814  Cellular Spectrum for IoT

814.1 Introduction

⏱️ ~8 min | ⭐⭐ Intermediate | πŸ“‹ P08.C16C.U01

While unlicensed spectrum (Wi-Fi, Bluetooth, LoRa) dominates many IoT deployments, cellular IoT technologies like NB-IoT and LTE-M leverage licensed spectrum to provide wide-area coverage with carrier-grade reliability. This chapter explores how cellular spectrum has evolved and why specific bands matter for IoT applications.

NoteLearning Objectives

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

  • Trace the evolution of cellular spectrum from 1G to 5G
  • Explain how multiple access techniques (FDMA, TDMA, CDMA, OFDMA) improved spectral efficiency
  • Compare low-band, mid-band, and mmWave frequencies for IoT applications
  • Understand why NB-IoT and LTE-M use specific cellular bands
  • Evaluate cellular vs unlicensed spectrum trade-offs for IoT deployments

814.2 Prerequisites

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

Series Navigation: - Previous: IoT Frequency Bands and Licensing - Next: Propagation and Design

Cellular IoT Deep Dives: - Cellular IoT Fundamentals - NB-IoT, LTE-M architecture - NB-IoT Fundamentals - Deep dive into NB-IoT spectrum and power modes - LPWAN Fundamentals - Comparison with unlicensed alternatives

814.3 Cellular Spectrum Allocation for IoT

⏱️ ~15 min | ⭐⭐ Intermediate | πŸ“‹ P08.C16C.U02

814.3.1 Evolution of Cellular Spectrum

The cellular industry has evolved through multiple generations, each requiring new spectrum allocations and improving spectral efficiency. Understanding this evolution is critical for IoT designers choosing between cellular IoT technologies (NB-IoT, LTE-M, 5G) and unlicensed alternatives (LoRaWAN, Sigfox).

%% fig-cap: "Cellular generations: representative spectrum ranges and channelization (illustrative)"
%% fig-alt: "Diagram showing representative cellular bands and channel bandwidths by generation: 1G analog in ~800–900 MHz with narrow channels, 2G GSM in 900/1800/1900 MHz with 200 kHz carriers, 3G WCDMA in 850/900/1700/1900/2100 MHz with 5 MHz carriers, 4G LTE in 700–2600+ MHz with 1.4–20 MHz carriers, and 5G NR in FR1 (600 MHz–6 GHz) and FR2 (~24–52 GHz), emphasizing that actual bands vary by region and operator"
%%{init: {'theme': 'base', 'themeVariables': { 'primaryColor': '#2C3E50', 'primaryTextColor': '#2C3E50', 'primaryBorderColor': '#16A085', 'lineColor': '#16A085', 'secondaryColor': '#E67E22', 'tertiaryColor': '#ECF0F1'}}}%%
graph TB
    subgraph SPECTRUM["Cellular Spectrum Evolution"]
        direction TB
        A["1G (Analog)<br/>~800–900 MHz<br/>Narrow channels"] --> B["2G (GSM)<br/>900/1800/1900 MHz<br/>200 kHz carriers"]
        B --> C["3G (WCDMA)<br/>850/900/1700/1900/2100 MHz<br/>5 MHz carriers"]
        C --> D["4G (LTE)<br/>700–2600+ MHz<br/>1.4–20 MHz carriers"]
        D --> E["5G (NR)<br/>FR1: 600 MHz–6 GHz<br/>FR2: ~24–52 GHz"]
    end

    subgraph TREND["Evolution Trends"]
        F["Higher Frequencies"] --> G["More Bandwidth"]
        G --> H["Better Spectral Efficiency"]
        H --> I["More Devices per Cell"]
    end

    style A fill:#7F8C8D,stroke:#2C3E50,stroke-width:2px,color:#fff
    style B fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style C fill:#E67E22,stroke:#16A085,stroke-width:2px
    style D fill:#2C3E50,stroke:#16A085,stroke-width:3px,color:#fff
    style E fill:#16A085,stroke:#E67E22,stroke-width:3px,color:#fff
    style F fill:#ECF0F1,stroke:#16A085,stroke-width:2px
    style G fill:#ECF0F1,stroke:#16A085,stroke-width:2px
    style H fill:#ECF0F1,stroke:#16A085,stroke-width:2px
    style I fill:#ECF0F1,stroke:#16A085,stroke-width:2px

Figure 814.1: High-level evolution of cellular generations and spectrum usage (1Gβ†’5G), including where LTE‑M/NB‑IoT fit.

814.3.2 Cellular Frequency Band Allocations

The following table summarizes representative cellular bands and typical channel bandwidths by generation (actual allocations vary by region and operator):

Generation Example bands (varies) Typical channel bandwidth Multiple access (typical) IoT Relevance
1G ~800–900 MHz ~30 kHz channels (analog) FDMA (AMPS) Historical only
2G 900 / 1800 / 1900 MHz 200 kHz carriers (GSM) TDMA (GSM) Legacy M2M modules (sunset in many regions)
3G 850 / 900 / 1700 / 1900 / 2100 MHz 5 MHz carriers (WCDMA) CDMA/WCDMA Being phased out
4G/LTE 700–2600+ MHz 1.4–20 MHz carriers OFDMA NB-IoT, LTE-M (in LTE bands)
5G NR (FR1) 600 MHz–6 GHz 5–100 MHz carriers OFDMA RedCap, mMTC (coverage-focused)
5G NR (FR2) ~24–52 GHz 50–400 MHz carriers OFDM + beamforming High throughput, limited range
NoteKey Insight: More Spectrum vs Better Efficiency

The wireless industry faces a fundamental choice: 1. Get more spectrum - Acquire new frequency bands (expensive, limited availability) 2. Improve spectral efficiency - Send more bits per Hz (technical innovation)

Each cellular generation has pursued BOTH strategies, but spectral efficiency improvements provide greater long-term value.

814.3.3 Spectral Efficiency Evolution: FDMA β†’ TDMA β†’ CDMA β†’ OFDMA

The evolution of multiple access techniques shows how the industry has dramatically improved spectral efficiencyβ€”the number of bits transmitted per second per Hz of bandwidth.

Note: The spectral-efficiency figures below are illustrative; real-world efficiency depends heavily on modulation/coding, MIMO, and radio conditions.

%% fig-cap: "Evolution of multiple access techniques from 1G FDMA to 5G OFDMA"
%% fig-alt: "Comparison diagram showing four multiple access techniques: FDMA (1G) divides spectrum into fixed frequency channels with 1 user per channel; TDMA (2G) adds time slots allowing multiple users per frequency; CDMA (3G) overlays all users on the same frequency using unique codes with soft capacity; OFDMA (4G/5G) uses orthogonal subcarriers dynamically allocated to users, providing higher spectral efficiency (values are illustrative and vary by conditions)"
%%{init: {'theme': 'base', 'themeVariables': { 'primaryColor': '#2C3E50', 'primaryTextColor': '#2C3E50', 'primaryBorderColor': '#16A085', 'lineColor': '#16A085', 'secondaryColor': '#E67E22', 'tertiaryColor': '#ECF0F1'}}}%%
graph LR
    subgraph FDMA["1G: FDMA"]
        F1["Frequency<br/>Division"]
        F2["30 kHz channels<br/>1 user/channel<br/>~0.03 bits/s/Hz"]
    end

    subgraph TDMA["2G: TDMA"]
        T1["Time + Frequency<br/>Division"]
        T2["200 kHz Γ— 8 slots<br/>8 users/channel<br/>~0.5 bits/s/Hz"]
    end

    subgraph CDMA["3G: CDMA"]
        C1["Code Division<br/>Spread Spectrum"]
        C2["5 MHz channels<br/>Soft capacity<br/>~1-2 bits/s/Hz"]
    end

    subgraph OFDMA["4G/5G: OFDMA"]
        O1["Orthogonal<br/>Subcarriers"]
        O2["Dynamic allocation<br/>MIMO support<br/>~5-10 bits/s/Hz"]
    end

    FDMA --> TDMA --> CDMA --> OFDMA

    style F1 fill:#7F8C8D,stroke:#2C3E50,stroke-width:2px,color:#fff
    style T1 fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style C1 fill:#E67E22,stroke:#16A085,stroke-width:2px
    style O1 fill:#2C3E50,stroke:#16A085,stroke-width:3px,color:#fff

Figure 814.2: Multiple access methods (FDMA/TDMA/CDMA/OFDMA) and how they share spectrum among users and devices.

How Each Technique Works:

  1. FDMA (Frequency Division Multiple Access) - 1G
    • Divides available spectrum into fixed frequency channels (30 kHz each)
    • One user occupies one channel for entire call duration
    • Simple but highly inefficient use of spectrum
    • Spectral Efficiency: ~0.03 bits/s/Hz
  2. TDMA (Time Division Multiple Access) - 2G GSM
    • Combines frequency and time division
    • Each 200 kHz channel divided into 8 time slots
    • 8 users share one frequency channel
    • Spectral Efficiency: ~0.5 bits/s/Hz (16Γ— improvement over FDMA)
  3. CDMA (Code Division Multiple Access) - 3G
    • All users transmit on same frequency simultaneously
    • Each user assigned unique spreading code
    • Receivers filter out other users using correlation
    • Spectral Efficiency: ~1-2 bits/s/Hz with soft capacity limits
  4. OFDMA (Orthogonal Frequency Division Multiple Access) - 4G/5G
    • Thousands of orthogonal subcarriers (15 kHz each in LTE)
    • Dynamic allocation based on channel conditions
    • Enables massive MIMO and spatial multiplexing
    • Spectral Efficiency: ~5-10 bits/s/Hz (or higher with MIMO)

814.4 Trade-offs: Range vs Bandwidth vs Penetration

⏱️ ~12 min | ⭐⭐ Intermediate | πŸ“‹ P08.C16C.U03

The choice of cellular frequency band involves fundamental physics trade-offs that directly impact IoT deployment strategies:

%% fig-cap: "Cellular frequency trade-offs for IoT: range, bandwidth, and building penetration"
%% fig-alt: "Trade-off triangle diagram showing: lower frequencies (600-900 MHz) offer excellent range (10+ km) and building penetration but limited bandwidth; mid-band frequencies (1700-2600 MHz) balance range and capacity; high frequencies (mmWave ~24–52 GHz) provide massive bandwidth but very limited range (100-300m) and no building penetration, making them unsuitable for most IoT unless line-of-sight is guaranteed"
%%{init: {'theme': 'base', 'themeVariables': { 'primaryColor': '#2C3E50', 'primaryTextColor': '#2C3E50', 'primaryBorderColor': '#16A085', 'lineColor': '#16A085', 'secondaryColor': '#E67E22', 'tertiaryColor': '#ECF0F1'}}}%%
graph TB
    A["Cellular Frequency<br/>Selection for IoT"] --> B["Low Band<br/>600-900 MHz"]
    A --> C["Mid Band<br/>1700-2600 MHz"]
    A --> D["High Band/mmWave<br/>~24–52 GHz"]

    B --> B1["βœ“ Excellent range (10+ km)<br/>βœ“ Deep indoor penetration<br/>βœ“ Wide area coverage<br/>βœ— Limited bandwidth"]
    C --> C1["βœ“ Good balance<br/>βœ“ Higher capacity<br/>βœ“ Urban coverage<br/>β—‹ Moderate penetration"]
    D --> D1["βœ“ Massive bandwidth<br/>βœ“ High data rates<br/>βœ— Very short range (100-300m)<br/>βœ— No building penetration"]

    B --> B2["Best for: Rural IoT<br/>Smart agriculture<br/>Asset tracking"]
    C --> C2["Best for: Urban IoT<br/>Connected vehicles<br/>Smart cities"]
    D --> D2["Best for: Fixed wireless<br/>Industrial AR/VR<br/>NOT typical IoT"]

    style A fill:#2C3E50,stroke:#16A085,stroke-width:3px,color:#fff
    style B fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style C fill:#E67E22,stroke:#16A085,stroke-width:2px
    style D fill:#7F8C8D,stroke:#16A085,stroke-width:2px,color:#fff

Figure 814.3: Frequency trade-offs for IoT: lower bands improve range/penetration; higher bands improve bandwidth but increase path loss.

Quantitative Trade-offs:

Note: These are order-of-magnitude examples; real-world cells vary widely with spectrum, site density, terrain, and network load.

Parameter Low Band (700-900 MHz) Mid Band (2.5 GHz) mmWave (28 GHz)
Cell Radius 10-30 km 1-5 km 100-300 m
Building Penetration Excellent (through concrete) Good (through drywall) None (blocked by glass)
Bandwidth per Carrier 5-10 MHz 20-100 MHz 100-800 MHz
Typical Data Rate 5-20 Mbps 50-300 Mbps 1-10 Gbps
IoT Suitability Excellent Good Limited

Think of radio frequencies like different types of roads:

Low frequencies (700-900 MHz) are like country highways: - They travel far with few obstacles - Not many lanes (limited data capacity) - Great for reaching remote areas - IoT example: NB-IoT sensors in rural water treatment plants

Mid frequencies (2-3 GHz) are like city roads: - Good balance of reach and capacity - Can handle moderate traffic - Work well in urban environments - IoT example: LTE-M connected cars in cities

High frequencies (mmWave) are like high-speed express lanes: - Extremely fast but very short - Blocked by almost everything - Only work with direct line of sight - IoT example: Factory robots with guaranteed clear paths (rare for IoT)

The key insight: Most IoT devices benefit from lower frequencies because they need: - Wide coverage (sensors spread over large areas) - Building penetration (indoor sensors) - Low power (small batteries)

This is why NB-IoT and LTE-M typically operate in the 700-900 MHz bands when available, even though 5G mmWave offers much higher speeds.

814.4.1 Implications for IoT Deployments

The cellular spectrum landscape directly impacts IoT technology selection:

NB-IoT (Narrowband IoT) - Uses just 180 kHz bandwidth (one LTE resource block) - Typically deployed in 800-900 MHz bands for maximum coverage - +20 dB link budget improvement over LTE - Ideal for: Deep indoor sensors, smart meters, remote agriculture

LTE-M (LTE Cat-M1) - Uses 1.4 MHz bandwidth - Supports voice and higher data rates than NB-IoT - Deployed across multiple LTE bands (700, 850, 1700, 1900 MHz) - Ideal for: Asset trackers, wearables, connected vehicles

5G IoT (RedCap/NR-Light) - Reduced capability 5G for IoT applications - Lower complexity than full 5G but higher than LTE-M - Expected to use FR1 bands (sub-6 GHz) - Ideal for: Industrial IoT, video surveillance, AR glasses

NoteRelated: Cellular IoT Deep Dive

For detailed coverage of cellular IoT technologies, see:

814.5 Summary

This chapter covered cellular spectrum allocation for IoT:

  • Cellular spectrum evolution from 1G (800-900 MHz) to 5G (sub-6 GHz and mmWave) reflects increasing bandwidth and spectral efficiency
  • Multiple access techniques evolved from FDMA (0.03 bits/s/Hz) to OFDMA (5-10 bits/s/Hz)
  • Low-band cellular (700-900 MHz) is optimal for IoT due to range and penetration
  • NB-IoT and LTE-M leverage existing LTE infrastructure in IoT-friendly bands
  • 5G mmWave offers high bandwidth but is unsuitable for most IoT applications due to limited range

814.6 What’s Next

Continue with the next chapter in this series:

  • Propagation and Design: Path loss calculations, interference mitigation, and practical band selection framework

814.7 Knowledge Check

  1. For battery-powered sensors requiring multi-kilometer range in rural terrain, which cellular band choice is generally most favorable?

Lower frequencies typically have lower path loss and better diffraction/penetration, making them better suited to long-range, low-power rural deployments. NB-IoT commonly uses 800-900 MHz bands.

  1. Which multiple access technique provides the highest spectral efficiency?

OFDMA achieves ~5-10 bits/s/Hz through orthogonal subcarriers, dynamic allocation, and MIMO support, far exceeding earlier techniques like FDMA (~0.03 bits/s/Hz).

  1. Why is 5G mmWave (28 GHz) generally unsuitable for most IoT applications?

mmWave signals have very short range (100-300m) and are blocked by buildings, glass, and even rain. Most IoT applications need wide coverage and building penetration, making low-band cellular more suitable.