Page 332 - From GMS to LTE

318 From GSM to LTE-Advanced Pro and 5G

For the calculation it was assumed that the device has a battery with a capacity of 5
Watt hours. This is about a third of the battery capacity that is built into a smartphone
today. The chapter further contains an assumption about how much power the device
draws in different states. In the ‘idle’ state, the state that a device is in most often, power
consumption is assumed to be 0.015 mW.
If the device was in idle state all the time, the battery could power the device for 38
years. This does not include battery self‐discharge, however. According to the Varta
handbook of primary lithium cells [38], self‐discharge of a non‐rechargeable lithium
battery is less than 1% per year and thus significantly less than the self‐discharge rate of
rechargeable batteries.
Obviously, a device is not always in idle state and when transmitting the device is
assumed to use 500 mW of power. With this power consumption, the battery would
only last 10 hours. As these two values are significantly different, the 3GPP study looked
at different transmission patterns. If 200 bytes are sent once every 2 hours, the device
would run on that 5 Wh battery for 1.7 years. If the device only transmits 50 bytes once
a day the battery would last for 18.1 years.

4.19.11 NB‐IoT – High Latency Communication
One of the main requirements of many IoT scenarios is very deep in‐house coverage
with a very low signal level. NB‐IoT addresses this issue by repeating data transfers
many times to give the receiver the opportunity to combine the signal energy received
during each transmission. The downside of this approach is that transmitting even a
small IP packet takes a significant amount of time. An interesting calculation that can
be found in an Ericsson paper on the topic shows that transferring a small UDP packet
can require up to 7 seconds under very low signal conditions where the system repeats
each individual transmission for system access, bandwidth assignment, user data trans-
fer and acknowledgement several dozen times [39].
Another main requirement of many IoT scenarios is the exchange of extremely low
device power consumption for the sending and receiving of very little data and very
long intervals in which no data is exchanged at all. If a device does not need to react
instantly to incoming requests it does not make sense to periodically activate the radio
module to check Paging messages. If, for example, it is enough to check once every half
hour for incoming IP packets, the radio module can be completely switched off for most
of this time, which saves a significant amount of energy. The downside is, of course, that
in the worst case it takes 30 minutes for a device to respond to an incoming IP packet.
To address such scenarios the 3GPP specifications were extended by a number of fea-
tures for ‘High Latency Communication’, as described below.

Extended Idle Mode Discontinuous Reception (eDRX)
When a mobile device is in idle state it has to listen on the LTE paging channel for
incoming Paging messages. These are sent when no active radio link is established and
IP packets arrive from the Internet for the device. The device then answers the paging,
a radio bearer is established and the IP packets are delivered. A typical paging interval
in LTE networks today is 1.28 seconds, i.e. the radio chip of a device has to wake up once
every 1.28 seconds and check the paging channel. While for smartphones the amount
of power required to check for incoming Paging messages once a second is negligible