822  Mobile Wireless: Scenario-Based Analysis

822.1 Learning Objectives

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

  • Analyze Agricultural Deployments: Plan smart agriculture networks with multi-year battery life
  • Mitigate 2.4 GHz Interference: Resolve Zigbee/Wi-Fi coexistence issues using spectrum analysis
  • Calculate Indoor Link Budgets: Determine coverage viability for multi-story buildings
  • Apply Trade-off Reasoning: Balance range, power, interference, and cost in real scenarios

822.2 Prerequisites

Required Chapters: - Mobile Wireless Technologies Basics - Core concepts - Cellular Network Architecture - Cellular IoT selection - Networking Fundamentals - Basic networking

Technical Background: - Path loss and link budget concepts - Frequency band characteristics - ISM band regulations (duty cycle, power limits)

Estimated Time: 45 minutes

What are these scenarios? Real-world wireless deployment problems that require trade-off analysis. Each scenario presents constraints and asks you to reason through solutions.

How to approach them: 1. Read the scenario and constraints carefully 2. Think about the questions before revealing the answer 3. Study the “Key Insight” sections for important principles 4. Use “Verify Your Understanding” to test your reasoning

Why scenarios matter: Multiple-choice questions test recall. Scenarios test understanding - the ability to apply principles to new situations you haven’t seen before.

822.3 Scenario 1: Large-Area Agriculture Wireless Design

Scenario: You’re deploying soil moisture sensors across a 200-hectare farm (1.4 km x 1.4 km). Sensors must transmit 100-byte readings every 15 minutes and run on batteries for 5+ years without replacement. The farm has crops, equipment, and varying terrain that will obstruct line-of-sight.

Think about: 1. How does radio frequency affect range when penetrating vegetation and soil? 2. What battery capacity is needed for 5 years if transmitting 96 times per day? 3. Why might infrastructure cost matter less than battery replacement labor over 5 years?

Key Insight: Range vs Frequency

Rules of thumb: - At the same distance, 868/915 MHz has approximately 9 dB less free-space path loss than 2.4 GHz - Lower frequencies are often more forgiving with foliage and non-line-of-sight paths, but range is still site-dependent (antenna height, terrain, noise floor, regulations)

Path Loss Calculation:

Using the free-space path loss formula: FSPL(dB) = 20log(d_km) + 20log(f_MHz) + 32.45

Frequency FSPL at 1 km Difference
868 MHz 91.2 dB baseline
915 MHz 91.7 dB +0.5 dB
2.4 GHz 100.0 dB +8.8 dB

The approximately 9 dB advantage means sub-GHz can reach similar distances with roughly 8x less transmit power (or achieve greater range with equal power).

Key Insight: Battery Life Reality Check

  • 100 bytes every 15 minutes averages approximately 0.9 bps, so bandwidth is not the limiting factor
  • Battery life is dominated by time-on-air, receive windows, and retries - not just payload size
  • A useful estimate: I_avg = (I_tx x t_tx + I_rx x t_rx + I_sleep x t_sleep) / 24h, then compare against battery capacity (and include temperature/aging margins)

Example Power Budget (LoRaWAN Class A):

State Current Duration Energy/Day
Transmit (14 dBm) 120 mA 80 ms x 96 = 7.68 s 0.26 mAh
RX Windows 12 mA 500 ms x 96 = 48 s 0.16 mAh
Sleep (PSM) 2 uA 86,344 s 0.05 mAh
Total 0.47 mAh/day

With a 19,000 mAh lithium battery (common D-cell): 19,000 / 0.47 = 40,000+ days theoretical

Reality factors (temperature, self-discharge, aging): 5-10 year battery life is achievable.

Key Insight: Likely Architectures

Choose based on your specific constraints:

Sub-GHz LPWAN (LoRaWAN): - Good fit when you can deploy/operate gateways and tolerate shared-spectrum constraints - Typical: 1-3 gateways cover 200 hectares with proper antenna placement - No per-device subscription fees

Licensed Cellular LPWAN (NB-IoT/LTE-M): - Good fit if coverage exists and subscriptions/lock-in are acceptable - Reduces your gateway operations burden - Carrier manages network infrastructure

2.4 GHz Mesh (Zigbee/Thread): - Can work if you can place powered/solar routers - Avoid making battery sensors route traffic - Range limitations in outdoor/vegetated environments

Wi-Fi: - Typically needs power and denser infrastructure - Best when throughput is the priority - Not ideal for multi-year battery operation

Verify Your Understanding:

  1. If 2.4 GHz adds approximately 9 dB of FSPL vs 868/915 at the same distance, what does that imply for range in free space (n=2) vs a cluttered environment (n > 3)?

  2. Which deployment choices increase link margin without raising transmit power (gateway height, antenna choice, payload interval, data rate)?

  3. Where do you expect the operational cost to land: field visits for batteries vs installing/maintaining gateways?

Answer (sketch): Approximately 9 dB corresponds to roughly 8x power. In free space that’s roughly 2.8x range (since range scales with sqrt(power) for n=2), and less in cluttered environments. Mesh routing shifts cost to always-on routers and maintenance. The payload rate is tiny, so the real battery drivers are airtime, retries, and idle current.

822.4 Scenario 2: 2.4 GHz Interference Mitigation

Scenario: Your Zigbee smart building deployment on Channel 20 (2450 MHz) is failing. RSSI measurements show: - Desired Zigbee signal: -65 dBm - Wi-Fi Channel 6 (2437 MHz): -55 dBm (10 dB stronger!) - Wi-Fi Channel 11 (2462 MHz): -60 dBm - Microwave oven: periodic -40 dBm spikes (25 dB stronger than Zigbee!)

Think about: 1. How wide is a Wi-Fi channel vs a Zigbee channel in MHz? 2. Which Zigbee channels avoid Wi-Fi Channel 6 and 11 overlap? 3. Can you eliminate microwave interference by changing channels?

Key Insight: Channel Bandwidth Mismatch

  • Wi-Fi channels: 22 MHz wide, centered at 5 MHz intervals
  • Zigbee channels (15-26): 2 MHz wide, centered at 5 MHz intervals
  • Wi-Fi Channel 6 (2437 MHz) spans 2426-2448 MHz
  • Zigbee Channel 20 sits between Wi-Fi channels 6 and 11, but strong Wi-Fi on both sides can still cause adjacent-channel interference. It’s also near the microwave oven center frequency (approximately 2.45 GHz).

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graph LR
    subgraph "2.4 GHz ISM Band"
        direction LR
        WIFI1["Wi-Fi Ch1<br/>2401-2423<br/>22 MHz"]
        WIFI6["Wi-Fi Ch6<br/>2426-2448<br/>22 MHz"]
        WIFI11["Wi-Fi Ch11<br/>2451-2473<br/>22 MHz"]
    end

    subgraph "Zigbee Channels"
        ZB15["Ch15<br/>2425"]
        ZB20["Ch20<br/>2450"]
        ZB25["Ch25<br/>2475"]
    end

    MICRO["Microwave<br/>~2.45 GHz<br/>Broadband"]

    WIFI1 -.-> ZB15
    WIFI6 -.-> ZB15
    WIFI6 -.-> ZB20
    WIFI11 -.-> ZB20
    MICRO -.-> ZB20

    style WIFI1 fill:#E67E22,stroke:#2C3E50,color:#fff
    style WIFI6 fill:#E67E22,stroke:#2C3E50,color:#fff
    style WIFI11 fill:#E67E22,stroke:#2C3E50,color:#fff
    style ZB15 fill:#16A085,stroke:#2C3E50,color:#fff
    style ZB20 fill:#16A085,stroke:#2C3E50,color:#fff
    style ZB25 fill:#16A085,stroke:#2C3E50,color:#fff
    style MICRO fill:#7F8C8D,stroke:#2C3E50,color:#fff

Figure 822.1

Key Insight: Frequency Separation Analysis

Zigbee Ch Center (MHz) Delta to Wi-Fi Ch6 (2437) Delta to Wi-Fi Ch11 (2462) Practical note
15 2425 12 MHz 37 MHz Often good when Wi-Fi uses 6/11 heavily
20 2450 13 MHz 12 MHz “Squeezed” between 6 and 11 when both are active
25 2475 38 MHz 13 MHz Above Wi-Fi 11; often a solid first choice
26 2480 43 MHz 18 MHz Also above Wi-Fi 11, but near band edge (power/regulatory constraints vary)

Key Insight: Best Practice - Wi-Fi Coexistence

  1. Use a spectrum scan to pick a Zigbee channel with the lowest observed interference (common candidates: 15, 20, 25)
  2. In this scenario (strong Wi-Fi on 6 and 11 + microwave spikes near 2.45 GHz), Zigbee channel 25 is often a good first try
  3. If you control the Wi-Fi network, moving high-throughput traffic to 5 GHz reduces 2.4 GHz congestion for Zigbee

Key Insight: Microwave Reality

  • Microwave ovens leak noise centered around approximately 2.45 GHz that can impact multiple nearby channels (often worst around Zigbee channel 20)
  • Channel selection can reduce impact, but you should still expect periodic retries during microwave use
  • Mitigation: Zigbee’s CSMA/CA automatically retries when clear
  • Microwaves typically run less than 5 minutes; Zigbee tolerates brief outages

Verify Your Understanding:

  1. Why does Wi-Fi Channel 6 span 2426-2448 MHz if it’s “centered” at 2437 MHz?
  2. If you can’t change Zigbee channel, what Wi-Fi channels would reduce interference?
  3. Why is -55 dBm Wi-Fi worse for Zigbee than -65 dBm desired signal?

Answer: Wi-Fi uses approximately 20-22 MHz channels, so +/-10-11 MHz from center. If Zigbee must stay on channel 20, move Wi-Fi away from channels 6/11 (or move Wi-Fi to 5 GHz). The -55 dBm Wi-Fi signal is approximately 10 dB stronger than Zigbee, so the receiver’s signal-to-interference ratio is poor even if the Zigbee link budget is “fine.”

822.6 Scenario Analysis Framework

When analyzing wireless deployment scenarios, use this systematic framework:

822.6.1 Step 1: Identify Constraints

Category Questions
Range Maximum distance? Obstacles? Indoor/outdoor?
Power Battery life requirement? Power source available?
Data Payload size? Transmission frequency? Latency tolerance?
Environment Interference sources? Regulatory region?
Cost Per-device budget? Infrastructure investment?

822.6.3 Step 3: Select Technology

Match constraints to technology capabilities:

If You Need… Consider…
Long range, low power, infrequent data Sub-GHz LPWAN (LoRaWAN, Sigfox)
Long range, moderate data, mobility Cellular (LTE-M, NB-IoT)
Short range, low power, mesh 802.15.4 (Zigbee, Thread)
Short range, high data Wi-Fi (2.4/5/6 GHz)
Indoor, deep penetration Sub-GHz or NB-IoT

822.6.4 Step 4: Validate and Iterate

  • Conduct site surveys before deployment
  • Measure actual path loss vs calculated
  • Test under realistic interference conditions
  • Plan for worst-case scenarios

Sammy Sensor: “Wireless problems are like detective cases! You gather clues (measurements), analyze evidence (link budgets), and solve the mystery!”

Lila the Light Sensor: “When Wi-Fi and Zigbee fight over the same frequencies, it’s like two people trying to talk at the same time - someone needs to move to a different conversation!”

Max the Motion Detector: “Buildings are like mazes for radio waves. Walls slow them down, floors really slow them down, and metal stops them almost completely!”

Bella the Button: “The best wireless detective always checks the crime scene (site survey) before guessing what happened!”

822.7 Summary

This chapter practiced scenario-based wireless analysis:

Agriculture Deployment: - Sub-GHz frequencies provide 8-9 dB link budget advantage over 2.4 GHz - Battery life depends on time-on-air, not payload size - LoRaWAN or cellular LPWAN are typical solutions for multi-year battery life

Interference Mitigation: - Wi-Fi channels are 22 MHz wide; Zigbee channels are 2 MHz wide - Channel selection requires understanding frequency overlap, not just channel numbers - Move 2.4 GHz traffic to 5 GHz when possible

Indoor Link Budget: - Floor penetration adds significant loss (15+ dB) - Multi-floor coverage from single AP is often marginal - Design with appropriate fade margin (10-20 dB)

General Principles: - Always calculate before deploying - Validate with site surveys - Plan for worst-case scenarios

822.8 What’s Next

Continue your mobile wireless review with: