1133  NB-IoT Power Optimization

PSM, eDRX, and Battery Life Engineering

Prerequisites: NB-IoT Architecture | NB-IoT Applications

This enables: NB-IoT Practical Guide | NB-IoT Lab Simulation

1133.1 Learning Objectives

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

• Configure PSM timers: Set T3412 and T3324 values for optimal battery life
• Design eDRX cycles: Balance power savings with downlink responsiveness
• Select power modes: Choose between PSM-only, eDRX-only, and combined modes
• Calculate battery life: Account for sleep current, TX duration, and CE repetitions

1133.2 Power Saving Mode (PSM) Deep Dive

PSM is the foundation of NB-IoT’s 10+ year battery life promise. When in PSM, the device’s radio is completely off, reducing current consumption to 3-10 uA.

1133.2.1 PSM Timer Configuration

Two timers control PSM behavior:

T3412 (TAU Timer - Extended Periodic Tracking Area Update)

• Determines how long the device can sleep while remaining registered
• Range: 2 seconds to 310 hours (configurable in 3GPP Release 13+)
• Device must send TAU before T3412 expires or be de-registered
• Typical values: 24-72 hours for smart metering

T3324 (Active Timer)

• Determines how long device stays in active mode after data transfer
• Range: 2 seconds to 186 minutes
• Device enters PSM when T3324 expires
• Typical values: 10-60 seconds (just long enough for ACK)

Mermaid diagram

Figure 1133.1: NB-IoT PSM Mode 24-Hour Sleep Timeline with T3412/T3324 Timers

1133.2.2 PSM Power Consumption

State	Current	Duration	Notes
PSM Sleep	3-10 uA	Hours/days	Radio completely off
Waking	50 mA	~100 ms	Restoring context
RRC Connected	40-80 mA	0.5-2 s	Signaling exchange
TX Active	200-300 mA	0.2-2 s	Data transmission

1133.2.3 PSM Limitations

Important: During PSM, the device is completely unreachable.

• Network cannot page the device
• Downlink messages are queued until device wakes
• Emergency commands cannot be delivered
• Firmware updates must wait for scheduled wake-up

This is why PSM is ideal for uplink-only applications (metering, environmental monitoring) but problematic for applications requiring downlink responsiveness.

1133.3 Extended Discontinuous Reception (eDRX)

eDRX provides a middle ground between always-connected (high power) and PSM (unreachable). The device periodically wakes to check for paging messages.

1133.3.1 eDRX Configuration

eDRX Cycle: How long between paging windows

• Range: 5.12 seconds to 2.91 hours (NB-IoT specific)
• Longer cycles = better battery life, slower downlink response

Paging Time Window (PTW): How long device listens during each cycle

• Range: 2.56 seconds to 40.96 seconds
• Longer PTW = better downlink reception, higher power

1133.3.2 eDRX Power Consumption

eDRX Cycle (20.48 seconds example):

|----Sleep period-----|--PTW--|----Sleep...
|     (0.5 mA avg)    | 2.5s  |
|                     |       |
|<--- 20.48s -------->|       |
|                     |       |
|  Cannot receive     | Check |
|  downlink           | page  |

Average current during eDRX:

Sleep current: 0.5 mA (light sleep, maintaining timing)
Active during PTW: 40 mA
PTW duration: 2.56 seconds every 20.48 seconds

Average current = (17.92s x 0.5mA + 2.56s x 40mA) / 20.48s
                = (8.96 + 102.4) / 20.48
                = 5.4 mA average

Compare to:
- PSM: 0.003 mA (1800x lower!)
- Always connected: 40 mA (7x higher)

1133.4 PSM vs eDRX: Mode Selection

Flowchart diagram

Figure 1133.2: Comparison of PSM and eDRX power modes showing key characteristics and trade-offs

1133.4.1 Mode Selection Guide

Use Case	Uplink Frequency	Downlink Needed?	Recommended Mode
Smart meter	Daily	Rare (monthly firmware)	PSM + scheduled wake
Asset tracker	Hourly	Yes (change reporting interval)	eDRX (wake every 20 min)
Smart parking	Continuous (occupancy change)	No	PSM (wake on sensor trigger)
Remote control	Low	Yes (real-time commands)	eDRX or RRC Connected

1133.5 Worked Example: PSM vs eDRX for Fleet Tracker

Scenario: A fleet management company deploys NB-IoT trackers in delivery vehicles. The vehicles operate 10 hours per day (7 AM - 5 PM) with varying requirements.

Given:

• Vehicle operation: 10 hours/day active, 14 hours/day parked
• Active tracking: Location every 5 minutes
• Parked mode: Location every 2 hours + instant wake on motion
• Downlink requirements:
  • During operation: Route updates every 30 minutes
  • Parked: Theft alerts (immediate delivery required)
• Battery: 10,000 mAh (replaceable annually)
• Target battery life: 12 months minimum

1133.5.1 Design: Mode Transitions

Mermaid diagram

Figure 1133.3: Vehicle tracker state machine showing transitions between PARKED (PSM), DRIVING (eDRX), and THEFT ALERT (Connected) modes

1133.5.2 Calculation: DRIVING Mode Power

Active hours: 10 hours/day = 36,000 seconds

Position updates:
- Frequency: Every 5 minutes = 120 per day (active)
- GPS fix: 1 second at 30 mA = 30 mAs
- NB-IoT TX: 0.5 second at 200 mA = 100 mAs
- Total per update: 130 mAs

eDRX listening:
- eDRX cycle: 20.48 seconds
- Paging windows per hour: 176
- Listen duration: 10 ms at 40 mA = 0.4 mAs each
- Per hour: 176 x 0.4 = 70.4 mAs
- 10 hours: 704 mAs

Sleep between eDRX:
- Duration: ~35,802 seconds
- Current: 10 uA
- Energy: 358 mAs

Daily DRIVING consumption:
- Position TX: 120 x 130 = 15,600 mAs = 4.33 mAh
- eDRX listening: 704 mAs = 0.20 mAh
- Sleep: 358 mAs = 0.10 mAh
- Total DRIVING: 4.63 mAh/day

1133.5.3 Calculation: PARKED Mode Power

Parked hours: 14 hours/day = 50,400 seconds

Position updates (every 2 hours):
- Updates: 7 per day (parked period)
- GPS + TX: 130 mAs each
- Total: 910 mAs = 0.25 mAh

PSM sleep:
- Duration: ~50,390 seconds
- Current: 3 uA
- Energy: 151 mAs = 0.04 mAh

Accelerometer (always-on for theft detection):
- Current: 5 uA continuous
- 24 hours: 432 mAs = 0.12 mAh

Daily PARKED consumption:
- Position TX: 0.25 mAh
- PSM sleep: 0.04 mAh
- Accelerometer: 0.12 mAh
- Total PARKED: 0.41 mAh/day

1133.5.4 Calculation: Total Battery Life

Total daily: 4.63 + 0.41 = 5.04 mAh

With 10,000 mAh battery:
Theoretical life = 10,000 / 5.04 = 1,984 days = 5.4 years

Apply derating:
- Usable capacity (80%): 8,000 mAh
- Temperature factor (90%): 7,200 mAh
- Aging over 1 year (85%): 6,120 mAh

Practical life = 6,120 / 5.04 = 1,214 days = 3.3 years

RESULT: 12-month target easily met with 2+ years margin!

Flowchart diagram

Figure 1133.4: NB-IoT theft alert optimization: accelerometer-triggered wake achieves less than 5-second alert latency vs 2-hour delay with pure PSM, with minimal battery impact

Result: Use hybrid PSM/eDRX mode with accelerometer wake. During driving (7 AM - 5 PM): eDRX with 20.48s cycle for route updates. While parked (5 PM - 7 AM): PSM with 2-hour T3412, accelerometer-triggered immediate wake for theft alerts. This achieves 5-minute tracking during operation, <5 second theft alerts when parked, and 3.3-year battery life.

Key Insight: PSM and eDRX are not mutually exclusive - design a state machine that transitions between modes based on operational context. The accelerometer is critical - it enables instant theft detection while allowing PSM’s ultra-low power sleep.

1133.6 Knowledge Check

Question: An NB-IoT device is configured with PSM timer T3412 = 24 hours and Active Timer T3324 = 10 seconds. The device sends one uplink message per day. What is the expected behavior when the cloud server needs to send a firmware update to this device?

Explanation: PSM creates long unreachable periods - the server must wait or trigger wake-up:

PSM Timeline:

1. Device sends daily reading
2. T3324 (10 seconds) starts - device reachable
3. T3324 expires - device enters PSM (unreachable)
4. Device sleeps for up to 24 hours
5. Only wakes when: sensor event, RTC timer, or T3412 expiry

Server downlink options:

1. Wait for next wake - up to 24 hours
2. Device triggering via SCEF - network pages device (if supported)
3. Schedule updates - know when device wakes

Why other options are wrong:

• A: PSM specifically disables always-on connectivity
• C: eNodeB doesn’t buffer user data for hours
• D: Question states PSM, not eDRX mode

NoteQuestion: Power Saving Modes - PSM vs eDRX

You’re deploying 50,000 smart water meters using NB-IoT. Meters report water usage once per day at midnight. Utility company wants to occasionally send firmware update commands to meters (approximately once per month).

Which power-saving mode should you use?

1. PSM only (deep sleep, wake up to send data, no downlink listening)
2. eDRX only (periodic wake-ups to check for downlink messages)
3. PSM + eDRX combination (deep sleep, but brief wake-ups for downlink)
4. No power saving (always connected for instant downlink)

Answer and Detailed Explanation

Correct Answer: C) PSM + eDRX combination

1133.6.1 Understanding the Trade-offs

Mode	Sleep Current	Downlink Capability	Battery Life	Use Case
No power saving	100mA	Instant (always listening)	20 hours	Not viable for battery devices
eDRX only	1-5uA	Periodic (every 2.91 hours max)	2-5 years	Fair compromise
PSM only	3-10uA	None (device unavailable)	10-15 years	No downlink possible
PSM + eDRX	5-15uA	Periodic during wake-ups	5-10 years	Best balance

1133.6.2 Why PSM + eDRX Combination is Optimal

Hybrid Approach:

Normal operation (29 days/month):
- PSM sleep at 10 uA
- Daily wake for meter reading
- No eDRX overhead

Firmware update month (1 day):
- Enable eDRX for 24 hours
- Device wakes every 2.91 hours to check for downlink
- Download firmware in chunks
- Return to PSM after successful update

Battery Life with Hybrid:

Normal month: 55.4 mAh
Firmware update month: 74 mAh
Annual total: 683 mAh

Battery life with 8,000 mAh battery:
8,000 / 683 = 11.7 years

1133.6.3 Why Other Options Fall Short

A) PSM Only: Cannot receive firmware updates (device unreachable) B) eDRX Only: Higher power consumption (wakes even when no downlink needed) D) No Power Saving: 20-hour battery life (completely unusable)

Real-World Example: Kamstrup Water Meters

• Default mode: PSM only (10-15 year battery life)
• Firmware update mode: Enable eDRX temporarily
• Update success rate: 99.2% (no technician visits needed)