Page 286 - From GMS to LTE

272 From GSM to LTE-Advanced Pro and 5G

● Start of interfrequency and inter‐RAT (Radio Access Technology) search in SIB
3. Defines the signal quality level of the serving cell at which the mobile device should,
in addition, start looking for neighboring cells on other LTE frequencies and cells of
other RATs such as GSM, UMTS and CDMA. Usually, this is set at a somewhat lower
value than the intrafrequency search value since finding an LTE cell is preferred.
Neighbor cell information in SIB 4–8. These System Information messages contain
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further details about neighboring cells on the same frequency and on other frequen-
cies, and other RAT cells. Table 4.4 at the beginning of the chapter contains additional
details. SIB 4 with intra‐cell neighbor information is optional. If not present, the
mobile device performs a blind search.

4.7.3 Mobility Management and State Changes in Practice

In practice, many factors influence how network operators configure the air interface
connection to a mobile device and when reconfigurations take place. On the one hand
the mobile being in a fully connected state without DRX results in the fastest response
times and generates no signaling overhead between the base stations and the core
network. On the other hand, being in a connected state even when no data is transferred
is inefficient on the mobile side as observing the downlink control channels and con-
tinuously transmitting control information in the uplink requires significant mobile
power, thus draining the battery quickly. The disadvantage on the network side is the
reduced capacity in the uplink direction due to many devices transmitting control
information in parallel. A compromise therefore has to be found for how long a mobile
device is in fully connected state before it enters connected DRX and how long it takes
afterward before the network sets the mobile device into the idle state. The following
examples show how networks are typically configured in practice today:
Network 1:
Time until DRX is enabled: 100 ms;
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DRX short cycle time: 80 ms;
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DRX long cycle time: 200 ms;
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On‐duration: 10 ms;
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Time alignment: 10.2 seconds;
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Time until idle: –.
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Network 2:
Time until DRX is enabled: 200 ms;
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DRX short cycle time: 40 ms;
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DRX long cycle time: 320 ms;
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On‐duration: 10 ms;
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Time alignment: infinity;
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Time until idle: –.
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The two networks above are configured very similarly. DRX mode is entered very
quickly after only a fraction of a second and devices have to listen to downlink assignments
for 10 milliseconds during each cycle. Both networks have very long time‐alignment
timers, the first one over 10 seconds and the other one has even set it to infinity. During
those times the mobile has to keep transmitting status and measurement information in
the uplink direction so power saving is significantly reduced.