76  Fading, Multipath, and RF Interference

76.1 Learning Objectives

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

• Explain how multipath propagation causes signal fading
• Differentiate between slow fading (shadowing) and fast fading (multipath)
• Calculate appropriate fading margins for different deployment environments
• Identify common sources of RF interference in IoT deployments
• Implement interference mitigation strategies including channel selection and frequency hopping
• Select appropriate frequency bands based on application requirements

76.2 Prerequisites

Before diving into this chapter, you should be comfortable with:

• Radio wave basics: Radio Wave Basics for IoT
• Path loss and link budgets: Path Loss and Link Budgets

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76.3 Fading and Multipath

76.3.1 What is Multipath?

Radio waves reflect off surfaces (walls, ground, buildings). The receiver sees multiple copies of the signal arriving at slightly different times:

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flowchart LR
    TX["Transmitter"]

    subgraph Paths["Signal Paths"]
        D["Direct Path<br/>(Line of Sight)"]
        R1["Reflected<br/>(Wall)"]
        R2["Reflected<br/>(Ground)"]
        R3["Diffracted<br/>(Corner)"]
    end

    RX["Receiver"]

    TX --> D --> RX
    TX -.-> R1 -.-> RX
    TX -.-> R2 -.-> RX
    TX -.-> R3 -.-> RX

    style TX fill:#16A085,stroke:#0D6655
    style RX fill:#2C3E50,stroke:#1A252F

Figure 76.1: Multipath Signal Propagation with Direct, Reflected, and Diffracted Paths

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sequenceDiagram
    participant TX as Transmitter
    participant RX as Receiver

    Note over TX,RX: Signal "Hello" sent at t=0

    TX->>RX: Direct path arrives t=10ns
    Note over RX: First copy received

    TX-->>RX: Wall reflection t=15ns
    Note over RX: Delayed copy (+5ns)

    TX-->>RX: Ground reflection t=18ns
    Note over RX: Another copy (+8ns)

    TX-->>RX: Building reflection t=25ns
    Note over RX: Late copy (+15ns)

    Note over RX: Result: Multiple overlapping<br/>"Hello" signals interfere<br/>Can add (boost) or<br/>cancel (fade)!

Figure 76.2: Alternative view: Multipath as Time-Delayed Copies - This sequence diagram shows multipath from the receiver’s perspective over time. The transmitter sends one signal, but the receiver gets multiple copies arriving at different times. The direct path arrives first (10ns), followed by wall reflection (15ns), ground reflection (18ns), and building reflection (25ns). These overlapping copies can constructively add (boost signal) or destructively cancel (cause fading). This time-domain view helps explain why simply moving a few centimeters can dramatically change signal strength. {fig-alt=“Sequence diagram showing multipath propagation over time from Transmitter to Receiver. Signal Hello sent at t=0. Direct path arrives at t=10ns (first copy received). Wall reflection arrives at t=15ns (delayed copy +5ns). Ground reflection arrives at t=18ns (another copy +8ns). Building reflection arrives at t=25ns (late copy +15ns). Final note at receiver explains that multiple overlapping Hello signals interfere and can add to boost signal or cancel to cause fading.”}

76.3.2 Types of Fading

Type	Cause	Time Scale	Mitigation
Path Loss	Distance	Static	Increase power, better antennas
Slow Fading (Shadowing)	Obstacles	Seconds-minutes	Fading margin in link budget
Fast Fading (Multipath)	Reflections	Milliseconds	Diversity, spread spectrum
Frequency-Selective Fading	Different delays per frequency	Varies	OFDM, wideband techniques

76.3.3 Fading Margin

To account for unpredictable fading, we add a fading margin to the link budget:

Application	Typical Fading Margin
Indoor, stationary	10-15 dB
Indoor, mobile	15-20 dB
Outdoor, urban	15-25 dB
Outdoor, rural	10-15 dB
Critical/safety	25-30 dB

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76.4 Frequency Band Selection

TipMinimum Viable Understanding: RF Interference Patterns

Core Concept: RF interference occurs when multiple signals occupy the same frequency band simultaneously, causing packet loss, reduced throughput, or complete communication failure - and the 2.4 GHz ISM band (used by Wi-Fi, Bluetooth, Zigbee, and microwave ovens) is the most crowded spectrum in IoT.

Why It Matters: Interference is the hidden cause of most “mysterious” IoT connectivity problems. A sensor network that works perfectly during testing may fail during business hours when Wi-Fi traffic peaks, or when the break room microwave runs. The 2.4 GHz band can see 40-60 overlapping networks in dense urban environments, while sub-GHz bands (868/915 MHz) typically have less than 5 competing signals.

Key Takeaway: Design for interference from day one using the “3C strategy”: Choose frequencies wisely (sub-GHz for critical sensors, avoid 2.4 GHz channels 1-6-11 overlap with Wi-Fi), use Clear Channel Assessment (listen-before-talk), and implement Channel hopping (frequency diversity). If your sensor node reports intermittent failures but RSSI looks good, suspect interference - measure SNR (signal-to-noise ratio) instead of just signal strength.

76.4.1 Choosing the Right Band for Your Application

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flowchart TD
    Start["What are your<br/>requirements?"]

    Start --> Range{"Need > 1km<br/>range?"}
    Range -->|Yes| SubGHz["Consider Sub-GHz<br/>LoRa, Sigfox, NB-IoT"]
    Range -->|No| Data{"Need > 1 Mbps<br/>data rate?"}

    Data -->|Yes| WiFi["Consider Wi-Fi<br/>2.4/5 GHz"]
    Data -->|No| Power{"Ultra-low power<br/>critical?"}

    Power -->|Yes| BLE["Consider BLE<br/>2.4 GHz, low duty cycle"]
    Power -->|No| Mesh{"Need mesh<br/>networking?"}

    Mesh -->|Yes| ZigThread["Consider Zigbee/Thread<br/>2.4 GHz"]
    Mesh -->|No| Default["Default: BLE<br/>or Wi-Fi depending on range"]

    SubGHz --> Licensed{"Licensed spectrum<br/>acceptable?"}
    Licensed -->|Yes| Cellular["Consider NB-IoT/LTE-M"]
    Licensed -->|No| Unlicensed["LoRa or Sigfox"]

    style Start fill:#2C3E50,stroke:#1A252F
    style SubGHz fill:#16A085,stroke:#0D6655
    style WiFi fill:#E67E22,stroke:#AF5F1A
    style BLE fill:#27AE60,stroke:#1E8449
    style ZigThread fill:#3498DB,stroke:#2471A3
    style Cellular fill:#9B59B6,stroke:#7D3C98
    style Unlicensed fill:#16A085,stroke:#0D6655

Figure 76.3: IoT Wireless Frequency Band Selection Decision Flowchart

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graph TB
    subgraph SUBGHZ["Sub-GHz (868/915 MHz)"]
        S1["Range: 5-15 km"]
        S2["Walls: Excellent penetration"]
        S3["Data: 0.3-50 kbps"]
        S4["Apps: Agriculture, Smart City"]
    end

    subgraph GHZ24["2.4 GHz"]
        T1["Range: 50-200 m"]
        T2["Walls: Good penetration"]
        T3["Data: Up to 2 Mbps"]
        T4["Apps: Smart Home, Wearables"]
    end

    subgraph GHZ5["5 GHz"]
        F1["Range: 30-100 m"]
        F2["Walls: Poor penetration"]
        F3["Data: Up to 1 Gbps"]
        F4["Apps: Video, High-speed"]
    end

    LOWER["Lower Frequency<br/>Longer range<br/>Better penetration<br/>Slower data"]

    HIGHER["Higher Frequency<br/>Shorter range<br/>Worse penetration<br/>Faster data"]

    LOWER --> SUBGHZ
    SUBGHZ --> GHZ24
    GHZ24 --> GHZ5
    GHZ5 --> HIGHER

    style S1 fill:#16A085,stroke:#2C3E50,stroke-width:1px,color:#fff
    style S2 fill:#16A085,stroke:#2C3E50,stroke-width:1px,color:#fff
    style S3 fill:#16A085,stroke:#2C3E50,stroke-width:1px,color:#fff
    style S4 fill:#16A085,stroke:#2C3E50,stroke-width:1px,color:#fff
    style T1 fill:#E67E22,stroke:#2C3E50,stroke-width:1px,color:#fff
    style T2 fill:#E67E22,stroke:#2C3E50,stroke-width:1px,color:#fff
    style T3 fill:#E67E22,stroke:#2C3E50,stroke-width:1px,color:#fff
    style T4 fill:#E67E22,stroke:#2C3E50,stroke-width:1px,color:#fff
    style F1 fill:#2C3E50,stroke:#E67E22,stroke-width:1px,color:#fff
    style F2 fill:#2C3E50,stroke:#E67E22,stroke-width:1px,color:#fff
    style F3 fill:#2C3E50,stroke:#E67E22,stroke-width:1px,color:#fff
    style F4 fill:#2C3E50,stroke:#E67E22,stroke-width:1px,color:#fff
    style LOWER fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style HIGHER fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff

Figure 76.4: Alternative view: Frequency Band Spectrum Comparison - Instead of a decision tree, this diagram arranges frequency bands on a spectrum from low to high, showing the fundamental physics trade-off. Lower frequencies (Sub-GHz, teal) travel farther and penetrate walls better but carry less data. Higher frequencies (5 GHz, navy) carry more data but have shorter range and struggle with obstacles. The 2.4 GHz band (orange) sits in the middle, offering a balance. Example applications are listed for each band. This helps students understand that frequency selection is fundamentally about physics trade-offs, not just protocol choice. {fig-alt=“Horizontal spectrum comparison of three IoT frequency bands from lower to higher frequency. Sub-GHz 868/915 MHz section (teal): Range 5-15 km, Walls excellent penetration, Data 0.3-50 kbps, Applications agriculture and smart city. 2.4 GHz section (orange): Range 50-200 m, Walls good penetration, Data up to 2 Mbps, Applications smart home and wearables. 5 GHz section (navy): Range 30-100 m, Walls poor penetration, Data up to 1 Gbps, Applications video and high-speed. Labels explain that lower frequency means longer range, better penetration, slower data while higher frequency means shorter range, worse penetration, faster data.”}

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76.5 RF Interference Analysis and Mitigation

Real-world IoT deployments rarely operate in isolation. Understanding and mitigating RF interference is essential for reliable wireless communication, especially in crowded ISM bands.

76.5.1 Sources of Interference

Co-Channel Interference (Same Frequency):

Source	Affected Technologies	Severity
Wi-Fi networks	BLE, Zigbee, Thread (2.4 GHz)	High
Microwave ovens	All 2.4 GHz devices	Very High (localized)
Other IoT networks	LoRa, Sigfox (sub-GHz)	Medium
Bluetooth audio	BLE sensors	Medium
Baby monitors	2.4 GHz mesh networks	High

Adjacent-Channel Interference:

Channel allocation example (2.4 GHz):
Wi-Fi Ch 1 -----------------
           |  Overlap Zone  |
Zigbee Ch 11-14 -------------
           |  Overlap Zone  |
Wi-Fi Ch 6 -----------------

76.5.2 Interference Measurement Techniques

Spectrum Analysis:

# Pseudo-code for interference survey
def conduct_rf_survey(center_freq, bandwidth, duration_min):
    """Measure RF environment over time"""
    samples = []

    for t in range(duration_min * 60):  # Sample every second
        reading = spectrum_analyzer.measure(
            center_freq=center_freq,
            span=bandwidth,
            rbw=100000  # 100 kHz resolution bandwidth
        )
        samples.append({
            'timestamp': time.time(),
            'peak_power_dbm': reading.peak,
            'avg_power_dbm': reading.average,
            'channel_occupancy': reading.duty_cycle
        })
        time.sleep(1)

    return analyze_interference(samples)

def analyze_interference(samples):
    """Classify interference patterns"""
    return {
        'avg_noise_floor': np.mean([s['avg_power_dbm'] for s in samples]),
        'peak_interference': max([s['peak_power_dbm'] for s in samples]),
        'duty_cycle': np.mean([s['channel_occupancy'] for s in samples]),
        'interference_events': count_peaks_above_threshold(samples, -60)
    }

Key Metrics to Capture:

Metric	Threshold	Interpretation
Noise floor	> -90 dBm	Elevated background interference
Peak power	> -40 dBm	Strong interferer present
Duty cycle	> 50%	Channel heavily occupied
Interference events/hour	> 100	Bursty interference source

76.5.3 Channel Planning and Avoidance

2.4 GHz Coexistence Strategy:

Non-overlapping channel sets:

Wi-Fi-friendly Zigbee channels:
- Zigbee 15, 20, 25, 26 (avoid Wi-Fi 1, 6, 11 overlap)

Recommended layout:
+--------------------------------------------------+
| 2400    2412    2437    2462    2480 MHz         |
| |       | Wi-Fi1 | Wi-Fi6 | Wi-Fi11|        |    |
| |       +--------+--------+--------+        |    |
| |                                           |    |
| +-----Zigbee 15-----Zigbee 20-----Zigbee 25-----|
|         (2425)        (2450)        (2475)       |
+--------------------------------------------------+

Sub-GHz Channel Selection (LoRa/Sigfox):

Region	Frequency Band	Duty Cycle Limit	Best Channels
EU868	868-868.6 MHz	1%	868.1, 868.3, 868.5
US915	902-928 MHz	No limit (FCC)	Use all 64 uplink
AS923	923-923.5 MHz	Varies	923.2, 923.4

76.5.4 Adaptive Frequency Hopping (AFH)

Bluetooth AFH Implementation:

Standard hopping: 79 channels, 1600 hops/sec

AFH channel map (example with 30% blocked):
Good channels:  [0,1,2,5,6,7,8,9,12,13,...]  (55 channels)
Bad channels:   [3,4,10,11,40,41,42,...]     (24 blocked)

Update frequency: Every 30 seconds based on:
- Packet error rate per channel
- RSSI measurements
- Blacklist from master

Implementation Considerations:

1. Minimum channels: Bluetooth requires at least 20 good channels
2. Update latency: Changes propagate to all slaves within 6 slots
3. Hysteresis: Don’t thrash channels based on single errors

76.5.5 Physical Layer Mitigation

Antenna Techniques:

Technique	Benefit	Application
Directional antenna	6-15 dB rejection of off-axis interference	Fixed outdoor links
Antenna diversity	3-6 dB gain against multipath fading	Indoor gateways
Polarization isolation	20-30 dB between cross-polarized signals	Collocated systems
Spatial separation	6 dB per doubling of distance	Antenna placement

Power Control Strategy:

def adaptive_power_control(current_rssi, target_rssi, current_tx_power):
    """Adjust transmit power to minimize interference contribution"""
    margin = 6  # dB safety margin

    if current_rssi > target_rssi + margin:
        # Signal too strong, reduce power
        new_power = current_tx_power - (current_rssi - target_rssi - margin)
    elif current_rssi < target_rssi - margin:
        # Signal too weak, increase power
        new_power = current_tx_power + (target_rssi - current_rssi)
    else:
        new_power = current_tx_power

    # Clamp to regulatory limits
    return max(min(new_power, MAX_TX_POWER), MIN_TX_POWER)

76.5.6 Protocol-Level Mitigation

Clear Channel Assessment (CCA):

Before transmit:
1. Listen for energy on channel (ED threshold: -62 dBm for 802.15.4)
2. If busy, defer and backoff
3. After backoff, check again
4. Maximum retries: typically 4-5

Backoff algorithm:
backoff_time = random(0, 2^BE - 1) x unit_period
where BE starts at 3, increments to max 5 on collision

Listen Before Talk (LBT) for Sub-GHz:

Required in EU868: - Minimum listen time: 5 ms - Threshold: -80 dBm (or technology-specific) - Penalty: Must wait 1 second if channel busy

76.5.7 Troubleshooting Interference Issues

Symptom: High Packet Error Rate

1. Measure RSSI and SNR (signal-to-noise ratio)
2. If RSSI good but SNR poor -> interference present
3. Conduct spectrum sweep during peak interference
4. Identify interferer by frequency, duty cycle, modulation

Symptom: Intermittent Connectivity

1. Log connection events with timestamps
2. Correlate with known interference sources (work schedules, microwave use)
3. Check for hidden node problem in mesh networks
4. Verify antenna orientation hasn’t changed

Symptom: Reduced Range

1. Verify transmit power setting
2. Check antenna connection (2 dB loss from poor SMA connection)
3. Measure noise floor vs. commissioning baseline
4. Look for new interference sources (new Wi-Fi APs, industrial equipment)

76.5.8 Site Survey Checklist

Pre-Deployment Survey:

• Measure noise floor at each sensor location
• Identify all existing RF systems (Wi-Fi SSIDs, alarm systems)
• Check for industrial equipment (motors, welders, inverters)
• Map microwave oven and elevator locations
• Test during peak occupancy times (weekday afternoons)
• Document channel plan for Wi-Fi and IoT coexistence
• Verify regulatory compliance (power, duty cycle, LBT)
• Plan antenna placement for polarization diversity

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76.6 Visual Reference Gallery

NoteStatic Diagram: IoT Frequency Band Selection Flowchart

Frequency Band Selection Decision Tree

A practical decision tree to help select the appropriate wireless technology based on application requirements including range, data rate, power consumption, and networking topology.

NoteStatic Diagram: Multipath Signal Propagation

Multipath Signal Propagation

Visualization of multipath propagation showing how radio signals take multiple paths from transmitter to receiver through reflection, diffraction, and direct line-of-sight.

NoteVisual: Fresnel Zone Clearance

Fresnel Zone

Fresnel zone clearance is critical for reliable line-of-sight wireless links, especially for long-range IoT deployments.

NoteVisual: Antenna Radiation Pattern

Antenna Pattern

Antenna pattern selection affects coverage area, range, and interference characteristics of IoT deployments.

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76.7 Summary

Concept	Key Points
Multipath	Multiple signal copies arrive at different times, causing fading
Slow Fading	Shadowing from obstacles, seconds-minutes time scale
Fast Fading	Multipath reflections, milliseconds time scale
Fading Margin	Add 10-30 dB depending on environment and criticality
2.4 GHz Interference	Wi-Fi, Bluetooth, microwaves compete for same spectrum
Sub-GHz Advantage	Less crowded, better penetration, longer range
CCA/LBT	Listen-before-talk protocols reduce collisions
Frequency Hopping	Spread interference across channels

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76.8 What’s Next

With fading and interference understood, continue to:

• Practical Wireless Lab - Hands-on experiments with RSSI and packet loss

NoteRelated Chapters

• Radio Wave Basics for IoT - Frequency bands and trade-offs
• Path Loss and Link Budgets - Calculate link margins
• LPWAN Introduction - Long-range IoT technologies
• Protocol Selection Framework - Choose the right protocol