
Excess Margin:         32.15 - 13 = **19.15 dB** ✓ EXCELLENT

Verdict: Same-floor corner has good headroom (19 dB excess). Can tolerate moderate fading.

Link budget formula: \(\text{Margin} = P_{\text{TX}} + G_{\text{TX}} - \text{FSPL} - L_{\text{walls}} + G_{\text{RX}} - S_{\text{RX}}\) where all values in dB. Worked example: +20 dBm TX + 5 dBi gain - 69.35 dB FSPL - 10 dB walls - 0.5 dB cable + 2 dBi RX - (-85 dBm) sensitivity = 32 dB margin. With 13 dB required (fade + interference), excess = 32 - 13 = 19 dB.

Scenario 2: Floor Above (Directly Above AP)

Path Description:

  • Sensor directly above AP (one floor up)
  • Distance: 4 meters vertical
  • Obstacles: 1 concrete floor (15 dB loss)

Link Budget Calculation:

TX Power (AP):        +20 dBm
TX Antenna Gain:       +5 dBi
EIRP:                 +25 dBm

FSPL (4 meters):
  FSPL = 20log₁₀(0.004) + 20log₁₀(2400) + 32.45    [d in km, f in MHz]
       = -47.96 + 67.6 + 32.45
       = 52.09 dB

Additional Losses:
  Concrete floor:       -15 dB
  Total Loss:           -67.09 dB

RX Antenna Gain:       +2 dBi
RX Power:              25 - 67.09 + 2 = -40.09 dBm
Link Margin:           -40.09 - (-85) = 44.91 dB
Excess Margin:         44.91 - 13 = **31.91 dB** ✓ EXCELLENT

Verdict: Floor above (best case, directly overhead) has 32 dB excess margin. Very healthy.

Scenario 3: Floor Above + Corner (Worst Case)

Path Description:

  • Sensor in corner of floor above
  • Distance: √(25² + 15² + 4²) = 29.41 meters (3D diagonal)
  • Obstacles: 1 concrete floor (15 dB), 2 drywall partitions (10 dB)

Link Budget Calculation:

TX Power (AP):        +20 dBm
TX Antenna Gain:       +5 dBi
EIRP:                 +25 dBm

FSPL (29.41 meters):
  FSPL = 20log₁₀(0.02941) + 20log₁₀(2400) + 32.45    [d in km, f in MHz]
       = -30.63 + 67.6 + 32.45
       = 69.42 dB

Additional Losses:
  Concrete floor:       -15 dB
  Drywall (2 walls):    -10 dB
  Total Loss:           -94.42 dB

RX Antenna Gain:       +2 dBi
RX Power:              25 - 94.42 + 2 = -67.42 dBm
Link Margin:           -67.42 - (-85) = 17.58 dB
Excess Margin:         17.58 - 13 = **4.58 dB** ⚠️ MARGINAL

Verdict: Floor above + corner is marginal with only 4.6 dB excess. Reliability concerns in production.

Scenario 4: Two Floors Below (Marginal Case)

Path Description:

  • Sensor two floors below AP
  • Distance: 8 meters vertical + 10m horizontal offset = √(8² + 10²) = 12.8 meters
  • Obstacles: 2 concrete floors (30 dB), 1 drywall partition (5 dB)

Link Budget Calculation:

TX Power (AP):        +20 dBm
TX Antenna Gain:       +5 dBi
EIRP:                 +25 dBm

FSPL (12.8 meters):
  FSPL = 20log₁₀(0.0128) + 20log₁₀(2400) + 32.45    [d in km, f in MHz]
       = -37.86 + 67.6 + 32.45
       = 62.19 dB

Additional Losses:
  Concrete floors (2):  -30 dB
  Drywall:              -5 dB
  Total Loss:           -97.19 dB

RX Antenna Gain:       +2 dBi
RX Power:              25 - 97.19 + 2 = -70.19 dBm
Link Margin:           -70.19 - (-85) = 14.81 dB
Excess Margin:         14.81 - 13 = **1.81 dB** ⚠️ MARGINAL

Verdict: Two floors down is marginal with under 2 dB excess. High risk of dropouts during fades.

Summary Table:

Scenario Distance Obstacles RX Power Margin Excess Status
Same floor, corner 29m 2 walls + cable -53 dBm 32 dB 19 dB Excellent
One floor up, center 4m 1 floor -40 dBm 45 dB 32 dB Excellent
One floor up, corner 29m 1 floor + 2 walls -67 dBm 18 dB 5 dB Marginal
Two floors down 13m 2 floors + 1 wall -70 dBm 15 dB 2 dB Marginal

Design Recommendations:

1. AP Placement Strategy:

  • 1 AP per floor minimum (two-floor penetration is marginal)
  • AP spacing: 40-50m on same floor (ensures 20+ dB margin in corners)
  • Ceiling-mount critical (reduces drywall penetration losses)

2. Coverage for 3-story building (50m × 30m per floor):

Floor area: 50m × 30m = 1,500 m²
AP coverage radius: 25m (with 10+ dB margin)
Coverage area per AP: π × 25² = 1,963 m²
APs per floor: 1,500 / 1,963 ≈ 1 AP (centrally placed)

Total APs: 3 floors × 1 AP = **3 APs minimum**

3. Enhanced Design (for redundancy):

  • 2 APs per floor (one at each end)
  • Provides seamless roaming (sensors never drop below -50 dBm)
  • Failover if one AP dies (self-healing coverage)
  • Total: 6 APs

Cost Comparison:

Design APs Hardware Cost Coverage Quality
Minimum (3 APs) 3 3 × $250 = $750 Marginal at cross-floor edges, 2-5 dB excess
Standard (4 APs) 4 4 × $250 = $1,000 Good, 15+ dB excess everywhere
Redundant (6 APs) 6 6 × $250 = $1,500 Excellent, 25+ dB excess + failover

Recommendation: 4 APs (1-2 per floor) balances cost and reliability. 3-AP design is cutting it too close (only 6 dB margin in worst case).

Key Insight: Concrete floors are the dominant loss factor (15 dB each). Directly above an AP is fine (32 dB excess), but floor + corner path drops to only 5 dB excess – insufficient for production. Two floors drops to under 2 dB. Always design for at least 10 dB excess margin after fade + interference budgets. The $250 cost difference between 3 and 4 APs is trivial compared to troubleshooting intermittent connectivity issues.

23.7 Concept Relationships

Concept Relationship Key Insight
Floor Penetration ↔︎ Margin 15 dB concrete floor loss Adding one floor + 2 walls takes 19 dB excess to 5 dB marginal
Frequency ↔︎ Wall Attenuation Higher freq = more absorption 2.4 GHz: 12.5 dB/wall, 868 MHz: 6 dB/wall
Link Margin ↔︎ Reliability 10 dB minimum for production < 5 dB excess = marginal, 15+ dB = robust
Sub-GHz Advantage ↔︎ Battery Life 28 dB = 630× less power Translates to 4-6× longer battery life

Common Pitfalls

EIRP = TX power + antenna gain - cable losses. Long cable runs between radio and antenna lose significant signal. A 10-meter RG-58 cable at 2.4 GHz loses ~3 dB — canceling half the gain of a 6 dBi antenna. Always account for all cable and connector losses in the link budget.

Receiver sensitivity is only meaningful at a specific bit error rate (BER). Sensitivity at BER=10⁻³ may be 6 dB better than at BER=10⁻⁶. Application requirements determine the required BER, which then determines which sensitivity figure to use in the link budget.

A factory floor may have open areas (free space, n=2), shelving aisles (obstructed, n=4), and metal rooms (high reflection, variable). Using one path loss model for the entire facility produces inaccurate coverage predictions. Segment the environment and apply appropriate models.

Link budgets that only account for noise (thermal + receiver noise figure) but not interference overestimate real-world performance. In shared ISM bands, interference can raise the effective noise floor by 10-20 dB. Include a 5-10 dB interference margin for unlicensed band deployments.

23.8 Summary

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

  1. Multi-floor Coverage: Floor penetration (15 dB) plus wall losses significantly impact adjacent-floor corner coverage; under 5 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

23.9 See Also

23.10 What’s Next

If you want to… Read this
Practice frequency selection scenarios Quiz: Frequency Bands
Apply link budgets to real deployments Mobile Labs: Coverage Planning
Learn propagation and spectrum fundamentals Spectrum & Propagation
Study cellular and LoRaWAN regulations Quiz: Cellular & LoRaWAN Regulations
Review all mobile wireless for exam Mobile Wireless Review