1154  Cellular IoT Power Optimization

1154.1 Learning Objectives

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

  • Understand Power Save Mode (PSM) and configure T3412/T3324 timers
  • Configure extended Discontinuous Reception (eDRX) for power/latency balance
  • Analyze cellular signaling overhead and its impact on battery life
  • Optimize radio state machine behavior for minimum power consumption
  • Apply Time-Dependent Pricing strategies for cost optimization

1154.2 Prerequisites

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

1154.3 Power Consumption Fundamentals

NoteKey Takeaway

In one sentence: Battery life in cellular IoT is dominated by idle power consumption, not transmission power, so PSM and eDRX enable 10+ year battery life by reducing sleep current from 15 mA to 5-10 µA.

Remember this: PSM = unreachable deep sleep (5 µA); eDRX = light sleep with periodic wake (15 µA to 1 mA).

⏱️ ~18 min | ⭐⭐⭐ Advanced | 📋 P09.C18.U04

Battery life is critical for cellular IoT. Power Save Mode (PSM) and extended Discontinuous Reception (eDRX) are key technologies that enable 10+ year battery life.

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stateDiagram-v2
    [*] --> Idle: Device powers on

    Idle --> Connected: Data to transmit
    Idle: 15 mA current<br/>Listening for pages<br/>High power drain

    Connected --> Active: Transmitting
    Connected: Radio synchronized<br/>10-20 mA

    Active --> Connected: TX complete
    Active: 200-300 mA<br/>Sending/receiving data<br/>10 seconds typical

    Connected --> eDRX: Enable eDRX
    eDRX --> Idle: Wake for paging
    eDRX: 15 µA sleep<br/>Wake every 2.56-43 minutes<br/>Check for messages<br/>Still registered

    Idle --> PSM: Enable PSM
    PSM --> Idle: TAU timer expires
    PSM: Deep Sleep State<br/>5-10 µA current<br/>Sleep 1-24+ hours<br/>Remain registered<br/>No paging listening<br/>✓ 10+ year battery life

Figure 1154.1: Cellular IoT Power States: Idle, eDRX, and PSM Deep Sleep Modes

{fig-alt=“State diagram showing cellular IoT power modes and transitions. Device starts in Idle state (orange) consuming 15 mA while listening for pages. Transitions to Connected state (orange) at 10-20 mA when data needs transmission. Enters Active state (orange) at 200-300 mA for actual transmission lasting typically 10 seconds. From Idle or Connected, device can enable eDRX mode (teal) reducing current to 15 microamps, waking every 2.56 to 43 minutes to check for messages while remaining registered to network. Alternatively, device enters PSM deep sleep mode (navy) consuming only 5-10 microamps for 1 to 24+ hours between wake cycles, remaining registered but not listening for pages. PSM enables 10+ year battery life by eliminating idle power consumption that dominates power budget. eDRX provides balance between responsiveness and power savings. Diagram shows power consumption decreasing from Active (300 mA) to Idle (15 mA) to eDRX (15 µA) to PSM (5 µA), demonstrating 1500× to 60,000× power reduction.”}

1154.4 Power Save Mode (PSM)

PSM enables devices to enter deep sleep while remaining registered to the network, achieving battery life of 10+ years for IoT applications.

1154.4.1 PSM Timer Configuration

Two timers control PSM behavior:

Timer Name Range Purpose
T3412 TAU (Tracking Area Update) 1 hour - 413 days How long device can sleep before re-registering
T3324 Active Timer 0 - 186 minutes How long device stays awake after transmission 

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sequenceDiagram
    participant DEV as IoT Device
    participant NET as Cellular Network

    Note over DEV,NET: PSM Configuration
    DEV->>NET: Request PSM<br/>T3412=24h, T3324=30s
    NET->>DEV: Accept PSM<br/>(may adjust values)

    Note over DEV: Data Transmission
    DEV->>NET: Send sensor data
    NET->>DEV: Acknowledge

    Note over DEV: T3324 Active Period (30s)
    DEV->>DEV: Stay awake for downlink
    DEV->>DEV: Listen for network commands

    Note over DEV: T3324 Expires
    DEV->>DEV: Enter PSM<br/>Deep sleep (5 µA)

    Note over DEV: PSM Sleep (up to 24h)
    DEV->>DEV: Radio OFF<br/>Unreachable<br/>Timer running

    Note over DEV: Wake Event (sensor trigger or T3412)
    DEV->>NET: TAU (if T3412 expired)<br/>or Data transmission

Figure 1154.2: PSM Timer Operation: T3412 and T3324 Interaction

{fig-alt=“Sequence diagram showing PSM timer operation between IoT device and cellular network. Device requests PSM with T3412=24 hours and T3324=30 seconds. Network acknowledges (may adjust values). After data transmission, device enters T3324 active period for 30 seconds, staying awake for potential downlink. When T3324 expires, device enters PSM deep sleep at 5 microamps with radio OFF and unreachable. Device remains in PSM up to 24 hours (T3412 period). Wake occurs on sensor trigger or T3412 expiry, triggering TAU (Tracking Area Update) if timer expired, or direct data transmission otherwise.”}

1154.4.2 PSM Configuration AT Commands

# Enable PSM with specific timers
AT+CPSMS=1,"","","01000011","00000101"
           |         |        |
           |         |        +-- T3324 (Active Timer)
           |         +----------- T3412 (TAU Timer)
           +--------------------- Enable PSM

T3412 encoding (Extended T3412):
  Bits 5-7: Timer unit
    000 = 10 minutes
    001 = 1 hour
    010 = 10 hours
    011 = 2 seconds
    100 = 30 seconds
    101 = 1 minute
    110 = 320 hours
    111 = Deactivated

  Bits 0-4: Timer value (0-31)

Example: "01000011" = 001 (1 hour) + 00011 (3) = 3 hours

T3324 encoding:
  Bits 5-7: Timer unit
    000 = 2 seconds
    001 = 1 minute
    010 = 6 minutes

  Bits 0-4: Timer value (0-31)

Example: "00000101" = 000 (2 seconds) + 00101 (5) = 10 seconds
CautionPitfall: Enabling PSM Without Configuring T3324 Active Timer

The Mistake: Developers enable PSM with AT+CPSMS=1 but leave T3324 at the network default (often 0 or very short). The device enters PSM immediately after transmission, and firmware updates or configuration commands never arrive.

Why It Happens: T3324 (Active Timer) defines how long the device stays in RRC_IDLE state (listening for downlink) before entering PSM deep sleep. If T3324=0, the device goes directly from transmission to PSM with zero downlink window. The AT command format AT+CPSMS=1,"","","TAU","ACTIVE" requires explicit T3324 configuration.

The Fix: Always set T3324 to at least 10-60 seconds to allow downlink reception:

AT+CPSMS=1,"","","01000011","00000101"
                  ^^^^^^^^  ^^^^^^^^
                  T3412=24h T3324=30s

T3324 encoding (rightmost 5 bits): 00001=2s, 00010=4s, 00101=10s, 01010=20s, 11111=60s. T3324 multiplier (bits 6-8): 000=2s units, 001=1min units. Example: 00000101 = 5x2s = 10 seconds active window. Power cost: 30s @ 10mA idle = 0.083 mAh per wake - negligible compared to attach cost (0.3 mAh) but enables reliable downlink.

1154.5 Extended Discontinuous Reception (eDRX)

eDRX provides a middle ground between always-on connectivity and PSM deep sleep:

Mode Sleep Current Reachability Latency Best For
Always Connected 15 mA Instant 10-100 ms Real-time applications
eDRX 15 µA - 1 mA Periodic 2.56s - 43 min Moderate latency tolerance
PSM 5-10 µA On wake only Hours - days Delay-tolerant sensors

1154.5.1 eDRX Configuration

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graph LR
    subgraph "eDRX Cycle"
        SLEEP["Sleep Period<br/>(Radio OFF)<br/>2.56s - 43 min"]
        PTW["Paging Time Window<br/>(Radio ON)<br/>1.28s - 20.48s"]
    end

    SLEEP -->|"Cycle repeats"| PTW
    PTW -->|"No downlink"| SLEEP
    PTW -->|"Downlink received"| DATA["Data Transfer"]
    DATA --> SLEEP

    style SLEEP fill:#16A085,stroke:#2C3E50,color:#fff
    style PTW fill:#E67E22,stroke:#2C3E50,color:#fff
    style DATA fill:#2C3E50,stroke:#16A085,color:#fff

Figure 1154.3: eDRX Cycle: Sleep Period and Paging Time Window

{fig-alt=“Diagram showing eDRX (extended Discontinuous Reception) cycle operation. Sleep period (teal) with radio OFF lasting 2.56 seconds to 43 minutes, followed by Paging Time Window PTW (orange) with radio ON lasting 1.28 to 20.48 seconds. During PTW, device checks for downlink messages. If no downlink, returns to sleep. If downlink received, enters data transfer (navy) then returns to sleep. Cycle repeats continuously, providing balance between power savings and reachability.”}

CautionPitfall: Using LTE-M eDRX Values for NB-IoT (and Vice Versa)

The Mistake: Developers copy eDRX configuration from LTE-M documentation to NB-IoT devices (or vice versa), not realizing the eDRX cycle ranges differ significantly. Result: either the command fails silently, or the device uses an unexpected cycle length.

Why It Happens: AT+CEDRXS uses the same command format but different AcT-type values and different eDRX cycle encodings: - LTE-M (AcT-type 4): eDRX cycles from 5.12s to 2621.44s (43.7 minutes) - NB-IoT (AcT-type 5): eDRX cycles from 20.48s to 10485.76s (2.91 hours) The same 4-bit eDRX value means different cycle lengths depending on technology!

The Fix: Use correct AcT-type and verify cycle encoding:

LTE-M:   AT+CEDRXS=2,4,"0101"  → 81.92s (1.36 min)
NB-IoT:  AT+CEDRXS=2,5,"0101"  → 327.68s (5.46 min)

NB-IoT eDRX values: 0000=5.12s, 0001=10.24s, 0010=20.48s, 0011=40.96s, 0100=61.44s (1 min), 0101=81.92s, … 1111=10485.76s (2.91 hours). LTE-M values differ! Always verify with AT+CEDRXRDP to read the network-assigned eDRX parameters after configuration.

1154.6 Radio State Machine Optimization

1154.6.1 3G vs LTE Radio State Machines

Comparison of 3G UMTS and LTE radio state machines showing power consumption and state transitions

Graph diagram
Figure 1154.4

3G UMTS Complexity: - Four states with gradual power reduction: Cell_DCH (continuous) → Cell_FACH (shared) → PCH (paging) → Idle - Multiple transitions = higher signaling overhead (each transition requires network messaging) - Longer state hold times: Device stays in high-power states longer due to intermediate FACH and PCH states - Signaling cost: Each state transition consumes network resources and battery

LTE Simplification: - Two states only: ECM_CONNECTED (active) → ECM_IDLE (sleep) - Direct transition: No intermediate states, faster power reduction - Lower signaling overhead: 60% fewer state transitions compared to 3G - Better for IoT: Simpler state machine means lower complexity and cost

Impact on IoT: - 3G IoT devices: Higher signaling overhead = 30-40% more expensive data plans - LTE-M/NB-IoT: Simplified state machine + PSM/eDRX = 10+ year battery life - Migration urgency: 2G/3G sunset forces move to LTE-based technologies

1154.7 Signaling Optimization Strategies

Three-layer signaling optimization showing network, OS, and application design strategies

Flowchart diagram
Figure 1154.5

1154.7.1 Layer 1: Network Design - Adaptive DRX

Discontinuous Reception (DRX) cycles determine how often devices wake to check for paging messages. Adaptive DRX adjusts cycles based on traffic patterns:

  • Aggressive DRX (1.28s cycle): High-traffic devices, real-time applications, ↑ power but ↓ latency
  • Moderate DRX (2.56s cycle): Balanced for typical IoT (periodic sensor reports)
  • Conservative DRX (10.24s cycle): Low-traffic devices, delay-tolerant, ↓ power but ↑ latency

Signaling reduction: 30% fewer paging messages by matching DRX to actual device activity patterns

1154.7.2 Layer 2: OS Design - Push Notification Management

Operating system controls app wake-ups and radio usage:

  • Batch notifications: Coalesce multiple app notifications into single wake-up event
  • Alignment: Schedule transmissions at DRX cycle boundaries to avoid extra wake-ups
  • Quotas: Limit background app network access to prevent signaling storms

Example: Android Doze mode, iOS Low Power Mode reduce signaling by 40-60% through aggressive batching

1154.7.3 Layer 3: Application Design - Traffic Optimization

Application Resource Optimizer (ARO) analyzes cellular traffic patterns to identify inefficiencies:

Traffic Analysis: - Identifies “chattiness”: Apps sending data every 30s instead of batching hourly - Detects tail states: Radio stays active 5-10s after transmission, wasting power if next transmission arrives during tail - Recommends batching: Send 5 hourly transmissions as 1 batch every 5 hours (5x reduction)

Alignment Strategies: - Good: Transmit at 0, 15, 30, 45 minutes (aligned to typical 15-minute DRX cycles) - Bad: Transmit at 7, 22, 37, 52 minutes (misaligned, forces extra wake-ups)

1154.7.4 RACH Overload Reduction

Random Access Channel (RACH) handles connection setup. Massive IoT deployments can overwhelm RACH capacity:

  • Strict Separation: Dedicate RACH resources exclusively for IoT (prevents smartphone signaling from blocking IoT access)
  • Soft Separation: Priority classes where IoT gets guaranteed minimum RACH resources
  • Hybrid Separation: Dynamic allocation adjusting IoT vs. smartphone RACH split based on real-time demand

Real-World Impact: Smart meter deployments with 100,000+ devices require RACH optimization to prevent “signaling storms” at peak hours (e.g., all meters reporting at midnight)

1154.8 Time-Dependent Pricing (TDP)

Time-dependent pricing research showing networking, economic, and HCI perspectives

Flowchart diagram
Figure 1154.6

1154.8.1 Research Context

Time-Dependent Pricing (TDP) for cellular IoT explores how dynamic pricing can shape network demand, reduce costs, and optimize resource utilization.

1154.8.2 Networking Perspective

How can pricing algorithms balance demand with network capacity?

Approach: - Implement time-of-day pricing: Peak hours (2x base rate), off-peak hours (0.5x base rate) - Monitor network utilization before and after TDP implementation - Measure peak demand reduction and capacity freed for latency-sensitive IoT

Results: - 30% peak reduction: Delay-tolerant IoT (smart meters, environmental sensors) shifted to off-peak hours - Capacity gains: Freed 30% of peak capacity for real-time IoT (autonomous vehicles, industrial control) - Network stability: Reduced congestion-related packet loss from 2% to 0.3%

1154.8.3 Economic Perspective

What pricing structures maximize revenue while maintaining user adoption?

Price Elasticity Findings: - 2x price increase → 30% demand decrease (elastic demand for delay-tolerant IoT) - 0.5x price discount → 20% demand increase (moderate response, users have limited control over IoT device schedules)

Unexpected “Sales Day Effect”: - When carriers announced promotional pricing days (24-hour windows with 0.3x rate), usage surged 130% - Users scheduled firmware updates, bulk data uploads, and non-urgent transmissions for promotional windows - Lesson: Predictable pricing promotions can create new demand peaks

Revenue Impact: - TDP revenue initially flat (peak reduction offset by off-peak discount) - Long-term revenue +8% due to higher overall data consumption (users more willing to deploy IoT knowing costs are controllable)

1154.8.4 Combined Results

Metric Before TDP After TDP (1 year) Change
Peak demand 100% baseline 70% baseline -30%
Off-peak demand 40% baseline 52% baseline +30%
Total data usage 100% baseline 108% baseline +8%
User satisfaction 3.2/5 3.8/5 +19%
Network CapEx $1M/year $700K/year -30%

1154.8.5 Strategic Lessons for IoT

  1. TDP enables demand shaping: 30% peak reduction validates dynamic pricing as network management tool
  2. Promotion timing matters: “Sales day” surges show users will respond, but unpredictable spikes can negate benefits
  3. Developer tools essential: Auto-scheduling APIs increase TDP effectiveness by 40% (users lack time/expertise to manually optimize)
  4. Savings threshold: 10-20% cost reduction needed to drive behavior change
  5. Industrial IoT best fit: Automated systems can optimize transmission schedules without human intervention

Time-Dependent Pricing connects to broader IoT monetization strategies. For business model frameworks and pricing psychology, see:

TDP demonstrates that technical optimization (signaling reduction) directly impacts business economics (30% lower data costs). Understanding both layers enables effective IoT solution design.

1154.9 Summary

  • Power-saving modes (PSM and eDRX) reduce sleep current from 15 mA to 5-15 µA, enabling decade-long battery life through deep sleep with periodic network registration
  • PSM provides deepest sleep (5 µA) with T3412 (TAU) timer controlling sleep duration and T3324 (Active) timer controlling downlink window
  • eDRX balances reachability and power with configurable cycles from 2.56 seconds to 43 minutes for LTE-M, or up to 2.91 hours for NB-IoT
  • Signaling optimization through adaptive DRX, notification batching, and RACH management reduces network overhead by 30-60%
  • Time-Dependent Pricing enables 30% peak demand reduction and 8% total cost savings through dynamic pricing strategies

1154.10 What’s Next

Continue your cellular IoT optimization journey: