1130  NB-IoT Coverage Enhancement and Deep Indoor Deployment

1130.1 Learning Objectives

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

• Design for Deep Coverage: Understand MCL enhancement and repetition schemes for basement/indoor scenarios
• Calculate Link Budgets: Compute path loss and coverage margins for different deployment environments
• Select Coverage Classes: Choose appropriate CE levels based on signal quality measurements
• Optimize Deployment Strategy: Balance coverage, battery life, and infrastructure investment

1130.2 Prerequisites

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

• NB-IoT Fundamentals: Understanding basic NB-IoT concepts, deployment modes, and system architecture
• NB-IoT PSM and eDRX: Knowledge of power saving modes helps understand coverage-battery trade-offs
• NB-IoT Channel Access: Understanding uplink configurations provides context for repetition mechanisms

NoteRelated Chapters

Deep Dives: - NB-IoT PSM and eDRX - Power saving modes and timer configuration - NB-IoT Channel Access - Uplink tone configurations - NB-IoT Labs and Implementation - AT command configuration

Comparisons: - Cellular IoT Comprehensive Review - NB-IoT vs LTE-M coverage - LoRaWAN Architecture - Alternative LPWAN coverage strategies

1130.3 Getting Started: Coverage Enhancement (For Beginners)

1130.3.1 Why NB-IoT Works in Basements

Analogy: Coverage enhancement is like shouting louder by repeating yourself:

• Normal conversation: “The meeting is at 3 PM” (said once) - works in quiet room
• Noisy environment: “The meeting is at 3 PM! The meeting is at 3 PM! The meeting is at 3 PM!” (repeat 3 times) - person hears through noise
• NB-IoT deep coverage: Repeat message up to 2048 times - works through concrete walls, basements, underground parking

How repetition improves coverage:

Signal quality improvement:
- 1 transmission:     0 dB SNR (barely detectable)
- 10 repetitions:    +10 dB SNR (each repetition improves ~3 dB)
- 100 repetitions:   +20 dB SNR (can penetrate walls)
- 2048 repetitions:  +33 dB SNR (extreme deep coverage)

Practical benefit:
+20 dB coverage gain = penetrate 4-5 additional concrete walls
(or 10-15 km extra range in rural areas)

Real-world example: Water meter in basement

Scenario: Water meter 3 floors underground (concrete ceiling above)

Signal path loss:
- Free space loss (1 km distance): -90 dB
- 3× concrete floors: -60 dB (20 dB each)
- Wall penetration: -10 dB
Total loss: -160 dB

NB-IoT link budget:
- Device TX power: +23 dBm
- Base station RX sensitivity: -141 dBm (with max repetitions)
- Link budget: 23 - (-141) = 164 dB ✅

Margin: 164 dB - 160 dB = 4 dB (connection possible!)

Without coverage enhancement (normal GPRS):
- Link budget: 144 dB
- Required: 160 dB
❌ Connection fails (would need to be above ground)

Trade-off: Coverage vs Battery Life

Coverage level → Repetitions → Battery impact

Normal coverage (good signal):
- Repetitions: 1-2
- Message time: 2 seconds
- Battery life: 15 years ✅

Extended coverage (basement):
- Repetitions: 50-100
- Message time: 100 seconds (50× longer!)
- Battery life: 10 years ⚠️ (acceptable)

Extreme coverage (deep underground):
- Repetitions: 1000-2048
- Message time: 2,000 seconds (33 minutes!)
- Battery life: 3-5 years ❌ (may need larger battery)

Design rule: Place devices where good NB-IoT coverage exists to minimize repetitions and maximize battery life.

1130.3.2 Why NB-IoT Has Better Coverage Than Wi-Fi or LoRaWAN

Three reasons: Lower bandwidth + Repetition + Licensed spectrum:

Coverage comparison (164 dB link budget):

1. Narrow bandwidth (180 kHz vs 20 MHz Wi-Fi)
   → Concentrates power in narrow band
   → +15 dB gain vs wideband

2. Repetition (up to 2048×)
   → Each repetition improves SNR by ~3 dB
   → +33 dB gain with max repetitions

3. Licensed spectrum (carrier-managed)
   → No interference (Wi-Fi/LoRaWAN share unlicensed spectrum)
   → Consistent performance

Total advantage: +48 dB vs Wi-Fi (164 dB vs 116 dB)

Practical impact:
- Wi-Fi range urban: 50-100 meters
- NB-IoT range urban: 1-5 km (10-50× farther!)
- NB-IoT penetration: +20 dB (4-5 extra walls)

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1130.4 Coverage Enhancement Mechanism

NB-IoT achieves 164 dB Maximum Coupling Loss (MCL), which is 20 dB better than GPRS:

1130.4.1 Repetition Mechanism

NB-IoT uses message repetition to achieve deep coverage:

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sequenceDiagram
    participant Device as NB-IoT Device<br/>(Basement)
    participant eNB as Base Station

    Note over Device,eNB: Message sent 1 time (normal)
    Device->>eNB: Transmission #1 (SNR: -5 dB)
    Note over eNB: Too weak to decode ❌

    Note over Device,eNB: Repetition Mode (CE Level 2)
    Device->>eNB: Transmission #1
    Device->>eNB: Transmission #2
    Device->>eNB: Transmission #3
    Note over eNB: Combine signals<br/>SNR improved to +4 dB
    Note over eNB: Successfully decoded ✓

    Note over Device,eNB: Deep Coverage (2048 reps)
    loop 2048 repetitions
        Device->>eNB: Transmit same message
    end
    Note over eNB: Coherent combining<br/>+33 dB SNR gain<br/>164 dB MCL achieved

    Note over Device: Trade-off:<br/>2048× longer airtime<br/>Higher power consumption<br/>But reaches basement!

Figure 1130.1: Message repetition for deep coverage enhancement

{fig-alt=“NB-IoT repetition mechanism sequence diagram showing coverage enhancement. Single transmission at SNR -5dB is too weak to decode. With repetition mode (CE Level 2), device sends message 3 times and base station combines signals achieving +4dB SNR for successful decode. For extreme deep coverage (164dB MCL), up to 2048 repetitions provide +33dB SNR gain through coherent combining, enabling basement/underground communication at cost of 2048× longer airtime and higher power consumption.”}

Coverage classes: - Normal coverage (CE0): No repetition or minimal - Extended coverage (CE1): Moderate repetitions (10-100) - Extreme coverage (CE2): Maximum repetitions (up to 2048)

Trade-off: - More repetitions = Better coverage - More repetitions = Higher latency - More repetitions = Higher power consumption

1130.4.2 Coverage Comparison

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graph TB
    subgraph "Maximum Coupling Loss Comparison"
        GPRS["GPRS<br/>144 dB MCL"]
        LTEM["LTE-M<br/>156 dB MCL<br/>(+12 dB vs GPRS)"]
        NBIOT["NB-IoT<br/>164 dB MCL<br/>(+20 dB vs GPRS)"]
    end

    GPRS --> RANGE1["Urban: 1-2 km<br/>Rural: 5-10 km<br/>Indoor: 1-2 walls"]
    LTEM --> RANGE2["Urban: 2-4 km<br/>Rural: 10-15 km<br/>Indoor: 3-4 walls"]
    NBIOT --> RANGE3["Urban: 3-6 km<br/>Rural: 15-25 km<br/>Indoor: 5-6 walls<br/>Basement: B3-B4"]

    GAIN["Coverage Gain:<br/>+20 dB = 10× area<br/>3.2× range"]

    NBIOT --> GAIN

    style GPRS fill:#7F8C8D,color:#fff
    style LTEM fill:#E67E22,color:#fff
    style NBIOT fill:#27AE60,color:#fff
    style GAIN fill:#3498DB,color:#fff

Figure 1130.2: NB-IoT vs GPRS vs LTE-M maximum coupling loss comparison

{fig-alt=“Cellular IoT Maximum Coupling Loss comparison showing coverage evolution. GPRS baseline at 144 dB MCL provides 1-2km urban, 5-10km rural, 1-2 walls indoor penetration. LTE-M at 156 dB MCL (+12dB) achieves 2-4km urban, 10-15km rural, 3-4 walls. NB-IoT at 164 dB MCL (+20dB vs GPRS) reaches 3-6km urban, 15-25km rural, 5-6 walls, and B3-B4 basement levels. The +20dB gain translates to 10× coverage area and 3.2× range increase, enabling previously impossible indoor/underground deployments.”}

Maximum Coupling Loss (MCL) explained: \[MCL = TX_{power} - RX_{sensitivity} + Antenna_{gains}\]

Example for NB-IoT: - Device TX power: +23 dBm - eNB RX sensitivity: -141 dBm (after processing gain) - Antenna gains: +0 dB (0 dBi each) \[MCL = 23 - (-141) + 0 = 164 \text{ dB}\]

This 164 dB budget allows for significant path loss and penetration.

CautionPitfall: Forcing Maximum Repetitions (2048x) for “Guaranteed Coverage”

The Mistake: Developers configure all devices for CE Level 2 with maximum 2048 repetitions, thinking “if it works in the worst basement, it works everywhere.” They hardcode AT+CEDRXS=2,5,"1111" and wonder why battery life drops to 6 months.

Why It Happens: Misunderstanding that coverage enhancement is a sliding scale, not an on/off feature. Each repetition multiplies transmission time and power consumption proportionally. A device with good signal (-90 dBm) forced to use 2048 repetitions wastes 2047 redundant transmissions.

The Fix: Use network-controlled adaptive repetitions (the 3GPP default behavior). The eNodeB automatically assigns CE level based on measured RSRP during RACH: - CE Level 0 (good signal, -90 to -100 dBm): 1-4 repetitions, 2-5 second TX - CE Level 1 (moderate, -100 to -120 dBm): 8-64 repetitions, 10-30 second TX - CE Level 2 (poor, -120 to -140 dBm): 128-2048 repetitions, 1-20 minute TX Do NOT override with AT+NCONFIG=“CR_0354_0338_SCRAMBLING”,TRUE unless you’ve verified actual signal conditions require it. Monitor with AT+CESQ to check signal quality during pilot deployment.

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1130.5 Deep Dive: Coverage Enhancement Techniques

TipHow NB-IoT Achieves +20 dB Coverage Gain

Understanding Maximum Coupling Loss (MCL):

Maximum Coupling Loss represents the total signal attenuation that a system can tolerate while still maintaining communication.

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graph LR
    subgraph "Link Budget Calculation"
        A["Device TX Power<br/>+23 dBm"]
        B["Path Loss<br/>-140 dB"]
        C["Base Station RX<br/>-141 dBm sensitivity"]
    end

    A -->|"Transmitted"| D["Signal travels"]
    D -->|"Attenuated"| B
    B -->|"Received"| C

    E["MCL = TX Power - RX Sensitivity<br/>= 23 - (-141) = 164 dB"]

    style A fill:#16A085,stroke:#2C3E50,color:#fff
    style B fill:#E67E22,stroke:#2C3E50,color:#fff
    style C fill:#2C3E50,stroke:#16A085,color:#fff
    style E fill:#7F8C8D,stroke:#2C3E50,color:#fff

Figure 1130.3: NB-IoT link budget calculation for 164 dB MCL

Coverage Class Breakdown:

Coverage Class	RSRP Range	Repetitions	MCL	Scenario
Normal (CE0)	> -108 dBm	1-4×	144 dB	Outdoor, line-of-sight
Extended (CE1)	-108 to -128 dBm	8-128×	154 dB	Indoor, 2-3 floors penetration
Extreme (CE2)	< -128 dBm	256-2048×	164 dB	Deep basement, underground parking

How Repetitions Improve SNR:

Each repetition improves Signal-to-Noise Ratio (SNR) by approximately 3 dB:

Mathematical relationship:
SNR_improvement_dB = 10 × log10(N)

Where N = number of repetitions

Examples:
- 10 repetitions:  10 × log10(10)  = 10 dB gain
- 100 repetitions: 10 × log10(100) = 20 dB gain
- 1000 repetitions: 10 × log10(1000) = 30 dB gain

Why this works:
- Each repetition adds signal energy coherently
- Noise adds incoherently (random)
- After N repetitions, signal power increases N×
- Noise power increases √N×
- SNR ratio improves by N/√N = √N → 10×log10(N) dB

Coverage Enhancement Techniques:

1. Repetition (most important):
   • Uplink: NPUSCH repeated up to 128× per coverage class
   • Downlink: NPDSCH repeated up to 2048×
   • Control channels: NPDCCH repeated up to 2048×
2. Narrow bandwidth concentration:
   • NB-IoT: 180 kHz (vs LTE: 1.4-20 MHz)
   • Power concentrated in narrow band - +13 dB gain
   • Formula: Gain_dB = 10×log10(BW_LTE / BW_NB-IoT)
3. Low coding rate:
   • Turbo coding with rate 1/3 (vs normal 1/2 or 2/3)
   • More redundancy = better error correction
   • Trade-off: Lower data rate, higher reliability

Real-World Coverage Examples:

Scenario 1: Water meter in basement (3 floors underground)

Path loss calculation:
├─ Free space loss (1 km): -90 dB
├─ Building penetration: -20 dB (exterior wall)
├─ Floor 1 penetration: -20 dB (concrete/rebar)
├─ Floor 2 penetration: -20 dB
├─ Floor 3 penetration: -20 dB
Total loss: -170 dB

Can NB-IoT reach it?
├─ Device TX: +23 dBm
├─ Required at base station: -141 dBm (extreme coverage)
├─ Link budget: 164 dB
├─ Margin: 164 - 170 = -6 dB ❌ Not enough!

Solution: Deploy indoor small cell OR relocate meter one floor up
- With 2 floors: -150 dB path loss
- Margin: 164 - 150 = +14 dB ✅ Works!

Scenario 2: Parking sensor underground (1 level)

Path loss:
├─ Free space: -90 dB (1 km)
├─ Building penetration: -20 dB
├─ Underground ceiling: -25 dB
Total: -135 dB

Link budget check:
├─ Required: 164 dB
├─ Actual: 135 dB
├─ Margin: +29 dB ✅ Excellent!
├─ Coverage class: Extended (16-32 repetitions)
├─ Message time: 10-30 seconds
└─ Battery life: 12+ years

Coverage vs Power Trade-off:

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graph TD
    A["RSRP Signal Quality"] --> B{"-108 dBm threshold"}

    B -->|"Better signal<br/>(> -108 dBm)"| C["Normal Coverage"]
    B -->|"Worse signal<br/>(< -108 dBm)"| D["Extended/Extreme"]

    C --> E["Repetitions: 1-4×<br/>TX time: 2-5s<br/>Battery: 15+ years"]
    D --> F["Repetitions: 8-2048×<br/>TX time: 10s-30min<br/>Battery: 2-10 years"]

    E --> G["Best deployment:<br/>Good cell coverage"]
    F --> H["May need:<br/>Small cells or<br/>device relocation"]

    style A fill:#2C3E50,stroke:#16A085,color:#fff
    style C fill:#5cb85c,stroke:#2C3E50,color:#fff
    style D fill:#E67E22,stroke:#2C3E50,color:#fff
    style E fill:#16A085,stroke:#2C3E50,color:#fff
    style F fill:#d9534f,stroke:#2C3E50,color:#fff

Figure 1130.4: Signal quality impact on coverage class and battery life

Deployment Design Rules:

To maximize battery life and minimize latency:

1. Target RSRP > -108 dBm for normal coverage
   • Deploy small cells if needed
   • Cost: 10k-20k EUR per small cell
   • ROI: Avoids frequent battery replacements (>10 year life)
2. Accept extended coverage (-108 to -128 dBm) where economical
   • Battery life: 8-12 years (acceptable)
   • Applications: Smart meters, asset tracking
3. Avoid extreme coverage (< -128 dBm) for battery-powered devices
   • Battery life: 2-5 years (frequent replacement needed)
   • Better solution: Relocate device OR deploy small cell

Key Insight: NB-IoT’s +20 dB coverage advantage comes from three factors: 1. Narrowband concentration (+13 dB) 2. Repetition (up to +33 dB with 2048 repetitions) 3. Low coding rate (+5 dB)

Total potential gain: +51 dB over wideband systems, enabling penetration through 5-7 additional concrete floors or reaching 50-100× farther in rural areas.

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1130.6 Knowledge Check

Test your understanding of NB-IoT coverage enhancement:

NoteQuestion: Coverage Enhancement Trade-offs

You’re deploying NB-IoT water meters in a high-rise apartment building (15 floors). Meters are installed in: - Basement (3 floors underground): 50 meters - Ground to 5th floor: 150 meters - 6th to 15th floor: 200 meters

Your carrier reports NB-IoT signal quality: - Basement: -130 dBm (extreme coverage required) - Ground-5th: -100 dBm (normal coverage) - 6th-15th: -85 dBm (excellent coverage)

Question: Which strategy best balances coverage and battery life without changing building infrastructure?

Explanation: B is the default best practice: it adapts repetitions per device based on signal quality so well-covered meters aren’t penalized. Small cells can help in extreme basements, but that’s an infrastructure decision.

Answer & Detailed Explanation

Correct Answer: B) Use Adaptive Coverage Enhancement (network-controlled repetitions)

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1130.6.1 Understanding NB-IoT Coverage Classes

NB-IoT defines three coverage classes based on signal quality:

Coverage Class	Signal Quality (RSRP)	Repetitions	Message Time	Use Case
Normal	> -108 dBm	1-4×	2-5 seconds	Outdoor, good signal
Extended	-108 to -128 dBm	8-128×	10-120 seconds	Indoor, moderate penetration
Extreme	< -128 dBm	256-2048×	3-30 minutes	Deep basement, underground

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1130.6.2 Why Adaptive Coverage Enhancement is Optimal

1. Battery Life Impact by Coverage Class

Basement meters (Extreme coverage: -130 dBm)

Required repetitions: ~512× (for -130 dBm)

Message transmission time:
- Single transmission: 2 seconds
- With 512 repetitions: 512 × 2s = 1,024 seconds (17 minutes!)

Battery consumption per reading:
- Transmit time: 17 minutes
- Current: 200 mA (TX mode)
- Energy: 1,024s × 200mA = 56.9 mAh per reading

Daily readings (once per day):
- Total: 56.9 mAh/day
- Plus PSM sleep: 0.12 mAh/day
→ 57 mAh/day total

Battery life: 10,000 mAh ÷ 57 = 175 days = **0.5 years** ❌

Ground-5th floor meters (Normal coverage: -100 dBm)

Required repetitions: 2-4×

Message transmission time: 4-8 seconds

Battery consumption per reading:
- Transmit time: 6 seconds (average)
- Energy: 6s × 200mA = 0.33 mAh per reading

Daily: 0.33 + 0.12 (PSM) = 0.45 mAh/day

Battery life: 10,000 ÷ 0.45 = 22,222 days = **60 years**! ✅

6th-15th floor meters (Excellent coverage: -85 dBm)

Required repetitions: 1-2×

Message transmission time: 2-4 seconds

Battery consumption: 0.22 mAh/day (transmit) + 0.12 (PSM) = 0.34 mAh/day

Battery life: 10,000 ÷ 0.34 = 29,411 days = **80 years**! ✅

Key insight: Battery life varies 120× between basement and upper floors due to coverage differences!

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1130.6.3 Comparison of Strategies

Option A: Maximum repetitions for all meters (2048×)

Problem: Forces ALL meters to use extreme coverage mode

Battery life:
- All 400 meters: 0.5 years (unusable!)

Cost over 10 years:
- Battery replacements: 400 meters × 20 replacements × $50 = **$400,000** ❌

Why this fails:
- 87.5% of meters (350/400) have good signal but forced to waste energy
- Repetitions configured statically, can't adapt

Option B: Adaptive Coverage Enhancement ✅

Network dynamically assigns repetitions based on signal quality:

- 50 basement meters: 512× repetitions → 0.5 year battery → replace every 6 months
- 150 ground-5th meters: 4× repetitions → 60 year battery → never replace
- 200 upper floor meters: 2× repetitions → 80 year battery → never replace

Cost over 10 years:
- Basement replacements: 50 × 20 × $50 = **$50,000**
- Other floors: $0
Total: **$50,000** (87.5% cost reduction vs Option A!)

How network adapts:
1. Device reports signal quality (RSRP) during attach
2. eNodeB assigns appropriate coverage class
3. If signal degrades, network automatically increases repetitions
4. If signal improves, reduces repetitions → saves battery

Benefit: Each meter uses MINIMUM repetitions needed for reliable delivery

Option C: Deploy indoor small cell in basement

Equipment cost:
- NB-IoT small cell (pico eNodeB): $5,000-10,000
- Installation: $2,000
- Backhaul (fiber/ethernet): $1,000
- Monthly connectivity: $50/month × 12 × 10 years = $6,000
Total 10-year cost: **$14,000-19,000**

Benefit:
- Basement meters now have excellent coverage (-85 dBm)
- Battery life: 80 years (no replacements needed)

Cost comparison vs Adaptive Enhancement:
- Small cell: $14,000-19,000 upfront
- Adaptive + replacements: $50,000 over 10 years

Winner: Small cell is cheaper if you have 50+ basement meters!

BUT: Requires building owner permission, installation complexity, ongoing maintenance

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1130.6.4 How Adaptive Coverage Enhancement Works

Network-side algorithm:

// Simplified coverage class assignment (eNodeB logic)

float rsrp = measure_rsrp_from_device();  // Signal quality in dBm

if (rsrp >= -108) {
    coverage_class = NORMAL;
    repetitions = 2;  // Minimal repetitions
    notify_device("Use normal coverage mode");
}
else if (rsrp >= -128) {
    coverage_class = EXTENDED;
    repetitions = calculate_repetitions(rsrp);  // 8-128 based on rsrp
    notify_device("Use extended coverage mode");
}
else {  // rsrp < -128
    coverage_class = EXTREME;
    repetitions = calculate_repetitions(rsrp);  // 256-2048 based on rsrp
    notify_device("Use extreme coverage mode");
    log_warning("Device in extreme coverage - consider infrastructure improvement");
}

// Device reports RSRP during attach
float measured_rsrp = measure_rsrp_from_cell();

// Network assigns appropriate repetitions
int uplink_reps = calculateRepetitions(measured_rsrp);
configure_npusch_repetitions(uplink_reps);

// Re-assess periodically (every TAU or when coverage changes)
schedule_rsrp_update(TAU_period);

Device behavior: - Reports signal quality during attach and TAU - Follows network’s repetition instructions - Automatically adapts to coverage changes (e.g., neighboring cell added)