808  Quiz: Frequency Band Selection for IoT

808.1 Introduction

This chapter provides assessments focused on frequency band selection and interference mitigation for IoT deployments. You’ll work through scenarios involving smart agriculture, 2.4 GHz coexistence, and technology selection based on range and power requirements.

NoteLearning Objectives

By completing this chapter, you will be able to:

  • Apply frequency band selection criteria to agricultural and outdoor IoT scenarios
  • Analyze interference patterns in the 2.4 GHz ISM band
  • Evaluate technology trade-offs between LoRaWAN, Zigbee, and Wi-Fi
  • Calculate link budget advantages of sub-GHz vs 2.4 GHz frequencies

808.2 Prerequisites

Before attempting these assessments, you should have completed:

808.3 Knowledge Check: Frequency Band Selection

A smart agriculture deployment needs to monitor soil moisture sensors across 200 hectares of farmland. Sensors report every 15 minutes and transmit 100 bytes per report. Battery life must exceed 5 years. Which frequency band and technology combination is most appropriate?

Explanation: Sub-GHz LPWAN (LoRaWAN) is built for km-scale coverage and multi-year batteries with small, infrequent payloads. Wi-Fi is power/infrastructure heavy, and Zigbee mesh adds forwarding overhead and many relay nodes at this scale.

Answer: B) 868/915 MHz LoRaWAN

Explanation:

This scenario requires careful analysis of range, power consumption, and data rate requirements.

Requirements Analysis:

Output:

Smart Agriculture Network Requirements:
  Field size: 1414m × 1414m
  Maximum sensor distance: 1414m (1.4 km)
  Data throughput: 9600 bytes/day per sensor
  Daily power consumption: 0.23 mAh/day
  Required battery capacity: 421 mAh (0.4 Ah)

Battery options for 5 years:
  2× D-cell (20,000 mAh): 4748.9% sufficient
  4× AA (10,000 mAh): 2374.4% sufficient

Technology Comparison:

Criterion 2.4 GHz Zigbee 868/915 LoRaWAN 5 GHz Wi-Fi 2.4 GHz Wi-Fi
Range 100-300m (mesh) 2-15 km (star) 50-100m 100-250m
Coverage Requires many routers 1-2 gateways sufficient Many APs needed Many APs needed
Power Medium (mesh hops) Very low (sleep mode) High (always on) High
Battery Life 1-2 years 5-10 years Days Months
Infrastructure Mesh nodes Single gateway Many APs Multiple APs
Data Rate 250 kbps (sufficient) 50 kbps (sufficient) 100+ Mbps (overkill) 1+ Mbps (overkill)
Cost Medium Low per node High High

Why LoRaWAN wins:

  1. Range: Sub-GHz frequency provides 1-2 km+ range, easily covering 200 hectares from 1-2 gateway locations
  2. Path loss advantage: 8-9 dB better than 2.4 GHz (demonstrated in earlier examples)
  3. Battery life: Ultra-low power sleep mode enables 5-10 year battery life with AA batteries
  4. Infrastructure cost: Single gateway vs. dozens of mesh routers or APs
  5. Penetration: Better penetration through soil, vegetation, farm equipment
  6. Data rate: 50 kbps more than adequate for 100-byte reports

Why other options are unsuitable:

  • A (Zigbee): Requires many mesh router nodes across 200 hectares, increasing cost and complexity. Battery life challenges due to mesh forwarding duty.
  • C (5 GHz Wi-Fi): Extremely short range, would need dozens of APs. High power consumption (battery life measured in days, not years). Massive overkill for 100-byte payloads.
  • D (2.4 GHz Wi-Fi): Better range than 5 GHz but still requires many APs. Power consumption prevents multi-year battery life.

Cost Analysis:

LoRaWAN Option:
  1× Gateway: $300
  100× Sensors with LoRa: $25 each = $2,500
  Total: $2,800

Zigbee Option:
  20× Router nodes: $40 each = $800
  100× Sensors with Zigbee: $20 each = $2,000
  Total: $2,800
  BUT: Routers need power/solar, increasing installation cost

Wi-Fi Option:
  15× Outdoor APs: $200 each = $3,000
  Ethernet/power infrastructure: $5,000+
  100× Wi-Fi sensors: $30 each = $3,000
  Total: $11,000+
  Battery life: Not feasible

Best Practice: For large-area, low-data-rate, battery-powered sensor networks, sub-GHz LPWAN technologies (LoRaWAN, Sigfox, NB-IoT) are the appropriate choice. The physics of radio propagation strongly favor lower frequencies for range and power efficiency.

808.4 Knowledge Check: Interference Mitigation

Your IoT deployment is experiencing poor performance in the 2.4 GHz band due to interference from neighboring Wi-Fi networks, Bluetooth devices, and a microwave oven. Current setup uses Zigbee on Channel 20 (2450 MHz). RSSI measurements show:

  • Zigbee RSSI: -65 dBm (desired signal)
  • Wi-Fi on Channel 6 (2437 MHz): -55 dBm
  • Wi-Fi on Channel 11 (2462 MHz): -60 dBm
  • Microwave interference: periodic -40 dBm spikes

What is the best mitigation strategy?

Explanation: Zigbee channels 15/16 (low end) and 25/26 (high end) often reduce overlap with Wi-Fi Ch1/6/11. Power increases don’t remove interference, Zigbee doesn’t operate at 5 GHz, and Zigbee uses DSSS (not Bluetooth-style FHSS).

Answer: B) Switch Zigbee to Channel 15 (2425 MHz) to avoid Wi-Fi overlap

Explanation:

This requires understanding channel allocation in the 2.4 GHz ISM band and interference avoidance strategies.

Channel Overlap Analysis:

Output:

======================================================================
2.4 GHz INTERFERENCE ANALYSIS
======================================================================

Current Configuration:
  Zigbee Channel: 20 (2450 MHz)
  Wi-Fi Channels: [6, 11]
  Interference Score: 98.0 (higher = worse)

======================================================================
CHANNEL ANALYSIS (Zigbee Channels 15-26)
======================================================================
Channel    Frequency       Interference Score   Status
----------------------------------------------------------------------
15         2425            0.0                  ✓ BEST
16         2430            0.0
17         2435            20.0
18         2440            40.0
19         2445            60.0
20         2450            98.0                 CURRENT
21         2455            88.0
22         2460            78.0
23         2465            68.0
24         2470            58.0
25         2475            28.0
26         2480            8.0

======================================================================
RECOMMENDATION: Switch to Channel 15 (2425 MHz)
======================================================================
Interference reduction: 98.0 → 0.0
Improvement: 100.0%

Frequency Separation Analysis:
  Wi-Fi Ch6 (2437 MHz):
    Current separation: 13 MHz
    Recommended separation: 12 MHz
  Wi-Fi Ch11 (2462 MHz):
    Current separation: 12 MHz
    Recommended separation: 37 MHz

Generating spectrum visualization...

Why each option is right or wrong:

A) Increase transmit power - WRONG - Doesn’t solve interference problem - Creates more interference for neighbors - May violate regulatory limits - Increases power consumption - Interference is still present at receiver

B) Switch to Channel 15 - CORRECT - Channel 15 (2425 MHz) has maximum separation from Wi-Fi Ch6 (2437 MHz) and Ch11 (2462 MHz) - 12 MHz separation from Ch6 edge, 37 MHz from Ch11 - Completely avoids Wi-Fi overlap - No regulatory or power consumption changes needed - Zigbee channels 15-16 are specifically recommended for Wi-Fi coexistence

C) Move to 5 GHz - WRONG - Zigbee/802.15.4 doesn’t operate in 5 GHz band - Would require completely different hardware - Higher cost, shorter range, more power consumption - Not a practical solution

D) Frequency hopping on current channel - WRONG - Zigbee uses DSSS (Direct Sequence Spread Spectrum), not FHSS - Can’t implement frequency hopping without protocol change - Still wouldn’t solve the interference if staying in same frequency range - Bluetooth uses FHSS, but that’s a different protocol

Best Practice for 2.4 GHz Coexistence:

Recommended Zigbee channels when Wi-Fi is present: - Wi-Fi using Ch1, 6, 11: Use Zigbee Ch15-16 or Ch25-26 - Wi-Fi using Ch1: Use Zigbee Ch21-26 - Wi-Fi using Ch11: Use Zigbee Ch11-16

Microwave Oven Note: - Microwaves emit broadband noise across 2.4-2.48 GHz - Channel selection won’t eliminate microwave interference - Solution: Physical distance or timing (microwaves typically run <5 minutes) - Zigbee’s CSMA/CA will automatically retry during microwave off periods

808.5 Summary

This quiz covered two critical frequency selection scenarios:

  1. Smart Agriculture: Sub-GHz LoRaWAN provides the optimal combination of range (2-15 km), battery life (5-10 years), and cost for large-area, low-data-rate sensor networks
  2. Interference Mitigation: Channel selection is the first-line defense against 2.4 GHz interference; Zigbee channels 15-16 and 25-26 minimize Wi-Fi overlap

Key Takeaways:

  • Lower frequencies (sub-GHz) provide 8-9 dB path loss advantage over 2.4 GHz
  • Battery life is primarily determined by sleep current, not transmit power
  • Interference mitigation through channel selection is more effective than power increases
  • Technology selection must match application requirements (range, power, data rate)

808.6 What’s Next

Continue testing your wireless knowledge:

Related Chapters: - Design Considerations and Labs - Practical frequency selection frameworks - LoRaWAN Overview - Sub-GHz LPWAN deep dive