Page 285 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 271

process of moving the device through the different activity states starts from the beginning.
The following list summarizes the different activity states:

RRC connected state with an observation of the control region for assignment grants
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in every subframe.
RRC connected state with an observation of the control region for assignment grants
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in a short DRX cycle pattern. The receiver is switched off for short periods of time.
RRC connected state with an observation of the control region for assignment grants
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in a long DRX cycle pattern. The receiver is switched off for longer periods of time.
RRC connected state in DRX, time alignment expired, no uplink status transmissions.
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RRC idle state in which the mobile scans only periodically for incoming Paging messages.
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While the mobile device is in RRC idle state, it decides on its own as to when to
change the serving cell. If a new cell is in the same tracking area as the previous cell, no
interaction with the network is required. If the new cell is in a new tracking area, the
mobile device needs to perform a tracking area update. For this purpose, a temporary
RRC connection is established with the eNode‐B, which is then used to perform the
tracking area update with the MME. Once the update is finished, the RRC connection
is released and the mobile device goes back into full RRC‐idle state, only observing
incoming Paging messages at the end of each Paging interval.
While the mobile device decides on its own when to change cells without interaction
with the network, the parameters used for the decision are given to the mobile device
by the eNode‐B via System Information (SI) messages. Each eNode‐B may have a differ-
ent cell reselection configuration. When the device changes from one cell to another, it
not only has to check if the new cell is in a new tracking area but also has to read the
System Information messages and decode all messages that contain parameters for the
cell reselection mechanism. Only afterward can it go back to the idle state and monitor
the paging channel for incoming paging messages. For cell reselection, the following
parameters are important:
Cell barring status in SIB 1. If the cell is barred, the mobile device must not use it as
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its new serving cell.
A serving cell hysteresis in SIB 3. The degree by which the current cell should be
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preferred to neighboring cells (in dB).
Speed state selection in SIB 3. Depending on the speed of the mobile, that is,
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whether it is stationary or in a car, train, etc., different cell reselection parameter
settings can be defined. When the mobile is moving, the cell search mechanism
could be started while the reception level is still relatively high to prevent loss of
coverage due to fast cell changes and lack of time to make appropriate measure-
ments. When the mobile is stationary, the cell search could be started when recep-
tion levels are lower. Thresholds can be set higher to prevent unnecessary cell
changes. As neighbor cell measurements consume additional power when the
mobile device is stationary, there is a good possibility of being able to reduce the
energy consumption when reception conditions are good. It should be noted at this
point that while speed dependent cell reselection parameters seem interesting they
are not widely used in practice today.
Start of intrafrequency search in SIB 3. Defines the signal quality level of the serving
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cell at which the mobile device should start looking for neighboring cells.