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PSM vs eDRX-Difference between PSM and eDRX GSM,LTE-M,NB-IoT

This page compares PSM vs eDRX and mentions difference between PSM and eDRX used for power savings in EC-GSM, LTE-M and NB-IoT technologies. It also mentions power consumption and battery life evaluation in LTE-M and NB-IoT systems.

Introduction:
PSM and eDRX have been introduced in 3GPP Rel.12 as solutions to optimize the device power consumption. We will understand these modes with respect to various technologies such as GSM/EDGE, LTE-M and LTE-NB.

PSM (Power Saving Mode)

• In PSM mode, device enters into power saving state in which it reduces power consumption to a bare minimum.
• In the power saving state, mobile does not monitor paging. Hence it becomes unreachable for mobile terminated call (MTC) services. This mode is beyond typical IDLE mode in which mobile performs certain tasks e.g. monitors neighbour cells , maintains reachability etc.
• The device leaves PSM mode when trigger comes from upper layers for MO (Mobile Originated) access for tasks such as uplink data transfer, TAU (Tracking Area Update), RAU (Routing Area Update) etc.
• After MO access and data transfer is completed, device using PSM mode starts active timer. Here device remains reachable for MT (Mobile Terminated) traffic by monitoring paging channel until active timer expires. After the timer expires device re-enters PSM mode and hence unreachable until the next MO event.
• Advantages:
➤Instead of turning OFF the device, PSM supports MT reachability via RAU or TAU.
➤In PSM mode, device stays registered in the network and maintains connection configurations.
➤When leaving PSM mode due to MO event, it is not needed to first attach to the network in order to setup connection. This reduces signaling overhead and optimizes power consumption consecutively.

PSM Operation

eDRX (Extended Discontinuous Reception) Mode

• It has been introduced in release-13 for both GSM and LTE.
• The basic principle for eDRX is to extend DRX cycles in order to allow device to remain longer in power saving state between "Paging Occasions". This will consecutively minimize its energy consumption.
• Advantages:
➤Device remains periodically available for MT services.
➤Moreover it is not required first to perform RAU or TAU to trigger limited period of reachability.

eDRX Cycle

Allowed eDRX cycle length values are 20.48 sec, 40.96 sec, 81.92 sec, 163.84 sec, 327.68 sec, 655.36 sec, 1310.72 sec, 2621.44 sec, 5242.88 sec and 10485.76 sec.

LTE Rel-13 specifies IDLE mode eDRX cycles ranging from 1 to 256 hyperframes. 1 hyperframe corresponds to 10.24 seconds and 256 hyperframes corresponds to 43.5 minutes. Following table mentions 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.


MODE GSM LTE
IDLE eDRX 52 min, 13.44 s 43 min, 41.44 s
CONNECTED, eDRX - 10.24 s
PSM > 1 Year (RAU) > 1 Year (TAU)

• Use of PSM or eDRX depends on applications.
• As per analysis and evaluation, for given triggering interval eDRX performs better than PSM mode when shorter reachability is needed.
• PSM performs better than eDRX when reachability requirement is in same range as trigger interval.
• Using eDRX over PSM reduces reachability periods in LTE compare to GSM/EDGE.

In LTE-M RRC resume procedure is considered for battery life evaluations. Packet sizes used on top of PDCP layer for evaluations are 50 bytes or 200 bytes for uplink report and 65 bytes for DL application acknowledgment (ACK) where as arrival rate is 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 various parameters such as reporting interval (Once every 2 Hrs or 24 Hrs), Downlink (DL) packet size (65 bytes), Uplink (UL) packet size (50 bytes or 200 bytes) and MCL (Maximum Coupling Loss). Typical values of MCL are 144 dB/154 dB/ 164 dB.
For example, with reporting interval of every 2 Hrs, DL packet size of 65 bytes, UL packet size of 50 bytes, MCL of 154 dB Battery life can be about 13.9 years. Following table mentions battery life for LTE-M device for various values of above parameters.

LTE-M Battery Life

LTE-NB or NB-IoT Battery Life

LTE-NB IoT battery life evaluation depends on various parameters such as message type, its size and arrival rate. UL report size can be either 200 bytes or 50 bytes where as DL application ACK size is about 65 bytes. Arrival rate is same as LTE-M which is once every 2 Hours or once every 24 Hours. 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").

LTE-NB IoT Battery Life

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


NB-IoT Related Stuff


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