PSM vs eDRX: Power Saving Differences in GSM, LTE-M, and NB-IoT

This article compares Power Saving Mode (PSM) and extended Discontinuous Reception (eDRX), highlighting the differences between them when used for power saving in EC-GSM, LTE-M, and NB-IoT technologies. It also discusses power consumption and battery life evaluation in LTE-M and NB-IoT systems.

Introduction

PSM and eDRX were introduced in 3GPP Release 12 as solutions to optimize device power consumption. We’ll examine these modes in the context of GSM/EDGE, LTE-M, and LTE-NB.

PSM (Power Saving Mode)

• In PSM mode, a device enters a power-saving state, reducing power consumption to the absolute minimum.

• While in this state, the mobile device does not monitor paging channels. Consequently, it becomes unreachable for Mobile Terminated Call (MTC) services. This mode goes beyond the typical IDLE mode, where the mobile performs tasks such as monitoring neighboring cells and maintaining reachability.

• The device exits PSM when triggered by upper layers for Mobile Originated (MO) access, such as uplink data transfer, Tracking Area Update (TAU), or Routing Area Update (RAU).

• After MO access and data transfer are complete, a device using PSM starts an “active timer.” During this period, the device remains reachable for Mobile Terminated (MT) traffic by monitoring the paging channel until the active timer expires. After the timer expires, the device re-enters PSM mode and becomes unreachable until the next MO event.

• Advantages:

  • Instead of turning the device OFF, PSM supports MT reachability via RAU or TAU.
  • In PSM mode, the device stays registered in the network and maintains connection configurations.
  • When exiting PSM due to an MO event, the device doesn’t need to first attach to the network to set up a connection. This reduces signaling overhead and optimizes power consumption.

eDRX (Extended Discontinuous Reception) Mode

• Introduced in Release 13 for both GSM and LTE.

• The fundamental principle of eDRX is to extend DRX cycles, allowing the device to remain in a power-saving state for a longer duration between “Paging Occasions.” This minimizes energy consumption.

• Advantages:

  • The device remains periodically available for MT services.
  • It is not required to first perform RAU or TAU to trigger a limited period of reachability.

Allowed eDRX cycle lengths are 20.48 seconds, 40.96 seconds, 81.92 seconds, 163.84 seconds, 327.68 seconds, 655.36 seconds, 1310.72 seconds, 2621.44 seconds, 5242.88 seconds, and 10485.76 seconds.

LTE Rel-13 specifies IDLE mode eDRX cycles ranging from 1 to 256 hyperframes. One hyperframe corresponds to 10.24 seconds, and 256 hyperframes correspond to 43.5 minutes.

The following table outlines mobile terminated (MT) reachability periodicities for GSM and LTE in different states, such as IDLE mode eDRX, connected mode eDRX, and PSM with RAU/TAU-based reachability.

• The choice between PSM and eDRX depends on the specific application.
• Analysis suggests that eDRX performs better than PSM for a given triggering interval when shorter reachability is needed.
• PSM performs better than eDRX when reachability requirements align with the trigger interval.
• Using eDRX over PSM reduces reachability periods in LTE compared to GSM/EDGE.

In LTE-M, the RRC resume procedure is considered for battery life evaluations. Packet sizes used on top of the PDCP layer for evaluations are 50 bytes or 200 bytes for uplink reports and 65 bytes for DL application acknowledgments (ACK), with an arrival rate of once every 2 hours or once every 24 hours.

LTE-NB IoT power consumption values are 500 mW (in Tx, 23 dBm), 80 mW (in Rx), 3 mW (in Light sleep), and 0.015 mW (deep sleep).

LTE-M Battery Life

LTE-M device battery life depends on parameters such as reporting interval (every 2 hours or 24 hours), Downlink (DL) packet size (65 bytes), Uplink (UL) packet size (50 bytes or 200 bytes), and Maximum Coupling Loss (MCL). Typical MCL values are 144 dB, 154 dB, and 164 dB.

For example, with a reporting interval of every 2 hours, a DL packet size of 65 bytes, a UL packet size of 50 bytes, and an MCL of 154 dB, battery life can reach approximately 13.9 years.

The following table shows battery life for an LTE-M device under different parameter configurations.

LTE-NB or NB-IoT Battery Life

LTE-NB IoT battery life evaluation depends on parameters such as message type, size, and arrival rate. The UL report size can be either 200 bytes or 50 bytes, while the DL application ACK size is about 65 bytes. The arrival rate is the same as LTE-M: once every 2 hours or once every 24 hours.

The following table mentions battery life for LTE-NB IoT device for various values of above parameters in stand-alone (“S”) , guard band (“G”) and in-band (“I”).

Like LTE-M, the packet flow of NB-IoT is used during battery life evaluation.