636 Material Attenuation and RSSI Localization

636.1 Learning Objectives

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

Quantify signal attenuation through common building materials
Calculate cumulative path loss for multi-obstacle indoor paths
Understand frequency dependence of material penetration
Interpret RSSI values for signal quality assessment
Estimate distance from RSSI for BLE beacon and Wi-Fi localization
Apply trilateration and fingerprinting techniques for indoor positioning

636.2 Introduction

Building materials significantly reduce wireless signal strength. Understanding material attenuation is critical for indoor IoT deployments. This chapter covers material penetration losses and how RSSI (Received Signal Strength Indicator) is used for device localization.

Time: ~15 min | Difficulty: Intermediate | P07.C15.U05b

636.3 Signal Attenuation Through Materials

Building materials significantly reduce wireless signal strength. Understanding material attenuation is critical for indoor IoT deployments.

Material Attenuation at 2.4 GHz (Wi-Fi, Bluetooth, Zigbee):

Material	Attenuation (dB)	Effect on Range
Air (free space)	0 dB	Reference (no loss)
Window glass	2-3 dB	Minimal impact
Interior drywall	3-5 dB	~30% range reduction
Wood door	3-6 dB	~40% range reduction
Brick wall (4”)	6-10 dB	~60% range reduction
Concrete wall (6”)	10-15 dB	~75% range reduction
Concrete floor	12-18 dB	~85% range reduction
Metal partition	20-30 dB	~95% range reduction
Elevator shaft	30-40 dB	Complete blockage

Cumulative Attenuation Example:

Smart home Wi-Fi scenario:
- Router in living room (ground floor)
- Sensor in bedroom (second floor)

Path: Router -> Drywall (4 dB) -> Concrete floor (15 dB) -> Drywall (4 dB) -> Sensor
Total attenuation: 4 + 15 + 4 = 23 dB

If free-space path loss at 10m = 60 dB:
Total path loss = 60 dB (FSPL) + 23 dB (materials) = 83 dB

With TX power 20 dBm:
RX power = 20 - 83 = -63 dBm (still above -90 dBm sensitivity check)

636.4 Frequency Dependence of Attenuation

Higher frequencies experience MORE attenuation through materials:

Material	900 MHz	2.4 GHz	5 GHz	60 GHz
Drywall	2 dB	3-5 dB	5-8 dB	8-12 dB
Concrete	6-8 dB	10-15 dB	15-25 dB	30-50 dB
Wood	2-4 dB	3-6 dB	5-10 dB	10-20 dB

Why LoRaWAN (915 MHz) Penetrates Buildings Better Than Wi-Fi (2.4 GHz): - Lower frequency results in less attenuation through walls - LoRa 915 MHz experiences ~40% less material loss than Wi-Fi 2.4 GHz

636.5 RSSI Interpretation for IoT Localization

RSSI (Received Signal Strength Indicator) measures received power in dBm. It’s commonly used for distance estimation in BLE beacons, Wi-Fi positioning, and Zigbee networks.

RSSI Ranges and Interpretation:

RSSI (dBm)	Signal Quality	Typical Distance	Use Case
-30 to -50 dBm	Excellent	0-5 meters	Close proximity detection (BLE beacons)
-50 to -70 dBm	Good	5-20 meters	Normal Wi-Fi operation
-70 to -80 dBm	Fair	20-50 meters	Acceptable for low-data IoT
-80 to -90 dBm	Weak	50-100 meters	Minimum usable (high error rate)
< -90 dBm	Very Weak	>100 meters	Connection drops, unreliable

636.6 Distance Estimation from RSSI

Using the log-distance model, we can estimate distance from RSSI measurements:

\[\text{RSSI}(d) = \text{RSSI}_0 - 10n\log_{10}\left(\frac{d}{d_0}\right)\]

Solving for distance:

\[d = d_0 \times 10^{\frac{\text{RSSI}_0 - \text{RSSI}(d)}{10n}}\]

636.6.1 BLE Beacon Localization Example

Scenario: iBeacon advertising at 2.4 GHz. You measure RSSI = -65 dBm at unknown distance.

Given: - RSSI_0 = -50 dBm (calibrated at d_0 = 1 meter) - Path loss exponent: n = 2.5 (indoor office) - Measured RSSI = -65 dBm

Calculate distance:

\[d = 1 \times 10^{\frac{-50 - (-65)}{10 \times 2.5}}\] \[d = 10^{\frac{15}{25}}\] \[d = 10^{0.6}\] \[d = 3.98 \text{ meters}\]

Accuracy Limitations: - Plus or minus 2-5 meters typical error due to multipath, shadowing, and interference - Fluctuations of plus or minus 5-10 dBm in RSSI readings are common - NOT suitable for precise positioning (<1m accuracy) - Good for room-level localization (e.g., “user is in kitchen, not bedroom”)

636.7 Improving RSSI-based Localization

636.7.1 1. Multiple Beacons (Trilateration)

Beacon A: RSSI = -60 dBm -> d_A = 5.0m
Beacon B: RSSI = -55 dBm -> d_B = 2.5m
Beacon C: RSSI = -70 dBm -> d_C = 10.0m

Intersection of three circles -> position estimate

636.7.2 2. Kalman Filtering

Smooth out RSSI fluctuations over time
Reduces noise from plus or minus 10 dBm to plus or minus 2-3 dBm
Requires continuous measurements

636.7.3 3. Fingerprinting

Pre-map RSSI values at known locations
Match measured RSSI to database
More accurate than distance calculation (1-3m error)

Localization Accuracy Comparison

Method	Accuracy	Setup Effort	Best For
Single beacon	3-10 m	Low	Proximity detection
Trilateration	2-5 m	Medium	Room-level tracking
Fingerprinting	1-3 m	High	Indoor navigation
UWB ranging	10-30 cm	Medium	Precision tracking

636.8 Summary

Material attenuation is cumulative - each wall, floor, or obstacle adds 3-20 dB loss
Higher frequencies experience more material loss - 5 GHz Wi-Fi penetrates worse than 2.4 GHz, which penetrates worse than 915 MHz LoRa
RSSI provides distance estimates but with 2-5 meter accuracy limitations
Trilateration with multiple beacons improves positioning accuracy
Fingerprinting databases provide the best accuracy (1-3m) but require site surveys
Room-level localization is practical with RSSI; precision tracking requires UWB or other technologies

636.9 What’s Next

Continue to the Link Budget and Coverage Planning chapter to learn how to design wireless links and compare protocol coverage across different environments.