1129  NB-IoT Channel Access and Uplink Optimization

1129.1 Learning Objectives

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

  • Analyze Channel Structure: Describe NB-IoT NPUSCH, NPDSCH, and control channel operations
  • Configure Tone Modes: Select optimal single-tone vs multi-tone uplink configurations
  • Understand Frequency Hopping: Explain how frequency diversity improves reliability
  • Optimize for Power Efficiency: Apply tone selection strategies based on signal quality and payload size

1129.2 Prerequisites

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

Deep Dives: - NB-IoT PSM and eDRX - Power saving modes and timer configuration - NB-IoT Coverage Enhancement - Repetition mechanisms for deep coverage - NB-IoT Labs and Implementation - AT command configuration

Comparisons: - Cellular IoT Comprehensive Review - NB-IoT vs LTE-M channel comparison

Related Topics: - LoRaWAN Architecture - Alternative LPWAN channel access

NoteQuick Reference: NB-IoT Physical Channels
  • NPDSCH (Downlink Shared Channel): User data to device.
  • NPDCCH (Downlink Control Channel): Scheduling and control information.
  • NPBCH (Broadcast Channel): Essential system information for initial access.
  • NPUSCH (Uplink Shared Channel): Uplink user data from device.
  • NPRACH (Random Access Channel): Initial access / connection requests.

1129.5 Deep Dive: Single-Tone vs Multi-Tone Optimization

NB-IoT Uplink Tone Configurations:

%%{init: {'theme': 'base', 'themeVariables': { 'primaryColor': '#2C3E50', 'primaryTextColor': '#fff', 'primaryBorderColor': '#16A085', 'lineColor': '#16A085', 'secondaryColor': '#E67E22', 'tertiaryColor': '#7F8C8D', 'noteTextColor': '#2C3E50', 'noteBkgColor': '#FFF9E6'}}}%%
graph TD
    A["NB-IoT Uplink<br/>NPUSCH"] --> B["Multi-Tone Options"]
    A --> C["Single-Tone Options"]

    B --> D["12 tones @ 15 kHz<br/>180 kHz, 160 kbps"]
    B --> E["6 tones @ 15 kHz<br/>90 kHz, 80 kbps"]
    B --> F["3 tones @ 15 kHz<br/>45 kHz, 40 kbps"]

    C --> G["1 tone @ 15 kHz<br/>15 kHz, 16 kbps"]
    C --> H["1 tone @ 3.75 kHz<br/>3.75 kHz, 5 kbps"]

    D --> I["Best for: Large payloads<br/>Good signal (-85 to -100 dBm)"]
    G --> J["Best for: Extended coverage<br/>Moderate signal (-108 to -125 dBm)"]
    H --> K["Best for: Extreme coverage<br/>Poor signal (< -125 dBm)"]

    style A fill:#2C3E50,stroke:#16A085,color:#fff
    style D fill:#5cb85c,stroke:#2C3E50,color:#fff
    style G fill:#16A085,stroke:#2C3E50,color:#fff
    style H fill:#E67E22,stroke:#2C3E50,color:#fff

Figure 1129.3: NB-IoT uplink multi-tone and single-tone options

Power Concentration Principle:

When using fewer tones, transmit power is concentrated in narrower bandwidth:

12-tone configuration:
β”œβ”€ Total TX power: 200 mW (23 dBm)
β”œβ”€ Bandwidth: 180 kHz (12 tones Γ— 15 kHz)
β”œβ”€ Power spectral density: 200 mW / 180 kHz = 1.11 mW/kHz
└─ Power per tone: 200 mW / 12 = 16.7 mW per tone

1-tone @ 15 kHz configuration:
β”œβ”€ Total TX power: 150 mW (22 dBm)
β”œβ”€ Bandwidth: 15 kHz (single tone)
β”œβ”€ Power spectral density: 150 mW / 15 kHz = 10 mW/kHz
└─ Power concentrated: 150 mW in ONE tone

Link budget comparison:
- 12-tone: 16.7 mW per tone = 12.2 dBm per tone
- 1-tone: 150 mW in one tone = 21.8 dBm
β†’ Single-tone has +9.6 dB advantage per tone!

This +9.6 dB translates to:
- 2Γ— better range in same coverage class
- OR half the repetitions needed for same reliability

Detailed Performance Comparison:

Configuration Bandwidth Data Rate TX Power Coverage Repetitions (RSRP = -115 dBm) Battery Impact
12 tones @ 15 kHz 180 kHz 160 kbps 200 mA Normal 16-32Γ— High (packet loss)
3 tones @ 15 kHz 45 kHz 40 kbps 180 mA Extended 8-16Γ— Medium
1 tone @ 15 kHz 15 kHz 16 kbps 150 mA Extended+ 4-8Γ— Low
1 tone @ 3.75 kHz 3.75 kHz 5 kbps 120 mA Extreme 1-4Γ— Lowest

Choosing the Right Configuration:

%%{init: {'theme': 'base', 'themeVariables': { 'primaryColor': '#2C3E50', 'primaryTextColor': '#fff', 'primaryBorderColor': '#16A085', 'lineColor': '#16A085', 'secondaryColor': '#E67E22', 'tertiaryColor': '#7F8C8D', 'noteTextColor': '#2C3E50', 'noteBkgColor': '#FFF9E6'}}}%%
graph TD
    A["Select Tone<br/>Configuration"] --> B{"Signal Quality<br/>(RSRP)?"}

    B -->|"> -100 dBm<br/>(Excellent)"| C["Multi-tone<br/>(6 or 12 tones)"]
    B -->|"-100 to -108 dBm<br/>(Good)"| D["3 tones<br/>@ 15 kHz"]
    B -->|"-108 to -125 dBm<br/>(Extended)"| E["1 tone<br/>@ 15 kHz"]
    B -->|"< -125 dBm<br/>(Extreme)"| F["1 tone<br/>@ 3.75 kHz"]

    C --> G["Fast uploads<br/>Low battery life priority"]
    D --> H["Balanced<br/>performance"]
    E --> I["Extended coverage<br/>Good battery life"]
    F --> J["Maximum coverage<br/>Best battery life"]

    style A fill:#2C3E50,stroke:#16A085,color:#fff
    style C fill:#7F8C8D,stroke:#2C3E50,color:#fff
    style D fill:#5cb85c,stroke:#2C3E50,color:#fff
    style E fill:#16A085,stroke:#2C3E50,color:#fff
    style F fill:#E67E22,stroke:#2C3E50,color:#fff

Figure 1129.4: Tone configuration selection based on signal quality

Battery Life Calculation Example:

Scenario: 100-byte message, 4 transmissions per day, RSRP = -115 dBm

Configuration comparison:

12-tone @ 15 kHz:
β”œβ”€ Data rate: 160 kbps
β”œβ”€ Base transmission time: (100 Γ— 8 bits) / 160 kbps = 5 ms
β”œβ”€ Repetitions needed: 32Γ— (poor link budget at -115 dBm)
β”œβ”€ Total TX time: 5 ms Γ— 32 = 160 ms
β”œβ”€ Retransmissions (30% packet loss): 3 attempts
β”œβ”€ Effective time: 160 ms Γ— 3 = 480 ms
β”œβ”€ TX power: 200 mA
β”œβ”€ Daily energy: 4 Γ— 0.48s Γ— 200mA = 0.384 mAh/day
└─ Battery life: 10,000 mAh / (0.384 + 0.12 PSM) = 19,841 days = 54 years
   BUT: High packet loss (30%) = unreliable ❌

1-tone @ 15 kHz:
β”œβ”€ Data rate: 16 kbps
β”œβ”€ Base transmission time: (100 Γ— 8) / 16 kbps = 50 ms
β”œβ”€ Repetitions needed: 8Γ— (better link budget from power concentration)
β”œβ”€ Total TX time: 50 ms Γ— 8 = 400 ms
β”œβ”€ Retransmissions: Minimal (99% delivery)
β”œβ”€ Effective time: 400 ms Γ— 1.01 = 404 ms
β”œβ”€ TX power: 150 mA
β”œβ”€ Daily energy: 4 Γ— 0.404s Γ— 150mA = 0.242 mAh/day
└─ Battery life: 10,000 / (0.242 + 0.12) = 27,624 days = **75 years** βœ…

Key insight:
- 12-tone seems faster (160 kbps vs 16 kbps)
- But needs 4Γ— more repetitions due to weaker per-tone power
- 1-tone uses less total energy AND more reliable!

Design Recommendation:

For battery-powered NB-IoT devices with <500 byte payloads:

  1. Good signal (> -108 dBm): Use 3-tone @ 15 kHz
    • Balanced speed and power
    • Battery life: 12-18 years
  2. Extended coverage (-108 to -125 dBm): Use 1-tone @ 15 kHz
    • Optimal power efficiency
    • Battery life: 10-15 years
    • 99%+ reliability
  3. Extreme coverage (< -125 dBm): Use 1-tone @ 3.75 kHz
    • Maximum coverage
    • Battery life: 8-12 years
    • Last resort option

Avoid 12-tone mode for battery-powered devices unless: - Excellent signal (> -100 dBm) AND - Large payloads (> 500 bytes) AND - Battery life < 5 years acceptable

1129.6 Knowledge Check

Test your understanding of NB-IoT channel access:

NoteQuestion 1: Single-Tone vs Multi-Tone Uplink

You’re optimizing an NB-IoT sensor deployment for long battery life and reliable delivery. Each sensor sends 50-byte messages every 6 hours. Your carrier’s NB-IoT network supports multiple uplink tone configurations.

Signal quality at the sensor location: RSRP β‰ˆ -115 dBm (extended coverage)

Question: Which uplink tone configuration is typically best for battery-powered devices at this signal level?

Explanation: C concentrates energy into a single 15 kHz tone, improving link budget and reducing retries in extended coverage. The 3.75 kHz option is usually reserved for extreme coverage.

Answer & Detailed Explanation

Correct Answer: C) 1 tone @ 15 kHz (best balance for -115 dBm signal)

1129.6.2 Battery Life Calculation

Scenario: 50-byte payload, 4 transmissions per day, RSRP = -115 dBm (extended coverage)

Option C: 1 tone @ 15 kHz (15 kHz bandwidth) βœ…

Data rate: 16 kbps
Transmission time: 400 bits / 16,000 bps Γ— 15 (overhead) = 0.375 seconds

Coverage enhancement (8 repetitions needed - single tone has better link budget):
- Total: 0.375s Γ— 8 = 3 seconds

TX power (1 tone): 150 mA (lower power for single carrier)
Daily energy:
- 4 Γ— 3s Γ— 150mA = 12s Γ— 150mA = 0.5 mAh/day
- PSM sleep: 0.12 mAh/day
Total: 0.62 mAh/day

Battery life: 10,000 Γ· 0.62 = 16,129 days = **44.2 years** βœ…

Reliability: Excellent (99%+ delivery) βœ…

Why this is optimal:
- Single tone concentrates power β†’ better coverage per repetition
- Needs HALF the repetitions vs 12-tone mode (8Γ— vs 16Γ—)
- Lower TX power (150mA vs 200mA)
- Best reliability at -115 dBm signal level

Option D: 1 tone @ 3.75 kHz (3.75 kHz bandwidth)

Data rate: 5 kbps
Transmission time: 400 bits / 5,000 bps Γ— 15 (overhead) = 1.2 seconds

Coverage enhancement (only 2 repetitions needed - extreme coverage capability):
- Total: 1.2s Γ— 2 = 2.4 seconds

TX power (1 tone narrow): 120 mA (lowest power)
Daily energy:
- 4 Γ— 2.4s Γ— 120mA = 9.6s Γ— 120mA = 0.32 mAh/day
- PSM sleep: 0.12 mAh/day
Total: 0.44 mAh/day

Battery life: 10,000 Γ· 0.44 = 22,727 days = **62.3 years** βœ…βœ…

Wait, this seems best! Why not choose D?

Problem: 3.75 kHz mode is ONLY for extreme coverage (< -125 dBm)
- Your signal: -115 dBm (extended coverage, not extreme)
- Using 3.75 kHz wastes time (1.2s vs 0.375s per transmission)
- Longer transmission = more vulnerable to interference
- Should reserve 3.75 kHz for basement/underground devices

Verdict: Overkill for -115 dBm signal

1129.6.3 Tone Configuration Decision Matrix

Signal Quality (RSRP) β†’ Optimal Tone Configuration

Excellent (-85 to -100 dBm):
β”œβ”€ Large payloads (>500 bytes): 12 tones @ 15 kHz
β”œβ”€ Medium payloads (100-500 bytes): 3 tones @ 15 kHz
└─ Small payloads (<100 bytes): 3 tones @ 15 kHz

Good (-100 to -108 dBm):
β”œβ”€ Large payloads: 3 tones @ 15 kHz
└─ Small payloads: 1 tone @ 15 kHz

Extended (-108 to -125 dBm):
└─ All payloads: **1 tone @ 15 kHz** βœ… (your scenario!)

Extreme (< -125 dBm):
└─ All payloads: 1 tone @ 3.75 kHz (maximum coverage)

Your sensor location: -115 dBm β†’ 1 tone @ 15 kHz is perfect match

NoteQuestion 2: Frequency Hopping in Single-Tone Mode

You’re deploying NB-IoT sensors in an industrial area where there is intermittent narrowband interference in parts of the uplink band. Your devices use single-tone NPUSCH for coverage.

Question: What is the primary reason NB-IoT uses frequency hopping in single-tone mode?

Explanation: C is the key benefit: hopping reduces the risk that one bad frequency (interference or a deep fade) breaks the uplink. It complements, not replaces, repetitions.

Answer & Detailed Explanation

Correct Answer: C) Frequency diversity against interference/fading

1129.6.4 Why hopping helps (even with one tone)

In single-tone mode, the uplink uses one narrow subcarrier at a time. If the device stayed on the same tone, two common real-world problems can break the link:

  • Narrowband interference: A jammer or industrial emitter might occupy only a small slice of the band.
  • Frequency-selective fading: Due to multipath, some frequencies can be in a deep fade while nearby frequencies are not.

By hopping the tone index across symbol groups/subframes, NB-IoT spreads the transmission across different frequencies over time. This provides frequency diversity, which improves decoding probability and can reduce the need for retransmissions.

1129.6.5 What hopping does not do

  • It does not raise transmit power above regulatory/radio limits (TX power is unchanged).
  • It does not replace coverage-enhancement repetitions; it complements them.
  • It does not change PSM/eDRX sleep current; those are power-mode behaviors.

1129.7 Adaptive Tone Configuration

Some NB-IoT implementations support dynamic tone selection:

// Device-side adaptive configuration (pseudo-code)

float rsrp = measure_rsrp();
int payload_size = get_message_size();

if (rsrp >= -100 && payload_size > 200) {
    // Excellent signal + large payload β†’ use multi-tone for speed
    configure_npusch(12_TONES, 15_KHZ);
}
else if (rsrp >= -108) {
    // Good signal β†’ 3-tone balance
    configure_npusch(3_TONES, 15_KHZ);
}
else if (rsrp >= -125) {
    // Extended coverage β†’ single-tone @ 15 kHz
    configure_npusch(1_TONE, 15_KHZ);
}
else {
    // Extreme coverage β†’ single-tone @ 3.75 kHz
    configure_npusch(1_TONE, 3_75_KHZ);
}

// Transmit message
send_npusch_message(payload);

// Log performance for analysis
log_transmission(rsrp, tone_config, repetitions, success);

Illustrative Optimization Example (Hypothetical):

Large rural sensor deployment:

Initial configuration (suboptimal):
β”œβ”€ Default: 3 tones @ 15 kHz (carrier default)
β”œβ”€ Coverage: many devices have RSRP < -108 dBm (extended coverage)
└─ Issue: retries and reduced battery life in marginal signal

Optimization:
β”œβ”€ Segment devices by RSRP (good vs extended coverage)
β”œβ”€ Reconfigure: switch to 1 tone @ 15 kHz for RSRP < -108 dBm
β”œβ”€ Results:
β”‚   β”œβ”€ Fewer retries and more reliable delivery
β”‚   └─ Longer battery life in extended coverage

Key lesson:
Carrier default configurations are optimized for average case,
not worst-case coverage. Request single-tone mode for extended
coverage deployments to maximize battery life.

1129.8 Why Single-Tone is More Power-Efficient in Extended Coverage

Power concentration principle:

12-tone transmission (180 kHz bandwidth):
- Total TX power: 200 mA
- Power per tone: 200mA / 12 = 16.7 mA per 15 kHz tone
- Energy per bit: 200mA / 160 kbps = 1.25 mA per kbps

1-tone transmission (15 kHz bandwidth):
- Total TX power: 150 mA
- Power for single tone: 150 mA (all power concentrated!)
- Energy per bit: 150mA / 16 kbps = 9.4 mA per kbps

Link budget comparison:
- 12-tone: Power spread across 180 kHz β†’ -10 dB per tone
- 1-tone: Power concentrated in 15 kHz β†’ 0 dB (reference)
β†’ Single tone has +10 dB advantage per tone!

Result:
- 1-tone mode needs 8 repetitions for -115 dBm
- 12-tone mode needs 16 repetitions for -115 dBm
β†’ Single tone is 2Γ— more efficient in marginal coverage

1129.9 Summary

1129.10 What’s Next

Build on your NB-IoT channel access knowledge: